Rulemaking for the interface of wind power stations with electricity networks is all the vogue. Four countries, Spain, Britain, Ireland and the United States, have recently added, or are about to add, specific sections on wind power to their main grid codes. Denmark's transmission system operators (TSOs) introduced their rules for wind nine months ago and Germany's TSOs published theirs in 2003. The reason for the sudden rush to impose technical requirements on wind power is down to its success.
First, wind energy is now meeting significant percentages of demand on many networks and so has the potential to cause operational problems if the technology fails to treat its host with respect. Second, larger wind farms are these days connecting directly into high voltage transmission networks where stringent conditions already exist for thermal plant. The boundary between low and high voltage wires is system dependent; it can be as low as 110 kV, or as high as 275 kV.
In general, wind power stations are expected to follow the grid code for all generation, with some specific requirements or exemptions built-in for wind's particular characteristics. These revolve around five major issues: the ability of wind turbines to: "ride through" faults on the grid; increase or decrease power output at the request of the TSO; provide reactive power to support grid stability; automatically adjust power output in response to frequency changes on the network; and restart a power system from scratch after severe blackouts, known as a black start.
The grid codes have no choice but to accept that wind has some limitations imposed by the availability of the resource. A wind turbine cannot guarantee that at all times it can increase power output in response to falling frequency; the wind might not be blowing. Neither is it certain that a wind station can kick a whole power system into action from a black start. In both cases, other generating technologies are better suited to meeting the requirements. Otherwise, wind plant are increasingly expected to toe the line and contribute to keeping the grid stable and ensuring secure supplies to customers.
To trip or not
What the grid codes all share in common are relatively new requirements for wind turbines to contribute to grid stability by "riding through" momentary network faults while at the same time providing grid support to keep power flowing smoothly. Where the codes differ is in the specifics of how big a fault a wind station is expected to cope with and the speed with which grid support is provided.
The fault ride through capability of wind turbines has been debated at some length and is still the subject of controversy. TSOs are concerned that numbers of wind turbines might shut down following even a split second disturbance on the system. In an area with a large concentration of wind turbines, such as the north-west of Denmark or Germany, the consequent loss of generation could lead to a fall in voltage and/or frequency elsewhere on the system, leading to further wind turbine shutdowns and a possible "cascade" effect that could be serious. Most system faults, however, are cleared in a split second. TSOs are now insisting that during that brief period, wind plant stay connected, even though the system voltage may have collapsed. A typical requirement is that loss of voltage for up to one-sixth of a second does not cause a wind turbine to disconnect.
Several wind turbine manufacturers are confident their technology can already meet this requirement. Others are asking for time to make their machines compliant. Testing whether a wind turbine can stay connected during a momentary loss of voltage is difficult, but type certification is now being offered to manufacturers which include real-life tests on isolated sections of a network (page 54)
Responding on demand
Another issue related to stability has led to rules that require generating plant to continue operating when the system frequency deviates markedly (more than 5%, say) from its nominal value (50 Hz in Europe, 60 Hz in the United States). The requirement is fairly easy to meet, involving adjustments to appropriate sensors on the wind turbines, but there is a further related issue that is more contentious.
When system frequency is above or below the nominal value, it means there is too much or too little generation. TSOs like to have plant available and ready to respond to frequency change by reducing or increasing their output automatically as soon as sensors register a need. Most systems work satisfactorily with these automatic controls on thermal plant and TSO's only require about 2-3% of their entire generating fleet to stand-by to offer extra frequency response should it be needed.
Stepping up generation is difficult -- and at times an impossible requirement for wind plant to meet. The wind cannot be boosted in the same way that steam supplies can be increased. The technical solution, just as for steam plant, is for some wind plant to be operated at less than optimum output, so that adjustments to blade pitch can be made to boost generation when network frequency falls.
The economics of such a requirement are unlikely to stack up, however. While it seems increasingly likely that wind turbines can operate to provide frequency response, the financial consequences of operating at reduced output are likely to be more severe for wind plant than for coal or gas plant. If a market is functioning properly, incentives will exist to make sure the technology best able to supply the service at least cost is the one doing the job. Yet another strand in the discussion is that wind plant will add to the requirements for frequency response plant and so it is only right that it should make a contribution, or be charged for the service, others are paid to provide.
Many of the same arguments apply to wind power's ability to contribute to the reserves of generation that all TSOs require. Spinning reserve is provided by plant that are ready to adjust their output up or down in response to instructions. Market mechanisms are more highly developed to meet the need for spinning reserve than for frequency response, with the result that the forces of supply and demand are naturally selecting the generating technologies that can provide the cheapest reserve. It is unlikely wind plant will be among them.
All power systems need reactive power to function and many systems, particularly in remote areas, suffer from shortages. Grid codes have long demanded that generating plant provide reactive power to support the network. For synchronous generators, which rotate at a speed fixed by the system frequency and are used with most thermal power plant, supplying reactive power is not a problem. But for the induction generators used on many wind turbines it is. Induction generators are favoured because they allow a generator to rotate at different speeds and thus cushion the changes in wind force. Their downside is that they draw reactive power from the system, exacerbating any shortages in the process.
To deal with the difficulty, the updated grid codes demand that all generating plant must be able to provide or absorb reactive power. It is not a new hurdle for wind power and ways around the problem exist. In the early days, wind turbines restricted their reactive power needs using capacitors. Later, more sophisticated systems enabled reactive power to be provided. Most recently, the newest types of mostly variable speed generator, such as the double-fed induction type, allow for very precise control of the reactive power.
One requirement that it would seem reasonable to exempt wind power from is black start capability -- the ability to start up without drawing power from the network. After a major disturbance on a power system, sections of the network can become isolated. Restoration of power supplies necessitates that some generating plant can start from scratch. Once voltage and frequency on the "island" are consistent with parameters on the rest of the network, it can join the "mainland" again.
Although it may be theoretically possible for wind turbines to provide this service, there would need to be other thermal plant on the "island" to provide the necessary stable supplies of power. For this reason, there seems little point in requiring wind turbines to have black start capability. Neither is it likely to be a requirement in a properly functioning market, which would naturally give the black start role to the generating technology able to provide it at least cost.
Slow and easy
A requirement that is becoming increasingly common in grid codes is restriction of generation "ramping rates," the allowable rate of change in power output. The restriction can be on megawatt change per minute, per ten minutes or per hour. Typical values are 30 MW per minute, 100 MW per every ten minutes and 600 MW per hour. Restrictions on ramping rates vary considerably between the codes. Some, such as the British code, do not mention them at all, others set out criteria that are influenced by the size of the wind farm and the size of the network.
To a degree, standardisation seems to be emerging. The TSO for northern Germany, E.ON Netz, and Ireland's national grid operator limit the allowable change in production within 10 minutes to the rated output of the wind station. When the wind speed is increasing, the limit presents no technical difficulties, but may mean some modification to shut down routines in high winds. Broadly speaking, most of ramping rate requirements do not seem unduly onerous, although they may lead to some small curtailment of energy production.
Constraints on wind
The most sensitive issue to have arisen out of the rash of grid code activity is that of constraining output from wind plant. In markets where by law all wind power has to be accepted onto the network, such as Denmark, Germany, Ireland and Spain, TSOs have insisted on a clause that requires wind plant to be able to constrain (curtail) their output on request. In liberalised markets such as Britain and the United States, which operate to the rules of supply and demand, the grid codes make no specific mention of output constraint and wind power, leaving it to be dealt with in power purchase contracts.
All generation is subject to the provision that their plant may be "constrained off" at certain times, due to transmission bottlenecks or when security or safety of supply is threatened. But the special emphasis place on constraining wind power in markets where the TSO is legally obliged to accept it casts an unwelcome shadow on the wind business.
For investors and financiers, the spectre of TSOs being at liberty to curtail wind power output -- and hence earnings -- is a worrying one, particularly in markets where the "unbundling" of utility businesses into separate entities for grid operation and generation may have been achieved in theory, but hardly in practice. In Germany, TSO E.ON Netz is part of an empire that includes a generation division in direct competition with independent wind power producers. Yet E.ON Netz freely admits that it frequently curtails wind power output to secure grid stability (Windpower Monthly, July 2005).
Furthermore, since only pitch regulated wind turbines, or those able to feather their blades, can exercise control over power output, the grid codes in mainland Europe effectively ban sales of stall regulated turbines (with fixed pitch). On a site with good interconnection and sufficient customer demand, however, curtailment could be unnecessary and the use of simpler stall regulation preferred. At one time, stall regulated machines made up the bulk of the world's wind power capacity. Although they have been going out of fashion, they are still found in company product lines (page 48).
There is one circumstance, which applies exclusively to wind, when constraint may be necessary. If wind generation is high, but system demand is low, the TSO may have to call up excessive amounts of spinning reserve, at a cost, to maintain stable operations. The more economic approach could be achieved by curtailing some wind output. In practice, low system demands are unlikely to occur at the same time as high wind generation. This is particularly the case in northern Europe, where only very rarely will it be necessary to constrain wind power to meet the overall aim of secure supplies of electricity at least cost.