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daptive

relaying

new direction in power system protection

Modern electric power systems can

deliver energy to users very reliably.

Protective relays in the power system

play an important role in assuring this

continuous service. Relays monitor the

status of the system continuously and

detect failures or abnormalities within

their assigned zone of protection. The

control action takes place by opening a

minimum number of circuit breakers to

isolate the defective element. An ele-

ment that would have otherwise caused

excessive damage or possibly collapse

o f the power system.

Although protective relays should

detect all system abnormalities quickly,

other considerations might detract from

this primary objective. In general, a

relay system is designed to achieve the

highest levels of speed, reliability,

selectivity, simplicity, and economics.

Since it is impractical to satisfy all

requirements simultaneously, compro-

mises must be made.

A typical conflictory objective is

embedded in the reliability of a relay

system. The dependability and security

of a relay system establish its reliabili-

ty. Dependability is a measure

of

the

relay system to perform properly in

removing system faults. Security is a

measure of the relay tendency in

not

initiating an incorrect trip action. There

is always a compromise between secu-

rity and dependability. The dependabili-

ty or security can be enhanced

significantly by utilizing redundant

relays. If the contact of the redundant

relay is connected in parallel with the

original relay, then the dependability is

increased. On the other hand, if the con-

tacts are connected in series, the securi-

ty is enhanced.

With conventional relays, the pro-

tective system design is either biased

toward the dependability or the secur-

ty. Therefore, the highest levels of

dependability and security cannot be

achieved at the same time.

In

addition, the compromises among

the desirable characteristics will lead to

a reIay system design which is far from

optimum. Performance degradation will

become more transparent

as

the network

topology changes. general, the ten-

dency

of

a relay system not perfomsing

in an optim~~rnanner is attributed to:

1.

Evolving relaying philosophies

over the past

80

years.

2.

Designs heavily relying

on

electromechanical technologies.

3.

Limitation in use

of

local vari-

ables, such as current or voltages,

as

the

relay inputs.

The major weaknesses are the hard-

wares inadequacy and limited capabili-

ty in adapting

to

the changing

environment of a power system.

Adaptive relaying

Adaptive relaying considers the fact

that the status of a power system can

change. Thus, the setting of relays will

be changed on-line

to

accommodate

these changes.

The adaptive relaying concept

requires the microprocessor-based digi-

tal relays. The digital relays are pro-

grammable devices with extensive

logic, memory, data transfer, communi-

cation, and reporting capabilities. These

features make them excellent candi-

dates for implementing the adaptive

relaying concepts.

However, this concept poses new

challenges in developing algorithms

that allow proper adaptability to

changes in system conditions. Addi-

tionally, since

a

power system is highly

integrated, it might not be possible to

detect all system loading and topologi-

cal changes at a local bus within the

power system. Therefore, system-wide

communication capability might

become a fundamental requirement.

Application areas

A recent paper summarized the

results of a survey on satisfaction of

practicing relay engineers with the

existing relays. In addition, this paper

investigated the areas where improve-

ments are desirable, and reliability

enhancements which can be made by

incorporating the adaptive features.

The following summarizes the

16

iden-

tified areas:

1. Operating time as a function of

the distance to fault,

28

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IEEE POTENTIALS

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2.

Mutual coupling compensation in

3. High source impedance ratio

4. Remote-end open-breaker detec-

5. Load flow compensation,

6.

Fault type changing speed of oper-

7.

Multi-terminal distance relay cov-

8.

Variable breaker failure timing,

9. Permissive reclosing,

10. Adaptive reclosing,

11. Sympathy trip reclosing,

12. Adaptive synchronism check

13. Proactive load shedding,

14. Adaptive transformer differential

15. Voltage change supervision of

16. Bus protection restraint for

ground impedance protection,

changing,

tion for high-speed sequential tripping,

ation,

erage,

angle for reclosing,

protection,

differential unit,

arrester applications.

Adaptive digital

distance protection

In a digital relaying scheme, voltage

and current samples are taken at the

relaying point and used to compute the

apparent impedance of the line seen by

the relay. If the impedance is inside a

predetermined boundary, the decision is

made to disconnect or trip the line. This

system works well for

a

zero-resistance

fault situation. The voltage and current

samples are taken and the apparent

impedance is determined to be the

impedance of the line from the relay

point to the fault. If this impedance is

less than the expected line impedance,

the line is tripped.

The problem occurs in the case of

non-zero resistance fault situations. The

voltage that is sampled is the sum of the

line voltage and the fault voltage. The

voltage drop across the fault is a func-

tion of the current from the relay termi-

nal and the current from the remote-end

terminal. The current contribution from

the remote end cannot be measured at

the relaying point. It is possible to mea-

sure the remote-end current and send it

back to the relay end by a high speed

communication channel; however, this

has not been very practical.

Traditional systems only incorporat-

ed a margin of error to account for the

unknown current in order to keep the

relay from overreaching. This resulted

in a certain amount of the line at the far

end not being protected by the first zone

of protection. In

order to protect the

line properly, the

amount of unpro-

tected line must be

minimized.

An alternative

method is to deter-

mine the apparent

line impedance as a

function of known

parameters such as

positive and zero

45

-

40 .

I

50 1 15 200

R ohms)

sequence imped-

Fig. R Xplane

ance components,

terminal voltages, and the unknown

fault resistance. Computer simulation

may then be performed to determine an

ideal trip boundary, for several fault

resistance values. A typical example of

these boundaries is shown in Fig. 1.

Multi-terminal lines can be protected

in a similar manner

as

two terminal

lines. The difference is that the apparent

impedance as seen at a relay location is

not just a function of the parameters of

one line and two terminals, but a func-

tion of two or more lines and three or

more terminals. These line and terminal

parameters can be determined in

advance. Computer simulation may

then be used to determine ideal trip

boundaries for several fault resistances

in different parts of the line. These

boundaries do change with changing

system conditions.

Thus, the adaptive approach of mea-

suring system conditions and updating

the ideal trip boundaries can be very

useful. The protection algorithm will

measure the voltage and current sam-

ples at the relay location. The apparent

impedance is then calculated and the

computer refers to the most recent trip

boundaries and determines occurrence

of a fault and its locations.

Relays should adapt to ever chang-

ing system conditions, whether it is a

two-terminal or multi-terminal line. By

using a computer or microprocessor

based detection scheme, the reliability

and system stability is greatly

improved.

During the normal operation of a

system, unexpected events can affect

the overall performance of the system.

If an abnormal condition should arise,

such as frequency deviations, the pro-

tective devices may not be prepared to

handle the obscurity of parameter

changes due to the pre-set inputs.

A solution is to use real-time data to

reset any relay input settings. Therefore,

of

distance relay

it is possible to develop control rules for

automatically adapting to the system

changes. Components are added to the

control law aimed at unpredictable fac-

tors that affect the states of the protected

line. This improves the effectiveness of

a distance protection scheme.

Power system frequency deviations

are expected within certain limits. Two

undesirable consequences of frequency

excursions in digital distance protection

are the influence on sampling period

and the computed value of the reac-

tance. To translate the input signals

properly, the digital signals after sam-

pling should be sinusoidal sequences

with a period

of

N when line currents or

voltages are sinusoidal. In the case of

frequency deviations, the sampIe signals

will not belong to a 50 or 60 Hz signal.

As

a

result, the computations will be in

error.

Also, when the frequency of the line

varies by a certain percentage, the reac-

tance of the line will also change by a

proportional amount. To correct for this,

adaptive revising of sampling period

and line reactance calculation

on

the

basis of frequency measurement should

be considered. This is accomplished by

calculating the period increment. The

sampling period is then adaptively

revised for use of the next cycle of the

waveform. Accordingly, a new setting

can be computed. The reactance setting

can also be revised adaptively.

Single-phase to ground faults may

have a major effect on the performance

of the protective devices. The most seri-

ous and instantaneous faults are faults in

the vicinity of the switch-gear in the

protected direction, and faults in the

vicinity

of

the end of the protected line.

Faults in these areas may lead to the

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operation

of

the wrong relays.

To

avoid

these false operations, an adaptive

method will change the operating char-

acteristics of the relays on-line.

Power swings may cause improper

operation of distance protection. An

adaptive method can be used to immo-

bilize the protection during power sys-

tem swings. This method is called the

incremental rate discrimination of

instantaneous current values. The prin-

ciple is that the rate of change of instan-

taneous value of line current has a limit

under normal conditions but increases

suddenly when faults occur. According

to such a difference in changing rates,

one may distinguish the fault condition

erally, thiis inrush current

is

prevented

from being recognized as a fault condi-

tion by the fact that the inrush current is

dominated by the second harmonic. The

rnagnitudle of the second harmonic

depends

on

residual magneiism and the

voltage switching angle.

Current differential relays, however,

are affected by factors such

as

i m s h

current, ewer excitation, transformer

taps, and current transformer mismatch-

es. A digital scheme for differential pro-

tection would be the ideal way to

account for these

affects,

and to control

ratio mismatches. Such a digital system

also could be faster, and can make deci-

sions that

;Ire

much more secure.

then the algorithm transfers control to

the two state filter.

Once the transformer is operating

under normal conditions, a two state

Kalman filter samples the primary and

secondary currents and creates a refer-

ence phasor for each. These phasors are

rotated with respect to time and then

compared to new current measurements.

If

a

significant difference exists, a tran-

sient is detected and control is trans-

ferred to the three state Kalman filter

that uses estimates from the two state fil-

ters. The three state filter creates esti-

mates of the fundamental component of

the differential current. These estimates

are then used in a differential motection

from a no fault condition.

The rate-of-change of cur-

rent feature can classify the sta-

tus of the power system to

normal operation, swing condi-

tion, and faulted condition.

During normal conditions, the

current and its rate of change

both vary sinusoidally, and thelr

amplitude values are level with

each other. The current is near-

ly periodic under power swing

conditions. The amplitude of

the rate change per cycle varies

slowly

as

an envelope curve of

the current.

For a fault, the current

increases suddenly and the rate

change will be large. The adap-

method

suggests

that

a

detecting unit be used and its

setting adjusted adaptively in real-time

according to the change in current

amplitude Therefore, the protection

will be able to differentiate between the

faults and power system swings. This

will involve measunng and also memo-

nzing the amplitude of the load current

per cycle, and adaptively resetting the

protecbon devices.

equation similar to the one used

by the five state filter. If no

fault is detected, new samples

are taken and the process is

repeated by the three state filter

until

a

set time period has

elapsed. If no trip signal is

issued, the control is transferred

back to the two state filter.

By changing monitoring

states, the algonthm can adapt to

different operating conditions

and apply the precise model

needed. Test results for such a

scheme have been promsing on

a 1 KVA,

120

VI120 V single

phase transformer.

Different voltage taps were

used at the secondary of this

transformer to simulate internal

faults in conjunction with an

electronic switch to select different taps.

Current and voltage transducers were

used to supply current signals to a data

acquisition system and host computer.

The algorithm was executed at 16 sam-

ples/cycle. A fault decision had to be

consistent for at least three samples

before trip signals were issued. In six-

teen test cases involving internal faults

across the secondary during switching

Fig

2 ix

step

loud

shedding scheme

Recent work has proposed such a

computer al go ri th that

is

adaptive and

utilizes Kalman filtering. The computer

algorithm relies

on

the monitoring of

transformer currents to determine the

state of thle transformer. Different order

Kalman filters a re then initialized

depending on the state of the trans-

former. A de-energized transformer is

monitored until switchng occurs. Upon

Protecting power

transformers

Differential protection has become

the standard protection method for

power transformers over 10 MVA. It is

well known that the fundamental com-

ponent of the differential current during

an internal fault becomes much greater

than during normal loading conditions.

Another aspect of differential protec-

tion deals with the transformer during a

switching period. During this switching

period, inrush current will also create a

discrepancy in the current balance. Gen-

switching, a five state Kalman filter is

used to estimate the dc, fundamental,

and second harmonic components of the

current.

Tlhe

fundamental component is

used for the differential protection. If

the differential current exceeds a pre-

determined percentage of the through

current, a ]possible fault condition exists.

The second harmonic component of the

current is then compared to the funda-

mental of the differential current. If the

second harmonic content of the current

is not high enough, then an internal fault

has occuned and a trip signal is issued.

If the second harmonic criteria is met,

conditions, the algorithm initiated a trip

signal in 3/4 to 1 cycle. In nearly one

hundred tests involving internal faults,

turn-to-tum faults, and capacitor switch-

ing operations, the average correct oper-

ating time for the algorithm was

5

ms

with minimum and maximum times of

3 ms and 9 ms, respectively.

Adaptive reclosing

Despite attempts to maintain impec-

cable reliability in a high voltage trans-

mission system, faults will and do

occur.

To

minimize their effect and the

consequent interruption of service, the

3 IEEE POTENTIALS

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system must be brought back on-line as

quickly and effortlessly as possible. It is

in this capacity that automatic reclosing

of circuit breakers are employed.

If a fault was determined to be per-

manent, the recloser would lockout after

a predetermined number of operations,

eventually isolating the faulted segment.

However, the vast majority of faults are

temporary in nature. The recloser usual-

ly will close the line without having to

lockout.

The two main methods of reclosing

are high speed reclosing

HSR)

and

delayed reclosing. In the first case, the

line is reclosed as quickly as possible

with no checking of voltage magnitudes

or phase angles. The only delay intro-

duced in this fashion is that required for

extinguishing the arc.

On the other hand, delayed automat-

ic reclosing imposes more of a delay.

This allows for adequate checking to

determine if desirable system conditions

are present. Although both methods

have fulfilled their purposes, the advent

of the digital computer can take this

ability one step further.

Conventional methods are limited by

their inability to adjust their actions to

real-time changes in the system. Adaptive

reclosing allows a safer and more easily

monitored method of closing onto faulted

sections of a system. By applying adap-

tive techniques, the voltage of a faulted

system can be utilized to determine the

severity as well as the location of the

fault, and further, whether or not the sys-

tem should be brought back online.

In

fact, adaptive relaying could effectively

prohibit closing into any fault unless it is

a line to ground or line to line fault.

Adaptive reclosing involves detect-

ing the nature and location of a fault by

re-energizing an unfaulted (or presum-

ably unfaulted) phase and observing the

phase voltages on the other unenergized

lines. Simple logic can be employed to

determine which phase should be ini-

tially reclosed. Any given fault can be

classified into one of the following four

categories:

1. three-phase,

2. line-to-line,

3. line-to-ground,

4. internal.

In the case of a three-phase fault, the

selection of the initial phase to reclose is

purely arbitrary. When a line-to-line

fault occurs, obviously the third, unin-

volved, phase would be chosen. In the

third case, either of the two other phases

could be re-energized. The final case

implies a permanent fault, which would

warrant the recloser to lockout, isolating

the defective equipment.

After the single-phase reclosing

action, the voltages of the other phases

can be monitored to determine the nature

and seventy of the fault.A variety of sce-

narios can arise after the initial reclosing:

a. If the reclosed line voltage equals

the source voltage and the voltage on

either of the remaining phases is zero,

then there is obviously a line-to-ground

fault on that respective phase.

b. If the reclosed line voltage equals

the source voltage and the voltage on

either of the other two is equal to the

reclosed line, then we can assume a

line-to-line fault between this phase and

the initial reclosed line.

c. If the reclosed line voltage equals

the source voltage and the voltage of the

other two phases are equal, then a line-

to-line fault exists between them.

d. If the reclosed line voltage equals

the source voltage and the voltages on

the other two phases non zero and nei-

ther equal to each other or to the re-

energized phase, then the system is

normal and the phases can be reclosed

immediately.

Since these measurements can be

made almost instantaneously, the effect

of an incorrect trip can easily be mini-

mized. Another issue is not only which

phases to reclose, but at what time to

reclose them. In order to minimize the

dc offset in case of permanent fault, the

initial reclosing should occur at maxi-

mum voltage across the breaker. On the

other hand, the other phases should be

reclosed at the minimum voltage across

the breaker to eliminate the arc and pro-

long the life of the contacts.

Adaptive reclosing also helps protect

the breakers upon opening. Digital

relays allow precise control of the open-

ing as well as the reclosing operations

of circuit breakers.

If

the breakers are

opened when the current

is

zero, this

eliminates not only system transients,

but also eradicates arcing which mini-

mizes maintenance and prolongs the life

of the device. Also, digital control aids

in disbursing the workload among vari-

ous breakers to balance out the wear

and tear imposed on each. This is most

advantageous in the ring bus situation,

where it is very difficult to apportion

the reclosing action equally by the con-

ventional methods.

Underfrequency protection

As a consequence of switching large

loads or

loss

of generation, a power sys-

tem can experience a decay in the sys-

tem frequency. When the total load is

greater than the total generation, the

generator speed will decrease causing

the system frequency to decrease.

One major concern during low fre-

quency periods is the safety of the tur-

bine-generators. Operating in low

frequency regions for a prolonged peri-

od of time will damage the turbine

blades. Electric utility companies use

underfrequency load shedding relays to

prevent drastic drops in the system fre-

quency.

The current method of preventing the

frequency decay is to shed a predeter-

mined amount of load when the system

frequency

drops

below a preset value.

Typically 57 Hz will be used as the

lowest safe operating level. (This value

was chosen from an industry survey in

1966 by the IEEE Power System Relay-

ing Committee-57

Hz

being the most

popular value.)

The problem with shedding

a

pre-set

amount is that each system disturbance

is different. For a particular disturbance,

the amount of load that needs to be shed

to correct the problem may be different

than what the company has specified for

other conditions.

If the amount of load shed is not

enough to correct the problem, the fre-

quency will continue to decline until the

next load shedding step is initiated. Due

to the large inertia of the turbine-gener-

ators, the frequency will continue to

decline for a short time after the abnor-

mality has been corrected. This may

cause the frequency to decay past the

minimum value, even though the

amount of load shed is sufficient. Fast

and efficient load shedding is required

to account for both the generator inertia

and various system disturbances.

If the frequency is not monitored

throughout the system, load shedding

outside the disturbance area will reduce

the system reliability. Another problem

with local frequency measurement is

that the frequency at one end of the sys-

tem could be below 57 Hz, while the

frequency at the other end could be

above

57

Hz.

A large integrated system consists of

many interconnected systems. When

there is a major disturbance in one of

the

smaller systems, the

last

line of

defense is to isolate this system from

the main system via the existing inter-

ties. As a result, an island is formed.

The smaller system that experienced an

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S.H. Horowitz, A.G. Phadke, and

J.S. Thorp, Adaptive Transmission

Relaying, I E E E T r ans ac t i ons

on

Power Delivery Vol. 3, No. 4, October

M. Adamiak, et. al, Feasibility of

Adaptive Protection and Control, IEEE

PES 1992 Summer Meeting Paper.

Y.Q.

Xia,

K.K.

Li, A.K. David,

Adaptive Relay Setting for Stand-

Alone Digital Distance Protection,

IEEE PES 1993 Winter Meeting.

Zhizhe, Zhang, Deshu, Chen, An

Adaptive Approach in Digital Distance

Protection, I E E E T r ans ac t ions on

Power Delivery Vol. 6,

No.

1, January

Chang, W.B., Girgis, Adly A., and

Hart, D. David, An Adaptive Scheme

for Digital Protection of Power Trans-

formers, IEEE Transactionson Power

Delivery Vol. 7

No. 2,

April 1992.

Girgis, Adly A., Brown, R. Grover,

Adaptive Kalman Filtering in Comput-

er Relaying: Fault Classification Using

Voltage Models, IEEE Transactions

on Power Apparatus and Systems Vol.

PAS-104, No.

5,

May 1985.

P.M. Anderson and M. Mirheydar,

1988, pp. 1436-1445.

1991, pp. 135-141.

An Adaptive Method

fo r

setting

Underfrequency Load Shedding

Relays, Transactions on Power Sys-

tems Vol. 7 No.

2

May 1992.

Jampala A.K., Venkata S.S.

Damborg M.J., Adaptive Transmission

Protection: Concepts and Computation-

al Issues, IEEE TransactionsonPower

Delivery Vol. PD-4, January 1989, pp.

177-185.

About the authors

Jamie Codling is an undergraduate

student at Rose-Hulman Institute of

Technology. He will be receiving a B.S.

in Electrical Engineering and would like

to work in the power engineering field.

Spencer House is a senior Electrical

Engineering student at Rose-Hulman

Institute of Technology. Upon complet-

ing his degree he would like to obtain

employment in the power industry. Joe

Joice is a member of IEEE and a mem-

ber of IEEE Power Engineering Soci-

ety. He will receive his B.S. degree in

Electrical Engineering in May of 1994

from Rose-Hulman Institute of Tech-

nology. Upon graduation he wishes to

work for a power engineering consult-

ing

fm

Kenneth M. Labhart is a senior

at Rose-Hulman Institute of Technology

pursing a B.S. in Electrical Engineering.

He is an IEEE student member and a

member

of

the IEEE Power Engineer-

ing Society. Jon Richards is a student of

Electrical Engineering at Rose-Hulman

Institute of Technology. After complet-

ing his schooling, he would like to work

in the power engineering field. John

Tenbush is an Electrical Engineering

student at Rose-Hulman Institute of

Technology. After graduation, he wish-

es to work for an automotive company

in the area of product design and devel-

opment. Matthew D. Tullis is presently

finishing his undergraduate studies at

Rose-Hulman. After receiving a B.S.

degree in Electrical Engineering, he

plans to work for a public utility. Todd

Wilkerson is presently finishing his B.S.

in Electrical Engineering at Rose-Hul-

man Institute of Technology. Upon his

graduation, he plans to join the Techni-

cal Services division of Anderson Con-

sulting in Chicago, IL. Dr. Rostamkolai

is the Associate Professor of Electrical

Engineering at Rose-Hulamn Institute

of Technology.

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