QUANTITATIVE DESIGN OF ACTIVE CONTROL FOR SELF EXCITED INDUCTION GENERATORS IN GRID ISOLATED OPERATION

The use of induction machines as electric generator with renewable energy sources is fostered by its simple connection to the power grid, and by its intrinsic safety. In grid isolated operation the induction generator requires a proper excitation. A suitable machine excitation can be achieved relying on a Current Controlled Voltage Source Inverter (CC-VSI) with a proper capacitors bank in the DC side. In this way, higher efficiency, higher control dynamic and a very robust behavior in case of transient variation of the prime mover speed is accomplished. A suitable capacitors bank is still used on the AC side, in order to reduce the ratings of the CC-VSI and to avoid pre-charging. This paper presents the quantitative design of the CC-VSI and of capacitors bank used for the power management in grid isolated operation. The aim is to achieve the minimum ratings doing to operate the CC-VSI in two quadrants. The quantitative procedure starts from the load requirements that lead to the choice of induction generator. Laboratory experimental results, obtained by an air-gap flux field oriented control, show good behaviors both in transient and in steady state conditions