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Variable speed variation

The first variable speed wind turbines from a Danish company have been installed in an offshore wind farm off the island of Gotland in Sweden (Windpower Monthly, September 1996). But Optimal Speed Control (OSC), the termed used by Wind World, is variable speed with a difference. The novel concept uses a power converter rated at about one-third of the turbine rating, which is used for power conversion at low wind speeds only. The turbine runs at variable speed at winds below about 8 m/s, and at higher outputs it runs at constant speed, with power fed to the grid via an induction generator.

Wind World argues that its OPC concept for stall regulated turbines overcomes four major problems: flicker caused by cut-in and cut-out operations at low wind speeds; demands for reactive power by induction generators, which can be substantial; flicker in the supply grid caused by dynamic fluctuations of the electrical power (both active and reactive components) due to fluctuations in the wind; and production of harmonics, which can produce up to 35% of the energy in a range of frequencies.

According to Wind World, generator control reduces flicker at cut-in by establishing a connection to the grid through the frequency converter as soon as the turbine is operating. The rotor is used as a dynamic energy storage reservoir and there is no more cutting in and out, only a very smooth variation of active power. The only situation where a cut-in and cut-out effect arises is at the infrequent transition between variable and fixed speed.

Consumption of reactive power is controlled by the frequency converter, which is used to produce controlled amounts of reactive power. The result is that there is no reactive power consumption with variable or fixed speed operation.

Flicker in the supply grid caused by periodical fluctuations in power depends on the characteristics of the grid. A typical grid connection for medium to large wind turbines is at 10-60 kV, where the grid impedance is mainly inductive. Thus the major part of the flicker in the grid is not produced by fluctuations in active power, but by fluctuations in reactive power. A wind turbine where the consumption of reactive power is dynamically compensated will not produce periodic fluctuations in reactive power and therefore the flicker will be reduced to a level corresponding to the active power fluctuations. This can be 10-20% of the flicker of a wind turbine without dynamic reactive power compensation.

Production of harmonics is limited, as the optimal speed frequency converter is a well filtered IGBT producing a three-phase 50 Hz (sine alternating current) for the supply grid. The amount of harmonic fluctuations is approximately 1%, with frequencies so high as to be of no importance to the grid and the power quality.

An OSC Wind World turbine, the makers argue, combines the advantages of the simple and reliable construction of the stall regulated turbine with the high power production of the pitch regulated turbine and the power quality of the full converter power turbine.

An additional feature of the wind farm, due to the weak grid in Gotland, is a demand from the utility that the wind turbines never produce more than their nominal output (500 kW). To comply with this, a dump load, with a 100 kW electric heat exchanger in the mono pile, filled up with salt water, has been installed -- on the same principle as in an immersion heater. Ashore the control computer is connected to each wind turbine through an optical fibre link. The computer measures the voltage on the grid and can send instructions to the wind turbines. When the computer registers that the grid cannot take off more current, it will connect a dumpload sub-system of 25 kW, as relief. There are four such 25 kW elements per turbine. If one is not sufficient, more sub-systems can be connected and if that is not sufficient the wind turbines can be disconnected one by one.

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