thermionic1
Electrical
- Nov 30, 2018
- 323
From time to time we test HV CCVT's (115kV-345kV). We have found that when using a TTR or similar, the measured ratio is nowhere near the calculated (5-15%). We have used TTR kits that claim to be able to accurately measure the ratio and have varied the voltage up to 100V on the primary.
What we have found that works is using a Power Factor test set as a source (10kV) and then measure the secondary voltage with a DMM. The measured ratios are then within an acceptable range. Unfortunately with this method, we can't verify the polarity in one step, like a TTR does, nor does the calculation / documentation get done electronically.
I am thinking there is either some voltage dependant issue or that the TTR does not have enough "uumph". We've used the same TTR to ratio test 138kV Iron/Copper VT's with no issue, but have issues with a 138kV CCVT. We have used a rental (Omicron) test set on 345kV CCVT's at 2kV and the ratios were spot on. I've experimented with a variac and a 120:480V VT (backfed) and that didn't seem to help.
I'm wondering what others do and if there is a reason for the large deviations at lower voltages.
Thanks.
What we have found that works is using a Power Factor test set as a source (10kV) and then measure the secondary voltage with a DMM. The measured ratios are then within an acceptable range. Unfortunately with this method, we can't verify the polarity in one step, like a TTR does, nor does the calculation / documentation get done electronically.
I am thinking there is either some voltage dependant issue or that the TTR does not have enough "uumph". We've used the same TTR to ratio test 138kV Iron/Copper VT's with no issue, but have issues with a 138kV CCVT. We have used a rental (Omicron) test set on 345kV CCVT's at 2kV and the ratios were spot on. I've experimented with a variac and a 120:480V VT (backfed) and that didn't seem to help.
I'm wondering what others do and if there is a reason for the large deviations at lower voltages.
Thanks.