# Experimental Investigation of Controlled and Uncontrolled Rectifiers for Low-Power Wind Turbines
## 2 Overview of Small Wind Turbines  
The small-scale WE systems are usually rated under 50 kW and have a hub height of less than 20 m, and offer alternate clean energy to domestic low-power consumers [10]. The small wind energy (SWE) systems couple the mechanical and electrical units. The mechanical unit consists of turbine blades, mechanical assembly, and a gearbox to drive the alternator shaft [11,12]. In comparison, the electrical system consists of a turbine gener  
Figure 1: Electricity generated by wind energy in 2021.  
ator, a rectification unit, and a converter or inverter. An electrical generator is the core of wind energy systems. Numerous generator types exist, including permanent magnet synchronous generators, self or separately-excited induction generators, multiple pole generators, and many more [11; 13]. In addition, an auxiliary excitation power supply is needed for self/separately excited generators. The research in modern alternators for WE systems is focused on designing the generators without additional excitation circuits; therefore, multiple-pole permanent magnet synchronous generators are preferred [14]. Further, by reducing or omitting the gearbox assembly, the losses in the mechanical coupling between the rotor and the generator shaft can be significantly reduced. However, the mechanical power at higher wind speeds or at the system's resonance may destabilize the rotor-generator assembly [9; 13]. Therefore, feedback control techniques to stabilize the mechanical assembly are often a part of the design process in WE systems [12]. For example, furling control or soft-stall control are renowned techniques to limit the rotational frequency and control the mechanical power of a small wind turbine. Other advanced control techniques include passive blade pitching or electronic feed-back controllers are also popular in high-performance small wind energy systems. In addition, the rotor's efficiency can also be improved with effective yaw control [9; 13]. The most straightforward yaw control can be implemented by hinging the tail vane of the rotor's assembly. During high winds, the hinged tail vane rotates the whole assembly, thus reducing the aerodynamic power to remain stable in high winds. However, although the furling technique can achieve power regulation and over-speed protection, the output AC power curve may have some spikes [6]. The power spikes and furling hysteresis may degrade the performance because the wind speed has to drop considerably to unfurl and resume its regular operation. Therefore, an adaptive furling approach is adopted to tackle this problem, where the furling is done electrically in small steps. Additionally, a damper may be required to avoid the chattering when engaging and disengaging the furling action. In contrast to furling, a soft-stall control offers better control in high wins but at a higher cost because an additional DC-DC converter is required between the rectifier and the load [11]. The rectifier and the DC-DC converter are designed to work over a wide range of turbine rpm. Some studies also present hybrid furling and soft-stall control to achieve even better controllability in highly windy conditions [15]. The variable speed wind turbine is coupled to a synchronous/asynchronous generator, followed by a two-stage full power conversion is shown in Figure 2[8; 16]. The generator unit produces variable AC power proportional to the stochastic wind speed. The rotor speed can be directly measured from the generated AC voltage since the stochastic wind speed produces variable AC amplitude and frequency [7; 10]. Therefore, the dynamics of the mechanical parts should be decoupled from the electrical parts. Next, the obtained variable voltage, variable frequency AC, is converted into a DC equivalent to feed a DC load or maintain the DC link voltage for a second-stage converter or inverter. An uncontrolled diode-bridge circuit is the most common rectifier topology for an SWE system [12; 13].