Determining settings for capacitor bank protection

As the electric power grid is pushed to its limits, efficiencies can be gained by properly using shunt capacitor banks. Protective relaying must be provided for these banks that will protect the system from abnormal conditions that could be caused by the capacitor bank as well as provide protection to the capacitor bank from abnormal conditions caused by system conditions or capacitor failed elements. Failed capacitor elements can cause failure of the entire bank due to overvoltage on the individual failed elements. As elements fail, the subsequence overvoltage caused by the failure increases the risk of further failures. The optimum solution is to recognize the condition of failed elements with alarms and perform maintenance, replacing the failed elements, when the bank is least needed by the system. Trip functions must also be provided to protect healthy elements from the overvoltage caused by the failed elements. Conditions associated with failed elements can easily be calculated and can be measured by microprocessor-based relays. This paper will examine the calculation of protective settings necessary to completely protect a shunt capacitor bank. After a brief review of capacitor bank design and failure mechanisms, the paper will examine and demonstrate calculations for both grounded and ungrounded banks. The general setting calculations to be examined include: phase overcurrent function, negative sequence overcurrent, bank overvoltage, and bus overvoltage. Additionally, calculations will be shown for current differential and voltage differential for alarm points for failed elements and for trip points for failed elements.