Use of Three-Level Power Converters in Wind-Driven Permanent-Magnet Synchronous Generators with Unbalanced Loads

Ming-Hung Chen

doi:10.3390/electronics4020339

Abstract

This paper describes the design and implementation of three-level power converters for wind-driven permanent-magnet synchronous generators with unbalanced loads. To increase voltage stress and reduce current harmonics in the electrical power generated by a wind generator, a three-phase, three-level rectifier is used. Because a synchronous rotating frame is used on the AC-input side, the use of a neutral-point-clamped controller is proposed to increase the power factor to unity and reduce current harmonics. Furthermore, a novel six-leg inverter is proposed for transferring energy from the DC voltage to a three-phase, four-wire AC source with a constant voltage and a constant frequency. The power converters also contain output transformers and filters for power buffering and filtering, respectively. All three output phase voltages are fed back to control the inverter output during load variations. A digital signal processor is used as the core control device for implementing a 1.5 kV, 75 kW drive system. Experimental data show that the power factor is successfully increased to unity and the total current harmonic distortion is 3.2% on the AC-input side. The entire system can attain an efficiency of 91%, and the voltage error between the upper and lower capacitors is approximately zero. Experimental results that confirm the high performance of the proposed system are presented.

Highlights

Wind energy is a type of clean and renewable energy, unlike fossil fuel energy
The machine and electric parameters of the permanent-magnet synchronous generators (PMSGs) are presented in the Appendix
If the PMSG is operated above 75 rpm or below 50 rpm, the control system is shut down to avoid damaging the PMSG

Summary

Introduction

Wind energy is a type of clean and renewable energy, unlike fossil fuel energy. Wind is a nonpolluting and renewable source of energy that is referred to as “white coal in the sky”, and it is expected to play a crucial role in providing a sustainable power supply without harming the environment [1]. A type A configuration involves a fixed-speed controlled wind turbine with a squirrel cage induction generator (SCIG) directly connected to the grid through a transformer. Type B involves a limited variable-speed controlled wind turbine with an opti-slip induction generator (OSIG) and pitch control. By varying the rotor resistance connected in series externally, the slip and output power are controlled to make the dynamic speed range 0%–10% above synchronous speed, typically. Both types A and B require starting equipment. Type C is a variable-speed controlled wind turbine with a doubly fed induction generator (DFIG) and pitch control. The stator is directly connected to the grid, while the rotor is connected through a power converter that is typically only 25%–30% of the generator rated power

Methods

Results

Conclusion