Power System 1 EPO520
Chapter 4 – Power System Protection (Distance)
Impedance Relays
Commonly used to protect buses and transmission lines.
These relays operate based on voltage-to-current ratio, which in turn produces impedance.
o It is also called a distance relay because the impedance is proportional to the distance in
transmission lines.
o The reach of an impedance relay denotes how far down the line the relay detects fault.
o For example, an 80% reach means that the relay will detect any (solid three phase) fault
between the relay and 80% of the line length.
Impedance relay block and trip regions are shown in Figure 10.28.
o The relay trips for │Z│<│Zr│
Where Z is the voltage-to-current ratio at the relay location
Zr is an adjustable relay setting.
Figure 10.28: Impedance relay block and trip region
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Power System 1 EPO520
Consider an impedance relay for breaker B12 in Figure 10.27, for which Z = V1 / I12
Figure 10.27: 345kV transmission loop
o During normal operation,
Load currents are usually much smaller than fault currents, and the ratio Z has a large
magnitude.
Therefore Z will lie outside the circle of Figure 10.28, and the relay will not trip during
normal operation.
o During a three-phase fault at P1, however,
Z appears to relay B12 to be the line impedance from the B12 relay to the fault.
If │Zr│ in Figure 10.28 is set to be larger than the magnitude of this impedance, then
the B12 relay will trip.
o Also, during a three-phase fault at P3,
Z appears to relay B12 to be the negative of the line impedance from the relay to the
fault
If │Zr│ is larger than the magnitude of this impedance, the B12 relay will trip.
o Thus, the impedance relay of Figure 10.28 is not directional; because a fault to the left or
right of the relay can cause a trip.
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Power System 1 EPO520
Two ways to include directional capability with an impedance relay are shown in Figure 10.29.
o In Figure 10.29 a), an impedance relay with directional restraint is obtained by including a
directional relay in series with an impedance relay
o In Figure 10.29 b), a modified impedance relay is obtained by offsetting the centre of the
impedance circle from the origin. This modified impedance relay is called a Mho relay.
o If either of these relays is used at B12 in Figure 10.27, a fault at P1 will result in a trip
decision, but a fault at P3 will result in a block decision.
a) Impedance relay with directional restraint b) Modified impedance relay (mho relay)
Figure 10.29: Impedance relay with directional capability
It is common practice to use three directional impedance relays per phase, with increasing
reaches and longer time delays.
For example, Figure 10.27 shows three protection zones for B12.
o Zone 1 relay is typically set for an 80% reach and instantaneous operation, in order to
provide primary protection for line 1-2.
o Zone 2 relay is set for about 120% reach, extending beyond bus 2, with a typical time delay
of 0.2 to 0.3 seconds.
o Zone 2 relay provides backup protection for faults on line 1-2 as well as remote backup for
faults on line 2-3 or 2-4 in zone 2.
o Reach for the zone 3 B12 relay is typically set to extend beyond buses 3 and 4 in order to
provide remote backup for neighbouring lines.
o Therefore, zone 3 reach is set for 100% of line 1-2 plus 120% of either line 2-3 or 2-4
whichever is longer, with an even larger time delay, typically one second
D. Johari, FKE UiTM 3
Power System 1 EPO520
Typical block and trip regions are shown in Figure 10.30 for both types of three-zone,
directional impedance relays.
a) Impedance relay with directional restraint b) Modified impedance relay (mho relay)
Figure 10.30: Three-zone, directional relay
Relay connections for a three-zone impedance relay with directional restraint are shown in
Figure 10.31.
Figure 10.31: Relay connections for a three zone directional impedance relay
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Power System 1 EPO520
Example 8 (Glover 10.8)
Table 10.8 gives positive sequence line impedances as well as CT and VT ratios at B12 for the
345kV system shown in Figure 10.27.
a) Determine the settings Zr1, Zr2 and Zr3 for the B12 three-zone, directional impedance relays
connected as shown in Figure 10.31. Consider only solid, three-phase faults.
b) Maximum current for line 1-2 during emergency loading conditions is 1500 A at a power
factor of 0.95 lagging. Verify that B12 does not trip during normal and emergency loadings.
Line Positive Sequence Impedance
1-2 8 + j50
2-3 8 + j50
2-4 5.3 + j33
1-3 4.3 + j27
Breaker CT Ratio VT Ratio
B12 1500:5 3000:1
Table 10.28: Data for Example 8
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Power System 1 EPO520
Solution for Example 8
a) Denoting V LN as the line to neutral voltage at bus 1 and IL as the line current through B12,
the primary impedance Z viewed at B12 is
V LN
Z
IL
Using the CT and VT ratios given in Table 10.8, the secondary impedance viewed by the
B12 impedance relays is
3000
VLN
Z ' 1 Z
1500 10
IL
5
We set the B12 zone 1 relay for 80% reach, that is, 80% of the line 1-2 (secondary)
impedance:
Z r1 0.80(8 j 50) / 10 0.64 j 4 4.0580.9 Secondary
Setting the B12 zone 2 relay for 120% reach:
Z r 2 1.2(8 j 50) / 10 0.96 j 6 6.0880.9 Secondary
From Table 10.8, line 2-3 has larger impedance than line 2-4. Therefore, we set the B12
zone 3 relay for 100% reach of line 1-2 plus 120% reach of line 2-4.
Z r 3 1.0(8 j 50) / 10 1.25.3 j 33 / 10
1.44 j8.96 9.0780.9 Secondary
b) The secondary impedance viewed by B12 during emergency loading, using
V LN 345 / 3 0 199 . 2 0 kV and
I L 1500 cos 1 0.95 1500 18.19 A , is
199.2 10 3
Z ' Z 10 10 13.2818.19 Secondary
1500 18.19
Since this impedance exceeds the zone 3 setting of 9.0780.9ºΩ, the impedance during
emergency loading lies outside the trip regions of the three zone, directional impedance
relay.
D. Johari, FKE UiTM 6
