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1. Copyright © SEL 2007
Out-Of-Step Protection
Fundamentals
Demetrios Tziouvaras
Schweitzer Engineering Laboratories, Inc.
IEEE PES San Francisco Chapter
December 13, 2007

2. Introduction
The aim of this presentation is to explain
The fundamentals of out-of-step (OOS)
protection
Discuss which relays and relay systems are
prone to operate during power swings
Share experiences and lessons learnt from the
past to avoid making the same mistakes

3. Introduction
Interconnected systems experienced an
increased number of large disturbances in
the last 15 years
Protective relay systems are often involved
during major disturbances
In many cases they prevent further propagation
of the disturbance
In some cases undesired relay operations have
contributed to cascading blackouts

4. Outline
Out of step (OOS) protection fundamentals
Relay performance during OOS conditions
Transmission lines
Generators
System design and protection
improvements
Conclusions

5. What Is a Power Swing?
Variation of power flow which occurs when
generator rotor angles are advancing or
retarding relative to each other in response to:
System faults
Line switching
Major load switching
Loss of large generation
Major system disturbances

6. Stable and Unstable Power Swings
Definitions
A power swing is considered stable if the
generators do not slip poles and the system
reaches a new state of equilibrium, i.e. an
acceptable operating condition
An unstable power swing results in a
generator or group of generators
experiencing pole slipping or loss-of-
synchronism for which some corrective
action must be taken.
Out-of-step is the same as an unstable power
swing.

7. Power Swings can Cause Undesired
Protective Relay Operation
Power swings can cause undesired relay
operation that may lead to:
Undesired tripping of power system elements
at undesired network locations
Weakening of the power system
Possible cascading outages and shutdown of
major portions of the power system
Damage of circuit breakers due to uncontrolled
tripping
Loss of human life

8. Unstable Power Swings
Damage System Integrity
Pole slipping may damage generators and
turbines
Low voltage conditions experienced during
unstable power swings may cause:
Motor stalling
Generator tripping
Damage to voltage-sensitive loads
Prolonged low voltages could cause instability of
smaller areas within a utility’s system

9. Need for Out-of-Step Protection
Generators operating asynchronously
with the rest of the power system cannot
regain stability as a result of any
excitation or regulator action
Asynchronous power system areas must
be separated in a controlled fashion to
avoid:
Equipment damage
Widespread outages in the power system

10. Philosophy of Power-Swing Protection
Detect both stable and unstable power
swings
Block tripping of relay elements prone to
operate during power swings
Differentiate between stable and unstable
power swings
Separate the system into islands during
out-of-step conditions

11. Philosophy of Power-Swing Protection
Separate the system at locations that
provide good balance of load/generation
in the resulting system islands
Trip only at pre-selected network locations
and block tripping at all other locations
Trip only under controlled transient
recovery voltages or with low current

12. Power System Stability
Brief Review

13. Power System Stability
Definition
The ability of the electric power system
to regain a state of operating equilibrium
after being subjected to disturbances
such as faults, line switching, load
rejection, loss-of-excitation, and loss of
generation.

14. Power Flow
Two-Machine System
1 2
3 4
Line 1
VS VR
Line 2
X
δ
⋅
= sin
X
VV
P
RSVS
VR
δ

15. Effect of Fault Type on Power Transfer
δ
P
Three-Phase Fault
Phase-Phase-Ground
Fault
Phase-Phase Fault
Single-Line-Ground Fault
Normal System
1 2
3 4
VS VR
Line 1
Line 2

16. Transient Stability Concepts
1 2
3 4
Prefault state (Both lines in service)
Fault state
Fault state with breaker 3 open
Post-fault state (Line 2 out)
VS VR
Line 1
Line 2

17. Equal-Area Criterion
δ180°
P0
P
Fault (one
breaker
open)
Fault
Prefault
Post-Fault
1
0
2
3
4
5
6
Area 2Area 1
1 2
3 4
VS VR
Line 1
Line 2

18. Effect of Fault Clearing Time
Unstable System
Fault
Post-Fault
δ
Prefault
P

19. Stable and Unstable Power Swings
Rotor Angle
Stable System
Unstable System
t
δ
δ0
δ1

20. Angular Instability
Distinguishing Features
Large voltage variations
Large power oscillations
Loss of synchronism
Zero voltage at the electrical center
Frequency excursions

21. Angular Instability
Large Voltage Variations
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
0
1
Voltage MagnitudePerunit
Seconds
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
0.5
1
1.5
PerUnit
Seconds

22. Angular Instability
Large Power Oscillations
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1000
-500
0
500
Real and Reactive Power
MW
Seconds
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-400
-200
0
200
400
600
MVar
Seconds

23. Angular Instability
V1 and Angle of V1 / I1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0
0.2
0.4
0.6
Positive-Sequence Voltage Magnitude
PerUnit
Seconds
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-200
0
200
400
Angle of (V1 / I1)
Degrees
Seconds

24. Relay Elements Prone to Operate
During Power Swings

25. Stable and Unstable Power Swings
Impedance Trajectories
Stable Swing
Unstable Swing
R
X

26. Relay Elements Prone to Operate
During Power Swings
Instantaneous phase overcurrent
Directional and non-directional
Undervoltage
Short time or instantaneous
Zone 1 distance
Zone 2 distance used in POTT scheme

27. Line Relays Prone to Operate
During Angular Instability
Zone 1 distance or overreaching distance
elements applied in DCB or POTT schemes
Potential reasons are:
Lack of PSB function or improper settings of
the PSB function
Lack of frequency tracking and long memory
polarizing voltage
Phasor measurement errors due to large
excursions of system frequency during
islanding

28. Relay Systems Unresponsive
to Power Swings
Phase comparison
Line current differential
Pilot-wire

29. Impedances Measured
by Distance Relays
21
V I
ZS
ZL ZR
VS VR
( ) SRLS
RS
S
ZZZZ
VV
V
IVZ −++
−
==

30. Impedance Locus for k = ES / ER = 1.0
δ
ZL
ZR
X
R
δ increases
δ decreases
δ=180°
ZS
0.5ZT
Z
R
S
S
T
Z
2
cotj1
2
Z
Z −⎟
⎠
⎞
⎜
⎝
⎛ δ
−=
R
S
E
E
k =

31. Two-Machine System Impedance Locii
R
X
S
k > 1
ES > ER
Rk = 1
k < 1
ES < ER

32. Power-Swing Protection
Relay Functions

33. Clarify the Terminology
Unstable power swing
Out of step (OOS)
Out-of-step blocking (OSB)
Power-swing blocking (PSB)
Out-of-step tripping (OST)
Pole-slip tripping

34. Power-Swing Protection
Relay Functions
Power-swing blocking (PSB)
Detects both stable and unstable power
swings
Prevents operation of protection elements
Out-of-step tripping (OST)
Detects unstable power swings or OOS
Separates system into islands with good
generation / load balance

35. Conventional PSB Scheme
Load Region
X
R
A
B
Z1
Inner Z
ElementDistance
Element
Outer Z
Element

36. Conventional OST Scheme
Load Region
X
R
A
B
Z1
Inner Z
ElementDistance
Element
Outer Z
Element

37. Disadvantages of
Conventional PSB Scheme
Needs detailed system information
Requires extensive system stability studies
It is difficult to set for long lines with heavy
loads
May fail after severe disturbances on
marginally stable systems
May fail during swings with high slip
frequency

38. Long Line With Heavy Load
ZL => ZΣ
A R
ZR
ZS
Z2
X
B
Swing Locus
Trajectory ZL

39. Short Line With Light Load
ZL << ZΣ
A R
ZR
ZS
Z2
X
B
Swing Locus
Trajectory
ZL

40. Unstable Swing
After Severe Disturbance
A
R
ZR
ZS
Z2
X
Swing Locus
Trajectory
B
ZL

41. New Zero-Setting PSB Function
Uses swing-center voltage (SCV)
Has no user settings
Does not need system parameters
Does not require system stability studies
Provides PSB during pole open
Detects evolving faults during power swings

42. Swing-Center Voltage (SCV)
( ) ( ) ( )
⎟
⎠
⎞
⎜
⎝
⎛δ
⋅⎟
⎠
⎞
⎜
⎝
⎛ δ
+ω=
2
t
cos
2
t
tsinE2tSCV
o'
o
o"
Z1S•I Z1L•I
VS
ER
δ
SCV
Z1R•I
ES
VR

43. SCV During System OOS Condition
Swing-Center Voltage
seconds
voltage(pu) SCV Amplitude
0 0.05 0.1 0.15 0.2
-1
-0.5
0
0.5
1

44. Local Estimate of SCV: Vcosϕ
ϕ⋅≈ cos|V|SCV S
o'Z1S•I Z1R•Iθ
VS
ES ϕ
SCV
ER
I
Vcosϕ
VR
AnglepedanceImSystem:θ

45. Local Estimate of SCV: Vcosϕ
⎟
⎠
⎞
⎜
⎝
⎛ δ
⋅=
2
cos1E1SCV
( )
dt
d
2
sin
2
1E
dt
1SCVd δ
⎟
⎠
⎞
⎜
⎝
⎛ δ
−=

46. Vcosϕ for 1-Rad/Sec OOS Condition
0
0 90 180 270 360
E1
E1/2
d (SCV1) / dt
δ
SCV1

47. Benefits of SCV for PSB Application
Independent of system source and line
impedance
Bounded:
Lower limit: zero
Upper limit: close to one per unit
Relates directly to angle difference of two
sources, δ

48. Transmission Line Relay Performance
During Out-of-Step Conditions

49. 500 kV System
Station C
Station D
Station E
Line 1
Line 3
Line 2
Unit 1
Unit 2
Lines 4, 5, and 6
Intertie
System B
System A

50. Angular Instability
Z1 Trajectory – Line 2 at Station C
-10 -5 0 5 10
-10
-8
-6
-4
-2
0
2
4
6
8
10
23
25
27 29 31 33 35 37 39
41
43
45
Positive-Sequence Impedance (Z1) Locus
Im(Z1)ohm
Re(Z1) ohm

51. EHV System – Northern California
Station C
Station D
Station E
Line 1
Line 3
Line 2
Unit 1
Unit 2
Lines 4, 5, and 6
Intertie
System B
System A

52. Zone 1 Operation During OOS
PSB Function not Enabled

53. Zone 1 Distance Is Blocked
First Slip Cycle

54. Zone 1 Distance Operates
Second Slip Cycle

55. Zone 1 Distance Operates
Second Slip Cycle
Fast slip frequency change
Setting fine-tuning could have prevented
operation of Zone 1
Concentric zone settings
Separation between concentric zones
PSB timer
All of these settings are difficult to make
Long heavy loaded lines
Require large number of stability studies

56. Proper Blocking of Distance Elements
by Zero Setting PSB
0 5 10 15 20
-1
-0.5
0
0.5
1
SCV1 (Solid), dSCV1/dt (Dash)
(pu),(pu/cyc)
0 5 10 15 20
Cycle
S
PSB
DPSB
67QUB
3PF Reset
PSB Reset
SLD Set
Start-Zn
SSD Set

57. Generator Relay Performance
During Disturbances

58. Units 1 and 2 Operations
Station C
Station D
Station E
Line 1
Line 3
Line 2
Unit 1
Unit 2
Lines 4, 5, and 6
Intertie
System B
System A

59. One Unit Trips by Undervoltage
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
-1
-0.5
0
0.5
1
Station C Voltage
Perunit
Seconds

60. Unit 1: Three-Phase P and Q
During OOS
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-2000
0
2000
Real Power
MW
Seconds
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1000
0
1000
Reactive Power
MVar
Seconds

61. Units 1 and 2 Trip
Inst. Dir. Phase OC Relay
-20 -10 0 10 20
-40
-30
-20
-10
0
10
1 3 5 7 9
11
13
15
17
19
21
23
25 27 29
Positive-Sequence Impedance (Z1) Locus
Im(Z1)ohm
Re(Z1) ohm

62. Protection System
and Other System Improvements
to Preserve System Stability

63. Proper System and Protection Design
Preserve System Stability
Prevent the occurrence of out-of-step
conditions
Install sufficient transmission capacity
Maintain adequate reactive reserves
Apply high-speed relaying systems and
high-speed reclosing
Apply single-phase tripping and reclosing
Apply wide-area stability controls

64. Wide-Area Stability Controls or SIPS
Preserve System Stability
Wide-area stability controls
Generator dropping
Direct load dropping
Fast valving
Insertion of breaking resistors
Series and shunt capacitor insertion
Use FACTS devices

65. Protection System
and Other Improvements
Improvements in transient stability
High speed fault clearing
Single phase tripping and reclosing
Apply local breaker failure protection on all
EHV and critical HV substations
Special protection systems
Controlled system separation
UVLS and UFLS

66. Protection System Improvements
Apply dual pilot protection relay systems on
all EHV and critical HV systems with
PSB capability, or with systems that are
immune to stable or unstable power swings
Replace secondary non-pilot line relay
systems in non-critical HV lines with:
A relay system that has similar functionality
with the main pilot protection system
Consider switching the communications
channel to the secondary relay system when
the Main 1 is out of service

67. Conclusions
Utilities must take every action economically
justifiable to preserve system stability
Out-of-step tripping should be applied and
operate only as a last resort to preserve
system stability
OST and PSB should be applied based on an
inter-regional controlled system separation
philosophy

68. Conclusions
OOS tripping must separate the system
at predetermined locations to minimize
the effect of the disturbance
OOS blocking compliments OOS tripping
by blocking relay elements prone to
operate and ensures a controlled system
separation
Controlled separation schemes provide
a safety net to lessen the impacts of
major disturbances

69. References
Out-Of-Step Protection Fundamentals and Advancements
http://www.selinc.com/techpprs/6163.pdf
Zero-Setting Power-Swing Blocking Protection
http://www.selinc.com/techpprs/6172_ZeroSetting_20050302.pdf
Relay Performance During Major System Disturbances
http://www.selinc.com/techpprs/6244_RelayPerformance_DT_20060914.pdf

70. Thank You