The 15 minute withstand voltage of 53kV DC of a 15kV, 200A loadbreak elbow was exceeded. A 65kV DC MV cable test voltage was used and the test failed. The cable and connector were subsequently retested at 53kV and passed. The operating voltage for these cables will be 13.2kV.
I'm hoping someone could explain what happens to the connector insulation during this type of failure, how it or any other component of the connector is possibly compromised (damaged) and how this would affect the service life?
As you are aware, the guiding lights for DC hipot are:
IEEE 400,
Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems Rated 5 kV and Above
and
IEEE 400.1, Guide for Field Testing of Laminated Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current Voltage
15 minutes duration suggest the test was acceptance as opposed to maintenance.
Couple of questions:
Did the test perform a continuous rise on test voltage over 60 Sec?
Or was a step increased used, (generally one minute per step)?
I'm surprised at the voltage. IEEE400.1 suggests 56KV for acceptance tests. 65KV is pretty high. How come so high?
So, just guessing on my part:
At 65kV, the current did not drop during the 15 minute duration. But the current likely did not climb.If the test was the step variety, and the stabilized current was recorded, a plot of voltage/current should show a knee - as in the slope goes down. This would have happened above 53kV. The knee indicates the insulation resistance dropped.
For the second test, I would expect the V/I slope was lower than the first test and the test was stopped before the knee - before the resistance exhibited anther drop.
If any of this is true, then the 65KV Hipot test broke down the insulation - not a lot but a bit.
In my opinion:
Any time the hipot goes far enough there is a knee in the V/I slope, the insulation is degraded.
The cable survived the 65KV and passed the 53KV. I would not expect a reduction in the service life.
The next time this is tested, it will be a maintenance test. If you don't have a VLF AC machine by then, pay attention to the IEEE400.1 maintenance limits, for a 15KV cable, that would be 46KV.
And if none of this is true, well, I've been known to all wet more than once before.
No, I am not a fan of DC hipot. They are too close to destructive.
The Worm
