# Experimental Investigation of Controlled and Uncontrolled Rectifiers for Low-Power Wind Turbines
## 3 Rectification Types
### Classification on the Basis of Half or Full AC Wave  
When the rectification is performed only for a half-AC cycle, it is said to be a half-wave rectification. Half-wave rectification is a process that allows unidirectional current flow [20]. Half-wave rectification is easily accomplished by basic PN junction diodes, which ensure that during the positive AC cycle, the anode of the PN-junction diode is at a higher voltage potential than the cathode, making the current flow from anode to cathode. On the other hand, during the negative half-cycle, the PN junction's anode is at a lower voltage potential than the cathode, so the diode blocks the negative AC cycle [22].  
Figure 3: Classification of wind energy systems.  
The rectification is called full-wave rectification if two or more semiconductor switches are connected so that the rectifier circuit makes the bi-directional AC current flow unidirectional for both positive and negative AC [14]. It is evident that full-wave rectification is more efficient, but the efficiency comes with the disadvantage of high cost. The output of the full-wave rectification is also influenced by load reactance and output filtering. The full-wave rectification can be interfaced through two types of input transformers, that is, a center-tapped transformer with two switches, to attain full-wave rectification, and a simple transformer with four switches to attain full-bridge rectification. In addition to the inherent advantage of producing a pulsating DC output in positive and negative AC cycles, the bridge rectifier topology produces almost twice the output voltage compared to the center-tap topology [18]. Therefore, the application of transformers is beneficial, as they can step the AC voltage up or down and provide galvanic isolation to enhance safety. However, transformers also bring additional losses, which may degrade the overall efficiency of the wind energy system.  
The ripple factor (RF) is one of the parameters used to benchmark the performance of different types of rectifiers [19]. All the rectifier units convert AC into pulsating DC. Thus, the ratio of the RMS value of the rectified pulsating DC to the average value of pulsating DC is called a ripple factor. The ripple factor is always less than unity, but usually, lower RF indicates smooth and ripple-free output; therefore, a low value of RF is desired [23]. Many studies have reported the theoretical and experimental ripple factors for all the rectifier topologies. For example, the ripple factor of the half-wave rectifier is 1.21, while that of the full-wave rectification is recorded as 0.482. The low-ripple performance can be further improved using three-phase rectifiers.  
Peak inverse voltage across the switches of the rectifier circuit indicates reliability [17]. As the number of switches increases, the PIV decreases because the series-connected switches share the peak inverse voltage (PIV). As a result, the half-wave switches face the highest PIV. In contrast, the center tap rectifier configuration PIV is twice that of a bridge rectifier. Therefore, the bridge rectifier configuration suits the application, which has a high DC-link voltage rating.