Coping with the wind resource1 May 2007
Can the European electricity grids cope with the output from more windfarms? Robert Freer summarises a report by the European transmission system operators that tackles this very question.
The idea of generating electricity from the wind is superficially attractive. The fuel is free, wind turbines use established technology and they can be built quickly enough for a government minister to see something happen during the time he or she is in office. But the practical reality is rather different.
EU Directive 2001/77/EC on the promotion of Renewable Energy Sources (RES) expects wind capacity in Europe to increases from 41 MW in 2005 to nearly 67 MW in 2008, and increase still more by 2015. But in a recent study (European wind integration study. Towards a successful integration of wind power into European electricity grids. ETSO Final Report, 15 January 2007) by the European Transmission System Operators, who are the operators of the main European grid systems serving some 18 countries, attention is drawn to the technical and financial problems which may arise from such expansion and from the attempts to integrate the output from windfarms into the various national grids. The study makes recommendations that are an attempt to solve these problems. Many of the proposed new windfarms will be in Germany but some will be in Spain, Portugal and Great Britain.
ETSO’s study is focused on the high voltage grid and investigates the steady state and stability conditions in the synchronous areas covered by the operators UCTE, NORDEL, UKTSOA and ATSOI for the expected wind farms installations in the year 2008. The study identifies a number of major problems in developing wind electricity if it is intended to make a useful contribution both to national supplies and to the European electricity system. There are technical solutions to these problems but they increase the costs of what is already an expensive form of generation.
A general problem with large wind turbines is the well known one of availability – they generate electricity only when there is a strong wind blowing, and these occasions are unpredictable, intermittent and do not necessarily occur when there is a demand for the electricity. Also the average electrical output is small. For instance in the UK the annual output from all the wind turbines contributes less than 1% to the national electricity supply of 375 Twh per year, and without back-up by conventional power they cannot contribute to ensuring the secure supply of energy which the UK prime minister now recognises is the most important of the “immense challenges” facing Britain, and without which “we could not function as an economy or modern society”. (Department for Trade and Industry, The Energy Challenge – Energy Review Report 2006.)
Wind farms are expensive and not economically competitive. In the UK the government provides a generous subsidy under the Renewables Obligation scheme, without which it is unlikely that anyone would build them. “ ... the subsidy for wind power until 2020 will be some £30 billion due to the freedom of multiple departmental committees to reach consensus conclusions in a policy vacuum with no effective ministerial leadership”. (Lord Tombs, House of Lords debate 23 June 2005.)
The main technical problems that arise from trying to integrate wind energy into electricity grids arise from their location, their variable output and the method of connecting them to the grid. The ETSO study looks at all these problems.
For instance many wind farms are built on sites which have high average wind speeds but are remote from the main load centres. New overhead lines are therefore necessary to transport the surplus electricity to the regions where it is consumed. Lengthening the transmission lines increases the grid losses (in Germany the active grid losses are reportedly doubled by large amounts of wind power from Northern Europe) and also leads to a higher load factor which consumes more reactive power. ETSO says that more reactive power generation at high voltage will need to be installed before 2008 to meet the EU’s development plan for an increase in wind power.
ETSO considers that the cost of these new investments and extra work should be met by the developers of the wind farms. Their argument is that if these costs had to be met by the system operator they would become part of the tariff paid by the customer and there would be no incentive for the windfarm developer to reduce the cost of integration with the grid.
Another problem ETSO looked at was the effect of an unexpected gale on a large remote wind farm which could generate a large surplus of wind power and cause a temporary power surge through neighbouring grids. Such unscheduled surges, together with the rapid increase and decrease in output as the wind speed rises and falls, could reduce system stability and increasingly affect trading capacities.
ETSO’s modelling was predicated on a base case of moderate winds throughout Europe and two scenarios – Scenario North and scenario South – characterised by high winds in the northern and southern parts of Europe.
The main effect of Scenario North is the switch from import to, to large export from, Germany owing to its high wind power production. Large deviations from the exchange schedules occur expecially on the axis Germany/The Netherlands/Belgium/France on the one hand and Germany/Poland/Czech Republic/Austria on the other. Although there is large scheduled import from Germany to France in UCTE Scenario North, the physical flow is from France to Germany. The impact of Scenario South is less dramatic, the main effect being the reversing of exports to France from Spain which could lead to decreased damping of interarea oscilllations owing to lower power station acticity in Spain, and therefore some instability.
ETSO expects that problems are likely to get worse as more wind farms are built and estimates that by 2008 large surges of wind power from the proposed new developments will cause bottlenecks in the internal and cross border transmission lines in Northern Europe.
High wind power generation in northern Germany together with conventional power and imports from the NORDEL grid would result in large North-South power flows through the transmission systems of Germany and neighbouring countries. ETSO say that in these circumstances single circuit outages could cause internal overloads in these countries, and if a circuit is unavailable due to a disturbance in the grid the remaining lines can be overloaded up to 180%.
The security of the grid can be put at risk by the way wind farms are connected to it. Conventional power stations do not disconnect from the grid even after a grid failure but ETSO says that many of the wind farms so far built disconnect themselves even in the event of a minor brief voltage dip. This could lead to a serious power failure on the system.
They recommend that manufacturers ensure that their machines are designed to support system stability even in the event of a fault.
ETSO also considered the economic impact of wind energy. Most European system operators are required to give priority of dispatch to renewable electricity sources including local sources of wind energy, but by doing so they may cut out some cost-effective generation from conventional plant.
In the UK there is no priority of dispatch for wind power but as a consequence of the financial support mechanisms wind power is more expensive to curtail than conventional generation for congestion management purposes.
ETSO makes five main recommendations. First they recommend establishing a Europe-wide rational allocation of renewable energy sources. This should ensure a more even spread of wind power installations and would avoid concentrations of output and make better use of the most efficient sites. Agreement on this allocation could replace the different national support schemes in Europe. Secondly, licensing approvals for both the renewable sites and the grid infrastructure should go hand in hand to avoid delaying the expansion of the grid. Approval procedures for the grid infrastructure are reported to be taking too long. ETSO also sees a need for an adjustment of market rules for imbalance management to ensure that the generation portfolio provides the balance power capacity needed at any time and at a high level of security. They recommend that the windfarm promoters should be made responsible for any imbalances they create.
To ensure proper protection of the grid and to avoid a potential black-out in the event of a sudden disconnection of a significant number of wind generators ETSO recommends that all power generators, including wind power generators, should be obliged to meet certain operational requirements such as fault ride through capability or voltage support. At present ETSO is concerned that with some wind power generators a slight drop in voltage or frequency can cause them to disconnect from the grid.
Finally ETSO recommends that the priority rules for the transmission of electricity from renewable sources should be re-examined to avoid discrimination against conventional electricity and against other renewable sources from other EU member states
Concentration of installed wind power, Europe, 2008. Most countries have plans that, combined, will increase total installed wind power capacity from 41 GW in 2005 to 67 GW in 2008 Base case. The ETSO study covers two wind situations with major impact on the operation and security of the European transmission network. In Scenario North maximum wind production in the northern UCTE countries is assumed, and in Scenario South, maximum The maps show the changed power flows in the North and South scenarios. Projected power flows in the North scenario. Projected power flows in the South scenario