The need for extra transmission and distribution connections for wind energy has led critical observers to argue that the associated cost is a hidden extra. What is frequently forgotten, however, is that other forms of electricity generation also come with a grid cost.
Electricity consumers worldwide need additional transmission and distribution connections for new generation, partly to replace ageing fossil-fired plant, and partly to meet the growing demands for electricity.
It can therefore be misleading to look at the requirements for wind in isolation.
One key difference between wind energy and conventional generation is that the power output from wind farms varies and only achieves its full rated level for a modest proportion of the time.
Although a single turbine might, with the right wind conditions, achieve its rated output power for, say, 7% of the time, when wind turbines are assimilated in a wind farm, the rated power is produced for a lower proportion of the time.
This is a consequence of the effects of so-called spatial diversity – the fact that the wind does not blow evenly on the collected turbines at any given moment – and wakes – the effect the front row of turbines has in reducing the output of machines located downwind.
With a small wind farm, for example, the output exceeds 97% of the rated power for only 1% of the time. Data from the large concentration of wind turbines in western Denmark shows that the maximum output from 2.44GW of plant at any one time in 2008 was 2.17GW, or 89% of the total capacity (see Fig 1, opposite).
Given that the wires are so seldom used at full capacity, local and regional grid reinforcements constructed primarily to export wind power do not need to be rated at the full capacity of the wind turbines connected to them.
The economically optimum rating for the wires in fact depends on the reinforcement costs. Some wind energy may occasionally need to be rejected, or constrained off the network, to use the technical term, but the value of this energy is less than the cost of building connections to convey the full output of the plant.
Optimise power line ratings
If a distribution connection is not to be rated at the full output of the wind farms to which it is linked, some energy may at times go to waste.
That energy has a value. The lower the rating of the power line, the higher the volume of wasted energy. That waste needs to be balanced against the cost of the line. These costs rise with power rating.
Connection costs vary by location and annual costs also vary, depending on the regulatory regime.
The total capital costs of connection are typically around €140 per megawatt-kilometre – some €12/MW-kilometre on an annual basis, including maintenance.
A 40 kilometre line typically costs around €480/MW/year.
The value of the wasted energy along a line with a rating of, say, 60%, of the output of the wind farms in a region, can be derived by looking at the number of hours that the power output of the wind farms exceeds this level.
The data in Fig 1 suggest this is about 11% of the time. With an energy purchase price of, say, €60/MWh, the value of the lost energy is around €10,000/year per MW.
Adding the value of the wasted energy to the cost of the connection gives an estimate of the total annual cost of the link.
With assumptions based on the above figure, the most economic rating of the line is around 70% of the rating of the wind capacity (see Fig 2). The case is fairly typical for wind farm connections.
If the connection costs are high, then the optimum rating is likely to be lower – say around 65% of the wind farm power rating but if the value of the energy is higher, then the desirable rating may also be higher – around 75% of the wind farms’ rating.
With a short or cheap connection, the optimum rating may also be higher – 85 to 90%, depending on the energy payments.
Similar trade-offs between additional reinforcement costs and a modest curtailment of wind farm output arise when new wind farms need to connect into existing networks.
The demand pattern in the local area also needs to be taken into account.
According to British grid consultancy Senergy Econnect it is possible to connect a 15MW wind farm to a 106MW line even when the line is already taking electricity from 106MW of hydro plant and 28MW of wind and the minimum demand is 23MW.
An analysis by Senergy Econnect of the power flows in this case show that it is much cheaper to connect all the generation and subsequently manage energy flows when they exceed the capacity of the wires.
Over a year, the 15MW wind farm may have to constrain output for 238 hours, the analysis found.
Wind plant can also share connections with conventional thermal generation.
The UK Supergen Consortium gives an example in which 800MW of conventional plant, with an 85% load factor, and 700MW of wind can share a connection rated at 1000MW.
Too much wind?
If there is an appreciable amount of wind in a region and little demand, it is quite possible for power flows on the low-voltage distribution network to reverse their normal direction from centralised power producer to consumer and travel "backwards", possibly into the high-voltage transmission network.
Although concerns are sometimes expressed about the problems that may result, the electricity system can handle such reverse flows.
An early study for the UK government, by distribution network operator South Western Electricity Plc, of the impact of a 4MW wind farm on a rural network found that during periods of high wind power output, the total demand on the local substation fell below the wind farm output.
On these occasions, the wind farm not only supplied the complete substation load, but also fed into the 33kV network to help supply other substations in the area.
The transformers coped with this situation in a satisfactory manner and no voltage problems were experienced.
The study also found an apparent reduction in the number of automatic tap-change operations on the 33/11kV transformers.
These electromechanical devices adjust transformer outputs to keep voltages within prescribed limits so less activity leads to lower maintenance costs.
The overall conclusion was that the wind farm inputs to the substation had a major influence in keeping the voltages on the 11kV system within the desirable range.
In other words, and perhaps contrary to public perception, the wind farm impacts on local distribution networks can be beneficial and the analysis suggests that wind can be connected using links with ratings that are lower than the output power of the wind plant.
David Milborrow is economics and technology consultant for Windpower Monthly