Grid-connected Current Control Research Articles

Current-Prediction-Controlled Quasi-Z-Source Cascaded Multilevel Photovoltaic Inverter

To address problems that traditional two-stage inverters suffer such as high cost, low efficiency, and complex control, this study adopts a quasi-Z-source cascaded multilevel inverter. Firstly, the quasi-Z-source inverter utilizes a unique impedance network to achieve single-stage boost and inversion without requiring a dead zone setting. Additionally, its cascaded multilevel structure enables independent control of each power unit structure without capacitor voltage sharing problems. Secondly, this study proposes a current-predictive control strategy to reduce current harmonics on the grid side. Moreover, the feedback model of current and system state is established, and the fast control of grid-connected current is realized with the deadbeat control weighted by the predicted current deviation. And a grid-side inductance parameter identification is added to improve control accuracy. Also, an improved multi-carrier phase-shifted sinusoidal PWM method is adopted to address the issue of switching frequency doubling, which is caused by the shoot-through zero vector in quasi-Z-source inverters. Finally, the problems of switching frequency doubling and high harmonics on the grid side are solved by the improved deadbeat control strategy with an improved MPSPWM method. And a seven-level simulation model is built in MATLAB (2022b) to verify the correctness and superiority of the above theory.

A Control Parameters Self-Adjusting Method for photovoltaic inverter considering the variation of inductance

The variation of inductance is the reason for the instability of photovoltaic (PV) inverter system. To this end, a control parameters self-adjusting method considering the variation of inductance is proposed to enhance the anti-interference ability. The basic control method is to use grid-connected current control and capacitor-current-feedback method to suppress resonance. Based on the Routh Criterion (RC), a simple Routh Array enumeration rule is proposed. Discrete Routh Criterion (RC) is selected as the stability analysis method to obtain the stability range of the control parameters. The control parameters self-adjusting method can enhance the anti-interference ability of system under variations of inductance, which is proved by comparative experiments.

MPC-based three-phase unbalanced power coordination control method for microgrid clusters

The microgrid cluster group-level bus voltage is high, the capacity is large, and the interconnection bus is connected to various microgrid subsystems. It is challenging for the microsource inverters in the subsystems to directly participate in the imbalance regulation of the group-level bus voltage, so it is currently difficult to solve the three-phase imbalance of the group-level bus voltage. In this paper, a particular structure of a series microgrid is applied to the cluster system, and a three-phase unbalanced control strategy based on model predictive control for the cluster system is proposed. First, the grid-connected current prediction control model of the series microgrid inverter using an LCL filter is established, a medium-voltage high-capacity three-level neutral point clamped (NPC) power electronic converter is introduced into the system and used as the central inverter of the DC microgrid, and the model prediction control method is applied to this central inverter. Second, considering possible load asymmetry operating conditions of the group bus, the p-q-o method is used to calculate the load imbalance current component that must be compensated, which is used as a part of the output current of the series microgrid or DC microgrid, based on which the three-phase unbalanced power coordinated control method between two heterogeneous microgrids is studied. The simulation results show that the series microgrid and DC microgrid exhibit good three-phase unbalance suppression under load unbalanced conditions, and the coordinated control between them can effectively improve the three-phase unbalance suppression capability of the system, which confirms that the proposed control strategy can achieve high-quality voltage output of the microgrid cluster system in multiple scenarios.

Adaptive Current Control for Grid-Connected Inverter with Dynamic Recurrent Fuzzy-Neural-Network

The grid-connected inverter is a vital power electronic equipment connecting distributed generation (DG) systems to the utility grid. The quality of the grid-connected current is directly related to the safe and stable operation of the grid-connected system. This study successfully constructed a robust control system for a grid-connected inverter through a dynamic recurrent fuzzy-neural-network imitating sliding-mode control (DRFNNISMC) framework. Firstly, the dynamic model considering system uncertainties of the grid-connected inverter is described for the global integral sliding-mode control (GISMC) design. In order to overcome the chattering phenomena and the dependence of the dynamic information in the GISMC, a model-free dynamic recurrent fuzzy-neural-network (DRFNN) is proposed as a major controller to approximate the GISMC law without the extra compensator. In the DRFNN, a Petri net with varied threshold is incorporated to fire the rules, and only the parameters of the fired rules are adapted to alleviate the computational workload. Moreover, the network is designed with internal recurrent loops to improve the dynamic mapping capability considering the uncertainties in the control system. In addition, to assure the parameter convergence in the adaptation and the stability of the designed control system, the adaptation laws for the parameters of the DRFNN are deduced by the projection theorem and Lyapunov stability theory. Finally, the experimental comparisons with the GISMC scheme are performed in an inverter prototype to verify the superior performance of the proposed DRFNNISMC framework for the grid-connected current control.

Repetitive Predictive Control for Current Control of Grid-Connected Inverter Under Distorted Voltage Conditions

A repetitive predictive control for grid-connected inverter current control scheme is presented in this paper under voltage harmonic distortion in the stationary reference frame. Predictive control is an approach that uses a receding horizon to achieve the optimal track for the reference. In this approach, the repetitive controller behavior is added to the predictive control to increase its performance under distorted voltage conditions. In addition, to apply the controller in the generation system, controller is designed from the perspectives of the power system. The controller is designed considering the state-space model in the stationary reference frame. The controller performance was verified using a laboratory experimental setup under distorted grid voltage condition. Experimental results confirm that the proposed controller can effectively suppress harmonics under both normal operation and distorted grid voltage conditions, and also satisfy the requirements stipulated in IEEE Std. 1547.2-2008.

A Novel Grid-Connected Current Control Method Based on Frequency Adaptive Proportional Resonance

In this paper, a novel quasi-proportional resonance (quasi-PR) controller for grid-connected current control is proposed. For the traditional quasi-PR controller, the performance decreases in cases when a large gird frequency deviation is going through. To solve the problem, a frequency adaptive structure is applied in the current control method to achieve online correction on the resonance frequency, so that a high control accuracy is maintained and the robustness of the control system is enhanced. The feasibility of the proposed method has been verified by experimental results.

A strategy of PI + repetitive control for LCL-type photovoltaic inverters

Due to the traditional grid-connected current control method of single Proportional Integral (PI) and Repetitive Control (RC) strategies, the photovoltaic inverter output current will have a distortion problem, which can not only maintain the stability of the whole photovoltaic system, but also the current quality of the photovoltaic inverter grid-connected system is reduced in the case of high-order LCL photovoltaic inverter control system operation. So, a strategy of PI + repetitive control in two-phase stationary frame is proposed. The introduction of the weighting coefficient m of the PI controller branch can enhance the adjustment ability of the PI link and accelerate the responding speed to meet the dynamic characteristics of the entire operating system, while the other weighting coefficient n on the repetitive controller branch can increase the correction capability and eliminate the steady-state error to meet system steady-state requirements. The scheme not only simplifies the coordinate transformation and decoupling calculation process, but also improves the harmonics suppression ability of the photovoltaic inverter and reduces the total harmonic distortion rate of the grid-connected current. At the same time, with the load operation mode change, the system can also retain better stability and keep a fast dynamic response capability, and the grid current can be able to return to a stable state in one cycle. Finally, the effectiveness of theoretical analysis and the strategy is verified by simulation.

Operation scheme for MMC-based STATCOM using modified instantaneous symmetrical components

Modular multilevel converters (MMCs) are characterized by modularization and multielectric equality, and the application of this structure to a high-voltage large-capacity static synchronous compensator (STATCOM) shows good potential. In this paper, a modified instantaneous symmetrical component method is proposed for positive- and negative-sequence decomposition, i.e. the critical part of the control device, which can accurately detect the instantaneous value of each sequence component in three-phase asymmetrical phasors in real time. Then, based on this method, the paper proposes a control method for MMC-based STATCOMs that solves the problems of multi-DC (direct current) voltage balance, grid-connected current control, and circulation suppression. Finally, the proposed detection and control methods are verified by performing simulations and experiments. The results show that the proposed method can quickly and accurately detect the positive- and negative-sequence reactive component that should be compensated by the device, thus realizing real-time reactive compensation for MMC-based STATCOMs.

Duty-Cycle Predictive Control of Quasi-Z-Source Modular Cascaded Converter Based Photovoltaic Power System

A duty-cycle predictive control is proposed for dc grid integration of front-end isolated quasi-Z-source modular cascaded converter (qZS-MCC) photovoltaic (PV) power system. The post-stage qZS half-bridge dc-dc converter deals with PV maximum power point tracking, dc grid integration, and dc-link voltage balance; whereas, the front-end isolation converters operate at a constant duty cycle of 50%. Thus, it saves control hardware resources while overcoming challenges from PV-panel voltage variations and dc-bus voltage limit. The proposed control uses the derived circuit model to predict the global active-state duty cycle for grid-connected current control and predict the shoot-through duty cycles for dc-link voltage balance, achieving a fast and accurate tracking target. The proposed control method has advantages of: i) eliminating weighting factors that exist in conventional model predictive control (MPC), ii) no sophisticated loop parameters design that exists in proportional-integral (PI) control, iii) operating at constant switching frequency that is different from the conventional MPC with variable switching frequency. Simulation and experimental tests are carried out to verify the effectiveness of the proposed control method and compare with the PI-based control system.

Coupling Influence on the dq Impedance Stability Analysis for the Three-Phase Grid-Connected Inverter

The dq impedance stability analysis for a grid-connected current-control inverter is based on the impedance ratio matrix. However, the coupled matrix brings difficulties in deriving its eigenvalues for the analysis based on the general Nyquist criterion. If the couplings are ignored for simplification, unacceptable errors will be present in the analysis. In this paper, the influence of the couplings on the dq impedance stability analysis is studied. To take the couplings into account simply, the determinant-based impedance stability analysis is used. The mechanism between the determinant of the impedance-ratio matrix and the inverter stability is unveiled. Compared to the eigenvalues-based analysis, only one determinant rather than two eigenvalue s-function is required for the stability analysis. One Nyquist plot or pole map can be applied to the determinant to check the right-half-plane poles. The accuracy of the determinant-based stability analysis is also checked by comparing with the state-space stability analysis method. For the stability analysis, the coupling influence on the current control, the phase-locked loop, and the grid impedance are studied. The errors can be 10% in the stability analysis if the couplings are ignored.

Multi-Step Predictive Current Control for NPC Grid-Connected Inverter

Aiming at improving the quality of output current when the inverter is connected to the grid, this paper proposes a control strategy of neutral point clamped (NPC) grid-connected inverter based on radial basis function neural network (RBFNN) multi-step predictive control. Firstly, the predictive control model for output current of NPC inverter is constructed by $\alpha \beta $ coordinate transformation. Then the future values of output current are predicted for each voltage vectors, using the RBFNN prediction model. After the predicted values are obtained, a cost function $f$ is calculated for each voltage vectors and the optimal voltage vector which minimizes cost function is selected. The switch state corresponding to the optimal voltage vector is applied to the inverter in the next sampling period. Thereby the control of grid-connected output current can be achieved. The simulation results show that the RBFNN multi-step predictive control method improves the capability of current tracking and reduces the harmonic rate of output current. The proposed method can balance output current under the condition of grid voltage imbalance.

Current Control Method in Doubly Fed Induction Generator Under Low Switching Frequency

The present current control method in doubly fed induction generator cannot realize the segmented grid-connected current control, it’s hard to effectively control the current in doubly fed induction generator. Therefore, a current control method in doubly fed induction generator under low switching frequency is proposed in this paper. Which means to build a mathematical model of the doubly fed induction generator under low switching frequency to analyze the parameters of doubly fed induction generator filter. Then the parameter values of the filter can be obtained. The current in generator can be predicted by adopting the double-sampling predict method. And the current control in generator can be improved according to dead beat control. Then the on-line identification of current parameters by least square method is needed to finish the current control method in doubly fed induction generator under low switching frequency. The experimental results show that the proposed method realized the segmented grid-connected current control in doubly fed induction generator.

Asymmetric H-Bridge Single-Phase Seven-Level Inverter Topology with Proportional Resonant Controller

ABSTRACT This paper presents an asymmetrical H-bridge single-phase seven-level inverter topology with modified gating scheme for reducing the number of high-frequency switches. Due to shortcomings like steady-state error and problems in removing low-order harmonics associated with proportional integral controller, proportional resonant controller is used for grid-connected converter current control. A practical application of proportional resonant current controller is developed using a low-cost dsPIC33EP256MC202 microcontroller to keep the current injected in to the grid. The validity of proposed inverter and control scheme is verified through simulation and implemented for low-voltage laboratory prototype.

Multifunctional Control Strategy for Asymmetrical Cascaded H-Bridge Inverter in Microgrid Applications

A multifunctional control strategy for a single-phase asymmetrical cascaded H-bridge multilevel inverter (ACHMI), suitable for microgrid systems with nonlinear loads, is presented. The primary advantage of ACHMI is to produce a staircase output voltage with low harmonic content utilizing unequal dc voltages on the individual H-bridge cells. In a grid-connected mode of operation, the control strategy of the ACHMI is based on the conservative power theory, providing selective disturbing current compensation besides injecting its available energy. In autonomous mode of operation, two different control methods along with a damping resistor in the filter circuit are developed for regulation of the ACHMI instantaneous output voltage in a variety of load conditions. The first method is a single-loop voltage control scheme without the need of any current measurement. The second one is a multiloop voltage control scheme with a load current feedforward compensation strategy and preservation of the grid-connected current control scheme. The steady-state response and stability of both voltage control schemes are analyzed, and based on the application requirement, the control schemes are implemented individually. The effectiveness of each control strategy is experimentally verified using a hardware-in-the-loop setup with the control algorithm implemented in the TMSF28335 DSP microcontroller.

Comparison of Dynamic Characteristics Between Virtual Synchronous Generator and Droop Control in Inverter-Based Distributed Generators

In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides inertia support for the system.

Simulation of Grid-Connected Solar Micro-inverter Based on Fuzzy PI Controller

Grid-connected solar micro-inverter is a highly nonlinear and time-varying system, so it is difficult to achieve good control effect using traditional PI controller. In this paper, small signal analysis model of micro-inverter is established, grid-connected current control strategy composed of fuzzy PI controller and grid voltage feed-forward is put forward, and the initial parameters of PI controller is optimized using the genetic algorithm. Simulation results show that the control strategy has the virtues of good robustness, small dynamic deviation, and could reduce the harmonic content of grid-connected current.

Asymmetry Three-Level Gird-Connected Current Hysteresis Control With Varying Bus Voltage and Virtual Oversample Method

Grid-connected current control plays an important role in the renewable grid-connected system. This paper investigates the grid-connected current hysteresis control of the asymmetry three-level inverter. In order to effectively utilize asymmetry three-level inverter and conquer the disadvantages of traditional hysteresis control, the multistage hysteresis control and current predict hysteresis control are developed. In the multistage hysteresis control method, the switching times of main switches can be declined by regularly adjusting the bus voltage without altering the inductor and bandwidth. In addition, the current predict hysteresis control is developed, which adapts to proper bus voltage without dividing bus switching time. In order to overcome the invalidation issue of the bandwidth in hysteresis control, virtual sample technology is used without increasing real sample frequency. Both simulation and experimental tests have been conducted to verify the efficacy of the proposed methods.

A Control Strategy for Single-phase Grid-Connected Inverter with Power Quality Regulatory Function

A single-phase grid-connected inverter system based on LCL filter is established，which combines the features of inverter and active power filter. A composite control strategy for grid-connected inverter with the function of implementing reactive power compensation and harmonic compensation in the grid-connected power generation is proposed. Firstly, grid-connected inverter system structure and model is analyzed. A quasi-Proportional Resonant control method to gain the control of grid-connected fundamental wave current containing reactive power compensation current as well as the control of harmonic compensation currents is put forward; then the calculation methods of composite control command current based on both second order generalized integrator-quadrature signal generator (SOGI-QSG) and instantaneous reactive power theory are given. Finally, the effectiveness of the control strategy proposed in this paper is verified by simulation. DOI : http://dx.doi.org/10.11591/telkomnika.v12i1.3911

Analysis on Single Photovoltaic Grid-Connected Inverter Control System

nverter can operate with power grid in parallel and can return the electricity generated by photovoltaic array with high power factor to power grid. The paper analyzes and compares several grid-connected current control methods, determines that the current control mode used in the design is improved SPWM current virtual value tracking control technology. And the paper analyzes and studies the transfer functions of key links of the inverter, and gives the derivation process of transfer functions. The control system uses dual-loop structure of inner current loop and outer voltage loop, and proposes the design methods of voltage loop and current loop PI regulator.

Design and Implementation of 30kW Three-Phase PV Grid-Connected Inverter

This paper developed a mathematical model of three-phase PV grid-connected inverter, and studied the grid-connected current control method based on PI control in synchronous rotating reference frame. Simulation and experimental results from the prototype of 30kW three-phase PV grid-connected inverter proved the correctness and the feasibility of the control strategy, and this grid-connected inverter can operate at the unity power factor state with a nice dynamic performance, and the output current has high sinusoidal and low harmonic content as well as good symmetry.