1. Introduction
Distributed generation (DG) can be defined as small-scale, bidirectional generating units that are connected to the power system at the distribution level. These units are generally considered a sustainable source of electrical power, which have a low environmental impact and power losses and can balance demand and production at the end-user side. DG units could be renewable and non-renewable resources that work together. Renewable energy sources (RESs) play a major role in DG. Using different power sources such as photovoltaic systems, wind turbines, biomass, fuel cells, and other RESs, DG aims to cover load capacity at the distribution level under different operating conditions with minimum losses, dispatch costs, and gas emissions [
1]. The incessant increase in electrical energy demand leads to RES penetration in DG increment, which can affect the power quality (PQ) parameters in the utility grid, so it is important to determine the optimal size and location for DG units under certain constraints in points of common connection such as voltage profile, power flow limits, and nominal voltage values [
2]. The high penetration of non-linear loads and power electronics is the main cause of voltage and current harmonics in power systems. As well, the high penetration of renewable energy systems such as PV systems, wind turbines, etc., which are installed at distribution power grids, increases every day because of power systems structure development [
3]. This penetration presents new power quality challenges for distributed generation systems. Additionally, low power quality can negatively affect the performance of sensitive loads connected to the grid. Many researchers are making efforts to improve power quality. Several solutions have been found for PQ improvement, such as the series active power filter (SAPF) to handle voltage harmonics and the parallel active power filter (PAPF) to eliminate current harmonics caused by non-linear loads [
3]. In [
4], a harmonic active filter has been used with four wind turbines connected with a 33 kV utility grid. The current THD was 7.44% when all the wind turbines operated together and 15.95% with only one wind turbine. Additionally, a passive filter has been used as a conventional solution [
5]. Different types of passive filters were applied with a small-scale wind turbine which operates with different wind speed values (6–13 m/s). The lowest THD value was 11.42% for the current and 5.84% for the voltage. In [
6], hybrid filters were used with four grid-connected wind turbines, and the proposed hybrid filter consists of a series active filter and a passive filter, which was considered a good solution to minimize the installation cost for APF.
A shunt active filter can be used to eliminate current harmonics provided by non-linear loads. An SAF consists of an inverter and a DC source. There are many topologies to generate pulses for electronic switches in an inverter, such as pulse-width modulation or hysteresis. The DC source can also be a renewable energy-generating unit such as PV or fuel cell. In [
7], using a fuzzy logic controller lowers the current THD from 28% to 4.85%. For improving active power filter performance, conventional controllers can be used, or expert systems controllers such as fuzzy logic to detect current harmonics and drive the APF, especially with the presence of RES where the generated power changes rapidly according to the resource nature. In [
8], the current THD for grid-connected 60 W PV was 3.54% with the non-linear load.
One of the most effective tools for addressing problems with voltage and current quality is the unified power quality conditioner (UPQC). A UPQC consists of parallel active power filters and series active power filters. The UPQC is a power conditioning tool that may correct various power quality issues [
9]. A non-linear load’s harmonics are eliminated by a shunt active filter, while supply voltage flicker and imbalance from the load terminal voltage are removed by the series active filter component.
The UPQC filters harmonics and reduces current and voltage power quality issues. Therefore, the UPQC is seen to be a very excellent solution to power quality issues, and there is no need to employ several installations in the distribution system. D-Q theory may also be applied in the controllers for the series and shunt compensator. The UPQC can regulate voltage amplitude, and it has been used in [
10] with an 11 kV utility grid, where it reduced voltage sag and swell by 73%.
The UPQC cannot compensate for voltage interruption without an energy storage system or an indented power-generating unit to feed the DC link voltage of back-to-back inverters in the UPQC. By using this installation, the UPQC can provide power to the sensitive loads during islanding, which may occur during faults in DG [
11]. It can also regulate the power quality parameters on the load side regardless of the changes on the grid side. A smart grid has both direction power flow and information, monitoring and reacting to variations in everything from power plants to distinct consumer appliances. Therefore, with the high implementation of distributed generating systems (DGSs), the quality of power supply has become a notable problem for micro-grids or smart grids.
Table 1 shows the PQ problems in distribution power systems [
12].
The synchronous reference frame approach is used in a series of active filters to determine the fundamental component of voltage. However, in parallel active filters, the instantaneous reactive power theory (IRPT) is used to extract the fundamental current. This method was proposed in [
13] to reduce voltage THD to 3.8% and current THD to 4.5% under unbalanced load conditions. In a [
14] study that compared STIA, PI, and PID controllers in the UPQC under different operating conditions, voltage THD was 10.3% with PI, 7.5% with PID, and 0.2% with STIA.
Several studies have proposed different control techniques for generating reference voltage and current, such as gate-pulse generation using PWM controllers and a capacitance-balancing or hysteresis current controller (HCC), multilevel vector-based hysteresis current controller (MVHCC), and adaptive hysteresis current controller (AHCC) [
15]. Furthermore, the controller for series and shunt inverters can be based on a variety of methodologies, including instantaneous power theory, the load-equivalent conductance approach, and fuzzy logic controllers [
16], etc. Ref. [
17] proposes a single-phase universal power compensator (UPC) system with equal reactive power sharing between the series and parallel APF of a UPC to increase PQ and alleviate voltage and current concerns. The equal VAR sharing function makes it possible for series and shunt active power filter (APF) inverters to have the same rating. To achieve equitable VAR sharing amongst the active filters, a synchronous reference frame based on direct power angle computation is employed. This power angle calculation for current parameters taken from the d and q axes is used to create the reference signal for a shunt active power filter. When compared to other standard approaches, the synchronous reference frame method is quite beneficial for power calculations. The current THD was reduced to 3.5%. In [
18], an intelligent control technique for maximum PQ improvement in a hybrid grid-tie power system comprising PV panels, a battery bank, and a wind turbine is proposed. In a grid-tied renewable energy system, a UPQC with active and reactive power (UPQC-PQ) includes atom search optimization based on a fractional-order PID controller. The suggested approach attempts to reduce the THD by controlling the voltage at the point of common connection (PCC), lowering loss, and mitigating harmonics. The suggested technique additionally uses atom search optimization based on a fractional-order PID controller to alleviate PQ difficulties. Ref. [
19] describes a compensation method for non-active voltage/current components that use a UPQC and is controlled by a conductance signal. The active power of the corrected load can be used to generate the conductance signal. Two variables, load power and the supply voltage, can be used to estimate this signal. It is possible to avoid the difficult ways of evaluating voltage or current waveforms by decomposing them into several components by utilizing the conductance signal as the reference for the UPQC. When combined with the UPQC, the conductance technique permits bidirectional power transfer with a unity power factor from the source to the load and vice versa when the load can create power. This allows the UPQC to be used as an energy buffering and distribution center, which may be highly useful in smart microgrids. The UPQC may be employed with grid-connected renewable energy systems such as solar PV systems and wind turbines [
20,
21], and the DC-AC converter can influence the THD for protected loads such as half-bridge or multilayer inverters [
22]. Ref. [
23] investigated a nine-level converter for the planned UPQC. The UPQC connects the PV system to the electrical grid. Then, a novel method for generating pulses for transistors on both UPQC inverters is presented. This is based on an adaptive hysteresis band (AHB) defined by FLC to obtain the output voltage with the least amount of THD. The goal of a UPQC connected to a wind turbine is to compensate for reactive power to control voltage and eliminate the THD in the PCC. The UPQC may be said to enhance the PQ at the PCC for distribution grids [
24,
25]. In [
26], a control approach for compensation by employing a UPQC coupled with a DFIG on a poor distribution system is described. The suggested compensatory approach enhances the system PQ by taking advantage of DC-bus storage and active power-sharing on UPQC inverters. The findings demonstrated a satisfactory reaction to AP fluctuation caused by the “tower shadow effect” and voltage regulation caused by a rapid load connection. In this paper, we attempt to improve all power quality issues for voltage and current (harmonics, voltage sag, swell, and drop) for the protected load, which is connected to the electrical grid with a wind turbine. The UPQC is used to improve power quality at the PCC for sensitive loads with wind turbine–grid power systems during different disturbances. We then compare the results for PWM and hysteresis-pulsing-generating methods.
7. Results Discussion
A UPQC is used to deliver electrical power to loads with high quality and without interruptions. The results can be divided into four sections to be discussed:
There are many parameters that affect the voltage amplitude, such as the change in the generated power from wind turbines, faults, load nature, and its value. Most standard emphases require a 5% variation in voltage amplitude at the distribution level. It is clear that the voltage amplitude on the load side is constant in all cases with two different pulsing techniques, despite the changes in the grid side, as we can see from the results figures when the fault happened. Additionally, removing it as a transit state in the system did not affect voltage amplitude.
- 2.
Voltage THD
Voltage harmonics usually come from the grid side to the load, and we can see that in the first case of the non-linear load, where the THD on the grid side was high, the UPQC should have lowered the THD by injecting the same amount of voltage harmonics at the PCC. As we know, the THD is a very important parameter in determining power quality. We used the THD as a point of compression between different control techniques.
Table 2 shows compression among the results of different cases of simulation when the non-linear load is on the grid side or the load side.
Table 3 shows the compression of the proposed method compared with the papers in the literature review for the voltage THD.
- 3.
Current THD
Current harmonics essentially come from non-linear loads and go toward other near loads or utility grids. The shunt part of the UPQC is responsible for illuminating these harmonics before it goes into the system. The injected harmonics from the non-linear load should be illuminated by the UPQC, no matter where the non-linear load is connected.
Table 4 shows the load current THD in different simulation cases.
Table 5 shows the compression of the proposed method with the papers in the literature review for the current THD.
- 4.
DC Link voltage
Using an independent voltage source is important, because if we use a rectified voltage from the grid, the UPQC performance will be affected by the changes on the grid side. The proposed structure was able to regulate voltage in the DC link between the inverters to ensure power injection to the system. By using Clack’s transformations and a PI controller, the DC voltage was adjusted with 1PU under different compensating cases, which improved the UPQC total performance.