Power Pulsation Research Articles

Review of power decoupling methods for micro-inverters used in PV systems

In single-phase photovoltaic(PV) systems, power mismatch between input and output terminal leads to power pulsation, seriously affecting maximum power point tracking (MPPT). Paralleling an electrolytic capacitor(e-cap) across the PV panel to smooth power pulsation seems to be an unreliable method owing to its short lifespan. Various power decoupling methods used in micro-inverters have been proposed to solve the problem. These methods were categorized. Some of them were introduced in this paper regarding efficiency, cost and technical features. Special novel methods proposed in recent years were presented to show the development trend of micro-inverter. Then, conclusion and comparisons were made. Finally, a potential technological route was proposed by author for further study on power decoupling.

Design and Analysis of the Low Device Stress Active Power Decoupling for Single-Phase Grid Connection for a Wide Range of Power Factor

Single-phase power converters suffer from the double frequency power pulsation. In voltage source inverters, the dc bus capacitor is designed to supply this double frequency current. The stringent dc bus voltage ripple specifications call for larger capacitance. This implies larger electrolytic capacitors. However, the electrolytic capacitor suffers from short-term life span, which consequently brings down the lifetime of the converter. As an alternative, active power decoupling (APD) ensures significant reduction in dc buffering requirement, thereby reducing the bus capacitance. This can make use of the film capacitors with better lifespan. In this paper, an APD scheme is designed and developed for the grid-connected system. Another important feature of the proposed scheme is that, it reduces the stress on the switches significantly while compared with existing APD solutions for all load power factors. The system is analyzed further to arrive at APD control conditions including the effect of grid filter inductance. The simulation and experimentation have been done to validate the proposed method. Additionally, the stress on the switches as a function of grid filter inductance is predicted theoretically and verified practically. The average stress on the switches is shown to be lesser than the existing solutions.

A Power Decoupling Control Method for an Isolated Single-Phase AC-to-DC Converter Based on Direct AC-to-AC Converter Topology

This paper proposes a new power decoupling control method for a direct ac-to-ac converter. The ac-to-ac converter consists of two half-bridge converters, two input filter capacitors, and a series-resonant circuit, which enables it to convert the single-phase line-frequency ac input to the high-frequency ac output directly. The proposed power decoupling control method stores input power pulsation at double the line frequency in the input filter capacitors. Thus, the proposed control method realizes a unity power factor in the line-frequency ac input and a constant amplitude current in the high-frequency ac output without any additional switching device or energy storage element. This paper theoretically discusses the principle and operating performance of the proposed control method and confirms the effectiveness of the proposed control method in experiments using an isolated ac-to-dc converter based on the direct ac-to-ac converter. As a result, the proposed power decoupling control method effectively improved the displacement power factor of the line-frequency ac input current to more than 0.99 and reduced the voltage ripple in the dc load to 4%.

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.

Power Decoupling for Single-Phase PV System Using <inline-formula> <tex-math notation="LaTeX"> $\acute{\mathrm{C}}$</tex-math> </inline-formula>uk Derived Microinverter

This paper reports an elegant approach to address two major challenges of transformerless grid-connected microinverters, common-mode earth leakage current and double-frequency power pulsation across the photovoltaic (PV) terminal. The proposed solution uses an embedded decoupling capacitor in a doubly grounded boost–buck ( $\acute{\mathrm{C}}$ uk) derived topology. An intelligent switching scheme ensures that a large swing of the decoupling capacitor voltage is easily accommodated. Thus, film capacitors can be used instead of electrolytic capacitors, which helps increase equipment lifetime. Details of circuit operation are provided along with dynamic modeling and controller design for close-loop operation. Simulation and experimental results are presented for performance validation of the microinverter.

Quasi-Z-Source Three-to-Single-Phase Matrix Converter and Ripple Power Compensation Based on Model Predictive Control

A quasi-Z-source three-to-single-phase matrix converter is proposed with low-frequency ripple power compensation. The employed impedance network between the source grid and the matrix converter extends the voltage gain; meanwhile, it has filtering function. Therefore, voltage step-up devices and an additional input filter are avoided. In addition, for a conventional three-to-single-phase matrix converter, input current of the source grid contains high harmonic components because of double-line-frequency pulsating power produced in single-phase side. A model predictive control-based ripple power compensation system is proposed in this paper to eliminate the low-order harmonic components from the three-phase input currents and voltages. Thus, the impedance parameters could be small in size and value. Simulation and experimental results verify the analysis and control of the proposed topology.

Heat transfer in a tapered fluidized bed of biomass particles with pulsed gas flow

Bed-to-surface heat transfer of pure biomass particles in a pulsed fluidized bed with a tapered bottom section was investigated. Three biomass species — Douglas fir, pine, and switchgrass — were studied under various operating conditions. Their heat transfer coefficients were found to be closely associated with hydrodynamics dominated by gas pulsations. A higher superficial gas velocity generally yielded better gas–solid contact and higher heat transfer rates. A moderately increasing pulsation frequency promoted convective heat transfer of particles but also reduced pulsation intensity, leading to undesired flow behaviours such as channelling and partial defluidization. The study of the pulsation duty cycle revealed that, for cohesive particles, a smaller duty cycle was preferred to generate powerful pulsations to break up inter-particle forces. Moreover, a duty cycle increase allowed higher gas throughput as long as a suitable fluidization was maintained. The addition of finer particles to a coarse fraction increased particle mobility, and subsequently heat transfer, which also explained the higher heat transfer coefficients of switchgrass as it contained more fines compared with fir and pine. Experimental results in the tapered bed were also compared with those of non-tapered geometry where a 10%–20% increase in heat transfer was observed.

The features of the use of energy storage devices in the railway power supply system

The pulsating mode of the power consumption in the railroad power supply network is the cause of the occurrence of voltage pulsations in the contact network. This leads to a decrease in the efficiency of the energy system. The purpose of this work is to develop means to improve the quality of the electrical energy of the railroad power supply system and formulate recommendations for their use. To compensate power fluctuations in the electric power supply network of the railway, it is proposed to use hybrid electric energy storage devices. The use of two-channel additional traction power station with an appropriate control system was proposed. The basis of the mathematical model is the data of real measurements. To suppress the power pulsations in the power supply system of the railway, it was proposed to use two-channel additional traction power station. The paper presents DC-DC converter with CF and VF terminals. As a result, dynamic losses are reduced and higher operating frequencies could be achieved. The possibility to transfer power in both directions allows the topology to be applied in high power battery applications. The possibility of using a single low-pass filter for the generation of control signals for both low-frequency and high-frequency channels of system is proven. The use of effective methods of balancing voltage levels in accumulators’ stack makes it possible to reduce the power of losses in the converter by 10-15%

VLA radio observations of AR Scorpii

Aims. AR Scorpii is unique amongst known white dwarf binaries in showing powerful pulsations extending to radio frequencies. Here we aim to investigate the multi-frequency radio emission of AR Sco in detail, in order to constrain its origin and emission mechanisms. Methods. We present interferometric radio frequency imaging of AR Sco at 1.5, 5 and 9 GHz, analysing the total flux and polarization behaviour of this source at high time resolution (10, 3 and 3 s), across a full 3.6 h orbital period in each band. Results. We find strong modulation of the radio flux on the orbital period and the orbital sideband of the white dwarf’s spin period (also known as the “beat” period). This indicates that, like the optical flux, the radio flux arises predominantly from on or near the inner surface of the M-dwarf companion star. The beat-phase pulsations of AR Sco decrease in strength with decreasing frequency. They are strongest at 9 GHz and at an orbital phase ~0.5. Unlike the optical emission from this source, radio emission from AR Sco shows weak linear polarization but very strong circular polarization, reaching ~30% at an orbital phase ~0.8. We infer the probable existence of a non-relativistic cyclotron emission component, which dominates at low radio frequencies. Given the required magnetic fields, this also likely arises from on or near the M-dwarf.

One-Cycle Control for Electrolytic Capacitor-Less Second Harmonic Current Compensator

The input power of the single-phase power factor correction ac–dc converter pulsates at twice the line frequency, while the output power of the single-phase dc–ac inverter pulsates at twice the output frequency. The pulsating power will result in second harmonic current (SHC) in the ac–dc converter and dc–ac inverter. In this paper, electrolytic capacitor-less second harmonic current compensator (SHCC) is presented to compensate the SHC. The SHCC has two operating modes, namely, charging mode and discharging mode. To achieve an excellent SHC compensation performance, a hybrid one-cycle control (OCC) is proposed to regulate the port current of the SHCC, and the SHCC can stably operate in both the charging mode and discharging mode. To avoid the mode detection required in the hybrid OCC and ensure seamless transition between the two modes, the OCC with dc bias is further proposed. Besides, a peak voltage control is proposed to regulate the storage capacitor voltage in the SHCC for reducing the power losses of the SHCC at light load. A 1-kW two-stage inverter with the SHCC is fabricated and tested, and the experimental results are provided to verify the effectiveness of the proposed control schemes.

Optimal preview stator voltage‐oriented control of DFIG WECS

In this study, a new optimal preview control (OPC) is proposed for stator active and reactive power controls of a doubly fed induction generator (DFIG) wind energy conversion system (WECS). These powers are controlled by rotor currents to avoid the stator active and reactive power pulsations. This OP controller is designed based on linear quadratic regulator (LQR) approach. In view of efficient superior tracking and disturbance rejection ability, OPC is chosen to control active and reactive powers of WECS. The proposed OPC is designed in order to minimise the error due to cross-coupling-induced electromotive force (emf) and emf-induced due to stator flux using feedforward compensation loops. This controller is developed for both sub-synchronous and super-synchronous modes of DFIG with stator voltage-oriented control (SVOC). An augmented error system is derived for tracking the desired response (active and reactive power controls) of DFIG WECS. Then, the constraints in error state equation are minimised by selecting quadratic performance index in which weighing factors are used for achieving faster rotor current control dynamics. Finally, pulse width modulation switching scheme for voltage-source converters has been designed with outputs of rotor current control loop dynamics. Extensive simulations are conducted using MATLAB/Simulink and then experiments are carried out using OPAL-RT and hardware set-up developed in the laboratory for DFIG WECS for validating the efficacy of the proposed control algorithm. The performances of the proposed LQR-OPC are compared with that of sliding mode field-oriented control (SM-FOC) and direct torque control (SM-DTC). From the comparison, it is observed that the LQR-OPC algorithm with SVOC technique applied to DFIG WECS provides excellent steady-state stability and zero steady-state error in face of transient disturbances both on rotor side and grid side compared with traditional FOC and DTC methods.

Активное подавление гармоник во входном токе силового преобразователя электропривода компрессора

The article considers the problem connected with interference that may occur at certain frequencies between the current consumed by the underground train carriage compressor unit electric motor and the signals used in the collision avoidance system. In upgrading the power converter supplying power to the compressor’s induction motor, the input filters were made more compact for achieving more reliable operation and for reducing the cost. This modification entailed a significantly higher consumption of current at the 275 and 325 Hz frequencies, which are used in the collision avoidance system. The problem was analyzed, and a method for solving it was proposed. The proposed method implies continuous control of the power consumed by the compressor electric drive. As a result, the 25 Hz current harmonic component generated by the load torque becomes smaller, due to which the 300 Hz ripple generated by the rectifier at the traction substation does not interfere with the power pulsation associated with the compressor mechanical characteristics. Therefore, the 275 and 325 Hz current harmonic components become smaller and do not affect the performance of the collision avoidance system. The proposed control method has been implemented, and the obtained experimental results are presented.

A Primary Full-Integrated Active Filter Auxiliary Power Module in Electrified Vehicles With Single-Phase Onboard Chargers

In single-phase ac high-voltage (HV) battery chargers, as the input current is enforced to be varying sinusoidally in phase with the input voltage, the pulsating power at two times of the line frequency will be seen on the dc-link. Bulky capacitor bank or extra active filter (AF) circuits are needed to assimilate this harmonic current, which become a major barrier in terms of power density and cost. Sinusoidal charging method can be applied, while this might affect the charging efficiency and a deep study is still needed to further investigate on the impact to the Lithium-ion battery. An active filter auxiliary power module (AFAPM) based dual-mode dual-voltage charging system for vehicle application has been proposed. The AFAPM converter has two modes: the HV active filtering mode, in which the vehicle is connected to the grid and the converter assimilates the significant second-order harmonic current; and the low-voltage (LV) battery charging mode, in which the vehicle is running and the converter charges the LV battery from HV battery. However, a relay and inductors are still required in that converter to achieve the dual-mode operation. This paper proposes a primary full-integrated AFAPM for electrified vehicle applications with single-phase onboard chargers. The proposed AFAPM converter is composed of a two-phase bidirectional buck converter to work as an AF and a dual-active-bridge to operate as a LV battery charger auxiliary power module (APM). With the proposed converter, only an extra active energy storage capacitor is needed to achieve the active filtering. All the switches and inductors on the primary stage are shared between the AF and APM. Therefore, the use of a bulky capacitor bank or an additional AF circuit is avoided and, thus, the cost, size, and weight of the dual-voltage charging system in the electrified vehicle applications can be reduced. A 720-W prototype has been built to confirm the effectiveness of the proposed converter.

A 2-kW Single-Phase Seven-Level Flying Capacitor Multilevel Inverter With an Active Energy Buffer

High-efficiency and compact single-phase inverters are desirable in many applications such as solar energy harvesting and electric vehicle chargers. This paper presents a 2-kW, 60-Hz, 450-V $ _{\text{DC}}$ -to-240-V $_{\text{AC}}$ power inverter, designed and tested subject to the specifications of the Google/IEEE Little Box Challenge. The inverter features a seven-level flying capacitor multilevel converter, with low-voltage GaN switches operating at 120 kHz. The inverter also includes an active buffer for twice-line-frequency power pulsation decoupling, which reduces the required capacitance by a factor of 8 compared to conventional passive decoupling capacitors, while maintaining an efficiency above 99%. The inverter prototype is a self-contained box that achieves a high power density of 216 W/in $^3$ and a peak overall efficiency of 97.6%, while meeting the constraints including input current ripple, load transient, thermal, and FCC Class B EMC specifications.

Control of DFIG Wind Power Generators in Unbalanced Microgrids Based on Instantaneous Power Theory

This paper presents a model and a control strategy for a doubly-fed induction generator (DFIG) wind energy system in an unbalanced microgrid based on instantaneous power theory. The proposed model uses instantaneous real/reactive power components as the system state variables. In addition to the control of real/reactive powers, the controllers use the rotor-side converter for mitigating the torque and reactive power pulsations. The control scheme also uses the grid-side converter for partial compensation of unbalanced stator voltage. The main features of the proposed control method are its feedback variables are independent of reference frame transformations and it does not require sequential decomposition of current components. These features simplify the structure of required controllers under an unbalanced voltage condition and inherently improve the robustness of the controllers. A power limiting algorithm is also introduced to protect power converters against over rating and define the priority of real/reactive power references within the control scheme. The performance of the proposed strategy in reducing torque ripples and unbalanced stator voltage is investigated based on the time-domain simulation of a DFIG study system under unbalanced grid voltage.

A High Power Density Series-Stacked Energy Buffer for Power Pulsation Decoupling in Single-Phase Converters

A high-efficiency, high-power-density buffer architecture is proposed for power pulsation decoupling in power conversion between dc and single-phase ac. We present an active decoupling solution that yields improved efficiency and reduced circuit complexity compared to existing solutions. In the proposed architecture, the main energy storage capacitor is connected in series with an active buffer converter across the dc bus. The series-stacked capacitor blocks the majority of the dc bus voltage to reduce the voltage stress on the buffer converter, such that fast, low-voltage transistors can be employed for the buffer converter. Moreover, the series capacitor provides the majority of the power pulsation decoupling through a wide voltage swing, and the buffer converter only needs to process a small fraction of the total power of the entire architecture, allowing a very small active circuit volume and very high system efficiency. The circuit operation and design constraints are analyzed in detail. In the proposed buffer architecture, the series stacking of a nearly lossless capacitor and a lossy converter presents a challenge of capacitor voltage balancing and power loss compensation. We propose a control scheme exploiting the small ripple in the bus voltage and dc input current to compensate for the power loss in the buffer converter while maintaining the voltage balance. Light-load techniques are also introduced to ensure that the buffer architecture meets strict ripple requirements while providing sufficient loss compensation. A 2-kW hardware prototype based on low-voltage GaN switches has been built to demonstrate the performance of the proposed solution. A power density of 25 W/cm $^3$ (410 W/in $^3$ ) by rectangular box volume and an efficiency above 98.9% across a wide load range has been experimentally verified.

Improved Power Pulsation Suppression of DFIG for Wind Shear and Tower Shadow Effects

This paper analyzes and simulates wind shear and tower shadow effects in doubly-fed induction generators, which are known to induce a low-frequency oscillation. Such oscillation is called a 3p oscillation, which can cause shaft torsion load aggravation and shaft torque oscillation, it inevitably lead to electromagnetic torque and power fluctuation in the generator and further affect the whole wind power system. By analyzing a two-mass model, it can be known that a flexible drive train has a low-damping characteristic and an amplification of the input of low-frequency wind torque disturbances. Based on this point, this paper presents a new control scheme to restrain 3p oscillation is presented, which is a damping torque observation. A novel torque observer is designed to ascertain the required damping torque. A small-signal model is established for the system and used to analyze the system's static and dynamic characteristics. The effectiveness of the proposed strategy is finally verified by simulation and experimental results.

Investigation and Suppression of Current Zero Crossing Phenomenon for a Semicontrolled Open-Winding PMSG System

The semicontrolled open-winding PMSG system has been widely investigated due to its simplified topology and easy control implementation. Nevertheless, compared with the conventional controllable converter structure, the current zero crossing phenomenon will last a longer time in the semicontrolled configuration, which is determined by the phase displacement between voltage and current. Since a long current zero crossing duration will lead to voltage pulse disturbance, serious electromagnetic interference, and output power fluctuation, it is necessary to reduce the zero crossing duration to improve the open-winding PMSG performance. This paper gives a detailed analysis on the zero crossing phenomenon of the semicontrolled open-winding PMSG system in the 3-D space vector modulation perspective. Meanwhile, a third harmonic injection method is suggested to reduce the zero crossing duration. It is also demonstrated that the torque and power pulsation are effectively improved, while the zero-sequence current amplitude does not increase. A proportional resonant controller is used to regulate the desired zero-sequence current. The proposed method is validated in a 1-kW semicontrolled open-winding PMSG experimental system with $i_{d} = 0$ control method.

Front-End Isolated Quasi-Z-Source DC–DC Converter Modules in Series for High-Power Photovoltaic Systems—Part II: Control, Dynamic Model, and Downscaled Verification

This paper is the continuation of Part I in which a quasi-Z-source modular cascaded converter (qZS-MCC), comprising front-end isolated qZS half-bridge dc–dc converter submodules (SMs), for dc integration of high-power photovoltaic (PV) systems is proposed. The qZS-MCC-based PV system features modular structure, high-voltage dc collection of PV power, simple control with a unified and constant duty cycle for the front-end isolation converter of all SMs, and low qZS impedance due to no double-line-frequency pulsating power. Here, control scheme of the qZS-MCC PV system integrated into the dc collection grid is investigated. Dynamic model of the system is established for controllers design and time-domain transient simulation. Experimental tests are carried out on the downscaled prototype as a proof-of-concept of the proposed control and modeling, demonstrating the validity of the proposed approaches.

A High‐Efficiency Single‐Phase Utility Interactive Inverter with an Active Power Decoupling Function and its Control Method

SUMMARYThe present paper introduces a single‐phase utility interactive inverter with a power decoupling function. In a conventional single‐phase inverter, power pulsation at twice the grid frequency appears in the input power. Hence, electrolytic capacitors having large capacitances have been connected to the DC input terminal to stabilize the input DC‐bus voltage. Because the lifetime of the electrolytic capacitor is relatively shorter than that of another component, the lifetime of the inverter is affected by the capacitor. In order to prevent such a problem, a novel single‐phase inverter circuit with an active power decoupling function is introduced. The pulsating power on the input DC‐bus line and the pulsated energy on the input DC capacitor is transferred to the energy in a small film capacitor. Hence, the extension of the inverter lifetime can be expected by substituting a small film capacitor for the large electrolytic capacitors. In addition, the loss in the power decoupling circuit is very small; hence, the reduction in the overall conversion efficiency of the inverter can be minimized. The effectiveness of the proposed method is verified using a 300 W experimental setup.