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This is the first part in the series of “Types of Relays.” This video series will focus on The most common power system protection element which is ‘Relay’ that are used in the Electrical Power System.
In this Part we will introduce you to the Design and working Mechanism of a Relay based on protection system.
Now What is a Relay?
It is a device that detects a fault and initiates the operation of the circuit breaker to isolate the defective part of the system.
The relays detect an abnormal condition in the electrical power system by constantly measuring the voltage and current through instrument transformers that are, the (CTs/PTs) which are different under normal and faulted conditions.
The electrical quantities which may change under faulted conditions are voltage, current, frequency and phase angle…. for example if there is a short circuit, a very high current will flow in the line or if there is a voltage sag, line voltage will drop…. Now, through the changes of one or more of these quantities, the protective relays are able to detect the abnormality (typically short circuits) and their presence, type, and location, and other very useful information. Having detected a fault, through the instrument transformer, the relay operates “enable” the trip circuit of the circuit breaker. This results in the opening of the breaker and disconnecting of the faulty segment of the line.
Now, Let’s move forward to see a typical relay protection scheme and discuss how it works”. This diagram shows a 3-phase system with a single line for simplicity. And we can divide this protection scheme into three parts:
- The First part is the primary winding of a current transformer or a (C.T.) which measures the line current.
- The Second part consists of secondary coil of C.T and the relay operating coil.
- Third part is the tripping circuit which may be either a.c. or d.c. It consists of a source of supply, the trip coil of the circuit breaker and the relay stationary contacts.
Now, It is may be noted here that the tripping coil of the breaker is solely responsible for the tripping the circuit breaker. Now, If the trip coil of the circuit breaker fails, then tripping will no longer take place. This the reason, two trip coils are normally provided in the circuit breaker.
The relay is connected to the CT. A typical CT is basically a hollow core through which the wire whose current is to measured is passed. The wire is known as the primary winding of the CT. When this current carrying wire is passed through a hollow core, magnetic flux is produced in it. The hollow core of the CT is wrapped around with a winding that is known as the secondary winding or coil of the CT. When the flux is produced in this core, it interacts with the secondary winding and as per Faraday’s law, and a current is produced in this secondary winding. This secondary winding is connected with an ammeter through which the current is measured.
When a fault occurs at point F on the transmission line, the current flowing in the line increases to very high value. It could be 2 times the maximum load current and up to 20 times or greater in certain cases.
It is not possible for a relay to measure currents and voltages of such high magnitudes due to its low pickup ratings. That’s why the relay is connected to the CT and senses the input quantity before sending the tripping signal to the CB which are measured by instrument current transformers. That is why the relay is connected to the cts or pts and senses the input quantity before sending the tripping signal to the circuit breaker. This results in a heavy current flow through the relay coil, causing the relay to operate by closing its contacts.
This in turn operates the trip circuit of the CB, causing it to open and isolate the faulty section of the power line from the rest of the system. In this way, the relay ensures the safety of the equipment from damage by isolating the faulted part and stability of the overall system which continue to operate in normal condition. If the faulty section is not cleared quickly (typically measured in cycles or 1/60th of a second) then the chances of cascading outages increases which is detrimental to the power system.
That’s all from Part 1a of this series. In the next part we will talk about some of the basic design requirements that should be considered when designing a protective relay
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