balevadu said:
Thank you very much for your response jim.
neutral is not grounded accidentally. the neutral was never connected to the grounding resistor, from which the 51N gets activated when ever there is flow of current through the neutral. as the neutral is not connected the 51N relay never got activated.
Ahhh, so we did not have a flash-boom after all. I feel better, thanks.
what i was thinking was the neutral grounding resistor will dissipate the energy in case of a ground fault.
Yes, that is its purpose. It limits the current that flows through a phase-ground fault to perhaps ten or twenty amps so there's not much arc-welding inside the machine.
Now - ten amps at 13.8kv is 138kw, so the resistor is physically quite large.
The voltage across an arc is probably only forty volts or so, so at twenty amps you're putting less than a kilowatt into damaging the machine.
if there is no neutral grounding resistor then there might be a huge explosion if the neutral is touching the ground during a ground fault in the generator. Correct me if i am wrong.
You are exactly right. During a fault...nothing limits the current except the machine constants as i described above.
Vaporized copper expands by the same gas laws as dynamite vapor.
You've heard the "BOOM" from electrical explosions?
Now - let's think about what situation i think you are saying you had, the neutral wire grounded instead of tied to resistor.
That's just bypassing the neutral resistor, eh?
So this question becomes '...what if neutral resistor is bypassed, ie your neutral wire touching touching ground
in absence of a phase-ground fault?
What current flows then ?'
Well, let us think about it - aloud.
First thought step will be Kirchoff's Current Law:
The current that flows into the neutral from ground is
the sum of the currents that flows out of the individual phases into ground.
What path do those currents take?
Those currents flow mostly through the distributed capacitance of your generator windings, transformer windings and interconnecting wiring. That's the only connection...
So the dominant impedance is that capacitance.
Next thought step - Tesla's gift to EE's- remember the three phases are 120 degrees out, so those capacitive currents add to zero.
So neutral current will be quite small so long as there's not much imbalance. I'd wager it's less than an amp. Well, maybe two... carry on...
Next thought step: Fourier's gift to EE's - remember your generator does not make a perfect sine wave, it has harmonic content like any other real genertor. Probably two or three percent in a machine that small.
And much if not all of that will be third harmonic.
Aha ! Third harmonic - that's 180 hz, only 5.555 milliseconds for a whole cycle , 360 degrees at 180 hz.
Aha^2 !
5.555 milliseconds is also 120 degrees (1/3 cycle)
at 60 hz.
What that means is this - the third harmonic of each generator phase is
in phase with the third harmonic of the other two phases.
Aha^3 ! So the third harmonic currents flowing into the neutral do not cancel out as did the fundamental, they directly add.
SO ------- in normal operation, the current returning in the neutral is largely third harmonic.
I have measured this with a spectrum analyzer and find neutral current to be maybe thirty percent third harmonic, maybe the majority like 85%. It doesn't look a lot like the 60 hz sinewave produced by the machine.
Using the amplitudes and ratios of fundamental to third harmonic in both phase voltage and neutral current, you can get a pretty good guess at what is the distributed capacitance of your generator-transformer system.
>>> Don't Give Up On Me Yet - I haven't forgot your question...<<<
I'm going to guess that in normal operation you'll find an amp or two of neutral current , at least 50% of it third harmonic. Harmonic content goes up with excitation.
Now - what happens if you bypass the neutral resistor with no phase fault present?
Basically, nothing. The dominant impedance in the circuit is still the distributed capacitance. Neutral current will change very little.
Incidentally that's how you size a grounding resistor, make it a few times smaller than the impedance of your distributed capacitance.
This is all described very clearly in IEEE's "Green Book", standard 142 i think it is.
Every power plant guy needs to have his own personal copy. Ask your company to get you one.
Well, thanks for listening. Helps an old guy feel useful.
I'm sorry the answer turned out so long -
it's just that i was trained by a stern mentor to always "answer the question that was asked"
and i hope this seemingly endless post helped.
old jim
