DC-link Capacitor Voltage Research Articles

Advanced nonlinear controller of single‐phase shunt active power filter interfacing solar photovoltaic source and electrical power grid

This paper addresses the nonlinear controller design and the stability analysis of a shunt active power filter (SAPF) interfacing a solar photovoltaic (PV) system that consists of a couple of PV panels, a single-phase two-stage IGBT half-bridge SAPF and two identical DC link capacitors. We seek for the considered system the fulfillment of three control objectives: (a) extracting the maximum PV power using an appropriate maximum power point tracking (MPPT) algorithm; (b) regulation of the DC link capacitor voltages to the reference provided by the MPPT controller hence ensuring the exchange of power between the PV source and the AC power grid; (c) compensation for reactive power and undesired harmonics caused by the nonlinearity of electronic power loads, that is, performing power factor correction (PFC). The control objectives are achieved using a new two-loop cascaded controller. The Backstepping approach is applied in the inner loop to perform PFC. A filtered proportional-integral (PI) controller is applied in the outer loop to ensure tight regulation of the PV voltage. The MPPT control is implemented using a perturb-and-observe (PO) algorithm. The performance of the closed-loop system under the two-loop control strategy is formally analyzed through the Lyapunov approach applied to the averaged model. Numerical simulations are performed in MATLAB/SimPowerSystems environment to validate the design methodology and to confirm the theoretically predicted performances of the system under the proposed nonlinear controller.

A new seven level boost-type ANPC inverter topology for photovoltaic applications

Developing of new photovoltaic inverter topologies is received more attention in the last few years. In particular, designing an active neutral-point-clamping inverter type structure is quite popular for PV applications. The output voltage is always half of the input voltage (vin), which further increases the voltage rating of dc-link capacitors in the conventional three-level ANPC. To rectify the above problem and increase the output voltage by reducing dc-link capacitors voltage rating, a new boost type seven-level ANPC inverter topology is proposed. The proposed topology consists of seven switches and one floating capacitor. The floating capacitor voltage is self-balanced, and the output voltage is 1.5 times higher than the input voltage. A detailed comparison for some power components, power loss and cost with other existing topologies are presented. Further, the proposed topology is validated in a prototype hardware setup for different load values.

Lyapunov based harmonic compensation and charging with three phase shunt active power filter in electrical vehicle applications

With the increasing proliferation of high-tech loads and electrical vehicle (EV) charging stations, shunt active power filters (SAPFs) are contemplated to play a growingly substantial role in power distribution and delivery. In this regard, this paper proposes a Lyapunov based harmonic compensation and charging with the operation of three-phase SAPF as interface EV applications and the electric grid. The proposed control algorithm provides effective harmonics mitigation capability under various non-linear loads (NLLs) groups and grid disturbances. A noteworthy contribution of this paper is that a Lyapunov based proportional integral with anti-windup (PI with AW) control is proposed to regulate oscillations of the DC-link voltage without increasing DC-link capacitor size. The proposed control algorithm offers comprehensive solutions with several improvements, novelties and features as: (i) keeping the total harmonic distortion (THD) of source currents below recommended IEEE-519 limits, (ii) extraction of reference current and voltage signals based on Lyapunov estimator (LE) with effective and good harmonic rejection capability, (iii) achieving good dynamic performance with utilizing a Lyapunov based PI with AW control for the DC-link capacitor voltage and (iv) eliminating the necessity of any high-pass or low-pass filters with utilizing LE based orthogonal signal generation (OSG). Besides, comparative analyses with previous studies are handled to examine improvements provided by the proposed control algorithm. The feasibility and effectiveness of the proposed control algorithm are confirmed by PSCAD/EMTDC environment and DSP based PIL quasi-real time results.

Simple Capacitor Voltage Balancing for Three-Level NPC Inverter Using Discontinuous PWM Method With Hysteresis Neutral-Point Error Band

Three-level neutral-point-clamped (3L-NPC) voltage source inverters are widely used in many low- and medium-power applications. However, the 3L-NPC inverter has an inheritance issue of the neutral-point (NP) voltage unbalancing due to the deviation of dc-link capacitor voltages causing distortions for the output waveform quality. Generally, discontinuous pulsewidth modulation (DPWM) is used to diminish the stress on power transistors and prolong their lifespan, however; it is not capable of solving the issue of NP voltage unbalancing. Therefore, this article proposes a simple voltage balancing control based on DPWM with a hysteresis NP voltage band. The balancing control method is only activated once the capacitance voltage error exceeds the hysteresis band by generating a momentary offset on the opposite direction of the DPWM offset. In this way, both top and bottom capacitance voltages will converge within a predefined error band. Various hysteresis error bands are investigated by analyzing the power losses, total harmonic distortions, and common mode voltage. The advantages of this proposed method are its simplicity and ease of control while maintaining the features of DPWM. The effectiveness of the proposed method is validated using simulation and experimental results.

Minimum Harmonic Distortion Losses and Power Quality Improvement of Grid Integration Photovoltaic-Wind Based Smart Grid Utilizing MOPSO

Increased usage of combined PV-Wind renewable energy sources is seen as a positive step toward reducing air pollution and carbon emissions. However, since non-linear loads have increased dramatically, voltage quality and harmonic distortion concerns have arisen, affecting the operation of combined PV-Wind RES and smart-grid electrical transmission structures. This study shows how a Shunt active power filter may improve energy quality in a microgrid structure at the distribution level. The major goal of this article is to find an appropriate controller approach for improving the shunt active power filter's compensating capacity. This paper simulates a PV-Wind hybrid renewable energy system that operates in the presence of unpredictably variable solar and wind energy resources. The objective is to allow the construction of an electrical control structure that produces the right duty cycle. It will aid in the regulation and stabilization of voltages at dc/dc energy conversion plant. Simulation is used to assess the proposed control system's ability to enhance power quality. The device's compensating capability is mostly determined by the DC link capacitor voltage control. The closed loop functioning of a proportional integral controller is used to attain this voltage regulation in the past. To increase the functioning of a shunt active power filter, the MOPSO procedure approach has been presented. The performance of suggested approaches and the comparison of different pulse generating strategies have been validated in the SIMULINK/MATLAB model environment. The suggested technology successfully improves power quality on the grid and maintains a steady voltage on the grid despite variations in RE output and load.

DC-Link Capacitor Voltage Balancing Control of a Five-Level Regenerative AC Electronic Load Using One-Cycle Control

High voltage electric power equipment requires rigorous regulation testing to specific standards which ensure proper and safe operation in the grid. Manufacturers conduct these tests in order to prove standard compliance and product liability. Variable linear or nonlinear loads are necessary for testing medium voltage (MV) high power AC power converters. Generally, those AC power converters or power supplies require performance validation, burn-in and/or lifetime testing under different load conditions, defined by the end-user or standards for the given applications. For flexible and efficient MV verification testing, this paper presents a five-level multilevel converter-based MV regenerative AC electronic load with one-cycle control (OCC), which is based on five-level diode-clamped multilevel converters with back-to-back structure and can emulate any impedance load. In this paper, especially the dc-link capacitor voltage balance of the proposed multilevel MV regenerative AC load is deeply analyzed. Simulation and experimental results are presented to verify the dc-link voltage balance performance of the proposed multilevel MV regenerative AC electronic load.

Two-level inverter and three-level neutral point diode clamped inverter for traction applications: a comparative analysis study

<span>This paper presents an analytical comparison between two-level inverter and three-level neutral point diode clamped inverters for electric vehicle traction purposes. The main objective of the research is to declare the main differences in the performance of the two inverter schemes in terms of the switching and conduction losses over an entire domain of the modulation index and the phase angle distribution, steady-state operation, transient operation at a wide range of speed variation, and the total harmonic distortion THD% of the line voltage output waveform. It also declares the analysis of the three-level neutral point diode clamped inverter (NPCI) obstacle and the unbalance of the DC-link capacitor voltages. The introduced scheme presents an Induction Motor (IM) drive for electric vehicle (EV) applications. Considering the dynamic operation of the EV, the speed of the three-phase induction motor is controlled using a scalar V/Hz control for the full range of the IM power factor (PF). A comprehensive MATLAB/Simulink model for the proposed scheme is established.</span>

Enhanced Direct Torque Control for a Three-Level T-Type Inverter

Direct torque control (DTC) is a widely used approach for motor drives due to its distinctive advantages, e.g., fast dynamic response and highly robust against motor parameters uncertainties and external disturbances. To achieve a lower torque ripple, a higher switching frequency is usually required, which, however, can reduce the drive efficiency. Using the emerging silicon carbide (SiC) devices, which can operate efficiently at a much higher switching frequency than their silicon counterparts, thus, it could be feasible to achieve high efficiency and low torque ripple simultaneously. In this work, a novel multilevel DTC for a SiC T-type inverter is proposed to significantly reduce the torque and flux ripples while retaining the fast-dynamic response. The unbalanced dc-link is a common issue of the T-type inverter, which may also lead to a higher torque ripple. To address this issue, the proposed DTC algorithm only uses the real voltage space vectors and virtual space vectors (VSVs) that do not contribute to the neutral point current such that an inherent dc-link capacitor voltage balancing can be achieved without using any active dc-link voltage controls or additional dc-link voltage and/or current sensors. The feasibility and effectiveness of proposed methods are verified by using both simulation and experimental studies.

Analysis of sliding mode controller based DSTATCOM for power quality improvement in distribution power system

In this paper, Sliding Mode Controller-based Distribution Static Compensator (DSTATCOM) is proposed for enhancement of power quality in the distribution network. The DSTATCOM plays a vital role to enhance the quality of power in the distribution network with linear and nonlinear loads under non-stiff sources. During the load disturbance in the systems, there is a sudden variation of voltage in DC link capacitor voltage connected with compensating device. The performance of DSTATCOM purely depends on the capacitor voltage level during load variation. It should be maintained within the prescribed limits. In general, the conventional controller is used in the converter to control dc-link capacitor voltage. Through, the conventional controller like PI transient response is still slow. Due to this reason, the harmonic elimination, load balancing, and reactive power compensation process are not satisfactory during different load variations in the distribution systems. To overcome these problems sliding mode controller is proposed in the control circuit for the proper operation of the compensator. The sliding mode controller regulates the DC link capacitor voltage to generate required reference currents. The reference currents play a vital role for switching pulse generation and control of DSTATCOM during various load disturbances. Sliding mode controller is designed to limit variation of DC-link voltage within limits. The proposed technique is modelled and its performance is simulated in MATLAB/SIMULINK platform and results are shown to validate the design and control of DSTATCOM under various load disturbance.

Grid Interfaced Windfarm Using Dynamic Voltage Restorer for Power Compensation with Pv Fed Sepic Converter

In this paper, the wind energy generation farm is connected with the grid system. The dynamic voltage restorer (DVR) is installed to control the grid side’s under and overvoltage. The photovoltaic (PV) system is installed in the DVR system, and the voltage of the grid over and under voltages is also addressed to achieve the proposed system’s high efficiency. The grid voltage compensation is harvested from PV and injected through the DVR. The boost converter is used in the existing systems to improve the available power from PV. In this system, the SEPIC converter is used to achieve high voltage gain from the PV plant to improve DC link capacitor voltage which is the inverter input source. The DVR based wind energy farm using PV with SEPIC system results are achieved in MATLAB/Simulink.

Harmonics Mitigation of a Solar PV-Fuel Cell Based Microgrid System using a Shunt Active Power Filter

The increased penetration of distributed energy resources is inspiring the entire design of conventional electrical power systems. A Microgrid (MG) includes distributed generation, loads, energy storage, and a control system capable of operating in grid-connected mode and/or island mode. The power quality (PQ) issue is one of the main technical challenges in an MG power system. To improve PQ, it is necessary to analyze the harmonic distortion of the system. Moreover, harmonic distortion in MG networks has significantly reduced PQ, affecting the stability of the system. The shunt active power filter (SAPF) has been extensively used to diminish the current harmonics and verified as being the best solution. Hence, in this paper, the impact of PQ issues in an adopted standalone MG system (comprising solar and fuel cell based renewable energy sources) is investigated in the presence of SAPF. The SAPF is realized using a conventional synchronous reference frame (SRF) technique for current generation with a pulse-width modulation voltage source inverter technique to generate pulses for the inverter along with a PI controller to regulate the DC-link capacitor voltage. The proposed model is developed in MATLAB/SIMULINK and the results validate the superiority of the proposed technique over others in terms of harmonic elimination.

Study of Toroidal Core Multilimb Transformer (TCMLT) for High-Power DC Application

Multimodule isolated converters incorporating excellent features of galvanic isolation, bidirectional power flow, and simple control are widely adopted in medium-voltage (MV) high-power dc-dc applications. However, the converter size and dc-link capacitor voltage balancing are significant concerns in galvanically isolated cascaded dc-dc converters. This article proposes a new design of an integrated transformer implementing toroidal cores named toroidal core multilimb transformer (TCMLT), employed in a dc-dc modular isolated bidirectional dual-active-bridge (IB-DAB) resonant converter topology. The modular design structure of the proposed transformer can be conveniently constructed and implemented for more number of participating modules conforming the compactness constraint. Moreover, an external coupling winding is implemented in the TCMLT structure to resolve the dc-link voltage balancing issue. This coupling winding method substitutes the expensive complex control circuits, which consequently results in cost reduction and reliability enhancement. Besides, the reluctance model of the proposed structure is analyzed. Based on the reluctance model, a common mode-difference mode (CM-DM) electrical circuit is realized for the proposed scheme. Furthermore, unlike the conventional approach of implementing U, UI, and EI cores for multilimb transformer (MLT) structure with separate limbs, a synthesis method of TCMLT in toroidal form is proposed. The comparative study of the proposed design with the conventional U-core MLT confirms that the proposed TCMLT offers less leakage inductance with significant size reduction. Finally, to validate the efficacy of TCMLT, a three-limb laboratory-scale prototype is employed in a 6-kW cascaded IB-DAB resonant converter topology.

Single‐source multilevel inverter based on flyback DC‐DC converter

A new multilevel inverter based on flyback converter is presented in this paper. By employing the DC-DC flyback converter the voltage of DC-link capacitors is adjusted. In the proposed structure, the required input DC voltages are provided with a single-input multi-output flyback DC-DC converter. The proposed topology can decrease total harmonics distortion (THD) of the injected grid current. Also, the proposed topology is suitable for photovoltaic (PV) applications. In addition, the proposed inverter can be connected to grid for injecting a sinusoidal current under desired power factors (leading, lagging and unity). In this case, the suggested inverter is able to control the active and reactive powers. This leads to generate a tightly controlled current with a high quality that can be injected to the grid utilizing a single renewable energy resource. It should be noted that the experimental results of the proposed inverter will be obtained when the topology is connected to the local grid. In addition, in the proposed topology by using flyback converter the local grid is isolated from PV panel, so the generated leakage current can be eliminated completely. The operation modes, design consideration, comparison results and finally 620 watt built prototype results are presented.

Improved Model Predictive Current Control for Three-Phase Three-Level Converters With Neutral-Point Voltage Ripple and Common Mode Voltage Reduction

An improved two-stage model predictive current control (MPCC) for three-phase three-level converters in the paper, which can simultaneously achieve common-mode voltage (CMV) reduction, effective dc-link capacitor voltage control, and decreased computational burden, is proposed. Compared to the other current control strategies, it has the capability of handling multiple control objectives and possesses a simple structure. Furthermore, it greatly reduces the CMV since the peak CMVs of all the candidate voltage vectors are constrained to be less than one sixth of the dc-link voltage. In addition, an optimal two-stage voltage vector selection mechanism is employed in the proposed method. It helps to significantly reduce the computational complexity. Simulation and experimental results are provided to verify the effectiveness of the proposed method.

An Optimized Hybrid PWM Strategy for Five Level NPC VSI With Unequal DC-Links in a PV System

This article presents an optimized hybrid pulsewidth modulation (PWM) strategy for five-level neutral point clamped (5L-NPC) inverter with unequal dc-link capacitor voltages fed by differential power processing (DPP)-based photovoltaic (PV) system. The generation of balanced output from the 5L-NPC inverter with the unequal dc-link capacitor voltages becomes necessary in a PV system. The unequal dc-link capacitor voltages cause low order harmonics in inverter output voltage. A hybrid PWM strategy combining the advantages of space-vector PWM (SVPWM) and the nearest level modulation (NLM) methods is proposed to address the issues associated with unequal dc-link capacitor voltages in a five-level NPC inverter. The proposed hybrid PWM is optimized to achieve balanced output voltage with less total harmonic distortion at fundamental frequency. In-depth theoretical analysis of sector identification algorithm and dwell time computation for the proposed optimized hybrid PWM method are described. Simulation and experimental results are illustrated for several combinations of unequal dc-link capacitor voltages to validate the functionality and performance of the proposed optimized hybrid PWM method.

Fault-Tolerant Predictive Torque Control Design for Induction Motor Drives Based on Discrete Space Vector Modulation

Four-switch three-phase inverters (FSTPIs) are generally applied as fault-tolerant topologies or cost-effective topologies for induction motor (IM) drives. However, conventional predictive torque control (PTC) for FSTPI-fed IM suffers from poor steady-state performance and tedious weighting factor assignment. In this article, a PTC based on discrete space vector modulation (PTC-DSVM) for an FSTPI-fed IM drive is proposed. First, the relationship between the dc-link capacitor voltage offset (CVO) and the load current is derived, and a stator voltage compensation strategy is proposed to suppress the CVO. Meanwhile, a reference-voltage-vector-error-based cost function that combines the reference stator flux vector control, the stator voltage compensation strategy, and the deadbeat control is designed to fully eliminate the weighting factors in the cost function of the conventional PTC. In addition, a DSVM scheme is developed to increase the number of admissible voltage vectors and improve the steady-state performance of IM drives. A preselection algorithm based on the offset voltage vector and sector division is designed to reduce the number of candidate voltage vectors. Extensive simulations and experimental results with a 2.2-kW IM are demonstrated to validate the effectiveness of the proposed control strategy.

Experimental validation of new self-voltage balanced 9L-ANPC inverter for photovoltaic applications

Multilevel inverters play an important role in extracting the power from renewable energy resources and delivering the output voltage with high quality to the load. This paper proposes a new single-stage switched capacitor nine-level inverter, which comprises an improved T-type inverter, auxiliary switch, and switched cell unit. The proposed topology effectively reduces the DC-link capacitor voltage and exhibits superior performance over recently switched-capacitor inverter topologies in terms of the number of power components and blocking voltage of the switches. A level-shifted multilevel pulse width modulation scheme with a modified triangular carrier wave is implemented to produce a high-quality stepped output voltage waveform with low switching frequency. The proposed nine-level inverter’s effectiveness, driven by the recommended modulation technique, is experimentally verified under varying load conditions. The power loss and efficiency for the proposed nine-level inverter are thoroughly discussed with different loads.

Improved Model Predictive Control With New Cost Function for Hybrid-Inverter Open-Winding PMSM System Based on Energy Storage Model

Conventional model predictive control (C-MPC) usually leads to considerable torque and current ripples since only one voltage vector is applied. In addition, the C-MPC applied in the hybrid-inverter) driven open-winding permanent magnet synchronous motor (OW-PMSM) suffers from complex tuning work of weighting factors and iterated evaluation work of all potential vectors because the capacitor voltage constraint in cost function is related to the switching states. In this article, an improved three-vector MPC based on energy storage model is proposed for the HI-OW-PMSM system. Firstly, a new prediction model of dc-link capacitor voltage based on energy storage model is proposed thus the capacitor voltage constraint is decoupled to the three-phase switching states. Furthermore, a novel restructured cost function without weighting factors can be obtained. Accordingly, by using the new prediction model and the novel cost function, the voltage reference can be easily obtained and a three-vector based prediction strategy without sector division can be carried out to achieve optimal voltage vectors selection and duty cycle calculation. Therefore, the system performance is improved and the calculation burden is also reduced. Finally, comparative experimental studies are carried out to validate the effectiveness of the proposed improved three-vector MPC method.

Analysis of an interleaved control scheme employed in split source inverter based grid‐tied photovoltaic systems

In conventional grid-tied renewable energy applications, the dc–dc converter is inevitable. An increased cost, size, and reduced efficiency are the demerits of incorporating a dc–dc converter. A recently introduced split source inverter (SSI) is capable to perform both dc–dc as well as dcac at a time. The main attractive controllable feature of SSI is its modulation index. By utilizing this parameter, an interleaved control structure is proposed for the SSI in grid-tied photovoltaic applications. The main objectives of the work are to track peak power and to maintain a stabilized voltage across the dc-link capacitor at a variable load and grid disturbances. The proposed peak power controller performs two important actions: (i) the peak power is traced at all irradiance and (ii) generates a duty cycle to enhance the dc-link capacitor voltage. The proposed scheme is essentially investigated with simulation. To validate experimentally, a 1 KW photovoltaic system is integrated into a grid-tied 10 kVA SSI prototype. A Xilinx based Spartan field-programmable gate array (FPGA) is used to provide a control algorithm for the SSI. Few case studies at different conditions are performed and investigated. Experimental and simulation results are presented to prove the robustness of the control strategy.

A Lyapunov-Function Based Controller for 3-Phase Shunt Active Power Filter and Performance Assessment Considering Different System Scenarios

Shunt active power filter (SAPF) belongs to the class of custom power devices (CPDs) and offers compensation to harmonics originated owing to customer side nonlinear loads, reactive power and unbalance in the distribution power networks functioning in current control mode (CCM). The performance of a SAPF as a harmonic compensator entirely relies on the control technique i.e. the precise detection of the harmonic current components of load that are necessary to be compensated. In the present work, a 3-phase SAPF, inspired by a Lyapunov function based control approach, has been designed for compensation of harmonics resulted in the feeder current owing to the customer side nonlinearity. A control law is determined in the proposed strategy which makes the derivative of the Lyapunov function consistently a negative one for an entire set of stable states. The DC-link capacitor voltage is regulated at constant reference through the proportional-integral (PI) controller. In this method rating of the shunt active power filter is considerably reduced than the other two broadly employed conventional methods. Furthermore, the harmonic compensation efficacy of the proposed Lyapunov function based SAPF is compared with the one based on other two conventional approaches under four different system scenarios namely a simple nonlinear load with and without utility side voltage distortion, a modified IEEE 13 bus test distribution system loaded with a 3-phase chopper fed direct current (DC) motor drive at a single bus and last especially for increasing the harmonic-constrained penetration level of renewable energy. Results obtained through simulation performed in MATLAB/Simulink shows that total harmonic distortion (THD) of source current and dynamic, as well as steady-state performance with Lyapunov function based controller, is significantly improved than the other two conventional methods. Also, the robust compensation performance of the SAPF empowers it to deal with the high penetration of renewable energy.