Protection Application Handbook (англ. яз.)

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SUB-DIVIDED PROTECTION SYSTEMS. WHY?

Sub-divided Systems

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1. SUB-DIVIDED PROTECTION SYSTEMS.

WHY?

The single failure criteria can be fulﬁlled with a protection system

including back-up protection. The back-up protection must be

able to detect all primary faults detected by the primary protec-

tion, primarily intended to clear the fault. There are two different

ways of arranging back-up protection:

Remote bac

k-up is the common way of arranging back-up in a

distribution network. This means that the back-up protection is

available at another breaker. Remote back-up is normally

achieved by a time grading where the back-up protection is given

higher time setting than the primary protection.

Local bac

k-up is used when remote back-up is difﬁcult to achieve

or when time grading to achieve selectivity is not acceptable due

to thermal limits or network stability reasons. Local back-up

means that at the breaker location, a second protection relay de-

tecting the same faults as the primary relay, is included. The two

systems are operating in parallel on the same breaker.

The principle with local back-up is mainly used in the HV trans-

mission network where the fault current is fed from many different

directions.

When the local back-up is operating with same time and principle

as the primary protection, the protection system consists of re-

dundant protection.

2. SUB-DIVIDED PROTECTION SYSTEMS.

HOW?

As mentioned above the task of the protection system is to clear

the primary faults and the abnormal service conditions. The total

fault clearance chain consists of different parts as shown in below

ﬁgure.

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Figure 1. The different parts of the fault clearance chain

As noted the secured clearance of the fault includes correct be-

havior of many different elements, none of which is allowed to fail.

All sections can thus be said to have the same importance which

puts stringent requirement on the plant design.

When remote bac

k-up is used the fault clearance is secured with

two different breakers including protection relays capable of de-

tecting same faults. The important care needed to be taken in

such cases is to ensure that the same DC supply, or at least bat-

tery feeder, is not used for two protection relays where one is be-

ing back-up to the other.

When local bac

k-up, with or without redundant protection, is used

the fault clearance chain will normally be as shown in ﬁgure 3.

Meas

relay

Aux

relay

Breaking

element

Breaker

mech

Trip

coil

supply

Cable

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Figure 2. Fault clearance chain at redundant protection systems

A common practices used at redundant systems is:

- Different cores are used on the instrument transformers.

Duplication of apparatus is very unusual due to the high cost involved

and the very good operating statistic.

- The circuit breakers are not duplicated of cost reason.

However dupli-

cated trip coils are used.

- The DC supply is separated as far as feasible.

For HV system the use of redundant batteries is common practices

- Physical separation is performed as far as realistic.

Often cables are laid in different cable ducts.

As the circuit breakers are not duplicated, the failure of a breaker

to operate will mean that the fault is not cleared. In order to clear

the faults the surrounding breakers are then required to operate

instead. For applications where local back-up is required due to

failure of “remote” protection relays to detect the fault, a breaker

failure relay is thus essential. The breaker failure relay measures

the fault current still fed through the breaker after expected trip-

ping and will then trip all surrounding circuit breakers.

An example of the protection system including redundant protec-

tion, for a HV line, is shown in Enclosure 1 under Protection Line.

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Instrument transformer circuits

The instrument transformers are normally not duplicated but only

provided with different cores for the two sub-systems. The redun-

dant protection systems are connected to different cores and any

mixture, where one current transformer core is feeding both

sub-systems, should be avoided. This also includes feeding an-

other type of protection e. g. the Busbar protection, when this is

included in the other sub-system, from the same core.

Any current loop feeding more than one panel should be avoided

due to the risk of open circuits at terminating point or in the ter-

minal itself (specially when this has a disconnectable link).

Both sub systems are normally connected to the same CT con-

nection box. Physical separation in different CT boxes is normally

not possible.

For voltage transformer circuits a common way is similarly to use

separate secondary windings for the two sub-systems. In some

cases two windings are not available and in such cases the cir-

cuits should be separated on different fuse groups in the mar-

shalling box. This is normally a fully acceptable solution due to

the high availability proven for voltage transformers.

Physical location in panels

- When sub-divided systems are used the physical location in panels of

the protection relays and of necessary auxiliary relays is of main impor-

tance.

- The common practices on the highest voltages is to use different panels

for sub 1 and sub 2 equipment. The reason for physical split-up is to pre-

vent problems due to e. g. cable ﬁre or mechanical damage in a panel.

However in some cases where the sub-systems takes small

place it must be considered acceptable to put the equipment for

both sub-systems in the same panel.

The risk of cable ﬁre or other mechanical damage which could in-

ﬂuence two systems in the same panel must be considered neg-

ligible. Problems with mixture of circuits and mistake at erection,

commissioning or maintenance must be considered and steps

taken to ensure that this will not happen.

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Two different objects in the same panel will give similar problems

and need careful split-up of the equipment both on panel front

and inside the panel

to prevent service and maintenance person-

nel from doing mistakes.

Figure 3. The front layout of a relay panel for a HV line where the sub-

systems are with panel separation. The equipment included is as shown on

block diagram for a HV line, see Line protection section.

Figure 4. Arrangement of two sub-systems in a common panel.

From above follows that an important part at panel design is to

clearly indicate to which object the equipment belongs. When

sub-divided systems are used the sub-system belonging shall

also be clearly indicated. This includes clear separation of the

equipment and clear labelling

of different sections, also on sepa-

ration parts between sub-systems or objects. The separation

must also be performed inside the panel by physically separ

ating

the terminal groups for the different equipment in the panel.

Figure 3 Figure 4

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A terminal numbering system allowing for visibility of sub-system

and circuit belonging is then an important part to simplify and en-

sure a correct design. ABB SUB/T has set up rules for terminal

numbering in sub-divided systems.

Please refer to Control System structure section for this number-

ing system.

All equipment used in the protection system and also other aux-

iliary relays connected to the same battery system, or fuse

groups, shall be grouped together and clearly labelled.

Remember to include the equipment for closing, interlocking etc.

which is located in the protection panel.

DC supply

The DC supply is one of the key parts at arranging back-up pro-

tection both when local and remote back-up are used. Many trip

failures causing large equipment damage have been caused by

DC failures.

The principle which must be followed is to ensure that the Main

and the Back-up protection are not fed from the same DC supply.

The level of independence can be different. On the highest volt-

age completely redundant systems are used which then includes

two separate batteries, two to three battery chargers etc.

For simpler applications it can normally be considered sufﬁcient

to separate the battery main distribution in two sections and feed

the main and back-up protection from different sections or at

least different main fuses on the battery distribution.

A commonly used principle is to minimize the sub 2 DC distribu-

tion as much as possible. Preferable it should only be used in re-

lay panel and on breaker trip coil.

An example showing the design of a DC system with redundant

batteries is available in Appendix 1.

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2.1 ARRANGEMENT OF TRIP CIRCUITS

Another main part of the protection system is the trip circuits.

A failure of the trip circuit will of course give a failure to trip the

breaker. As mentioned above the breaker itself is not duplicated

of cost reason and the only realistic means of improving depend-

ability is to use two trip coils on the breaker. This will cover prob-

lems with DC supply, the wiring or the coil but will not give any

advantage for mechanical problems.

Duplication of the trip coil is today possible on all EHV, HV and

MV circuit breakers and to a low cost.

Another mean of further improving dependability of the trip circuit

is to include trip circuit supervision relay/s. These can supervise

the circuit from the relay panel to the breaker and will detect open

circuit or loss of DC supply. The common principle used is to feed

a small current through the circuit. The current path will be broken

at open circuit or DC problems and an alarm is given.

It should be ensured that all wiring is included in the supervision.

Specially care must be taken when some trip contacts are provid-

ed in a different panel which can happen for transformer bays or

generator bays.

2.2 EXCHANGE OF INFORMATION BETWEEN THE

TWO SUB-SYSTEMS

The main principle used when sub-divided systems are utilized is

to avoid exchange between the two systems. As far as possible

the systems shall operate completely independent of each other.

This gives the highest total security as problems at design, main-

tenance etc. due to human or equipment failure are avoided.

In most cases however some signals are required to go to other

sub-system. The most common signals are:

- Start and block of auto-recloser.

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Auto-recloser is normally not duplicated due to speed problems with two

units and due to the minor consequence a failure to auto-reclose will mean.

A number of signals will be required between the sub-systems to give au-

to-reclose also when only the protection in the other sub-system operat

es.

- Start of Breaker failure protection

Similar to Auto-recloser the Breaker failure function (BFR) is not duplicated

in redundant systems. The reason for this is however completely different.

The Breaker failure function has extremely high requirements on the securi-

ty against unnecessary trippings and duplication will mean an increased de-

pendability but the security will decrease. As the risk of breaker failure

statistically is very low the security aspect is most important and the BFR

function is only provided in one of the systems, normally in the primary pro-

tection system

The exchange of information between the systems should be

handled with outermost care to prevent problems e. g. when

maintenance is done in one of the systems.

Interface relays should be put inside or on front on a preferable

separate labelled area in one of the panels only.

The terminals should also be clearly indicated inside the panel in

order to simplify the ﬁnding and opening of the correct terminals

when the sub-systems are to be separated.

Interface relays should be separately mounted and preferably in

sub 2 panel. This will limit the distribution of the sub 2 DC supply

as mentioned above.

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Figure 5. Sub 1 -Sub 2 interface for a HV overhead line

An example of how an interface unit for sub 1-sub 2 interface can

be arranged for a HV line is shown in Figure 5. The example is

same as on Block diagram in Enclosure 1 to Protection Line sec-

tion and on the panel layouts Figure 3 and 4. The terminal num-

bering used is based on the standard terminal numbering

system, refer to section Control System Structure.

2.3 CABLING

A common requirement for sub-divided protection system is to

use different cable ways for the two systems.

Often this requirement is limited to e. g. use of different cable lad-

ders in the same cable duct. The sub 1 and Sub 2 signals must

under all circumstances be in different cables.

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2.4 ALARMING AND TESTING

The protection system is designed according to “single failure cri-

teria”. This means that the “single failures” in the protection sys-

tem must be discovered within a reasonable time. An alarming of

secondary system faults is thus of same importance as the

alarming of primary faults. The alarms must be connected to give

remote alarms as well.

Some faults are not detected by the supervision equipment. To

detect these faults regular system testing and post fault distur-

bance analysis are of vital importance.

Testing and analyzing of protection system behavior gives a pos-

sibility to detect “hidden faults” such as failure in some parts of a

protection relay or problems with selected settings or with the

used measuring principle.

A correctly performed alarming and regular testing of the protec-

tion system will ensure that faults in the secondary circuits are

discovered and the fault clearance at an occurring primary fault

secured.

2.5 GENERAL ASPECTS ON SECURITY AND DE-

PENDABILITY

As mentioned the reason for redundant systems is to ensure the

local back-up i. e. secure fault clearance according to the “single

failure criteria”. This is fulﬁlled with the two independent systems

where one system can fail in any component of the fault clear-

ance chain but the fault clearance is still secured.

Adding extra equipment and/or cross tripping etc. to cover even

more than a “single failure” should be avoided as the security and

the dependability are in opposition and an increased dependabil-

ity will always lead to a decreased security.

More equipment, a more complicated and advanced protection

system, cross tripping, where protection relays in each systems

are connected to trip also in the other system, shall be strictly