INTRODUCTION

PILOT WIRE & CARRIER CURRENT PROTECTION

PILOT WIRE PROTECTION INRODUCTIONPilot relaying scheme consist of three type protection scheme.A) pilot wire protection B) carrier current protection C) microwave In pilot wire protection scheme two wire are used to carry information signal from one end to the protection line to other.A wire pilot may be buried cable or a pair of overhead auxiliary wire other than the power line conductor.

BASIC PRINCIPALThe differential pilot-wire protection is based on the principle that under normal conditions, the current entering one end of a line is equal to that leaving the other end. As soon as a fault occurs between the two ends this condition no longer holds and the difference of incoming and outgoing currents is arranged to flow through a relay which operates the circuit breaker to isolate the faulty line.

WORKINGThe scheme is unit type protection and operates or work on the principal of differential protection .The comparison is made between the C.T secondary at the two end of line,as the pilot channel is very expensive a single phase current is derived from three phase current at each end of the line.

There are several differential protection schemes for the lines. They areciA) circulating current B) balance voltage (or oppose voltage scheme) C) Translay scheme

Circulating Current SchemeIn this scheme two C.T are connected in each protected one at each end under healthy or external fault condition the secondary current are equal and circulate in pilot wires the relay is connected between equipotential point of pilot wire and the differential current of two C.T is zero .

Balance Voltage SchemeIn this scheme current does not normally circulate through the pilot wire The operating coil of relay is placed in series with the pilot wire Current does not flow through the pilot wire under normal condition In case internal fault the polarity of the remote end C.T is reversed and hence current flow through the pilot wire and operating coil of relay.

Transley SchemeIn this scheme the induction disc type relay is used at each end of the protected line section and secondary windings of relay is interconnected in opposition as a balance voltage system .In normal condition no operating torque is produced In case internal fault the relay torque is produced from the interaction of the two flux one of which produced directly through the upper magnet of the relay the second flux is produced by the current flowing through the lower magnet.

TRANSLEY SCHEME

APPLICTIONPilot-wire relays are used for protection of short to medium length cable The standard relay can be applied to circuits with more than two line ends. Therefore three or more terminal applications are possible The most sensitive earth-fault detection of the relay is 25% of rated current. This may be fed

ADVANTAGEThey are simple because of their simplicity They are more reliable They have maximum stability od the system Minimum damage to the line

DISADVANTAGEAdditional expenditure of pilot wire They are economical only up to length 16 kmThey require continuous supervision .

Carrier Current Protection

INTRODUCTION In carrier current scheme a low voltage, high frequency signal is used to transmitt information from one end of the line to the other.In this scheme, the pilot signal is coupled directly to the same high voltage line which is to be protected .This type of pilot is also called a power line carrier.

Operating PrincipleThere are two operating techniques for carrier current protection :-

Phase Comparison Technique

Directional Comparison Technique

Phase Comparison Protection

In this , phase angle the current entering one end is compared with the phase angle of the current leaving the other end of line.If currents at both ends are in phase , there is no fault(or external fault).In case of internal fault two currents will be 180 degree out of phase.In this scheme carrier signal is employed as a blocking pilot.

Directional Comparison Technique

In this scheme the direction of power flow at the two ends of the protected line section is compared.During normal condition or external fault the power must flow into the protected line section at one end and out of it at other end.In case of internal fault power flows inwards from both ends

ADVANTAGEFast, simultaneous operating of circuit-breakers at both ends.Fast clearing prevents shocks to systemsFor simultaneous faults, carrier current protection provides easy discriminationcarrier current relaying is best suited for fast relaying in conjunction with modern fast circuit breaker.

APPLICATIONStation to station communication. In power station, receiving stations and sub-stations telephones are provided. These are connected to carrier current equipment and conversion can be carried out by means of "Current Carrier Communication".Remote control of power station equipment by carrier signalsTelemetering.

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