## 1 Introduction
Therefore, these converters are well-suited for the voltage stabilization of a sensitive load in the case of voltage fluctuation. For low-voltage issues, the rms value of the line voltage is dropped approximately to 80% to 20% of its rated rms values. Thus, to tackle the load voltage, a compensation voltage is added to the line voltage, as shown in Figure 2a. This can only be realized if the  
Figure 1: Conventional voltage compensation techniques employed (**a**) with and (**b**) without energy storage devices. output voltage of a DVR is in phase with the input phase voltage. Therefore, an AC-AC converter has to be operated in non-inverting mode. In the same way, during the voltage swell issue, the line or load rms voltage is increased to 120 to 180% of its rated value. In this case, the compensator's voltage is to be subtracted from the line voltage to stabilize them, as may be seen in Figure 2b. Thus, the required operating mode of the AC-AC compensator employed in DVR is inverting. Consequently, a variable bipolar voltage gain is required to tackle the issues of voltage sag and swell. The input of the AC voltage controllers present in the DVRs employed in a three-phase system may be connected in the healthy phases rather than the faulty phase. One such approach is called the interphase technique, developed in [21], which is shown in Figure 3a. The DVR connected in the faulty phase consists of two AC voltage controllers, as depicted in Figure 3b. The input of one AC voltage controller is connected to one of the non-faulty phases, while the input of the other one is taken from the other healthy phase. For example, the inputs of voltage compensator 1 connected in phase '_a_' are connected to phase '_b_' and '_c_', as can be seen in Figure 3a,b. The outputs of these two AC voltage controllers are not only out of phase with respect to each other but are also not aligned with the phase voltage of the faulty line [21]. Here, the voltage injection in the faulty phase is a challenging issue. This problem is tackled by adding the outputs of the two AC voltage controllers through the series connection of two transformers. The resulting output voltage of this setup has two components, where one component is in phase with the voltage of the faulty phase, but the other component is quadrature with respect to this phase. The second component of the injected voltage is unwanted and its value is made zero with the adjustment of the duty cycles of the controllers. Therefore, adjusting the voltage gain of the desired component with the cancelation of the quadrature component is a very challenging issue. Moreover, the use of the two series voltage injection transformers and two AC voltage controllers increases the overall size, cost, and losses. These issues may be tackled if the AC voltage controller connected in the faulty phase takes the input power from the same phase where it is connected. This approach is depicted in Figure 4, ensuring the use of a single AC-AC converter with a line frequency transformer for voltage injection. The overall arrangement lowers the size and cost of the DVR. However, with this configuration, the value of the required voltage gains for the problem of voltage sag or swell has a high depth level, normally more than one. This necessitates the development of a converter that may invert or non-invert output with voltage step-up capabilities. Figure 2: Voltage compensation during (**a**) voltage sag event and (**b**) voltage swell event. The voltage stabilization features may also be obtained with buck-boost controllers. There is a power quality issue associated with the input current as it is discontinuous. High conversion losses result from high values of the switching current and voltage. The discontinuity in the input current is tackled in [22; 23; 24] by adding more passive components and semiconductor devices. This may result in high costs and conversion losses due to the conduction of a large number of components and switching devices.