Stability and Security of Power

NetworksG. 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

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(||||)(

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).

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.

xx

xx

x xx

xx

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

SPSSLOCAL

MEASUREMENTS

REGIONAL MEASUREMENTSREGIONAL MEASUREMENTS

Wide area robust power system stability control

Low Earth Orbit Low Earth Orbit Satellites LEOSSatellites LEOS

Hierarchical robust power system controller

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 SystemPower System

Input DataInput Data ControlControl

Voltage Regulator With PSS and SPSS

GeneratorExcitation

System

+

-

Ref

Generator Field

Gen

Vt

+

PSS ,f, or Pa

VoltageRegulator

Remote SignalsSPSS

G1 G3

G2 G4

0.011+j0.11

Load 1 Load 2

Area 1 Area 2

Time (sec.)

Am

plitu

de

SYS impulse response--1st input to outputs

-1

0

1From: 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.)

Am

plitu

de

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+LMI

1 open-loop transfer function Bode graph

-400

-200

0From: U(1)

-500

0

500To: Y

(1)

-400

-200

0

100 105-1000

-500

0

To: Y

(2)

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)

Intentional Intentional human actshuman acts

NetworkNetwork MarketMarket

Information & Information & decisionsdecisions

CommunicationCommunicationsystemssystems

Natural calamitiesNatural calamities

InternalInternalSourcesSources

ExternalExternalSourcesSources

Security refers to the ability of the system to

respond only to intended operator

commands, blocking all unintended

operations

Security

Electric power system is vulnerable to failure due to

Natural disasters

Deliberate attack

Equipment failures

Operator error

Accidents

Tree-related events

High load periods

Software failures

PMU

Sensor Systems

Transformers

Substations

Monitoring of electric power networks

Advanced

Underground

Transmission Lines

Overhead Transmission

Lines

EMS

Energy management systems

Archiving

E M S

Operator interaction

State estimator

Generator controls

Sensory information

Command and control

Virtual Sensor Present

Virtual Data

EMS

Network vulnerability reduction through virtual sensor utilization

EMS

Network Data Lost

No Data!

EMS

Tradeoffs betweenvirtual and physical sensors

$$$$

$$$ $

$ $

Low Cost

Less Accurate

Physical Sensors

Virtual Sensors

High Cost

Greater Accuracy

V IZ = [H] X

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

Failure

Analysis

Information

&

Sensing Vulnerability

Assessment

Self Healing

Strategies

Strategy

Deployment

GP

S

Sate

llit

e

LE

O

Sate

llit

eIn

tran

etIn

tern

et

Strategic Power Infrastructure Defense System

Communication system for strategic power infrastructure defense

Satellite dish

Protective device

Gateway

Strategic power infrastructure main system

Time synchronization (GPS) / Self healing / Info. Exchange (LEO)

IntranetEthernet 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

GPS or LEO satellite communication

Internet based communication channelInternet based or more direct and faster communication channel

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 causesFailure 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

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

GW

Ins

talle

d

in F

ive

Ye

ar P

erio

d

A hindsight view of the past building boom

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

GW

Inst

alle

d i

n F

ive

Yea

r P

erio

d

Generation Building Boom Follows the Baby Boom Labor Force Entry

17.93

29.41

11.93

19.23

11.69 Per

cen

t C

han

ge

in L

abo

r F

orc

e

0

5

10

15

20

25

30

35

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

By 2020, the U.S. will need 1300 new power plants at 300 MW each

Total System Generation Capacity

Cumulative Additions

GW

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 sourcesEnvironmental 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

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)

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

100 % circuitcorrelation

1% circuitcorrelation

Zero circuitcorrelation

Rel

iabi

lity

expr

esse

d as

num

ber

of 9

s

Number of circuitfeeders

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.

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, + 1feeder FOR = 1%

Two feeders FOR =1%, Dependence10%

1 day in 20years

1 day in 200 years

3 feeders FOR = 1%,Dependence 10%

Threegenerators, FOR= 1%

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

LIFE

Probability, N

97.368, 1.6

99.99995, 6.3

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

PROTON EXCHANGE

MEMBRANE FUEL CELL - 7.5 kVA

PHOSPHORIC ACID

250 kVA FUEL CELL

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

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

U. S. Power engineering undergraduate enrollments

1960 1980 20000

500

1000

1500

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

un

der

gra

du

ate

d

egre

e r

ecip

ien

ts

U. S. Power engineering graduate enrollments

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 & Energy Magazine, vol.1, issue 1, Jan/Feb 2003, pp 38-45

gra

du

ate

d

egre

e r

ecip

ien

ts

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.