Capacitor switching is one of the most operationally demanding tasks for medium-voltage switch switches, contactors and breakers, characterized by high-frequency current transients and significant voltage stress. Utilizing IEEE C37.06, IEEE C37.09, IEEE 37.011, IEEE C37.012, as design foundations. This page answers frequently asked technical questions about capacitor bank switching and switching transients and VarStec’s system-fit™ design strategies that account for the physical realities of capacitor switching, including inrush, outrush, and transient recovery voltage (TRV).

Technical Q&A

When a capacitor stage is energized while adjacent stages are already in service, a “back-to-back” switching event occurs. The energized stages act as a low-impedance source, discharging accumulated energy into the incoming stage at extremely high frequencies, often exceeding several thousand Hertz. Without mitigation, this peak inrush current can exceed the “making” and “latching” capacities of the switching device, leading to contact welding or catastrophic failure.

VarStec utilizes the Capacitor Bank Peak Inrush Analysis Tool to calculate the peak magnitude and frequency of these transients. By quantifying the Ipeak and finrush,we determine the precise inductance required for current-limiting reactors to bring the transient within the switching device’s rated capability while maintaining compliance with IEEE C37.06.

Outrush current is the high-frequency, high-magnitude discharge of a capacitor bank into a low-impedance fault. As established in IEEE C37.012 and further detailed in IEEE PES-TR16, a critical concern arises when a “Victim Breaker”, a feeder or bus-tie breaker located between the capacitor bank and the fault, closes into a pre-existing fault.

In this scenario, the Victim Breaker must withstand the combined stresses of the power system’s short-circuit current and the high-frequency capacitor bank outrush current. The VarStec Capacitor Bank Outrush & Close-in Fault Analysis Tool calculates the peak outrush magnitude (Ipeak) and frequency (fo) to ensure these values do not exceed the breaker’s high-frequency current withstand or rated making current. Failure to limit these transients can result in contact welding or catastrophic structural failure of the Victim Breaker. When limits are exceed, VarStec Analysis tool calculates the required inductance to bring the “victim breaker” into compliance with its rating.

While reactors are essential for limiting inrush and outrush, they introduce a secondary challenge: they can significantly increase the Transient Recovery Voltage (TRV) seen by a circuit breaker during fault interruption. When a breaker opens to clear a fault, the stored energy in the reactor produces a high-frequency voltage oscillation across the breaker contacts. If the rate of rise of this recovery voltage (RRRV) exceeds the dielectric strength of the opening gap, the arc may reignite, causing a failure to interrupt.

VarStec addresses this using the Reactor Limited Fault TRV & Breaker Duty Assessment Tool. This tool analyzes the interaction between the reactor’s inductance and the system’s stray capacitance to predict the TRV profile. By coordinating this analysis with IEEE C37.06 TRV envelopes, we ensure that the selected circuit breaker has the necessary withstand capability or recommend the use of surge capacitors or specialized snubber circuits to soften the TRV slope.

The integration of a new capacitor bank into an existing substation with legacy oil circuit breakers (OCBs) requires transient analysis because these breakers often have limited high-frequency current withstand capability. Many legacy OCBs were designed and tested before the modern requirements of IEEE C37.06 and IEEE PES-TR16.

When a legacy OCB, or as VarStec calls it, a “victim breaker,” is exposed to a close-in fault, it can be subjected to high-frequency, high-magnitude outrush currents that exceed the mechanical and thermal limits of the oil-interruption mechanism. In addition, the presence of the capacitor bank alters the Transient Recovery Voltage (TRV) profile during fault interruption. Using the VarStec Reactor Limited Fault TRV & Breaker Duty Assessment Tool, we analyze the increased rate-of-rise of recovery voltage (RRRV) to help verify that the legacy equipment can successfully clear the fault without arc re-ignition.

Related Technical Topics

How VarStec Addresses Capacitor Switching Transients

VarStec designs capacitor banks with full consideration of capacitor switching duty and the transient stresses imposed on the electrical system and switching equipment. We evaluate inrush, outrush, TRV, and related switching effects as part of the application and design process, helping ensure that each project is installed and operated without transient-related problems. Backed by practical experience in medium-voltage capacitor bank design and manufacturing, VarStec delivers system-fit™ solutions engineered for reliable field performance.