I understand that by adding a capac 1

[kitha]

I understand that by adding a capacitor bank to a low power factor motor control center or main switchboard(less effective) greatly reduces the current input, lowers the cable ratings and the cost of electricity. But what happens to the ampere interrupting rating of the downstream protective devices and how do we take the effect of this capacitor bank when calculating fault levels at the downstream.

 

[cuky2000]

Dear Kitha:

Capacitors do not add or reduce fault level. However, there are transient surges during switching conditions downstream.

Upstream, capacitor have additional benefits mitigating the surge effect of travelling wave created by lightning or starting devices.

Capacitor along or in combination with surge arrester help to protect dry insulated units such as motor or dry insulated transformer lowering the slope of the surge travelling wave. This reduce the dielectric stresses (lower turn-to-turn voltage) in the winding and increase the likelihood of extend the life of the motor and other unit connected to this circuit.

I hope this could help.

 

[GRPatel]

Short circuit currents have three fold effect.
1. Thermal effect on the lines and components
2. Dynamic forces due to the peak of the current and the frequency and
3. System stability.
The capacitors tend to increase the peak of the current at high frequency during the initial period of the short circuit. This is a characteristic of capacitors as they discharge energy in the system.
This high currents at high frequency had effect on the components due to the dynamic forces due to very large di/dt.

 

[jbartos]

Suggestion to cuky2000 (Electrical) Oct 17, 2002 marked ///\\Capacitors do not add or reduce fault level. However, there are transient surges during switching conditions downstream.
///Capacitive storage may add to the short circuit level. However, the capacitive energy somewhat cancels the inductive energy contribution supplied from rotating machinery, e.g. induction motors. It just depends on the power distribution system and fault locations.\\