Brief Overview of Relay Operating Principles

8/13/2019 Brief Overview of Relay Operating Principles

1/19

[email protected]


2/19

Level Detection

Magnitude Comparison

Differential Comparison

Phase Angle Comparison Distance Measurement

Pilot Relaying

Harmonic Content

Frequency Sensing


3/19

Simplest of all operating principles.

Fault Current magnitude is always greater then the

normal load current.

Example of 2000HP Motor Connected to 4kV powersystem. Full load current of motor is 245A.Emergency

overload capability allowed is 25 percent. Hence 306 A

level is set as a safety margin for faults inside the zone

of protection.


4/19

R

2000 HPMotor

4KV


5/19

The Relay should operate and trip the circuit breaker

for all currents above the setting.

In case of manual operation of circuit breaker, the relay

may be connected to sound an alarm. The level above which the relay operates is called

pickup setting of relay.

For currents smaller than the pickup value, the relay

takes no action.


6/19

However viceversa is also possible as in undervoltage

relay.

Operating Characteristics of Overcurrent Relay.(I/Ip vs

Operating time)

I/Ip is known as normalized current.

I/Ip< 1.0, then operating time is infinite.

I/Ip> 1.0, then relay operates.


7/19

This operating principle is based upon the comparison

of one or more quantities with each other.

Example of current balance relay which compares the

current in one circuit with the current in another circuit.

These currents are equal in normal operation. The relay

will operate when these currents vary by given

tolerance.


8/19

If |Ia|>|Ib|+E (where E is tolerance) and line B is not

open,the relay will declare a fault in line A and trip it.

Similar logic is used to trip line B.


9/19

Most sensitive and effective method to provide

protection against faults.


10/19

Example of a generator winding. Since winding is

electrically continuous that is current entering one end

is equal to current leaving at another end in normal

operation.

For abnormal operation the currents are no longer equal

and relay has now current equal to difference of them.


11/19

Both level detection and magnitude comparison

techniques are used in it and protection is termed as

differential protection.

Differential protection is capable of detecting very

small magnitudes of current.

Its drawback is that it requires current from the

extremities of a zone of protection which restricts it to

power apparatus such as generators, transformers,motors etc.


12/19

This relay compares the relative phase angles between

two A.C quantities.

It is used to determine the current direction with respect

to reference quantity.

For Example In a normal power flow, the phase angle

between voltage and current varies around its power

factor angle ( 30).


13/19

When power flows in opposite direction, then the angle

will be 180 30.

For fault in forward or reverse direction, the phase

angle of current with respect to voltage is - or 180-

.

where is the impedance angle.


14/19

The most reliable and positive type of protection

compares the current entering the circuit with the

current leaving it. But on transmission lines and

feeders, the length, voltage and configuration of line

may make this principle uneconomical.

Instead of comparing the local line current with the far

end line current, the relay compares the local current

with the local voltage. This gives the impedancemeasurement of line.


15/19

An impedance relay relies on the fact that the length of

the line for a given conductor diameter and spacing

determines the impedance.


16/19

Certain relaying principles are based on the information

obtained by the relay from a remote location.

The information is usually in the form of contact

status.(open or closed)

The information is sent over a communication channel

using power line carrier, microwave, telephone circuits.


17/19

Current and Voltages usually have sinusoidal waveform

of fundamental power system frequency.

There are deviations from pure sinusoid such as third

harmonic voltages and current produced by the

generators during normal operation.

During abnormal operation, odd harmonics associated

with transformer saturation or transient component

caused by energization of the transformers.


18/19

These abnormalities are detected by sensing the

harmonic content through filters in electromechanicalor solid state relays.


19/19

Normal power system operation is at 50 or 60 Hz.

Any deviation from these values indicates a fault.

Frequency can be measured by filter circuits.

Frequency sensing relays are used to take correctiveactions which will bring the system frequency back tonormal.

Brief Overview of Relay Operating Principles

Documents

Transcript of Brief Overview of Relay Operating Principles