A Study on the Direct Currect Injection Controlled Statcom for Voltage Regulation of a Grid-Connected Induction Generator

As compared to the synchronous generators, the induction generators (IG) have robust construction, lower initial and run-time cost and therefore are becoming popular with the proliferation of such nonconventional energy sources as wind, micro-hydro projects, etc. While the IG is connected to the power grid, the lagging var demand of the IG is supplied by the grid utility and hence, the stator frequency is dominated by the grid frequency, irrespective of the rotor speed. This makes why it is suitable for variable speed applications such as low-head hydro and wind energy conversion systems. However, since the IG is rurally located, its terminal voltage may experience severe voltage fluctuation while there is a step change in load or rotating speed. To mitigate voltage fluctuation resulted form various loading conditions for an IG system, the utilization of the shunt capacitor is rather economic in providing the required reactive power of the IG. However, the shunt capacitor is unable to continuously regulate the reactive power and the random switching of the capacitor banks may give rise to undesirable transients in line current and voltage. In addition, when the IG is disconnected from the power grid due to fault events, the IG excited with inappropriate shunt capacitor would be subjected to over- or under-voltage. The main configuration of the proposed direct current injection static synchronous compensator (STATCOM) is composed of a voltage-sourced inverter (VSI) and a DC capacitor. Only a little electrical power is needed to sustain the capacitor voltage such that the inverter output voltage can be modulated to compensate the reactive power continuously for the IG. In this project, a novel voltage space vector modulation technique, which can follow the phase of the IG terminal voltage and direct control the STATCOM current, is proposed. In order to speed up the synchronization of the IG with the power grid after the fault event and reduce the impact of the reclosing transient on the power system, the electronic load governor (ELG) (NSC 96-2218-E-182-001) granted by National Science Council in 2007 will be integrated with this project such that the magnitude, frequency and phase of the terminal voltage will coincide with power grid before switching in the IG to the power system. Since all material of the proposed system is made in Taiwan, the superiority, such as high efficiency and low cost, of the system over the traditional IG system in the market will be expected.

Project ID:PB9907-12639
External Project ID:NSC99-2628-E182-053