Carrier current protection
Carrier current protection is the most widely used scheme for the protection of transmission lines. This protection is used for the protection of EHV and UHV POWer lines. In this scheme carrier currents of the high-frequency range are transmitted and received with the help of transmission lines for protection. The carrier signal is directly coupled to the same high voltage line which is to be protected.
Carrier signal of frequency range 50 KHz to 700 KHz is used in this scheme, because below this range, the size, the cost of coupling equipment becomes high and above this range, signal attenuation and transmission loss is considerable. The power level is about 10 to 20 w.
As the development in power system is growing and large interconnected systems becoming very essential for high-speed protective schemes, carrier current protection is becoming suitable for EHV and UHV power lines. They are faster and superior to distance protection schemes. They are also economical ‘and provide both primary and back-up protection. In conventional time-stepped distance protection, circuit breakers at both ends of the line don‘t trip simultaneously when a fault occurs at one end of the end zones of the protected line section. This causes instability in the system sometimes. The carrier current protection does not suffer from the disadvantages of distance protection. The carrier current protection is capable of providing high-speed protection for the whole length of the protected line.
The carrier signal can be used either to prevent or initiate the tripping of a protective relay.
The carrier scheme is of two types:
In the blocking carrier scheme, the presence of carrier blocks the tn’pping signal, when a fault occurs the carrier starts. Under normal condition, there is no carrier. If the fault is internal as sensed by directional elements, the carrier stops and there is tripping. The presence of carrier blocks tripping of relays. For extreme faults, only one of the directional relays will stop the carrier while the other directional relay will maintain the carrier and there is no tripping.
In the case of interdependent tripping or intertripping (or transferred tripping) the presence of the carrier gives the trip signal. In figure 3.1 (a) after the occurrence of a fault, the zone-I (Z 1) unit of the relay at the end nearest to the fault causes local tripping and also sends a carrier to the other bus at the other end of the protected line. The relay at this bus after getting carrier signal causes immediate tripping even though the fault may be outside its zone of reach.
A substitute of carrier intertripping is carriers acceleration as shown in figure 3.1 (b), whereafter the receipt of the carrier by the relay, the zone-l unit is automatically extended to zone-II (22) two unit. This helps in rapid fault clearing in the zone-ll unit of the relay.
The carrier signal may be jointly utilized for telephone communication, supervisory control, telemetering and relaying.
Carrier current scheme is only possible in the case of overhead lines and not in the case of underground cable because of the capacitance of the cable. The capacitance of the cable attenuates the carrier signal to a very low value.