Overcurrent Protection

1. BEF43303
POWER SYSTEM ANALYSIS AND PROTECTION
WEEK 13
OVERCURRENT PROTECTION

2. 13.0 CONTENTS
13.1 Reclosers and fuses
13.2 Directional relays
13.3 Protection of two-sources system with directional relays

3. 13.1 RECLOSER AND FUSES
• Automatic circuit reclosers are commonly used for distribution
circuit protection.
• It is a self-controlled device for automatically interrupting and
reclosing an AC circuit with a preset sequence of openings and
reclosures.
• More than 80% of faults on overhead distribution circuits are
temporary.
• The automatic tripping-reclosing sequence clears these temporary
faults and restores service with only momentary outages.

4. 13.1 RECLOSER AND FUSES
Example 1
For the system of Figure 1, describe the operating sequence of the
protective devices for the following faults:
a) A self-clearing, temporary, three-phase fault on the load side of tap
2.
b) A permanent three-phase fault on the load side of tap 2.
The data for the system is given in Table 1 and the coordination
between the protective elements in the system is given in Figure 2.

5. 13.1 RECLOSER AND FUSES
Table 1 – Single line diagram of a 13.8kV radial distribution feeder with fuse,
recloser and relay protection

6. Table 1 – Data for Figure 1
13.1 RECLOSER AND FUSES

7. 13.1 RECLOSER AND FUSES
Table 2 – Time current curves for the radial distribution circuit

8. 13.1 RECLOSER AND FUSES
Example 2
Solution:
a) From Table 1, the three-phase fault current at bus 2 is 1500 A. From Figure 2, the
560A fast recloser opens 0.05s after the 1500-A fault current occurs, and then
recloses 1/2 s later. Assuming the fault has self-cleared, normal service is restored.
During the 0.05-s fault duration, the 100 T fuse does not melt.
b) For a permanent fault the fast recloser opens after 0.05 s, recloses 1/2s later into
the permanent fault, opens again after 1/2 s, and recloses into the fault a second
time after a 2-s delay. Then the 560-A delayed recloser opens 3 seconds later.
During this interval the 100 T fuse clears the fault. The delayed recloser then
recloses 5 to 10 s later, restoring service to loads 1 and 3.

9. 13.2 DIRECTIONAL RELAY
• Consider the directional relay
D in Figure 3.
• The contacts of the OC relay
and the D relay are connected
in series.
• This is to ensure that the
breaker trip coil is energized
only when the input current:
 Exceed the OC pickup value.
 In the forward tripping direction.
Figure 3 – Directional relay in series with overcurrent
relay

10. 13.2 DIRECTIONAL RELAY
• The directional relay has two
inputs:
 Reference voltage 𝑉𝑉 = 𝑉𝑉∠0°
 Current 𝐼𝐼 = 𝐼𝐼∠𝜙𝜙°
• The relay trip and block
regions, shown in Figure 4.
Figure 4 – Directional relay block and trip region in
complex plane

11. 13.3 PROTECTION OF TWO-SOURCES
SYSTEM WITH DIRECTIONAL RELAYS
• It becomes difficult to coordinate overcurrent relays when there are
two or more sources at different locations.
• Consider the system with two sources shown in Figure 5.

12. 13.3 PROTECTION OF TWO-SOURCES
SYSTEM WITH DIRECTIONAL RELAYS
• Suppose there is a fault at P1, B23 and B32 need to operate to clear
the fault so that service to the three loads continues without
interruption.
• Using time-delay overcurrent relays, set B23 faster than B21.
• Now consider a fault at P2 instead, B23 will open faster than B21,
and load L2 will be disconnected.
• When a fault can be fed from both the left and right, overcurrent
relays cannot be coordinated.
• However, directional relays can be used to overcome this problem.