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RC failure

RC failure

RC failure

(OP)
Here’s the problem :

Network :
- 132/6,6kV 20MVA 10% Yd (earthed via NECR) supply transformer, giving LV fault level of 192MVA.
- 6x6,6kV 150mm^2 400m PILC feeders to Mill Substation. Mill SS feeds 2 x 5,5MW mills.
- Speed control is achieved by slip recovery via Alstom PWM drive in rotor circuit (IGBT chopper on machine and supply side).
- Fed back to 6,6kV via 950kVA 6,6kV/580V 4% Dy recovery transformer.
- Line reactors of 250uH (on machine and supply sides).
- Carrier freq, is 3kHz.
- ZORC suppressors (RC with ZnO arrester across resistor) are on the 6,6kV side of the recovery transformer. R=30ohm (about 100W), C=0,2uF.

Problem :
- Machines have been running for 2 years, no problem.
- Client installed 6MVAr detuned 5th filter (L=1.02mH, C=420uF/ph) PFC bank.
- ZORC on one machine failed (exploded after about four days).
- Installed new ZORC, lasted about 5 minutes.

My thinking :
- When looking from the supply side chopper, cable resonance occurs near to 90th harmonic (2nd order of the carrier) with PFC out.
- With PFC in, cable resonance occurs near to the 2nd order of the carrier freq. i.e. 120th harmonic or 6kHz.
- Everything seems to point towards resonance being the problem.
However, my gut feel asks the following :
- Why did the second ZORC not fail. Maybe it has already and just did not self destruct. We will check when we get a shut.
- With such large line reactors and the recovery transformer the series resonance amplification is very low according to my model.
- Is there enough power at this frequency to cause the ZORC to fail.

Anyone with any other ideas ? Hope I have given enough info.



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