Figure 4. Top – Replay overview of a single phase excitation test on a phase
(Red Line – HV Phase-to-Ground Voltage; Green Line – Partial Discharges);
Bottom – Phase – resolved PD patterns
The transformer was then excited by supplying all three phases as per service condition which proved that partial discharges originated from the A phase were also cross-coupled PD to other phases in relatively small magnitude (Figure 5).
Acoustic sensors were positioned in various locations on the transformer tank during the three-phase excitation tests. Figure 6 shows the acquired acoustic signals with the sensors positioned 450 mm from the top lid of the tank on HV side against each phase. The calculated distances from a potential PD source for B & C phases in relation to the signal received by the sensor on the A phase were:
- 43 m for B phase sensor and
- 55 m for C phase sensor.
As one can see from Figure 7, the A phase signal has a sharper front rise in comparison with more attenuated signals coming from other phases. It means that the signals received by other phases are more remote and include both longitudinal and transverse waves propagated through transformer oil and tank.
Familiarity with internal structure of the transformer was presented by the asset management team to determine the most likely source of partial discharges. The investigation continued on the A phase assuming a direct acoustic path of PD signals coming from either a flux deflector on top of the A phase winding or from bottom of the HV bushing.