Different protective schemes and the choice
The overhead transmission and distribution lines are more prone to faults because of their length and exposure to atmospheric conditions. There are several protective schemes for the protection of these lines and feeders viz. over-current protection, distance protection and pilot protection. The choice of a particular protection scheme depends upon the cost of the scheme, type of the feeder, length of the feeder, method of operations etc.
Pilot relaying protection
Pilot relaying protection is a form of unit protection used for the protection of transmission line sections. In these protection schemes some electrical quantities such as the phase angle of current, direction of power flow etc. at the two ends of the transmission lines are compared. Some form of interconnecting channel, called pilot, is required to transmit information from one end to the other. The three different types of interconnecting channel or pilots used are wire pilot, carrier-current pilot, and microwave pilot.
For long overhead lines the power line itself may be used as the interconnecting channel between the terminal equipments. Carrier-current protection is the most widely used scheme for the protection of Extra High Voltage (EHV) and Ultra High Voltage (UHV) power lines. The carrier signal is directly coupled to the power line itself which is to be protected. Carrier-current protection is faster and superior to distance protection schemes and is more reliable when used for long transmission lines, although the terminal equipments are more expensive and complicated. In addition to protection the carrier signals can also be used for communication, supervisory control and telemetering.
In carrier-current protection or any other unit protection, the circuit breakers at both the ends of the line trip simultaneously when a fault occurs at one of the ends of the protected line sections. This helps in improving the stability. The carrier signals can be used either to initiate or to prevent the tripping of a protective relay according to which they are classified. When a carrier signal is used to initiate tripping of relay, the scheme is known as carrier inter-tripping, or transfer tripping or permissive tripping scheme. The scheme is known as carrier-blocking scheme when the carrier signals are used to prevent the operation of a relay.
Different operating techniques used in carrier-current protection
The two operating techniques mainly used in carrier-current protection are:
1. Phase comparison technique, and
2. Directional comparison technique.
In phase comparison technique, 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 section. During normal operating conditions or in case of an external fault, the currents at both the ends of the protected line are in phase. In case of an internal fault i.e. fault in the protected section, the currents at the two ends will be 180o out of phase.
The direction of power flow at the two ends of the protected sections is compared in the directional comparison technique. During normal conditions or external faults, the power flows into the protected section at one end and leaves at the other end. During internal faults, the direction of power flow is inwards at both the ends.
The signals generated in a carrier-current protection scheme are at a frequency between 50 and 500 kHz. Below 50 kHz the size and cost of the coupling equipments would be too high and above 500 kHz the line losses and therefore the signal attenuation would be too high on long lines. Carrier-current protection can be used only on overhead lines and cannot be used for underground cables as the capacitance of a cable would attenuate the carrier signals appreciably.