Dc-link Voltage Balance Research Articles

Closed-Loop Control and Performance Evaluation of Reduced Part Count Multilevel Inverter Interfacing Grid-Connected PV System

Multilevel inverters (MLIs) have drawn tremendous attention in the power sector. Application of MLI has grown extensively to improve the power quality and efficiency of the photovoltaic (PV) system. For an MLI interfacing PV system, the size, cost and voltage stress are the key constraints of the MLI that need to be minimized. This paper presents a novel reduced part count MLI interfacing single-stage grid-tied PV system along with a closed-loop control strategy. The proposed MLI consists of n repeating units and a level boosting circuit (LBC) that assists in generating 4n + 7 voltage levels instead of 2n + 3 levels. Three different algorithms are proposed for a proper selection of dc-link voltages to enhance the levels further. A comparative analysis is carried out to confirm the superiority of the developed MLI. The workability of the proposed MLI is investigated with a 1.3 kW PV system. The closed-loop control strategy ensures the maximum power tracking, dc-link voltage balancing, satisfactory operation of the MLI and injection of clean sinusoidal grid current under any dynamic changes. Comprehensive simulation analysis is carried out considering a 15-level MLI structure. Experimental tests further confirm the practicality of the topological advancement for a PV system under different dynamic conditions.

Duty-Cycle Predictive Control of Quasi-Z-Source Modular Cascaded Converter Based Photovoltaic Power System

A duty-cycle predictive control is proposed for dc grid integration of front-end isolated quasi-Z-source modular cascaded converter (qZS-MCC) photovoltaic (PV) power system. The post-stage qZS half-bridge dc-dc converter deals with PV maximum power point tracking, dc grid integration, and dc-link voltage balance; whereas, the front-end isolation converters operate at a constant duty cycle of 50%. Thus, it saves control hardware resources while overcoming challenges from PV-panel voltage variations and dc-bus voltage limit. The proposed control uses the derived circuit model to predict the global active-state duty cycle for grid-connected current control and predict the shoot-through duty cycles for dc-link voltage balance, achieving a fast and accurate tracking target. The proposed control method has advantages of: i) eliminating weighting factors that exist in conventional model predictive control (MPC), ii) no sophisticated loop parameters design that exists in proportional-integral (PI) control, iii) operating at constant switching frequency that is different from the conventional MPC with variable switching frequency. Simulation and experimental tests are carried out to verify the effectiveness of the proposed control method and compare with the PI-based control system.

Model Predictive Current Control for Fault-Tolerant Bidirectional Voltage Source Converter With Open Circuit Fault and Unbalanced Grid Voltage

To improve the fault-tolerant operation capability of bidirectional voltage source converter (BVSC), an improved model predictive current control (IMPCC) for fault-tolerant BVSC, is proposed with balanced DC-link capacitor voltage under unbalanced grid voltage. The proposed method can maintain continuous operation even if power device faults and unbalanced grid voltage faults occur together. A current predictive model of the fault-tolerant BVSC is established. By using grid voltages and 90° lagging signals in the $\alpha \beta $ stationary coordinate system, the reference current calculation method is designed for BVSC to eliminate power ripple under unbalanced grid voltage, which can avoid the complex positive and negative sequence extraction. DC-link split capacitor voltage balancing is achieved by the improved cost function. Based on the current predictive model and improved cost function, the optimal space voltage vectors are selected for fault-tolerant BVSC. Compared to the existing predictive methods, the proposed IMPCC can balance DC-link capacitor voltage and eliminate the power ripples for fault-tolerant BVSC under unbalanced grid with simple implementation. The experimental results validate the effectiveness of the proposed control scheme.

Fuzzy Based Combined Feed-Forward and Feed-Back Controlled Dynamic Voltage Restorer for Enhanced Fault Ride through Capability in DFIG based Wind Turbines

In this paper, Fault ride through (FRT) capability is enhanced using dynamic voltage restorer (DVR) in a wind turbine-driven doubly fed induction generator (DFIG). For effective control of the DVR, Fuzzy based combined feed-forward and feed-back (CFFFB) voltage control is used. The performance of DVR using CFFFB with FLC control is observed during balanced and unbalanced conditions in terms of voltage sag mitigation, reduced THD, fault current control and dc-link voltage balancing. The advantage of control strategy is validated utilizing simulation of 1.5MW DFIG-WT and results shows better reactive power support during faults condition.

DC-link voltage balance control method considering the wide regulation range in SOR for single-phase cascaded H-bridge rectifier

In this paper, a DC-link voltage balance control method that considers the wide regulation range in the steady-state operating region is proposed for a single-phase cascaded H-bridge rectifier. A mathematical model of the DC-link voltage is also established while considering the imbalance caused by the inconsistency of the active power absorbed and consumed by the cell. On the basis of the steady-state operating region of the cascaded H-bridge rectifier, the operating loci of different balance methods are analyzed and a novel balance method is proposed. The DC-link voltage regulation range of the proposed method is then compared with that of two other commonly used methods based on the operating locus in the steady-state operating region. The comparison results show that the proposed method has the widest regulation range in the steady-state operating region wherever the cascaded H-bridge rectifier is operated. The experimental results also verify the effectiveness and superiority of the proposed method.

Power Quality Performance Evaluation of Multilevel Inverter With Reduced Switching Devices and Minimum Standing Voltage

Multilevel inverters (MLIs) have been extensively employed to improve the power quality of the photovoltaic (PV) systems. However, the need for large number of components, higher standing voltage, and high harmonic content in the output of a conventional MLI greatly affects the system efficiency. Asymmetrical MLIs have been therefore developed as a suitable alternative to address these issues. This article aims at developing such a hybrid asymmetrical structure suitable for PV application that has a high level per component ratio and minimum standing voltage. The proposed MLI is assembled using a reduced switch H-bridge-based (RSHB) MLI structure with n asymmetrical repeating units and different level doubling circuit (LDC) combinations. The two dc sources used in the repeating units are in the ratio of 1: n voltage ratio, and using n such units, the proposed MLI structures, i.e., PS1 and PS2 can synthesize 4 n + 5 and 4 n + 7 levels, respectively, at the output instead of 2 n + 3 levels with only RSHB MLI. Comparative analysis reveals that both PS1 and PS2 have fewer switches, low standing voltage, less power loss, and lower cost. A 3.9-kW standalone solar PV system is considered for performance evaluation of the PS1 structure applying both the selective harmonic elimination and carrier-based pulsewidth modulation control schemes. In light of this, dc-link voltage balancing and self-voltage balancing mechanism of the LDC are warranted. Extensive simulation of the proposed MLI is performed in MATLAB/Simulink platform under a change in modulation index, sudden load change, frequency change, and step change in solar insolation. Furthermore, theoretical and simulation findings are validated experimentally by performing similar tests on a prototype of the proposed 17-level MLI.

DC-Link Voltage-Balancing Strategy Based on Optimal Switching Sequence Model Predictive Control for Single-Phase H-NPC Converters

In this article, a model predictive control (MPC) strategy based on the optimal switching sequence (OSS) concept for a single-phase grid-connected H-bridge neutral-point-clamped (H-NPC) power converter is presented. The proposed OSS-MPC algorithm considers both the grid current tracking error and the dc-link capacitor voltage balance. Special emphasis is placed on the power converter control region in order to design suitable switching sequence candidates for this multiobjective control problem. Additionally, based on an analysis of the weighting factor effect over closed-loop performance, it is possible to demonstrate that this controller parameter is relatively easy to adjust. In fact, the weighting factor only affects the peak current during transients, with no effect over the steady-state performance. As a result, the proposed OSS-MPC provides a fast closed-loop dynamic to the H-NPC converter, which operates with a fixed switching frequency at all times. This predictive control strategy is experimentally validated in a 3.5-kVA laboratory setup.

Novel space vector-based control scheme with dc-link voltage balancing capability for 10 switch converter in bipolar hybrid microgrid

Abstract Bipolar hybrid microgrid application is enhancing in power systems since this type of microgrid owns a DC and AC bus simultaneously, feeds local loads with local resources, and presents two dc voltage levels to users and resources. Interlinking converter (ILC) is the most important part of the microgrid structure due to its essential responsibilities in various applications. In this paper, a 10 switch converter is presented as an ILC for bipolar hybrid microgrid that advantages from both two and three-level converters’ benefits simultaneously, plus it is applicable to a wide variety of power ranges. This paper presents a novel space vector modulation (SVM) for 10 switch converter in order to reduce total harmonic distortion (THD) and increase the dc-link voltage utilization in comparison to prior carrier-based modulation methods. Also, a simplified, full-scale dc-link voltage balancing strategy is proposed for this converter. The performance of existing dc-link voltage balancing strategies for carrier-based modulation method which are an extension to the general control structure of a grid-tied ILC is compared to the method presented in this paper that shows the superiority of the proposed robust full-scale balancing strategy and the proposed SVM method. Moreover, the structure of grid-tied ILCs is upgraded to eliminate low order harmonics.

An integrated interlinking converter with DC-link voltage balancing capability for bipolar hybrid microgrid

The bipolar hybrid microgrid is a newly emerged structure which has gained increasing attention during the last decades. The heart of this structure is the interlinking converter that plays a crucial role in this system and has three main functionalities: (1) bidirectional connection between AC and DC subsystems, (2) sinusoidal current injection with desired power factor to AC side and (3) providing two buses with equal voltages in DC side. This paper applies a recently proposed ten-switch inverter as an interlinking converter in the bipolar hybrid microgrid that can increase efficiency, reliability and power quality on AC and DC sides in such systems. Ten-switch converter has lower cost, volume and size compared to the conventional interlinking converters. A new modulation and control strategy is proposed for ten-switch converter in the bipolar hybrid microgrid which, unlike conventional systems, does not need any extra hardware as a voltage balancer converter to balance DC-link pole voltages. The effectiveness of the proposed modulation and control strategy of ten-switch converter is evaluated and compared with conventional structures in a simulation study. Adaptive power loss calculations and cost analysis are conducted and then compared with three interlinking converters candidates to show the superiority of the proposed interlinking converter.

Single-Phase Two-Stage Seven-Level Power Conditioner for Photovoltaic Power Generation System

This paper presents a new power conditioner with the inherent benefits of boosting, generation of seven-level output voltage with minimum leakage current in a grid-connected photovoltaic (PV) power generation system. The proposed power conditioner is an upgrade of a front-end multioutput dc–dc boost converter and an asymmetrical seven-level inverter. A high-frequency transformer (HFT) employed in front-end converter produces balanced dc-link voltages to generate the seven-level output voltage. The leakage current caused by the parasitic capacitance of the PV panel is minimized by providing a common-mode conducting path to the inverter. This results in a reduction of the leakage current well below the VDE0126-1-1 grid standards. Furthermore, the proposed configuration utilizes a minimum number of devices for every level generation, which reduces the control complexity and also improves the system efficiency. The dynamic performance of the system is tested for intermittent changes in the PV characteristics for grid-connected operation. The proposed power conditioner is simulated using MATLAB software, and a laboratory prototype of 750 W is developed to validate its feasibility. Finally, a comparison is made with other recently proposed seven-level inverters to highlight the benefits of this power conditioner over others.

Control of asymmetrical cascaded multilevel inverter for a grid‐connected photovoltaic system

This article presents a generalised asymmetrical cascaded multilevel inverter (MLI) for a single-phase grid-connected photovoltaic (PV) system and their control strategy. The control strategy, including maximum power tracking along with a suitable interface, is implemented for maximum power transfer from the PV source to the single-phase low-power grid. The balancing of DC-link voltages for an asymmetrical MLI under variable solar parameters as well as grid parameter variation is implemented using the proposed control strategy. The voltage controllers maintain the constant DC-link voltage ratio, whereas the current controller injects the sinusoidal current into the grid at unity power factor and track the grid voltage under variation of grid voltage using grid tracker. Stability analysis of the proposed grid-connected asymmetrical inverter system is also incorporated. The whole grid-tied PV system is simulated in the MATLAB/SIMULINK environment and the exhaustive simulation results of the system under different transient conditions are presented. In addition, a laboratory prototype for a low-power grid-tied PV system has been developed and implemented using DS1103. The performance of the system is also tested at varying irradiance conditions and the corresponding experimental results are also presented.

Centralised model‐predictive decoupled active–reactive power control for three‐level neutral point clamped photovoltaic inverter with preference selective index‐based objective prioritisation

This study presents a single-stage grid-tied three-level neutral point clamped photovoltaic inverter with a centralised model-predictive decoupled active–reactive power control. The proposed centralised model predictive control (CMPC) incorporates the constraints of maximum power extraction, dc-link capacitor voltage balancing and active–reactive power tracking in a single objective function. The dc-link voltage of the inverter is regulated to its reference for extracting the maximum power. In order to eliminate the impact of reactive power exchange on floating dc-link voltage regulation, a decoupled active–reactive power control is used in the CMPC. Furthermore, a preference selective index-based dynamic weighting factor selection approach is introduced to maintain the relative importance between the power tracking and dc-link capacitor voltage balancing. The proposed control approach eliminates the outer dc-link voltage control loop and also, the empirical approach required for the selection of weighting factors. As a result, it ensures an optimal control action in each sampling period to improve the steady-state and dynamic tracking performance of the control objectives. The proposed control approach is experimentally verified by using a 1.2 kW laboratory-scale prototype and the results are presented to demonstrate its effectiveness compared to the classical proportional–integral-based model predictive control.

Individual DC Voltage Balance Control for Cascaded H-Bridge Electronic Power Transformer With Separated DC-Link Topology

For the electronic power transformer, the dc voltage balance is an important issue. Unbalanced dc voltages will degrade the performance of the device and may trigger the dc voltage protection. In this paper, an overall individual dc voltage (including high-voltage and low-voltage dc-link voltages) balance strategy is proposed for the electronic power transformer with separated dc-link topology. The strategy adjusts active powers flowing through the isolation and output stages in different power modules to enhance the dc voltage balance capability. Through the strategy, the high-voltage and low-voltage dc-links can be well balanced when unbalance occurs among different power modules (e.g., component parameter mismatches or some of the high-voltage or/and low-voltage dc-links are connected with renewable energy sources or/and dc loads). The proposed strategy is analyzed and supported by experimental validation.

A Novel Control Strategy for DC-Link Voltage Balance and Reactive Power Equilibrium of a Single-Phase Cascaded H-Bridge Rectifier

The dc-link voltage balance and reactive power equilibrium of the cascaded H-bridge rectifier (CHBR) are the prerequisites for the safe and stable operation of the system. However, the conventional PI (Proportional-Integral) control strategy only puts emphasis on the CHBR dc-link voltage balance without taking into account its reactive power equilibrium under capacitive and inductive working conditions. For this reason, this paper has proposed a novel control strategy for the CHBR that can not only balance dc-link voltage, but also achieve reactive power equilibrium and eliminate the coupling effect between the voltage-balancing controller (VBC) and original system controller (OSC). The control strategy can achieve dc-link voltage balance and the reactive power equilibrium of the CHBR through modifying the active duty cycle by closed loop control, and adjusting the reactive duty cycle relatively according to the modifiable amount of the active duty cycle. Moreover, the strategy can eliminate the coupling effect between the VBC and OSC by the open loop control modification of the active and reactive duty cycle of any H-bridge module in CHBR. Simulations and experiments have shown that the proposed control strategy is feasible and effective in performing the CHBR dc-link voltage balance and reactive power equilibrium under all working conditions and load variations.

Direct RFOC Strategies Aimed to Symmetrical Two-Phase IM Drives: Comparison Between B4- and B6-Inverters in the Stator

This paper deals with an investigation of the steady-state and transient performance of the direct rotor flux-oriented control (RFOC) strategies implemented in symmetrical two-phase induction motors (2PIMs). Such an investigation is structured within a comparison study considering: 1) the case of a two-leg voltage source inverter (B4-inverter), and 2) the case of a three-leg voltage source inverter (B6-inverter), in the stator of the 2PIM. Following a presentation of both drives, the principle of the RFOC is briefly recalled. Then, a special attention is paid to the implementation of the direct RFOC on current-regulated B4- and B6-inverters. The control of the power switches is achieved by two two-level hysteresis regulators in the B4-inverter rather than three in the B6-inverter. Initiated by a simulation-based analysis of the selected steady-state features of both 2PIM drives under direct RFOC strategies, the comparison study is then extended to a deep experimental investigation of the steady-state operation with emphasis on the dc-link capacitor voltages, the stator line-to-line and phase voltages, stator phase currents, the rotor flux, and the electromagnetic torque. The established results reveal three major limitations of the B4-inverter fed 2PIM drive, such as: 1) unbalanced dc-link capacitor voltages at low speeds; 2) a reduced speed range due to the low level of the phase voltages; and 3) an injection of harmonics due to the $d$ – $q$ transformation. While the B6-inverter exhibits balanced dc-link capacitor voltages in the total constant torque region with higher ripple of the stator phase currents and of the electromagnetic torque, the experiment-based comparison is achieved by an investigation of the dynamic behaviors of both drives under step- and ramp-shaped reversals of the reference speed. It highlights the poor dynamic behavior of the B4-inverter fed 2PIM drive under both shapes of speed reversal.

Low-Complexity Model Predictive Control of Single-Phase Three-Level Rectifiers With Unbalanced Load

The neutral-point potential fluctuation in single-phase three-level rectifiers leads to coupling between the line current regulation and dc-link capacitor voltage balancing, deteriorating waveform quality of the line current. For addressing this issue, this paper proposes a low-complexity model predictive current control (MPCC) with constant switching frequency, which achieves a decoupling control of the line current and the neutral-point potential under the unbalanced load condition. The switching frequency is fixed by combining the MPCC scheme with predefined switching sequences. The boundary of the durations for voltage vectors in switching sequences is presented to balance the capacitor voltage without worsening the line current quality. The optimal switching sequence and the corresponding optimal durations of voltage vectors in the switching sequence are readily derived without the assessment of the cost function, which dramatically simplifies the complexity of the MPCC scheme. Finally, simulations and experimental results are conducted to verify the effectiveness of the proposed MPCC scheme.

Power and Voltage Control for Single-Phase Cascaded H-Bridge Multilevel Converters under Unbalanced Loads

The conventional control method for a single-phase cascaded H-bridge (CHB) multilevel converter is vector (dq) control; however, dq control requires complicated calculations and additional time delays. This paper presents a novel power control strategy for the CHB multilevel converter. A power-based dc-link voltage balance control is also proposed for unbalanced load conditions. The new control method is designed in a virtual αβ stationary reference frame without coordinate transformation or phase-locked loop (PLL) to avoid the potential issues related to computational complexity. Because only imaginary voltage construction is needed in the proposed control method, the time delay from conventional imaginary current construction can be eliminated. The proposed method can obtain a sinusoidal grid current waveform with unity power factor. Compared with the conventional dq control method, the power control strategy possesses the advantage of a fast dynamic response. The stability of the closed-loop system with the dc-link voltage balance controller is evaluated. Simulation and experimental results are presented to verify the accuracy of the proposed power and voltage control method.

Selective finite‐states model predictive control of grid interfaced three‐level neutral point clamped photovoltaic inverter for inherent capacitor voltage balancing

A selective finite states model predictive control is proposed for a grid interfaced three-level neutral point clamped solar photovoltaic inverter. The proposed control approach eliminates the weighting factor selection for dc-link capacitor voltage balancing and reduces the computational burden for real-time implementation. The switching states required for the prediction and objective function optimisation are selected based on the position of reference voltage vector in the space vector plane, inverter current directions and the charge status of the dc-link capacitors. As a result, the selection of optimal switching state is fast, easy to implement and eliminates the selection of weighting factor for capacitor voltage balancing. The feasibility of the proposed control approach is verified through simulation and laboratory-scale experimentation. The results confirm that the proposed method attains the inherent dc-link capacitor voltage balance and also retains the dynamic and steady-state current tracking in comparison with the classical finite control-set model predictive control.

Single-Phase AC–DC–AC Multilevel Converter Based on H-Bridges and Three-Leg Converters Connected in Series

This paper investigates an ac–dc–ac multilevel power converter. The studied configuration is composed of two single-phase ac–dc–ac three-leg modules and series-connected H-bridges in the shared part of system. Because the proposed converter has shared legs between the input and output, it is employed in applications with same input and output frequency. Uninterrupted power supply and unified power quality conditioner are application examples for this converter. Such multilevel topology has lower dc-link voltage rating, which, consequently, presents low switch blocking voltages when compared to conventional topologies. System model, a space-vector pulsewidth modulation (PWM) strategy to symmetrical and asymmetrical dc-link voltages, and an overall control strategy to adjust the system variables are presented. A power flux analysis shows the operation zone in which the individual dc-link voltage balancing is possible. PWM and control strategies are developed to reduce the semiconductor total losses, harmonic distortion, and switching stress. Two ac–dc–ac multilevel conventional structures are used for comparison. Simulation and experimental results demonstrate the feasibility of the studied converter.

DTC of Three-Level NPC Inverter Fed Five-Phase Induction Motor Drive With Novel Neutral Point Voltage Balancing Scheme

In this paper, direct torque control (DTC) of five-phase induction motor (FPIM) is implemented using three-level neutral point clamped (TL-NPC) inverter. One of the advantages of three-level inverter over two-level one for DTC operation is the low torque ripple. Also TL-NPC inverter through space vector modulation technique gives low $ dv/dt$ transition with better voltage waveform. By applying conventional lookup table for DTC, the TL-NPC inverter does not ensures lower $dv/dt$ transition. In this paper, a novel switching scheme for DTC of FPIM using TL-NPC inverter is proposed that ensures the low $ dv/dt$ transition and balancing of dc-link capacitor voltages of TL-NPC inverter. To form the lookup table for DTC operation, instead of using voltage vectors directly, virtual vectors (VVs) are utilized. Two switching states are used in one sample time to generate a VV in $\alpha \beta$ plane, which gives zero resultant voltage in $ xy$ plane. The switching strategy ensures low number of transitions to reduce the switching losses. The switching state redundancies are used in a novel way to balance the dc-link capacitor voltages without using any additional hardware. The proposed technique to balance the dc-link capacitor voltage gives lower switching frequency. The MATLAB/Simulink environment is used for the simulation and the results are validated through experiments.