SELF BALANCE SYSTEM
This is very sensitive type of earth fault detection scheme, which necessarily consists of two connecting cables connected to the two ends of each phase and the cables are passed through the circular aperture of the ring type C.T as shown in fig 5. Under normal working conditions the current carried by two cables in the central aperture of the C.T will be in opposite direction resulting in no magnetization of the C.T. If there is an earth fault on any one of the phases. The fault current will pass only once through the circular ring of the C.T of that particular phase, this results into production of flux and thereby an e.m.f is induced in the relay circuit, which will trip the circuit breaker. The relay will also trip for any fault between two phases. It does not provide protection when fault occur between the turns of the same phase for ordinary generator winding: but when the winding of each phase is designed in two parallel circuits this type of protection is possible.
disadvantages
- Since the cables carry normal full voltage, under heavy short circuit currents large electromagnetic forces are produced.
- The scheme necessitates a different layout of the cable leads.
- In order to protect the cable from circuit breaker to stator terminal the ring C.T.s should be placed close to the circuit breaker but it’s cumbersome. This difficulty can be overcome by providing earthed metallic sheet around the cable. If the cable lead punctures anywhere, it will cause current to flow through the sheet resulting in energisation of relay.
Written by arjun on March 19th, 2009 with
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