Ungrounded Transformer Causing VS Drive Problems
Ungrounded Transformer Causing VS Drive Problems
(OP)
Transformer is rated 3500kVA 25kV/480Volts Delta-Delta.
Ungrounded delta secondary is causing variable speed drive problems.
Please recommend best means to ground this transformer.
Ungrounded delta secondary is causing variable speed drive problems.
Please recommend best means to ground this transformer.






RE: Ungrounded Transformer Causing VS Drive Problems
RE: Ungrounded Transformer Causing VS Drive Problems
RE: Ungrounded Transformer Causing VS Drive Problems
Can you please provide some details on a suitable grounding transformer so we can at least consider this option? Some thoughts: primary - wye connected solidly grounded, secondary - broken delta with resistor?
RE: Ungrounded Transformer Causing VS Drive Problems
RE: Ungrounded Transformer Causing VS Drive Problems
RE: Ungrounded Transformer Causing VS Drive Problems
Is this a fact or a suspicion? I assume you've had the drive manufacturer involved?
David Castor
www.cvoes.com
RE: Ungrounded Transformer Causing VS Drive Problems
Limit the ground current to 5 amps.
5 Amps at 480 Volts will be 2400 KVA.
Connect 3 2500 KVA dry type transformers in wye delta.
Connect the 480 Volt rated Wye connected windings to the 480 Volt delta supply.
The primaries will be 480 Volts and the secondary voltage is not important.
In normal operation, each 480 Volt winding will see 277 Volts. In the event of a ground fault, the windings on the unfaulted phases will each see 480 volts.
Connect the wye point to ground with a suitable impedance to limit the current to 5 amps at 480 Volts. (96 Ohms or the nearest easily obtained value)
In the event of a ground fault, the system will become a corner grounded system and the VFDs must be suitable for operation on a corner grounded system.
Suggestion #2:
If you want a solidly grounded system, omit the resistor and size the dry type transformers to withstand ground fault currents for a suitable clearing time.
Bill
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"Why not the best?"
Jimmy Carter
RE: Ungrounded Transformer Causing VS Drive Problems
There is another scheme in which the grounding transformer primary is solidly grounded wye but the secondary is connected broken delta with a grounding resistor which also produces an effective HRG system. In your opinion which would be best for this application of trying to stabilize voltages and limit transient overvoltages that the drives may see:
1. Your suggestion - grounding transformer primary connected wye with resistor in neutral to limit GF current to 5 Amps. Grounding transformer delta secondary windings shorted together.
2. Grounding transformer primary connected wye but neutral soldily grounded. Grounding transformer secondary connected broken delta with resistor in the broken delta to limit GF current.
RE: Ungrounded Transformer Causing VS Drive Problems
That said, my final choice may be conditional on the availability of a suitable resistor.
Note, when you said "Grounding transformer delta secondary windings shorted together." the windings are NOT shorted together. The secondary is connected in delta and no further connections are made.
Bill
--------------------
"Why not the best?"
Jimmy Carter