Tettex MIDAS 2881 Operating Instructions Manual

Operating Instructions

HAEFELY TEST AG

MIDAS 2881

Mobile Insulation

Diagnosis &

Analysing System

Version 2.0

(LAN version only)

4843930

Art.Nr. 4843930 Operating Instructions MIDAS 2881

Date Version Responsible Changes/Reasons

04.16 2.0 SG 2881 LAN Version,

replace the firewire model

WARNING

Before operating the instrument, be sure to read and fully

understand the operating instructions. This instrument

produces hazardous voltages. It is the responsibility of the user

to ensure that the system is operated in a safe manner.

This equipment contains exposed terminals carrying hazardous

voltages. There are no user serviceable components in the unit.

All repairs and upgrades that require the unit to be opened must

be referred to HAEFELY TEST AG or one of their nominated

agents.

This Manual is only for Midas 2881 with Ethernet (LAN)

connection to computer, for those equipped with FireWire

connection type refers to the corresponding manual.

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 equipment.

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 specified in the miscellaneous folder under the setup menu.

Unauthorized opening of the unit may damage the EMI protection of the system and will reduce its

resistance to interference and transients. It may also cause the individual unit to be no longer

compliant with the relevant EMC emission and susceptibility requirements. If the unit has been

opened, the calibration will be rendered invalid.

Note

HAEFELY TEST AG has a policy of continuing improvement on all their products. The design of this

instrument will be subject to review and modification over its life. There may be small discrepancies

between the manual and the operation of the instrument, particularly where software has been

upgraded in the field.

HAEFELY TEST AG retains the right to modify the functionality, specification or operation of the

instrument without prior notice.

electronic without the prior written permission of HAEFELY TEST AG.

2016, HAEFELY TEST AG, Switzerland

Manual Conventions

In the manual, the following conventions are used:

Indicates a matter of note.

If it refers to a sequence of operations, failure to follow the instructions could result in errors in

measurement.

Indicates hazards.

There is a risk of equipment damage or personal injury or death. Carefully read and follow the

instructions. Be sure to follow any safety instructions given in addition to those for the site at which

tests are being performed.

Foreword

Welcome as a new user of the Insulation Diagnosis System MIDAS 2881. 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 6

3.1General ........................................................................................................... 6

3.2Personnel Safety ............................................................................................ 6

3.3Safety Features .............................................................................................. 7

3.4Safety Precautions .......................................................................................... 7

3.5Summary ........................................................................................................ 8

4Theory 9

4.1Why is Insulation Tested? ............................................................................... 9

4.2What is Loss Factor? ...................................................................................... 9

4.3What is Dissipation Factor tan ? ................................................................. 10

4.4The Difference between Power Factor and Dissipation Factor ..................... 11

4.5Apparent Power, Real Power, Reactive Power ............................................. 11

4.6Test Instruments ........................................................................................... 12

4.7Evaluation of Test Results ............................................................................ 12

4.8Supplementary Test Methods ....................................................................... 16

4.9Standard Capacitor, Measuring Current & Limits .......................................... 16

4.10Parallel & Series Equivalent Circuits ............................................................. 17

5Functional Description 18

5.1System Overview .......................................................................................... 18

5.2V-potential point and Guarding ..................................................................... 19

5.3Test Modes ................................................................................................... 21

5.4Interference Suppression .............................................................................. 23

6Operation Elements 24

6.1Individual Parts ............................................................................................. 24

6.2Laptop as System Controller......................................................................... 25

6.2.1Interfaces ...................................................................................... 25

6.3Computer connection .................................................................................... 25

6.4Instrument Side Panel .................................................................................. 26

6.4.1Measuring Inputs .......................................................................... 26

6.4.2High Voltage and Power Outputs .................................................. 27

6.5HV GND Connection Surveillance ................................................................ 29

II Introduction

6.6Safety Ground (Earthing) .............................................................................. 29

6.7Emergency Stop ........................................................................................... 30

6.8Warning Lamp Bar ........................................................................................ 30

7Software 31

7.1General ......................................................................................................... 31

7.1.1Start-up ......................................................................................... 31

7.1.2Main Window ................................................................................ 32

7.1.3Title bar ......................................................................................... 33

7.1.4Alarm Messages ........................................................................... 34

7.1.5Function Keys ............................................................................... 35

7.2File Manager ................................................................................................. 35

7.2.1File Selector Dialog ....................................................................... 36

7.2.2Report ........................................................................................... 37

7.2.3Data Format Files ......................................................................... 39

7.3Display of Measurement Values ................................................................... 39

7.4Tab sheet SETUP ......................................................................................... 43

7.4.1Menu DUT Info .............................................................................. 44

7.4.2Menu Conditions (Temperature correction)................................... 45

7.4.3Menu Settings ............................................................................... 47

7.4.4Menu Options ................................................................................ 50

7.4.5Menu Auxiliary .............................................................................. 53

7.5Tab sheet MANUAL ...................................................................................... 55

7.5.1Tools Window ............................................................................... 57

7.5.2Definition of Columns for Measuring Spreadsheet ........................ 58

7.5.3Formulas in Measuring Grids ........................................................ 59

7.5.4Menu Signal Analysis .................................................................... 61

7.6Tab sheet SEQUENCE ................................................................................. 65

7.6.1Definition of Spreadsheet Sequence ............................................. 65

7.6.2Sequence Measurement ............................................................... 68

7.6.3Edit Sequence Limiters ................................................................. 70

7.6.4Starting Sequence ........................................................................ 72

7.6.5Sequence with External AC Power Source ................................... 74

7.7Tab sheet ANALYSIS ................................................................................... 75

7.7.1Spreadsheet Measurement ........................................................... 75

7.7.2Graphic Analysis ........................................................................... 76

7.7.3More Analysis ............................................................................... 77

7.8Remote Operation ........................................................................................ 78

7.8.1Characteristics of the interface ...................................................... 79

7.8.2General Commands ...................................................................... 82

7.8.3System control commands ............................................................ 84

7.8.4Measurement commands ............................................................. 86

7.8.5Alarms ........................................................................................... 88

8Accessories and Options 90

8.1Office Software ............................................................................................. 90

8.2Safety Strobe Light ....................................................................................... 90

8.3Oil Test Cell .................................................................................................. 90

8.4Resonating Inductor ...................................................................................... 91

8.4.110kV Resonating Inductor 5288A ................................................. 92

8.4.215kV Resonating Inductor 5289 .................................................... 95

8.5Current Booster .......................................................................................... 102

8.6Care and Maintenance ............................................................................... 104

Introduction III

9Instrument Storage 105

10Packing and Transport 106

11Recycling 107

12Trouble Shooting 108

12.1Software Updates ....................................................................................... 108

13Conformity 109

Appendix 110

14Applications Guide 111

14.1Bushings ..................................................................................................... 111

14.1.1Spare Bushings ........................................................................... 113

14.1.2Installed Bushings ....................................................................... 114

14.1.3Measuring Data Interpretation ..................................................... 118

14.2Transformers .............................................................................................. 119

14.2.1Power and Distribution Transformers .......................................... 119

14.2.2Shunt Reactors ........................................................................... 125

14.2.3Current Transformers .................................................................. 126

14.2.4Voltage Transformers ................................................................. 127

14.2.5Short Circuit Impedance ............................................................. 128

14.2.6Excitation Current Measurement ................................................. 130

14.3Rotating Machines ...................................................................................... 132

14.3.1Test Procedure ........................................................................... 132

14.3.2Measuring Data Interpretation ..................................................... 133

14.3.3Measuring high cap values using the Resonating Inductor ......... 135

14.3.4Operation of Resonating Inductor with Midas ............................. 136

14.4Liquid Insulation .......................................................................................... 137

14.4.1Test Procedure ........................................................................... 137

14.4.2Measuring Data Interpretation ..................................................... 138

14.5Cables ........................................................................................................ 139

14.5.1Test procedures on different cables ............................................ 139

14.5.2Test Procedure Example ............................................................ 140

14.5.3Measuring Data Interpretation ..................................................... 141

14.6Capacitors .................................................................................................. 141

14.6.1Measuring high cap values using the Resonating Inductor ......... 142

14.7Circuit Breakers .......................................................................................... 143

14.7.1Dead Tank Breaker ..................................................................... 145

14.7.2Live Tank Breaker ....................................................................... 146

14.7.3Measuring Data Interpretation ..................................................... 147

14.8Surge (Lightning) Arresters ......................................................................... 147

14.8.1Test Levels .................................................................................. 147

14.8.2Test Procedures .......................................................................... 148

14.8.3Measuring Data Interpretation ..................................................... 150

Index 151

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 it’s 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)

Cables in rugged trolley including

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

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

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

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

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

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

HV connection hook, used with HV supply cable

HV connection clamp, used with HV supply cable

Handheld Safety Switch with cable, LEMO plug, 10m

Small set of accessories

1 Operating Instruction (this manual) and Test Certificate

1 USB Memory Stick with software and manual in electronic format

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 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

HV Power Supply

MIDAS 2881 MIDAS 2881G

Output Voltage Up to 12 kV Up to 15 kV

Output Voltage Regulation  1 % rdg  1 V  1 % rdg  1 V

Output Frequency @ Nom Volt 45Hz …. 70 Hz 45Hz …. 70 Hz

Output Frequency @ 5kV 15 Hz … 1000 Hz 15 Hz … 1000 Hz

Output Current

1

@ Nom Volt 115 mA continuous , > 275mA max 1 min. 115 mA continuous , > 275mA max 1 min.

Output Current @ 10kV 165 mA continuous , > 400mA max 1min. 165 mA

continuous , > 400mA max 1 min.

Output Power

1

max > 4000 VA > 4000 VA

Output PD level

2

max  500 pC  500 pC

Duty cycle  1650VA 1650 .. 2000 VA 2001 .. 3000 VA >3001

continuous 30 min. ON / 1h 5 min. ON / 1h 1 min. ON / 1h

1 Can be expanded with optional Resonating Inductor

2 Can be reduced with additional filters on coupling capacitor

Measuring Unit (MIDAS 2881 & MIDAS2881G)

Range Resolution Accuracy

Test Voltage < 1MV 1 V  0.2 % rdg  1 V

Dissipation Factor (tan ) 0 … 10’000.% 0.001%  0.5 % rdg 0.01.%

Power Factor (cos ) 0 ... 100.% 0.001%  0.5 % rdg 0.01.%

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

Capacitance 0.01pF

 0.2 % rdg  0.2pF

Inductance 0.1mH

 0.5 % rdg  0.5 mH

Test Current Input A, B & HVGND 20.uA … 15 A 0.1 uA  0.1 % rdg  1 uA

Ref Current Input Cn ext 20.uA … 300 mA 0,1 uA  0.1 % rdg  1 uA

Test Frequency 15 Hz … 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

Accuracy @ frequency 15Hz…100Hz ; In & Ix > 20A ; Ix/In: 0.01 .. 10’000 ; With Internal Cn

Internal reference capacitor

Capacitance 100 pF

tan  < 0.00002

Capacitance constancy < 0.01.% / year

Temperature coefficient < 0.01.% / K

Mains Power supply

Voltage / Frequency 90 VAC … 264 VAC / 50 … 60 Hz

Input Power 1 kW

Environmental

Operating Temperature -10 .. 50 °C

Storage Temperature -20 .. 70 °C

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

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

Technical Data 5

Mechanical

Weight & Dimensions Instrument 58 kg (128 lbs) 34 x 47 x 104 cm (13.5” x 18.5” x 41”)

Trolley 11 kg (25 lbs) 33 x 68 x 112 cm (13” x 26.8” x 44”)

General

Control unit Lap top Computer Windows 7 based.

Measuring unit With Internal and external Power supply and nominal capacitor

Interfaces Ethernet connection to external laptop

Recorded Values DF(tan ), DF(tan )@20°C , DF%(tan ), DF%(tan )@20°C , PF(cos ), PF(cos

)@20°C , PF%(cos ), PF%(cos )@20°C, QF (quality factor), QF (quality factor)

@20°C 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 @2.5 kV, Real Power @10

kV, TemperatureAmbient 5, TemperatureInsulation 5 , Rel. Humidity 5,

Temp.Corr.Factor K, Connection mode, Settings, all Notes and Comments,

Time, Date

Software Windows 7

Data format XML, CSV

5 measured by external temperature/humidity probe

6 Safety 6

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.

Safety 7

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.

8 Safety 8

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 !

Theory 9

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 20°C compatible values. Any results that are now acquired

at different Test Levels and temperatures are recalculated for 10kV and / or 20°C 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.

10 Theory 10

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

d

A





C

where:

A electrode face

d distance between the electrodes

C capacitance

0 dielectric constant of air (0=8,8542•10-12 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)

RCR

X

I

Q

PC

C

R

C

R









1

tan

Power Factor

PF I

I

P

S

RR

C





cos tan

tan





12

UTest applied test voltage

IC current through capacitance

IR current through resistance (insulating material)

C ideal capacitance

R ideal resistance

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Tettex MIDAS 2881 Operating Instructions Manual

Tettex MIDAS 2881 Operating Instructions Manual

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