Instructions Manual MIDAS 288x

Operating Instructions

HAEFELY TEST AG

MIDAS 288x Mobile Insulation

Diagnosis & Analysing System

Version 1.9

Art.Nr. 4840602 Operating Instructions MIDAS 288x

Date Version Responsible Changes/Reasons Sept 2004 1.0.0 ThG, RF, LWA, MF Initial version June 2005 1.0.1 LWA More connection schematics

inserted July 2005 1.0.3 ThG RS 232 Options Sept 2005 1.0.4 ThG Additonal Remote

Commands Oct 2005 1.0.5 ThG Software Update Info Nov 2005 1.06 ThG Inductor Instruction

Febr 2006 1.07 RSch Text added and changed

March 2006 1.1 LWA 2881 integrated May2006 1.2 RSch 5289 updated June 2006 1.3 ThG Extended Noise Reduction Sept 2006 1.4 ThG Signal / Noise Ratio March 2007 1.5 LWA 5289 & base load July 2009 1.6 ThG Parallel Capacitance

Detection Nov 2009 1.7 ThG Record Options / Formula in

Grid Sept 2011 1.8 ThG Cn Current Oct 2012 1.9 ThG Chinese language

Remember - Hazardous voltage can shock, burn or cause death !

This warning sign is visible on the MIDAS. Meaning: This equipment should only be operated after carefully reading the user manual which is an integral part of the instrument.

HAEFELY TEST AG and its sales partners refuse to accept any responsibility for consequential or direct damage to persons and/or goods due to none observance of instructions contained herein or due to incorrect use of the MIDAS.

Further be aware that Safety is the responsibility of the user !

Any correspondence regarding this instrument should include the exact type number, instrument serial number and firmware version number. With the exception of the firmware version number, this information can be found on the registration plate on the right panel of the instrument. The firmware version written on the About Menu.

The design of this instrument will be continuously reviewed and improved where possible. Therefore there may be small differences between the operating manual and the actual instrument. Although all efforts are made to avoid mistakes, no responsibility is accepted by HAEFELY TEST AG for the accuracy of this operating manual.

HAEFELY TEST AG accepts no responsibility for any damage that may be caused during use of this document. We reserve the right to amend the operation, functionality and design of this instrument without prior notice. If discrepancies are noticed between the on-line help provided by the instrument and the operating manual, then the on-line help should be followed.

All rights reserved. Any use of this manual other than for operation of the instrument requires prior written authorization from HAEFELY TEST AG.

2012, HAEFELY TEST AG, Switzerland

Foreword Welcome as a new user of the Insulation Diagnosis System MIDAS. Thank you for placing your confidence in our product.

With the purchase of this measuring instrument you have opted for all the advantages that have built a world-wide reputation for a Tettex Instrument: Robustness, performance and quality assured. As a result this instrument provides a solution which achieves the optimal combination of traditional know-how and leading edge technology.

This operating manual is designed for completeness and easy location of the required information. Customers who already have experience with this kind of equipment will find this document to be of assistance as an extended help. A keyword index at the end of the operating manual greatly eases use.

If you find a mistake or inconsistency in the operating manual then please feel free to inform our Customer Support department with your corrections so that other users may benefit.

Abbreviations, definitions Wherever possible the corresponding IEC definitions are used. The following abbreviations and definitions are used in this manual:

CN Standard capacitor (Measurement reference, built-in the instrument)

CX Test object capacitance (e.g. power transformer, generator, motor etc.)

HV High voltage

cos Power Factor PF Power Factor

tan Dissipation factor DF Dissipation factor

DUT Device Under Test (Test Object)

ppm Parts per million

Introduction I

Contents 1 Introduction 1

1.1 Receiving Instructions..................................................................................... 1 1.2 General ........................................................................................................... 1 1.3 Hardware ........................................................................................................ 1 1.4 Software ......................................................................................................... 2 1.5 Scope of Supply.............................................................................................. 2 1.6 Optional Accessories ...................................................................................... 3

2 Technical Data 4

3 Safety 7 3.1 General ........................................................................................................... 7 3.2 Personnel Safety ............................................................................................ 7 3.3 Safety Features .............................................................................................. 8 3.4 Safety Precautions.......................................................................................... 8 3.5 Summary ........................................................................................................ 9

4 Theory 10 4.1 Why is Insulation Tested?............................................................................. 10 4.2 What is Loss Factor? .................................................................................... 10 4.3 What is Dissipation Factor tan ? ................................................................. 11 4.4 The Difference between Power Factor and Dissipation Factor ..................... 12 4.5 Apparent Power, Real Power, Reactive Power............................................. 12 4.6 Test Instruments ........................................................................................... 13 4.7 Evaluation of Test Results ............................................................................ 13 4.8 Supplementary Test Methods....................................................................... 16 4.9 Standard Capacitor, Measuring Current & Limits.......................................... 17 4.10 Parallel & Series Equivalent Circuits............................................................. 18

5 Functional Description 19 5.1 System Overview.......................................................................................... 19 5.2 V-potential point and Guarding ..................................................................... 20 5.3 Test Modes................................................................................................... 22 5.4 Interference Suppression.............................................................................. 24

6 Operation Elements 25 6.1 Individual Parts ............................................................................................. 25 6.2 Touchscreen PC head as System Controller ................................................ 26

6.2.1Interfaces .............................................................................................. 26 6.2.2Touchscreen calibration........................................................................ 26 6.2.3Strip Printer........................................................................................... 27

6.3 Laptop as System Controller......................................................................... 28 6.3.1Interfaces .............................................................................................. 28

6.4 Instrument Glove box ................................................................................. 28

II Introduction

6.5 Instrument Side Panel .................................................................................. 29 6.5.1Measuring Inputs .................................................................................. 29 6.5.2High Voltage and Power Outputs.......................................................... 30

6.6 HV GND Connection Surveillance ................................................................ 31 6.7 Safety Ground (Earthing).............................................................................. 31 6.8 Emergency Stop ........................................................................................... 32 6.9 Warning Lamp Bar........................................................................................ 32

7 Software 33 7.1 General......................................................................................................... 33

7.1.1Start-up................................................................................................. 33 7.1.2Main Window ........................................................................................ 34 7.1.3Title bar................................................................................................. 35 Alarm Messages ........................................................................................... 36 7.1.4Function Keys ....................................................................................... 38

7.2 File Manager................................................................................................. 38 7.2.1File Selector Dialog............................................................................... 39 7.2.2Report ................................................................................................... 41

7.3 Display of Measurement Values ................................................................... 42 7.4 Tab sheet SETUP......................................................................................... 46

7.4.1Menu DUT Info ..................................................................................... 47 7.4.2Menu Conditions (Temperature correction) .......................................... 49 7.4.3Menu Settings....................................................................................... 52 7.4.4Menu Options ....................................................................................... 55 7.4.5Menu Auxiliary ...................................................................................... 58

7.5 Tab sheet MANUAL...................................................................................... 59 7.5.1Definition of Columns for Measuring Spreadsheet................................ 63 7.5.2Formulas in Measuring Grids................................................................ 64 7.5.3Menu Signal Analysis ........................................................................... 65

7.6 Tab sheet SEQUENCE................................................................................. 70 7.6.1Definition of Spreadsheet Sequence .................................................... 70 7.6.2Sequence Measurement....................................................................... 73 7.6.3Edit Sequence Limiters ......................................................................... 75 7.6.4Starting Sequence ................................................................................ 77 7.6.5Sequence with External AC Power Source........................................... 78

7.7 Tab sheet ANALYSIS ................................................................................... 79 7.7.1Spreadsheet Measurement................................................................... 79 7.7.2Graphic Analysis................................................................................... 80 7.7.3More Analysis ....................................................................................... 81

7.8 Remote Operation ........................................................................................ 83 7.8.1Characteristics of the interface ............................................................. 83 7.8.2General Commands.............................................................................. 86 7.8.3System control commands.................................................................... 88 7.8.4Measurement commands ..................................................................... 90 7.8.5Alarms .................................................................................................. 92

8 Accessories and Options 94 8.1 Office Software ............................................................................................. 94 8.2 Safety Strobe Light ....................................................................................... 94 8.3 Oil Test Cell .................................................................................................. 94 8.4 Resonating Inductor...................................................................................... 95

8.4.110kV Resonating Inductor 5288A ......................................................... 96 8.4.215kV Resonating Inductor 5289............................................................ 99

8.5 Current Booster .......................................................................................... 105

Introduction III

8.6 Transformer Turns Ratio Meter TTR........................................................... 107 8.7 Frequency Response Analyser FRA........................................................... 108

9 Care and Maintenance 109

10 Instrument Storage 110

11 Packing and Transport 111

12 Recycling 112

13 Trouble Shooting 113 13.1 Software Updates ....................................................................................... 113

14 Conformity 114

Appendix 115

15 Applications Guide 116 15.1 Bushings..................................................................................................... 116

15.1.1Spare Bushings ................................................................................ 118 15.1.2Installed Bushings............................................................................. 119 15.1.3Measuring Data Interpretation .......................................................... 122

15.2 Transformers .............................................................................................. 123 15.2.1Power and Distribution Transformers ............................................... 123 15.2.2Shunt Reactors ................................................................................. 128 15.2.3Current Transformers ....................................................................... 129 15.2.4Voltage Transformers ....................................................................... 130 15.2.5Short Circuit Impedance ................................................................... 131 15.2.6Excitation Current Measurement ...................................................... 133

15.3 Rotating Machines ...................................................................................... 135 15.3.1Test Procedure ................................................................................. 135 15.3.2Measuring Data Interpretation .......................................................... 136 15.3.3Measuring high cap values using the Resonating Inductor............... 138 15.3.4Operation of Resonating Inductor with Midas ................................... 139

15.4 Liquid Insulation.......................................................................................... 140 15.4.1Test Procedure ................................................................................. 140 15.4.2Measuring Data Interpretation .......................................................... 141

15.5 Cables ........................................................................................................ 142 15.5.1Test procedures on different cables.................................................. 142 15.5.2Test Procedure Example .................................................................. 143 15.5.3Measuring Data Interpretation .......................................................... 144

15.6 Capacitors .................................................................................................. 144 15.6.1Measuring high cap values using the Resonating Inductor............... 145

15.7 Circuit Breakers .......................................................................................... 146 15.7.1Dead Tank Breaker........................................................................... 148 15.7.2Live Tank Breaker............................................................................. 149 15.7.3Measuring Data Interpretation .......................................................... 150

15.8 Surge (Lightning) Arresters......................................................................... 150 15.8.1Test Levels ....................................................................................... 150 15.8.2Test Procedures ............................................................................... 151 15.8.3Measuring Data Interpretation .......................................................... 153

Index 155

IV Introduction

Introduction 1

1 Introduction

1.1 Receiving Instructions When taking delivery, any possible transport damage should be noted. A written record should be made of any such damage. A suitable remark should be recorded on the delivery documents.

A claim for damage must be reported immediately to the transport company and to the Customer Support Department of HAEFELY TEST AG or the local agent. It is essential to retain the damaged packing material until the claim has been settled.

Check the contents of the shipment for completeness immediately after receipt (See chapter "Scope of Supply). If the shipment is incomplete or damaged then this must be reported immediately to the transport company and the Customer Support Department of HAEFELY TEST AG or the local agent. Repair or replacement of the instrument can then be organised immediately.

1.2 General The MIDAS provides determination of the capacitance and dielectric loss of liquid and solid insulation. Measurements can be carried out on solid insulation such as cables, capacitors, power transformers, generators, motors, bushings etc. and on insulating oil with an optional oil test cell.

Operation is achieved via a sunlight readable touch screen and offers optimal user friendliness. Operation is simple thanks to the user dialogue system and on-line help.

The system is ideal for high and low voltage measurements over a wide frequency range. The test setup has been specially developed for efficient use in maintenance measurements, production and quality control. Thanks to its high precision, the system is also ideal for laboratory and development use. Once a measurement on a specific device is done I can be recalled and repeated in the same way and results can be compared graphically. So the system shows fast and easy a trending analysis of your equipment under test.

1.3 Hardware The measuring instrument is fully automatically balanced and the measurement values are calculated and displayed. The measuring instrument is provided with two measurement inputs, HV Supply, HV Ground and Safety Ground. Over 20 various parameters can be measured respectively calculated. The instrument , as a double vector meter, recognises the type of test object ( inductive / capacitive) and determines and displays its values automatically.

Advanced noise reduction is provided for field measurements where the measurement results might otherwise be falsified due to interferences.

The combination of the measuring instrument with built-in high voltage supply, standard capacitor CN and all suitable connection cables provides the user with a complete measurement system.

2 Introduction 2

1.4 Software With the powerful operating software manual measurements and automated test sequences can be defined and executed. The user simply enters the desired high voltage values and frequencies. The measurement values are then automatically read and displayed on the computer screen in curve and tabular format. With the occasion to integrate bitmaps, comments and hints, a guided step by step measurement macro can be done. So it is possible to grant a reproducible defined measurement procedure.

1.5 Scope of Supply The standard scope of supply includes the following items:

Qty Description

1 MIDAS Instrument trolley with measuring hardware and HV supply * * MIDAS standard type supplies 12kV, MIDAS G type supplies 15kV

Control Computer

1 Fully installed Laptop in shell case (MIDAS 2881) or Embedded, fully installed touchscreen-operated PC with thermo-printer (MIDAS 2880)

Cables

1 Rugged shell case

1 Shielded measuring cable, unipolar, LEMO plug, 20m, with clamp, indicator blue

1 Shielded measuring cable, unipolar, LEMO plug, 20m, with clamp, indicator white

1 HVGND measuring cable, unipolar, LEMO plug, 20m, with clamp, indicator yellow

1 HV supply cable, unipolar, double shielded, 20m, yellow

1 Safety Ground cable, 20m, with locking pliers, yellow/green

1 Country-specific mains cord, 2P & E, 10A , 2m

1 HV connection hook, used with HV supply cable

1 HV connection clamp, used with HV supply cable

1 Handheld Safety Switch with cable, LEMO plug, 10m

Accessories

1 Operating Instruction (this manual) and Test Certificate

1 USB Memory Stick with release key for optional software (if ordered)

Introduction 3

1.6 Optional Accessories This chapter describes the accessories that can be used in conjunction with the MIDAS. Contact us directly if you have a special application, as the following list is only a part of the broad range of accessories available.

Type / Series Description

288x TTR Embedded 3 phase Turns-Ratio meter

288x FRA Embedded Frequency Respose Analyser

288x TEMP Laser infrared, contact-less Thermo/Hygrometer. For determination of: tank(oil) temperature, air temperature and air humidity.

288X CASE Rugged field case for transportation of MIDAS

288X RACK Mechanical kit for 19 rack mounting of MIDAS

288x SAFE Safety Strobe Light with magnetic base for mounting e.g. on a transformer tank, providing additional visual warning of high voltage presence.

MIDAS OFFICE MIDAS software for customer personal office PC. Used for data visualisation, staff education, test planning and preparation, reporting.

5287 Current Booster to increase test current (while voltage decreases), especially for short circuit impedance testing of power transformers to diagnose transformer winding deformation.

5288A 10kV Resonating inductor (creates LC parallel resonant circuit) to increase test current up to max. 4.4A. Used for testing of high capacitance values up to 1uF

5289 15kV Resonating inductor (creates LC parallel resonant circuit) to increase test current up to max. 6A. Used for testing of high capacitance values up to 1.56 uF

6835 Mobile Test cell for on-site maintenance measurements on liquid insulations (10kV max.)

4 Technical Data 4

2 Technical Data

MIDAS with built-in Cn and built-in AC supply Range Resolution Accuracy

Dissipation Factor tan

0 .. 100 (0 .. 10000%)

0.0001 (0.01%)

0.5 % rdg 0.0001 ( 0.5 % rdg 0.01%) 1

Power Factor cos

0 .. 1 (0 .. 100%)

0.0001 (0.01%)

0.5 % rdg 0.0001 ( 0.5 % rdg 0.01%) 1

Quality Factor 0.01 .. 10000 0.0001 0.5% rdg 0.0001 1

Capacitance Range 3 @ 50Hz

6.5 pF .. 56 nF @ 15kV 8.1 pF .. 88 nF @ 12kV 1.2 nF .. 13 uF @ 80V

0.01 pF 0.3 % rdg 0.3 pF

Capacitance Range 3 @ 60Hz

5.4 pF .. 47 nF @ 15kV 6.8 pF .. 73 nF @ 12kV 1.0 nF .. 10.8uF @ 80V

0.01 pF 0.3 % rdg 0.3 pF

Inductance Range 4 @ 50Hz

140 H .. 1600 kH @ 15kV 112 H .. 1280 kH @ 12kV 0.75 H .. 8.5 kH @ 80V

0.1 mH 0.5 % rdg 0.5 mH

Inductance Range 4 @ 60Hz

117 H .. 1334 kH @ 15kV 93 H .. 1067 kH @ 12kV 0.62 H .. 7 kH @ 80V

0.1 mH 0.5 % rdg 0.5 mH

Test Voltage 15 kV (12 kV) 1 V 0.3 % rdg 1 V 2 Test Current (Input A & B) 30uA .. 15 A 0,1 uA 0.3 % rdg 1 uA Ref Current (Input Cn ext) 30uA .. 300 mA 0,1 uA 0.3 % rdg 1 uA Test Frequency 15 .. 400 Hz 0.01 Hz 0.1 % rdg 0.1 Hz Apparent Power S 4000 VA 0.1 mVA 0.8 % rdg 1 mVA Real Power P 1100 W 0.1 mW 0.8 % rdg 1 mW Reactive Power Q 4000 var 0.1 mvar 0.8 % rdg 1 mvar Output Voltage 80 V .. 15 kV MIDAS 288x G 80 V .. 12 kV MIDAS 288x

Output Frequency 15 .. 400 Hz Output power derating beyond 40..70Hz

Output Current 350 mA intermittent , 150 mA continuous

PD level max 500 pC together with connected resonating inductor 5289 or 5288A Output Power max 4000 VA

Internal Cn (Reference)

100 pF, tan 0.00002 Capacitance constancy < 0.01% / year Temperature coefficient < 0.01% / K

1500VA 1501 .. 2000VA 2001 .. 3000VA > 3000VA Output Power duration continuous 30 min.

1h pause 5 min. 1h pause

1 min. 1h pause

Technical Data 5

1 Accuracy valid @ 50..60Hz 2 Accuracy valid for voltage > 1000 V 3 Can be expanded with optional Resonating Inductor 4 Can be lowered with optional Current Booster

MIDAS as measuring instrument with external Cn and external AC source

Range Resolution Accuracy

Dissipation Factor tan

0 .. 100 (0 .. 10000%)

0.0001 (0.01%)

0.5 % rdg 0.0001 ( 0.5 % rdg 0.01%) 1

Power Factor cos

0 .. 1 (0 .. 100%)

0.0001 (0.01%)

0.5 % rdg 0.0001 ( 0.5 % rdg 0.01%) 1

Quality Factor 0.01 .. 10000 0.0001 0.5% rdg 0.0001 1 Capacitance 2 0.1 pF 0.01 pF 0.3 % rdg 0.3 pF Inductance 2 2000 kH 0.1 mH 0.1 % rdg 3 mH Test Voltage 300mA / Cn 1 V 0.3 % rdg 1 V 3 Test Current (Input A & B) 30uA .. 15 A 0,1 uA 0.3 % rdg 1 uA Ref Current (Input Cn ext) 30uA .. 300 mA 0,1 uA 0.3 % rdg 1 uA Test Frequency 15 .. 1000 Hz 0.01 Hz 0.1 % rdg 0.1 Hz Apparent Power S 1 MVA 0.1 mVA 0.8 % rdg 1 mVA Real Power P 1 MW 0.1 mW 0.8 % rdg 1 mW Reactive Power Q 1 Mvar 0.1 mvar 0.8 % rdg 1 mvar

1 Accuracy valid @ 50..60Hz 2 Range is limited by test current and voltage of used power source 3 Accuracy valid for Voltages > 20uA / Cn Max. Load Capacitance

fUkVAC

*2*3max 2

where U Test Voltage f Test Frequency

the C range can be increased with optional resonating inductor

Power Supply

Input Power 100 .. 240 VAC, 50 / 60 Hz, 1kW, active PFC (IEC61000-3-2)

Main Fuse 10A slow-blow with high breaking capacity

Environmental Conditions

Operating Temperature -10 .. 50C

Storage Temperature -20 .. 70C

Relative Humidity 5 .. 95 % r.h.

Protection classes IP22, IEC 61010, CE mark, general IEC 61326-1, IEC 61000-4-X, 61000-3-X, EN 55011, ANSI/IEEE C37.90

6 Technical Data 6

Weight and Dimensions

Weight Instrument box 58 kg (174 lbs) PC head 7.5kg (17 lbs) Trolley 11kg (25 lbs)

W x D x H Instrument box 34 x 47 x 104 cm (13.5 x 18.5 x 41) PC head 30 x 42 x 26 cm (12 x 16.5 x 10) Trolley 33 x 68 x 112 cm (13 x 26.8 x 44)

Recorded Values DF(tan), DF(tan)@20C , DF%(tan), DF%(tan)@20C , PF(cos), PF(cos)@20C , PF%(cos), PF%(cos)@20C, QF (quality factor), QF (quality factor) @20C

CP (ZX= CP RP), RP (ZX= CP RP), CS (ZX= CS + RS), RS (ZX= CS + RS) LS (ZX= LS + RS), RS (ZX= LS + RS), LP (ZX= LP RP), RP (ZX= LP RP), Standard capacitor Cn,

URMS, URMS 3, ITest eff, IRef eff, Im, IFe Impedance Zx, Phase-angle (Zx), Admittance Yx, FrequencyTest, FrequencyLine App. Power S, Real Power P, Reactive Power Q, Real Power @2.5kV, Real Power @10kV TemperatureAmbient 1, TemperatureInsulation 1 , Rel.Humidity 1, Temp.Corr.Factor K, Connection mode, Settings, all Notes and Comments, Time, Date

1 measured by external temperature/humidity probe

Miscellaneous

Measuring Time 0.3 sec / measurement @ averaging = 1

Interfaces (PC head) USB, Ethernet, RS232, Mouse, Keyboard, Thermal printer

Data format XML, CSV

Calibration Interval 2 years recommended

Safety Specification VDE 0411/part 1a , IEC/EN 61010-1:2002

Safety 7

3 Safety

This warning sign is visible on the MIDAS. Meaning: This equipment should only be operated after carefully reading the user manual which is an integral part of the instrument.

Haefely Test AG and its sales partners refuse to accept any responsibility for consequential or direct damage to persons and/or goods due to none observance of instructions contained herein or due to incorrect use of the MIDAS.

Further be aware that Safety is the responsibility of the user !

Remember - Hazardous voltage can shock, burn or cause death !

3.1 General Safety is the most important aspect when working on or around high voltage electrical equipment.

Personnel whose working responsibilities involve testing and maintenance of the various types of high voltage equipment must have understood the safety rules written in this document and the associated safety practices specified by their company and government. Local and state safety procedures should also be consulted. Company and government regulations take precedence over Tettex recommendations.

The MIDAS generates high voltage and is capable of causing serious even lethal electrical shock. If the instrument is damaged or it is possible that damage has occurred, for example during transportation, do not apply any voltage.

The instrument may only be used under dry operating conditions. The use of MIDAS is prohibited in rain or snow.

Do not open the MIDAS, it contains no user replaceable parts.

Do not switch on or operate a MIDAS instrument if an explosion hazard exists.

3.2 Personnel Safety The MIDAS should not be operated by a crew smaller than two people. Their function can be described as follow:

Test Operator The person who is making the test connections and operates the MIDAS. He must be able to have a clear view of the device under test and the area where the test is performed.

Safety Observer The person who is responsible for observing the performance of the test, seeing any safety hazard, and giving warning to crew members.

Both persons should perform no other work while the MIDAS is energized.

While making the various types of connections involved in the different tests, it may be necessary for personnel to climb up on the equipment, but no one should remain on the equipment during the test itself.

8 Safety 8

Non-test related persons who are working in proximity to the area where testing is performed must be informed. Consistent visual and verbal signals should be agreed and followed.

Perform only one job at a time on any equipment. The situation in which two crews are doing different tasks with the same equipment at the same time is an open invitation for confusion, trouble, and danger to the personnel.

People with heart pacemakers should not be in the vicinity of this system during operation.

3.3 Safety Features Beside an Emergency Stop switch the MIDAS is equipped with an external Safety Switch (spring-release type or a 'dead man' type). The Safety Switch should be controlled by the second test crew member (safety observer). Without the Safety Switch the instrument can not be activated.

Prior to making the first measurements, the Safety Switch operator should verify the correct operations of the switch.

It is recommended that the Safety Switch be the last switch closed. It must remain open until all personnel are safely in the clear. If unauthorized personnel should enter the area, or if some other undesirable situation should develop, the Safety Switch operator should release the switch immediately, and then notify the MIDAS operator.

The Safety Switch should be used at all times. Never short circuit it and do not use fixed mechanical locking devices for depressing the switch button. The switch button must be manually operated at all times.

For visual warning of high voltage presence a warning lamp bar is located on the top rear side of the instrument. Optional a strobe light is delivered which can be mounted on the device under test.

The MIDAS is equipped with a HV GND connection surveillance. The high voltage can only be switched on when the earth circuit is properly connected. The instrument indicate the status Grounded or Open by LED and by software.

A separate green/yellow earth cable is provided for the purpose of safety grounding the instrument. The earth cable should be connected to the Earthing Screw on the back of the MIDAS at one end and to the station grounding system at the other end.

The green / yellow safety ground cable should be the FIRST lead to be connected to the set.

3.4 Safety Precautions All tests must be performed with the device under test completely de-energized and isolated from its power systems. The equipment, its tank or housing must be disconnected from all buses and properly earthed, so that all induced voltages or trapped charges are neutralized. Only when the measurement procedure is actually being performed the grounds should be temporarily removed.

The MIDAS must be solidly earthed with the same ground as the device under test. When the MIDAS is permanently housed in a vehicle, the MIDAS ground should be bounded to the vehicle chassis, which in turn is grounded.

Exposed terminals of equipment should not normally be allowed to 'float'. They should be grounded directly or through the low voltage leads (INPUT V) of the MIDAS, unless otherwise specified.

Testing of high voltage equipment involves energizing the equipment through the MIDAS. This can produce dangerous levels of voltage and current. Care must be taken to avoid contact with the equipment being tested, its associated bushings and conductors, and with the MIDAS cables. Especially the high voltage test cable should not be held during energization of the MIDAS. Flashover of the test specimen or the MIDAS can generate transient voltages of sufficient magnitude to puncture the insulating jacket of the high voltage test cable.

Safety 9

It is strongly recommended that the test crew make a visual check to ensure that the equipment terminals are isolated from the power system. If there is real possibility that the device under test fails precautions such as barriers or entrance restrictions must be taken against harm in the event of violent failure.

Proper clearance between the test equipment and the device under test must be ensured during the presence of high voltage. Barriers and safety tapes can be established around the test area to prevent unintentional entry into the live area. It must also be guaranteed that extraneous objects like ladders, buckets, etc. can not enter the test area.

After the MIDAS is properly grounded, the remaining test leads and the High Voltage Test Cable are plugged into their receptacles. Do not connect test leads to the equipment terminals until after the leads are connected to the MIDAS.

The proper procedures for connecting the MIDAS leads to the device under test is described in detail in chapter "Accessories and Options" The safety observer should supervise this procedure at all times.

The MIDAS operates from a single-phase power source. It has a three wire power cord and requires a two-pole, three terminal, live, neutral and ground type connector. Do not bypass the grounding connection. Any interruption of the grounding connection can create electric shock hazard. The power input connection should be the last step in setting up the instrument.

After the tests are completed, all test leads should be disconnected first from the device under test and earthed before they are disconnected from the instrument.

The green / yellow safety ground cable should be the LAST lead to be disconnected from the set.

Do not disconnect the voltage cables from unless the MIDAS Voltage is set to HV OFF, and the Safety Switch is released. Attempts to disconnect leads while the MIDAS is energized may result in a serious and possibly lethal electrical shock.

3.5 Summary Note: Many accidents that happen around high voltage equipment involve personnel who are familiar, and perhaps too familiar, with high voltage equipment. Staying alert and ever watchful requires constant training and awareness of the inherent hazards. The greatest hazard is the possibility of getting on a live circuit. To avoid this requires constant vigilance - for oneself and for one's fellow workers.

In addition to the obvious dangers, personnel should be alert to recognize subtle dangers as well. For example, during transformer excitation-current tests, the floating terminals may have significant voltages induced in them by simple transformer action. Therefore, all terminals of a device under test, unless grounded, should be considered to be live while the test is in progress.

When potential transformers or any transformers are interconnected, voltage can be back-fed through the secondary windings to produce high voltage on the primary although the primary is seemingly isolated from the power system. This entail a second important rule - all terminals of a device under test should be completely isolated.

Finally it should be noted that the MIDAS is relatively heavy. We recommend that at least two people are used to slide the MIDAS and three to lift it. Special care must be taken in lifting or sliding the Instrument into or from a vehicle so as not to incur bodily injury.

Remember - Safety, FIRST, LAST, ALWAYS !

10 Theory 10

4 Theory

4.1 Why is Insulation Tested? All transformers, high voltage switchgear, motors and electrical equipment accessories have a high voltage lifespan. From the first day of use the equipment is subject to thermal and mechanical stresses, foreign particle ingress and variations in temperature and humidity. All of these influences raise the working temperature of the equipment when switched on. This heating accelerates chemical reactions in the electrical insulation, which result in a degradation of the dielectric characteristics. This process has an avalanche character i.e. the changing electrical characteristics of the insulation increase the loss factor and produce heating which further degrades the insulation. If the loss factor of the insulation is periodically monitored and recorded, it is possible to predict and / or avoid catastrophic failure of the electrical equipment.

At the beginning of the public electricity supply industry, methods and processes were sought to avoid unexpected losses caused by equipment defects. One method that provided repeatable data and offered simple on-site measurement was the measurement of capacitance and loss factor (power factor) of the equipment insulation.

In cases where loss factor tests were regularly carried out, and the relevant test results compared with earlier results, the deterioration of the insulation was noted and necessary preventative measures were carried out. Based on this groundwork, a series of test procedures were developed and described in various IEEE, ANSI and IEC documents and standards to specify the insulation quality for various types of electrical equipment.

In order to define acceptable loss factor values, a data record service was developed based upon statistical data that related to specific equipment types and models. Standard measurements of capacitance and loss factor of the electrical insulating medium were carried out to ensure that the data was comparable. The loss factor was calculated and the results were corrected by energy comparison to a value for a test voltage of 10kV. Some test results were further multiplied by a temperature correction factor to produce 20C compatible values. Any results that are now acquired at different Test Levels and temperatures are recalculated for 10kV and / or 20C and then recorded and compared. In this way the degradation of the insulation characteristics over a specified period of time can be determined. With the test result history an experienced engineer is able to take the necessary maintenance actions based upon changes in the value of loss factor.

4.2 What is Loss Factor? Loss factor is the total energy that will be used by the equipment during normal service. In particular, the insulation loss factor is any energy that is taken by the flow of current through the resistive component of the insulation. The earth path varies according to the type of electrical equipment. For example, switchgear will probably develop tracking to earth at right angles to the floor connections. In transformers paths can develop in the insulation resistance between the windings or between the windings and housing (tank). In all cases the result is a loss factor in the form of heating.

Note: In this text loss factor (losses, watts) is referred to, in contrast with total loss factor. Total loss factor is normally used to describe the total losses of the transformer under load and should not be confused with the energy that is lost due to degradation of the insulation.

Theory 11

4.3 What is Dissipation Factor tan ? To specify the insulation loss factor, the test object must be considered in the test arrangement as a capacitor. Consider all test objects e.g. transformers, bushings, busbars, generators, motors, high voltage switchgear etc. are constructed from metal and insulation, and therefore possess associated capacitive properties. Every test object consists of various capacitances together with the insulation and the internal capacitance to earth. The figure shows the components that comprise a capacitance and the diagram for a simple disc capacitor.

Figure 1 : Disc Capacitor

dA C

where:

A electrode face

d distance between the electrodes

C capacitance

0 dielectric constant of air (0=8,854210-12 F/m) r relative dielectric constant dependent upon material

= 0 r, dielectric constant

In an ideal capacitor the resistance of the insulation material (dielectric) is infinitely large. That means that, when an AC voltage is applied, the current leads the voltage by exactly 90 as it flows as pure current.

After further consideration it must be realized that every insulation material contains single free electrons that show little loss under DC conditions with P= U2/R. Under AC a behaviour called dielectric hysteresis loss occurs which is analogous to hysteresis loss in iron.

As losses therefore occur in every insulation material, an equivalent diagram of a real capacitance can be constructed as follows:

Figure 2 : Parallel equivalent diagram of a lossy

capacitance with vector diagram

Loss factor (Dissipation Factor)

RCRX

II

QP C

C

R

C

R

1tan

Power Factor

PF II

PS

R R

C cos

tan

tan 1 2

UTest applied test voltage

IC current through capacitance

IR current through resistance (insulating material)

C ideal capacitance

R ideal resistance

12 Theory 12

Because P = Q tan , the losses which are proportional to tan , will usually be given as a value of tan to express the quality of an insulation material. Therefore the angle is described as loss angle and tan as loss factor.

4.4 The Difference between Power Factor and Dissipation Factor

While Dissipation Factor tan is used in Europe to describe dielectric losses, the calculation used in the United States is Power Factor cos . The statistical data that have been collected in North America have been calculated using the loss factor cos (Power Factor) to specify the power losses in the insulation. Because the angles are complimentary it is unimportant whether tan or cos is used as with very small values the difference is negligible. However the conversion formulas are:

2tan1

tan

PF

21tan

PFPF

4.5 Apparent Power, Real Power, Reactive Power

The relationship between the various types of power is clarified in the following equations.

Apparent Power

S = UI

[VA]

Real Power P = UI cos [W]

Reactive Power Q = UI sin [var]

Figure 3 : Vector Diagram of Apparent Power, Real power and Reactive Power

Because most test objects are not a pure resistance and therefore have a phase angle between the test voltage and current, this phase shift must also be taken into consideration in the power calculation.

Theory 13

4.6 Test Instruments There are three basic kinds of capacitance, tan and Power Factor test instruments in use. Although the high accuracy Schering Bridge must be balanced manually and the balance observed on a null indicator, it has been widely sold and used for decades up until this day. The capacitance and dissipation factor can be calculated by reading the position of the balance elements.

The automatically balanced C tan measuring instrument performs measurement by the differential transformer method. The automatic balancing makes operation very easy.

The double vector-meter method is essentially an improvement of the differential transformer method.

All three methods are in current use for accurate and repeatable measurements of C tan on various test objects. The differences basically lie in the resolution and accuracy. Different instruments are generally developed specially for field or laboratory measurement.

Field instruments are specially constructed for rugged field requirements and are equipped with a mobile high voltage source. In addition, such instruments provide noise suppression for onsite use.

Laboratory instruments have been constructed for indoor use where high accuracy specifications are required. These test systems are built in a modular construction for higher Test Levels. The systems may be used for daily routine testing, for high precision long duration tests or for acceptance tests.

4.7 Evaluation of Test Results

Significance of Capacitance and Dissipation Factor

A large percentage of electrical apparatus failures are due to a deteriorated condition of the insulation. Many of these failures can be anticipated by regular application of simple tests and with timely maintenance indicated by the tests. An insulation system or apparatus should not be condemned until it has been completely isolated, cleaned, or serviced. The correct interpretation of capacitance and dissipation factor tests generally requires a knowledge of he apparatus construction and the characteristics of the types of insulation used.

Changes in the normal capacitance of insulation indicate such abnormal conditions as the presence of a moisture layer, short circuits, or open circuits in the capacitance network. Dissipation factor measurements indicate the following conditions in the insulation of a wide range of electrical apparatus:

Chemical deterioration due to time and temperature, including certain eases of acute deterioration caused by local overheating.

Contamination by water, carbon deposits, bad oil, dirt and other chemicals. Severe leakage through cracks and over surfaces. Ionization.

The interpretation of measurements is usually based on experience, recommendations of the manufacturer of the equipment being tested, and by observing these differences:

Between measurements on the same unit after successive intervals of time. Between measurements on duplicate units or a similar part of one unit, tested under the

same conditions around the same time, e.g., several identical transformers or one winding of a three phase transformer tested separately.

Between measurements made at different Test Levels on one part of a unit; an increase in slop (tip-up) of a dissipation factor versus voltage curve at a given voltage is an indication of ionization commencing at that voltage.

14 Theory 14

An increase of dissipation factor above a typical value may indicate conditions such as those showed above: If the dissipation factor varies significantly with voltage down to some voltage below which it is substantially constant, then ionization is indicated. If this extinction voltage is below the operating level, then ionization ma progress in operation with consequent deterioration. Some increase of capacitance (increase in charging current) may also be observed above the extinction voltage because of the short-circuiting of numerous voids by the ionization process.

An increase of dissipation factor accompanied by a marked increase of the capacitance usually indicates excessive moisture in the insulation. Increase of dissipation factor alone may be caused by thermal deterioration or by contamination other than water.

Unless bushing and pothead surfaces, terminal boards, etc., are clean and dry, measured values not necessarily apply to the insulation under test. Any leakage over terminal surfaces may add to the losses of the insulation itself and may give a false indication of its condition.

Dissipation Factor of Typical Apparatus Insulation

Values of insulation dissipation factor for various apparatus are shown in this table. These values are useful in roughly indicating the range to be found in practice; however, the upper limits are not reliable service values.

Equipment Dissipation factor @ 20C

Oil-filled transformer, New, HV ( > 115kV) 0.25% .. 1.0%

Oil-filled transformer, Age 15 years, HV ( > 115kV) 0.75% .. 1.5%

Oil-filled transformer, Age 15 years, LV, distribution 1.5% .. 5%

Circuit breakers, oil-filled 0.5% .. 2.0%

Oil-paper cables, "solid" (up to 27.6 kV) new 0.5% .. 1.5%

Oil-paper cables, HV, oil-filled or pressurized 0.2% .. 0.5%

Stator windings, 2.3 .. 13.8kV 2.0% .. 8.0%

Capacitors 0.2% .. 0.5%

Bushings, (solid or dry) 3.0% .. 10.0%

Bushings, compound-filled, up to 15kV 5.0% .. 10.0%

Bushings, compound-filled, 15 .. 46kV 2.0% .. 5.0%

Bushings, oil-filled, below 110 kV 1.5% .. 4.0%

Bushings, oil-filled, above 110 kV 0.3% .. 3.0%

Dissipation Factor and Dielectric Constant of Typical Insulation Materials

Typical values of 50/60Hz dissipation factor and permittivity (dielectric constant ) of some typically used insulating materials.

Material Dissipation factor @ 20C Acetal resin (Delrin) 0.5% 3.7

Air 0.0% 1.0

Askarels 0.4% 4.2

Kraft paper, dry 0.6% 2.2

Transformer oil 0.02% 2.2

Polyamide (Nomex) 1.0% 2.5

Polyester film (Mylar) 0.3% 3.0

Polyethylene 0.05% 2.3

Polyamide film (Kapton) 0.3% 3.5

Polypropylene 0.05% 2.2

Porcelain 2.0% 7.0

Theory 15

Rubber 4.0% 3.6

Silicone liquid 0.001% 2.7

Varnished cambric, dry 1.0% 4.4

Water 100% 80

Ice 1.0% @ 0C 88 Note: Tests for moisture should not be made at freezing temperatures because of the 100 to 1 ratio difference dissipation factor between water and ice.

Influence of Temperature

Most insulation measurements have to be interpreted based on the temperature of the specimen. The dielectric losses of most insulation increase with temperature. In many cases, insulations have failed due to the cumulative effect of temperature, e.g. a rise in temperature causes a rise in dielectric loss which causes a further rise in temperature, etc.

It is important to determine the dissipation factor temperature characteristics of the insulation under test, at least in a typical unit of each design of apparatus. Otherwise, all tests of the same spec should be made, as nearly as practicable, at the same temperature. On transformers and similar apparatus, measurements during cooling (after factory heat-run or after service load) can provide required temperature correction factors.

To compare the dissipation factor value of tests made on the same or similar type of equipment at different temperatures, it is necessary to correct the value to reference temperature base, 20C (68F). The MIDAS does that automatically. See also chapter "Software : Menu Conditions (Temperature correction).

The insulation material temperature for apparatus such as spare bushings, insulators, air or gas filled circuit breaker and lightning arresters is normally assumed to be the same as the ambient temperature. For oil-filled circuit breakers and transformers the insulation temperature is assumed to be the same as the oil temperature. The (transformer mounted) bushing insulation temperature can be assumed to be the midpoint between the oil and ambient temperatures.

The capacitance of dry insulation is not affected by temperature; however, in the case of wet insulation, there is a tendency for the capacitance to increase with temperature.

Dissipation factor-temperature characteristics, as well as dissipation factor measurements at a given temperature, may change with deterioration or damage of insulation. This suggests that any such change in temperature characteristics may be helpful in assessing deteriorated conditions.

Be careful making measurements below the freezing point of water. A crack in an insulator, for example, is easily detected if it contains a conducting film of water. When the water freezes, it becomes non-conducting, and the defect may not be revealed by the measurement, because ice has a volumetric resistivity approximately 100 times higher than that of water. Tests far the presence of moisture in solids intended to be dry should not be made at freezing temperatures. Moisture in oil, or in oil-impregnated solids, has been found to be detectable in dissipation factor measurements at temperatures far below freezing, with no discontinuity in the measurements at the freezing point.

Insulating surfaces exposed to ambient weather conditions may also be affected by temperature. The surface temperature of the insulation specimen should be above (never below) the ambient temperature to avoid the effects of condensation on the exposed insulating surfaces.

Influence of Humidity

The exposed surface of bushings may, under adverse relative humidity conditions, acquire a deposit surface moisture which can have a significant effect on surface losses and consequently on the results of a dissipation factor test. This is particularly true if the porcelain surface of a bushing is at temperature below ambient temperature (below dew point), because moisture will probably condense on the porcelain surface. Serious measurement errors may result even at a relative humidity below 50% when moisture condenses on a porcelain surface already contaminated with industrial chemical deposits.

It is important to note that an invisible thin surface film of moisture forms and dissipates rapidly on materials such as glazed porcelain, which have negligible volume absorption. Equilibrium after a sudden wide change in relative humidity is usually attained within a matter of minutes. This excludes thicker films which result from rain, fog, or dew point condensation.

16 Theory 16

Surface leakage errors can be minimized if dissipation factor measurements are made under condition where the weather is clear and sunny and where the relative humidity does not exceed 80%. In general, best results are obtained if measurements are made during late morning through mid afternoon. Consideration should be given to the probability of moisture being deposited by rain or fog on equipment just prior to making any measurements.

Influence of Surface Leakage

Any leakage over the insulation surfaces of the specimen will be added to the losses in the volume insulation and may give a false impression as to the condition of the specimen. Even a bushing with voltage rating much greater than the test voltage may be contaminated enough to cause a significant error. Surfaces of potheads, bushings, and insulators should be clean and dry when making measurement.

It should be noted that a straight line plot of surface resistivity against relative humidity for an uncontaminated porcelain bushing surface results in a decrease of one decade in resistivity for a nominal 15% increase in relative humidity.

Electrostatic Interference

When tests are conducted in energized sub stations, the readings may be influenced by electrostatic interference currents resulting from the capacitance coupling between energized lines and bus work to the test specimen.

The measurement difficulty, which is encountered when testing in the presence of interference, depend not only upon the severity of the interference field but also on the capacitance and dissipation factor of the specimen. Unfavorable weather conditions such as high relative humidity, fog, overcast sky, and high wind velocity will increase the severity and variability of the interference field. The lower the specimen capacitance and its dissipation factor, the greater the difficulty, with possible reduction in accuracy, in making measurements. It is also possible that a negative dissipation factor reading may b obtained so it is necessary to observe the polarity sign for each reading. The MIDAS interference suppression feature minimizes the influences but however, the influences may be minimized considerably by:

Using the maximum voltage of the test set if possible. Disconnecting and grounding as much bus work as possible from the specimen terminals. Making measurements on a day when the weather is sunny and clear, the relative humidity is

less than 80%, the wind velocity is low, and the surface temperature of exposed insulation is above the ambient temperature.

If the test set is energized from a portable generator when conducting tests in an energized substation the readings may fluctuate over a significant range. This results from the frequency of test mains being out of synchronization with the electrostatic interference field. If it is not possible to synchronize the frequency of the two voltage systems, disconnect and ground as much bus work as possible from the specimen terminals. This will decrease both the interference pickup and the reading fluctuation.

Negative Dissipation Factor

It is believed that a complex tree network of capacitances and resistances, which exists within a piece of equipment, cause the negative dissipation factor phenomenon. Error currents may flow into the measuring circuit in instances where phantom multiple terminals or a guard terminal appear in the measurement system. It is also believed that a negative dissipation factor may be produced by currents flowing into a tee network as a result of space coupling from electrostatic interference field.

4.8 Supplementary Test Methods As of today there exists no other test method that can replace the currently used C tan test. Nevertheless, several measurement methods exist which compliment dissipation factor measurement and assist in localization of defects in the test object.

Theory 17

Partial Discharge Measurement is unprotected against external electromagnetic disturbances and on-site measurement presents quite a lot of problems.

Oil Analysis Measurements provide useful information about the insulating oil in transformers and oil-paper insulation systems.

The Recovery Voltage Meter RVM provides information about the aging condition of the oil-paper insulation. This method cannot currently be used for testing synthetic insulation.

4.9 Standard Capacitor, Measuring Current & Limits

To evaluate the expected values of test current, standard capacitor current, the corresponding limiting parameters and the resulting load range use these basic conditions and rules:

(1) Maximum test voltage shall be less than the nominal voltage of the standard capacitor. UTestmax UCN

Current through standard capacitor CN NTestCN Cf2UI (2) Minimum current through standard capacitor CN ICN min 30 A Note: Minimal current through CN (internal or external) to ensure accuracy

(3) Maximum current through standard capacitor CN EXT ICN max 300 mA Note: Maximum input current of the CN EXT INPUT to avoid overload *

(4) Maximum test voltage ** U If C

TestCN

Nmax

max 2

(5) Minimum test voltage U If C

TestCN

Nmin

min 2

Test current IX through test object CX I U f CX Test x 2 (6) Maximum Test current through test object CX IX max 15 A *** Note: Maximum input current of the INPUT A, B, HVGND to avoid overload

(7) Minimum Test current through test object CX IX min 30 A Note: Minimal input current of the INPUT A, B, HVGND to ensure accuracy

(8) Limitations based on Technical Data (e.g. max supply power, current etc.)

Note: These calculations are valid for capacitive test objects (tan = 0). They can also be as a close approximation for test objects with a tan value < 0.01. * Maximum current trough CN INTERNAL is limited by 15kV / 100pF 470A @ 50Hz ** The max. output power can also limit the maximum test voltage

*** Test current higher than 15A cant be measured with this test system. External current dividers (current comparators) would affect functionality and/or accuracy of the midas288X. For range extension please contact TETTEX customer support.

18 Theory 18

Example:

UTest =10kV, CX=80nF (tan < 0.01), f=50Hz, internal AC source used, built-in CN used IX = 251mA within IX range OK > 150mA - Only intermittent testing allowed due to max. supply output current limit

Psupply = 2513VA within P range OK ICN = 314 uA within ICN range OK

4.10 Parallel & Series Equivalent Circuits The MIDAS measures and displays both - the parallel and/or series equivalent circuit values. The following formulas describe the calculation of the value conversion parallel series :

Figure 4 : Parallel equivalent circuit Cp-Rp

RpCp

1

tan * *

* measured values

Figure 5 : Series equivalent circuit Cs-Rs

Cs Cp * ( tan *)1 2 Rs Rp

tan *tan *

2

21

* measured values

Functional Description 19

5 Functional Description

5.1 System Overview

To be able to execute correct and reproducible measurements it is essential to understand how the MIDAS measuring system works.

The MIDAS measuring system is based on the double vector-meter method which relies upon the measurement of the current IN through the known reference capacitor CN and the measurement of the current IX through the unknown test object CX.

Both branches are energized by the built-in HV AC power source (UTest) and both currents are measured by the adjustable high accurate shunts RX and RN and then digitised. By using IEEE 1394 fire wire data bus technology each digitised value is time stamped. With this technology not only the values but also the time information (phase displacement) between IN and IX can be measured very fast and highly accurate.

The digitised data streams are fed into the PC and over the known standard capacitor all other desired measuring values can now be determined online.


Figure 6 : Function Schematics

I X Current trough Device Under Test CX

I N Current trough known Standard Capacitor CN

I RX Losses of the Device Under Test CX

CX Test Object (ideal capacitance)

CN Standard capacitor (with tan < 10-5)

RX Measuring shunt for I X , CX

RN Measuring shunt for I N , CN

V Low voltage point of the HV supply and reference point of the measurement

ADC Analogue to Digital Converter

t1, t2 Time stamps of the measured values

5.2 V-potential point and Guarding This measuring system is able to measure capacitances with highest accuracy to determine trending analysis of insulating materials. In the range of normal insulation capacitances the always existent stray capacitances - measured together wit the DUT - are influencing the measuring values significant. So these unwanted stray capacitance effects have to be eliminated.

This is realized by the so called guarding of the relevant elements. That means that the complete high voltage source, the supply and measuring cables have to be shielded with the so called V-potential which is the low voltage point (reference) of the high voltage supply. All capacitances connected to this reference point are bypassed and are therefore not influencing the measuring value. Several parts have to be double shielded (Guard and Ground) to compensate other side effects and to ensure the specified measuring accuracy. Due to this guarding concept the supplied shielded coax measuring cables (for High Voltage Supply, Input A and Input B) have to be used always. If the system is connected with normal unshielded cables the measuring values will be incorrect.

To keep in mind for the user of the system is that capacitances related to the V-point are bypassed. Make sure that all unwanted capacitances are related to the V-potential point and their current is flowing directly into the V-point and not through the measuring shunt RX.

This has to be evaluated for every measuring setup. The most common ones are described in this manual for the other ones the user has to make sure that only the desired capacitances are measured with the chosen test setup. Most cases can be solved by setting the internal Test Mode Switch matrix correctly which sets unused measuring cables and connected parts to the V-potential automatically.


The V-potential point is accessible over a 4mm plug on the instruments side panel where the user can connect external parts of a test setup.

Example: Bypass the leakage current on bushing surface with guarding.

Figure 7 : Measurement without Guard (V-Potential)

Normal connection in GST gA+B mode to measure high voltage winding to tank CHG . But with this connection the stray capacitance Cstray (surface leakage current on bushing surface) is measured in parallel and therefore causes a minor error on the measurement. The measured value is CHG. + Cstray

Figure 8 : Measurement with connected V-Potential point to the powered bushings (guarding)

Normal connection in GST gA+B mode to measure high voltage winding to tank CHG. With guard collars mounted on the bushings surface close to the tank (not touching). These electrodes, connected to the V-potential point bypass now the leakage current and therefore also the stray capacitance Cstray The measured value is now only CHG. and the best accuracy is reached.

Note: As guard collar you can use any conducting material as aluminium foil, copper band, etc


5.3 Test Modes When measuring transformers and other test objects the problem often arises that, in addition to the normal ungrounded capacitances, capacitances with one side grounded must also be measured (e.g. capacitance between a winding and an earthed housing). Conventional measurement systems require the external test setup (cable connections) to be changed for such measurements. This involves a lot of work and time, especially when on-site measurements are being performed on large power transformers.

Using the Test Modes, the test object only has to be connected once for measurement and all relevant capacitances can be measured by switching the test mode as required.

The selected Test Mode connects the DUT current path(s) to the internal current measuring shunt RX and the other (not measured) connected leads to the V-potential (reference point) of the system. All capacitances connected to this reference point are bypassed and are not influencing the selected measurement.

Figure 9 : Measuring setup on a single phase transformer with two low voltage windings. The Test Mode

Switch is set to UST A resulting in a measurement of the capacitance CHL1.

Note: The connection between HV GND on the measuring instrument and the earth point of the test object is a normal measuring channel as well. A good clean contact is essential.

Test Mode

Explanation

INPUT A connected to (S1)

INPUT B connected to (S2)

HV GND connected to (S3)

Actual measured CX

UST A Ungrounded Specimen Test, A used as measuring channel, B and HV GND connected to V- potential point (bypassed)

RX V V CHL1

UST B Ungrounded Specimen Test, B used as measuring channel, A and HV GND connected to V- potential point (bypassed)

V RX V CHL2

UST A+B Ungrounded Specimen Test, A and B used as measuring channels, HV GND connected to V- potential point (bypassed)

RX RX V CHL1 + CHL2

GST A+B Grounded Specimen Test, A and B and HV GND used as measuring channels. RX RX RX CHL1 + CHL2 + CHG


GSTgA Grounded Specimen Test with guarding (V-potential) connected to A (bypassed). HV GND and B are used as measuring channels.

V RX RX CHL2 + CHG

GSTgB Grounded Specimen Test with guarding (V-potential) connected to B (bypassed). HV GND and A are used as measuring channels.

RX V RX CHL1 + CHG

GST gA+B Grounded Specimen Test with guarding (V-potential) connected to A and B (bypassed). Only HV GND is used as measuring channel.

V V RX CHG

Note: For testing the insulation secondary winding tank, the HV cable and measuring cables shall be exchanged. The HV shall be connected to the secondary winding(s) and the measuring cable to the primary winding. The measured capacitances in the table will change accordingly.

Test Mode UST for ungrounded test objects

This test mode is the most common situation when measuring capacitance and dissipation factor. Various ungrounded capacitances can be measured using this mode, providing that the maximum test current of the measuring instrument is not exceeded. When measuring power transformers and HV current transformers, this configuration determines the capacitance and dissipation factor between the various winding groups.

In this mode the highest measurement accuracy is reached.

Test Mode GST for grounded test objects

This test mode enables the measurement of capacitances that are normally earthed on one side when in operation. When measuring transformers this configuration measures the capacitance and dissipation factor between the HV winding and all other windings and the transformer housing.

Test Mode GST g for grounded test objects with guarding (V-potential)

This test mode directly measures the capacitance between the HV terminal and the housing (which is grounded). The partial capacitances that are undesirable for the measurement are connected to the V-potential point and thereby rendered ineffective. When measuring transformers this configuration measures the capacitance and dissipation factor between the various winding groups and the transformer housing. The windings which are not used for measurement are connected to the v-potential of the measuring system via the A (or B) measuring cable and the internal Test Mode Switch.


5.4 Interference Suppression The presence of power line frequency fields induce spurious voltages and currents (interference inductions) onto the test object and therefore causes an error on the measuring signal i meas(t). To determinate that error, the induced interference signal i noise(t) is measured from the corresponding (normally phase related to nearby power line) mains of the instrument or by an external antenna.

At test start the internal AC source is switched off and the i meas(t) signal shows only the error signal (induced interference). Now the ratio (phase and amplitude) between this error signal and the additional measured noise signal i noise(t) is calculated and stored (calibration of the noise path to the measuring path).

During the actual measurement the i noise(t) (corrected by the stored ratio) signal is continuously subtracted from the i meas(t) signal. So the interference signal is suppressed online with relation to its actual amplitude value which can shift. This patented method allows getting highly accurate results under high interference conditions and allows measuring at or very close to power line frequency.

"Method And Equipment For Measuring Impedance Of Electrical Component Under High Interference Conditions" Patent No. US2005075076.

Figure 10 : Interference suppression schematics.

Operation Elements 25

6 Operation Elements

6.1 Individual Parts The MIDAS system can be dismounted to individual, transportable parts . The system contains of the following elements:

Instrument box Trolley Touchscreen operated PC head

or Laptop

These parts can be mounted / dismounted by opening the related fastener.

Figure 11 : Individual Parts


6.2 Touchscreen PC head as System Controller

6.2.1 Interfaces The head with integrated PC, touchscreen (7) and thermal strip printer (6) has the following accessible interfaces:

1 External Mouse (PS2)

2 External Keyboard (PS2)

3 USB connector

4 RS232 connector

5 Ethernet connector

6 Strip printer

Three connection cables are located on the bottom plate of the PC head to interconnect with the instrument box:

Power (Supply) IEEE1394 (Control of MIDAS functions) USB (Control of TTR and/or FRA option) The instrument box connection are located in the glove box on the backside of the instrument. Also longer power cables can be easily stored in there.

Figure 12 : PC head

6.2.2 Touchscreen calibration Do calibrate the touch screen positioning follow these steps:

Close or minimize MIDAS software by pressing the Minimize Button or the Close Button

The calibration software tool can be started by pressing the reladed icon in the Windows bottom task bar (depending on the mounted touchscreen type).

Pen Mount touchscreen

LiyiTouch touchscreen

Press the Symbol and the calibration tool opens. Follow the instructions.


6.2.3 Strip Printer

The strip printer has a press button for paper feed and a light that indicates when the paper has run out. The printer uses thermal sensitive paper. Never print without paper.

The following thermal sensitive paper shall be used: Type GPR T01-057-031-007-060A Width 57.5 0.5mm Density 60 g/ m2

Figure 13 : Option Strip Printer

Printouts

Logo, can not be changed, this part of the printout will only printed after start-up, or if you have selected a new measuring file. Type of Instrument Date of Measurement Unit No. of Measurement

Site, Object, Serial-No.

Dataset #1

Pressing the Record Button produces this printout. The printed measuring data correspond with the selected measuring values

Dataset #2

Next Data Set.

etc.

Figure 14 : Printout example

For operating the printer see chapter . "Software: Menu Options"


6.3 Laptop as System Controller

6.3.1 Interfaces The supplied Laptop is fully installed with operating system and MIDAS software.

The interconnection with the instrument box is done with the following links:

Power (Supply) IEEE1394 (Control of MIDAS functions) USB (Control of TTR and/or FRA option) The connection link sare located in the glove box on the backside of the instrument. Also the connection cables and the laptop power supply can be easily stored in there.

6.4 Instrument Glove box On the rear top side of the instrument box there is a kind of a

glove box where the interconnection with the PC part is located.

On the inner side panels of that glove box the following connectors are accessible:

(left side of the box)

1 Power connector for PC touchscreen head

2 Power connector for Laptop (90..230V AC as Mains)

(right side of the box)

3 USB connector (for TTR and FRA)

4 IEEE1394 firewire connector (for MIDAS control)


6.5 Instrument Side Panel

Figure 15 : Side panel

The instruments right side panel is devided into four main parts:

1 Measuring inputs

2 FRA connections (optional)

3 TTR connections (optional)

4 High Voltage and power outputs

6.5.1 Measuring Inputs

Figure 16 : Side panel top

1 EMERGENCY STOP BUTTON

2 EXT WARNING LAMP Plug receptacle for connecting optional external safety strobe lamp

3 MEASUREMENT INPUT A Plug receptacle for connecting the low voltage test lead A

4 MEASUREMENT INPUT B Plug receptacle for connecting the low voltage test lead B

5 MEASUREMENT INPUT CNEXT Plug receptacle for connecting an external Standard Capacitor (if used)

6 LOW VOLTAGE POINT V 4mm plug for connecting all parts witch capacitance shall not be measured (with this reference v potential the HV transformer and the entire HV circuit is enclosed Guard. Its also the low voltage point of the HV supply NOT the system earth)

7 SAFETY SWITCH INPUT Plug receptacle for connecting the handheld Safety Switch.


The Safety Switch should be used at all times. Never short circuit it and do not use fixed mechanical locking devices for depressing the switch button. The switch button must be manually operated at all times.

8 MEASUREMENT INPUT HV GND This test lead is the low voltage connection of the HIGH VOLTAGE OUTPUT. It has to be connected to the ground-point of the device under test. (See also HV GND Connection Surveillance)

9 LED INDICATOR Open & Grounded OPEN indicates an open or defective grounding of the system. GROUNDED indicates a correct grounded HV GND INPUT ( HV ON is enabled) 10 WARNING LAMP BAR Red HV ON indication lamp

11 MAINS Socket for the supplied mains cable, mains power switch and mains fuse

The instrument must be disconnected from all voltage sources if a fuse has to be replaced. When replacing the mains fuse always use a replacement fuse with the same specifications and current rating.

6.5.2 High Voltage and Power Outputs

Figure 17 : Side panel bottom

12 LOW VOLTAGE OUTPUT TO BOOSTER Receptacle to connect the optional current booster type 5287 to extend the supplied test current ( for Inductance testing and short circuit impedance testing)

13 HIGH VOLTAGE OUTPUT Plug receptacle for connecting the high voltage output cable (yellow) respectively the test objectNote: The matching plug has a locking ring that should be fastened by hand

14 RESONATING INDUCTOR CONTROL Receptacle to connect the control link to the optional 15kV resonating inductor 5289. (Not used for 5288A)

15 RESONATING INDUCTOR SUPPLY Receptacle to connect the link to the parallel power supply in the optional 15kV resonating inductor 5289 to drive the additional load of the inductor. (Not used for 5288A)

16 SAFETY GROUND Wingnut terminal for safety ground connection


6.6 HV GND Connection Surveillance The HV GND connection to system ground is kept under constant surveillance. As shown in below, relay Re1 can only close when the test system is supplied with mains power and the test object is earthed via a separate earth connection. Therefore, the high voltage can only be switched on when status Grounded is shown on the instrument side panel (and on the display as well).

Should the HV GND connection being interrupted the actual test (HV) will be switched off immediately.

Figure 18 : Functional principle of the safety circuit for earth connection monitoring

6.7 Safety Ground (Earthing)

Wing nut ground terminal for connecting the safety ground lead to earth ground

(connected to the instruments housing and the ground pin from the mains connector, there is no measuring or AC supply functionality)

Figure 19 : Side panel Safety Ground

A separate green/yellow earth cable is provided for the purpose of safety grounding the instrument. The Safety Ground cable should be connected to the Earthing Screw on the back of the MIDAS at one end and to the station grounding system at the other end.

For safety reasons the earth cable should be the FIRST lead to be connected to the set and the LAST to be disconnected.

The device under test, its tank or housing, and the MIDAS must be solidly and commonly grounded or earthed. This also applies to any mobile equipment being tested.


When the MIDAS is permanently housed in a vehicle, the MIDAS ground should be bounded to the vehicle chassis, which in turn is grounded.

Exposed terminals of equipment should not normally be allowed to 'float'. They should be grounded directly or through the measuring leads of the MIDAS, unless otherwise specified.

6.8 Emergency Stop

When the Emergency Button is pressed the test is automatically terminated (high voltage is switched off and it is not possible to switch the high voltage on until the button is released)

Note: The emergency stop switch is directly integrated in the safety interlock circuit (hardwired) without any interaction of the built-in PC or software.

Figure 20 : Emergency Stop Button

6.9 Warning Lamp Bar

The red warning lamp bar located on the instrument side panel indicates the actual high voltage state of the MIDAS.

No light The system is in a safe state. The emergency stop is pressed, the Safety Switch is released or HV-GND lead is not connected.

Illuminated

Caution: High Voltage possible anytime! The system is ready to switch high voltage on anytime. You have only to press the High Voltage ON button to switch high voltage on. The emergency stop is released, the Safety Switch is pressed and the HV-GND lead is connected.

Blinking

Warning: High Voltage is live! The High Voltage is switched ON and active.

Figure 21 : Red warning lamp bar

Never attempt to disconnect the High Voltage Test Cable or the Low Voltage Lead(s) from either the terminals of the test specimen to which they are connected at the outboard end, or from the receptacles on the Instrument at the inboard end unless the MIDAS Voltage is set to HV OFF (No warning lamp light), and the Safety Switch is released. Attempts to disconnect leads while the MIDAS is energized may result in a serious and possibly lethal electrical shock.

Software 33

7 Software The MIDAS Software is running on Microsoft Windows XP Embedded Operation System. The software is designed to control all operation and inputs by a touch screen. For additional operation like installing LAN connectivity, printer etc., a standard PC mouse (PS2) and/or a standard PC keyboard (PS2) can be connected to the system for easier operating.

7.1 General

7.1.1 Start-up Once the system has been started, the following Start-up window comes, in which two options could be selected for different operations.

Select to start the system in Manual Mode. The default file for storing measuring data will be used.

Select to launch the file manager in which you have the possibility to select a file for operation, to load a previous file, or to create a new file. See section File Manager for more information.

34 Software

7.1.2 Main Window The main window consists of four parts accessible over the related tabs on the right-hand side.

The function of these tab sheets are:

Tab Sheet Setup

Pressing this button provides access to the definition of the Device under Test (DUT), measuring conditions and auxiliary information.

See section Tab sheet SETUP for details.

Tab Sheet Manual

This sheet is used for manual operation, such as setting the measuring connection, voltage and frequency and storing the measuring results in a spreadsheet.

See section Tab sheet MANUAL for details.

Tab Sheet Sequence

This tab sheet defines the test sequence and create complex test cycles. The measured data are automatically stored in a spreadsheet, which can be used for additional analysis afterwards.

See section Tab sheet SEQUENCE for details.

Tab Sheet Analysis

This sheet is used to sort and analyse the measured data in a graphical way. Trends or different comparisons can be generated without an extraordinary effort. As a result, you may predict the actual state of your equipment.

See section for details.

Software 35

7.1.3 Title bar The title bar (header line) has following structure:

Device Name

Status if operation mode is Simulated

Path and name of the actual active file

Status field Help Button Minimize Button

Close Button

The functional descriptions of the title bar elements are:

Device Name

Name of the controlled device (MIDAS).

The colour of the title bar will change to red, if high voltage is switched ON.

Simulation Mode

If the MIDAS OFFICE software is used on a standalone PC or laptop to prepare sequences, connection diagrams, display measurements etc. the Simulated status is shown in the title bar.

The Simulated mode provides the same functionality as on the MIDAS itself, but no system hardware is needed. The measuring values are simulated.

Document Name

The actual active (loaded) test file and its path is shown here. All data are stored in this file.

Status Field

In this field you find the actual status of the system.

Help Button

Pressing this button an explorer with help screen will open

Minimize Button

The display of the software will be minimized and you have access to the Windows OS desktop.

This button can only be pressed while the HV is switched off.

Close Button

Press Exit to Windows button to terminate the MIDAS software and exit to Windows Operating System. Press Shut Down button to terminate the MIDAS software and shut down the system.

36 Software

Its strongly recommend to shut down the system correctly before switching the main power off.

Alarm Messages Alarm messages of the system is displayed in the status field. The High Voltage could not be switched on if an alarm messages appears in the status field.

The alarm messages can be described as follow:

Alarm Emergency

The emergency button on the right hand side of the screen panel is pressed. Release the Emergency Button to remove this alarm (rotate to unlock).

Alarm HV Ground not conn.

The HV GND Input is

Instructions Manual MIDAS 288x

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Transcript of Instructions Manual MIDAS 288x