Relay Co-ordination and Discrimination

Presentation by Group 6

October 2024
Relay discrimination

▶ Discrimination is the ability of protective devices to

distinguish between faults within their protection zones and
faults outside their zones.
▶ This ensures that only the device protecting the faulty section
of the system trips, avoiding unnecessary disconnection of
healthy sections.
Overcurrent Coordination

▶ Overcurrent Coordination refers to the systematic application

of current-actuated protective devices in the electrical power
system, which, in response to a fault or overload, will remove
only a minimum amount of equipment from service.
▶ The coordination study of an electric power system consists of
an organized time-current study of all devices in series from
the utilization device to the source.
▶ This study is a comparison of the time it takes the individual
devices to operate when certain levels of normal or abnormal
current pass through the protective devices
Why Coordination

▶ To isolate only the faulty circuit or apparatus from the system.

▶ To prevent tripping of healthy circuits or apparatus adjoining
the faulted circuit or apparatus.
▶ To prevent undesirable tripping of other healthy circuits or
apparatus elsewhere in the system when a fault occurs
somewhere else in the system.
▶ To protect other healthy circuits and apparatus in the
adjoining system when a faulted circuit or apparatus is not
cleared by its own protection system.
Co-ordination Procedure

▶ Information required to achieve proper co-ordination are;

1. A single line diagram of the power system.
2. The impedance of transformers, feeders, motors etc. in Ohms/
p.u. or % ohms.
3. The maximum peak load current in feeders and full load
current of transformers etc, with permissible overloads.
4. The maximum and minimum values of short circuit currents
that are expected to flow.
5. The type and rating of the protective devices and their
associated protective transformers.
6. Performance curves or characteristic curves of relays and
associated protective transformers.
Protection Co-ordination Principles

▶ Whenever and wherever possible, use relays with the same

characteristics in series with each other.
▶ Set the relay farthest from the source at minimum current
settings.
▶ For succeeding relays approaching the source, increase the
current setting or retain the same current setting.
▶ Instantaneous units should be set so they do not trip for fault
levels equal or lower to those at bus bars or elements
protected by downstream instantaneous relays.
▶ The maximum and minimum values of short circuit currents
that are expected to flow.
▶ Time-delay units should be set to clear faults in a selective
and reliable way, assuring the proper coverage of the thermal
limits of the elements protected.
Relay Co-ordination Methods
▶ Discrimination by time
▶ Discrimination by current
▶ Discrimination by both time and current

The common aim of all three methods is to give correct

discrimination i.e. each one must isolate only the faulty
section of the power system network, leaving the rest of the
system undisturbed.
Discrimination by Time

▶ In this method, selectivity is achieved by introducing time

intervals for the relays.
▶ The operating time of the relay is increased from the farthest
side towards the generating source. This is achieved with the
help of definite time delay over current relays.
▶ When the number of relays in series increases, the operating
time increases towards the source.
Disadvantage

▶ Longest clearing time for faults closest to the source

Discrimination by current

▶ The principle is based on the fact that the fault current varies
with the position of the fault because of the difference in
impedance values between the source and the fault.
▶ The relays are set to pick up at progressively higher currents
towards the source.
▶ The current grading is achieved by high set over current relays
and different current tap positions in the over current relays.
Illustration of current grading
Limitations of the Current Graded System

▶ The magnitude of the fault current cannot be accurately

discriminated since all the circuit parameters may not be
known exactly and accurately.
▶ There may be variations in the fault level depending upon the
source generation, thereby necessitating the frequent change
in the settings of the relay.
▶ The discrimination by current alone is not a practical
proposition for exact grading, thus may be used with a time
graded system.
Discrimination by both time and Current

▶ Due to limitations of either current or time graded protection,

time and current grading is employed.
▶ Over current relays with inverse time characteristics are used
to achieve co-ordination.
▶ In such relays the time of operation is inversely proportional to
the fault current level and the actual characteristics is a
function of both time and current settings.
▶ The most widely used is the IDMT characteristic where
grading is possible over a wide range of currents and the relay
can be set to any value of definite minimum time required.
Continuation

▶ There are other inverse relay characteristics such as very

inverse and extremely inverse which are also sometimes
employed.
▶ If the fault current reduces substantially as the fault position
moves away from the source, very inverse or extremely inverse
relays are used instead of IDMT relays.
Inverse Definite Minimum Time (IDMT) Operating
Characteristic
Time and Current Graded System
Factors that Determine Coordination Time Interval

▶ Circuit Breaker Interruption Time:

The total time elapsed from when the circuit breaker contacts
open until the arc is extinguished and the relay is energized.
Modern circuit breakers in EHV ranges have operating times
of 3 to 5 cycles, while those in HV and MV ranges can take
up to 8 cycles.
▶ Overshoot:
A brief continuation of the circuit breaker’s operation after
the relay is de-energized. This is due to the dissipation of
stored energy. Predominant in electromagnetic relays but not
in static relays.
Continuation

▶ Allowance for Errors:

All devices involved in protection, such as relays and current
transformers (CTs), have inherent errors. Relay grading must
account for these errors by assuming the accuracy of the
measuring devices or by incorporating a margin for error.
▶ Safety Margin:
An intentional allowance introduced to compensate for errors
and delays in breaker operating time. The setting for the over
current relay (OCR) can be as high as 150 to 200% of full
load current or 130%, depending on the station philosophy.