The tuning are purposely a little bit incorrect, in order not to get a too low impedance for the harmonic, to which it is tuned. The capacitor banks usually are connected in double Y-connection with the neutral of the halves connected. The current between the two neutrals are supervised by an overcurrent (unbalance) relay. 1. Unbalance relay
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.
Capacitors of today have very small losses and are therefore not subject to overload due to heating caused by overcurrent in the circuit. Overload of capacitors are today mainly caused by overvoltages. It is the total peak voltage, the fundamental and the harmonic voltages together, that can cause overload of the capacitors.
As you already know, capacitor banks are normally used in medium voltage networks to generate reactive power to industries etc. Capacitor banks are, almost always, equipped with a series reactors to limit the inrush current.
3. Short circuit protection In addition to the relay functions described above the capacitor banks needs to be protected against short circuits and earth faults. This is done with an ordinary two- or three-phase short circuit protection combined with an earth overcurrent relay.
Since the capacitors mostly are connected in series with a reactor it is not possible to detect overload by measuring the busbar voltage. This is because there is a voltage increase across the re- actor and the harmonic currents causing overvoltages will not in- fluence the busbar voltage.