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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