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REL352AN-93L-02

Phase Comparison Relay REL352 Current Pickup Calculation

Introduction

This note describes how to calculate the current pick-up level for different types of faults.

IT current

REL352 uses sequence filters to obtain positive, negative and zero sequence currents. These currents arethen combined into one quantity:

021 021 ICICICIT ++=

The positive, negative and zero sequence current is computed from the phase currents by the use ofClarkes components based on sample currents that can mathematically be expressed as:

3

3

43.104983.043.134983.015966.0

3

43.134983.043.104983.015966.0

0

2

1

CBA

CBA

CBA

IIII

IIII

IIII

++=

++=

++=

The advantage of using Clarkes components is that they give a close approximation of symmetricalcomponents while not having the need of using complex numbers.

Note that all sequence component currents are referenced to phase A current.

General calculation of IT

Based on input currents and C-settings, the IT current is calculated as follows:

Settings

0

2

1

C

C

C

ABBApplication NoteSubstation Automation and Protection Division


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Phase Comparison Relay REL352 Current Pickup Calculation AN-93L-02

2

Input currents

C

B

A

I

I

I

IT

3

)(

3

)43.104983.043.134983.015966.0(

3

)43.134983.043.104983.015966.0(

02

1

002211

CBACBA

CBA

T

T

IIICIIIC

IIICI

ICICICI

+++

+++

+++

=

=++=

Example 1a, phase A to ground fault

With settings

0.1

7.0

1.0

0

2

1

=

=

=

C

C

C

and input currents

0

0

00.5

=

=

=

C

B

A

I

I

AI

IT becomes:

ACCC

I

ICICICI

T

T

65.23

)0.5(

3

)15966.00.5(

3

)15966.00.5( 021

002211

=+

+

=

=++=


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3

Example 1b, phase B to ground fault

With settings

0.1

7.0

1.0

0

2

1

=

=

=

C

C

C

and input currents

0

1200.5

0

=

=

=

C

B

A

I

I

I

IT becomes:

ACCC

I

ICICICI

T

T

33.13

)1200.5(

3

)12043.134983.00.5(

3

)12043.104983.00.5( 021

002211

=

+

+

=

=++=

That the IT current is different for a phase B to ground fault compared to phase A to ground is due to thefact that Clarkes symmetrical component computations are made referenced to phase A.

REL352 Trip Criterion

REL352 operates when the IT RMS current exceeds the LP setting, assuming that the relay is connected inloop-back or back-to-back (with the same current fed into the two relays):

thresholdoperatingset

currentcomposite

=

=

>

LP

I

where

LPI

T

T

It is also assumed that all other settings (IPL, IGL, ITA1, ITA2) are set so that they do not restrict tripping atthe set LP level.


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Sometimes, ITA2 is the limiting threshold, and then the operating criterion will be:

thresholdoperatingset2

currentcomposite

2

=

=

>

ITA

I

where

ITAI

T

T

Pickup calculation

To determine the theoretical pickup current for different types of fault, we need to determine that the outputfrom the trip criterion exceeds the set operating threshold.

Set-up in loop-back or back-to-back is assumed so that IR= IL= IT, i.e. the infeed current from both ends areequal in magnitude and in phase. This represents an internal fault.

In order to determine the required current threshold for operation for different types of faults the expressionsabove for ITand sequence currents need to be entered into the formula, solving the phase current(s).

Phase A to ground fault

Input currents

0

0

0

=

=

=

C

B

aA

I

I

AII

Pickup current phase A

)15966.015966.0(

3

3

15966.015966.0

021

021

002211

CCCLPI

LPICICIC

LPICICIC

LPI

a

aaa

T

++>

>++

>++

>


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Phase B to ground fault

Input currents

0

120

0

=

=

=

C

bB

A

I

II

I

Pickup current phase B

)12043.254983.057.15983.0(

3

3

12043.254983.057.15983.0

021

021

002211

++>

>++

>++

>

CCCLPI

LPICICIC

LPICICIC

LPI

b

bbb

T

Phase C to ground fault

Input currents

=

=

=

120

0

0

cC

B

A

II

I

I


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Pickup current phase C

)12043.224983.043.14983.0(

3

3

12043.224983.043.14983.0

021

021

002211

++>

>++

>++

>

CCCLPI

LPICICIC

LPICICIC

LPI

c

ccc

T

Phase A to B fault

Input currents

0

180

0

=

=

=

C

abB

abA

I

II

II

Pickup current phases A and B

)]57.45983.015966.0()43.284983.015966.0([

3

3

)57.45983.015966.0()43.284983.015966.0(

21

21

002211

+++>

>+++

>++

>

CCLPI

LPIICIIC

LPICICIC

LPI

ab

abababab

T


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Phase B to C fault

Input currents

=

=

=

60

120

0

bcC

bcB

A

II

II

I

Pickup current phases B and C

)]43.164983.043.254983.0()43.74983.057.15983.0([

3

3

)43.164983.043.254983.0()43.74983.057.15983.0(

21

21

002211

+++>

>+++

>++

>

CCLPI

LIICIIC

LPICICIC

LPI

bc

bcbcbcbc

T

Phase C to A fault

Input currents

=

=

=

120

0

60

caC

B

caA

II

I

II


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Pickup current phases C and A

)]43.224983.075966.0()43.14983.075966.0([

3

3

)43.224983.075966.0()43.14983.075966.0(

21

21

002211

+++>

>+++

>++

>

CCLPI

LPIICIIC

LPICICIC

LPI

ca

cacacaca

T

Three phase ABC fault

Input currents

=

=

=

120

120

0

abcC

abcB

abcA

II

II

II

Pickup current phases A, B and C

43.224983.043.254983.015966.0()43.14983.057.15983.015966.0(

3

3

)43.224983.043.254983.015966.0(

3

)43.14983.057.15983.015966.0(

21

2

1

002211

+++++>

>++

+++

>++

>

CCLPI

LPIIIC

IIIC

LPICICIC

LPI

abc

abcabcabc

abcabcabc

T

Theoretically, there should be no negative sequence current for a three phase fault, but Clarke introduces asmall value. However, the formula for three phase fault pickup current can be simplified by removing the C2term. The error by doing so will result in less than 10% error. Note however, that the below formula gives a10% highertheoretical result (i.e. the actual test current required for operation is lower).


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Example 2b, Phase B to ground fault

With input currents:

0

120

0

=

=

=

C

bB

A

I

II

I

and settings:

0.10

7.02

1.01

5.0

=

=

=

=

C

C

C

OTH

the required phase B current becomes:

63.5)1200.143.254983.07.057.15983.01.0(

3

3

1200.143.254983.07.057.15983.01.0

002211

=++

>

>++

>++

>

LPI

LPIII

LPICICIC

LPI

b

bbb

T

That the pickup current is higher for a phase B to ground fault compared to phase A to ground is due to thefact that the symmetrical component computations of IT are made referenced to phase A.

Contributed by:Solveig WardRevision 0, 03/25/02

ABB Inc.7036 Snowdrift RoadAllentown, PA 18106800-634-6005 Fax 610-395-1055Email: [email protected]: www.abb.com/substationautomation

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