Great Britain's new grid code rules for wind farms will provide greater certainty for manufacturers and wind farm operators, says the wind industry. But the jury is still out on just how the new requirements will impact wind projects and how much it will cost. Manufacturers and developers warn that extra costs will be incurred to comply with the grid code.
As more and larger wind projects line up to connect to the UK electricity system, particularly in Scotland where the network is remoter and weaker, energy regulator Ofgem has published changes to the national grid code to ensure that wind farms do not adversely impact the security and stability of supplies. The new grid code comes three years after system operators first began to push for changes to the rules to accommodate the rush to wind.
Richard Ford from the British Wind Energy Association welcomes the clarity the new code brings to the industry. "The most important thing is not the details of the grid code; it's having it settled at last so that manufacturers and developers can get on with projects, knowing what the ground rules are."
But what the rules are, exactly, depends largely on where in the UK the generator is located. The grid code applies to all "large" and "medium" power plant with a generation licence and to all plant connected to the transmission system. In England and Wales this is the network of 275 kV and 400 kV lines. Since practically all wind generators are small, unlicensed, and embedded in the distribution network, the code, therefore, need not apply. To complicate matters though, some smaller plants connected to the distribution network have an agreement with grid operator NGT which requires them to comply with certain elements of the grid code.
In Scotland, however, the transmission system starts at power lines of 132 kV and above. Moreover, the definition of "large" and "medium" also varies from Scotland to the rest of Britain. In England and Wales "large" is considered to be over 100 MW capacity, while "medium" is between 50 MW and 100 MW. Yet in southern Scotland "large" is above 30 MW and "medium" from 5 MW to 30 MW, and in the north of Scotland where the system is weakest there is no concept at all of "medium" power stations and large refers to any plant over 5 MW.
This all means that many more wind generators in Scotland than in the rest of the UK will be required to abide by the code. "It is almost exclusively a Scottish issue," says Ford. Offshore projects, however, because of their size, which is an average 90 MW in Britain, could also be required to comply with the code.
The most contentious aspect of the new rules, as far as the wind industry is concerned, is the capability demanded of wind operators to continue generating through system faults. The industry argues that the "fault ride-through" requirement is more onerous than similar grid code conditions in Germany and Denmark. In particular, the active power recovery rate demanded of wind plants is tough, says BWEA. Ofgem counters that the German and Danish power systems are part of the much larger European grid which can withstand loss of generation better than the British system.
In addition, the wind industry points out, the British code requires wind generators to withstand drops in voltage down to zero at the point of fault for 140 milliseconds, compared with, say, the Irish grid code which allows generators to trip off the system if the transmission voltage goes below 15% -- although wind turbines must remain connected at that voltage for as long as 625 milliseconds.
Ford says that providing fault ride-through capability will be challenging for manufacturers. "There may be increased costs in applying that particular bit of the grid code," he says. "Manufacturers have all said they can meet it, but the proof will be in the delivery."
Simon Cowdroy of consultants Econnect agrees. "It remains to be seen how the manufacturers will comply with fault ride through," he says. He points out that they already have to meet similar requirements in parts of Europe. But it may take time to develop control systems compatible with the new British grid code. He adds that it is the wind farm that is required to comply with many of the new requirements -- not individual wind turbines. This means that an alternative approach to ensure compliance with these requirements could be to install equipment separate from the wind turbines.
Developers, however, would appear to prefer turbines to be compatible with the new rules. "It is simpler and less risky for developers to say to manufacturers: make your turbines able to ride through faults," says Joe Duddy from development company Renewable Energy Systems. He accepts this will be potentially expensive depending on which turbine technology developers opt for. "There are cost implications, not only for providing compatible equipment, but there is also an administrative cost for us in demonstrating compliance," he explains.
From wind turbine manufacturer Vestas Celtic, Mark Powell says: "We can comply with the grid code but it adds to the overall cost to the client." He stresses that under the new rules the ability to ride through faults is only needed by larger wind farms connected at the transmission level. Moreover, different wind plants will have different fault ride-through requirements depending on their location in the system. "Every wind farm will have different fault ride-through definitions," he says. "To come up with the most cost effective solution you need the co-operation of the wind turbine manufacturer, the developer, their electrical contractor and the network operator to ascertain what the grid code limits are for that particular wind farm."
Other provisions in the new grid code require wind generators to operate at frequencies above the usual 50 Hz, respond to changes in network frequency by increasing or decreasing power output, and produce and absorb reactive power to support the grid.
The ability of wind generators to increase or decrease their output automatically in response to frequency changes on the system -- known as frequency response -- is not needed at current levels of wind penetration, notes Ofgem. It argues, however, that this will become necessary as wind projects increase in size and number. But as with reactive power, whether or not wind farms are required to provide this service will depend on their size and location in the network.
Ford believes that under Britain's renewables obligation (RO) support system, the higher cost to be paid by the system operator for requiring wind plant to provide frequency response compared with other generation will make it unlikely that wind farms will be called upon to provide the service. "Wind farm operators would want a higher recompense because they would be foregoing not only the lost income from lower energy output, but also the income from ROCs [renewables obligation certificates]," he explains. "We suspect that in the short-to-medium term it will be less economic to dispatch downwind generators compared with other generators."
The new grid code came into effect on June 1. Cowdroy claims that so far he is not aware of any projects that have had their development timetable adversely impacted by the extra requirements. "But it is going to be challenging to make sure that the wind farms that are coming forward are compliant," he says.