Medium Voltage WYE-DELTA grounding

[bubbafat1155]

I have a deisel power plant about 40 meg. One bank of gens is 11Kv wye with an earthing resistor connected to the neutral of each generator. another bank of gens is 416v steppped up thru trannies to 11Kv delta, ungrounded. these are all connected to the same bus.
We have some small earth leakage out in the distribution and also intermittant ground fault from birds, rain and whatever. My concern is that the ground fault that is returning to the wye gens is not a problem in itself, but since some of the current originates from the Delta source, and is trying to get home via the other two phases, it is doing so by passing thru the alternators at the wye source. This in itself might not be a problem except that it is known that groundfaults on an ungrounded system (consider the Delta source gens) have a tendency to build up capacitance which will release at 7 or 8 times the normal voltage. We are experiencing frequent fires in the termination boxes of the WYE source alterators. I think that it is caused by the capacitance discharging and then a path has been created for the rest of the burnout. Has anyone else tied wye and delta together like this or am I a guinea pig? does my theory hold water?

 

[Kiribanda]

bubbafat1155,

Interesting issue!!

Could you please upload a copy of the Single Line Diagram of your network with correct transformer vector symbols for every body's use?

 

[rbulsara]

Just thinking out loud, I would suspect that terminal box fires are unrelated to the grounding scheme. The wye gens themselves are grounded, regardless of delta ones.

Are the fires related to ground fault incidents?

How old are the terminations? May be the terminations are not of high quality or workmanship?





Rafiq Bulsara

http://www.srengineersct.com

 

[PQ101]

I recommend that you set power quality analyzer to determine the cause. And, check your facility- grounding system. If there is arcing involved or any transient issues, etc. it will show up on your voltage waveforms 100%. Once you have data to establish the facts, you will be able to git rid of that bird, and resolve the situation.

Monitoring power generation plants I can tell you that's the only way you are going to determine the cause-origin. Otherwise it's all heresay, and lot of guesswork and finger pointing, and none of them is gonna amount to anything-actual/factual data is improtant to investigate the issue,

Maintaining electrical system integrity is the key issue here as you know and that's what you need to check for.

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[pwrengrds]

If the 11 kV generators are on line with the delta connected transformers there will not be a buildup of voltage, the neutral of the generator would provide the center point. Do you have lighting arrestors in the junction boxes of the 11kV? If so what are they rated for? (Line to Line voltage or Line to neutral voltage) What exactly is catching on fire? Do you monitor the voltage or current on the neutral? If it's zero then the voltages should be balanced and centered.

 

[PQ101]

To pwregrds-

You are correct as for voltage unbalance between phases travel thru generator Neutral, but you never-ever get zero volts on the neutral because the load keeps changing every cycle- zero is only on textbooks.

Moving on: With no picture attached on termination boxes or monitoring equipment, it is difficult to say anything factual, but typically in power quality world the cause of fire is usually due arcing, or overheat due to bad connections, bonding, harmonics, bad house keeping, lack of maintenance, etc.

One way you can approach this is to utilize thermal imagers (FLIR, FLUKE 20-35-45 or others alike) to identify the hot spots in your system/term boxes, etc. I bet you will resolve the case with a little investigative effort.

Power quality solutions that empower you!!!

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[pwrengrds]

Typically you would have a neutral over voltage (59N) on a neutral high resistance ground. If this is a low resistance ground it would be an 51N relay. I would expect the neutral on the generator to hold the delta on an attached transformer centered. Is there a 59N relay on the neutral of the generator?

The direction of the inquiry was to see if there are lightning arrestors in the cabinets that are rated less than 11kV that could fail if exposed to phase to phase voltage if the neutral goes toward 6kV and drives two phase voltages from a nominal 6kV to 11kV.

Every neutral voltage I have measured (on the secondary of the neutral PT) was at 0 volts (or at least < 1 vac), with some third harmonic voltage. I don't understand why you would "never-ever" get zero volts on the neutral. It's grounded via a resistor, with a balanced 3 phase current, the neutral current would be 0.

 

[PQ101]

To pwrengrds.

I was pointing to the generator nuetral (in gereral). The Phase voltage & current (RMS) keeps on changing every cycle, so in the field you never get zero, because there is always residual flow thru system neutral. If you get zero, it means you have a bad meter/or not as sensitive to show cycle by cycle monitoring.

Going back to the real issue: "frequent fires in the termination boxes of the WYE source alterators", it appears this is a generic-frequent problem that affects one specific bank of generators (not all).

Based on troubleshooting experience I'd say start with inspection on all terminal boxes, see if the symptoms are exact-same on each location, run a thermal scan, make sure all terminations are good-tight, set the power analyzer, collect data, and you will identify the cause.







Power quality solutions that empower you!!!

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