About 6000 MW of Germany's 18 GW of wind power is already connected to E.ON's network in just two German states, the windy northern regions of Lower Saxony and Schleswig-Holstein. By 2020, the utility expects the national wind capacity to have reached 50 GW, with a significant proportion from offshore facilities. "Modification of the existing grid code for connection and operation of wind farms in the high voltage network will be necessary," states the company's Wilhelm Winter in a yet to be published report.
Winter describes the addition of a "Short Term Interruption" amendment to the existing grid code, which requires wind turbines to stay online to support grid recovery if voltage suddenly collapses (Windpower Monthly, September 2005). Extensive monitoring of wind plant to ensure they fulfil new fault ride-through requirements is also to be introduced, according to Winter's report. Moreover, wind plant built before the existing grid code came into force in 2003 -- which are today exempted from the code -- could be subject to the new code.
In a second report, Winter proposes subjecting older wind turbines to a minimal retrofit so they too can withstand voltage dips and avoid tripping after network faults. Germany was unusual in its past requirement for wind turbines to trip offline and stay offline in times of trouble rather than stay online to supply grid support. E.ON Netz clearly regrets that short-sighted strategy today.
Meanwhile, E.ON Netz is introducing a new system to monitor transmission line temperatures and wind conditions with the aim of improving its use of transmission capacity in the medium 110 kV network in northern Schleswig-Holstein. The greater the flows of electricity, the hotter the wires become, setting the capacity limit. Careful temperature monitoring allows the wires to be used to their full capacity.
According to E.ON's Urban Keusser, improved monitoring will provide precise information on when the wires are reaching critical temperature levels enabling transmission mangers to know when the system is safe enough to accommodate more wind capacity. If the monitoring proves successful, it would reduce calls made by E.ON Netz for curtailment of wind output in windy periods of low demand. To be connected, wind plant owners have had to go along with the demands.
"We are confident the new system works technically, but we must see how big the effect will be on generation management. We have to replace a lot of components in the system," says Keusser. He declines to say when the roll out can begin for other regions where transmission capacity is limited. "That needs to be analysed. It's too early to fix a schedule."
A similar system could be adopted for the high voltage network, although Keusser notes this would be more complicated. "The high voltage 380 kV is more complicated since what we do there directly affects our neighbours across the borders," he says. "Whether we can take this further depends on results of discussions with our neighbours."