General about capacitor banks
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.
Harmonic filters, for thyristor controlled reactors, are also variations of capacitor banks having the reactor inductance together with the capacitor capacitance tuned for series resonance at a certain frequency.
The tuning is 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
Capacitor Bank Protection
1. Unbalance relay
This overcurrent relay detects an asymmetry in the capacitor bank caused by blown internal fuses, short-circuits across bushings, or between capacitor units and the racks in which they are mounted.
Each capacitor unit consists 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. If this level is exceeded, or if the capacitance of the faulty unit has decreased below 5/6 of the nominal value, the capacitor bank must be taken out of service.
In normal service when all capacitor units are healthy the unbalance current is very small. With an increasing number of blown internal fuses, the unbalance current increases and the unbalance relay will give an alarm. The alarm level is normally set to 50% of the maximum permitted level.
The capacitor bank then should be taken out of service to replace the faulty units. If not the capacitor bank will be tripped when the maximum allowed unbalanced current level is exceeded.
2. Capacitor bank overload relay
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 an overload of the capacitors.
The capacitor can withstand 110% of the rated voltage continuously. The capability curve then follows an inverse time characteristic where withstand is approximately 1 second -180%, 10 cycles -210%.
!!! 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
For example, ABB Transmit Oy has designed a relay that measures the current in the capacitor bank and transforms this into a voltage that corresponds to the voltage across the elements in the capacitor bank.
This relay is called SPAJ 160C and includes unbalance protection, overload protection and undercurrent relay. The undercurrent function is used to prevent the charged capacitor bank to be reconnected when a short loss of supply voltage occurs.
The connection of the relay is shown in figure 2.
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.