INTRODUCTION TO RELAYS: PART 1B
*This is the Second part in the series of “Types of Protective Relays and Design Requirements”
In this part we will have a discussion on the factors that need to be considered when designing a relay based protection system. *
The purpose of protective relaying system is to rapidly remove any of the component of the power system when they starts to operate in an
abnormal way. To do this effectively and accurately, -- a protective relay scheme should be designed by considering the following
factors. We can refer to them as the relay design requirements similar to most textbooks and technical papers. These design
requirements are: Selectivity, Reliability, Sensitivity, Speed, Simplicity, Economy
*(So the first 1 is Selectivity)* Selectivity:
A well designed and efficient relay should be selective. For example it should be able to detect the point at which the fault occurs and
causes the circuit breaker that is closest to the fault to trip first.
So, let’s suppose we have a fault near point 3 as shown in this diagram and we have breakers and relays placed at several locations in the
network which are marked by numbers, so which breakers should trip for a fault near point 3?
In this case relay number 3 and 5 should trip first as opposed to 1, or 4 or 7 , if that happens the relay protection scheme is selective – which
means it causes the closest relay to trip without disturbing the remaining system, so that the fault is cleared with minimum system outage.
*(Now we move on to the next one which is Reliability)*
Reliability: A relaying system should be reliable, which means that the system should not operate unnecessarily.
Without reliability, the protection would be rendered largely ineffective and could even become a liability.
Now, Let’s suppose that generator 1 was running and circuit breaker #3 closed to synchronize with the grid. But immediately after
closing it tripped because a high current flowed due to energization of the transformer, and if that happens the scheme is not reliable. This is
just one example of reliability. There are multiple examples that can be given.
*(Number 3 is Sensitivity)* Sensitivity: It is the ability of the relay system to be sensitive enough to detect even a small
amount of fault current. It means that the relay is able to sense and respond to minimal changes in the measured
parameters and the Current is the most common measured parameter.
For example, let’s suppose insulation damage in the underground cable link after breaker 14 causes current leakage. A sensitive relay
should detect the current leakage which may eventually turn into a ground fault. If the protection system is able to detect the small
abnormality, then it is sensitive enough to function properly.
*(Number four is Speed)* Speed: To maximize safety, and minimize
equipment damage and system instability, a fault should be cleared as rapidly as possible. This indicates that a relay should quickly attain
a decision and the circuit breaker operation should be fast enough to isolate a fault.
If the relay is able to disconnect the faulty system as fast as possible, equipment damage that may result from flow of heavy fault current
can be prevented and further escalation of faults can be prevented.
Looking back at our previous network let’s suppose we have a fault in the transformer winding, relay 3 and 4 must be able to quickly
disconnect the system before the transformer catches fire and destroys itself completely.
*(The next one is Simplicity)* Simplicity: The relaying system should be
made simple so that it can be easily engineered and maintained. Reliability is closely related to simplicity. Generally speaking, the simpler the
protection scheme, the greater the reliability. A simple system is probably the one which
uses the least amount of relays to give optimal protection.
*(And the final one is Economy)* Economy: The most significant factor of a particular protection system is the economy.
Sometimes it is economically unjustified to use an ideal scheme of protection that protects all elements of the system with full reliability and a
compromise method has to be adopted. In our network we have a total of 15 relays that protects the system however it will be costly to
install and maintain all of 15 relays, hence a choice is to be made, we could easily remove relay No. 9 in favor of a tie switch and to
minimize the costs without compromising on protection reliability.
Else, relays 7 and 8 could be replaced with more advanced relays so that the need for a relay at point number 5 or 6 is eliminated. It is
the core job of a design engineer to reach an optimum point in the trade-off between costs and reliability.
However, when the apparatus to be protected is of utmost importance (e.g. generator, main transmission line etc), economic considerations
are generally subordinated to reliability. *That’s all from this part of the series*
*Now in the next part of this video series we will be discussing the most common different types of Relays that are used in
Electrical Power systems, their mechanism and their applications as well* We hope that you have a continued interest in this
topic and in this series as either a student or a professional. And we also hope that you find this content useful and enlightening, please
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