Normally, we do not consider the effects of shunt capacitor bank during calculating the short circuit level. We normally consider 90° lagging power factor for the short circuit currents with respect to the voltage phasor.
The utilities generally install shunt capacitor banks in receiving substations. The capacitor banks improve power factor and voltage regulation. Shunt capacitor banks reduce fault level by supplying current at leading power factor.
Suppose, there is a three phase symmetrical busbar fault at a particular voltage level. The capacitor banks connected to that busbar contribute fault MVA at leading power factor. That means it opposes the fault level supplied by the source. Therefore, the effective fault level becomes the difference between the normal fault \(MVA_n\) and leading \(MVA_c\) contributed by the shunt capacitor banks. The effective fault level \(MVA_e\) at a point in a three phase system is,
Symmetrical Faults and Current Limiting Reactors
The concept of Effective Short Circuit Level and Effective Short Circuit Ratio are two useful terms. Utilities use these concepts in evaluating the strength of AC Power System to incorporate HVDC System. The AC MVAR supplied by capacitor banks in HVDC Substations remains about 60% of convertor MVA load. Therefore, SCR ESCR are considered at the planning stage of HVDC Project for determining suitability of SC System to accommodate the HVDC System.
Normal Short Circuit Ratio
\[\text{Normal Short Circuit Ratio} = \frac{\text{Normal Fault Level of AC Bus \(MVA_n\)}} {\text{Rated Power of HVDC System MW}} \]
Effective Short Circuit Ratio
\[\text{Effective Short Circuit Ratio} = \frac{\text{Effective Fault Level of AC Bus}} {\text{Rated Power of HVDC System MW}} \]
The effective short circuit ratio of AC System at the AC substation busses should be more than 5 for planning the HVDC system connection.