Protection: Objectives of distribution system protection Types of common faults and procedure for fault calculations Protective devices: Principle of operation of fuses Circuit reclosures Line sectionalizes and circuit breakers. Coordination: Coordination of protective devices: General coordination procedure Residual current circuit breaker RCCB (Wikipedia).

1. LENDI INSTITUTE OF ENGINEERING AND TECHNOLOGY
Jonnada, Andhra Pradesh- 535005
UNIT – 4
Protection & Coordination
Presented by
Dr. Rohit Babu, Associate Professor
Department of Electrical and Electronics Engineering

2. SYLLABUS
Department of Electrical and Electronics Engineering
Protection:
 Objectives of distribution system protection
 Types of common faults and procedure for fault calculations
 Protective devices: Principle of operation of fuses Circuit reclosures
 Line sectionalizes and circuit breakers.
Coordination:
 Coordination of protective devices: General coordination procedure
 Residual current circuit breaker RCCB (Wikipedia).

3. Coordination of protective devices
Department of Electrical and Electronics Engineering
• The process of selecting overcurrent protection devices with certain time–current settings
and their appropriate arrangement in series along a distribution circuit in order to clear
faults from the lines and apparatus according to a preset sequence of operation is known
as coordination.
• When two protective apparatus installed in series have characteristics that provide a
specified operating sequence, they are said to be coordinated or selective.
• The apparatus that furnishes backup protection but operates only when the protecting
device fails to operate to clear the fault is defined as the protected device.

4. Coordination of protective devices Contd.
Department of Electrical and Electronics Engineering
Properly coordinated protective devices help
(1) to eliminate service interruptions due to temporary faults,
(2) to minimize the extent of faults in order to reduce the number of customers affected
(3) to locate the fault, thereby minimizing the duration of service outages.

5. Coordination of protective devices Contd.
Department of Electrical and Electronics Engineering
Since coordination is primarily the selection of protective devices and their settings to develop
zones that provide temporary fault protection and limit an outage area to the minimum size
possible if a fault is permanent, to coordinate protective devices, in general, the distribution
engineer must assemble the following data:
1. Scaled feeder-circuit configuration diagram (map)
2. Locations of the existing protective devices
3. TCC curves of protective devices
4. Load currents (under normal and emergency conditions)
5. Fault currents or megavoltamperes (under minimum and maximum generation
conditions) at every point where a protective apparatus might be located

6. Coordination of protective devices Contd.
Department of Electrical and Electronics Engineering
• Usually, these data are not readily available and therefore must be brought together from
numerous sources.
• For example,
the Time-current characteristic curves (TCCs) of protective devices are gathered from the
manufacturers;
the values of the load currents and fault currents are usually taken from computer runs
called the load flow (or more correctly, power flow) studies and fault studies, respectively.

7. Coordination of protective devices:
General Procedure
Department of Electrical and Electronics Engineering
A general coordination procedure, regardless of whether it is manual or computerized, can be
summarized as follows:
1. Gather the required and aforementioned data.
2. Select initial locations on the given distribution circuit for protective (i.e., sectionalizing) devices.
3. Determine the maximum and minimum values of fault currents (specifically for three-phase, line-
to-line [L–L], and line-to-ground faults) at each of the selected locations and at the end of the feeder
mains, branches, and laterals.
4. Pick out the necessary protective devices located at the distribution substation in order to protect
the substation transformer properly from any fault that might occur in the distribution circuit.
5. Coordinate the protective devices from the substation outward or from the end of the distribution
circuit back to the substation.
6. Reconsider and change, if necessary, the initial locations of the protective devices.

8. Coordination of protective devices:
General Procedure
Department of Electrical and Electronics Engineering
7. Reexamine the chosen protective devices for current-carrying capacity, interrupting capacity, and
minimum pickup rating.
8. Draw a composite TCC curve showing the coordination of all protective devices employed, with
curves drawn for a common base voltage (this step is optional).
9. Draw a circuit diagram that shows the circuit configuration, the maximum and minimum values of
the fault currents, and the ratings of the protective devices employed.

9. Coordination of protective devices:
General Procedure
Department of Electrical and Electronics Engineering
There are also some additional factors that need to be considered in the coordination of protective
devices (i.e., fuses, reclosers, and relays) such as
(1) the differences in the TCCs and related manufacturing tolerances,
(2) preloading conditions of the apparatus,
(3) ambient temperature, and
(4) effect of reclosing cycles.
These factors affect the adequate margin for selectivity under adverse conditions.

10. Coordination of protective devices:
Types of co-ordination
Department of Electrical and Electronics Engineering
1. Fuse-to-Fuse Coordination
2. Recloser-to-Recloser Coordination
3. Recloser-to-Fuse Coordination
4. Recloser-to-Substation Transformer
5. Fuse-to-Circuit-Breaker Coordination
6. Recloser-to-Circuit-Breaker Coordination

11. Fuse-to-Fuse Coordination
Department of Electrical and Electronics Engineering
Figure 1. Coordinating fuses in series using TCC curves of the
fuses connected in series.
• Fuse A is called protected fuse.
• Fuse B is called protecting fuse.
• For perfect coordination fuse B must melt And clear the
fault before fuse A
The selection of a fuse rating to provide adequate
protection to the circuit beyond its location is based upon
several factors.
A fuse is designed to blow within a specified time for a
given value of fault current.

12. Recloser-to-Recloser Coordination
Department of Electrical and Electronics Engineering
The need for recloser-to-recloser coordination may arise due to any of the following situations that
may exist in a given distribution system:
1. Having two three-phase reclosers
2. Having two single-phase reclosers
3. Having a three-phase recloser at the substation and a single-phase recloser on one of the branches
of a given feeder
The required coordination between the reclosers can be achieved by using one of the following
remedies:
1. Employing different recloser types and some mixture of coil sizes and operating sequences
2. Employing the same recloser type and operating sequence but using different coil sizes
3. Employing the same recloser type and coil sizes but using different operating sequences

13. Recloser-to-Fuse Coordination
Department of Electrical and Electronics Engineering
Figure 2. Typical recloser tripping characteristics.
In Figure, curves represent the instantaneous, time-delay,
and extended time-delay (as an alternative) tripping
characteristics of a conventional automatic circuit recloser.
Here, curves A and B symbolize the first and second
openings and the third and fourth openings of the
recloser, respectively.

14. Recloser-to-Fuse Coordination Contd.
Department of Electrical and Electronics Engineering
Figure 3. Recloser TCC curves superimposed on fuse
TCC curves. (From General Electric Company,
Distribution System Feeder Overcurrent Protection,
Application Manual GET-6450, 1979.)
Figure shows a portion of a distribution system where a
recloser is installed ahead of a fuse.
The figure also shows the superposition of the TCC curve
of the fuse C on the fast and delayed TCC curves of the
recloser R.
If the fault beyond fuse C is temporary, the instantaneous
tripping operations of the recloser protect the fuse from
any damage.
If the fault beyond fuse C is a permanent one, the fuse
clears the fault as the recloser goes through a delayed
operation B.

15. Recloser-to-Fuse Coordination Contd.
Department of Electrical and Electronics Engineering
Figure 4. Temperature cycle of fuse link during
recloser operation.
Figure 4 illustrates the temperature cycle of a fuse link during
recloser operations.
As can be observed from the figure, each of the first two
(instantaneous) operations takes only 2 cycles, but each of the last
two (delayed) operations lasts 20 cycles. After the fourth operation,
the recloser locks itself open.
Figure 5. Recloser-to-fuse coordination
(corrected for heating and cooling cycle).
Figure 5 illustrates a practical
yet sufficiently accurate
method of coordination.

16. Recloser-to-Substation Transformer
Department of Electrical and Electronics Engineering
• Usually, a power fuse, located at the primary side of a delta–wye-connected substation transformer,
provides protection for the transformer against the faults in the transformer or at the transformer
terminals and also provides backup protection for feeder faults.
• These fuses have to be coordinated with the reclosers or reclosing circuit breakers located on the
secondary side of the transformer to prevent the fuse from any damage during the sequential
tripping operations.
• The effects of the accumulated heating and cooling of the fuse element can be taken into account by
adjusting the delayed-tripping time of the recloser.

17. Fuse-to-Circuit-Breaker Coordination
Department of Electrical and Electronics Engineering
• The fuse-to-circuit-breaker (overcurrent-relay) coordination is somewhat similar to the fuse-to-
recloser coordination.
• If the minimum-melting time of the fuse is approximately 135% of the combined time of the circuit
breaker and related relays, the coordination is achieved.
• In summary, when the circuit breaker is tripped instantaneously, it has to clear the fault before the
fuse is blown.
• Therefore, it is necessary that the relay characteristic curve, at all values of current up to the
maximum current available at the fuse location, lie above the total-clearing characteristic curve of
the fuse.
• A sectionalizing fuse installed at the riser pole to protect underground cables does not have to
coordinate with the instantaneous trips since underground lines are usually not subject to transient
faults.

18. Recloser-to-Circuit Breaker Coordination
Department of Electrical and Electronics Engineering
• The reclosing relay recloses its associated feeder-circuit breaker at predetermined intervals (e.g., 15,
30, or 45 s cycles) after the breaker has been tripped by overcurrent relays.
• If desired, the reclosing relay can provide an instantaneous initial reclosure plus three time-delay
reclosures.
• If the relay used is of an electromechanical type, rather than a solid-state type, it starts to travel in
the trip direction during the operation of the recloser.
• If the reset time of the relay is not adjusted properly, the relay can accumulate enough movement
(or travel) in the trip direction, during successive recloser operations, to trigger a false tripping.

19. Residual Current Circuit Breaker (RCCB)
Department of Electrical and Electronics Engineering
• A Residual Current Circuit Breaker
(RCCB) is an important safety measure
when it comes to protection of electrical
circuits.
• It is a current sensing device, which can
automatically measure and disconnect the
circuit whenever a fault occurs in the
connected circuit or the current exceeds
the rated sensitivity.

20. Residual Current Circuit Breaker (RCCB) Contd.
Department of Electrical and Electronics Engineering
Principle behind RCCB
• RCCB works on the principle of Kirchhoff’s law, which states that the incoming current must be
equal to the outgoing current in a circuit.
• RCCB thus compares the difference in current values between live and neutral wires.
Benefits of RCCB
• Provides protection against earth fault as well as any leakage current
• Automatically disconnects the circuit when the rated sensitivity is exceeded
• Offers possibility of dual termination both for cable and busbar connections
• Offers protection against voltage fluctuation as it includes a filtering device that guards against
transient voltage levels.

21. Residual Current Circuit Breaker (RCCB) Contd.
Department of Electrical and Electronics Engineering
Sensitivity of RCCB
• RCCBs are designed to look for small changes in residual current.
• In cases where protection from fire is sought, RCCBs are also used to track higher changes in
residual current of up to 300mA.
Classification of RCCB
RCCBs are of two types; the 2 Pole RCCB and 4 Pole RCCB.
• 2 Pole RCCB: This is used in case of a single-phase supply connection that has only a live and a
neutral wire.
• 4 Pole RCCB: This is used in case of a three-phase supply connection.
Rating from 10 Amp ….100 Amp
Sensitivity 30,100,300 m Amp

22. Residual Current Circuit Breaker (RCCB) Contd.
Department of Electrical and Electronics Engineering
Limitations of RCCB
While RCCB has many advantages, it has some limitations as well:
• RCCB does not guarantee to operate if none standard waveforms are generated by loads.
• There might be some unwanted tripping of RCCB.
• RCCB does not protect from current overload.
• RCCB does not protect against line-neutral shocks.
• RCCB does not protect from the overheating that strike if conductors are not properly screwed
into terminals.

23. Thank You