Soft Starter

From Wind wiki

The soft-starter is a simple and cheap electrical component used in fixed-speed wind turbines during their connection to the grid. The soft-starter’s function is to reduce the in-rush current by building up the magnetic flux slowly in the generator, thereby limiting the disturbances to the grid. Without a soft-starter, the in-rush current can be several times the rated current, which can cause the generator to have a dangerously high starting torque that, in turn, causes severe voltage disturbances on the grid.

The soft starter contains two thyristors as commutation devices in each phase. They are connected antiparallel for each phase. The smooth connection of the generator to the grid, during a predefined number of gird periods, is achieved by adjusting the firing angle of the thyristors. The relationship between the firing angle and the resulting amplification of the soft-starter is highly nonlinear and is additionally a function of the power factor of the connected element. After the in-rush, the thyristors are bypassed in order to reduce the losses of the overall system.

If you switched a large wind turbine on to the grid with a normal switch (without the thyristors), the neighbors would see a brownout (because of the current required to magnetize the generator) followed by a power peak due to the generator current surging into the grid. You may see the situation in the drawing in the accompanying browser window, where you see the flickering of the lamp when you operate the switch to start the wind turbine. The same effect can possibly be seen when you switch on your computer, and the transformer in its power supply all of a sudden becomes magnetized. Another unpleasant side effect of using a "hard" switch would be to put a lot of extra wear on the gearbox, since the cut-in of the generator would work as if you all of a sudden slammed on the mechanical brake of the turbine.