**1. Introduction**

From the last few decades, power sectors are considering accommodating clean and sustainable energy sources to minimize the adverse effect of non-sustainable energy. Among the sustainable energy sources, wind energy has become a globally more attractive solution for high power electricity production [1]. According to a report, the State of Alaska Legislature has committed to supplying 50% of its energy from renewable sources by 2025 [2]. Meanwhile, China has also set her target to rise supply from renewable energy up to 20% in the national energy consumption by 2030 [3]. Therefore, with the passage of time, the demand for high power generator in manufacturing and installing industries is also growing for high power production.

Wind energy conversion systems (WECSs) have been installed in 80 countries and their total expected capacity will reached 800 GW by 2021 [4,5]. Nowadays, the most commonly deployed high capacity wind energy conversion systems occurs in the form of onshore and o ffshore wind farms. Such a developing tendency in wind energy area attracts the manufacturing companies to build individual high power wind turbine generator. Therefore, from the last few years', generator designing industries are manufacturing high power generators up to 10 MW, which will become double after few years [6,7].

For the wind energy conversion system, a power electronics converter plays an important role as a machine side frond-end converter [8,9]. Generated alternating energy from the wind needs to be converted into stable direct current for appropriate use in numerous applications, such as standalone and grid-connected systems [10,11]. Therefore, generator side converters are considered the most accountable part for e fficiency, power density, control circuitry, and cost of the entire wind energy conversion system.

Among power electronics converters, there are various sorts of rectifiers that have used for generator side energy conversion, such as, passive, active, and hybrid rectifier, as reviewed in [12,13]. Among the family of passive rectifiers, diode full-bridge rectifier remained more common in the past as a front end rectifier with harmonics and power factor problems, but these issues lower the e fficiency of the whole system. Therefore, to overcome the aforementioned problems, another two-level six switch active rectifier, as shown in Figure 1a, became more popular as a front end rectifier whereas both discussed types of rectifiers were best designed for low power applications up to KVs [13]. Thus, for medium or high power applications, multilevel converter as well as parallel configuration of neutral point clamped (NPC) converter, as shown in Figure 1b, became more attractive in wind energy conversion systems (WECSs) to reduce the voltage and current stress on the semiconductor devices [14–16].

Nowadays, a three-level NPC converter as a front end rectifier is commonly used with a few hundreds of KW wind generator. While, in multi-MW wind generator, parallel arrangemen<sup>t</sup> of 3L-NPC converters, as shown in Figure 2, became a more attractive solution for reducing the current stress on power devices by equal sharing of total current in all parallel-connected converters. Otherwise, high rated semiconductor devices would be required that can bear thousands of amperes and KVolt, which are expensive and not common in markets. Besides, the parallel configuration of NPC converters also increases the complexity in aspects of a large number of active switches and control circuitry, which a ffects cost and e fficiency. Therefore, another type of active rectifier, known as the Vienna rectifier analogous to a t-type inverter, as shown in Figure 1c, has been considered as a generator side unidirectional rectifier that is based on numerous advantages, as mentioned in [17].

**Figure 1.** (**a**) Two-level converter; (**b**) Three-Level NPC converter; (**c**) Three-level Vienna rectifier.

In the aspects of power flow direction, NPC has bi-directional, whereas the Vienna rectifier has unidirectional power flow options. In the case of wind energy integration with the electric grid, occasionally voltage sag occurs because of rapid change in rotor speed, which demands stabilizing power operation. In addition, it also sometimes requires a little power for field excitation of the wind generator. Accordingly, a full-scale bi-directional power flow converter helps to deal with such problems [18–21].

A new bi-directional configuration of converter has been proposed for high power offshore WEC applications by keeping in view the above-mentioned issues for bidirectional power flow in multi-MW WECS. In this fresh topology, a single module of three-phase/three-level NPC converter connected in parallel with 'n' number of three-phase Vienna rectifiers has been designed. In this configuration of converters, NPC works as a bi-directional power flow, while Vienna rectifiers work as a unidirectional active-rectifier having a lower number of active switches and lower cost as compared to 3L-NPC [22]. The functioning of a deliberated parallel-connected converters ratifies for a higher range of wind generator applications by dropping current stress on power devices in each parallel converters [23,24]. Hence, the overall proposed hybrid configuration uses less number of switches with high power density as compared to existing parallel-connected three-level neutral point clamped converters. The major advantages of the considered system and its control scheme are:


Different configurations of machine side converters for wind energy transformation are discussed in Section 2; the proposed bi-directional hybrid converter scheme is presented in Section 3; the control scheme and working of the proposed system are discussed in Section 4; and, the MATLAB based simulation results of suggested hybrid converter are shown in Section 5. In Section 6, the DSP based experimental results are presented. Finally, in Section 7, the conclusion is discussed.

**Figure 2.** Conventional scheme of 3L-NPC converter with multi-MW wind generator.

### **2. Front End Active Rectifiers for WECS**

In this section, the most commonly used active rectifiers and number of devices used by them have been studied. Due to the inefficient and low power applications of passive rectifiers, they are not considered as a good option in WEC applications.

Therefore, active rectifiers that are based on control switches became more captivating for efficient energy conversion in WECSs [13,25]. Among the family of active rectifiers, the two-level converter was most commonly used as a front end rectifier in energy conversion progression. On the other hand, such a low power application rectifier has higher harmonics and EMI. Although, in the case of medium or high power wind generators up to 10 MW or more, it cannot endure the voltage and current stress, because of the low power handling capability of the power devices [15,24]. Hence, predefined multilevel or parallel configuration of converters was considered as a vigorous solution, as discussed in Section 1, to overcome these issues. From the last few years, a three-level back-to-back neutral point diode clamped (NPC) converters are extensively using as a front end rectifier as a machine

side converter. This type of dual-mode converter can work as an inverter as well as a rectifier mode, which mainly depends on its modulation signal. Additionally, a three-level NPC converter or its parallel configuration also handle more power with low power handling devices [26–29].

The rest of NPC circuit, there is another three-level unidirectional rectifier having less number of active switches and easy to implement as a machine side rectifier that is called a Vienna rectifier. It also has a boosting and continuous input current ability. According to the efficiency point of view, a comparative analysis of all three topologies of Vienna rectifier has been discussed in [30], which shows that three-phase Vienna with a t-type inverter shape with six control switches, as shown in Figure 1c, has more efficiency than all other topologies. This simplest three-phase power circuit shows less number of power switches, easy to control, and cost-effective circuit. It has several advantages, such as high efficiency, operation at high power factor, and being reliable to implement with high power density [22,30].

Table 1 mentions the number of devices used by all discussed circuits.


**Table 1.** Comparison of the number of devices used.

Based on a comparative study of devices used by discussed converters, as mentioned in Table 1 and aforementioned problems of converters, two parallel combinations of 3L-NPC converters use almost twice the number of power devices as compared to two parallel-connected Vienna rectifiers. Therefore, a parallel configuration of two 3L-NPC will make the circuit complex, expensive, and with low power density. Thus, a new hybrid bi-directional converter for high power applications with high power density and cost-effective is recommended, as shown in the next section.
