This video was brought to you by GeneralPAC.com, making power systems Intuitive, Open and Free for Everyone,
Everywhere. Consider subscribing and supporting through patreon.com/GeneralPAC. This is a mechanism for you to support us
financially so we can continue making high quality power system video tutorials. Our corporate sponsor for this topic
is AllumiaX.com from Seattle, Washington. Contact them for industrial and commercial power system studies.
Types of Protective Relays and Design Requirements. PART 1A: INTRODUCTION TO RELAYS:
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 coil, 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 CTs or PTs and senses the
input quantity before sending a tripping signal to the CB. 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. 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
consider subscribing to generalpac.com or becoming a patron on patreon.com. GeneralPAC.com making power systems
intuitive, open and free to everyone, everywhere, Thank you.