Circulating Currents in 13.8 KV Parallel Feeders 3

[sishah2011]

We have our two 13.8KV substations (A & B), connected through 2 x cable feeders. Each feeder comprised of 3 x single core power cables per phase.
Electrical power is being fed to Substation B from Substation A (Substation A is receiving electric power from 3 x Steam Turbine driven generators, running in synchronism).
Since commissioning, we are experiencing circulating current in both feeders, which is causing tripping of feeder breakers on Ground Fault protection. The circulating current usually remains around 25 Amps, however, sometime it increases to 38 Amps without any change in load or operating condition. We are not sure what is causing this increase. Appreciate if someone can advise us the possible cause(s) and its remedial action.
Regards.

 

[krisys]

a) In the power generator environment, it is very important to know is circulating current is purely reactive component or active component. Due to the uneven reactive power generation it could happen.

b) If the generators are directly connected to the bus, (i.e. without the unit transformer), some harmonics (like 3rd, 5th etc., I don't remember the exact harmonic orders)may be present in the current. This could be circulating. If it contains 3rd harmonic component, certainly, it would reflect in the earth path.

This may need a good amount of field investigation also.

 

[waross]

We have been assuming equal generation parameters and equal loads.
If either the voltage or the fuel supply of TG1 is not exactly equal to TG2 then uneven load sharing will may the appearance of circulating current. In the SLD, the direction of the top arrow will be reversed.
Uequal loads may show the same effect. (More load on the left than on the right)

Bill
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