This paper presents an optimal predictive controller for a multilevel converter-based dynamic voltage restorer (DVR), which is able to improve the voltage quality of sensitive loads connected to the electrical power network. The optimal predictive controlled multilevel DVR can restore sags and short interruptions while reducing the total harmonic distortion (THD) of the ac line voltages to values lower than 1%. The DVR is based on a three-phase neutral point clamped converter to dynamically inject a compensation voltage vector in series with the line voltage, through series-connected transformer secondary windings. To assure high-quality voltages for sensitive loads, we devise optimal predictive control laws for the injected compensation ac voltages. A suitable quadratic weighed cost functional is used to choose the voltage vector, minimizing both the ac voltage errors through current injection and the dc side capacitor voltage unbalancing. The performance of the proposed predictive controller is compared to classical proportional integral (PI): synchronous frame and stationary frame (P+resonant) controllers. The line-side filter capacitor topology is compared to the regular converter-side filter capacitor. Obtained experimental results show that the ac voltages are almost sinusoidal in steady-state operation when facing balanced and unbalanced sags and short interruptions with unbalanced loads. Voltage THD is reduced to values lower than 1%; the DVR is behaving also as a series active power filter for the ac voltages.

Multilevel Optimal Predictive Dynamic Voltage Restorer

A comprehensive review of dynamic voltage restorers

A novel control scheme for the dynamic voltage restorer (DVR) is proposed to achieve fast response and effective sag compensation capabilities. The proposed method controls the magnitude and phase angle of the injected voltage for each phase separately. Fast least error squares digital filters are used to estimate the magnitude and phase of the measured voltages. The utilized least error squares estimated filters considerably reduce the effects of noise, harmonics, and disturbances on the estimated phasor parameters. This enables the DVR to detect and compensate voltage sags accurately, under linear and nonlinear load conditions. The proposed control system does not need any phase-locked loops. It also effectively limits the magnitudes of the modulating signals to prevent overmodulation. Besides, separately controlling the injected voltage in each phase enables the DVR to regulate the negative- and zero-sequence components of the load voltage as well as the positive-sequence component. Results of the simulation studies in the PSCAD/EMTDC software environment indicate that the proposed control scheme 1) compensates balanced and unbalanced voltage sags in a very short time period, without phase jump and 2) performs satisfactorily under linear and nonlinear load conditions.

A Fast and Effective Control Scheme for the Dynamic Voltage Restorer

The static series compensator (SSC) is suited to protect sensitive loads against voltage dips. Because most of the power system faults are single- or double-phase, the control algorithms of the SSC should be adapted for unbalanced dips. This paper proposes two control strategies to improve the dynamic performance of the SSC. The first strategy uses a fast technique for separating positive and negative sequence components of the supply voltage, which are then controlled separately. Thus, two controllers are implemented for the two sequences, each based on vector control. The second strategy is based on using only a positive sequence controller and increasing the switching frequency. Consequently, the negative sequence due to the unbalanced dip is transformed into variations in the positive sequence. As the switching frequency increases, the ability of the controller to follow those variations improves. The validity of the proposed strategies is demonstrated through PSCAD/EMTDC simulation and experimental measurements carried out on a 10-kV SSC, when the grid is subjected to unbalanced voltage dips.

Mitigation of unbalanced voltage dips using static series compensator

This paper introduces a novel method for the mitigation of the voltage sag and voltage flicker by using Kalman filter and its derivatives (adaptive, and extended). The Kalman filter is used as a tool to extract both the instantaneous envelope of the voltage sags, and to extract the Instantaneous Flicker Level (IFL) of the voltage flicker. Also, this paper demonstrates the advantages of using the Kalman filter instead of the existing tools for tracking and extracting voltage disturbances. Digital simulation results are presented to illustrate the mitigation of unbalanced voltage sags, and the compensation of the cyclic and noncyclic voltage flicker by employing the proposed algorithm.

Unified approach for mitigating voltage sag and voltage flicker using the DSTATCOM

The role of the inverter-side filter used in the dynamic voltage restorer (DVR) is examined. Using the circuit analysis method, it is shown that the selection of the filter parameters can affect the DVR inverter rating. Furthermore, with the DVR filter-series injection transformer combination, the loading effect of the filter on the restorer and the primary supply system has been quantified under normal as well as voltage sag conditions. A systematic filter design method is then proposed, the primary objective of which is to achieve specific harmonic performance while ensuring that the DVR inverter rating and the loading effect are minimized. Illustrative examples are also included.

Li

Papers

Design and Analysis of the Inverter-Side Filter Used in the Dynamic Voltage Restorer