Download Power Oscillations – Definition (1)

New Setting Free Algorithm for
Out of Step Tripping
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Sept. 2009 MOSCOW
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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:

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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
Electrical
.
center
Vs
Vr
Zs
Zr
OST condition :
I=(Vs -Vr )/ ZT =(Vs -(-Vr ))/ZT ~2 Vn /ZT
ZT = Zs +Zline +Zr
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Power Oscillations – Definition (4)
 Elliptic shape:
recoverable
swing
 Circle:
OST condition
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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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Disadvantages

Conventional methods:
 Conventional
methods
use
blinders to determine speed of
impedance crossing the ∆R
region (R6-R5). They may
predict or detect OST condition.
+jX
Z6

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
Z5
Predictive Out of
step trip
ZL
Recoverable swing
Out of step trip
R6'
R5
R5'

R6
R

Disadvantages
 Difficulties to set blinders due to
heavy loading
 Setting dependant on system
topology, thus settings may be
inaccurate
Z5'
Z6'
 Comprehensive system study
required
–
increases
engineering time
the
 Prone to unstable operation in
series
compensated
during MOV operation
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lines
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New Algorithm
 New algorithm provides:
 Setting free OST detection
 CB tripping at a favourable angle
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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:
 Non- recoverable swings:
jX
∆R changes polarity when ∆I changes polarity
∆R doesn’t change polarity when ∆I changes polarity
∆I=IN+1-IN
∆R=RN+1-RN
I=positive sequence current
R=positive sequence resistance
RECOVERABLE SWING
:
Point when both, ∆I and
∆R change polarity
R
NON RECOVERABLE SWING
:
Point when ∆I changes
polarity and ∆R polarity
remains unchanged
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Setting Free OST Detection – Principle (2)
Recoverable Swings
Pole Slips
Delta I and Delta R change
polarity around same time
Recoverable Swing
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When Delta I changes
polarity , Delta R does
not
Pole Slip
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Tripping Angle Control
Circuit breaker tripping angle control
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Tripping Angle Control
 Current locus during oscillation is a circle
0
Vs
Vr
90 
90 
270 
180 (minimum Z)
180 
90 
270 
Electrical Centre locus
Vr locus
180 
270 
Current Locus (I)
X
 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
90
180
0
Imax
I240
240
270
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Tripping Angle Control (2)
 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:
I trip=IMAX sin (240/2)=0.866 IMAX
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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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Test Results (3)
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Swing /Pole Slip I
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507
760 1013 1266 1519 1772 2025 2278 2531 2784 3037 3290 3543 3796
OST
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Test Results (4)
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Test Results (2)
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Conclusions
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Advantages
 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
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Thank You
Questions?
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