Study notes on Switch-Gear & Protection 2 For Electrical Engineering Students

By Yash Bansal|Updated : May 25th, 2021

In this article, you will find the study notes on Switch-Gear & Protection-2 which will cover the topics such as Power System Protection and Switch-gear, Over-current Relay, Type of Over Current Relay, Universal Relay Torque Equation, Differential Protection, Distance Protection, Impedance Relay, Reactance Relay, Mho Relay, Time Multiplier Setting, Distance protection Scheme, Zones of Protection, Impedance Characteristic, Impedance Characteristic, Mho Characteristic, Significance of R-X Diagram ,Sampling Comparator, Amplitude Comparator, Phase Comparator, Carrier Current Protection Scheme, Generator Protection Scheme, Transformer Protection Scheme & Bus Bar Protection Scheme.

In this article, you will find the study notes on Switch-Gear & Protection-2 which will cover the topics such as Power System Protection and Switch-gear, Over-current Relay, Type of Over Current Relay, Universal Relay Torque Equation, Differential Protection, Distance Protection, Impedance Relay, Reactance Relay, Mho Relay, Time Multiplier Setting, Distance protection Scheme, Zones of Protection, Impedance Characteristic, Impedance Characteristic, Mho Characteristic, Significance of R-X Diagram ,Sampling Comparator, Amplitude Comparator, Phase Comparator, Carrier Current Protection Scheme, Generator Protection Scheme, Transformer Protection Scheme & Bus Bar Protection Scheme.

Power System Protection and Switch-gear

An electric power system should ensure the availability of electric power without interruption to every load connected to the system. One of the sources of trouble to continuous supply is the shunt fault or short circuit which produces a sudden and sometimes violent change in the system.

Protective relays and relaying system detect abnormal conditions like faults in electric circuits and operate automatic switchgear to isolate faulty equipment from the system as quickly as possible.

Over Current Relay:

A relay that operates or picks up when it’s current exceeds a predetermined value (setting value) is called Over Current Relay.

Over current protection protects electrical power systems against excessive currents which are caused by short circuits, ground faults, etc. Over current relays can be used to protect practically any power system elements, i.e. transmission lines, transformers, generators, or motors.

Over current includes short-circuit protection. Short circuits can be Phase faults, Earth faults, Winding faults, Differential and distance protection.

Over current protection is useful for the following:

  • Detect abnormal conditions
  • Isolate faulty part of the system
  • Speed Fast operation to minimize damage and danger
  • Discrimination Isolate only the faulty section
  • Dependability / reliability
  • Security / stability
  • Cost of protection / against cost of potential hazards

Type of Over Current Relay:

  1. Instantaneous Over Current (Define Current) Relay
  2. Define Time Over Current Relay
  3. Inverse Time Over Current Relay (IDMT Relay): Moderately Inverse, Very Inverse Time, Extremely Inverse
  4. Directional over Current Relay.

Application of over current relay: Motor protection, Transformer protection, Line protection, Distribution protection

 Universal Relay Torque Equation

The universal relay torque equation can be given as

12-Principles (1)

where I = RMS value of current in current coil

V = RMS value of voltage fed to the voltage coil

ϕ = Electrical angle between V and I

T = The maximum torque angle K1, K2

and K3 = Relay constant

K = Mechanical restraining torque

Differential Protection:

It is used for transformer and generator protection. It simultaneously compares the phaser difference & magnitude of the current entering & leaving the protected zone. Differential protection is a unit protection, relay works on the principle of Kirchoff's current Law. Measuring element is Current Transformer. The differential current measured between the incoming current and Outgoing current must be negligible current during stable and through fault condition. In case of in zone fault or unstable condition (due to CT saturate) relay will sense the differential current and issue the trip signal.

Applications of Differential protection: Transformer, Generator and Cable Protection.

Distance Protection: Distance protection is widely used in the transmission network, its also called impedance protection because relay operates with respect to fault impedance of the transmission line (Z=V/I).

It calculates the apparent impedance of a line with the help of voltage & current input connected to the relay. If measured impedance falls below set impedance trip command is issued to clear the fault.

Measuring element is Current Transformer and Voltage Transformer. Usually relay having 4 zones of the transmission line.

Zone1, Zone2, Zone3 will be forward zones (Towards the Line) and zone 4 will be Reverse zone (Towards the Source).

Impedance Relay

From the universal torque equation, putting K3 = 0 and giving negative sign to voltage term, it becomes,


(neglecting spring torque)

An impedance relay is a voltage restrained over current relay.

12-Principles (2)

12-Principles (3)

Reactance Relay

In universal torque equation, putting k2 = 0

12-Principles (4)

The reactance relay is direction restrained over current relay.

12-Principles (5)

12-Principles (6)

Mho Relay

In this relay, the operating torque is obtained by the V-I element and

12-Principles (7)

12-Principles (8)

The mho relay is voltage restrained direction relay i.e., this relay has property of inherently directional.

12-Principles (9)

Time Multiplier Setting (TMS)

The time multiplier setting for an inverse time relay is defined as,

12-Principles (10)

Where T = Required time of operation

Tm = The time obtained from the relay characteristics

Plug Setting Multiplier (PSM)

Current setting is adjusted by means of a topped plug bridge, hence known as PSM

12-Principles (11)

Distance protection Scheme

A distance relay has the ability to detect a fault within a pre-set distance along a transmission line orpower cable from its location. Every power line has a resistance and reactance per kilometer relatedto its design and construction so its total impedance will be a function of its length. A distance relaytherefore looks at current and voltage and compares these two quantities on the basis of Ohm’s law.


Zones of Protection

In Zone of Protection, Careful selection of the reach point settings and tripping times for various zones of measurement enables correct coordination between distance relays on a power system. Basic distance protection will comprise one instantaneous (Zone 1) and one or more time delayed zones (Zone 2, Zone 3, Zone4 …). Typical reach and time settings for a 3-Zone distance protection are shown below:


  • Zone 1: this is set to protect between 80% of the line length AB and operates without any time delay. This “under-reach” setting has been purposely chosen to avoid “over-reaching” into the next line section to ensure selectivity since errors and transients can be present in the voltage and current transformers. Also manufacturing tolerances limit the measurement accuracy of the relays.
  • Zone 2: this is set to protect 100% of the line length AB, plus at least 20% of the shortest adjacent line BC and operates with time delay t2. (≈0.5s) It not only covers the remaining %20 of the line, but also provides backup for the next line section.
  • Zone 3: this is set to protect 100% of the two lines AB, BC, plus about 25% of the third line CD and operates with time delay t3. (≈1.5s)

Impedance Characteristic:

If the relay’s operating boundary is plotted on an R-X diagram, its impedance characteristic is a circle with its center at the origin of the coordinates and its radius will be the setting (the reach point) in ohms. The relay will operate for all values less than its setting i.e. for all points within the circle. This type of relay, however, is non-directional. It can operate for faults behind the relaying point. It takes no account of the phase angle between voltage and current. It is also sensitive to power swings and load encroachment due to the large impedance circle.


Mho Characteristic:

The limitation of the impedance characteristic can be overcome by a technique known as self polarization. Additional voltages are fed into the comparator in order to compare the relative phase angles of voltage and current, so providing a directional feature. This has the effect of moving the circle such that the circumference of the circle passes through the origin. Angle 𝜃 is known as the relay’s characteristic angle. It appears as a straight line on an admittance diagram.



Significance of R-X Diagram

  • In general, all electro-mechanical relays respond to one or more of the conventional torque producing input quantities: (a) voltage, (b) current, (c) product of voltage, current and the angle θ between them and (d) a physical or design force such as a control spring.
  • Similar considerations hold for solid-state relays as well. For distance relay, analyzing the response of the relay for all conditions is difficult because the voltage varies for each fault, or varies for the same fault but with different system conditions.
  • To resolve this difficulty, it is common to use an R–X diagram. By utilizing only two quantities, R and X (or Z and θ), we avoid the confusion introduced by using the three quantities E, I and θ. There is an additional significant advantage in that the R–X diagram allows us to represent both the relay and the system on the same diagram.


The subscript ‘p’ represents primary and ‘s’ represents secondary quantities. In terms of the secondary quantities of voltage and current transformers, the relay sees Zfp & Zfs as 


where ni and ne are the current transformer (CT) and voltage transformer (VT) turns ratios.

Sampling Comparator

There are two methods of comparison: the amplitude and phase comparison techniques.

In amplitude comparison technique, the compactor produces an output whose amplitude is proportional to the amplitude difference of the input quantities;

While in phase comparison technique, the Comparator the phase angles of the input quantities and produces pulses whose width is proportional to the phase difference of the input quantities. The amplitude Comparator can be used as phase Comparator and vice versa, if certain modifications are made.


Instantaneous Comparator (Directing Amplitude Comparator) – Averaging Type

This is then compared with the peak value of operating signal, which may or may not be rectified but is smoothened. The tripping signal is provided if the operating signal exceeds the level of the restraint.Since this method involves smoothening, the operation is slow. A faster method is phase splitting the wave shapes of instantaneous amplitude comparator are shown in fig below before rectification and the averaging circuit can be eliminated.


Phase Comparator

Phase comparison technique is the most widely used one for all practical directional, distance, differential and carrier relays.If the two input signals are S1 and S2 the output occurs when the inputs have phase relationship lying within the specified limits.
Both the input must exist for an output to occur. The operation is independent of their magnitudes and is dependent only on their phase relationship. The figures below show that the phase comparator is simple form. The function is defined by the boundary of marginal operation and represented by the straight lines from the origin of the S-plane.
The condition of operation is β1 < θ < β2.
θ is the angle by which S2 lands S1. If β1 = β2 =90o, the comparator is called cosine comparator and if β1=0 and β2=180o, it is a sine comparator.
In short, a phase comparator compares two input quantities in phase angle (vertically) irrespective of the magnitude and operates if the phase angle between them is < 90o.

The Carrier-Current Protection

  • Carrier current protection scheme is mainly used for the protection of the long transmission line. In the carrier, current protection schemes, the phase angle of the current at the two phases of the line are compared instead of the actual current. And then the phase angle of the line decides whether the fault is internal and external.
  • The main elements of the carrier channel are a transmitter, receiver, coupling equipment, and line trap.Line trap is inserted between the bus-bar and connection of coupling capacitor to the line. It is a parallel LC network tuned to resonance at the high frequency. The traps restrict the carrier current to the unprotected section so as to avoid interference from the with or the other adjacent carrier current channels. It also avoids the loss of the carrier current signal to the adjoining power circuit.



Generator Protection -Introduction

No international standards exist regarding the extension of the protective schemes for different types and sizes of generators. The so called "common standard" varies between different countries and also between power companies within the same country, depending on their past experience and different ways in which fault statistics may be interpreted. A relay manufacturer working on the international market should, therefore, be able to offer a protective system which can be easily modified to meet different requirements from different users.

Under certain situations like internal faults, the generator has to be quickly isolated (shut down), while problems like loss of field problem requires an ‘alarm' to alert the operator. Following is a descriptive list of internal faults and abnormal operating conditions.

Internal Faults

  • Phase and /or ground faults in the stator and associated protection zone
  • Ground faults in the rotor (field winding)

Abnormal Operating Conditions

  • Overload.
  • Overvoltage.
  • Loss of field.
  • Unbalanced Operation e.g. single phasing.
  • Under and over frequency.
  • Motoring or loss of prime mover.
  • Subsynchronous oscillation.
  • Loss of synchronization (out of step).

Protective Relays Used

  • Over voltage protection
  • Over fluxing protection
  • Low forward power and reverse power protection
  • Dead machine protection
  • Stator over current protection
  • Rotor Over current protection
  • Generator Loss of excitation protection
  • Generator negative phase sequence protection
  • Back up distance protection
  • Shaft damage protection
  • Under frequency protection
  • Generator pole slipping protection
  • UAT breaker failure protection

Protection Schemes Used 

  • Generator differential
  • Generator and generator transformer overall differential
  • Transformer (GT-UAT) differential
  • Inter turn fault
  • Generator rotor earth fault
  • Generator stator earth fault
  • UAT differential
  • Generator transformer restricted earth fault
  • Fire protection for GT and UAT

Stator Earth Fault Protection 

Earth fault protection must be applied where impedance earthing is employed that limits the earth fault current to less than the pick-up threshold of the overcurrent and/or differential protection for a fault located down to the bottom 5% of the stator winding from the starpoint. The type of protection required will depend on the method of earthing and connection of the generator to the power system.

Step-up Transformer:Differential Protection

The generator stator and step-up transformer can be protected by a single zone of overall differential protection. This will be in addition to differential protection applied to the generator only. The current transformers should be located in the generator neutral connections and in the transformer HV connections. Alternatively, CT’s within the HV switchyard may be employed if the distance is not technically prohibitive. Even where there is a generator circuit breaker, overall differential protection can still be provided if desired.

byjusexamprepInternal faults in oil filled transformers

In oil filled transformers, internal faults may be classified as follow:

  • Faults generating production of gases, mainly:
    • Micro arcs resulting from incipient faults in the winding insulation
    • Slow degradation of insulation materials
    • Inter turns short circuit
  • Faults generating internal over pressures with simultaneously high level of line over currents:
    • Phase to earth short circuit
    • Phase to Phase short circuit.

These faults may be the consequence of external lightning or switching over voltage.

Depending on the type of the transformer, there are two kinds of devices able to detect internal faults affecting an oil filled transformer.

  • The Buchholz Relay dedicated to the transformers equipped with an air breathing conservator.
The buchholz relay is installed on the pipe connecting the tank of he transformer to the conservator. It traps the slow emissions of gasses and detect the flow back of oil due to the internal over pressures


Unit Transformer Differential Protection

The current taken by the unit transformer must be allowed for by arranging the generator differential protection as a three-ended scheme. Unit transformer current transformers are usually applied to balance the generator differential protection and prevent the unit transformer through current being seen as differential current.

Overloads and internal faults in dry type transformers

The dry type transformers are protected against over-heating due to possible downstream overloads by a dedicated relay monitoring thermal sensors embedded in the windings of the transformer.

The internal faults, mainly inter turns and phase to earth short circuits occurring inside a dry type transformers are cleared either by the circuit breaker or the fuses installed on the primary side of the transformer. The tripping of the circuit breakers when used is ordered by the phase to phase and phase to earth over current protections.

Inter turns faults need a dedicated attention:

  • They generally generate moderate line over currents. As an example when 5 % of a HV winding are short circuited the line current of the transformer does not exceed 2 In, for a short circuit affecting 10 % of the winding the line current is limited around 3 In.

Busbar Protection 

Busbars have often been left without specific protection, for one or more of the following reasons:

  • The busbars and switchgear have a high degree of reliability, to the point of being regarded as intrinsically safe
  • It was feared that accidental operation of busbar protection might cause widespread dislocation of the power system, which, if not quickly cleared, would cause more loss than would the very infrequent actual bus faults.
  • it was hoped that system protection or back-up protection would provide sufficient bus protection if needed.

Types of Protection Scheme
A number of busbar protection systems have been devised:

  • System protection used to cover busbars
  • Frame-earth protection
  • Differential protection
  • Phase comparison protection
  • Directional blocking protection

Single-Busbar Frame-Earth Protection

This is purely an earth fault system and, in principle, involves simply measuring the fault current flowing from the switchgear frame to earth. A current transformer is mounted on the earthing conductor and is used to energize a simple instantaneous relay.

  • The principal earth connection and current transformer are not shunted, thereby raising the effective setting. An increased effective setting gives rise to the possibility of relay maloperation. This risk is small in practice.
  • Earth current flowing to a fault elsewhere on the system cannot flow into or out of the switchgear frame via two earth connections, as this might lead to a spurious operation.


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