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.
Carrier-current
protection
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.
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