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Pre-commissioning Test —
The objective of the pre-commissioning tests is to confirm that the transformer has not suffered any
damage during transit and also check any inadvertent slips in the factory tests.
(Measurement of Insulation Resistance) The IR values are measured between the following.

  1. HV to LV and earth connected together.
  2. LV to HV and earth connected together.
  3. HV&LV
  4. HV and LV connected to w.rt. carth.

IR value shall be measured with all windings earthed except, the one being measured by a 5/2.5/k megger.
Before measurement all bushings shall be cleaned and all external lines, cables and lighning arrestor isolated
from the transformer. The oil temperature prevailing at the time of measurement has to be noted since insulation resistance varies inversely with temperature by a factor as noted below.

Difference in temperature Factor K
10 1.65
20 2.6
30 4.2
40 6.6
50 10.5

Typical values of Insulation Resistance

Rated voltage Minimum Minimum IR value (in Mega-Ohms)
30°C 40°C 50°C 60°C
66kv & above 600 300 150 75
33kv & 22kv 500 250 123 65
11kv & 6.6 kv 400 200 100 50
Below 6.6 kv 200 100 50 25


PI Insulation condition
Less than 1 Dangerous
1 to 1.1 Very poor
1.1 to 1.25 Questionable
1.25 to 2 Satisfactory
above 2 Good

IR values are taken at the end of 1 minute, 10min, 15 min, & 60

Polarisation Index  =     IR value at the longer interval
                                           IR value at the shorter interval

=      PI value at 10 min
PI value at 1 min

PI Value greater than 1.5 in considered good.

Ratio test – 3 phase 415 VA.C. in applied on the H.V. terminals and the voltage on LV and Tertiary (IV) terminals
are measured for all tap positions. The ratio obtained by this in compared with assigned ratio. The readings can
also be taken with the help of ratio meter.

Magnetic balance Test : –

Detects shorted turns. If there is a shorted turn it will oppose establishment of fluxin that limb because of circulating current within the shorted turn.

Step-1 Apply 415V or 240V AC to say, U phase, measure voltage in V & W phase and also the current in U
phase. In case of all phases healthy the voltage in V phase and W phase shall be 70% & 30% of U phase.

Step-2 Next energise W phase & measure voltage in U & V readings should be same as in step-1. (Precautions)
a. Transformer neutral should be disconnected from ground.
b. No winding terminal should be grounded.

Step – 3 Apply voltage in V phase, voltage in U & W should be equal. In case one of the phases has shorted
turns, it will draw comparatively large current den energized. When other phases are energized
voltage in the shorted phase will be significantly reduced. Similar test also be done on LV side.

Short circuit test :- 

LV terminals of the transformer are shorted using copper or aluminium wires of minimum150 sqm e3 phase
415V is applied on HV and Current is measured. V/lgives the short circuit impedance of transformer at that tap. Readings should be taken at all taps and it should tally with the impedance (% impedance) figures given in the name plate.

Floating Neutral Voltage measurement :-

Apply 3F 415 V Ac to HV after disconnecting transformer neutral from ground. For a healthy transformer voltage between neutral and ground in zero or negligible. But in case there is a short circuited winding voltage between neutral and ground is appreciable. This test will help in detecting gradual deterioration or development of fault in winding while in service.

Magnetizing Current Measurement :-

Apply 415 V, 3 Ph AC to HV keeping LV open and tap switch in normal position. Measure the magnetizing currents in HV, LV & IV, compare with factory results. Apply 3 ph 415 AC to HV. Measure voltage in LV & IV in all taps find ratio, compare with factory test reports.

Measurement of winding Resistance (DC Resistance) :- 

The transformer winding resistance should be below 1ohm. The DC resistance can be measured by:
1. Appling suitable DC voltage across the winding and measuring current flow.
2. By Kelvin double bridge or Wheat stone bridge.
During measurement temperature should be noted.
Temperature correction of resistance,
R75 =  Rt(235+75) / (235+t)
t = temperature at which measurement is made.
For Delta connected winding measurement shall be done between pairs of line terminals.
Resistance per winding = 1.5 x measured resistance.

Measurement of BDV :-

Oil samples should be collected separately from main tank, conservator tank OLTC & cable box link chamber.
Their BDV shall be 30 KV (rms) minimum. If not oil shall be filtered and BDV after filtration shall be 60 KV (rms)

Testing of Protective Devices :-

Actuation of Top float and bottom float of Bucholtz relay shall be checked by admitting dry nitrogen of
adequate pressure at the bottom of the tank. The alarm & trip circuits of Magnetic oil level gauge, MOLG
pressure relief device, PRD temperature indicators and surge relay shall also be tested. Alter watery air can be
pumped into Bucholtz relay chamber and by keeping the conservator tank side valve closed for the above check.
Parallel Operation – Conditions to be fulfilled by incoming transformer
1 . Vector group shall be compatible i.e.
a. Same inherent phase angle difference.
b. Same polarity.
c. Same phase rotation.
2 . Same voltage ratio on all taps.

Also Read : General Problems in Transformers  



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