Conventional primary control employs outer-loop droop and inner-loop cascaded linear control to realize local voltage regulation and power-sharing of an islanded ac microgrid. However, it has a complex structure, limited dynamic response, and a rapid rate of change of frequency when disturbances occur. This article resolves these issues by proposing a model predictive control-based virtual synchronous generator (VSG-MPC). An improved finite-set MPC is first proposed for the inner loop, achieving simplified control structure, faster dynamic response, enhanced bandwidth and stability, as well as improved current limitation. In the outer control loop, a simplified VSG without a phase-locked loop is employed to realize active power-sharing and inertia emulation. The merits above are verified by a description function of MPC and the frequency-domain response of the overall VSG. Simulation and experimental results verify the feasibility of the proposed method.

A typical finite-set model predictive control (FS-MPC) scheme for LC-filtered voltage source inverters (VSIs) requires measurements of capacitor voltage and inductor current as well as load current measurement or estimation, which increases the system complexity and cost. To reduce the number of sensors in typical FS-MPC, this paper proposes a new current-sensorless FS-MPC scheme for LC-filtered VSIs. First, based on the inner relationship of the predictive matrices, the predictive model is exactly simplified with the capacitor voltage and its current as the state variables, making it suitable for current-sensorless control. Then, to eliminate the current sensors for cost reduction and reliability enhancement, the dynamic model is reconstructed and an easily-implemented capacitor current estimator is designed, which can achieve a comparable performance with typical FS-MPC scheme. Considering the inevitable control delay in digital implementation, the delay compensation is inherently obtained by using the proposed estimator. In addition, the proposed control scheme is flexible to various cost functions and can reduce the computational cost. The feasibility of the presented control scheme under the load variations and model mismatches are verified by the comparative simulation and experimental results with typical FS-MPC.

Voltage source inverters with output LC filter enable a sinusoidal output voltage with low harmonics, suitable for islanded ac microgrid or uninterruptible power supply applications. Conventional finite-set model predictive voltage control (MPVC) applies only a single switching vector per control period, leading to a variable switching frequency and significant output ripple. This article resolves these issues by proposing an improved MPVC with optimal switching sequence (OSS-MPVC). First, an improved vector switching sequence is defined, aiming to reduce the output-voltage ripple with a constant switching frequency. Then, to tackle the difficulty in extending the OSS to high-order systems due to the coupling effect of the output filter, a generalized “one-step estimation” solution is proposed, which directly associates the control-variable gradients with the vector switching sequence. To further enhance the output-voltage tracking accuracy, intersample dynamics are taken into account in the cost function. The control delay and dead-time compensation are also considered. Simulations and experimental results verify the feasibility of the proposed method.

This paper proposes a constrained modulated-model predictive control (M $^2$ PC) strategy for an $LC$ -filtered voltage source converter (VSC). To tackle the coupling effects of the state variables in a second-order $LC$ filter, a dual-objective cost function (CF) is used to explicitly track both capacitor voltage and inductor current references, which can achieve improved voltage quality. To handle the state and control input constraints of VSCs, a constrained M $^2$ PC scheme is proposed with an ‘online post-correction’ constraint-handling technique. First, the unconstrained optimal voltage vector (OVV) of the VSC is derived. It is generated by seeking the minimum analytical solution of the CF offline, simplifying the online implementation. Then, the ‘online post-correction’ strategy is employed by reconsidering the constraints to correct the precalculated OVV online, which guarantees the future states within the allowed range. Finally, the corrected OVV is synthesized by the space vector modulation (SVM), resulting in a fixed switching frequency and low harmonics. Compared with the typical constrained MPC, the presented scheme has the advantages of improved control performance, flexible constraint-handling ability and lower computational cost. Additionally, analytical procedures for weighting factor (WF) selection in the CF are given. Comparative experiments are investigated to verify the presented control strategy.

To address the stability issues aroused by the negative-resistance effect of constant power loads (CPLs), this paper proposes a composite robust discretized quasi-sliding mode control (DQSMC) scheme for stabilization of buck-converter fed dc microgrids with CPLs. In the outer control loop, a robust DQSMC voltage controller is proposed. First, a discrete integral sliding surface is designed to obtain a fast and robust dynamic response of dc-bus voltage. Then, to tackle the varying load disturbances and model uncertainties, a second-order sliding mode disturbance observer is embedded in the voltage controller for disturbance estimation and compensation. The resulting composite DQSMC scheme features enhanced disturbance rejection, inherent chattering suppression, and guaranteed dynamics. Meanwhile, a PI current controller is retained in the inner control loop to realize the current control and limitation. Robustness and stability analysis of the whole composite controller are proved to assure large-signal stability. Simulation and experimental results confirm the superiority of the presented approach.