# Performance Analysis of Grid-Connected Distributed Generation System Integrating a Hybrid Wind-PV Farm Using UPQC

Authors: Tongfei Lei; Saleem Riaz; Noor Zanib; Munira Batool; Feng Pan; Shaoguo Zhang
Journal: Complexity
 (2022)

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## Abstract

This work presents a distributed generation system (DG) that combines system of a wind turbine (WT) and photovoltaic (PV) using a unified power quality conditioner (UPQC). Along with providing active power (AP) to the utility grid, Wind-PV-UPQC improves PQ indicators, for example, voltage drops/surges, harmonics of grid voltages, and PF. Since Wind-PV-UPQC depends on dual compensation scheme, the parallel converter works as a sinusoidal voltage source, while the series converter works as a sinusoidal current source. In this way, a smooth transition from grid operation to island operation and vice versa can be achieved without load voltage transitions. In addition, in order to overcome the problems through abrupt solar radiation or wind speed variations, a faster power balance is achieved between the wind turbines, the PV array, and the grid, as FFCL pursue the production of the current references of series converter. Consequently, the dynamic reactions of the converter currents and the voltage of dc bus are enhanced. A comprehensive analysis of flow of the AP through the converters is done to ensure a proper understanding of how Wind-PV-UPQC works. Finally, the simulation results are shown to estimate the dynamic and static performance of Wind-PV-UPQC in conjunction with the power distribution system.


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## Body

## 1. Introduction

Today, electricity generation via renewable energy sources (RESs) has increased significantly, especially due to the increasing needs for electrical energy as well as intense worldwide struggles to reduce the dangerous environmental impact of contaminant energy sources, for example, coal, oil, gas, and others [1–6]. As a result, greenhouse effect and the global warming have enforced several states to sign a contract in Paris at the 2015 Climate change conference this century to toughen international measures to keep global temperature growth well below 2 degrees Celsius. Countries around the world tend to use renewable energy as a clean, useable, and cheaper energy source to achieve a high degree of integration of twenty percent by 2020 [7–10].In this context, RES-based distributed generation systems (DG) have helped to discover new modern way out for developing traditional energy structures. The use of DGs is more attractive as it improves the quality of the system, reduces CO2 emissions, and reduces losses in transmission and distribution systems [1, 11–15]. Though the core purpose of DGs is to provide real power to the grid, its multifunctionality may be extended to include the active power line conditioning targets [16–22]. In this situation, in addition to generating energy, an integrated DGs can also perform, instantaneously, active power line compensation, thus contributing to the improvement of the power quality indicators (PQ) [23]. Photovoltaic and wind energy included in this scenario are potential sources of DGs, as they simply require sunshine and wind to produce energy wherever assets are plentiful, comparatively clean, and free [24, 25]. The weakness of wind turbines and photovoltaics, in addition to being able to generate electricity, creates a series of current and voltage harmonics caused through the existence of different sorts of photovoltaic devices and wind turbines and power inverters. In addition to the rise in the amount of nonlinear loads associated with the network, it ultimately leads to a deterioration in the quality of the network.UPQC has been proposed to improve and overcome power quality because of the presence of nonlinear loads and the incorporation of photovoltaic and wind turbines in the utility grid. UPQC is used to recompense for quality problems of supply voltage, e.g., harmonics, flicker, sag, swell, and quality issues of load current such as harmonics, unbalances, reactive currents, and neutral current. As a general rule, steady-state (voltage imbalances and voltage harmonics) or nonpersistent (voltage sag and swell) PQ events related to line voltages may be resolved or alleviated by series active power filters, so-called dynamics voltage restorers [26, 27]. In contrast, PQ issues produced thru load configuration or even nonlinear properties of load currents may often be overwhelmed via using parallel active power filters. On the other hand, single-phase or three-phase arrangements of the UPQC are further suitable to simultaneously reduce PQ events from mains and load. In other words, systems of UPQC may perform series active power filters and parallel active power filter functionality at once. Two compensation schemes, “traditional” and “dual/inverted,” were chosen to control UPQC. These schemes are described according to the control methods of UPQC’s serial and parallel converters. With traditional UPQC, the controlled variables (current and voltage) are nonsinusoidal, so the series converter works as a nonsinusoidal voltage source, whereas the parallel converter works as a nonsinusoidal current source. Variables controlled by dual UPQC are always sinusoidal. Hence, the series converter works as a sinusoidal current source, whereas the parallel converter works as a sinusoidal voltage source. It should be noted that the dual UPQC controls the relevant variables (output voltages and input currents) because the series converter directly controls the input currents to remain in phase, symmetrical, and sinusoidal with the mains voltages while the parallel converter controls the balanced output voltages to remain sinusoidal and regulated [28–37]. Iterative control method is used to locate the specific position and time of the fault by its repetitive property. The data produced in each iteration is used for the next iteration for improvement in control system. The modeling of iterative control method for linear and nonlinear system is difficult work. The optimal control scheme based on optimization improves performance of the iterative control algorithm. For system optimization, convergence speed of iterative control algorithm is important and is improved by applying optimal control scheme [38–43].Typically, grid-connected photovoltaic systems can be used with either a single stage (S-S) or two-stage energy conversion [44, 45]. In a single stage system of photovoltaic, maximum power point tracking (MPPT) monitoring must be done by a DC/AC converter [46, 47], while in a two-stage system of photovoltaic, this job is typically taken over via a DC/DC boost converter [48]. Whatever the topology of the photovoltaic structure, the power balance between the system of photovoltaic and the electrical grid is achieved thru regulating the dc bus voltage of the inverter.In [49], a 3-Φ three-wire (3P3W) single stage system of photovoltaic layout is presented by utilizing two 3-wire voltage source inverters working using UPQC. However, experimental results are presented only considering its work like a dynamic voltage restorer. The elimination of harmonics from the mains voltage and the parallel active power filters process are not considered. In [50], the double stage PV system was implemented on a system of three-phase three-wire using simple computer simulations. However, harmonic suppression is not considered for line voltage or load current. A combined study of UPQC using distributed generation is described in [26]. UPQC power quality degradation in microgrids powered by wind turbines and photovoltaic energy system was implemented in [51]. As a result, PI and FLC were able to improve the power supply quality and reduce degradation in output power.In this article, the features of Wind-PV-UPQC can be highlighted as follows:(1)
Improvement of PQ indicators through series-parallel conditioning capabilities of power lines. Therefore, poor voltage harmonics and voltage regulation can be overcome because of the capability of the series active power filtering.(2)
Furthermore, effectual power factor (PF) correction may be made by capability of the active power filtering in parallel. It should be noted that parallel-series active filtering may continue in operation even if photovoltaic wind system is out of service due to maintenance work or during the night.(3)
Injecting the energy generating from a wind and photovoltaic energy system to the grid. Single-phase and three-phase consumers may or may not be connected to the system in this scenario. When there is no load, the system works similarly to traditional DGs in that universal active filtering can be disabled.The control loop of dc bus regulates the sinusoidal currents amplitudes supplied to network via the series inverter, for which the voltage reference of dc bus is determined through the MPPT procedure. Usually, the reaction of dynamics of the dc bus regulation loop should be slower as compared to the current regulation loops of the grid-connected inverter. It is therefore absolutely necessary to adequately adjust the gains of the two controllers mentioned, as the control circuit of dc bus should not degrade the current control loops performance, in order to ensure that undeformed currents are supplied to the network. On the other hand, sudden fluctuations in solar radiation or in wind speed can cause significant fluctuations in the circuit voltage of dc bus [46], which may change the correct balance of Wind-PV-UPQC power and thus affect the current references calculation of inverter.To solve this issue, this article too recommends using a feedforward control loop (FFCL) working in combination with the control loop of dc bus to speed up the control of current references of series converter. As a result, even with rapid changes in solar irradiation or wind speed, the dc bus voltage fluctuations are sufficiently damped, accelerating the system’s power equalization. The response time of the voltage of dc bus and overvoltage/undervoltage are significantly reduced as the dynamics are also improved.This paper is structured as follows: second part presents the explanation of the system of Wind-PV-UPQC along with the control structures including two NPC inverters connected back to back. Also, the strategies are used to create the input references of the serial and parallel converters as well as the FFCL and its core purpose. Third part analyzes the flow of energy via the Wind-PV-UPQC, whereas fourth part shows the results of advanced simulation to feature the double compensation scheme and FFCL achievement. Lastly, fifth part describes conclusions.


## 2. Wind-PV-UPQC System Configuration

Figure1 illustrates the power circuit’s block diagram of 3-phase 4-wire Wind-PV-UPQC used during the simulation tests. It is consisting of two three-level neutral point clamped (NPC) inverters that are connected back to back. According to the assumed double compensation strategies, the LC and L filters are coupled to the output of the respective parallel and series PWM converters. The NPC series converter is connected to the utility grid via three single-phase coupling transformers. A neutral point clamped module is frequently known as a three-level module. Every branch of the neutral point clamped inverter has four transistors that could be controlled, providing an entire of 24=16 possible conditions. However, only 3 of these conditions could be realized, because others produce short circuits on the dc link. The three possible conditions result in three various output voltages: 0 V, VDC/2, −VDC/2. The center point of the DC link capacitor is connected to the neutral point of the 3P4W power supply and the load, so that charge imbalances can be compensated.Figure 1
Block diagram of the Wind-PV-UPQC System.


### 2.1. Operation of UPQC with Dual Compensation Strategy

With the double compensation scheme, the series inverter must be regulated as a sinusoidal current source. In this study, the symmetrical and sinusoidal currents of the series inverter are regulated to be directly in phase with the fundamental positive sequence constituents of the mains voltages. This compensates for load asymmetries and reactive power and provides efficient power factor correction by eliminating harmonic currents. Also, the high impedance path generated via the sinusoidal current controlled series inverter enforces load current harmonics to stream via parallel inverter.In contrast, the parallel inverter must be regulated as a sinusoidal voltage source to deliver regulated, symmetrical, and sinusoidal voltages to the load. In this article, the output voltages are regulated so that they are always in phase with the positive sequence fundamental constituents of the mains voltages. In this situation, when the input and output voltage amplitudes are different from each other, only real power may be taken through the utility grid or injected into the utility grid with the series inverter in order to sustain the power stability of the system. Furthermore, the low impedance path formed through the voltage controlled sine wave parallel inverter permits current harmonics of load to flow thru the parallel inverter.It may be noted that the static (voltage asymmetries and harmonics) and unstable (voltage spikes and dips) power quality actions occur through the series transformers, leading to indirect voltage suppression/compensations.Because input references are sinusoidal, it is not necessary to calculate nonsinusoidal input references using specific techniques typically used in conventional compensation strategy. Also, the dq input references in the synchronous rotating reference frame are continuous because the regulated voltage and current values in the SRF abc are sinusoidal. This permits the utilization of a conventional proportional-integral (PI) controller through zero static error.


### 2.2. Series Converter References Current and Parallel Converter Output References Voltage

The signal flowchart presented in Figure2(a) shows the control loop of series current. First, load currents iL_a,iL_b,iL_c are dignified and converted to the dq axis of the SRF from the stationary reference frame of the abc axis, so as to the current iL_d may be attained directly as(1)iL_d=23cosθ−12cosθ+32sinθ−12cosθ+32sinθiL_aiL_biL_c.Figure 2
Signal flow diagrams of control loops of the series and parallel converters. (a) Series currents reference generation and control loops; (b) parallel reference voltages generation and control loops.
(a)(b)The currentiL_d consists of the harmonic and active constituents of the load currents, where cosθ and sinθ are the coordinates of the rotating unit vector. A PLL [52] is utilized to evaluate the phase angle of the line voltage θ = θpll.The average value ofiL_diL_ddc is obtained with a low pass filter (LPF), so that iL_ddc in SRF are active constituents of abc load currents. Lastly, the input current reference of the series converter on the d-axis is specified as(2)ise_d∗=iL_ddc+idc−iff,where idc is the PI controller DC link voltage output signal and iff is the expected current. The variable idc characterizes the quantity of active power processed by the series NPC inverter to ensure the power stability of the UPQC and thereby control the voltage of dc bus. In other words, idc adjusts the amplitude of ise_d∗ to regulate the energy flow thru the system of Wind-PV-UPQC to perform energy balancing. Together with idc, the quantity iff will accelerate the balance of power as described in the next section. Since symmetrical and sinusoidal currents are anticipated in the utility grid, both the quadrature current ise_q∗ and the zero component ise_0∗ are set to zero.The signal flow diagram presented in Figure2(b) shows control loop of parallel voltage. As mentioned earlier, parallel converter’s input voltage references are set to VL_abc∗ in the SRF (abc axes) and VL_d∗ directly on the SRF axis (d-axis) as presented in Figure 2(a). Since symmetrical and sinusoidal voltages are supplied to load, the quadrature voltage VL_q∗ and the zero sequence component VL_0∗ are set to zero [19].


### 2.3. Calculation ofiff

In the case of particularly sudden changes in wind speed or solar radiation, voltage fluctuations occur in the dc bus of the inverter, which interfere with the calculation of inverter current references and the correct operation of the inverter. Therefore, FFCL is necessary to reduce the amplitude of these oscillations because FFCL allows to speed up the calculation of input current references during the occurrence of rapid changes in wind speed or solar irradiance [18, 19, 48]. The action is represented by FFCL iff shown in Figure 2(a).Suppose that the supply voltage grid and the grid-connected series inverters currents are symmetrical. The wind photovoltaic system is also assumed to operate under ideal conditions, so system losses are negligible. In this scenario, all the active power drawn from the photovoltaic wind turbine is fed into the grid withPac=Pwind−pv, that is,(3)Pwind−pv=vwind−pviwind−pv=vdc1+vdc2iwind−pv=3Vs_pkIs_pk2,where vdc1 and vdc2 represent the dc bus voltage of NPC inverter, while iwind−pv represents the current of the wind photovoltaic system; and Vs_pk and Is_pk are peak magnitude of mains voltage and current, correspondingly, so that Vs_pk is assessed using the PLL scheme [52].(4)Is_pk=2vdc1+vdc2iwind−pv3Vs_pk.Lastly, the currentiff characterized in the dq rotating frame is specified by(5)iff=32.Is_pk.So,iff in (5) is used to calculate the input current reference series inverter ise_d∗ in the dq axis.As can be noted, the currentiff therefore makes it possible to improve the dynamic response of both the voltage of DC link and the currents of the utility grid-connected series inverters.


## 2.1. Operation of UPQC with Dual Compensation Strategy

With the double compensation scheme, the series inverter must be regulated as a sinusoidal current source. In this study, the symmetrical and sinusoidal currents of the series inverter are regulated to be directly in phase with the fundamental positive sequence constituents of the mains voltages. This compensates for load asymmetries and reactive power and provides efficient power factor correction by eliminating harmonic currents. Also, the high impedance path generated via the sinusoidal current controlled series inverter enforces load current harmonics to stream via parallel inverter.In contrast, the parallel inverter must be regulated as a sinusoidal voltage source to deliver regulated, symmetrical, and sinusoidal voltages to the load. In this article, the output voltages are regulated so that they are always in phase with the positive sequence fundamental constituents of the mains voltages. In this situation, when the input and output voltage amplitudes are different from each other, only real power may be taken through the utility grid or injected into the utility grid with the series inverter in order to sustain the power stability of the system. Furthermore, the low impedance path formed through the voltage controlled sine wave parallel inverter permits current harmonics of load to flow thru the parallel inverter.It may be noted that the static (voltage asymmetries and harmonics) and unstable (voltage spikes and dips) power quality actions occur through the series transformers, leading to indirect voltage suppression/compensations.Because input references are sinusoidal, it is not necessary to calculate nonsinusoidal input references using specific techniques typically used in conventional compensation strategy. Also, the dq input references in the synchronous rotating reference frame are continuous because the regulated voltage and current values in the SRF abc are sinusoidal. This permits the utilization of a conventional proportional-integral (PI) controller through zero static error.


## 2.2. Series Converter References Current and Parallel Converter Output References Voltage

The signal flowchart presented in Figure2(a) shows the control loop of series current. First, load currents iL_a,iL_b,iL_c are dignified and converted to the dq axis of the SRF from the stationary reference frame of the abc axis, so as to the current iL_d may be attained directly as(1)iL_d=23cosθ−12cosθ+32sinθ−12cosθ+32sinθiL_aiL_biL_c.Figure 2
Signal flow diagrams of control loops of the series and parallel converters. (a) Series currents reference generation and control loops; (b) parallel reference voltages generation and control loops.
(a)(b)The currentiL_d consists of the harmonic and active constituents of the load currents, where cosθ and sinθ are the coordinates of the rotating unit vector. A PLL [52] is utilized to evaluate the phase angle of the line voltage θ = θpll.The average value ofiL_diL_ddc is obtained with a low pass filter (LPF), so that iL_ddc in SRF are active constituents of abc load currents. Lastly, the input current reference of the series converter on the d-axis is specified as(2)ise_d∗=iL_ddc+idc−iff,where idc is the PI controller DC link voltage output signal and iff is the expected current. The variable idc characterizes the quantity of active power processed by the series NPC inverter to ensure the power stability of the UPQC and thereby control the voltage of dc bus. In other words, idc adjusts the amplitude of ise_d∗ to regulate the energy flow thru the system of Wind-PV-UPQC to perform energy balancing. Together with idc, the quantity iff will accelerate the balance of power as described in the next section. Since symmetrical and sinusoidal currents are anticipated in the utility grid, both the quadrature current ise_q∗ and the zero component ise_0∗ are set to zero.The signal flow diagram presented in Figure2(b) shows control loop of parallel voltage. As mentioned earlier, parallel converter’s input voltage references are set to VL_abc∗ in the SRF (abc axes) and VL_d∗ directly on the SRF axis (d-axis) as presented in Figure 2(a). Since symmetrical and sinusoidal voltages are supplied to load, the quadrature voltage VL_q∗ and the zero sequence component VL_0∗ are set to zero [19].


## 2.3. Calculation ofiff

In the case of particularly sudden changes in wind speed or solar radiation, voltage fluctuations occur in the dc bus of the inverter, which interfere with the calculation of inverter current references and the correct operation of the inverter. Therefore, FFCL is necessary to reduce the amplitude of these oscillations because FFCL allows to speed up the calculation of input current references during the occurrence of rapid changes in wind speed or solar irradiance [18, 19, 48]. The action is represented by FFCL iff shown in Figure 2(a).Suppose that the supply voltage grid and the grid-connected series inverters currents are symmetrical. The wind photovoltaic system is also assumed to operate under ideal conditions, so system losses are negligible. In this scenario, all the active power drawn from the photovoltaic wind turbine is fed into the grid withPac=Pwind−pv, that is,(3)Pwind−pv=vwind−pviwind−pv=vdc1+vdc2iwind−pv=3Vs_pkIs_pk2,where vdc1 and vdc2 represent the dc bus voltage of NPC inverter, while iwind−pv represents the current of the wind photovoltaic system; and Vs_pk and Is_pk are peak magnitude of mains voltage and current, correspondingly, so that Vs_pk is assessed using the PLL scheme [52].(4)Is_pk=2vdc1+vdc2iwind−pv3Vs_pk.Lastly, the currentiff characterized in the dq rotating frame is specified by(5)iff=32.Is_pk.So,iff in (5) is used to calculate the input current reference series inverter ise_d∗ in the dq axis.As can be noted, the currentiff therefore makes it possible to improve the dynamic response of both the voltage of DC link and the currents of the utility grid-connected series inverters.


## 3. Flow of Active Power through the Parallel and Series NPC Inverter

The flow of active power via the system of Wind-PV-UPQC is shown in Figure3. It is determined by considering the following: (1) the difference among the RMS network and the voltage of load; (2) the amount of energy produced by the wind and photovoltaic power system; and (3) the volume of energy consumed through the load. To create analysis, the following active powers are used in Figures 3(a) to 3(c): grid power Ps, load power PL, wind turbine and photovoltaic generator power Pwind−pv, power of series converter Pse, and power of parallel converter Ppa.Figure 3
Flow of active power by the system of Wind-PV-UPQC. (a)Pwind−pv>PL; (b) Pwind−pv>PL; (c) PL=0.
(a)(b)(c)In an ideal case, whenVs=VL, all the energy is controlled via parallel converter and there is no flow of real power via the series converter.In Figure3(a), as PL≠0W, Pwind−pv>PL, and Vs>VL, in this scenario, the amount of the energy generated by the wind and photovoltaic energy system is sent to the utility grid via the parallel and series converter and the rest is delivered to load via parallel converter. If Vs>VL, in this scenario, the amount of the energy generated by the wind and photovoltaic energy system is supplied to the grid through parallel converter and the rest is directed to load.In Figure3(b), as PL≠0W, Pwind−pv<PL, and Vs>VL, in this scenario, all energy generated through the wind and photovoltaic energy system is supplied to load via parallel converter and the rest is exhausted from the utility grid. As can be noted, amount of the energy provided thru the utility grid to the load proceeds through parallel and series converters. If Vs>VL, in this scenario, the energy generated through the wind and photovoltaic energy system is supplied to load via the parallel and series converters and the rest is exhausted from the utility grid.In Figure3(c), as PL=0W and Vs>VL, in this scenario, all energy generated through the wind and photovoltaic energy system is supplied to the grid via the parallel and series converters. The bulk of energy is constantly controlled via the parallel converter. If Vs<VL, in this scenario, all energy generated through the wind and photovoltaic energy system is supplied to the utility grid via the parallel NPC inverter. The real power consumed through the series converter proceeds by the parallel converter.Table1 presents the conditions that are used to regulate the power flow through the NPC inverter.Table 1
Conditions assumed to regulate the flow of active power by the Wind-PV-UPQC.
ModesRMS voltagesLoad powerWind-PV power1aVs>VLPL>0Pwind−pv>PLbVs<VL2aVs>VLPL>0Pwind−pv<PLbVs<VL3aVs>VLPL=0Pwind−pv>0bVs>VL


## 4. Simulation Results

The steady-state and dynamic performance of the Wind-PV-UPQC system are analyzed by simulating the system in MATLAB/Simulink software. The simulation of the system is performed based on the block diagram of circuit shown in Figure1. Sinusoidal Pulse Width Modulation (SPWM) technique is utilized in series and parallel NPC inverters [53]. For the simulation 5.0505e − 6 s solver step size is used. Detailed system parameters and three-phase nonlinear load are given in Appendix.


### 4.1. Performance of Wind-PV-UPQC in Steady-State Condition

This subsection describes the steady-state results where the Wind-PV-UPQC works in three operating modes (OPM). These OPMs are illustrated in Figure4 taking into account the various operating conditions to which the Wind-PV-UPQC is exposed. In OPM 1, the system of Wind-PV-UPQC works without load and injects only active power into the utility grid. OPM 2 happens while there is instantaneous power of wind turbine and solar irradiation is needed by native loads linked to the system of Wind-PV-UPQC, so that Pwind−pv<PL. In OPM 3 that arises at night (without solar radiation), with Wind-PV-UPQC coupled to asymmetrical and distorted utility grid with Pwind−pv = 0 W, the system only works as a UPQC.Figure 4
Operating modes described in the steady-state simulation results.Figures5 and 6 show steady-state results for a system of Wind-PV-UPQC supplying real power to the utility grid, taking into consideration a 3-Φ full-bridge rectifier followed via an R load. In Figure 5, the load is detached and only active power is interjected into the utility grid (OPM 1). Therefore, all the power generated through the system of wind and photovoltaic is interjected into the utility grid and the system losses are updated, where Pwind−pv is approximately 3500 W. As can be noticed, the currents of source are sinusoidal and reverse to the phase according to the particular mains voltages, although three-phase output voltages keep regulated, symmetrical, and sinusoidal. It has also been noted that almost all of the real power is interjected into the utility grid via parallel inverter.Figure 5
Only active power interjection is performing by Wind-PV-UPQC into the grid withPL = 0 W and Pwind−pv = 3500 W. (a) Grid voltage currents, (b) load voltages, (c) load voltage and parallel converter currents.
(a)(b)(c)Figure 6
Active filtering along with active power interjection is performed by Wind-PV-UPQC withPwind−pv<PL. (a) Grid voltage and current, (b) load voltage, (c) grid currents, (d) load current, (e) parallel converter current.
(a)(b)(c)(d)(e)OPM 2 happens whilePwind−pv<PL, as presented in Figure 6. Note that the Wind-PV-UPQC system feeds the load in conjunction with the grid. Since wind and photovoltaic power is less than the actual power required by the load, the system draws power from the utility grid. As can be seen, the system of Wind-PV-UPQC in OPM 2 performs active power line conditioning. By eliminating current harmonics and compensating for load asymmetries, sinusoidal and symmetrical mains currents are obtained. Moreover, regulated and practically sinusoidal and symmetrical load voltages are attained.In OPM 3, Figure7 shows the operation of Wind-PV-UPQC. In this situation, it is connected to a programmable AC power supply that delivers biased input voltages, for example, voltage imbalances, harmonics, and faults. As you can see, parallel and series inverters both are performing active line filtering. The output voltages vL_abc are consistently balanced, free of harmonics, and regulated even under the distorted voltage conditions. Since the voltages of output are controlled sinusoidally and symmetrically, the difference among the voltages of input and output occurs in series coupling transformers vTse_abc.Figure 7
Wind-PV-UPQC coupled to asymmetrical and distorted utility gridPwind−pv = 0 W; (a) voltages of grid; (b) load voltages; (c) grid voltages of Phase “a” intended for voltage sags (25%), (d) output voltage of Phase “a” intended for voltage sags (25%).
(a)(b)(c)(d)Table2 shows the total harmonic distortion (THD) and RMS values of currents and voltages connected to the utility grid and the load. As can be noticed, THDs of output voltages and mains currents are reduced in all operating modes. It is also noticed that the THD values of current and voltages are within limits described in IEEE 519 Standard. In addition, Table 2 presents the PF for the network and the load. It should be noted that effectual PF corrections are attained. From the steady results shown in Figures 5–7 and Table 2, it may be proved that the system can work in various operating mode and provides regulated, balanced, and sinusoidal voltages to the loads. Also, the system can efficiently feed/drain power to/from the grid with high PF.Table 2
Conditions assumed to regulate the flow of real power via the Wind-PV-UPQC.
Operating conditionsRMS values of the currents (A) and voltages (V)vsavsbvscisaisbiscvLavLbvLciLaiLbiLcOPM11271271278.028.018.04128128128———OPM 21271271274.344.354.361261261267.187.197.18OPM 31261161067.187.167.141151161166.396.406.40Total harmonic distortion (%)OPM11.81.91.92.52.62.42.41.91.6———OPM 21.71.81.75.55.54.82.52.21.7292928OPM 310.69.99.94.53.93.92.22.01.5292829Power factor (PF)Utility gridLoadPhase “a”Phase “b”Phase “c”Phase “a”Phase “b”Phase “c”OPM1−0.99−0.99−0.99———OPM 20.990.980.980.960.960.96OPM 31.00.980.990.960.960.96


### 4.2. Performance of Wind-PV-UPQC in Dynamic Condition

The dynamic performance of the system of Wind-PV-UPQC has been tested taking into account sudden changes in wind speed and solar radiation such as 100% to 0% and 0% to 100%. The dynamic consequences of wind speed changes and sudden changes in solar radiation are shown in Figures8 where the total dc bus voltage Vdc, grid voltage vsa, and current isa across PCC 1, the voltage of load vLa across PCC 2, dc bus controller output signal idc, feedforward current iff, and power of wind and photovoltaic system Pwind−pv are shown. The tests were carried out by disconnecting and reconnecting the wind and photovoltaic systems from the dc bus. Figure 8 illustrates the outcomes when iff is accustomed to speed up the production of series converter current references. According to the results, if iff is used during transitions in wind and photovoltaic systems, the dc bus voltage fluctuations are reduced.Figure 8
Active filtering along with active power interjection is performed by Wind-PV-UPQC withPwind−pv>PL. Voltage of dc bus Vdc, Phase “a” grid current and grid voltage, load voltage, iff current, idc current, and Wind and Photovoltaic power Pwind−pv for sudden wind and solar irradiance variations using FFCL.As you can see, Wind-PV-UPQC performs active filtering while the wind and photovoltaic system is off. By eliminating current harmonics and compensating for load asymmetries, sinusoidal and symmetrical mains currents are obtained. Moreover, regulated and practically sinusoidal and symmetrical load voltages are attained. The Wind-PV-UPQC system then provides approximately 3500 W of real power to the load and the grid when the wind and photovoltaic system are connected. It also illustrates that the grid current dynamic response is quick when the wind and photovoltaic system is exposed to sudden power changes, because of using FFCL.


## 4.1. Performance of Wind-PV-UPQC in Steady-State Condition

This subsection describes the steady-state results where the Wind-PV-UPQC works in three operating modes (OPM). These OPMs are illustrated in Figure4 taking into account the various operating conditions to which the Wind-PV-UPQC is exposed. In OPM 1, the system of Wind-PV-UPQC works without load and injects only active power into the utility grid. OPM 2 happens while there is instantaneous power of wind turbine and solar irradiation is needed by native loads linked to the system of Wind-PV-UPQC, so that Pwind−pv<PL. In OPM 3 that arises at night (without solar radiation), with Wind-PV-UPQC coupled to asymmetrical and distorted utility grid with Pwind−pv = 0 W, the system only works as a UPQC.Figure 4
Operating modes described in the steady-state simulation results.Figures5 and 6 show steady-state results for a system of Wind-PV-UPQC supplying real power to the utility grid, taking into consideration a 3-Φ full-bridge rectifier followed via an R load. In Figure 5, the load is detached and only active power is interjected into the utility grid (OPM 1). Therefore, all the power generated through the system of wind and photovoltaic is interjected into the utility grid and the system losses are updated, where Pwind−pv is approximately 3500 W. As can be noticed, the currents of source are sinusoidal and reverse to the phase according to the particular mains voltages, although three-phase output voltages keep regulated, symmetrical, and sinusoidal. It has also been noted that almost all of the real power is interjected into the utility grid via parallel inverter.Figure 5
Only active power interjection is performing by Wind-PV-UPQC into the grid withPL = 0 W and Pwind−pv = 3500 W. (a) Grid voltage currents, (b) load voltages, (c) load voltage and parallel converter currents.
(a)(b)(c)Figure 6
Active filtering along with active power interjection is performed by Wind-PV-UPQC withPwind−pv<PL. (a) Grid voltage and current, (b) load voltage, (c) grid currents, (d) load current, (e) parallel converter current.
(a)(b)(c)(d)(e)OPM 2 happens whilePwind−pv<PL, as presented in Figure 6. Note that the Wind-PV-UPQC system feeds the load in conjunction with the grid. Since wind and photovoltaic power is less than the actual power required by the load, the system draws power from the utility grid. As can be seen, the system of Wind-PV-UPQC in OPM 2 performs active power line conditioning. By eliminating current harmonics and compensating for load asymmetries, sinusoidal and symmetrical mains currents are obtained. Moreover, regulated and practically sinusoidal and symmetrical load voltages are attained.In OPM 3, Figure7 shows the operation of Wind-PV-UPQC. In this situation, it is connected to a programmable AC power supply that delivers biased input voltages, for example, voltage imbalances, harmonics, and faults. As you can see, parallel and series inverters both are performing active line filtering. The output voltages vL_abc are consistently balanced, free of harmonics, and regulated even under the distorted voltage conditions. Since the voltages of output are controlled sinusoidally and symmetrically, the difference among the voltages of input and output occurs in series coupling transformers vTse_abc.Figure 7
Wind-PV-UPQC coupled to asymmetrical and distorted utility gridPwind−pv = 0 W; (a) voltages of grid; (b) load voltages; (c) grid voltages of Phase “a” intended for voltage sags (25%), (d) output voltage of Phase “a” intended for voltage sags (25%).
(a)(b)(c)(d)Table2 shows the total harmonic distortion (THD) and RMS values of currents and voltages connected to the utility grid and the load. As can be noticed, THDs of output voltages and mains currents are reduced in all operating modes. It is also noticed that the THD values of current and voltages are within limits described in IEEE 519 Standard. In addition, Table 2 presents the PF for the network and the load. It should be noted that effectual PF corrections are attained. From the steady results shown in Figures 5–7 and Table 2, it may be proved that the system can work in various operating mode and provides regulated, balanced, and sinusoidal voltages to the loads. Also, the system can efficiently feed/drain power to/from the grid with high PF.Table 2
Conditions assumed to regulate the flow of real power via the Wind-PV-UPQC.
Operating conditionsRMS values of the currents (A) and voltages (V)vsavsbvscisaisbiscvLavLbvLciLaiLbiLcOPM11271271278.028.018.04128128128———OPM 21271271274.344.354.361261261267.187.197.18OPM 31261161067.187.167.141151161166.396.406.40Total harmonic distortion (%)OPM11.81.91.92.52.62.42.41.91.6———OPM 21.71.81.75.55.54.82.52.21.7292928OPM 310.69.99.94.53.93.92.22.01.5292829Power factor (PF)Utility gridLoadPhase “a”Phase “b”Phase “c”Phase “a”Phase “b”Phase “c”OPM1−0.99−0.99−0.99———OPM 20.990.980.980.960.960.96OPM 31.00.980.990.960.960.96


## 4.2. Performance of Wind-PV-UPQC in Dynamic Condition

The dynamic performance of the system of Wind-PV-UPQC has been tested taking into account sudden changes in wind speed and solar radiation such as 100% to 0% and 0% to 100%. The dynamic consequences of wind speed changes and sudden changes in solar radiation are shown in Figures8 where the total dc bus voltage Vdc, grid voltage vsa, and current isa across PCC 1, the voltage of load vLa across PCC 2, dc bus controller output signal idc, feedforward current iff, and power of wind and photovoltaic system Pwind−pv are shown. The tests were carried out by disconnecting and reconnecting the wind and photovoltaic systems from the dc bus. Figure 8 illustrates the outcomes when iff is accustomed to speed up the production of series converter current references. According to the results, if iff is used during transitions in wind and photovoltaic systems, the dc bus voltage fluctuations are reduced.Figure 8
Active filtering along with active power interjection is performed by Wind-PV-UPQC withPwind−pv>PL. Voltage of dc bus Vdc, Phase “a” grid current and grid voltage, load voltage, iff current, idc current, and Wind and Photovoltaic power Pwind−pv for sudden wind and solar irradiance variations using FFCL.As you can see, Wind-PV-UPQC performs active filtering while the wind and photovoltaic system is off. By eliminating current harmonics and compensating for load asymmetries, sinusoidal and symmetrical mains currents are obtained. Moreover, regulated and practically sinusoidal and symmetrical load voltages are attained. The Wind-PV-UPQC system then provides approximately 3500 W of real power to the load and the grid when the wind and photovoltaic system are connected. It also illustrates that the grid current dynamic response is quick when the wind and photovoltaic system is exposed to sudden power changes, because of using FFCL.


## 5. Conclusion

In this paper, a DG system named Wind-PV-UPQC is proposed, which combines a 3P4W grid-connected wind and photovoltaic energy conversion system using a unified power quality conditioner and also performs the duties of an UPQC dual compensation scheme, along with FFCL. The arrangement, called Wind-PV-UPQC, consists of two 3-level NPC inverters connected back to back. In addition to supplying active energy from the arrangement of wind photovoltaic, the wind photovoltaic-UPQC arrangement was capable of realizing series and parallel conditioning of power lines. The dynamic and steady-state performance of the system was assessed in distorted/disturbed line voltage situations, containing dips, asymmetries, and harmonics. In addition to the series compensation, an effective compensation of the reactive power is obtained through compensating the reactive power of the load by eliminating the harmonic currents of the load. The efficiency of the FFCL acting on the current references of the series converter was correctly evaluated for sudden changes in wind speed and solar irradiance. Wind-PV-UPQC can be seen to be a good solution for modern distribution systems via integrating distributed generation with improved power quality.



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*Source: 4572145-2022-03-18.xml*