Is harmonics blowing up my capacitors??

[imisspla07]

I have a power factor correction capacitor bank rated 600kVAR. It is in parallel with three 1250hp motors on soft start systems used for gas compression. The bank has a contactor that closes only when 2 or three motors are running and is unlatched when the soft start is ramping up the motors. So, basically, it is only latched in when 2 or 3 motors are running full speed. The first time we used the cap bank it blew the fuses upstream of the contactor (connected to the bus). We changed out the fuses to different fuses and the second time it ran it blew the cap bank. It was suggested we have harmonics issue, but the utility did a power quality study and showed .05% harmonics. Seems low to me. Also, it was mentioned it might be resonance. But when I went out there with the utility we only latched in 300kVAR; half of the full amount. This should have gotten us away from any resonance issue. Now, the last thing to note is that when we did latch in the 300kVAR, it made a very eerie sound, like someone was waving a light saber in front of you (wa, wa, wa, wa). Definitely didn't sound right, so we latched it back out. I have no idea what's going on with this thing. It's been noted maybe it's from the utility. Someone mentioned perhaps an open NGR at the utility. Any ideas would be GREATLY appreciated!

 

[Skogsgurra]

There's definitely something that shoudn't be there on page 9 and 10, channel 2.

The frequency is 35 - 40 times fundamental and doesn't show in the report on harmonics because the highest harmonic registered is the 13th.

You have a sharp resonance there. What kVA rating and % uk does the transformer have? It looks like the leakage reactance and the capacitors form a high-Q resonance circuit.

I really cannot understand why the utility didn't tell you about this. It is quite obvious.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[imisspla07]

The transformer data is: 7.5MVA, Delta-Wye, 8.23% Z, 138kV/4.16kV. Channel two was attached to 'B' phase. The way this particular PT is set up is phase B is actually grounded and it uses phases A and C as reference to 'assume' the voltage on phase B. That all being said, it is curious that there is anything on phase B since it was attached to ground? I will definitely do some research on the high-Q resonance circuit; not something I'm familiar with.

 

[YMF]

From the report, it seems like the soft-starter is not bypassed when the motor is at the rated speed and it is supply the required voltage to the induction motors right ?

If that is the case, from the report, there is a number of high frequency voltage 900 Hz to 1200Hz. If the capacitor is not tuned properly, this capacitor bank will act as a sink that draws this high frequency current. And that can be a huge amount as the reactance is very small. That explains the blown fuses, and funny humming noise.

 

[LionelHutz]

If this system is only running 3 motors then this system be fairly well balanced. I see different voltages, currents and power factors on each phase so I personally would have a hard time believing the report.

I would say you need someone who knows how to run his power metering equipment to do some measurements at different points on the system.