Solidly Grounded Wye Generator

[tunemeister]

We have a (installed 35 years ago) 13.8 kV 25 MVA generator (peaker) resistively grounded through a grounding transformer and resistor with a voltage relay to detect ground faults down to 5% of the generator winding which is typical of utility grid connected generator units. The generator feeds a 13.8 kV breaker and then only a generator stepup transformer (GSU) to 120 kV. Unfortunately, we intalled a standard distribution substation transformer with the delta connection on the 120 kV and a wye connection on the 13.8 kV and have solidly grounded the 13.8 kV (the standard practice for 13.2 kV distribution substation transformer installations). Typically, GSU transformers are delta on the low side to keep the generator ungrounded and wye on the high side to act a ground source into the bulk power system with grounding connected as needed. A high end generator ground fault current contribution from our system is calculated to be 7700 amps and should be cleared with a relay and breaker time of 7 cycles. What kind of damage can be expected for a ground fault in the high end of the generator with this fault amperage and clearing time.

I am trying to present a case to unground the 13.8 kV transformer neutral and install ground fault relaying on the transformer side of the generator breaker to se ground faults when the breaker is open. If this (or some other fault) will do significant damage to the generator to require more extensive repairs (steel core damage requiring core rebuilding or more expensive rewinding) then the changing of the transformer neutral connection is warranted.

 

[ScottyUK]

The normal method of connewcting a generator is via a dYn* transformer as you have already noted. The generator neutral point is usually connected to earth through a resistance which is sized to limit the stator ground fault current to about 10A or so, which is enough to give reliable detection of a fault but is low enough to prevent severe burning of the generator core in the event of a fault. Usually this achieved by earthing the neutral point via a single-phase distribution transformer with the LV side loaded by a burden resistor sized to pass the required HV fault current to earth.

You didn't mention whether the neutral point of the generator is grounded or not. Standard practice is to have only one ground on the neutral.

You run the real risk of a core wreck if you have a stator earth fault. Your installation is frankly a little alarming from the generator's perspective. Even very fast protection is unlikely to prevent serious damage to the laminated core in the event of a major fault, and your existing protection is not especially fast. Your first stator earth fault would in all likelihood also be the last one for that machine. In terms of damage, burning to the core caused by relatively low currents shorts the laminations together causing hotspots when in operation. These can be carefully ground out by someone with a lot of skill and experience if they are caught early enough. If the source of the damage is of much higher energy then you would expect metal loss and localised welding between laminations, metal vapour deposition, metal spatter from the fault site, carbon and soot, etc.

I think you are correct to unground the transformer neutral and ground the generator in the conventional manner. You will have to review the protection applied to the transformer and generator very carefully to understand how these changes will affect existing protection and then modify it accordingly. Your earth fault protection requirements are likely to be very different.

Aside: how on earth did the wrong transformer get ordered? Amazing.


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I don't suffer from insanity. I enjoy it...

 

[tunemeister]

Your description with my ending of "earthing the neutral point via a single-phase distribution transformer with the LV side loaded by a burden resistor sized" to allow a voltage relay to operate down to 5% of the winding is how the generator neutral is grounded and how the generator manufacturer relayed this portion of it. We apparently purchased a a generator without a step up transformer and supplied our own. Either we or a consultant used a standard distribution station transformer which has a groundable wye connection on the low side. This is a standard transformer size for us and we have maybe twenty or thirty on our system so it seems that we just used what already was being used in large numbers and missed this detail of keeping the low side ungrounded. We should have been aware of the standard large generator neutral relaying system with a delta low side on the GSU transformer as discussed above since we have as of 1970 perhaps 15 large machines connected this way. But, they were all complete generator/transformer packages so we were not directly involved with all the equipment selection details. However, we also had enough relaying instruction pamphlets to show us how to do it right. We just did not do it right and I am trying to explain the ramifications and the cost of correction to our peaker group who is right now quite disbelieving.

Your comments are on machine damage are helpful. Thanks!