busbar protection abm

The electrical power system starts at Power Station. Electrical power is generated by converting potential energy such as coal gas & oil into electrical energy The powerconverting potential energy such as coal, gas & oil into electrical energy. The power station generates electrical energy at lower voltage level of 11kV to 20kV. The electrical power generated is then transferred to power grid for transmission to load center. This is done either directly or in this case through step up power transformer, depending on the generated voltage and required voltage at the transmission power grid, i.e. for TNB system at 132kV, 275kV and 500kV. Next step, the power is transmitted via transmission lines, normally over long distances to high voltage substations (in TNB, the main intake substation is called PMU pencawang masuk utama) at load center. The substation is tapped directly from the power grid and normally interconnected with other substations pp y p g yto improve reliability of supply. The power is again transmitted to distribution substations, which form the distribution network. The power is also transmitted to heavy industry at high voltage, e.g. 132kV. At distribution substations, the transmission voltage will be transformed form high voltage to lower voltage (for e.g. 33kV or 11kV in TNB) for distribution purposes. Main distribution substation supply to clearly defined distribution network, e.g. to light industrial areas. The power voltage is also transformed to suitable lower voltages (240V/415V) to urban areas (towns and cities), and residential areas (houses). It can be seen that equipment failures at GTD will cause supply interruption to consumersinterruption to consumers.

If not properly planned and designed, failure of this system may lead to long interruption of supply or worst lead to blackout of power system. This essentially interrupts the transfer of electrical energy to our customers. One important components of power system is the electrical substation.

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This slide shows the TNB national grid, i.e. power system connectivity for transmission system in Peninsular Malaysia. For TNB, the voltage levels for transmission system include 132kV, 275kV and 500kV. The medium voltage levels include 11kV and 33kV.

From the national grid, snapshot of Klang Valley, i.e. Selangor, WilayahPersekutuan, & parts of Perak, Negeri Sembilan & Pahang, is displayed.

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From the earlier topological snapshot of national grid, the system connectivity single line diagram is derived as shown. The system or the network single line diagram will be used as reference, for TNB utility personnel to remember the interconnectivity among power stations and substations in TNB transmission system. The diagram will also be used to study and analyse the system, for example when conducting system studies or post fault analysis.

The voltage levels and type of stations are shown as per the legend ForThe voltage levels and type of stations are shown as per the legend. For example, the Janamanjung Power Station as shown generated power and transmitted it through 500kV system to the National Grid. The rectangular boxes are various substations interconnected as part of the National Grid.

For example, at load centre, the power are distributed to the load via substation such as KPCK (Kampung Chempaka 132/33kV) substation.( p g p / )

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This slide zooms to the earlier Kampung Chempaka 132/33kV substation as h i thi b t ti i l li di Wh t th f ti f b t tishown in this substation single line diagram. What are the functions of substation

in an electrical power system?

Substations form vital nodes in high voltage power systems, and in this case form part of the transmission power system that is transporting high power over large distances at extra high voltage EHV (e.g. 500kV and 275kV). The power is then transported to high voltage HV (e.g. 132kV) transmission system and distribution networks covering shorter distances and lower voltages (e g 11kVdistribution networks covering shorter distances and lower voltages (e.g. 11kV and 33kV). Substations are also points in the network where circuits or feeders are connected together through circuit breaker and isolators.

The role of substations are:• In-feed points for generation capacity (normally the voltage level is stepped-up via transformer) or delivery point for electrical power consumption (normally the

l l l d d ) h h l l l d dvoltage level is stepped-down). In this case, the voltage level is stepped-down from 132kV to 33kV via power transformers.• Permit alterations of power system or network configuration during operation. This allows the control of power flow in the network and general switching operation during maintenance.• Information (power system parameters, e.g. current and voltage) from the substation high voltage primary system is collected and processed by substation

d t ( h t ti d t l t ) f d i i kisecondary system (such as protection, and control systems) for decision making and monitoring. The faulty components of power system are automatically detected by protection system and switched off or tripped off by circuit breaker.•Local voltage control through transformer tap change or local shunt capacitor bank

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One of the main components of a substation is the busbar. The substationbusbar definition is as above. Busbars can be open air insulated busbars in outdoor switchyard, can be insulated by gas inside enclosures, can be inside a metal-clad cubicle restricted within a limited enclosure with minimum phase-to-phase and phase-to-earth clearance. Busbar forms the “electrical node” where many circuits interconnected together, feeding in and sending out power. The number of circuits to be connected depends on the power system requirements.

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The slide shows the layout (from top view) of an open terminal Air Insulated Switchgear (AIS) substation. The busbar consists of tubular aluminium alloy busbar supported by post insulators (to support live part from earth or from another live part). Insulation by air.

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Cross sectional view of AIS substation.

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3-Dimensional view of the substation components. The three phase tubular aluminium busbar is shown. Post insulators are shown to support the busbar from live part to earth or from another live parts.

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Actual picture of busbar in AIS transmission substation.

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Another view of busbar.

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Some of the substations are insulated by gas, i.e. SF6 (Sulphur Hexafluoride) gas. Live parts of the substation are contained in gas tight enclosures. Some of the advantages and disadvantages of GIS are as listed above.

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Picture of GIS substation. Live busbar in GIS enclosures. Example of phasesegregated GIS busbar.

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Medium voltage are normally AIS Metal-clad switchgear.

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Busbar in metal-clad switchgear. Normally using copper busbar.

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Not economically feasible to design and manufacture electrical equipment that will never fail in service. Equipment will and do fail, and the only way is to limit further damage, and to restrict danger to human life, is to provide fast, reliable power system protection. The protection system detects the abnormal conditions and faults, initiate tripping command to circuit breaker, and selectively remove the affected faulty equipment from service, from the rest of power system. Localised the faults.

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1. Minimize further damage to the equipment. If the fault in the equipment, such as f i l d f h h i b blpower transformer, is not cleared fast enough, the equipment may not be able to

withstand or sustain the high fault current. This may damage the equipment beyond repair.

2. Minimize the time of supply interruption. If fault is not cleared fast enough, more backup protection will operate. This will lead to more and wider supply interruption. Furthermore, the damaged equipment will also require longer repair time. This incur cost and time to repair.

3 Mi i i th ff t f f il l d t S dd l f hi h f lt3. Minimize the effect of failure upon people and property. Sudden release of high fault energy may lead to explosion of the equipment. Fire and flying fragments may lead to consequential risks - on the safety of the personnel working nearby or risk of damaging the surrounding healthy equipment.

4. Minimize the stress on other equipment. Substation equipment is rated to carry fault current for only a short time. High fault current flowing in the healthy equipment may cause unnecessary stress to that equipment.

5 Mi i i i t f di t b t t t bilit d lit F5. Minimize impact of disturbances to power system stability and quality. For transmission system, longer fault clearance may lead to loss of generator synchronism, with widespread loss of supplies. If this is not arrested, it may cause cascaded tripping and also lead to total system blackout. The present customers demand not only the availability of supply but also the quality of supply. Power system dips may cause interruption of factory that produce microprocessor chips.

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Fault Clearance System – “A system to detect fault and isolate it discriminately”

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Functions of Fault Clearing System:1 CT/VT P id bilit t d t t f lt i id it f t ti ( bilit t “ ”1. CT/VTs - Provide ability to detect a fault inside its zone of protection (ability to “see”

the power system)2. Relays - Logic element which make decision to initiate tripping3. DC System - Power the FCS subsystems and provide continuity of service in the

event of loss or interruption of AC supplies, for a certain duration of time4. Circuit Breakers - Provide the ability to physically isolate the fault by disconnecting all

equipment inside the zone5. Telecommunication - Provide/transmit information from remote location to

i l t t i i f ti h f lt i ithi th f t tiimplement tripping function when fault is within the zone of protection

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Impact of busbar faults if busbar protection is not available:

1. Cascaded tripping

2. Slow fault clearing time (delayed operation of remote backup such as distance protection zone 2 (300 to 450 millisecond) and zone 3 (1 to 3 second), or overcurrent protection time)

3. Massive loss of supply at faulted substation and surrounding areas/substations

4. Interruption of supply to customers

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Impact of busbar faults if busbar protection is available:

1. Selective busbar tripping. Only the faulted busbar zone is tripped to clear the fault. The other healthy busbar remain in service, still connected to the power system.

2. Fast fault clearance. Total fault clearing time of within 100ms to 150msec.

3. Minimal load loss. Total interruption of substation supply is prevented. Part of the load is supplied from different source.

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Protection Criteria - “ Criteria to be considered for developing or designing a trustworthy protection system”

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FAST:Th f lt l t b f t h ( t t f t b f th l k tThe fault clearance must be fast enough (not too fast before the relay can make an accurate decision based on information from the power system) to limit the damage at fault point and to limit the effect of fault. Even more important for busbar protection as the fault level at busbar are generally larger than that of a circuit or feeder. Circuits connected to the busbar contribute to the high fault level.System Stability Analysis:Maximum Fault Clearing Time (MFCT) for main protection – e.g. in TNB; 500kV & 275kV =

100msec, 132kV=100msec.Critical Fault Clearing Time (CFCT) – based on Transient Stability (~500ms for 500/275kV)Critical Fault Clearing Time (CFCT) – based on Transient Stability (~500ms for 500/275kV)Backup FCT - Equipment Ratings (1s or 3s)Fault Energy Analysis: Thermal Capability and Mechanical Forces Characteristics.

DEPENDABLE:Can be achieved through Optimum design of protection schemes. Redundant & Duplicate system. E.g. one of two operations voting criteria. Application of appropriate settings. However, increase in dependability causes decrease in security.

SECURE OR STABLE:Must not operate for fault outside the zone (external fault). Stability must be guaranteed. Even more important for busbar protection, as protection mal-operation causes tripping of a number of circuits resulting in wide spread supply interruption. The protection must be stable during load or through fault condition. E.g. introduction of “check feature”. Two of two operations voting criteria (discriminating zone and check busbar protection). However, increase in security causes decrease in dependability.

SELECTIVE OR DISCRIMINATION:Ability to isolate only the affected section of the faulted system (i.e., minimize extent of interruption). Introduce concept of “zone of protection”, where components of power system such of busbars are divided into zones. Only the faulted component will be tripped and isolated from the rest of power system.May require: 1. Correct location of CB2. Appropriate location of CTs & VTs3 Correct application of relay types

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SENSITIVEAbilit t di i i t b t l d d f lt diti f l th f tiAbility to discriminate between load and fault condition for any length of time.Protection must not operate under load condition, but must operate even under minimum fault current. The level of sensitivity can be set through application of protection setting.May require:1. Sensitive settings but does not offset the requirements of selectivity2. Protective relays that overreach other relays such that they operate as fast as

possible within their primary zone but have delayed operation in their backup zone

3. Relay coordination

SIMPLEMinimum number of elements and associated circuitry.

ECONOMICALMaximum coverage at minimum cost (Justify initial cost against long term costs) MayMaximum coverage at minimum cost (Justify initial cost against long term costs). May need to perform “Cost Benefit Analysis”.

CONSISTENCYStandardization of scheme & drawings, design & coordination and equipment’s nomenclature.

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Example of “Zone of Protection” concept. All major components of power system are divided and selectively protected within its Zone of Protection. The protection must be able to discriminate between “zones”, and also between phases of the fault. For busbar protection, it must also be able to discriminate between sections of the busbar. For example, fault on main busbar must trip only the main busbar. The remaining busbar must be stable and remain in service.

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The busbar arrangement selections depend on;

1. Minimising the number of circuits that must be opened for a bus fault.

2. Economics e.g. more expensive if more circuit breakers and isolators are employed.

3. Flexibility of system operation and maintenance.

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The busbar arrangements involves circuit connection and configurations of busbar(s), circuit breaker(s), isolators/disconnectors. Isolators are used to provide means of isolating circuit breaker and other power system components after fault or for maintenance. Isolators are NOT meant to interrupt load or fault currents.

During this lesson, the emphasis is more on busbar arrangement implementation in TNB The following comparison of various busbar arrangements is based onin TNB. The following comparison of various busbar arrangements is based on common 4 circuits connection to the busbar(s).

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Various types of busbar protection techniques or methods will be briefly touched. The major part of the lessons will emphasise more on the high impedance differential protection. This high impedance protection as well as low impedance protection are widely used in TNB especially in transmission system. The differential protection can be considered and applied for all type of busbar arrangement.

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Frame to Earth (Leakage) Protection•Only an earth fault system•Involves measuring fault current from switchgear frame to earth•Switchgear insulated by standing on concrete plinth•Only one earthing point allowed on switchgear•C.T. mounted on single earth conductor used to energise instantaneous relayAll bl l d t b i l t d•All cable glands must be insulated

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Method of Sensing

Detection of light (arcing) + Overcurrent = Trip

Sensing of arc using photo sensors or bare fibre

Sensing of overcurrent using current input from OCEF

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Method of Sensing

•Detection of light (arcing) + Overcurrent = Trip

•Sensing of arc using photo sensors or bare fibre

•Sensing of overcurrent using current inputfrom OCEF

One sensor in each compartment Easily accessibleOne sensor in each compartment. Easily accessible.

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Fast

Modular scheme design allows relays to relate to each circuit and function of the protection. This enables the user to easily understand the principles of application.

High sensitivity for phase and earth faults. Protection for each phase can be relatively independent.

Earlier schemes were less stable than high impedance schemes. Modern h i i d d l blschemes incorporate saturation detectors and are extremely stable.

Duplicate measuring circuits are included.

Current transformers can be of different ratio, relatively small output and be shared with other protections.

Current transformer secondary circuits are not switched.

Continuous supervision of CT circuits and constant monitoring of vital circuits are included.

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FM – Feeder Module

BCM – Bus Coupler Module

BSM – Bus Section Module

Z1, Z2, Z3, Z4 - Busbar Protection Zone 1, 2, 3, and 4

ZCK – Check Zone

F1, F2, F3, F4 – Feeder 1, 2, 3, and 4

BC1 BC2 B C l 1 d 2BC1, BC2 – Bus Coupler 1 and 2

BS – Bus Section

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Prevent protection operation due to CT mismatch and CT saturation during external fault.

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Differential Protection is a unit protection. Differential protection compares/calculates the sum of all currents flowing into and out of the protected object. Based on Kirchhoff’s Current Law (KCL), the sum of all currents at any node in a circuit must equal to zero. Thus, current “IN” must equal current “OUT” of the node.

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Example of Unit Protection principle.

When applying differential protection, current transformers (CTs) are used to measure the current vectors of all currents entering and leaving a circuit. If no faults exist within this differential “zone,” the sum of the current vectors will equal to zero. The protected area or zone of protection is between/among CTs.

The convention used states that the currents flowing into the protected zone/object are positive, while the currents leaving the protected zone are negative.

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The slide shows the behavior of the simplest differential scheme (using an instantaneous overcurrent relay) during an external fault. If the CTs are considered ideal and identical, the primary and secondary currents at both sides of the protected equipment are equal. There will not be any differential current Idiff flowing through the relay. Ideally, the differential current is equal to zero. Currents will circulate around the secondary CT circuits. Therefore, the scheme is also called Circulating Current scheme.

The Protection is stable for external fault or during load condition.

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For an internal fault, the primary fault currents allow the secondary currents to produce a differential current or spill current through the relay. If this differential current is larger than the relay’s pickup setting, the relay will trip both circuit breakers instantaneously. The relay setting determine the sensitivity of the protection.

In summary, the differential protection;

i d i i /l i h f i h• Measure current entering and exiting/leaving the zone of protection. The currents are measured at the boundary of the zone of protection.

• If currents are not equal, a fault is present.

• Provides high selectivity, i.e. the differential protection only protect the circuit or zone between their current transformers. Therefore, it is categorised as unit protection scheme.

D t id f b k t ti f th t f th• Do not provide a measure of back-up protection for other parts of the network. The protection operation to clear the fault is relatively high speed.

• Provides high sensitivity (depending on the settings). In this example, say the relay setting Isetting= 0.2A secondary. When the differential current Idiff exceed this setting (e.g. 0.3A), the relay will operate.

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The same concept applies for busbar differential protection. However, in this case, all circuits connected to the bus must be involved.

It compares the total current entering the busbar zone with the total current leaving the zone.

For a normal load condition or for an external fault (through fault), the sum of current entering the busbar is equal to sum of current leaving the busbar. Ideally,

h ld i l d h C i i d diff i l h ldcurrents should circulate around the CT circuit, and no differential current should flow through the relay. Relay will not pickup. However, all current transformers in busbar protection cannot be identical even from the same manufacturer. This is due to difference in manufacturing tolerances. There will also be CT mismatched due to variation in magnetising current and CT resistance RCT. There will be spill current flowing through the relay during normal load condition. The relay can be desensitised (decrease the sensitivity) by increasing the relay setting value.

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For internal fault condition, the protection also compares the total current entering the busbar zone with the total current leaving the zone. In this case, there will be differential current flows through the relay. The relay will operate when the differential current Idiff exceeds the setting.

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In most cases, more than two circuits or feeders are connected to the busbar. For these multi-circuits (more than two), circuits are operating with different energy levels, often with different characteristics. For example in this case, not all circuits have the same current, although all the circuits contribute to the fault current.

Again, for an internal busbar fault, differential current Idiff flows through the relay As the I (30A secondary) is very much higher than the relay settingrelay. As the Idiff (30A secondary) is very much higher than the relay setting (0.2A), the relay will operate very fast and send tripping signals to all the circuit breaker to clear the fault.

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For multi-circuits (more than two), circuits are operating with different energy levels, often with different characteristics.

Ideally, for external faults, the current entering the busbar zone of protection is equal to current leaving the zone. In this case, theoretically, there should not be any differential current Idiff flowing through the relay. The relay should remain stable and should not operate.

However, is this true in practical? Should the busbar protection operate?

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Again for this multi-circuits condition, the circuits currents are operating with different fault levels.

Ideally, for external faults, the current entering the busbar zone of protection is equal to current leaving the zone. This may be true for currents at high voltage primary equipment in the system. However, in actual case, for busbar protection, these primary currents may not be accurately reproduced at secondary side by the current transformers CT performance influence the reproduction of thethe current transformers. CT performance influence the reproduction of the primary fault current to the secondary fault current. By right, this CT for the faulted circuit should reproduce potentially high magnitude secondary current with sufficient accuracy to match the other CT secondary circuits. This is to prevent the differential busbar protection from mal-operation. Protection must restrain from operating for all external faults. Therefore, the CT performance is very important for a differential protection scheme.

The most critical condition for CT performance is an external fault just outside the differential zone. The CT associated with the faulted circuit receive the sum of all the currents from the other circuits. There is a possibility where this CT carrying the faulted current were to completely saturate while all other circuits CTs were to transform correctly/accurately. The CT may saturate due to saturation on symmetrical ac current input (from CT characteristics and y p (secondary load) and/or due to saturation by the dc offset of the primary ac current. Due to this CT mismatch, there will be differential current Idiff flowing through the relay. When this Idiff (in this case 2A) exceeds the setting (in this case 0.2A), the protection mal-operates for an external fault.

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One of the solution of the spill/differential current problem from external fault is to use high impedance busbar protection. For the high impedance busbar protection scheme, most of the differential current is force to flow through the magnetising branch of the saturated CT.

The characteristics of a high impedance busbar protection scheme are as above.

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This technique loads the CTs with high impedance to force differential current through the CTs (the magnetising branch of the saturated CT) instead of relay operating coil.

For three phase power system, three sets of high impedance busbar protection relays are required for each phase.

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The CT secondary circuits are connected so that through fault current circulates around the CT connection. The differential relay is connected as a shunt, and is made up of a sensitive current relay in series with a high resistance. The current relay and series resistance basically correspond to a voltage relay.

During load and through fault conditions, the voltage across the shunt is small (theoretically zero) if the internal resistances of the CT secondary winding resistance (R ) and the CT connecting pilot cable lead burdens (R ) on bothresistance (RCT) and the CT connecting pilot cable lead burdens (RL) on both sides of the resulting bridge circuit are equal. The CTs must all have the same ratio.

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Assume one of the CT is completely saturated under heavy through fault (external fault) condition The saturated CT can simply be substituted by its(external fault) condition. The saturated CT can simply be substituted by its secondary internal resistance (magnetising impedance). If the series resistance is high compared with the CT internal resistance RCT + connecting lead cable/wiring resistance RL, then the voltage distribution would be as shown (RHS of diagram).

The voltage across the shunt path then is:VR = IFAULT.(RCT+RL)

If relay voltage pick up VS is set higher than VR, stability is achieved even in the event of most extreme CT saturation. The highest through fault current that can arise should be determined and used in the calculation. Busbar protection stability limit is based on maximum through fault current.

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The equivalent circuit of the high impedance busbar protection scheme (in this case,h i i d h b b ) d i l f l i h bthree circuits are connected to the busbar) during external fault is shown above.

Base on the basic operating principle discussed earlier and the above equivalent circuit, the required high impedance busbar protection stability limit voltage setting VS can be determined. Busbar protection stability limit is based on maximum through fault current.Normally, the rated short circuit withstand current (three phase fault) of the busbar is used as the maximum fault current.

For an external fault, the maximum voltage across the relay VR will occur if the CT on the faulted circuit is completely saturated, while the other CTs are not (other CTstransform accurately). This is the worst case, as in practice not all CTs may saturate on external light fault, and will have varying degree of saturation for heavy faults. All CT magnetising impedance of all other circuits except the circuit carrying full fault current are infinite. The faulted circuit CT is assumed to be fully saturated and its magnetising i d t N t t f th CT t i it A th f lt d i it Thimpedance goes to zero. No output from the CT at circuit A, the faulted circuit. The maximum voltage across the relay VR is equal to the maximum through fault secondary current IF times the faulted circuit CT resistance RCT plus the pilot lead resistance RL.

The busbar protection stability limit voltage setting VS is normally set above or slightly more than this VR value to achieve stability on external fault.

VS = Ifault x (RCT + RL), where Ifault is the secondary fault current. The maximum total S fault CT L faultvalue of (RCT + RL) is also chosen in the calculation. A slight higher value of VS is normally applied above the limit as safety to achieve stability for external fault.

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The busbar protection stability limit voltage setting VS also depends on the maximum lead resistance RL from the relay shunt points to the CT. Normally, the maximum lead resistance is selected from the furthest bay CT to the relay (assume same type of secondary multicore conductor size). The pilot lead resistance RL value is also different for different type of faults, i.e. RL for three phase fault, and 2 RL for phase to earth fault.

The pilot lead burdens between various sets of CT’s must be kept low.

Above is a typical route for TNB application as referred to an actual substation layout as follows:-

• CT’s to bay marshalling kiosk

• Bay marshalling kiosk to isolator auxiliaries

• Bay marshalling kiosk to common busbar marshalling kiosk

• Common busbar marshalling kiosk to relay panel

The lead multicore conductor size is normally 4mm2. A bigger size of 6mm2 if lower lead burden is required.

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There are two types of high impedance relay implementation, i.e. voltage operated relay and current operated relay. Typical operating time the relays are in order of 20 – 30msec.

The voltage operated relay generally have a very high resistance and will operate at a constant current of perhaps 20mA over the available voltage range of perhaps 25-375volts. The scheme may require a separate parallel shunt resistor to reduce the relay sensitivity (desensitise the relay) to prevent relay operationto reduce the relay sensitivity (desensitise the relay) to prevent relay operation under load current when the CT secondary open circuited.

The current operated relay by itself is a low impedance relay, but when applied in series with a resistor (typically 0 to 470 ohm), the required high impedance scheme results. The series resistor is known as the stabilising resistor RS. Combination of the relay current and stabilising resistor will produce the desired y g pscheme voltage limit voltage. As a guide, the setting current should not exceed 30% of the minimum fault current. In TNB, normally the setting secondary current of the relay is set at 0.2A.

Normally, the relay has special feature to block d.c. offset component in fault current. If not, high setting maybe required, e.g. a factor of 2.

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For the current operated relay, the busbar protection stability limit voltage setting is derived from the formula VS ≥ IFAULT.(RCT+RL). Normally, the relay operating current IR is set at 0.2A secondary. The stabilising resistor RS is normally a continuous adjustable type and normally set at site once the stability limit voltage setting has been derived from the actual maximum total resistance (RCT+RL).

At stability limit voltage setting the stability is balanced between stable andAt stability limit voltage setting, the stability is balanced between stable and unstable condition.

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If the relay impedance is high in comparison with CT resistance and lead wiring resistance, the above approximation is true. To ensure full stability under external through fault current, choose relay voltage setting in access of the calculated voltage.

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In terms of physical CT arrangement and related single line diagram, the CT location is as shown. In three phase system, it is necessary to use three CTs, at one per phase.

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Physical CT arrangement and location in an actual AIS open terminal HV substation. E.g. of top head live tank current transformer.

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Transformer Theory

Transformer is an electromagnetic device that has two or more mutually coupled windings, i.e.share a common magnetic circuit or core. It allows transformation of voltage, current orimpedance. The transformers are used to either increase or decrease the magnitude of analternating current. Most transformers also provide electrical isolation (except forautotransformer).

As illustrated above, the basic components or structure of a typical transformer are two or morecoils/windings called the primary winding N and secondary winding N linked by an iron orcoils/windings, called the primary winding Np, and secondary winding Ns, linked by an iron ormagnetic core which is used to concentrate the magnetic field. When an alternating primarycurrent Ip flows through the primary coil or winding, it produces a magnetic flux in the magneticcore. This magnetic flux links both windings. The magnetic flux in the magnetic core circuitgenerates emf in the secondary winding. Therefore, alternating current (a.c.) in primary windinginduces emf in the secondary winding, thus generating secondary current Is to flow in thesecondary winding. The value of secondary emf or secondary current, and whether thetransformer is a step up or step down type depend on the transformer turn-ratio. Thistransformer operating principle is called mutual induction.

For ideal transformer, where power consumed by the secondary load is equal to power suppliedby primary winding, Ep/Np = Es/Ns or EpIp = EsIs. Ampere-turn balance is maintained betweenwindings. For ideal transformer, core is also assumed to be lossless with infinite permeability, noleakage flux, and no winding losses.

A current transformer is an instrument transformer intended to have its primary windingconnected in series with the conductor carrying current to be measured or controlled. The ratioof primary to secondary current is roughly inversely proportional to the ratio of primary toof primary to secondary current is roughly inversely proportional to the ratio of primary tosecondary turns ratio, i.e. Ip/Is = Ns/Np Normally, the CT primary turns ratio is a single windingor bar conductor. The CT is usually arranged to produce 5A or 1A in the secondary circuit whenrated current (e.g. 1200A) is flowing in the primary.

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A simple equivalent circuit of a current transformer is shown above. In this case,the magnetising impedance (Rc & Xm) is usually represented at secondarywinding side (small value at primary). Leakage reactance of a currenttransformer is small and is negligible. With evenly distributed winding theleakage reactance is very low and usually ignored. CT secondary resistance (inohm) can be represented by CT winding resistance RCT.

CT is a “series connected” transformer where the burden is effectively connectedCT is a series connected transformer, where the burden is effectively connectedin series with power system load. The secondary impedance is very smallcompared with the power system load. Thus, the current flows in the CT(primary and secondary) is effectively determined by the primary power systemload. The secondary current is virtually independent of the burden on thesecondary (total burden – parallel combination of burden Zb and magnetisingimpedance Zm).

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All CTs should have the same ratio and operate on the full winding. Auxiliary CTsare not recommended. Normally CT Accuracy Class PX of IEC 60044-1 is used for high impedance busbar protection scheme. Class PX is similar to Class X of BS standard.

Class PX is chosen because of how the CT Class is specified. Class PX current transformer is specified bases on the rated knee point voltage Vkp, maximum CT secondary winding resistance R small turn ratio error (±0 25%) Basicallysecondary winding resistance RCT, small turn ratio error (±0.25%) Basically, these parameters are very usefully to calculate and determine the high impedance stability limit voltage VS.

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The protection CTs are required to maintain their accuracy up to several timesthe rated primary current. Accuracy class 5P and 10P are intended to coversimpler form of protection such as IDMT overcurrent & earth fault, currentdifferential, biased differential, etc. The primary current at which the limits oferror of 5P start to be exceeded is known the “accuracy limit primary current”.The rated accuracy limit factor ALF = the ratio of; (Accuracy limit primarycurrent)/(Accuracy primary current).

The “5” referred to for 5P class CT describes the percentage error between theideal and the actual secondary current at the accuracy limit primary current.Accuracy limit factor of CT may increase with the reduction of CT burden.

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Low leakage reactance CT. Equivalent to CT class X in BS 3938 standards. The CTclass PX, in accordance to IEC 60044-1, is used for special purpose applications.The performance specification is defined in term of rated primary current, turnsratio (not exceed ± 0.25%) , rated knee-point EMF Ek or Vkp, maximum excitingcurrent Ie at rated knee-point EMF, and resistance of secondary winding at 75degree.

The class PX CTs are usually applied when a high knee point is required to avoidThe class PX CTs are usually applied when a high knee point is required to avoidsaturation of CT core. Example applications for high-impedance circulatingcurrent scheme.

Dimensioning factor (Kx) is based on relay manufacturer recommendation insteadof using factor (1 + X/R). Normally the factor is derived from manufacturerdynamic simulation or laboratory testing. Reduce the value required wheny y g qspecifying CT performance.

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Marking required for Class PX CTs.

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Knee point voltage is defined as a point on the CT magnetisingcharacteristics/curve at which a further increase of 10% of secondary voltagerequires a 50% increase in the excitation current Ie. Beyond this point, CT isconsidered to be in saturation condition where the secondary RMS current valueand wave shape are no longer correspond to those of the primary current.

CT must not be driven into saturation.

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For the scheme to operate and trip correctly for a fault within the protected it i ti l th t ll CT h k i t lt V t l t t i thzone, it is essential that all CTs have a knee point voltage Vkp at least twice the

relay stability limit setting voltage VS, i.e. Vkp > 2VS.. Knee point voltage Vkp of up to 5 times (preferably Vkp to be 3 to 5 times VS) may be required for faster or improve relay operating time. However, this requires bigger CT dimension and more expensive.

Other that the knee point voltage Vkp and CT winding resistance RCT, CT performance is not really critical CT Class PX is selected since the knee pointperformance is not really critical. CT Class PX is selected since the knee point voltage Vkp and CT winding resistance RCT are required for determination of protection setting.

Other important CT requirements for high impedance busbar are;• Class PX of IEC 60044-1 standard.• CTs must have an identical transformation ratio and magnetisingCTs must have an identical transformation ratio and magnetising

characteristics. CTs operate on the full winding.• If possible, CTs with identical construction.

• The secondary leakage reactance should be negligibly small

• The secondary internal resistance (winding resistance) must be kept as small a possible

• Small magnetising current to ensure minimum effective sensitivitySmall magnetising current to ensure minimum effective sensitivity

• Auxiliary CTs are not recommended.

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Typical possible distortion in CT secondary current resulting from DC saturation: (a) large resistive burden; (b) small resistive burden. (Figure 3 of IEEE 76-CH1130-4, PWR Transient Response of Current Transformers.)

The CT may saturate due to;

1. Saturation on symmetrical ac current input (from CT characteristics and secondary load). Most critical at point of relay decision. For busbar differential

i h d i i i i h C l f l lprotection, the decision point is at the CT nearest an external fault. Fault on one side of CT (internal fault), protection must operate (i.e. dependable). But fault on the other side (external fault), the relay must not operate (i.e. stable or secure). External fault is often large, and requires good performance CT.

2. Saturation by the dc offset of the primary ac current. Dc offset temporarily restrict the output of CT. Critical for differential busbar protection where several CTs are involved in fault decision. Depend on the power system time p p yconstant. If time constant is short, when dc offset occur, it decays rapidly.

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In the case of internal faults (fault inside the protected zone), all CTs try to drive current into the high impedance relay. The high impedance differential relay RRplus stabilising resistance RS forces very high voltage across it. This results in CT saturation and potentially dangerous high voltages, both for personnel and equipment, across the CT secondary winding. The effect of this is to cause CT secondary insulation breakdown and/or panel wiring insulation breakdown. A non-linear ZnO resistor or varistor or metrosil (“metrosil” is a brand name) is connected in shunt with the high impedance relay to limit secondary voltages to a safe value.

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Limiting CT Secondary Voltages during Internal Fault ConditionFor internal faults, the high impedance differential relay RR plus stabilising resistance RS forces very high voltage across it. A non-linear ZnO resistor or varistor or metrosil (“metrosil” is a brand name) is connected in shunt with the high impedance relay to limit secondary voltages to a safe value. The “metrosil” draws

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The overall high impedance busbar protection sensitivity can be confirmed or h k d b d t i i th ff ti tti i ti t Ichecked by determining the effective setting or primary operating current IPOC,

i.e. verifying the protection pickup sensitivity by means of the determined pick-up threshold of the differential relay. The effective primary operating current is given by;IPOC= CT Ratio (IRelay + IMetrosil + N.IMag)where :-

IRelay = Relay setting currentIMag = CT magnetising current (one CT at relay setting voltage

V )VS)N = Number of paralleled CT’sIMetrosil = Non linear resistor current at relay setting voltage

The parallel connection of CT magnetising impedances, relay resistance, and metrosil resistance is shown in the above equivalent circuit. Since in each zone of protection there are several CT’s in parallel with the relay and each other, the combined CT magnetising currents will increase the effective primary operatingcombined CT magnetising currents will increase the effective primary operating current IPOC.

The value of effective primary operating current should not exceed 30% of the minimum fault current. This is to ensure sufficient relay current during internal fault conditions for high speed operation.

Busbar protection stability limit is based on maximum through fault currentBusbar protection stability limit is based on maximum through fault current. Generally this value is derived from the rated busbar short circuit withstand current irrespective of existing fault level, since it can be expected that system will develop up to limit of rating.

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High Impedance for Double Busbar• Provides high flexibility by system operation.

• Widely used in TNB transmission system

• Any circuit can be operated from either busbar (via busbar isolators)

• Both busbars can be operated together or independently (via bus coupler).

• Requires complicated secondary current switching of busbar protection zones.

F lt b b i t i i f ll i it t d t th b b t• Faults on one busbar require tripping of all circuit connected to the busbar at the time.

• Fault on bus coupler must trip both busbar zones.

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The problem encountered in applying the high impedance busbar protection scheme for a double busbar substation arrangement is the CT secondary current circuit switches correctly during main isolator change over or live transfer process. Busbar isolator is used to switch the circuit/feeder from one busbar to the other. At the same time, the isolator auxiliary contact will switch the secondary circuit current. The isolator auxiliary contacts are used to switch the CT secondary circuit in harmony with switching of the primary system. If auxiliary switching does not occur before the isolator primary switching, spurious operation of both busbar schemes may occur.

Switching of primary connection from Main Busbar to Reserve Busbar requiresswitching of secondary current via isolator auxiliary switch. The application of check zone busbar protection (which is not switch using isolator auxiliary contacts) together with the discriminating zone busbar protection (2 of 2 operation) will prevent mal operationoperation), will prevent mal-operation.

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The pictures display typical busbar isolators or disconnectors. They provide a visible isolating distance in air isolated gap. Above is a center-break disconnector which is the most frequently used type. The disconnector base supports the operating mechanism and two rotating porcelain support insulators. The current path arms, which are fixed to the insulators, open in the center. Contact fingers with silver plated contact surface provide maximum conductivity.

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Exact coordination between the isolator main and auxiliary contacts is virtually impossible to achieve. However, the busbar protection scheme will remain stable if early make and late break isolator auxiliary contacts are employed. Ensure the isolator switching of CT secondary current are “early make” and “late break”.

Normally Open NO Contact (open when isolator open), Early make contact.Closes on the closing stroke of the isolator, but makes before the pre-arcing distance is reached It should open on the opening stroke of the isolator afterdistance is reached. It should open on the opening stroke of the isolator after the arc is extinguished.

Normally Closed NC Contact (closed when isolator open). Early break contact. This contact should open on the closing stroke of the isolator before the pre-arcing distance is reached. It should close on the opening stroke of the isolator after the arc is extinguished.g

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Alternative approach to prevent tripping on CT secondary open circuit is to install second busbar check scheme. This check zone high impedance busbar protection zone covers the whole busbars. Both main and checks must operate to trip busbar. Increase in security will cause decrease in dependability especially for double busbar arrangement.

Usually provided by duplication of primary protection using second set of CTs on all circuits other than bus section and coupler units Check system forms oneall circuits other than bus section and coupler units. Check system forms one zone only, covering whole of busbar systems and not discriminating between faults on various sections.

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Usually provided by duplication of primary protection using second set of CTs on all circuits other than bus section and coupler units. Check system forms one zone only, covering whole of busbar systems and not discriminating between faults on various sections.

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One tripping relay (device 94) is required for each feeder breaker and two for each bus section or bus coupler circuit breakers. Both main and check relays must be energised before tripping relays trip all breakers associated with zone.

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Open circuit connections between CT’s and relay circuit result in unbalance t hi h t th t ti Th i ibilit th t thicurrents which may operate the protection. There is a possibility that this

condition could exist without being discovered. The result could be relay damage as relay is not continuously rated. Supervision is applied by a voltage relay (96BBS) across differential relay circuit.

Supervision relay 96BBS is another high impedance relay installed parallel with main busbar protection relay 87B. However, the difference is that 96BBS setting range is small (typically 2 14volts) The voltage setting is selected base on therange is small (typically 2-14volts). The voltage setting is selected base on the primary operating current. Example of setting – base on primary operating current IPOC of 50% of the minimum feeder load (to ensure operate even under light load conditions, open CT circuit is detected). For example in this case, if SR=330, RR=25, Voltage develop across relay is 0.25x(330 + 25) = 0.25(355)=88.75V. Select the CT Supervision 96BBS to 50% of this value, i.e. 44V. Verify that this value is implement able in a relay.

The CT supervision will also operate during fault condition on the busbar. Therefore, CT supervision should NOT operate within 0 to 3 seconds. For internal fault, the busbar protection should clear the fault.

In the case of no genuine fault, the shunt resistor or busbar check zone will inhibit spurious tripping. Therefore, greater than 3second, CT supervision operate (most probably CT wiring problem)operate (most probably CT wiring problem).

An additional function often implemented after time delay is for the CT supervision to short out the CT secondary wiring. Dangerous/damaging CT secondary voltages are eliminated. The busbar protection is now out of service. Possibility of future spurious tripping on external fault is prevented. However, it is suggested that the shorting of CT secondary wiring is acceptable if duplicated busbar protection scheme is implemented. Remote backup like distance relay (Z2

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Three phase high impedance busbar protection including CT Supervision in three phase system.

Open circuit connections between CT’s and relay circuit result in unbalance currents which may operate the protection. Supervision is applied by a voltage relay (96BBS) across differential relay circuit. Supervision relay is time delayed, gives alarm and also shorts out bus wires to protect differential relay circuit.

CT supervision scheme is in operation to prevent spurious tripping on open circuit CT secondary. There is a possibility that this condition could exist without being discovered. The result could be relay damage as relay is not continuously rated. CT supervision relay is another high impedance relay, is connected in parallel with the busbar zone protection relay. Its setting is low, to detect open circuit CT on minimum load current. The action is delayed for 3 second to permit y pgenuine bus faults to be cleared. When times out, indicative of an open circuit, it will either initiate an alarm, or initiate an alarm and short out the CT secondary circuitry, thus preventing damage to relays as well as taking the busbar protection out of service.

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busbar protection abm

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