Capacitor Voltage Control Research Articles

An Extend Control Algorithm in PV-ZSI Capacitor-Assisted with Shoot-Through Allowable Boundary on Different Load Cases

This paper proposed an extend controller on Capacitor Voltage Control (CVC) for Z-Source Inverter (ZSI) network when it been applied at the PV generation. The improved controller is been combined with the existing MPPT algorithm. Although, the existing CVC has some drawback as it produces an unnecessary shoot through signal period which cause the capacitor voltage at ZSI cannot be constantly maintained. Therefore, the modification has been included a deviation of the duty cycle of the shoot through. It is to enhance the existing CVC performance and to increase the efficiency of the whole PV generation whenever the input PV irradiation is changed. At the same time, it is also able to maintain a constant dc-link voltage at ZSI. This proposed control algorithm has been verified throughout simulations tests using linear and non-linear loads by observing the constant dc-link capacitor voltage. This proposed controller has also been compared with the existing conventional CVC with the same load conditions. At the end, the improved controller has shown the ability to maintain the dc-link inverter at PV generation and to limit the allowable boundary of the capacitor voltage to not overflow from the rated value. In terms of quality at the output the proposed controller is able to reduce the total harmonic distortion at the load connections.

Parameters Design and Optimization for LC-Type Off-Grid Inverters With Inductor-Current Feedback Active Damping

In this paper, the stability of LC -type voltage source inverter is investigated, with the emphasis focused on the LC resonance. It has been found that the traditional capacitor-voltage inductor-current dual-loop control is unstable when the resonance frequency is higher than one-sixth of the sampling frequency. In addition, due to the coupling between current inner loop and voltage outer loop, it is difficult to design control parameters to ensure the system stability in traditional dual-loop control. In order to develop a systemic design principle, the capacitor-voltage control with inductor-current feedback active damping evolved from traditional dual-loop control is analyzed in detail. The mathematic model is established to reveal the reason of instability, and the stable regions are identified. Moreover, the optimal damping coefficient is also derived to enhance the system damping performance. Finally, the experimental results are provided to verify the effectiveness of the proposed method.

Research on the modulation and control of multilevel matrix converter

This paper discusses the effects of different space vector modulation (SVM) hexagon partition on common mode voltage suppression at high modulation index and presents an improved capacitor voltage balance strategy for multilevel matrix converter. Matrix converter is a compact designed converter which allows direct AC–AC power transfer without an intermediate DC link. Multilevel matrix converters, which utilises a multilevel structure on a traditional matrix converter, expand these characters to high-voltage and high-power situations. This paper is concerned with SVM of three-level matrix converter and concentrates on the comparison between different hexagon partitions; the improved balancing strategy is employed to improve the performance of flying capacitor voltage control. Simulation from Matlab/Simulink and results from a small-scale experimental prototype are presented to validate the theoretical results.

Feedback Linearization-Based Current Control Strategy for Modular Multilevel Converters

Modular multilevel converters (MMCs) are multi-input multi-output (MIMO) nonlinear systems. The control systems for MMCs are required to simultaneously achieve multiple control objectives, e.g., output current regulation, submodule capacitor voltage control, and circulating ripple currents suppression. Existing cascaded control strategies for MMCs achieve those control objectives with relatively complex controllers, and the controller parameter design is normally difficult for such nonlinear systems with highly coupled states. In view of this, a feedback linearization-based current control strategy is proposed for an MMC system in this paper. The nonlinear state function model of the MMC is presented and transformed to a linearized and decoupled form with the help of the input-output feedback linearization technique. Based on the linearized system, simple linear controllers are employed to regulate the output and inner differential currents of the MMC, which significantly reduces the difficulty in controller design. The stability of the proposed control strategy is analyzed. The experimental verification results show that, compared to the conventional cascaded control strategies for MMCs, the proposed feedback linearization control strategy is able to achieve improved steady-state and dynamic performances. The robustness of the proposed control strategy against parametric uncertainties is experimentally investigated.

Modern control scheme for Z-Source inverter based PV power generation systems

Z-source inverter (ZSI) based photovoltaic power conversion system (PV-PCS) provides both the DC link voltage boost and DC-AC inversion in single stage with added features. Traditional maximum power point tracking (MPPT) control algorithm generates the required shoot-through interval to output maximum power to the Z-network. At this instant, the voltage across Z-source capacitor is equal to the MPP voltage of PV array. The capacitor voltage cannot be further increased if it is demanded by the load. This paper presents an improved MPPT control algorithm along with modified MPPT algorithm to achieve both the MPPT as well as capacitor voltage control at the same time. Development and implementation of the proposed algorithm has been carried out by Matlab/Simulink software and the results are provided.

A Novel Diode-Clamped Modular Multilevel Converter With Simplified Capacitor Voltage-Balancing Control

Multilevel converters have become very attractive for high-voltage-level power conversion in renewable power generation applications. The converter topology is an important issue in the studies of multilevel converter. Many multilevel topologies have been developed, but few of them are qualified with capacitor voltage self-balancing capability. This paper proposes a novel diode-clamped modular multilevel converter with simplified capacitor voltage-balancing control. In this topology, low-power rating diodes are used to clamp the capacitor voltages of the converter. Only the top submodule in each arm of the converter requires capacitor voltage control. Consequently, very few voltage sensors are needed for voltage control and the control computation burden is reduced greatly when the quantity of the submodules is high. A simple voltage-balancing control method with carrier phase-shifted modulation strategy is developed for this topology. Experiments based on a laboratory prototype were carried out and the results validated the capacitor-balancing performance of the proposed topology.

Five-Level Active-Neutral-Point-Clamped DC/DC Converter for Medium-Voltage DC Grids

This paper proposes a five-level active-neutral-point-clamped (5L-ANPC) dc/dc converter for applications in medium-voltage dc grids. A modulation strategy is proposed for the 5L-ANPC dc/dc converter to generate multilevel voltage waveforms, which can effectively reduce voltage change rate dv/dt, reduce voltage stress on the transformer, and thus reduce the electromagnetic interference and increase reliability. An elimination method for the dead time effect is also proposed along with the proposed modulation strategy by employing a switch in series with the flying capacitor, which can effectively eliminate high voltage leaps caused by the dead time effect. In addition, a capacitor voltage control strategy is proposed for the 5L-ANPC dc/dc converter to ensure the balanced flying capacitor voltage and desired five-level voltage waveforms. Finally, simulation and experimental studies are conducted, and the results have verified the proposed converter and control strategies.

DC Capacitor Voltage Control of MMC for HVDC using Negative Sequence Current

DC Capacitor Voltage Control of MMC for HVDC using Negative Sequence Current

Derivation of Voltage Source Multilevel Converter Topologies

Multilevel converters have gained popularity in both medium voltage and low voltage applications. To find out the connections between various multilevel voltage-source converter topologies and to reveal how to obtain new topologies, this paper has presented four methods to derive multilevel converter topologies. Many existing topologies as well as new topologies can be derived with the methods presented in this paper. The fundamental characteristics of the multilevel converters which determine their usability such as dc-link neutral point voltage balancing and flying capacitor voltage control are also investigated in this paper with a mathematical model and an example. It is expected that more new topologies will be invented based on the work in this paper for emerging applications.

A Transformerless Bipolar Multistring DC–DC Converter Based on Series-Connected Modules

This paper proposes a transformerless bipolar multistring dc–dc converter based on series-connected submodules. It is intended for converting dc voltage and managing power flow for medium-voltage high-power dc networks. This new converter has a flexible structure of multistrings and each string consists of four valves with their middle points crossly connected through two cost-effect dc capacitors to form power balancing channels between valves. Its transformerless solution and single-stage configuration greatly reduce the converter volume, weight, cost, and power loss. In this paper, voltage ratio range and its influence on submodule design are analyzed. Moreover, a control strategy with focus on current sharing control and submodule capacitor voltage control is proposed. A simulation model rated at ±10 kV 2 MW verifies the feasibility of the proposed dc–dc converter. Experimental results obtained from a 100-V 1.5-kW laboratory setup also confirm the validation of the proposal.

Model Predictive Control of Capacitor Voltage Balancing for Cascaded Modular DC–DC Converters

Input-series output-series (ISOS) modular dc–dc converters are suitable for applications involving high input and output voltages. Input voltage sharing (IVS) and output voltage sharing (OVS) of the constituent modules must be guaranteed for an ISOS system. A duty cycle-based model predictive control (DC-MPC) scheme is proposed in this paper to achieve IVS and OVS for an ISOS system. The employed switching frequency in the DC-MPC scheme is fixed; duty cycles are optimized and then determined by minimizing a cost function, which is derived from an ISOS system aimed at achieving IVS and OVS. IVS and OVS can, therefore, be obtained by employing the optimized duty cycle within each sampling period. In addition, challenges and disadvantages of directly using traditional MPC to achieve IVS and OVS is briefly discussed, while the proposed DC-MPC can effectively avoid them. Compared with the existing proportional integral-based schemes in the literature, the proposed DC-MPC is better for IVS and OVS because it is a very simple and intuitive approach with lower cost and higher overall dynamic performance. Simulation and experiments are conducted to verify that the proposed DC-MPC scheme works well during both steady and transient states.

A Novel Flexible Capacitor Voltage Control Strategy for Variable-Speed Drives With Modular Multilevel Converters

Modular multilevel converter (MMC) is very popular in medium-voltage applications, such as high-voltage direct current transmission, static Var generator, and motor drives. The arm capacitor voltage of MMC is usually constant and the output voltage is adjusted by regulating the pulse width modulation (PWM) index for switches in previous studies. In this paper, a novel flexible capacitor voltage control strategy for MMC as motor drives was proposed, with which the arm capacitor voltage was controlled flexibly according to the motor speed, by regulating the dc component and ac component of the PWM reference signal, and unsymmetrical control was used for the control of the upper arm and the lower arm in MMC. With the proposed control strategy, the circuit efficiency and the precision of the output voltage of the converter could be improved when the motor speed was low, and d i /d t and d v /d t of the motor windings could be decreased. These proposals were implemented on a laboratory prototype, and experiments were carried out. The validity of the control strategy under both steady and dynamic states was verified.

Y結線モジュラーマルチレベル変換器STATCOMにおける対称座標法を用いた直流キャパシタ電圧制御法

STATCOMs have attracted attention as var compensators on electric power distribution systems. In this study, a star-connected modular multilevel converter (MMC) was applied to STATCOM. The voltage across the strings of the star-connected MMC was 1/√3 times that of a delta-connected MMC. MMCs can be interconnected without a transformer because they have high equivalent carrier frequency through phase-shifted pulse-width modulation. The dc capacitor voltage control of a star-connected MMC is relatively complex compared with delta connected MMCs, which have a circulation current as a control parameter. Relevant literature suggests that conventional dc capacitor voltage controls of MMCs for STATCOMs might be difficult to operate when the output reactive power is zero var. This paper presents a new dc capacitor voltage control method using symmetrical components. The overall control block mainly consists of two parts: average dc and dc capacitor voltage controls. The proposed dc capacitor voltage control and reactive power control were verified through experiments.

Load Current Distribution between Parallel Inverters based on Capacitor Voltage Control for UPS Applications

Load Current Distribution between Parallel Inverters based on Capacitor Voltage Control for UPS Applications

A Cascaded NPC/H- Bridge Inverter with Superior Harmonic Content and Improved DC Link Voltage Control for Grid Interface Application

Modeling of Cascaded Multilevel converter has put more emphasize on conventional cascaded H- bridge inverter. First a new phase shifted SPWM control algorithm for cascaded NPC/H-bridge inverter is proposed. Its Superior harmonic suppression is verified by comparing its harmonic spectrum with that of the conventional multicarrier approach. For DC- bus capacitor voltage control, a new, balance circuit is proposed. Because of its modularity, it is simple and can be easily incorporated in cascaded inverter model with any number of levels. Feedback controller design is presented with main focus on grid voltage regulation and overall system capacitor voltage balance. The validity of grid interface model and its control strategies is substantially confirmed by the extensive simulation results.

DC voltage control strategy of chain star STATCOM with second‐order harmonic suppression

Chain static synchronous compensators (STATCOMs) with star structure produce second-order harmonic fluctuations on the DC side. This effect is often ignored and only the average voltage value is taken into consideration when describing this system for control purpose. In this study, the unbalance of per-phase DC voltages caused by active power absorbed and consumed by the chain is analysed. The steady-state expressions regarding the average capacitor voltage balancing between phase clusters as well as the suppression of second-order harmonic fluctuation are obtained, respectively, based on the injection of fundamental and third-order harmonic zero-sequence voltages. Although the expressions are helpful to understand the essence of DC capacitor voltage control, this method has complicated calculation process and the calculation results may be beyond the modulation limit. Therefore, it is not conducive in real-time control. For this reason, a new control strategy based on the instantaneous zero-sequence voltage injection is proposed. The evaluation function with the transient variables is established to get the instantaneous value of the zero-sequence voltage including fundamental and third-order harmonic components. The fundamental and third-order harmonic zero-sequence voltages are automatically allocated to fully use the remaining modulation capacity. Finally, simulation and experimental results are provided, which verifies the correctness of the theoretical analysis and the high performance of the proposed method.

Control and Analysis of the Modular Multilevel DC De-Icer With STATCOM Functionality

De-icing of overhead transmission lines is necessary to prevent damage to the transmission system and outages. In this paper, a modular multilevel dc De-Icer (M2D2) is introduced, based on the structure of the modular multilevel converter with full-bridge submodules. The M2D2 exhibits a lot of salient features: high modularity, scalability, compactness, high efficiency, wide dc voltage range, and no harmonic filter requirements, etc. Moreover, during good weather, the M2D2 can be configured as a static synchronous compensator, providing reactive power support to the grid and alleviating power quality problems. Operating principle of the M2D2 is explained, harmonic features under phase-shifted carrier modulation are analyzed, and selections of the phase-shifted angle are derived to achieve best harmonic performance. Control schemes for the M2D2 are also developed, including control of de-icing current, capacitor voltages, and reactive power. Finally, experimental results, utilizing a downscaled prototype, confirm the validity of this topology and effectiveness of the proposed control strategies.

A DC Capacitor Voltage Control Method for Active Power Filters Using Modified Reference Including the Theoretically Derived Voltage Ripple

This paper proposes a new dc capacitor voltage control method suitable for active power filters equipped with a small dc capacitor. The proposed control method calculates the theoretical dc capacitor voltage ripple and provides it to the dc capacitor voltage feedback controller as the reference. As a result, the proposed method has the capability to regulate the mean value of the dc capacitor voltage under a large voltage ripple condition. The proposed method also allows us to use a high feedback gain for suppressing the capacitor voltage fluctuations without any performance deterioration in the harmonic compensation. Experimental results confirm a good harmonic compensation and an effective voltage regulation performance under a small dc capacitor condition. The results also demonstrate a significant improvement in the capacitor voltage regulation even when a sudden load change occurs.

A Predictive Capacitor Voltage Control of a Hybrid Cascaded Multilevel Inverter With a Single DC-Link and Reduced Common-Mode Voltage Operation

For cascaded multilevel inverter topologies with a single dc supply, closed-loop capacitor voltage control is necessary for proper operation. This paper presents zero and reduced common-mode voltage (CMV) operation of a hybrid cascaded multilevel inverter with predictive capacitor voltage control. Each phase of the inverter is realized by cascading two- three-level flying capacitor inverters with a half-bridge module in between. For the presented inverter topology, there are redundant switching states for each inverter voltage levels. By using these switching state redundancies, for every sampling instant, a cost function is evaluated based on the predicted capacitor voltages for each phase. The switching state that minimizes cost function is treated as the best and is switched for that sampling instant. The inverter operates with zero CMV for a modulation index upto 86%. For modulation indices from 86% to 96%, the inverter can operate with reduced CMV magnitude ( $V_{\rm dc}/18$ ) and reduced CMV switching frequency using the new space-vector pulsewidth modulation (SVPWM) presented herein. As a result, the linear modulation range is increased to 96% as compared to 86% for zero CMV operation. Simulation and experimental results are presented for the inverter topology for various steady state and transient operating conditions by running an induction motor drive with open loop $V/f$ control scheme.

Multilevel converter with capacitor voltage actively balanced using reduced number of voltage sensors for high power applications

Capacitor voltage balance control has attracted increasing attention in the studies of cascaded multilevel converters and modular multilevel converters. This study proposes a novel multilevel converter topology based on diode clamped half-bridge cascaded converter for medium/high voltage and high power applications. In this converter, capacitor voltage balancing using clamping diodes is achieved. Thus very little capacitor voltage control code has to be executed by the digital controller. Furthermore, only two voltage sensors are required for the capacitor voltage control, and the control scheme can be designed as simple as that of a two-level converter. The phase shifted pulse width modulation (PWM) method is employed for the converter, and a control strategy for the converter utilised as a static Var generator is presented. The proposed converter and control strategy were simulated with PSIM. Experiments were also carried out with a laboratory prototype. Results showed that the proposed converter topology with capacitor voltage balancing could work effectively. The applied PWM and control method were also validated.