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

These requirements apply to a reactor whether it is operational, or in the initial period of shutdown, when fission product decay heating occurs, during the posttrip cooling period.

AdeCluate post-trip cooling must be available for all credible faults and accidents that can be sustained by the reactor, and sufficient redundancy and diversity of mechanical plant and systems ensures this. Clearly, electrical equipment, where required as the power source for the mechanical plant and systems, must also be capable of meeting the redundancy and diversity requirements.

Having satisfied the demands of reactor safety, the electrical system must also enable the station to be

operated at full or part output with the best possible efficiency and operational flexibility. To achieve all of the above objectives, the electrical system is structured into two parts:

(a)The main electrical system.

(b)The essential electrical system.

The main electrical systems of all magnox stations are based on the unit and station system principle. This has already been described in Section 3 of this chapter.

In the case of the rnagnox stations however, different voltage levels (e.g., 6.6 kV) and sometimes discrete systems for a particular purpose (e.g., gas circulator

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supplies) are used. Although the details of the design are different in each case, the objectives are the same, viz safe and economic operation of the power station as a whole.

The essential electrical system is an integral part of t he main electrical system, but is designed so that it

can operate independently of it.

The safety of the reactor is dependent on the essential electrical system as all items of plant necessary for post-trip cooling are connected to this system, e.g., emergency boiler feed pumps, pony motors for gas circulators, necessary auxiliaries, etc.

The main electrical system provides the preferred supply to the essential electrical system, via unit or station auxiliary transformers. If there is a loss of grid causing loss of supplies to the essential electrical system, then on-site generation will provide supplies to the essential system only. Usually, gas turbine or

diesel generators are used for this purpose.

There are certain items of plant which can tolerate a short interruption of supplies, and these will be connected to AC switchboards supplied by the on-site generation. The short break in supplies due to the starting period of the on-site generation is acceptable

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from reactor safety considerations. Plant items which cannot tolerate any break in supplies (e.g., instrumentation, controls and indications) are connected to supplies derived from a DC system, i.e., batterybacked. Normally the DC supplies are provided from the essential system AC switchboards via rectifier units, the battery being maintained in a fully charged state.

Following a loss of supplies to the essential system, the battery maintains supplies to the plant items connected to the no-break system during the short period of time while the on-site generation is starting up. When the on-site generation is fully available, the DC supplies are again provided from this source, and the battery is recharged to a fully charged state.

The main and essential electrical systems thus provide supplies for both reactor and plant safety, and economic operation of the station when supplying power to the National Grid.

3.3.3 AGR nuclear power stations

The choice and ratings for the auxiliaries systems for the AGR stations are similar to the fossil-fired stations

and provide supplies in the same way on a unit and station basis. Although the electrical auxiliaries system is a single integrated design, it has two major constituent parts namely the main electrical system and the essential electrical system.

The most recent of the CEGB AGR stations is at Heysham 2 and its design has followed CEGB design philosophies evolved over the period since the first AGRs were designed. Heysham 2 as described represents the latest design aspects of the AGRs. The main electrical system function is primarily to operate the station in producing its output, while the essential system is to ensure that the CEGB meets the required safety criteria in supplying safety related reactor auxiliaries plant both following a reactor trip and in the general longer term. Heysham 2 (2 x 660 MW) is the largest auxiliaries power system for a two-unit station installed by the CEGB. Considering first the main electrical system, the auxiliaries power system for the two units, numbered 7 and 8, are illustrated in Figs 1.9 and 1.10.

Unlike Heysham I, the grid connections have been made to both the 400 kV and 132 kV grid substations.

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This enabled the setting up of an electrical system which matched the reactor quadrant concept and provided higher integrity grid connections. Each reactor/ generator has four 11 kV switchboards; A, B, C and D. Station boards A and B deriving supplies from the 132 kV system and unit boards C and D from the 400 kV system.

In this way, there is an 11 kV switchboard associated with each reactor quadrant and it is at this level that the four-trained electrical system starts and is continued to the lower voltages.

The auxiliaries associated with each quadrant all derive supplies down through the various voltage levels from the same 11 kV source. The lower voltage levels are 3.3 kV and 415 V. The drives connected at each voltage level are:

•415 V up to and including 150 kW.

•3.3 kV up to and including 1500 kW.

•11 kV above 1500 kW.

The gas circulators (5220 kW), CW pumps (1700 kW) and emergency boiler feed pumps (10 500 kW) are therefore supplied at 11 kV.

At the 3.3 kV level, two systems are established, the X system for cooling the reactor by forced circu-

lation and the Y system to feed water into the main boilers, after a trip. The 415 V system continues with X and Y systems.

In addition, at 3.3 kV there are auxiliaries associated with the turbine-generator for which the electrical needs are similar to most 660 MW units, and a unit auxiliary and station auxiliary system has been created by deriving supplies from the appropriate 11 kV level (D and B). This follows through to the 415 V level and in addition provides supplies for the reactor services auxiliaries.

Each generator output at 23.5 kV passes to the grid via a generator switch and a generator transformer to 400 kV. The generator also feeds its auxiliaries via t wo unit transformers (23.5/11 kV).

Each unit transformer normally supplies one II kV board but it is rated (60 MVA) to be capable of supplying the normal loads of two 11 kV boards, via the 11 kV interconnectors. It can be seen that each unit board is interconnected to a station board, i.e., D to A and C to B. This makes possible either a 400 kV or 132 kV derived source. Likewise the two station transformer secondaries are each rated to supply the normal load of a unit/station board.

The two station transformers are also interconnected at 11 kV. Again each secondary winding is rated