Protection 2

7/30/2019 Protection 2

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Principles of Protection Part 21

PRINCIPLES

OF POWER SYSTEM

PROTECTION- Part 2 -

PRINCIPLESPRINCIPLES

OF POWER SYSTEMOF POWER SYSTEM

PROTECTIONPROTECTION-- Part 2Part 2 --

Bob Coulter

Power System Protection


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Protection Function - ComponentsProtection FunctionProtection Function -- ComponentsComponents

CB trip coilTr

Communications LinkPCL

Man-machine interfaceHMI

DC Auxiliary supplyDC Aux

Voltage TransformerVT

Current TransformerCT

Protected ItemEquip

Circuit BreakerCB

Protection RelayPR

Bus

CBCT

P

C

L

Equip

PR

Tr

VT

DC Aux HMI

Basic Arrangement of a

Protection Scheme

Control


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Methods of Detecting FaultsMethods of Detecting FaultsMethods of Detecting Faults

Magnitude of current Overcurrent protection

Magnitude of current in earth or neutral Earth Fault protection

Magnitude and Phase Angle of current Directional Overcurrent protection

Magnitude and Phase Angle of current in earth or neutral Directional Earth Fault

protection

Magnitude and Angle of Impedance (Ratio V/I) Impedance protection

Difference between two or more currents Differential protection

Difference between Phase Angles of two currents Phase Comparison protection

Magnitude of negative sequence current

Magnitude of Voltage Overvoltage or Undervoltage protection

Magnitude of Frequency Over or Underfrequency protection

Temperature Thermal protection

Specials i.e. transformer gas protection,


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OVERCURRENT and EARTH FAULT

PROTECTION

OVERCURRENT and EARTH FAULTOVERCURRENT and EARTH FAULT

PROTECTIONPROTECTION

Bob Coulter


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Principle of an Overcurrent RelayPrinciple of anPrinciple of an

OvercurrentOvercurrent

RelayRelay

Ip

Is

I

Iset Tset

IIset TimeDelay

Generator

Current

Level

Detector

Output

Auxiliary

Relay

+ve

To Circuit Breaker

Trip Coil

Primary

current

CT secondary

current

Operating

Current

Setting

Time Delay

Setting

Ip

Is

Iset

Tset

Operate Zone

Iset

Tset

Time

Current

Definite Time Characteristic

Time

Current

Inverse Time Characteristic

Operate Zone

Iset 10xIset

Tset


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Connection of Overcurrent Relays for Phase andEarth Fault ProtectionConnection ofConnection of OvercurrentOvercurrent Relays for Phase andRelays for Phase andEarth Fault ProtectionEarth Fault Protection

Secondary

current

Is

Primary fault

current

Ip

Earth Fault

Overcurrent

Relay

EF

Phase

Overcurrent

Relay

OC

.Current

Transformer

CTCT

OC OC OC

EF

Ip(R)

Ip(W)

Ip(B)

Is(B) Is(W) Is(R)


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Current Flow for Phase-to-Phase FaultCurrent Flow for PhaseCurrent Flow for Phase--toto--Phase FaultPhase Fault

Secondary

current

Is

Primary fault

current

Ip

Earth Fault

Overcurrent

Relay

EF

Phase

Overcurrent

Relay

OC

.Current

Transformer

CTCT

OC OC OC

EF

0

Ip(W)

Ip(B)

Is(B) Is(W) 0


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Current Flow for Phase-to-Earth FaultCurrent Flow for PhaseCurrent Flow for Phase--toto--Earth FaultEarth Fault

Secondary

current

Is

Primary fault

current

Ip

Earth Fault

Overcurrent

Relay

EF

Phase

Overcurrent

Relay

OC

Current

Transformer

CTCT

OC OC OC

EF

Ip(R)

0

0

0 0 Is(R)


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Four-Wire Systems - 1FourFour--Wire SystemsWire Systems -- 11

Secondary

current

Is

Primary fault

current

Ip

Earth Fault

Overcurrent

Relay

EF

Phase

Overcurrent

Relay

OC

Current

Transformer

CTCT

OC OC OC

EF

Neutral


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Four-Wire Systems - 4FourFour--Wire SystemsWire Systems -- 44

Secondary

current

Is

Primary fault

current

Ip

Earth Fault

Overcurrent

Relay

EF

Phase

Overcurrent

Relay

OC

Current

Transformer

CTCT

OC OC OC

EF

Neutral

RCD


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Setting Overcurrent Protection - 1SettingSettingOvercurrentOvercurrent ProtectionProtection -- 11 Load current to be carried safety margin of 30 to 50%

Minimum fault current to be detected

Phase to phase or phase to earth

Allowance for fault resistance

Back-up for failure of adjacent protection

Maximum fault current to be detected

Short-time rating of protected equipment

Time coordination margin between adjacent protection schemes


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Load Modeling ConsiderationsLoad Modeling Considerations Network loads usually recorded

as 15 or 30 minute averagedvalues

OK for thermal, load accountingand setting slow control schemepurposes

Real and reactive powercomponents varyinstantaneously and to somedegree independently

Not much known about loadvariation over an averaging timeframe of seconds

MW

MVAr

hh:15 hh:30 hh:45 Time Interval


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Statistical distribution of short-time load variationStatistical distribution of shortStatistical distribution of short--time load variationtime load variation


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Example of short-time load variationExample of shortExample of short--time load variationtime load variation

10 second load values - output from simulation

0 100 200 300 400 500 600 700 800 900 100012

12.2

12.4

12.6

12.8

13

13.2

Time in seconds

Load

MWs


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Time Coordination Margin betweenOvercurrent RelaysTime Coordination Margin betweenTime Coordination Margin between

OvercurrentOvercurrent RelaysRelays

Operating time interval between the operation of two adjacent

overcurrent relays has to allow for:

Fault current interrupting time of the relevant circuit breaker

Overshoot time of the relay

Errors in current transformer ratio, relay operating time and calculated

fault current magnitude

Example 1: Electromechanical Relay and Oil Interruption CB

Time Coordination Margin = 0.4 seconds

Example 2: Digital Relay and Vacuum Interruption CB

Time Coordination Margin = 0.25 seconds


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IMPEDANCE PROTECTION

PRINCIPLES

IMPEDANCE PROTECTIONIMPEDANCE PROTECTION

PRINCIPLESPRINCIPLES

Power System Protection


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Impedance or Distance ProtectionImpedance or Distance ProtectionImpedance or Distance Protection

CB trip coilTr

Communications LinkPCL

DC Auxiliary supplyDC Aux

Voltage TransformerVT

Current TransformerCTProtected ItemLine

Circuit BreakerCB

Impedance RelayZ0

Protected

EquipmentIp1 Ip2

Is1 Is1 Is2 Is2

N1

N2

Is1 Is2

ICurrent

Measuring Relay

FaultIf

Consider ideal current transformer performance: If = Ip1+ Ip2

Therefore Is1 Is2 Therefore I = Is1+Is2 0, magnitude ofI> 0 Current measuring relay operates


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Basic Current Differential Protection Internal Fault 2Basic Current Differential ProtectionBasic Current Differential Protection Internal Fault 2Internal Fault 2

Consider ideal current transformer performance: If = Ip1 as Is2 = 0 ie no fault current infeed from one side

Is2 = 0

Therefore I = Is1 0, magnitude ofI> 0 Current measuring relay operates

CT 1 CT 2

|I|>0

Protected

EquipmentIp1

Ip2 = 0

Is1 Is1

N1

N2

Is1

ICurrent

Measuring Relay

FaultIf

Is2 = 0


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Consider ideal current transformer performance: If = (Ip1- Ip2) > 0

Therefore Is1 Is2 Therefore I = Is1- Is2 0, magnitude ofI> 0

Current measuring relay operates

CT 1 CT 2

|I|>0

Protected

EquipmentIp1 Ip2

Is1 Is1 Is2 Is2

N1

N2

Is1 Is2

ICurrent Measuring

Relay

High Impedance

Fault

If


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Consider ideal current transformer performance: Ip1 = Ip2

Therefore Is1 = Is2

Therefore I = 0, magnitude ofI = 0 Current measuring relay does not operate

CT 1 CT 2

|I|>0

Protected EquipmentIp1 Ip2

Is1 Is1 Is2 Is2

N1

N2

Is1 Is2

ICurrent Measuring

Relay

Short-circuitedturns


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Summary Current Differential PrinciplesSummary Current Differential PrinciplesSummary Current Differential Principles

Protection zone defined by current transformer locations

Measures quantities for fault detection at two or more

locations

Faults between current transformer locations are called

internal faults and faults outside of zone between current

transformers are called external or out of zone faults

In principle can have very high sensitivity, i.e. operation for

high impedance type faults possible

Will not operate for load current

Will not operate for internal series type faults


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Balanced Voltage Differential Protection

External Fault

Balanced Voltage Differential ProtectionBalanced Voltage Differential Protection

External FaultExternal Fault

Consider ideal current transformer performance:

Ip1 = Ip2 Vs1 = Vs2 and Is1 = Is2 = 0

Therefore magnitude ofI = 0

Current measuring relay does not operate

CT 1 CT 2

Protected

EquipmentIp1 Ip2

Is1 Is1 Is2 Is2

Current Measuring Relay

|I|>0

Vs1 Vs2


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Balanced Voltage Differential Protection

Internal Fault

Balanced Voltage Differential ProtectionBalanced Voltage Differential Protection

Internal FaultInternal Fault

Consider ideal current transformer performance:

Vs1 Vs2 Therefore magnitude ofI 0 Current measuring relay operates

CT 1 CT 2

Protected

EquipmentIp1 Ip2

Is1 Is1 Is2 Is2

Current Measuring Relay

|I|>0

Vs1 Vs2

FaultIf


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Practical Current Differential CircuitPractical Current Differential CircuitPractical Current Differential Circuit

CT 1 CT 2

|I|Iset

Protected

EquipmentIp1 Ip2

Is1

Is1 Is2

Is2

N1

N2

Is1 Is2I

Current

Measuring

Relay

RL

RLRL

RL

Z

Rc Rc

Xm Xm

Note: Operating criteria for protection now |I|Iset instead of |I|>0

Iset = Current setting of measuring relayZ = Input impedance of current measuring

relay

RL = secondary circuit wiring resistancesRc = resistance of current transformer

secondary winding

Xm = Magnetising reactance of current transformer, note this is non-linear


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45

Principle of Biased Current Differential RelayPrinciple of Biased Current Differential RelayPrinciple of Biased Current Differential Relay

C is a current magnitude comparator - operates forIs1-Is2 k[Is1+Is2]k[Is1+Is2[ input is called restraint or bias input, [Is1-Is2] input is called

operate input. k is less than 0.5, typically 0.1 to 0.4,and is called the

bias setting.

Is1

CT 1 CT 2

Is1 Is2 Is2

Is1 Is2

Protected

EquipmentIp1 Ip2

Is1-Is2

Is1-Is2

Is1 Is2

k[Is1+Is2]

k

1 1

1 1

C


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46

Biased Current Differential Relay External FaultBiased Current Differential RelayBiased Current Differential Relay External FaultExternal Fault

Ip1 = Ip2

Is1 and Is2 will be similar in magnitude and phase therefore

Is1-Is2 will be small and Is1+Is2 will be large.

Is1-Is2 < k[Is1+Is2]so no operation

Is1 Is1 Is2 Is2

Is1 Is2

Protected

EquipmentIp1 Ip2

Is1-Is2

Is1-Is2

Is1 Is2

k[Is1+Is2]

k

1 1

1 1

C

CT 2CT 1


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47

Biased Current Differential Relay Internal Fault 1Biased Current Differential RelayBiased Current Differential Relay Internal Fault 1Internal Fault 1

Primary fault current If = Ip1+Ip2

Is1-Is2 will be large and Is1+Is2 will be small.

Is1-Is2 > k[Is1+Is2]so operation achieved

Is1 Is1 Is2 Is2

Is1 Is2

Protected

EquipmentIp1 Ip2

Is1-Is2

Is1-Is2

Is1 Is2

k[Is1+Is2]

k

1 1

1 1

C

CT 2CT 1 FaultIf


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48

Biased Current Differential Relay Internal Fault 2Biased Current Differential RelayBiased Current Differential Relay Internal Fault 2Internal Fault 2

Primary fault current If = Ip1

Operate input will be equal to Is1, restraint input will be k[Is1]

Is1-Is2 > k[Is1+Is2]so operation achieved

Is1 Is1 0 0

Is1 0

Protected

EquipmentIp1

Ip2=0

Is1

Is1

Is1

k[Is1]

k

1 1

1 1

C

CT 2CT 1 FaultIf


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49

Arrangement for Phase by Phase Protection

Protected

Equipment

I

I

I

R Relay

B Relay

W Relay

R

W

B

R

W

B


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50

Arrangement for Earth Fault Protection Only

Protected Equipment

I

RW

B

N

Restricted Earth

Fault Protection

Date post:	14-Apr-2018
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