DC Capacitor Voltage Research Articles

A Virtual Space Vector Model Predictive Control for a Seven-Level Hybrid Multilevel Converter

This article proposes a virtual space vector (VSV) model predictive control (MPC) for a three-phase seven-level (7L) hybrid multilevel converter (HMC), where each phase consists of an active-neutral-point-clamped converter with a floating H-bridge. To achieve the best current tracking, which is the primary goal of the proposed algorithm, a novel geometrical positioning approach is proposed to select the optimal voltage vector among all the real-space vectors and VSVs. Then, all the possible switching sequences that belong to the optimal voltage vector are evaluated to realize the dc capacitor voltage balancing and common-mode voltage reduction. Through an external modulator, the optimal voltage vector can be synthesized by using either one-, three-, or seven-segment switching sequence. Compared with the conventional MPC, the proposed VSV-MPC can reduce not only the computational burden but also the current THD. Both simulation and experimental results obtained on silicon carbide based 7L-HMC prototype are presented to validate the feasibility and effectiveness of the proposed VSV-MPC strategy.

Performance Improvement of DC Capacitor Voltage Balancing Control for Cascaded H-Bridge Multilevel Converters

In this article, an improved dc capacitor voltage balancing control is proposed for single-phase cascaded H-bridge (CHB) multilevel converters. Compared with conventional voltage balancing control methods, the proposed scheme has a comprehensive consideration of three aspects while balancing the capacitor voltages among H-bridges: coupling effect elimination, reactive power balancing, and dynamic performance improvement. First, the system mathematical model in the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d–q</i> reference frame is derived, and the relationship among duty cycle modifications of H-bridges is revealed to eliminate the coupling effect of voltage balancing control on the current control loop. Then, based on the H-bridge power model, a quantitative solution for reactive power balancing is presented to design the reactive duty cycle modifications. In order to enhance the dynamic performance of voltage balancing control, a dynamic references design method is adopted to evaluate the active duty cycle modifications. Finally, both simulation and experimental results are presented to validate the effectiveness of the proposed scheme.

Three-Phase Three-Level Neutral Point Clamped Rectifier with Predictive Control Method without Employing Weighting Factor

A predictive control method using injected offset voltage to achieve neutral point (NP) voltage balance of three-phase three-level neutral point clamped (NP) rectifiers, without employing a weighting factor, is proposed in this study. One of the biggest problems with the three-level NP rectifiers is the dc link capacitor voltage imbalance. Therefore, it is necessary to maintain the balance of the NP voltage in addition to synthesize the three-phase sinusoidal input current by control methods. Conventional predictive control methods for the NP rectifiers have used a weighting factor in a cost function that determines the control ratio of the input currents and the capacitor voltage balance. As a result, it is burdensome to empirically redesign the weighting factor when the rectifiers’ parameter values and control conditions change. Unlike the conventional methods, the proposed approach without the weighting factor can significantly eliminate differences between two DC capacitor voltages by utilizing an offset voltage, which is generated by using the difference between the upper and lower capacitor voltages. Consequently, the proposed approach using the offset voltage injection can control the input currents and retain the balance of NP voltage. Simulation and experiments are presented to verify the correctness of the NP voltage balancing of the proposed control method.

An improved power regulation method for a three-terminal hybrid AC/DC microgrid during module failure

To reduce the number of power conversion stages and achieve the direct connection to medium-voltage (MV) grid without line-frequency transformer integration, a three-terminal hybrid AC/DC microgrid with one AC port and two DC ports has attracted great interests. It is composed of a cascaded H-bridge (CHB) converter based AC grid port and two dual active bridge (DAB) converters based DC microgrid ports. The large number of power modules increases the probability of module failure, which may cause unbalanced capacitor voltages of CHB converter and unstable DC microgrid voltages. This paper proposes an improved power regulation method with a combination of zero-sequence voltage (ZSV) and negative-sequence current (NSC) injection. When module failure occurs, first, ZSV is injected to adjust the modulation margin in the faulty phase; then, based on three-phase power analysis, NSC injection is activated to realize DC voltage balancing control in three clusters. In addition, the constraints ofZSV and NSC injection are respectively investigated, and the selection of injected ZSV and the derived NSC are analyzed. In a three-terminal hybrid AC/DC microgrid, the simulation waveforms validate the effectiveness of the proposed fault-tolerant operation scheme with the stable DC microgrid voltages and balanced DC capacitor voltages. Experimental results on a CHB converter also verify the effectiveness of the proposed scheme.

Adaptive fuzzy controlled hybrid shunt active power filter for power quality enhancement

A novel switching pulse generation methodology based on adaptive fuzzy hysteresis current controlled hybrid shunt active power filter (A-F-HCC-HSAPF) is presented in this paper for compensating reactive power and harmonics in distribution network. The harmonic problems are mainly evolved because of extensive use of nonlinear loads in industry and domestic sectors. There are some adverse effects of harmonics such as: malfunctioning of sensitive equipment, resonance issues, conductors heating, power losses and reduced efficiency in distribution system. To mitigate harmonics issues passive, filters are used but when the harmonic component increases the design of passive filters is complex and becomes bulky. With the advancement in power electronic, the active power filter has been designed. Generally, the rating of the active filter is very high for some applications; hence hybrid shunt active power filter (HSAPF) is proposed by low-rated shunt active power filter (SAPF) and low-cost shunt passive filter. Proportional-integral or fuzzy logic controller is used to estimate the reference current and to regulate the dc capacitor voltage. To generate the switching pulse for the voltage source inverter of the SAPF, a novel adaptive fuzzy hysteresis current controller (A-F-HCC) is adopted. The performance of the proposed A-F-HCC-HSAPF is investigated during steady-state and transient conditions using MATLAB/Simulink and real-time environments.

Development and Verification Test of the 6.6-kV 200-kVA Transformerless SDBC-Based STATCOM Using SiC-MOSFET Modules

This article discusses development and verification test results of the 6.6-kV 200-kVA transformerless static synchronous compensator (STATCOM). This STATCOM is characterized by the use of a modular multilevel single-delta bridge-cell (SDBC) converter and silicon carbide metal-oxide-semiconductor field-effect transistor (MOSFET) modules. The article discusses a control method for the 6.6-kV system with focus on dc-capacitor voltage control. The voltage control presented in this article is different from the ones presented earlier. It consists of intercluster balancing control and intracluster balancing control. The former, also known as cluster balancing control, eliminates the requirement of the separate overall dc-voltage control present in conventional methods, whereas the latter, also known as individual balancing control, is replaced with a new technique based on the control of a dead time of each bridge cell. The article makes a detailed description of the latter. Experimental results obtained from the 6.6-kV 200-kVA verification test equipment validate the effectiveness of the control method. Moreover, successful test results confirm the efficacy of the system for grid-voltage regulation and load compensation.

Model predictive power control for a fault‐tolerant grid‐connected converter using reconstructed currents

The reliability and stability of a grid-connected converter is vital to the power conversion system. The faults of switching device and current sensor lead to operation failure for a grid-connected converter. An improved model predictive power control (MPPC) based on reconstructed currents is proposed for fault-tolerant operation of a grid-connected converter. Firstly, the three-phase currents are reconstructed using the sampled dc current and normal sensor information according to the relationship between the voltage vector and output currents. Secondly, the predictive power model is built based on fault-tolerant three-phase four-switch topology. Then the reconstructed current is used for the MPPC method. To keep the balance of dc capacitor voltages, the cost function is optimised and the switching vector, which minimises the cost function, is applied to the fault-tolerant converter. Finally, the experiment platform and simulation models are built to verify the proposed method. The results show that the proposed MPPC using reconstructed currents can keep the fault-tolerant operation of the grid-connected converter with sinusoidal currents and stable output power. The proposed method achieves the fault-tolerant operation under switching device and ac current sensor fault, which improve the reliability of the system.

Power Quality Improvement using Intelligent Fuzzy-VLLMS Based Shunt Active Filter

The power quality problem in the power system is increased with the use of non-linear devices. Due to the use of non-linear devices like power electronic converters, there is an increase in harmonic content in the source current. Due to this there is an increase in the losses, instability and poor voltage waveform. To mitigate the harmonics and provide the reactive power compensation, we use filters. There are different filters used in the power system. Passive filters provide limited compensation, so active filters can be used for variable compensation. In this paper, a shunt active filter has been made adaptive using a Variable Leaky Least Mean Square (VLLMS) based controller. Proposed adaptive controller can be able to compensate for harmonic currents, power factor and nonlinear load unbalance. DC capacitor voltage has been regulated at a desired level using a PI controller and a self-charging circuit technique. But, this scheme has two disadvantages such as, tuning issues of current controller pre-requisites the traditional PI controller, which is controlled by intelligent based Fuzzy-Logic controller for achieving good performance features. The design concept of proposed intelligent Fuzzy controller for shunt active filter has been verified through simulation analysis and results are presented with proper comparisons.

Adaptive Nonlinear Control of Three-Phase Series Active Power Filters with Magnetic Saturation

The problem of controlling three-phase series active power filters (series APF) is addressed in the presence of nonlinear loads. In previous works, the control design for series APF is generally based on standard models that assume the involved magnetic coil to be linear. In reality, the magnetic characteristics of these components are nonlinear. In this paper, a new model for series APF load system, taking into account for the nonlinearity of coil characteristics, is developed. Based on the new model, a nonlinear adaptive controller is developed, using the backstepping design. The control objectives is twofold: (i) compensating for the harmonic and disturbed voltages components at the point of common coupling, this objective is referred to network voltage quality; and (ii) regulating the inverter DC capacitor voltage. Moreover, the controller is made adaptive for compensating the uncertainty on the switching loss power. The performances of the proposed adaptive controller are formally analyzed using tools from the Lyapunov stability and average theory. The supremacy of the proposed controller with respect to standard control solution is illustrated through simulation.

Improved auto‐synchronisation of grid‐connected PV inverter based on the DC capacitor voltage control

Based on inherent dynamics similarity between synchronous generator (SG) and DC capacitor power port, this study proposes an improved synchronisation control method of grid-connected PV inverter based on DC capacitor voltage control where a DC-link capacitor voltage is regulated to provide the synchronisation angle. The proposed control method uses the estimated grid angle feedforward to improve the damping performance of the system under disturbances. To assess the robustness of the proposed control method, a two-step procedure is followed. Firstly, a reduced-order model of the system was derived. Secondly, the performance and stability of the proposed control method were fully investigated through the eigenvalue analysis. The offline simulations and real-time simulations are carried in PLECS software and PLECS RT Box, respectively in order to validate and demonstrate the effectiveness of the proposed control method. It has been found that the proposed control method yields a large band controller parameter setting while maintaining system stability. The results showed that the proposed method improved the operating capability and enhanced the performance of the system under disturbances and weak grid operation.

Virtual Flux Predictive Direct Power Control of Five-level T-type Multi-terminal VSC-HVDC System

This paper proposes a Virtual Flux Predictive Direct Power Control (PDPC) for a five-level T-type multi-terminal Voltage Source Converter High Voltage Direct Current (VSC-HVDC) transmission system. The proposed PDPC scheme is based on the computation of the average voltage vector using a virtual flux predictive control algorithm, which allows the cancellation of active and reactive power tracking errors at each sampling period. The active and reactive power can be estimated based on the virtual flux vector that makes AC line voltage sensors not necessary. A constant converter switching frequency is achieved by employing a multilevel space vector modulation, which ensures the balance of the DC capacitor voltages of the five-level t-type converters as well. Simulation results validate the efficiency of the proposed control law, and they are compared with those given by a traditional direct power control. These results exhibit excellent transient responses during range of operating conditions.

Two-Phase Current Injection Method for Single Line-to-Ground Fault Arc-Suppression With Revised STATCOM

To suppress the arc current in the single line-to-ground (SLG) fault in neutral isolated distribution network, a two-phase current injection method is proposed for the revised static synchronous compensator (STATCOM), as an extra function during the SLG fault. Therefore, the traditional roles of the dedicated passive coil or power electronic based arc-suppression devices can be taken by this multi-purpose STATCOM. This article first introduces the principles and the models of the arc-suppression current injection during SLG fault. Then, different options of arc-suppression current injection by the revised STATCOM are analyzed in terms of stabilizing the floating dc capacitor voltages of the cascaded H-bridges (CHB) converter. It is discovered that the two-phase current injection is the only viable option to maintain the floating dc capacitors voltages. Then, the arc-suppression current controller and the dc capacitors voltages controller are proposed, and the design process of the control parameters adapting to the varying transitional (grounding) resistances computed online is also introduced. The proposed methods are first validated by simulations. Then, an MVA-rating CHB converter prototype is constructed, and 10 kV feeder SLG fault and arc-suppression experiments are performed to demonstrate the effectiveness of the proposed method.

Adaptive Backstepping-Based H${_{\infty}}$ Robust Controller for Photovoltaic Grid- Connected Inverter

To improve the robustness and stability of the photovoltaic grid-connected inverter system, a nonlinear backstepping-based H ∞ controller is proposed. A generic dynamical model of grid-connected inverters is built with the consideration of uncertain parameters and external disturbances that cannot be accurately measured. According to this, the backstepping H ∞ controller is designed by combining techniques of adaptive backstepping control and L2-gain robust control. The Lyapunov function is used to design the backstepping controller, and the dissipative inequality is recursively designed. The storage functions of the DC capacitor voltage and grid current are constructed, respectively, and the nonlinear H ∞ controller and the parameter update law are obtained. Experimental results show that the proposed controller has the advantage of strong robustness to parameter variations and external disturbances. The proposed controller can also accurately track the references to meet the requirements of high-performance control of grid-connected inverters.

Sliding-Mode Control Strategy for Three-Phase Three-Level T-Type Rectifiers With DC Capacitor Voltage Balancing

A sliding mode control (SMC) strategy with dc capacitor voltage balancing is proposed for three-phase three-level T-type rectifiers. The proposed SMC strategy is designed in the abc frame rather than the dq frame. In this case, the necessity of three-phase current transformations is eliminated. The proposed SMC is based on the errors of the line currents. The amplitude of line current references is generated by controlling the dc voltage using a proportional-integral (PI) controller. In order to obtain unity power factor, the generated reference amplitude is multiplied by the corresponding sinusoidal waveform obtained from the phase locked loop (PLL) operating with grid voltages. The dc capacitor voltage balancing is achieved by adding a proportional control term into the line current reference obtained for each rectifier leg. The performance of the proposed control strategy is validated by simulations and experiments during steady-state, transients caused by load change, and unbalanced grid conditions. The results show that the proposed control strategy offers excellent steady-state and dynamic performances with low THD in the line currents, zero steady-state error in the output voltage, and very fast dynamic response.

A Dual VSG-Based M3C Control Scheme for Frequency Regulation Support of a Remote AC Grid Via Low-Frequency AC Transmission System

This paper proposes a low-frequency ac (LFAC) transmission system capable of providing frequency regulation support for a remote ac grid aiming at enhancing its frequency stability. The LFAC transmission system using modular multilevel matrix converters (M3Cs) for a direct ac-ac conversion is receiving noticeable attention as an alternative solution for a long-distance transmission system. One of its potential applications is the system connection between a large ac grid and a remote ac grid, whereas the latter usually suffers from a fluctuant frequency due to lack of inertia. The core of this work is addressing a novel control scheme of the remote ac grid-side M3C, which makes both interactions with the LFAC system and the remote ac grid behave like a synchronous generator using a dual virtual synchronous generator (VSG) control scheme. This control scheme can enhance the frequency regulation in the remote ac grid, which the existing control schemes cannot provide. Average dc capacitor voltage versus active power (V ave -P) droop control is proposed to coordinate the power flow between the VSG controls applied in both sides of the M3C. To demonstrate the problem of the existing control scheme, as well as the benefits offered by the proposed control scheme, transient performance including load variation and fault events in the remote ac grid is studied and examined in the EMTDC/PSCAD software environment.

Hybrid Controller with Fuzzy Logic Technique for Three Phase Half Bridge Interleaved Buck Shunt Active Power Filter

This paper addresses a new control of three phase half bridge interleaved buck shunt active power filter (HBIB-SAPF). We aim for a control strategy achieving simultaneously, the two following objectives: i) compensation of harmonic currents and reactive power absorbed by nonlinear loads for satisfying a power factor correction; ii) regulation of the HBIB converter DC capacitor voltage. To meet the above objectives, the proposed controller structure consists of two loops. The inner loop is designed using a hybrid dynamical approach to model the system, the hybrid automaton is proposed to deal with the compensation topic by switching between the different operative modes, which is conditioned by some invariance and transition conditions. The outer loop is built up based on fuzzy logic control (FLC), applied to regulate the DC bus voltage of three phase HBIB-SAPF. It is confirmed, via simulation results in Matlab/ SimPowerSystems & Stateflow toolbox that the proposed controller actually achieves the objectives it is designed for.

Hardware Implementation of 15-Level Cascaded Multilevel Inverter using Pic16f877a

Multilevel inverters can manufacture a high- power, high- voltage inverter with a multilevel structure to control the voltage of the device. A symmetrical multilevel cascaded standard inverter requires 'n' DC sources for' 2n+1' levels that require isolated DC sources for power conversions. The objective of this paper is to increase the number of levels by reducing the number of dc sources. The proposed scheme is to use a multilevel asymmetrical inverter with a separate DC power supply. The analysis is extended to the use of the single DC power source with the remaining ' n-1 ' DC source being a capacitor and simultaneously maintains the capacitor 's DC voltage level and selects a fundamental frequency switching pattern to produce an almost sinusoidal output. Matlab simulink simulation is performed to verify the performance of the Asymmetrical Multilevel Inverter using isolated Dc source. The results of simulation and hardware are presented and discussed in this paper.

A New Variable Frequency Control of 49-Level Cascaded Packed U-Cell Voltage Source Inverter

Requirement of large number of levels with lower number of switching devices has made asymmetrical converters more popular than the symmetrical ones. Asymmetrical cascaded multilevel inverters (ACMLI) can achieve high efficiency by combining switching devices with different voltage ratings and technologies. The proposed ACMLI cascades two or more units of packed U-Cell (PUC) inverters using two or more isolated dc link supplies. In this article, one of the PUC unit is controlled using high switching frequency while the other PUCs are operated in a step mode at low switching frequencies, thus operating them in a variable frequency control mode. The cascading of two 7-level PUC inverters with dc link voltage ratio of 1:7 can produce an output voltage with 49 (7x7) levels. The multilevel output voltage waveform is nearly sinusoidal with very low THD content, and the low switching frequency operation leads to lower power dissipation and greater system efficiency. However, each PUC module requires two dc voltage sources. To address this concern, in this article, each PUC module consists of one dc voltage source and one dc bus capacitor. With the cascaded PUC topology and proposed control algorithm, load current and dc bus capacitor voltage control is achieved simultaneously. The proposed converter and its control technique lead to the breaking of the design tradeoff rule between switching frequency (efficiency) and filter size. This is very useful in various applications such as uninterruptible power supplies and grid-tie inverters. The converter and its control technique are simulated using MATLAB/Simulink software and simulation results for both open loop and closed loop are discussed. Hardware results are obtained by developing a 1-kW experimental prototype. Simulation and experimental results confirm the usefulness and effectiveness of the proposed topology and its control technique.

Parameters Tuning of Photovoltaic Power Generation System with Static Synchronous Compensator Based on Chaos Orthogonal Particle Swarm Optimization

China is a country rich in solar energy resources, with good natural conditions for the utilization of solar energy. At present, the main application mode of photovoltaic power generation system is grid-connected photovoltaic power generation. In order to maintain the voltage stability of the common connection point (PCC), STATCOM is mainly added to the PCC of photovoltaic power station. It can be regarded that the photovoltaic system and STATCOM constitute a whole and are integrated into the grid, and the unified coordination and optimization of the overall control system becomes particularly important. In this paper, the parameter tuning strategy of PI controller for photovoltaic grid-connected system with STATCOM is studied. Taking the optimal overall performance of DC capacitor voltage and PCC point voltage of photovoltaic inverters as the optimization objective, chaotic orthogonal particle swarm optimization algorithm is used to optimize the parameters of PI controller of the system, and simulation is carried out under the condition of voltage variation to verify the effectiveness of the optimization strategy.

An Improved Grid Current and DC Capacitor Voltage Balancing Method for Three-Terminal Hybrid AC/DC Microgrid

In this paper, a three-terminal ac/dc hybrid microgrid with two dc terminals and one ac terminal is proposed. The proposed system consists of cascaded H-bridge (CHB) converters based ac grid interface and two dual active bridge (DAB) converters based dc subgrid interface that connects two isolated dc buses. In order to reduce the number of power conversion stages and power devices, the DAB converters are directly connected to CHB dc rails according to the system operation requirement. To overcome the imbalanced grid currents and dc rail voltages issues caused by this modified system configuration with only two power conversion stages, an improved method is proposed through the zero-sequence voltage injection in the CHB converters. In addition, to avoid the conflicts between zero-sequence voltage injection and the voltage/current regulation of the system, the impacts of the control parameters to the system stability and dynamic response are investigated. Evaluation results from both three-terminal and five-terminal hybrid ac/dc microgrids show that the generalized effectiveness of the proposed three-phase ac current and dc rail voltage balancing method.