conform with that concept, the implied commitment has been recognised to the extent of maintaining parallelism of the station centre-lines in the north­ south direction.

The arrangement of the main plant was governed t)y the need to keep the cooling water culverts and the 400 kV cables as short as possible. The CW pumphouse was located in the harbour alongside the Stage 1 pump­ house and the Stage 2 400 kV transmission lines leave the station area in a north easterly direction. It was therefore economically advantageous to site the turbine hall and generator transformers at the northern end of the available area and the nuclear island to the south of the turbine house.

This arrangement led to the location of the active effluent treatment plant and the solid waste store in a position to the south of the reactor building with access from the nuclear island by means of a bridge.

The essential supplies buildings were strategically positioned around the main building envelope to satisfy the safety requirements.

The block containing the administration, welfare and workshop facilities was located so as to satisfy a number of conditions, namely:

•To fit into a rational pattern of personnel movement.

•To occupy kind unsuitable for other uses.

•To permit the movement of station personnel and visitors to and from the block with little or no contact with other station activities.

Sea water for cooling purposes is drawn from the harbour and is discharged in a westerly direction from the turbine hall through culverts and tunnels to an outfall to the bay.

The 400 kV and 132 kV substation is located about 0.6 km east of the power station site and is separated from it by a golf course. Two parallel double-circuit 400 kV overhead lines carry the outputs of Stages 1 and 2 to the substation by a route which leaves from the north east corner of the site, while 132 kV supplies are cabled to the station transformers by a southerly route.

4 Pumped storage

4.1 Introduction.

It is not feasible to store electricity directly but the CEGB heeds to have a reserve capacity to cope with its system requirements such as:

•Immediate reserve which has a response of only a few seconds to cope with rare major breakdowns.

•Sustained breakdown reserve, available in about five minutes to cover the period until replacement plant

can be synchronised.

• Reserve for variations in demand on all timescales to cope with load prediction errors.

Pumped storage

The inherent ability for rapid loading of hydro and pumped storage plant and then flexibility in changing from one operating mode to another (e.g., pumping to generating), makes it ideally suited to providing the required reserve capacity.

The geological formation of the UK means that there are relatively few true hydro sites and the majority of those that do exist have already been developed. Pumped storage offers the capability of increasing the hydro capability by pumping water from a lower reservoir to an upper reservoir at times of low demand (e.g., during the night), and then allowing the water to fall back to the lower reservoir to drive turbines and hence generators during periods of peak demand or in order to meet a reserve requirement. The hydraulic machines can be separate as at Ffestiniog where there is a separate pump and turbine on the same shaft, or combined into a reversible pump/turbine as at Dinor­ wig. The latter reflects current world practice.

In siting a pumped storage power station there are a number of important features as follows.

4.2 Suitable topology

The basic requirements for a pumped storage scheme comprise the following, which are showtFift^^JTS^ and relate specifically to the DijjjJrgfig jVojeeK L’

•High pressure penstock tunnels.

•Machine hall.

•Tailrace tunnel.

•Lower reservoir.

The most conventional arrangement of pumped storage scheme utilises an existing lake as the lower reservoir and a convenient mountain topology which would allow a high level feature such as a cirque or corrie to be transformed into a reservoir by the addition of a dam. Economics and environmental issues will dictate whether the hydraulic system and conduits are aboye or below ground. There are two other arrangements which have been considered. Underground reservoir pumped storage uses a nominal sea level reservoir as the upper reservoir and some arrangement of tunnels or caverns as the lower reservoir. Salt caverns, disused coal or mineral mines and purpose-built tunnel systems have been considered for this type of duty. The other possibility is the use of the sea as one of the reservoirs. In this case there is a significant potential difficulty with contamination ol Iresli water sources with salt water.

Pumped storage has only been shown to be economic when the available head between the two reservoirs is

relatively high (about 300 m) and in order that the hydraulic losses can be contained within reasonable bounds, the two reservoirs should be horizontally as close together as possible. The position of the power station is obtained by projecting a 10% gradient access tunnel line from the surface down to the level required for pump runner submergence and then checking that there is sufficient rock cover of suitable quality to withstand the operating pressures. This will locate the minimum length of the low pressure end of the system, and an economic evaluation must then be done to estimate the cost changes arising from further extension of the low pressure and access tunnels, in order to shorten the more expensive high pressure tunnels.

This'requirement for high vertical displacement and small horizontal separation is only satisfied, in England and Wales, in the mountainous districts to the west of the country and in the Pennines, although a lower head scheme has been examined in North Devon. The studies in the early 1970s which led to the selection of the Dinorwig site, investigated three sites in detail. All of them were in the Snowdonia area of North Wales and were either wholly or partly inside the National Park.

4.3 Ground conditions

The requirements here are for ground that is either impervious to water leakage or which can be made impervious, e.g., by the injection of a chemical or concrete grout. All of the sites mentioned have slate as the underlying rock. These would be classified as hard rock sites allowing the construction of significant underground galleries without bracing and which can be made sufficiently watertight. The reservoir works would be developed using rockfill embankment dams.

In the early planning stages, assumptions must be made about the suitability of the ground and rock based on a very limited amount of data. Some data can be obtained by strategically placed boreholes and sup­ ported by at least one exploratory tunnel down into the power house location. Excavation costs can be esti­ mated and a balance must be made between minimising the size of underground chambers and the difficulties which this strategy creates in terms of constructability and access for maintenance during station operation.

4.4 Site capacity

Pumped storage schemes are designed according to the following criteria:

•Pumping capacity and the time it takes to refill the upper reservoir, assuming all the pumps are in operation at full output.

•The return amount of energy which can be generated as a percentage of the pumping energy (charge energy factor).

Pumped storage

•The requirements for quick start-up in the gener­ ating mode.

The first two criteria determine the size of pumping plant and the capacity of the upper reservoir, whilst the third has important effects on the hydraulic layout of the system. “r' I

As an example, the Dinorwig power station ‘is' j designed for a pumping period of 6 hours each night?? ‘ the six reversible pump-turbines are each nominally rated at 300 MW and the charge energy factor is 0.78J This allows the station to meet a generating criterion of full minimum head output for 5.4 hours per day. In order to meet this storage requirement at Dinorwig, the , working volume of the reservoirs is 7 million cubic : metres.

4.5 System and transmission requirements

Pumped storage can be economic in a number of scenarios, for example, to meet a reserve capacity and to limit the two-shifting operation of large units.

The requirement for reserve provides the biggest economic benefit because it allows expensive running of large plant at part load to be limited. In general, for a large modern pumped storage scheme, this reserve requirement would be.met by the first station. Subse­ quent pumped storage schemes would be more difficult to justify until such time as the nuclear contribution (which is difficult to load cycle) exceeds the night-time trough demand.

Ideally, for the scheme to provide the best reserve capacity, it should be located close to the major load centres so as not to be at risk from grid disconnections. Unfortunately, the geology of the UK does not allow this and the Dinorwig station, which provides the system reserve, is located in a relatively isolated region. This places significant pressure on the security of the grid connections and the potential faults which occur during split grid operation. Particular care is therefore required in designing the electrical systems to protect the plant against dynamic oscillations and pole slipping.

Large generating stations in the UK would normally be connected to the grid by at least three circuits. In the case of Dinorwig, this was reduced to two single circuits by using underground cables for much of the routes. This avoided the problems of double circuit faults caused by storm conditions. The solution of using underground cables also solved the amenity problems of taking overhead lines through a National Park.

4.6 Hydraulic system requirements j

The design of the hydraulic system and the layout | features which need to be accommodated are deterr • mined to a large extent by the system requirements, |

For a plant to meet the system reserve capability, the requirement is for a quick response.

At Dinorwig the requirement is for the plant to generate 1320 MW in 6 to 10 seconds. The plant is also designed to meet the other spinning reserve require­ ments and provide a frequency regulating duty. This imposes a requirement for up to 40 mode changes per day and 400 000 pressure cycles over the station life, so that fatigue is one of the design criteria for the high pressure parts of the system.

Fast start-up and mode changing is best achieved with as short a hydraulic system as possible to limit the pressure surge effects. The hydraulic machinery can be arranged for fast load pick-up in several ways:

•Running the system in hydraulic short-circuit with some of the units pumping, while the other units are generating. Load pick-up is achieved by tripping the pumping plant and rapidly bringing the turbines to full load.

•Running the gvncralois synvhioniscd with the system but generating no load. The operation of the inlet guide vanes is then the critical factor in achieving the required loading rate.

•Spinning in air is similar to the foregoing item except that the machines are motored from the system, with

•the pump-turbine dewatered by means of com­ pressed air to reduce losses.

turbines during the loading process. The operation of the main inlet valve is critical to this method of fast load pick-up which is the one adopted at Dinorwig.

In order to simplify construction, the power station complex should be located as close as possible to the lower reservoir. It must also have sufficient sub­ mergence at minimum water levels to avoid cavitation at the pump-turbine inlet. The power system conduits must have a very smooth profile in order to minimise operating friction losses and the design and construc­ tion process is simplified by maintaining the conduit operating pressures within a small number of fixed envelopes. The HP penstocks should be as short as possible as they represent the most highly stressed section of the hydraulic system and provision must be made for a surge shaft as close upstream of the power complex as possible to alleviate the upsurge during loss of generation. The size of the various hydraulic struc­ tures is optimised following a detailed surge study of all the normal transient events, as well as the credible sequences of events which can intensify the surge effects. These might include two successive station trips from full load pumping with'the second station trip timed to give the worst level variations in the surge chamber.

At Dinorwig, a number of alternative tunnel schemes were examined including: a single high pressure shaft

The first scheme was the most economical, and after civil and reliability engineering design studies had shown it would give the required availability and taking account of tunnel inspection times, the arrangement shown in Fig 1.37 was adopted. Maximum station water demand is 420 m3/s. The power/time criterion was the most important factor in tunnel sizing because of the need to accelerate the 2 km water column from stand­ still to full flow in six seconds. The system velocities were then checked and fixed by balancing the cost of various tunnel sizes and their energy losses, within the limits of previous experience, to ensure a satisfactory tunnel lining integrity and acceptable pressure surge levels. Figure 1.38 shows the optimised surge shaft design and gives the nominal design conduit velocities. Figure 1.39 shows the flow and water hammer press­ ures following complete trip of six turbines from full load.

4.7 Heavy load access

Because of its size and weight the generator motor is often built in-situ so that the largest loads brought to

System velocities corresponding to the extreme maximum

Station generating flow of 420 cubic metres per second are