Switched Inductor Quasi-Z-Source Based Back-to-Back Converter for Variable Speed Wind Turbines with PMSG

摘要

Sustainability is the main aspect that forces the renewable energy sources to be implemented for electric energy generation instead of fossil ones. Wind energy is quite attractive among other sources because of its commercial potential (72 TW) that is five times higher than world energy demand in all forms [1]. However, the installed capacity in 2009 was only 159 GW [2]. Large turbines dominate the market, but there is also a demand for small turbines in the power range up to 15 kW as the power source for micro generators. A micro generator is an electrical energy source that includes all interface units and operates in parallel with the distribution network. Some energy sources can be connected directly to the distribution network, but in the case of DC power sources or variable speed wind turbine (VSWT) systems it is necessary to use a power converter that interfaces the source and the grid. Generally, VSWT based micro generators consist of a wind turbine, a generator and an interface converter (Fig. 1). Wind turbines capture wind energy and convert it to rotational mechanical energy. Variable speed operation of the wind turbine allows extraction of higher energy from wind than constant speed systems [3]. The generator converts mechanical energy into electricity. Different types of generators can be used in wind energy conversion systems (WECS), but permanent magnet synchronous generators (PMSG) dominate the market. The main advantage of PMSG is the possibility of multipole design that offers slow speed operation and the possibility of gearless WECS construction. Another advantage is maintenance free operation since there are no brushes. The main drawback of PMSG is the dependence of its output voltage on the rotation speed. The difference between the minimum and the maximum voltage can reach four times in VSWT applications [2]. This drawback can be easily overcome with the help of an appropriate interface converter. The interface converter rectifies the input AC with variable voltage and frequency, adjusts voltage levels and inverts DC voltage into AC with grid voltage and frequency. Additionally, it should have maximum power point tracking (MPPT) functionality to extract more power from wind. Different topologies of the interfacing converter are discussed in the literature [3-8]. Basically, they can be categorized as topologies with HF isolation (Fig. 1a) [2] and [4] and those without isolation (Fig. 1b) [5-8]. This paper is devoted to the transformerless topologies, i.e. topologies without isolation. Most of the topologies without isolation have an intermediate DC link. It means that on the generator side there is a rectifier, but an inverter is placed on the grid side. Some low pass filter can be added to the inverter to fulfill standard requirements. The overall performance of an interface converter is highly affected by the properties of the combination of the generator side and grid side converter.