One of the
important protections in Power System Protection is Feeder Protection.
Different types of relays are used for feeder protection like electromagnetic type relays, static type relays etc.
But now a day we are using Numerical relays for all protections.
The benefits of Numerical relays are,
1.
Accurate
tripping,
2.
Less
tolerance,
3.
Fault events
and counter storage
4.
Display of
fault parameters on screen
(Fault
parameters means current, voltage, resistance and reactance values during fault and fault
distance, Numerical relays can store thousands of tripping events).
The main inputs needed for distance protection is Voltage and Current from corresponding feeder PT and CT.
According to site conditions we will set certain impedance values in relay settings (i.e., R and X values) for fault
detection. Relay will monitor Current and Voltage in the feeder line (PT and CT
secondary), and from these values, the relay will calculate Impedance value Z.
i.e., Z = V/I. In normal load condition the impedance values on line will be
high. But when fault comes on the feeder line, the impedance will decrease and
becomes less than the impedance setting in the relay, then the distance relay
will trip with in 40 ms in zone 1 (different zones are there and that will be
explained later) and isolate power equipments from fault. I.e., during fault,
relay will trip and show the fault parameters like fault current, voltage,
reactance, resistance and fault distance on relay screen.
Suppose if the
fault is on 25 km, then relay will show Fault Distance (FD) = 25km, and thus it
becomes easy to identify the location where there is fault. For Distance protection now a day Quadrilateral
characteristics is used. We already discussed that for fault identification we
have to set different parameters in relay. i.e.,
1.
Forward and
Backward Resistance (RF, RB),
2.
Forward and
backward Reactance (XF, XB),
3.
RCA (Relay
Characteristics Angle) and
4.
Line Impedance
per km.
These
parameters are used for making Quadrilateral characteristics. Suppose if RCA =
70o and by using
parallelogram characteristics (Quadrilateral), we can plot the graph by setting
Forward Resistance (RF) value in positive X
axis, Backward Resistance (RB) in negative X axis, Forward Reactance (XF) value in positive Y axis, and Backward Reactance
(XB) in negative Y axis and
plot parallelogram with a slope of RCA angle.
Thus we will get a parallelogram graph and the protection zone is inside the
parallelogram. Means during fault the impedance will reach inside the
parallelogram then the relay will trip. In graph there are 4 quadrants of
operation
- First quadrant (R and X values + ve) : If the load is inductive and the fault is in forward direction from Relay, then the relay will trip in this quadrant values.
- Second quadrant (R – ve and X + ve) : If the load is capacitive and the fault is in reverse direction from Relay, then the relay will trip in this quadrant values.
- Third quadrant (R – ve and X – ve : If the load is inductive and the fault is in reverse direction from Relay, then the relay will trip in this quadrant values.
- Fourth quadrant (R + ve and X – ve) : If the load is capacitive and the fault is in forward direction from Relay, then the relay will trip in this quadrant values.
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