There are as many different rulebooks for connecting and integrating wind power into national electricity systems as there are countries with wind farms deployed. While in some countries transmission system operators (TSO) issue no more loose guidelines for how wind plant should be made to behave, others introduce stringent wind-specific grid codes dictating in detail what network support each single turbine must provide. The complex tangle of approaches to integration, especially irksome for wind turbine manufacturers and wind farm operators doing business in a multitude of national markets, was the main focus of Techwind grid, an international wind integration conference held recently in Madrid.
Underlining the global spread of integration rules, the conference coincided with the publication of a new wind-specific grid code in China (box). Delegates also got a glimpse of a revised, more exacting draft code for Spain and learned how operators in Germany are coping with that country's new code. "With all this going on, the conference couldn't have happened at a better time," said César Guardamino of German turbine manufacturer REpower, the conference sponsor. The event was organised by Spain's wind power association, Asociación Empresarial Eólica (AEE), and drew 70 delegates, mainly from Spain, Europe and the United States.
Many speakers alluded to Spain as an international reference for wind integration, supporting AEE's step towards "linguistic integration" by holding the event in English, said Guardamino. According to AEE, Spain is the first to nationally require wind plant operators to provide the TSO with real time monitoring and emergency override control of output for all wind capacity online. All Spanish turbines must be able to "ride though" momentary voltage dips on the network. For new turbines, the rule has been in force since January 2008. All older turbines must be retrofitted by end-2009. Previously, turbines automatically tripped offline to protect them from the electromechanical forces of such disturbances, "a luxury the system can ill afford with the clusters of hundreds of megawatts of wind capacity typical of Spain," says AEE's Alberto Ceña.
Regardless of Spain's national lead, similar requirements exist locally in some European and American states and regions. Since January, Germany requires all new wind turbines connected to the grid to ride through voltage dips. Other countries, delegates learned, go a step further than Spain in some aspects by requiring wind farms to be equipped with voltage and frequency control, such as in the UK. From Spanish wind turbine supplier Ecotècnia, Montserrat Mata demonstrated how ten different ride-through codes had more variations than similarities to one another. Wind turbines suppliers are developing ride-through controls with as many variables as possible in order to meet the differing demands, adding to the cost of the equipment.
A European working group, with AEE's participation, has been set up to promote a single European grid code for wind, reported Ceña. "But the TSOs are extremely reluctant even to collaborate in establishing a list of issues -- which is where we are now -- let alone negotiate solutions," he says. The group is led by Danish turbine supplier Vestas. Other participants include wind farm operators, turbine manufacturers and national wind associations as well as the European Wind Energy Association (EWEA).
By complying with grid codes, wind farm operators raise their stature in the conventional power sector. "They improve the risk reputation of wind power in the eyes of the TSO," said Javier Ruiz from Acciona, a globally active wind plant owner and turbine supplier. The lower perceived risk of operating a system with a variable supply from wind farms, the higher the TSO is likely to push its penetration limit for wind energy. Business opportunities in wind integration are expanding rapidly, especially in the field of voltage ride through.
Testing the theory
Demonstrating the ability of wind farms to ride through grid voltage dips requires creating an "artificial" voltage drop in the field. For this purpose, transportable "voltage dip generators" are used. In essence, these are huge 25 ton container lorries stuffed with power electronics that are plugged into a wind farm's network link. Voltage dip generator trucks, which are few and far between, trundle from wind farm to wind farm, testing whether the wind turbines not only stay linked to the grid during a momentary voltage dip, but also help the grid recover by shooting a jolt of extra power into the wires.
"There are only 20-25 voltage dip trailers in the world," says Jörge Gütschow from German company Windtest GmbH, based in Grevenbroich, south-west of Düsseldorf. "The turbine manufacturers themselves own half. The rest belong to test labs like Windtest, which has eight." At EUR 40,000-70,000 for each commissioned testing of a wind farm's voltage ride-through ability, errors in when to carry out the test and how to go about it are costly, points out Gütschow: "For a start, you need to make sure you are not sitting around for days waiting for the wind to blow." Windtest has been serving the wind industry since 1996.
Some speakers reiterated that because turbines are electromechanical machines, interference with power electronics has a mechanical impact and mechanical disruption effects the power electronics. This is especially true of torque on drive train components, particularly on gearboxes and generators, two of the most costly components for maintenance. Demand for simulating turbine response is growing fast, in part to test the impact on turbines of different grid connection demands, such as voltage dip ride-through capability or frequency and voltage control, and to help wind operators design reinforcements or redesign maintenance programs. They also help grid operators understand the impact of their own grid codes.
Delegates heard how simulation of how wind turbines work in the field is a niche market with huge growth potential. German software group DigSilent has already sewn up a large chunk of the market, selling its PowerFactory simulation software in the UK, Germany and China (box). But for newcomers, "designing simulators is not easy," warns José San Leandro Ros of Automated Computing Machinery, a Spanish firm specialising in industrial software which has just produced a simulator for the Bonus 1300 turbine.
The first step, data logging, is complicated by the reduced space available and the distance between the large number of components and devices. While the generator, gearbox and yaw mechanism are perched 100 metres above the ground, the transformer and other power electronics are located at the base of the tower. Each of those requires a variety of measurements, like temperature, vibration, rotational speed, voltage, current, frequency, and so on. The data logger, meantime, works with 46 analogue and 63 digital signals from different extremes of the turbine. "Then you have to combine all that within a user friendly interface," says Leandro.
In Spain, wind farm operators are struggling to meet the end year deadline for 100% compliance with the ride-through regulation. By the end of January, 9500 MW of Spanish wind power capacity, or about 57% of the 16,740 MW online at the time, had certified ride-through capability, said Ceña. By end-2009, he expects 13,000 MW to be in compliance. "It's all been a race against time, much tougher than anticipated and a lesson for all countries planning something similar," says Acciona's Javier Ruíz.
The incentive for wind plant operators to retrofit their turbines for ride-through capability is a EUR 3.8/MWh bonus paid for power produced by machines equipped with the necessary technology. But that bonus runs out at the end of 2012. Ceña said AEE has asked the government for a two-year extension, both on meeting the legal requirement and on the bonus. "We need to recoup costs," said Ruíz, indicating that in Germany, the five-year compensation is EUR 7/MWh for new turbines and old turbines do not have to comply with the regulation
The delays in Spain mainly involve retrofits of older variable speed turbines with double-fed, asynchronous generators, which make up about 2500 MW of Spanish wind capacity, mostly made up of Gamesa 660 kW machines from when Gamesa made Vestas turbines. From Spanish engineering firm Wind to Power Systems (W2PS), Santiago Arnaltes said the firm is finalising a retrofit solution for that technology. W2PS supplied its Coverdip patent to the 18 MW Careón wind plant, which uses Vestas-Nordtank 600 kW turbines. Back in December, Careón became the first retrofitted wind plant to receive official certification for ride-through compliance. While turbine manufacturers continue producing their own solutions, AEE says there are a handful of other independent products similar to Coverdip now appearing on the market.
Business opportunities aside, grid codes are "shifting sands," according to Ana Morales of engineering firm DigSilent. At the conference, Morales revealed details of the draft grid code proposed by Spanish TSO Red Eléctrica de España (REE). It is requesting the wind industry's comments on the proposed new rules, which if agreed on will be applicable by 2012.
The existing Spanish code requires turbines to ride through drops of 80% of nominal voltage across the point of connection of the turbine for 500 milliseconds, without absorbing reactive power (that is the power the generator would normally absorb to energize the magnetic field needed to excite the stator). "Now, REE's draft asks turbines to ride through 100% voltage dips lasting one second while providing reactive power at the same time," said Morales. Additionally, REE wants turbines to provide the grid with frequency and voltage control, not currently required in Spain. "REE's asking a lot all round," said Ceña. "We've got some tough debating ahead."
Perhaps most drastically, REE is proposing that wind turbines operate below their full capacity (and full earnings potential) to keep some "reserve power" available for the system to draw on should demand suddenly escalate or the wind start to take off unexpectedly. "From what we can tell, REE is contemplating reserve wind capacity for a few critical periods in the year and even then it seems tentative on the idea," says Morales.
Spain however, will need 40-44 GW of wind power capacity, up from 17 GW today, to help meet its share of the EU legal commitment to reaching 20% renewable energy supply by 2020, pointed out REE's Miguel de la Torre. Under Spain's energy plan, wind will supply 22.4% of an expected demand of 380,000 GWh. To maintain secure electricity supplies in Spain, wind farms will need to provide an increasing number of grid services, far beyond the requirements established for REE's 29 GW wind capacity expectation to 2016.
Improved interconnection of the Spanish grid with the rest of Europe and more hydro pumped storage capacity would help with balancing ever greater volumes of wind power. At the moment, the capacity of each is limited to 3 GW. Accurate forecasting of wind power output is another help for operating a power system with 20% wind. De la Torre called for better forward schedules of wind power output.
Although he praised Spanish wind operators for meeting an obligation to forecast and schedule their generation 24 hours ahead of delivery, he claimed that REE's own scheduling of wind, through its centralised wind forecasting model in Madrid, Sipreolico, had a more accurate track record, compared with relying on the sum of distributed forecasts by individual wind operators across the country. Delegates from the floor expressed disagreement. They insisted that distributed, high resolution forecasting is more accurate
Scheduled delivery of power deviated from the forecasts, they argued, because on the wholesale electricity market the penalty for not delivering promised power is greater than the penalty for delivering more power than promised. If delivery does not deviate more than 10% either side of forecast generation, there is no penalty. But beyond that deviation, there is more risk connected with delivering short on a schedule than delivery long. For that reason, wind farm operators will err on the side of caution when bidding into the system.
"It's hard to criticise operators for shading their forecasts in the most profitable way," said Nicholas Miller of US turbine manufacturer General Electric. "But it would be useful for high accuracy distributed forecasting to impact system balancing more positively." Ceña highlighted progressive improvements: "Five years ago, we saw some forecasters producing errors of 30% against power delivered. The same models, now fine tuned, are producing errors of just 5% over the same period," he said.
De la Torre said REE supports the government initiative to get one million electric vehicles on the road by 2014, believing that when stationary at night they can be recharged using any excess wind energy not needed by industry or domestic dwellings.
Not all delegates were convinced that electric vehicles are more than an excuse for keeping fossil fuel generating capacity online, but this view was shot down by Jose Santamarta of Reve, an electric vehicle research consortium led by AEE: "President Obama is not sceptical, Gordon Brown is not sceptical. The governments of Israel, Denmark, Austria or France are not sceptical. General Motors, Ford, Mitsubishi, Renault-Nissan and Toyota are not sceptical; that is all that matters."