blackout avoidance & undervoltage load shedding

BLACKOUT AVOIDANCE &

UNDERVOLTAGE LOAD SHEDDING



Chuck MozinaConsultant

Beckwith Electric

Chuck MozinaConsultant

Beckwith Electric

BLACKOUTSHow and why they occur

Why voltage rather than frequency is the leading edge indicator of system collapse

How blackout conditions effect generators and generator protection

Undervoltage load shedding

Blackout Avoidance

RECENT BLACKOUTS

2003 - East Coast Blackout2003 - Italian Blackout2002 - Swedish Blackout1997 - PJM Disturbance1996 - West Coast Blackout1995 - PECO Disturbance1987 - City of Memphis

Blackout Avoidance

Root Cause of Recent Blackouts

VOLTAGE COLLAPSE – WHY?

Today, major generation sources are remote from load centers. This was NOT the case 35 years ago.

This makes the power system very reliant on transmission system to transport power to load centers.

Blackout Avoidance

VOLTAGE COLLAPSE – WHY?

Purchase power from remote sources to save $$$.

New generation built remotely from load centers.

Little new transmission being built.

Utility loads are increasingly made up of air conditioning motors susceptible to stall conditions due to transmission system faults.

Root Cause of Recent Blackouts

Blackout Avoidance

As lines trip between remote generation and load center, the reactance increases.

This increases the reactive (VAR) losses-reducing the voltage at the load center.

The voltage phase angle between thegenerators at the load center andremote generators also increase.

REMOTEGENERATION LOCAL LOAD

CENTER

LINE 1

LINE 2

LINE 3

LINE 4

LINE 5

LINE 6

How Voltage Collapses Occur

Blackout Avoidance

POWER TRANSFEREXAMPLE

POWER TRANSFEREXAMPLE

Blackout Avoidance

Real Power (MW) Flow Example

SYSTEMA

SYSTEMB SYSTEM

C

Eg O

Es O

POWERFLOW

Blackout Avoidance

Pe = Eg Es Sin ( 0g- 0s ) X

Where: Eg = Voltage at the Load Center Generation Es = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0g = Votage Angle and Local Generation

0s = Voltage Angle at Remote Generation

POWER TRANSFER EQUATION

Where: Es = Voltage at the Load Center

Eg = Voltage at the Remote Generation

Pe = Electrical Real Power Transfer

X = Reactance Between Local and Remote Generation

0s = Voltage Angle and Local Generation

0g = Voltage Angle at Remote Generation


Where: Eg = Voltage at the Load Center Generation Es = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0g = Voltage Angle and Local Generation



SYSTEMA

Load = 5000 MWGEN. =5000 MW

SYSTEMB

Load = 5000 MWGen. = 5000 MW

SYSTEMC

Load = 5000 MWGen. = 5000 MW

Eg O

Es O

POWERFLOW - 0 MWTRANSFER


Blackout Avoidance








Where: Eg = Voltage at the Load Center Generation Es = Voltage at the Remote Generation Pe = Electrical Real Power Transfer X = Reactance Between Local and Remote Generation 0g = Voltage Angle and Local Generation




SYSTEMA

Load = 5000 MWGEN. = 7000 MW

SYSTEMB

Load = 5000 MWGen. = 5000 MW

SYSTEMC

Load = 5000 MWGen. = 3000 MW

Eg O

Es O

POWERFLOW - 2000 MW

TRANSFER

Blackout Avoidance







SYSTEMA

SYSTEMB SYSTEM

C

Eg O

Es O

REACTIVEPOWER

FLOW SMALL

1. To make reactive power flowyou need to have a differencein voltage magnitude betweenEg and Es.

2. Voltage on a power systemcan only be varied +/- 5%which is not enough differenceto result in a significant VARSflow.

3. Thus VARS cannot betransmitted over longdistances and must begenerated locally near theload.

Reactive Power (Mvars) Flow

Blackout Avoidance

VAr Support must be provided at the load center.

Two major sources of VAr support:

Capacitor Banks – Double-Edge Sword. Vars go down with the square of voltage.

Generators/ Synch. Condensers – A dynamic source of Vars. Can adjust VAr output rapidly during contingencies.

Sources of Reactive (Var) Support

Blackout Avoidance

LOAD RESPONSE TO

LOW VOLTAGE

LOAD RESPONSE TO

LOW VOLTAGE

Blackout Avoidance

How Load Responds to Low VoltageBasic Power System

RemoteGeneration

Transmission System Load Center

ResistiveLoad

MotorLoad

VARSupport

LocalGeneration

Ps PL

QLVL

Blackout Avoidance

Resistive load current decreases as voltage goes down Helping the system.

Motor loads are constant KVA devices and increase their load current as voltage decreases-Hurts the system.

During “Heat Storm” conditions, most load is motor load making blackout more likely.

How Load Responds to Low Voltage

Blackout Avoidance

Example of Voltage Recovery From a Transmission Fault- Rapid Voltage Collapse

R e s id e n tia l V o lta g e R e co ve ry fo r P h o e n ix A re a In c id e n t o n Ju ly 2 9 , 1 9 9 5

Blackout Avoidance

RECENT BLACKOUTS REVISITED

2003 - East Coast Blackout2003 - Italian Blackout2002 - Swedish Blackout1997 - PJM Disturbance*1996 - West Coast Blackout1995 - PECO Disturbance*1987 - City of Memphis*

* Rapid Voltage Collapse

Blackout Avoidance

POWER SYSTEM INSTABILITY

POWER SYSTEM INSTABILITY

Blackout Avoidance

POWER SYSTEM INSTABILITIES

Four Types of Instability:Voltage* Steady State *Transient Dynamic

*Involved in Recent Blackouts

Blackout Avoidance

REMOTEGENERATION LOCAL LOAD

CENTER

LINE 1

LINE 2

LINE 3

LINE 4

LINE 5

LINE 6

Eg O Es O

POWER FLOW

Voltage Collapse Scenario

Blackout Avoidance

0 1800

Max.Power

TransferAll Lines in Service

Line 1Tripped

Line 2Tripped

Pe

Pmax = Eg Es X

0g - 0s90o0 1800

Max.Power


Line 1Tripped

Line 2Tripped

Pe

Pmax = Eg Es X

0g - 0s90o


POWER TRANSFER EQUATION REMOTEGENERATION LOCAL LOAD

CENTER

LINE 1

LINE 2

LINE 3

LINE 4

LINE 5

LINE 6

Eg Og Es Os

POWER FLOW

0 1800

Max.Power


Line 1Tripped

Line 2Tripped

Pe

Pmax = Eg Es X

0g - 0s90o

Steady State Instability

Blackout Avoidance

Generator

G

GSU SystemReactance

VXd XT

XS

V2 __ 1____ + 1 2 XT + XS Xd

Per Unit MW

Per UnitMVAR

V2 ___1___ 12 XT + XS Xd

a) MW - MVAR PER UNIT PLOT

X

R

Xd + XT + XS 2

XT + XS

Xd - XT + XS 2

b) R-X DIAGRAM PLOT

Xd

Steady State Instability

Blackout Avoidance

GENERATOR RESPONDSTO BLACKOUTCONDITIONS

GENERATOR RESPONDSTO BLACKOUTCONDITIONS

Blackout Avoidance

Gen.

AVR ExcitationTransformer

Generator Step-upTransformer

CT VTGenerator

Field

StaticExciter

Generator Excitation & AVR Control

Blackout Avoidance

The Generator AVR (Automatic Voltage Regulator) Controls Field Current into the Rotor Which in Turn Controls Terminal Voltage and VAr Output/Input.

G

Reactive Powerinto System

Reactive Powerinto Generator

Real Powerinto System

+MVAR

Overexcited

Underexcited

0

-MVAR

G

UnderExcitation

Limiter(URL)

+ MW

MW

MVARS

OverexcitationLimiter (OEL)

RotorWindingLimited

StatorWindingLimited

Stator EndIron Limited

MW

MVARSystem

SystemNormal OverexcitedOperation

UnderexcitedOperation

How Generators Provide Vars to the System

Blackout Avoidance

DEPENDS ON THE MANUFACTURER:

Some Limiters Change as the Square of the Voltage – 90% Voltage Results in 81% of Setting

Some Proportional to Voltage – 90% Voltage result in 90% Setting

Some Do Not Change with Voltage at all

AVR Limiters Response During Low Voltage

Blackout Avoidance

0 1800

Max.Power


Breakers 1 and 2Tripped

PM = Pe

Pmax = Eg Es X

0g - 0s90o

A2

0C

A1

Transient Instability

Blackout Avoidance

Typical Out-of-Step Impedance LOCI

Blackout Avoidance

Occurs when fast acting AVR control amplifies rather than damps small MW oscillations.

Occurs when generators are remote from load

Solution is AVR Power System Stabilizers(PSS) – Low Freq. Filter

Required by WECC in the Western USA for generators larger than 30 MVA

Dynamic Instability

Blackout Avoidance

GENERATOR PROTECTION RESPOND TO BLACKOUT

CONDITIONS

GENERATOR PROTECTION RESPOND TO BLACKOUT

CONDITIONS

Blackout Avoidance

Loss of Field (40)

Overexcitation (24)

Overexcitation (24) System Backup (21 & 51V)

Under Frequency (81)

Out of Step Protection (78)

Key Generator Protection Functions Effected by Major System Disturbances

Blackout Avoidance

Generator Protection Effected by Major System Disturbances

Loss of Field (40) – Must be Coordinated with AVR Control, Steady State Stability Limit and Secure Under Low Voltage.

Overexcitation (24) - Coordinated with AVR Control.

System Backup (21 & 51V) – Secure on Stable Power Swings and System Low Voltage. Must be Coordinated with TransmissionProtection.

Blackout Avoidance

Under Frequency (81) – Coordinated with System Load Shedding.

Out of Step Protection (78) – Set to trip the Generator if it Losses Synchronism.

Generator Protection Effected by Major System Disturbances

Blackout Avoidance

+X

-X

+R-R

- Xd’ 2

Xd

GeneratorCapability

Under ExcitationLimiter (UEL)

Heavy Load Light Load

Impedance LocusDuring Loss of Field

1.0 pu

Steady StateStability Limit

Zone 1

Zone 2

Loss of Field (40) – Must be Coordinated with AVR Control, Steady State Stability Limit and Secure Under Low Voltage.

Blackout Avoidance

Transformation From Mw-Mvar to R-X Plot

Blackout Avoidance

G

Reactive Powerinto System

Reactive Powerinto Generator

Real Powerinto System

+MVAR

Overexcited

Underexcited

0

-MVAR

G

UnderExcitation

Limiter(URL)

+ MW

MW

MVARS

OverexcitationLimiter (OEL)

RotorWindingLimited

StatorWindingLimited

Stator EndIron Limited

Steady StateStability Limit

MW

MVARSystem

SystemNormal OverexcitedOperation

UnderexcitedOperation

Overexcitation (24)-Coordinated with AVR Control

Blackout Avoidance

Generator (IEEE/ANSI C-50.12 &13)1.05pu V/Hz on Gen. Base

Transformers (IEEE/ANSI C-57.12 )1.05pu V/Hz loaded at output1.10pu V/Hz unloaded

Overexcitation Gen./Trans. Capability

Blackout Avoidance

Figure #4C

Overexcitation Operating Limits

Blackout Avoidance

Dual Set-point Definite Time1.18pu V/Hz – 2-6 Sec. Delay1.10 pu V/Hz – 45-60 Sec. Delay

Inverse Time Curve1.10pu V/Hz Pickup with CurveSelection to Match Gen./Trans V/Hz Capability Curve

Typical V/Hz 24 Relay Settings

Blackout Avoidance

J X

R

Z2

Z1

RPFA

Max.TorqueAngle

GeneratorCapability

Curve

Z2 Reach at 50 to 67% ofGenerator Capability Curve

Z2 Reach 120% of Longest Line butMust be Less than 80 to 90%

of Capability Curve

System Backup (21 & 51V) – Secure on Stable Power Swings and System Low Voltage. Must be Coordinated with Transmission Protection.

Blackout Avoidance

J X

R

Z2

Z1

Max.TorqueAngle

GeneratorCapability

Curve

Z2 Reach at 50 to 67% ofGenerator Capability Curve

Z2 Reach 120% of Longest Line butMust be Less than 80 to 90%

of Capability Curve

Z3 out of step blocking

Load EncroachmentBlocking

RPFA

Z3

Security Enhancements for Generator Distance Backup Protection

Blackout Avoidance

Voltage Controlled Overcurrent Relays- Voltage control set below emergency system

operating voltage.- Current pickup set at 30-40% of full load (Xd).- Time delay set to coordinate with transmission

backup.

Voltage Restrained Overcurrent Relays- Current pickup varies proportional to voltage and set

150% of gen. Rating at gen. Rated voltage.- Time delay set to coordinate with transmission

backup.

Generator Voltage Overcurrent (51V) Backup

Blackout Avoidance

Coordinate Under Frequency Tripping of Generator With North American Electric Reliability Council (NERC) System Load Shedding Regions – WECC, ECAR, ERCOT, PJM, Others.

Hydro Generators not affected by Under Frequency

Gas Turbines Controls Run Back Mw Output When Frequency Drops

Under Frequency (81) – Coordinated with System Load Shedding

Blackout Avoidance

Out of Step Protection (78) – Set to trip the Generator if it Losses Synchronism

Blackout Avoidance

Undervoltage Condition Not Itself Harmful To Synchronous Generators – V/Hz is a Low Limit.

Auxiliary System is Effected By Low Voltage –Auxiliary Motor Tripping Can Shut Down Gens.

U.S. Nuclear Plants Have Second Level Voltage Separation Relays on Auxiliary System.

Automatic Generator Control (AGC) can Cause Problems when the Power System Breaks-up into Islands.

Undervoltage Power Plant Trippings Brought About By System Var Deficits

Blackout Avoidance

SYSTEM UNDERVOLTAGE LOAD SHEDDING

(UVLS)

SYSTEM UNDERVOLTAGE LOAD SHEDDING

(UVLS)

Blackout Avoidance

Utility Undervoltage Load Shedding (UVLS)

ATTEMPT TO BALANCE MVAR LOAD WITH MVAR SOURCES BY SHEDDING LOAD.

TWO TYPES OF UVLS SCHEMES:

Decentralized - Relays Measure Voltage at load to be shed.

Centralized – Relays Measure Voltage at Key locations. Voltage transmitted to Central Location and combined with other System Information. Schemes Called SPS or Wide Area Protection.

Blackout Avoidance

Status Of Utility Undervoltage Load Shedding (UVLS)

NERC- UVLS Not Mandatory- Recognized as a Cost-effective Method to Address

Voltage Collapse.- Allowed Region to Establish Policy

WECC- Most Aggressive in UVLS- Established UVLS Guidelines

Blackout Avoidance

North American Electric Reliability Council (NERC) Regional Areas

Blackout Avoidance

UVLS AT Utilities

Centralized Decentralized- BC Hydro - Puget Sound- Hydro Quebec - First Energy- Entergy - TXU- Public Service of NewMexico

- PG&E

Blackout Avoidance

Blackout Avoidance

DESIGNING A SECURE UVLS

SCHEME

DESIGNING A SECURE UVLS

SCHEME

Selection of Voltage Relays for UVLS

Measure all Three Voltage or Positive Sequence Voltage.

Use Low Voltage Cutoff.

Consider Negative Sequence Blocking.

Start Timer only if Voltage is Within Window.

Use Relay with High Reset Ratio.

Digital Voltage Relays are ideal for This Application.

Blackout Avoidance

Three-Phase UVLS Logic

27

27B

47B

Va ≤Vb

Vc

Setpoint #1

≤≤

Setpoint #1

Setpoint #1

AND

AdjustableTimer

UndervoltageTrip

AND

SINGLE PHASEUNDERVOLTAGE

Va ≤VbVc

Setpoint #2

≤≤

Setpoint #2

Setpoint #2

OR xUNDERVOLTAGE

BOCK

V2 Setpoint #3≥ xNEGATIVE

SEQUENCEOVERVOLTAGE

BLOCK

Blackout Avoidance

Positive Sequence UVLS LogicV1 = 1/3 ( Va + aVb +a2Vc )

Where: Va,Vb,Vc are line-to-neutral voltages

a = 1l120o

a2 = 1l240o

Balanced Conditions:V1=Va=Vb=Vc.

27

27B

47B

V1 ≤Setpoint #1

AdjustableTimer

UndervoltageTrip

AND

POSITIVESEQUENCE

UNDERVOLTAGE

Va≤VbVc

Setpoint #2

≤≤

Setpoint #2

Setpoint #2

OR xUNDERVOLTAGE

BOCK

V2 Setpoint #3≥ xNEGATIVESEQUENCE

OVERVOLTAGEBLOCK

Blackout Avoidance

Point of Voltage Measurement

UTILITY TRANSMISSIONSYSTEM

27

81

A C

Trip Selected Circuits(A-D)

Typical DistributionSubstation Transformer with

LTC

B D

27 = Undervoltage Relay81= Underfrequency Relay

Blackout Avoidance

UVLS SETTING CONSIDERATIONS

UVLS SETTING CONSIDERATIONS

Blackout Avoidance

UVLS Setting Considerations

Relay Engineers Must Work Closely With System Planning Engineers to Design UVLS.

Planning Engineers Have the Load Flow Data Required to Determine the Voltage Measurement Locations and Amount of Load to Shed.

They also develop the P-V (Nose Curve) that will determine the Voltage Relay Pickup Setting.

Time Delay for UVLS are Typically in the 2-10 Sec. Range – not in Cycles Range for UFLS.

Blackout Avoidance

Undervoltage Relay Pickup

MW LOAD

VOLTAGE

VcollapseSetting Margin

Relay and VT Accuracy Band

V settingAllowableOperating Area

Operating Margin

Blackout Avoidance

Coordinating UVLS Relay Pickup

Blackout Avoidance

NEW M-3401 BECKWITH LOAD SHEDDING RELAY

NEW M-3401 BECKWITH LOAD SHEDDING RELAY

Blackout Avoidance

NEW M-3401 RELAY

Blackout Avoidance

Blackout Avoidance

NEW M-3401 RELAY

M-3401 Protective Functions

Blackout Avoidance

4-Step Phase Undervoltage (27) Protection, single-phase and positive sequence

4-Step Phase Undervoltage, selectable as single phase or positive sequence responding, with Negative sequence overvoltage and single phase undervoltage supervision

Phase Overvoltage (59) Protection

Four-Step Over/Under Frequency (81) protection

Rate of Change of Frequency (81R) protection

IPSlogicTM takes the contact INPUT status and function status and generates OUTPUTS by employing (OR, AND, and NOT) boolean Logic and a timer

Three-Phase UVLS Logic

27

27B

47B

Va ≤Vb

Vc

Setpoint #1

≤≤

Setpoint #1

Setpoint #1

AND

AdjustableTimer

UndervoltageTrip

AND

SINGLE PHASEUNDERVOLTAGE

Va ≤VbVc

Setpoint #2

≤≤

Setpoint #2

Setpoint #2

OR xUNDERVOLTAGE

BOCK

V2 Setpoint #3≥ xNEGATIVE

SEQUENCEOVERVOLTAGE

BLOCK

Blackout Avoidance

Positive Sequence UVLS LogicV1 = 1/3 ( Va + aVb +a2Vc )

Where: Va,Vb,Vc are line-to-neutral voltages

a = 1l120o

a2 = 1l240o

Balanced Conditions:V1=Va=Vb=Vc.

27

27B

47B

V1 ≤Setpoint #1

AdjustableTimer

UndervoltageTrip

AND

POSITIVESEQUENCE

UNDERVOLTAGE

Va≤VbVc

Setpoint #2

≤≤

Setpoint #2

Setpoint #2

OR xUNDERVOLTAGE

BOCK

V2 Setpoint #3≥ xNEGATIVESEQUENCE

OVERVOLTAGEBLOCK

Blackout Avoidance

Blackout Avoidance

5 Programmable Outputs, 2 programmable inputs, and 1 self-test outputOscillographic Recording (COMTRADE file format)Time-Stamped Sequence of Events (SOE) recording for 32 eventsMetering of Voltage and FrequencyPorts – one RS-232 port (COM1) on front and one RS-232 and 485 port (COM2) on rearSetting Software – M-3812 IPScom® For WindowsTM Communica-tions SoftwareModbus ProtocolRelay Voltage Inputs Can Be Directly Connected (no VT required) for voltages < 480 V acContinuous Self-Diagnostics

M-3401 Standard Features

CONCLUSIONS

Voltage Collapse is the Major Cause of Blackouts in US Power Systems.

UVLS is a Viable Method of Providing Protection to Avoid Slow Voltage Collapse Situations.

UVLS May be too Slow to respond to Rapid Fault Induced Voltage Collapses.

UVLS Requires close Cooperation Between Planners and Relay Engs.

UVLS Schemes are More Difficult to Design and Set than UFLS.

Blackout Avoidance

Generator Protection Needs to be Made More Secure During Low Voltage Conditions and be Coordinated with Generator Controls.

Methods to do this are Scattered in Various Text Books and Manufactures Literature.

My Paper Provides a Single Document with this Information.

My Paper Highlights the Important Role the AVR Plays During Major Disturbances.

CONCLUSIONS

Blackout Avoidance

blackout avoidance & undervoltage load shedding

Engineering