Capacitors overvoltages

When harmonic filter circuits are connected to a power system with saturated reactors (Figure 27.2(e)) resonance may occur between the reactor and the filter capacitors to give rise to overvoltages. 

Overcurrent will also mean an overvoltage across the capacitor units, which would inflict greater dielectric stresses on the capacitor elements. 

If a large induction motor is switched on such a system it is possible that its rotor may lock up at the sub-synchronous speed and keep running at higher slips. This situation is also undesirable, as it would cause higher slip losses in addition to higher stator current and overvoltage across the series capacitors. 

For large capacitor banks using a number of small identical units per phase the configuration of the units must be such that a fault or a failure in one unit of a phase should not result in an overvoltage of more than 10% across the remaining healthy units in that phase. This requirement becomes more specific when the capacitor units are used... 

In LT capacitors controlled through automatic p.f. conection relay it i s recommended that the capacitors are not switched more than 1000 times during a year or, say, three or four switchings per day. More switchings will mean more frequent overvoltages and inrush currents which may reduce the life of a capacitor. In HT capacitor banks, how ever. such a situation may not be relevant as HT capacitors inay be switched only once or twice a day. 

Then the overvoltage developed in the event of a failure of any one unit (assuming the capacitor units to have external type of fuses) as determined in the said example... 

Rapid reinsertion of the capacitors as soon as the fault coiiditions are removed is an important requirement to retain the stability of the system. This can be achieved with the use of an additional ZnO. non-linear resistance (ZnO being the latest in this field compared to SiC. which was used earlier), across the capacitor banks (Figure 26.12). Generally, the ZnO resistor will be adequate to dampen the fault current without initiating the spark gap. and w ill limit the overvoltage across the capacitors. It will also permit automatic reinsertion of the capacitors as soon as the fault conditions are removed without causing a delay. The spark gap will serve as a backup to the ZnO resistor in the event of very severe faults. 

Concentration of capacitor banks at one point in a system may cause amplification of harmonics, selfexcitation of machines, overvoltages due to switching and unsatisfactory working of audiofrequency remote-control apparatus. To overcome this, a concentration of capacitor banks at one point must be avoided as far as possible. These may be installed at different locations in the system. 

This is an alternative to the ageing test and is applicable for shunt capacitors of 1000 V and above. It is an in-house process test and is carried out on the capacitor elements before they are assembled into a capacitor unit to ascertain their dielectric stability against repeated overvoltages. For the test procedure refer to lEC 60871-2. 

Unnecessary loading of the breakdown fuse through transient overvoltages can be avoided by connection to a r element which consists of a length choke and transverse capacitors. So-called iron core chokes are most conveniently used for series chokes, which are usual in power electronics. A damping element with a 61-fjF capacitor is advised at the input and output of the r element. 

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