Post-mortem of SCR trigger board failure

Yesterday I was at the facility where I do design work for high current circuit breaker test sets, and they had experienced a rather dramatic failure of an SCR controller board I had designed some time ago. The SCRs are connected to a 480 VAC mains supply, and the output is a multiple section high current transformer with one section connected to a variable autotransformer (Powerstat), and the other sections are either shorted or connected directly to the SCR output. The secondary consists of a heavy copper bus bar rated at about 6000 amps continuous and able to produce currents approaching 100,000 amps into a short.

In this case, they had installed a retrofit which replaced the instrumentation and the unreliable SCR trigger can that was in the unit.

Part of the testing involves initiating the unit with an open circuit and reading output voltage with various settings of the variable autotransformer (vernier), and coarse taps from 1 to 9. Maximum output voltage is about

12 VAC. During this test, high surge currents were observed on the input from the AC line, and it would trip a 30 amp service breaker on tap 2 where the second section is connected to the output of the SCRs. The waveform indicated high peaks that looked like saturation. We have experienced similar problems with this type test set before, and it has seemed to be

remanent magnetism from short AC pulses with unequal positive and negative half-cycles, causing a net DC component. And when the unit is connected to a higher rated mains supply, it usually survives the high initial surge (several thousand amps) and then works OK. It is particularly bad with an open circuit secondary. With a load, the waveform looks good, and the controller is designed to initiate at about 70 degrees phase angle, and provides 5 full cycles so no significant DC component is present.

However, in this case, when they connected the test set to the 400 ampere

480 VAC mains supply, it caused a severe overcurrent event which resulted in the SCR pucks shorted and the tracks blown off the bottom of the trigger

board and a plating of vaporized metal on the surface. The gate supplies are totally separate and isolated by means of split bobbin transformers and

3500V rated optoisolators. These components did not appear to be damaged, and the only visible damage was the vaporized tracks on the bottom of the board. So I think something caused a high voltage to be imposed on the gate circuit itself, which connects to the gates of the SCRs through twisted pairs of probably #18AWG wires. There is no fusing on these wires, and it appears that the PCB tracks functioned as a fuse.

After replacing the SCRs and the controller board, they had resumed testing, but connected the power section to a low-current isolated variable supply. We observed similar high current spikes as we attempted open circuit testing. Then they connected a shunt as an output load, and after one or two pulses with some high current surges, the output current looked normal and there were only occasional and brief high current transients on the input which resolved after about 20 cycles, and was indicative of DC offset and remanent magnetism. So it appears that drawing current from the transformer in a controlled manner demagnetized it.

My opinion is that the SCRs may have been previously damaged and weakened, and when they encountered the high current spike from the remanent magnetism, one of them failed partially open, and briefly imposed much of the 480V mains voltage across the gate circuit, causing it to vaporize, and then finally failed as a short circuit, which removed the high power from the gate circuit and then simply fed the transformer primary with normal

voltage and current, although it may also be that the 400A main breaker tripped.

This is the first time I know of that an SCR board has failed in this manner, and I wanted to know if anyone else has experienced anything like this. In a post a few years back I brought up the possibility of fusing the gate circuit, and I have found some fast-acting SMT fuses that are rated at

500 VAC, so perhaps they would be a good safety measure. It may also be necessary to add a TVS zener across the gate circuit downstream of the fuses to clamp the voltage and provide a low impedance current path for the fuses to act on.

Any thoughts?

Thanks,

Paul

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P E Schoen
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