Single-phase grid-connected inverters are widely used to connect small-scale distributed renewable resources to the grid. However, unlike a three-phase system, control for a single-phase inverter is more challenging, especially when the inverter is used with an LCL filter. This paper proposes a novel recurrent neural network-based vector control method for a single-phase inverter with an LCL filter. The neural network is trained based on adaptive dynamic programming principle, and the objective of the training is to approximate optimal control. The Levenberg–Marquardt plus forward accumulation through time algorithm is developed for training the proposed recurrent neural network controller. The neural network vector control approach is compared with the conventional control methods, including the conventional PI-based vector control method and the PR-based control technique for single-phase inverters. Both the simulations and hardware experiments demonstrate the great advantages of the proposed neural network vector control over the conventional control methods. Compared with conventional control methods, the neural network control allows for low sampling rate and low switching frequency, while maintaining high performance in controlling a single-phase inverter. In addition, no specific damping policy is required to implement the proposed neural network vector control for an LCL -filter based single-phase inverter. The study shows that the neural network vector control is a robust control method, and can provide better control performance even when facing system parameter changes, while under this case, both the conventional PI-based vector control and the PR-based control failed to yield the acceptable results.

Control of Single-Phase Grid-Connected Converters With LCL Filters Using Recurrent Neural Network and Conventional Control Methods

This work proposes a linear generalized predictive current control (GPCC) for grid-connected voltage-source inverters, which presents a fast response to current reference step changes, parameter variation robustness, and low distortion at the output currents, with reduced computational effort. Experimental results on a 10-kW converter connected to a real grid are shown, and the proposed controller is compared against a classical proportional resonant controller.

Generalized Predictive Current Control (GPCC) for Grid-Tie Three-Phase Inverters

This article presents a two-stage T-type hybrid five-level transformerless inverter (TLI) for grid-connected photovoltaic (PV) applications. The proposed T-type hybrid five-level inverter and its level-shifted pulsewidth modulation scheme offers reduced leakage current by eliminating the high-frequency variations and sudden transitions in the voltage across PV parasitic capacitance C PV and a path for the negative current in all the modes of operation under unity and nonunity power factor conditions of the grid without degrading the waveform quality. Moreover, in this article, the proposed inverter is integrated with a traditional three-level boost converter (3LBC) for boosting the lower PV voltage to higher dc-link voltage and also to extract maximum power from the PV source. The 3LBC provides high efficiency and reduced input inductor size for the same power rating over the conventional boost converter. The simulation results of the proposed system are presented using MATLAB software, and a 500-W experimental prototype is constructed and tested in the laboratory to validate the feasibility of the proposed configuration. Finally, a comparison of the proposed inverter with other five-level TLI topologies is presented to highlight its merits.

A Two-Stage T-Type Hybrid Five-Level Transformerless Inverter for PV Applications

This paper proposes an extended Kalman filter (EKF) based control strategy for fault ride-through operation in two-stage grid-connected photovoltaic (GPV) system. Unlike the conventional controllers for ride-through operation, the proposed strategy does not compromise with power quality improvement features in the system while enabling ride-through operation. The controller accounts for nonlinear loads in the system, grid harmonic-currents elimination, and grid-currents balancing even during the harmonic/distorted grid voltages. The IEEE Standard-1547.4 compels the distributed resource to ride-through during voltage disturbances caused by faults. For the ride-through operation, a limit is imposed on PV active power injection to prevent inverter overcurrents and dc-link energy aggregation, which reduces the lifetime of dc-link capacitor. The reactive power is fed to the grid, as per the depth in voltage-sag. The derated PV array power is supplied in cases where the inverter cannot handle the utmost PV-power. The power quality improvement is ensured using EKF state estimator, which precisely estimates the fundamental load currents. In distribution network with modern nonlinear loads, especially at far radial ends, the grid voltages are prone to huge diversions and the proposed controller provides a possible solution to maintain active/reactive power support and maintain power quality in the network. The effectiveness of the strategy is demonstrated through simulations and experiments. Under all disturbances, the harmonic content in grid currents is observed within limits, in accordance with the IEEE Standard-519.

Fault Ride-Through Strategy for Two-Stage Grid-Connected Photovoltaic System Enabling Load Compensation Capabilities

https://vbn.aau.dk/ws/files/288595959/IJEPES_Modeling_and_Design_of_a_Multivariable_Control_System_for_Multi_Paralleled_Grid_Connected_Inverters_with_LCL_Filter.pdf

Modeling and design of a multivariable control system for multi-paralleled grid-connected inverters with LCL filter

The Proportional Resonant (PR) current controller provides gains at a certain frequency (resonant frequency) and eliminates steady state errors. Therefore, the PR controller can be successfully applied to single grid-connected PV inverter current control. On the contrary, a PI controller has steady-state errors and limited disturbance rejection capability. Compared with the L- and LC filters, the LCL filter has excellent harmonic suppression capability, but the inherent resonant peak of the LCL filter may introduce instability in the whole system. Therefore, damping must be introduced to improve the control of the system. Considering the controller and the LCL filter active damping as a whole system makes the controller design method more complex. In fact, their frequency responses may affect each other. The traditional trial-and-error procedure is too time-consuming and the design process is inefficient. This paper provides a detailed analysis of the frequency response influence between the PR controller and the LCL filter regarded as a whole system. In addition, the paper presents a systematic method for designing controller parameters and the capacitor current feedback coefficient factor of LCL filter active-damping. The new method relies on meeting the stable margins of the system. Moreover, the paper also clarifies the impact of the grid on the inverter output current. Numerical simulation and a 3 kW laboratory setup assessed the feasibility and effectiveness of the proposed method.

https://www.mdpi.com/1996-1073/7/6/3934/pdf

A Systematic Method for Designing a PR Controller and Active Damping of the LCL Filter for Single-Phase Grid-Connected PV Inverters

Sampling and computational delay are necessary in digital control, though they may alter the characteristics of the actual controlled plant when considered in the discrete domain. Accordingly, this article establishes a mathematical model of a single-phase grid-connected inverter with an LCL filter in the discrete domain. Three active damping control delay times: 0, Ts/2, Ts are analysed, considering the influences introduced by digital control of the plant. Further, the range of the capacitor current feedback coefficient used in active damping control is derived to guarantee that the controlled plant does not have poles outside the unit circle. Using Nyquist diagrams, the proper proportional-resonant (PR) controller parameters can be intuitively determined. The theoretical analysis is verified using MATLAB and a 3 kW laboratory prototype based on a digital signal processor (TMS320F2808).

Analysis of active damping of LCL filter used in single-phase PV system in discrete domain

As for high power plasma power supply, due to high efficiency and flexibility, multi-channel interleaved multi-stage paralleled Buck DC-DC Converter becomes the first choice In the paper, two-channel interleaved two- stage paralleled Buck DC-DC Converter powered by three-phase AC power supply is selected as research subject, dedicated for the use of plasma cutting machine of 2×45kW, and the passivity-based control over the converter is analyzed theoretically at first, then simulated by means of MATLAB/SIMULINK, finally implemented on the modular product prototype From the obtained results, it can be concluded that the passivity-based controller can be easily designed and implemented, which brings about the global stability and reluctance to outer perturbation, system parameter variation and load fluctuation, which also brings about better current sharing due to fast respond to the variations and the use of phase-shift driving technology and current sharing technology

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Research on two-channel interleaved two-stage paralleled Buck DC-DC Converter for plasma cutting power supply

The invention provides an automatic soft power-on rectifying circuit including five power diodes PD1-PD5, a unidirectional thyristor, an electrolytic capacitor, a thermosensitive resistor, a voltage-stabilizing diode, resistors R1-R6, and capacitors C1-C3. When a single-phase alternating current power supply is switched on, a rectifying bridge formed by the four power diodes PD1-PD4 charges the electrolytic capacitor EC1. The electrolytic capacitor EC1 and the thermosensitive resistor form a resistance-capacitance charging branch circuit. The voltage of the electrolytic capacitor EC1 rises slowly. When the voltage of the electrolytic capacitor EC1 rises and approaches a net voltage peak, a divider resistor R3 gets enough voltage. After rear stage voltage stabilization and current limitation, the current is formed and sent to a place between a gate pole and a negative pole of the thyristor TY1. The thyristor is conducted to short out the power-on current-limiting thermosensitive resistor. The rectifying circuit enters the full and natural rectifying state, and finishes the soft power-on. The automatic soft power-on rectifying circuit can be applied to the field in which an AC-DC convertor is used as a preceding stage circuit, and can simultaneously realize the automatic soft power-on process and the single-phase rectification process.

Automatic soft power-on rectifying circuit

The invention provides a single-phase rectification wide region power supply power-on circuit, comprising a rectification unit and a determination unit. The rectification unit comprises a power MOSFET; the determination unit includes four subunits with a completely same circuit, wherein each subunit comprises two voltage comparators; the inverting input terminal of a first voltage comparator is in connection with a first branch voltage branch, and the non-inverting input terminal is in connection with the rectification unit; the output terminal of the first voltage comparator is in connection with a RC branch input terminal; the non-inverting input terminal of a second voltage comparator is in connection with the output terminal of the RC branch, and the inverting input terminal of the second voltage comparator is in connection with a second branch voltage branch to obtain a second reference voltage; the output terminal of the second voltage comparator is in connection with a diode; the other end of the diode is in connection with the rectification unit and is used for driving the power MOSFET and completing soft start. The single-phase rectification wide region power supply power-on circuit can realize the single-phase rectifier soft start in a wide range of input voltage, and have the characteristics of simple circuit, complete function and low cost.

Tang Houjun

Papers

A Systematic Method for Designing a PR Controller and Active Damping of the LCL Filter for Single-Phase Grid-Connected PV Inverters

Analysis of active damping of LCL filter used in single-phase PV system in discrete domain

Research on two-channel interleaved two-stage paralleled Buck DC-DC Converter for plasma cutting power supply

Automatic soft power-on rectifying circuit

Single-phase rectification wide region power supply power-on circuit