Stability and Secirity of Power Networks

7/28/2019 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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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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Time: 8/14/03 04:10:58 PM EDT: 19 seconds later, the

disturbance had propagated to the eastern seaboard.


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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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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

Stability and Secirity of Power Networks

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