A Review on AC Voltage Variation Compensators in Low Voltage Distribution Network

Abstract

Power quality is one of the most important aspects of the power system, especially in the distribution system. This system can be affected by such disturbances as short-term voltage sags/swells, long-term voltage increases/decreases over the normative limits, harmonic distortion, interruptions, etc. Maintaining the electricity quality parameters in accordance with the applicable standards in the power distribution system will require the use of additional devices. The scientific literature on the subject indicates a very wide range of different topologies of AC voltage compensators. In this paper, different power electronics-based AC voltage compensator topologies are reviewed that are typically used with distribution energy systems. A summary of the properties and functionalities of the selected topologies has been presented. This review focuses mainly on the different topologies of the Unified Power Quality Controllers (UPQC) and Hybrid Transformers (HT). This review is divided into several sections. The first section presents the topologies used in UPQC and HT. The second section discusses selected control methods for these systems. The third section presents the basic properties of the systems related to compensating changes in the supply voltage and improvement of power quality. The fourth section presents the use of UPQC and HT systems. Finally, a summary and conclusions are presented.

1. Introduction

Reliability and quality are two of the most important aspects of the power system. Reliability of the power distribution system means ensuring continuity of power supply without any interruptions. The concept of power quality focuses mainly on maintaining a sinusoidal voltage at a frequency of 50 or 60 Hz [1]. From the customer’s perspective, a power quality problem is defined as any power problem manifested by voltage, current, or frequency variations that result in power failure or equipment failure [2].

The causes of power quality problems are complex and hard to trace. They most often occur within the energy distribution system due to various types of receivers connected to it, which are often non-linear and variable (Figure 1). In the medium and low voltage distribution network, the connection of Distributed Generation (DG) on a large scale has led to some negative effects on the permeability of this system, and it introduces the greatest disturbances. Unstable energy sources, such as photovoltaic (PV) and wind energy, are characterized by volatility and randomness, which affects the amount of power produced, is not always at the same level, and can cause, for example, overvoltage or undervoltage. In lines with a high saturation of prosumer renewable energy source installations, especially the PV, the simultaneous generation from different sources causes the local voltage level to increase above the level allowed by standards. On the other hand, in some lines, the reverse test is performed. Excessive loading of radial lines with long lengths can cause the voltage level to drop below the allowable standards. Inverters cooperating with DG produce harmonic distortion, thus affecting the reliability and quality of the power supply [3]. The non-linearity of the loads also causes the presence of harmonics in the system, which leads to overheating of the electrical equipment and then, to voltage fluctuations. Other causes are random, e.g., lightning. Many receivers connected to the network, especially those sensitive to the power quality, may react unfavorably and operate incorrectly, which may even lead to the destruction of these devices or serious faults [4]. In addition to undesirable effects related to the power quality (voltage sags/swells, asymmetry, harmonics, etc.), large financial losses are also generated. It is associated with, e.g., interruption of the production process, equipment damage, loss of data, waste of raw material, etc. [4,5].

Many review articles [1,2,3,4,6,7,8,9,10,11,12] have been written on the subject of power quality problems and how to eliminate or mitigate them. Therefore, various devices are used in the distribution power system. Hybrid transformer topologies were defined much later, so they were not described in review articles on power conditioners. Several review articles on HT have been published, including [10,11,12].

Table 1. List and comparison of review articles.

References	Description	Differences from This Article
[4]	Key technological issues for the construction of smart grids were presented with a special discussion of the most important elements of a smart grid, such as uninterruptible power supply (UPS), adaptive power compensator (AVC), static synchronous compensator (STATCOM), active power filter (APF), unified power quality conditioner (UPQC).	Other compensator topologies are discussed. Discussion of the topology as an element of the smart-grid system. There is no description of the control method in the context of compensation of distribution network voltage changes.
[6]	The topology and properties of the UPQC were reviewed in the context of improving the quality of electricity in the distribution network. Different configurations of the UPQC system for single-phase (two-wire) and three-phase (three-wire and four-wire) networks are shown.	This article mainly focuses on presenting various UPQC topologies, without detailing the control strategy. Only a description of the various system studies is presented.
[7]	This article mainly focuses on the presentation of various UPQC topologies and the details of the control strategy. In addition, it briefly summarizes the description of other works on UPQC systems.	Similar topics without taking into account the HT configuration.
[9]	A very short and general overview of energy quality improvement devices. Topology nomenclature and basic properties are defined.	Much shorter descriptions and also description of other topologies without showing the HT topology.
[10,11]	The TH concept and various possible configurations of the electromagnetic transformer are described. In addition, selected topologies of power electronic converters used in HT are listed.	Common part of this article describes the TH concept and various possible configurations of the electromagnetic transformer. This article does not discuss the converter topology in detail. The focus was only on the use of HT to compensate for voltage changes.
[12]	A very wide review of power electronic converters used in HT and their properties. Description of voltage distortion compensation methods and description of possible application places in the power system.	A more detailed description of the converter topology used in HT. AC/AC topology without DC energy storage is not included.

summarizes a short comparison of selected review articles on UPQC and HT systems with the subject of this article in order to highlight the differences between the articles.

1.1. Classification of Problems Related to the Power Quality

Power quality problems can be classified as shown in Figure 2 [4,6]. Figure 3 shows some of the variations that may occur in the voltage waveform [8].

Table 2. Description, cases, and consequences of chosen power quality problems [4,7].

Type of	Description	Causes	Consequences
Voltage sag/dip	Root Mean Square (RMS) voltage drop from 10–90% of nominal RMS voltage value lasting from a half cycle of a power frequency to one minute	Large loads (e.g., large motors), improper installation of equipment by the customer, short circuits in the installation	Failure of IT equipment and electromechanical relays, equipment malfunction, or damage
Voltage swell	RMS voltage escalation from 110–180% of nominal RMS voltage value, lasting from half cycle to one minute	Capacitor switching, start/stop heavy loads, source voltage variation, badly adjusted distribution transformers	Loss of data, increase in flickering of lights and TV screens, equipment damage
Very short interruptions	Total disturbance or power interruption lasting from a few milliseconds to one or two seconds	Insulation failure, lightning strike, opening and re-closing of power protection devices	Triggering of protection devices in the power system, stopping the operation of sensitive loads such as data processing instruments, PCs, PLCs, etc.
Long interruptions	Total power outage lasting more than 2 s	Thunderstorms or trees, that hit a transmission or distribution line or electric poles; fire in the distribution network or substations, failure of the protection devices	Suspension of operation of all devices connected to the distribution network
Voltage spike	Voltage value changes lasting from a few microseconds to a few milliseconds. The voltage value can jump up to thousands of volts	Lightning strike, disconnection of heavy loads, switching of power factor correction capacitors	Destruction of electrical equipment and insulation, loss of data, electromagnetic interference
Harmonic distortion	Non-sinusoidal voltage or current waveforms	Non-linear receivers, power electronic converters, arc furnaces, welding machines	Probability of resonance, overheating of lines and devices, interference in adjacent communication systems, errors in the signal measurements
Voltage fluctuations and flicker	Cyclical or random change or oscillation of the voltage waveform near its nominal value	Arc furnaces, repeated starts and stops of electric motors, oscillating electric loads	Flickering lights and TV screens
Overvoltage	Rise of 10–20% in the nominal RMS voltage value for more than one minute	Switching off heavy loads, energized capacitor bank	Damage household appliances
Undervoltage	Dip of 10–20% in the nominal RMS voltage value for more than one minute	De-energized capacitor bank, switching on loads	Increased loss and heating
Noise	High frequency signals on the frequency signal of the power system	Electromagnetic interference, improper grounding	Interference with sensitive electronic equipment
Voltage imbalance	In the three-phase system, the voltage values or phase angles between the three phases are not equal	Large single-phase loads, uneven distribution of loads over the three phases	Negative sequence voltage and current appears, which is detrimental to all three-phase loads

briefly discusses selected problems related to power quality. A short description of the causes and consequences of these disturbances is presented. It appears that about 92% of all undesirable events in the power system (or grid/distribution system) are voltage sags of 40–50% of the nominal RMS value, lasting from 2 to 30 periods [5].

1.2. Disturbance Mitigation in the Distribution Network

In order to increase both the reliability and the quality of energy supplied to consumers, static voltage regulators can be used in the energy distribution system, in which the tap settings on transformers are changed [13], or alongside the dynamic compensators using power electronic devices. Some of the voltage compensation devices are listed in Figure 4.

The concept of Custom Power (CP) was introduced by N. G. Hingorani [14]. This term refers to the use of electronic power controllers in a distribution system, which is mostly known as Custom Power Device (CPD) [1]. The CPDs are most commonly used in the power distribution system to ensure the proper power quality in accordance with the requirements of sensitive loads [6]. They work in unbalanced sinusoidal conditions [1]. The CPDs are divided into network reconfiguration devices and compensation devices. Compensation devices are used for active filtering, load balancing, power factor correction, and voltage regulation [1,9]. Compensation devices include Distribution STATCOM (DSTATCOM), Dynamic Voltage Restorer (DVR), and Unified Power Quality Conditioner (UPQC). The UPQC is typically used in the AC network [5].

DSTATCOM is a shunt connected to the power supply by a coupling transformer (CT) device. It is a static compensator based on a voltage source inverter (VSI) used to maintain voltage sags at the required level by supplying or receiving reactive power in the distribution system. The DVR is a static series compensator designed to inject a voltage that is dynamically controlled, in terms of amplitude and phase, into the distribution line through the CT to correct the load voltage [6,9]. The UPQC system is a combination of the two systems listed above. It is used in the power distribution system to perform shunt and series compensation simultaneously. It uses two VSIs that are connected to the DC energy capacitor. One of these two VSIs is series connected with the AC line while the other is shunt connected with the same line [1].

Hybrid transformer (HT) has been proposed as an efficient and economical solution to some problems caused by DG and new types of loads in modern power systems. The HT is a power converter and a conventional Low-Frequency Transformer (LFT) in one device. Different HT topologies with various control methods have been proposed in the scientific literature [12]. The most common configuration consists of a shunt converter powered by an Auxiliary Winding (AW) of the LFT and a series converter connected to the low-voltage network (Figure 5). This topology provides continuous load voltage regulation, DC link control, and power factor correction. The CT is not required in this configuration. The presence of the DC link allows for the integration of pure DC applications (PV, DC microgrids) with LFTs [15].

1.3. Article Contribution and Structure

In the above description, problems with the quality of electricity and selected ways of counteracting some problems have been indicated. The main contribution of this article is to present an overview of selected technical solutions of power electronic compensators of voltage changes in the AC power distribution grid. Among the many topologies described in the scientific literature, this article focuses on the properties of two groups of devices, the Unified Power Quality Controllers and Hybrid Transformers.

The motivation to write this article stems from the fact that hybrid transformer systems are often unambiguously called UPQC systems by some scientists. This article compares these two groups of systems in the context of the AC voltage change compensation service in the AC distribution network. The summary of the basic parameters and functionalities of the selected topologies is a significant contribution of this article. The presented lists of properties can be used as a knowledge base for possible implementation or implementation works of the analyzed compensators of AC voltage changes.

The first part of this article presents selected topologies of UPQC and HT solutions. The second section discusses selected control methods for these systems. The third section presents the basic properties of the systems related to compensating changes in the grid voltage and improvement of power quality. The fourth part presents the use of parameters and properties of selected UPQC and HT systems. Finally, a summary and conclusions are presented.

2. Topologies of the UPQC and the HT

This section provides an overview of selected system topologies used for compensating voltage variations and improving the power quality in the low-voltage distribution system. The overview includes the UPQC and the HT topologies.

2.1. UPQC

The two main components of the UPQC are series Active Power Filter (APF) and shunt APF. The general UPQC topology is shown in Figure 6. The other elements of this configuration are as follows [7]:

• DC link—a capacitor or an inductor depending on the UPQC converter topology;
• LC filter—a low-pass filter that reduces high system switching ripple caused by the series APF;
• series inductor—it is a high-pass filter that reduces waves during switching modes;
• injection transformer—supplies the series APF.

Figure 6. Basic topology of the UPQC, also named in the literature as UPQC-R.

Figure 6. Basic topology of the UPQC, also named in the literature as UPQC-R.

The series APF is focused on voltage compensation and regulation. It also ensures a separation of harmonics between the transmission and distribution systems [6,7,8,9]. The shunt APF controls the voltage of the DC link between the two APFs and compensates for the reactive power and harmonics of the load current in the power system [16].

The UPQC classification according to the type of converter topology, power system, system configuration, and voltage compensation is presented in Figure 7 [6,7,8,9].

2.1.1. Power Converter Topology

There are two topologies of power electronic converters that are used in the investigated UPQC systems, the VSI and the current source inverter (CSI). The VSI is used more often because it can be extended to a multi-level converter to improve efficiency [6,7,8,9]. The topology with VSI is shown in Figure 6. An inductance in the DC link instead of a capacitor is the difference between VSI and CSI (Figure 8). Both topologies are PWM-controlled.

2.1.2. Power Supply System

Due to the power supply system, the UPQC can be divided into single-phase (including two-wire (1P2W) (Figure 9a) and three-phase systems (including three-wire (3P3W) (Figure 9b) and four-wire (3P4W)). In the single-phase system, the main problems are the harmonics of the load current and the reactive current of the load. In the 3P3W system, there is also a current asymmetry and current in the neutral conductor. Both single-phase and three-phase systems can experience sags, swells, harmonics, and more. The voltage unbalance compensation is required in the three-phase system but is not required in the single-phase system [6].

2.1.3. System Configuration

There are many types of the UPQC, which can be divided according to the system configuration as follows;

• UPQC Right Shunt (UPQC-R) and UPQC Left shunt (UPQC-L)

The UPQC-R topology is shown in Figure 6 and is the most commonly used. In this configuration, the shunt APF minimizes current harmonics. The UPQC-L scheme is shown in Figure 10. It is used, e.g., to minimize difficulties between a passive filter and the shunt APF [8].

b.

UPQC Interline (UPQC-I)

The shunt APF and the series APF are used in the middle of the distribution feeders in the UPQC-I topology (Figure 11). The series APF is connected to one feeder, and the shunt APF is connected to the other. The UPQC-I can control the flow of active power between two feeders [6]. This is described in detail in [17].

c.

UPQC Multiconverter (UPQC-MC)

The UPQC-MC is used in multifeeder systems for continuous voltage and current correction. One shunt APF is coupled with two or more series APFs (Figure 12). DC link connects all the APFs, allowing for the power to be sent from one feeder to another to accommodate power quality issues [6,18,19]. The series APFs I and II compensate for the source voltage problems of the respective feeders (v_s1 or v_s2), while the shunt APF compensates for the load of current imperfections (reactive power, harmonics) of the main feeder 1. In case of a power failure of feeder 1, the shunt APF II compensates the source voltage by transferring the power from path 2 to path 1 through the series APF II [19].

d.

UPQC Modular (UPQC-MD)

The UPQC-MD topology is shown in Figure 13 [8]. It is obtained by connecting several H-bridges in a cascade for each phase [20].

e.

UPQC Multilevel (UPQC-ML)

In the multilevel UPQC, both series APFs and shunt APFs are multilevel, e.g., fifth-level, as shown in Figure 14. These multilevel APFs are connected by a common capacitor in the DC link [21].

f.

UPQC Distributed (UPQC-D)

The UPQC-D topology consists of the single series APF unit and many shunt APF units with the common DC link, which can form distributed units (Figure 15). The advantages of this structure include multiple shunt APFs to compensate voltage, reactive current, and harmonics (even for different harmonic levels) individually or collectively. By selecting the number of units, the efficiency of the UPQC-D can be maximized; hence, losses can be minimized. It is easy to integrate in a single or three-phase network with different compensating current requirements in different phases. It is also easy to be integrated with the DG through the DC link [22].

g.

UPQC Distributed Generation Integrated (UPQC-DG)

The DG sources, such as PV or wind energy, can be combined with the UPQC. The DG is connected to the UPQC through the DC link (Figure 16). The power generated by the DG can be stored as a reserve in the DC bus, which makes it possible to supply the load during power interruptions (S1, S3, and S4 are open; S2 is closed). The power from the DG is fed in the combined mode, i.e., to both the load and the network (S1, S3, and S4 are closed; S2 is open), or in the island mode, in which the power is transferred to selective receivers (S1, S3, and S4 are open; S2 is closed) [8]. In addition to maintaining voltage in the case of grid failure, the UPQC-DG can compensate for voltage sags and swells, current harmonics, and reactive power in both modes [19].

2.1.4. Voltage Sag Compensation

There are several types of UPQC circuits to compensate for voltage sags [7,8]. These are the following:

• UPQC-P is a type in which active power is controlled. The series APF introduces active power to mitigate the problem of voltage sags in the power supply;
• UPQC-Q is a type in which reactive power is controlled. The series APF introduces reactive power to mitigate the problem of voltage sags in the power supply;
• UPQC-S is a type in which active and reactive power is controlled. The series APF introduces both active and reactive power to mitigate the problem of voltage sags in the power supply;
• UPQC-VAm is a type that reduces the VA of load to compensate for voltage sags. In addition to the in-phase or quadrature of voltage injection by the series APF at minimum VA, the load voltage is injected with an optimal angle.

2.2. HT

In the literature, there are many HT configurations proposed by various authors, as well as many different topologies of power electronic converters have been used. In general, the HT consists of the LFT and power converter [10,11,12].

In Figure 17, Figure 18 and Figure 19, topologies of the HT depending on the power converter supplying method are presented. The power converters can be supplied from a capacitor (Figure 17), the LFT (primary or secondary winding) (Figure 18), or the AW (Figure 19). Moreover, there are different ways of the power converter connection to the grid. It may be connected in series without the CT, in series with the CT, in shunt, and through the LFT core. The HT configurations presented in Figure 17, Figure 18 and Figure 19 are supposed to show the general structure without precising the power converter topology or number of phases.

Depending on the HT and the power converter configuration, various types of compensation can be performed (

Table 3. Types of compensation provided by the HT topologies [12].

Type of Compensation	Energy Source
	Capacitor				AW			LFT
	(a)	(b)	(c)	(d)	(a)	(b)	(c)	(a)	(b)	(c)
	from Figure 17				from Figure 18			from Figure 19
Series without the CT	√				√			√
Series with the CT		√				√			√
Shunt				√				√	√	√
Magnetic			√		√	√	√

). These can be classified into series with the CT, series without the CT, shunt, and magnetic compensation [12].

The utilized configuration of the HT and the power converter topology determines the operating region, which is divided into the following [12]:

• reactive power injection;
• restricted active and reactive power injection;
• unrestricted active and reactive power injection.

In Figure 20, there is a classification of the topology of the power electronic converters that are considered in the literature [12]. The topologies are divided into AC/AC and DC/AC converter. The AC/AC converters are classified into direct converters without DC link, converters with DC link, and converters that provide the galvanic isolation. The DC/AC topologies are applied in the self-supported configurations only.

3. Control Methods of the UPQC and the HT

Effective control methods are essential to monitor and maintain power quality to ensure proper system operation. Selected control methods for the UPQC and the HT systems are presented in this section.

3.1. UPQC

One of the most important elements of any system is the control unit. Any control method should quickly detect any disturbance or error and react to it. In the UPQC, the control systems for the series APF and the shunt APF are independent. Each of them has a different task. The series APF should be able to detect voltage sags/swells, generate a voltage reference, and define a compensation voltage injection strategy [16,23]. The shunt APF should be able to control the DC link voltage, extract harmonics, generate reference current and gate signals for current compensation, and calculate reactive power [24].

The series APF is most often controlled by using the PWM method [16,25]. The shunt compensation usually uses the PWM with hysteresis control. The authors also use the above control methods enriched with a fuzzy controller [26,27] (Figure 21).

The main goals of the control methods used by the various authors whose work has been taken into account in this review are generally centered around the following:

• compensation of voltage sags/swells [28,29,30];
• reduction in harmonics in the output current [16,31];
• voltage stabilization of the DC link [31];
• system response to the non-linear load changes [31,32].

3.1.1. Voltage Sag/Swell and Harmonic Compensation

The series APF for voltage control needs the three-phase sine wave with the correct amplitude and phase. In [16], the authors used a phase-locked loop (PLL) to identify the phase of the input voltage. On this basis, three reference voltages are generated with a corresponding phase shift. The difference between the reference voltage and the normalized input voltage is used in the PWM generator to produce the gating signals for the series APF that will correct voltage fluctuations. In the shunt APF control, using the transformation of abc to quadrature zero (dq0), a reference current is generated. The amplitude and phase of the load voltage are used as inputs to this transformation. In order to improve the power factor, the voltage phase is used as the current reference phase.

To deliver active power from the series APF to the system during voltage sag, the input currents and output voltages must be in phase with the corresponding input voltages. The active power is absorbed by the shunt APF in order to keep the power balance of the system. In the case of, e.g., a voltage spike, the series APF absorbs the active power and delivers it to the load through the shunt APF. Therefore, the amplitudes of the UPQC input currents must be adequately controlled to regulate the voltage, prevent the voltage transients in the DC link, and perform the system power balance as soon as possible [28]. To mitigate voltage sags/swells in [33], the UPQC control is designed using a Synchronous Reference Frame (SRF) to generate a sine wave output of the desired magnitude. The SRF network draws the three-phase voltage and current reference signals. The transformation abc to dq0 is carried out to simplify the control design. The PLL is used to calculate the phase angle of the reference signal. The low pass filter removes the harmonics from the component of the direct axis current, and the PI controller calculates the magnitude of the generated pulses and transforms dq0 back to abc to obtain an equivalent three-phase output, which is fed to the PWM generator. The shunt and series APFs with their control circuits and Maximum Power Point Tracking (MPPT) algorithm based on Fractional Open Circuit Voltage (FOCV) are considered in the proposed system by [34]. The shunt APF injects current into the system and compensates the load currents to ensure a unity power factor. During a voltage sag, the voltage applied from the series APF is in phase with the source voltage, while during a voltage swell, the series APF delivers a voltage out of phase. In [30], a control method of the series APF to compensate for input voltage harmonics, sags/swells, and interruptions is proposed. It is based on a single-phase synchronous transformation of the dq reference system. Unlike previous methods that require a three-phase voltage measurement to detect sags, it fails to detect a voltage sag below a certain depth. The output voltage of the series APF is controlled to be in phase with the supply current.

3.1.2. DC Link Voltage Stabilization/Control

Stabilization of the voltage of the DC link is an issue investigated, especially in the case of the UPQC system operating with the DG, e.g., with PV. The UPQC must provide a continuous output voltage from the PV to stabilize the DC link voltage, but in reality, this voltage is unstable due to factors such as weather that affect the amount of power produced by the PV. For this reason, it is necessary to introduce power to the PV. In [35], MPPT control is implemented by the boost circuit, and then, a stable voltage value is introduced by the DC/AC converter. Two-loop control uses the inner voltage loop of the outer current loop to keep the DC voltage constant. Considering the problem of energy coupling between the series and shunt APFs, which reduces the stability of the DC link voltage, the Random Weighted Particle Swarm Optimization (RW-PSO) algorithm is additionally proposed in order to implement the dynamic online optimization of the parameters of the PI controller used in the control strategy.

A similar solution is presented in [24] for controlling a sensitive load in the UPQC with a PV system. The PV is coupled with the MPPT control algorithm, and the step-up converter is designed to supply energy to the DC link.

3.1.3. System Response to Non-Linear Load Changes

In order to adapt to the characteristics of real-time load changes in [32], an improved Fourier harmonics detection algorithm based on the principle of phase discrimination to eliminate the error caused by the delay of the detection link is proposed. In conjunction with the positive sequence tracking method, dynamic compensation is performed under non-linear load conditions. According to the different control characteristics of the voltage and current, the parallel carrier delta voltage control and the current tracking control of the improved time hysteresis comparison method are adopted accordingly, which not only solves the problem of excessive switching frequency but also ensures that the error control of the compensation system can converge quickly and has good dynamic and static performance.

3.2. HT

The power converter is able to perform various types of compensation depending on the connection in the HT. The shunt connection provides current mitigation (power factor correction, filtering of harmonics, etc.). The series connection provides mitigation of voltage sag/swell, voltage harmonics, and voltage unbalance. The power converter is usually controlled with the PWM method using the information of the output and input voltage values (Figure 22).

The main goals of the control methods used by the various authors whose work has been taken into account in this review are generally centered around the following:

• regulation of the reactive power [36]
• compensation of voltage sags/swells and harmonics [15,37,38,39,40,41,42,43,44];
• regulation of the DC link voltage [39,40]

In [43], the single-phase HT for improvement of the power quality for critical loads is under investigation. It consists of two secondary windings with the AC/AC power converter in one of them. The converter is controlled with the PWM. During the voltage sag, the duty cycle of the power converter increases to maintain the voltage value required by load level. In the case of the voltage swell appearance, the duty cycle of the power converter decreases. To compensate for the input voltage harmonics, the PWM converter injects the opposite distorted voltage.

For the voltage fluctuation mitigation, an integral controller with deadband and saturation limit is proposed in [38]. The voltage of the power converter is adjusted in each cycle when the voltage at the end of the feeder is deadband.

In [44], the three-phase voltage sag/swell compensator with the buck-boost matrix-reactance chopper is controlled with the use of the PWM method. To detect the voltage fluctuation, the peak detector is applied.

The series converter is controlled by the use of discrete-time state feedback to mitigate voltage sags/swells in [15]. The series converter supplies the LC filter on the primary side through the CT, and the shunt converter is connected to the secondary side with the LCL filter. By regulating the capacitor voltage of the LC filter, taking into account the state of the grid, the task is achieved. The estimation of the variables is necessary. A similar method is used for controlling the shunt power converter, i.e., a mutliresonant state feedback with computation of delay. It mitigates the load and capacitor harmonics and controls the DC link voltage.

In [40], the compensation of the voltage sag/swell and grid harmonics is performed with the utilization of the second-order generalized integrator PLLs (SOGI-PLLs). The input voltage and input current vectors are estimated. The compensation voltage vector is synthesized to mitigate the voltage sag/swell. To regulate the DC link voltage and compensate for the reactive power, the power converter controls the current in the differential mode.

The HT topology for voltage regulation and reactive power control in one device is proposed in [36]. This structure regulates the voltage and the reactive power for each feeder in the distribution system under various load power factors and input voltage values. The control strategy requires vector analysis and equations.

The power converter with a DC link is capable of providing only the reactive power compensation irrespective of the converter connection. For that reason, an additional active power control is required to regulate the voltage of the DC link to its rated value [13].

The two methods of modeling the HT are used in [39]. The first one is the single-phase multiwinding transformer method, and the other one is the mutual inductance transformer method. To calculate parameters, the finite element model of the HT is applied. The investigated methods are employed to establish the DC traction supply.

4. Basic Properties of the UPQC and the HT

In this section, the basic properties of the considered systems of the UPQC and the HT are discussed.

4.1. UPQC

The primary function of the UPQC is to compensate and mitigate variations in the supply voltage (voltage sags/swells, interruptions, flicker, harmonics, etc.) by the series APF by injecting voltage into the system and to compensate the load current and reactive power using the shunt APF by current injection [16,31]. The functions of individual APFs in the UPQC are listed below in

Table 4. Functions of the series APF and the shunt APF in the UPQC [6,7,8,9].

The Series APF	The Shunt APF
(1) The supply voltages balance by inserting equal and negative components to remove those present in the system;	(1) The source currents balance according to the load requirements by inserting equal and negative components;
(2) The load protection from harmonics by introducing voltage harmonics into the supply;	(2) The required harmonic current injection to compensate those present in the system;
(3) The required active and passive elements introduction to regulate the magnitude of the load voltage;	(3) The reactive current injection for power factor control;
(4) Adjustment of the input power factor.	(4) The DC link voltage management and synchronization.

[6,7,8,9].

In the articles selected for this review, in order to mitigate changes in the supply voltage and improve the power quality in the distribution network, research in many works focuses on reducing the Total Harmonic Distortion (THD) factor [16,23,24,25,26,29,30,32,33,45,46,47,48]. In the presented results from simulations and/or real experiments, the authors succeed in significantly reducing the THD value of voltage and current by using their proposed (new or modified) UPQC topology [16,23,24,25,26,32,33,46,48] or control method [29,30,45,47].

The THD factor may deteriorate in the case of the negative impact of non-linear or unbalanced receivers on the network. In article [30], the UPQC-R and the UPQC-L topologies for voltage and current harmonic compensation are presented. The performance of UPQC compensation is performed in the single feeder distribution system. Before compensation, the supply voltage THD is 16.98%; after the compensation, the results are similar for the UPQC-R and the UPQC-L, which reach the THD values of 1.968% and 1.986%, respectively. However, the supply current THD decreased from 14.87% to 0.38% for the UPQC-R and 0.46% for the UPQC-L. In [30], the UPQC 3P4W topology is used to reduce the harmonic content of the load current from 29.10% to 3.4%. For an unbalanced three-phase system, THD is reduced from 15% to 0.13%. The reactive power is effectively compensated, and the output power factor is close to 1. In [47], the UPQC with nine switches (NS-UPQC) is proposed. The NS-UPQC system uses two control approaches that are compared with each other in terms of the THD value for load voltage and source current; these are SRF and Real-Power Theory (P-Theory) with harmonic extraction method. Before the compensation, the THD was around 14.14% for load voltage and 15.95% for supply current. After the compensation, these values decreased to 5.62% and 5.07 using SRF method control, respectively. The P-Theory is observed to perform better than the SRF control. The THD values are 4.53% and 3.94% for load voltage and supply current, respectively.

The THD factor may worsen in the case of the negative impact of the non-linear or unbalanced receivers in the network. Another reason for the deterioration of this factor may be the occurrence of variations in the voltage value, such as voltage sags and swells [19,24,28,33,34,45,46,47,49]. In [46], the authors studied the UPQC-MD composed of two modules. It is shown that the two-module system can cause harmonic distortion of the load voltage of 10.10%, which is lower than the single UPQC model, which causes a THD of 26.70%. The comparison is presented for different types of voltage variations (sinusoidal or distorted sags, swells, and interruptions) and non-linear loads. The THD of the dual UPQC is almost at the same level or lower than for a single UPQC. The presence of the PV introduces active power into the grid, improving grid power quality and limiting load harmonic current, which is impossible to achieve with UPQC cooperating without it [25]. The results presented in [33] show that in the case of the voltage sag of 50%, the THD of the supply voltage is 4.07%, and the supply current is 16.1%. When using the UPQC system operating with PV, the THD of the load voltage drops to 0.09% and the load current to 0.12%. The case of the voltage swell is also investigated. The THD of the supply voltage is then 5.57%, and the supply current is 10.8%. After compensation with the UPQC-PV system, the THD of the load voltage drops to 0.09% and the load current to 0.09%. In [29], the authors pointed out that the conventional UPQC configuration using PV and the SRF-PLL control method does not keep the supply current and load voltage within the limits specified by the IEEE-519 standard. The source current THD value is 18.77%, and the load voltage THD value is 18.40%. However, the proposed approach, which includes an advanced CDSC-based PLL, effectively mitigates these problems. Mains current THD is reduced to 4.81%, and load voltage THD is reduced to 0.77%, both below IEEE-519 limits. The behavior of the three-phase UPQC with a PV system during the occurrence of grid voltage sags/swells, and unbalanced load shedding is investigated in [34]. The maximum power from the PV is obtained through a fractional open-circuit algorithm, which is not only easy to implement but also ensures a smooth and oscillation-free response. This system works as a multi-functional system, delivering clean energy to the grid with improved power quality. The voltage sags or spikes on the grid side are compensated by the series APF that operates in voltage regulation mode.

The only disadvantage of the UPQC is that it cannot maintain the voltage in the case of a power interruption [19]. The effective operation of the UPQC-MC has been demonstrated during voltage sags/swells in the supply lines and load changes or short circuits occurring in both supply lines in [49]. If the voltage sag occurs, the phase angle of the relevant bus voltage to which the shunt APF is connected is adjusted to supply the actual power demand of the connected load to the same bus. In this way, the UPQC-MC can overcome voltage sags/swells and interruptions in both power lines and solve power quality issues to protect the multi-utility system from them.

In addition to improving the power quality by compensating for variations in voltage and THD, an important aspect is also the generated power losses. In [25], the UPQC, UPQC-DG, and UPQC with separately integrated DG (UPQC-IDG) topologies were compared in terms of power losses. The comparison results for different cases (e.g., sag, swell, etc.). Looking at the overall losses of the UPQC and UPQC-DG, it turns out that the connection of the DG in the DC link of the UPQC leads to significant additional losses. The total power loss of the UPQC-DG is 5.80% and of the UPQC—2.17%. Despite the higher power losses, the UPQC-DG is more practical in real distribution systems because the combination of the PV and the shunt APF reduces the cost of the additional power circuit to a level that is economically feasible. In addition, in the UPQC configuration with separately integrated PV (not to the DC link), higher power losses are observed (15.11%), which proves the uselessness of its implementation in a practical distribution system. In [45], the methods used to compensate for voltage sags and swells in terms of power losses are compared. The total losses of the UPQC-P are found to be 2.04%, which is higher than the UPQC-S (1.9%) and the UPQC-Q (1.8%). The minimal losses during general compensation for different loads confirm the good performance of the UPQC-Q with minimal active power consumption and improved power angle. Nevertheless, the evaluated result also points to the inability of the UPQC-Q to compensate for voltage spikes.

Providing good power quality to users is very important, but the cost reduction aspect is interesting and is being researched, too [25,32,47]. In [47], the UPQC topology consisting of nine switches is proposed, which allowed for a reduction of their number by 33% compared to the conventional topology. In addition to a lower cost, the system is also smaller in size and results in lower switching losses. In [25], it is shown that the UPQC-DG system described above is more practical in real distribution systems because the combination of the PV and the shunt APF reduces the cost of the additional power circuit to a level that is economically viable.

Voltage stabilization in the common DC link is also an important aspect in ensuring the correct operation of the UPQC system. In [35], the focus is on voltage stabilization in the DC link. The RW-PSO algorithm based on swarm optimization of randomly weighted particles for dynamic optimization of online PI controller parameters is proposed. The RW-PSO method solves the problem of large fluctuations in DC voltage parameters.

4.2. HT

The power converter connection in the HT defines the type of compensation that is performed. Moreover, the connection type affects the LFT.

The shunt connection of the power converter in the HT provides current mitigation (power factor correction, filtering of harmonics, etc.). The location of the power converter has an impact on the LFT. If it is placed in the primary winding, the improvement of the power quality is possible. However, in that case, the mitigation is not performed on the secondary side of the LFT, which may cause the presence of the harmonics generated by the load and lead to the LFT destruction. If it is placed on the secondary side, the lifetime of the LFT is improved. The series connection of the power converter in the HT provides mitigation of voltage-based power quality problems, e.g., voltage sag/swell, harmonics, and imbalance. Furthermore, in this connection, the location of the power converter impacts the LFT. Improvement of the LFT performance, power quality, and active regulation on the load voltage are possible through the power converter connection to the primary side. If the converter is connected to the secondary side, the LFT is not protected from voltage disruption and causes malfunctioning of the grid. A mixed solution, which means the series and the shunt connection of the converter in one configuration, is possible. Again, depending on the location of the series converter (primary or secondary side), two scenarios are analyzed [12].

The transformer is one of the most reliable units in the distribution system. By applying only a fractional part of the power converter, the cost and switching losses are lower, and so the reliability increases. Using the power converter in the HT extends the regulation range of the voltage and the current. This regulation range depends on the power converter topology. By the connection of the series converter to the distribution system through the CT, the optimization and reduction in the DC link voltage is possible regardless of the series power converter location. The presence of the CT ensures an isolation between the LFT and the power converter.

In the articles selected for this review, in order to mitigate variations in the supply voltage and improve the power quality in the distribution network, many works focus on the following aspects:

• stability problem [15];
• regulation of the voltage and the reactive power [36,44];
• voltage sag/swell and harmonic compensation [15,40,43,44,45];
• power losses [12,37];
• cost reduction [43];
• dynamic response [44];
• disconnection in the case of fault appearance [43].

By using the HT, all of the listed above can be achieved.

In [14], the state feedback controller is used to solve the stability problems related to resonances in the LC and LCL filters. This approach enables improving the power quality (voltage sag/swell, harmonic, power factor). It reduces the THD of the voltage and current and extends the lifetime of the LFT.

The HT topology for voltage regulation and reactive power control in one device is investigated under various load power factors and input voltage values in [36]. This structure regulates the voltage and the reactive power for each feeder in the distribution system. In [44] the three-phase voltage sag/swell compensator based on the HT and the buck-boost matrix–reactance chopper is presented. The HT compensates for deep voltage sags (more than 50% drop in the nominal source voltage value) and overvoltages (up to 140% of the nominal source voltage value), keeping good dynamic properties. This topology provides a wide range of output voltage regulation (0.66–3.5 multiplies the voltage value of the source), good dynamics, and the galvanic separation between the load and the source. In [40], the HT with a partially rated series power converter is presented. The power converter connection provides a reduced power rating of the converter and an additional current loop. The voltage vector is synthesized to compensate for voltage sag/swell and harmonics of the grid. An improvement in the power factor and efficiency is achieved. The single-phase HT for improvement of the power quality for critical loads in [43] consists of two secondary windings with the AC/AC power converter in one of them. The proposed topology provides compensation of voltage sag/swell at the level of 50%, harmonic compensation to keep a low THD factor, and rapid disconnection of load in terms of fault. By applying the AC/AC converter, there is no bulky capacitor needed, which reduces the cost. Losses in the HT consist of load and no-load losses [13]. The system losses of the HT analyzed in [37] are 13.5% higher than in the LFT.

In [38], some characteristics of the HT in the distribution system are listed. The HT has rather low controllability (5–20% voltage/power), high efficiency (98.6–98.9%), long lifetime (30–40 years for the transformer; 20 years for the power converter), high reliability, low cost, and large size. The HT with integrated storage increases the PV capacity up to 490.91%, prevents reverse power flow, and mitigates voltage fluctuations.

5. Application of the UPQC and the HT

This section briefly discusses the application of the UPQC and the HT.

5.1. UPQC

Mainly, the UPQC is used in [14] the following:

• SVC—Static VAR compensation (reactive power);
• harmonic suppression;
• current balancing;
• active and reactive power regulation;
• voltage balancing;
• voltage compensation.

A significant amount of recent work is related to the UPQC topology working with energy from renewable sources (UPQC-DG). In such a system, energy from, e.g., PVs is connected to the DC link between the series APF and the shunt APF in the UPQC. The connection of PV or battery energy storage systems (BESS) is primarily intended to maintain a constant voltage value in the DC link [27] also in the case of grid failure [46], but in addition, they are able to compensate voltage sags and swells, current harmonics and reactive power too [46,50]. This ensures system stability and eliminates the (biggest) disadvantage of the UPQC topology not cooperating with the DG (UPQC-DG), which is the lack of voltage maintenance in the case of power interruption [19,46].

In the case of both the usual UPQC and the UPQC-DG topologies, the published works consider similar issues related to power quality and the compensation of certain quantities. Most often, the objectives of the conducted research focus on the compensation of disturbances from the network side (sags, swells) or from the load side (non-linear receivers) [8,27,28,29,31,33,46,47], harmonic compensation [16,31,47,49], regulation and stabilization of the DC link voltage [23,31,35], power-sensitive receivers [24], comparison of topologies with each other in terms of, e.g., power losses [25,45], efficiency [34], reactive power compensation, mitigation of voltage sags/swells, and number of switches [19,47].

In response to the above objectives, the authors propose various control strategies [27,28,29,30,31] or new/modified architectures of the UPQC conditioner [16,19,33,34,35,46,48].

5.2. HT

The HT is mainly used in:

• voltage compensation;
• harmonic compensation;
• active and reactive power regulation;
• voltage regulation.

The voltage compensation/regulation and the reactive power regulation are mostly the main tasks of the HT in the low voltage distribution system [36,40,42,44]. The presence of the power converter in the HT topology ensures the voltage regulation range.

The presence of the DG in the distribution system is lately the main interest of the researchers. Since it introduces serious problems to the system, the mitigation methods are investigated. In [37], a Monte Carlo study describes the impact of the HT on the low voltage distribution system considering PV generation and household demand. The analysis takes into account voltage disturbances, overloads of lines, and general system losses. The voltage regulation and reactive power control, including real losses of the system, are investigated under various power factor values. Information about the technical and economic impact of the HT can be found in [38]. An extensive study of net present value and internal rate of return under various PV and BESS (Battery Energy Storage Systems) capacities is performed.

Reducing the cost of the topology is always an important topic. It is investigated in the HT field [41,44]. Moreover, cooperating with critical loads and the reaction in case of the fault appearance are studied [43].

6. Conclusions

Power quality is one of the most important aspects of the power system, especially in the distribution system. This system can be affected by such disturbances as voltage sags/swells, long-term voltage increase/decrease in the normative limits, harmonic distortion, interruptions, etc. It should be emphasized that the problem of too high temporary voltage fluctuations results from the supply of electricity from RES, and too little of its consumption requires a structural approach related to both digitization and IT engineering, as well as the reconstruction of the electricity transmission and energy storage infrastructure [51]. The compensator topologies proposed in this article are one of the elements of this comprehensive approach.

This article presents a subjective review of selected voltage compensator topologies in the low voltage distribution network to improve power quality, focusing mainly on the UPQC and HT. This review is divided into several sections, which present selected topologies, control methods, basic properties, and the use of UPQC and HT systems. Both topologies are quite popular to be applied in the distribution system. They present a good performance, use a simple control approach, and are rather cheap and reliable. Mitigation of the voltage disturbances is successfully provided by the UPQC and the HT. Since the interest in improving the power quality due to the presence of the DG in the distribution network is high, those devices are investigated and optimized by a lot of researchers.