Parasitic Resistance

[jvjtech]

I may have a problem with parasitic resistance, not parasitic capacitance, in a customer's 600 Vac system. It appears that a parasitic resistance, in the order of a few 100 kOhms or single digit megohms, is appearing from the neutral to ground, effectively in parallel to the neutral grounding resistor (NGR).

In thread238-199182 Oldfieldguy says "I have found that in many cases there is sufficient capacitive and resistive path to ground, like a connected MCC's buswork, ... ". I would like to know what Oldfieldguy means by "sufficient".

I suspect that in my case, leakage in various MCC systems is cumulative and is appearing as a parasitic resistance from the neutral to ground.

I will add that while it is possible, I do not think that any bleeder resistors as discussed in thread238-45575 have been added to the connected equipment.

Any information as to what can be expected in the way of parasitic resistance is welcome.

Parasitic capacitance is not causing problems.

Thanks all.

 

[Bronzeado]

Hi folks,

May I resume this very interesting thread? I have a doubt that may be associated with this matter.

Our 69kV system is fed by the delta of 230/69kV transformers, wye-delta connected, with a zig-zag tranformer to provide a earth reference. In general, there are more than one 230/69kV transformer in our substation. So, when one transformer is returning from maintenance, it is energized by closing, first, the 230kV circuit breaker and then the 69kV breaker.

Question 1. In this case, before closing the 69kV breaker, the 69kV link between the transformer bushing and the breaker remains without earth reference. As we use PTs in this link, connected phase-to-earth, what kind of voltage measuring we will get? Is this measuring correct?

Question 2. In the case of normal configuration (all 230/69kV transformers on) but without the zig-zag transformer, the PTs will measure the voltage correctly?

Slava, I really appreciate the diagrams you posted. They are very didactic. I wonder if you could make one with the system described above, i.e., being fed by a delta instead a wye. Please, include the 3 PTs connected phase-to-ground. Thank you!

Best regards,

H. Bronzeado

 

[davidbeach]

The PTs will measure phase to ground voltages correctly, but that measurement may be meaningless. All it is is an indication of the balance of the parasitic capacitances. The phase-to-phase voltage is a much more meaningful value.

 

[Bronzeado]

Davidbeach,

Thank you for your reply.

I agree with you that the voltage measurement MAY BE meaningless.

However, these measurements are normally used to feed some protection systems, such as the residual voltage relays (3V0). Do you think that this protection may operate incorrectly due to these meaningless measurements?

Suppose that the parasitic capacitances/resistances of all phases are similar. In this case, do you think that the voltage measurement could be considered correct, as the "neutral point" formed by the capacitances/resistances may coincide with the "grounding point" of the PTs?

Regards,

H. Bronzeado

 

[davidbeach]

You will get an accurate 3V0, and that 3V0 will be much more meaningful than any single phase-to-ground voltage measurement. You won't get a zero value for 3V0 unless your bus work is fully transposed, which it won't be; but your 3V0 should be small. If you have a larger 3V0, what you might do depends on various factors, but you probably don't want to close into your grounded system while you have a significant 3V0 on the ungrounded system. The definition of "significant" will vary from system to system.

 

[slavag]

Hi David,Herivelto.
Good night in your part of world, good morning in our.
Please find attached file.

Herivelto.
Hope that help.
Best Regards.
Slava

 

[Bronzeado]

Hi Slava,

Thank you for posted your complete set of slides.

If you have some time, could you, please, make a electrical system fed by a delta winding, including 3 PTs connected phase-to-ground to see how will be the distribution of currents? Thank you in advance.

David, thank you for your though on this matter.

In Recife, Brazil, it is 3 AM.

Regards,

Herivelto

 

[slavag]

Hi Herivelto.

(In Recife, Brazil, it is 3 AM. and you think about delta trafo fed ).

Slides from 1 up to 8 are OK for both, not grounded wye and delta trafos. PT's aren't affect on the current distribution. You can use 3PT's phase to ground, but connect to relays, metering as phase to phase.
Best Regards.
Slava

 

[Bronzeado]

Hi Slava,

Thank you for your "smile face".

Regarding to ungrounded systems, I have discussed many times with colleagues on the "meaningless" measurements provides by the PTs connected phase-to-ground. However, I have not been able yet to convince them. So, your slides will help me in the further discussions.

Now, I start to think that the PTs will provide a "metalic" connection between phases through the windings of the power transformer. Perhaps, this could make the phase-to-ground voltage measurements be meaningful (?).

Also, if there are two sets of PTs on the same busbar, the circuit formed by the primary windings of the PTs connected phase-to-ground on the same phase could stabilize the measurements.

Thought on that will be very welcome.

Regards,

Herivelto

 

[slavag]

Hi Herivelto.
I read again your's and David's posts.
I'm not big theoretical specialist, maybe what I say now isn't right, I don't know.
I hope this thread also read guys from Finland or Sweden.
Them have a good experince with ungrounded systems and can help us.
What I think.
When we say ungrounded systems, we simlify situation, actually we haven't connection to ground reference point, but.. we still have capcitive coupling to ground ( motors, cables, lines, trafos). That means, when you connect phase to earth PT, via capacitive coupling you meas. phase to phase/ sqr 3 voltage.Not via trafo's winding, via capacitive coupling, but 3PT's connection point is flow, becouse capacitive.
I think you are right in your's:
"Suppose that the parasitic capacitances/resistances of all phases are similar. In this case, do you think that the voltage measurement could be considered correct, as the "neutral point" formed by the capacitances/resistances may coincide with the "grounding point" of the PTs?"
Sorry for English, I hope I'm was enough clear.
Regards.
Slava