DG trip on volts/hz
DG trip on volts/hz
(OP)
DG's are a little out of my area. I was asked to provide input on the following scenario:
700 kw 480 vac Kato / Reliance diesel generator.
Generator and system are ungrounded.
Generator was supplying MCC in islanded mode, with low load on the MCC, but load bank resistors were added to bring total generator load to approx 610KW for test purposes.
A large block of lighting fed from the bus auto-switched on. The amount may be equivalent of several hundred kva.. suspected reactive ballast inrush according to an engineer more familiar with the lighting system.
At approx the same time, a generator volts/hz trip occurred and undervoltage alarm occurred expected following generator trip).
No other protection or alarms occurred. The other protection/alarms include:
"Generator Overcurrent with voltage restraint" 51V
64F (field ground?)
overspeed, low-lube oil, high-cooling-water-temperature
It is suspected that the block of lighting load caused the diesel to slow resulting in low hz and therefore high volts/hz trip.
Any thoughts on this scenario?
I have 2 reservations:
1 - if it is truly inductive inrush load, then it probably would not slow down generator but would mess with the voltage control.
2 - Why didn't overcurrent trip? Would you generally expect to get an overcurrent 51V?... (I'm sure it may require knowledge of time current characteristics, and inertia etc, but maybe someone is familiar with typical behavior).
What testing would you recommend ?
What would be the purpose of voltage restraint of the overcurrent ?
700 kw 480 vac Kato / Reliance diesel generator.
Generator and system are ungrounded.
Generator was supplying MCC in islanded mode, with low load on the MCC, but load bank resistors were added to bring total generator load to approx 610KW for test purposes.
A large block of lighting fed from the bus auto-switched on. The amount may be equivalent of several hundred kva.. suspected reactive ballast inrush according to an engineer more familiar with the lighting system.
At approx the same time, a generator volts/hz trip occurred and undervoltage alarm occurred expected following generator trip).
No other protection or alarms occurred. The other protection/alarms include:
"Generator Overcurrent with voltage restraint" 51V
64F (field ground?)
overspeed, low-lube oil, high-cooling-water-temperature
It is suspected that the block of lighting load caused the diesel to slow resulting in low hz and therefore high volts/hz trip.
Any thoughts on this scenario?
I have 2 reservations:
1 - if it is truly inductive inrush load, then it probably would not slow down generator but would mess with the voltage control.
2 - Why didn't overcurrent trip? Would you generally expect to get an overcurrent 51V?... (I'm sure it may require knowledge of time current characteristics, and inertia etc, but maybe someone is familiar with typical behavior).
What testing would you recommend ?
What would be the purpose of voltage restraint of the overcurrent ?
=====================================
(2B)+(2B)' ?





RE: DG trip on volts/hz
64 = ground detection circuit associated with the generator ac output.
=====================================
(2B)+(2B)' ?
RE: DG trip on volts/hz
RE: DG trip on volts/hz
=====================================
(2B)+(2B)' ?
RE: DG trip on volts/hz
Yes, the load was definitely exactly 610kw and pf near 1.0 prior to the lights coming on. The load posed by the lights is somewhat unknown, but estimated as several hundred kva
=====================================
(2B)+(2B)' ?
RE: DG trip on volts/hz
rasevskii
RE: DG trip on volts/hz
That was an Electrical Tsunami...
rasevskii
RE: DG trip on volts/hz
51V is basically overcurrent protection which is high set at normal voltage, but reduces on undervoltage to a lower setting according to a curve. The time lag may have been such that the protection "started" but did not trip as the UV operated first.
rasevskii
RE: DG trip on volts/hz
RE: DG trip on volts/hz
we only had undervoltage alarm, not undervoltage trip. The only trip was volts/hz.
The preliminary plan is:
megger generator
check relay cal
Repeat dg run to check performance
=====================================
(2B)+(2B)' ?
RE: DG trip on volts/hz
rasevskii
RE: DG trip on volts/hz
The purpose of the voltage restraint is to allow the relay to operate on sustained fault contributions from the generator that are not cleared by other relays. The fault current produced by the generator will rapidly decay for an extended fault. Absent field-forcing, the current is ultimately limited by the synchronous reactance of the generators and this is often greater than 100%, meaning the fault current contributed by the generator is less than the full load current rating. At very low voltage, such as during a fault, the relay curve shifts to a more sensitive pickup, allowing the relay to operate on current that are less than load current. These relays generally will be fairly slow to operated for an external fault.
David Castor
www.cvoes.com
RE: DG trip on volts/hz
The UFRO feature typically allows a 3 Hz drop and then lowers the voltage as the frequency drops, avoiding a V/Hz trip. If the jumper is on the 50 Hz position you will drop 13 Hz before the voltage is reduced and if you have V/Hz protection it will probably trip.
I almost never see a V/Hz trip on a diesel set in that size range as the UFRO feature of the AVR maintains a safe V/Hz ratio.
Bill
--------------------
"Why not the best?"
Jimmy Carter