safe circuit breakker

7/29/2019 safe circuit breakker

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Safe Circuit Breaker Timing with New Technology

Linus Claesson, Zoran Stanisic, Heinz Wernli, Klas Pettersson

GE Energy, Test Equipment, Programma Electric, Eldarvgen 4, 187 75, Tby, Sweden

An oral presentation of this article was given at CMD2006, International Conference onCondition Monitoring and Diagnosis, Changwon, Korea, April 4, 2006. The article is available onthe proceedings CD. The conference is Technically Co-sponsored by CIGR and IEEE.

The article is under copyright by The Korea Institute of Electrical and Electrical MaterialEngineers and Korea Electrotechnology Research Institute.


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Safe Circuit Breaker Timing with New Technology

Linus Claesson, Zoran Stanisic, Heinz Wernli, Klas Pettersson

GE Energy, Test Equipment, Programma Electric, Eldarvgen 4, 187 75, Tby, Sweden

Abstract The best way to improve personnel safetywhen working in a substation is to increase the distancebetween personnel and devices with high voltage.Regulations and laws require all objects to be grounded onboth sides before any maintenance work is performed onthe object. For circuit breaker maintenance, the most basicand important test, that is main contact timing, isperformed today without this basic safety prerequisite.Additionally, there is no feasible technique for this testwith both sides grounded. A new technology that makesmain contact timing of high voltage circuit breaker

possible with both sides grounded is described. Capacitiveproperty of circuit breakers for high frequency signals isused. Increased safety and simplified work process are themain advantages. The method described applies to anycircuit breaker, but is best for GIS, generator circuitbreakers and for the highest voltages is where there is themost to gain.

Index Terms Safety, Circuit breaker testing, Circuitbreakers, Timing, Capacitance, Resonance.

I. INTRODUCTION

The use of electricity throughout society has increasedtremendously over the last century. In parallel, the

safety around all components and systems has increased.When Thomas Alva Edison lit the first light bulbs on theSwedish countryside, there was no protective insulationon the wiring. Today, we have a consumer system thatmakes utilization of electricity, in homes and industries,much safer.

In transmission and distribution systems, the power isstronger and safety systems less developed. In highvoltage substations, only highly trained and skilledprofessionals can be permitted to work. They have to bewell acquainted with the safety deficits. Still, there areaccidents with severe outcome for workers in substation

environments.Safety in electricity transmission and distribution is ahigh priority to all involved, from the political level tothe field engineer. With the evolution of electronics, itbecomes possible to improve substation safety. Theproposed method is one example.

II. SUBSTATION PERSONNEL SAFETY

Since testing of circuit breakers was started, safety forthe personnel has been a high priority for developmentof test methods and equipment. The best way to improvepersonnel safety, when working in a substation, is toincrease the distance between personnel and devices

with high voltage. When an object is taken out ofservice, there are three main cases when the objectbecomes dangerous due to high power electric potential.The first is if a fault occurs and an unwanted electricalpotential reaches the object. The second is lightningsomewhere on the lines coming in contact with theobject. The third is capacitive coupling from oneconductor with high voltage causing a dangerouspotential on the object in question. The voltage cancause a current of 20 mA, or even more, through a

human body. Currents above 3.5 mA are considereddangerous according to IEC EN 61010.The only way to make an object safe, for all the cases

mentioned, is to connect the object to a ground at allpoints where it is in contact with the surroundingsystem. In this way, any potential on the object will takethe shortest path to earth that is through the 120 mmgrounding cable. An object grounded in this way cannotbecome dangerous.

Regulations and laws require all objects to begrounded on both sides before any maintenance work isperformed on the object. There are approved exceptionsfor this safety prerequisite because it is not possible to

do all maintenance work with both sides grounded.Main contact timing of circuit breakers is performedwith only single side grounding.

Another property of substation test equipment that isimportant for safety is the user interface. Interactionshall be straightforward, fast and easy. Field engineersshall spend energy solving the task they are out to solve,not figuring out how the equipment works. Thisproperty is hard to put numbers on and is not covered instandards and regulations.

While typically safe, there are obvious risks topersonal safety inside a substation. The time in the

substation is not only expensive, it also means exposureto a less safe environment. This is another argument forchoosing user-friendly equipment in the name of safety.

III.CIRCUIT BREAKER TIMING

The most basic and important test for circuit breakerdiagnosis is main contact timing. The response time ismeasured from operation command until operation,close or open, is completed. If the main contact timingon an interrupter is deviating from the manufacturerrecommendation, the circuit breaker needs maintenance.Main contact timing also evaluates synchronizationbetween phases in a three-phase system and the


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synchronization of multiple contacts within one phase.Procedures for diagnostic test are described in [1].

A. Conventional Timing Method

Conventional technology for main contact timing is to

use ohms law with a DC test voltage over the circuitbreaker to distinguish the moment when the circuitbreaker state changes from open to close. This method isnot based on a standard and there is, therefore, noabsolute reference point. However, it is todays state ofthe art process and used throughout the industry.

With both sides of the circuit breaker connected toground, this method does not work. The DC test voltagewill only sense the connection over ground and not whathappens on the circuit breaker. Therefore, safe timing isnot possible.

To overcome this safety drawback, EDF, the state-

owned electricity company in France, uses an automaticgrounding device. A voltage sensor automatically closesa ground connection if there is a voltage potentialbetween the object and ground. This device is used onone side while the other is grounded with a cable. Thetiming test is more complicated and requires longer timewithin the substation.

B. On-line Indirect Timing

As an alternative to the above approach, it is possibleto derive timing data for a circuit breaker by sensing onthe auxiliary contacts during an open-close operation.This On-line Indirect Timing gives timing based on the

control circuits information. Usually the trig coilcurrent curve is also recorded as additional data.

The advantage is that the circuit breaker can stay inservice even though the open-close operation will causea disturbance on the power delivery. One drawback isthat timing values are not the real values and do notreflect the circuit breaker performance. The method isnot suitable for circuit breaker diagnosis. It is a quicktest where the primary take-away is that the circuitbreaker reacted on open and close commands. The testreveals about the same information as an opticalinspection. It is not advisable to base a diagnosis on this

kind of test.C. Dynamic Resistance Measurement (DRM)

With DRM, a high current is used to obtain theresistance during a circuit breaker operation. Theresistance trace is used primarily for advanceddiagnosis, but the contact time is readily available. Thedrawback with this method is the requirements onequipment to generate the strong current. Equipment isheavy and expensive. DRM is performed with only oneside grounded.

IV. BOTH SIDES GROUNDED TIMING

A new technology that makes main contact timing ofhigh voltage circuit breaker possible with both sidesgrounded is now described. Dangerous voltage can bekept at a safe distance from all personnel throughout thecircuit breaker timing test. A safe area around the circuitbreaker can be created and clearly marked with securityfencing in accordance with common recommendationsand regulations. Accidents with electric arc andelectrocution can be avoided. A schematic diagram ofthe application is given in Fig. 1. On the outside of thedouble grounding there will be potentially dangerousvoltages. On the inside, between the groundings, theelectrical potential is removed through the groundings.This creates the Safe area also shown in Fig. 1.

The increased safety is not based on the new methodor equipment. Safety comes from keeping both sides

grounded throughout the timing test.

Fig 1. Schematic hook-up diagram for timing with bothsides grounded. Between the grounding leads a safe area iscreated where personnel can be safe during a timing test.

The result of a main contact timing based on this newtechnology is in no way different for an interpreter andfully compatible with the conventional main contacttiming measurement. On the graphical user interface, thesame thin or bold line will appear representing open andclose. The new technology is embedded within thedevice without impacting user interaction. For the fieldpersonnel the way of working becomes somewhat easierand faster, but otherwise remains familiar.

The method is non-intrusive and does not affect thecircuit breaker in any way. The only connections arelow current sense leads to the bus bars. The mechanismis not touched and there is, therefore, no need for anymechanical adoptions or conversion tables for differentcircuit breaker types. No reference value from

manufacturing, commissioning or other beforehandinformation is required.


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With an easier and faster test, field personnel candedicate more attention to performing the actual task.The amount of time spent in the substation is alsoreduced.

V.APPLICATION AREA

The method is applicable on any circuit breaker.There are some cases where, for practicalconsiderations, the double grounding brings advantagesbeyond what has been described so far. These cases aredescribed briefly below.

A. High Voltage Circuit Breakers

For high voltage circuit breakers the advantages of thetechnique is interesting. This is because the safetyconcern increases with increasing voltage. The proposed

technique has no upper limit for voltage and is fullyapplicable on high voltage, ultra high voltage andextremely high voltage.

B. Generator Circuit Breaker (GCB)

The power outlet from a turbine in a power plant hasa high current. The short circuit current can be around900 kA at 11-33 kV. The high current requires verythick conductors to avoid overheating. The bus bars andcouplings are enormous, with hundreds of screws drawnto an accurate torque. Disconnection of the bus bar takesmore than one day. If one screw is drawn to the wrongtorque, there will be a spot of increased resistance that is

heated. If the heating is strong, the power plant needs tobe closed down for the loose bolt to be drawn. Knowingthis, it is easy to understand why power plant owners arereluctant in disconnecting the bolted bus bars for atiming measurement.

All service in the power plant is scheduled aroundplanned overhaul events. At this time, there is all kind ofpersonnel working on the turbine and power line. Safetyrequirements demand the GCB to be grounded at bothends during planned service events.

Timing of the main contacts with conventional timing,using a DC voltage to sense contact, is now only

possible if one bus bar is disconnected. The proposedmethod allows timing with both sides grounded. Thetime-consuming work to disconnect the bus bar is savedand the risk to have one loose bolt causing resistiveoverheating is eliminated.

C. Gas Insulated Substation (GIS)The major advantage with GIS is the reduction of

space required compared to air-insulated substations.The maintenance and test interval for circuit breakersinstalled in GIS is longer compared to air-insulatedinstallations. GIS have been installed since the 1980sand many are reaching the age for maintenance.

To diagnose a GIS circuit breaker, the most importantdiagnosis tool is the main contact timing describedpreviously. Conventional timing requires that at leastone side of the circuit breaker be grounded. An IsolatedGround Switch has a built-in feature that permits access

to the primary circuit without grounding it. An IsolatedGround Switch is, therefore, required on at least oneside for a conventional timing without a full dismantlingof the GIS.

Early GIS installations rarely have Insulated GroundSwitches installed. Usage has increased over thedecades, but it is still common to find new installationswithout Insulated Ground Switches. The dismantlingrequired for conventional timing is time consuming andexpensive. Also, if the bolts are not properly drawn,there will be a SF6 leakage.

The proposed method is capable of performing a

timing measurement of a GIS circuit breaker wherethere is not an Isolated Ground Switch available.

VIPROPOSED DIAGNOSTIC TECHNIQUE

A. Resonant Frequency Model

The fundamental physical property that the proposeddiagnostic technique is based upon is that a capacitanceis formed when two areas of conductor is separated byan insulation medium. In a circuit breaker, the contactsare the conductors and the insulation media is usuallyoil, air, vacuum or SF6. Any circuit breaker, therefore,

holds capacitance. When the contacts move, i.e. during aclose or open operation, the capacitance varies.

The capacitance in the circuit breaker is used as a partof a resonant circuit. The circuit also containsinductance and resistance. The inherent resistance andinductance of the cables used to connect the measuringdevice is used. See Fig. 2 for a model. The dashed boxis the interrupter part of the circuit breaker. Theinterrupter has a small resistance by design. The variablecapacitance is formed by the two separated connectorsand is variable with connector position, drawn as avariable capacitor in Fig.2. When the circuit breaker is

closed, the capacitance is zero. In Fig. 2 gradingcapacitors and pre- insertion resistors are excluded.The LRC resonant circuit has a distinct resonant

frequency. The resonant frequency is dependent on thevalue of the circuit breaker capacitor; therefore, theresonant frequency will vary with movement of themain contacts.


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Fig. 2: The resonant circuit model formed by the circuitbreaker (dashed box) and the connection cables from themeasurement unit. The grounding cables are also included inthe figure, even though they are not part of the resonantcircuit.

The ground connection has a very limited influenceon the resonant circuit for high frequency signalsbecause the impedance in the ground cables is too high.In special situations, like application on GIS, thegrounding connection requires an increase of impedanceon the ground connection. This is achieved by putting aferrite clamp around the ground connector. The ferriteclamp is two half-donuts that are easy to attach around acable or bridge.

B. Measurement Principle

A resonant circuit with the resonant frequencydepending on the contact position has been defined. Thefirst step in the measurement is to identify the resonantfrequency. A sweep scanning over different frequencies,in the MHz band, and comparing the response strengthreveals the resonant frequency. This is preferablyperformed on the closed circuit breaker.

The next step is to generate a signal with the resonantfrequency over the resonant circuit and record theresponse during an operation. The operation is eitheropen or close. The recorded response will beproportional to the capacitance of the circuit breaker.The diagnosis technique is to use the non-linear changein capacitance and stepwise change in resonance of thecircuit breaker with the major events through anoperation. Fig. 3 is showing a theoretical diagram for aclose operation. At t0 the circuit breaker is open. Alongthe contact motion, the response signal amplitudechanges. First contact is at t1, with a discontinuous jumpin response. The first contact time t1 is the measurementvalue that IEC [5] and ANSI/IEEE [4] standarddefines as the timing value. The bounces between t1 andt2 are due to touches of arcing contact and main contact

before a permanent contact is established. At t2 there is a

discontinuous jump in response taking place when themain contact closes. Thresholds are used to convert theresponse values into a traditional timing graph, shownbelow in Fig. 3, where the thin line represents open andthe bold line represents a closed circuit breaker.

The proposed method for diagnostic test is covered intwo patents [2] and [3].

Fig. 3: The response measured when applying the highfrequency AC test signal over a close operation. Below is thetraditional thin or thick line indication circuit breaker status.

C. Diagnosis

The result is interpreted in the same way as forconventional timing. All results are in accordance withthe established standards [4] and [5].

VII.FIELD EXPERIENCE

Field experience is still limited to trial tests withprototype product and prototype software. Results arestraightforward to verify, as conventional timing is wellunderstood. The first results are promising and verifiesthe application field described above. No field test hasbeen made on GCB.

VIIIFUTURE DEVELOPMENT

This technology holds promise for futuredevelopment to obtain even better analysis compared toconventional technology and what has been verified inthe field.

A. Velocity Without Transducer

It is desired to measure the velocity of moving partswithin the circuit breaker, like the mechanism, toconduct a condition diagnosis. The conventional way tomeasure the velocity is to physically mount a transduceron the accessible moving part and record the signal.


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With a conversion table, the contact motion can becalculated. The conversion table translates themovement on the accessible mechanical part where thetransducer is mounted into actual movement of theconnector.

With further development and research, there aretechnical openings to measure the contact movementvelocity without using a physical transducer. Theresonant properties that change with mechanismposition are taken advantage of. The measurement willneed calibration from a previous operation withtransducer-based velocity measurement. During thiscalibration, operation data is collected that can laterreplace the transducer. A conversion table is created thatis valid for this particular type of circuit breaker.

This method of measuring velocity is less invasive asno mechanical parts are touched. This improves safety

further because no objects are mounted on the circuitbreaker. It also reduces the need for mechanical adaptersand knowledge of conversion tables. The circuit breakertype does not have to be known and the easy procedureis identical, independent of circuit breaker type. Bothsides remain grounded.

B. Pre-Insertion Resistor (PIR)

In some circuit breaker constructions a resistor isconnected in parallel to the main interrupter. Aninterrupter in series connects the Pre-Insertion Resistor(PIR). The PIR is used to limit the current to beinterrupted. The PIR is in parallel to the dashed box and

not shown in Fig. 2.With a PIR in the circuit breaker, the resonant model

is changed. Depending on the PIR status, there are twodifferent resonant frequencies. A diagnosis of a circuitbreaker with PIR shall include timing measurement ofthe PIR connector. The proposed methods can detect thetransition of the PIR connection.

IX.CONCLUSIONS

The following conclusions can be drawn from thispaper:

1) The safety for field personnel can be substantiallyimproved during timing test of circuit breakers.2) The proposed method is general for all kinds ofcircuit breakers. Interpretation is not changed. Theamount of time required for measurement is reduced.The hook-up procedure is easier.3) The proposed method is non-intrusive and does notrequire any beforehand information. Timing andvelocity is measured without touching any mechanicalpart.

REFERENCES

[1] F. Salamanca, F. Borras, H. Eggert, W Steingrber,Preventive diagnosis on high-voltage circuit-breakersCIGR Symposium Berlin 1993, 120-02.

[2] Methods and apparatus for analyzing high voltagecircuit breakers, U.S. Patent 6 963 203, Nov.8, 2005.

[3] Methods and apparatus for analyzing high voltagecircuit breakers, U.S. Patent 6 850 072, Feb.1, 2005.

[4] IEEE Std C37.09-1999.[5] IEC Std 62271-100.

TRADEMARK NOTICES

IEC is a registered trademark of InternationalElectrotechnical Commission.

IEEE is a registered trademark of Institute of Electrical andElectronics Engineers

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