## 2 Hybrid Active Power Filter Concept  
Usually, most of the power electronics loads in the distribution network such as rectifiers, thyristors, etc., are non-linear and generate unsolicited harmonics. An individual inductor is the simplest structure as a filter that is not commonly used for medium and high-power converters due to problems such as voltage reductions in the DC side of the inverter, low dynamic converter efficiency, cost increment, and low power density [20; 21; 22]. The LCL filter is an attractive solution to overcome the problems mentioned, which improves the harmonic mitigation at lower switching frequency [23]. Nowadays, the most common device used to support the load voltage is a shunt capacitor, which could also improve the reactive power. However, they suffer from harmonics existing in the power grid, which makes them explode. A few later, the passive power filters (PPFs) were developed to compensate for the non-linearity components through series or parallel connections. Their advantages include simplicity in installation, but they were bulky and expensive. Therefore, the APFs have come into the industries to overcome all the drawbacks declared above. The most important issue in operating APFs is how to control and connect them to the power grid. Since in APFs, both the DC link voltage and load current harmonics must be fully controlled, consequently, several nested control loops are usually required to be designed and implemented. In addition, their switching and conduction losses are added to the total system losses, and if an appropriate switching algorithm is not selected, it may greatly reduce the overall efficiency. Various APFs have been introduced so far, with different applications depending on how they are controlled and connected to the network [24; 25; 26; 27; 28; 29]. A new type of compensator that uses  
Figure 1: The active power filter (APF) current compensation concept. both APFs and PPFs to improve performance is called hybrid APF, which has received much more attention than the conventional ones [30]. APFs are usually divided into different categories according to their operation modes, control strategy, series and parallelism connections, etc. Figure 2 shows the various topologies of APFs that have been presented so far [28]. Figure 2a shows a four-switch APF controlled by two proportional integrator (PI) controllers to provide gate pulses of power switches in each inverter legs. Another PI controller is used to adjust the DC link voltage to make the inverter output voltage regulated. This APF is categorized in a parallel classification that can supply the non-linear required load current. A nine-switch AC/AC converter with a quasi-DC link is presented in Figure 2b to compensate the non-linear load current. This APF topology is inflexible regarding controlling the power switches. A full bridge type of conventional APF with an eight-switch four-leg converter is shown in Figure 2c, in which the separated DC link voltage into two parts facilitates the zero-current component recognition. Figure 2: Cont. Figure 2d is dedicated a back-to-back two-level voltage source converter (VSC) to compensate for the load current harmonics. The quasi DC link voltage between the load and the AC source provides a better capacitive current injection. In Figure 2e, a three-switch three-capacitor multi-level inverter (MLI) is proposed as a parallel APF. This topology is capable of compensating for the non-linear load current, but the non-inductive properties of the APF are the drawbacks of the control method. A six-switch two-leg inverter is represented in Figure 2f with a hybrid connection between the AC source and the load. Figure 2g shows a 12-switch APF with a feedback harmonic detection scheme. This prototype is equipped with two PI controllers, which make the current harmonics feasible to detect. Finally, an eight-switch dual parallel full bridge inverter is proposed in Figure 2h, which is almost conventional to the current compensation concept.