When capacitor units in a capacitor bank fail, the amount of increase in voltage across the remaining units depends on the connection of the bank, the number of series groups of capacitors per phase, the number of units in each series group, and the number of units removed from one series group.
Each capacitor unit consist of a number of elements protected by internal fuses. Faulty elements in a capacitor unit are disconnected by the internal fuses. This causes overvoltages across the healthy capacitor units. The capacitor units are designed to withstand 110% of the rated voltage continuously.
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.
Capacitor banks require a means of unbalance protection to avoid overvoltage conditions, which would lead to cascading failures and possible tank ruptures. Figure 7. Bank connection at bank, unit and element levels. The primary protection method uses fusing.
Each unit should be rated 9.96 kV and 667 kvar. For a fuseless bank, capacitor units are only connected in series (illustrated in Figure 10); they are never placed in parallel like an externally or internally fused capacitor bank.
If the phases of the bank are constructed in distinct separate structures, a flashover within the capacitor bank will begin as a short circuit fault over of a single-series group. Such a fault produces very little phase overcurrent. For this type of fault, fast protection is provided by the unbalance protection.