Carrier Current Protection of Transmission Lines:

In modern high-power electrical systems it is necessary to have quick acting protections on long transmission lines. The requirements to be met by such protections are fully satisfied by the circulating current differential protection with its high sensitivity, quick action and independence upon the settings of the adjoining section protections. Not withstanding this, owing to the need for installing interconnecting conductors (cables) as already pointed out in previous Articles, circulating current differential protections are confined to lines up to 8 or 15 km long.

It is, however, possible to make use of the main line conductors as the interconnecting conductors of a circulating current differential protection. The need for special interconnecting conductors (cables) then disappears and it hence becomes possible to set up a circulating current differential protection on transmission lines of any length. This is the basis of what are called Carrier Current Protection of Transmission Lines. The essential difference between Carrier Current Protection of Transmission Lines and the voltage balance (Translay) pilot wire protection is that, in the former, only the phase angles of the currents at the two ends of a line are compared instead of actual currents as in the latter case and this phase angle decides whether the fault is internal or external.

To make possible the transmission of commercial frequency (50 Hz) load current, and at the same time use the main line conductors as the interconnecting conductors of the differential protection, it is necessary to use a current of higher frequency in order to be able to transmit current impulses from one end of the line to the other. High frequency signals in the range of 50 kHz to 400 kHz, commonly known as the carrier, are transmitted over the conductors of the protected line. To inject the carrier signal and to restrict it within the protected section of the line suitable coupling apparatus and line traps are used at both ends of the protected section. This obviously makes this carrier current protection of transmission Lines scheme quite expensive and justifies its application only in transmission lines of 110 kV and above.

The schematic diagram of carrier current protection of transmission Lines is given is Fig. 14.16. The main elements of the carrier channel are (i) transmitter (ii) receiver (iii) coupling equipment and (iv) line trap.

Here we need not to go through the details of carrier current transmitters or receivers, all we need to know is that when a voltage of positive polarity is impressed on the control circuit of transmitter, it generates a high frequency output voltage. This output voltage is impressed between one phase conductor of the transmission line and the earth, as illustrated schematically in Fig. 14.16.

Each carrier current receiver receives carrier current from its local transmitter as well as from the transmitter at the distant end of the line. In effect, the receiver converts the received carrier current into a dc voltage that can be used in a relay or other circuit to perform any desired function. The voltage is zero when carrier current is not being received.

Line trap unit is inserted between the bus-bar and connection of coupling capacitor to the line. It is a parallel LC network tuned to resonance at the high frequency. It hence presents a high impedance to the high frequency carrier current, but a relatively low impedance (less than 0.1 Ω) to the power frequency (50 Hz) current. Traps are employed to confine the carrier currents to the protected section so as to avoid interference with or from other adjacent carrier current channels, and also to avoid loss of the carrier current signal in adjoining power circuits for any reason whatsoever, external short circuit being a principal reason. Consequently, carrier current can flow only along the line section between the traps.

The coupling capacitor (CC) connects the high frequency (carrier) equipment to one of the line conductors and simultaneously serves to isolate the carrier equipment from the high power line voltage. It presents a relatively low reactance to the high frequency currents (about 150 Ω at 500 kHz) and a high reactance to the power frequency (about 1.5 MΩ at 50 Hz). To reduce impedance further a low inductance is connected in series with the coupling capacitors to provide a resonance at carrier frequency.

It is thus evident that the commercial frequency current will be able to flow only through the line conductors, while the high frequency carrier current will circulate, when the receiver-transmitter operate, over the line conductor fitted with the high frequency traps, through the coupling capacitors and through ground (the return conductor).

Advantages of Carrier Current Protection

Carrier current over the power line provides simultaneous tripping of circuit breakers at both ends of the line in one to three cycles. Thereby high speed fault clearing is obtained, which improves the power system stability. Besides there are several other advantages of carrier current protection. These are:

1. Fast, simultaneous operation of circuit breakers at both ends.
2. Autoreclosing simultaneous reclosing signal is sent thereby simultaneous (1 to 3 cycles) reclosing of circuit breaker is obtained.
3. Fast clearing prevents shocks to systems.
4. Tripping due to synchronising power surges does not occur, yet during internal fault clearing is obtained.
5. For simultaneous faults, carrier current protection provides easy discrimination.
6. Carrier current relaying is best suited for fast relaying in conjunction with modern fast circuit breakers.
7. No separate wires are required for signalling, as the power lines themselves carry power as well as communication Hence the capital and operating costs are smaller.

The main application of power line carrier has been for the purpose of supervisory control, telephone communication, telemetering and relaying.