Digital control of voltage and frequency of induction generator in isolated small hydro system

This paper presents a new control algorithm for voltage and frequency control (VFC) of an isolated induction generator in small hydro power system. The VFC is a combination of a voltage source converter (VSC), a dc link capacitor, a dc link chopper and an auxiliary load on the dc bus. A zigzag/star transformer is used to attenuate the neutral current and zero-sequence harmonic currents on the generator side thus reducing the kVA rating of VSC and also to adjust the dc voltage of VSC to an optimum level. It serves as a neutral current compensator by converting three-phase three-wire system to three-phase four-wire system by connecting the load neutral terminal to a zigzag transformer neutral terminal. The VFC also acts as a harmonic eliminator and a load balancer under three phase four wire nonlinear balanced/unbalanced loads. In comparison to other controllers, the proposed controller provides additional attenuation to harmonics of load currents with lower harmonic distortion, and improves the voltage and current profile which is demonstrated through test results.