Grid-connected Photovoltaic Converter Research Articles

Transient Synchronous Stability Analysis of Grid-Forming Photovoltaic Grid-Connected Inverters during Asymmetrical Grid Faults

Compared with the traditional grid-following photovoltaic grid-connected converter (GFL-PGC), the grid-forming photovoltaic grid-connected converter (GFM-PGC) can provide voltage and frequency support for power systems, which can effectively enhance the stability of power electronic power systems. Consequently, GFM-PGCs have attracted great attention in recent years. When an asymmetrical short-circuit fault occurs in the power grid, GFM-PGC systems may experience transient instability, which has been less studied so far. In this paper, a GFM-PGC system is investigated under asymmetrical short-circuit fault conditions. A novel Q-V droop control structure is proposed by improving the traditional droop control. The proposed control structure enables the system to accurately control the positive- and negative-sequence reactive current without switching the control strategy during the low-voltage ride-through (LVRT) period so that it can meet the requirements of the renewable energy grid code. In addition, a dual-loop control structure model of positive- and negative-sequence voltage and current is established for the GFM-PGC system under asymmetrical short-circuit fault conditions. Based on the symmetrical component method, the composite sequence network of the system is obtained under asymmetrical short-circuit fault conditions, and positive- and negative-sequence power-angle characteristic curves are analyzed. The influence law of system parameters on the transient synchronous stability of positive- and negative-sequence systems is quantitatively analyzed through the equal area criterion. Finally, the correctness of the theoretical analysis is verified by simulation and hardware-in-the-loop experiments.

Smooth switching control strategy with complete process of frequency support for photovoltaic grid-connected converter

The continuous increase in the penetration of renewable energy sources such as photovoltaics (PV) brings operational challenges to the traditional distribution network. With a large-scale PV integration, the power system behaves as a weak grid, which leads to more frequent and wider changes in system strength. In order to improve the system stability, the PV system is required to operate stably while providing necessary system support under different operating conditions. However, there are two issues for the PV system needs to address to satisfy this requirement. Nowadays, there are two types of control strategies for PV systems: the grid-following (GFL) control and the grid-forming (GFM) control. The two control strategies differ in inertia support capability and control stability under different operating conditions. To guarantee the grid connection stability as well as the operating stability under AC systems with different strengths, the smooth switching between the GFL and GFM is an urgent topic. In addition, how to maintain the system frequency support performance during the control switching of the PV system is also a challenge. In this paper, an advanced smooth switching control strategy is proposed to enhance the operational stability and flexibility of the PV system while maintaining the frequency support performance during the switching process. Firstly, the small-signal stability of typical control strategies of GFL and GFM under different system strengths is compared to analyze the necessity of the control switching between the two control modes. Then, a smooth switching control strategy is proposed to realize the smooth switching when AC grid strength changes. More importantly, to improve the system frequency performance and realize continuous frequency support during the switching process, a power correction function is developed on the smooth switching control strategy. The effectiveness and universality of the proposed control strategy are verified by case studies on a modified IEEE 33 bus distribution network with high PV system integration.

Low Frequency Ripple Reduction in Input DC Current and AC Line Currents With 240$^{\circ }$ Clamped Space Vector PWM in Grid-Connected Photovoltaic Converters

Low Frequency Ripple Reduction in Input DC Current and AC Line Currents With 240$^{\circ }$ Clamped Space Vector PWM in Grid-Connected Photovoltaic Converters

Single and Double Input DC Sources Multilevel Inverter Topologies With Reduced Components Counts and Voltage Boosting Property for Grid-Connected Photovoltaic Converters

This article proposes a single-source nine-level multilevel inverter topology that handles a reduced number of components. Moreover, it utilizes the virtual dc sources in terms of the switched capacitors to reduce the number of the active dc source. Those capacitors have a self-charging with naturally balancing capability. The proposed topology can be upgraded to a 19-level configuration. This modification is performed by adding a single supply and a pair of switches. To highlight the good features of the proposed structures, a set of comparative studies are held with other structures based on some of the performance evaluating parameters. The comparison results prove the superiority of the proposed configurations over the other comparative references. A simple method is presented to estimate the boundary for the capacitance of capacitors. The performance of the proposed configurations is validated through the practical prototypes for these structures, which are implemented and tested based on the dSPACE (DS-1103) hardware. For further validation, the proposed topology is integrated with a photovoltaic (PV) system to study its performance with a high output power system.

Control and Performance of 240-Clamped Space Vector PWM in Three-Phase Grid-Connected Photovoltaic Converters Under Adverse Grid Conditions

This article evaluates the performance of a relatively new pulsewidth modulation (PWM) method i.e., 240 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> -clamped space vector PWM (240CPWM), in three-phase grid-connected photovoltaic (PV) converters under various grid voltage conditions. 240CPWM is a minimum switching loss PWM method that reduces the switching loss by 85% at unity power factor and has better total harmonic distortion (THD) as compared to conventional space vector PWM. A unique six-pulse dynamically varying dc-link voltage is required for 240CPWM. Typically, a three-phase grid-connected PV system consists of a dc–dc stage followed by a dc–ac stage. In this article, the dc–dc stage is operated in closed-loop control to shape the dynamic dc-link voltage required for 240CPWM. The dc–ac stage is operated in the current control mode using a proportional–integral controller along with a harmonic compensator to ensure sinusoidal grid currents along with maximum power point tracking. The modulation index of the dc–ac stage is uncontrolled and fixed at the maximum value, and the primary control mechanism changes the magnitude, phase, and wave shape of the dc-link voltage. The coordinated control of dc–dc and dc–ac stages is ensured for better dynamic performance and grid support functions. Finally, the performance of the grid-connected PV converter with 240CPWM is validated using a three-phase 3-kW 208-V hardware prototype under nonunity power factors, voltage unbalance, and voltage sag/swell conditions for the first time. The implemented control with 240CPWM achieves smooth operation under nonunity power factor and grid voltage disturbance conditions with THD as low as 3.5% and peak combined efficiency of dc–dc and dc–ac stages as 96.4%.

L2 Norm Enabled Adaptive LMS Control for Grid-Connected Photovoltaic Converters

This article presents an L2 norm-enabled variable step-size adaptive least mean square controller for grid-connected photovoltaic converter. The variable step size is adopted to damp out the weight oscillations and to enable faster convergence under exposure to disturbances in the load and solar insolation. More precisely, to achieve oscillations free response, a nonlinear relationship is developed between step size and estimated error. This way, it significantly improves the convergence speed and stability. The iteration function of the presented variable step-size feature includes the input signal to study the impact of harmonically polluted load current on the weight extraction performance. Besides, the designing procedure of step size considers both current and previous error states, thereby ensuring noise immunity. With the inclusion of these features, the devised controller is applied to accurately extracting the fundamental weight component of the load current and in turn enabling various ancillary services. The proposed control algorithm is designed in <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">matlab</small> /Simulink software and its effectiveness is examined under both steady-state and transient conditions during grid disturbances. Furthermore, the practicability of the proposed controller is verified and compared with conventional controls using ARM cortex M4 microcontroller-based laboratory test bed. In a nutshell, the results confirm the oscillations free weight component with higher convergence and better stability under various operating conditions.

240$^\circ$-Clamped PWM Applied to Transformerless Grid Connected PV Converters With Reduced Common Mode Voltage and Superior Performance Metrics

Transformerless voltage source inverters (VSIs) are one of the popular topologies for photovoltaic (PV) grid-connected applications due to the lowest component count and simple design. Because of the absence of galvanic isolation in such systems, problem of electromagnetic interference (EMI) due to high leakage current is highly pronounced. Recently various reduced common mode voltage (CMV) PWM (RCMV-PWM) methods which address these issues of CMV, and leakage current have been proposed. These schemes typically function at the expense of increased total harmonic distortion (THD), higher switching loss, high DC link current stress along with limited modulation index range. In this work, 240<inline-formula><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> clamped PWM (240CPWM) is selected as an ideal candidate for grid connected VSIs in terms of all round performance of switching loss, THD, DC link current stress, CMV and leakage current. 240CPWM is a relatively new space vector based PWM method which can attain much superior performance by avoiding the zero states. After a brief overview of the 240CPWM concept, the paper provides a detailed comparison of 240CPWM with conventional space vector PWM (CSVPWM) and discontinuous PWM (DPWM1) methods in terms of each of the above metrics. A 3-phase 208 V 3 kW silicon IGBT based hardware prototype is built to validate the performance of 240CPWM and compared with other popular schemes under grid-connected mode. Experimental results show a 66.7&#x0025; reduction in the peak CMV and 50&#x0025; reduction in leakage current with the proposed scheme as compared to CSVPWM. Also, THD, DC link current stress and inverter switching loss are greatly reduced as an added advantage using the proposed method. A new, combined performance index is proposed to compare the performance of different PWM schemes, and it is shown that the 240CPWM achieves the best value for this index among the PWM methods studied.

Research on Synchronous Control Method for Suppressing Nonlinear Impulse Perturbation of Photovoltaic Grid-Connected Inverter

In this paper, the synchronous control problem of photovoltaic grid-connected inverter with nonlinear impulse perturbation is studied. The mathematical model of photovoltaic grid-connected converter is established by using Kirchhoff’s law. The nonlinear impulse perturbation characteristics of grid-connected current are studied and analyzed. Controllers are designed to suppress nonlinear impulse perturbation. The effectiveness of the controller is verified by Lyapunov stability theory. Finally, the simulation design is carried out by simulink in Matlab and the test platform is constructed. In this paper, simulation verification and data comparison are carried out by using various control methods for suppressing nonlinear impulse perturbation of grid-connected currents. Compared with PI controller, PR controller and other controller, the output harmonic content of photovoltaic inverter can be controlled within 1%, which can effectively reduce the harmonic content. The power quality of grid-connected current is improved. The feasibility and effectiveness of the control strategy proposed are verified by simulation and experiments.

A robust control scheme for grid-connected photovoltaic converters with low-voltage ride-through ability without phase-locked loop

A robust control scheme with low-voltage ride-through ability is presented for grid-connected photovoltaic converters that operate under harsh conditions such as voltage sags and unknown disturbances and parameters. The proposed strategy allows for flexible active and reactive power injection into the grid during asymmetrical voltage sags without using a phase-locked loop or positive and negative sequence components. In addition, the same controllers are used under both normal operation and voltage sags, thus, lowering the control system’s complexity. The scheme is conceived by combining uncertainty-and-disturbance-estimation compensation and repetitive control for the dc voltage and phase currents, respectively. Controller design is carried out systematically and closed-loop stability is proven through Lyapunov’s analysis. Several simulation case studies are presented using the SimPowerSystemsTM toolbox of MATLAB/Simulink computing environment to demonstrate the performance of the proposed control system under voltage sags, unknown disturbances, abrupt changes in operating conditions, and parametric uncertainties.

Circulating Current Suppression of Parallel Photovoltaic Grid-Connected Converters

The photovoltaic (PV) grid-connected converter (GCC) is a critical interface between solar energy and the power grid. Improved efficiency is possible if several PV GCCs have a common array bus. However, the dc side shunt causes the circulation of a zero-sequence current, which will distort the GCC or even damage semiconductor devices. The zero-sequence current includes high- and low-frequency components. A novel passive filter is proposed to increase the order of the zero-sequence circulating current transfer function to reduce the high-frequency component without increasing the cost. Furthermore, an advanced finite-time controller is introduced to mitigate the low-frequency component with fast regulation performance and strong disturbance rejection. The test results for two 10-kW GCCs are presented to confirm the performance of the method.

Active Common-Mode Filter for Ground Leakage Current Reduction in Grid-Connected PV Converters Operating With Arbitrary Power Factor

This paper proposes a solution for reducing the ground leakage current in transformerless single-phase grid-connected photovoltaic converters. This is obtained with the introduction of an active common-mode filter able to compensate for variations of the output common-mode voltage of the power converter. The active common-mode filter is applied to a widespread and efficient full-bridge driven by a three-level pulse width modulation, allowing the power converter to operate with low ground leakage current and with an arbitrary power factor. After showing the desired voltage waveform for common-mode voltage compensation, this paper presents the design guidelines for the needed additional magnetic component together with the power loss considerations for all the devices added for the proposed solution. Experimental results show the performance of the proposed solution in terms of ground leakage current reduction, effectiveness of dead-time compensation, total harmonic distortion of the injected grid current, and power losses.

A Nine-Level Grid-Connected Converter Topology for Single-Phase Transformerless PV Systems

This paper presents a single-phase transformerless grid-connected photovoltaic converter based on two cascaded full bridges with different dc-link voltages. The converter can synthesize up to nine voltage levels with a single dc bus, since one of the full bridges is supplied by a flying capacitor. The multilevel output reduces harmonic distortion and electromagnetic interference. A suitable switching strategy is employed to regulate the flying-capacitor voltage, improve the efficiency (most devices switch at the grid frequency), and minimize the common-mode leakage current with the help of a novel dedicated circuit (transient circuit). Simulations and experiments confirm the feasibility and good performance of the proposed converter.

Grid-connected photovoltaic converters: Topology and grid interconnection

With the advances in high power converter technologies, photovoltaic (PV) power has been integrated into the utility system in greater pace in recent years. This paper aims to provide a broad overview of the various grid-connected PV system configurations and their power converter technologies for applications in single- and three-phase utility power systems. Grid-connected PV systems presented include module-integrated, string, and centralized configurations. Single- and three-phase power converters discussed include full-bridge (with an additional switch in the DC link and back-to-back IGBT on the AC terminals), and neutral point clamped converters. The synchronization techniques for grid-connected PV applications are discussed in this paper. An improved method to detect the grid frequency and voltage magnitude for single-phase system is introduced. More importantly, harmonic distortions resulting from the operation of grid-connected photovoltaic systems are addressed and analyzed. Methods for reducing harmonic currents using three types of passive filters, L, LC, and LCL, are presented. The LCL-filter design guidelines for a specific PV system are presented in detail. The effectiveness of these filters is demonstrated on 3 kW and 50 kW photovoltaic systems.