VARIABLE SPEED VERSUS FIXED SPEED

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Initially, most wind turbines operated at fixed speed when producing power. In a start-up sequence the rotor may be parked (held stopped), and on release of the brakes would be accelerated by the wind until the required fixed speed was reached. At this point, a connection to the electricity grid would be made and then the grid (through the generator) would hold the speed constant. When the wind speed increased beyond the level at which rated power was generated, power would be regulated in either of the ways previously described, by stall or by pitching the blades.

Wind generator

Subsequently, variable speed operation was introduced. This allowed the rotor and wind speed to be matched, and the rotor could thereby maintain the best flow geometry for maximum efficiency. The rotor could be connected to the grid at low speeds in very light winds and would speed up in proportion to wind speed. As rated power was approached, and certainly after rated power was being produced, the rotor would revert to nearly constant speed operation, with the blades being pitched as necessary to regulate power. The important differences between variable speed operation, as employed in modern large wind turbines and the older conventional fixed speed operation are:

· Variable speed in operation below rated power can enable increased energy capture; and
· Variable speed capability above rated power (even over quite a small speed range) can substantially relieve loads, ease pitch system duty and much reduce output power variability.

The design issues of pitch versus stall and degree of rotor speed variation are evidently connected.

In the 1980s, the classic Danish, three-bladed, fixed speed, stall-regulated design was predominant. Aerodynamicists outside the wind industry (such as for helicopters and gas turbine) were shocked by the idea of using stall. Yet, because of the progressive way in which stall occurs over the wind turbine rotor, it proved to be a thoroughly viable way of operating a wind turbine. It is one of the unique aspects of wind technology.
Active pitch control is the term used to describe the control system in which the blades pitch along their axis like a propeller blade. Superficially, this approach seemed to offer better control than stall regulation, but it emerged through experience that pitch control of a fixed speed wind turbine at operational wind speeds that are a lot higher than the rated wind speed (minimum steady wind speed at which the turbine can produce its rated output power) could be quite problematic. The reasons are complex, but in turbulent (constantly changing) wind conditions it is demanding to keep adjusting pitch to the most appropriate angle and under high loads, and excessive power variations can result whenever the control system is ‘caught out’ with the blades in the wrong position.
In view of such difficulties, which were most acute in high operational wind speeds (of say 15-25 m/s), pitch control in conjunction with a rigidly fixed speed became regarded as a ‘challenging’ combination. Vestas initially solved this challenge by introducing OptiSlip (which allows a certain degree of variable speed using pitch control in power limiting operation, which allows about 10 speed variation using a high slip induction generator). Suzlon presently use a similar technology, Flexslip, with a maximum slip of 17 per cent. Speed variation helps to regulate power and reduces demand for rapid pitch action.
Variable speed has some attractions, but also brings cost and reliability concerns. It was seen as a way of the future, with expected cost reduction and performance improvements in variable speed drive technology. To some extent this has been realised. However, there was never a clear case for variable speed on economic grounds, with small energy gains being offset by extra costs and also additional losses in the variable speed drive. The current drive towards variable speed in new large wind turbines relates to greater operational flexibility and concerns about power quality of traditional stall regulated wind turbines. Two-speed systems emerged during the 1980s and 1990s as a compromise, improving the energy capture and noise emission characteristics of stall regulated wind turbines. The stall-regulated design remains viable, but variable speed technology offers better output power quality to the grid and this is now driving the design route of the largest machines. Some experiments are under way with the combination of variable speed and stall regulation, although variable speed combines naturally with pitch regulation. For reasons related to the methods of power control, an electrical variable speed system allows pitch control to be effective and not over-active

Wind turbine

Another significant impetus to the application of pitch control, and specifically pitch control with independent pitching of each blade, is the acceptance by certification authorities that this allows the rotor to be considered as having two independent braking systems acting on the low speed shaft. Hence, only a parking brake is required for the overall safety of the machine.
Pitch control entered wind turbine technology primarily as a means of power regulation, which avoided stall when stall, from the experience of industries outside wind technology, was seen as problematic if not disastrous. However, in combination with variable speed and advanced control strategies, stall offers unique capabilities to limit loads and fatigue in the wind turbine system and is almost universally employed in new large wind turbine designs. The load-limiting capability of the pitch system improves the power to weight ratio of the wind turbine system and compensates effectively for the additional cost and reliability issues involved with pitch systems.