CARRIER CURRENT PROTECTION
In this type of protection, transmission lines are used to carry protective currents at carrier frequency (30 to 200 kc/s) or at an ultra high frequency (above 900 mega cycles). When the transmission lines carry protective currents at carrier frequency it is called carrier current-pilot protection and when it carries protective current at ultra high frequency its called as microwave pilot wire protection. As no separate pilot wires are used, the transmission lines are used to carry both power current as well as protective carrier currents it causes a great saving.
Fig 14 represents a phase blocking system schematic arrangement of the equipment required at both ends of the transmission line. Each end of the transmission line consists of a network which transforms CT output currents into a single- phase sinusoidal output voltage, carrier current transmitter and a receiver, carrier-current blocking network, comparer and a tripping relay.
The transmitter generates high frequency output voltage when a positive voltage (positive half of sinusoidal voltage is applied to the control circuit of the transmitter. The high frequency output is applied between one phase and the ground. Each receiver receives high frequency currents from its local transmitter as well a, currents from the transmitter at the other end and converting it into a d.c. voltage, which is applied to the comparer circuit. The output of the converting network is also applied to the comparer circuit. The comparator controls the operation of the tripping relay, which in turn controls the operation of the circuit breakers at each end of the line. The blocking network forms a resonant circuit, which offer negligible impedance to power-frequency currents, but offers very high impedance to carrier-currents.
The schematic arrangement of the comparer is shown in Fig. 15, it consists of a vacuum tube having operating grid fed from local converting network and a restraining grid fed from carrier current receiver. When a positive potential is applied to the grid of the tube, the tube conducts and if during the next half of the cycle if the restraining voltage is not received from the other end of the line the tube conducts and energizes an auxiliary relay which trips the circuit breaker.
When fault occur at points outside the protective zone, the output voltages of 3-phase to single phase converting networks with ends A and B are 180° out of phase and as the transmitter currents are transmitted only during the positive halves of the output voltage so the transmitted currents are displaced in time equal to half of the time period. Thus there is always a carrier current. When there is a fault within the protective zone both ends A and B feed the fault current, so there no longer occurs a phase difference of 180° in the converting network output voltages of ends A and B. This results in carrier currents to be in phase and there will be no signal from either of the station every half cycle. This system of protection essentially compares the phase of the carrier current and there is no tripping whenever the carrier currents are displaced in time.
Written by arjun on March 22nd, 2009 with
2 comments.
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Manjider singh
#1. October 27th, 2009, at 6:28 PM.
very nice and effective discussion may i know from which book u took ? i willl study that book