In recent years, photovoltaic (PV) systems have gained significant attention as a renewable energy solution, contributing to the global efforts towards sustainable development and combating climate change. Central to the efficient functioning of these systems are inverters, which play a crucial role in converting the direct current (DC) generated by solar panels into alternating current (AC) that can be used to power electrical devices and feed energy back into the grid [1,2]^[1][2]. The rapid growth of PV systems as a clean and sustainable energy solution has sparked immense interest in improving the components of these systems, due to its main properties:

• Inherent short-circuit protection;
• Low current and voltage harmonics.

One of the topologies that has gained an increasing importance in the field of PV systems is the current source inverter (CSI). CSIs offer several advantages over other inverter technologies, making them a popular choice for both residential and utility-scale PV installations. Interconnected systems are categorized according to the quantity of power processing stages, utilization of transformers, transformerless configurations, and the type of commutation. Consequently, topologies relying on the number of stages in energy processing are classified into single- and multistage systems, as illustrated in Figure 1.

The single-stage CSI is a straightforward and cost-effective solution, suitable for small-scale PV installations. It offers higher efficiency but may require additional filtering to address voltage harmonics. On the other hand, multi-stage CSI allows for more flexibility and control over the output waveform, making it a preferred choice for larger PV systems where power quality is crucial. However, it involves more complex circuitry and has a slightly lower overall efficiency due to additional conversion stages. The choice between single-stage and multi-stage CSI depends on the specific requirements and priorities of the PV system application.

2. Performance of CSIs in Photovoltaic Systems

Inverter performance is critical in determining the overall efficiency and effectiveness of PV systems. Among various inverter technologies, CSIs have emerged as a reliable solution for converting DC power from solar panels into AC power suitable for grid connection. This article aims to comprehensively examine the performance of CSIs in photovoltaic systems, highlighting important parameters such as conversion efficiency, stability, energy quality, power factor, dynamic response, and variation management of solar radiation.

• Interface with PV modules: Effective interface between CSIs and PV modules is crucial for optimized power conversion and energy extraction. Challenges may arise from module-level variations, partial shading conditions, or differences in maximum power points. Advanced techniques such as distributed maximum power point tracking (DMPPT) [65]^[25], module-level power electronics, and innovative bypass diode configurations can address these challenges. These solutions enable CSIs to efficiently interface with PV modules and extract maximum energy, ensuring optimal system performance;

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• and a non-SiC diode in series, as well as legs equipped with SiC MOSFETs and a SiC Schottky diode in series
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A comparative analysis of PV-powered VSI and CSI converters argues that for gradual load changes, the CSI performs worse than other types of inverters in maintaining power quality if the load is variable [56]^[13]. But, this might not be a problem in grid-connected CSIs, since the load can be adjusted more gradually. Based on an analysis of the performance of the three-phase inverter in the solar PV system under dynamic load conditions, it is evident that the power quality of the CSI is inferior to that of the VSI [57]^[14]. Efficiency-wise CSI microinverters still tend to have efficiencies below 98%, as most conventional single-phase PV inverters use switching frequencies below 20 kHz [58]^[15]. Compared to the VSI, the CSI has the intrinsic drawback of not being able to withstand the open circuit faults present. Unlike VSI, where an open circuit fault can be detected and managed relatively easily, CSI are challenging in this regards. In a CSI, the current source input implies that when an open circuit fault occurs, the inverter cannot inherently limit or control the output voltage. This limitation can result in overvoltage conditions, posing risks to connected equipment and the grid. Additionally, it can lead to reduced system efficiency and compromised power quality.

3. Control of CSIs in Photovoltaic Systems

The control strategies employed in CSIs in PV systems focus more on techniques such as MPPT control, predictive control strategies, and more (Figure 25). Potential solutions for control mechanism optimization in CSIs are:

• Current sensing and monitoring: Implementing precise current sensing and monitoring techniques is crucial in CSIs. Real-time current measurements can detect open circuit faults promptly. When an open circuit fault is detected, the inverter can respond by reducing its output voltage, limiting the risk of overvoltage conditions;
• Active voltage control: Advanced control algorithms can be employed to actively manage the output voltage of CSIs. By modulating the switching patterns of the inverter in response to system conditions, the voltage output can be controlled within safe limits, even in the presence of open circuit faults;
• Feedback mechanisms incorporate feedback mechanisms that continuously assess the output voltage and current. These feedback loops can adjust the operation of the inverter to ensure that the voltage remains within predefined boundaries, reducing the risk of overvoltage;
  In Figure 26 M is the modulation index, and the current control loop operates significantly faster than the voltage control loop;
• Fault detection algorithms develop fault detection algorithms specifically designed for CSIs. These algorithms can quickly identify open circuit faults and trigger protective actions to prevent overvoltage. They can also distinguish between genuine faults and transient conditions.

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• . The research findings indicate that the parasitic capacitance of the series diode significantly impacts the switching performance, limiting the achievable switching frequencies. 
• Reliability and durability: The performance of CSIs is also influenced by their reliability and durability. These inverters are designed with robust components, thermal management systems, and advanced protection mechanisms to withstand various environmental conditions, such as temperature variations and humidity. Reliability features, including fault detection and protection against voltage spikes metal-oxide-semiconductor varistors (MOVs) or current surges
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• ]^[11], contribute to the long-term performance and durability of CSIs in photovoltaic systems. A reliability study of CSI and Voltage Source Inverter (VSI) systems connected to a transformerless power grid [19]^[12] concludes that the CSI topology is the most reliable. Furthermore, among the CSI topologies, the four-leg one has a reliability greater than 98% (Table 102).

The challenges introduced by PV applications can be addressed through a combination of advanced control strategies, current sensing, feedback mechanisms and protection devices. Preventing open circuit failures and dealing with overvoltage problems is indeed a problem worthy of attention, and continuous research and innovation in power electronics continues to provide solutions to improve the reliability and safety of CSI-based systems (Table 113). These potential solutions aim to optimize the control mechanisms in CSIs, enabling improved performance, power quality, and grid integration. The choice of solutions to implement should be based on specific system requirements, objectives, and the complexity of the application in which CSI are deployed. As far as the control of the converter is concerned, different techniques can be used:

• MPPT control is a key control technique used in CSIs to optimize the PV array output power. MPPT algorithms continuously monitor operating conditions and dynamically adjust the operating point to extract maximum power from the solar arrays. Various MPPT algorithms, such as Perturb and Observe (P&O), Incremental Conductance (IC), and Fractional Open Circuit Voltage (FOCV), are commonly employed in CSIs to ensure efficient energy conversion, as in Figure 2

• 7.

• Predictive control strategies offer advanced control capabilities for CSIs in PV systems. Model Predictive Control (MPC) and Direct Predictive Control (DPC) are examples of predictive control techniques used to optimize the performance of CSIs. These strategies use mathematical models of the system and predictive algorithms to make control decisions in real time, ensuring optimal power extraction, rapid response to changing conditions and improved stability. Using MPPT control data, the three state vectors for an SVM modulation in a three-phase CSI can be calculated [46]^[16]. In [15]^[17], an artificial neural network-based fuzzy logic controller (FLC) [59]^[18

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• , coupled with a nonlinear sliding mode control (SMC) for power grid connection demonstrates the capability to achieve lower THD compared to the SMC approach.

• Integration with energy storage systems: The integration of Energy Storage Systems (ESSs) with PV systems is gaining traction to enable efficient energy management and grid support. CSIs can be tailored to facilitate seamless integration with ESSs. Challenges in ESS integration include bidirectional power flow, battery management, and control coordination. Advanced control algorithms, bidirectional power converters, and communication protocols enable CSIs to effectively interface with ESSs, allowing for optimized energy utilization, peak shaving, and grid ancillary services;
• Monitoring and control systems: Reliable monitoring and control systems are essential for efficient operation, performance evaluation, and fault detection in PV systems with CSIs. Challenges involve real-time data acquisition, system diagnostics, and remote control capabilities. Advanced monitoring and control solutions, such as supervisory control and data acquisition (SCADA) systems, IoT-based technologies, and predictive maintenance algorithms, offer comprehensive monitoring, precise control, and effective system management. These solutions enhance the performance and reliability of PV systems with CSIs;

• Efficiency is a critical performance metric for CSIs, as it directly impacts the energy conversion process. CSIs are known for their high conversion efficiencies, resulting in minimal power losses during the DC to AC conversion. Factors contributing to their efficiency include advanced control algorithms, optimized switching techniques, and low conduction and switching losses. High-efficiency CSIs ensure maximum power generation from the PV system, thereby maximizing the overall system performance and energy yield;
• Stability is a critical performance parameter for CSIs, ensuring their reliable and consistent operation. By maintaining stable voltage and frequency levels, CSIs contribute to a robust and secure grid integration. Advanced control strategies, including grid synchronization techniques and voltage regulation algorithms, enable CSIs to adapt to varying operating conditions and maintain stability even under fluctuating solar irradiance levels. Dynamic modeling and stability analysis of a three-phase PWM-based CSI for standalone applications, based on an analytical and experimental investigation, verify that the examined topology can effectively operate as a boost converter with a VLL_rms/Vdc ratio exceeding 3, while ensuring a THD below 5% [49]^[3]. This characteristic is particularly advantageous for applications utilizing low voltage DC links;
• Power quality: Maintaining high-power quality is essential for PV systems to ensure reliable and stable operation. CSIs excel in this aspect by offering superior power quality features. Their precise current control capability allows for low harmonic distortion and reactive power compensation, ensuring compliance with grid standards and reducing the risk of grid disturbances. One of the techniques for reactive power control of the grid-connected photovoltaic microinverter is based on third-harmonic injection [7]^[4] to achieve better overall power quality (Figure 22).
  The circuit is controlled by a phase-locked loop (PLL)-based controller as shown in Figure 23.
  Additionally, CSIs exhibit an excellent dynamic response, enabling seamless load adaptation and grid synchronization. A particular single stage solar inverter using a unique active filter that replicates the behavior of a conventional second order LC mains filter was presented in [50

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  . This solution (Figure 24) offers the flexibility to adjust the cutoff frequency, allowing emulation of the desired reactance for efficient filtering. Additionally, the proposed system incorporates active closed-loop filtration to continuously monitor and improve power quality. An active power filter for grid connection that uses a shunt active power filter that can be used even when the PV array is not sending energy to the grid was proposed in [51]^[6]. The control is used simultaneously by MPPT and harmonic compensation.
  The compensation of the harmonics introduced by the network takes place through the introduction of a reactive power q and the two passive elements, L_dc and C_dc, can be controlled via a switching strategy to act as an active LC filter. To improve the control of the active power shunt filter connected to a photovoltaic system is to directly control the power by selecting the combination of switches to be applied based on a switch [52]^[7]. The potential solutions for reducing harmonics in current source inverters are summarized in Table 91.
  These potential solutions address the challenge of reducing harmonics in CSIs and improving the quality of the output voltage. The choice of solution(s) to implement will depend on specific system requirements, performance objectives, and constraints in the photovoltaic or other applications using CSIs;
• Power factor: Maintaining a high power factor is crucial for efficient power transmission and utilization. CSIs offer excellent power factor control, ensuring a near unity power factor during grid connection. By actively managing the power factor, CSIs improve system efficiency and minimize losses, enhancing the overall performance of the PV system;
• Dynamic response of CSIs refers to their ability to quickly and accurately respond to load changes and variations in solar irradiance levels. CSIs exhibit excellent dynamic response characteristics, allowing them to adapt to rapid changes in load demand and maintain stable grid integration. Their fast response time and advanced control algorithms enable smooth transitions and reliable operation under dynamic conditions;
• Management of solar irradiance variations: CSIs are designed to handle variations in solar irradiance levels effectively. Through advanced MPPT algorithms, they optimize the power output from solar panels, maximizing energy harvest even under varying solar conditions. This effective management of solar irradiance variations ensures optimal performance and energy production throughout the day;
• Grid integration: The seamless integration of PV systems with the electrical grid is a key performance requirement. CSIs facilitate smooth grid integration through their ability to regulate voltage and frequency, support reactive power control, and provide anti-islanding protection. By actively interacting with the grid, CSIs ensure stable and reliable operation, minimizing the risk of grid instability or disruptions. This feature is particularly important in utility-scale PV systems where grid compliance and grid support functionalities are essential. An article presents and examines a prototype of a Silicon Carbide (SiC) current source inverter CSI that analyzes the switching performance of legs constructed with SiC MOSFETs [53,54]^[8

Furthermore, an FLC controller can be used to quickly locate the MPPT [42]^[19] and demonstrates the ability to meet network requirements. The control structure of the system consists of an MPPT, a current loop and a voltage loop to improve the system performance during normal and variable conditions, and a PLL for grid connection Figure 28. Different MPPT methods, such as P&O, IC, SMC and FLC, applied to a two-stage grid-connected PV system, show different THD. From a comparison on the THD level of the currents injected into the connection network, the FLC and SMC-based MPPT methods, the injected current’s THD was 1.34% and 1.99% [60]^[20]. Ref. [61]^[21] proposes a technique that generates SPWM using Clark–Park transformations applied to the desired output current waveform allows for combining the desired amplitude with waveform data to generate the modulating wave for SPWM. In addition, the CSI can be used in three-level voltage boost converters that use three-level logic SPWM [62]^[22]. Controlling CSIs in PV systems presents some challenges that must be addressed for optimal performance. One challenge is the mitigation of grid voltage fluctuations and harmonic distortions caused by the operation of CSIs. Advanced control algorithms and filtering techniques can be implemented to solve these problems and maintain high power quality. Considering that CSIs exhibit slower response to load variations and generally produce higher THD when operating with variable loads, a nonlinear control strategy for single-phase PWM current source inverters can better address dynamic control shortcomings. An adaptive control strategy that updates gain values based on feedback data, aiming to better compensate for load variations, is presented in [63]^[23]. Another challenge of PV converters is managing partial shading conditions, which can lead to multiple maximum power points and inefficient power extraction. The optimization of the discrete-time PI controller for a single-phase grid-connected current source inverter involves sizing the controller by approximating the behavior of the inverter. One way is to approximate it as an LC filter during the initial tuning stage. Subsequently, the controller gains are calculated based on the desired crossover frequency and phase margin [64]^[24]. Furthermore, the control of CSIs in PV systems requires robust fault detection and protection mechanisms to ensure system reliability and safety. Comprehensive fault detection algorithms and reliable protective measures can be implemented to mitigate risks and prevent equipment damage. Additionally, coordinating communication and control between multiple inverters in a PV system poses challenges, especially in large-scale installations. Advanced control architectures and communication protocols enable effective coordination and ensure optimized system performance.

4. Integration of CSIs in Photovoltaic Systems

Integrating CSIs into photovoltaic systems presents challenges and strategies for interfacing with PV modules, energy storage systems, monitoring and control mechanisms, and more. The most relevant ones are:

• Regulatory and grid connection requirements: Integrating CSIs into PV systems requires compliance with regulatory and grid connection standards. These standards may include grid codes, safety regulations, and certification requirements. Challenges arise in meeting grid compliance, anti-islanding protection, and grid support functionalities. CSIs should incorporate protection mechanisms, voltage and frequency control algorithms, and communication protocols to ensure compliance with regulatory and grid connection requirements. Adhering to these standards ensures safe and reliable operation while supporting the stability and integrity of the electrical grid.