jghrist
Electrical
- Jul 16, 2002
- 4,225
Our client has a 2000 kW diesel generator connected to the 34.5 kV system through a 2500 kVA, 480 volt grd-wye to 34.5 kV delta, transformer. The generator is wye connected with the neutral connected to the transformer neutral, grounded at the transformer only. There are no phase-to-neutral connected loads on the 480 volt system.
There is a considerable amount of neutral current, even though there is no zero-sequence connection to the 34.5 kV system and no phase-to-neutral load. Current measurements at the transformer were:
X1 - 663A, X2 - 665A, X3 - 630A, X0 - 81A
The third harmonic neutral current is 75% of the fundamental (60% of total rms). There is little other harmonic current. There is very little harmonic current in the phases (1% third, 2.5% fifth, 1.7% seventh).
We are attributing the third harmonic current to circulating current between the generator and the transformer, similar to third harmonic circulating current between two paralleled generators.
Could unbalanced cable impedances account for fundamental neutral currents?
The neutral current is proportional to load current, so that it is about 300A at full load (2400A phase current). Do you see any reason to be concerned with this level of neutral current? Ground fault protection is provided with a CT in the single ground connection, and will not see the circulating neutral current.
There is a considerable amount of neutral current, even though there is no zero-sequence connection to the 34.5 kV system and no phase-to-neutral load. Current measurements at the transformer were:
X1 - 663A, X2 - 665A, X3 - 630A, X0 - 81A
The third harmonic neutral current is 75% of the fundamental (60% of total rms). There is little other harmonic current. There is very little harmonic current in the phases (1% third, 2.5% fifth, 1.7% seventh).
We are attributing the third harmonic current to circulating current between the generator and the transformer, similar to third harmonic circulating current between two paralleled generators.
Could unbalanced cable impedances account for fundamental neutral currents?
The neutral current is proportional to load current, so that it is about 300A at full load (2400A phase current). Do you see any reason to be concerned with this level of neutral current? Ground fault protection is provided with a CT in the single ground connection, and will not see the circulating neutral current.