This document discusses overcurrent protection and radial system protection. It describes different types of overcurrent relays, including instantaneous and time-delay relays. Instantaneous relays trip immediately when current exceeds the pickup setting, while time-delay relays introduce an intentional delay based on how many times the pickup current is exceeded. The document includes examples of selecting settings for time-delay relays in a radial power system to coordinate protection among circuit breakers while maintaining a minimum coordination time interval between devices.

1. BEF43303
POWER SYSTEM ANALYSIS AND PROTECTION
WEEK 12
OVERCURRENT PROTECTION

2. 12.0 CONTENTS
12.1 Overcurrent relays
12.2 Radial system protection

3. 12.1 OVERCURRENT RELAYS
• Figure 1 shows an overcurrent
protection scheme.
• It shows that the CT secondary
current 𝐼𝐼𝐼 is the input to the
overcurrent relay operating
coil.
Figure 1 – An overcurrent protection scheme

4. 12.1 OVERCURRENT RELAYS
• An instantaneous overcurrent
relays respond to the
magnitude of input current.
• Figure 2 shows the trip and
block regions of an
instantaneous overcurrent
relay.
• 𝐼𝐼𝐼 > 𝐼𝐼𝑝𝑝 (pickup current) →
relay contact close → CB trip
coil energized, instantaneously.
Figure 2 – An instantaneous overcurrent relay block
and trip regions

5. 12.1 OVERCURRENT RELAYS
• Time-delay overcurrent relays
respond to the magnitude of
input current, with an
intentional time delay.
• Figure 3 shows the
characteristics of this relay.
• For smaller multiples of
pickup, the relay trips after a
longer time delay.
Figure 3 – Time delay overcurrent relay block
and trip region

6. 12.1 OVERCURRENT RELAYS
• Typically, overcurrent relays have two settings:
 Current-tap setting: The pickup current in amperes.
 Time-dial setting: The adjustable amount of time delay.
• The characteristic curves are usually shown with the operating time
delay in second versus relay input current as a multiple of the
pickup current as shown in Figure 4.
• The inverse time characteristic can be shifted up and down by
adjusting the time-dial setting.

7. 12.1 OVERCURRENT RELAYS
Figure 4 – CO-8 time-delay overcurrent relay characteristic

8. 12.1 OVERCURRENT RELAYS
Example 1
The CO-8 relay with a current tap setting of 6 amperes and a time-dial
setting of 1 is used with the 100:5 CT. Determine the relay operating
time for each of the following cases.
a) 𝐼𝐼𝐼 = 5𝐴𝐴 (Ans.: Relay does not operate)
b) 𝐼𝐼𝐼 = 8𝐴𝐴 (Ans.: 6s)
c) 𝐼𝐼𝐼 = 15𝐴𝐴 (Ans.: 1.2s)

9. 12.1 OVERCURRENT RELAYS
• Figure 5 shows the time-
current characteristic of five
CO time delay overcurrent
relays.
• The differences depends on the
sensitivity of the relay towards
higher fault currents.
• The choice of relay time-
current characteristic depends
on the sources, lines and loads.
Figure 5 – Comparison of CO relay characteristics

10. 12.2 RADIAL SYSTEM PROTECTION
• Typically, radial system are protected by time-delay overcurrent
relays.
• Adjustable time delays are selected such that the breaker closest to
the fault opens, while other upstream breakers with larger time
delays remain closed.
• The relays are coordinated to operate in sequence so as to interrupt
minimum load during faults.
• Coordination of overcurrent relays usually limits the maximum
number of breakers in a radial system to five or less.

11. 12.2 RADIAL SYSTEM PROTECTION
• Consider a radial system as shown in Figure 6.
Figure 6 – Single line diagram of a 34.5kV radial system

12. 12.2 RADIAL SYSTEM PROTECTION
• The coordination time interval is the time interval between the
primary and remote backup protective device.
• Precise determination of relay operating time is difficult due to
several factors such as:
 CT errors.
 DC offset component of fault current.
 Relay overtravel.
• Thus, typical coordination time intervals from 0.2 to 0.5s are
selected to account for these factors.

13. 12.2 RADIAL SYSTEM PROTECTION
Example 2
Data for the 60-Hz radial system of Figure 6 are given in Tables 1, 2,
and 3. Select current tap settings (TSs) and time-dial settings (TDSs) to
protect the system from faults. Assume three CO-8 relays for each
breaker, one for each phase, with a 0.3s coordination time interval. The
relays for each breaker are connected as shown in Figure 7, so that all
three phases of the breaker open when a fault is detected on any one
phase. Assume a 34.5-kV (line-to-line) voltage at all buses during
normal operation.

14. 12.2 RADIAL SYSTEM PROTECTION
Example 2
Table 1 – Maximum loads
Table 2 – Symmetrical fault currents

15. 12.2 RADIAL SYSTEM PROTECTION
Example 2
Table 3 – CT and relay data

16. 12.2 RADIAL SYSTEM PROTECTION
Example 2
Figure 7 – Relay connections to trip all three phases