jghrist
Electrical
- Jul 16, 2002
- 4,226
One of our municipal utility clients had a ground differential trip on their peak shaving generating plant recently. The generator is diesen engine driven, 1825 kW, paralleled with two other identical generators and with the utility system. The trip occurred 25 minutes into the run and was restarted successfully. The generators were operating in Peak Shave/Base Load mode and were set to generate a constant 1825 kW.
Generators are 4.16 kV with 400A grounding resistors. Three generators are paralleled and connected to the utility 12.5 kV substation with a 5000/6250 kVA delta-grd Y transformer, delta on the generator side.
I believe that the ground differential is set too sensitively and that the residual current from the phase CTs has enough noise to trip the relay. The relay is a Beckwith M-3240. Phase CTs are 400:5. Neutral CT is 50:5. Pickup is 0.2 amps (minimum setting) with a time delay of 2 cycles. The CT ratio correction factor is 7.99 (maximum setting).
Oscillography of the event is on the attached Word document.
What really has me wondering is the current dip and oscillations starting at cycle 34. Current was about 270A (1960 kVA) before the dip and settled to 150A (1090 kVA)after the dip, before tripping. There was no dip or increase in voltage. Note that there seems to be a frequency deviation also. Phase C peaked at 32.2 cycles and at 33.2 cycles. After the current dip, Phase C peaked at 34.7 cycles and 35.7 cycles.
As far as we can tell from records, there was no trip of any distribution feeder at the time, and the substation is connected to a 100 kV transmission system that would absorb any excess generation.
Any ideas?
Generators are 4.16 kV with 400A grounding resistors. Three generators are paralleled and connected to the utility 12.5 kV substation with a 5000/6250 kVA delta-grd Y transformer, delta on the generator side.
I believe that the ground differential is set too sensitively and that the residual current from the phase CTs has enough noise to trip the relay. The relay is a Beckwith M-3240. Phase CTs are 400:5. Neutral CT is 50:5. Pickup is 0.2 amps (minimum setting) with a time delay of 2 cycles. The CT ratio correction factor is 7.99 (maximum setting).
Oscillography of the event is on the attached Word document.
What really has me wondering is the current dip and oscillations starting at cycle 34. Current was about 270A (1960 kVA) before the dip and settled to 150A (1090 kVA)after the dip, before tripping. There was no dip or increase in voltage. Note that there seems to be a frequency deviation also. Phase C peaked at 32.2 cycles and at 33.2 cycles. After the current dip, Phase C peaked at 34.7 cycles and 35.7 cycles.
As far as we can tell from records, there was no trip of any distribution feeder at the time, and the substation is connected to a 100 kV transmission system that would absorb any excess generation.
Any ideas?