[chusker]

My company recently acquired some new facilities. When I went to check one of the 69Kv - 34.5 KV power stations, at the new facilities, I noted that the neutral wire, of the solidly-grounded 34.5Kv system, was routed from the transformer XO bushing down the exterior of the transformer through a 1 inch rigid metal conduit, for approximately 25 feet in length, attaching to the ground grid of the substation. I am curious as to the induction effects of the wire in the ferrous conduit during a heavy fault.

We have had two 34.5Kv, KYLE (Cooper Power Systems) type WVE38X reclosers to fail in the last four months. They have failed internally and the resultant arcing pressurized the tank and hot/flaming oil spewed out the tank gasket/O-ring upward and outward for approximately 30 feet. We dropped the tanks and found in both cases the 34.5Kv closing coil circuitry had failed with resulting arcing from phase A to the tank and the internal framework...it was also noted that a heavy flashover had occurred between the external bushing of phase B and phase C on the source side of both reclosers. There are no surge arresters on either the source side or load side of the reclosers.

I am curious as to the effective high impedance, of the neutral wire in the conduit, possibly created during a heavy fault limiting fault current and shifting the effective neutral point and creating high voltages on the phases that are not grounded. Has anyone encountered this scenario? Do you recommend the changing of the rigid metal conduit to nonmetallic rigid conduit or is the induced effect possibly minor to non-existent?

 

[Semi-Ret Electrician]

Yes, to PVC with no coils or sharp bends. Treat it like lightning.

But, Code says grounding the end of the metal conduit is the answer.

But IMO all that does is create a loop (pick-up-coil), which makes it even worse.

 

[backstay]

I find it hard to believe the wire from your grounding ring being in metal is causing a flashover on the line side of a 34k recloser. Did the line side bushings get cleaned up after the first failure? And have you identified your fault? Sounds more like your downstream OC system is lacking.

 

[chusker]

The two failures involved two different reclosers.

 

[chusker]

In both cases there was no evidence of a fault on the system after the failures...the circuits were checked and nothing obvious was noted....in both cases the circuits powered up without issue when the reclosers were replaced.

 

[macmikeman]

Chuckster, your job makes me happy to drill holes in 2x4's . Stay safe.

 

[Big John]

Even completely losing a neutral wouldn't cause a flashover, because the poles are designed to withstand the full phase-to-phase system voltage continuously.

If you're getting lightning strikes and they're getting into the controls, my money says they would take out more than just the close coil, so if you're not seeing any other damage, that's not my first theory.

It could be a lot of causes: The coils themselves could be smoking because a jammed up mech, they contaminate the oil sufficiently to allow flashover, the carbon from the blowout allows phase-to-phase faults.

What protection do you have on the system? Is there anything indicating a fault? Any damage to the controllers? How old are they? How often do they operate?

 

[chusker]

Do you think ferroresonance could be playing a part as to the flashovers, where as, the "choke effect" of the ferrous conduit causing the circuit to be impedance grounded versus being solidly grounded?

 

[Big John]

Do you think ferroresonance could be playing a part as to the flashovers, where as, the "choke effect" of the ferrous conduit causing the circuit to be impedance grounded versus being solidly grounded?

No idea, I don't know enough about ferroresonance. All I can say is I wouldn't expect any failure with the neutral to impart more than line-to-line voltage, so if your equipment is designed to handle that but you're still seeing flashover failures, it's not the first thing I'd look at.

Why are your closing coils burning up? That's where I'd start.