Defects detected in power transformers with the MIKO-2.3 device

16 september 2013

The most typical defects detected by ohmic resistance measurements of transformer windings include:

Break of one or several parallel wires in the taps-off;
Soldering defects;
Poor connection of winding taps-off to input terminals;
Poor contact in the off-circuit tap changers (CTC) or in the on-load tap changers (LTC);
Incorrect adjustment of CTC drive;
Break of current-limiting LTC resistors.

Such defects during measurement of ohmic resistances of transformer windings can be detected using the MIKO-2.3 instrument.

In the milliohmmeter mode the instrument allows resistance measurement in the range of 10^-4 - 10³ Ohm. In this mode the instrument allows resistance measurement of windings connected following the Od (single winding), D (triangular), Y (star), YO (start with zero) diagrams, as well as reduction of resistances to temperature (with account of the winding metal), conversion of resistances into resistances of phase windings, computation of differences between phases in per cent.

Resistance measurement requires determination of the transformer windings temperature. This can be done by the MIKO-2.3 instrument using the thermometer mode and a special thermometer available with the instrument kit. For transformers that has been idle for at least 20 hrs and were not heated the winding temperature is assumed to be equal to the temperature of upper oil layers. For transformers with the capacity of 1MV*A the measurements in this case should be taken minimum 30 minutes after oil filling, and for transformers of higher capacity - two (2) hours after.

Prior to measuring the ohmic resistance of transformer windings, check the availability of local heatings on the transformer tank using infrared devices, and analyze the oil samples. Defect location is detailized by comparing the results of an infrared imager, chromatography analysis of dissolved gases (DGCA), by determination of idle run losses at low single-phase excitation, and by measuring the winding turns ratio.

It should be mentioned that it is difficult to detect the contact system break on the HV winding side using the thermography method. This is because the local heating sources are deep in the oil thickness whose volume is estimated at several tens of tons, and centers of heating are weak. For this reason the ohmic resistance is used. Examples of defects detection in power transformers using the winding ohmic resistance measurement are given in Table 1.

Table 1. Examples of defects of HV windings of transformers

#	Transformer type	DGCA	Deviation R	Defect
1	TRDCN-63000/220/10	СН₄- 0.0139 С₂Н₄- 0.017	Deviation of ohmic resistance as against other phases on HV winding in Position 5 of phase B was 3%.	On the 5th branch line of Phase B (LTC of PC-20 type) the bolt was half unscrewed. There are traces of partial 2 cm²discharges on the locking washer. The bolt was not protected by a locking washer. Manufacturer's defect.
2	TRDCN-63000/220/10	С₂Н₂- 0.0014	Resistance of Phase B on all the taps-off 1..9 differs from 12...19 taps-off by 5%.	There are traces of 1-2 cm² melting on the contact of LTC of PC-4 type reverse.
3	TDN 16000/110/10	С₂Н₄- 0.0014 С₂Н₂- 0.0019	Resistance of Phase A on the 4th tap-off differs from other LTC taps-off by 4.3%.	After transformer opening the loosened fixing of the 4th tap off of HV Phase A winding was detected.
4	TDTN-40000/110/35/10	С₂Н₂- 0.0011	On Phase A in the second position of the HV LTC winding the difference from other phases at the same LTC position was 7%.	After transformer opening the loosened fixing of the 2nd tap-off of HV winding switch of phase A was detected.

Table 2 presents cases of defects detected by ohmic resistance measurement in low voltage (LV) windings..

Table 2. Examples of defects of LV windings of transformers

#	Transformer type	Infrared vision control	DGCA	Deviation R	Defect
1	TDNG-16000/110/6	Fluorescence in the mentioned LV phases.	СН₄- 0.058 С₂Н₄ - 0.091 С₂Н₆ - 0.026	Deviation of ohmic resistance on Phase b as against other phases was 17.1%.	Loosened output of phase b and input pin; the washer was 20% burnt off, an isolation tube was coaled. Two copper plates were not tinned. Manufacturer's defect.
2	TDN 16000/110/10		С₂Н₄ - 0.039 С₂Н₂ - 0.81 СО₂ - 0.63	Deviation of ohmic resistance at phase c as against other phases was 9.6%.	A loosened nut on the LV winding input pin.
3	TDN 16000/110/10		С₂Н₄ - 0.025	Deviation of ohmic resistance on phase c as against other phases was 30%.
4	TDTN-16000/110/6	Local heating was detected in the phase area.	Not revealed	Deviation of ohmic resistance at phase c as against other phases was 5%.

For transformers with LTC devices there is an option for quick measurement of winding resistance on all the taps-off. It is done using the MIKO-2.3 in the mode of measurements of a single winding (Od) (Fig.1).

For reducing the current and voltage stabilization time in a winding the measurements should be started with LTC position when minimum number of winding turns is connected.

miko-2.3

Fig. 1. A board of the MIKO-2.3

On a powerful transformer the first measurement can be rather extended. The measured resistance value is entered into the protocol and, without interrupting the measurements, LTC is switched over to the next tap-off. Now it takes less time for the current and voltage to stabilize as only a small portion of winding turns is added. LTC device ensures interruption-free switchover of turns. Therefore, measurement of winding resistance at each LTC position should be done without current interruption, and values of measured resistances should be entered into the protocol. Current should be interrupted only after measurement on the last tap-off is over, and measurement should be stopped by pushing the START button of the MIKO-2.3.

In the off-circuit tap changers the switchover from a tap-off to a tap-off is accompanied by the circuit break and, hence, by stabilized current interruption. Therefore, measurement of resistance on every tap-off should be every time started with current stabilization across the winding. And in this case it is impossible to reduce the time of measurements across all the taps-off.