Parasitic Resistance
Parasitic Resistance
(OP)
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: Odd voltage to ground 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: The Physics of Elecrtrical Failure: Ungrounded Systems 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.
In thread238-199182: Odd voltage to ground 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: The Physics of Elecrtrical Failure: Ungrounded Systems 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.






RE: Parasitic Resistance
On a 600V MCC a Meg ohm from line to ground would result in about 1/3 ma leakage current. How did you detect a meg ohm in parallel with the grounding resistance? On a 600 volt system a 347 ohm neutral resistor will limit the ground fault to 1 amp. 1 meg ohm in parallel will result in a combined resistance of 347.88 ohms. This will result in a ground fault current of 1.000347 amps.
At 480 volts the difference will be less.
Have I misunderstood something here?
What is leading you to believe you have a problem?
respectfully
RE: Parasitic Resistance
My apologies, but I have to withhold some information that would shed more light on the situation. I hope this helps a bit.
Thanks again.
RE: Parasitic Resistance
Regards. JVJ
RE: Parasitic Resistance
Can you give us enough information (eg. make and model #) of your grounding device and relay that we may google up a circuit drawing?
What is the ohmic value of your neutral grounding device? What ground current is allowed to flow during a ground fault?
Thanks
respectfully
RE: Parasitic Resistance
Thanks again. JVJ
RE: Parasitic Resistance
What are the inputs to your NGR monitor? You mentioned the neutral connection. Are there others? Voltage? Current? Or is it just monitoring the resistance of the NGR?
Where are you opening the NGR circuit?
RE: Parasitic Resistance
Yours
Bill
RE: Parasitic Resistance
When system power is on and if the relay detects measurable current through the NGR it uses this value with the input from the sense resistor to determine the resistance of the NGR and then see if it is within tolerance.
When system power is off there is, or there no current through the NGR, the relay injects a signal through the sense resistor to determine if there is continuity through the NGR to ground.
When the connection from the NGR to the GND is removed, with the other connection left intact, the relay does not indicate an NGR failure as expected. However, if this connection is left intact and the connections to the X0 terminal and through it to the plant wiring and are removed the relay does indicate an NGR failure as expected.
This leads me to believe that the presence of plant wiring is appearing as as a high impedance/resistance to ground mirroring continuity through the NGR to ground. Admittedly, this would be a high, very high, resistance in the range of a few 100 kOhms or single digit megohms.
Thanks all and safe travels Bill. jvjtech
RE: Parasitic Resistance
Where are you disconnecting the NGR from ground? My understanding is that the detection is to detect a failure of the NGR resistor not intentional disconnection. I may be wrong here. Is the ground lead of the resistor connected to the ground end of the NGR or to a separate ground connection?
You may have an inadvertent neutral ground connection bypassing the NGR.
Have you considered connecting a known resistor from one line to ground. Something in the range of 7000 ohms or higher.
From there, measure the voltage across your test resistor and the NGR. They should sum to 347 volts. If not,there may be some reactance in the NGR. From the voltages and the known resistance of the test resistor you should be able to calculate the current and the impedance from line to ground to neutral of the NGR plus any parallel resistance. I'm still quite tired. I may make more sense tomorrow.
respectflly
RE: Parasitic Resistance
Other tests at the site, with a low resistance intentional fault, showed a current of 800 mA at the point of the fault. Without an NGR the fault should have drawn significantly more than 800 mA but was limited by the NGR. As the current at the NGR would be 100 mA and if my math is correct, this shows the impedance of the parasitic capacitance to be 27517 Ohms, ignoring the contribution of the suspected parasitic resistance. The parasitic capacitance works to to be 0.010 uF.
I will do some more work on the vector math involving the combination of the suspected parasitic resistance, the NGR, the fault resistance and the calculated impedance of the parasitic capacitance above to see what to expect in the voltage divider presented by these elements. This work should also show what to expect if one element is varied as described above. And, from that, how to discern if there is indeed a parasitic resistance in the system.
The intentional removal of the NGR to ground connection is intended to simulate a failure of the NGR for the purposes of testing the relay's capability of detecting the failure. This connection is made through a wire at a terminal strip. I do not think that there is any other path to ground, e.g. chassis, mountings, etc. in the mix except for the parasitic impedances in the system, i.e. capacitance and resistance.
I have read guidelines on the net that state that in tests of the insulation of plant wiring, conducted with a megger, the plant wiring should present a resistance to ground in the order of 100s of megohms. These guidelines also state that these these tests should be conducted with the MCC equipment disconnected.
Thanks. JVJ.
RE: Parasitic Resistance
347V / 800 ma = 434 Ohm.
1 / ([1 / 434] - [1 / 3470]) = 496 ohm
Reality check:
100 ma = 3470 ohm
700 ma = 3470/7 = 495.7 ohm.
You are looking for an impedance in the order of magnitude of 496 Ohms.
You may want to intentionally ground each phase in turn.
Approximately equal currents indicate leakage. One phase much higher than the others indicates a single point contact.
With a ratio of 7:1 between known and unknown impedances, the unknown impedance will predominate, even if it is reactive.
By far the easiest and quickest, possibly by days, is to reapply your intentional ground, and de-energize your system, step by step until the leakage current drops.
More later
respectfully
RE: Parasitic Resistance
RE: Parasitic Resistance
RE: Parasitic Resistance
Something not seems in this scheme. Why 3Ico? Why Ir=3Ico?
Capacitive current in faulty phase must be=0
May be attached document can help you.
Regards.
Slava
RE: Parasitic Resistance
Please see attached.
Hope it help.
Regards.
Slava
RE: Parasitic Resistance
Please note that the graphic I posted shows a series impedance XL in the feeder which may serving to isolate IF from 3xIco. I will study this and try to reconcile the differences.
I noted that your diagram also shows the resistive leakage (R0) that prompted my original post. The accompanying text notes that resistive leakage losses are neglected for the purposes of the analysis. Regards.
RE: Parasitic Resistance
From time to time we try found some "big" problem and don't put attention on small things.
I would like recommend you check installation of monitor and all devices included, that they are installed according to all requerements of vendor.
Once I had such problems in other system, only change of
wiring of VT connection from 6mm^2 to 35mm^2 helped me.
Injection module ( that check resistance to grounding) was 3 meter from monitor instead 0.5meter.
Try check.
Good Luck.
Slava.
Could you please send us link to this device.
RE: Parasitic Resistance
The source of the resistive leakage path to ground has been identified as a resistor network in a piece of equipment that, by design, includes a High R path to ground. This High R leakage resistance is in the order of a few hundred kOhms. As Slava suggested, we had to look for a more practical root of the problem and not some of the more elusive possible causes such as “parasitic resistance”.
Once again I hope you will forgive me for withholding some information as there are many players involved including various suppliers, manufacturers, integrators and, of course, the customer.
Thanks again all.
Waross: Congrats on being voted TipMaster of the Week.
Regards.
RE: Parasitic Resistance
Thanks for the feed back.
Good Luck.
Slava
RE: Parasitic Resistance
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
RE: Parasitic Resistance
RE: Parasitic Resistance
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
RE: Parasitic Resistance
RE: Parasitic Resistance
Good night in your part of world, good morning in our.
Please find attached file.
Herivelto.
Hope that help.
Best Regards.
Slava
RE: Parasitic Resistance
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
RE: Parasitic Resistance
(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
RE: Parasitic Resistance
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
RE: Parasitic Resistance
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