Carrier-Current Protection of Transmission Lines | Electrical Engineering

In this article we will discuss about:- 1. Introduction to Carrier-Current Protection 2. Methods of Carrier-Current Protection 3. Advantages.

Introduction to Carrier-Current Protection:

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. Notwithstanding this, owing to the need for installing interconnecting conductors (cables), 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 protections. The essential difference between carrier current protection 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 compare 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 protection scheme quite expensive and justifies its application only in transmission lines of 110kV and above.

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.

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 high impedance to the high-frequency carrier current, but 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 M Ω 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).

Methods of Carrier-Current Protection:

There are different methods of carrier current protection and basic forms of carrier protection are:

(i) Directional Comparison Protection and

(ii) Phase-Comparison Carrier Protection.

(i) Directional Comparison Protection:

The protection operates on the basis of comparison of the fault-power flow directions at the two ends of the protected line. Operation takes place only when the flow of power at both ends of the line is in the bus-to-line direction, a condition which will evidently only arise in event of a fault on the protected section of the line. With directional- comparison relaying, the carrier pilot informs the equipment at one end of the line how a directional relay at the other end responds to a short circuit.

The conditions for internal and external faults are illustrated in Fig. 5.17. The relays at both ends of the protected section respond to fault power flowing away from the bus (tripping direction). For faults in the protected section, power flows in the tripping direction at both ends. For external faults power flow will be in opposite directions. A simple signal through carrier pilot is transmitted from one end to the other during faults.

The pilot scheme can be employed for transmitting either blocking, or permitting signals. Thus possible carrier protections are of two types viz., carrier-blocking scheme and carrier-permitting scheme.

In a carrier-blocking protection scheme, the presence of carrier prevents or blocks operation of the protection. Carrier is, therefore, transmitted only upon the occurrence of a fault and is employed to prevent tripping in the event of an external fault. In carrier-permitting scheme the presence of carrier permits operation of the protection. The carrier blocking scheme is more reliable than carrier- permitting scheme. This is because a failure in the carrier-permitting signal equipment will mean a failure in isolating the fault, whereas a failure in carrier-blocking signal equipment isolates the section on which no fault exists. However, such false operation is preferable to the failure to clear a faulted section.

In a carrier-permitting protection scheme, normally no pilot signal is transmitted from any terminal. Should a short circuit occur in an immediately adjacent line section, a pilot signal is transmitted from any terminal where short-circuit current flows out of the line (i.e., in the non-tripping direction). While any station is transmitting a pilot signal, tripping is blocked at all other stations. But should a short circuit occur on the protected section of the line, no pilot signal is transmitted and tripping occurs at any terminal where short-circuit current flows. Therefore, the pilot is blocking pilot, since the reception of a pilot signal is not required of permit tripping.

Directional comparison protection scheme (carrier blocking type) is illustrated in Fig. 5.18. The operation of the directional element provided on each breaker is indicated by the arrow arid the non- operation by the letter O. Occurrence of fault activates relays on each of the breakers near the fault. This relay unless blocked from operation, causes tripping of breakers. The blocking signal is controlled by the directional relays on each breaker, and is transmitted from one end of a protected section to the other by carrier.

If a directional element determines that the fault is external to the protected section, a signal is transmitted blocking the operation of breakers at both ends of the section. In case the directional elements at both ends determine that fault is in the protected section, no blocking signal is transmitted from either end, and both breakers trip. The sequence of event for a fault at F is made clear by illustration in Fig. 5.18.

At breaker 1 the directional element shows that the fault may be in the section 1-2. This breaker trips if no blocking signal is received. No blocking signal is transmitted to breaker 2. At breaker 2 the directional element shows that the fault is not in section 1-2. A carrier signal is transmitted that blocks tripping of both the breakers 1 and 2.

At breakers 3 and 4 the directional elements show that the fault may be in section 3-4. No blocking signal is transmitted and after a very short time delay (1 to 3 cycles), both the breakers 3 and 4 trip.

(ii) Phase-Comparison Carrier Protection:

Phase-comparison relaying equipment uses its pilot to compare the phase relation between current entering in the protected zone and the current leaving the protected zone. The current magnitudes are not compared. Phase comparison protection provides only main or primary protection, backup protection must be provided in addition.

Figure 5.19 shows schematically the principal elements of the equipment provided at both ends of a two-terminal transmission line, employing a carrier-current pilot. As in ac wire-pilot protection, the transmission-line CTs feed a network that transforms the CT output currents into a single-phase sinusoidal output voltage. This voltage is applied to a carrier-current transmitter and to a ‘comparer’.

The output of a carrier-current receiver is also applied to the comparer. The comparer controls the operation of an auxiliary relay for tripping the transmission-line circuit breaker. These elements provide means for transmitting and receiving carrier-current signals for comparing at each end of the relative phase relations of the transmission-line current at both ends of the line.

For examining the relations between the network output voltages at both ends of the line and also the carrier-current signals that are transmitted during external and internal fault conditions refer to Fig. 5.20. For an external fault at point D in Fig. 5.20, the network output voltages at stations A and B (waves a and c) are 180° out of phase.

This is because the connections of CTs at the two stations are reversed. Since an ac voltage is used to control the transmitter, carrier-current is transmitted only during the half cycles of the voltage wave when the polarity is positive. The carrier-current signals transmitted from stations A and B (waves b and d) are displaced in time, so that there is always a carrier-current signal being sent from one end or the other.

For internal fault at point C, the network output voltage at station B reverses because of reversal of power-line currents at station B, the carrier-current signals (waves b and d) are concurrent, and there is no signal from either station every other half cycle.

Phase-comparison relaying acts to block tripping at both terminals whenever the carrier- current signals are displaced in time so that there is little or no time interval when a signal is not being transmitted from one end or the other. When the carrier-current signals are approximately concurrent, tripping will occur whenever there is sufficient short-circuit current flowing. This is shown in Fig. 5.21 where network output voltages are superimposed, and the related tripping and blocking tendencies are illustrated.

As shown in Figs. 5.20 and 5.21, the equipment at one station transmits a blocking carrier-current signal during one half cycle, and then stops transmitting and tries to trip during the next half cycle, if carrier current is not received from the other end of the line during this half cycle, the equipment operates to trip its breaker. But, if carrier current is received from the other end of the line during the interval when the local carrier-current transmitter is idle, tripping does not occur.

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. Auto-reclosing 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 signalling. 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.