A Five Level Modified Cascaded H-Bridge Inverter STATCOM for Power Quality Improvement

Multilevel converters have received serious attention on account of their capability of high voltage operation, high efficiency, and low electromagnetic interference. It has many advantages compared to conventional two-level inverters such as high dc-link voltages, reduced harmonic distortion, fewer voltage stresses, and low electromagnetic interferences. The multilevel converters have been used for STATCOM widely as it can improve the power rating of the compensator to make it suitable for medium or high-voltage high power applications. While deploying multilevel STATCOMs, designer’s role is to reduce the number of switching devices since, the total switching losses are proportional to the number of switching devices. The reduction in the count of switching devices also reduces the size and cost. In this paper, a five-level modified cascaded H-bridge inverter STATCOM is proposed for mitigation of harmonics. Modified Five-level CHB configuration is the most suitable as with lesser number of switches, give better performance resulting in a compact system. The PQ theory-based controller is developed for control of STATCOM operation. MATLAB simulation results are presented to demonstrate mitigation of harmonics

Power Balancing in Cascaded H-Bridge and Modular Multilevel Converters Under Unbalanced Operation: A Review

Multilevel Voltage-Source Converters (VSC) based on modular structures are envisioned as a prominent alternative for grid and industry applications. Foremost among these are the Cascaded H-Bridge (CHB) and the Modular Multilevel Converter (MMC). In this context, depending on the application and the power conversion structure, unbalanced operating conditions can be asked to the converter. Previous investigations regarding the operation and the solutions for modular structures under unbalanced conditions have already addressed this topic, but information is dispersed over a wide number of sources. This paper identifies, classifies, and analyzes the intercluster active power balancing strategies for the adequate operation of the most commonly used modular structures in some typical unbalanced operating scenarios: the Static Synchronous Compensator (STATCOM) under unbalanced voltage and/or current conditions, the unequal power generation in large-scale photovoltaic (PV) power plants, and the uneven power distribution in a battery energy storage system (BESS). Each of the applications has been independently studied so as to provide a comprehensive analysis of the alternative techniques found in the specialized literature, clearly explaining their respective strengths and drawbacks. Several future challenges have been identified during the study, which will involve greater research effort in this key research topic

A hybrid multilevel converter for medium and high voltage applications

This paper investigates the suitability of the hybrid multilevel converter for medium and high voltage application. The converter operation, modulation, and capacitor voltage balancing method are described in detail. The ability of the hybrid multilevel converter to operate with different modulation indices and load power factors is investigated. It has been established that the hybrid multilevel converter is capable of operating independent of load power factor. Operation with variable modulation index increases voltage stresses on the converter switches and does not alter the fundamental voltage magnitude as in all known voltage source converter topologies. The viability of the hybrid multilevel converter for medium and high voltage applications is confirmed by simulations

Transformer-Less Cascaded Voltage Source Converter Based STATCOM

In this work, a transformer-less voltage source converter (VSC) based STATCOM is proposed with a combination of cascaded conventional three-phase voltage source inverters. This modular structure provides multilevel operation with reduced switch count and independent DC-link capacitors. The actual contribution of this paper is the transformer-less configuration of a conventional cascaded voltage source converter which provides reduced cost and volume as compared to other transformer-less converter configurations. The system provides reactive power compensation with better power quality when connected to the nonlinear power electronics load also. A simple control system is provided for balancing the Dc link capacitor voltage and reactive power compensation. The validation of the proposed model is analyzed with simulation using MATLAB/SIMULINK software and the results are obtained with different linear and nonlinear load configurations

Direct control of D-STATCOM based on 23-level cascaded multilevel inverter using harmonics elimination pulse width modulation

The distribution static synchronous compensator (D-STATCOM) is primarily used for solving power quality problems. Normally, the phase-shifted pulse width modulation (PS-PWM) switching is employed in conjunction with the direct control of the D-STATCOM. However, the PS-PWM exhibits high switching losses. To alleviate this problem, a direct control scheme for D-STATCOM based on the harmonic elimination PWM (HEPWM) switching is developed. Due to the difficulty in solving the equations for the HEPWM angles, no work is reported on the direct control for a multilevel voltage source inverter (MVSI) D-STATCOM with more than 15-levels. Thus, the main contribution of the work is the application of HEPWM for 23-level cascaded MVSI using a wide modulation index (MI) range (i.e. 5.40 – 8.15 p.u). The main motivation to utilize the high number of level is to allow for the output voltage of the D-STATCOM to be sufficiently high, thus avoiding the use of step-up transformer. Furthermore, the achieved MI keeps the total harmonic distortion of the MVSI output voltage below the IEEE 519 Standard (5%) over the entire operating range. The eleven HEPWM switching angles were computed using an optimization technique, known as the differential evolution. Since the angles were computed offline, they were retrieved from a look-up table whenever the output voltage of the MVSI was to be constructed. The HEPWM-based direct control was benchmarked against the popular PS-PWM using ± 6.5MVAr/11kV D-STATCOM modelled in MATLAB-Simulink and PLECS software. For the same switching frequency, the proposed HEPWM switching exhibited superior harmonic spectra, hence had lower losses. Furthermore, the size of the series coupling inductor can be reduced to at least half. Dynamically, the steady state value of the reactive current was reached in less than one mains cycle when a transition from the full inductive to full capacitive modes was imposed. In addition, the proposed D-STATCOM controller mitigated the swell and sag problems in less than one cycle

Cascaded Multilevel Inverter-Based Asymmetric Static Synchronous Compensator of Reactive Power

The topology of the static synchronous compensator of reactive power for a low-voltage three-phase utility grid capable of asymmetric reactive power compensation in grid phases has been proposed and analysed. It is implemented using separate, independent cascaded H-bridge multilevel inverters for each phase. Every inverter includes two H-bridge cascades. The first cascade operating at grid frequency is implemented using thyristors, and the second one—operating at high frequency is based on the high-speed MOSFET transistors. The investigation shows that the proposed compensator is able to compensate the reactive power in a low-voltage three-phase grid when phases are loaded by highly asymmetrical reactive loads and provides up to three times lower power losses in the compensator as compared with the situation when the compensator is based on the conventional three-level inverters implemented using IGBT transistors.publishedVersio

Unbalanced and Reactive Load Compensation Using MMCC-Based SATCOMs With Third-Harmonic Injection

Modular multilevel cascaded converter-based STATCOMs are scalable to higher voltages without requiring a step-up transformer with multiple windings and can be realized using a low switching frequency, giving lower harmonic content and hence a reduced filtering requirement. The paper presents a new injection technique to extend the operating ranges of MMCC STATCOMs when used for negative sequence and reactive current compensation. A non-sinusoidal voltage or current containing a fundamental and its third harmonic component is injected to achieve phase cluster voltage balance. This technique reduces the maximum dc-link voltage for the star configuration, and the maximum converter phase circulating current for the Delta case, compared to applying only sinusoidal zero sequence components for mitigating the same degree of load unbalance. The analysis is confirmed experimentally, showing that the third harmonic injection can allow a significant improvement of STATCOM capability for simultaneous compensation of unbalanced load and reactive current