Binfeng Zhang

Grid-voltage feedforward based control for grid-connected LCL-filtered inverter with high robustness and low grid current distortion in weak grid

For grid-connected LCL-filtered inverters, recent applications often observe a weak grid at the point of common coupling (PCC) with non-negligible grid impedance. In this case, the previous control methods would perform poor or even cause the inverter protection. Especially when the feedforward of PCC voltage is designed to suppress grid current harmonics, phase and gain margins will be largely reduced and rejections of grid current harmonics will be aggravated if a weak grid is connected. In fact, the grid voltage feedforward in the weak grid introduces a positive feedback loop related to the grid impedance, and then the unexpected feedback loop makes the inverter dynamic badly influenced by the grid impedance at low frequencies. Therefore, this study proposes to feed forward only the fundamental grid voltage through a second-order generalized integrator (SOGI) and to design the harmonic resonant controller with adaptability to large grid impedance, in order to avoid the undesired dynamic interactions at low frequencies. With the proposed design and control, the grid-connected LCL-filtered inverter is capable of realizing high robustness and low grid current distortion in the weak grid. Comparative experiments verify the effectiveness of the proposed control method.

Stability Analysis and Improvement of the Capacitor Current Active Damping of the LCL Filters in Grid-Connected Applications

For grid-connected LCL-filtered inverters, dual-loop current control with an inner-loop active damping (AD) based on capacitor current feedback is generally used for the sake of current quality. However, existing studies on capacitor current feedback AD with a control delay do not reveal the mathematical relation among the dual-loop stability, capacitor current feedback factor, delay time and LCL parameters. The robustness was not investigated through mathematical derivations. Thus, this paper aims to provide a systematic study of dual-loop current control in a digitally-controlled inverter. At first, the stable region of the inner-loop AD is derived. Then, the dual-loop stability and robustness are analyzed by mathematical derivations when the inner-loop AD is stable and unstable. Robust design principles for the inner-loop AD feedback factor and the outer-loop current controller are derived. Most importantly, ensuring the stability of the inner-loop AD is critical for achieving high robustness against a large grid impedance. Then, several improved approaches are proposed and synthesized. The limitations and benefits of all of the approaches are identified to help engineers apply capacitor current feedback AD in practice.

Analysis and suppression of the aliasing in real-time sampling for grid-connected LCL-filtered inverters

Real-time sampling and feedback can effectively reduce the sampling delay and improve the robustness of the active damping control for grid-connected LCL-filtered inverter. However, the practicability of this approach would be limited by the aliasing problem of sampling. In order to solve the aliasing problem, a systematic analysis is done to figure out the relation between the sampling instant of digital signal processor (DSP) and the level of aliasing by establishing an aliasing analysis model based on the actual DSP working mode. Results show that the aliasing of traditional synchronous sampling method is lighter than that of the real-time sampling. Besides, combining with the demand of signal sampling for active damping, two methods including the multiple sampling and the real-time sampling with additional high-pass filter are presented to suppress the aliasing. Finally, simulation and experimental results show that the multiple sampling increases the difficulty of the software and hardware implementation, and real-time sampling with additional high-pass filter is simple and practical.

An adaptive algorithm for grid-connected inverter to suppress current harmonics and instabilities due to grid impedance and distortion

For grid-connected inverters, the feedforward of the grid voltage at the point of common coupling (PCC) highly suppresses the grid current harmonics caused by the grid distortion; however, when the PCC grid is weak with non-negligible grid impedance, the feedforward of the PCC voltage arouses serious current harmonics or even instability. In order to maintain the stability in the weak grid case, the typical robust design usually had to compromise the dynamic performance or the harmonics rejection performance, while the typical adaptive control had to measure the grid impedance precisely through injecting extra harmonics into the system. Both solutions are not satisfactory enough. Therefore, this study proposes a novel adaptive control algorithm which has overcome the drawbacks of typical solutions. In the propose algorithm, the band-pass filters at harmonics frequencies are used to detect the variation of the grid impedance as well as to facilitate the adaptive PCC voltage feedforward. It is not necessary to inject any extra harmonics to estimate the grid impedance. In this paper, the realization and logic of the proposed control algorithm are briefly explained, and selected waveforms are provided to demonstrate the effectiveness of the proposal.

An improved gate driver based on magnetic coupling for crosstalk suppression of SiC devices

Silicon carbide (SiC) devices attract widespread attention of scholars because of their superior characteristics. However, the interaction between the upper and lower devices during the switching process in the bridge circuit will affect the safe operation of SiC devices. To suppress the crosstalk spurious voltage, this paper proposes a gate driver circuit based on magnetic coupling for SiC devices to adjust the gate-source voltage to the safe range when the positive and negative spurious pulse voltages appear. Compared with the existent gate drivers for crosstalk suppression, the proposed circuit can realize fully galvanic isolation and generate the positive-negative gate-source voltage without auxiliary power supplies. As a result, the gate driver circuit is simplified and conducive to the integration. Based on the experimental results, the performance of the proposed circuit is evaluated.

A fast fault diagnosis method for submodule failures in modular multilevel converters

To improve the reliability of modular multilevel converter (MMC), the fault submodules must be isolated rapidly. In this paper, a new fast fault diagnosis method is proposed containing two steps: fault detecting and fault locating. Fault detection step finds failures by improved predictive model which detects the fault by comparing the difference between the predictive value and the measured one of the arm current with the threshold value. Then the failures are located by the proposed location method which contains two process: feature mining and fault location. The proposed method could locate multiple submodule failures without additional sensors rapidly and can be easily implemented with a microprocessor. The proposed method has been validated by the simulation results.

Circulating resonant current suppression for current-controlled inverters based on output impedance shaping

Current-controlled LCL-filtered inverters are usually paralleled together for interconnecting the renewable energy resources into the grid. When the inverter output impedance is not well designed, the harmonic circulating currents out of the control bandwidth may be easily aroused. To reveal this resonant phenomenon and to enhance the stability of the parallel system, this paper builds the parallel system model with the consideration of the asynchronous carrier wave. Aiming at optimizing the inverter output impedance around the resonant frequency, a simple control method, which cascades the classical active damping loop with a proportional resonant controller with phase compensation, is proposed for the parallel inverter system. Each inverter is carefully designed with a minimum phase behavior by tuning the control parameters. Both the high modulus and the passivity of the output impedance are guaranteed to mitigate the circulating resonant currents. The effectiveness of the proposed control method is experimentally validated by a three-parallel inverter system.

Model predictive control for single phase T-type neutral point clamping ZCZ-filtered inverters

The model predictive control (MPC) predicts all the possible behaviors and selects an optimal switching state which minimizes the predefined cost function at the next step, and is emerging as a powerful control method in power electronics. However, for a single phase T-NPC inverter with the conventional MPC method, only three different voltage vectors can be generated, which significantly impairs the system performance. Therefore, aiming at improving the current quality, a real-time duty ratio calculation scheme based on the MPC control is proposed. Moreover, the error of the two capacitor voltages is embedded into the current reference for balancing the capacitor voltage. The improved MPC control strategy achieves the balanced capacitor voltage and the minor steady-state error by providing another control degree of freedom. The effectiveness of the proposed MPC strategy is validated on a T-NPC LCL-filtered inverter by both the simulation and experimental results.

Robust control and design based on impedance-based stability criterion for improving stability and harmonics rejection of inverters in weak grid

The impedance-based stability criterion is usually used to evaluate the performance of a current-controlled inverter in the weak grid with considerable grid impedance at the point of common coupling (PCC). The stability of the real inverter-grid system is the same as the virtual system where the ratio of the grid impedance and the inverter output impedance is the open-loop transfer function. However, not only the stability but also the rejection of the grid current harmonics is critical to the real inverter-grid system. So far, existing studies have not revealed the relation between the stability margin of the virtual system and the grid current harmonics rejection of the real system. Therefore, this study focuses on the control and design based on impedance-based stability criterion for improving not only the stability but also the grid current harmonics rejection. The design guideline of the phase margin of the virtual system is derived through mathematical derivations. A case study based on an LCL-filtered inverter has been provided to verify the analysis. Then, an improved control for the LCL-filtered inverter has been proposed. Selected waveforms have been provided to verify the improved control and design.

A Magnetic Coupling Based Gate Driver for Crosstalk Suppression of SiC MOSFETs

Silicon carbide (SiC) devices have attracted widespread attention because of their superior characteristics. However, not only the higher slew rate of drain–source voltage but also the higher slew rate of reverse recovery current can result in a more serious crosstalk problem than Si-based devices in a half-bridge application. Crosstalk suppression should be integrated into the gate driver to ensure the safe operation of SiC devices. Therefore, a specific mathematical analysis is done in this paper to figure out the crosstalk phenomenon. The limitations of the existing suppression methods are illustrated. Thus, a gate driver based on the magnetic coupling is proposed to ensure the gate–source voltage within the safe range, even when the positive and negative spurious pulse voltages appear. The proposed gate driver uses three ring transformers to insulate the control signal and driver power supply. So, it is feasible to drive a half-bridge circuit in the medium or high power applications. By saving optical couplers and isolated drive power supplies, the gate driver can realize fully galvanic isolation and generate the positive and negative gate–source driving voltage simply. The results derived from the proposed mathematical analysis and the effectiveness of the proposed driver in suppressing the spurious pulse voltage are verified by the experiments.