DC offset during fault in capacitive circuit 2

[TS Liew]

Hi buddies, I have always wondered if a large capacitor bank in a 3 phase high voltage system would contribute a DC offset to a fault near it. We have had many studies of the same for inductive circuit but have not seen any on fault of capacitive circuits.

TS Liew (Electrical testing, Commissioning, Power system studies)

 

[magoo2]

A large capacitor bank produces a high frequency discharge into the fault. This will offset the fundamental frequency current but I'm not sure if is considered DC offset. It is really fundamental frequency plus natural frequency (of capacitor bank).

I did find a reference that might be helpful:

http://www.ece.mtu.edu/faculty/bamork/EE5223_S09/mip99.pdf

See outrush transient.

 

[magoo2]

One further comment. DC offset is mostly a function of closing angle, i.e., the point of wave on which the fault occurs. So, whether you have a capacitor bank on the circuit or not, you will get DC offset strictly based on closing angle.

 

[cuky2000]

One way to mitigate the effect on the switching device, is to use synchronous control on the breaker to capture the wave near or at the zero closing. The objective is to maintain the switching effect within the rated transient recovery voltage (TRV) of the circuit breaker.

 

[TS Liew]

Hi fellas, thank you for the posts you have put up. I now understand that with the cap bank in the network, with the L, R & C switching in and out, this would cause significant transient to the network at high frequencies depending on f=1/√LC. What if in a situation where the cap bank is already switched in at steady state condition, and a fault should occur as in the picture below:

Would both the generation source and cap bank be feeding into the fault. How would the sub-transient fault current look like at the generator protection & the cap bank protection side? What's the influence of fault time on the contribution to the total fault current from the generator & cap bank?

TS Liew (Electrical testing, Commissioning, Power system studies)

 

[cuky2000]

A difference of a series capacitor, a shunt capacitor bank contributes very little to the short circuit currents.

However, high frequency in the kHz range caused by the capacitor outrush current component could impact the yard protective devices transient TRV rating during short line fault (SLF).

Special consideration should be observed in the case of reignition to avoid exceeding the circuit breakers duty rating particularly for old breakers classified as a general-purpose either SF6 or more carefully for oil-filled type.