16-04-2023

Things discussed so far in Industrial switchgear :

• Voice over ppt shared on BBB and 20 discussion meets related to Switchgear.
• Power system Protection: Introduction.
• Differential relay & Protection scheme: Basics
EE422 Principles of Switchgear and Protection • Biased Differential relay & Protection scheme
• Transformer protection relays and schemes.
L36-Topic: Phase unbalance, single phasing, -ve phase • (i) Busbar differential and (ii) Induction motor protection
sequence relay for Induction motor.

Course Instructor: Gururaj S Punekar

Dept of E & E NITK Surathkal. • Today we will discuss: Phase unbalance, single phasing, -ve phase
sequence relay for Induction motor.

Induction Motor: Faults and abnormal operation

Motor Protection
• Most commonly used motor in industries is Induction Motor.
• Ratings: Induction Motors from factional horsepower to MW rating
motors are in use.
• General classification: Small == <1 kW
Medium == >40 kW but <1 MW
Large == > 1 MW

• Protection schemes for a motor depends on the size and importance

of the motor.
• No general scheme is possible.
Starter Related faults

IM-Starting current and abnormality : I-t curves

1. Faults related to motor

2. Abnormal operating condition from Mechanical side

3. Abnormal operating conditions from supply side

1
 16-04-2023

Faults related to motor: % Differential relay and protection scheme

A] Faults on motor terminals
• High-set instantaneous over-current relay.
B] Phase faults inside the motor:
1) HRC Fuse protection
2) Phase-fault and ground-fault over current relay and protection

C] Ground faults inside a motor: Core Balance CT (CBCT) Abnormal operating condition from Mechanical side
TRIP-LAW: Summation (vectorial) of Ia+Ib+Ic == 0 implies RESTRAIN Else A] Failure of bearings and rotor jam
TRIP
• Excessive stator current: But there are many other causes which can
lead to excessive stator current.
B] Overload
Thermal over load relays scheme
Over current relays and protections scheme
C] Resistance Temperature Detector Relays (RTDs): windings &
Bearings: (For large motors
& embedded)
D] Inter-turn faults:
Result in hot-spots and detected using embedded temperature D] Thermal replica relays
detectors.

Abnormal operating conditions from supply side

Motor Protection A] Reduction in supply voltage: Torque reduces (T is preparational to square of voltage)
Under voltage relays
• Most commonly used motor in industries is Induction Motor.
B] Unbalanced supply voltage: -ve sequence components
• Ratings: Induction Motors from factional horsepower to MW rating Phase unbalance relay (i) voltage based (ii) current based
motors are in use.
• General classification: Small == <1 kW C] Single phasing
Medium == >40 kW but <1 MW -ve phase sequence or phase unbalance relay can be used.

Large == > 1 MW D] Reversal of phases: -ve phase sequence

Instantaneous trip in case of phase reversal.
• Protection schemes for a motor depends on the size and importance
of the motor. Cases B], C], D] require segregation/filtering of Negative-sequence components.
• No general scheme is possible.

2
 16-04-2023

Sequence components
Phase unbalanced relay: Negative phase sequence relay
• Unbalanced current in IM == overheating and ageing.

• Phase unbalanced relay has filter circuit and it operates based on the
–ve sequence or zero-sequence component.

• Earth fault relay provides phase-earth protection and ph-ph fault

protection can be for –ve-ph-sequence relay.

1. What is phase sequence?

2. What is balance and unbalanced supply?
3. What are implications of unbalanced load?
4. What are phase components when we have balanced supply?

Negative phase sequence relay @ Node-A: Phasor diagrams of current indicate that MN bisects IR; ON= IR/2
From tringle ONM Cos30=ON/OM=(IR/2)/I2…….|IR|/|I1|=|IR|/|I2|=Sqrt(3)
Also I1 leads IR by 30 degrees & I2 lags IR by 30 degrees.

@ Node-C: Phasor diagrams of current indicate that PQ bisects IB; OP= IB/2
From tringle OPQ Cos30=OP/OQ=(IB/2)/I2…….|IB|/|I3|=|IB|/|I4|=Sqrt(3)
Also I3 leads IB by 30 degrees & I4 lags IR by 30 degrees.

In the bridge
Z1 & Z3 are purely resistive branches
Z2 & Z4 are combinations of R & L
Current in branch Z2 & Z4 lag by 60 degrees w.r.t
current in Z1 & Z3.
Arm BD has Inverse O/C relay with Zrelay (assumed)=0.
@Node-A: IR=I1+I2 @ Node-C: IB=I3+I4
@Node-B: I1+I4+IY=Irelay
Set |I1|=|I2| and |I3|=|I4| @Node-B: I1+I4+IY=Irelay For +Ve sequence I1+I4+IY=Irelay=0 For -Ve sequence I1+I4+IY=Irelay=IY

For Zero sequence currents Things to learn and information to gather.

• How a have a relay for –ve sequence currents only?

• How to have digital relays for phase unbalance?

@Node-B: I1+I4+IY=Irelay For Zero sequence currents I1+I4=IY IY+IY=2IY=Irelay

18