From: Goyal, AjayDate: 01/14/06 17:15:50

To: mskkumar@igcar.ernet.inSubject: Diagnostic Insulation Testing

Department: Kind Attn: Mr.M.S. KarthikumarDiagnostic Insulation TestingIntroductionA major cause of equipment failure is breakdown of the insulation. A range of diagnostic techniques can be applied to monitor insulation condition in order to take timely maintenance action. Each test gives one perspective on the overall condition of the equipment and their combination can form a more complete picture than is provided by a single test. In the practical operational world there is not the time to completely analyse a piece of equipment and choices have to be made on the appropriate test strategy for different types of equipment.Insulation DegradationThere are 5 basic initiators for degradation of the insulation; electrical and mechanical stress, chemical attack, thermal stress and environmental contamination. Normal cycles of operation will lead to 'aging' through these mechanisms. The aging of insulation is a slow process of degradation as these factors interact with each other in a gradual spiral of decline. The rate of this insulation deterioration is of increasing interest to engineers dealing with electrical supply networks such as those in Europe and the USA, where large parts of the network were installed 30 or 40 years ago in a burst of investment in the infrastructure.TestingAs insulation deteriorates leakage current may increase, the dielectric loss characteristic will change and the degree of polarisation will alter. Insulation tests look for a change in one of these responses to indicate the deterioration.Go/NoGo Diagnostic Insulation TestsThe traditional insulation resistance test is the simplest way to gain an overall indication of the condition of the insulation. Although the Insulation Resistance test can be applied as a simple Go/NoGo test, it can also be used to give more extensive diagnostic information. The most common voltages applied for non-destructive d.c. insulation tests are 2.5 and 5kV (defined in BS6266 and IEEE43).'Spot' TestThis is the simplest insulation test, giving a reading of Insulation Resistance in M. The test is applied for a short, but specific period of time, after which a reading is taken. The time is typically 60 seconds (allowing the capacitive charging current to reduce). On installation of the equipment, these readings will be compared to the required minimum specification. Insulation resistance is significantly temperature dependent and correction factors can be applied to show a trend more clearly from a history of insulation tests.The factor to correct the temperature to 40C is given by; Rcorrected = Kt x Rtest where Kt,, the compensation factor, doubles for each 10C rised.c. Insulation Test CurrentsThe test current in the body of the insulation can be split into three components; the capacitance charging current, the polarisation (or absorption) current and the conduction or leakage current. In addition there may exist a surface leakage current, which it may be advantageous to remove from the test result using the guard terminal of the Insulation Tester. Capacitive Current This is initially large, but goes to zero as the test piece is charged. Polarisation Current Caused by charges in the insulation material moving under the effect of the electric field or by molecular di-poles lining themselves up with the applied field (orientational polarisation). It is greatly affected by moisture or contamination in the insulation, as the water molecule has additional orientational polarisation. This process takes much longer than the capacitive charging.Leakage current Steady (resistive) current through the insulation, which is usually represented by a very high resistor in parallel with the capacitance of the insulation.Figure 1. Insulation Test Currents

It is immediately apparent that the Insulation Resistance reading is time dependent. In general, it is mainly the capacitive current that is seen in the first seconds after the test is started, as the needle on your MEGGER(r) Insulation Tester climbs. Then, at the one minute period most commonly used for a 'spot' test, the current is a combination of polarisation & leakage current. By 10 minutes you are reading mainly leakage current, though it can take up to 30 minutes for polarisation effects to be complete.Discharge CurrentsDuring the discharge phase the reverse occurs with the exception of the leakage current. There is no test voltage so the leakage current is insignificant. The capacitive current decays quickly and the re-absorption current takes several minutes, and perhaps hours, to reduce to zero.Time-Resistance Tests (PI & DAR)Time-Resistance tests take successive readings at specified times and have the great advantage of being independent of temperature. They also help in the situation where past test records are sketchy, as, being a ratio, they are also independent of the size of the equipment, although it is always more valuable if a trend can be established. This is a lot easier with the time-resistance (Polarisation Index and Dielectric Absorption Ratio) tests than with Spot Tests, as temperature correction is unnecessary.Good insulation generally shows an increase in resistance over the 10 minute period, but with contaminated insulation the polarisation effects are masked by high leakage currents and the readings are flat.Table 1. Test Result AnalysisInsulation ConditionPIDARTCDD

Definition10 minute1 minute60 seconds15 secondsResistance x CapacitanceDischarge current (at 60 s)/(V x C)

Poor< 1 4> 1.6>2000

These values can give a guide to condition of insulation, although the figures are better interpreted in the context of the equipment history. If a PI falls by 30% or more, then remedial action such as cleaning, or further investigation, should be considered.Time Constant (TC)The time constant of the insulation is related to the basic characteristics of dielectric constant and resistivity, as well as having some influence from polarisation. Trending this simple parameter allows degradation to be monitored, with a reduction in TC showing problems in the insulation.Discharge based testsThere are a range of techniques looking at the response of the insulation during its discharge. These tests have all targeted the polarisation behaviour of the insulation, which is very sensitive to moisture. As all three components of current (charging, polarisation and leakage currents), are present during the charging phase of an insulation test, the determination of polarisation or absorption current is potentially confused by the presence of the capacitive and leakage currents. The discharge phase of the test can more rapidly remove these effects, giving the possibility of interpreting the degree of polarisation of the insulation and relating this to moisture and other polarisation effects. The Dielectric Discharge (DD) TestThis test operates during the discharge of the dielectric, but is set up to be a simple and practical test. The DD test was developed for generators, by EdF in France.The insulator is charged for a sufficient time to be 'stable' (usually 30 minutes) This means that charging and polarisation are complete and the only remaining component of the current is leakage current. The insulator is then discharged and the resulting current is measured. This current constitutes the capacitive discharges and the 'reabsorption currents', combining to give the total 'dielectric discharge'.In order to make comparisons between equipment, the current is measured after a standard time of 1 minute, which is much greater than the primary time constant of the capacitive discharge. The result is not affected by surface leakage, which is effectively short-circuited. The resultant current is dependent on the overall capacitance (C), the final test voltage (V) and the degree of polarisation of the dielectric. During the discharge, the voltage and capacitance of the equipment are measured so that the 'Dielectric Discharge' can be quoted as a simple number; DD = I1 min / V x C (mA/V/F)

The Dielectric Discharge can identify absorbed moisture in an insulation as this affects the absorption behaviour of the dielectric and is masked by leakage effects if we try to measure it on the charging cycle.a.c. based testsIn a.c. testing, the polarisation current never has time to die away and, together with capacitive current, predominates in the total dielectric loss. It is therefore also sensitive to moisture and degradation of the insulation material.Some a.c. tests do not require a high voltage test signal and so do not stress the insulation. This means there is very little chance of damaging the dielectric.Tan-Delta (Power Factor) TestingThe loss angle / tan delta / power factor of the insulation is more sensitive to small changes in the condition of the insulation than the raw d.c. insulation resistance and in some circumstances this may be important. It is therefore a very useful test for monitoring insulation condition from initial installation. Comparing previous results is essential to be able to spot deterioration; a single result cannot give much information.The use of an a.c. source (often 2.5 or 10kV) makes the Power Factor test set heavier and more expensive than a d.c. tester. Also there is a limit to the capacitance that can be tested, but the early warning given by monitoring this parameter makes it invaluable.SummaryDiagnostic insulation tests allow more information to be gained from an insulation test which may prove invaluable in a maintenance program. An a.c. test also provides early information that can point to future insulation problems. Each type of test helps to give a more complete picture of the condition of the insulation.Table 2. Summary of Diagnostic Insulation TestsTest TypeDefinitionApplication

d.c. charging testsInsulation Resistance (Spot test) Single insulation value, often after 60 sGeneral condition, need temperature compensation

Polarisation Index (PI)Ratio of 10 min.:1 min. insulation valuesLevel of dirt/moisture, less temperature dependent

Dielectric Absorption Ratio (DAR)Ratio of 60 s:30 s insulation valuesQuicker version of PI

Time ConstantResistance CapacitanceFigure of merit

Step Voltage (SV)Comparison of insulation at different test voltagesPresence of cracks & holes

d.c. discharge testsDielectric Discharge (DD)Discharge current after 60s/(Volts Capacitance)Level of absorbed dirt/moisture

Recovery Voltage /Polarisation Spectrum AnalysisDischarge for 50% of charge time. Measure peak, initial slope and recovery voltageState of transformer insulation including moisture in oil and paper

Isothermal Relaxation Current AnalysisDischarge characteristic after 60 s charge at 1 kVXLPE cable insulation condition

EDA TestCurrent & voltage characteristics + capacitanceMotor & generator insulation condition

a.c. testsFlash (Hi-Pot)Leakage current (a.c.)Type tests with go/no-go trip levels

Partial DischargeMeasures small charges caused by voltage stressUsually used for component testing

Power Factor/'Tan Delta' Difference in the in-phase and out-of-phase currentVery sensitive to insulation moisture and degradation

VLF (Very Low Frequency)Insulation value at 0.1 HzNon-destructive cable testing

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Best RegardsAjay GoyalInternational Regional Manager - Southern AsiaMegger P.O.Box No.12052 Mumbai - 400 053T + 91 22 2631 5114. (Direct)F + 91 22 2632 8004. M + 91 9820300932.E ajay.goyal@megger.com www.megger.comThe information contained in this electronic mail message is confidential. It is intended solely for the use of the individual or entity to whom it is addressed and others authorised to receive it. If the reader of this message is not the intended recipient, you are hereby notified that any use, copying, dissemination or disclosure of this information is strictly prohibited.Archcliffe Road, Dover Kent, CT17 9EN, UKTel: +44 (0)1304 502100 Fax +44 (0)1304 207342

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