Tough targets on reducing carbon emissions are driving an increase in renewable power generation. However, it is the network operators that perhaps face the toughest challenge: connecting the distributed generators to the network while maintaining power quality standards. One UK utility has devised an innovative solution.

Jeff Douglas, Central Networks, UK

The way we choose to generate our electricity and the impact this has on our environment remains high on the political agenda. In its last White Paper on energy, the UK government set out some tough targets on reducing carbon emissions and increasing renewable power generation.

Electricity generated from renewable sources now accounts for around three per cent of the UK’s supply, with more planned, including an increase in the amount generated from offshore and onshore wind farms.

But, as renewable operators face the concerns of stakeholders sceptical about having a wind farm in their community, electricity distributors face the challenge of connecting those wind farms to their networks and ultimately transporting the power into homes.

The energy regulator Ofgem is well aware of this challenge and, in support of the UK government’s tough renewable power targets, decided to tackle it by outlining an incentive initiative, called the Registered Power Zone (RPZ), to all electricity distribution companies. The initiative encourages distributors to develop and implement innovative solutions to connect distributed generators such as wind farms to their networks.

Central Networks is part of E.ON UK and is the second largest electricity distribution company in the UK, serving central England. Distributing electricity to 4.8 million customers, it covers an area from the Welsh borders to the Lincolnshire coast and from the Peak District to the outskirts of Bristol.

Saturation point

Finding the right location for a wind farm can prove problematic but there are some obvious choice locations.

The UK’s east coast may be a popular tourist haunt but its position is also proving popular with many renewable developers because of its wind resources and the general proximity of the electricity network.

Central Networks has received several applications and requests for connection to its network in the area, which have resulted in a reduction in the spare capacity available for future applications. Put another way, the network has almost reached saturation point.

Figure 1. Geographical plan of 132 kV network post Bicker Fen GSP
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It is expected that there could be between 320 MW and 1570 MW of renewable power connected in this area within the next ten years. The capacity of the existing local network in terms of generation is only 226 MW. Clearly, something had to be done to allow these projects to have any chance of success.

Traditionally, generation connection offers have been made on a ‘fit and forget’ basis using minimum loads and minimum circuit ratings. With these current forecasts and applications, a considerable amount of new infrastructure, including 132 kV reinforcement for the lower estimate or a new grid supply point (GSP) for the higher estimate, would be needed.

Another concern for the engineers was the danger that any new 132 kV circuits built now to serve the minimum case requirements could later become ‘stranded assets’ should a new GSP be required.

Faced with this issue, Central Networks’ engineers considered all their options and took a closer look at the actual output of wind farms.

Number crunch

Wind farms in general will only operate at maximum rating for a relatively short period of time, say ten per cent of the year and this is more than likely to occur in the cold winter months when the network has a greater thermal capacity and higher system load.

On average, wind farms operate at about 30 per cent of maximum capacity and this is where the significant portion of their income comes from.

It was this realisation that led engineers at Central Networks to consider constraining a generator back at peak generation periods by providing an opportunity where a suitable balance could be achieved between the generators’ capital outlay and the restriction placed on the income from exporting the power.

Central Networks concluded that, by better utilising its existing assets and exploiting the network capacity currently not used – the higher winter ratings and higher network loads – it could connect more renewable generation.

To this end, engineers developed an arrangement that uses an automated control system that is capable of analysing the network and sending signals to the customer’s generator to control output to best use the existing network, so avoiding installing costly new infrastructure.

The RPZ automated control system will be embedded in the network primary control system. It will use real time data, including load and ambient temperature.

The control system calculates in real time the maximum operating characteristics of the network infrastructure and ensures that the network continues to operate within the safe operational limits of its circuits and equipment.

As the east coast is an area of significant development in both onshore and offshore wind farms it was decided that a scheme should be developed that focused on this section of the network.

This RPZ area comprises the 132 kV network between the Walpole GSP and Boston bulk supply point (BSP), the proposed Bicker Fen GSP and Boston BSP and between the Boston BSP and Skegness BSP, including all the associated lower voltage networks.

The innovative scheme relies on active export management control and active circuit ratings, and is event programmable and backed-up by failsafe protection.

Network circuit ratings are traditionally based on assumed winter or summer temperatures. Distributors use these ratings to determine and plan acceptable power flows and system security for their networks. However, by taking real site ambient temperatures, an enhanced circuit rating, based on the prevailing conditions, can be derived.

The second element of the solution takes account of the actual output of the connected generators in real time. This is combined with the derived circuit rating within Central Networks’ control system in order to generate control messages to the wind farms requesting them not to increase output levels, or to effect predetermined output reductions.

This approach enables generators to have flexibility over their connection, and to balance the higher cost of ‘fit and forget’ connection against the potential loss of some output for a connection using the RPZ approach.

Figure 2. Existing network post Bicker Fen GSP
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When the proposed Bicker Fen GSP is installed it will supply the Skegness BSP via the busbars at Boston, with Boston being supplied from Walpole. Figure 2 shows how the Bicker Fen GSP will be connected into the 132 kV network.

Bicker Fen GSP is due to be commissioned at the end of 2007. The development of the RPZ is currently underway and Central Networks anticipates that it will be available in time to allow connection of new renewable generation from 2007.

A shared solution

Of course, while innovative, this technique can only permit a modest incremental increase in connection capacity. On the other hand, it does enable the connection of generation that would otherwise be precluded by high network costs and could potentially be replicated in other situations where the thermal ratings of circuits are constraining connection.

Once the system is fitted and working, Central Networks will be able to share the approach with the other network operators across the UK and to potentially help to solve a tricky problem for renewable generators striving to meet government targets.