Unit III

Apparatus Protection

Topics to be Covered

• Current transformers and Potential transformers

• Applications of CTs and PTs in protection schemes

• Protection of Transformer

• Protection of Generator

• Protection of Motor

• Protection of Busbars

• Protection of Transmission line.

Transformer Protection

• The ratings of transformers used in transmission and distribution

systems range from a few kVA to several hundred MVA.

• The type of protection scheme used depends on the size and

importance of the transformer in the power system.

• For small transformers, simple fuses are employed.

• Overcurrent relays employed for medium size transformers.

• Differential protection is required for the protection of large

transformers.

Types of Faults Occur in Transformers

1. External or Through Faults

2. Internal Faults

Short circuits in the transformer winding and connection

Incipient faults

Winding Fault

Differential Protection of Transformers

• It is used for the protection of large power transformers havingratings of 5 MVA and above.

• This scheme protects the transformer against internal short circuitfaults. It will not detect incipient faults.

• Factors to be considered for differential protection of transformers

a) Magnetizing inrush current

b) Phase shift in star – delta transformer

c) C.T. connections

d) Selection of C.T. taps

e) Effect of transformer taps

Differential Protection of Transformers

• In order to avoid the difference in the magnitude of currents and also to

compensate for phase shift, certain rule has to be followed while making

C.T. connections as shown in the table below.

Sl. No.

Power Transformer Current Transformers

Primary SecondaryOn the

primary side

On the

secondary side

1. Star Star Delta Delta

2. Star Delta Delta Star

3. Delta Star Star Delta

4. Delta Delta Star Star

Differential Protection of Transformers

Harmonic Restraint• There is an inrush current due to the initial energizing of thetransformer.

• This inrush current flows only in the primary winding and hencethe relay may consider it as internal fault.

• Harmonic contents in inrush current are different from normalcurrent.

• The harmonic restraint unit filters out the second harmonic currentpresent in the inrush current.

• This filtered current will be made to flow through the restrain coilof the relay.

• Hence the relay operation under magnetizing inrush current can beprevented.

Buchholz’s Relay

Minor Fault Major Fault

Protection of AlternatorsStator Faults

▪ Insulation failure of stator winding (L-L or L-G)

▪ Inter-turn faults in any of the phases

▪ Unbalanced loading and heating of alternator

▪ Overloading

▪ Failure of prime mover

▪ Overvoltage

Rotor Faults

▪ Excitation failure

▪ Rotor earth fault

▪ Overcurrent in rotor circuit

Protection of Alternators

Stator Protection

▪ Protection against L – G and L – L faults

▪ Stator inter turn fault protection

▪ Overloading protection

Rotor Protection

▪ Loss of excitation relay

▪ Rotor earth fault protection

▪ Field Suppression

Protection Against Stator Faults

Earth Fault Protection (L – G)

Phase Fault Protection (L – L)

Protection Against Stator Inter Turn Faults

Protection Against Unbalanced Loading

• Unbalanced loading of the stator winding produces negative

sequence currents in it.

• This gives rise to additional heating of the stator winding.

• A negative sequence relay is used to sense this problem and hence

the alternator is protected.

Protection Against Overloading of Alternator

• Normally all generators are designed with a capability of supplying some

additional load for some period of time to get additional power from the

machine in case of loss of other generators.

• This may be associated with a loss of voltage which leads to an increase of

current in the stator.

• Voltage restrained IDMT relays are used for protection.

• Relay operation is possible if both overcurrent and under-voltage elements

are operating simultaneously.

Protection Against Failure of Prime Mover

• In case of turbine failure, the alternator does not get mechanical

power.

• But it is continuously connected to grid and becomes a synchronous

motor with field excitation existing in the rotor.

• Now the machine undesirably starts to run as synchronous motor and

takes power from the grid.

• This is prevented by using a reverse power relay which is similar to a

directional current relay.

Protection Against Overvoltage

• It is unsafe to run the alternator for long time with overvoltage across

its terminals.

• An overvoltage relay is used to sense this problem.

• This relay will trip the C.B. when the voltage exceeds 110 – 130% of

rated voltage.

• Generator main C.B and exciter C.B must be tripped to save the

system from this overvoltage problem.

Protection Against Rotor Faults

Loss of Excitation Relay

• When a synchronous generator loses its excitation, it speeds up

slightly and run as an induction generator.

• Instead of delivering reactive power, it absorbs the reactive power

from the system as much as 2 to 4 times the rated current.

• Stator draws magnetizing current from the supply and it causes the

stator to get overheated.

• Rotor is overheated quickly due to the induced currents in it.

• An under current relay can be used to identify this fault.

• An impedance or mho type relay can also be used to find this type of

fault.

Rotor Earth Fault Protection

• If more than one earth fault occur, shortcircuit between the faulty points on thewinding occurs.

• This may cause unbalance magnetic field andmechanical damage may occur in the bearingof the machine.

• During earth fault in the field winding, the DCvoltage across the bridge rectifier will beapplied across the voltage relay.

• The relay will pick up and trips the circuitbreaker.

Field Suppression

• It is a method of discharging stored energy in the field through a

resistor.

• Whenever a fault occurs in the alternator winding, its C.B is tripped.

• But fault remains there since the voltage is induced in the generator

with the help of field.

• Hence the field C.B needs to be opened and the stored energy in the

field winding is to be discharged through a resistor.

• This discharge resistor is to be connected in parallel to the field

winding before the field C.B is opened.

AC Motor Protection

• HRC or rewirable fuse is normally used as protective device formotors.

• Rewirable fuse is used for motors ranging from FHP to 5 HP.

• HRC fuses are used for motors ranging above 5 HP.

• But HRC fuse cannot provide protection for overloading, singlephasing and earth faults.

• Thermal overloading relays are used for protection againstoverloading.

• For high capacity induction motors, a comprehensive motorprotection is used.

Protection Scheme for Motors

Short Circuit Protection

• Attracted armature type relay is connected in each phase.

• Starting current during DOL starting may be 4-5 times full loadcurrent.

• Normally the fault current will be 7 times the full load current.

• Hence the relays are set to operate for currents of 5-7 times full loadcurrent.

Earth Fault Protection

• Earth fault relays (IDMT type) with a setting of 10% to 40% ofthe rated current is used.

• This is a single relay for all the 3 phases.

Earth Fault Protection

• IDMT type overcurrent relays are connected in each phase with

setting of 110% - 125% of rated current.

Stalling Protection

• Due to some mechanical problem or overload, the motors mayrefuse to start.

•Motor will draw a heavy current for an indefinite period. This isreally dangerous and undesirable.

• Definite time relays with a current setting equal to the startingcurrent at stall are used to protect the motors for this case.

Single Phasing Protection• It is undesirable to run the motor when one of the phases isdisconnected.

• It is called single phasing and it needs to be identified so that themotor should be disconnected from supply.

• It is done by single phasing preventer circuits.

Undervoltage Protection

• Operation of motor on undervoltage generally cause overcurrent

which can be sensed by overload devices or temperature sensitive

devices.

• But still a separate undervoltage relay is used to protect against a 3

phase voltage drop.

• A time delay is given for the relay operation to prevent tripping by a

transient voltage drop.

Phase Reversal Relay• Phase reversal occurs when the supply connections are changed afterrepairs. The motor will run in wrong direction.

• In applications like elevators, cranes & hoists, the phase reversal isdangerous.

• A phase reversal relay working based on the electromagneticprinciple is used.

• For a correct phase sequence (RYB), the disc present in the relayproduces a positive torque so that its contacts are closed. Hencemotor runs.

•When phase reversal happens (RBY), the disc present in the relayproduces an opposite torque so that its contacts are opened.

• Hence the magnetic coil of the started is de-energized or circuitbreaker can be tripped.

Busbar

• A busbar is a metallic strip or bar that conducts electricity insidea substation.

• It is made of copper or aluminium.

Busbar Faults

• Failure of insulation due to material deterioration.

• Failure of circuit breaker

• Earth fault

• Flashover due to overvoltages

• Errors in the operation of switchgear

• Accidents due to foreign bodies falling on the busbars

• Flashover due to heavily polluted insulator

Protection of Busbars

•Frame leakage protection

•Circulating current protection

Frame Leakage Protection

Circulating Current Protection

Distance Relays Vs Overcurrent Relays

• The fault current depends on the type of fault and sourceimpedance.

• If overcurrent relays are used in transmission lines, it will tripthe C.B irrespective of the location of fault.

• Hence a relay which operates based on the location of fault isrequired.

• Location is identified by the ratio of voltage to current(selectivity).

Directional Comparison Carrier Protection

Phase Comparison Carrier ProtectionA B

IF1

F2IF2 IF2

IF1

F1

Current flowing into line AB at A

Current flowing into line AB at B

Carrier Transmission at A to A & B

Carrier Transmission at B to A & B

Sum of carriers received at A rectified

Sum of carriers received at B rectified

Internal Fault External Fault