RELAY PROTECTION

Current Transformer
Main tasks of current transformer are
I1
Measurement of Current
Isolation between High voltage and Low
Voltage
Inputs to Relays & Protection System I2

I1 N1 I1 = N2
I2 = N1
I2 N2 Protection cores :
1 core - Main-1
2nd Core- Main-2 & LBB
3rd core - Metering
4th & 5th core - Busbar
 Voltage Transformer
Main tasks of Capacitance Voltage transformer
Measurement of Voltage
Isolation between High Voltage & Low Voltage.
Inputs to Relay/Protection systems
PLCC (Power line Carrier Communication)

CVT – 220kV , 400kV and above

Potential transformer : upto 132kV

Protection cores :
1st core - Main-1
2nd core - Main-2
3rd core - Metering
PLCC – Power Line Carrier Communication
 PLCC
• Protection , Data & Speech signal : 0 – 4Khz
– Speech : 0.3kHz-2kHz
– Protection & Data : 2kHz-3.4kHz
– Carrier Signal : 100-500 Khz

• No. of PLCC panels

– 02 Protection & 01 speech

• Amplitude Modulation : Carrier & low frequency signal

• Pilot frequency & Guard frequency

• Inter-trip & Direct Trip Signal

 Tx = 256KHz Tx = 258KHz
Rx= 258 Khz Rx= 256 Khz

carrier PLCC
RTU Data
Coaxial
cable
Modulator Amplifier Hybrid
Telephone
Handset Set Speech (2stages)

Relay Protection
 Electrical faults in the power system

• Transmission lines 85%

• Busbar 12%
• Transformer/ Generator 3%
 Input to relays
• Relays response to one or more electrical
quantities such as current, voltage or
frequency.

• Aux. Power supply

(220 v / 110 v DC) : Relays

50V : PLCC
 Fault Clearance System

Protection System

CT Circuit Breaker

Circuit
Protection Breaker
VT Trip
Equipment Mecha-
Coil
nism
TE

DC-System
• Unit Protection
– protection whose operation and section selectivity are dependent on
the comparison of electrical quantities at each end of the protected
section

• Non-Unit protection
– protection whose operation and section selectivity are dependent on the
measurement of electrical quantities at one end of the protected section
by the measuring relays and, in some cases, on the exchange of logic
signals between the ends
 Transmission line protection

Transmission lines are to be protected against :

• Lightening strokes
• Over Voltage
• Faults (earth / phase faults) : Ph- Earth , Ph-Ph & 3
Ph faults, 3 Ph faults are most severe.

Distance scheme:
• Main 1/ Main 2 protection- to increase the
reliability.
One and Half Breaker Scheme

Bus-2

Bus-1

21

Ckt-1 Ckt-2
The principle of distance protection
• Power lines have impedances of size 0,3- 0,4 ohm/ km and normal angles of 80 -
85 degrees in a 50Hz systems.

A B
ZL=R+jX

Z< Z<

• The line impedance must be converted to secondary values with the formula:

VTsec CTprim
Zsec= x x Zprim
VTprim CTsec
 ZK=Uk/ Ik

Uk Uk=0

metallic fault

Zk
A Ik B

Z<
Mho Quadrilateral
x

ZL

R
•Better fault resistance coverage
•No Load encroachment problem
Measuring loop for three- phase faults
• The distance protection relays are always set based on the phase
impedance to the fault

Zs RL XL
IL1
UL1 IL2
UL2 IL3
UL3

The measured impedance is equal to the positive sequence

impedance up to the fault location
 Measuring loop for two- phase faults

• The distance protection relays are always set

based on the phase impedance to the fault
Zs RL XL
IL1
UL1-L2 IL2

The measured impedance is equal to the positive sequence

impedance up to the fault location
 Measuring loop for earth faults
• The distance protection relays are always set
based on the phase impedance to the fault
Zs RL XL
IL1
UL1

RN XN
IN

The measured Impedance is a function of positive and zero

sequence impedance
Distance Protection schemes shall have …

• Three independent zones

• Separate measurements for all Phase to Phase & Phase to

ground faults.

• Single and three pole tripping capability.

• Directional characteristics for Zone-1, Zone-2 and Zone-3.

• Capability of operation for switch on to faults.

• Accuracy of better than or equal to 5% of set value for reach

measurement in Zone-1 & Better than equal to 10% of set value
for Zone-2 & Zone-3.
 400kV Transmission Line Protection

M-1 Z3A Unprotected section from

Z2 remote end (Z-3), now
M-2
Z1

Protected Line
Longest Line from Remote Bus Bar
Z L1
Z L2
A C
B D

CONSTRAINED SETTING 3 x 240MVA;12%

Z 1 = 0.8 x Z L1 ; Z 2 = 1.2 x Z L1 ; Z 3A = (Z L1 + Z BT)* : where,

Z L1 = Impedance of protected line section,
Z BT = Impedance of the Transformers at remote Bus Bar (B)= 80 / 3 ohms
* In order that the Zone-3 protection does not see a fault in 220kV side of ICT
 CARRIER SCHEME
• Z2 fault will be cleared after Z2 time which is
not acceptable for internal fault.

• Carrier schemes facilitates faster clearing of

fault:
o Direct Transfer Trip
o Permissive Under Reach
o Permissive Over Reach
o BLOCKING
o Acceleration
o Z1 Extention
 Direct transfer Trip
Zone – 1 contact arranged to send signal to remote end
and receive relay contact at that end is connected in the
tripping circuit
Z1A Z2A
A B A B

Z1B Z1B
 Permissive Under reach
Z1A
Z1A
A B
A B

Zone – 1 trip Z1B Z1B

Zone – 1 trip
C.S
Zone – 1 trip C.S Z2 + C.R = Trip
C.S

A B
In a permissive underreaching scheme
the communication signal is sent
from a zone that underreaches the
remote end.
Not sense
Z2 + time
delay= Trip
 Permissive Over reach
Z2A
Z1A

Z1B
Z2B

In short transmission lines Zone-1 over reaches.

Zone-2 sends carrier instead of Zone-1

In a permissive overreaching scheme the communication signal is sent

from a zone that overreaches the remote end
 Blocking Scheme
Z1A
Z1A
A B
A B

Zone – 1 trip Z1B

Zone – 1 trip Z2 + no carrier =
Zone – 1 trip instantaneousTrip

Carrier signal (CS) is sent to remote

end when the fault is detected in the
A B
reverse direction. Tripping is
achieved when this blocking signal is
not received within a time T0 (20-40
ms) and the local relay has detected
a fault in the forward direction.
C.S Z2 + C.R + time
delay= Trip
 Acceleration Scheme
Z1A
Z1A
A B
A B

Zone – 1 trip Z1B

Zone – 1 trip C.R activates
C.S
Zone – 1 trip C.S Zone-2 and
C.S trips
immediately
without
waiting for
time delay

• Similar to permissive under reach scheme.

• Distance relays having same measuring unit for Zone-1 & Zone-2. Zone-1 element
initiates a signal to remote end along with tripping of local C.B. The receive relay
operates range change relay which extends the reach of measuring unit from
Zone-1 to Zone-2 immediately instead of at the end of Zone-2 time delay
Zone-1 extentention

Z1A = 80% Z1A = 120%

A B

Z1B =120% Z1B = 80%

 Auto Reclosing scheme

• Setting criteria of auto reclosure

– Dead time : 1 sec (long enough to completely
de-ionise the arc)
– Reclaim time : 25 Sec sufficient for the CB closing
mech to reset & get ready for next reclosing
mechanism
• Protections which shall start auto reclosing
– Main-I Protection
– Main-II Protection
 Auto Reclose Blocking

• Protections which block auto reclosing:

– LBB
– Line reactor Protection
– Over Voltage Protection
– Bus Bar Protection
– Circuit breaker problem
– Carrier fail – both protection channels
– Receipt of Direct Trip signal.
 Over Voltage Protection

• monitors all three voltage and has associated

timer.
• Two stages
• Low set stage : Ideally-1.1Vn,Timer of 5s
• High set stage: Ideally-1.5Vn,Instataneous
 Over Voltage Protection Logic

• Over Voltage Protection shall be connected

to:
– Trip connected line breaker(s)
– Start LBB
– Block auto reclose
– Send direct trip command
 Over Voltage Protection-Settings

• Low set stage at 110% with typical time

delay of 5 seconds.
• High set stage at 150% with time delay of
100mSec
Open jumper protection:
Detects the negative sequence current
Alarm/ Trip issued when I2/I1 exceeds a set limit and after a set time delay
Time delay shall be greater than the single phase dead time of A/R and a
considerable margin

Fuse failure protection

Detects the sec fuse failure of VT and blocks the voltage operated protections
In modern numerical distance relays, VT fuse failure switch on an O/C feature

Back up Earth fault protection

Provides Back for lines where Zone-3 setting are limited due to Transformers at
other end. IDMT chrs
 Switch On To Fault (SOTF)
• When energizing a power line onto a
forgotten earthing no measuring voltage will
be available and the directional measuring can
thus not operate correctly.

SOTF conditon can either be taken

from the manual closing signal
U=0 V activating the (BC) input or it can be
Z< detected internaly by a logic.
 Stub Protection
Bus A It is not possible for the distance
+ >Z protection relay to measure
impedance when the line
disconnector is open. Not to risk
incorrect operation the distance
protection must be blocked and a
Stub protection is released.
The Stub protection is a simple
current relay.

25ms
line dis c
Bus B open & trip

I S TU B >
 Power Swing
• Power swing phenomenon is quite unpredictable
and temporary in nature.

• Occurs when Heavy load is introduced into the

system or thrown out

• 3 ph symmetrical voltage & current variation

• Rate of change of impedance is lower than fault

condition

• Distance relay are to be blocked for power swing

condition
 Power Swing

X
Z3
Recoverable
Z2
Swing

Z1

Load
R Area
 Breaker Failure Protection
Primary Protection detects fault and issues trip command
to the concerned (Primary) Breaker

If the Breaker fails to trip, the BFR detects the condition by

monitoring the current in the circuit (preferred option) after
the elapse of sufficient time for execution of Breaker trip
(typically 200ms)

BFR issues trip command to all the other Breakers

connected to the Bus to which the failed Breaker is
connected.
 PRIMARY TRIP
RELAY OPTD

200
& TRIP BUS BAR
CURRENT ms
SENSOR I >

REPEAT TRIP TO
PRIMARY BREAKER

RECENT MODIFICATION
 Breaker protection relays
• Pole Discrepancy relay

• Anti pumping relay

• Trip circuit supervision relays

• SF6 Alarm & Lockout relay

• Air/oil pressure alarm & lockout relay

 DIRECT TRIP SEND LOGIC
• Manual trip of main bkr when tie bkr is open
• Manual trip of tie bkr when main bkr is open
• BFR of main CB
• BFR of Tie CB
• 3 ph Trip relay of Gr. A & B
• B/B Protection
• O/V Protection
Transformer Protection
 Types of Internal Faults
• Earth faults

• Short-circuits

• Inter turn Faults

• Core Faults

• Tank Faults

• Reduced cooling
 Abnormal Conditions
• Overload

• Over voltage

• Reduced system voltage

• Over-excitation
Differential Protection
 Differential Protection
• Classical application of differential protection

• Interposing current transformers for phase

shift and transformation ratio correction.

• Influence from OLTC.

• Harmonic restrained differential protections.

 Earth Fault Protection
• Restricted earth fault protection
– High-impedance restricted earth fault protections
– Low-impedance restricted earth fault protections
Earth Fault Protection
 Over excitation or Over Flux
protection

• Overexcited transformers become

overheated and damaged

• Especially required for Generator

Transformers

• V/F ratio is monitored

 Impedance Protection
• Back-up protection of the local bus-bars

• Back-up protection of the outgoing feeders

• Back-up protection of the busbar at the remote

end

• Plain under-impedance protection/Distance

protection
• 51UT (UT Back-Up Over Current protection).
– Back up against un-cleared L.V system short
circuits. Time graded, with the down stream short
circuit protection.
• 51NUT (UT Back-Up Earth Fault protection).
– Time graded definite time low set earth fault relay
at the UT neutral CT as back up for un-cleared
faults.
 A
400KV

51

400/220 KV
Transformer

51

220KV B
 Monitors
Monitors are very important devices which detect faults and
abnormal service conditions which may develop into fault.
 Temperature Monitoring
• The oil thermometer: Conditions like sustained over load or
loss of cooling will cause abnormal temperature rise which can reduce the
life of the transformer. Mercury switches mounted on the temperature
indicating dial gives alarm and trip outputs.

• The winding thermometer : Top oil temperature

corrected for I2R heating by proportional current in replica resistance
 Oil and Gas Monitoring
• Sudden gas pressure protection

• Buchholz protection

• Oil level monitoring

• Gas Operated (Buchholz’s) protection
– Operates by the gases produced by Electrical
discharges/arcing inside the transformer.
• Alarm element operates by the slow collection of gas in
the gas trap of the relay while the gases are escaping from
the transformer tank to the conservator. The collected
gases displaces oil downward and the float switch
operates.
• Sudden rush of gases and oil to the conservator, for a
severe fault moves a flap in the flow path to operate the
trip element.
• Also trips for very low oil level in Transformer.
• Oil Surge protection:
• Normally provided for OLTC Chamber
• Similar to Buchholtz’s relay, but with only oil surge element (no gas
collection chamber and float).

• Oil level (MOG) alarm:

• Operates on Conservator oil level low, before Buchholtz’s relay trip
occurs, to give early warning

• Pressure Relief Device:

– Operates for excessive pressure rise and / or rate of rise of
pressure.
– Avoids costly rupture of tank in case of an internal fault
•
 Reactor Protection

• Differential Protection
• Restricted earth fault Protection
• Reactor Back-up protection
• Buchholz relay
• Winding temperature
• Oil Temperature
• Pressure relief
• Oil Level
Bus-Bar Protection
• Need for Bus bar protection

In its absence fault clearance takes place in
Zone-II of distance relay by remote end
tripping.

This means slow and unselective tripping and
wide spread black out.

• Effect of delayed clearance

Greater damage at fault point.

Indirect shock to connected equipments like
shafts of Generator and windings of
transformer.
Bus-Bar Protection
Single Bus Bar with Sectionalizer
 Ckt-1
Ckt-2
R
Y
B
87 Check Zone N

87
Main Zone
R
Y
B
N
LOGIC OF MAIN AND TRANSFER BUS SYSTEM
 Type
• High impedence
– a) PVD, b) CAG34, c) FAC, d) RADHA

• Low Impedence
High Impedence Differential Protection
BUS
BUS

Line -1 Line -2

Line -1 Line -2

• Stabilizing resistor

87 • Non Linear resistor (Metrosil)

(Vp>3kV)

• Alarm relay : CT supervision

 Vs

Vk>2Vs Vs - stabilizing voltage

Vs>0.5 If(Rct+2RL) Vk - Knee point voltage
If - Max through fault current
Rstab = Vs/Ir Rct - CT resistance
RL - Max Lead resistance
Ir - relay operating current
High Impedence Differential Protection
• The Measuring Circuit comprises a High
impedance stabilising Resistor (Metrosil)
connected across the circulating current
arrangement of all the CT’s in parallel.

• The Value of Stabilising Resistor chosen such

that the voltage drop across the relay circuit is
insufficient to operate the relay for faults
outside the protection zone.
 Limitations of High impedance
differential relay
• Puts stringent requirements on CTs

Need for dedicated CTs

Identical CT ratios , magnetising impedances

Aux CTs not acceptable
Operating Torque proportional to spill current = I1+I2

Restraining Torque proportional to Fault current= I1 + I2

3.0
Operate

2.0

Non-Operate
1.0

0.2

1.0 2.0 3.0 4.0

Through Bias Current
Distributed Centralized
FREE OF ANY NEED FOR MATCHED CT FREE OF ANY NEED FOR MATCHED
CT CHARACTERISTIC OR RATIOS.CHARACTERISTIC OR RATIOS.

OTHER PROTECTIVE RELAYS CAN BE INCLUDED INOTHER PROTECTIVE RELAYS CAN

BE INCLUDED INTHE SAME CIRCUIT.

STABLE FOR INFINITE FAULT LEVEL.

INSENSITIVE TO CT SATURATION.

DETECTS FAULTS WITH IN 1DETECTS FAULTS WITH IN 1--2 m sec AND INITIATES2 m
sec