Translating "Electrical" Into English

[vpl]

I figured this might be the best place for this as, even though it's a work-related question, I'm not an electrical engineer.

I'm reviewing a document that seems to be mixing KVARs, ohms and watts. The person who wrote it is a good electrical engineer, but not a native English speaker. I'm not well-enough versed in "electrical" to make sure I'm "translating" correctly.

The paragraph that I'm currently scratching my head over reads: "The calculation concluded that maximum grounding resistor value of 4.225 ohms was required to provide the required KW loss equal to or greater than the charging KVAR and therefore, the installed resistor value of 4.2 ohms was adequately sized to protect the components from damage. This only provided a 0.025 ohms margin which allowed excess charging of 0.025 KVAR. The calculation results were based upon using the cabling and circuits for component-11 which showed that the calculated charging KVAR value was 4.545 and the resistor KW loss for the 4.2 ohms resistor was 4.57."

I've read this paragraph several time and keep coming to the conclusion that KVARs are being subtracted from KWs to get ohms. I speak enough "electrical" to know that's not right.

So the question is does this make sense to you electricals? Is it just happenstance that the resistor margin (0.025 ohms) is equivalent to the excess charging(0.025 KVAR)? Is the 4.57 loss in the last sentence in KW or KVAR?

Thanks!

Patricia Lougheed

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[davidbeach]

Maybe, maybe not. I think I sort of follow.

What I think is happening, is that a grounding resistor is being discussed and that it needs to supply enough current at a power factor of 1 (Watts) to offset a parasitic capacitance of some amount (VArs - k or otherwise). Unfortunately Watts, VArs, and VA all have the same units dimensionally, amps * volts. All you have to do is through in an assumed value of volts and ...

The whole thing could make sense, or it could just be a bunch of mumbo-jumbo. Without seeing the system in question, I can't say one way or the other, but the verbage isn't all that outrageous.

 

[waross]

Not to contradict David but to try to present it another way:
It looks to me as if the system has charging currents. This is common for long circuits. Expressed as VARs or KVARs and, after dividing by the voltage, measured in Amps.
A grounding resistor is to be installed which, under ground fault conditions, will limit the ground fault current in Amps to a level determined by the voltage and by the value of the resistor. This will have losses expressed in Watts and, after dividing by the voltage, measured in Amps.
It is much easier to discriminate between charging currents and ground fault currents if the ground fault currents are larger than the charging currents.
But without seeing the system and seeing some details of the protection devices???

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[vpl]

Thanks both of you. I don't have any details of the actual system; just the engineer's writeup based on his inspection at a nuclear plant. The system has both grounding resistors and charging currents and the underlying issue that he's describing (in other paragraphs) is that the measured field resistance values were above the maximum resistance assumed in the design calculation. The rest of the writeup made sense -- it was just this one paragraph that I was having trouble grasping.



Patricia Lougheed

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[dpc]

I think I understand what is being said, but it could be written more clearly. I think the two 0.025 values is probably just a coincidence but I didn't check the calculations.

The discussion appears intended to confirm the proper sizing of the generator grounding resistor. In a high-resistance grounding scheme it is sized to allow ground fault current that is at least equal to the capacitive current in the generator system in order to reduce the magnitude of overvoltages during ground faults. (The turns ratio of the grounding transformer must be factored in.)

Normally, the generator system capacitance is calculated (guessed at, really) and the resistor is sized to match with a slight safety margin. The resistance of the grounding resistor (referred to the grounding transformer primary) is directly comparable to the capacitive reactance. I've never seen kW and kVAR used in this calculation or evaluation. It isn't wrong, but seems needlessly complex and indirect.

 

[BJC]

IT's bad. "The calculation concluded ....".
The calculation doesn't conclude anything. The engineer concluded based on a calculation. From there on it's not organized and logical.