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New Setting Free Algorithm for Out of Step Tripping

Sept. 2009 MOSCOW

H Kang – ART Areva T&D

B Cvorovic, P Horton- SAS Areva T&D

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Introduction

Recoverable and non-recoverable power oscillations

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Power Oscillations - Causes

What causes power oscillations?

Imbalance in generation and load

Faults (internal and external)

Load/Line switching

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Power Oscillations – Definition (1)

Nature and definition of power oscillations

Power oscillation that leads to system split is called:

Out of step condition or pole slip or non-recoverable swing

Power oscillation that will not cause system split are called:

Stable swings or Recoverable swings

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Power Oscillations – Definition (3)

Out of step condition

Occurs when two internal voltages of equivalent sources are in opposite direction

At that point the phase (swing) current is maximum

The position of the electrical centre will depend on Zs/Zr ratio

Recoverable swings

Two voltages typically oscillate between up to 120deg

VrVsZs Zr

OST condition :

I=(Vs -Vr )/ZT =(Vs -(-Vr ))/ZT ~2 Vn /ZT

ZT =Zs +Zline +Zr

Electrical. center

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Elliptic shape: recoverable swing

Circle: OST condition

Power Oscillations – Definition (4)

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Power Oscillations – Definition (5)

Recoverable

Non-Recoverable

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Traditional Out of Step Detection Methods

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Conventional methods:

Conventional methods use blinders to determine speed of impedance crossing the ∆R region (R6-R5). They may predict or detect OST condition.

If polarity of ‘R’ has changed on exiting Z5, it is Out of Step condition (already happened)

If positive sequence impedance crosses ∆Z region faster than ‘delta T’ set time the predictive OST is declared

Disadvantages

Difficulties to set blinders due to heavy loading

Setting dependant on system topology, thus settings may be inaccurate

Comprehensive system study required – increases the engineering time

Prone to unstable operation in series compensated lines during MOV operation

Out of step tripR

Z5

ZL

Z6

Z5'

Z6'

R5 R6R5'R6'

Predictive Out of step trip

+jX

Recoverable swing

Disadvantages

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New algorithm provides:

Setting free OST detection

CB tripping at a favourable angle

New Algorithm

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New Algorithm - Principle

Setting Free OST Detection Principle

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Setting Free OST Detection – Principle (1)

OST detection principle:

Recoverable swings: ∆R changes polarity when ∆I changes polarity

Non- recoverable swings: ∆R doesn’t change polarity when ∆I changes polarity

I=positive sequence currentR=positive sequence resistance

RECOVERABLE SWING:Point when both, ∆I and∆R change polarity

Point when ∆I changes polarity and ∆R polarity

remains unchanged

NON RECOVERABLE SWING:

R

jX ∆I=IN+1-IN∆R=RN+1-RN

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Recoverable Swings

Delta I and Delta R change polarity around same time

Pole Slips

When Delta I changes polarity , Delta R does not

Recoverable Swing Pole Slip

Setting Free OST Detection – Principle (2)

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Tripping Angle Control

Circuit breaker tripping angle control

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Tripping Angle Control

Vs Vr

Current Locus (I)

90

180 (minimum Z)

Vr locus Electrical Centre locus

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180

90

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270

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Current locus during oscillation is a circle

Drawing taken from Westinghouse book

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Tripping Angle Control (1)

Current during oscillation can be defined as:

I swing=IMAX sin (θ/2)

where θ is the angle between internal voltages of sources

Imax

I240

90

240

180

270

0

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Tripping Angle Control (2)

I trip=IMAX sin (240/2)=0.866 IMAX

Maximum phase (swing) current is recorded at the point when ∆I changes polarity (that point corresponds to minimum impedance)

Favourable (safe) split angle entered, for example 240 degrees

Tripping command is issued when phase current drops to:

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Supporting Elements(1)

Power Swing Detection and Blocking

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Supporting Elements(2)

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Supporting Elements(3)

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Proof of Concept

Pole slip COMTRADES captured by the relays for various system tests were used to prove that the basic principle was sound

Modifications were made to the original principle to make it more robust.

Logic implemented to account for difference between the frequency of I and V during swings

Logic to make the algorithms immune to system disturbances and faults

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Test Results (1)

Numerous cases from different systems were applied

Algorithm remains stable during power system faults or recoverable swings

Both, balanced and open pole oscillation tested

No mal-operation recorded during evolving faults, sudden change of power flow, cross country faults and frequency variations

Angle set tripping compared with actual angle across the breaker proved to be accurate

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S w i n g / P o l e S l i p I

O S T

Test Results (3)

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Test Results (4)

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Test Results (2)

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Conclusions

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Setting free

All conventional methods require system studies and comprehensive settings

No blinders, no starters, thus no constraints on operating characteristics versus loading

Immune to topology changes

Security – Provides control over the angle at which the system is to be split.

Minimises chances of breaker opening at voltage maximum

Advantages

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Thank You

Questions?