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Grid voltage transients and, in particular, grid voltage swells pose control challenges for static compensators (StatComs). This article presents an efficient control strategy for changing the dc value of capacitor voltages, which is especially useful for operating a low-capacitance StatCom (LC-StatCom) under grid voltage swells. In the proposed strategy, the capacitor voltages are changed as a function of the grid voltage magnitude in each phase. This article details all the required control stages for implementing this control strategy. The effectiveness of this strategy to deal with grid voltage swells is compared with other strategies both in conventional StatComs and LC-StatComs using a simulated 36-MVA 11-level cascaded H-bridge converter in MATLAB Simulink. The comparison is based on current transient performance and an analytical model of switching losses. Finally, the feasibility of this strategy is confirmed by experiments on a 1 kVA prototype. [ABSTRACT FROM AUTHOR]

The cascaded H-bridge (CHB) low-capacitance static compensator (LC-StatCom) has a limited inductive region compared to a conventional StatCom, both in balanced and unbalanced conditions, due to the inherent large oscillations on the capacitor voltages. This article presents a detailed analysis of the viable operating region in these converters that takes into account the inherent large oscillations. The analysis facilitates sizing of the dc-link capacitors to ensure that the StatCom is able to inject a given range of positive- and negative-sequence current. Though the analysis is generic and applicable to the CHB StatComs with relatively large capacitance values, it is more relevant in the LC-StatComs. In addition, the article derives reference steady-state signals that can be used as feedforward terms in LC-StatCom control loops to enhance the control performance. The article presents simulation and experimental results, in balanced and unbalanced conditions to corroborate the proposed analysis. [ABSTRACT FROM AUTHOR]

This article presents a discontinuous modulation (DM) strategy for static compensators (StatComs) based on a cascaded H-bridge (CHB) converter with a star configuration. The proposed DM strategy considers the capacitor voltage oscillations at twice the fundamental frequency and the effect of zero-sequence voltage injection on the capacitor voltages. Considering these effects is especially important in CHB-StatComs with large capacitor voltage ripples (low-capacitance StatComs), where the assumption of a constant dc-link voltage, which is the basis of conventional DM strategies, does not apply. This article also describes a coherent set of steady-state waveforms for CHB-StatComs under DM. In addition to the well-known benefit of reducing the switching losses, the proposed DM also reduces the dc-link capacitors size and extends the operating range. The viability of the proposed DM strategy is verified experimentally on a small-scale prototype. In addition, simulation results are obtained using a real-scale system to study feasibility of the DM under unbalanced conditions. [ABSTRACT FROM AUTHOR]

This article presents a constrained model predictive control (MPC) strategy for a three-phase low-capacitance static compensator (LC-StatCom) with delta configuration. The controller consists of an outer loop that provides dynamic references for the active grid current component and the circulating current; whereas, the inner predictive loop minimizes the quadratic error of state variables subject to operating limit conditions, thus providing optimal control signals that produce a fast response while guaranteeing the prescribed safe operating limit conditions. The article also proposes an enhanced discrete-time model, which uses intersample values, that improves the accuracy of the model predictions when the sampling rate is not high enough to assume constant state variables. As a result, it facilitates the implementation of MPC on less powerful processors compared to the conventional modeling. The proposed control is especially suited for the LC-StatComs since it incorporates analytical computation of desired steady-state trajectories, which takes into account the induced oscillation on the capacitor voltages that are inherent to the LC-StatComs. The proposed approach has been verified by simulation and experimentally with a laboratory prototype. [ABSTRACT FROM AUTHOR]

A low-capacitance static compensator (LC-StatCom) based on a cascaded H-bridge (CHB) multilevel converter can be an efficient and low-cost solution for reactive power compensation in power grids. However, the reduced capacitance, and, therefore, faster capacitor voltage dynamics, makes it challenging to control it. Classical cascade control schemes, in which the bandwidths associated with the inner current loop and the outer capacitor voltage loop need to be separated enough, are the most reported approaches. Nevertheless, for the LC-StatCom case, the voltage loop can be almost as fast as that one of the current loop. For this reason, this article proposes a control approach that does not separate voltage and current dynamics. Concretely, we propose a multi-input linear time-variant control law, based on incremental passivity theory for CHB multilevel converters. The proposed control guarantees stability despite saturation in the control signal. In addition, it provides capacitor voltage balance without the need to use a specific balancing stage. This article, first, reviews the average model of the converter and, then, analyses the steady-state dynamic behavior to determine the desired and coherent reference signals. Then, it applies an incremental passivity control approach to drive the LC-StatCom state variables to the desired references. After introducing the proposed control law, experimental results on a seven-level 1-kVA CHB LC-StatCom are shown to demonstrate its excellent performance in steady state and transients. [ABSTRACT FROM AUTHOR]