reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

decrease in the range 49.5-47 Hz. Operation below 48.8 Hz will be very infrequent and for periods not longer than 15 minutes.

(b)A fault, including a fire, in any section of any auxiliaries system shall not cause more than one main generator to trip under all normal operating conditions.

(e)The plant auxiliaries systems shall remain stable for three-phase faults of duration up to 200 ms

on 'close-up' sections of the supergrid and grid busbars and the adjacent system, over a specified range of operating conditions.

(d)The plant auxiliaries supply arrangements shall be designed to meet all the operating flexibility requirements, e.g., two-shifting, part-loading and

load rejection.

(e)The plant auxiliaries system shall satisfactorily withstand any internally generated switching or other

transient overvoltages.

(1)The plant auxiliaries system shall accommodate the generator operating with a terminal voltage in the range of 95% to 105% of the rated value.

In addition to the needs that the auxiliaries electrical system must meet as requirements of the STPs, the designer may incorporate system features to improve availability by supplementing those required by the STPs. For example, the incorporation of alternative supplies to selected switchboards could reduce outage ti me and consequently lost revenue from a main generator following an electrical fault; such a design feature will be subjected by the designer to economic justification. This and other 'additional requirements' will be explained in more detail in Section 4 of this chapter.

2.2 Grid system operation criteria

While the power station has specified operating criteria, the grid system into which it generates also has such criteria defined for it. The significant ones are those associated with frequency, voltage and total or partial loss of the grid connections in the vicinity of the power station.

The frequency ranges have been outlined in Section 2.1 of this chapter. In addition however, looking from the grid into the power station, it must be remembered that below 47 Hz the auxiliaries system may be protected by an automatic trip, although an excursion of this sort has a very low probability. There are also the onerous transient frequency excursions as a result of full-load rejection and possibly periods of steady high frequency up to 52 Hz, which the auxiliaries system will be expected to withstand without tripping for periods not exceeding 15 minutes. To put the likelihood of local frequency excursions

in perspective, outside the range of 50.5 Hz, it is generally expected at the following estimated rates:

•Greater than 50.5 Hz, I incident per year.

•Greater than 52 Hz, 0.2 incidents per year.

In addition to the frequency ranges above, the auxiliaries system will be required to accommodate the network voltage variations. The 400 kV supergrid system voltage will normally remain within the range 400 kV +5%. The maximum voltage which can arise is 440 kV, but this condition would not be permitted to last longer than 15 minutes. The 132 kV system voltage can vary between the limits of 132 kV ± 10 070.

Internally generated switching or other transient overvoltages on the auxiliaries system were mentioned above, but added to these will be any transferred surges from the grid system. The amplitude of step changes of voltage on the 400 kV system are not expected to exceed +6%.

The effects of total or partial loss of the grid connections to power stations vary depending on the type of station, the most significant effects being on nuclear power stations. It is essential to re-establish AC supplies within known timescales in these instances to maintain nuclear safety. This is described more fully in Section 2.4 of this chapter. In the case of conventional power stations, the safety of plant and personnel is normally taken care of by the DC systems if AC supplies are lost. Re-establishing the AC supplies does not usually require the same emphasis other than for returning the main generators to service.

2.3 Plant and personnel safety needs

It will be appreciated that maximising the output from power stations must be achieved within recognised standards, codes of practice and rules to ensure the safety of the power station plant and personnel. In electrical system design terms, adequate safeguards must be incorporated to meet the statutory requirements of the Electricity Regulations and the Health and Safety at Work Act, relating these to safety rules. The CEGB Safety Rules set out the mandatory requirements for establishing the safety of persons at work. The electrical systems must build-in means of achieving operational and maintenance regimes to comply with all necessary safety requirements.

Operationally, the major considerations are to ensure that the normal and abnormal duties and prospective fault capabilities for circuits and system configurations are not exceeded. The circuits must be rated for required voltage and for normal and fault currents calculated during the design, and in the case of the switchgear must be capable of making and breaking current during normal and fault operations. Interlocking or monitoring schemes need to be incorporated to ensure that ratings are not exceeded due to operator

error. Descriptions of such schemes are contained later in this chapter. For maintenance of plant and equipment there is a CEGB mandatory requirement to isolate and earth all circuits at voltage levels of 3.3 kV and above before work can commence. At 415 V and below, proof of isolation must be established. Details of these features are described later in this chapter.

Protection of the plant must be arranged to prevent damage without resulting in an increased loss of availability. For example, should a turbine-generator trip as a result of loss of AC supplies or trip and cause loss of AC supplies, the lubricating oil systems are maintained by means of DC motor-driven pumps.

Other means of maintaining the safety of plant will be described later in this chapter when considering what safeguards need to be incorporated into the system.

2.4 Nuclear hazard needs

The electrical systems provided at nuclear power stations must relate to the plant and equipment required to prevent the release ultimately of radioactivity to the atmosphere. Initially the favoured source of electrical supplies to these safety systems would be derived from the grid network. This network has finite li mits of its own, the voltage and frequency limits having been described in Section 2.2 of this chapter. However, when these limits are exceeded they can, particularly in the case of nuclear power stations, be regarded as being the equivalent to a total loss of grid supplies. The likelihood of this 'total loss' must be considered in relation to the time factors associated with maintaining nuclear safety. For example, in the case of the Sizewell B pressurised water reactor (PWR) station ti me bands of 0 to 2 hours, 2 to 12 hours and greater than 12 hours have been considered. The probability can be related to the required stage by stage availability of the plant needed to meet the safety case.

Consideration of the needs of the safety related plant to the probabilities of losing grid supplies invariably leads to the provision of a supplementary AC source of supply by means of on-site generation. In most cases this has been provided by either gasturbine or diesel-driven generators. The choice between the two will depend on several factors including the rating and availability of the auxiliary generation required. For example, the CEGB have utilised gasturbine generators of 17.5 MW rating on earlier AGR nuclear power stations. For the later Heysham 2 AGR power station, diesel generators up to 8 MW rating have been installed. A significant factor regarding the Heysham 2 diesel generators was the need for a fast start-up/loading requirement. This influenced the generator parameters, e.g., a low subtransient reactance value was chosen to achieve fast start-up while still containing the prospective fault contribution to an acceptable level. The manner in

which each has been utilised is described later in Section 3 of this chapter.

3 System descriptions

The generating units of each power station deliver their electrical output to the National Grid via connections at 400 kV or 275 kV, although at some older generating stations the generators are connected to the grid at 132 kV. As part of the design of new power stations, dependent on the network and capacity requirements of the transmission system in the area, consideration may be given to building a new 400 kV substation at locations where existing generating plant is connected at lower voltages, i.e., 275 kV or 132 kV. The present policy is to use SF6 insulated metalciad 400 kV switchgear, often mounted indoors, particularly on coastal or polluted sites. If extensions to existing substations is the economic method of connecting new generators, then 'open' busbar design would be employed using SF6 circuit-breakers.

The stations require supplies to be available at all ti mes for supplying 'station' auxiliaries and depending on the system design, for providing a supply to the 'unit' auxiliaries for starting up and shutting down of the units as shown in Fig 1.1. In the cases where generators are connected to the grid via a generator voltage switch, the units are normally started and shutdown via the generator/unit transformer route, though a separate source for 'station' supplies would still be provided for the station auxiliaries and for standby to the unit transformer as illustrated in Fig 1.2. If available, this would normally be derived from a 132 kV source since, for the rating of 500 MVA and below, the transformers are well proven, economic and the switchgear is cheaper. If however, 132 kV is not available on the site, to create a 132 kV substation might require long cable routes or overhead lines and possibly provide additional intergrid reinforcement. This may be more costly than considering station transformers connected at 400 kV.

3.1 Main generator and station systems

3.1.1 Main generators

Generators of 660 MW (776 MVA) rating having a nominal output voltage of 23.5 kV. The output of the machine to the generator transformer is via phase isolated connections, naturally air cooled and rated at 20 000 A, either directly connected or switched by purpose built generator voltage switchgear, depending on the auxiliaries system design. Details of the generator main connections and generator voltage switchgear are given in Chapters 4 and 5 respectively.

At present these are the largest generating sets installed in the UK. Designs are being developed for generators rated at 900 MW, in which case the gen-

erator terminal voltage will probably increase to 26 kV. The present style of main generator connection arrangements will be capable of carrying the increased output current, although forced cooling by either air or water may be required. The limiting factor for naturally cooled connections would be accommodation of the greatly increased size, particularly within the generator terminal centres. This is fully described in Chapter 4.

3.1.2 Generator transformers

Each generator is connected to the grid system via a generator transformer with the appropriate voltage ratio. The CEGB fit on-load tap changers to accommodate the grid voltage variations and the voltage operating range of the generator. It has established a 'registered design' of generator transformer for the 660 MW generating units rated at 800 MVA and made

up of 3 single-phase units. The intention of the registered design is to achieve a high level of reliability by avoiding all but essential change to proven systems in detail design, materials or components (see Chapter 3).

The 800 MVA rating is based on taking the main generator 776 MVA rating plus the possible 44 MVA output from a gas-turbine generator, contributing from the 11 kV level via a unit transformer, less a minimum unit auxiliaries load of 20 MVA. For the generating units being considered at 900 MW (nominal) rating, a generator transformer rated at 1145 MVA, also in 3 single-phase tanks, is being developed, taking into account an overload capability from the main unit. As with the 800 MVA rating, on-load tapchangers will be fitted for the same reasons.

There have been instances on nuclear power stations using generator voltage switchgear, where the on-load tapchanger has been arranged with an auto-

matic feature. This has been done to deal with the problem that arises when a generator trip results in the opening of' the generator voltage switch but retains the grid connection. Under these circumstances, the 11 kV switchboard voltage could fall to a level such that the direct on line starting of a boiler feed pump may not be achieved. The auto-tap facility raises the voltage in a timescale and to a level capable of achieving a pump start thus securing an initial boiler feed without relying on the emergency feed pumps. Should this scheme not achieve boiler throughput, the emergency pumps connected to the 3.3 kV system will still ensure reactor safety.

3.2 Electrical auxiliaries systems

These systems provide the power for the station auxiliaries and are nowadays almost always designed on the unitised principle. In the past, in some cases, particularly early magnox and AGR stations, this principle was not strictly followed, the former consortia

•ho built the stations having used criteria different from present day practice.

The general arrangements for electrical auxiliaries systems are described below, and in their design due comisance is taken of the limits and constraints imposed by the equipment commercially available which is or could be type approved for the particular application.

In the unit principle, all auxiliaries associated with

starting and running the unit at CMR output are connected to the unit electrical system. This must be designed such that one fault including a fire, does not lose the output of more than one generating unit. Similarly, plant which does not have an immediate effect on the running of the unit, will be connected to the station electrical system. It is required that a fault on this plant will not immediately propagate into the unit electrical system and affect unit output.

To achieve this, the electrical and mechanical plant, switchgear and cabling is segregated between the units, and between the two halves of the station system, (normally known as 'station A' and 'station B'). Segregation is not normally provided between the unit and station systems.

Standard voltage levels of 11 kV, 3.3 kV and 415 V have been selected to accommodate the very wide range of plant drives and equipment.

In the case of the 11 kV unit system, a major constraint is the thermal current rating of the largest circuit-breaker commercially available and approved for use on CEGB systems. The present nominal rating is 3150 A which, when calculated in relation to the transformer standard BS171 requirements, relates to an incoming transformer rating of approximately 60 MVA. Therefore, if the unit load is in excess of this, two unit transformers are required. The fault interrupting capability of the switchgear also adds a constraint, which is discussed under 'system choice' in Section 5 of this chapter.

Supplies to the unit board are derived from a unit transformer on the basis of one per switchboard, the primary of which is teed-off the generator voltage main

connections system.

All auxiliaries requiring electrical motor drives, whose combined operation is necessary to keep the unit venerating, are connected to the unit system. Large motors rated at 1500 kW and above are generally connected to the 11 kV system, e.g., electrical boiler feed pump , circulating water pumps, gas circulators at nuclear power stations and boiler fans at fossil-fired

power staticns.

In the case of nuclear power stations, particularly early AGRs (and some magnox), the 11 kV unit systems form part of the nuclear safety case. This is because the 11 kV provides a preferred source of supply to the essential system, and in some cases feeds essential plant directly, e.g., gas circulators. In this context, essential plant is that which is required following a reactor trip to shut down the reactor safely and remove post-trip decay heat. Back-up emergency generating facilities are provided at the appropriate voltage level should the grid connection fail. If the emergency drive is greater than 1500 kW and therefore requires connecting to the II kV system, emergency generation at II kV is provided. This is described in more detail later in this section.

The 11 kV unit switchboard as well as supplying large motors also provides a feed to the 3.3 kV unit system via 11/3.3 kV oil-filled unit auxiliaries transformers located outdoors. For nuclear stations there is, in addition, an 'essential system' which normally derives supplies from 11/3.3 kV 'essential transformers'. If grid derived supplies are not available, the essential system is supplied by on-site generation (gas turbines or diesels). The thermal current limits applicable to unit transformer incoming circuit-breakers apply equally to the auxiliaries and essential transformers, which are limited by the largest approved circuit-breaker at 3.3 kV, rated at 2400 A, giving the largest practical size of transformer rating of around 14.5 MVA.

Unit related auxiliaries in the range 150-1500 kW are connected at 3.3 kV, although motors outside this range may be connected for special cases.

The 3.3 kV system also provides feeds to the 415 V unit system via 3.3/0.415 kV unit services transformers. These transformers are normally naturally air cooled 'AN' type and mounted in the switchboards. The 415 V system is distributed around the power station, with switchboards located in switchrooms as close to the load as possible. Motors up to approximately 150 kW are connected to this system although motors above this rating may be considered in special cases. To maintain a high availability of electrical supplies to auxiliaries, duplicate feeds are supplied to each switchboard. This may be achieved by two incoming supplies and a bus section switch, or by one incoming supply and a cabled interconnector to another switchboard which has its own incoming supply. For both these

methods, each transformer feeding the switchboards must be rated to include the standby requirement of the other transformer.

Those auxiliaries which are common to two or more units or are not necessary to maintain unit output, are connected to a 'station' electrical system, i.e., not specifically associated with any one unit.

The 11 kV station system has several duties, and the rating chosen will reflect the duty it is called on to perform. It does however share the same constraints as the 11 kV unit system brought about by the circuitbreaker ratings.

The station system may be required to provide a source of supply to large 11 kV 'unit' drives, directly in some system arrangements or in a standby mode in others to cater for a unit transformer outage. The station transformer rating must be chosen accordingly. The different duties expected of station transformers are outlined in Section 3.2.1 of this chapter.

Feeds from the 11 kV station system are provided to a station 3.3 kV system via 11/3.3 kV station auxiliary transformers. Common station services, such as coal handling plant at fossil-fired power stations, would be supplied at this voltage level. The rating of motors at 3.3 kV would be on the same basis as the unit system.

Station supplies at 415 V are derived from the 3.3 kV switchboards via 3.3/0.415 kV services transformers, usually of the 'AN' type, mounted in the switchboards and located in switchroorns as near to the load centre as layout permits.

On some nuclear power stations while the station system is not required ultimately in the safe shutdown case, in many cases it may provide grid derived supplies to nuclear plant and relieve the demands on the essential system. For example, at Heysham 1 AGR, the 132 kV grid derived supplies can be made available from the station system to the main gas circulator motors via converters following a reactor trip. As such it would be required to be engineered with this duty in mind.

3.2.1 Auxiliaries system transformers

Unit transformers

As mentioned above, the supply to the II kV unit board is via a dedicated 23.5 kV/11 kV unit transformer, with a rating chosen to match the unit load, but limited to 60 MVA due to the largest approved rating of 11 kV circuit-breaker. Another consideration the designer must take into account is the choice of transformer impedance. A unit transformer has a typical impedance of approximately 15% on rating. When this value is used in the analysis of the station's electrical auxiliaries system, it may require alteration. For example, the electrical system regulation may be too high, making the starting of large 11 kV squirrel-cage induction motors direct-on-line (DOL) difficult. The maximum rating suitable for DOL starting at 11 kV

is about 11 MW. Also, unacceptable voltage conditions may be experienced at the lower voltage levels. In this case the impedance may need to be reduced. In con11-1 with this, too low an impedance may give rise to unacceptable fault levels on the 11 kV system, especially when unit and station supplies are paralleled during station start-up and shutdown procedures. The subject of parallel operation is discussed more fully

Section 4 of this chapter.

Solutions to this conflict are seldom easy and almost always cause complications and additional expenditure. The options are:

•Use assisted starting techniques, i.e., 'soft' starting, for the largest motors by utilising static or rotary converters. This may also be combined with woundrotor motors, rather than squirrel-cage.

aUse automatic fast transfer systems when switching between unit and station supplies to reduce the transfer time to one or two cycles. This permits break before make without allowing the speed of running motors to drop below recovery times. If make before break is ever regarded as an acceptable option, it would limit the time during which prospective fault levels exceed ratings.

•Use generator voltage switchgear to provide start-up supplies via the generator and unit transformer.

•Use HV connected unit transformers, with HV disconnection of the generator/generator transformer combination.

•Increase the system voltage to say 15 kV thereby increasing the possible transformer rating. This is not being pursued in present designs, mainly because it would mean either creating a 15 kV system with a separate unit transformer for the very large drives only or raising the voltage for all the motors catered for at II kV, e.g., induced and forced draught fans, CW pumps.

These problems have become more pronounced with he proposed introduction of larger generating sets, c.g., 900 MW, without steam-turbine-driven boiler feed pumps and relying on large full duty electric feed pumps in a 3 x 50c% configuration each rated at 13.5 MW. It should be noted that past practice has beLn to design systems whereby unit and station systems arc capable of being paralleled for start-up/shutdown and 'or standby duty without exceeding fault levels (sce Section 4 of this chapter).

Slur on transformers

The supply for the 11 kV station boards is via a 132 kV, 275 kV or 400 kV/11 kV station transformer, the rating of which is chosen to provide a starting facili- t ■, for the unit, and standby capacity to the unit transformer in the case of its being unavailable, due to an outage.

The station transformers' duties may be summarised as follows:

•Supply the total 'station' load (due to outage of the other station transformer) as well as supplying the starting load of a unit.

•Supply its proportion of the station load and the CMR unit load when acting as replacement for a unit transformer.

It should be noted that to accommodate the single fault criteria (that one fault should not lose all station supplies), a minimum of two station transformers will be required for the station. The above duties become more complex when more than two station transformers are used on multi-unit stations. However, the above principles remain the same.

Similar to the principles outlined in the section on unit transformers, the impedance of the station transformer must be chosen to enable paralleling with the unit transformer, for start-up and shutdown and to allow the largest electric motor (normally the feed pump) to be started. As mentioned above, the use of higher rated sets may preclude paralleling and alternative methods may be required to achieve successful methods of changeover for start-up and shutdown supplies.

It should be noted that the above criteria are a general guide, and each proposed electrical system is designed with the particular requirements of the station addressed specifically. More information is given in the following sections.

3.2.2 Interconnection

To enable flexibility of operation and to cater for planned or forced outages, interconnection between different switchboards at the same voltage levels is normally provided. These are usually cabled interconnections with circuit-breakers at each end. The associated circuit-breakers are arranged such that one is normally closed. This energises the cable permanently, so that any cable fault is detected and cleared by the protection immediately, rather than when the circuit is energised just prior to being required. Interconnection may be one of two distinct types:

•Where the two supplies may be paralleled, thereby giving continuity of supply.

•Where an alternative supply is required but the two sources may not be paralleled due to a paralleled fault level in excess of the switchgear certified rating.

Where interconnection is provided between supplies which may be paralleled (as there is no fault level restriction) but may be out of phase and frequency, check synchronising facilities will be provided at the normally open circuit-breaker. Where interconnection

would produce unacceptable fault levels at the switchboard, an indication or interlocking system is provided to ensure that the circuit-breaker is not closed. Indication and interlocking systems are discussed further in Section 8 of this chapter.

3.2.3 Essential systems

All power tations require essential systems, but a fundamental difference exists between fossil and nu-

clear plant.

Fossil plant only requires essential electrical systems to maintain unit output and to protect plant from damage following a loss of supply. The consideration of these systems only needs to examine economic and personnel safety issues, and the systems are designed to achieve these objectives.

Nuclear plant requirements are much more onerous, due to the fission product decay heat which requires removal to avoid an unacceptable risk of a radiological hazard and expensive plant damage.

Essential systems for all the power stations are based on additional on-site prime movers, either diesel generators or gas-turbines, together with batteries and chargers providing no-break supplies. Present designs also use uninterruptable power supplies (UPS) to provide instrumentation and power supply requirements which are battery-backed. These systems are also used in normal operation since they provide a stable voltage and frequency supply 'isolated' from the transients experienced by the main auxiliaries system. They are based on centralised schemes of static or rotary inverters, with a battery backing for a timescale in the region of 30 minutes to cater for loss of the battery charger or its AC supplies. For more details on UPS see Section 6 of this chapter.

The DC system voltage levels are chosen for selected duties such as emergency drives and emergency lighting at 250 V, switchgear with the higher current closing solenoids at 220 V, protection, direct control and switchgear tripping at 110 V and telecommunications, remote control and indications at 48 V. The batteries are usually of the lead-acid Plante type.

The DC systems are described later in Section 7 of this chapter, and the batteries and chargers in Chapter 9.

3.2.4 Emergency generation

As mentioned previously, on-site generation is provided for emergency supplies to the auxiliaries system on all power stations. There are many differing types, dependent on the type of station and the needs which have to be met. Generators may be powered by gas

turbines, or diesels and may be at voltages of 11 kV or 3.3 kV.

On-site generation for large fossil-fired stations since the early 1960s has been provided by gas turbines at 11 kV, and has satisfied the following needs:

•To provide an independent supply to the auxiliaries of the main steam units in the event of unacceptably low frequency on the Grid system.

•Use as output plant capacity to meet system requirements. In this mode of operation the gas turbines will normally be used for 'Peak generation' purposes, and will also act as 'hot standby'.

•Ability to start-up a station without external Grid supply.

•To provide an independent supply in order to ensure the operation of essential drives, such as the main bearing lubricating oil, in the event of loss of normal supplies. This duty is, in effect, a back-up to the DC battery system.

On-site generation for nuclear power stations assumes a more important role as it becomes part of the nuclear safety case. All plant required to safely shut down and cool the reactor is normally supplied from an essential system, which derives its preferred supply from the grid supply. Failure of the off-site connection requires the on-site generation to connect, usually automatically, to the essential system. The large quantities of decay heat in the reactor core/boiler system cause prolonged requirements for feedwater, steam dumping and reactor core cooling after the turbine-generator has been tripped.

3.3 Types of stations

The CEGB have a wide variety of power stations from base load coal-fired and nuclear power stations to oil-fired, hydro, pumped-storage and gas turbine types, and gas-fired and wind power pilot installations. The bulk of the demand is of course met by the base load stations which this section will concentrate on. The present design policy to take the CEGB into the t wenty-first century is to have both large coal-fired stations and nuclear stations of the PWR design. Combined cycle gas-turbine (CCGT) installations are also a future possibility.

The coal-fired stations will be at the 2 x 900 MW size and the first PWR will be at Sizewell B with a single reactor and 2 x 660 MW turbine-generator units.

With the increasing concern for controlling the emissions from coal-fired stations, retrofitting of Flue Gas Desulphurisation (FGD) plant is taking place at selected existing coal-fired stations and included at the design stage for the new 2 x 900 MW designs. The additional loading imposed by FGD on the auxiliaries system is very significant, resulting in the designers assessing different schemes for meeting the various methods of providing FGD plant. FGD is an international problem being tackled in various ways, but initially the CEGB are employing the limestone/ gypsum method. The additional auxiliaries system load-

it-1g for this process at a 2 x 900 MW station is of the order of 45 MW for the entire plant.

Considering now the auxiliaries systems for the various types of stations, this section describes the different aspects associated with each.

3.3.1 Fossil-fired power stations

The majority of existing CEGB fossil-fired plant is fuelled by coal or residual oil with a small number capable of being fired by either. There are a few gas turbine stations with units of about 70 MW using distillate fuel, and Hams Hall C power station which is dual-fired, using coal or natural gas. The latter example using natural gas was a pilot conversion scheme to assess its feasibility.

Basically for a given location and station output, the electrical auxiliaries system for a coal-fired or oil-fired station would differ only in respect of the loads associated with the fuel handling and combustion plant. For a coal-fired station, this plant consists of the unitised draught plant (induced draught, forced draught and primary air fans), coal mills and feeders and the precipitators together with the common services associated with coal handling, dust handling and ash disposal systems. For a 2000 MW coal-fired station of 4 x 500 MW units, operating at CMR, the auxiliaries load is typically 31 MVA per unit plus a station load of 20 MVA. The comparative figures for a similar sized oil-fired station are 20 MVA and 13 MVA respectively since the unit load will not have the PA fans, precipitators and coal mills and the station loads will not have the coal, ash and dust handling systems. The fuel oil system does not make the same load demands as the coal fuel systems.

Take as an example, the electrical auxiliaries system provided for the 2000 MW (3 x 660 MW) Littlebrook D oil-fired station. The outline of the auxiliaries system is shown in Figs 1.3 and 1.4. An important consideration in the adoption of the most economic station supplies arrangement was the availability of an existing 132 kV substation on the site.

One of the SIP requirements was for the output from the three gas turbines, for system reasons, to be available to the grid independent of the operation of the main units. Each gas turbine generator rating is 35 MW, which required three station transformers, since one transformer circuit (maximum rating 60 MVA) could not accommodate more than one gas turbine generator for thermal reasons nor could the auxiliaries system for prospective fault level reasons. The use of three station transformers however does lend itself to a simpler and more flexible system configuration than is possible with the more general two station transformers scheme. At the 11 kV level, the station/ station interconnections maximise the availability of the station transformers across all three units and their gas turbines. These interconnections are an example of how an auxiliaries system design can economically

justify providing facilities beyond what could be provided to meet only the STP requirements.

Each 3.3 kV unit auxiliaries board is supplied by duplicate 8 MVA transformer feeders, each capable of supplying the 3.3 kV auxiliaries load and thereby providing standby to each other. The transformer i mpedance was chosen to enable both transformers to be in service at the same time. There was no need therefore to provide any unit/station interconnection at the 3.3 kV level.

At the 415 V level, a sectionalised unit services/ station services switchboard was introduced, each section fed from its respective 3.3 kV auxiliaries board. This provides a better utilisation of transformer capacity at this level than having separate unit and station 415 V boards with duplicate supplies for each from the respective 3.3 kV unit or station auxiliaries system; thus reducing the cost, space and maintenance requirements. Transfer of loads from one transformer to its standby is carried out off-load since the prospective fault level at 415 V does not permit carrying this out on-load.

For the fossil-fired stations, slightly different auxiliaries systems have evolved as the CEGB moved from the 500 MW unit period to the 660 MW units of the late 1960s. All systems used the unit/station principle. Most of the stations with 500 MW units had four units each with two station transformers, typically shown in Fig 1.5.

Drax power station was designed as a 6 x 660 MW unit station, with three units initially installed, followed in the early 1980s with the three remaining units. The station auxiliaries system catered for the six units from the outset by providing four station transformers. Despite the long time interval between the construction of the first and second halves of the station, there was great emphasis placed on replication wherever possible for the completion phase to ensure the operational and maintenance convenience of the station as a whole. The outline of the auxiliaries system for the six-unit station (to 11 kV level) is shown in Fig 1.6.

The auxiliaries systems for the present 900 MW unit coal-fired station designs are being assessed as for past stations against their STP requirements and economics. The alternative systems considered include using generator voltage switchgear, which the CEGB first used at Hartlepool and Heysham AGR stations, but which to date has not been used at fossil-fired stations.

All generator voltage switchgear used by the CEGB on their modern large units has been the 3 x singlephase airblast type, designed and manufactured by Brown Boveri. A description of the design, construction and performance of the types used by the CEGB is given in Chapter 5.

It has not been a requirement at fossil-fired stations to make grid supplies available via the generator transformer, and generator voltage switchgear has not been economically justifiable compared with a unit/ station transformer scheme.

The introduction of FGD plant follows the CEGB policy decision to reduce the overall sulphur emission from its power stations. To achieve this, it is proposed in the first instance to retrofit FGD equipment to existing coal-fired stations starting with Drax. In addition, the CEGB will be providing FGD equipment on all their new coal-fired stations. For a 2000 MW station burning 2% sulphur content coal, the load consumption of the FGD plant using the limestone/ gypsum process is of the order of 53 MVA. When compared with a nominal station load of 51 MVA, this represents 104% additional auxiliary power required, which constitutes a significant increase in capital and through-life costs for the station. The electrical auxiliaries system currently proposed for a 2 x 900 MW subcritical coal-fired station design, which includes the FGD plant, is shown in Figs 1.7 and 1.8. It will

be seen that the FGD plant electrical supplies have been derived from the unit/station electrical systems. All voltage levels of 11 kV, 3.3 kV and 415 V are required to accommodate the loads, including large booster fans fed at II kV.

Cabling system design is made more complex with this arrangement since the unit/station system is determined by the layout of the generator, station and unit transformers and major switchboards. These are located at the opposite end of the station to the FGD auxiliaries and plant.

Alternatively, the FGD plant can be considered as a separate entity, giving rise to the provision of a dedicated FGD electrical auxiliaries system centred on a location adjacent to the FGD plant and with its own Grid connections. The comparison between the two approaches is mainly one of economics. For new

rata, ia 0 yd .,. .0 a.

1

3

77.■ 71711, 1 1174 11 0/..0:

4..7 .111aC.0 •aal )a

projects the most economic approach utilises the unit/ station electrical system, although the separate electrical system has clear benefits for retrofit FGD schemes.

eyed differently for each station and differently from those which would be adopted today. However the fundamentals for reactor safety remain the same. They are:

3.3.2 Magnox nuclear power stations

The CEGB has eight magnox reactor nuclear power stations. These stations were commissioned over a period spanning eleven years, from Bradwell in 1962, to Wylfa in 1973. The stations were built by different consortia as 'turnkey' contracts, and hence have many differences in terms of output and design. The design measures which ensure reactor safety, which is the most onerous requirement on system design, are achi-

•To ensure a main coolant flow over the reactor internals, so cooling the reactor core and fuel.

•To ensure a flow of feedwater to extract the heat developed in the reactor, and hence provide steam to power the main turbine-generators.

•To provide reactor auxiliaries and services, e.g., pressure vessel cooling water flow.

•To provide controls and indications for the above.