Transcript Document

Stability and Security of
Power Networks
G. T. Heydt
Arizona State University
ECEDHA 2004 Annual Meeting
March, 2004
Orlando, Florida
Outline
 Stability and security: a general discussion
 Weaknesses and strengths of the North American grid
 Some theoretical considerations
 Solutions: short range and long range
 Propaganda: power engineering education
 Conclusions
Stability
Power system stability basically refers to the
ability of operating an AC network with all
generators in synchronism, retaining
synchronism even after a large disturbance
Stability
Each synchronous generator has a ‘Newton’s
law’ second order nonlinear differential equation
that describes the machine angle – and control
systems (e.g., power system stabilizers) also
contribute a higher order nonlinear controller to
the dynamics
P  (T ) 
| E f || Vt | sin( )
xs
A large interconnection (WECC, e.g.) may have
about 200 generators + 150 PSSs = about 1000 to
10000 order nonlinear differential equations
Stability
The basic analysis technique is state space analysis /
eigenvalues for the linearized system, or simulation for the
nonlinear system. Typically, the dimension is very high –
in the 1000 – 10,000 range. The interconnection is modeled
as Ibus = Ybus Vbus which is reduced to eliminate the nondynamic nodes (i.e., remove the non-generation nodes).
Power system stabilizers
A PSS is a controller that uses (usually local)
measurements to provide a signal to one
generator so that damping torque is produced by
the machine field winding. The basic concept is
that a linear controller is used with standard
feedback control technology to place the poles of
the linearized system solidly in the LHP. Virtually
all large generating units in North America are
fitted with PSSs.
Power system stabilizers
The main weaknesses of this approach are that
the nonlinear system may respond poorly, and
also dynamics external to the generator + PSS
are not modeled (nor included in the
measurements). Therefore modes that result
from inter area dynamics may not be damped.
x
x
xx
x
xx
x
x
Wide area robust power system
stability control
By injecting the appropriate
signals from distant
measurements in the
system, transmitted through
LEOS, the controller is able
to obtain superior
performance in terms of
damping interarea
oscillations compared to use
of conventional local
signals. The main concept is
to use interarea signals for
interarea controls
Low Earth Orbit
Satellites LEOS
REGIONAL MEASUREMENTS
LOCAL
MEASUREMENTS
SPSS
Hierarchical robust power system
controller
 Execution Level
Management Level
Signal pre-processor
Actuator / Distributor
Operation Level
 Operation Level
System modal identifier
 SPSS damping loop
 Management Level
Fuzzy logic based parameter tuner
Execution Level
Input Data
Control
Power System
Voltage Regulator With PSS
and SPSS
Vt
SPSS
Remote Signals
PSS
,f, or Pa
+

Ref+
-
Voltage
Regulator
Generator
Excitation
System
Gen
Generator Field
Area 1
G1
Area 2
G3
0.011+ j0.11
Load 2
Load 1
G2
G4
SYS impulse response--1st input to outputs
SYS bode graph--1st input to 1st-2nd outputs
From: U(1)
0
-1
0
14
28
42
56
0
-200
0
-200
-400
200
0
70
-200
0
-500
-1000
-2
0
10
2
10
10
Time (sec.)
Frequency (rad/sec)
SYS+LMI1 impulse response--1st input to outputs
SYS+LMI1 open-loop transfer function Bode graph
From: U(1)
To: Y(2)
-0.2
0.2
0
-0.2
0
5
10
15
Time (sec.)
20
25
30
-200
To: Y(1)
To: Y(1)
0
0
-400
500
0
-500
0
-200
To: Y(2)
Phase (deg); Magnitude (dB)
From: U(1)
0.2
Amplitude
200
To: Y(1)
To: Y(1)
0
-1
1
To: Y(2)
Amplitude
1
To: Y(2)
Phase (deg); Magnitude (dB)
From: U(1)
-400
0
-500
-1000
0
10
Frequency (rad/sec)
5
10
Key issues
• Full scale nonlinear solution (transient stability study)
• Eigenvalues of the linearized system near the operating
point (small signal stability)
• Line and component ratings
• Voltage ratings (maximum and minimum)
• Coherency - groups of generators swinging together
• Synchronizing torque, PSSs
• Acceptable operating conditions (including operation
within about 50 mHz of 60 Hz)
Security
Market
Network
Internal
Sources
Communication
systems
Information &
decisions
Intentional
human acts
Natural calamities
External
Sources
Security refers to the ability of the system to
respond only to intended operator
commands, blocking all unintended
operations
Electric power system is vulnerable to
failure due to
Natural disasters
Deliberate attack
Equipment
failures
Tree-related events
Operator error
High load periods
Accidents
Software failures
Monitoring of electric power networks
Underground
Transmission Lines Advanced
Substations
PMU
Transformers
Overhead
Transmission Lines
Sensor Systems
EMS
Energy management systems
Archiving
Sensory
information
Command and
control
EMS
Operator
interaction
State estimator
Generator
controls
Network vulnerability reduction through
virtual sensor utilization
No Data!
Network Data
Lost
EMS
Virtual Data
Virtual Sensor
Present
Tradeoffs between
virtual and physical sensors
Low Cost
Less Accurate
High Cost
Greater Accuracy
Z = [H] X
$
$
$
I
$ $ $
Physical Sensors
$
V
$
$
$
Virtual Sensors
What is needed to enhance both security
and stability
•Ability to acquire and interpret extensive real-time
information from diverse sources, ranging from sensors to
satellites. Sensory data used in Hx = z state estimators to
enhance system performance.
•Ability to quickly evaluate system vulnerability with
respect to catastrophic events in a market environment
involving competing, self-serving agents
•Ability to adapt protective device performance based on
system-wide and external system assessment
•Ability to reconfigure the power network to minimize
system vulnerability
•Ability to develop system restoration plans to minimize
the impact of disruption
Strategic Power Infrastructure Defense
System
Communication system for strategic power
infrastructure defense
Time synchronization (GPS)
/ Self healing / Info.
Exchange (LEO)
GPS or LEO satellite
communication
Internet based
communication
channel
Internet based or more
direct and faster
communication
channel
Satellite dish
Protec ti ve devi ce
Gateway
S ate
llite
Strategic power
infrastructure main system
Intranet
Ethernet or model based network
is used in the Intranet. Each
Intranet can have a “gateway” that
handles IP addresses in the
Intranet
Internet or any other
communication channel
for a number of Intranets
The North American grid
NERC: policies, rules, reliability, plans,
synchronous interconnections
North American Electric Reliability Council
• Sets standards for the reliable operation and planning
• Monitors, assesses and enforces compliance with standards
• Provides education and training
• Assesses, analyzes and reports on bulk electric system adequacy
• Coordinates with Regional Reliability Councils
• Coordinates the provision of applications, data and services
• Certifies reliability service organizations and personnel
• Coordinates critical infrastructure protection
• Enables the reliable operation by facilitating information
exchange and coordination among reliability service
organizations
• Administers procedures for appeals and conflict resolution
Weaknesses and strengths of the North
American grid
• Basic transmission design is over 40 years old. Some
basic distribution circuits are over 60 years old.
• Never designed to handle high levels of bulk power
• Both transmission and generation constrained
• The impact of market driven exchange of power has
stressed the transmission grid
• The transition to market based infrastructure has
stressed the newly created control entities (e.g., ISOs)
– in an industry that is rapidly loosing corporate
memory
The Northeast blackout of 2003
Time 8/14/2003 4:09:57 PM EDT: The first significant events
were initially recorded in Michigan and Ohio
The Northeast blackout of 2003
Time: 8/14/03 04:10:39 PM EDT: The disturbance was then
recorded all over Michigan , Ohio , and the city of Buffalo, NY
The Northeast blackout of 2003
Time: 8/14/03 04:10:58 PM EDT: 19 seconds later, the
disturbance had propagated to the eastern seaboard.
The Northeast blackout of 2003
Main causes
Failure of state estimator in MISO to model ‘external’
system changes
Combination of heavy power exchanges, high
reactive power flows, planned outages of
transmission circuits and planned outage of a main
generating facility (none of which are unusual)
Operator error / training of MISO operators /
imprudent operation of an Ohio utility (generation
outages)
Unplanned unit and line outages
The Northeast blackout of 2003
The Northeast blackout of 2003
Generation building boom of the past
200
180
160
140
120
100
80
60
40
20
0
1950
1955
1960
1965
1970
Coal
1975
Oil
1980
Gas
1985
Nuclear
1990
Other
1995
2000
A hindsight view of the past building
boom
Generation Building Boom Follows the Baby Boom Labor Force Entry
35
200
180
160
25
140
120
20
19.23
100
17.93
15
80
11.93
60
10
11.69
40
5
20
0
0
1950
1955
1960
1965
1970
Coal
1975
Oil
Gas
1980
Nuclear
1985
Other
1990
1995
2000
Percent Change in Labor Force
30
29.41
Generation building boom of the future
1400
Total System Generation Capacity
1200
GW
1000
By 2020, the U.S. will need
1300 new power plants at
300 MW each
800
600
400
Cumulative Additions
200
0
2000
2005
2010
2015
2020
2025
2030
Employment at major IOU’s
TRANSMISSION
DISTRIBUTION
The N9s problem
 Electric power quality
 Extreme bus voltage reliability, for example 'five
nines' (i.e., 0.99999 availability), or six nines or
even higher
 Utilization of new transmission and distribution
technologies for improvement of reliability
 Utilization of distributed energy sources (DERs)
to improve reliability
 Working with manufacturers of information
technology
equipment
to
reduce
load
vulnerability
24/7 UTILIZATION OF POWER SYSTEM ULTRA
HIGH RELIABILITY
INFORMATION PROCESSING, FINANCIAL
SERVICES, AIRLINES, POLICE, MILITARY
Reliability enhancement
Distributed rather than
concentrated loads
Loop circuits for distribution
systems
Information Technology and
sensitive manufacturing loads
Independence of energy sources
Environmental issues
AS A RESPONSE TO THE 1993 TERRORIST BOMBING OF THE WTC, THE
PRIMARY DISTRIBUTION SYSTEM IN THE BUILDING WAS IMPROVED TO
KEEP THE POWER ON IN THE CASE OF SEVERE DISRUPTION OF THE
SUPPLY / INTERRUPTION OF THE IN-BUILDING PRIMARY
DISTRIBUTION. THERE WERE TEN SUBSTATIONS IN EACH WTC
TOWER, ON FLOORS 7, 41, 75, AND 108, AND THE SOUTH TOWER HAD
AN ADDITIONAL TENANT OWNED DOUBLY FED SUBSTATION ON
FLOOR 43
THE USE OF MULTIPLE
FEEDS, MULTIPLE
SUBSTATIONS, AND
ISOLATED POWER SUPPLIES
KEPT THE POWER ON IN
MOST OF THE WTC FOR 102
MINUTES AFTER THE INITIAL
STRIKE. IT IS BELIEVED THAT
THIS WAS THE MAIN FACTOR
IN SAVING THE LIVES OF AS
MANY AS 18,000 PEOPLE
WHO ESCAPED FROM THE
TOWERS BEFORE COLLAPSE
Independence of sources
LOAD
1-P = (1-P1)(1-P2)
TWO FEEDERS
RELIABLE LOAD BUS
The dependence of the sources will result in a
much higher outage rate than (1-P1)(1-P2)
Modeling dependence of sources
The dependence effect of multiple sources can be
modeled using a difference equation of the form
qn+1 = Cqn+(1-C)(q1)1/n qn
where qn = 1-pn = outage rate of circuit upon addition
of nth feeder, C is a correlation coefficient
The (q1)1/n term is called a discounting term and it
accounts for increased potential for dependence for
cases of large n (large numbers of feeders)
Discounted model
C = 0 indicates no correlation between multiple
feeders
C = 1 indicates the feeder outages among
several feeders are ‘common mode’
Reliability of multiple feeds
Reliability expressed as
number of 9s
Zero circuit
correlation
10
8
1% circuit
correlation
6
4
100 % circuit
correlation
2
0
0
1
2
3
4
Number of circuit
feeders
5
The addition of
feeders to improve
reliability has a
diminishing effect.
For practical cases,
use of more than
three ‘independent’
feeders of 100%
capacity is counterproductive.
One generator, + 1
feeder FOR = 1%
Two feeders FOR =
1%, Dependence
10%
Three
generators, FOR
= 1%
Two generators,
FOR = 1%
1 day in 200 years
One
generator,
FOR = 1%
3 feeders FOR = 1%,
Dependence 10%
1 day in 20
years
0.9
1
0.99
2
0.999 0.9999
3
4
0.99999 0.999999 0.9999999 0.99999999 0.999999999
6
7
8
9
5
Probabilities of uncommon events
POWER SYSTEM
RELIABILITY
COMMON (?) LIFE
Event_______ Probability, N
Loosing at roulette
Reliability
N
Outage time
99.9
3
8h 45 min / yr
99.998631 4.9
1 day / 200 yrs
99.999
5
5 min 15 s / yr
99.99999
7
3.2 s / yr
99.999999
8
99.9999999 9
18.9 cycles / yr
1.8 cycles / yr
97.368, 1.6
in Las Vegas – bet
on 00
Loosing the
PowerBall
99.99995,
6.3
lottery
FAA design
criteria for
aircraft
0.999999999
0.999999999999,
9 to 12
Solutions: short range
Distributed generation
Added small generation units at all levels
Conservation / electronic control of loads
Investment in distribution systems
Sharp increase in research in both transmission
and distribution engineering
Recruiting of students to the power area at all
levels
Improvement of software tools
PHOSPHORIC ACID
250 kVA FUEL CELL
PROTON
EXCHANGE
MEMBRANE FUEL
CELL - 7.5 kVA
Microturbines
 Low capacity, high speed units with
electronic interface with 60 Hz bus
 Alternative fuel sources (e.g., biogas,
gasifier, pyrolysis, fuels that have less than
10% of heat content compared to fossil
fuels)
 Catalytic combustor to reduce nitrous
oxide production
 Heat recovery
 Lower capacities -- e.g.,
5 - 300 kVA
 High efficiency small units
 New IEEE standard requires disconnection
from the distribution system within a few
cycles during low voltage or outage events
Solutions: long term
Added generation in larger units
Local solutions for high reliability
requirements
Added capacity in distribution systems
Adaptive islanding of interconnected
systems
Coordinate national energy policy with
system realities
The educational aspect of the
problem
undergraduate degree
recipients
U. S. Power engineering
undergraduate enrollments
2000
1500
1000
500
0
1960
1980
2000
Source: G.T. Heydt and V. Vittal, “Feeding Our Profession,” IEEE Power & Energy
Magazine, vol.1, issue 1, Jan/Feb 2003, pp 38-45
U. S. Power engineering graduate
enrollments
200
graduate degree
recipients
M.S.E.E.
150
100
50
Ph.D.
0
1970 1975 1980 1985 1990 1995 2000
year
Source: G.T. Heydt and V. Vittal, “Feeding Our Profession,” IEEE Power & Energy
Magazine, vol.1, issue 1, Jan/Feb 2003, pp 38-45
The general electrical engineering
reality
 There is a certain ebb and flow to the enrolments
in engineering nation-wide; since the all-time low
in undergraduate engineering in 1998, there has
been an uninterrupted growth in enrolments
 In many electrical and computer engineering
programs, the growing tendency to select the
computer engineering option has resulted in the
majority of students seeing little or no subject
matter relating to energy and power
The general electrical engineering
reality
Given the decreasing number of electrical
engineering undergraduates, there is good
progress in stopping the precipitous decline in
the undergraduate power engineering enrolments
to the point where many power programs are
experiencing record levels
Encouraging developments on the
curriculum front
 A determined movement away from the old
straight jacket curriculum to new enriched course
offerings with broadened choice
 New developments are evident in three principal
thrusts
 addition of microeconomic/finance elements
 introduction of energy, environment and
public policy aspects
 wider array of power systems, power
electronics and machines/drives courses
The impact of recent events
 Restructuring of electricity and the California
crisis sharpened public interest in electricity
 The September 11, 2001 tragedy brought to
prominence the issue of the security of the North
American interconnected power system
 The 2003 mega-blackout produced keen interest
in the reliability of the interconnected grid
Conclusions
Stability of power systems is a well understood
phenomenon, but complex numerical problem.
Stability enhancement controls are very complex
to design, but the present research thrusts and
engineering practice have yielded in-service
designs (or designs nearly in-service) that are
suitable to the task
The transition to a market based energy
infrastructure may not have been well thought
out, and system implications are just now being
remedied
Conclusions
Distribution engineering, long a step-child of power
engineering, is a focus of research – mainly related to
high reliability, cost reduction, and distributed generation
sources
System security is a point of focus in contemporary
power engineering
Research on sensory systems is needed to enhance
system security
Power engineering education and the production of
power engineers at all levels seems to have a significant
impact on the health of the national power system. It is
unclear that the number of engineers needed will be
attained by US educational institutions.