1 1
- 4
- 3
- 2
- 1
0
1
2
3
4
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
2 2
Introduction
Recoverable and non-recoverable power oscillations
3 3
Power Oscillations - Causes
What causes power oscillations?
Imbalance in generation and load
Faults (internal and external)
Load/Line switching
4 4
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
6 6
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
7 7
Elliptic shape: recoverable swing
Circle: OST condition
Power Oscillations – Definition (4)
8 8
Power Oscillations – Definition (5)
Recoverable
Non-Recoverable
9 9
Traditional Out of Step Detection Methods
10 10
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
11 11
New algorithm provides:
Setting free OST detection
CB tripping at a favourable angle
New Algorithm
12 12
New Algorithm - Principle
Setting Free OST Detection Principle
13 13
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
14 14
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)
15 15
Tripping Angle Control
Circuit breaker tripping angle control
16 16
Tripping Angle Control
Vs Vr
Current Locus (I)
90
180 (minimum Z)
Vr locus Electrical Centre locus
0
180
90
180
90
X
270
270
270
Current locus during oscillation is a circle
Drawing taken from Westinghouse book
17 17
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
18 18
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:
19 19
Supporting Elements(1)
Power Swing Detection and Blocking
20 20
Supporting Elements(2)
21 21
Supporting Elements(3)
22 22
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
23 23
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
24 24
- 3
- 2
- 1
0
1
2
3
1 2 5 4 5 0 7 7 6 0 1 0 1 3 1 2 6 6 1 5 1 9 1 7 7 2 2 0 2 5 2 2 7 8 2 5 3 1 2 7 8 4 3 0 3 7 3 2 9 0 3 5 4 3 3 7 9 6
S w i n g / P o l e S l i p I
O S T
Test Results (3)
25 25
Test Results (4)
26 26
Test Results (2)
27 27
Conclusions
28 28
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
29 29
Thank You
Questions?