Contents Introduction
Nowadays, with the high penetration of distributed generation resources in low voltage networks and with the increase of non-linear and unbalanced loads that have a major portion of the total load of a small-scale system creates many challenges in terms of power quality in these networks which requires extensive research in this field. Since the systems based on centralized production are facing two limitations, the lack of fossil fuels and the need to reduce pollution; Therefore, the importance of distributed generation resources has increased by connecting renewable energy systems to grids. In order to make optimal use of distributed generation resources, microgrids have been widely considered. Microgrids are local networks that include distributed generation sources, energy storage systems, and loads that can be operated in two grid-connected and island modes. The most important challenges facing microgrids is power fluctuation control, voltage control, power distribution control, and maintaining power quality in both grid-connected and island modes [1].
In general, power quality problems in the microgrid can be divided into two separate categories. First, the problems that are related to the voltage delivered at the point of common coupling and include voltage harmonics, overvoltage, undervoltage, voltage swell, voltage sag, voltage imbalance, voltage fluctuations, outages, etc. Second, the problems related to the current drawn from the network by non-linear loads such as electric arc furnaces, uninterruptible power supplies, speed control systems, etc., which can lead to power quality complications, including improper power factor, high reactive power, harmonic currents, unbalanced currents, etc. Therefore, in order to improve the level of power quality in a network, the proposed solutions should solve the power quality problems from both perspectives and try to ensure that under any conditions of network problems, the voltage delivered to the load is standard [2]. A reinforcement learning-based decision system is provided for the selection of electricity pricing plans, which minimize the electricity payment [3]. A method from the demand response (DR) optimization proposed in distribution markets consisting of a retailer and multiple demand response aggregators [4]. The importance of renewable energy is presented in [5]. The hierarchical reinforcement learning (HRL) is developed to handle the distributed online economic dispatch problem [6]. The main purpose of this paper is to review power quality issues, challenges and solutions presented to improve it in power system.
Focusing on voltage unbalance and harmonic disturbances, which are among the most important challenges of power quality in microgrids.
The importance of Flexible Alternating Current Transmission System (FACTS) devices and control approaches in improving the power quality of microgrids.
The rest of the paper is organized as follows. Section II discusses FACTS devices. Section III summarizes common power quality disturbances in microgrids. Section IV reviews compensation of voltage unbalance. Section V highlights compensation of harmonic disturbances. Section VI addresses different types of integrated power quality compensations. In Section VII, different control strategies for Unified Power Quality Conditioner (UPQCs) are examined. Section VIII provides experimental results and discussions. Finally, Section IX concludes this survey.
Facts Devices
In addition to active and reactive power regulation, voltage control in the microgrid is required to ensure microgrid stability and dependability. Many approaches are utilised to improve the power quality of distribution networks, the most efficient and successful of which is the employment of FACTS devices [7]. FACTS devices are modeled as equipment to evaluate voltage sag correction in distribution networks. The performance of this equipment in correcting power quality indicators depends significantly on the performance of the control system [8]. The task of FACTS devices is to restore and maintain the voltage when voltage drops at the point where sensitive loads are connected to the network [9]. Microgrids that have a large number of microsources may suffer reactive power fluctuations and enter instability without correct voltage control. In this case, to stabilize the network can use a variety of compensators of FACTS devices or a power system stabilizer. FACTS devices are one of the most prominent tools of researchers’ interest in increasing system power quality [10]. Fig. 1 shows the types of compensation of FACTS devices which include:1-Series compensator: thyristor controlled series capacitor (TCSC), thyristor controlled series reactor (TCSR), thyristor switched series capacitor (TSSC) and static synchronous series compensator (SSSC). 2-Shunt compensator: static reactive power compensator (SVC), thyristor controlled reactor (TCR), thyristor switched capacitor (TSC) and thyristor switched reactor (TSR) [11].
FACTS and Distributed Flexible AC Transmission System (D-FACTS) devices play a main role in increasing the power quality of modern and conventional networks. Table. 2 shows summary of power quality improvements by FACTS devices. In the following, two main challenges related to power quality including voltage unbalance and harmonic disturbances will be presented.
A. Series Controllers
Series controllers are used to create series voltage with line voltage. This controller includes a capacitor or reactor (impedance devices). These types of controllers are used to generate or consume variable reactive power. When the transmission line load is higher, it needs additional reactive power. In this situation, a capacitor is used to provide reactive power. When the load of the transmission line is very light, the voltage at the end of the line will increase compared to the voltage at the beginning of the line, due to the lower demand of reactive power. In this situation, reactive power consumption is used with the help of an inductor. In most cases, capacitors are installed at the end of the line to compensate for reactive power demand. For this purpose, thyristor controlled series capacitors (TCSC) and synchronous static series compensators (SSSC) are used.
Thyristor Controlled Series Capacitor (TCSC): In this method, the capacitive reactance is connected in series with the power system and includes a capacitor bank in which several capacitors are connected in series-parallel. The capacitor bank is connected in parallel with the thyristor controlled reactor and is used to create a smooth variable series capacitance. Thyristors are used to control system impedance. The impedance of the entire system can be controlled by controlling the firing angle of a thyristor.
Thyristor Controlled Series Reactor (TCSR): This device is a series compensating device that provides a smooth variable inductive reactance. TCSR is the same as TCSC, except that the capacitor is replaced by a reactor. When the fire angle is 180 degrees, the reactor will stop working, and when the fire angle is less than 180 degrees, it will be in circuit.
Thyristor Switched Series Capacitor (TSSC): This compensation method is similar to TCSR. In TCSR, the power is controlled by controlling the firing angle of a thyristor. Hence, it provides step-by-step control. But in the case of TSSC, the thyristor can only be on or off and there is no angle of fire. Thus, the capacitor is either fully connected or fully disconnected from the line. This causes the cost of the thyristor and the cost of the controller to decrease.
Static Synchronous Series Compensator (SSSC): SSSC method is a series compensation method used in transmission system. Power distribution through the transmission line is adjusted by controlling the equivalent impedance of the transmission line. The output voltage of the SSSC is fully controlled and independent of the line current. Therefore, by controlling the output voltage of SSSC, it is possible to control the impedance of the line. The static synchronous series compensator is considered as a static synchronous generator connected in series with the transmission line. The purpose of this system is to increase or decrease the voltage drop along the line. Hence it controls the power distribution through the transmission line. This system injects voltage perpendicular to the line current. If the supply voltage is leading with respect to the current, it performs capacitive compensation and if the supply voltage is lagging with respect to the current, inductive compensation is suppiled.
B. Shunt Controllers
This type of equipment is used to inject current into the power system at the connection point. Similar to series controllers, this type of controller also consists of variable impedance such as capacitor and inductor. When a capacitor is used for parallel connection with the power system, this method is known as shunt capacitor compensation. When the transmission line consists of a dominantly inductive load, it operates with a lagging power factor. This method is used to absorb the leading current compared to the source voltage to compensate the lagging load with the help of the shunt capacitor. When an inductor is used to connect to the power system, this method is called shunt inductive compensation. Generally, this method is not useful in case of transmission network. But if there is a very large transmission line, which is possible to cut off the load or act in no-load or low-load conditions, due to the ferranti effect, the voltage at the end of the line increases and then the voltage at the beginning of the line increases. To avoid these conditions, a shunt inductive compensator is used.
Static Reactive Power Compensator (SVC): In static reactive power compensator, a fixed capacitor bank is connected in parallel with thyristor controlled inductor. The thyristor firing angle controls the reactor and the inductor voltage.
Thyristor Controlled Reactor (TCR): In this type of controller, the reactor is connected in series with the thyristor. The control circuit is used to pulse the thyristor every half wave. The firing angle of the thyristor controls the delay of the power supply. Generally, TCR is used in EHV high voltage transmission line to provide reactive power during light load or no load condition.
Thyristor Switched Capacitor (TSC): In the presence of heavy load, the reactive power demand increases. In this situation, TSC is used to respond to reactive power demand. Usually, TSC is used in very high voltage or EHV transmission line and is used under heavy load conditions. TSC is the same as TCR, only the capacitor is replaced by the reactor.
Thyristor Switched Reactor (TSR): The TSR is similar to TCR. In TCR, thyristor firing angle is controlled to control device current. But in TSR, the thyristor is completely on or off and there is no phase control. Therefore, due to the lack of control of the firing angle, the cost of the thyristor is lower and the switching losses are reduced.
Static Synchronous Compensator (STATCOM): A STATCOM is a power electronics-based voltage converter that can regulate power in a transmission system with a reactive power source. It is also used to provide active power to the transmission line. Generally, STATCOM is used in transmission line which has very low power factor and poor voltage regulation. Static synchronous compensator is the most common device to improve power system voltage stability.
C. Series-Shunt Controller
This type of controller is used to generate voltage in parallel using a shunt controller and also to generate current using a series controller. But both controllers must act in harmony.
In the following, the types of these compensators are presented.
Unified Power Flow Controller (UPFC): This controller is a combination of STATCOM and SSSC with a common DC voltage link and a three-phase controllable bridge pair is used to generate the current. This current enters the transmission line using a transformer. UPFC is used to improve voltage stability, power angle stability and system damping.
D. Series-Series Controller
In multi-line transmission lines, a combination of separate series controllers is used in tandem to provide independent series reactive compensation for each line. But it is possible to transfer active power with lines through power link. Or integrated controllers can be connected where the DC terminals of the converters are connected together. This helps to transfer active power to the transmission line.
Interline Power Flow Controller (IPFC): In this type of compensation method, a number of converters are connected with a common DC link and each converter is connected to a separate line. Therefore, this compensation technique is only used in a multi-line transmission system. Converters are also capable of transmitting active power. Therefore, in this method, both active and reactive power can be transferred between the lines.
Common Power Quality Disturbances in Microgrids
Basically, power quality disturbances can have many and varied reasons. On the other hand, the mechanism of effect and disruption in the network and consumers can be different depending on the nature of the disturbance. For this reason, in the studies of power quality and the compilation of relevant standards, it is tried to categorize the aforementioned disturbances based on their nature [26]. With the development and advancement of technology in the field of power electronic devices, a variety of non-linear loads with non-sinusoidal nature have appeared in the power network. The presence of these loads causes the waveform to deviate from the ideal state at rated frequency and causes power quality distortion. On the other hand, nowadays many loads are controlled by sensitive electronic and microprocessor systems. These systems are susceptible to network disruptions and their performance might be quickly disturbed [27], [28]. Power quality is an indicator for evaluating distortion in voltage, current and frequency quantities. Nowadays, due to the increase in the sensitivity of electrical equipment, the increasing importance of improving the overall efficiency of the power system, power quality has been more and more considered. In order to monitor the quality of electrical power, standards for power quality indicators have been defined, which can be referred to the international standards of IEC, IEEE and ANSI [29]. In general, the common disturbances in the waveform are: the presence of DC value in the alternating network, harmonics, intermediate harmonics, notch and noise. Table. 1 shows a summary of power quality issues.
Compensation of Voltage Unbalance
The problem of voltage unbalance is mainly due to the unbalanced distribution of single-phase and nonlinear loads. This phenomenon occurs for various reasons, including the occurrence of unbalanced short circuit in the network and unbalanced connection of single-phase loads in different phases of the network. Once a single-phase is connected to grid, a current is drawn from that phase, which causes the network current to become unbalanced [40]. Furthermore, this unbalanced current due to passing the network impedance causes the voltage to be unbalanced at the connection of loads, which will have adverse effects on the performance of loads connected to the network, especially induction motors and equipment with electronic power converters. With the recent development of the use of renewable energy, this issue has taken on new dimensions [41].
In general, voltage unbalance can be compensated by using various equipment, including shunt or series active filters and series-shunt combined filters. The series active filter is one of the best compensators for improving power quality, which is able to isolate between the load and the voltage source and the method of determining the reference voltage in this filter is very important [42]. By selecting the reference voltage determination method, the desired compensation and the overall structure of the filter can be determined. The shunt filters also reduce the imbalance by injecting a negative component, the most common types of which are synchronous static compensators and reactive power static compensators [43]. Fig. 2 shows an overview of two series and shunt filters. One of the series-shunt hybrid compensators is an integrated power quality compensator that improves the power quality by injecting both current and voltage of the negative component [44], [45]. Another option is a synchronous static compensator that modulates voltage by injecting reactive power into the grid [46], [47], [48], [49]. Despite the various methods offered to improve the unbalance, the cost of purchasing and installing this equipment is one of the obstacles and challenges in using them. In addition, the control of each of them has complexities that lead to an increase in the complexity of the network and its control [50].
The effects of voltage unbalance are quite severe for electric machines, electronic power converters, etc. Therefore, many microgrid interface systems have been studied to how well they can act properly under voltage unbalance conditions. A group of electronic power converters are specially designed for the purpose of voltage regulation, including voltage unbalance compensation. One of the most widely used is the dynamic voltage regulator and static series compensator. The dynamic voltage regulator injects compensatory voltages through series connection transformers [51], [52]. In [53], the proposed method based on distributed source control is presented as a negative sequence conductor to compensate for voltage unbalance in the microgrid. In this method, based on the reactive power of the negative sequence, a reference is generated for the negative sequence conductor. This conductor is then applied to generate the compensating reference current. In this case, the compensation reference is applied to the output of the voltage control loop. To overcome this problem, the method of direct change of reference voltage to compensate for voltage unbalance in the microgrid is presented [54]. In this method, the compensating reference signal is considered as a command for the voltage controller. The control system is designed within a static reference framework and the main loop include: voltage and current controllers, virtual impedance loop, active and reactive power controllers, and voltage unbalance compensator [55]. The positive sequence components and negative voltage and current have been applied to calculate the positive active and reactive powers of the positive and negative sequences. The positive sequence powers are applied by power controllers to generate the frequency and amplitude of the output voltage of the units as well as reactive power of the negative sequence to generate a voltage unbalance compensation reference. In order to compensate for the unbalance in the microgrid, a reactive-conductance power droop loop is proposed for the regulation of negative sequence currents among distributed inverters [56], [57].
Compensation of Harmonic Disturbances
Another issue related to power quality in distribution networks is harmonic disturbances that negatively affect the performance of electrical equipment. The odd harmonics are more likely to exist in power systems than even harmonics. Furthermore, higher-order harmonic magnitudes are generally modest and in most situations, these harmonics are filtered in the system. Because their magnitudes are large, low-order odd harmonics can be more dangerous. In general, when the magnitudes of higher-order harmonics decrease, the third harmonic in power system would be the dominating harmonic component which distorts the voltage or current waveform. Furthermore, third harmonic currents in a three-phase system are not removed in the neutral conductor, and in this case, the neutral conductor will draw a cumulative third harmonic current, which is too dangerous since it may cause electric equipment and protective systems to malfunction. In other words, the third harmonic has a negative impact on the generators, transformers, and induction motors found in every electric system. Among these effects, the following cases can be mentioned:
In a grounded generator with a resistor, the third harmonic may cause continual heating (without fault) of the earthing resistor and cause the protective system to malfunction.
In the case of induction motors, the winding connection design is critical because the current third harmonic loops in the delta and does not reflect at the source side. This harmonic raises the current RMS value, resulting in increased copper and core losses. Because of the increased losses, motor power de-rating should be considered to avoid any damage. It means that output will be lowered to match the permissible temperature rise, and efficiency will suffer as a result of losses.
Third harmonic can increase core and copper losses, and core saturation can decrease transformer efficiency and operating power.
In most cases, the third harmonic (third, ninth, etc.) is dominant among harmonics, and other harmonics can be ignored.
A. Active Filter Classification
The classification criteria of active filters are different. An example of this classification is provided with the following criteria (Fig. 3):
Configuration
Power range and response speed
Compensation variables
Control strategies
In weak grids, harmonic disturbances have more serious effects due to the high impedance of the network [61], [62], [63]. To eliminate harmonic disturbances, passive and active filters have been considered by power engineers, which can be used near nonlinear loads to prevent the flow of harmonic currents in the rest of the network. Although passive filters have long been considered to reduce harmonic voltages, these devices increase system costs and are sensitive to load specifications. In addition, resonance phenomena can occur if inactive filters act parallel to a busbar [64]. To overcome these problems, active filters have been used, which have more flexibility than inactive filters. Active and reactive filters evaluate harmonic loads or network currents based on certain techniques and inject compensatory currents into the networks [65], [66], [67], [68], [69], [70]. A proportional resonance control strategy for voltage source inverters is proposed to compensate for harmonic components in the microgrid. This strategy is implemented with a PI controller to adjust the amplitude of currents based on harmonic disturbances [71]. Considering that LCL filters used in electronic interface systems have inherent resonance, this resonance can amplify current and voltage harmonics and lead to power quality and stability problems. An active resonance damping method is proposed to neutralize the undesired resonance, which uses a filter-based approach with inherent stability property to reconstruct and compensate the undesired resonance harmonics [72]. The effect of harmonic current in single-phase microgrid in two microgrid operation modes has been evaluated in [73]. The influence of the output impedance of the studied major control loops on the output harmonic of the voltage source converter has been thoroughly investigated [74]. A virtual admittance loop is proposed to reduce current harmonics in grid-connected operation. It also takes into account the harmonic current distribution and the harmonic voltage generated in the Point of Common Coupling (PCC) bus [75]. A capacitive virtual impedance loop is utilised in the PCC bus to enhance harmonic current distribution and minimise voltage harmonics [76]. In order to distribute reactive power and harmonic current in the microgrid, a coordinated controller for voltage and current controlled sources is presented. The voltage controlled sources (VCS) play a role in harmonic compensation by using the virtual impedance of a capacitor that can fully compensate the inductive effect of LCL filters. In addition, an adaptive virtual admittance based on the residual capacity of current controlled sources (CCS) is employed [77]. An approach for harmonic compensation based on Lyapunov using active shunt filter is presented. The proposed strategy has the ability to reduce harmonics under non-linear loads and network disturbances. Keeping the total harmonic distortion within the permissible range, extracting voltage and current signals based on the Lyapunov estimator, achieving proper performance by using the PI controller based on Lyapunov are some of the prominent features of this study [78]. A sliding mode based adaptive linear neuron (ADALINE)-proportional resonant (PR) approach has been presented to increase the performance of Vienna rectifier, AC/DC converter under non-linear loads. The ADALINE-PR controller is proposed to regulate the source current fault by real-time adaptive of the control gains. Compared with other recent studies, this approach has proper steady-state performance that can eliminate current source harmonics and ripples in active power, DC-link voltage [79].
Different Types of Integrated Power Quality Compensators
In this section, an overview of various integrated power quality compensating systems is provided. It is worth mentioning that the types of integrated power quality compensators are classified into 12 different categories. The details of the classification of integrated power compensators based on their structure are described below [80]:
A. Structure R-UPQC and L-UPQC Configuration
Since UPQC has two inverters connected back-to-back, it can be classified based on the location of the parallel inverter compared to the location of the series inverter. The parallel inverter can be placed on the right side or on the left side of the series inverter. Figs. 4 and 5 show the R-UPQC and L-UPQC confiurations respectively. Among the two provided structures, R-UPQC is used more. In R-UPQC, the current passing through the series transformers, independent of the load current conditions of the system is almost constant. Because in this structure, the parallel converter compensates all the harmonic, reactive and load imbalance currents. Therefore, R-UPQC is generally more efficient compared to L-UPQC. The L-UPQC structure is sometimes used in special situations, for example to avoid performance interference between parallel converters and passive filters [81].
B. Integrated Compensator of Power Quality Between Lines (UPQC-I)
Fig. 6 shows the UPQC-I system structure. In this type of compensator, two inverters are integrated power quality compensator between two distribution feeders. One of the inverters is connected in series with one feeder and the other is connected in parallel with another feeder. With such a structure, it is possible to regulate the voltage simultaneously for both feeders. In addition, UPQC-I can control and manage active power distribution between two feeders. However, the mentioned structure has certain limitations and can only be used in certain circumstances. Power quality problems related to current (such as harmonics and load imbalance) can only be compensated in a feeder whose inverter is installed in parallel. Similarly, power quality problems related to grid voltage can be compensated only in the feeder where the series inverter is installed [82]. To maintain voltage levels, a boosting transformer is connected between the series converter and the transmission system. IUPQC can function as a smart circuit breaker and power flow controller between grid and microgrid to repay the series converter’s active and reactive power references [83] and [84].
C. Integrated Multi-Converter Power Quality Compensator (UPQC-MC)
Due to switching frequency limits, single converters cannot perform harmonic filtering in high power applications. Therefore, in order to improve the efficiency of the UPQC system, some researchers have investigated the possibility of adding a third converter to support DC bus. The third converter can be connected in different ways, for example in parallel with the same feeder or in series or parallel with the adjacent feeder. Fig. 7 shows an example of UPQC-MC structure [84]. The UPQC-MC system has the ability to compensate voltage and current fluctuations in a multi-feeder system by active power sharing. The converter-III optimizes the harmonics of the load current on the load side, it also compensates reactive power and adjusts the DC link voltage. While the converter-I and converter-II reduce voltage sag/swell [85]. The UPQC-MC can control and manage active power distribution between multi feeders. The UPQC-MC structure offers the following advantages [86]:
Power transmission between two adjacent feeds to compensate sag/swell and interruptions;
Compensation for interruptions without the necessity for a battery storage system and, as a result, without the limits of storage capacity;
Sharing power compensation capabilities between two unconnected adjacent feeders.
D. Integrated Modular Power Compensator (UPQC-MD)
The structure of integrated modular power compensator is shown in Fig. 8. This structure is formed by series connection of several full H-bridge modules for each phase. The H-bridge modules for the parallel part are connected in series by means of a multi-winding transformer, while the H-bridges for the series part without a series transformer are directly connected to the distribution line. As the number of H-bridges increases, the voltage divided over each H-bridge decreases, which can be beneficial for achieving higher power levels in medium voltage applications [87].
E. Integrated Multilevel Power Quality Compensator (UPQC-ML)
The integrated multilevel power quality compensator structure is shown in Fig. 9. A three-level UPQC structure, like the UPQC-MD structure, can be enabled to operate at higher voltages and powers by increasing the switching levels. Based on the requirement, UPQC-ML can be made in three-level, five-level, seven-level or higher types [88].
F. Integrated Power Quality Compensator for Converting Three-Wire to Four-Wire Distribution System (UPQC-D)
A four-wire three-phase distribution system is usually obtained by providing a neutral conductor next to the three-phase lines from the substation or from the star center of the delta-star distribution transformers. A new structure for creating four-wire distribution systems is to use UPQC-D integrated power compensators as shown in Fig. 10. In this case, the neutral of the UPQC series transformers is used as the connection point for the neutral wire of the four-wire system. Therefore, even if the feeding system is three-wire, four-wire system can be obtained by using the UPQC-D design. In this design, by using the fourth leg in the UPQC-D design, the neutral current is also compensated [81].
G. Integrated Power Compensator Connected to Distributed Generation Source (UPQC-DG)
The distributed generation sources such as solar and wind are alternative sources for electricity generation. These sources can deliver their produced energy to the system by connecting to the integrated compensation system. The structure of such a system is shown in Fig. 11. The output of the distributed generation sources is connected to the DC bus of the UPQC-DG compensator. The output power of distributed generation source can be adjusted and managed through UPQC for delivery to consumer loads. Also, the operation of compensating power quality problems of voltage and current can be done simultaneously. In addition, a battery can be connected to the DC bus to store the excess energy produced by the distributed generation source. In the event of a voltage outage, the UPQC-DG system can continue to feed the load power [89].
H. Integrated Power Quality Compensators With Voltage Dip Compensation Capability
Voltage dip is known as one of the most important effects of power quality. Therefore, the way of compensating voltage dips by UPQC is also considered as a factor to categorize these compensators. In this regard, the four main categories of UPQC compensators with the ability to compensate voltage dips include UPQC-P, UPQC-Q, UPQC-VAmin and UPQC-S [90].
I. Voltage Dip Compensation Based on Active Power Control (UPQC-P)
In this method, active power is used to compensate the voltage dip and the injected voltage is in phase with the system voltage through the series transformer. In order to achieve effective voltage dip compensation, the parallel inverter absorbs the power required by the series inverter and UPQC losses from the system. In this regard, in the UPQC-P system, the current drawn from the network always increases in the condition of voltage dip compensation [91].
J. Voltage Dip Compensation Based on Reactive Power Control (UPQC-Q)
In this method, reactive power injection is used to compensate the voltage drop, and the injected voltage is perpendicular to the system voltage through the series transformer, So that the sum of the source voltage and injection voltage is equal to the desired voltage in the consumer bus. In order to compensate the voltage dip, unlike UPQC-P, the parallel inverter does not need to absorb active power for compensation purposes and always keeps the load power factor at the same value. But in this type of compensator, to compensate for the same dip, a much larger voltage injection is required, which raises the level of the series inverter equipment accordingly. On the other hand, these types of compensators are not able to solve the problem of voltage spikes and there is always some phase shift in the compensation conditions. Therefore, due to the limitations of this compensator, UPQC-P compensator is usually used [92].
K. Voltage Dip Compensation Based on Minimum Apparent Power (UPQC-VAmin)
In this method, in order to compensate the voltage dip, it is tried to minimize the apparent power consumption of UPQC during voltage dip compensation, regardless of active and reactive power injection. In this method, the injection voltage is injected into the system through a series transformer and with an optimized angle compared to the source current. In order to achieve the minimum UPAC load in this design, in addition to controlling the injection voltage by series transformers, the parallel inverter current is also included in the power optimization process [93].
L. Voltage Dip Compensation Based on Apparent Power Control (UPQC-S)
In this method, similar to the UPQC-VAmin method, the active and reactive powers is simultaneously injected into the system by the series transformer. But unlike the UPQC-VAmin method, in this method, the effort is to maximize the power of the series inverter. In this design, the series inverter has the task of simultaneously controlling the dip/sag of the voltage and participating in supplying part of the reactive power requirement of the load. Since in this UPQC design, active and reactive power is injected into the system simultaneously by the series converter, this system is named UPQC-S. The UPQC-S control system includes several control loops and is relatively complex to implement [95]. Table 3 shows a summary of the performance of each type of compensator along with its advantages and limitations. Most existing controllers have limitations that make it difficult to use them as an integrated power controller for a hybrid microgrid. It also has a complex control system, the impossibility of simultaneous compensation of the active component of power quality disturbances in two converters, The lack of compensation performance in island conditions, the lack of control of power fluctuations, the lack of proper efficiency in voltage dips, the lack of control of two-way power sharing, etc.
Different Control Strategies for UPQCs
Control methods play a important role in any system based on power electronics. In fact, it is the responsibility of the control system to decide on the behavior and performance of a particular system. The efficiency of a UPQC system is directly dependent on its control algorithm [96], [97]. The UPQC control method determines the current and voltage reference signals and the switching times of the inverters. Several control methods have been presented in the literature that have been successfully applied to UPQC systems. The two control methods that have been used the most to control UPQCs include the instantaneous power theory pq and the synchronous reference frame theory dq. These methods transfer the voltage and current signals from a three-phase system in an abc reference frame to components in a stationary pq reference frame or the synchronous rotating dq reference frame and separate them into main and harmonic components. In pq theory method, active and reactive instantaneous powers are calculated, while in dq theory, current and voltage are calculated independently of each other. The active and reactive powers corresponding to the main component in pq theory and the main component of currents and voltages in dq theory are dc values. Therefore, these values can be easily extracted by using low-pass filters [98], [99], [100].
A. Compensator Control System Based on PQ Power Theory
The pq theory is based on a collection of time-domain instantaneous powers. There are no constraints on voltage or current waveforms, and it may be utilised for three-phase systems with or without a neutral wire, in both steady and transient states. This theory is very efficient and flexible in designing controllers for power converter systems based on electronic switches. The pq theory first converts voltages and currents from abc coordinate to 
\begin{align*} \begin{bmatrix} V_{\alpha }\\ V _{\beta }\\ V _{0} \end{bmatrix}=&{2/3}\begin{bmatrix} 1&-1/2&-1/2\\ 0&\sqrt {(}3) /2 & -\sqrt {(}3) /2\\ 1/2&1/2&1/2 \end{bmatrix}\begin{bmatrix} V_{a}\\ V _{b}\\ V _{c} \end{bmatrix} \tag{1}\\ \begin{bmatrix} V_{a}\\ V _{b}\\ V _{c} \end{bmatrix}=&\begin{bmatrix} 1&0&1\\ -1/2&\sqrt {(}3) /2&1\\ -1/2&-\sqrt {(}3) /2&1 \end{bmatrix}\begin{bmatrix} V_{0}\\ V _{\alpha }\\ V _{\beta } \end{bmatrix} \tag{2}\\ \begin{bmatrix} I_{\alpha }\\ I _{\beta }\\ I _{0} \end{bmatrix}=&{2/3}\begin{bmatrix} 1&-1/2&-1/2\\ 0&\sqrt {(}3) /2 & -\sqrt {(}3) /2\\ 1/2&1/2&1/2 \end{bmatrix}\begin{bmatrix} I_{a}\\ I _{b}\\ I _{c} \end{bmatrix} \tag{3}\\ \begin{bmatrix} I_{a}\\ I _{b}\\ I _{c} \end{bmatrix}=&\begin{bmatrix} 1&0&1\\ -1/2&\sqrt {(}3) /2&1\\ -1/2&-\sqrt {(}3) /2&1 \end{bmatrix}\begin{bmatrix} I_{0}\\ I _{\alpha }\\ I _{\beta } \end{bmatrix} \tag{4}\end{align*}
Accordingly, by considering (1) and (3), the three-phase voltages and currents can be referred to the \begin{align*} e=&v_{\alpha }+jv_{\beta } \tag{5}\\ i=&i_{\alpha }+ji_{\beta } \tag{6}\end{align*}
\begin{align*} S=&e.i^{\ast}=(v_{\alpha }+jv_{\beta })+(i_{\alpha }-ji_{\beta }) \\=&(v_{\alpha }i_{\alpha }+v_{\beta }i_{\beta })+j(v_{\beta }i_{\alpha }-v_{\alpha }i_{\beta }) \tag{7}\end{align*}
\begin{align*} \begin{bmatrix} p\\ q \end{bmatrix}=\begin{bmatrix} v_{\alpha }&v_{\beta }\\ -v_{\beta }&v_{\alpha } \end{bmatrix}\begin{bmatrix} i_{\alpha }\\ i _{\beta } \end{bmatrix} \tag{8}\end{align*}
\begin{equation*} p=\bar {p}+\tilde {p}, q=\bar {q}+\tilde {q} \tag{9}\end{equation*}
\begin{align*} \begin{bmatrix} i_{\alpha }^{\ast}\\ i _{\beta }^{\ast} \end{bmatrix}=&\frac {1}{v_{\alpha }^{2}+v_{\beta }^{2}}\begin{bmatrix} v_{\alpha }&v_{\beta }\\ v _{\beta }&-v_{\alpha } \end{bmatrix}\begin{bmatrix} \tilde {p}\\ \tilde {q} \end{bmatrix} \tag{10}\\ \begin{bmatrix} v_{\alpha }^{\ast}\\ v _{\beta }^{\ast} \end{bmatrix}=&\frac {1}{v_{\alpha }^{2}+v_{\beta }^{2}}\begin{bmatrix} v_{\alpha }&v_{\beta }\\ v _{\beta }&-v_{\alpha } \end{bmatrix}\begin{bmatrix} \tilde {p}\\ \tilde {q} \end{bmatrix} \tag{11}\end{align*}
B. UPQC-DG Compensation Control System
The structure of UPQC-DG control system is shown in Fig. 13. The three main elements of this structure include positive component detector, series inverter control and parallel inverter control. The system acts in the condition of the simultaneous presence of the main power supply and distributed generation in the grid-connected mode, but if the main power supply is interrupted, the system acts in island mode. The structure of the positive component detector is shown in Fig. 14. The source voltage is used to calculate the current component \begin{align*} \begin{bmatrix} V_{s\alpha }^{\prime} \\ V _{s\beta }^{\prime} \end{bmatrix}=\frac {1}{i_{\alpha }^{\prime }{^{2}}+i_{\beta }^{\prime }{^{2}}}\begin{bmatrix} i_{\alpha }^{\prime} i_{\beta }^{\prime} \\ i _{\beta }^{\prime} -i_{\alpha }^{\prime} \end{bmatrix}\begin{bmatrix} \bar {p}^{\prime} _{s}\\ \bar {q}^{\prime} _{s} \end{bmatrix} \tag{12}\end{align*}
\begin{equation*} V_{c}^{\ast}=\Big [K_{PI}\Big \{ (V_{ref}^{\ast}-V_{s})-V_{F} \Big \}-I_{SF}\Big]*K+V_{F}^{\ast} \tag{13}\end{equation*}
\begin{align*} \begin{bmatrix} i_{c\alpha }^{\ast}\\ i _{c\beta }^{\ast}\end{bmatrix}=\frac {1}{v_{s\alpha }^{\prime }{^{2}}+v_{s\beta }^{\prime }{^{2}}}\begin{bmatrix} v_{s\alpha }^{\prime} -v_{s\beta }^{\prime} \\ v _{s\beta }^{\prime} v_{s\alpha }^{\prime} \end{bmatrix}\begin{bmatrix} -\tilde {p}+p_{loss}\\ -q \end{bmatrix} \tag{14}\end{align*}
Experimental Results and Discussions
In order to verify the performance of the various UPQC topologies, comprehensive simulations studies have been carried out with the help of power system simulation package MATLAB/SIMULINK. Figs. 15 and 16 show the active power drawn by series and parallel compensators for R-UPQC and L-UPQC. As can be seen, the parallel compensator is activated at t=0.1s to maintain the DC link voltage at a constant level, while the series compensator operates at t=0.2s in each topology. It is worth mentioning that the active power of series and parallel compensating for R-UPQC topology shows almost a constant value of zero compared to L-UPQC topology.
Figs. 17 and 18 show the reactive power drawn by series and parallel compensators for R-UPQC and L-UPQC. As can be seen, the compensation of the series is activated at t=0.2s to maintain the DC link voltage at a constant level. It is worth noting that R-UPQC topology act only in zero power injection/absorption mode [102]. Fig. 19 shows the active and reactive powers of the load,
Fig. 20 shows the three-phase voltage disturbances of distribution network. In the first section (a), the system voltage does not provide optimal system conditions and include disturbances due to the non-linear loads. The second section (b) shows the network voltage along with the M-UPQC module. It can be see that the voltage waveform has improved considerably [104]. The network voltage unbalance is other aspect provided in Fig. 21. In the first section (a), the voltage waveform has unbalanced with a variable amplitude of 1.5 pu. In the second section (b), by M-UPQC, the system unbalanced be improved to an acceptable level in 1 pu value. The another factor in the ‘PQ’ issue is harmonics which is defined as a total harmonic distortion (THD) component in electrical standards [104]. Conventional power angle control (PAC) methods lead to reactive power circulation and increased UPQC loading. These problems can be solved based on Synchronous Reference Frame (SRF) theory [105]. In contrast to traditional PAC, where series APF supply all reactive power, suggested PAC just compensates balanced reactive power. As a result, the suggested power angle and magnitude of series voltage are less than those in the traditional PAC. Source current and source voltage are in phase in both scenarios, indicating that there is no net exchange of reactive power with the grid, although there is reactive power circulation in the conventional case. Voltage waveforms in steady state for proposed PAC and conventional PAC are shown in Fig. 22.
Three phase voltage disturbances; (a) without the M-UPQC module, (b) with the M-UPQC module.
Three phase unbalanced output voltage; (a) without the M-UPQC module, (b) with the M-UPQC module.
Steady state waveforms of UPQC-DG, (a) with proposed method, (b) with conventional method.
Conclusion
In recent years, the reduction of fossil energy sources, the increase in the price of energy carriers, and the environmental consequences of energy production and consumption have made the power quality improvement more important than ever. This paper presents the most recent findings and compelling arguments about the power quality issues applicable to distribution network with the presence of distributed generation resources. The main purpose of this paper is to identify the most common power quality problems occurring in the distribution network and to provide a wide prospect to researchers and engineers who deal with disturbances and power quality issues. Innovative and revolutionary advancements in the field of grid-connected inverters have been described in order to improve the power quality of the DER. The main tendency of the researchers in this connection includes topologies of APF, UPQC and FACTS devices as power quality improvement technologies. APFs are established solutions that operate as a strong interface between harmonics pollution and DERs. UPFCs are one of the most widely used types of FACTS devices, which is used as an element for active power control, voltage stability, fluctuation damping, reactive power compensator, fault current limiter, power quality improvement in the power system.