Stability and Secirity of Power Networks
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Transcript of Stability and Secirity of Power Networks
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Stability and Security of
Power NetworksG. T. Heydt
Arizona State University
ECEDHA 2004 Annual Meeting
March, 2004
Orlando, Florida
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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
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Stability
Power system stability basically refers to theability of operating an AC network with all
generators in synchronism, retaining
synchronism even after a large disturbance
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Stability
Each synchronous generator has a Newtonslaw 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
A large interconnection (WECC, e.g.) may have
about 200 generators + 150 PSSs = about 1000 to
10000 order nonlinear differential equations
s
tf
x
VETP
)sin(||||)(
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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 non-dynamic nodes (i.e., remove the non-generation nodes).
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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.
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Power system stabilizers
The main weaknesses of this approach are thatthe 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.
xx
xx
x xx
xx
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By injecting the appropriatesignals from distantmeasurements in thesystem, transmitted throughLEOS, the controller is ableto obtain superiorperformance in terms ofdamping interarea
oscillations compared to useof conventional localsignals. The main concept isto use interareasignals for
interareacontrols
SPSSLOCALMEASUREMENTS
REGIONAL MEASUREMENTS
Wide area robust power systemstability control
Low Earth Orbit
Satellites LEOS
Hi hi l b
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Hierarchical robust power systemcontroller
Execution Level
Signal pre-processor
Actuator / Distributor
Operation Level
System modal identifier
SPSS damping loop
Management Level
Fuzzy logic based parameter tuner
Management Level
Operation Level
Execution Level
Power System
Input Data Control
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Voltage Regulator With PSS
and SPSS
Generator
Excitation
System
+
-
Ref
Generator Field
Gen
Vt
+
PSS ,f, or Pa
Voltage
Regulator
Remote SignalsSPSS
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G1 G3
G2 G4
0.011+j0.11
Load 1 Load 2
Area 1 Area 2
Time (sec.)
Amplitude
SYS impulse response--1st input to outputs
-1
0
1
From: U(1)
To:Y(1)
0 14 28 42 56 70-1
0
1
To:Y(2)
Frequency (rad/sec)
Phase(deg);Magnitude(dB
)
SYS bode graph--1st input to 1st-2nd outputs
-200
0
200From: U(1)
-400
-200
0
To:Y(1)
-200
0
200
10-2
100
102
-1000
-500
0
To:Y(2)
Time (sec.)
Amplitude
SYS+LMI1 impulse response--1st input to outputs
-0.2
0
0.2From: U(1)
To:Y(1)
0 5 10 15 20 25 30
-0.2
0
0.2
To:Y(2)
Frequency (rad/sec)
Phase(deg);Magnitude(dB)
SYS+LMI1 open-loop transfer function Bode graph
-400
-200
0From: U(1)
-500
0
500
To:Y(1)
-400
-200
0
10
0
10
5-1000
-500
0
To:Y(2)
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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)
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Intentional
human acts
Network Market
Information &decisions
Communication
systems
Natural calamities
Internal
Sources
External
SourcesSecurity refers to the ability of the system to
respond only to intended operator
commands, blocking all unintended
operations
Security
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Electric power system is vulnerable tofailure due to
Natural disasters
Deliberate attack
Equipment
failures
Operator error
Accidents
Tree-related events
High load periods
Software failures
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PMU
Sensor Systems
Transformers
Substations
Monitoring of electric power networks
Advanced
Underground
Transmission Lines
Overhead
Transmission Lines
EMS
http://theoak.com/rick/phasor.gif -
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Energy management systems
Archiving
E M S
Operator
interactionState estimator
Generatorcontrols
Sensory
information
Command and
control
N t k l bilit d ti th h
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Virtual Sensor
Present
Virtual Data
EMS
Network vulnerability reduction throughvirtual sensor utilization
EMS
Network Data
Lost
No Data!
EMS
T d ff b t
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Tradeoffs betweenvirtual and physical sensors
$$$$
$$$
$
$ $
Low Cost
Less Accurate
Physical Sensors Virtual Sensors
High Cost
Greater Accuracy
V IZ = [H] X
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What is needed to enhance both securityand stability
Ability to acquire and interpret extensive real-time
information from diverse sources, ranging from sensors tosatellites. Sensory data used in Hx = zstate estimators to
enhance system performance.
Ability to quickly evaluate system vulnerability with
respect to catastrophic events in a market environmentinvolving 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
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Strategic Power Infrastructure Defense
System
C i ti t f strategic po er
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Communication system forstrategic powerinfrastructure defense
Satellite dish
Satellite
Protective device
Gateway
Strategic power
inf rastructure main system
Time synchronization (GPS)
/ Self healing / Info.
Exchange (LEO)
Intranet
Ethernet or model based network
is used in the Intranet. Each
Intranet can have a gateway that
handles IP addresses in theIntranet
Internet or any other
communication channel
for a number of Intranets
GPS or LEO satellite
communication
Internet based
communication
channel
Internet based or more
direct and faster
communication
channel
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The North American grid
NERC: policies, rules, reliability, plans,
synchronous interconnections
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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
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Weaknesses and strengths of the NorthAmerican 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
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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
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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
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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.
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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 oftransmission 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
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The Northeast blackout of 2003
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The Northeast blackout of 2003
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Generation building boom of the past
0
20
40
60
80
100
120
140
160
180
200
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
Coal Oil Gas Nuclear Other
A hindsight view of the past building
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A hindsight view of the past buildingboom
0
20
40
60
80
100
120
140
160
180
200
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
Coal Oil Gas Nuclear Other
Generation Building Boom Follows the Baby Boom Labor Force Entry
17.93
29.41
11.93
19.23
11.69 PercentChangeinLaborForce
0
5
10
15
20
25
30
35
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Generation building boom of the future
0
200
400
600
800
1000
1200
1400
2000 2005 2010 2015 2020 2025 2030
By 2020, the U.S. will need
1300 new power plants at
300 MW each
Total System Generation Capacity
Cumulative Additions
GW
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Employment at major IOUs
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TRANSMISSION
DISTRIBUTION
Th N9 bl
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The N9s problem
Electric power qualityExtreme bus voltage reliability, for example 'five
nines' (i.e., 0.99999 availability), or six nines oreven higher
Utilization of new transmission and distributiontechnologies for improvement of reliability
Utilization of distributed energy sources (DERs)to improve reliability
Working with manufacturers of informationtechnology equipment to reduce loadvulnerability
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24/7 UTILIZATION OF POWER SYSTEM ULTRAHIGH RELIABILITY
INFORMATION PROCESSING, FINANCIALSERVICES, AIRLINES, POLICE, MILITARY
R li bilit h t
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Reliability enhancement
Distributed rather thanconcentrated loads
Loop circuits for distribution
systemsInformation Technology andsensitive manufacturing loads
Independence of energy sources
Environmental issues
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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
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THE USE OF MULTIPLEFEEDS, 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 ESCAPEDFROM THE TOWERS BEFORE
COLLAPSE
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Independence of sources
The dependence of the sources will result in a
much higher outage rate than (1-P1)(1-P2)
TWO FEEDERS RELIABLE LOAD BUS
LOAD
1-P = (1-P1)(1-P2)
M d li d d f
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Modeling dependence of sources
The dependence effect of multiple sources can bemodeled using a difference equation of the form
qn+1= Cqn+(1-C)(q1)1/n qn
whereqn= 1-pn= outage rate of circuit upon addition
ofnth feeder, C is a correlation coefficient
The(q1)1/nterm is called a discount ingterm and it
accounts for increased potential for dependence for
cases of large n(large numbers of feeders)
Di t d d l
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Discounted model
C= 0 indicates no correlation between multiple
feeders
C= 1 indicates the feeder outages among
several feeders are common mode
R li bilit f lti l f d
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Reliability of multiple feeds
100 % circuit
correlation
1% circuitcorrelation
Zero circuit
correlation
Reliabilityexpressedas
number
of9s
Number of circuit
feeders
0 1 2 3 4 50
2
6
8
4
10
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 counter-
productive.
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0.9 0.99 0.999 0.9999 0.99999 0.999999 0.9999999 0.99999999 0.9999999991 2 3 4 5 6 7 8 9
Onegenerator,
FOR = 1%
Two generators,
FOR = 1%
One generator, + 1
feeder FOR = 1%
Two feeders FOR =1%, Dependence
10%
1 day in 20
years
1 day in 200 years
3 feeders FOR = 1%,
Dependence 10%
Threegenerators, FOR
= 1%
P b biliti f t
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Probabilities of uncommon events
COMMON (?)Event_______
Loosing at roulette
in Las Vegas bet
on 00
Loosing the
PowerBall
lottery
FAA design
criteria for
aircraft
POWER SYSTEM
RELIABILITY
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 18.9 cycles / yr
99.9999999 9 1.8 cycles / yr
LIFEProbability, N
97.368, 1.6
99.99995, 6.3
0.999999999
0.999999999999,
9 to 12
Solutions: short range
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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
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PROTONEXCHANGE
MEMBRANE FUEL
CELL - 7.5 kVA
PHOSPHORIC ACID
250 kVA FUEL CELL
Microturbines
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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
Microturbines
Solutions: long term
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Solutions: long term
Added generation in larger unitsLocal solutions for high reliability
requirements
Added capacity in distribution systems
Adaptive islanding of interconnected
systems
Coordinate national energy policy with
system realities
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The educational aspect of the
problem
U. S. Power engineering
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U. S. Power engineeringundergraduate enrollments
1960 1980 20000
500
1000
1500
2000
Source: G.T. Heydt and V. Vittal, Feeding Our Profession, IEEE Power & EnergyMagazine, vol.1, issue 1, Jan/Feb 2003, pp 38-45
undergradua
te
degree
recipi
ents
U. S. Power engineering graduate
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U. S. Power engineering graduateenrollments
1970 1975 1980 1985 1990 1995 20000
50
100
150
200
year
M.S.E.E.
Ph.D.
Source: G.T. Heydt and V. Vittal, Feeding Our Profession, IEEE Power & EnergyMagazine, vol.1, issue 1, Jan/Feb 2003, pp 38-45
graduate
degree
recipi
ents
The general electrical engineering
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The general electrical engineeringreality
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
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The general electrical engineeringreality
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
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cou ag g de e op e ts o t ecurriculum 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 principalthrusts
addition of microeconomic/finance elements
introduction of energy, environment andpublic policy aspects
wider array of power systems, power
electronics and machines/drives courses
The impact of recent events
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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
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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-servicedesigns (or designs nearly in-service) that are
suitable to the task
The transition to a market based energyinfrastructure may not have been well thought
out, and system implications are just now being
remedied
Conclusions
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Conclusions
Distribution engineering, long a step-child of power
engineering, is a focus of research mainly related tohigh 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 ofpower 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