High Voltage Circuit Breakers - DiVA portal

Master of Science Thesis KTH School of Industrial Engineering and Management

Energy Technology EGI_2017-0012 MSC EKV 1178 Division of Heat & Power SE-100 44 STOCKHOLM

Multi-physical Simulations of High Voltage Circuit Breakers

Julien POUILLY

Master of Science Thesis EGI_2017-0012 MSC EKV 1178

Multi-physical Simulations of High Voltage Circuit Breakers

Julien POUILLY

Approved

2017-03-30

Examiner

Miroslav Petrov - KTH/ITM/EGI

Supervisor

Miroslav Petrov Commissioner

General Electric Grid Solutions

Contact person

Cyril GREGOIRE; Nadia HASNAOUI

Abstract

This report is the outcome of a 6-month Master’s Thesis carried out at General Electric Grid Solutions in Villeurbanne within the LTDT team in the Circuit Breaker and Bay Development (CB2D) department of the Apparatus Research Center (ARC). It presents both the innovative methodology adopted and the results obtained along the project.

The Master’s Thesis deals with high voltage circuit breakers and is focused on one project, which will be called CB2 for confidentiality reasons. The aim of the project is to design a new circuit breaker, the CB2, based on an existing circuit breaker, with a cost reduction of 25% in order to adapt the device to the Indian market. To do so, numerical simulations on a software platform called MC3 were implemented. Simulations are indeed quicker and much cheaper than real tests. MC3 is a 2D axisymmetric solver, thus it is necessary to calculate 2D equivalent sections and volumes of the circuit breaker.

The first step of the project is to calibrate the MC3 tool for the original device, called CB1. Before implementing the tests on MC3, a mechanical study is led in order to study both the wear and the velocities in these different tests. The conclusion of the study asserted that the velocities are lower than expected on almost all of the tests, however, the levels of wear are sometimes critical. The calibration – comparison of the overpressure in the thermal volume – is carried out based on one parameter, the FRAC (fraction of the radiative flux involved in the ablation process); each default and arcing time is thus tested with two FRAC values: 0.8 and 1.1. The first step was invalidated due to the high wear of the circuit breaker. FRAC 1.1 shows more accurate results than FRAC 0.8. However, the pressure differentials are lower in all MC3 calculations than in real tests, which may be due to the high wear and low velocities (reduced by 3 m/s) compared to the old device.

Finally, some modifications of the CB1’s geometry are recommended based on different studies in order to improve the design of CB2. Those modifications will be tested on MC3 by the next interns at ARC.

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SAMMANFATTNING Denna rapport är resultatet av en sex-månaders examensarbete utförd vid General Electric Grid Solutions i Villeurbanne, Frankrike, inom LTDT laget av Circuit Breaker Bay Development (CB2D) avdelningen som ingår i Apparatus Research Center (ARC). Både den innovativa metoden som använts och de resultat som uppnåtts under arbetets gång presenteras härmed.

Examensarbetet behandlar högspänningsbrytare och är inriktad på ett projekt som kommer att kallas för CB2 av sekretesskäl. Syftet med projektet är att utforma en ny brytare, CB2, baserad på den befintliga brytaren CB1 med en kostnadsminskning på 25% för att anpassa enheten till den indiska marknaden. För att göra detta, numeriska simuleringar av strömbrytarens interna termomekaniska processer har genomförts på en programvara som kallas för MC3. Simuleringar är faktiskt snabbare och mycket billigare än riktiga tester. MC3 använder sig av 2D-rotationssymmetriska beräkningar, därför är det nödvändigt att avspegla flertalet 2D avsnitt i motsvarande mängd för att simulera strömbrytaren.

Det första steget i projektet är att kalibrera MC3 verktyget på den ursprungliga enheten, strömbrytaren CB1, med sina välbeprövade parametrar. Innan simuleringsprocessen för CB2 genomförs på MC3 har en mekanisk studie inletts för att studera både slitaget och hastigheterna i den nya enheten. Slutsatsen av studien tillstyrker att hastigheterna blir lägre än väntat på nästan alla drivrutiner, dock kan slitagenivåerna bli kritiska. Simulationsprocessen för CB2 – nämligen en jämförelse av övertrycket i den termiska volymen - genomförs baserat på en varierande parameter, FRAC (fraktion av strålningsflödet som påverkar slitaget genom ablationsprocessen); varje tidsberoende verkan inom enheten har beräknats med två FRAC värden: 0,8 och 1,1. Ett första steg ansågs ogiltigt på grund av den höga slitaget av effektbrytaren, således FRAC 1,1 visar mer exakta resultat än FRAC 0,8. Däremot är tryckökningen lägre i alla MC3 beräkningar än i verkliga tester, vilket kan bero på den det höga slitaget och den låga hastigheterna (som minskar med 3 m/s) jämfört med den ursprungliga strömbrytaren.

Slutligen har några specifika ändringar på CB1:s geometri föreslagits för att bättre utforma CB2, baserat på olika studier. Dessa modifikationer kommer att simuleras igen på MC3 plattformen av nästföljande praktikanter vid ARC.

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Contents 1 INTRODUCTION ............................................................................................................................. 1

1.1 Presentation of the service and of the company: .................................................................... 1

1.1.1 Presentation of the company: GE group ......................................................................... 1

1.1.2 Takeover of some Alstom entities by GE ....................................................................... 1

1.1.3 GE Grid Solutions .............................................................................................................. 2

1.1.4 The site of Villeurbanne ..................................................................................................... 3

1.2 Scientific background .................................................................................................................. 6

1.2.1 Main elements of a high voltage line ................................................................................ 6

1.2.2 Basic principle of a high voltage circuit breaker ............................................................. 7

1.2.3 Types of circuit breakers .................................................................................................... 8

1.2.4 Main parameters of a circuit breaker and typical values ................................................ 9

1.2.5 Usual architecture of a line circuit breaker and its operating principle ..................... 10

1.2.6 Opening of a circuit breaker – various steps ................................................................. 11

1.2.7 SF6: the insulating gas ...................................................................................................... 13

1.2.8 Default types and tests ...................................................................................................... 14

1.2.9 Notion of arcing time ....................................................................................................... 15

1.2.10 Case of non-clearance ....................................................................................................... 16

1.3 The MC3 software ..................................................................................................................... 17

1.3.1 Presentation ........................................................................................................................ 17

1.3.2 Self-adaptive mesh ............................................................................................................ 18

1.3.3 Calculation hypotheses in MC3 ....................................................................................... 18

1.4 Methodology .............................................................................................................................. 18

2 GEOMETRY PREPARATION AND 3D/2D MODELLING .............................................. 19

2.1 Introduction to my work, objectives and strategy ................................................................ 19

2.2 Geometry preparation ............................................................................................................... 19

2.2.1 Importation of the 3D model .......................................................................................... 20

2.2.2 Dimensions ........................................................................................................................ 20

2.2.3 2D modelling ..................................................................................................................... 20

2.2.4 Parametric study of the two valves ................................................................................. 23

3 MECHANICAL STUDY OF REAL TESTS ............................................................................... 31

3.1 First step of calibration: signal treatment ............................................................................... 33

3.2 Separation and release of the nozzle neck ............................................................................. 33

3.3 Wear study .................................................................................................................................. 34

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3.4 Velocity study ............................................................................................................................. 34

4 FIRST RESULTS AND CALIBRATION OF MC3................................................................... 38

4.1 The FRAC................................................................................................................................... 38

4.2 Comparison criteria ................................................................................................................... 38

4.3 Calibration of the MC3 tool ..................................................................................................... 38

4.3.1 T30 study ............................................................................................................................ 40

4.3.2 T100s study ........................................................................................................................ 44

4.3.3 Conclusions on the study ................................................................................................. 49

4.3.4 L75 test ............................................................................................................................... 51

4.3.5 Remark on the tests and sensors ..................................................................................... 51

4.3.6 Calibration based on the FRAC parameter ................................................................... 52

4.4 MC3 database on the CB1 ........................................................................................................ 54

5 DESIGN OF THE CB2 .................................................................................................................. 56

5.1 Optimization of the thermal volume ...................................................................................... 56

5.2 Optimization of the exhausts ................................................................................................... 58

6 REMAINING STEPS AND CHALLENGES ............................................................................ 59

CONCLUSION ......................................................................................................................................... 60

REFERENCES .......................................................................................................................................... 61

APPENDIX 1 ............................................................................................................................................ 62

T30 study ................................................................................................................................................. 63

T100s study ............................................................................................................................................. 67

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List of Figures Figure 1: GE's activities and corresponding revenues ............................................................................ 1 Figure 2: Joint-venture of GE Grid Solutions ......................................................................................... 2 Figure 3: Departments of the site of Villeurbanne .................................................................................. 3 Figure 4: Organization of the LT/DT team............................................................................................. 4 Figure 5: CERDA ......................................................................................................................................... 4 Figure 6: Test laboratories and climatic chamber .................................................................................... 5 Figure 7: Organization of the CB2 project at ARC ................................................................................. 5 Figure 8: Schema of an electrical network ................................................................................................ 6 Figure 9: Air Insulated Switchgear ............................................................................................................. 8 Figure 10: Gas Insulated Switchgear ......................................................................................................... 8 Figure 11: Dead tank .................................................................................................................................... 9 Figure 12: Generator circuit breaker ......................................................................................................... 9 Figure 13: The three phases of current breaking ................................................................................... 10 Figure 14: Usual architecture of a line circuit breaker .......................................................................... 11 Figure 15: Flow of current in the breaking chamber in closed position ............................................ 11 Figure 16: Breaking chamber during the commutation ........................................................................ 12 Figure 17: 3D picture of the breaking chamber at the arc creation .................................................... 12 Figure 18: Pictures of permanent contacts, arcing contacts and nozzle ............................................ 13 Figure 19: Dielectric resistance of SF6 compared to other materials in a uniform field ................. 13 Figure 20: The six gases targeted by the Kyoto protocol; and the Lewis structure of SF6 ............. 14 Figure 21: Modelling of a high voltage line ............................................................................................ 14 Figure 22: Arcing time and clearing window .......................................................................................... 16 Figure 23: Thermal non-clearance ........................................................................................................... 16 Figure 24: Dielectric non-clearance ......................................................................................................... 17 Figure 25: Self-adaptive mesh in MC3 .................................................................................................... 18 Figure 26: Main dimensions of a breaking chamber ............................................................................. 20 Figure 27: 3D/2D modelling of flow dividers in the thermal volume ............................................... 21 Figure 28: 3D/2D modelling of width openings ................................................................................... 22 Figure 29: 3D/2D modelling of a belcrank ............................................................................................ 22 Figure 30: Thermal and compression volumes in MC3 ....................................................................... 23 Figure 31: Thermal valve – different steps in the opening of the circuit breaker ............................. 24 Figure 32: Thermal valve in 3D................................................................................................................ 24 Figure 33: Thermal valve – main sensitive sections .............................................................................. 24 Figure 34: Thermal valve – Configuration 0 .......................................................................................... 25 Figure 35: Thermal valve – Configuration 1 .......................................................................................... 25 Figure 36: Thermal valve – Configuration 2 .......................................................................................... 26 Figure 37: Thermal valve – Configuration 3 .......................................................................................... 26 Figure 38: Thermal valve – Configuration 4 .......................................................................................... 27 Figure 39: Thermal valve – Configuration 5 .......................................................................................... 27 Figure 40: Discharge valve – main sensitive sections ........................................................................... 28 Figure 41: Discharge valve – Configuration 0 ........................................................................................ 29 Figure 42: Discharge valve – Configuration 1 ........................................................................................ 29 Figure 43: Discharge valve – Configuration 2 ........................................................................................ 30 Figure 44: All tests on the CB1 – mock-up 309 .................................................................................... 31 Figure 45: MC3 calibration – inputs and outputs .................................................................................. 32

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Figure 46: Shape of the current in MC3 ................................................................................................. 33 Figure 47: Total v15 – no loads ................................................................................................................ 35 Figure 48: Total v7 - tests .......................................................................................................................... 35 Figure 49: Total v7 – no loads .................................................................................................................. 36 Figure 50: Total v15 – tests ....................................................................................................................... 36 Figure 51: CB1 – displacement curves .................................................................................................... 37 Figure 52: CB1 – displacement curves (zoom) ...................................................................................... 37 Figure 53: T30 – Simulated arcing times ................................................................................................. 39 Figure 54: T100s – Simulated arcing times ............................................................................................. 39 Figure 55: T30 Tmin – overpressure in the thermal volume – simulation of Pole C ...................... 40 Figure 56: T30 Tmin – overpressure in the thermal volume – simulation of Pole B ...................... 41 Figure 57: T30 Tmax – overpressure in the thermal volume – simulation of pole C ...................... 42 Figure 58: T30 Tmax – overpressure in the thermal volume – simulation of Pole B ...................... 42 Figure 59: T30 Tmed – overpressure in the thermal volume – simulation of pole B ...................... 43 Figure 60: T100s Tmin – overpressure in the thermal volume – simulation of pole C ................... 45 Figure 61: T100s Tmin – overpressure in the thermal volume – simulation of pole B ................... 46 Figure 62: T100s Tmax – overpressure in the thermal volume – simulation of pole B .................. 47 Figure 63: T100s Tmed – overpressure in the thermal volume – simulation of pole B .................. 48 Figure 64: T100s Tmed – overpressure in the thermal volume – simulation of pole B .................. 48 Figure 65: T30 and T100s – maximum overpressure on the first loop .............................................. 49 Figure 66: T30 and T100s – maximum overpressure on the second loop ........................................ 49 Figure 67: T30 and T100s – overpressure at I = 0 ................................................................................ 50 Figure 68: T30 and T100s – difference between the maximum overpressure and the overpressure at I = 0 ......................................................................................................................................................... 50 Figure 69: L75 Tmed – overpressure in the thermal volume .............................................................. 51 Figure 70: Calibration of MC3 – maximum overpressure on the second loop ................................ 52 Figure 71: Calibration of MC3 – maximum overpressure on the first loop ...................................... 52 Figure 72: Calibration of MC3 – overpressure at I = 0 ........................................................................ 53 Figure 73: Calibration of MC3 - difference between the maximum overpressure and the overpressure at I = 0 .................................................................................................................................. 54 Figure 74: L90 Tmed – overpressure in the thermal volume .............................................................. 55 Figure 75: T100a Tmin – overpressure in the thermal volume ........................................................... 55 Figure 76: Simulation model for hot and cold gas in the thermal volume ........................................ 56 Figure 77: Gas temperature distribution at current zero for different A with the same volume ... 57 Figure 78: Old calculations – wrong assumptions ................................................................................. 62 Figure 79: New calculations – correct assumptions .............................................................................. 62 Figure 80: Initially simulated tests ............................................................................................................ 63 Figure 81: Old calculations – overpressure in the thermal volume – T30 Tmin .............................. 64 Figure 82: Old calculations – overpressure in the thermal volume – T30 Tmax ............................. 66 Figure 83: Old calculations – overpressure in the thermal volume – T30 Tmed ............................. 67 Figure 84: Old calculations – overpressure in the thermal volume – T100s Tmin .......................... 68 Figure 85: Old calculations – overpressure in the thermal volume – T100s Tmax .......................... 69 Figure 86: Old calculations – overpressure in the thermal volume – T100s Tmed ......................... 70

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List of Tables Table 1: Characteristic quantities of a circuit breaker ............................................................................. 7 Table 2: T30 Tmin – Wear and velocities ............................................................................................... 40 Table 3: T30 Tmax – Wear and velocities .............................................................................................. 41 Table 4: T30 Tmed – Wear and velocities .............................................................................................. 43 Table 5: T100s Tmin – Wear .................................................................................................................... 44 Table 6: T100s Tmax – Wear ................................................................................................................... 46 Table 7: T100s Tmed – Wear ................................................................................................................... 47 Table 8: Old calculations – Wear before T30 Tmin (kA2s) .................................................................. 63 Table 9: Old calculations – Wear before T30 Tmin (mm) ................................................................... 64 Table 10: Old calculations – Wear before T30 Tmax (kA2s) ............................................................... 65 Table 11: Old calculations – Wear before T30 Tmax (mm) ................................................................ 65 Table 12: Old calculations – Wear before T30 Tmed (mm) ................................................................ 66 Table 13: Old calculations – Wear before T100s Tmin (mm) ............................................................. 67 Table 14: Old calculations – Wear before T100s Tmax (mm) ............................................................ 68 Table 15: Old calculations – Wear before T100s Tmed (mm) ............................................................ 69

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Abbreviations and Nomenclature

A, B, N, M, P, etc.: letters replacing confidential values

ARC: Apparatus Research Center AGK: German production center AIS: Air Insulated Switchgear CB: Circuit Breaker CERDA: Centre d’Essais et de Recherche de l’Appareillage (Apparatus Tests and

Research Center) CIGRE: Conseil International des Grands Réseaux Electriques (International Council on

Large Electric Systems) Dielectric: A material is dielectric if he does not contain electrical charges able to move

in a macroscopic way. It is thus an electrically insulating medium. DT: Dead Tank FRAC: Fraction of the Radiative Flux involved in the ablation process GCB: Generator Circuit Breaker GIS: Gas Insulated Switchgear IEC: International Electro-technical Commission, in charge of the standardization of the

electrical and electronical sectors and related technical activities IEEE: Institute of Electrical and Electronics Engineers HVM: Indian production center LT: Live Tank MC3: Modelling and Computation of Circuit-breaker Chamber Plasma: Gaseous conducting medium made of free electrons, ions and neutral atoms or

molecules. The proportions of the different particles types induce that at the macroscopic scale the medium is neutral.

Puffer: Technique used by some circuit breakers to blast the electrical arc. The blasting pressure is obtained by the compression of a volume thanks to a piston.

Restrike: When the natural frequency of the charge circuit is high (>700 Hz), short-time arcing restrikes may happen. Indeed, if the current interruption is possible with a low distance between arcing contacts, the dielectric withstand is sometimes insufficient. An initial restrike is generally followed by other restrikes, leading to over-voltages, until the distance between arcing contacts is sufficient to ensure the dielectric withstand of the TRV.

SF6: Sulfur hexafluoride, insulating gas used in high voltage circuit breakers SLF (L75, L90, and so on): Short Line Faults Substation: Place of a high voltage network where circuit breakers, switchers,

transformers and so on are gathered TF (T30, T100s etc.): Terminal Faults TLF: Transformer Limited Fault TRV: Transient Recovery Voltage

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ACKNOWLEDGMENTS

First of all, I would like to thank my supervisor, Nadia HASNAOUI for her help and patience all along the Thesis. The time she spent with me contributed a lot to the work I have achieved. I would also like to thank Quentin ROGNARD for his daily availability to answer my questions on the MC3 software.

Besides, I want to thank Cyril GREGOIRE, Georges GAUDARD and François BIQUEZ, respectively manager of the LTDT team, manager of the Circuit Breaker and Bay Development department (CB2D), and director of the Apparatus Research Center (ARC) for having enabled me to join their teams.

Furthermore, I thank Miroslav PETROV, professor at the KTH Royal Institute of Technology of Stockholm for having accepted to be my supervisor, and for his regular follow-up of my Master’s Thesis.

Finally, my warmest thanks go to the whole LTDT team and more generally all the engineers, technicians, assemblers and interns of ARC for their sympathy, their precious advice and explanations as well as for the great atmosphere they have brought to my Master’s Thesis.

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1 INTRODUCTION

1.1 Presentation of the service and of the company

1.1.1 Presentation of the company: GE group This Master’s Thesis was carried out at GE (General Electric) Grid Solutions in Villeurbanne, France. General Electric is one of the biggest companies in the world in terms of number of employees. This American multinational conglomerate corporation was originally founded in 1892 from the fusion of Edison General Electric Company and Thomson-Houston Electric Company. The corporation numbered 305,000 employees in 2015. GE divides itself into 8 branches in 2016: Power, Energy Connections, Renewable Energy, Oil & Gas, Aviation, Transportation, Healthcare, Appliances and Lighting, with their revenues presented in Fig. 1 below (General Electric, 2015). When it comes to energy, GE delivers devices for the production, transport and distribution of electricity. The company notably deals with Smart Grids, thermal power plants based on steam or gas turbines, wind turbines and grid management software (SmallWorld). GE is also a major actor in the desalination and in the sewage water treatment.

Figure 1: GE's activities and corresponding revenues

1.1.2 Takeover of some Alstom entities by GE In June 2014, Alstom accepted the offer of General Electric to buy its energy technology entities – Grid, Thermal Power, and Renewable Power – for a total of 12.35 billion euros. That change enabled the French company to keep its most profitable hub – Alstom Transport – and to ensure the future of its sites and its energy-related service activities.

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An agreement between the two companies was settled, thus creating 3 joint-ventures, i.e. companies owned half by General Electric and half by Alstom. Those ventures include the wind and hydroelectric sectors as well as the nuclear, steam turbines and electric grids related entities. GE’s offer also includes the cession of its signalization branch to Alstom Transport for 600 million euros.

1.1.3 GE Grid Solutions During this Master’s Thesis, the author was an intern at GE Grid Solutions. This entity of GE Energy Connections numbers around 20,000 employees spread among 80 countries and delivers worldwide most of the devices necessary to the transport of energy from the production plant to the consumer. GE Grid Solutions’ clients are 90% of the electricity providers in the world. The activity of this entity represents around $6.2 billion. It has several objectives: to satisfy a growing electricity demand, to increase the safety of networks and their energy efficiency, to improve the existing infrastructures and to encourage the diversified production of renewable energy. While GE Grid Solutions delivers various products and services at its various locations (cf. Fig. 2), the site where the author worked only deals with high voltage equipment (General Electric, 2015).

Figure 2: Joint-venture of GE Grid Solutions

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1.1.4 The site of Villeurbanne The site of Villeurbanne where the author carried out this Thesis exists since 1912 and currently employs around 600 people. The site is specialized in conceiving and building high voltage circuit breakers and generator circuit breakers. Through its three departments (AHT, QCD and ARC) the Villeurbanne site designs, produces and sells a variety of products, as shown in Fig. 3.

1.1.4.1 ARC and CB2D ARC (Apparatus Research Center) is the research and development center of high voltage circuit breakers for GE Grid Solutions. It is the worldwide research and tests center dedicated to the high voltage equipment. This department numbers around 120 people, among whom 45 are certified experts in simulation and design; 12 people are also involved in standardization bodies such as IEC, IEEE and CIGRE. ARC includes several research units, notably the “Circuit Breakers and Bay Development” department (CB2D) where this Thesis was carried out. The CB2D – whose budget was of 18M$ in 2015/2016 – is divided into five teams:

Simulation;

Generators Circuit Breakers (GCB);

Innovation;

Gas Insulated Switchgears (GIS);

Live Tank / Dead Tank (LT/DT). Their main missions are:

Development of new technologies;

Development of new products by support to Product Lines;

Testing and product qualification.

Figure 3: Departments of the site of Villeurbanne

•High Voltage Apparatus: it is one of the production centers

•Develops 3 types of products: generator circuit breakers, line circuit breakers and lightning arresters

AHT•Mechanical prototyping

•Mechanical testing

•Mechanical endurancesQCD

•The Apparatus Research Center

•The entity in which the author carried his internshipARC

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During this project, the author joined the LT/DT team of ARC, which is directed by Cyril GREGOIRE. The local manager was Nadia HASNAOUI, and the author also worked under the supervision of Quentin ROGNARD. The organization of the team is described in Fig. 4.

Figure 4: Organization of the LT/DT team

1.1.4.2 CERDA The studies and developments made by the R&D Engineers of ARC are made together with real tests. The presence on the site of a testing laboratory eases the interaction between numerical studies and tests.

CERDA (French acronym for Apparatus Tests and Research Center, cf. Fig. 5 and Fig. 6) is composed of:

2 power testing stations of 2500 MVA that can be coupled in parallel;

2 synthetic tests stations;

2 dielectric testing laboratories;

2 thermal heating testing labs;

1 lightning protectors testing lab;

1 climatic chamber (-60°C to +60°C).

Figure 5: CERDA

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Figure 6: Test laboratories and climatic chamber

In the project of the Thesis, two other international units were involved: one based in Germany (AGK) and the other based in India (HVM). They are two production units of General Electric for high voltage circuit breakers.

1.1.4.3 Organization of the project Fig. 7 shows the organization of the CB2 project at ARC.

Figure 7: Organization of the CB2 project at ARC

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1.2 Scientific background During the Master’s Thesis, the author used a great part of the knowledge acquired all along his studies. The multi-physical aspect of the simulations makes it compulsory to have a developed knowledge in a wide range of scientific domains such as fluid mechanics, electro-magnetism, radiation, thermodynamics and mechanics. Therefore, the internship started with a 2-week-long training about the physical principles of high voltage circuit breakers and about the use of MC3, the numerical simulations software developed by GE. Here will be presented the fundamental phenomena that govern the functioning of a high-voltage circuit breaker, in particular the physical principle of current breaking.

1.2.1 Main elements of a high voltage line Electricity is delivered to most of the European households at 230V single phase. Some industrial companies require a 400V three-phase current. The most important losses in electrical lines are the Joule effect losses:

∗ ∗ /

In order to limit the line losses, it is thus interesting to transport electricity with a higher voltage. Indeed, the higher the voltage in the transmission lines the lower the losses. This is why electric current is transported at, depending on the distance, medium voltage of 20 to 63 kV or high voltage of 230 kV to 600 kV.

A typical electrical network is represented in Fig. 8 below (Clean Technica, 2016):

Figure 8: Schema of an electrical network

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Several devices are used in the network depicted above, notably in the transformation centers:

The circuit breakers, necessary to the network protection; The circuit switchers, which cannot clear the current, but which are used to secure

and materialize the opening of a line, for instance to carry out an operation of maintenance;

The power transformers, whose role is to raise or lower the voltage level during the transport and to electrically isolate the different parts of the network;

The measuring transformers, which allow for control of the voltage and of the intensity in the network.

1.2.2 Basic principle of a high voltage circuit breaker A circuit breaker is an electrical device whose role is to establish, withstand and interrupt currents under its assigned voltage (Robin Jouan, 2016). One talks about high voltage when the voltage is between 400 and 600 kV. Several current interruption conditions can be depicted:

‐ Normal service conditions, for instance to connect or disconnect a line in an electricity network to carry out an operation of maintenance;

‐ Unusual specified conditions, notably to eliminate a short-circuit.

The circuit breaker is often associated to a command device that detects the default and elaborates orders to eliminate it automatically. The command device must also allow for a reactivation of the circuit when the default is fugitive or has been eliminated by another circuit breaker.

Table 1 summarizes the typical values of some of the main characteristic parameters of a high voltage circuit breaker:

Table 1: Characteristic quantities of a circuit breaker

Physical quantity Order of magnitude Permanent current Up to 5 kA

Voltage 70 to 1200 kV Short-circuit voltage 30 to 80 kA

Opening velocity 3 to 20 m/s Command energy 200 to 12 000 J

Arcing time 10 to 25 ms Gases flow velocity 500 to 2000 m/s

Max overpressure in the thermal volume (with arc) 100 bars Temperature of the SF6 (with arc) 20 000 K

Filling pressure 6 to 9 bars Thermal volume 1 to 3 L

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1.2.3 Types of circuit breakers Circuit breakers can be divided into two main categories:

Line Circuit Breakers, whose role is to secure high voltage networks lines;

Generator Circuit Breakers (GCB) that are placed just after an electric power plant. Those circuit breakers have a very high breaking capacity.

Three types of Line Breakers exist:

Live Tanks (LT) or Air-Insulated Switchgears (AIS)

The Live Tanks, as shown on Fig. 9 (V.R.V., 2015), are “open” devices. That is to say, the insulation between the phases is ensured by air. The conducting parts of each of the 3 phases are insulated by means of a dielectric gas whose insulating capacity is superior to the air capacity. AIS circuit breakers have a large footprint and are usually installed in rural areas near high voltage lines because they require a large site surface.

Armored breakers or Gas Insulated Switchgears (GIS)

The GIS (cf. Fig. 10 (Power Engineering International, 2012)) are called “armored” because the phases are entirely contained in metallic envelopes filled with a pressurized dielectric gas. Thus, the size of the substations is considerably reduced compared to AIS. Furthermore, the GIS are more secure during maintenance operations; which favors their utilization in urban areas or in limited spaces. However, the cost of a GIS is higher than for an AIS by around 30%.

Figure 9: Air Insulated Switchgear

Figure 10: Gas Insulated Switchgear

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Hybrid circuit breakers or Dead Tanks (DT) Dead Tanks (see Fig.11 (V.R.V., 2015)) are hybrid between GIS and AIS. The insulation of the breaking chamber is done thanks to a dielectric gas inside a metallic tank whereas the other active parts are insulated by air. The circuit breaker is linked to the network thanks to insulating bushings. The DT technology is mainly used in North America and in China.

Generator Circuit Breakers (GCB) are another type of circuit breakers.

Generators circuit breakers (cf. Fig. 12 (V.R.V., 2015)) are connected between an alternator and a voltage step-up transformer. Those circuit breakers are generally used at the output of high power generators (100 MWA to 1800 MWA) to protect them in a safe and rapid way.

They operate with higher currents (63 kA to 275 kA) but lower voltages (max 38 kV) than line circuit breakers.

The LTDT team works exclusively on Live Tanks and Dead Tanks, and the circuit breakers studied in this report were all Live Tanks.

1.2.4 Main parameters of a circuit breaker and typical values The main parameters of a circuit breaker are listed below:

Assigned voltage (Ur) It represents the maximum value of voltage of the network in which the circuit breaker can work. Voltage values have been harmonized between standards CEI and ANSI for high voltage.

Assigned frequency

Bushing

Current transformer

Breaking chamber

Moving mechanism

Figure 11: Dead tank

Figure 12: Generator circuit breaker

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The assigned frequency corresponds to the network frequency in which the circuit breaker is integrated: 50 Hz for Europe, Asia and Africa; 60 Hz for America.

Assigned current in permanent regime The assigned current in permanent regime corresponds to the current the device can withstand indefinitely in normal service conditions. Under the effect of permanent current, the different parts of the device tend to heat up, but they must not overcome the values fixed by the standards. These values are determined in order to maintain the performances of the device. Above those values, irreversible degradations can happen, inducing a thermal runaway and major failures (intern arc). The IEC standard assigns a maximum current of 6300 A.

Breaking capacity in short circuit (Ir) It is the highest value of the short-circuit current that the circuit breaker should be able to interrupt in the normal functioning conditions specified by the standards.

Transient Recovery Voltage (TRV, or TTR in French) Current breaking depends on the characteristics of the voltage recovery that happens between contacts after the interruption of the current. We can distinguish two phases in the voltage recovery (cf. Fig. 13 (Robin Jouan, 2016)):

a) The Transient Recovery Voltage (TRV), which is applied to the terminals of the device as soon as the current is interrupted. This phase lasts several hundreds of microseconds.

b) The voltage recovered to the industrial frequency, which remains after the end of the transitory regime on all poles. It is expressed in terms of effective value. For the standard, it has to be maintained for at least 100 ms in order to test the dielectric withstand of the device all along the voltage recovery.

Figure 13: The three phases of current breaking

1.2.5 Usual architecture of a line circuit breaker and its operating principle

Most of the line circuit breakers rely on the same architecture and use the same functioning principle. In general, we find the components shown on Fig. 14 (GE Grid Solutions, 2016). It is important to mention that most of those components are axisymmetric and can thus be modeled

Recovered voltage

Current

Transient recovery voltage

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in 2D. However, some components don’t have that property and are thus harder to model in 2D; that will be discussed later.

Figure 14: Usual architecture of a line circuit breaker

A great number of techniques have been tested to interrupt a high-voltage current (>50 kV). While former technologies used air or oil (some even tried with void), new techniques use an insulating gas called SF6 as breaking agent. Research is also led in order to replace it with a less harmful gas for the planet, called g3. Two technologies have been developed for circuit breakers that use SF6: pneumatic blast and self-blast. The circuit breakers studied during this project operate with SF6 and self-blowing, which is the most used technology nowadays. The author will thus focus here on the presentation of that technique.

1.2.6 Opening of a circuit breaker – various steps When a fault in the electrical transmission line is identified, the circuit breaker opens to separate the contacts and cut off the current. The opening operation can be divided into 5 main steps.

Step 1: Operation under rated current – closed position

In the normal functioning conditions, the current goes through the permanent contacts. These are dimensioned so as to withstand the assigned current of the device. When a fault is detected, an opening order is given to the command. The movement of the device starts (cf. Fig. 15).

Figure 15: Flow of current in the breaking chamber in closed position

Permanent contacts

Mobile part

Fixed (or semi-fixed) part

Pressurized gas

Current

Discharge valve

Compression volume

Thermal volume

Thermal valve

Permanent contacts

Pin

Thermal channel Cap

Tulip Nozzle

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Step 2: Pre-arcing period – commutation

When a fault is detected, the device moves to separate the permanent contacts. The current then goes through other contacts, especially conceived and dimensioned to withstand intense heat-ups. These contacts are called arcing contacts, and the corresponding phase – shown in Fig. 16 – is called commutation.

Figure 16: Breaking chamber during the commutation

Step 3: Arc creation

The movement continues until the arcing contacts separate. We then observe an ionization of the gas. As a consequence, the current goes through a conductive channel in the gas phase called plasma. The electrical arc is thus created (cf. Fig. 17 (GE Grid Solutions, 2016)). In the case of high voltage circuit breakers, the arc has a considerably high energy, reaching local temperatures up to 20 000 K. To withstand such extreme heating, the arcing contacts generally have edges made of tungsten. The intense radiation in the inter-contact zone provokes a hot gas flow inside the circuit breaker. Due to the very high level of energy, the arc cannot be cut off just by the distance between the arcing contacts created by their displacement. It is then necessary to cool the arc down at the moment when the alternating current reaches zero, i.e. when its energy is lower.

Figure 17: 3D picture of the breaking chamber at the arc creation

Step 4: Filling

The parts made of Teflon (PTFE), i.e. the nozzle and the cap, allow for canalizing the arc, thus forming the arcing zone. The gas contained in that zone is subject to an important heating that leads to a high pressure rise. Besides, the ablation of the Teflon parts – due to the thermal radiation – increases the pressure rise. This mechanism is called thermal effect.

A channel, called thermal channel, connects the arcing zone to a closed cup called thermal volume. In this closed volume appears a pressure rise because of the thermal effect. The

Fixed part

Electrical Current

Direction of displacement of the mobile part

Pressurized gas

Mobile part

Path of the el. current

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pneumatic pressure rise thanks to an adjacent volume, the compression volume, also contributes to the pressure rise in the thermal volume.

Step 5: Arc extinction

The considered electrical current being alternating, the Ohm law implies that the power delivered by the arc is zero when the current reaches zero. The pressure accumulated in the thermal volume enables to ensure a cold gas flow to be forced onto the arcing zone: this mechanism is called blast. The cold gas flow ensures the cooling and blow-off of the electrical arc, leading to its extinction. After the blast, the gas situated in the inter-contact zone is entirely replaced by a colder gas, which enables the medium to retrieve its insulating properties.

The mobile arcing contact is called tulip, and the fixed arcing contact is called pin (cf. Fig. 18).

Figure 18: Pictures of permanent contacts, arcing contacts and nozzle

1.2.7 SF6: the insulating gas All the current circuit breakers are filled with an insulating gas, SF6. The SF6 gas has some advantages compared to other gases or oils when used as a dielectric gas, see Fig. 19 (Budapest University of Technology and Economics Department of Electric Power Engineering, n.d.).

Indeed, one may observe that the voltage at which an arc is created at 5 bars – the devices are gas-filled at a pressure between 5 to 9 bars – as a function of the distance between the electrodes is well above the other commonly used materials shown on the graph. Only a

vacuum (void) has better dielectric resistance when the electrodes are still close to each other.

Besides, the SF6 gas behaves as a perfect gas until around 1500 K. After that, a change in the properties of the gas appears. It then behaves as a real gas and loses at the same time its insulating properties.

Figure 19: Graph of the dielectric resistance of SF6 compared to other materials in a uniform field

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However, SF6 is among the six worst types of greenhouse gases described by the Kyoto protocol as shown in Fig. 20. Indeed, its global warming potential is 23 900 times superior to CO2.

For 1 kg of SF6 released or leaking into the atmosphere, the equivalent of 24 tons of CO2 can be released for the same greenhouse effect, therefore representing potentially the most powerful greenhouse gas on Earth. Research is now conducted on replacing SF6 with another, less harmful, insulating gas: the g3.

1.2.8 Fault types and tests The faults are mainly characterized by an over-intensity because of a short-circuit in the network. This over-intensity may provoke a deterioration of devices (transformers, generators and so on) if the current is not interrupted quickly enough by the circuit breaker. These defaults are detected by electronic detection systems.

The type of default mainly depends on where it happens on the line compared to the position of the circuit breaker (cf. Fig. 21). A difference is also noticeable between single-phase, two-phase and three-phase line faults. Furthermore, one may observe that around 90% of current defaults have a current lower than 75% of the breaking capacity of the circuit breaker.

We distinguish two types of short-circuit defaults: terminal faults and short-line faults.

Figure 20: The six gases targeted by the Kyoto protocol; and the Lewis structure of SF6

Figure 21: Modelling of a high voltage line

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Terminal fault (TF)

This default appears at a terminal of the device. The single-phase default is the most common one even though the two- or three-phase defaults also exist. The maximum current that must be interrupted by the circuit breaker happens in the three-phase case.

Since it is too costly to check the correct clearance of all currents by testing all those currents, 4 values of current have been selected for tests by the CEI and IEEE standards: terminal faults at 10% (T10), 30% (T30), 60% (T60) and 100% (T100) of the breaking capacity of the device in short-circuit. Moreover, the T100 test differ between symmetric (T100s) and asymmetric current (T100a).

Short-line fault (SLF)

This default – sometimes called kilometric default – happens on a line close to the circuit breaker, from around 100 meters to a few kilometers. In those conditions, a voltage wave appears on the line side, and is added to the TRV on the source side. The default is thus characterized by an oscillation of the TRV. One may observe a high du/dt during the first instants of recovery of the TRV, which induces that the fault is particularly difficult to clear.

The performance of a circuit breaker when it comes to breaking short-line faults can however be improved by adding a capacitor between the terminals or at the head-end. This implies a decrease in the oscillating frequency of the TRV and thus the increase speed of the voltage, but also to increase the time delay of the TRV. The standard tests corresponding to this type of default are the L90 (90% of the breaking capacity in short-circuit) and the L75 (75% of the breaking capacity in short-circuit).

1.2.9 Notion of arcing time As mentioned above, the electrical arc is blasted when the current crosses zero, i.e. when the power delivered by the network is zero. Nevertheless, in order to blast efficiently, the distance between the pin and the tulip must be sufficient to ensure the dielectric resistance of the medium.

The minimum arcing time is thus defined as the minimal arc duration the circuit breaker must be able to break. Thus, any current that crosses zero after that time must be cleared by the device. One thus defines a clearing window of the circuit breaker, whose extent corresponds to half a period of the current loop, minus 1 ms. For a frequency of 50 Hz, it is thus a 9 ms window.

Furthermore, the maximum arcing time corresponds to the minimum arcing time plus the clearing window. The mean arcing time corresponds to the median time between minimum and maximum arcing times. All these notions are depicted in Fig. 22.

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Figure 22: Arcing time and clearing window

1.2.10 Case of non-clearance Two main cases of non-clearance exist:

Thermal non-clearance As explained before, the pressure accumulated in the thermal volume together with the mixture of hot and cold gases in that volume enable the electrical arc blast. The main characteristics of that blast are the blast flow rate, the gas temperature as well as its density. Indeed, the objective is to blast a high flow rate of cold gas with a high density of SF6 inside. However, when the blast does not have all those properties, the device may not clear the arc. This is called “thermal non-clearance” (cf. Fig. 23) because at that moment the most important parameter is the temperature between the arcing contacts, which is not low enough for the arc to be extinguished.

‐1,5

‐1,0

‐0,5

0,0

0,5

1,0

1,5

0 5 10 15 20 25

Current [p.u]

Time [ms]

Tmed

Tmax

Tmin

Exemple

Clearing window

Min arcing time

Med arcing time Max arcing time

a

Figure 23: Thermal non-clearance

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Dielectric non-clearance Dielectric non-clearance happens once the arc is extinguished and the current is equal to zero. As explained earlier in this report, at the extinction of the arc, a strong transient voltage, the TRV, is applied. This tension as well as the electrical field and the state of the gas – still ionized – may lead to a dielectric non-clearance. This phenomenon is shown in Fig. 24. Then, a restrike of the arc may happen between different parts such as the arcing contacts, the permanent contacts or other metallic elements having a difference of electrical potential. An electrical arc is thus re-created and damages – sometimes destroys – the parts that are not dimensioned to withstand its presence. This can also lead to the destruction of the entire circuit breaker. This non-clearance is called “dielectric” because it comes from the fact that the device does not withstand the assigned voltage. It is often due to a hot gas pocket moving in the circuit breaker and preventing the medium to retrieve its insulating properties.

1.3 The MC3 software

1.3.1 Presentation MC3 – an acronym for “Modelling and Computation of Circuit-breaker Chamber” – is a numerical simulation software specially developed for the study of the breaking chamber of high voltage circuit breakers. MC3 is developed by the Center of Research in Applied Calculation (CERCA, based in Montreal) and is exclusively used at ARC. It uses the finite volume method in order to discretize the partial elliptical differential equations used in the modelling of the various phenomena that take place inside the breaking chamber (Vassilev, Feb. 2016).

The complexity of unsteady, compressible and dielectric gaseous flows coupled with the opening movement of the circuit breaker make it necessary to use a numerical scheme that needs to adapt spatially and temporally to the evolution of the flow. This is why MC3 includes an auto-adaptive mesh that adapts continually to the temperature or pressure gradient in order to optimize the precision of the solution.

Figure 24: Dielectric non-clearance

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1.3.2 Self-adaptive mesh The adaptive mesh shown in Fig. 25 allows for triangle sizes in agreement with the local behavior and the gradient of the gas. MC3 uses the finite volume method, which enables one to study the flows passing through the mesh – and not only the values at the nodes as it is the case for the finite element method. Thus, the finite volume method gives more accurate results when it comes to fluid mechanics. Furthermore, the auto-adaptive mesh is able to reduce the calculation errors in high-gradient regions but also to optimize the calculation since the mesh is only refined in the regions that

require some refining. This induces a reduction in the calculation time.

1.3.3 Calculation hypotheses in MC3 To realize the simulations, MC3 includes several important hypotheses that are to be taken into account:

The flows as well as the geometry are axisymmetric. That is to say, all the features of the geometry must present an axial symmetry to be represented. One thus understands that non-axisymmetric parts have to be modified in order to be represented in the 2D geometry. That is why we introduce the concept of equivalent surface, detailed elsewhere in the report.

Simulated gases are considered non viscous. Movement boundary layers are thus neglected. The resolved equations for fluid mechanics are the Euler equations.

Turbulence is not taken into account, which is quite a strong assumption given the nature of the simulated flows.

The electrical arc is modelled as a perfect fluid.

1.4 Methodology To carry out this Master’s Thesis, the author started by learning the functioning of a high voltage circuit breaker. After two weeks of initial training, a 3D model of the circuit breaker was studied in detail in order to properly understand the movement of the device and the links between the different parts. Moreover, the author had access to several internal documents at GE explaining the various physical phenomena that occur in a circuit breaker.

The rest of the work was on the development and optimization of the new circuit breaker CB2, based on the geometry of an existing one. Thus, all the available data on the former circuit breaker (dimensions, geometry, tests) was gathered and studied to build a MC3 calculations database on that device. After that, modifications of geometry were initiated to develop the new device.

Figure 25: Self-adaptive mesh in MC3

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2 GEOMETRY PREPARATION AND 3D/2D MODELLING

2.1 Introduction to the work, objectives and strategy To meet the different international standards (notably IEEE and IEC), it is necessary for high voltage circuit breakers to clear different types of defaults and arcing times. This induces a high number of tests, which are costly and take time to implement. Furthermore, some tests – if they fail – can severely damage the tested devices. The current challenge is thus to reduce the number of tests and to replace them by numerical calculations, which are much less costly. The other advantage of numerical calculations is that they allow for a greater number of configurations to be tested: it is of great help for the design of new circuit breakers and for the optimization of old ones.

During the Master’s Thesis, the author worked on one main project: the CB21. The goal was to modify an existing circuit breaker to make it 25% cheaper for the Indian market. The basis of the study was the CB1, which is a Live Tank with an assigned voltage of U kV and a breaking capacity in short circuit of Isc kA.

The first goal of the project is to calibrate MC3 numerical calculations on real tests for the CB1 (mock-up 309). To do so, it is necessary to simulate several defaults with high and low currents as well as different arcing times.

In order to carry out MC3 calculations, several assumptions have to be made. First, MC3 is a software that works with 2D geometries. As a result, equivalent sections and volumes have to be calculated. Also the choice is to use real tests displacement, current and voltage laws. Finally, the wearing laws are calculated thanks to a macro developed by N. HASNAOUI and based on the wear observed in real tests.

The next step of the project is, in agreement with AGK and HVM, to implement modifications of the geometry of the CB1 to make it cheaper without deteriorating its performances. The obtained circuit breaker is called CB2.

2.2 Geometry preparation The first step of the project consisted in preparing a 2D model in order to simulate the behavior of the CB1 device in MC3. The author first got acquainted with the 3D geometry of the circuit breaker by looking at both the drawings and the CREO model of the device. That enabled him to understand the external and internal kinematics as well as the different fluid pathways taken by the gas during the operation of the circuit breaker. He was also able to measure the relevant dimensions of the circuit breaker for the modelling in MC3. Furthermore, the CREO model makes it easier to realize whether a part of the model is axisymmetric or not, which is crucial for the 2D modelling of the device.

1 CB stands for Circuit Breaker. All the projects and devices names are confidential. The same applies for most numerical values.

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2.2.1 Importation of the 3D model The first step of the circuit breaker 2D modeling is to export the 3D CREO model and to import it in the MC3 software. To do so, a cut view of the 3D model is created in a plan containing the revolution axis. The curves and reference points of the model are displayed and 2D IGES model of the device is generated. Then, that model is imported in MC3 and modified there. Since CREO and MC3 were not created together, some modifications are necessary on the model to make the simulation on MC3 possible and realistic.

2.2.2 Dimensions In order to simulate the behavior of a high voltage circuit breaker, it is necessary to know some dimensions of the device, shown on Fig. 26.

Figure 26: Main dimensions of a breaking chamber

L0: Geometrical overlap (axial distance between the end of the pin and the end of the tulip)

Distance between geometrical and electrical separation

MC3 distance between pin and fictive electrode

L0 + L1: Distance between pin and cap

L0 + L1 + L2: Distance between pin and nozzle

L3: Length of the nozzle neck

L4: Length of the nozzle divergent The CREO model was used to measure those dimensions thanks to several sectional views of the geometry.

2.2.3 2D modelling If some parts of the geometry are axisymmetric and are directly represented on the 2D view, other parts need to be modified to be modelled accurately. The principle of equivalent sections is

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to be used for that purpose. That is to say, the 2D section represented on MC3 should be equal or as close as possible to the value of the 3D section. Those sections can be divided into two major categories:

Diameter openings (ex.: flow dividers): the value of the 2D sections can be determined with the following formula: ∗ ∗ . One thus determines the equivalent 2D axial length of the section, keeping the same inner diameter and width as in 3D. It is then important to decide which axial position we choose to keep to represent the physical phenomena as accurately as possible. For instance, on Fig. 27 below the author chose to keep each opening’s right side position in order not to delay the gas exhaust.

Figure 27: 3D/2D modelling of flow dividers in the thermal volume

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Width openings (ex.: “beans” openings on valves): the value of the 2D sections can be calculated with the formula: ∗ ∗ . We keep the same mean radius as in 3D, and determine “a”, radial width of the opening (cf. Fig. 28).

Figure 28: 3D/2D modelling of width openings

For reasons of confidentiality, the author will not show here the 7 other parts that were not axisymmetric and had to be modelled the way presented above.

An equivalent volume also had to be modelled for the belcrank, as shown below on Fig. 29:

Figure 29: 3D/2D modelling of a belcrank

3D complex shape

2D equivalent: cylinder

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2.2.4 Parametric study of the two valves As mentioned earlier in the report, two valves exist in the breaking chamber and have to be modelled. They are represented on Fig. 30. Those regions of the geometry are strategic since they influence to a great extent the flow and pressure distributions inside the chamber. Furthermore, it is not possible to ensure that all the 2D and 3D sections have the same value. One thus determines critical sections on which the error has to be minimized.

Figure 30: Thermal and compression volumes in MC3

2.2.4.1 Parametric study of the thermal valve The thermal valve is an essential part of self-blast circuit breakers, like the CB1.

Functioning principle and utility of the thermal valve:

‐ The detection of the default generates an opening order by the command part, which creates a pressure rise in the compression volume by means of a swabbing effect. This effect is created by a combined movement of the mobile part. The thermal valve is in close position (cf. step 1 of Fig. 31)

‐ When the overpressure in the compression volume overcomes the overpressure in the thermal volume, the thermal valve opens, creating a flow of gas from the compression volume towards the thermal volume (cf. step 2 of Fig. 31)

‐ After the contacts separation, an electrical arc is generated. Its energy produces a high overpressure in the thermal volume, which overcomes the overpressure in the compression volume. As a result, the thermal valve closes (step 3 of Fig. 31)

‐ The pressure accumulated in the thermal volume creates a gas flow towards the arc when its energy decreases, close to a zero of the current. This gas flow leads to the extinction of the electrical arc (step 4 of Fig. 31).

In this case, the pressure difference between the two volumes for the valve to open is set at N bar.

Exhausts

Thermal volume Compression volume

Thermal valve Discharge valve

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Figure 31: Thermal valve – different steps in the opening of the circuit breaker

We can here identify two different fluid pathways:

Pathway 1: S1 S2.1 S3.1

Pathway 1: S1 S2.2 S3.2 3D sections calculations (the valve is represented in 3D and 2D respectively in Figs. 32 and 33, with some confidential details concealed for the purpose of this report) give that S2.1 and S2.2 are the critical sections of respectively pathways 1 and 2. Since the modelling of all the sections

Step 4 Step 1 Step 2 Step 3

Figure 32: Thermal valve in 3D Figure 33: Thermal valve – main sensitive sections

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cannot be perfect, the priority is put on S2.1 and S2.2, smaller (and thus limiting) sections. The main assumptions taken into account are that S1 and Splate are kept constant for all configurations. Indeed, since fluidic forces are applied on those two surfaces, it is important to represent them accurately. When the plate is closed, gas that is inside the compression volume will apply forces on the thermal plate through the beans section (S1) and gas that is inside the thermal volume will apply forces on all the plate’s section (Splate, see above).

Six configurations are implemented, shown in Figs. 34, 35, 36, 37, 38 and 39.

Config 0 based on the 3D model

As expected (it is one of th assumptions), there is no error on S1. S2.1 is smaller than reality and S3 is much bigger. Config 1 is implemented to reduce the error on S2.1.

Config 1 increase in ,

Figure 34: Thermal valve – Configuration 0


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This configuration indeed induces a better S2, but it is a bit costly (4% variation in , and the plate is off-center by 1.6 mm). S3 is bigger than in reality. Config 2 is then implemented to reduce again the error on S2 globally.


This configuration is good for S2 but not for each fluid pathway. Thus, it is rejected and configuration 3 is created to study the influence of the plate thickness on S2.1 and S2.2.

Config 3 decrease in

Decreasing the thickness of the plate induces a lower error both on S2.1 and S2.2. It seems that there is no reason why the thickness of the plate could not be changed in simulation. Indeed, its mass is not taken into account. After that, Config 4 is implemented to reduce the error on S3.



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Config 4 changing and

This configuration is very good for S3 (error lower than 0.5 %). However, it induces a change of 1.6 % in dmin and a change of 0.6 % in dmax. Those changes will impact the gas mixture and therefore the blasted gas quality, which is not acceptable since it is a critical aspect of the calculation. Thus, the choice is not to change S3.1 nor S3.2 since those sections are not limiting. Config 5 is made to reduce the error on both S2.1 and S2.2.

Config 5 increase in and ,

This configuration combines the advantages of the plate thickness variation and of the change in dbeans,moy. A very low error (less than 0.2%) is thus obtained for both S2.1 and S2.2. The configuration is a bit costly (5.2 % variation in dbeans,moy and the plate is off-center by 2.1 mm) but it is still acceptable. This is the chosen configuration for the MC3 calculations.



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2.2.4.2 Parametric study of the discharge valve In breaking conditions, the arc between contacts will generate a consequent energy that is traduced by a thermal expansion, especially inside the thermal volume. When the thermal effect is sufficient, pressure inside the thermal volume exceeding the pressure inside the compression volume, the thermal plates closes. The compression volume that is then isolated can become problematic since it can generate a pneumatic damping and decelerate consequently the circuit breaker. There is a real need to purge this volume; this is why a discharge valve is implemented.

Unlike the thermal valve, the discharge valve is equipped with springs that determine the discharge. This is why the MC3 modelling of the discharge valve is different.

The main sensitive sections are shown on Fig. 40. Again, one can identify two different fluid pathways:

Pathway 1: S1 S2.1 S3.1

Pathway 1: S1 S2.2 S3.2 3D sections calculations give that S2.1 and S3.2 are the critical sections of respectively pathways 1 and 2. Since the modelling of all the sections cannot be perfect, the priority is put on S2.1 and S3.2, limiting sections. The main assumptions taken into consideration again are that S1 and Splate are kept constant for all configurations. Furthermore, the 3D sections calculation gives that S3.1 is much bigger than the other sections. The choice is to neglect this section.

Like for the thermal valve, we carried out a parametric study to determine which configuration was the best for the discharge valve. We compare 3 configurations (cf. Fig. 41, 42 and 43).

Figure 40: Discharge valve – main sensitive sections

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Config 0 based on the 3D model

As expected (cf. assumptions), no error exists on S1. However, S2.1 is smaller by 3.5 % compared to 3D and there is an important error on S3.2 as well (around 12.5 %). The goal of configuration 1 is to reduce the error on S2.1.


Figure 41: Discharge valve – Configuration 0


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This configuration allows for a low error on S2.1, but still the same high error on S3.2. Moreover, the mean radius of the plate is changed by 1.7 mm. Config 2 is then implemented to reduce the error on S3.2 as well.

Config 2 increase in , and decrease in ,

This configuration induces very good results for both S2.1 and S3.2. The plate is off-center by 1.9 mm, which is still acceptable. This configuration is chosen for the calculations.

Conclusion on this section

One of the main issues at stake for the MC3 calculations is to represent accurately the circuit breaker geometry. Indeed, it is a microscopic software coupling several physical phenomena and based mainly on the principles of fluid mechanics. The shapes of the components, the cross-sectional areas and the gas expansion volumes are extremely important. Therefore, a good geometry construction is necessary. It is even more difficult considering that the software is 2D-axisymmetric while one wants to model are 3D geometries. A real work of research, analysis and choice of the most important hypothesis has been carried out. The aim of all this work is to model as accurately as possible the real physical phenomena and to be as predictive as possible.


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3 MECHANICAL STUDY OF REAL TESTS After the geometry is prepared, the following part of the CB2 project is to build a reference in terms of numerical calculations. To do so, the basis is the geometry of the CB1– not from the CB0 because it works with higher velocities than the two other devices – in order to compare tests results and numerical calculations. The database to build is going to include different defaults, high and low currents, as well as different arcing times.

As a first step, all the tests results of CERDA are gathered to simulate some of them on MC3. The calibration will rely on pressure measurements recorded during breaking tests inside the thermal volume. The other sensors represent the strokes, the current, the separation and the TRV. But several tests carried out on the device do not include pressure sensors, nor sometimes cursors for the strokes either. All the tests led on the CB1 mock-up 309 are shown in Fig. 44 below. Also, the inputs and outputs of the MC3 calibration are shown in Fig. 45.

Figure 44: All tests on the CB1 – mock-up 309

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The decision was to focus on DE 7248 (L75 tests on pole A), DE 7251 (T30 and T100s tests on pole B) and DE 7296 (T30 and T100s tests on pole C) because they are the only tests including pressure sensors. For T30 and T100s, our aim is to simulate a Tmin, a Tmed and a Tmax. For L75, a mechanical study led by my supervisor shows that the most critical test is a Tmed: Tmin or Tmax will not be simulated for this default.

The main characteristics of the tests are described below:

The device is a double motion and the strokes for all the tests are X mm for the pin side and Y mm for the tulip side. The strokes cursors are placed on non-tested poles.

The rotational cursor, sometimes placed on the tested pole, is not accurate and thus not used.

The absolute filling pressure if Z bar. Between each tests series (DE), the breaking chamber is changed: no wear is to be taken

into account at the beginning of each DE. A table was built and used to define which test is able to be simulated correctly on MC3.

Figure 45: MC3 calibration – inputs and outputs

Inputs• Strokes

• Wear

• Current

• TRV

MC3 calibration

• FRAC parameter

• 2D modelling

Outputs•Pressure insidethe thermal volume

•Other quantitiesfor the database

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‐10000

0

10000

20000

30000

40000

50000

60000

70000

80000

0 10 20 30 40

Curren

t (A)

Time (ms)

MC3 current

MC3 current

3.1 First step of calibration: signal treatment The curves received from CERDA require to be analyzed and modified. For the strokes, each time a moving average on 2000 points is applied to reduce the noise of the measurement that could lead to calculation problems on MC3. This curve treatment may however lead to 0.5 mm error on the stroke. The next steps of the treatment were the following:

“amplitude to target” to start effectively at zero “amplitude scale” to make sure that the total stroke is correct “temporal to target” in order to have t =0 when the tulip side movement starts Trend curve on Excel to have the same time increment between pin and tulip to generate

the fictitious electrode law. This may lead to an error of 0.5 mm As a result, the curve treatment can lead up to 1 mm error at a precise instant. We consider that this is a good approximation, but we must keep it in mind in case of inaccuracy in the results.

The treatment applied for the current and voltage laws do not induce errors on the results. The only noticeable thing is that before implementing an MC3 calculation, the current is kept above a certain value apart from at the vicinity of the extinction time (cf. Fig. 46 below)2.

3.2 Separation and release of the nozzle neck To calculate the separation time3, two methods can be chosen:

From the theoretical electrical overlap: assuming the circuit breaker is exactly in its closed position at the beginning of the motion, the separation time corresponds to the time to reach the theoretical electrical overlap. This assumption is too strong since we never know the exact position of the device at the beginning of the test. Thus, we never used this method.

2 The numerical values on the Y-axis are confidential 3 we call “separation” the separation of the arcing contacts

Figure 46: Shape of the current in MC3

Arc

Sep

Extinction

-34-

From the Iinj/refsep curve: assuming there is no temporal offset between that curve and the stroke curves, the separation time corresponds to the difference between the extinction time (easily noticeable on the Iinj/refsep curve) and the arcing time. We chose this method, but we have to keep in mind that the strokes are not exactly the same in the tested pole.

The nozzle neck release time can be calculated from the total stroke curve. However, that time will be different between real tests and MC3 simulations, since the device is not always in the same initial position in reality and since the cursors are not placed on the tested pole:

Release of the nozzle neck in real test: The stroke between electrical separation and release of the nozzle neck is known, and so is the stroke at the electrical separation. One can therefore determine the nozzle neck release time. The time we obtain using this method is enormous and does not seem to be real.

Release of the nozzle neck in MC3: we know that the device is exactly in its closed position (geometrical overlap of Z mm) at the beginning of the motion. The geometrical distance between the closed position and the release of the nozzle neck is known, so we calculate the time to release. MC3’s ability to simulated tests where the nozzle neck is not released at the extinction is questionable. This time is thus really useful for us since it determines whether MC3 will be reliable or not.

3.3 Wear study The wear happens mainly on the tulip, on the cap and on the nozzle. In real tests, it is not possible to put a sensor directly on the parts to measure the wear. However, the inner diameters of those parts are measured before and after the tests series. On MC3, the wear will be represented as a uniform and linear change in diameter on the whole part. The macro (excel file) created by N. HASNAOUI enables to calculate the wear (in kA2s) for each test. To calculate the initial diameters for each test, a proportional law is applied: one has access to the final wear at the end of the test series, as well as the corresponding diameters. One can thus determine the diameters for each test.

3.4 Velocity study An important analysis job has been made on the velocity in order to simulate correctly the different defaults but also to understand the breaking conditions for the CB1. Once again, it is necessary to keep in mind that the transducers were not placed on the tested poles but on the pole next to it (tripolar). This induces that different magnetic effects and the acceleration of the pin due to electrical breaking conditions are neglected. For the mechanical study, two main velocities have to be taken into account:

The mean velocity between the separation of the arcing contacts and 7 ms later. This velocity, called v7, is meant to determine if the device is efficient in capacitive tests. Its target for our project is A m/s.

-35-

The mean velocity between separation and 15 ms later. This velocity, called v15, indicates if the nozzle neck is released after 15 ms in order to clear minimum arcing time, especially for short line faults. The target is set at B m/s for this project.

The next graphs (Figs. 47, 48, 49 and 50) show v7 and v15 for different no loads and tests. For no loads, one clearly observes that the new mock-up induces higher v7 and v15 for the capacitive tests, but those performances are rapidly deteriorated after those tests. For breaking tests, the velocities are low compared to the objectives (v7 = A m/s and v15 = B m/s). As a result, the nozzle neck is not released for the Tmin tests. Since MC3’s ability to simulated tests where the nozzle neck is not released at the extinction is questionable, we decide to simulate as well Tmin tests with the extinction happening 1 ms after the release of the nozzle neck.

Figure 47: Total v7 - tests

Figure 48: Total v15 – no loads

-36-

The next graphs (Figs. 51 and 52) show that the damping happens too early on the tests; that makes it complicated to simulate Tmin on MC3.

Figure 49: Total v7 – no loads

Figure 50: Total v15 – tests

-37-

Figure 51: CB1 – displacement curves

Figure 52: CB1 – displacement curves (zoom)

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4 FIRST RESULTS AND CALIBRATION OF MC3 The aim of this part is to present the first aspect of the project: the calibration of numerical calculations on real tests for the CB1.

4.1 The FRAC The chosen parameter for the calibration of MC3 on real tests is the FRAC. It is a MC3 parameter corresponding to the fraction of the radiative flux causing ablation during the arc. Each default and arcing time was thus simulated with two different FRAC: 0.8 and 1.1. FRAC 0.8 is the default FRAC in MC3, whereas FRAC 1.1 has shown great results for former projects, even though its value is not physical (a fraction cannot overcome 1).

4.2 Comparison criteria In general, tests include a measure of the arcing voltage, which is a very good comparison criterion. However, for our tests the arcing voltage is not available. Since only pressure measurements in the thermal volume are available, 4 criteria are chosen for the comparison:

Maximal overpressure during the first current wave; Maximal overpressure during the second wave; Overpressure at I = 0; Difference between the maximal overpressure of the second wave and the overpressure

at I = 0.

4.3 Calibration of the MC3 tool A first batch of calculations gave very questionable and non-coherent results. An analysis showed that it was probably due to very important values of wear during the simulated tests. Another batch of calculations has been led on a second T30 and T100s test series. The results were closer to what is known from the apparatus by experience. An important output from this study was that MC3 ability to simulate breaking tests with consequent values of wear is questionable (see APPENDIX 1).

Since the tests on Pole C were considered questionable, the decision is to compare their wear and velocities with the same tests on Pole B. All the compared tests are shown below in Fig. 53 and Fig. 54.

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Figure 53: T30 – Simulated arcing times

Figure 54: T100s – Simulated arcing times

-40-

4.3.1 T30 study

4.3.1.1 T30 Tmin (around 16 ms) Table 2 points out the wear and velocities for the T30 Tmin of Poles B and C. A green box means that the parameter is better, red means worse. The grey boxes mean that the values are confidential.

Table 2: T30 Tmin – Wear and velocities

Test Wearing

before test (kA^2s)

Section under cap before test (mm^2)

Section under nozzle before test (mm^2)

Section under tulip before test (mm^2) v7 v15

No wear Before test

No wear Before test

No wear Before test

Value %wear Value %wear Value %wear

Pole C – 7296 (16.2 ms) 1.20 0.34 0.10 4.5 4.1

Pole B – 7251 (16 ms)

6.48 1.55 0.50 4.7 4.4

One remarks that the wear is lower in percentage for Pole C. Nonetheless, the wear is low in both cases (less than 7% for the sections), and the wear on Pole B is only bigger by 6% for the section under the cap. Furthermore, the velocities are higher for Pole B. Thus, we decided to simulate both tests and to compare them (cf. Fig. 55 and 56).

The test values on pole C are very high compared to both Pole B and numerical calculations, even on the first loop while the current amplitude is lower than on Pole B. Thus, that test will not be considered.

Figure 55: T30 Tmin – overpressure in the thermal volume – simulation of Pole C

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The numerical calculations on pole B give results quite close to the real test. On the first loop, a pressure difference of about n bar is noticeable for both FRAC 0.8 and FRAC 1.1. On the second current loop, FRAC 1.1 is better for the maximum (pressure difference lower than n bar) and the discharge.

As a conclusion, FRAC 1.1 is better for T30 Tmin.

4.3.1.2 T30 Tmax (around 25 ms) Table 3 points out the wear and velocities for the T30 Tmax of Poles B and C. A green box means that the parameter is better, red means worse.

Table 3: T30 Tmax – Wear and velocities

Test Wearing before

test (kA^2s)



Section under tulip before test (mm^2)

v7 v15

No wear Before test

No wear Before test

No wear Before test


Pole C – 7296 (25.3 ms)

7.01 1.93 0.58 4.5 4.2

Pole B – 7251 (25.2 ms)

11.82 2.80 1.00 4.7 4.3

The wear is lower for Pole C (DE 7296), but the section under the cap is bigger for Pole B by only 4%. Furthermore, the wear is low in general (lower than 12% for both tests), and the

Figure 56: T30 Tmin – overpressure in the thermal volume – simulation of Pole B

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velocities are higher for Pole B. As a consequence, the decision is to simulate both tests (cf. Fig. 57 and 58).

The test values of Pole C are high compared to pole B and MC3 calculations on the first loop (around 2c bars versus c bars). On the first loop, FRAC 0.8 and 1.1 are similar, whereas one cannot conclude on the second loop. Indeed, the second current loop is very different between the two tests.

Figure 57: T30 Tmax – overpressure in the thermal volume – simulation of pole C

Figure 58: T30 Tmax – overpressure in the thermal volume – simulation of Pole B

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When it comes to Pole B, FRAC 0.8 and 1.1 are similar for the first loop whereas FRAC 0.8 is more accurate on the second loop.

Thus, FRAC 0.8 fits better for T30 Tmax.

4.3.1.3 T30 Tmed (around 20 ms) Table 4 shows the wear and velocities for the T30 Tmed of Poles B and C.

Table 4: T30 Tmed – Wear and velocities

Test Wearing

before test (kA^2s)




v7 v15

No wear Before test

No wear Before test

No wear Before test


Pole C –

7296 (19.6 ms)

11.09 3.05 1.00 4.5 4.1

Pole B – 7251 (20.2

ms) 14.35 3.40 1.05 4.5 4.2

The wear is lower on Pole C but the wear remains low for both tests. Moreover, the difference in the wear is slight and the velocities are bigger for pole B. Thus, only Pole B is simulated. The results are shown below on Fig. 59.

Figure 59: T30 Tmed – overpressure in the thermal volume – simulation of pole B

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On the first loop, FRAC 0.8 and 1.1 are equivalent, whereas FRAC 1.1 is more accurate on the second loop. The discharge is slower on MC3 for both tests.

As a result, FRAC 1.1 is chosen for T30 Tmed.

4.3.2 T100s study

4.3.2.1 Remarks on the velocities and pressure sensors No cursors were available for the T100s tests of Pole B (DE 7251). As a result, one cannot compare the velocities between the two poles as for T30 tests. Furthermore, when simulating the T100s tests of pole B, the strokes and velocities of Pole C will be used.

During the T100s tests, the pressure sensors heat up and they end up indicating negative pressures, which is clearly not physical. It is very difficult to analyze and conclude on such results.

4.3.2.2 T100s Tmin (around 16 ms) Table 5 shows the wear on the device for Poles B and C.

Table 5: T100s Tmin – Wear

Test Wearing

before test (kA^2s)




No wear Before test

No wear Before test

No wear Before test


Pole C – 7296 (16.2 ms)

14.67 4.00 1.18

Pole B – 7251 (16.2 ms)

38.28 8.72 2.77

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One notices that the device is more worn for Pole B. The wear is quite high for that test (the section under the cap is bigger by 38% than for a new pole). The section under the cap is also bigger than on pole C by more than 20%. However, since the measurement seems more reliable on Pole B, both tests are simulated. The results are depicted below on Fig. 60 and 61.

For pole C, the test values are again very high compared to pole B, and one notices a negative pressure at the end of the discharge.

Figure 60: T100s Tmin – overpressure in the thermal volume – simulation of pole C

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The simulation of pole B gives more accurate results. Indeed, FRAC 0.8 and FRAC 1.1 induce a low error on the first current loop. On the second loop, FRAC 1.1 follows the pressure rise observed in reality until a certain point, marked by the rose ellipse. In the end, FRAC 1.1 is better for T100s Tmin, even though the discharge is slower in MC3 and even though a negative pressure is observed on the real test measures.

4.3.2.3 T100s Tmax (around 25 ms) Table 6 sums up the wear for the T100s Tmax for both poles.

Table 6: T100s Tmax – Wear

Test Wearing

before test (kA^2s)




No wear Before test

No wear Before test

No wear Before test


Pole C – 7296 (25.2 ms)

86.57 21.52 6.14

Pole B – 7251 (25.1 ms)

59.60 13.19 4.04

It is clear that the wear is lower on Pole B. Pole C induces an important wear: the section under the cap is for instance 90% bigger than on a new pole. Consequently, the decision is to simulate only the test on pole B (cf. Fig. 62).

Figure 61: T100s Tmin – overpressure in the thermal volume – simulation of pole B

-47-

The shapes of pressure rise are completely different between real tests and MC3 calculations. Thus it is not possible to conclude here on the FRAC to implement.

4.3.2.4 T100s Tmed (around 20 ms) On Table 7, the wear for the T100s Tmed is shown.

Table 7: T100s Tmed – Wear

Test Wearing

before test (kA^2s)




No wear Before test

No wear Before test

No wear Before test


Pole C – 7296 (19.2 ms)

142.47 33.59 9.41

Pole B – 7251 (20.4 ms)

93.10 19.81 6.09

Again, the wear is higher for pole C: the section under the cap is bigger by 142% than for a new pole. The decision is thus to simulate only the test on pole B.

Figure 62: T100s Tmax – overpressure in the thermal volume – simulation of pole B

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One notices on Fig. 63 that on the first loop, FRAC 1.1 seems to be more accurate. On the second loop, FRAC 1.1 induces a good pressure rise before a dropout. Furthermore, the discharge is slower in MC3, but the discharge in real test measures ends with negative pressures as shown below (Fig. 64).

Figure 63: T100s Tmed – overpressure in the thermal volume – simulation of pole B

Figure 64: T100s Tmed – overpressure in the thermal volume – simulation of pole B

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0,020,040,060,080,0

100,0T30 Tmax

T30 Tmed

T30 Tmin

T100s Tmax

T100s Tmed

T100s Tmin

Max overpressure 1st loop

DE 7251 ‐ Pole B ‐FRAC 0.8


DE 7296 ‐ Pole C ‐FRAC 0.8


0,0100,0200,0300,0400,0500,0

T30 Tmax

T30 Tmed

T30 Tmin

T100s Tmax

T100s Tmed

T100s Tmin

Max overpressure 2nd loop





4.3.3 Conclusions on the study As mentioned earlier in the report (section 4.2), 4 comparison criteria (all of them concerning the overpressure in the thermal volume) are used. The figures below (Fig. 65, 66, 67 and 68) point out the percentage of error for numerical calculations compared to real tests for those 4 criteria.

Figure 65: T30 and T100s – maximum overpressure on the first loop

Figure 66: T30 and T100s – maximum overpressure on the second loop

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0,0100,0200,0300,0400,0500,0600,0700,0

T30 Tmax

T30 Tmed

T30 Tmin

T100s Tmax

T100s Tmed

T100s Tmin

Overpressure at I = 0





0,0

20,0

40,0

60,0

80,0

100,0T30 Tmax

T30 Tmed

T30 Tmin

T100s Tmax

T100s Tmed

T100s Tmin

Difference overpressure: max 2nd loop ‐ I = 0





Figure 67: T30 and T100s – overpressure at I = 0

Figure 68: T30 and T100s – difference between the maximum overpressure and the overpressure at I = 0

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Based on those criteria (notably the two first ones that are the most important and accurate), it is clear that MC3 simulations are more accurate for Pole B (DE 7251) than for Pole C (DE 7296). The MC3 tool now has to be calibrated with the FRAC parameter.

4.3.4 L75 test The only L75 implemented in MC3 for the calibration is the L75 Tmed because it has been identified as a critical default for the CB1 circuit breaker in a former study. The results are shown on Fig. 69.

One may point out that FRAC 1.1 is more accurate than FRAC 0.8 for L75 Tmed. On the first current loop, the difference in the maximum value for FRAC 1.1 is about m bars, but on the second loop an error of approximatively n bars is observed.

4.3.5 Remark on the tests and sensors No pressure sensors were available in the thermal volume for the T100a and L90 tests, that is why the calculations associated to these tests will only be part of the database of the CB1 for future modifications of the geometry. The calibration of the MC3 tool is only done with the T30, T100s and L75 tests. For each simulated default and arcing time, two FRAC are implemented: 0.8 and 1.1. The FRAC is the parameter that enables to calibrate MC3 (cf. section 4.1).

Figure 69: L75 Tmed – overpressure in the thermal volume

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4.3.6 Calibration based on the FRAC parameter As mentioned earlier, the calibration of MC3 for the CB1 circuit breaker is made with the FRAC parameter (0.8 or 1.1). Furthermore, seven tests are used: three T30 (Tmin, Tmax and Tmed), three T100s (Tmin, Tmax and Tmed) and one L75 (Tmed). The results are depicted below (Fig. 70, 71, 72 and 73), for the same 4 comparison criteria as before.

Figure 71: Calibration of MC3 – maximum overpressure on the first loop

Figure 70: Calibration of MC3 – maximum overpressure on the second loop

0,010,020,030,040,050,0T30 (25.2 ms)

T30 (20.2 ms)

T30 (16 ms)

T100s (25.1ms)

T100s (20.4ms)

T100s (16.2ms)

L75 (20.9 ms)

Max overpressure 1st loop

FRAC 0.8

FRAC 1.1

0,010,020,030,040,050,060,0

T30 (25.2ms)

T30 (20.2ms)

T30 (16 ms)

T100s (25.1ms)

T100s (20.4ms)

T100s (16.2ms)

L75 (20.9 ms)

Max overpressure 2nd loop

FRAC 0.8

FRAC 1.1

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Figure 72: Calibration of MC3 – overpressure at I = 0

0,010,020,030,040,050,060,070,0

T30 (25.2ms)

T30 (20.2ms)

T30 (16 ms)

T100s (25.1ms)

T100s (20.4ms)

T100s (16.2ms)

L75 (20.9ms)

Overpressure at I = 0

FRAC 0.8

FRAC 1.1

-54-

The results give that on the first current loop, FRAC 1.1 is more accurate than FRAC 0.8 for almost all the tests. It is difficult to conclude on the accuracy of each FRAC on the second loop. This is notably due to the fact that in real tests the sensors are heated up, leading to a discharge until negative pressures, which is not physical. It is also difficult to conclude on the two last criteria. As a result, it seems that FRAC 1.1 gives slightly better results than FRAC 0.8. That was also the case on former projects of the LTDT team. The choice is therefore to use FRAC 1.1 for the calculations.

4.4 MC3 database on the CB1 In addition to the T30, T100s and L75 used for the calibration of MC3, several other tests are simulated. A L90 Tmed is simulated because it is a critical test. Furthermore, capacitive tests (LC1 and LC2) and T100a Tmin and Tmax are simulated to complete the database. For each of those tests, some values are gathered to compare with later in the project.

As a result, Figs. 74 and 75 are obtained for respectively the L90 and T100a, and a whole database is built with various criteria.

Figure 73: Calibration of MC3 - difference between the maximum overpressure and the overpressure at I = 0

0,020,040,060,080,0

100,0T30 (25.2 ms)

T30 (20.2 ms)

T30 (16 ms)

T100s (25.1ms)

T100s (20.4ms)

T100s (16.2ms)

L75 (20.9 ms)

Difference overpressure: max 2nd loop -I = 0

FRAC 0.8

FRAC 1.1

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Figure 74: L90 Tmed – overpressure in the thermal volume

Figure 75: T100a Tmin – overpressure in the thermal volume

‐10000

0

10000

20000

30000

40000

50000

‐2

0

2

4

6

8

10

12

14

16

‐10 0 10 20 30 40

Current (A)

Ove

rpressure in

the therm

al volume (bar)

Time (ms)

L90 Tmed (20.2 ms) ‐ overpressure in the thermal volume

Pth

I total

‐10000

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

‐2

0

2

4

6

8

10

12

‐10 0 10 20 30 40

Current (A)

Ove

rpressure in

the therm

al volume (bar)

Time (ms)

T100a Tmin real (15.3 ms) ‐ overpressure in the thermal volume

Pth

I total

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5 DESIGN OF THE CB2 As mentioned earlier in the report, the final aim of the project is to design a new device, 25% cheaper than the CB0 in order to adapt it to the Indian market. This device, called CB2, is based on the CB14 on which the geometry is to be modified and optimized.

5.1 Optimization of the thermal volume An article (Yoshida, et al., 1997) studies the influence of the shape of the thermal volume. As mentioned in the article, the mixture of SF6 inside the thermal volume has a great influence upon the interrupting capability. The authors state that the efficiency of the thermal volume for mixing the gases correctly relies on a dimensionless parameter called A, as shown in Fig. 79 (Yoshida, et al., 1997), with:

√

With L axial length of the volume and S cross-sectional area.

The study points out that a thermal volume with 0.5 gives a good gas mixture. Indeed, this enables the hot gases to expand close to the end wall, and thus to push the cold gases towards the entrance of the thermal volume. As a result, at current zero the gases that flow back towards the arc are cold gases. This is shown in Fig. 80 (Yoshida, et al., 1997).

4 Reminder: the CB1 is a cheaper CB0

Figure 76: Simulation model for hot and cold gas in the thermal volume

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However, an internal study carried out by Quentin ROGNARD points out that the circulation of gases is also determined by some other features of the thermal volume. For the CB2, an optimization needs to be done on the thermal volume based on both studies. Until now, the thermal volume of the CB1 is very long and the hot gases don’t always reach the end of the volume. Four main technical solutions are proposed and delivered by HVM.

The CB1 is a double-motion circuit breaker, whereas for cost killing reasons the CB2 will have a single motion. The mobile part stroke is thus almost doubled between the two devices. As a consequence, it is necessary to reduce the cross-sectional area of the compression volume from M cm2 to N cm2. Otherwise, the pistoning effect would be too important and the resulting forces would slow the movement of the device.

Four technical solutions are proposed:

Solution A1: to implement the same cross-sectional area of N cm2 for both the compression and the thermal volumes. The advantage of this solution is that it reduces the outer diameter of the parts and thus allows for a decrease in the cost. However, it leads to an increase in the parameter A up to X.

Solution A2: to implement a cross-sectional area of N cm2 for the compression volume and a cross-sectional area of P cm2 for the thermal volume. In this case one gets A = Y.

Figure 77: Gas temperature distribution at current zero for different A with the same volume

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Solution B1: to implement a cross-sectional area of N cm2 for the compression volume while keeping an area of M cm2 for the thermal volume. This solution is a backup solution

Solution B2: to implement the same sections as for B1, but instead of decreasing the outer diameter of the compression volume we increase the inner diameter of that volume.

In addition to those solutions, various geometrical features are implemented in MC3 based on studies led by engineers of ARC during former projects. The aim of such features is once again to ensure a good gas mixture in the thermal volume.

5.2 Optimization of the exhausts Based on several former studies, four other solutions have been proposed by the author but are deemed confidential and could only be treated inside the company structures. These technical solutions will be tested on MC3 by the following interns of the LTDT team.

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6 REMAINING STEPS AND CHALLENGES All along this project, various difficulties have been faced and several problems have been solved. However, some actions are still to be led in order to complete the started studies.

Concerning the basis of the study, the geometry of the CB1 was modelled correctly. A mechanical study has been carried out and a MC3 database of the CB1 tests has been completed. Since the pressures were quite low for the MC3 calculations, the decision was to compare the CB1 simulations to the CB0. This study needs to be finalized; this is why it is not presented in this report.

The next step of the project is clearly now the optimization of the CB2 design on MC3. If the geometry is being created at the moment, the studies must be carried out. Some key points, like the thermal channel and the hot gas tube and the exhausts, have been changed compared to the CB0. The next simulations should focus on these modifications too. The team will be reinforced by two interns to work on the MC3 studies.

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CONCLUSION This Master’s Thesis has brought an important contribution to one industrial project, which aims at designing a new lower-cost high voltage circuit breaker: referred to herein as CB2. Indeed, the performed work has marked the beginning of the project development in terms of MC3 numerical calculations. The strategy chosen for the project was to modify the geometry of an existing circuit breaker and to make it 25% cheaper in order to adapt it to the Indian market.

To begin with, the first important step consisted in gathering all the tests data on the existing circuit breaker (CB1). A mechanical study has then pointed out that the velocities were lower than expected for almost all those tests, and that some tests induced a very high level of wear.

After that, the MC3 tool was calibrated. The implementation of several defaults, current levels and arcing times in MC3 showed that a FRAC parameter of 1.1 was more adapted to the CB1 device. A complete database of the CB1 was built with this value of the parameter. Nevertheless, the pressure rises were much lower in MC3 than in reality. To validate the reliability of the MC3 calculations, a comparative study is currently being carried out with the CB0. The next step of the project aims at modifying the geometry of the CB1 in order to design the CB2. This has been started – several configurations have been implemented – but the MC3 simulations will be done by the next interns.

The results of this project are expected to have a great impact in terms of their importance for the provision of affordable electrical engineering hardware for developing countries; and for the ability to simulate the complex thermo-chemo-mechanical & electrical phenomena inside a circuit breaker with the help of a tailored software.

On a more personal level, this thesis has represented a perfect complement to the author’s field of studies and professional development. Being a part of a dynamic and innovative team in which the author has been able to deepen his knowledge in several energy-related engineering domains – such as fluid mechanics, electromagnetism and radiation – and in the use of complex computational simulation models, is an irreplaceable experience. Furthermore, it has been really motivating to work with several international units of the GE company on very complex technical challenges with the objective to remain at the competitive edge of technology improvement.

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REFERENCES Budapest University of Technology and Economics Department of Electric Power Engineering.

(n.d.). Electrical switching devices and insulators . Retrieved from tankonyvtar: http://www.tankonyvtar.hu/en/tartalom/tamop425/0048_VIVEM174EN/ch06s02.html

Clean Technica. (2016). How The Grid Works, & Why Renewables Can Dominate. Retrieved from Clean Technica: https://cleantechnica.com/2015/12/16/how-the-grid-works-why-renewables-can-dominate/

GE Grid Solutions. (2016). Architecture of a line circuit breaker.

General Electric. (2015). Benefits of the "GE Store". USA: StockTwits.

General Electric. (2015). GE Grid Solutions joint-venture. Retrieved from http://www.gegridsolutions.com/GS_Brochure/files/assets/basic-html/page-1.html

Power Engineering International. (2012). ABB launches compact 420 kV gas insulated switchgear. Retrieved from Power Engineering International: http://www.powerengineeringint.com/articles/print/volume-20/issue-5/regulars/equipment-roundup/abb-launches-compact-420-kv-gas-insulated-switchgear.html

Robin Jouan, P. (2016). MC3 Formation - Theory. Villeurbanne.

V.R.V. (2015). CIRCUIT BREAKERS - HT & EHT. Retrieved from LinkedIn: https://www.linkedin.com/pulse/circuit-breakers-ht-eht-v-r-v

Vassilev, A. (Feb. 2016). Easy MC3. Villeurbanne.

Yoshida, D., Ito, H., Kohyama, H., Sawada, T., Kamei, K., & Hidaka, M. (1997). Evaluation of Current Interrupting Capability of SF6 Gas Blast Circuit Breakers. Amagasaki, Hyogo, JAPAN: Mitsubishi Electric Corporation.

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APPENDIX 1

At first, it was decided to simulate the T30 and T100s of DE 7296. However, the fact that the breaking chamber is changed after each DE was not taken into account. As a result, the wear of the whole pole C was simulated (see Fig. 81).

After having post-treated the numerical calculations, the author realized that the correct way was to simulate only the wear due to the T30 and T100s of DE 7296 (see Fig. 82). The difference between both cases is important mainly for the first tests, since the final diameters after T100s are the same.

Figure 78: Old calculations – wrong assumptions

Figure 79: New calculations – correct assumptions

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In the next section, we explain the difference in the wear for each simulated test between the old and new calculations, as well as the consequence of that difference on the results.

At first, the simulated tests are those shown on the picture below. As mentioned earlier, the first calculations included an error in the wear. Furthermore, the discharge valve, which is represented as a dynamic valve in MC3, was wrongly defined. The decision was to restart the calculations with a correct definition of the discharge valve and a correct wear (see Fig. 83).

T30 study T30 Tmin (real)

Tables 8 and 9 summarize the wear for the old and new calculation for the T30 Tmin. One observes that the wear is much lower in the new simulation.

Table 8: Old calculations – Wear before T30 Tmin (kA2s)

Test

Wearing before test (kA2s)

Old calc New calc %

difference

T30 Tmin 98.04

Figure 80: Initially simulated tests

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The table below shows the wear for both the old and new calculation. The color code concerns the difference in the value of the section between a new pole and the pole before the concerned test:

‐ Green for a difference < 10 % ‐ Orange for 10 % < difference < 20 % ‐ Red for a difference > 20%

Table 9: Old calculations – Wear before T30 Tmin (mm)

Test




No wear

Old calculation

New calculation

No wear

Old calculation

New calculation

No wear

Old calculation

New calculation

Value %

wear Value %wear

Value

% wear

Value %wear

Value %

wear Value %wear

T30 Tmin

47.17

1.20 12.28

0.34 3.56

0.10

The overpressure for the new simulation is accurate on the first loop but lower than in test on the second one (z bar at the maximum versus almost 1.9z bar in real test, see Fig. 84). It is necessary to restart the simulation with a correct definition of the thermal valve.

Figure 81: Old calculations – overpressure in the thermal volume – T30 Tmin

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T30 Tmax

Tables 10 and 11 compare the wear for the old and new calculations for the T30 Tmax.

Table 10: Old calculations – Wear before T30 Tmax (kA2s)

Test

Wearing before test (kA2s)

Old calc New calc %

difference

T30 Tmax 90.05

Table 11: Old calculations – Wear before T30 Tmax (mm)

Test




No wear

Old calculation

New calculation No

wear

Old calculation

New calculation No

wear

Old calculation

New calculation

Value %

wear Value %wear Value

% wear

Value %wear Value %

wear Value %wear

T30 Tmax

51.99

7.01 13.45

1.93 3.89

0.58

It is noticeable that the new calculation is made with much lower levels of wear. This induces a higher pressure rise (max at 1.2y bar instead of y bar, for the FRAC 1.1) in the thermal volume. However, the discharge valve still does not open: there is still a mistake in its definition and the calculations will have to be started once again.

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The overpressure in the thermal and compression volumes for the new calculation are shown on Fig. 82 below, for each simulated FRAC, and compared to the test measures.

The red dotted line indicates the set calibration of the discharge valve. Thus, normally when the overpressure in the compression volume (blue line) overcomes that value, the valve opens and the pressure decreases. One clearly notices that here again, the discharge valve does not open.

The non-opening of the discharge valve may decrease a little the pressure rise on both waves, but cannot explain the difference in the shape of the first pressure rise. Indeed, while in the numerical calculation the pressure rise is around x bar with a FRAC 1.1, it is about 2x bar in the real test.

The main conclusion is that it is necessary to restart the simulation.

T30 Tmed

The T30 Tmed of Pole C (DE 7296) was not simulated at first because the velocities seem too low on that test. That is why we decided to simulate the T30 Tmed of Pole B (DE 7251) instead. For that test, the wear was correct, so the old and new calculations are the same. It is noticeable on Table 12 below that the wear is quite low.

Table 12: Old calculations – Wear before T30 Tmed (mm)

Test




No wear

New calculation No wear

New calculation No wear

New calculation


T30 Tmed 14.35 3.40 1.05

Figure 82: Old calculations – overpressure in the thermal volume – T30 Tmax

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On Fig. 83 below, one may point out that the shape of the overpressure in the thermal volume is very similar between test and simulation, and that the simulation in quite accurate. However, since the thermal valve still does not open, the simulation will have to be relaunched.

T100s study T100s Tmin

The wear for the two calculations series is shown on Table 13.

Table 13: Old calculations – Wear before T100s Tmin (mm)

Test




No wear

Old calculation

New calculation No

wear

Old calculation

New calculation No

wear

Old calculation

New calculation

Value %


% wear

Value %wear Value %

wear Value %wear

T100s Tmin

58.28

14.67 14.96

4.00 4.32

1.18

One notices that the wear is much lower in the new calculation, especially in terms of section under the cap. Fig. 84 below shows the overpressure in the thermal volume for the new calculation and both FRAC. The overpressure in the thermal volume is too low for both cases, even though the error is lower for FRAC 1.1. Relaunching the calculation with a correct definition of the discharge valve may solve the problem.

Figure 83: Old calculations – overpressure in the thermal volume – T30 Tmed

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T100s Tmax

Table 14 summarizes the wear values in terms of diameters for the T100s Tmax.

Table 14: Old calculations – Wear before T100s Tmax (mm)

Test




No wear

Old calculation

New calculation No

wear

Old calculation

New calculation No

wear

Old calculation

New calculation

Value %


% wear

Value %wear Value %

wear Value %wear

T100s Tmax

113.91

86.57 27.56

21.52 7.80

6.14

The wear is a bit lower on the new calculation but the difference is low. As for the overpressures, they are shown on Fig. 85 below. One observes that the overpressure in the thermal volume is lower on MC3 than in real tests, even for FRAC 1.1. The decision that is taken is thus to restart the calculation with a correct definition of the discharge valve.

Figure 84: Old calculations – overpressure in the thermal volume – T100s Tmin

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T100s Tmed

Table 15 below shows the wear for the T100s Tmed tests. One remarks that the wear is a little lower for the new calculations but the difference is slight.

Table 15: Old calculations – Wear before T100s Tmed (mm)

Test




No wear

Old calculation

New calculation No

wear

Old calculation

New calculation No

wear

Old calculation

New calculation

Value %


% wear

Value %wear Value %

wear Value %wear

T100s Tmed

154.45

142.47 36.07

33.59 10.07

9.41

Fig. 86 points out that the pressure rise is much lower even on the first current loop for both FRAC. However, the negative pressures observed in tests may partly explain this phenomenon. This test is thus relaunched on MC3.

Figure 85: Old calculations – overpressure in the thermal volume – T100s Tmax

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Conclusions of that calculations series

The main conclusion of the calculations series is that the discharge valve is still wrongly defined, making it necessary to restart the calculations since that parameter has a great influence on the overpressure in the thermal volume, at least for the T30 tests.

Furthermore, the assumptions for the wear were correct for all simulations but the overpressure in the thermal volume is still low compared to real tests. The only test that gives a good correlation between test and simulation is the one coming from Pole B (DE 7251). Thus, measurements of Pole C (DE 7296) may be questioned giving the important wear before each simulated test. Finally, the ability of the MC3 tool to deal with high wear may be questioned.

The decisions taken were to start new calculations for the T30 and T100s on pole B (DE 7251) to compare with the same tests done on Pole C (DE 7296) and to start other calculations (Capacitive tests, T100a, L75, L90) to complete the database of the CB1. This is what was presented in the report.

Figure 86: Old calculations – overpressure in the thermal volume – T100s Tmed

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