Capacitor Voltage Balancing Research Articles

A Modified Model Predictive Power Control for Grid-Connected T-Type Inverter with Reduced Computational Complexity

This study proposed a modified power strategy based on model predictive control for a grid-connected three-level T-type inverter. The controller utilizes the mathematical model to forecast the performance of the grid current, the balance of DC-bus capacitor voltages and switching frequency. The proposed method outlines a new technique to formulate a control objective. The control objective includes the absolute error of the inverter voltage reference and its possible values instead of the grid current error. By using the modified equivalent transformations in the cost function, the execution time was reduced 22% compared to the traditional model predictive control while maintaining the high dynamic performances of the power and low total harmonic distortion of the current. A comparative investigation showed that the proposed method obtains a high-performance control compared with the classical power control scheme with linear PI controllers and space vector modulation. The feasibility of the proposed method was verified by the simulation and experimental results.

Research of control strategies of MMC based on full bridge sub‐modules

Although full bridge sub-module topologies require more power devices than half bridge topology, it can offer flexible control of positive, negative, and zero voltage output characteristics. Here, full-bridge MMC with three kinds of operating modes as positive, negative voltage output with low voltage operation, and DC-link fault current suppression with zero voltage output are studied. To deal with the problem of insulation reduction when long-distance overhead line is applied and reduced DC voltage operation is required when LCC is connected with MMC, the effect of constant power and reduced power transmission on the storage capacitor voltage balance are studied. What is more, the zero voltage output characteristic is also studied not only to avoid the problem that the sub-module capacitor voltage may diverge due to long time blocking converter during DC line fault but also to supply reactive power for AC grid. Control strategies are designed under different operating modes to achieve full bridge MMC flexible operation. Finally, a simulation model is built in PSCAD/EMTDC to simulate the output characteristics of full-bridge MMC in different operation modes.

Switched-Capacitor-Based Active-Neutral-Point-Clamped Seven-Level Inverter With Natural Balance and Boost Ability

This paper proposes a new active-neutral-point clamped (ANPC) seven-level inverter based on switched-capacitor technique. The proposed seven-level inverter employs only nine switches and one floating capacitor. With the input amplitude of V DC , the peak output of the proposed topology can reach 1.5V DC . Its boost ability indicates a wider range of output compared with other APNC converters. Needless for auxiliary control methods, the voltages of capacitors in dc link can naturally maintain at 0.5V DC , and the floating capacitor can get self-balance at V DC . The mechanism of natural balance for capacitors and the operation principle of the proposed topology are described in detail. The merits of the proposed topology on reduced components, lower overall voltage stress, natural balance, and boost ability are also demonstrated through the comparison against the latest similar topologies, showing its suitability for distributed generation, electric vehicle, and other applications. Definitively, the simulation and experimental prototype are implemented to verify the feasibility and performance of the proposed topology. The voltage balance of capacitors has been fully demonstrated through multiple transient experiments.

A novel three-phase multilevel inverter structure using switched capacitor basic unit for renewable energy conversion systems

This paper presents a new three phase multilevel inverter based on switched capacitor basic units. Each phase of the proposed topology consists of a number of switched capacitor basic units and a leg of two-level voltage source inverter in cascade. The proposed topology is supplied by only one DC power supply and it has the ability to boost the input supply to the desired level. By employing simple switching strategy, capacitor voltage balancing can be achieved. A multi-carrier phase disposition pulse width modulation has been developed for switching the proposed topology. The proposed topology can generate the three phase output waveforms with less number of power supplies and semiconductor devices compared to other recently developed three phase multilevel inverters. For proving the effectiveness of the proposed topology, a seven level line to line voltage inverters is simulated in MATLAB/simulink and a laboratory prototype of the same is developed and tested for experimental investigation.

An Optimal Voltage-Level Based Model Predictive Control Approach for Four-Level T-Type Nested Neutral Point Clamped Converter With Reduced Calculation Burden

Four-level T-type nested neutral point clamped (NNPC) converter is a newly developed multilevel voltage source converter for higher power density systems in low and medium voltage applications. The T-type NNPC converter can operate over a wide range of voltages and own less proportion of components compared with other four-level topologies, which makes it attractive among multilevel converters. In this paper, an optimal voltage-level based model predictive control (OVL-MPC) with reduced calculation burden is proposed for the T-type NNPC converter. The main objectives of this paper are to achieve the T-type NNPC converter load current control and the flying capacitors voltages balancing while remaining computationally feasible. In our proposed method, the output voltage levels are considered as the control options rather than the switching states in the conventional MPC scheme. Meanwhile, a simplified capacitor voltage balancing approach is employed to regulate the voltages of the capacitors at their desired values without punishing the computational complexity. The contribution of this paper lies in the following two aspects: first, the finite control set is decreased in the optimization processes. Second, a simplified capacitor voltage balancing strategy is introduced for the T-type NNPC converter. The performance of the proposed OVL-MPC approach for the T-type NNPC converter under steady-state, transient-operation, and unbalance mode are verified by the simulation results.

A novel three-phase multilevel inverter structure using switched capacitor basic unit for renewable energy conversion systems

This paper presents a new three phase multilevel inverter based on switched capacitor basic units. Each phase of the proposed topology consists of a number of switched capacitor basic units and a leg of two-level voltage source inverter in cascade. The proposed topology is supplied by only one DC power supply and it has the ability to boost the input supply to the desired level. By employing simple switching strategy, capacitor voltage balancing can be achieved. A multi-carrier phase disposition pulse width modulation has been developed for switching the proposed topology. The proposed topology can generate the three phase output waveforms with less number of power supplies and semiconductor devices compared to other recently developed three phase multilevel inverters. For proving the effectiveness of the proposed topology, a seven level line to line voltage inverters is simulated in MATLAB/simulink and a laboratory prototype of the same is developed and tested for experimental investigation.

Neural network-based three-phase NPC rectifier for DC bus capacitor voltage balancing under perturbed mains supply conditions

This paper presents an artificial neural network-based modified space vector pulse width modulation control approach for better performance of three-phase neutral-point clamped rectifier using optimised switching sequences. Use of optimised switching sequences even under ideal supply conditions, it is depicted that source side and load side parameters deviate from acceptable limits and a DC-bus capacitor voltage unbalance occurs. Under the influence of disturbed supply, source side and load side parameters deviate more beyond acceptable limits which causes a very large unbalance in DC bus capacitor voltages. This non-ideal performance of the converter is responsible for the deterioration of quality of source currents and a large stress on power semiconductor devices. The proposed control scheme employs a three-layer feed-forward neural network at different stages for capacitor voltage balancing of a three-phase three-level neutral-point clamped converter with improved power quality. According to the supply conditions, the neural network varies the speed of the reference vector and forms a required trajectory while passing through the most effective regions of SVPWM hexagon. The proposed controller scheme is modelled in MATLAB/Simulink software. Simulation results show that the proposed implementation of neural-networks controller in three-phase NPC converter displays better performance under ideal and disturbed mains conditions.

Parallel Operation of Unity Power Factor Rectifier for PMSG Wind Turbine System

Offshore wind power has inspired the fields of high voltage direct current (HVdc) for advantages of high power transmission in long distance. Hefty wind generators are making advanced multilevel rectifier and parallel operation of rectifiers popular choice of research with the aim to accommodate higher power. Issues of reliability and complexity of control are associated with active power electronic devices at such high power. This paper focuses on the novelty of operation of parallel three-phase diode rectifiers each with auxiliary bidirectional switching blocks (BSB) to improve their performance. For the system, maximum percentage power is commuted through the three-phase diode rectifier and paralleling further lessens the current stress on the semiconductor switches present in the BSBs. Competence of continuous operation at times of any specific branch failure provides the proposed system with higher modularity and reliability. The paralleled topology is controlled to obtain unity power factor at the generator output with low generator output-current harmonics using hysteresis current control. The dc-link voltage is controlled at a reference value, for various dynamic variations along with balanced capacitor voltages. The permanence of the proposed system is simulated for scenarios associated with variation of wind speed and load demands while various operation modes are tested on a 2.5-kW experimental setup.

A New Single Phase Single Switched-Capacitor Based Nine-Level Boost Inverter Topology With Reduced Switch Count and Voltage Stress

Based on the concept of switched-capacitor based multilevel inverter topology, a new structure for a boost multilevel inverter topology has been recommended in this paper. The proposed topology uses 11 unidirectional switches with a single switched capacitor unit to synthesize nine-level output voltage waveform. Apart from the twice voltage gain, self-voltage balancing of capacitor voltage without any auxiliary method along with reduced voltage stress has been the main advantages of this topology. The merits of proposed topology have been analyzed through various comparison parameters including component counts, voltage stresses, cost and efficiency with a maximum value of 98.3%, together with the integration of switched capacitors into the topology following recent development. Phase disposition pulse width modulation (PD-PWM) technique and nearest level control PWM (NLC-PWM) have been used for the control of switches. Different simulation and hardware results with different operating conditions are included in the paper to demonstrate the performance of the proposed topology.

A Simple and Novel Precharging Control Strategy for Modular Multilevel Converter

Modular multilevel converter (MMC) has been widely concerned due to its inherent excellent performance in recent years. However, in practical engineering, one of the technical challenges of MMC operation control is how to complete the submodule (SM) capacitors precharging safely and effectively, and reduce the impact of overcurrent phenomenon on the converter during the start -up process. In this paper, a simple and novel precharging control strategy is proposed. It can precharging all SMs of the converter at the same time, without the need for capacitor voltage balance algorithm, extra measurements and additional power supplies. It takes advantage of the symmetry of the grid voltage and the structural characteristics of MMC to form boost circuits between the arms by specific switching operations. Furthermore, the proposed method is not limited by the SM topology. The most commonly used SM topologies can also be converted into boost circuits and have the similar precharging process. By selecting appropriate control parameters, the precharging speed and efficiency of the converter can be improved. The effectiveness of the proposed control strategy is verified by simulations and experiments.

Unipolar Double-Star Submodule for Modular Multilevel Converter With DC Fault Blocking Capability

Adopting overhead lines is an effective way to reduce the cost of flexible high voltage direct current (HVDC) systems. However, the dc short-circuit fault is easily occurred and the development of the overhead lines based HVDC system is hampered. This paper proposes a unipolar double-star submodule (UDSSM) with the dc fault blocking capability. Under the dc fault condition, the UDSSM based modular multilevel converter (MMC) could block the fault current by switching all power switching devices off to protect the MMC system. In each proposed submodule, two capacitors work under almost the same condition and could be in series or parallel connection to output different voltage levels. Then two voltage sensors of the two capacitors are redundant each other, which improves the reliability of the MMC system. Additionally, voltage balancing methods for the capacitors under both normal operation and sensor fault conditions of the MMC submodule are developed in this paper. The effectiveness of the proposed submodule and the capacitor voltage balancing strategy are validated by the simulation and experiment results.

Novel Frequency-Doubling Modulation and Control Strategy for Three-Level Full Bridge based Power Electronic Traction Transformers

Power electronic traction transformers (PETTs) are widely investigated to substitute bulky line frequency transformers in railway traction system presently. This paper adopts the three-level topology for both grid-side cascaded converters and primary-sides of LLC resonant converters. The cascade number is reduced by half as well as isolated transformers compared to the traditional two-level configuration. Less cascade number of modular converters leaves relatively more space for electric insulation implementation, which is beneficial to PETTs in terms of the limited installation space in the train. A frequency-doubling modulation strategy for LLC resonant converters is proposed, which can enable the operating frequency of isolated transformers two times higher than the switching frequency to further reduce the transformer size without increasing the switching loss. Meanwhile the proposed modulation strategy can realize the capacitor voltage balance autonomously as well. Additionally, the control strategy of cascaded rectifiers and key parameter design guideline of LLC resonant converters are also introduced.

Neural network-based three-phase NPC rectifier for DC bus capacitor voltage balancing under perturbed mains supply conditions

Neural network-based three-phase NPC rectifier for DC bus capacitor voltage balancing under perturbed mains supply conditions

Analysis of Maximum Load Unbalancing Limits of Cascaded H-Bridge Rectifier Under Unity Power Factor

In a cascaded H-bridge rectifier (CHBR), the dc capacitor voltages need to be controlled to balance by the voltage balance controller when the loads are unbalanced. However, the voltage balance controller will fail to balance the dc capacitor voltages when the load unbalancing range exceeds a certain limit under unity power factor; that limit is constrained by the AC voltage and transmission power of each H-bridge. First, the relationships in CHBR were analyzed. Then, the expression between AC voltage and the power of each H-bridge was derived. Then the load unbalancing limits to maintain the dc capacitor voltage balance of each H-bridge were further deduced. In addition, the influence of ripple power and total load power on the load unbalancing limits was investigated. Finally, an experimental prototype of a single-phase three-cell cascaded H-bridge rectifier was built and the correctness of the proposed load unbalancing limits was verified by the experiment. The proposed load unbalancing limits are universal, which provides theoretical support for the stable operation and power transmission of CHBR.

A Simplified Model Predictive Control for T-Type Inverter with Output LC Filter

In this paper, a model predictive control scheme for the T-type inverter with an output LC filter is presented. A simplified dynamics model is proposed to reduce the number of the measurement and control variables, resulting in a decrease in the cost and complexity of the system. Furthermore, the main contribution of the paper is the approach to evaluate the cost function. By employing the selection of sector information distribution in the reference inverter voltage and capacitor voltage balancing, the execution time of the proposed algorithm is significantly reduced by 36% compared with conventional model predictive control without too much impact on control performance. Simulation and experimental results are studied and compared with conventional finite control set model predictive control to validate the effectiveness of the proposed method.

Soft Switching DC Converter for Medium Voltage Applications

A dc-dc converter with asymmetric pulse-width modulation is presented for medium voltage applications, such as three-phase ac-dc converters, dc microgrid systems, or dc traction systems. To overcome high voltage stress on primary side and high current rating on secondary side, three dc-dc circuits with primary-series secondary-parallel structure are employed in the proposed converter. Current doubler rectifiers are used on the secondary side to achieve low ripple current on output side. Asymmetric pulse-width modulation is adopted to realize soft switching operation for power switches for wide load current operation and achieve high circuit efficiency. Current balancing cells with magnetic component are used on the primary side to achieve current balance in each circuit cell. The voltage balance capacitors are also adopted on primary side to realize voltage balance of input split capacitors. Finally, the circuit performance is confirmed and verified from the experiments with a 1.44 kW prototype.

Capacitor Voltage Balancing and Current Control of a Five-Level Nested Neutral-Point-Clamped Converter

The five-level nested neutral-point-clamped (5L-NNPC) converter is one of the most promising topologies for medium-voltage (2.3–7.2 kV) high-power applications such as medium-voltage drives, wind energy conversion systems, and grid-connected systems. The 5L-NNPC requires a fewer number of switching devices, freewheeling and clamping diodes, and flying capacitors compared to the existing five-level multilevel converters. In the 5L-NNPC topology, each flying capacitor voltage is regulated at one-fourth of the dc-bus voltage to obtain the five-level operation. Due to the lack of redundant switching states, it is difficult to control the flying capacitor voltages by using the pulse-width-modulation-based classical control methods. This paper proposes a model-predictive current control (MPCC) approach to control the flying capacitor voltages along with the output currents of the 5L-NNPC converter. The discrete-time model of 5L-NNPC is developed to implement the MPCC scheme. The simulation and experimental studies are conducted to verify the dynamic and steady-state performance of 5L-NNPC with the MPCC scheme. The performance of the proposed MPCC approach is compared with the conventional space-vector-modulation-based voltage-balancing approach. Furthermore, the flying capacitor voltage control capability of MPCC is verified at different load power factors.

A New Diode-Clamped Multilevel Inverter With Balance Voltages of DC Capacitors

This paper proposes a new diode-clamped multilevel inverter that can effectively balance the voltages of dc capacitors. For the proposed inverter, the inductors are inserted between the diodes and the main switches. Also, the voltage balance of the dc capacitors can be guaranteed by controlling the opening time of the corresponding main switches in particular modes. Thus, the parallel circuits on the dc side in the conventional inverters are canceled. Consequently, the proposed topology requires fewer switches, diodes, and capacitors, although it adds the compensation currents to the corresponding main switches. Both the simulation and experimental results demonstrate the suitability and applicability of the proposed topology.

Operation and Control Scheme of a Five-Level Hybrid Inverter for Medium-Voltage Motor Drives

This paper proposes a control method for a five-level hybrid flying-capacitor (5L-HFC) inverter, of which structure stems from the conventional five-level active neutral-point-clamped (5L-ANPC) topology by dividing the dc-link stage into three series-connected capacitors. In this inverter, the voltage stress on the power switches connected to the dc link is reduced by a half compared with that of the 5L-ANPC topology, thus the lower number of equally voltage-rated power devices can be employed. Also, the power losses in the 5L-HFC inverter are more evenly distributed than in the 5L-ANPC. In order to balance the dc-link capacitor voltages, a third-order harmonic offset injection is applied. When a diode rectifier is used to supply the dc bus voltage, the balancing method is not effective if the modulation index is higher than 0.64. Thus, an auxiliary circuit is needed to support the balancing of the dc-link capacitor voltages. However, the unbalancing problem can be overcome in a full-range operation without the auxiliary circuit if the back-to-back configuration is utilized. Finally, simulation and experiment results have verified the performance of the 5L-HFC inverter with the proposed control method.

Model Predictive Control of Four-Leg Three-Level Flying Capacitor Converter: Influence of Model Parameter Mismatch

Abstract This paper presents a control method for a four-leg three-level flying capacitor converter (FCC) operating as a shunt active power filter (SAPF), based on model predictive control with finite number of control states (FS-MPC). Current control and capacitor voltage balancing are described. Influence of the mismatch of the inductive filter model parameters on the current control precision is analysed. Results are supported by the experimental waveforms obtained with a 10kVA set-up.