Effect of Reactive Power Capability of the PV Inverter on the Power System Quality

ABSTRACT


INTRODUCTION
The depletion of conventional energy reserves, rising environmental concerns such as pollution and global warming, in addition to the volatility of oil prices, and the increasing demand for electricity worldwide have led to the need to find alternatives to traditional energy sources to overcome the challenges of a sustainable energy supply [1].Renewable energy-based distributed generation systems have garnered a lot of interest as an alternative energy source that can be used to provide electricity to small or medium-sized businesses and homes.Solar energy has gained global attention for its immense potential and inexhaustible nature [2].The amount of energy from the sun that reaches the Earth's surface in one day is ten times the total energy consumed by all the people on this planet in one year.PV, which is clean and pollution-free, has great potential for the supply of energy with minimal impact on the environment, in addition to several advantages, including the possibility of generating energy near to consumers to reduce energy losses, increased voltage levels, improved energy quality and reliability, reduced operating costs, and the creation of a more stable network [3] ، [4].
In recent years, the integration of a solar energy system with traditional electrical networks has witnessed a tremendous growth [5], reaching about 700 GW at the end of 2020 [6].According to the renewable energy capacity statistics of the International Renewable Energy Agency (IRENA) (2022), as shown in Figure .1,China is currently the main producer of solar energy, followed by the United States, Japan and then, Germany.Also, the installed solar energy is estimated by the area shown in Figure .2. In Asia, the installed solar energy is about 57.07% of the global total, and in Europe it is estimated at 22.79% of the global total, while the Middle East has the lowest installed solar energy of about 1.13% of the global total.The interconnection of these DGs to the conventional grid is usually achieved through the use of power converters.The use of power converters provides significant benefits such as optimal operation and flexible control [7], in addition to the benefits offered by DGs, which include voltage support, reduced losses, high power quality and system reliability.Depending on the characteristics of both the system and the DG, customer bills can also be reduced by reselling the additional energy generated.Nevertheless, the introduction of DGs into the electrical network creates many problems [8] in terms of protection and safety issues, harmonic distortion, and transient issues [9].
In addition, PV systems efficiently inject power into the grid, thereby providing an effective solution to the increasing demand for energy [10].On the other hand, the irregular supply of Photovoltaic (PV) energy can severely affect the complete distribution network, and also hinder the quality, reliability and availability of the network.Furthermore, if the production of the PV energy is higher than the domestic energy requirement [11], the complete distribution system loses control, and it is likely to reverse the power circulation through a distribution feeder or even a local substation, thereby damaging the utility grid.This can further affect some of the customer utilities that are powered by this distribution circuit, owing to unstable voltage [12].
Reactive power compensation is an actual solution to the challenges of voltage fluctuations and better power quality.Thus, PV inverters can be used as on-site VAR compensators that are responsive to changing loads to provide the reactive power to be absorbed by inductive loads.This could help in decreasing the voltage regulation problems, improving the power factor (PF) and grid stability, along with maximising the power transmission capacity after decreasing the power transmission losses.The customers could also save a lot of money as they would not have to invest in the equipment for controlling the PF [13].From this perspective, and to study the effect of using a PV inverter to improve the power quality and power factor, and to reduce the Total Harmonic Distortion (THD) of a proposed distribution network, a solar photovoltaic system with a capacity of 100.725KW was modeled.Using this model, the reactive power of the inverter was investigated in the mode of effective power injection, reactive power injection/absorption and in various environmental conditions using two stages, including the use of a DC-DC boost converter and an AC-DC inverter with a sinusoidal pulse-width modulation (SPWM) controller.
This paper is structured as follows: Section I is the introduction; Section II explains the classification of photovoltaic systems; Section III shows the importance of reactive power in the power grid; Section IV introduces a PV system model; Section IV represents the reactive power capability of the PV inverter; Section V discusses the simulation and results for different cases and conditions of operation; and finally, Section VI extracts the outcomes and gives the conclusions of this research.

GRID-CONNECTED PV SYSTEMS
PV systems are divided into 2 categories, The off-grid and on-grid PV systems.The off-grid systems do not depend in their work on the utility network.In the past few years, the on-grid PV systems have become more popular than the off-grid PV systems, thereby accounting for 99% of the overall capacity of the installed PVs.This is attributed to the fact that they can operate parallel to the electric utility grid and require no energy storage systems.Additionally, they resupply the power to the utility grid if the power that is generated exceeds the load requirements [14].
Thus, it is noted that the production of PV energy decreases the utilisation of other sources of energy supplied to the grid, like fossil fuels or hydroelectricity.Furthermore, The on-grid systems are more inexpensive and do not require a lot of maintenance and reinvestment, since they do not use batteries [15].
Solar PV systems that are connected to the grid are divided into two categories: (1) Single-stage; and (2) Double-stage power conversion systems.The single-stage power conversion system has a single control over the voltage amplification, grid-injected current, and maximal power point tracking systems.It makes use of a line frequency transformer for adjusting the power based on the weight and size of the device.However, the design of the inverter in the single-stage power conversion system is severely affected by poor power quality, low input voltage, and higher switching stress.
In this study, the researchers adopted the Double-stage power conversion system, wherein Stage 1 included the DC-DC boost converter that increased the PV voltage.Meanwhile, Stage 2 included a VSI voltage source conversion.As Stage 1 boosted the DC-DC converter, no transformer was needed during power conversion.Furthermore, for ensuring that maximal power was extracted in any situation, the researchers implemented a Maximum Power Point Tracking (MPPT) algorithm during the DC-DC converter phase.The output generated in this step was linked to a DC-AC voltage stage inverter that controlled the power transmission based on the requirements of the network [16].

IMPORTANCE OF REACTIVE POWER
It is essential to apply the most favourable reactive power compensation process in the electric power generation system for maintaining the optimal engineering and economic conditions in the power system.The concept of reactive power compensation includes a wide and diverse range of system and customer-related issues, which are generally related to the power quality.The recent development of power 783 electronics technology has led to the application of several non-linear loads that have increased the fluctuations in network voltage, thereby decreasing the power quality and affecting the proper working of the equipment.Hence, the reactive power compensation helps in enhancing the network's power factor, stabilising the voltage, decreasing voltage fluctuations, increasing the long-distance transmission ability and reducing power losses.The majority of the power quality issues can be handled by effectively controlling the reactive power [17].
The inductive load consumes reactive power (lagging PF), resulting in a drop in the network voltage profile.The angle between the active power and reactive power (θ) increases with the consumption of reactive power (-Q) by the inductive load, thereby lowering the power factor (Cos θ).This increases the load current and decreases the voltage and a simultaneous loss in network regulation.For compensating the reactive power consumption, reactive power (+Q) is injected, which decreases the value of the angle (θ) to a minimal value (i.e., increase in power factor) and reduces the load current, improving network regulation.It is worth noting that, for deriving the full benefits from the network, the reactive power compensation is done locally [16], as presented in Figure.The reactive power (leading PF) injection, which is generally done with a capacitor bank or FACTS device, elevates the voltage in the network.To maintain the grid voltage as a constant value, an electrical utility should always stabilise the reactive power consumption, locally.The wind power and PV systems are frequently combined to build hybrid power systems for lowering the installation costs.In this scenario, the low-power windmills use induction generators that utilise a lot of reactive power.As a result, a reactive power generation system is needed to improve the system power handling capacity, maintain grid voltage control, and compensate for the reactive power requirements by the wind generators and utility networks.

PV SYSTEM COMPONENTS 4.1. PV Array Model
A basic component of a photovoltaic system is the photovoltaic cell, which is a simple P-N junction diode that converts solar radiation into electricity.A group of photovoltaic cells are electrically connected to form a photovoltaic module (PV module).These PV modules are connected to form a panel.In a gridconnected PV system, a group of PV modules are configured in parallel and in series (Nss x Npp modules) to produce a PV array to generate the required current and voltage.The output of the PV array varies according to the inputs represented by solar radiation and temperature, where the standard inputs according to standard test conditions (STC) are 1000 W/m 2 of solar radiation at a temperature of 25 o Celsius [18].In this study, the researchers modelled the PV cell circuit after combining the source current (Iph), parallel resistance (Rp), and the series resistance (Rs).Furthermore, for overcoming the limitations of modelling the PV single diode at the high and low voltage values, a photocell was expressed using two diodes, (D1) and (D2).Figure .5 depicts the equivalent circuit, wherein D1 reflects the diffusion process; D2 denotes the space charge area in the Junction; Rs highlights the initial losses that are caused due to the current flow and contact leads; while Rp indicates the modelled reverse saturation current.The voltagecurrent properties of the solar cell are depicted using the equations below:

DC-DC Boost Converter
The output of the PV array served as the input for the DC-DC boost converter to increase the voltage and decrease the current [19].Here, the researchers used a boost converter to achieve Maximum Power Point Tracking (MPPT) for enhancing and managing the PV array output voltage, as illustrated in Figuer.8.Theoretically, maximum power is transferred from the source to the load when the load impedance matches the impedance of that source.This was achieved by adjusting the duty cycle of a DC-DC boost converter.A duty cycle is described as a ratio of the switch on time to the overall switching period.For extracting the maximal power from a PV array, the duty cycle of a DC-DC boost converter is adjusted to the variable weather conditions.Compared to the different configurations, the boost converter offers many advantages, like cost-effectiveness, simple structure, high efficiency and very reliable.

MPPT Technique
The efficiency of a PV power generation system can be improved after controlling the PV array such that a maximal amount of available power is generated under varying environmental conditions.The Maximum Power Point Tracking (MPPT) technique is seen to be a self-optimisation procedure that uses algorithms for ensuring that a PV array is operational at the MPP [20].MPPT is described as the non-linear feature of a solar PV module.It comprises a single maximum power point that is based on the intensity of the temperature and radiation in the cell.The solar panels do not display a high efficiency.Since the PV voltage  ISSN: 2089-3272 IJEEI, Vol. 10, No. 4, December 2022: 780 -795 786 and current increase at higher radiation levels, it also leads to an increase in the output power of the PV array.However, if the temperature of the cell increases at a steady solar radiation intensity, the PV current increases marginally with a sudden decrease in the PV array voltage which further decreases its output power.Hence, MPPT can be used for enhancing solar panel efficiency.The MPPT is used in the DC-DC converters that connect the load and PV array for optimising the alignment of the system.The MPPT controller has 2 inputs, i.e., output voltage and output current in the solar array.These inputs help in generating the control signals that offer power to the IGBT switches in the DC-DC converter.In the past, several researchers have applied the Perturbation and Observation (P&O) and the Incremental Connectivity (IC) algorithms for MPPT modelling owing to their basic structure.The IC-MPPT algorithm can track the varying conditions faster compared to the P&O MPPT algorithm, however, it needs an expensive and complicated controller and also produces a slightly unstable output power.On the other hand, the output power in the P&O algorithm fluctuates around MPP [21].In this study, the researchers have used the P&O technique.Figuer.9 presents the P&O algorithm that is used in the MPPT technique for controlling the PV generator.It displays a basic structure, is cost-effective, easily implemented, requires fewer parameters and can be easily improved to become more efficient [22].This algorithm helps in determining the connection between the output voltage and power in the PV module.In this technique, the voltage of the PV module terminal is periodically disturbed and the output power of the PV array is compared to the earlier perturbation cycle [23].When the operating voltage in the PV module is changed and output power increased, the operating point in the control system shifts in that direction; otherwise, it shifts in the opposing direction.The current and voltage values in the PV array are estimated and the voltage and current output help in determining the actual power of this PV module.Thereafter, it determines the state and assesses if P = 0 or not.When this condition is satisfied, the trigger point is seen to be the MPP.On the other hand, if the condition is not satisfied, it checks a different condition, i.e., if ΔP > 0. If this condition gets fulfilled, the algorithm determines if ΔV > 0. When that condition is also fulfilled, it is noted that the operating point lies on the left side of MPP.If the condition of

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ΔV > 0 is unsatisfied, the operating point is seen to lie on the right side of the MPP.This procedure is constantly repeated until MPP is reached, as depicted in Figure 10.

Voltage Source Inverter (VSI) Model with Controller Model
In the case of Double-stage grid-connected PV systems, the DC-AC inverter or VSI is primarily used for converting the PV array's output power from the DC to AC, synchronising the grid while also controlling the DC output voltage in the boost converter.A 3-phase Voltage Source Inverter (VSI) in the ongrid PV system is equipped with an Insulated Gate Bipolar Transistor (IGBT), Gate Turn-Off thyristor (GTO) or a Metal Oxide Semiconductor Field-Effect Transistor (MOSFET).The inverter topologies are categorised into 2 types, i.e., single-level and multi-level inverters.The single-level inverter presents many advantages like cost-effectiveness, higher efficiency, better performance, more reliability and a basic structure.However, the multi-level inverter can accept the input voltage variations, Although it, displays an isolated and complicated structural topology with high-frequency transformers that extract power from the source, despite the low DC voltage.
Figuer.11 presents the design of an inverter controller system that generated the Pulse-Width Modulation (PWM) for synchronising the grid and PV system.The control system included a DC voltage regulator, Phase Lockup Loop (PLL) block, current controller block, and the PWM signal generation block.
The PLL could convert the 3-phase time-varying system into all components of a DC system, with voltage components ([Vd, Vq] and current components [Id, Iq].The system measured the DC components in addition to the phase angle of the grid that is used for grid synchronisation [24].The DC voltage regulator generates the current reference of Id, which further produced an instantaneous PV power from an inverter as the active power.Controlling the grid-side converter (3-phase PV inverter) was vital in regulating the DC link voltage so that the power balance between the power grid and PV array can be maintained.This also ensured the power quality that is generated after controlling the injected current in the grid.The current controller output produced the Vd and Vq references depending on the Id references and Iq references using the measured Id, Iq and Vd, Vq values from PLL.The PWM signal is produced depending on 3 modulating signals that are produced by Vd and Vq.The Iq current reference is generated from a reactive power controller for regulating the voltage at the Point of Common Coupling (PCC), within admissible limits, after generating a relevant reactive power from the inverter [12].The reactive power controller guides the inverter for offering or absorbing reactive power depending on the necessary conditions.The reactive power control is related to the voltage behaviour at PCC and is affected by the drawbacks of this inverter [25].The voltage at PCC is measured constantly and the algorithm estimates the accurate Iq reference based on the instantaneous PV power for determining the voltage and improving the PF at PCC.The researchers set the value of the reactive power reference at 0 in the algorithm since the power is injected at the power factor of 1 [16].

Inverter with Reactive Power Capability
The power grid can be made more reliable and efficient through the use of advanced inverters, combined with distributed generation.The PV system provides reactive power for 24 hours, irrespective of the time of the day, without decreasing the real power output.
As long as the provision of reactive power is properly offset, there will be an important shift in the driving of these utilities as reactive power is dynamically supplied where it is most needed near the loads.The researchers in Oak Ridge National Lab noted that the distributed voltage control was significantly better than the centralised voltage control.Regulation of the distributed voltage presents an efficient system, without any power outages.The advanced inverters are programmed to override the minor voltage fluctuations, which eliminates the need for additional network disconnections.As the advanced inverters were cost-effective compared to the conventional voltage regulation options, the customers and utilities are served better by the implementation of a novel technology.
In this work, the power quality of the inverter was investigated by analysing its reactive power injection/consumption capability under different operating conditions.Figuer.12shows the reactive power capability of the inverter.The inverter was able to operate in the (+Q) mode when there was a shortage of reactive power generation by the grid where reactive power is required.The inverter was also able to operate in the (-Q) mode to reduce the grid voltage, where P, Q, and S were the active, reactive and apparent power, respectively of the inverter.The inverter produced a maximum active power (S = Pmax) when the reactive power was zero at Unity Power Factor (UPF).The inverter was only able to produce a maximum reactive power (S = Qmax) when the active power was zero and θ = 90 degrees.
The reactive power could be controlled by changing the value and angle of the inverter output voltage, and the directional control of the inverter voltage was attained by changing the Iq reference to control the reactive power output of the inverter.One of the most important advantages of using an inverter to generate/absorb reactive power is the possibility of operating the inverter in the VAR mode at low levels of solar radiation subject to climatic conditions.Thus, it improves the use of solar photovoltaic energy and reduces the additional cost to compensate for the reactive capacity.
The P-Q relationship of the inverter is given in Equation ( 4), where  .is the capacity of the inverter,   is the instantaneous PV power of the PV array, and  .is the reactive power of the inverter.

SIMULATION AND RESULTS
The model of the Double-stage grid-connected photovoltaic system in MATLAB/Simulink is shown in Figuer.13.The model consisted of a 100-KW PV array [SUNPOWER SPR-305E-WHT-D] connected to a DC boost transformer for maximum power point tracking using a P&O algorithm.Then, the system was connected to a 3-level NPC inverter with two capacitors connected in series with a value of 12,000 µF for each capacitor to form a DC-link between the two stages, and to reduce distortion in the inverter output voltage, An LC filter was used.The output of the inverter was connected to the distribution network via a transformer (260/25K) V.   2 presents the influence of the reactive power injection mode/absorption mode in the inverter on the power factor, as presented in Figure .21.The voltage total harmonics distortion (THD) also turned out to be within the permissible limit of the IEEE STD 519-1992 as shown in Figure .22.The current harmonics were high if the reactive power reference was set at 0 (at UPF), and this was decreased if the system was operated in a reactive power injection mode /absorption mode as shown in Figure .23.Thus, it was noted that the inverter's reactive power Effectively affected on the grid output's power quality.    2 and Table 3 that the PV that was connected to a inverter was able to inject the reactive power at a maximum inverter capacity when the PV power was available or in a night mode (PV power was unavailable).

Figure. 3
depicts the design of the Doublestage solar energy system.

Figure 3 .
Figure 3. Double Stage Power Conversion Effect of Reactive Power Capability of the PV Inverter… (R. A. Othman and O. S. Al-Deen Al-Yozbaky) 4.

Figure 5 .
Figure 5. Equivalent Circuit of Solar PV Panel

Figure 6 .
Figure 6.I-V Characteristic Curve of a PV Module

Figure 8 .
Figure 8.The Schematic of the DC-DC Boost Converter

Figure 9 .
Figure 9. Flowchart of P&O MPPT Technique Effect of Reactive Power Capability of the PV Inverter… (R. A. Othman and O. S. Al-Deen Al-Yozbaky)

Figure 13 .Figure 15 .
Figure 13.Simulation Model of The Three-Phase Grid-Connected PV System At STC, and without the reactive power of the inverter having any effect on the grid, Figuer.14 shows the per phase (RMS) voltage and current at PCC, Figuer.15shows the inverter output voltage, and Figuer.16 shows the active and reactive power at PCC

Figure 24 .
Figure 24.Relation Between QRef.(Injection/Absorption) & PCC Voltage at STC It was clear from Table2and Table3that the PV that was connected to a inverter was able to inject the reactive power at a maximum inverter capacity when the PV power was available or in a night mode (PV power was unavailable).

Table 2
Analysis of The Reactive Power Ability of the Grid-Connected PV Inverters in The Different Inverter Modes at STC Conditions

Table 3
Reactive Power Capability Analysis of Grid-Connected PV Inverter in Different Inverter Mode at Night Mode Irradiation