Control Strategy For Grid-connected Inverter Research Articles

Impedance remodeling control strategy of grid-connected inverter with inertia-damping phase-locked loop under extremely weak grid

The operation of the grid-connected inverter (GCI) in weak grid conditions presents a risk of instability due to the presence of high grid impedance and the negative impedance effect of the phase-locked loop (PLL). In response to this issue, this paper introduces an impedance remodeling control strategy for the GCI utilizing the inertia-damping phase-locked loop (ID-PLL). Firstly, the proposed ID-PLL exhibits certain inertia and damping properties in comparison to the traditional PLL (T-PLL), resulting in an enhanced phase margin of the GCI system's equivalent output impedance. Consequently, the adaptation range of the grid impedance increases from 12mH (SCR = 3.00) to 24mH (SCR = 1.37). Next, from the perspective of impedance analysis, this study remodels the PLL impedance and inverter control loop impedance, further improving the amplitude and phase margin of the system's output impedance. As a result, the adaptation range of the grid impedance extends to 33mH (SCR = 1.00), and the transition of grid-connected currents becomes smooth and free of overshoot. Finally, the effectiveness of the proposed control strategy is validated through theoretical analysis and experimental verification.

Enhancing grid-connected inverter performance under non-ideal grid conditions: a multi-functional multiplexing control strategy

Compared to the traditional thermal power generation, new energy sources, such as wind and photovoltaic systems, are more vulnerable to the effects of non-ideal power grids due to their limited capacity. This susceptibility can jeopardize the safe operation of power equipment, degrade power output quality, and lead to non-compliance with grid-connected specifications. The LCL-type grid-connected inverter is a typical nonlinear system that weakens the controllability of the grid-connected energy. To address these challenges, this study employs feedback linearization theory to transform the inverter into a standard linear system. Subsequently, it utilizes linear system methodologies to develop robust control laws, ultimately introducing a multi-functional multiplexing control strategy for grid-connected inverters based on feedback linearization and Hamilton-Jacobi-Issacs inequality. Simulation results demonstrate that this multi-functional strategy outperforms traditional grid-connected inverter control schemes, effectively mitigating issues related to low short-circuit ratios, voltage fluctuations, imbalances, harmonics, and other non-ideal grid conditions. Furthermore, it significantly expands the system’s adaptability to varying weak grid impedances.

Research on a Three-Phase Energy Mutual-Aid Strategy for a Grid-Connected Inverter Based on Constructed Negative Sequence Current Control

With the increased grid-connected capacity of a single-phase distributed power supply, three-phase power unbalance is more likely to occur in a power grid. Three-phase power unbalance can further lead to three-phase voltage unbalance, which can have adverse effects on power quality and power supply reliability. Therefore, there is a need to build a three-phase power transmission channel to realize power exchanging among phases. In this paper, a novel grid-connected inverter control strategy for three-phase power exchanging is proposed based on constructed negative sequence current control. A completed negative sequence current control loop is added to a conventional three-bridge inverter to realize the decoupling control of three-phase grid current, and then three-phase power exchanging is realized. On this basis, this paper further puts forward a strategy for three-phase power exchanging aimed at three-phase voltage balance. Correspondingly, the three-phase grid current is controlled according to the feedback of the three-phase voltage. Then, three-phase voltage balance is achieved by three-phase power exchanging. The simulation and experimental results show that the proposed strategy is suitable for a three-phase unbalanced power grid, which can realize three-phase power exchanging, and further, can achieve three-phase voltage balance. The proposed strategy can help to improve power quality and supply reliability.

Sliding mode control strategy for grid-connected inverters based on boundary layer

A sliding mode controller is designed to effectively track the reference current for the characteristics of conventional grid-connected controllers that cannot perform static-free tracking. and aiming at the jitter phenomenon of sliding mode controller, a boundary layer-based sliding mode controller is designed, which adopts traditional linear control inside the boundary layer and sliding mode control outside the boundary layer. the error is limited to a certain range and a compensation term is used to improve the problem of slow control speed. The proposed strategy is validated by simulation using matlab simulation software, and the simulation results prove that the proposed strategy can achieve grid current and grid voltage in the same frequency and phase, which proves the effectiveness of the proposed strategy.

Grid-connected Inverter Control Strategy of New Energy Microgrid

The traditional power system is composed of a synchronous generator. Because the rotor of the synchronous generator has the characteristics of the moment of inertia and damping, it can provide or absorb excess energy when the system’s frequency fluctuates. The inertia and damping of synchronous generators determine the frequency dynamic response process of the power grid, which further affects the operation, control, and protection of the whole power grid. However, because renewable energy is connected to the power grid by power electronic equipment, it does not have mechanical inertia and damping characteristics. With the increase of installed capacity of new energy, the whole power system shows low inertia characteristics. In this case, the power grid is more sensitive to interference, which may further lead to load shedding or system instability, or even system collapse. Virtual synchronous generator technology can solve this problem.

Control strategy for grid-connected inverters based on STATCOM reactive power compensation Research

The problem of grid-connected inverter reactive power compensation becomes a decisive factor for grid-connected power quality. To ensure the system unit power factor operation, this paper designs a reactive power compensation control strategy for grid-connected inverters based on STATCOM. On the basis of detailed analysis of the working principle of STATCOM, the grid-connected inverter model is built with the help of MATLAB simulation environment. The experimental results show that the use of STATCOM for grid-connected inverter reactive power compensation ensures stable grid-side output voltage of the grid-connected inverter and can improve the power factor of the system at the same time, which has certain reference significance for upgrading the grid-connected inverter design.

An H∞ filter based active damping control strategy for grid-connected inverters with LCL filter applied to wind power system

Since the LCL filter has good performance to attenuate high frequency harmonics, it is widely used in wind power inverters. But it can cause high-frequency oscillations and instability, which limits its application in wind power system. For an LCL-type grid-connected inverter, the conventional capacitor-current-feedback type active damping control strategy can retain the high-frequency characteristics of LCL filter while suppressing the resonant peak effectively, with the drawback of requiring additional current sensors. Therefore, an H∞ filter based active damping strategy of LCL grid-connected inverter is proposed. The state space model and the process noise model of the LCL inverter are built, and then the general model of the standard H∞ control problem is derived, which helps to solve the H∞ filter. The H∞ filter can estimate the filter capacitor current according to sample values of the injected current and the voltage of the point of common coupling, and the estimation accuracy can be guaranteed even if parameter perturbations exist in the LCL filter. The proportional resonant controller is adopted to track the grid current. Therefore, the proposed control strategy can effectively suppress the resonance peak of the LCL filter without additional current sensors and be insensitivity to system parameter perturbations. The simulation and experimental results verify the feasibility and superiority of the proposed method.

Power Control Strategy for Grid-Connected Inverters in Stationary Reference Frame

The control of grid-connected converters in stationary reference frame has been investigated in several works of literature. One of the challenges of this method is in the computation of the current computing current references as a function of power references. In some works, this issue has been solved based on instantaneous power theory equations described in the stationary reference frame. However, this approach can lead to steady-state errors in the injected power if there is an error in the current control loop, which is the focus of this investigation. Firstly, analytical expressions are derived to investigate the effect of current loop steady-state error in the active and reactive power injected into the grid. Then, this paper proposes a closed-loop power control for grid-connected inverters controlled in the stationary reference frame. This strategy is experimentally investigated in two power conversion system configurations for battery energy storage systems. The results indicate that the proposed scheme guarantees zero steady-state error in the injected power even when the current loop presents an amplitude and phase steady-state error.

Research on Control Strategy of Grid-connected Inverter with Active Filter Function

Grid-connected inverters incorporate distributed energy sources such as photovoltaics and wind energy into the micro-grid. Due to the volatility of distributed energy generation and the presence of a large number of non-linear loads in the micro-grid, harmonic wave seriously affect power quality. The grid-connected inverter with active filtering function is formed by adding the harmonic wave suppression function to the grid-tie inverter, which not only realizes the inverter grid connection but also achieves the purpose of harmonic wave compensation. The traditional model predictive control (MPC) has the characteristics of strong robustness and fast dynamic response, but its high switching frequency causes serious heating of the inverter. In response to the above problems, combined with hysteresis control theory, a model control strategy based on hysteresis is proposed in this paper, which improves the accuracy of grid-connected current of the grid-connected inverter with active filter function and inhibits the harmonic component of the system, verifying the effectiveness of the strategy.

Research on control strategy of grid-connected inverter based on three-loop structure

A grid-connected inverter system is deeply researched based on the two-stage single-phase PV grid-connected inverter. Full bridge structure is adopted in the backend inverter circuit. In order to realise the balanced flow of power, to improve the system's response speed and to eliminate the influence of the resonance peak of LCL filter, the three loop control strategy is adopted in the inverter system. At the same time, the reason of generation of the dead-time and its influence on the output of the grid-connected inverter are analysed in detail. And an improved dead-time compensation method is proposed for zero-current clamping phenomenon. The output voltage error of inverter is extended from two to seven in this method, which effectively solves the problems, such as zero-crossing distortion of the grid-connected current, additional odd harmonic content, higher distortion rate and lower amplitude of fundamental wave.

Research on control strategy of grid-connected inverter based on three-loop structure

Research on control strategy of grid-connected inverter based on three-loop structure

A Theoretical Concept of Decoupled Current Control Scheme for Grid-Connected Inverter with L-C-L Filter

This paper proposes a nonlinear decoupled current control scheme for a grid-connected inverter with LCL filter. Decoupling the active and reactive current control channels is one of the main demands in the control of inverters. For inverters with an L filter, the decoupling can be achieved by a proper feed-forward of grid voltages. However, the coupling of channels is a complex issue for converters with LCL filters. The resonance mode of the LCL filter may cause instability, which adds more complexity to the analysis. In this paper, state equations of the system are provided, which highlight the coupling between active and reactive currents injected into the grid. Accordingly, a non-linear control scheme is proposed which effectively decouples the channels and dampens the resonant modes of the LCL filter. The stability of the proposed control method is verified by the Lyapunov criterion. Independency of the system stability to the grid-impedance is another feature of the proposed approach. Moreover, only grid-side currents are needed for implementation of the proposed scheme, avoiding the need for additional current sensors for the output capacitor and grid-side inductor. For accurate modelling of the inverter, the computation and PWM sampling delays are included in the controller design. Finally, various case studies are provided that verify the performance of the proposed approach and the stability of the system.

Optimization of Var-Voltage Regulation Control Strategy for Grid-Connected Inverter of Photovoltaic Power

With the continuous increase of installed capacity of large photovoltaic power stations, the light intensity and temperature changes of photovoltaic power generation units themselves will cause grid-connected voltage fluctuation or even beyond the limit, so large photovoltaic power stations must participate in voltage regulation control. To solve this problem, the voltage distribution characteristics of the photovoltaic power station are firstly analyzed quantitatively, and the analysis shows the voltages at the photovoltaic inverter buses are relevant with the impedance of the collecting lines, the voltage at the PCC point, the location of the photovoltaic unit, and the output power of the photovoltaic unit. Furthermore, our work proposes the reactive power distribution strategy and voltage control strategy of photovoltaic power station based on particle swarm algorithm, and the correctness and feasibility of the proposed control strategy are verified by simulation, so as to ensure the rationality of the voltage control target of each photovoltaic inverters while taking into account the voltage of each photovoltaic unit.

Grid-connected Inverter Control Strategy Based on Capacitor Voltage Error Inertia Feedback

Firstly, the mathematical model of single-phase grid connected inverter is established.Then, the mathematical derivation is carried out accord to the state space model, and the control strategy of grid-connected inverter based on capacitor voltage error inertia feedback is proposed. In this strategy, the first-order inertial feedback of capacitor voltage error is adopted to suppress the resonance peak of LCL filter, the feedforward of capacitor reference voltage is adopted to improve the tracking performance of the system, and the proportional feedforward of grid-connected voltage is used to suppress the harmonic of power grid. In addition, Simulation verify the feasibility and effectiveness of the proposed control strategy.

Grid-connected Inverter Control Strategy Based On Capacitor Reference Voltage Feedforward

In view of the disadvantage that grid-connected power is not high and the phase margin of the grid-connected current is greatly reduced with the increase of impedance of many grid voltage feedforward control strategies. A grid-connected inverter control strategy based on capacitor reference voltage feedforward is proposed. In this control, the error between the filter capacitor voltage and its reference voltage is combined with the proportional controller as the inner loop control, the error between the inverter side current and its reference current is combined with the current controller as the outer loop control to improve the robustness of the system. The phase difference between grid-connected current and grid voltage is reduced by using capacitor reference voltage feedforward. In addition, by using the grid voltage resonance feedforward, the system can not only retains the ability to suppress harmonics, but also prevents the system’s phase margin from decreasing greatly under weak power grid. The proposed strategy has good dynamic performance, it can obtain good grid-connected current quality and high grid-connected power, and it has strong robustness to the change of LCL parameters and the fluctuation of grid voltag. At the same time, it has better adaptability to weak power grid.

Study on Energy Coordination Mechanism of Micro Photovoltaic Grid Connected Inverter

In this paper, the non isolated grid connected inverter control strategy is adopted. The simulation model of photovoltaic grid connected inverter is built in MATLAB. Realize the same frequency and phase of grid connected current and grid voltage, the sine degree of grid connected current waveform is better, and realize the islanding detection function. In the experiment platform, the control idea is further verified, and the simulation is proved to be correct.

Multiobjective Coordinated Control Strategy for Grid-Connected Inverter under Unbalanced Voltage Conditions

AbstractTo improve grid-connected inverter operation performance, a power oscillation and current harmonic suppression coordinate control strategy that considers the problem of overcurrent is propo...

Enhanced Current-Limiting Droop Controller for Grid-Connected Inverters to Guarantee Stability and Maximize Power Injection Under Grid Faults

Droop-controlled inverters are widely used to integrate distributed energy resources (DERs) to the smart grid and provide ancillary services (frequency and voltage support). However, during grid variations or faults, the droop control scheme should inherit a current-limiting property to protect both the inverter and the DER unit. In this brief, a novel structure of the recently developed current-limiting droop (CLD) controller is proposed to accomplish two main tasks: 1) guarantee current limitation with maximum power injection during grid faults and 2) rigorously guarantee asymptotic stability of any equilibrium point in a given bounded operating range of the closed-loop system for a grid-connected inverter. Since the maximum power of the DER unit can be utilized under grid faults with the proposed enhanced CLD and then inspired by the latest fault-ride-through requirements, it is further extended to provide voltage support to a faulty grid via the maximum injection of reactive power. This is achieved by simply adjusting the reactive power reference opposed to the existing control schemes that require adjustment of both the real and the reactive powers. Hence, a unified current-limiting control scheme for grid-connected inverters under both normal and faulty grids with a simplified voltage support mechanism is developed and experimentally verified in this brief.

Frequency Adaptive Current Control Scheme for Grid-connected Inverter without Grid Voltage Sensors Based on Gradient Steepest Descent Method

This paper presents a frequency adaptive grid voltage sensorless control scheme of a grid-connected inductive–capacitive–inductive (LCL)-filtered inverter, which is based on an adaptive current controller and a grid voltage observer. The frequency adaptive current controller is constructed by a full-state feedback regulator with the augmentation of multiple control terms to restrain not only the inherent resonance phenomenon that is caused by LCL filter, but also current harmonic distortions from an adverse grid environment. The number of required sensing devices is minimized in the proposed scheme by means of a discrete-time current-type observer, which estimates the system state variables, and gradient-method-based observers, which estimate the grid voltages and frequency simultaneously at different grid conditions. The estimated grid frequency is utilized in the current control loop to provide high-quality grid-injected currents, even under harmonic distortions and the frequency variation of grid voltages. As a result, the grid frequency adaptive control performance as well as the robustness against distorted grid voltages can be realized. Finally, an inverter synchronization task without using grid voltage sensors is accomplished by a fundamental grid voltage filter and a phase-locked loop to detect the actual grid phase angle. The stability and convergence performance of the proposed observers have been studied by means of the Lyapunov theory to ensure a high accuracy tracking performance of estimated variables. Simulation and experimental results are presented to validate the feasibility and the effectiveness of the proposed control approach.

A Novel Control Strategy for Grid-Connected Inverter Based on Iterative Calculation of Structural Parameters

A novel control strategy that is based on iterative calculation of structural parameters is proposed for grid-connected inverter in this paper. The proposed strategy has a good dynamic performance, which makes it particularly suitable for the application of PV grid-connected generation. First, a second-order discretization mathematical model of grid-connected inverter control is established in the dq frame. The corresponding relation between the control signal and the output current is deduced in formulas. Then, the values of structural parameters in the formulas can be obtained through iterative calculation, which can further reduce the amount of calculation. After several iteration cycles, the structural parameters are approximately equal to their actual values and the inverter can be controlled as an open-loop system with its dynamic performance optimized. At last, simulation and experiments are performed. The results show that the static performance of the proposed strategy is as good as that of the classical ones, but its dynamic performance is improved significantly.