In this article we will discuss about the functions of surge capacitor, reactor and absorber in a power system.
Damage caused by over-voltages depends not only on the amplitude of an incoming wave but also on the steepness of its wave front. The device, which reduces the steepness of the wave front of a particular surge and thus minimises the danger due to over-voltages is known as surge modifier or absorber.
A condenser when placed between the line and earth reduces the steepness of the wave front to a considerable extent and hence protects the other apparatus from damage due to overvoltage.
The condenser also provides protection against comparatively low-voltage, high-frequency waves. Since the impedance of a condenser is inversely proportional to the frequency, it is low at high frequencies and large at low frequencies.
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The normal frequency voltage produces only a small current in the condenser, so that negligible loss is caused during normal operation. A condenser is very efficient in relieving a line from the effects of high frequency disturbances because for high frequency discharges the system behaves as though the lines were connected direct to the ground.
The combination of a capacitor and an arrester can be very useful because they complement each other.
A pure condenser cannot dissipate the energy in the wave front of a travelling wave or in a high frequency discharge. It merely reflects the energy away from the apparatus under protection, and the energy is dissipated in the resistance of the line conductors and the earthing resistances.
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It thus appears desirable to place in series with the condensers a non-inductive resistance [Fig. 9.36(a)], so as to rapidly absorb the energy of a disturbance. Such a combination of resistance and capacitance is known as surge absorber.
Another type of surge absorber is obtained by having an inductance connected in parallel with a non-inductive resistance. The combination is placed in series with the line. The inductance offers a large impedance to higher frequency currents, which are forced to pass through the resistance, in which surge energy is dissipated. The normal-frequency currents find the inductance a low impedance path and pass through it without much loss. The arrangement is depicted in Fig. 9.36(b).
Another type of apparatus which has come into use is the Ferranti surge absorber, which consists of an air-cored inductor connected in series with the line and surrounded by, but insulated from, an earthed metallic sheet called a dissipator.
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For line voltages below 11 kV, it consists simply of a coil surrounded by a sheet-iron case; for voltages up to 33 kV there are two or more disc-shaped coils with dissipators in the form of metal sheets placed between the coils; for voltages exceeding 66 kV, the coil is disposed horizontally and is oil immersed, the dissipator being a metal cylinder inside the coil.
The principle of all the three forms is the same i.e., the coil can be considered as the primary of the transformer, and the dissipator as a short- circuited secondary of one turn. The energy of a surge is used up in the form of heat generated in the dissipator; firstly, due to the current set up in it by ordinary transformer action, and secondly, by eddy currents.
The apparatus is used largely for the protection of transformers, and since there is distributed capacitance between coil and dissipator, just as there is distributed capacitance between transformer windings and grounded core, it is possible to regard the surge absorber as an extension of the transformer winding specially arranged to relieve the main winding from the initial very steep potential gradient.
It is claimed that, by installing this apparatus, the voltage rise across the end turns of a transformer subjected to incoming high-frequency waves can be reduced to about 15% of the original value. A more recent form of the surge absorber is the ERA (Electrical Research Association, UK) surge filter incorporating a gap G and an expulsion gap E. The arrangement is depicted in Fig. 9.37.
When a high-frequency wave reaches the inductor L, a high voltage is induced across it and causes the gap G to breakdown and so the resistor R and the expulsion gap E are included in the circuit. An incoming wave is thus flattened by the inductor L and resistor R and its amplitude is reduced by the expulsion gap E.
Connection diagram for protection of a 3-phase transformer installed at a substation by surge absorber and diverter in Fig 9.3.8.
