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Capacitor bank switching study for a utility located in North America



The scope of work concerned a Medium Voltage/High Voltage 100MVA substation located at the end of a relatively long radial transmission line. A load increase was forecasted for the next years and it was planned to replace the actual capacitor bank by a 50MVAR unit to provide the additional power without a severe voltage dip.


The first step of the study consisted in modelling the substation using EMTP. Capacitor banks, current-limiting reactors and circuit breakers are modeled using ideal devices. In order to get the appropriate level of details required by capacitor bank switching analysis, bus work inductances are derived from typical values found in IEEE C37.012-2005. Surge arresters are modeled using the IEEE arrester model for fast front surge. Incoming transmission lines are modeled using a frequency-dependent model built using the power frequency impedance data and the EMTP Line Rebuild tool. The upstream network model is validated by comparing the load-flow results calculated by EMTP with the results provided by the client.

Four scenarios are analyzed using EMTP:

  • Capacitor bank energization. For the first two cases, a time-domain statistical analysis is performed to evaluate the distribution of overvoltages across the capacitor bank. It consisted in running 500 simulations with different closing times of the capacitor bank breaker. In the first analysis, the natural Gaussian distribution of closing time around the zero-voltage crossing is modeled. In the second analysis, a uniform distribution of closing time on one power cycle is added to take into account that the CB closes at any time during the power frequency cycle. The simulated overvoltages are compared with the withstand level.


  • Outrush Transient. The capacitor bank is already energized and operating in the steady state and a fault occurs on the 115kV bus. This discharge of the capacitor bank into the fault is called outrush current and can cause important stress on the circuit breaker. The product of the inrush current peak and transient inrush current frequency is compared with the limit defined in IEEE C37.06.


  • Transient Recovery Voltage (TRV). The current-limiting inductor reduces the severity outrush current but also increases Transient Recovery Voltage of the circuit breaker protecting the capacitor bank. To assess the capability of the capacitor bank circuit-breaker to open after a fault between the inductor and the capacitor bank, the TRV is analyzed with EMTP. The TRV prospective envelope is derived from IEC 62271-100 and compared with the TRV simulated in EMTP.


Simulation shows that the maximum switching overvoltage is kept below the limits and the system should be safe from the overvoltages originated from the switching of the capacitor bank. If a fault occurs between the capacitor bank and the current-limiting reactor, the TRV analysis confirms that the circuit breaker is able to open without any problem.

In the case of 3-phase fault on the 115kV bus, there was a small hazard of circuit breaker damage. The product of the inrush current peak and transient inrush current frequency was slightly higher the IEEE limit, which is an unnecessarily restrictive criterion.


Overvoltages (top) and in inrush current (bottom) during the capacitor bank switching



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