Protection Application Handbook (англ. яз.)

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MEASURING PRINCIPLES

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SIMULTANEOUS FAULTS In the quite common “parallel line appli-

cations” simultaneous faults can occur. A fault can occur between

“L1” and “L2” but the phases are at different lines. A full scheme

relay is a must to give correct operation for simultaneous faults as

the measuring loops for earth faults must detect one forward- and

one reverse fault in different phases and a logic must be provided

in the relay to get the correct operation. Many Distance protection

relays (even of new types), will risk maloperation at these faults

and care must also be taken when the relay zones and the phase

selection are set.

Switched schemes can due to the starter function not operate

correctly for simultaneous faults. The starter will select both in-

volved phases to the measuring element on both involved lines

distance protection relay. The measurement will then naturally be

incorrect.

Figure 21 shows the principle for simultaneous faults. The new

Distance protection relays from ABB Network Partner provides a

logic to cover the problem.

Figure 21. Simultaneous faults, in parallel lines, at the same tower.

SERIES CAPACITORS The use of forward impedance measurement

using the reactive characteristic of the transmission lines, implies

that Distance protection relays can’t be used in series compen-

sated lines without special care. Experts must be consulted for

such applications. This is also valid for surrounding lines in the

same station.

RN/SN

Forw

RN/SN

Forw

RN Forw

SN Rev

SN Forw

RN Rev

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3.5 TRAVELLING WAVE PROTECTION RELAYS

The travelling wave detector, was a revolution in relay technology,

when ﬁrst developed at the end of the seventies.

A fault in the power network, moves with the speed of light and

causes changes in the transmission network.

By detection of the transient changes in the current “∆i”, and in

the voltage “∆u”, and by comparing the signs of the two, the di-

rection to the fault can be determined. The fault detection can be

done very quickly. In the total trip time the communication time for

the communication with the remote end also must be included as

the protection forms a directional comparison scheme.

The travelling wave protection schemes are only used in EHV

networks and are very suitable to be used in series compensated

lines where a distance protection scheme has a draw-back.

Due to the rather limited use for EHV lines the protection will not

be discussed further in this document.

4. AUTO RECLOSING

4.1 THE PRINCIPLE OF AN AUTO RECLOSING DE-

VICE

For restoration of the normal service after a line fault an Auto re-

closing attempt is mostly made for Overhead lines. From fault sta-

tistics it has become accepted that as high as 95% of the faults

are of transient nature i.e. an Auto reclosing can be successful.

The three phase quick Auto reclosing attempt is mostly carried

out after a synchro check where the voltages on both sides of the

circuit breaker are checked and it is veriﬁed that they are not out

of phase due to e. g. a heavy power swing.

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For single phase Auto reclosing an identiﬁcation of the faulty

phase is necessary. This phase selection can not be achieved at

high resistive earth fault, when “R

>50-200 Ω”. Faults with higher

“R

” can be detected by zero sequence current protections but

can’t be single phase Auto-reclosed as the discrimination of the

faulty phase is difﬁcult. Special schemes have however been de-

veloped for cases where there is a deﬁnite need to disconnect

only one phase due to a very weak network.

4.2 GENERAL CONDITIONS FOR AUTO RECLOSING

The Auto reclosing shall be coordinated with various equipment

as indicated in Figure 22.

AR - Auto-recloser

BFR - Breaker failure relay

COM - Communication

COND - CB condition

DLC SC - Dead line check and synchronism check

LP - Line protection

MAN - Manual CB control

PD - Pole discordance relay

Figure 22. The Auto recloser and communicating equipment in a line bay.

Line protection relays.

There may be a single distance relay or duplicated protection. A

delayed back-up function is normally included. The Auto recloser

BFR

LP2

LP1

COND

Man

COM

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is started at relay tripping. At delayed tripping the recloser is nor-

mally blocked.

Communication.

Communication between the line relays is used to ensure quick

and simultaneous tripping at both ends. With relay cooperation in

the permissive mode there is a risk for delayed tripping at one

end if the communication is not working when a line fault occurs.

Dead time at quick Auto reclosing must be increased or recloser

start should be blocked at loss of communication.

Circuit breaker operating gear.

Energy is stored for close and trip operations. If there is insufﬁ-

cient energy stored for a close-trip sequence, recloser start shall

be prevented.

Circuit breaker failure relays. Circuit-breaker trouble detected by

breaker failure relay or pole discordance relay should block Auto

reclosing.

Dead-line check and synchronism check.

Depending on the power grid conﬁguration it may be necessary

to use a synchro-check relay to prevent Auto reclosing at loss of

synchronism. Dead line check is then made at one line end and

synchronism check at the other. In power grids with several reli-

able parallel circuits high-speed Auto reclosing is, however, often

arranged to work without any synchro-check function.

Manual CB operation.

At manual CB closing the recloser is blocked. Should there be a

fault on the line it is then tripped but not reclosed. At manual CB

tripping the recloser is not started.

Status of recloser.

The recloser status shall inﬂuence the tripping. In the recloser for

single-pole or three-pole reclosing there is a programming for se-

lection of reclosing mode. If the program is set to the position for

three-phase reclosing or reclosing off, or the recloser is blocked

by external conditions, all trippings shall be made as three-pole

operations. Sometimes, when investment in communication be-

tween distance relays is not made, the relays are set with a zone

1 overreach and thus cover more than the whole line at the ﬁrst

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tripping. At reclosing the zone 1 reach is shortened to ensure se-

lective clearance of persistent faults. When the recloser is

blocked or switched off, the zone 1 reach shall also be reduced

to give selective fault clearance already at the ﬁrst tripping.

The principle for an Auto reclose cycle

The principle of an auto reclosing cycle is shown in ﬁgure 23.

Figure 23. The operating principle of an Auto reclosing device.

The Auto reclosing device is providing a reclosing attempt, after

a dead interval with sufﬁcient length, to ensure that deionisation

of the fault area has taken place. The dead, and blocking time

must match the breaker capability. Normally the circuit breakers

fulﬁl the IEC duty cycle for circuit breakers O - 0.3 seconds CO -

3 min CO

FAULT TIMES AND DEIONISATION The time to achieve deionisa-

tion after the fault is dependent of several factors such as time to

fault clearance, fault current, wind, air humidity, capacitive cou-

pling to live parts etc.

The dead interval must be selected to give sufﬁcient time, to

deionisate the fault area. For instantaneously cleared faults

(<100ms), a dead time of 300-400ms can be sufﬁcient.

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If the fault clearance is delayed, say fault clearance times of 0.5-1

second, the dead interval must be longer e.g. 1 second.

When single phase Auto reclosing is used the dead time must

also be longer. Normally times of 0.8-1.2 seconds are used. The

reason for this is the capacitive coupling to live phases which will

maintain the arc. For long lines (>150km) it may even be neces-

sary to introduce a neutral reactor to the line (teaser reactor). The

teaser reactor shall have an inductance matching the line capac-

itive reactance to enable single phase auto reclose with reason-

able long dead time.

Auto reclosing schemes

There is a number of different practices, the most common ones

are:

- Three phase auto-reclosing at all type of faults.

- Three phase auto-reclosing only at single phase faults.

- Single phase auto-reclosing at single phase faults together with three

phase auto-reclosing at multi phase faults.

- Single phase auto-reclosing at single phase faults only.

Three types of dead interval are used:

- Quick Auto reclosing, with a dead interval less than 0.8 seconds. A quick

Auto reclosing requires communication between the line ends and an in-

stantaneous fault clearance at both ends.

- Quick auto-reclosing with a dead interval longer than 0.8-2 seconds can

be used when no communication exist and the fault clearance will or can

be made from the Distance protection second zone at one of the line

ends. The dead interval must then be increased.

- Delayed auto-reclosing, with a dead interval from 6 seconds up to sev-

eral minutes is mostly used in distribution networks but are also used at

transmission voltages in some countries.

4.3 BLOCKING

The Auto recloser must be blocked under the following condi-

tions:

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- Energizing of a line onto a fault. When a line is closed onto a fault the

Auto recloser is blocked to prevent unnecessary disturbances in the

power network. The chance of a persistent fault is high as the line has

not been carrying load and is therefor not a part of “normal conditions”.

- Auto-reclosing onto a persistent fault. An unsucessful auto reclosing

means that the fault probably is persistent. The auto-recloser is then

blocked to prevent new attempts. Solutions with several attempts how-

ever exists but the breaker duty cycle must then be carefully checked.

- “Breaker not ready” is used to block the auto-recloser. The breaker is

then not ready to perform any attempt and the breaker can not be auto-

matic reclosed.

- PLC (or communication channel) out of service must be connected to

block the Auto recloser if a quick auto reclose attempt is used. The fault

has not surely been cleared from the remote end and an auto reclose

might therefor not be successful.

5. AUTO RECLOSE FOR ONE-AND A HALF

BREAKER OR RING BUSBAR SCHEMES.

Auto reclosing of transmission line circuit-breakers is somewhat

more complicated in One- and a half breaker, 2–breaker and

Ring busbar stations than in simpler stations with a One line -

One breaker arrangement. At a line fault two CB’s shall be tripped

and reclosed at each end of the line and some coordination is re-

quired. In addition one circuit breaker (tie breaker) is shared by

two circuits in One- and a half breaker and Ring busbar stations.

A standardized Auto reclosing scheme designed by ABB com-

bines ﬂexibility with some particular features:

- High-speed or delayed Auto reclosing.

- Single-pole an/or three-pole Auto reclosing with independent setting of

dead times.

- Synchronism check feature as an option. It can be combined with de-

layed or high-speed three pole Auto reclosing.

- Cooperation with single- or duplicate line relays.

- Adapted to communicating line relays or Distance relays with overreach

of zone 1 before Auto reclosing.

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- Suitable for various station arrangements including One- and a half

breaker and ring busbar stations. The two line CB:s in a station can be

reclosed successively with a priority circuit.

- Only those CB:s which were closed before the line fault are reclosed.

- The dead line time is not inﬂuenced if one of the two line CB:s in a station

is out of service when the fault occurs.

- Correct behavior also for interline faults when parallel lines are used.

Figure 24. Auto reclosing of transmission line CB:s.

CB1

CB2

CB3

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INFLUENCE OF THE STATION ARRANGEMENT ON THE

AUTO RECLOSING

The type of switchgear arrangement will inﬂuence the introduc-

tion of Auto recloser and Synchro check devices. A number of

possible different arrangements are shown in below ﬁgure.

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Figure 25. Examples of multi breaker switchgear arrangements

MULTI BREAKER ARRANGEMENT

Application of Auto reclosing in an One- and a half breaker, Two

-breaker or a ring busbar station requires some particular atten-

tion. After a line fault two CB’s at each end of the line shall be re-

closed and need to be coordinated. One CB can be taken out of