TITLE Method for operating a frequency converter of a generator and wind energy turbine having a generator operated according to the method BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method for operating a frequency converter of a generator and, in particular, of a generator of a wind energy turbine, in the event of a substantial grid voltage drop. Moreover, the present invention also comprises a wind energy turbine having a generator operated accordingly. Finally, the present invention also provides for the operation of a generator under normal grid conditions with the generator not being in its operating state for supplying power to the grid, and a wind energy turbine having a generator operated accordingly. Related Prior Art Electrical power in public power grids is supplied by diverse energy sources converting mechanical energy into electrical energy. The main energy sources supporting public power grids are coal power plants and nuclear power plants. Several other types of energy sources, in particular power plants of regenerative energy like solar power plants, water power plants or wind energy turbines, also contribute to support the public power grid. In the past, In the event of a significant grid voltage drop in a public power grid, it was required that in such an event wind energy turbines are shut down automatically. However, due to the increased number of wind energy turbines 1t becomes more and more Important that these turbines support the public power grid in the event of a substantial grid voltage drop. In power generators like those used for wind energy turbines, it is known to couple the rotor windings of the generator via a frequency converter to the grid. In the frequency converter the frequency of the power generated by the generator is converted to the frequency of the grid. In order to support a public power grid, not only real power but also reactive power has to be supplied. Both the real and the reactive power has to be controlled by the frequency converter in the event of normal grid conditions. Methods for controlling the reactive power under normal grid conditions are described e.g. in DE-A-100 20 635, WO-A-01/20745, WO-A-02/086314, WOA- 02/086315, and EP-A-1 222 389. From the utility it is more and more required to provide support of the grid by supplying reactive current in the event of a substantial grid voltage drop or under normal grid conditions but with the stator windings of the generator being disconnected from the grid. SUMMARY OF THE INVENTION The present invention provides a method for operating a frequency converter of a generator in particular of a wind energy turbine, in the event of a substantial grid voltage drop, wherein the frequency converter comprises a generator-side power converter, to be connected to the generator, a grid-side power converter to be connected to the voltage grid, and a DC link circuit for connecting the generator-side power converter to the grid-side power converter, the method comprising the step of controlling the amount of reactive current to be supplied to the grid by controlling the frequency converter so as to generate reactive current also in the event of a grid voltage drop. According to a first aspect of the invention, in the event of a substantial grid voltage drop, the amount of reactive current to be supplied to the grid is controlled by controlling the frequency converter accordingly so as to generate reactive current. Depending on the type of generator used, either the generator-side power converter or the grid-side power converter or both power converters of the frequency converter is/are controlled. The generatorside power converter of a frequency converter often is referred to as a AC/DC converter or rectifier, while the grid-side power converter is referred to as a DC/AC converter or inverter. In a frequency converter, the generator-side and grid-side power converters are connected by a DC link circuit comprising at least one capacitor. If a double fed or dual-powered asynchronous generator is used, in the event of a substantial .grid voltage drop, according to the invention at least one of the generator-side power converter and the grid-side power converter is controlled so as to generate reactive current necessary to support the grid. A double fed asynchronous machine is an induction generator having its stator and rotor windings driven by the grid. In case of disconnection of the stator windings from the grid, according to the invention the reactive current can.still be controlled in the event of a substantial grid voltage drop, in that the gridside power converter is controlled. In case of the generator being an asynchronous or synchronous machine, controlling the amount of reactive current to be supplied to the grid is performed by controlling the grid-side power converter so as to generate reactive current. If such a generator in the event of a substantial grid voltage drop is not in its operating state for generating power, i.e. the stator is disconnected from the grid, when e.g. the wind speed is less thari the cut-in or a minimum wind speed or exceeds the cut-off of a maximum wind speed, the amount of reactive current to be supplied to the grid is generated by controlling the grid-side power converter. Most preferably and typically, the reactive current in the frequency converter according to the invention is controlled by controlling the performance factor (cos