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Directional Relay
Directional Control
The considerations discussed in previous section have all been related to
obtaining discriminative operation of relays associated with a simple radial
feeder
...
To
remove this failing it is necessary to feed power to both ends of the line as
shown in figure below
...
)
It is now necessary to insert circuit breakers at each end of each line section;
isolation of a section can then be achieved, leaving all substations connected
to one supply point or the other, and no loads are lost
...
In fact the arrangement may be regarded as the equivalent of two radial
feeders fed, respectively, from either supply point the two 'component feeders'
being superimposed in the actual line
It is necessary that relays shall operate only with current supplied from the
station which is the normal feed for the 'component feeder' to which they
belong
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(Definite-time protection of margin of 0
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A suitable reference is the system voltage
...
The induction relay is by far the
most widely used
...
A line through the phasor origin, perpendicular to the polarising voltage
phasor will divide the diagram into nominal operation and nonoperation
regions
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It is not given a calibrated setting, but
typical relays will operate with 1 -2% of rated power
...
Since the system voltage may fall to a low value during a short circuit, directional
relays should retain their directional properties down to a low voltage, typically 1 2% of rated value
...
Connections for directional phase-fault relays
Although wattmetric in principle, if directional relays responded to the actual
fault power, their operation would not always be satisfactory
...
This is, however, not the only issue
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For a single-phase
fault, it is the voltage across the short-circuited points which is collapsed
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At the
fault location the two phase phasors will coincide, with zero voltage across the
fault and half the initial phase voltage to earth
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The large degree of unbalance in currents and voltages would cause the
torque developed by wattmetric elements connected to each phase to vary
widely and even be of opposite sign as will be seen from the figure
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