Continuous finite‐time control of four‐leg inverter through fast terminal sliding mode control

This paper reports the slow- and fast-scale instabilities in the parallel-connected single-phase H-bridge inverters and discusses the two types of instabilities from the practical design viewpoint....

Slow-Scale and Fast-Scale Instabilities in Parallel-Connected Single-Phase H-Bridge Inverters: A Design-Oriented Study

Effective sinusoidal voltage regulation is of permanent importance for grid-forming converters. Usually, the following two types of schemes are employed to achieve a high level of performance: 1) single-loop voltage and 2) dual-loop voltage-current control. The performance of both schemes has comprehensively been evaluated in this work, based on the developed discrete-time model of the LC-filtered grid-forming converter. The challenges of insufficient stability margin, constraint bandwidth, and high sensitivity to parameter variation faced by the single-loop control scheme have been addressed, if the high-performance resonant controllers are employed for voltage regulation. Alternatively, the dual-loop control does not experience such issues with the inclusion of inner-current loop which provides active damping for the overall system. The essence of the inner-current loop is identified based on the discrete root locus analysis. Also, to obtain the highest damping and most enhanced stability, the criterion for current loop design has been addressed and a method for optimal tuning of the inner-current loop is developed, where the original plant with the one-sampling delay and the current gain are considered as the equivalent plant for the voltage controller. Experimental results have verified the effectiveness of the developed method for regulation of grid-forming converters.

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Optimal Tuning of the Current Loop for Dual-Loop Controlled Grid-Forming Converters Based on Active Damping Optimization

Effective voltage regulation is of utmost essence for grid-connected converters. The dual-loop voltage-current control approach have a preference for preferred for its excellent performance of providing active damping for the undamped equipment to avoid filter resonance. However, the tuning of the controller is commonly performed in the continues-time domain, which is not suitable for applications with a low pulse ratio, that is, the ratio of the switching frequency to the switching frequency of the reference signal. Therefore, this work focuses on the analysis and tuning the dual-loop control in the discrete-time domain. The discrete model of the plant is firstly developed, and then the tuning of the inner-loop gain is discussed based on the discrete root locus analysis, to achieve the maximum active damping for the equivalent plant. The experimental results have verified the validity of the methods proposed above for grid connected converter regulation.

Analysis and Tuning of Dual-loop Control for Grid-forming Converters in the Discrete-time Domain

DC distribution systems are a typical power electronic system with low inertia, low-rotational kinetic energy, and poor antidisturbance capability when loads fluctuate or parameters change. In this paper, a virtual inertia control with an additional first-order filtering link is proposed on the basis of P-Udc droop control. The results of the simulations and experiments verify that the additional inertia control reduces the voltage change rate and improves the system inertia by adjusting the virtual capacitance value on the DC side of the converter, which can achieve a smoother and more accurate voltage control and suppress the continuous voltage oscillation.

/pdf/a-virtual-inertia-method-for-stability-control-of-dc-1yi2fb2i.pdf

A Virtual Inertia Method for Stability Control of DC Distribution Systems with Parallel Converters

In order to improve the control performance of a train auxiliary inverter and satisfy the requirements of power quality, harmonics, and unbalanced factor, this paper proposed a design method of a double closed-loop control system based on a complex state variable structure. The method simplifies the design process and takes full account of the effects of coupling and discretization. In the current closed-loop process, this paper analyzed the limitations of the proportional integral (PI) controller and simplified to P controller. In the voltage closed-loop, the paper employed the PI controller plus the resonant controller, designed the parameters of the PI controller. and analyzed the optimal discretization method of the resonant controller under dq axis coupling. Finally, experiments and simulations were conducted to show that the proposed method can achieve the above improvements.

https://www.mdpi.com/1996-1073/12/20/4005/pdf

Control Strategy of Three-Phase Inverter with Isolation Transformer

In order to further provide theoretical support for the stability of an auxiliary inverter parallel system, a new model which covers most of control parameters needs to be established. However, the ability of the small-signal model established by the traditional method is extremely limited, so this paper proposes a new small-signal modeling method for the parallel system. The new small-signal model not only can analyze the influence of the droop parameters on the system performance, but also can analyze the influence of the output impedance of the inverter, the unbalanced and nonlinear loads, and the power calculation method and cut-off frequency of the low-pass filter on the system performance and stability. Based on this method, this paper carries out a comprehensive analysis on the performance of a parallel inverter system. And the correctness of the modeling method and analysis process of the system performance and stability are verified by the consistency of the simulation and experimental results.

/pdf/stability-analysis-method-of-parallel-inverter-1m5hdrceg6.pdf

Jun Li

Papers

Control Strategy of Three-Phase Inverter with Isolation Transformer

Stability Analysis Method of Parallel Inverter