Busbar Protn Notes

8/10/2019 Busbar Protn Notes

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INTRODUCTION

In the early days of the electricity supply industry, protective equipment for plantsconnected to a busbar installation was relied upon to clear busbar faults. This resulted intime delayed fault clearance by time graded protections such as distance relays orovercurrent time relays. With present day widely meshed power system networks withline sections varying in length and numerous intermediate infeeds, fault clearance byZone 2 or Zone 3 of distance relay can be difficult plus the impossibility of selectivetripping of different bus sections. In order to maintain system stability and minimisedamage due to high fault levels time delayed tripping for busbar faults is no longeracceptable. It is therefore necessary to detect busbar faults selectively with a unit form ofprotection system.

BASIC REQUIREMENT OF A BUSBAR PROTECTION SCHEME

i) It must be completely reliable, since the protection may only be called to operateonce or twice in the life of the switchgear installation and failure to operate under fault

conditions would be unacceptable.

ii) It must be absolutely stable under all through fault conditions since failure tostabilise would cause unnecessary widespread interruption of supply.

iii) It must be capable of complete discrimination between sections of the busbars toensure that the minimum number of circuit breakers are tr ipped to isolate the fault.

iv) It must possess high speed of operation to minimise damage and maintain systemstability.

TYPES OF BUSBAR PROTECTION SCHEME

Three types of busbar protection are commonly applied:1. Frame to Earth (Leakage) Protection2. Differential Protection3. Directional Comparison (Blocking Schemes) Protection

FRAME LEAKAGE PROTECTION

This is a simple and economical form of busbar protection which is ideal for the protectionof phase segregated indoor metalclad switchgear where earth fault protection only isrequired. The main basic requirement is that the frame of the switchgear must beinsulated from the true earth and between sections of the switchboard. This provision of

insulation between switchboard sections is the main disadvantage of this form ofprotection plus the fact that it is not possible to discriminate between faults on two sets ofbusbars running though common switchgear frames.


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Principle of Operation

The principle of operation of a frame leakage scheme is based on the fact that anybreakdown of the switchgear insulation will raise the potential of the frame to earth andcause a current to flow in the connection between the frame bonding bar and earth. Acurrent transformer connected between the bonding bar and earth will therefore measurethis earth fault current and operate a protective relay. An instantaneous current relay issufficient for this application.

The current transformer ratio used is not critical provided the necessary fault setting canbe obtained.

Outgoing

Switchgear

Switchgear

frame

Generator

Systemearthing

Earth

Frame-leakagecurrenttransformer

Earthing

electrode

Frame

insulation

IF = I1+ I2

I1+ I2

I1 I2I1


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Insulation Requirement and Frame Earthing

The switchgear must be insulated as a whole, usually by standing it on concrete, takingcare that the foundation bolts do not touch any steel reinforcement. No other earthconnections of any type including incidental connections to structural steelwork should bepresent. This is to ensure that :

The effective setting of the relay is not raised by any path shunting the principal earthconnection and current transformer.No spurious tripping will take place for an external earth fault with current flowing into orout of the switchgear frame.

The insulation achieved should be greater than 10 ohms to ensure stability under externalfault conditions.

All cable glands must be insulated to prevent circulation of spurious current produced byhigh voltages induced in the cable sheaths under through fault conditions causingflashover between gland and switchgear frame.

DIFFERENTIAL PROTECTIONTwo forms of differential protection are adopted for busbar protection, namely, `HighImpedance` and Low Impedance`.

High Impedance Differential Protection

This is a unit type protective scheme in which currents entering and leaving the busbarinstallation are compared continuously. The object is to provide fast operation at a lowfault setting on internal faults and yet retain stability up to the highest possible value ofshort circuit current on through faults. Current transformers on each of the busbar circuitsare connected in parallel which will produce a resultant current to operate a relay for

internal busbar faults only. Theoretically such a system is unaffected by through faults,but in practice the associated current transformer may not behave ideally when thecurrent exceeds a certain value. Errors in transformation due to saturation of the currenttransformer cores may be sufficient to cause maloperation if special precautions are nottaken. In order to ensure stability for external faults the current through the relay is limitedby the insertion of an external resistor in series with the relay. This resistor is oftenrefered to as a stabilising resistor.

The stability limit of a busbar protection scheme is based on the maximum through faultcurrent. In general this takes the value of the associated switchgear rating irrespective ofthe existing or anticipated fault levels.

Fault Setting Resistors

These are used to increase the effective primary fault setting by creating a shuntresistance across the relay circuit. They are useful where a standard relay with a givensetting is used for all the busbar installations to achieve a given primary fault settingthroughout.


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Check Feature

A second line of defence is considered good practice in most schemes of busbarprotection, not to give security against maloperation of the primary protection due toinherent defects but to prevent incorrect tripping as a result of damage to wiring andequipment from extraneous sources. A check feature is provided by duplication of theprimary protection using a second set of current transformers on all circuits other thanbus section and coupler units. The check system is arranged in a similar manner to theprimary protection but forms one zone only covering the whole of the busbars and doesnot discriminate between faults in the various sections of the busbars.

Use of Non-Linear Resistors (Metrosils) to Limit VoltageAcross Relay and Current Transformer Secondary Wiring

Under in-zone fault conditions, the high impedance relay circuit constitutes an excessiveburden to the current transformers, leading to the development of a high voltage thewaveform of which will be highly distorted with a peak value many times the nominalsaturation voltage. Non-linear resistors are used in parallel with the relay circuit to reduce

this voltage.

CT WIRING SUPERVISION

When a current transformer secondary winding or connections between currenttransformers and the relay circuit become open circuited, the resultant out-of-balancecurrent will flow through the parallel combination of relay, metrosil, fault setting resistorand current transformer magnetising impedance. This may cause the protection tooperate for load or through fault conditions depending on the effective primary setting.

The condition of an open circuit can be detected by measuring the voltage across therelay circuit by a sensitive voltage operated relay as shown in the following figure. This

relay is set to operate when the out-of-balance current equals about 10% of the leastloaded feeder connected to the busbars or 25 amperes whichever is the greater.

If accurate details of current transformer magnetising characteristics are available, therequired setting can be calculated. Checks should be done on site to ensure that therelay will not operate due to normal unbalance with the system and protection healthy.

CT1

Super

vision

relay

V ZM2 ZM3 ZM4

I1

I2 I3 I4


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Operation of the supervision relay is arranged to give an alarm that the busbar protectionis faulty and to short circuit the buswires if this is necessary to prevent damage to theprotective relay and stability resistors.

When the busbar protection has a fault setting below full load of the connected feeders itis very likely to operate due to an open circuit current transformer. In this case a checkfeature is required to prevent tripping. At the same time it is important that the buswiresare short circuited via the supervision relay to prevent thermal damage to the protectiverelay and stabilising resistors which would otherwise remain continuously picked up underload conditions.

The supervision relay must have a time delay to prevent its operation due to genuinebusbar faults. A time delay of about 3 seconds is used.

CURRENT TRANSFORMERS

Current Transformer Design

An important advantage of using high impedance relay in a circulating current system isthe ability to predict the protective scheme performance in terms of primary fault settingand through fault stability by calculation without heavy-current conjunctive tests. Thevalidity of the calculation is based on the assumption that all the current transformers areof low reactance type. A low reactance current transformer is defined as one of which aknowledge of the secondary exciting current, secondary winding resistance and turnsratio is sufficient for an assessment of its performance. This covers current transformerswith uniformly distributed windings or whose core leakage flux is negligible.

Current Transformer Wiring

With high impedance circulating current schemes, it is of the utmost importance that the

lead burdens between the various sets of current transformer be kept as low as possiblein order to obtain the required stability and sensitivity. It is therefore advisable to run thebuswires in the form of a closed ring between all the circuit breaker control cabinets. Thisavoids the need for numerous radial loops between the current transformers and the buszone panel which would be required if the buswires were formed in the bus zone panel.

A closed ring consisting of cores in multicore cables affords increased security againstmaloperation which may result from unbalancing of the protection due to inadvertentdisconnection of bus wires. It also provides easy extension of the protection when newcircuits are to be connected into the protection zone.

An example of running a multicore cable ring in the case of a double busbar arrangementis as follows :

i) current transformers to marshalling kiosk.

ii) marshalling kiosk to auxiliary switches in the busbar selector isolators.

iii) loop between marshalling kiosks.

The size of conductor normally used for the interconnecting pilots is 2.5 mm. However, itis occasionally necessary to use parallel cores to reduce the burden.


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BUSBAR SELECTOR AUXILIARY SWITCHES

In a lot of cases such as a double bus arrangement where on-load transfer of a circuit ispossible, current transformer outputs are switched to the correct buswires by means ofauxiliary switches on the selecting isolators. These auxiliary switches should close beforethe main isolator closes and should open after the main isolator opens to ensure stabilityduring switching operation. This is shown in the following figure.

Current Transformer Location

The three alternative arrangements as shown in the following diagram :

R

M

A B C D

a b c d

R

M

All C.T.s on line side of

circuit breaker

Circuit

protection

Busbar

protection

Overlapping C.T.s

Circuit

protectionBusbar

protectionInterlocked

overcurrentrelay

All C.T.s on Busbar side of

circuit breaker

Interlocked

overcurrent

relay

Circuit

Protection

Busbar

Protection

F1

F2

F3

F4

F1

F3

F4

F1

F3

F2


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i) current transformers for feeder and busbar protection overlapping the circuitbreaker

ii) all current transformers on line side of circuit breaker

iii) all current transformers on the busbar side of circuit breaker.

i) In this arrangement faults at F1and F2are cleared correctly by the busbar andfeeder protection respectively. Faults at F3between the circuit breaker andfeeder protection current transformers will be cleared by the busbar protectionand possibly also by the remote end of the feeder protection. No unnecessarydisruption to loads will result from this.

Faults at F4 will be seen by the feeder protection but also by the busbarprotection resulting in unnecessary tripping of the busbars for what isessentially a feeder fault. This is the main disadvantage of this arrangement.

ii) This is the most common arrangement where all the current transformers are

on the feeder side of the circuit breaker. However, there is a blind spot atpoint F3where faults are seen by busbar protection but not seen by the feederprotection. With this arrangement it is therefore required to intertrip theremote circuit breaker when busbar protection operates.

Intertripping can be achieved by unstabilising the feeder protection and can beinstantaneous or time delayed to allow clearance of faults on the busbar sideof the circuit breaker before intertripping.

Alternatively an interlocked overcurrent relay can be used to intertrip theremote circuit breaker. This relay is interlocked with the busbar protection.

iii) When all the current transformers are located on the busbar side of the circuit

breaker a fault at F3 between the current transformers and circuit breaker willcontinue to be fed from the busbars after the circuit breaker has been trippedby the feeder protection. An interlocked overcurrent relay which is interlockedwith the feeder protection is required to ensure that the busbars are onlytripped for this condition and not for faults on the feeder.

BUSBAR CONFIGURATIONS

Several switching schemes are available and there are many variants of each scheme.When selecting a suitable scheme consideration should be given to the ability to take outany circuit breaker or other equipment for maintenance without removing thecorresponding circuit from service, also the ability to isolate the busbar for maintenance,

some schemes being more flexible than others in this respect.

In addition to plain single and double busbar schemes, the following are some of the othermore popular arrangements:

1) Double Busbar with Transfer

With this double busbar variation, each feeder has isolators to enable switching to main orreverse/transfer bars, and also an additional isolator to enable the feeder breaker to bebypassed. The reverse bar may then function also as a transfer bar and the bus couplerbreaker takes over the function of the feeder breaker to free it for maintenance.


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To apply discriminative busbar protective, suitable auxiliary switches are required on eachisolator to select the CTs for the correct zone, and the trip circuits to the appropriaterelays.

2) Triple Busbar

This is a double busbar scheme with a third, transfer busbar.

Under normal conditions all bus section and bus coupler breakers are closed. Duringmaintenance of a feeder breaker, the transfer bus is energised from the selected main or

reserve bus by the transfer breaker and the feeder bypass isolator closed on the transferbar. All bus section and bus coupler breakers remaining closed. For busbar protectionisolator auxiliary switches are required as previously.

Main

Reserve / Transfer

By-pass

Isolator

By-pass

Isolator

Main

Transfer CBTransferReserve Transfer CB


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3) Mesh Busbar Scheme

The mesh busbar scheme is a frequently used EHV busbar configuration. A transformerand a feeder are linked at each corner of the mesh and four circuit breakers used tocomplete the mesh interconnection the arrangement being justified on the grounds ofeconomy.

The protection shown consists of a fully discriminative scheme with a relay at eachcorner. A fault at any corner trips the two breakers associated with that corner and alsoinitiates any intertripping necessary to open circuit breakers at remove ends.

T1

F1 F3

T4

T3

T2

F4 F2

T1

F1 F3

T4

T3

T2

87

R187

R3

87

R4

87

R2


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4) One and a Half Breaker Scheme

This is a very popular and economical scheme, three breakers and two feeders beingarranged between the two busbars. Under normal conditions all breakers are closed.During maintenance of a feeder breaker only that breaker would be kept open.

During maintenance of a busbar, all the breakers connected to that busbar would remainopen to isolate that busbar.

When busbar protection is required, then each busbar is considered individually and asingle busbar scheme applied to each as shown, as with the protection for the meshbusbar previously, the protection scheme does not require isolator auxiliaries for CT zoneselection or in the tripping circuits, the scheme being very simple, and this together withthe operational flexibility of this busbar configuration accounts for its popularity.


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BREAKER FAIL PROTECTION

Where breaker fail protection is applied to a system, back tripping of associated breakersis required in the event of breaker failure. Often, breaker fail protection is arranged inconjunction with busbar protection tripping circuits to initiate tripping of breakers on abusbar zone associated with the failed breaker.

LOW IMPEDANCE PROTECTION

Low impedance busbar protection has a number of advantages:

Fast

Modular scheme design allows relays to relate to each circuit and function of theprotection

High sensitivity for phase and earthfaults. Protection for each phase can be relativelyindependent

Extremely stable for external faults. This is achieved by using saturation detectors

Current transformers can be of different ratio, relatively smaller output and sharedwith other protective devices

The current transformer secondary circuits are not switched

Continuous supervision of CT circuits and constant monitoring of vital circuits can beincluded

87

87


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DIRECTIONAL COMPARISON (BLOCKING SCHEMES)

The use of numerical overcurrent relays enables busbar protection and backup protectionto be combined within the same unit. This allows the use of busbar protection at voltagelevels where the traditional high or low impedance protection would have been tooexpensive.

Typically the overcurrent relays would be time co-ordinate in the normal manner providingovercurrent and earthfault protection for the system. The instantaneous element in theincomer relay can be prevented form operating by the overcurrent relays on the outgoingfeeders. Upon detection of a feeder fault the associated feeder relay would operate astart contact, this contact would be wired to an opto isolator in the incomer relay whichupon energisation would block the instantaneous element of the incomer relay.

By using directional relays it is possible to provide zones of protection, thus only removingthe faulty section of a busbar.

IncomerIncomerIncomerIncomer

IF2

Block tBlock tBlock tBlock tBlock tBlock tBlock tBlock tBlock tBlock tBlock tBlock t

IF2

BacktBacktBacktBacktBacktBacktBacktBackt

Block tBlock tBlock tBlock tBlock tBlock tBlock tBlock tO CO CO CO C

O CO CO CO CO CO CO CO C O CO CO CO C

O CO CO CO C

BLOCK

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Busbar Protn Notes