IBRCAN
Electrical
- Feb 3, 2012
- 53
We are attempting to connect a temporary generator in a standby application. Site is equipped with 3x 1100 kw, 600V diesel generators which parallel using woodward 2301A LSSC, and Cat VR3 regulators utilizing droop. Generator pitch is .8.
One unit is off line for repair--in its place we connected a 1250 kw unit, pitch .7333, Cat VR6. I know that dissimilar generator pitch is not desirable, however a small amount of circulating current is acceptable in this temporary application. Temporary generator voltage is 480 which is stepped up to 600 using an autotransformer. Generator neutrals are solidly connected. Neutral to ground bond was removed from temporary unit. Load gain, frequency, voltage and voltage droop were adjusted to match existing units.
Temporary was closed to bus and approx 1000 kw of 600V resistive load applied (approx 1000A). We then synched one of the existing units to the bus and the temporary 600v breaker kept tripping on ground fault at the moment of syncronization, presumably due to circulating current on the neutral. Was told by a couple sources that an autotransformer is not desirable for this application. Upon a recommendation, the generator neutral was disconnected from transformer X0.
Above sequence was replayed, and breaker no longer tripped. Temporary and existing shared kw in proportion, power factors were approx. .96 lead and .95 lag indicating that voltage droop settings were close to being satisfactory. Other existing unit was closed to bus and this is where things went badly. Current on temporary increased to 1000A, existing were carrying approx 500A each, suggesting 1000A of circulating current. Voltage dropped to 550V. We opened breaker on last existing unit and readings returned to same as before. All existing units synchronized and shared kw and kvars very well before connection of the temporary.
Any comments on the autotransformer or any other suggestions?
One unit is off line for repair--in its place we connected a 1250 kw unit, pitch .7333, Cat VR6. I know that dissimilar generator pitch is not desirable, however a small amount of circulating current is acceptable in this temporary application. Temporary generator voltage is 480 which is stepped up to 600 using an autotransformer. Generator neutrals are solidly connected. Neutral to ground bond was removed from temporary unit. Load gain, frequency, voltage and voltage droop were adjusted to match existing units.
Temporary was closed to bus and approx 1000 kw of 600V resistive load applied (approx 1000A). We then synched one of the existing units to the bus and the temporary 600v breaker kept tripping on ground fault at the moment of syncronization, presumably due to circulating current on the neutral. Was told by a couple sources that an autotransformer is not desirable for this application. Upon a recommendation, the generator neutral was disconnected from transformer X0.
Above sequence was replayed, and breaker no longer tripped. Temporary and existing shared kw in proportion, power factors were approx. .96 lead and .95 lag indicating that voltage droop settings were close to being satisfactory. Other existing unit was closed to bus and this is where things went badly. Current on temporary increased to 1000A, existing were carrying approx 500A each, suggesting 1000A of circulating current. Voltage dropped to 550V. We opened breaker on last existing unit and readings returned to same as before. All existing units synchronized and shared kw and kvars very well before connection of the temporary.
Any comments on the autotransformer or any other suggestions?