Performance evaluation of Renewable Energy Sources based grid connected system considering Unified Power Flow Controller

The quality of power is a major concern for the utilities and the power industries. Since the 1980’s, the term power quality has become a very prevalent one. There are various factors that affect the power quality, such as sag, swell, and power factor variation. The main cause of these issues is the use of non-linear loads. In a RES based grid-connected system, harmonics variation is one of the main issues that can affect the quality of power. Hence to address this in this paper Unified Power Flow Controller (UPFC) is proposed for the grid connected system. Many papers were proposed with the various FACTS devices but the performance of the proposed system exhibits a better one. In this paper application of UPFC is implemented in MATLAB/Simulink on test cases of wind-based grid connected system and PV based grid connected system. The performance of the proposed UPFC is compared with conventional Voltage Source Inverter (VSI). In all the cases the proposed UPFC exhibits the improved performance. This ensures the reduction in the harmonics in the power system in future.


Introduction
New challenges are arising in the area of power quality compensation.The introduction of new technologies, such as energy storage systems and renewable energies, is expected to have a significant impact on the development of grids.Nevertheless, the development of grids leads to the power system's modernization [1].These systems are designed to meet the increasing demands of the modern population while ensuring environmental sustainability.
Aside from the technological advancements that are expected to be made in the field of power quality compensation, the various factors that are related to the development of grids are also expected to affect the efficiency of the system.One of the most common factors that can affect the cost of power quality compensation is the failure of equipment.Also, poor power quality can have a IOP Publishing doi:10.1088/1755-1315/1285/1/012009 2 significant impact on the cost of the system.It is estimated that the annual cost of implementing power quality compensation can reach up to $182 billion in the US and up to $150 billion in Europe [2].The goal of the regulation is to ensure that the electrical energy that is used is of the highest quality.
Various organizations have suggested different definitions of power quality.For instance, the International Standards Organization (ISO) has a set of standards that specify the methods and techniques that are used to test and measure the power quality of electrical equipment [3]- [5].Nonlinear loads, which are commonly used in industries, are known to cause a significant reduction in the power quality of electrical equipment.They are also prone to experiencing total harmonic distortion.These special loads are also regulated by the different power quality standards [6].
Many electrical installations are prone to experiencing issues with the power quality of their equipment.These include low power factor, transient phenomena, and issues related to connectors oversizing and energy efficiency [7].Power quality problems can lead to significant losses.In addition, they can interrupt the operations of the equipment, which can reduce its life span.The variations in the power quality between ac and DC are caused by the varying voltage levels in the power grid.For instance, the AC power grid has a constant voltage and an alternating voltage.This means that the issues with the power quality of electrical equipment will not have the same occurrence and severity [9].
The difference between the power quality issues encountered by electrical equipment when using AC and DC lines is caused by the varying voltage levels in the power grid.For instance, the DC power grid has a constant voltage, while the AC power grid has an alternating voltage [10].The power quality of electronic converters is also affected by the type of equipment.Different types of power quality problems are presented in the Fig. 1 Fig. 1 Power Quality Problems

Renewable Energy Sources and its Power Quality Issues:
The rapid emergence and evolution of renewable energy sources (RES) such as solar and wind power are expected to lead to the replacement of traditional energy generation with distributed generation systems (DG systems).However, there are still some technological difficulties that need to be overcome.These include the development of reliable and efficient energy storage systems.Currently, there are various initiatives focused on developing these technologies.Due to the technological advancements and the increasing number of people worldwide, electrical energy has become an essential resource for modern life.However, it is becoming harder to rely on traditional sources of electricity due to the scarcity of resources [11].
The activities involved in the extraction, transportation, refining, and use of fossil fuels have a negative impact on the environment.These activities contribute to the accumulation of greenhouse gases and other harmful substances in the atmosphere.It is believed that approximately 25% of the global warming emissions are caused by electricity generation.Renewable energy sources are an alternative to traditional energy sources that can help minimize environmental damage.Due to the rapid emergence and evolution of solar and wind power, the total amount of electricity generation from these sources has increased significantly [12].In 2015, Europe's electricity generation from renewable energy sources was 34.2%, up from 20.1%.
In 2019, the global total installed wind power capacity reached 60.4 gigawatts.China and the US were the main contributors to the growth of this industry, as they were responsible for over 60% of the total additions.Europe was also the leading offshore wind power producer, accounting for 59% of the total additions.Similarly, the global installed solar photovoltaics capacity reached 114.9 gigawatts.China was the leading contributor to the growth of this industry, as it was able to add 30.1 gigawatts, followed by the US with 13.03 gigawatts, and Japan with 7.7 gigawatts.At the end of the year, there were over 600 gigawatts of solar PV capacity globally [13].
Despite the advantages of renewable energy sources, they are still prone to intermittent production and pose a threat to the safety and stability of the electricity grid.An energy storage system can be used in different applications.Its various characteristics, such as its storage capacity and response time, can be compared with other technologies.The main indicators that are used to evaluate the performance of an energy storage system are the specific power and the maximum power [14].
The main objectives of this paper are i) Calculating the THD by the FFT analysis.
ii) Comparing the THD values of wind and PV grid connected systems for VSI and UPFC implementations.
The methodologies employed here are i) Usage of MATLAB Simulink models to design the system and run.
ii) FFT analysis is performed

Power Quality Improvement using FACTS:
A flexible AC transmission system can eliminate the effects of power quality problems.FACTS are mainly utilized in the distribution system.Due to its numerous advantages of power semiconductors these are used in the FACTS Devices.There are various types of devices used in the system, such as series connected, shunt connected, series and shunt connected devices.Various types of FACTS devices are presented in Fig. 2.

Unified Power Flow Controller (UPFC)
A Unified Power Flow Controller (UPFC) is a flexible AC transmission system (FACTS) device that is used to control and optimize the flow of electrical power in high-voltage power systems.The UPFC is a combination of two individual FACTS devices: the Static Synchronous Series Compensator (SSSC) and the Static Synchronous Shunt Compensator (STATCOM).The UPFC can control the power flow in a transmission line by regulating both the voltage magnitude and phase angle of the electrical signal.This is accomplished by using a combination of series and shunt-connected power electronics devices, such as inverters and transformers, which are controlled using a sophisticated control algorithm [15].
The series-connected SSSC component of the UPFC can inject a controllable voltage in series with the transmission line, which can either add or subtract from the voltage of the line.By controlling the magnitude and phase angle of this voltage, the SSSC can control the power flow in the line.The shuntconnected STATCOM component of the UPFC is connected in parallel with the transmission line and can provide a reactive power source to the system.By controlling the amount of reactive power provided, the STATCOM can regulate the voltage magnitude of the line.
The control algorithm for the UPFC is designed to optimize the power flow in the transmission line and can respond quickly to changes in the system.By controlling the power flow in this way, the UPFC can improve the stability, reliability, and efficiency of the power system.The UPFC is a versatile and effective tool for improving the performance of power systems, and it is commonly used in high-voltage transmission systems to control power flow and mitigate issues such as voltage instability, power oscillations, and transient disturbances.The schematic diagram of the UPFC is presented in Fig. 3.

Fig 3. Schematic Diagram of UPFC
Hence to address this in this paper Unified Power Flow Controller (UPFC) is proposed for the grid connected system.In this paper application of UPFC is implemented in MATLAB/Simulink on test cases of wind-based grid connected system and PV based grid connected system.The performance of the proposed UPFC is compared with conventional Voltage Source Inverter (VSI).In all the cases the proposed UPFC exhibits the improved performance.

Importance of usage of RES
According to COP26, the climate changes are encountered by the adequate usage of the cheapest means of power generation i.e., solar and wind power generations.This improves the global health by lowering the air pollution.It also creates employment to various people.The increase in usage of these renewable energy sources makes the possibility of power production at a lower cost.
Major Co2 emissions gets reduced because of the wind and solar power generations.Now-a-days a home-based solar power production is being used and it has been used in small scale industries.

Results and Discussions:
In this paper the proposed UPFC is implemented on the following cases   The FFT analysis is presented for both UPFC and VSI based wind grid connected system and presented in Fig. 10 and Fig. 11    For the above results is observed that UPFC based wind grid connected system exhibits the improved performance by reducing the THD from 0.40 % to 0.19 %.

Performance Evaluation of UPFC considering PV Grid Connected system
In this case the proposed UPFC is implemented on a PV based grid connected system and compared with conventional VSI in MATLAB/Simulink environment as shown in Fig. 12 and Fig. 13 respectively.The FFT analysis is presented for both UPFC and VSI based PV grid connected system and presented in Fig. 18 and Fig. 19 respectively.For the above results is observed that UPFC based PV grid connected system exhibits the improved performance by reducing the THD from 0.40 % to 0.18 %.

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Performance Evaluation of UPFC considering Wind Grid Connected system • Performance Evaluation of UPFC considering PV Grid Connected systemPerformance Evaluation of UPFC considering Wind Grid Connected systemIn this case the proposed UPFC is implemented on a wind-based grid connected system and compared with conventional VSI in MATLAB/Simulink environment as shown in Fig.4and Fig.5respectively.

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Fig. 12PV grid connected system Simulink implementation with UPFC.

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Fig. 13PV grid connected system Simulink implementation with VSI.