How 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

1. BLACKOUT
AVOIDANCE
&
UNDER VOLTAGE
LOAD
SHEDDING
Blackout Avoidance

2. BLACKOUTS
 How 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

3. RECENT BLACKOUTS
2003 - East Coast Blackout
2003 - Italian Blackout
2002 - Swedish Blackout
1997 - PJM Disturbance
1996 - West Coast Blackout
1995 - PECO Disturbance
1987 - City of Memphis
Blackout Avoidance

4. 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

5. Root Cause of Recent Blackouts
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.
Blackout Avoidance

6.  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 the
generators at the load center and
remote generators also increase.
REM OTE
GENERATION LOCAL LOAD
CENTER
LINE 1
LINE 2
LINE 3
LINE 4
LINE 5
LINE 6
How Voltage Collapses Occur
Blackout Avoidance

7. POWER TRANSFER
EXAMPLE
Blackout Avoidance

8. Real Power (MW) Flow Example
SYSTEM
A
SYSTEM
B SYSTEM
C
Eg O
Es O
POWER
FLOW
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

9. 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 = Voltage Angle and Local Generation
0s = Voltage Angle at Remote Generation
POWER TRANSFER EQUATION
SYSTEM
A
Load = 5000MW
GEN. =5000 MW
SYSTEM
B
Load = 5000MW
Gen. = 5000MW
SYSTEM
C
Load = 5000MW
Gen. = 5000MW
Eg O
Es O
POWER
FLOW - 0MW
TRANSFER
Real Power (MW) Flow Example
Blackout Avoidance
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

10. 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
0s = Voltage Angle at Remote Generation
POWER TRANSFER EQUATION
Pe = Eg Es Sin ( 0g- 0s )
X
Real Power (MW) Flow Example
SYSTEM
A
Load = 5000 MW
GEN. = 7000 MW
SYSTEM
B
Load = 5000 MW
Gen. = 5000 MW
SYSTEM
C
Load = 5000 MW
Gen. = 3000 MW
Eg O
Es O
POWER
FLOW - 2000 MW
TRANSFER
Blackout Avoidance
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

11. SYSTEM
A
SYSTEM
B
SYSTEM
C
Eg O
Es O
REACTIVE
POWER
FLOWSMALL
1. T o m a k e reactive p o w e r f l o w
y o u n e e d to h a v e a d i f f e r e n ce
in v o l t a g e m a g n i t u d e b e t w e e n
E g a n d E s .
2. V o l t a g e o n a p o w e r s y s t e m
c a n o n l y b e va r i e d +/- 5 %
w h i c h is n o t e n o u g h d i f f e r e n ce
to result in a significant V A R S
flow .
3. T h u s V A R S c a n n o t b e
t r a n s m i t t e d o v e r l o n g
d i s t a n c e s a n d m u s t b e
g e n e r a t e d locally n e a r t h e
l o a d .
Reactive Power (Mvars) Flow
Blackout Avoidance

12. Sources of Reactive (Var) Support
 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.
Blackout Avoidance

13. LOAD RESPONSE
TO
LOW VOLTAGE
Blackout Avoidance

14. How Load Responds to Low Voltage
Basic Power System
Remote
Generation
TransmissionSystem LoadCenter
Resistive
Load
Motor
Load
VAR
Support
Local
Generation
Ps PL
QLVL
Blackout Avoidance

15. How Load Responds to Low Voltage
 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.
Blackout Avoidance

16. Example of Voltage Recovery From a
Transmission Fault- Rapid Voltage Collapse
R e sid e ntia l V o ltag e R e cove ry fo r P hoe n ix A rea In cide nt on July 2 9 , 19 95
Blackout Avoidance

17. RECENT BLACKOUTS REVISITED
2003 - East Coast Blackout
2003 - Italian Blackout
2002 - Swedish Blackout
1997 - PJM Disturbance*
1996 - West Coast Blackout
1995 - PECO Disturbance*
1987 - City of Memphis*
* Rapid Voltage Collapse
Blackout Avoidance

18. POWER SYSTEM
INSTABILITY
Blackout Avoidance

19. POWER SYSTEM INSTABILITIES
Four Types of Instability:
 Voltage*
 Steady State *
 Transient
 Dynamic
*Involved in Recent Blackouts
Blackout Avoidance

20. REMOTE
GENERATION 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

21. REMOTE
GENERATION 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

22. Pmax = Eg Es
Max. X
Power All Lines in Service
Transfer
Line 1
Tripped
Pe
Line 2
Tripped
0 90o 1800
0g - 0s
Pmax = Eg Es
Max. X
Power All Lines in Service
Transfer
Line 1
Tripped
Pe
Line 2
Tripped
0 90o 1800
0g - 0s
Pe = Eg Es Sin ( 0g- 0s )
X
POWER TRANSFER EQUATION
Eg Og Es Os
POWER FLOW
REMOTE
GENERATION LOCAL LOAD
CENTER
LINE 1
LINE 2
LINE 3
LINE 4
LINE 5
LINE 6
0 1800
Max.
Power
Transfer
All Lines in Service
Line 1
Tripped
Line 2
Tripped
Pe
Pmax = Eg Es
X
90o
0g - 0s
Steady State Instability
Blackout Avoidance

23. Generator
G
GSU
System
Reactance
V
Xd XT
XS
V2
__ 1____ + 1
2 XT + XS Xd
Per Unit MW
Per Unit
MVAR
___1___ 1V2
2 XT + XS Xd
a) MW - MVAR PER UNIT PLOT
X
R
Xd + XT + XS
2
XT + XS
Xd - XT + XS
2
Xd
b) R-X DIAGRAM PLOT
Steady State Instability
Blackout Avoidance

24. GENERATOR RESPONDS
TO BLACKOUT
CONDITIONS
Blackout Avoidance

25. Gen.
AVR
Excitation
Transformer
CT VTField
Static
Exciter
Generator Excitation & AVR Control
Generator Step-up
Transformer
Generator
Blackout Avoidance

26. Reactive Power
into System
Reactive Power
into Generator
Real Power
into System
+
MVAR
Overexcited
Underexcited
0
-
MVAR
Under
Excitation
Limiter
(URL)
+ MW
MW
G
MVARS
Overexcitation
Limiter (OEL)
Winding
Limited
Stator
Winding
Limited
Stator End
Iron Limited
MW
G
MVAR
System
Normal Overexcited
Operation
Underexcited
Operation
System
How Generators Provide Vars to the
System
 The Generator AVR (Automatic Voltage Regulator)
Controls Field Current into the Rotor Which in Turn
Controls Terminal Voltage and VAr Output/Input.
Rotor
Blackout Avoidance

27. AVR Limiters Response During Low Voltage
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
Blackout Avoidance

28. 0 1800
Max.
Power
Transfer
All Lines in Service
Breakers 1 and 2
Tripped
PM = Pe
Pmax = Eg Es
X
90o
0g - 0s
A2
0C
A1
Transient Instability
Blackout Avoidance

29. Typical Out-of-Step Impedance LOCI
Blackout Avoidance

30.  Required by WECC in the Western USA for
generators larger than 30 MVA
Dynamic Instability
 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
Blackout Avoidance

31. GENERATOR PROTECTION
RESPOND TO BLACKOUT
CONDITIONS
Blackout Avoidance

32. Key Generator Protection Functions
Effected by Major System Disturbances
 Loss of Field (40)
 Overexcitation (24)
 Overexcitation (24) System Backup (21 & 51V)
 Under Frequency (81)
 Out of Step Protection (78)
Blackout Avoidance

33. 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 Transmission
Protection.
Blackout Avoidance

34. Generator Protection Effected by Major
System Disturbances
 Under Frequency (81) – Coordinated with
System Load Shedding.
 Out of Step Protection (78) – Set to trip the
Generator if it Losses Synchronism.
Blackout Avoidance

35. -X
+R-R
- Xd’
2
Xd
Generator
Capability
Under Excitation
Limiter (UEL)
Heavy Load Light Load
Impedance Locus
During Loss of Field
1.0 pu
Steady State
Stability Limit
Zone 1
Zone 2
Loss of Field (40) – Must be Coordinated with
AVR Control, Steady State Stability Limit and
Secure Under Low Voltage.
+X
Blackout Avoidance

36. Transformation From Mw-Mvar to R-X Plot
Blackout Avoidance

37. G
Reactive Power
into System
Reactive Power
into Generator
Real Power
into System
+
MVAR
Overexcited
Underexcited
0
-
MVAR
Under
Excitation
Limiter
(URL)
+ MW
MW
G
MVARS
Overexcitation
Limiter (OEL)
Rotor
Winding
Limited
Stator
Winding
Limited
Stator End
Iron Limited
Steady State
Stability Limit
MW
MVAR
System
Normal Overexcited
Operation
Underexcited
Operation
System
Overexcitation (24)-Coordinated with AVR Control
Blackout Avoidance

38. Overexcitation Gen./Trans. Capability
 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 output
1.10pu V/Hz unloaded
Blackout Avoidance

39. Figure #4C
Overexcitation Operating Limits
Blackout Avoidance

40. Typical V/Hz 24 Relay Settings
 Dual Set-point Definite Time
1.18pu V/Hz – 2-6 Sec. Delay
1.10 pu V/Hz – 45-60 Sec. Delay
 Inverse Time Curve
1.10pu V/Hz Pickup with Curve
Selection to Match Gen./Trans
V/Hz Capability Curve
Blackout Avoidance

41. J X
R
Z2
RPFA
Max.
Torque
Angle
Z1
Generator
Capability
Curve
Z2 Reach at 50 to 67% of
Generator Capability Curve
Z2 Reach 120% of Longest Line but
Must 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

42. J X
Z2
Z1
Max.
Torque
Angle
Generator
Capability
Curve
Z2 Reach at 50 to 67% of
Generator Capability Curve
Z2 Reach 120% of Longest Line but
Must be Less than 80 to 90%
of Capability Curve
Z3 out of step blocking
Load Encroachment
Blocking
RPFA
R
Z3
Security Enhancements for Generator
Distance Backup Protection
Blackout Avoidance

43. Generator Voltage Overcurrent (51V)
Backup
 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.
Blackout Avoidance

44. Under Frequency (81) – Coordinated with
System Load Shedding
 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
Blackout Avoidance

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

46.  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

47. SYSTEM UNDERVOLTAGE
LOAD SHEDDING
(UVLS)
Blackout Avoidance

48. 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

49. 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

50. North American Electric Reliability Council
(NERC) Regional Areas
Blackout Avoidance

51. Decentralized
- Puget Sound
- First Energy
- TXU
UVLS AT Utilities
Centralized
- BC Hydro
- Hydro Quebec
- Entergy
- Public Service of New
Mexico
- PG&E
Blackout Avoidance

52. Blackout Avoidance
DESIGNING A SECURE
UVLS
SCHEME

53. 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

54. Three-Phase UVLS Logic
27
27B
UNDERVOLTAGE
BOCK
47B
NEGATIVE
SEQUENCE
OVERVOLTAGE
BLOCK
Vc 
Va Setpoint #1
Vb Setpoint #1
Setpoint #1
AND
Adjustable
Timer
Undervoltage
Trip
AND
SINGLE PHASE
UNDERVOLTAGE
Va Setpoint #2
Vb Setpoint #2
Vc Setpoint #2
OR x
Setpoint #3V2  x
Blackout Avoidance

55. Blackout Avoidance
Positive Sequence UVLS Logic
V1 = 1/3 ( Va + aVb +a2Vc)
Where: Va,Vb,Vc are line-to-
neutral voltages
a = 1l120o
a2 = 1l240o
Balanced Conditions:
1V =Va=Vb=Vc.
27
47B
NEGATIVE
SEQUENCE
OVERVOLTAGE
BLOCK
1
V Setpoint #1
Adjustable
Timer
Undervoltage
Trip
AND
POSITIVE
SEQUENCE
UNDERVOLTAGE
Va Setpoint #2
Vb Setpoint #2
VcSetpoint #2
OR x
27B
UNDERVOLTAGE
BOCK
Setpoint#3V2  x

56. Point of Voltage Measurement
UTILITY TRANSMISSION
SYSTEM
27
81
A C
Trip Selected Circuits
(A-D)
Typical Distribution
Substation Transformer with
LTC
B D
27 = Undervoltage Relay
81= Underfrequency Relay
Blackout Avoidance

57. UVLS SETTING
CONSIDERATIONS
Blackout Avoidance

58. 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

59. Undervoltage Relay Pickup
MW LOAD
VOLTAGE
Relay and VT Accuracy Band
Setting Margin
Vcollapse
V setting
Allowable
Operating Area
Operating Margin
Blackout Avoidance

60. Coordinating UVLS Relay Pickup
Blackout Avoidance

61. NEW M-3401
BECKWITH LOAD
SHEDDING RELAY
Blackout Avoidance

62. NEW M-3401 RELAY
Blackout Avoidance

63. Blackout Avoidance
NEW M-3401 RELAY

64. M-3401 Protective Functions
 4-Step Phase Undervoltage (27) Protection, single-phase and
positive sequence
Blackout Avoidance
 4-Step Phase Undervoltage, selectable as single phase or
positive sequence responding, with Negative
overvoltage and single phase undervoltage supervision
sequence
 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

65. Three-Phase UVLS Logic
27
27B
UNDERVOLTAGE
BOCK
47B
NEGATIVE
SEQUENCE
OVERVOLTAGE
BLOCK
Vc 
Va Setpoint #1
Vb Setpoint #1
Setpoint #1
AND
Adjustable
Timer
Undervoltage
Trip
AND
SINGLE PHASE
UNDERVOLTAGE
Va Setpoint #2
Vb Setpoint #2
Vc Setpoint #2
OR x
Setpoint #3V2  x
Blackout Avoidance

66. V1 = 1/3 ( Va + aVb +a2Vc)
Blackout Avoidance
Positive Sequence UVLS Logic
Where: Va,Vb,Vc are line-to-
neutral voltages
a = 1l120o
a2 = 1l240o
Balanced Conditions:
V1=Va=Vb=Vc.
27
47B
NEGATIVE
SEQUENCE
OVERVOLTAGE
BLOCK
1
V Setpoint #1
Adjustable
Timer
Undervoltage
Trip
AND
POSITIVE
SEQUENCE
UNDERVOLTAGE
Va Setpoint #2
Vb Setpoint #2
VcSetpoint #2
OR x
27B
UNDERVOLTAGE
BOCK
Setpoint#3V2  x

67. Blackout Avoidance
M-3401 Standard Features
 5 Programmable Outputs, 2 programmable inputs, and 1 self-test
output
 Oscillographic Recording (COMTRADE file format)
 Time-Stamped Sequence of Events (SOE) recording for 32 events
 Metering of Voltage and Frequency
 Ports – one RS-232 port (COM1) on front and one RS-232 and 485
port (COM2) on rear
 Setting Software – M-3812 IPScom® For WindowsTM Communica-
tions Software
 Modbus Protocol
 Relay Voltage Inputs Can Be Directly Connected (no VT required)
for voltages < 480 V ac
 Continuous Self-Diagnostics

68. 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

69. CONCLUSIONS
 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.
Blackout Avoidance

70. BLACKOUT AVOIDANCE
&
UNDERVOLTAGE LOAD SHEDDING
THE END
Questions ?
Blackout Avoidance
©2008 Beckwith Electric Co., Inc.