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3. ALTERNATIVE LAYOUTS
3.1 Single Busbar
FIG. 2 Five circuit breaker single busbar arrangement.
FIG. 3 (a) Bypass isolator for circuit breaker maintenance. (b) Bypass
isolator facilities between two adjacent line bays.
The single busbar arrangement is simple to operate, places minimum reliance on signaling for satisfactory operation of protection and facilitates the economical addition of future feeder bays.
FIG. 2 illustrates a five circuit breaker single busbar arrangement with four feeder circuits, one bus section and ten disconnectors. Earth switches (not shown) will also be required.
1. Each circuit is protected by its own circuit breaker and hence plant outage does not necessarily result in loss of supply.
2. A fault on a feeder or transformer circuit breaker causes loss of the trans former and feeder circuit, one of which may be restored after isolating the faulty circuit breaker.
3. A fault on a bus section circuit breaker causes complete shutdown of the substation. All circuits may be restored after isolating the faulty circuit breaker and the substation will be 'split' under these conditions.
4. A busbar fault causes loss of one transformer and one feeder. Maintenance of one busbar section or disconnector will cause the temporary outage of two circuits.
5. Maintenance of a feeder or transformer circuit breaker involves loss of that circuit.
6. The introduction of bypass isolators between the busbar and circuit isolator ( FIG. 3a) allows circuit breaker maintenance facilities without loss of the circuit. Under these conditions full circuit protection is not avail able. Bypass facilities may also be obtained by using a disconnector on the outgoing ways between two adjacent switchgear bays ( FIG. 3b). The circuits are paralleled onto one circuit breaker during maintenance of the other. It’s possible to maintain protection (although some adjustment to settings may be necessary) during maintenance but if a fault occurs then both circuits are lost. With the high reliability and short maintenance times involved with modern circuit breakers such bypasses are not nowadays so common.
3.2 Transformer Feeder
The transformer-feeder substation arrangement offers savings in land area together with less switchgear, small DC battery requirements, less control and relay equipment, less initial civil works together with reduced maintenance and spares holding in comparison with the single busbar arrangement.
FIG. 4 shows the single line diagram for a typical transformer feeder, two transformer substation arrangement. A comparison of land area requirements between a conventional single busbar fully switched outdoor 33/11 kV distribution substation (2,150 m2), a fully switched one-storey indoor substation (627 m2) and for the transformer-feeder arrangement (420 m2) is shown in FIG.
The major practical service continuity risk for the transformer-feeder sub station occurs when the substation supply cables are both laid in the same trench and suffer from simultaneous damage. Much of the substation cost savings would be lost if the supply cables were laid in separate trenches since the civil trench work, laying and reinstatement costs are typically between 33% and 40% of the total supply and erection contract costs for 132 kV oil filled and 33 kV XLPE, respectively. In congested inner city areas planning permission for separate trenches in road ways or along verges is, in any case, seldom granted. The civil works trenching and backfill costs for two separate trenches (one cable installation contract without special remobilization) are typically 1.6 times the cost of a single trench for double circuit laying. The choice depends upon the degree of risk involved and the level of mechanical protection, route markers and warnings utilized. The cable routes for ring systems don’t normally present such problems since the feeder cables usually run in different directions and only come in close proximity adjacent to the substation.
A comparison of equipment requirements between a ring, hybrid and transformer-feeder arrangement is given in FIG.
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Overhead line incomers Line isolator (and earth switch) Protection CTs often located in transformer bushings
FIG. 4 Transformer-feeder arrangement.
--- 38.500 11 kv cable reservation 11 kV switchgear room Control room 22.000 T1 (b) Access road Site area = 627 m2 33 kV switchgear room 33 kV cable reservation T2 28.500 DC room WC 33 kV switchyard Access road Control building Site area = 2,150 m2 (a) 55.850 T2T1 21.000 20.000 11 kV switchgear room; Control room; Access road; Site area = 420 m2 (c) DC room WC 11 kV cable reservation
FIG. 5 Comparison of land area requirements for 33/11 kV substations.
(a) Conventional outdoor fully switched single busbar. (b) Fully switched
indoor. (c) Transformer feeder.
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The usual practice for the cable supplied transformer-feeder substation is to terminate the supply cables on outdoor sealing ends with bare busbar connections to the transformer HV bushings. On first examination it might appear more sensible to terminate the HV cables directly into a transformer cable box. This would reduce the length of exposed live conductor and hence reduce the likelihood of insulation failure due to pollution, debris, animals or birds, etc. However, difficulties arise with this solution when, say, after cable damage, isolation and earthing, repair and DC pressure testing is required. On lower voltage systems (11 kV) disconnection chambers may be specified on transformers but this is not practical at the higher voltage (36 kV and above) levels. With outdoor bushings and busbar it’s easy to apply portable earths and isolate the transformer or cable for maintenance, repair or test.
An isolator and earth switch may be added at the transformer HV connections depending upon the electrical supply company's operational procedures.
FIG. 6 Comparison of equipment requirements: (a) Ring system; (b) hybrid
system; (c) transformer feeder. Equipment requirements to maintain firm
capacity; ; No. of 33 kV switchgear bays; Cable circuits no. at capacity
With the development of metal-clad SF6 insulated equipment the possibility exists for provision of an HV isolator and earth switch all within an SF6 insulated environment connected directly to the transformer windings without the need for additional land space. With an overhead line fed transformer-feeder substation a line disconnector/earth switch is desirable since the probability of a fault (insulator failure, development of hot spots on connections, etc.) is greater than with a cable circuit.
In rural or remote areas a fault thrower may be needed. This is a means of deliberately introducing a phase-to-earth fault in order to ensure remote end tripping of a circuit breaker _ For example that feeding a transformer feeder substation _ if inter-tripping pilot wires are not available. In such rural distribution networks supplying remote areas, dedicated pilot wires or rented telecommunications cables are not always available for inter-trip ping and, in any case, would be expensive to install. In addition certain local transformer fault levels detected by the normal sensing protection -- such as a Buchholz relay or earth fault current relay -- may be too low to ensure that the remote feeder circuit breaker tripping takes place. Instead, such local protection initiates operation of the fault thrower that, in turn, generates sufficient fault current to cause a trip to take place. Fault throwers are available in ratings up to 145 kV, and 12 kA short circuit making current. They are more normally employed for ground or pole mounting in aluminum tanks on 36 kV systems with typically 25 kA short circuit ratings utilizing vacuum interrupter modules and overall SF6 insulation.
3.3 Mesh
An arrangement known as a three switch mesh substation is shown in FIG. 7a. It utilizes only three circuit breakers to control four circuits. The scheme offers better features and facilities than the single busbar without a bus section switch.
1. Any circuit breaker may be maintained at any time without disconnecting that circuit. Full protection discrimination will be lost during such maintenance operations. To allow for all operating and maintenance conditions all busbars, circuit breakers and disconnectors must be capable of carrying the combined loads of both transformers and line circuit power transfers.
2. Normal operation is with the bypass disconnectors or optional circuit breaker open so that both transformers are not disconnected for a single transformer fault.
3. A fault on one transformer circuit disconnects that transformer circuit without affecting the healthy transformer circuit.
4. A fault on the bus section circuit breaker causes complete substation shut down until isolated and power restored.
A development of the three switch arrangement for multiple circuit substations is the full mesh layout as shown in FIG. 7b. Each section of the mesh is included in a line or transformer protection zone so no specific separate busbar protection is required. Operation of two circuit breakers is required to connect or disconnect a circuit and disconnection involves opening the mesh.
Line or transformer circuit disconnectors may then be used to isolate the particular circuit and the mesh reclosed.
FIG. 7 (a) Three switch mesh. (b) Full mesh.
1. Circuit breakers may be maintained without loss of supply or protection and no additional bypass facilities are required. The particular circuit may be fed from an alternative route around the mesh.
2. Busbar faults will only cause the loss of one circuit. Circuit breaker faults will involve the loss of a maximum of two circuits.
3. Generally not more than twice as many outgoing circuits as infeeds are used in order to rationalize circuit equipment load capabilities and ratings.
Maximum security is obtained with equal numbers of alternatively arranged infeeds and load circuits. Sometimes banked pairs of feeders are arranged at mesh corners.
3.4 Ring
FIG. 8 Ring.
The ring busbar offers increased security compared to the single busbar arrangement since alternative power flow routes around the ring busbar are available. A typical scheme which would occupy more space than the single busbar arrangement is shown in FIG. The ring is not so secure as the mesh arrangement since a busbar fault causes all circuits to be lost until the fault has been isolated using the ring busbar isolators. Unless busbar disconnectors are duplicated maintenance on a disconnector requires an outage of both adjacent circuits. The inability of disconnectors to break load current is also an operational disadvantage.
3.5 Double Busbar
3.5.1 Transfer Bus
The double busbar arrangement is probably the most popular open terminal outdoor substation arrangement throughout the world. It has the flexibility to allow the grouping of circuits onto separate busbars with facilities for transfer from one busbar to another for maintenance or operational reasons. A typical transfer busbar arrangement is shown in FIG.
1. This is essentially a single busbar arrangement with bypass disconnector facilities. When circuit breakers are under maintenance the protection is arranged to trip the bus-coupler breaker.
2. The system is considered to offer less flexibility than the full duplicate double busbar arrangement shown in FIG. 10.
FIG. 9 Transfer busbar.
FIG. 10 Duplicate busbar (and wrap around arrangement).
3.5.2 Duplicate Bus
1. Each circuit may be connected to either busbar using the busbar selector disconnectors. On-load busbar selection may be made using the bus coupler circuit breaker.
2. Motorized busbar selector disconnectors may be used to reduce the time to recon figure the circuit arrangements.
3. Busbar and busbar disconnector maintenance may be carried out without loss of supply to more than one circuit.
4. The use of circuit breaker bypass isolator facilities is not considered to offer substantial benefits since modern circuit breaker maintenance times are short and in highly interconnected systems alternative feeder arrangements are normally possible.
5. A variant on the scheme uses a 'wrap around' busbar layout arrangement as shown in FIG. 10 in order to reduce the length of the substation.
3.6 1 1/2 Circuit Breaker
The arrangement is shown in FIG. 11. It offers the circuit breaker bypass facilities and security of the mesh arrangement coupled with some of the flexibility of the double busbar scheme. The layout is used at important high-voltage substations and large generating substations in the USA, Asia and parts of Europe where the cost can be offset against high reliability requirements. Essentially the scheme requires 1 1/2 circuit breakers per connected transmission line or transformer circuit and hence the name of this configuration.
FIG. 11 --- 1 1/2 circuit breaker.
1. Additional costs of circuit breakers are involved together with complex protection arrangements.
2. It’s possible to operate with any one pair of circuits, or group of pairs of circuits separated from the remaining circuits. The circuit breakers and other system components must be rated for the sum of the load currents of two circuits.
3. The arrangement offers high security of supply.
4. SPACE REQUIREMENTS
4.1 Introduction
Having selected the required substation single line diagram arrangement it’s then necessary to convert this into a practical physical layout. It’s essential to allow sufficient separation or clearances between substation equipment to withstand voltage stresses and to allow safe operation and maintenance of the equipment. The designer will have to consider the following.
Actual site selection. The substation configuration and number of circuits involved (including any allowance for future expansion) will largely determine the land area requirements. The ideal site will have the following characteristics:
1. Reasonably level and well drained so minimum surface dressing and civil ground works are required.
2. Low lying and not in a prominent position so that planning permission will be relatively easy to obtain. For an open terminal air insulated switchgear (AIS) switching substation a location as far from a built-up area as possible. However, for a primary distribution substation this will conflict with the technical and economic requirement to have the substation as close to the load centre as possible. Consider the use of indoor gas insulated switchgear (GIS), the cost of which in some locations will be largely offset by reduced land area costs.
3. Good access from public highways for easy transportation of materials and especially heavy items such as transformers to the site.
4. Good overhead line wayleave substation entry routes.
5. Pollution-free environment. If the substation is to be sited in a highly polluted industrial area (next to a quarry, cement works, etc.) or close to a coastal salty atmosphere then a meteorological study will be required to determine the prevailing wind direction. The substation should then be sited upwind of the pollution source. Again an indoor GIS arrangement should also be considered.
High or low level, catenary or solid, busbar arrangements. A high busbar is exposed and must span complete switchgear bays. Low busbars are more shielded, may be more suitable for connection of portable earths but may need frequent supports. They may be considered to be more visually or environmentally acceptable. Space savings are also possible from the use of different types of switchgear, For example by using pantograph instead of horizontal swivel isolators.
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TBL. 1 Safety Clearances to Enable Operation, Inspection, Cleaning, Repairs, Painting and Normal Maintenance Work to be Carried Out (BS7354)
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FIG. 12 Substation work section boundaries, section and ground clearances.
4.2 Safety Clearances
The safety distance means the minimum distance to be maintained in air between the live part of the equipment or conductor on the one hand and the earth or another piece of equipment or conductor on which it’s necessary to carry out work on the other. A basic value relates to the voltage impulse with stand for the substation. To this must be added a value for movements for all methods necessary to maintain and operate the equipment so that a safety zone may be determined. Section clearances and ground clearances based on British practice (BS7354) are given in TBL. 1. Figures 12 and 13 illustrate diagrammatically the clearances required between the different items of substation equipment for maintenance and safe working limits (Tables 2 and 3). Note that safety clearance must also be allowed to any necessary working platforms.
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Indicates section clearance required from position at which men may stand for work described in example numbered in circle to nearest live conductor or equipment.
Indicates section clearance required from ground, buildings, fences and permanent access ways to permit work thereon with substation alive.
Indicates ground clearance from ground or permanent access ways to nearest part of insulator carrying live conductor.
FIG. 13 Work section clearance, example.
==== TBL. 2 Necessary Operations for Maintenance Work on Different Items of Open Terminal Outdoor Substation Plant as shown in the Substation Layout, FIG. 13
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TBL. 3 International Practice _ Electrical Clearances for Open Terminal Outdoor Switchgear (BS 7354)
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TBL. 4 Phase_Phase and Phase_Earth Clearances (IEC 60071)
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CIGRE is an organization of electricity authorities which meets to discuss and exchange information on matters of electricity generation, transmission and distribution. Working groups study various problems and report back to various committees. Their work is published in Electra and excellent reports have been issued which form guides for the selection of substation clearances. CIGRE recommendations are technically coherent and essentially the same as BS7354 but slightly more difficult to apply. A basic curve is drawn on the layout drawings first and separate horizontal and vertical clearances added.
4.3 Phase_Phase and Phase_Earth Clearances
IEC 60071 deals with insulation co-ordination and proposes standard insulation levels and minimum air distances. BS7354 also specifies phase_phase and phase_earth clearances. Extracts from BS covering International practice are enclosed in TBL. 3 and from IEC in TBL. 4. Phase_phase clearances and isolating distances are usually specified as 10-15% greater than phase_earth clearances. The justification is that phase_phase faults or faults between equipment terminals usually have more serious consequences than phase_earth faults. It should be noted that the configuration of conductors and adjacent earthed structures and equipment also affects these clearances. Therefore care must be taken when applying these criteria. For example, the clearance required from an open contact on a disconnector to an adjacent structure will be greater than that from a continuous busbar to ground level in order to achieve the same insulation level.
Once the various minimum allowable phase_phase and phase_earth clearances have been chosen it’s necessary to ensure that the design maintains these at all times. Allowance must be made for movement of conductors in the wind and temperature sag effects. Under short circuit conditions, flexible phase conductors may first repel each other (reducing clearances to adjacent equipment) and then swing together (reducing phase_phase clearances). The coincidence of an overvoltage on one phase with an overvoltage or peak value of system voltage of opposite polarity on an adjacent phase can produce an increase in voltage between phases. The 10_15% margin in phase_phase clearances allows for a degree of protection against this occurrence.
At high altitudes the reduced air density lowers the flashover voltage and clearances should be increased by approximately 3% for each 305 m (1,000 ft) in excess of 1,006 m (3,300 ft) above sea level.
Allowances must also be made for variations in the level of the substation site and the positioning of foundations, structures and buildings. At lower voltages an additional margin may be added to avoid flashovers from birds or vermin. A common mistake is not to take into account the substation perimeter fence and thereby infringe phase_earth clearances.