Each capacitor consists of several series and parallel internally connected elements. The container is made from mild or stainless steel suitable for indoor and outdoor installation. Built-in discharge resistors reduce the residual capacitor voltage to 50 V or less within 5 minutes of de-energisation.
The most commonly used capacitors are:-
- Single Phase two bushing type capacitor with both terminals insulated.
- Single Phase one bushing type capacitor with one terminal insulated and the other interconnected to the housing.
Single phase capacitors with or without internal fuses rated up to 21,600 Volts are available.
Three phase capacitors 2,400 V up to 4,800 V are available for rating 25 to 200 kVAr.
Single phase and three phase capacitor banks are constructed with capacitors connected in a series and parallel combination to give the required output.
Capacitor Technologies manufacture various types of units to suit a wide variety of installations and conditions.
Capacitor Bank Construction Types available are
• Totally enclosed
• Cubicle mounted
• Open rack mounted
Totally Enclosed capacitor banks are suitable for power factor correction of motors where protection to the capacitors is provided by the motor fuses. Alternatively totally enclosed banks can be used for applications where external protection is available.
Open Rack type capacitor banks mounted on galvanized steel frame or Cubicle mounted type can also be supplied, ready for installing. The internal electrical connections are designed in accordance with the bank being constructed.
Protection of Capacitor Banks
Shunt capacitors must be protected against internal faults resulting in short circuiting and overload conditions. The protection can be categorised into two types :
• Individual Protection
• Overall Protection
Capacitors connected in star configuration may be provided with internal fuses which disconnect the faulty element instantaneously avoiding case rupture. As an alternative expulsion type fuses are used to protect and isolate the capacitors at the first fault. Expulsion fuses can be replaced where the capacitors have to be replaced after blowing internal fuses.
Special types of switchgear and protection systems suitable for switching and protecting power capacitors.are used. The HV contactors and circuit breakers used for capacitor bank switching must be re-strike free. Current rating of the switching device should be carefully selected to take care of the effect of harmonics in addition to the continuous overload of capacitors.
Delta connected capacitor banks are commonly protected with external current limiting HRC fuses.
Large three phase, high voltage capacitor banks are normally connected in double star configuration. In order to prevent the over-stressing of healthy capacitors due to successive fuse interruption in the circuit of other capacitors, a current transformer is connected between the two neutral points of the equally divided star groups. An overcurrent relay with time delay may be required to be installed to protect against an overload condition due to harmonics in the power system.
Harmonic Blocking Reactors
Since capacitor impedance decreases with increase in frequency, the presence of harmonics in the power system may cause an over voltage and over current in the capacitor bank. The condition may also exist that the capacitor bank starts resonating with the inductive elements of the supply transformer or load.
Air core and iron core high voltage reactors, known as Blocking Reactors, are used to reduce resonance in capacitive circuits.
Inrush Current Limiting Reactors
Capacitors draw a large inrush current during switching on. The peak of this inrush curent may rise to a high value if a new stage is switched in parallel with some capacitors already connected. In such cases, current limiting reactors are required to limit the high inrush current transient to an acceptable limit.
The voltage rating of capacitor banks are selected by first assessing the effect of voltage rise at the capacitor terminal, due to the installation of blocking reactors and the capacitors themselves. Consideration should also be given to the normal supply variation in the supply system. Capacitor units manufactured by Capacitor Technologies are designed to operate with 10% continuous over voltage.
Capacitors normally have an over-current capacity equivalent to 1.3 times the rated current corresponding to rated voltage and frequency. This over-current, due to harmonics, should be limited to this value by means of series detuned harmonic blocking reactors.