Dc-link Voltage Ripple Research Articles

Enhanced Metaheuristic Methods for Selective Harmonic Elimination Technique

Metaheuristic techniques have shown remarkable performance in a vast variety of engineering problems. Accurate solving of nonlinear equation set in selective harmonic elimination (SHE) has been one of the widely discussed numerical problems in power electronics. Current study aims at conducting an inclusive comparison of prominent stochastic methods adopted for SHE and modified SHE (MSHE) pulse width modulation techniques. The problem is defined as finding local optima of cascaded H-bride converter operation parameters. The survey investigates key indices of low-order harmonic components and weighted total harmonic distortion. Floating fundamental component is introduced for SHE and MSHE to achieve higher rate of flexibility for optimization techniques. Finally, an advanced modulation technique is proposed to address the dc-link voltage ripples. Simulation and experimental results are presented as a proof of concept.

Benchmarking of power control strategies for photovoltaic systems under unbalanced conditions

Unbalanced voltage sags are common phenomenon in power systems which affects the performance of grid-connected photovoltaic (PV) systems. The unbalanced voltage conditions results in double-line frequency power oscillations. These oscillations increase the dc-link voltage ripple leading to higher stresses in dc-link capacitors. In this case, the literature presents many power control strategies in order to improve the performance of PV systems during unbalanced conditions. Nevertheless, a deep comparison of these strategies has been missing. This work presents a complete analysis of the main power control strategies proposed in the literature. The energy storage requirements of the converter are analytically deduced and some guidelines for the dc-link design are presented. Furthermore, a case study based on a 11 kW PV system is used to compare the performance of the most interesting strategies in terms of dynamic behaviour, power oscillations and dc-link voltage ripple. Furthermore, the dc-link capacitors stress is discussed, considering the characteristics of reactive power injection, which is required by the modern grid codes. The power losses and the thermal stresses in dc-link capacitors are evaluated and the worst operational scenario is obtained for each power control strategy.

Repetitive Controller of Capacitor-Less Current-Fed Dual-Half-Bridge Converter for Grid-Connected Fuel Cell System

This paper proposes a repetitive controller (RC) for a capacitor-less current-fed dual-half-bridge (CF-DHB) converter for a grid-connected fuel cell system. The pulsating power caused by the ac grid makes the dc-link voltage and input current fluctuate with twice the grid frequency. To achieve zero-voltage-switching (ZVS) for the capacitor-less CF-DHB converter and to increase the lifetime of the fuel cell, we need to suppress the low-frequency dc-link voltage ripple and the low-frequency input current ripple. We first make use of the duty cycle to regulate the dc-link voltage, and of the phase-shift angle to regulate the input current. Because the transfer function from the duty cycle to the dc-link voltage has a right-half plane (RHP) zero, the conventional proportional-integral controller cannot achieve satisfactory performance at the dc-link voltage. To compensate for the phase lag due to the existence of the RHP zero, we propose to use an RC with phase-lead compensation for dc-link voltage control. Because the transfer function from the phase-shift angle to the input current has one left-half plane zero, the conventional RC is used to control the input current. In developing the proposed controller, we first derive the dynamic model of the CF-DHB converter in the grid-connected environment, and then use the model to design an RC. We also provide a detailed and practical design guideline to select the control parameters of the capacitor-less CF-DHB converter that can meet the desired performance. The proposed RC can reduce the input current ripple significantly and suppress the dc-link voltage ripple within the predetermined range and thereby achieves ZVS. Experimental results demonstrate that the proposed control scheme achieves desirable performance.

Effects and Mitigation of Nonzero DCM in Buck-Boost-Derived Hybrid Converter

Hybrid converter having ac and dc outputs, being single stage converter, is an interesting solution for dc nanogrid application. It offers independent control to both ac and dc output. The Buck-Boost-derived hybrid converter (BBDHC) proposed in this paper is capable of providing a wide range of buck/boost output voltage. For better understanding, a theoretical analysis of the converter in continuous current mode (CCM) and discontinuous current mode (DCM) is discussed. Unlike the conventional zero valued DCM, nonzero DCM (NZ-DCM) is a peculiar phenomenon observed in this converter. The NZ-DCM injects ripple in dc link voltage, deteriorating the performance and operation of BBDHC. To mitigate this, modified BBDHC (MBBDHC) is proposed. In MBBDHC, the NZ-DCM is replaced by forced CCM (FCCM). This modification permits the stable operation of MBBDHC under hybrid and standalone loads. Moreover, closed-loop study of MBBDHC is presented under load and voltage variations.

Variable Switching Frequency PWM Strategy of Two-Level Rectifier for DC-Link Voltage Ripple Control

The switching frequency is an important control parameter of pulse-width-modulation (PWM) rectifier to reduce switching losses and electromagnetic interference noise. This paper proposed a variable switching frequency PWM (VSFPWM) strategy for dc-link voltage ripple control in two-level rectifier. DC-link voltage ripple is determined by the dc-link current directly, and can be predicted synchronously with PWM signals. A real-time prediction model of dc-link voltage ripple is derived for a common voltage-oriented control PWM rectifier. Then, VSFPWM control is introduced, which changes the switching frequency cycle to cycle with a restriction of dc-link voltage ripple peak value. Furthermore, the dynamic behavior is also observed when the proposed VSFPWM control scheme is adopted. Detail simulation and experimental comparisons are carried out between VSFPWM and normal constant switching frequency PWM, which demonstrate the advantages of the proposed method.

Optimal Control of a Compact Converter in an AC Microgrid

This paper presents an optimal control method for a compact reduced switch count AC/AC converter in an AC microgrid. The AC/AC converter is adopted to interconnect dual three-phase renewable energy sources (RESs) and a three-phase permanent magnet synchronous motor (PMSM) to the grid to form an example of a mixed grid-feeding-drive system. For the adopted converter, a generalized pulse width modulation (PWM) scheme employing the third harmonic injection and a control method are proposed. Moreover, to achieve reduced dc link voltage ripple, good reference tracking and disturbance rejection, the gains of the controllers are optimized by minimizing a weighted sum of different objective functions. Each objective function represents a specific aspect to be minimized and the optimization problem is solved employing particle swarm optimization (PSO) method, while ensuring that the total harmonic distortion (THD) of the current at the points of common coupling (PCC) are less than 5%. For this mixed grid-feeding-drive system, simulation results under fast transient are provided to demonstrate the applicability of the adopted converter in the AC microgrid, the effectiveness of the proposed PWM, and the proposed optimal control method.

DC-Link Current and Voltage Ripple Analysis Considering Antiparallel Diode Reverse Recovery in Voltage Source Inverters

In this paper, a method is proposed to investigate the dc-link current and voltage ripple calculations in voltage source inverters by considering the reverse recovery of the antiparallel diodes. The impact of the diode reverse recovery transient on the dc-link current and voltage within the switching period is first analyzed. The analysis indicates that the current ripple rms value is affected by the value of inverter switching frequency, diode reverse recovery time, and current. However, the diode reverse recovery shows negligible impact on the dc-link voltage ripple calculation, since the difference between the voltage ripples calculated by the proposed and existing method is less than 1%. To evaluate the estimation accuracy of the dc-link current ripple rms value by the proposed method, the experiments are implemented and the comprehensive analysis is done under different operating conditions, including various power factors, modulation indexes, inverter output currents, and inverter switching frequencies. Experimental results show that the accuracy of dc-link rms current ripple calculation by the proposed method is in the range of $\pm$ 5% and increased by up to 7% compared with the existing method neglecting the antiparallel diode reverse recovery.

Development of DC to Single-Phase AC Voltage Source Inverter With Active Power Decoupling Based on Flying Capacitor DC/DC Converter

In the present, a power decoupling method without additional component is proposed for a dc to single-phase ac converter, which consists of a flying capacitor dc/dc converter (FCC) and the voltage source inverter (VSI). In particular, a small flying capacitor in the FCC is used for both a boost operation and a double-line-frequency power ripple reduction. Thus, the dc-link capacitor value can be minimized in order to avoid the use of a large electrolytic capacitor. In addition, component design, of, e.g., the boost inductor and the flying capacitor, is clarified when the proposed control is applied. Experiments were carried out using a 1.5-kW prototype in order to verify the validity of the proposed control. The experimental results revealed that the use of the proposed control reduced the dc-link voltage ripple by 74.5%, and the total harmonic distortion (THD) of the inverter output current was less than 5%. Moreover, a maximum system efficiency of 95.4% was achieved at a load of 1.1 kW. Finally, the high power density design is evaluated by the Pareto front optimization. The power densities of three power decoupling topologies, such as a boost topology, a buck topology, and the proposed topology are compared. As a result, the proposed topology achieves the highest power density (5.3 kW/dm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) among the topologies considered herein.

Mitigation of Current Distortion in a Three-Phase Microinverter With Phase Skipping Using a Synchronous Sampling DC-Link Voltage Control

To improve light load efficiency, a three-phase photovoltaic (PV)-based microinverter may operate in single-phase, two-phase or three-phase mode depending on the available input power. Therefore, its dc-link capacitor may be subjected to large peak-to-peak voltage ripple when operating in phase skipping mode. This paper determines the minimum value of the dc-link capacitor by allowing the maximum acceptable voltage ripple on the dc link. This allows for the use of film capacitors instead of electrolytic capacitors in order to improve the microinverter reliability. Unfortunately, higher dc-link voltage ripple introduces harmonic distortion on the inverter output current waveform if it is not compensated for by the dc-link voltage controller. This paper proposes a simple synchronous control method to regulate the dc-link voltage with the presence of large voltage ripple. The dc-link voltage controller is synchronized with the phase-locked loop and samples the average value of dc-link voltage without introducing unwanted harmonics into the voltage sense path. Experimental results were obtained from a 300-W three-phase half-bridge microinverter prototype to verify the proposed dc-link voltage control performance. Inverter output current total harmonic distortion of 1.6% was achieved even though the dc-link peak-to-peak voltage ripple reached as high as 87 V with 219-V split dc-link voltage. Dynamic response is fast enough to easily track the most extreme ramp change in input PV power.

Circulating current suppression with improved DC‐link power quality for modular multilevel converter

Despite having more advantages, a modular multilevel converter (MMC) has a circulating current as one of its main drawbacks that augments arm current greatly. However, circulating current suppression (CCS) of the conventional MMC will deteriorate the power quality of direct current (DC)-link voltage. This study proposes the CCS by applying the three-winding transformer and a DC-alternating current CCS inverter. The mathematical models of the proposed MMC and circulating current are analysed in detail. The parameter setting of the three-winding transformer is also discussed and is set to have an equivalent arm inductance of the conventional MMC to make comparisons. The comparing results validate that the proposed MMC can exhibit much better performance under steady and dynamic conditions. The significant DC-link voltage ripple that exists in the conventional MMC does not appear. The mean-square-error values of DC-link voltage have more than 90 per cent improvement under steady and dynamic conditions. Due to this contribution, the proposed MMC can provide better DC-link power quality.

High-Voltage Hybrid Generator and Conversion System for Wind Turbine Applications

This paper presents the design of a high-voltage hybrid generator (HG) and conversion system for wind turbine applications. The HG combines wound field (WF) and permanent magnet (PM) rotor excitations. At any given speed, the PM induces a fixed stator voltage, while the WF induces a variable controlled stator voltage. The HG alternating output is rectified via a passive rectification stage; hence, the machine net dc output voltage is controlled over a prescribed, but a limited range. The split ratio between PM and WF rotor sections is considered as varying from a fully WF rotor, or traditional synchronous generator, to some ratio of PM to WF excitation. The turbine operational characteristics and maximum wind velocity variations between turbines in a wind farm are used to define the WF to PM split ratio. Both a three-phase and a nine-phase stator winding design are investigated. The nine-phase winding results in 4.2% higher output RMS voltage that yields a more power dense solution. It further yields lower rectified dc-link voltage ripple. The HG mass, loss audits, and efficiency discussions are presented. In order to investigate the feasibility of the HG concept, a small scale laboratory prototype is designed, and operational test results presented that show good agreement with the simulation model results.

Analysis of dc-Link Voltage Switching Ripple in Three-Phase PWM Inverters

The three-phase voltage source inverter (VSI) is de facto standard in power conversion systems. To realize high power density systems, one of the items to be correctly addressed is the design and selection of the dc-link capacitor in relation to the voltage switching ripple. In this paper, effective formulas for designing the dc-link capacitor as a function of the switching voltage ripple amplitude are obtained, considering the operating conditions such as the modulation index and the output current amplitude. The calculations are obtained considering the requirements and restrictions referring to the high (switching)-frequency dc-link voltage ripple component. Analyses have been performed considering the dc source impedance (non-ideal dc voltage source at the switching frequency) and a balanced load. Analytical expressions are derived for the dc-link voltage switching ripple amplitude and its maximum value over the fundamental period. Different values of modulation index and output phase angle have been considered and different diagrams are presented. Analytical results were validated both by simulations and comprehensive experimental tests.

A Reduced Switch Count Three-Phase AC/AC Converter With Six Power Switches: Modeling, Analysis, and Control

Reducing the number of semiconductor switches in power electronics converters has been a continuing effort in recent years as a measure to enhance the system reliability and to decrease its size, weight, and component cost. For these reasons, a new reduced switch count three-phase ac/ac converter is being proposed. Being realized by only six active switches and antiparallel diodes, the proposed converter topology employs the minimum number of semiconductor devices amongst the converters of its kind. It also features unity power factor, regenerative operation, pulsewidth-modulated output voltage, and sinusoidal input current. The reduced number of switches results in a simplified associated gate drive circuit as well as cooling system which, in turn, may reduce the overall manufacturing cost and increase the reliability, especially in low-voltage and low-power applications. The modulation scheme of the new converter is developed, and a control algorithm is proposed for the converter’s rectifier side. Moreover, an analysis is performed on the dc-link capacitor sizing for the purposes of reducing the dc-link voltage ripple, and balancing the input current and lowering its total harmonic distortion. The simulation and experimental results corroborate the transient and steady-state performance of the proposed converter topology.

Theoretical and Experimental Investigation of Switching Ripple in the DC-Link Voltage of Single-Phase H-Bridge PWM Inverters

Direct current (DC)-link voltage ripple analysis is essential for determining harmonic noise and for DC-link capacitor design and selection in single-phase pulse-width modulation (PWM) inverters. This paper provides an extensive theoretical analysis of DC-link voltage ripple for full-bridge (H-bridge) inverters, with simulation and experimental verifications, considering a DC source impedance (non-ideal DC voltage source). The DC voltage ripple amplitude is theoretically estimated as a function of the output current, both amplitude and phase angle, and the modulation index. It consists of a switching frequency component and a double-fundamental frequency component (i.e., 100 Hz), thereby both components are considered in the analysis. In particular, the peak-to-peak distribution, maximum amplitude, and root mean square (RMS) values of the voltage switching ripple over the fundamental period are obtained. Based on the DC voltage requirements, simple and effective guidelines for designing DC-link capacitors are obtained.

Power quality control of voltage source converter-based HVDC system using particle swarm optimisation

Power quality control of voltage source converter (VSC)-based high voltage direct current (HVDC) transmission system is proposed in this paper. The particle swarm optimisation (PSO) technique is used for selection of optimum controller gain which results quick response to various transient conditions such as load perturbation, voltage sag, nonlinear load, and tapped load fault at points-of-common coupling (PCC). The conventional PSO approach is modified to get various power quality indices such as harmonic distortion, power factor, and DC-link voltage ripple within the limits for an 18-pulse VSC-based HVDC system. The effectiveness of proposed PSO approach is compared with conventional PI tuning, i.e., Zeigler-Nichols approach. The complete design and optimisation process are presented using illustrations, equations and flow charts in a simplified manner so as to provide a good support for readers.

Power quality control of voltage source converter-based HVDC system using particle swarm optimisation

Power quality control of voltage source converter (VSC)-based high voltage direct current (HVDC) transmission system is proposed in this paper. The particle swarm optimisation (PSO) technique is used for selection of optimum controller gain which results quick response to various transient conditions such as load perturbation, voltage sag, nonlinear load, and tapped load fault at points-of-common coupling (PCC). The conventional PSO approach is modified to get various power quality indices such as harmonic distortion, power factor, and DC-link voltage ripple within the limits for an 18-pulse VSC-based HVDC system. The effectiveness of proposed PSO approach is compared with conventional PI tuning, i.e., Zeigler-Nichols approach. The complete design and optimisation process are presented using illustrations, equations and flow charts in a simplified manner so as to provide a good support for readers.

Peak Current Limitation for Grid Side Inverter by Limited Active Power in PMSG-Based Wind Turbines During Different Grid Faults

Grid faults constitute the main challenge for distributed generation, especially permanent magnet synchronous generator (PMSG)-based wind turbines connected to the grid via back-to-back (BTB) converters. On the other hand, grid codes enforced wind turbines to aid network and accomplish low-voltage ride-through (LVRT) requirements during grid faults. DC-link overvoltage suppression without external devices and reactive current injection to the grid in symmetrical and asymmetrical faults were the main required subjects in different grid codes. Reactive current injection may lead to overcurrent in one or two phases during asymmetrical faults. This paper presents solutions to accomplish LVRT requirements during grid faults, including dc-link overvoltage suppression by improving BTB converter's controllers and the design of active power limitation to maintain the peak current of the grid side inverter in a safe range during different asymmetrical grid faults. In fact, by controlling generation of PMSG active power directly with grid side inverter, the peak current remained in the safe limit and required reactive current, including negative-sequence component can be injected to the grid. Furthermore, the grid side active power oscillation and dc-link voltage ripple can be suppressed by using the proposed controller. Simulation results showed the validity and efficiently of the proposed control approach in different conditions.

MOPSO-based predictive control strategy for efficient operation of sensorless vector-controlled fuel cell electric vehicle induction motor drives

This paper introduces an optimal control strategy of model-based predictive control (MPC) based on multiobjective particle swarm optimization (MOPSO) for a sensorless vector control induction motor, which is used in a fuel cell electric vehicle drive system. The proposed MPC-MOPSO algorithm is implemented to tune the weighting parameters of the MPC controller to tackle all the conflicting objective functions. The paper handles the following fitness functions: minimizing the speed error, minimizing the torque ripple, minimizing the DC-link voltage ripple, and minimizing machine flux ripple. Computer simulations studies have been completed utilizing MATLAB/Simulink with a specific end goal of assessing the dynamic performance of the proposed MPC-MOPSO optimal controller and comparing it with single-objective particle swarm optimization and traditional PI controllers. The simulation results demonstrate the good dynamic response of the proposed MPC-MOPSO optimal tuning strategy over the traditional PI controllers for more accurate tracking performance through the whole speed range, especially at starting conditions and load change disturbances.

Hybrid Single-Phase AC–AC Modular Multilevel DSCC Converters With Modulation and DC-Link Voltage Ripple Improvement

This paper proposes two hybrid topologies based on a single ac–ac modular multilevel cascade cascade converter (MMCC) with double-star chopper cells (DSCC), in each of which a voltage reference technique (VRT) is associated for generating the output modulating references. In doing so, either the grid-connected leg or the shared leg can be switched at the line frequency, thus reducing the switching losses. For proper operation of the VRTs in real-time, the input and output converter voltages must be in sync, hence a synchronization technique is proposed. Furthermore, when no dc-link capacitor is used in a DSCC converter, the dc-link is subjected to voltage ripples derived from the pulsewidth modulation (PWM) technique. In that case, high-frequency PWM techniques can produce high-frequency components across the arm inductors, which are reflected in the dc-link. Therefore, this paper focuses on the operability of the hybrid modular multilevel DSCC converters, but also on the investigation of the proposed VRTs in reducing the dc-link voltage ripple on the conventional DSCC converter using PWM with phase-shifted carriers (PSCs). Simulated and experimental results are presented to validate the proposed techniques and converters.

Lifetime Monitoring of Electrolytic Capacitor to Maximize Earnings From Grid-Feeding PV System

Electrolytic capacitors are popularly used in single-phase grid-feeding solar photovoltaic (PV) inverters to suppress the second harmonic and switching frequency voltage ripples. With aging equivalent series resistance (ESR) of capacitor increases and its capacitance value decreases, which lead to increase in dc-link voltage ripple. Oscillations in PV operating point around its maximum power point (MPP), results in reduction of average output power and revenue generated. To address this, frequent replacement of capacitors are required, which may lead to increased cost. Therefore, capacitors must be replaced at an optimal period to ensure maximum earnings. To realize this, a technique for monitoring of power extraction efficiency (PEE) is proposed in this paper. Further, criteria for replacement of capacitor based on the measured values of PEE is suggested. Mathematical model relating the capacitance and ESR values to the PEE is derived. Effect of variation in temperature and solar radiation on PEE is discussed. Detailed simulation studies are carried out using MATLAB-Simulink. A scaled down laboratory prototype of inverter is developed. The proposed technique is implemented in the existing digital processor/controller used for MPP tracking, thereby avoiding additional circuits/sensors. PEE estimated by simulation and experimentation are found to be within 1% of each other.