GSU Delta/Wye Floating Neutral & Line Imbalance
GSU Delta/Wye Floating Neutral & Line Imbalance
(OP)
I need some recommendations regarding high side voltage imbalance. The system is a 2.5MW 2400 delta gen feeding a delta/wye 2400/12,470 GSU. The neutral is floating on the highside wye. I have 15% ph-N voltage imbalance when running the generator isolated. I was called to the customers site to troubleshoot a failed DECS 200. I found the A-ph PT failed as well as the DECS. This is the second PT failure in less the a year.
THe sensing PT's are 60:1 wye.
The generator is used mostly for interruptable duty and runs minimally.
Why is the neutral floating? Recommended solution?
Thanks
THe sensing PT's are 60:1 wye.
The generator is used mostly for interruptable duty and runs minimally.
Why is the neutral floating? Recommended solution?
Thanks






RE: GSU Delta/Wye Floating Neutral & Line Imbalance
Consider grounding the neutral.
Bill
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"Why not the best?"
Jimmy Carter
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
I believe you are correct I need to run a ground strap from the xfmr Xo terminal. Funny thing is they have had this unit in service for 5 years. I don't know how they ever got it paralleled! They had to have had voltage offset issues at start-up.
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
Should definitely add grounding for island mode. Resistance grounding may be workable for both interconnected and island operation.
Alan
----
"It's always fun to do the impossible." - Walt Disney
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
In the event of a ground fault on the line, ground currents will be much higher. Should ground fault current damage the connection between the generator and the grid, the touch voltage on the generator may rise to lethal levels.
Connect the X0 directly to to grid.It is still possible for fault current to damage a connection to ground, but it is better to kill a PT as is happening now than to kill an employee.
I was looking at a large diesel installation in the third world. There was a large gauge cable from the station ground grid to the generator. The connections and workmanship looked good. However the cable was connected to the radiator assembly which was completely rubber mounted.
Bill
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"Why not the best?"
Jimmy Carter
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
If X0 is connected to the 12.47 kV neutral, then the transformer will be a ground source and current will flow in the transformer windings during a 12.47 kV system ground fault and if the 12.47 kV system has unbalanced loads. These currents will flow with the generator off if the 12.47 kV system is connected to the utility source. This may or may not be acceptable, depending on the source and transformer impedances, and on the transformer protection, but needs to be considered.
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
Thank you for sharing the final resolution with us.
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
Problem not solved. I reviewed the system and this AM and discovered the config is not as discribed. X0 not grounded, and it shouldn't be!
I spent today trying to troubleshoot the voltage imbalance/instability.
To begin with the GSU is a delta/delta and the PT's are Y. Gen is 2400Vac delta. GSU is 2400/12470. Utility nominal voltage is 12,900.
While gen is running. Primary=12,800Vac Secondary PT voltages are A-N =142 B-N =112 C-N =118 60:1 PT's.
If I manually raise the gen voltage up to match the Utility voltage it becomes very unstable and I recorded secondary peaks of almost 300Vac. If I drop the voltage down to 12,700 everything looks normal other than the seconday voltage imbalance. The Regulator output goes nutty once the voltage hits 12,900!
Am I seeing ferroresonance? Delta/delta GSU and Y PT's.
Shouldn't the PT's be Open Delta?
I'm at my wits end on this one!?!?!?
East
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
You will have trouble trying to serve Ø-grd loads on the 12.9 kV system if it is isolated from the grounded utility source. You shouldn't try to serve the system islanded without a grounding bank.
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
Could you send to us a single line diagram of your system, including the power transformer and VTs conections and, also, the voltages waveforms during the gen tests (voltage changes)?
Best regards,
H. Bronzeado
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
Trying to synch an ungrounded system to an other system, grounded or not, will be frought with difficulties as you won't be able to adjust out the standing voltages to ground of the two different systems. If you have a lowside breaker you might try synchronizing there, the voltage balance may be better.
For many reasons the highside should be a grounded wye, but it is too late for that now, so a grounding bank that let's you synch two grounded systems is your best bet. If the utility truly doesn't want you to be a ground source, then you can switch it out after closing the breaker.
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
21 Sep 09 20:31 post. At an indicated A-N =142 (118%) are we getting close to saturation of the PT?
Thanks
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
The gen is direct coupled to the GSU. Also this particular unit has a direct coupled 2400Vac 250 HP blower motor used for combustion air. The motor must begin to spin as the engine is started. The motor may be causing us problems. I'll get back to that in a minute.
The VT's are wye connected 7200:120 ratio. I believe they should have installed VT's rated for line to line. Ungrounded phases are subjected to 1.73 times rated in the event of a ground fault on the delta system. How does that square when the gen is paralleled to the wye system?
I agree with M. Beach it's just wrong to parallel a delta/delta GSU to a wye system.
It's my opinion that the A phase VT is saturating at around 12.9kV line to line volts. I recorded over 150 VT secondary volts at 12.9kV. The VT voltage is very irractic (saturation) above 12.9.
What would happen if the combustion air motor had a low grade ground fault?
I'm going to test the motor in the next few days.
BTW I will also download the 1/4 cycle events from the blue box. The voltage phase angles should indicate presence of a ground fault. Right?
Thanks for the input.
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
Another issue that may arise is synchronization. If you have much leakage to ground on the wrong phase it may put your phase reference out by as much as 30 degrees.
If you have access to some distribution transformers, connect the primaries in wye to the 12,470V system and connect the secondaries in delta. This will give you a stable neutral point on the 12,470V system. The wye point should be grounded to maintain the neutral point at ground potential. This is a good example of an instance where neutral and ground are NOT interchangeable. There are serious safety issues if the neutral rises above ground potential.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
The customer says they have paralleled this unit a few times. They must have been real lucky. I don't see how it ever worked!?!?
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
The biggest that I have seen paralleled at 30 degrees out was a pair of 600 kW sets. The bus PT supplying the syncroscope was line to neutral connected and the generator PTs were line to line connected. A permanent 30 degree error that had been there for several years. If the operators closed a little early the breaker tripped. If they closed a lot early they sheared a coupling, about once a year.
Closing with a voltage error doesn't concern me as much as the possible phase angle error. Neither are good.
Voltage error gives rise to reactive current that may be trimmed out with a voltage adjustment. An angle error results in a mechanical shock as well as a current transient.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance
RE: GSU Delta/Wye Floating Neutral & Line Imbalance