Three-phase Grid-connected Voltage Source Research Articles

Impedance Modeling of Three-Phase Grid-Connected Voltage Source Converters With Frequency-Locked-Loop-Based Synchronization Algorithms

With the increased installation of voltage source converters (VSCs) into the power grid, concerns about resonance/stability issues associated with interactions among converters, and between them and the ac grid is growing. The prevailing method for investigating these issues is the impedance modeling of VSCs, which involves linearizing all their control elements, such as their current control and grid synchronization system among others. In this process, most often a phase-locked loop (PLL) is considered for the grid synchronization of VSCs in the literature. The available options for the grid synchronization, however, are not limited to PLLs; a large number of frequency-locked loops (FLLs) for the grid synchronization of VSCs may also be found in the literature. The impedance modeling of three-phase VSCs equipped with FLL-based grid synchronization systems can be quite challenging as FLLs have different structures compared to PLLs. This article aims to address this difficulty. The general idea is obtaining the PLL counterparts of FLLs, which makes the VSC impedance modeling very straightforward. To make this idea clear, several case studies are presented and investigated.

Optimal design of proportional‐resonant controller and its harmonic compensators for grid‐integrated renewable energy sources based three‐phase voltage source inverters

This paper proposes new analytical and optimal design procedures of the proportional-resonant (PR) controller and its harmonic compensators (HCs) for three-phase grid-connected voltage source inverters (VSIs) powered by renewable energy resources. The modeling and analysis based on stationary reference frame are performed for VSIs collaborated with an L-type filter. The theoretical verification and simulation validation of the proposed design guidelines are done to approve its effectiveness and robustness. Particle swarm, grey wolf and Harris hawks' optimization techniques are applied and compared for a proper selection of the parameters of the proposed PR controller and its HCs. To accomplish this study, multi-objective error functions are employed and compared to minimize the total harmonic distortion of the grid output current. The proposed PR controller and its HCs are tested, using MATLAB/Simulink, along with the allowable changes of inverter output active and reactive powers, and also under the grid voltage distortion. Moreover, their performance is evaluated according to IEEE and IEC harmonics standards, and compared with the conventional PI controller based on reference frame. Furthermore, the experimental validation for the proposed controllers is done based on the hardware-in-the-loop real-time simulator using C2000TM-microcontroller-LaunchPadXL-TMS320F28377S kit.

Generalised average modelling of grid‐connected three‐phase VSC with closed‐loop vector control and regular‐sampled modulation

Based on the generalised averaging method, a comprehensive mathematical model for a three-phase grid-connected voltage source converter (VSC) with closed-loop vector control and symmetrical regular-sampled modulation is proposed here. The double Fourier series of switching functions is presented to construct the state average vectors in this model. To involve the conventional vector current control strategy, the coordinate transformation method of the state average vector is derived. The multiplication properties are also illustrated to calculate the product average vector for state variables. The proposed model is validated in both MATLAB simulations and hardware experiments. The results show that both the fundamental and the switching behaviours of the converter as well as the closed-loop dynamics of the controller can be accurately demonstrated by the derived model with fast simulation speed. The steady-state harmonic distribution can also be directly achieved by computing the equilibrium point of the system. The proposed model is appropriate for system-level studies of power electronic-based power systems and can provide a beneficial reference for the practical design and the performance assessment of the closed-loop VSC system, which indicates the broad prospect of the application of generalised average models in power system and power electronics fields.

An SiC MOSFET Based Three-Phase ZVS Inverter Employing Variable Switching Frequency Space Vector PWM Control

In this paper, a variable switching frequency space vector pulsewidth modulation control is proposed. It is used to achieve zero voltage switching (ZVS) for a three-phase grid-connected voltage source inverter with unity power factor. A wide range of ZVS can be realized without any additional sensors, auxiliary circuits, or current zero crossing detection circuits. The switching frequency can be easily calculated by a digital controller. The frequency variation range in a line cycle is only about 1.5 times at any specific load. An LCL filter is used to attenuate the high current ripples at the inverter side, and active damping is adopted to avoid the resonance and reduce the filter power loss. The switching loss can be significantly reduced by using silicon carbide mosfet s so that the conversion efficiency is high. The power density can also be improved due to the high switching frequency and the low inductance value of the filter. The operating principle, design considerations, and loss analyses are discussed in detail. A 3.5-kW simulation and experimental prototype interfacing a 350–400-V dc with a three-phase 110-V ac grid is developed to verify the performance of the proposed control strategy.

Optimal Energy Management Strategy and Novel Control Approach for DPGSs Under Unbalanced Grid Faults

Recently, the use of distributed power generation systems (DPGSs) based on renewable energy resources is increasingly being pursued as a supplement and a reliable alternative to the large traditional energy sources. For it, power-electronic interface technologies and control have also emerged as the most important key elements in the area of energy management and integrating DPGSs. The specification of a power-electronic interface is subject to several requirements that are related not only to the DPGS itself but also to its interactions with the power system especially where the utility grid is subject to events that can potentially lead to large-scale disturbances or even to its collapse if it operates near its capacity without fault margin. This study deals, first, with an optimized energy management strategy and, second, with a newly-conceived control strategy called symmetrical components control algorithm (SCCA) that was proposed for four-leg three-phase grid-connected voltage source inverter (VSI) used for DPGSs with wind–solar–battery sources. A mechanism of negative and zero sequences injection based on the control of ([Formula: see text]) current coordinates has been introduced. The performance of entire control system, to enhance the unbalanced fault ride-through capability of DPGSs, has been evaluated by time domain simulations with MATLAB/Simulink. Advantages of the combined active–reactive control ensuring both current and voltage controls have been achieved compared to the majority of already published strategies. The distinct features of the proposed SCCA strategy prove that it allows to meet the requirements for grid interconnection and the new stricter standards with respect to power quality, safe running, and islanding protection.

Apparent Impedance Analysis: A Small-Signal Method for Stability Analysis of Power Electronic-Based Systems

In this paper, a new method for power system stability analysis is introduced. The method is based on injection of a small voltage or current in an arbitrary point of a power system. The apparent impedance is then defined as the ratio between the voltage and current at the injection point. It is shown that the apparent impedance can be used to estimate the eigenvalues of the system that are observable from the injection point. The eigenvalues are obtained by applying system identification techniques to the measured set of apparent impedances. The method is similar to the well-established impedance-based stability analysis based on source and load impedance models. However, while the source/load impedance ratio is viewed as the minor-loop gain, the apparent impedance can be viewed as a closed-loop transfer function. It can also be expressed as the parallel connection of the source and load impedance. It is shown, in this paper, how the system eigenvalues can be extracted based on a set of apparent impedance values. The apparent impedance holds, therefore, complementary information compared with the existing impedance-based stability analysis. The method can also be used as a tool to validate analytically derived state-space models. In this paper, the method is presented as a simulation tool, while further work will extend it to include experimental setups. Two case studies are presented to illustrate the method: 1) a dc case with a buck converter feeding a constant power load and 2) a three-phase grid-connected voltage source converter with a current controller and a phase lock loop. The estimated (apparent) eigenvalues of the studied systems are equal to those obtained from the analytic state-space model.

Direct power control of grid connected voltage source inverters using port-controlled Hamiltonian system

This paper presents a new direct active and reactive power control (DPC) scheme for a three-phase grid connected voltage source inverter (VSI) based on the passivity viewpoint using the port-controlled Hamiltonian (PCH) system. The proposed controller consists of feedforward and feedback parts. The feedforward part (the reference inputs) is generated through the flatness of the dynamics of the VSI model, which makes the error dynamics in the form of PCH system. The nonlinear feedback part is designed to enhance the damping of the error dynamics by using its Lyapunov function. The proposed control method has an ability of the finite time reaching condition similar to sliding mode control (SMC). Moreover, the exponential stability and uniform performance are guaranteed over all operating points without need for reaching a certain manifold. The proposed method is validated by using an experiment through hardware-in-the-loop system with a digital signal processor. The experimental results for the proposed method are compared with those using SMC-DPC method. The proposed method significantly reduces the total harmonic distortion in the output current without deteriorating the transient response of the active and reactive powers. In addition, it provides robust performance against the line impedance variations and the grid voltage sag.

Power oscillation analysis and control of three‐phase grid‐connected voltage source converters under unbalanced grid faults

Control of three-phase grid-connected voltage source converter under unbalanced grid faults greatly depends on the active and reactive powers processed by the converter. The instantaneous active power theory with sequence decomposition is employed to analyse the instantaneous power components, especially the second-order oscillation power. Study shows that the second-order oscillation power comprises two quadrature components, the cosine and sine terms, which are contributed by the average active power and the average reactive power, respectively. This finding sheds insight on the regulation of oscillation power under unbalanced grid conditions. Based on this observation, the authors propose a positive and negative sequence conductance and susceptance control scheme, which enables simple regulation of the active power or reactive power oscillation with the average active power and reactive power control. In addition, the authors investigate the relationship between the positive/negative sequence conductance and susceptance distribution factors with power oscillation and peak current. A maximum current limitation scheme is embedded into the current reference generation block for overcurrent protection. Numerical simulations and prototype measurements verify the accuracy of the analysis and the effectiveness of the control scheme.

Optimal VF-PDPC of grid connected inverter under unbalanced and distorted grid voltages

In this article, an optimal virtual flux predictive direct power control (VF-PDPC) is proposed for three-phase grid connected voltage source inverter (VSI) operating under unbalanced and distorted grid supply voltage. The concept of virtual flux and extension pq theory is introduced in the conventional PDPC, where the former enhances its resilience toward distorted supply voltage while the latter enhances its resilience toward unbalanced supply voltage. The inherent advantages of the conventional PDPC scheme, such as simple implementation, fast dynamic responses, and no demand of modulators are retained. Getting rid of the complicated positive and negative sequence extraction of grid components, power compensation, or any tuning work, the proposed optimal VF-PDPC achieves excellent power tracking capability, and sinusoidal grid currents with low harmonics under various grid voltage conditions. Simulation results are presented to verify the effectiveness of the proposed control scheme when compared to the conventional PDPC, conventional VF-PDPC, and optimal PDPC.

Robust Fuzzy-Second Order Sliding Mode based Direct Power Control for Voltage Source Converter

This paper focuses on a second order sliding mode based direct power controller (SOSM-DPC) of a three-phase grid-connected voltage source converter (VSC). The proposed control scheme combined with fuzzy logic aims at regulating the DC-link voltage of the converter and precisely tracking arbitrary power references, in order to easily control the system’s power factor. Therefore measures are proposed to reduce the chattering effects inherent to sliding-mode control (SMC). Simulations performed under Matlab/Simulink validate the feasibility of the designed Fuzzy-SOSM. Simulation results on a 1kVA grid-connected VSC under normal and faulted grid voltage conditions demonstrate good performance of the proposed control law in terms of robustness, stability and precision.

Novel grid voltage estimation by means of the Newton–Raphson optimisation for three‐phase grid connected voltage source converters

This study presents a proportional resonant current control scheme for three-phase grid connected voltage source converters without any grid voltage sensors. Towards this goal, the grid voltage sensors are replaced by a voltage estimation scheme based on the Newton–Raphson algorithm. In the proposed method, the grid connected converter behaviour is converted to an optimisation problem which relates the instantaneous grid voltage value to the filter parameters, the generated converter voltage and the exchanged powers in each sampling period. The proposed optimisation problem is then solved in each sampling period by means of the Newton–Raphson optimisation technique. Afterwards, the reference current for the current control loop is generated from the estimated grid voltages. Using an estimator instead of the voltage sensors not only reduces the size and cost of the converter system but also improves its reliability and protects it from grid voltage disturbances and noises. The excellent performance of the proposed grid voltage sensorless scheme is confirmed through experimental and simulation results.

An LTCL Filter for Three-Phase Grid-Connected Converters

This paper presents a new high-order filter for three-phase grid-connected voltage source converters (VSCs) named the inductor-trap-capacitor-inductor (LTCL) filter, in which multiple LC trap filters are inserted in parallel with the branch of the capacitor in the traditional LCL filter to compose multiple series resonant circuits at the selected frequencies. Particularly, the LTCL filter can attenuate the harmonic currents around the multiples of switching frequencies and guarantee -60 dB/decade attenuation in the high-frequency band, leading to a decrease of the total inductance and volume. Furthermore, within half of the switching frequency range, an LTCL -filter-based grid-connected VSC has almost the same frequency-response characteristic as that with the traditional LCL filter. That is to say, the LTCL filter does not bring any extra control difficulties. The basic parameter selection criteria and the parameter design procedure of the LTCL filter are introduced. Moreover, the sensitivity analysis of the LTCL filter is analyzed and discussed in detail. The comparative analysis and discussion considering the LCL filter, the LLCL filter, the multituned traps filter, and the proposed LTCL filter, have been presented and evaluated through the experiments on a 65-kW three-phase grid-connected VSC prototype.

Improvement of DPC of Grid-Connected Inverter for Wind Power System

This paper analyzes the principle and properties of the traditional direct power control (DPC) system for the three-phase grid-connected voltage source inverter based on its mathematical model. Hysteresis comparators in power loop of DPC control system, leading to the switching frequency not fixed .the fluctuation of power and DC voltage in temporary state. This paper brought an improved program. The active and reactive powers are calculate by estimating the virtual flux. The AC side voltage sensors could be replaced. The SVPWM has been utilized instead of the power hysteresis and switching table of the traditional DPC system.

Exploring Inherent Damping Characteristic of LCL-Filters for Three-Phase Grid-Connected Voltage Source Inverters

This paper investigates the inherent damping characteristic of LCL- filters for three-phase grid-connected voltage source inverters. Specifically, it is found that when the converter-side current is used for implementing the feedback control, there will be an inherent damping term embedded in the control loop, which can neutralize the resonance introduced by LCL- filters. Theoretical analysis is then presented to suggest a general design guideline for choosing values of grid- and converter-side inductors, so that optimum damping can be naturally achieved by solely using converter current control, doing away with active damping, passive damping, or complex state observer. In cases where the design criterion is not fulfilled, the damping information contained in the converter current is extracted by a second-order notch filter, and then processed by a compensation gain to tune the system damping factor. The proposed compensation strategy requires no additional hardware and it will not cause an overmodulation problem due to its free of fundamental component. Both simulation and experimental results are finally provided to validate the theoretical findings developed in this paper.