Basics of Substation Protection

Said Salim PalayiAEE,

Electrical Division, KSEB Limited, Manjeri, Kerala, South India

sspalayi@yahoo.com

Protection - Why Is It Needed?

FAULT• Short circuit produced by failure of insulation.

PROTECTION IS INSTALLED TO :• Detect fault occurrence and isolate the faulted equipment.

SO THAT :• Damage to the faulted equipment is limited;• Disruption of supplies to adjacent equipment is minimized.• Danger to staff or the public is avoided

All Power Systems may experience faults at some time.

Faults Are Mainly Caused By Insulation Failure

Underground Cables

Diggers

Overloading

Oil Leakage

Ageing

Faults Are Mainly Caused By Insulation Failure

Overhead Lines

LightningKitesTreesMoistureSaltBirdsFailure of discsBroken Conductors

Faults Are Mainly Caused By Insulation Failure

Machines

Mechanical Damage

Unbalanced Load

Types of Faultabce

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Ø/Ø/E

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e

abc

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abc

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EQPT / FEEDER

BREAKER

RELAY

BATTERY

INSTRUMENT TRANSFORMERS

Types Of Protection - Principles

Most of the protective relays in substation works in the following principle.

1.Over current Protection 2.Differential Protection3.Pilot wire protection4.Distance Protection

1. Overcurrent Protection

Requires secure DC auxiliaryNo trip if DC fails

IF'IF

DCBATTERY

SHUNTTRIP COIL

51

1. Overcurrent ProtectionEarth Fault Relay Connection - 3 Wire System

Combined with OC relays

E/F OC OC OC E/F OC OC

Economise using 2x OC relays

Over current protection Parallel Feeders

Consider fault on one feeder :-

Relays ‘C’ and ‘D’ see the same fault current (I2). As ‘C’ and ‘D’ have similar settings both feeders will be tripped.

51 A 51C

51 B 51D

LOAD

I1 + I2I1

I2

Parallel FeedersSolution:- Directional Control at ‘C’ and ‘D’

Relay ‘D’ does not operate due to current flow in the reverse direction.

51 A 67C

51 B 67D

LOAD

I1 + I2I1

I2

Establishing Direction:- Polarising Voltage

The DIRECTION of Alternating Current may only be determined with respect to a COMMON REFERENCE.

The most convenient reference quantity is POLARISING VOLTAGE taken from the Power System Voltages.

Polarizing Voltage for Directional Over current Relay

IA

VA

90

VBVC

MAX SENSITIVITYLINEOPERATE

IA FOR MAXSENSITIVITYRESTRAIN 45

45

135

VA

VBC VBC

RELAY CURRENTVOLTAGE

A IA VBC

B IB VCA

C IC VAB

Residual Voltage for E/F RelayMay be obtained from ‘broken’ delta V.T. secondary.

VRES = VA-G + V B-G + VC-G = 3V 0

ABC

VRES

VC-GVB-GVA-G

Application of Overcurrent Protection

Overcurrent & Earth fault Protection is used.•HT lines•Transformers•Generators•Reactors•Capacitor Banks•Motors•Neutral displacement relays in capacitor banks•EHT lines (Directional overcurrent &E/F)•Breaker failure Protection

2. Differential Principle

It works on the principle of comparing the current entering and leaving a protected object. If there is a difference, It is assumed that there is some internal fault and relay operates according to the setting

Protected object

Relay

Differential Protection Principle (1)

ProtectedCircuit

R

P1 P2

S2 S1P1P2

S2S1

External fault - Current circulates between the HV & LV CTs; no current thro’ the relay No Trip

Differential Protection Principle (2)

ProtectedCircuit

R

For an internal fault the unbalanced current flows thro’ the relay So Relay operates

P1P1 P2P2

S1S1 S2S2

Differential Relays are used for Transformers Generators

Application of Differential Relay

Restricted Earthfault Protection Increased sensitivity for earth

faults REF elements for each transformer winding

CTs may be shared with differential element

Uses differential principle

6464

64

Restricted Earthfault Protection

P1S1

P2S2

P1S1

P2S2P1S1

P2S2

P1

P2

S1

S2

Stability level : usually maximum through fault level of transformer

REF Case I : Normal Condition

Under normal conditions no current flows thro’ RelaySo, No Operation

Restricted Earthfault ProtectionREF Case II : External Earth Fault

External earth fault - Current circulates between the phase & neutral CTs; no current thro’ the relay

So, No Operation

Restricted Earthfault ProtectionREF Case III : Internal Earth Fault

For an internal earth fault the unbalanced current flows thro’ the relay

So, Relay Operates

Bus bar protection Relay

• Bus bar protection works on the differential principle.

Single bus - Busbar Protection

BUSBARZONE

F1

• Fast clearance by breakers at the busbars

3. Pilot wire Protection using OFC communication

Relay at End ‘B’ measures current and transmits the value to Relay at end ‘A’ thru optical fibre cable. Relay ‘A’ compares measured value and the value recived from ‘B’.

If both values are same, relay keeps restraint state. If there is difference in values. Relays operates.

A B

R RRelayin

gPoint

Trip A

Trip B

CommunicationChannel

RelayingPoint

PGCIL Areakode S/S

Pilot wire protection

Used for – EHT cables – short distance EHT transmission lines

4.Distance• . Distance

For : Transmission and Sub-Transmission CircuitsAlso used as Back-up Protection for Transformers and Generators

Impedance Relay

Operate

IF

VF

Restrain

Spring

Trip

zF

Ampere Turns : VF IZ

Trip Conditions : VF < IFZ

jIX

IZ V1V2V3

IR

TRIP STABLE

Voltage to Relay = VCurrent to Relay = IReplica Impedance = ZTrip Condition : S2 <

S1where : S1 = IZ Z

S2 = V ZFIncreasing VR has a Restraining Effect VR called Restraining VoltageIncreasing IR has an Operating Effect

Basic Principle of Distance Protection

LOADLRRR Z Z V Zmeasured Impedance

RelayPT.

Normal Load

IR ZLZS

VRVS ZLOAD

The relay is set based on the line impedance.The measured ZR is more than the relay

setting Z , hence relay restrains

Basic Principle of Distance Protection

Fault

IRZS

VRVS ZLOAD

ZL

ZF

Impedance Measured ZR = VR/IR = ZF

Relay Operates if ZF < Zwhere Z = setting

Increasing VR has a Restraining Effect VR called Restraining Voltage

Increasing IR has an Operating Effect

• It is assumed that there will be an error up to 20% in distance relay measurements.

• Hence distance relay setting is divided into Zones • Normally 3 zones are for dist. Relay• Zone-1 is instantaneous and covers 80% of protected

line.• Zone- 2 covers 120% of line & is normally with 0.4 s time

delay• Zone -3 covers next line from the substation also & 0.75

seconds timedelay.

Distance Protection - Zones

Zones of Protection

RA

D

CB

Z1A

Z2A

Z3A

jX

Zones of Protection

Z2A Z2C

Z3A Z3C

Time

T3

T2

Z1CZ1A

Z1B DCA

Z2BT2

Z1A = 80% of ZAB

Z2A = 120% of ZAB

Z3A(FORWARD) = 120% of {ZAB + ZCD}

B

Distance Relay -features

POWER SWING BLOCKING Provides Stability during Power swing.

VT SUPERVISION Blocks tripping of Distance Relay when VT supply fails.

SOTF (Switch- On-To-Fault.) Function enables high speed tripping when line is energized to a persisting fault.

AUTORECLOSINGMAINTAINS STABILITY AND SUPPLY BY FAST RECLOSING OF THE TRIPPED FEEDER.

CARRIER INTERTRIPS are provided for fast clearance of faults for entire line.

Distance Protection- features

Distance Relay Applications

• Transmission lines• Sub transmission line• Backup protection for generators

Other type of relays used in substation

1.Under / Over Voltage Relays

– Used for protection of capacitor banks.– Under voltage relays are also used for interlocking of line

Earth switches.

2.Under Frequency Relays

– Monitors the frequency of Power system– Initiates commands for load shedding if system goes

below specified value.

3.DC supervision

DC supervision relay– Indicates the failure of DC supply to the panel.– DC source holds the flag in reset condition– When DC fails, the flag drops. – N/C contact is wired to the annunciator for alarm.– N/O contact is wired to the SCADA

4.AC supervision relay

• Indicates the failure of AC to the panel.• AC is necessary for the operation of space

heaters.

5.Overfluxing Relay

Low frequency High voltage Geomagnetic disturbances

Causes

Over fuxing = V/F

Overfluxing Relay

Transient Over fluxing - Tripping of differential element Prolonged Over fluxing - Damage to transformers

Over flux relay measures V/f ratio and in it gives alarm in stage-1 (usually set at 110%)

It gives trip signal in stage-2 (set at 120%)

Effects

6.Master Trip Relay• It will transfer the actuation of trip signal from

relays to the circuit breaker• A circuit breaker normally open (52a) contact

is used to interrupt trip coil current. This saves trip coil from burning out due to continuous current flow.

• Burning of trip coil may happen if this contact is faulty.

7.Trip circuit supervision

• It gives supervision of trip circuit healthiness• Pre close supervision checks the

healthiness of CB when the CB is open condition. (It is wired through the Normally Closed ( 52b) auxiliary contact of the CB).

• Post close supervision checks the healthiness of CB when the CB is in Closed condition. (it is wired though Normally Open ( 52b) auxiliary contact of the CB)

8.Pole discordance relay

• Used in CBs with single pole tripping• This relay confirm whether all poles are

Opened or Closed, if not it will generate a trip signal.

pole discordance relay scheme

9.Breaker Failure Protection (LBB)A PROTECTION WHICH IS DESIGNED TO CLEAR A SYSTEM FAULTY BY INITIATING TRIPPING OTHER CIRCUIT BREAKER(S) IN THE CASE OF FAILURE TO TRIP OF THE APPROPRIATE CIRCUIT BREAKER.

IN MODERN NETWORKS THE CRITICAL FAULT CLEARING TIME MAY BE LESS THAN 200ms. HENCE, IF THE FAULT IS NOT CLEARED DUE TO FAILURE OF THE PRIMARY PROTECTIVE RELAYS OR THEIR ASSOCIATED CIRCUIT BREAKER, A FAST ACTING BACK-UP PROTECTIVE RELAY MUST CLEAR THE FAULT.

LBB/BFR FLOW CHART

MAIN PROTECTIONOPERATED

YES

YES

TRIP MAINBREAKER

INITIATE BFR

WAIT FOR FAULT CLEARENCE

AND

FAULT CLEARED

YES

NO

RESET BREAKER FAILURE SCHEME

TRIP BACK-UP BREAKERS

10.Low SF6 alarm/lockout

Stage-1 : Alarm•Alarm stage indicates the inadequate gas pressure inside CB.Stage-2 :Lockout.•Lock out stage blocks the operation of CB. No tripping or closing will happen then.

11.Control Relays

• Auto-Reclose Relay Used to auto reclosing of EHT feeders

• Tap change control RelayUsed for regulating the output voltage of transformer by raising/lowering the tap

12. Transformer Accessories • 1) Buchholz Relay.• 2) Oil Temperature Indicator.• 3) Winding Temperature Indicator.• 4) Magnetic Oil Level Gauge ( MOLG).• 5) Pressure relief device ( PDR).

Oil temperature Indicator.

• Indicates the temperature of the oil inside transformer.

• Gives alarm/Trip signal

Wednesday, May 3, 2023

Sensing device

Temperature indicator

Capilary tube

Oil Temperature Indicator

Fitted on the Thermometer pocket.

Winding Temperature relay

• Give indications about the temperature of winding temperature of oil.

WTI with HOT spot simulation CT arrangement.

FAN CONTROL CUBICLE

S1=alarm S2=trip

S3=cooler control 1S4=cooler control 2

Winding temperature indicator

12.2 Pressure Relief Device (PRD)

• When pressure inside transformer exceeds the PRV will operate to release excessive pressure inside the transformer.

• It issues a trip command and visual indication by operation of a liver (from horizontal position to vertical position)

Spring Operated Pressure relief device

12.3 Buchholtz relay

• Mechanical relay which has two mercury switches.

• When gas enters the chamber the position of switches deflects and it will make alarm in stage-1 and trip in stage-2 in Main tank Buchholz relay.

• Only trip signal is provided in OLTC buchholz relay.

Buchholz Relay Installation

5 x internal pipediameter (minimum)

3 x internal pipediameter (minimum)

Transformer

3 minimum

Oil conservator

Conservator

Buchholtz Relay arrangement.

12.4 MAGNETIC OIL LEVEL GAUGE (MOLG)

• Mounting 15 0 Inclination.• The movement of the float is transmitted to

the pointer by using a magnetic coupling.• The follower magnet follows the driving

magnet.• The driving magnet remains inside the

conservator and the other magnet out side the conservator.

Wednesday, May 3, 2023

EMPT

Y

1/41/

23/

4FULL

MOLGFLOATING MEMBER

PERMANENT MAGNET

BEARING