Megger Testing In-Service CT or PT secondary winding

[Milliamp]

What are the thoughts on megger testing in-service CT or PT secondary wiring? What precautions would you follow. Would it be done at all at your company? Thanks.

 

[busbar]

An applicable reference may be §7.10 of NETA Maintenance Testing Specifications for Electrical Power Distribution Equipment and Systems 2001 Order info

http://www.NETAworld.org

 

[scottf]

Do you mean to test the windings while the units are energized?? Or do you mean to test the wiring from the units to the control house?

Assuming the wiring was disconnected from the units during the test (i.e. with shorting bars, knife switches, etc...) and from the connected devices, it should be no problem.

I don't believe it to be possible to megger test the units themselves while energized, or for it to be advisible to try!

 

[NormGA]

I have done high voltage (I usually used 1000 volts) megger tests of many, many current transformer circuits while in service. These include power circuit breaker line relaying currents from 12 kv to 230 kv, metering circuits at same voltages, bus differential circuits and bank differential circuits. The only problem I ever encountered was mechanical, when the ground was not the top wire on the stud. Even then you can usually find a way to get the ground disconnected. I left the ground off of a bank differential scheme for about a year because of a low resistance fault on one conductor. I will admit that I always opened the Bank or Bus Differential cutout switch before starting just in case there was a wiring error. But never had any indication that it was needed. Never tried a potential circuit. Always figured that if you have potential the cable can't be that bad.

Ichabod

 

[jbartos]

Suggestion: Additional megger test details including precautions might be available from CT and PT manufacturers tech support.

 

[scottf]

NormGA-

I can see how it's possible on CT circuits, but not on VT circuits.

Remember, if you apply a potential to the secondary of a VT (without is being shorted), a potential will be induced on the primary. If the unit is in-service, this will probably cause the VT to saturate, and potentially fail (i.e. blow-up).

 

[peterb]

Despite NormGA's experience, it is usually a very bad idea to lift the ground on a CT secondary circuit - the ground is there for safety reasons, to tie down the potential of the secondary circuit to ground. Otherwise, capacitive coupling between primary and secondary could float the secondary winding potential to ground up to high levels - dangerous to personnel and potentially damaging to equipment & wiring. I recommend against lifting the ground while the circuit is energized.

 

[NormGA]

Correct me if I am wrong, but aren't most if not all, CT's used at higher voltages shielded? The majority of my experience has been with higher voltage oil circuit breakers and I know that the current transformers are actually around the grounded portion of the bushing. On the 230 and 115 kv gas breakers that I have experience with the ct's are outside the tank of the breaker and you can actually reach up and touch them, even though the static is kind of harsh. But the ct's themselves are inside a shield. As a field test engineer, lifting the ground off a ct circuit is way down my list of hazards. Inserting a relay test plug into a GE directional overcurrent relay is far higher IMO, and a GE bus differential relay is about the same.

Norm

 

[Milliamp]

Thanks for the replies. We have been meggering secondary CT & PT circuits during troubleshooting when the circuits are in-service for several years with no problems. It is not always possible to take the bus or breaker out of service to megg the circuit while you are hunting for a second ground. We are now recommending that the technician wear their 600 volt gloves while removing the ground, during the test and while replacing the ground. We use a 1000 volt setting on the megger and have not had any problems with any of our relaying during the testing. We also megg the PT circuit when looking for a second ground. If there is a ground at the control house and another unintentional ground outside or at the PT junction box, it can cause problems with the directional units of the relaying. I wanted to see what the industry was doing. Thanks again.

 

[jbartos]

Suggestion: Any capacitive charges might be discharged by shorting cables prior to megger testing.

 

[scottf]

Milliamp-

Is the control wiring connected to the PT secondary during these tests? If so, lifting the ground could be big trouble!! If there is a system transient, etc... and there is no ground reference on the PTs secondary, the secondary can float to very high voltages and it could be very dangerous.

 

[Milliamp]

For Scottf - The neutral is never disturbed. The safety ground is lifted long enough for the test. This would not be done during inclement weather.

 

[scottf]

Milliamp-

It's up to you...

However...as a manufacturer of VTs and CTs, I wouldn't recommend it! Quite frankly, in my humble opinion, it's a good way to get somebody killed.

Inclement weather is not the only thing that can cause transients...line switching, equipment failure, etc...can all be causes of voltage rise or high-frequency transients on the line.

Also....isn't a megger test DC? Have you considered the effect of applying a DC voltage to the secondary of a VT with the primary energized?

 

[jbartos]

Suggestion: The some PTs have fuses on the primary and on the secondary. Then, the in service as explained in the author's second posting is posing no problem.
The CT meggering in service may become difficult if the CTs secondary windings are connected in grounded wye connection.

 

[scottf]

Jbartos-

Sorry....I missed any comments about removing fuses (and still don't see them)...I asked earlier if the vt was removed from the circuit and I understood milliamp to say that the connections to the VT were left in tact.

 

[jbartos]

Comment to the previous posting: Connections are normally related to conductors and terminals. Fuses are not considered connections.