Today was yet another rather uneventful day on the consulting project front. I guess the highlight was watching some electrical testing of the cable joints that were made earlier as shown in my previous posts about cable jointing.
A chap who specialises in testing cables for power grids and companies came out to our building (he was late) and began to test the joints that we had. First he did a low voltage test for capacitance and resistance to see if there were any shorts in the joints. By low voltage, it was 5000v. It ramped up and held at 5kv for one minute and registered about 3-400 G Ohms on the three phases.
The next stage was testing the connectivity for non-service range, which is normally done at 19kv. His little box was plugged in and hooked up appropriately, and it was generating a stable DC flow of 18.5kv, close enough to the 19kv standard it seems. The test ran for fifteen minutes on each phase. You couldn't see anything happening except on the screen of the device, but we were warned if you got close enough or touched the exposed cable on either end, it'd pretty much be the end of you since it was generating about 200mA, which is enough to stop your heart straight away (I believe you only need about 120mA across the chest?).
The most spectacular part of the testing though was the high voltage test. In an effort to deliberately cause a blowout of the joint, we decided to try to ramp the voltage to a staggering 75kv, which apparently to stress-test a cable you run 3x the cable spec, so for a 11kv cable, you would run it at 33kv etc. Our cable was a 33kv cable but because it is three phase, it means each cable is actually only a 11kv, so running 75kv is in extremes, and in theory should blow it out.
Let me tell you, electricity is frightening in the sense that you can't see it until it misbehaves. Everything was hooked up, and the voltage bumped up gradually, at about 35kv you can start to hear it hum, and then it just gets louder, and louder. At 42kv there was this almighty * crack * and the circuit breaker pops in the truck. Well, what had happened was the exposed cable had arced across a gap of about fifteen centimeters to the semiconductor/ground segment of the cable and shorted the system. It was quite startling. So, being engineers, we decided to push ahead and instead of stopping, clean off the burnt segment, and insulate it with #23 rubber tape and try again.
We have both ends bound so it wouldn't arc on either side, and run it again. 35kv hum, 40kv, 45kv, 50kv and it's very loud, 52kv and it pops again. The joint didn't fail, but once again, it had arced from the supply wire to the semiconductor layer..... Amazing since it was in open air that it had ionised enough air around it to jump a good 20cm or so. We then tested a second joint (which had been made under different conditions) and it too blew out by shorting at the 51kv mark, also through the rubber insulation..... Scary stuff.
This as interesting as it was did not help much in what we were trying to find, so the cables will be re-prepped for another electrical test run next month, and we will try to drive the joints all the way up to 75kv and get a cable joint failure instead. Should be interesting to see the damage it does on the inside when it goes.
A chap who specialises in testing cables for power grids and companies came out to our building (he was late) and began to test the joints that we had. First he did a low voltage test for capacitance and resistance to see if there were any shorts in the joints. By low voltage, it was 5000v. It ramped up and held at 5kv for one minute and registered about 3-400 G Ohms on the three phases.
The next stage was testing the connectivity for non-service range, which is normally done at 19kv. His little box was plugged in and hooked up appropriately, and it was generating a stable DC flow of 18.5kv, close enough to the 19kv standard it seems. The test ran for fifteen minutes on each phase. You couldn't see anything happening except on the screen of the device, but we were warned if you got close enough or touched the exposed cable on either end, it'd pretty much be the end of you since it was generating about 200mA, which is enough to stop your heart straight away (I believe you only need about 120mA across the chest?).
The most spectacular part of the testing though was the high voltage test. In an effort to deliberately cause a blowout of the joint, we decided to try to ramp the voltage to a staggering 75kv, which apparently to stress-test a cable you run 3x the cable spec, so for a 11kv cable, you would run it at 33kv etc. Our cable was a 33kv cable but because it is three phase, it means each cable is actually only a 11kv, so running 75kv is in extremes, and in theory should blow it out.
Let me tell you, electricity is frightening in the sense that you can't see it until it misbehaves. Everything was hooked up, and the voltage bumped up gradually, at about 35kv you can start to hear it hum, and then it just gets louder, and louder. At 42kv there was this almighty * crack * and the circuit breaker pops in the truck. Well, what had happened was the exposed cable had arced across a gap of about fifteen centimeters to the semiconductor/ground segment of the cable and shorted the system. It was quite startling. So, being engineers, we decided to push ahead and instead of stopping, clean off the burnt segment, and insulate it with #23 rubber tape and try again.
We have both ends bound so it wouldn't arc on either side, and run it again. 35kv hum, 40kv, 45kv, 50kv and it's very loud, 52kv and it pops again. The joint didn't fail, but once again, it had arced from the supply wire to the semiconductor layer..... Amazing since it was in open air that it had ionised enough air around it to jump a good 20cm or so. We then tested a second joint (which had been made under different conditions) and it too blew out by shorting at the 51kv mark, also through the rubber insulation..... Scary stuff.
This as interesting as it was did not help much in what we were trying to find, so the cables will be re-prepped for another electrical test run next month, and we will try to drive the joints all the way up to 75kv and get a cable joint failure instead. Should be interesting to see the damage it does on the inside when it goes.
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