Germany's first grid code to include rules for connection of wind plant to electricity networks was introduced by E.On Netz, the Transmission System Operator (TSO) for much of the windy north of Germany, in August 2003. Although a similar national code has come into being, in practice it is Germany's four TSOs that make the decisions. As the TSO with about 43% of Germany's 17,000 MW of wind power on its system, E.On Netz has led the way. Annual wind generation on the E.On Netz system was 11 TWh in 2004 and total sales of electricity 244 TWh.
Under its grid code, E.on Netz demands that wind turbines connecting to the high voltage networks (110 kV and above) must be able to curtail output on request -- and as the TSO requires. The requirement sits uncomfortably with the German renewable energy law, which stipulates that all green power generation must be accepted onto the network. In practice, curtailment occurs on a voluntary basis, with wind plant accepting their output may be constrained in return for a grid connection permit.
The ability to curtail wind output "is necessary for emergency situations," according to the research agency for electricity, Forschungsgemeinschaft für Elektrische Anlagen und Stromwirtschaft in Mannheim (FGH Mannheim). It cites a major blackout in Italy in September 2003 as an example of what can happen if frequency is pushed to critical levels by feeding too much power into the wires. FGH Mannheim provides certification of wind power plant to specific grid codes.
Wind turbines in the E.on Netz area must also be able to ride-through momentary faults on the wires and provide grid support, if required, by continuing to supply reactive power for up to three seconds during a drop in voltage to assist a return to stability. It is a reversal of a long running stipulation in Germany that wind turbines disconnect from the wires in the event of network voltage disturbances so as not to add to a problem.
Wind turbines are not, "for the time being," required by E.On Netz to be part of the fleet of power plant which automatically adjust power output to maintain system frequency. Neither do they have to offer "black-start" services -- the ability to start from scratch without the assistance of electricity from the grid.
Following the E.On Netz lead, Vattenfall Europe Transmission added nine lines of text to its high voltage grid code at the start of 2004 to deal with renewable energy in Germany. About 39% of Germany's wind power is connected to Vattenfall's network, which covers the east of the country. Wind supplies about 10 TWh into the Vattenfall system. Total Vattenfall sales in Germany were about 74 TWh in 2004.
Like E.on Netz, Vattenfall demands that renewable plant largely follow the grid code for all generation. But while E.on Netz exempts renewables from certain requirements, Vattenfall allows the exemptions only "under agreement." The exemptions for which a negotiated agreement can be achieved involve rules for a generating plant's internal consumption of power; grid support if part of the network unexpectedly disconnects from the main system; black starts; and the provision of primary balancing power (spinning reserve).
Demonstrating that wind power stations can meet the requirements of the grid code is a hurdle the wind industry is learning to negotiate. Last month, Germanischer Lloyd WindEnergie in Hamburg became the second German agency offering grid code certification after FGH Mannheim. It announced it had been authorised to offer certification of wind turbine types and complete wind stations to prove they comply with a series of national grid codes. So far GL WindEnergie offers compliance certification for grid codes in Germany, Denmark, Ireland, Britain and Australia.
FGH Mannheim was accredited to provide grid code certification in December 2004. It says the grid codes in the UK and Ireland closely follow the E.on rules. FGH has so far completed certification of Enercon turbine types and like GL Windenergie expects to certify grid compatibility of turbine types built by other manufacturers in the coming months.
FGH's first fault ride-through test was successfully carried out on an Enercon 2 MW turbine in Kronprinzenkoog near Brunsbüttel in early 2004. A short circuit lasting three seconds was created artificially in a section of the network that had been isolated for the experiment so as not to interrupt electricity supply to customers.
Enercon, which prides itself on being a front-runner in network matters, says its grid connection system "meets all the new connection stipulations for wind turbines" and that turbines are available for integration in both high voltage transmission grids and distribution grids -- even though no specific rules for wind energy are yet included in Germany's grid code at the level of the distribution network. "These will probably come within the next six months," says FGH Mannheim. Until now well over half of Germany's wind capacity has been connected at the level of the distribution grid.
Tough but okay
Opinions vary on how big a challenge the German grid code presents to turbine manufacturers. Meeting the specifications is difficult, according to FGH Mannheim. "It's not the ride-through capability itself that is tricky, but rather continuing to supply reactive power during the duration of the fault. The software is demanding but most turbine companies will have it within the year." GL Windenergie agrees. It says the changes required "represent a great challenge to the electrical systems of a wind turbine, and must be supported by adequate technical expertise."
But Danish turbine builder Vestas is taking the new requirements for design and manufacture in its stride. "Fortunately for all concerned, the technology to make fault ride-through possible was already well in hand," says the company. To comply with the emerging grid codes internationally, Vestas says it had to modify its power converters and SCADA (supervisory control and data acquisition) systems and also to increase the size of its generators slightly. "The extra cost in doing this is the negative aspect of the new codes, but it applies to every wind manufacturer and the fact that we now have to play by the same rules as conventional generators shows that wind power has come of age," says Vestas technical support manager Søren Plagborg.