Carrier Current Protection of Transmission Lines

Carrier current protection is mostly preferred protection scheme for long transmission lines because it is more reliable, cheaper, and provides protection at a faster rate. In this protection scheme, the transmission line serves two purposes, it transmits the power and at the same time, it acts as the medium for carrying information from one to the other end of the line i.e., the power lines themselves carry power as well as communication signals.


The carrier signal is produced when a voltage of positive polarity is influenced on the control circuit of a transmitter, which consists of an electronic oscillator and amplifier with an output power of about 15 to 20 watts at a high frequency of about 50 to 500 kHz. At the two ends of the protected section, the transmitter and receiver are placed and are tuned to the same frequency. So that each receiver responds to local as well as far-end transmitters.


The carrier signal which is responsible for performing both initiating and preventing tripping operations of the relay is directly fed into the power line circuit which is to be protected. The line traps and coupling capacitors are connected at both ends of the protected line.


Line traps are used to restrict the carrier currents within the protected section avoiding interference with or from the adjacent carrier current channels. Coupling capacitors provide an impedance (reactance) of low value to the high-frequency currents. They can also be used as potential dividers to supply reduced voltage to instruments.


Methods of Carrier Current Protection :

Carrier current protection can be performed by the following two techniques. They are,
  • Directional comparison protection, and
  • Phase comparison protection.

Directional Comparison Protection :

In this protection scheme, the fault in the protected section can be detected by comparing the power flow direction at both ends of the line. Modern directional comparison relaying scheme operates in conjunction with distance relays that acts as backup protection. The below shows the condition for external and internal faults.

Carrier Current Protection of Transmission Lines

The relay will operate when the direction of fault power is in the tripping direction. On the occurrence of a fault in the protected section, the flow of fault power will be in the tripping direction of the relays (i.e., away from the bus) on both sides of the protected section. Whereas in the case of external faults, power flow will be in opposite direction.


In this scheme, the pilot carrier signal informs the equipments at one end of the line as to how a directional unit at the other end responds to a short circuit. The pilot protection scheme used for the protection of transmission can be of two types,

  • Carrier Blocking Protection Scheme - In this scheme, the relay operation is restricted by the presence of a carrier signal. Carrier is therefore transmitted only upon the occurrence of a fault and is used to prevent tripping in the event of an external fault.
  • Carrier Permitting Blocking Scheme - Here the presence of the carrier initiates the tripping of the relay.

Phase Comparison Carrier Protection :

The phase comparison method operates on the principle of phase angle comparison. The phase angle of the current entering at one end is compared with the phase angle of the current leaving the other end of the protected line section. It should be observed that the magnitudes of the currents are not compared.


When current is flowing in the feeder under normal and external fault conditions, the current at both ends of the transmission line will be in phase. When there is a fault in the protected section, the currents at both ends will be 180° out of phase. The schematic diagram of the application of the phase comparison carrier protection scheme applied to a two-terminal transmission line is as shown below.

Carrier Current Protection of Transmission Lines

Current transformers at both ends of the transmission line feed the sequence network by which the output current of the current transformer is transformed into single-phase sinusoidal output voltage which is applied to both the transmitter and comparer. The comparer is also fed with the output of the receiver. The tripping relay operation is controlled by the comparer. The carrier current signal is then transmitted and received for comparing the phase positions of the currents at both ends of the protected section.


Advantages of Carrier Current Protection :

The advantages of carrier current protection over other schemes are listed below.
  • In this type of protection, the circuit breakers at both ends of the line reclose at a faster rate than in any other type of protection. This helps in quickly removing the faulted section from the power system due to which occurrence of shocks to the system is prevented.
  • In this type of protection scheme, the power line itself is used as the channel for carrying information between the two ends of the transmission line, since the power line themselves carry power as well as communicating signals. This eliminates the use of separate wires required for signaling and reduces the cost of the scheme.
  • For applications where fast relaying is required, a carrier current protection scheme in conjunction with modern fast circuit breakers is used.
  • This type of protection provides easy discrimination in case of simultaneous faults.
  • High-speed fault clearing is obtained through carrier current protection which improves the stability of the power system.

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