## 1 Introduction  
The development of the power grid toward smart-grid networks assumes their modernization in the following directions: smart metering of energy consumption and flow, decentralization of the electricity generation and storage process, and the introduction of more intelligent power grid control processes (transmission and distribution) [1]. It is expected that, in the next few years, the power grid will move from electromechanical to electronic control using power electronics. At the same time, the loads are changed to high-power loads with rapidly changing characteristics over time (e.g., electric vehicles). Moreover, energy from renewable energy sources has unstable characteristics over time. The new climate goals also assume a reduction in electricity production using fossil fuels [2]. All this may contribute to problems with the stability of the power system as well as problems related to power quality [3, 4]. Hybrid transformers (HTs) with a power electronic converter have been proposed as an efficient and economical solution to some problems caused by distributed energy sources and new types of loads in modern electric power systems. Various HT topologies with different regulatory capabilities have been proposed in the scientific literature. A few review articles have been published to explain the concept of how they work [5, 6, 7, 8, 9, 10]. The most frequently considered HT systems are topologies with a back-to-back converter [11, 12, 13, 14, 15, 16, 17], matrix converter [18, 19, 20, 21] or various types of AC/AC choppers [22, 23, 24]. HTs in most topologies are based on a conventional transformer with two pairs of three-phase output windings. Such a solution complicates the construction of a substation transformer. Existing substation transformers currently in use cannot be used in this concept. Another approach to regulating voltages in the power system or compensating for overvoltages, voltage sags or other voltage distortions is to use series switching voltage compensators [25; 26; 27; 28; 29; 30; 31; 32; 33]. One of the first concepts of such a compensator of voltage changes in the network is the topology described in the article [25], in which the concept of voltage dip compensator for supplying critical loads in electrical distribution systems is presented. In the proposed solution, the AC-AC converter with PWM type modulation cooperates with an autotransformer. During a voltage dip, the proposed compensator supplies the missing voltage and helps to keep the voltage at the rated level at the terminals of the critical load. This approach does not use any energy-storage electronic components such as capacitors or chokes, and provides a fast response to a fault using a low-cost power electronics converter. Over the years, many concepts of voltage compensators based on AC/AC converters that do not contain DC energy storage have been developed. An overview of selected topologies is presented in [26]. Interesting concepts of three-phase voltage compensators with AC/AC converters are topologies wherein the compensation voltage for a given line is generated by power electronic converters supplied from other power lines. Various topologies of AC-AC converters have been proposed: with four two-level AC chopper legs [27], with an AC chopper having two bidirectional connectors in a diode bridge and transistor configuration [28; 29], with a Cuk B2 matrix-reactance chopper [30] and with a bipolar AC-AC chopper [31; 32]. In this article, the research on the properties of the AC voltage compensator topology with bipolar AC-AC converters presented in the papers [31; 32] will be continued. In these two publications, only an outline of the concept of operation of such a compensator has been presented. In addition, the results of simulation and experimental tests operating in an open feedback loop for manually set pulse duty ratios of the control functions of individual converters have been shown. The proposed voltage compensator topology requires the use of additional input transformers.