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1. INTRODUCTION
Substations are the points in the power network where transmission lines and distribution feeders are connected together through circuit breakers or switches via busbars and transformers. This allows for the control of power flows in the network and general switching operations for maintenance purposes. This section describes the principal substation layouts, the effects of advancements in substation equipment, the modular design, the compact sub stations and the moves toward design and construction 'turnkey' contract work. The descriptions concentrate on air insulated switchgear (AIS) outdoor open terminal designs at rated voltages of 72 kV and higher. The design of distribution voltage switchgear and gas insulated switchgear (GIS) is described in Section 13, in which terminology is also defined.
2. SUBSTATION DESIGN CONSIDERATIONS
2.1 Security of Supply
In an ideal situation all circuits and substation equipment would be duplicated such that following a fault or during maintenance a connection remains avail able. This would involve very high cost. Methods have therefore been adopted to achieve a compromise between complete security of supply and capital investment. A measure of circuit duplication is adopted whilst recognizing that duplication may itself reduce the security of supply by, for example, providing additional leakage paths to earth.
Security of supply may therefore be considered in terms of the effect of this loss of plant arising from fault conditions or from outages due to maintenance. The British Code of Practice for the Design of High Voltage Open Terminal Substations BS7354 categorizes substation service continuity, recognizing that line or transformer faults destroy service continuity on the affected circuits.
Category 1: No outage necessary within the substation for either maintenance or fault; for example the 1 1/2 breaker scheme under maintenance conditions in the circuit breaker area.
Category 2: Short outage necessary to transfer the load to an alternative circuit for maintenance or fault conditions; for example the double busbar scheme with bypass isolator and bus-coupler switch under fault or maintenance conditions in the circuit breaker or busbar area.
Category 3: Loss of a circuit or section; for example the single busbar with bus section circuit breaker scheme for a fault in the circuit breaker or busbar area. The transformer-feeder scheme also comes under category 3 ser vice continuity and for this arrangement the addition of incoming circuit breakers, busbar and transformer circuit breakers does not improve the classification.
Category 4: Loss of substation; for example the single busbar scheme without bus sectionalization for a fault in the busbar area.
2.2 Extendibility
The design should allow for future extendibility. Adding bays of switchgear to a substation is normally possible and care must be taken to minimize the outages and outage durations for construction and commissioning. Where future extension is likely to involve major changes (such as from a single to double busbar arrangement) then it’s best to install the final arrangement at the outset because of the disruption involved. When minor changes such as the addition of overhead line or cable feeder bays are required then busbar disconnectors may be installed at the outset (known as 'skeleton bays') thereby minimizing outage disruption. The use of gas insulated switchgear (GIS) tends to lock the user into the use of a particular manufacturer's switchgear for any future extension work. In comparison an open terminal switchyard arrangement allows the user a choice of switchgear for future extension work.
2.3 Maintainability
The design must take into account the electricity supply company system planning and operations procedures together with a knowledge of reliability and maintenance requirements for the proposed substation equipment. The need for circuit breaker disconnector bypass facilities may therefore be obviated by an understanding of the relative short maintenance periods for modern switchgear. Portable earthing points and earthing switch/interlock requirements will also need careful consideration. In a similar way the layout must allow easy access for winching gear, mobile cranes or other lifting devices if maintenance downtimes are to be kept to a minimum. Similarly standard minimum clearances (see Section 4.2) must be maintained for safe working access to equipment adjacent to operational live switchgear circuits or switchgear bays, bearing in mind that some safety authorities now resist the use of ladder working and require access from mobile elevated working platforms or scaffolding.
2.4 Operational Flexibility
The physical layout of individual circuits and groups of circuits must permit the required power flow control. In a two transformer substation operation of either or both transformers on one infeed together with the facility to take out of service and restore to service either transformer without loss of supply would be a normal design consideration. In general a multiple busbar arrangement will provide greater flexibility than a ring busbar.
2.5 Protection Arrangements
The design must allow for the protection of each system element by provision of suitable CT locations to ensure overlapping of protection zones. The number of circuit breakers that require to be tripped following a fault, the auto-reclose arrangements, the type of protection and extent and type of mechanical or electrical interlocking must be considered.
For example a 1 1/2 breaker substation layout produces a good utilization of switchgear per circuit but also involves complex protection and interlocking design which all needs to be engineered and thus increases the capital cost.
See Sub-Section 8 regarding the use of circuit breakers with CTs in the bushings.
2.6 Short Circuit Limitations
In order to keep fault levels down parallel connections (transformers or power sources feeding the substation) should be avoided. Multi-busbar arrangements with sectioning facilities allow the system to be split or connected through a fault limiting reactor. It’s also possible to split a system using circuit breakers in a mesh or ring type substation layout although this requires careful planning and operational procedures.
2.7 Land Area and Wayleaves
In order to construct, operate and maintain an overhead line or substation at all voltage levels, land acquisition and plant access are required. Normally the rights to install such facilities and the rights to secure the associated rights of way, or wayleaves, are negotiated on a voluntary basis between the power line company and the land owner(s). Such negotiations will be required to take into account the due planning application laws and processes. However, because of the importance of a secure electricity supply to both public and industry, governments recognize that the power line companies have a public service role to play. Therefore, when necessary, such wayleaves may be obtained under compulsory purchase schemes where due compensation is determined and paid to the landowners in accordance with national regulatory processes. Where an application is made in relation to more than one overhead line (which may be of a multicircuit nature), each line is normally assessed on its own merits for planning approval purposes.
The cost of purchasing a plot of land in a densely populated area is considerable. Therefore there is a trend towards compact substation design. This is made possible by the use of indoor gas insulated switchgear (GIS) substation designs or by using such configurations as the transformer-feeder substation layout. In addition compact design reduces civil work activities (site preparation, building costs, requirements for concrete cable trenches, surfacing and access roads). Long multicore control cable runs and switchyard earth grid requirements are also reduced. The reduction in site work by using compact layouts and in particular by using modular elements results in an overall shorter substation project design and construction duration to the advantage of the client. FIG. 1 dramatically shows the reduction in land area required for an indoor GIS substation as a direct replacement for the previous conventional outdoor open terminal switchyard arrangement.
2.8 Cost
A satisfactory cost comparison between different substation layout designs is extremely difficult because of the differences in performance and maintain ability. It’s preferable to base a decision for a particular layout on technical grounds and then to determine the most economical means of achieving these technical requirements.
Busbar span lengths of about 50 m tend to give an economical design.
However, the gantry structures involved have a high environmental impact and the current trend is for low profile substations. Tubular busbars tend to offer cost advantages over tensioned conductor for busbar currents in excess of 3,000 A. Taking into account some of the factors mentioned and the savings in cost of land (see Section 2.7) resulting from a reduced 'footprint', manufacturers now consider that a 400 kV GIS substation may produce over all savings when compared to a conventional open terminal arrangement, although this varies greatly depending upon site and territory, and the reduced bay centers can result in clearance difficulties where there are incoming overhead lines.
FIG. 1 GIS substation replacement for conventional open terminal outdoor
arrangement. A striking comparison between land area requirements for a
conventional open terminal 132 kV double switchyard arrangement and replacement
indoor GIS housing to the top right-hand corner of the picture (Yorkshire
Electricity Group plc).
The use of circuit breakers with CTs in the appropriate bushings, available up to 275 kV, saves the use of separate post-CTs, with their associated plant, structural, civil and space costs, and may result in overall economy compared to the use of initially cheaper breakers without this facility.