i would be intrested to get a detailed feed on this !

15kV insulation testing perhaps will require voltage leakage test and also heat test if this is for transformer. With temperature the leakage resistance may change. What does the specifications of the transformer say and what is the standard you are supposed to implement says? Compare the standard with specs and also test within that limit.

There must be a comb voltage test pattern generator to slowly step up the voltage in pulses and to see if leakage in the insulation develops. On reaching a leakage limit or maximum selected test voltage limit this test need to stop before damage to insulation is done permanently.

I was doing voltage test for high voltage cable insulation and I found that some cable have high corona leakage even at 15kV. If insulation is of the order of 2cm or more then corona leakage is less likely for 15kV. My cables have only few mm of insulation if they have air or cracks then corona leaks is high.

Ther following is the list of pre-commissioning tests on a transformer:

i) IR Value test - with a 2.5kV or a 5kV Megger (HV to Earth, LV to Earth & HV to LV Values)

ii) Winding Resistance Mesaurement (on all taps) - may be with a Kelvin's Double Bridge)

iii) Ratio Test (on all taps)

iv) Magnetic Balance Test

v) No-load test (Magnetising current / No-load Loss Measurement)

vi) Short-circuit Test (To ascertain the percentage impedance and the full load losses of the trafo.)

vii) Vector Group Test

viii) Tests on Isnulating Oil (for Oil immersed trafo.)

ix) Tests on Control & Protection - for proper operation

I hope the above would do.

All the above are in order except magnetic balance and short circuit test which will difficult to carry out at site and not required.

It is not at all difficult to perform Magnetic Balance Test or Short Circuit Test at site.

Magnetic Balance Test:

i) Keep LV side open.

ii) Apply single-phase 415V across 1U-1V;

iii) Measure voltage across 1V-1W & 1W-1U;

iv) Now, apply single phase 415V across 1V-1W;

v) Mesaure voltage across 1W-1U & 1U-1V;

vi) Now, apply single phase 415V across 1W-1U;

vii) Measure voltage across 1U-1V & 1V-1W

Establish Mganetic balance with the above measurements.

Short Circuit Test:

i) Keep the LV side shorted.

ii) Apply 3-phase variable voltage to 1U-1V-1W.

iii) Slowly increase the voltage applied to the HV side in steps.

iv) Measure current at the LV side at each step of voltage applied.

v) Note down the applied voltage at which full load rated current flows in the short circuited secondary.

With this you can extrapolate the short circuit impedance of the trafo.

A word of caution would be: NEVER try to apply voltage to LV side; this would be dangerous as it would induce a very high voltage at the HV side, which may be fatal.

Site commissioning

Transport to site could well have involved a journey of many hundreds ofmiles, part possibly by sea. The transformer will have had at least two lots of handling. There is, however, very little testing which can be done at site which can demonstrate that it has not suffered damage.

It is therefore vital that such tests as can be carried out at site should be done as thoroughly andas carefully as possible. These may include:

1 Ratio measurement on all taps.

2 Phasor group check.

3 Winding resistance measurements on all taps.

4 Operation of tap changer up and down its range. Check the continuity of tapped winding throughout the operation.

5 Insulation resistance between all windings and each winding to earth.Insulation resistance core-to-earth, core-to-frame and core frame-to-earth.

6 No-load current measurement at reduced voltage; very likely this will bedone at 415 V and compared with the current obtained at the same voltage in the works.

7 Oil samples taken and checked for breakdown strength and moisture content.For a large, important transformer for which the oil is to be tested periodically for dissolved-gas content (doing dissolve gas analisys), this sample would also be checked for gas content and taken as the starting point.

8 All control, alarms, protection and cooler gear checked for correct operation.Alarm settings and protection trips set to appropriate level for initial energisation.

9 Tank and cooler earth connections checked as well as the earthing of the HV neutral, if appropriate.

Insulators outside the tank should be cleaned with a dry cloth.

The transformer tank and cover should be effectively earthed in a direct and positive mannerwhile, in order to comply with any statutory regulations, the low-voltage neutral point of substation and similar transformers should also be earthed.

In unattended substations it is an advantage to fit each transformer witha maximum indicating thermometer, so that a check can be kept upon the temperature rise.The setting of alarms is dependent on local ambient and loading conditions,but is usually based on the BS maximum oil temperature rise of 60°C.

Alarm thermometers, which depend upon oil temperature, might be set at 85 and90°C respectively to take account of the inherent time lag between maximumand top oil temperatures.

Winding temperature indicators, which more closelyfollow variations of winding temperature, are used for all large transformers and might have a warning alarm set at 105°C and a trip at 110°C: these values are similarly subject to local ambient and loading conditions.

It must be borne in mind that there will be a temperature gradient between the actual maximum temperature of the copper conductors and that registered in the top of the oil, the former, of course, being the higher. This accounts for the differences suggested between the permissible continuous temperatureand the alarm temperatures.Protection settings may be set to a lower level than the recommended permanent settings for the initial energisation.If the transformer is not required to operate in parallel with othert ransformers, the voltage may now be applied. It is desirable to leave the transformer on no-load for as long a period as possible preceding its actual use, so that it may be warmed by the heat from the iron loss, as this minimises the possible absorption of moisture and enables any trapped air to be dispelled by the convection currents set up in the heated oil. The same objective wouldb e achieved by switching in directly on load, but for transformers fitted withgas-actuated relay protection the supply may be interrupted by the dispelled gas from the oil actuating the relay, which could then trip the supply breaker.

If, however, the transformer has to operate in parallel with another unit, it should be correctly phased in, as described in the chapter dealing with parallel operation, before switching on the primary voltage. It is essential that the secondary terminal voltages should be identical, otherwise circulating currents will be produced in the transformer windings even at no-load.

Transformers of which the ratings are greater than three to one should not be operated in parallel.Switching in or out should be kept to an absolute minimum. In the case ofswitching in, the transformer is always subject to the application of steepfrontedtravelling voltage waves and inrush current, both of which tendto stress the insulation of the windings, electrically and mechanically, so increasing the possibility of ultimate breakdown and short-circuit betweenturns. From the point of view of voltage concentration it is an advantage,wherever possible, to excite the transformers from the low-voltage side,although, on the other hand, the heaviest inrush current are experienced when switching in on the low-voltage side. The procedure adopted will therefore be one of expediency, as determined from a consideration of voltage surges andheavy inrush current. If the protection settings have been put to a lower level for initial energisation, these should be returned to their recommended valuesfor permanent service.

Insulation resistance test

Insulation resistance tests are carried out on all windings, core and core clampingbolts. The standard Megger testing equipment is used,

Should it be required to determine exactly the insulation of each separate

winding to earth or between each separate winding, then the guard of the Megger should be used.

insulation test voltages

high-voltage transformers were first designed and manufactured insulation test levels were arbitrarily set at twice normal volts.

This represented a convenient factor of safety over rated conditions and ensured that equipment in service was never likely to be stressed to a level approaching that to which it had been tested.

Not big enough to worry about, plug it in. If she holds, check voltage on secondary, connect and call it a day.