Protection Application Handbook (англ. яз.)

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PRINCIPLES OF DIFFERENTIAL BUSBAR CONFIGURA-

Bus&Breaker Protection

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For single busbar arrangement no switching is done in the CT-cir-

cuits and a check zone is therefore not necessary.

Figure 1. A one zone differential relay for a single busbar.

At 1 1/2-breakers system arrangements a 2-zone protection must

be used se ﬁg. 2.

PRINCIPLES OF DIFFERENTIAL BUSBAR CONFIGURA-

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If the main CT:s have equal ratio, which is normally the case, aux-

iliary CT:s are not necessary.

Figure 2. A two-zone busbar differential relay for 1 1/2-breaker switchgear

At double busbar switchgear a 2-zone protection shall be used,

see ﬁg. 3. If the main CT:s have equal ratio auxiliary CT:s are not

necessary.

CT-circuits are switched depending on the position of the busbar

disconnectors. The current is either connected to busbar A or

busbar B:s differential protection. Switching is easiest performed

by using repeat relays (RXMVB 2), controlled via two auxiliary

contacts at each busbar disconnector, see ﬁg. 5.

PRINCIPLES OF DIFFERENTIAL BUSBAR CONFIGURA-

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Figure 3. 2-zone busbar differential protection, for double busbar switchgear.

Many cases when switching in CT-circuit is done requires a check

zone for the busbar protection, see ﬁg. 4. The check zone is

ﬁxed and has no switching of CT:s in all outgoing circuits and is

not connected at busbar sections and busbar couplers. Check

zone, will detect faults anywhere in the substation but can not dis-

tinguish in what part of the substation the fault is located. When

the check zone detects a fault it gives a release signal to the bus-

bar protection relays in all individual, discriminating zones. The

busbar protection discriminating zones will then trip the part of

the substation that is faulty.

The releasing when the check zone detects a fault is normally

done by sending out positive DC-voltage to the discriminating

zones. Another way to do the releasing is to connect the negative

DC-voltage to all trip relays from the check zone and to connect

the positive DC-voltage from the discriminating zones.

SWITCHING IN CURRENT CIRCUITS

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Figure 4. 2-zone busbar differential protection, for double busbar switchgear

(including check zone), releasing the main zones for busbar A and B.

3. SWITCHING IN CURRENT CIRCUITS

For busbar systems, where the object can be connected to more

than one busbar, double busbar systems, the current must be

switched to the correct zone busbar protection. This switching

must be done in such a way that no differential current develops

in any of the busbar protections concerned. The switching is done

by using two auxiliary contacts on the busbar disconnectors.

These auxiliary contact are energizing throw-over relays (RXM-

VB 2) which perform the switching in the CT-circuits. The discon-

nectors auxiliary contacts must be provided as in ﬁg. 5.

Following requirements must be fulﬁlled by the auxiliary contacts

“a” and “b” respectively:

PROTECTION OF MV BUSBARS IN DISTRIBUTION NET-

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- The auxiliary contact “a” must close minimum 100 ms before the main cont

can start to carry current.

- The auxiliary contact “b” must open before the auxiliary contact “a” clos-

es.

- The auxiliary contact “b” must not close before full insulation is secured

(normally at 80% of the full insulation distance).

Figure 5. Requirements and connection of busbar disconnector auxiliary

contacts, for switching of RXMVB 2 repeat relays.

4. PROTECTION OF MV BUSBARS IN DISTRI-

BUTION NETWORKS

For busbars in distribution networks Busbar protection can be

achieved mainly in two different ways, either by blockable over-

current protection at the incoming bays to the switchgear, or by

locating arc detectors inside the enclosure.

Blockable overcurrent protection is based upon the principle that

fault current is only fed by the incoming to the busbar. The incom-

ing circuit is equipped with an overcurrent relay that have a fast

step deﬁnite time-delay of approximately 100 ms (this step is

APPLICATION OF BUSBAR PROTECTION

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blocked if any of the overcurrent relays in the outgoing circuits

starts, se ﬁg. 6.

For metal enclosed busbars, arc detectors can be used as busbar

protection. The arc at a primary fault is detected by a arc sensitive

device, which trips all incoming circuit breakers. In order not to

risk maloperation due to photographic ﬂashes etc., it’s advisable

to also have an overcurrent interlock.

Figure 6. Blockable Overcurrent protection.

5. APPLICATION OF BUSBAR PROTECTION

Figure 7 gives a survey of different type of busbar protecti

Figure 7. Application of different types of busbar protection relays

3I>

3I>3I>

3I>

100 ms

BLOCK

TRIP

START

APPLICATION OF BUSBAR PROTECTION

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For busbars in distribution networks the protection can be

achieved mainly in two different ways. One way is by blockable

overcurrent protection at the incoming bays to the switchgear an-

other way is by locating arc detectors inside the enclosure.

High impedance protection needs separate CT-cores with equal

ratio at each feeder, bus-coupler and bus-section. This means

that other protection relays cannot be connected to the same

CT-core. This type of busbar protection is mainly for medium volt-

age levels.

RADSC is a new type of low impedance busbar protection that

can be used as an alternative to high impedance protection re-

lays. It has a maximum of 6 inputs and can therefore not be used

in larger substations. It can however use the same CT-cores as

other protection relays but requires auxiliary CT:s if the main

CT:s in the different bays have different ratio.

Summation type of busbar protection can be used for more eco-

nomical variants of protection. The Summation type Busbar pro-

tection relays has only one measuring unit which means that

there is no internal back-up for 2-phase and 3-phase faults. Pri-

mary operating current varies with a factor 4 depending on in

which phase the fault occurs. Phase indication cannot be ob-

tained.

RADSS and REB 103 are medium impedance type of busbar

protection relays, during internal faults, but low impedance pro-

tection during load and external faults. RADSS and REB 103 can

therefor be used together with other types of object protection on

the same cores. This kind of protection is intended for high (HV)

and extra high voltage (EHV) levels.

RADSS can be used without auxiliary CT:s if the main CT:s have

equal ratio. This gives a solution that is both economical and

saves space. The only restraint is that the maximum circulating

current through RADSS, power transferred through the zone,

must not exceed 4 A secondary. This could be a problem if the

BREAKER FAILURE RELAYS

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main CT:s both have 5 A secondary but can be overcome by in-

creasing the primary ratio of the main CT:s.

When main CT:s have different ratio REB 103 gives a more eco-

nomic solution since REB 103 is designed with auxiliary CT:s of

small physical sizes. REB 103 cannot be used without auxiliary

CT:s.

INX 5 is a low impedance busbar protection for high (HV), and ex-

tra high voltage (EHV) levels.

REB 500 is a new decentralized, or centralized, numerical Bus-

bar protection relay with a process bus connecting the different

bays. The relay is mainly intended for the HV and EHV voltages.

6. BREAKER FAILURE RELAYS

In order to take care of possible breaker failure, Circuit Breaker

Failure relays normally are installed in high voltage, and extra

high voltage systems.

All protection tripping (not manual opening) will start a current re-

lay measuring the current through the CB. If the current has not

disappeared within the set time, all adjacent CB:s will be tripped

to clear the fault.

BREAKER FAILURE RELAYS

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The time “split up” at a normal tripping and a breaker failure trip-

ping is shown in ﬁg. 8.

Figure 8. Time scheme for breaker failure relays

In normal cases the total fault clearing time will be the protection

relay operating time plus the CB interrupting time. Every time a

relay gives a trip order it will at the same time start the Circuit

Breaker Failure (CBF) relay. The CBF-timer will be running as

long as the current is ﬂowing through the CB. When the CB inter-

rupts the current the CBF-current relay will reset and the

CBF-timer will stop.

Fault

occurs

Normal

clearing

Failed

breaker

Protection

time

Breaker inter-

rupting time

Normal clearing time

Margin

Breaker failure timer

Breaker failure total clering time

Back-up breaker

interrupting time

Breaker

failure

tripping

POLE DISCORDANCE RELAYS

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The setting of the CBF-timer must be:

maximum interrupting time of the ordinary CB, plus the reset time

of the CBF-current relay, plus the impulse margin time of the

CBF-timer, plus a margin.

With “impulse margin time” is meant the difference between set

time and the time “to no return”. If the timer is set to 100 ms and

is fed during, lets say 98 ms, it will continue to operate since the

margin between the set time and the actual time is small. The

margin should be chosen to at least 50-60 ms to minimize the risk

of unnecessary tripping, specially considering the heavy impact

on the network a CBF tripping will cause.

7. POLE DISCORDANCE RELAYS

For circuit breakers with one operating device per pole, it is nec-

essary to supervise that all three phases have the same position.

This is normally done by using the auxiliary contacts on the circuit

breaker according to ﬁgure 9.

Figure 9. Connection of a Pole Discordance relay.