generator derating for unusual harmonic load
generator derating for unusual harmonic load
(OP)
thread237-278719: quiz: neutral current = sqrt(3)*phase currrent described a generator with an unusual load that draws current from each phase for only 1/3 of the cycle (all current returns on the neutral).
what kind of derating would be applied to the rms current to account for harmonic effects
Generator Nameplate information
500KVA, pf=0.8, 400KW, 260vac, 60hz, 1110Amps.
what kind of derating would be applied to the rms current to account for harmonic effects
Generator Nameplate information
500KVA, pf=0.8, 400KW, 260vac, 60hz, 1110Amps.
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RE: generator derating for unusual harmonic load
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(2B)+(2B)' ?
RE: generator derating for unusual harmonic load
The harmonics are probably inducing currents in the rotor but this size of machine is much more forgiving of rotor heating than larger machines.
The issues that I see are:
Possible AVR instability that may be resolved with a different model of AVR.
Possible rotor heating: If this machine is in service I would attempt to check or monitor rotor heating.
Oversizing? As a suggestion, this load has a lot in common with a UPS load or a VFD load. Possible 50% to 100% oversize. (Or 50% to 67% of rated load.)
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: generator derating for unusual harmonic load
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(2B)+(2B)' ?
RE: generator derating for unusual harmonic load
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(2B)+(2B)' ?
RE: generator derating for unusual harmonic load
Note this normal condition shown in the attached corresdponds to 100 – 125A true rms phase current steady state. Current can double from the existing load for long periods under worst case conditions and there are discussions of possibly higher. If I take a conservative approach, I would say the temperature rise above ambient will increase with load squared. Under that assumption, the existing ~50F rise (150F on skin minus 100F ambient) would quadruple to 200F when we double load, which would put the temperature close to 300F... above class B limits, even on the outside of the machine. Is that a reasonable or conservative conclusion?
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RE: generator derating for unusual harmonic load
By the way, we have four identical machines at same load level and same temperature pattern. But it may be nothing to do with the load.... there may be some recirculation of cooling air going on. These machines are s single-end ventilated at the inboard end. Looking from inboard end, the air discharges at 9:00 position, has intake at 7:00 position, discharges at 5:00 position, and has another intake at 3:00 position. FWIW From this end the machine would be rotating CCW. The support directly below the machine forms a sort of hollow pocket... I tend to think the 5:00 hot discharge air is going into that pocket and getting sucked right into the 7:00 intake. Would like to try to put a barrier in there to check if it causes improvement, but it's not easy for various reasons.
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Also, I don't think even harmonics are an issue here any more than odd (accounted for simply be rms). But I googled to see what I could find:
http://www
IEEE Table 10.3 Current Distortion Limits for General Distribution Systems
(120 V through 69 kV).... [Note] 1 Even harmonics are limited to 25% of the odd harmonic limits above.
Does anyone know why they have a lower limit for even harmonics?
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RE: generator derating for unusual harmonic load
Those phase conductors are protected by breaker which has an internal CT feeding an amp-tector unit for time overcurrent.
I assume the dc portion of the wave is lost. I can calculate the parameters of the CT secondary current (rms, peak, rectified average etc).
What I'm not sure about: what parameter does the amptector respond to? The rms?
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