Copyright © SEL 2007
Out-Of-Step Protection Fundamentals
Demetrios Tziouvaras
Schweitzer Engineering Laboratories, Inc.
IEEE PES San Francisco Chapter
December 13, 2007
Introduction
The aim of this presentation is to explainThe fundamentals of out-of-step (OOS) protection
Discuss which relays and relay systems are prone to operate during power swingsShare experiences and lessons learnt from the past to avoid making the same mistakes
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
Outline
Out of step (OOS) protection fundamentals
Relay performance during OOS conditionsTransmission lines
Generators
System design and protection improvements
Conclusions
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
Stable and Unstable Power SwingsDefinitions
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 conditionAn 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.
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
Unstable Power SwingsDamage 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
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 actionAsynchronous power system areas must be separated in a controlled fashion to avoid:
Equipment damageWidespread outages in the power system
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
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
Power System StabilityBrief Review
Power System StabilityDefinition
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.
Power FlowTwo-Machine System
1 2
3 4
Line 1VS VR
Line 2X
δ⋅
= sinXVV
P RSVS
VRδ
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 VRLine 1
Line 2
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 VRLine 1
Line 2
Equal-Area Criterion
δ180°
P0
P
Fault (one breaker open)Fault
Prefault
Post-Fault
1
0
2
34
5
6
Area 2Area 1
1 2
3 4
VS VRLine 1
Line 2
Effect of Fault Clearing TimeUnstable System
Fault
Post-Fault
δ
PrefaultP
Stable and Unstable Power SwingsRotor Angle
Stable System
Unstable System
t
δ
δ0
δ1
Angular InstabilityDistinguishing Features
Large voltage variations
Large power oscillations
Loss of synchronism
Zero voltage at the electrical center
Frequency excursions
Angular InstabilityLarge Voltage Variations
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
0
1
Voltage MagnitudePe
r uni
t
Seconds
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80
0.5
1
1.5
Per U
nit
Seconds
Angular InstabilityLarge 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 PowerM
W
Seconds
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-400-200
0200400600
MVa
r
Seconds
Angular InstabilityV1 and Angle of V1 / I1
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90
0.2
0.4
0.6
Positive-Sequence Voltage MagnitudePe
r Uni
t
Seconds
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-200
0
200
400Angle of (V1 / I1)
Deg
rees
Seconds
Relay Elements Prone to OperateDuring Power Swings
Stable and Unstable Power SwingsImpedance Trajectories
Stable Swing
Unstable Swing
R
X
Relay Elements Prone to OperateDuring Power Swings
Instantaneous phase overcurrentDirectional and non-directional
UndervoltageShort time or instantaneous
Zone 1 distance
Zone 2 distance used in POTT scheme
Line Relays Prone to OperateDuring 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
Relay Systems Unresponsiveto Power Swings
Phase comparison
Line current differential
Pilot-wire
Impedances Measuredby Distance Relays
21
V I
ZS ZL ZR
VS VR
( ) SRLSRS
S ZZZZVV
VIVZ −++−
==
Impedance Locus for k = ES / ER = 1.0
δ
ZL
ZR
X
Rδ increases
δ decreasesδ=180°
ZS0.5ZT
Z
R
S
ST Z
2cotj1
2ZZ −⎟
⎠⎞
⎜⎝⎛ δ−=
R
S
EEk =
Two-Machine System Impedance Locii
RX
S
k > 1ES > ER
Rk = 1
k < 1ES < ER
Power-Swing ProtectionRelay Functions
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
Power-Swing ProtectionRelay 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
Conventional PSB Scheme
Load Region
X
RA
BZ1
Inner ZElementDistance
ElementOuter Z Element
Conventional OST Scheme
Load Region
X
RA
BZ1
Inner ZElementDistance
ElementOuter ZElement
Disadvantages ofConventional 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
Long Line With Heavy LoadZL => ZΣ
A R
ZR
ZS
Z2
X
B
Swing LocusTrajectory ZL
Short Line With Light LoadZL << ZΣ
A R
ZR
ZS
Z2
X
BSwing Locus
Trajectory
ZL
Unstable SwingAfter Severe Disturbance
A R
ZR
ZS
Z2
X
Swing LocusTrajectory
B
ZL
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
Swing-Center Voltage (SCV)
( ) ( ) ( )⎟⎠⎞
⎜⎝⎛ δ⋅⎟
⎠⎞
⎜⎝⎛ δ+ω=
2tcos
2ttsinE2tSCV
o'
o
o"
Z1S•I Z1L•I
VS
ERδ
SCV
Z1R•I
ES
VR
SCV During System OOS ConditionSwing-Center Voltage
seconds
volta
ge (p
u)SCV Amplitude
0 0.05 0.1 0.15 0.2-1
-0.5
0
0.5
1
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
Local Estimate of SCV: Vcosϕ
⎟⎠⎞
⎜⎝⎛ δ⋅=2
cos1E1SCV
( )dtd
2sin
21E
dt1SCVd δ
⎟⎠⎞
⎜⎝⎛ δ−=
Vcosϕ for 1-Rad/Sec OOS Condition
0
0 90 180 270 360
E1
E1/2d (SCV1) / dt
δ
SCV1
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
Transmission Line Relay PerformanceDuring Out-of-Step Conditions
500 kV System
Station CStation D
Station E
Line 1
Line 3
Line 2
Unit 1
Unit 2
Lines 4, 5, and 6Intertie
System B
System A
Angular InstabilityZ1 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 3941
43
45
Positive-Sequence Impedance (Z1) LocusIm
(Z1)
ohm
Re(Z1) ohm
EHV System – Northern California
Station CStation D
Station E
Line 1
Line 3
Line 2
Unit 1
Unit 2
Lines 4, 5, and 6Intertie
System B
System A
Zone 1 Operation During OOS PSB Function not Enabled
Zone 1 Distance Is BlockedFirst Slip Cycle
Zone 1 Distance OperatesSecond Slip Cycle
Zone 1 Distance OperatesSecond Slip Cycle
Fast slip frequency changeSetting fine-tuning could have prevented operation of Zone 1
Concentric zone settingsSeparation between concentric zones
PSB timer
All of these settings are difficult to makeLong heavy loaded linesRequire large number of stability studies
Proper Blocking of Distance Elementsby Zero Setting PSB
0 5 10 15 20-1
-0.5
0
0.5
1SCV1 (Solid), dSCV1/dt (Dash)
(pu)
,(pu/
cyc)
0 5 10 15 20Cycle
S
PSBDPSB
67QUB3PF ResetPSB Reset
SLD SetStart-ZnSSD Set
Generator Relay PerformanceDuring Disturbances
Units 1 and 2 Operations
Station CStation D
Station E
Line 1
Line 3
Line 2
Unit 1
Unit 2
Lines 4, 5, and 6Intertie
System B
System A
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 VoltagePe
r uni
t
Seconds
Unit 1: Three-Phase P and QDuring OOS
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-2000
0
2000Real Power
MW
Seconds
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1000
0
1000Reactive Power
MVa
r
Seconds
Units 1 and 2 TripInst. Dir. Phase OC Relay
-20 -10 0 10 20-40
-30
-20
-10
0
10
1 3 5 7 9 1113
15
17
19
21
23 25 27 29
Positive-Sequence Impedance (Z1) LocusIm
(Z1)
ohm
Re(Z1) ohm
Protection Systemand Other System Improvements
to Preserve System Stability
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
Wide-Area Stability Controls or SIPSPreserve System Stability
Wide-area stability controlsGenerator dropping
Direct load dropping
Fast valving
Insertion of breaking resistors
Series and shunt capacitor insertion
Use FACTS devices
Protection Systemand Other Improvements
Improvements in transient stabilityHigh 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
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 systemConsider switching the communications channel to the secondary relay system when the Main 1 is out of service
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
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
References
Out-Of-Step Protection Fundamentals and Advancementshttp://www.selinc.com/techpprs/6163.pdf
Zero-Setting Power-Swing Blocking Protectionhttp://www.selinc.com/techpprs/6172_ZeroSetting_20050302.pdf
Relay Performance During Major System Disturbanceshttp://www.selinc.com/techpprs/6244_RelayPerformance_DT_20060914.pdf
Thank You