AC Voltage Magnitude Research Articles

Transformerless Inverter with Charge Pump Circuit Concept for PV Application

This paper introduces a new single phase transformerless photovoltaic (PV) inverter for grid connected PV systems using the concept of a charge pump. It consists of four power switches, two diodes, two capacitors and an LC filter at the output stage. The neutral of the grid is directly connected to the negative terminal of PV panel. This common ground creates a constant Common Mode (CM) voltage and zero leakage current. Meanwhile, the charge pump circuit generates the negative bus voltage for the inverter during the negative power cycle. A unipolar Sinusoidal Pulse Width Modulation (SPWM) technique is applied to the proposed inverter with three level output voltage to reduce the output current ripple and the filter requirements. The main advantages of the proposed inverter are (1) the negative polarity of the PV is directly connected to the grid, so the leakage current eliminated, (2) it has a compact size, (3) peak of output voltage is equal to input dc voltage (unlike Neutral Point Clamped (NPC) inverter, Active NPC (ANPC) and some topologies, which requires two times of the peak ac-voltage magnitude), flexible grounding configuration and high efficiency. The operating principle and analysis of the proposed topology are presented in details. Experimental results of a 500 W prototype are demonstrated to validate the proposed topology and the overall concept. The obtained results clearly verify the performance of the proposed inverter and its practical application for grid connected PV systems.

A Probabilistic Framework for the Robust Stability and Performance Analysis of Grid-Tied Voltage Source Converters

This paper proposed a probabilistic framework that could be used for the sensitivity assessment of grid-connected voltage source converters (VSCs), where uncertainties in the grid short circuit ratio (SCR) and operating point conditions, as well as control-loop interactions, were considered. The proposed method tried to broaden the available knowledge on the small-signal stability analysis of VSCs and provide a probabilistic point of view of this subject. It considered the probability of different operational conditions in order to obtain less conservatism and more accurate results. Based on uncertain inputs and the employed stability model, the proposed model produced statistical distributions of the critical mode and its damping factor and ratio, which were not accessible by existing deterministic methods. Crucial statistical information measures how much system stability and performance are maintained or changed over the system uncertainties and disturbances, as well as provides a clear insight into the system stability problem. For instance, as concluded in this paper, for the conventional control system design, fast dynamic parts of a VSC, such as the current controller and control delay, significantly impact the minimum damping ratio. Furthermore, slow dynamic parts, such as outer voltage control loops and the synchronization block, influence the maximum damping factor. For strong grids, the AC voltage magnitude controller (AVC) significantly impacts the maximum damping factor due to its lower bandwidth among all control loops. For weak grids, the damping factor of the critical mode is highly affected by interactions between the VSC, the power grid, and different control loops due to the synchronization mechanism. The other contributions of this paper were the introduction of robust stability and performance definitions and indices; explanations of the pros and cons of probabilistic assessment methods and their applicability; interpretation of the obtained results; and, finally, a link was provided between system stability and reliability, which will be crucial for future power system design.

A multiplexing alternate arm multilevel converter with high submodule utilization

The miniaturization requirement of the modular multilevel converter (MMC) has drawn great attention in the applications, e.g. distant offshore wind power integration system and flexible grid interconnection. Based on the alternate arm multilevel converter (AAMC), this paper proposes a multiplexing AAMC (M-AAMC) topology, which can reduce the number of submodules (SMs) significantly. The stacks of SMs in the middle arms of the proposed AAMC are drawn out to operate at time-division multiplexing modes by controlling the states of the additional arm selection switches. In this way, the M-AAMC can achieve the compact design of MMC with a 75% submodules (SMs) reduction. And the phase-shift switching (PSS) scheme is applied to the multiplexing conduction modes for energy balance of the proposed converter stacks, which permits the M-AAMC to operate at wide-range AC voltage magnitude variations. Finally, the simulation results within the MATLAB/Simulink have verified the feasibility of the proposed M-AAMC topology.

Analysis of bidirectional 15 MW current source DC/DC converter for series-connected superconducting-based 1 GW/100 kV offshore wind farm

A wind farm with series dc collection can potentially eliminate the costly offshore platform and its step-up components. Multiple series strings can be connected in parallel to extend the power capacity for a given voltage. In such arrangement, the DC/DC converters for individual generators are rated at only a fraction of the transmission voltage. The maturity of superconducting cable technology will eliminate the difficulty of transmission losses with high current low voltage systems. This paper presents a feasibility study on the DC/DC converter design for a parallel-series-connected wind farm. The isolated front-to-front (F2F) mixed voltage source converter (VSC) and current source converter (CSC) is examined with power back-feeding capability during wind calm periods for a 15 MW generator. A novel configuration using 3-level (3L) VSC and thyristor-based CSC with combinational control of AC voltage magnitude, frequency, and thyristor delay angle is proposed and evaluated for a 1 GW/100 kV parallel/series wind farm based on the superconducting cable. Detailed converter model including power loss estimation in PSCAD is presented to support the converter design recommendations.

Space-charge induced particle reflection between hybrid AC/DC biased electrodes

Space-charge limited current flow between DC biased electrodes is a widely applicable problem in many areas of physics. Recently, radio-frequency biasing, together with DC self-bias formation, has been studied as a new concept for the extraction of charged particles from an upstream plasma source. Here, we compare particle extraction between systems using this hybrid AC/DC biasing, with conventional DC biased electrodes, and identify important similarity parameters. The injection current first leading to particle reflection strongly depends on the applied AC frequency and voltage magnitude, as well as the initial particle injection velocity, and is in general lower than the DC case. For injection currents above the AC limit, the system becomes unstable, and self-excited space-charge oscillations are generated. A critical parameter is the ratio of the average particle transit time between the electrodes to the AC period, γ = t L/T. As long as γ ≫ 1, the onset of particle reflection can be sufficiently delayed that the extracted current approaches the DC limit.

Two-Level Grid-Side Converter-Based STATCOM and Shunt Active Power Filter of Variable-Speed DFIG Wind Turbine-Based WECS Using SVM for Terminal Voltage

Regulation of the terminal voltage of doubly fed induction generator (DFIG)-based wind energy conversion system (WECS) by modifying the control scheme of the grid side converter (GSC) is necessary compared to connecting alone flexible AC transmission systems (FACTS) and active power filter devices from economic point of view. An assessment study of GSC control strategy based both of static synchronous compensator (STATCOM) and shunt active power filter (SAPF) is introduced in this paper. STATCOM based AC voltage magnitude regulator and SAPF-based load calculations are controlled using space vector modulation (SVM) technique to maintain the terminal voltage of DFIG wind turbine constant when a grid disturbance happened. The system under study is modeled in MATLAB/Simulink environment. The simulation results under wind speed variation demonstrated that GSC-based STATCOM gives better results in regulation of the terminal voltage of DFIG compared to GSC-based SAPF and enhancement of the stability of this system.

A Reduced Arm DC–AC Modular Multilevel Converter Topology

ABSTRACT This paper presents a new DC to AC Modular Multilevel Converter (MMC) topology. The proposed topology requires lesser number of arms than the traditional MMC to generate equal magnitude and excellent quality of output AC voltage. The positive arm of the proposed converter is having submodules similar to traditional MMC. In contrast, passive tuned LC filters are present in the place of the negative arms in the proposed MMC topology. This helps to generate an equal magnitude of output AC voltage as traditional MMC with half of the input DC bus voltage magnitude. Additionally, traditional MMC requires large number of submodules to generate many voltage levels and hence create small steps in the output voltage matching close to a sine wave. The passive tuned LC filters in the proposed circuit topology are designed to absorb the switching harmonics generated from the positive arms. This makes the output AC voltage very close to a pure sine wave, with highly reduced higher order switching harmonics. This paper explains the operational principle along with the pre-charging of the submodule capacitors at start-up of the new converter. Simulation and experimental studies confirm the operation of the new converter topology.

A Consensus-Based Secondary Control Strategy for Hybrid AC/DC Microgrids With Experimental Validation

Secondary control strategies in hybrid ac/dc microgrids have been conventionally designed independently for each side of the microgrid, neglecting the interaction between the ac and dc sides due to the power transferred through the interlinking converter. This has a negative effect on the power sharing and dynamic response of the system. In this article, a novel distributed secondary control strategy for hybrid ac/dc-microgrids is presented which coordinates the control actions of the ac and dc sides. The proposed control scheme simultaneously regulates ac-voltage magnitude and frequency, as well as the dc-voltage magnitude, by including the interlinking converters in the control strategy via a distributed consensus approach. This improves the power-sharing accuracy and secondary control restoration of the variables in both ac and dc sides of the microgrid. Moreover, the proposed methodology avoids the circulating currents which are typically produced in systems with multiple interlinking converters. Several simulation and experimental tests, such as load impacts and plug-and-play capability, are presented to validate the performance of the proposed control strategy using a 24-kW hybrid ac/dc-microgrid laboratory prototype.

DQ Impedance Stability Analysis for the Power-Controlled Grid-Connected Inverter

For a grid-connected inverter requiring the ac voltage magnitude and the active power control, both vector control and power synchronization control can be applied. The stability comparison based on the dq impedance stability analysis between both control are carried out via three factors including the grid impedance, the inner current loop and the virtual impedance. The dq impedances of the inverter based on both control are derived. The determinant of the impedance ratio matrix is used for the stability analysis. The bode plot of the grid impedance and the inverter impedance are present to assist the stability analysis and explain their interactions. It is found that increasing the grid impedance and the cut-off frequency of the current loop stabilize the inverter with the power synchronization control, which is converse to the vector control. Furthermore, the inverter with the power synchronization control may suffer the instabilities when connecting to a strong grid. The virtual inductor and resistor are proposed to enhance the stability for the vector control and the power synchronization control respectively. The simulation validation using Matlab/Simulink is performed.

Design, Control, and Analysis of a Novel Grid-Interfaced Switched-Boost Dual T-Type Five-Level Inverter With Common-Ground Concept

Multilevel inverters with a common-ground structure and voltage step-up feature are beneficial for transformerless grid-connected photovoltaic applications. In this article, a novel topology of such converters with a five-level output voltage generation is presented. The proposed topology comprises of an integrated switched-boost (SB) module and a dual T-type (D2T) cell constructed with 10 power switches (six unidirectional and two bidirectional). An inductor in the integrated SB module is used to boost the voltage across two involved capacitors of the D2T cell. Therefore, a desirable ac voltage magnitude for the grid-connected application can be achieved over a wide range of input voltage changes. Current stress is also kept within a permissible input current range by the soft-charging operation of the involved capacitors through the inductor. A corresponding dead-beat continuous-current-control-set controller with a sinusoidal PWM modulator is implemented for controlling the real and reactive powers. The controller has an added benefit of achieving constant switching frequency for power switches. Circuit description, theoretical analyses, and comparative study are discussed. Simulation and experimental results are also presented to confirm the feasibility and correctness of the proposed topology.

Unified Modular State-Space Modeling of Grid-Connected Voltage-Source Converters

This article proposes a modular state-space modeling framework for grid-connected voltage-source converters, where the different control loops, including the ac current control, the phase-locked loop, the dc-link voltage control, and the ac voltage magnitude control, can be modeled separately as building blocks. Moreover, the mathematical relationship between state-space models in the rotating ( dq -) frame and the stationary ( αβ -) frame is explicitly established, and, thus, the modal analysis can be performed directly in the αβ -frame, which allows intuitive interpretation of voltage and current oscillation modes in the αβ -frame. Experimental tests of a 3-kW back-to-back converter system validate the effectiveness of the unified modular state-space modeling and analysis.

Frequency Control of Offshore Wind Farm With Diode-Rectifier-based HVdc Connection

This paper presents a direct frequency control for offshore wind farms (OWFs) connected to the diode-rectifier-based (DR-based) high voltage direct current (HVDC) link. The frequency control is guaranteed through reactive power balance at the DR station without a capacitor bank placed at the DR station; while AC-voltage magnitude is clamped by the DR, provided that the HVDC link voltage is imposed by the onshore inverter. The control system is implemented by a voltage source converter (VSC) connected to the DR station. The VSC is also used to compensate harmonic currents, leading to passive AC-filters being also removed from the DR station. The proposed control system provides comparative advantages in terms of DR station volume and OWF installation cost with other centralized controls where besides a VSC, a high-voltage large capacitor bank is needed to be placed at the DR station. A new average-value model (AVM) of the DR system is derived to test the proposed frequency control, while the stability of the system is evaluated through a small-signal analysis. The proposed control system performance and the accuracy of the proposed AVM are verified through a detailed switching model of the system using operation of a cascaded H-bridge VSC.

Bifurcation and Chaos in Size-Dependent NEMS Considering Surface Energy Effect and Intermolecular Interactions

The impetus of this study is to investigate the chaotic behavior of a size-dependent nano-beam with double-sided electrostatic actuation, incorporating surface energy effect and intermolecular interactions. The geometrically nonlinear beam model is based on Euler-Bernoulli beam assumption. The influence of the small-scale and the surface energy effect are modeled by implementing the consistent couple stress theory proposed by Hadjesfandiari and Dargush together with Gurtin-Murdoch elasticity theory. The governing differential equation of motion is derived using Hamilton’s principle and discretized to a set of nonlinear ODE through Galerkin’s method. Nonlinearities stemmed from different sources such as mid-plane stretching, electrostatic and interatomic forces lead to an intensive nonlinear dynamics in nano-electro-mechanical devices so that the systems exhibit rich dynamic behavior such as periodic and chaotic motions. Poincare portrait is utilized in order to present the system dynamic response in discrete state-space. Bifurcation analysis has been performed with a change in the magnitude of AC voltage corresponding to the various values of DC voltage and excitation frequency. Then, we compare some ranges of AC voltage amplitude, in which the system response becomes stable for these cases. Fast Fourier transformation is also carried out to analyze the frequency content of the system response.

Consecutive Commutation Failure Mitigation Caused by AC Filters Switching

AC filter bank switching in the HVDC inverter station may cause harmonics which leads to commutation failure. This paper analysis the characteristics of AC filter switching and build the transient progress circuit and mathematics model. And the time-response solution of AC filter overvoltage is acquired, the AC voltage magnitude decrease and distortion can be calculated. Commutation caused by these reasons was analyzed. And a consecutive commutation failure mitigation method is proposed. The algorithm was verified in the PSCAD/EMTDC.

Cell transport and suspension in high conductivity electrothermal flow with negative dielectrophoresis by immersed boundary-lattice Boltzmann method

The cell transport and suspension using AC electrokinetics is essential for cell patterning and other biomedical applications in microfluidics. To avoid the undue cellular stress and irreversible damage to cells caused by low conductivity media, direct manipulations of cells in physiological solution of high electrical conductivity without dilution becomes significant. The driving mechanism of alternating current electrothermal (ACET) flow makes it attractive for pumping the physiological conductivity solution and transporting cells through the electrohydrodynamic (EHD) force. In addition, negative dielectrophoresis (nDEP) force is induced on a cell when its electrical conductivity is lower than that of solution media. In this paper, the effectiveness of ACET flow and negative DEP force in high conductivity solution is novelly used simultaneously to achieve a successful long-range cell transport and suspension in the microfluidic chamber. An immersed boundary-lattice Boltzmann method (IB-LBM) is developed to investigate the cell transport and suspension mechanism with respect to AC voltage magnitude, electrical conductivities of cell and solution, cell initial position, and cell size. It is found that a sufficient DEP force is indispensable for stabilizing the cell transport process and anchoring cells by overcoming the cell-cell interaction. Based on this, the design of a lab-on-a-chip device to generate a large DEP force is essential for future research to realize an efficient AC electrokinetic-based cell transport and suspension in physiological fluids.

Analysis of DC Fault for Dual-Active Bridge DC/DC Converter Including Prototype Verification

This paper presents dc fault analysis for a dual-active bridge (DAB) dc/dc converter which comprises two active bridges and an internal medium frequency transformer. This topology provides galvanic isolation, voltage step up/down and bidirectional power transfer. The dc fault study assumes the dc terminal voltage of the converter is at zero. The steady-state fault current is limited to a low magnitude which is less than the rated value without any controller action depending on the design of the converter. The dc fault current magnitudes are analyzed with ac equivalent circuit where only the fundamental component of the ac voltages and inductor current are considered. Phase shift and ac voltage magnitude modulation control methods are selected. A detailed DAB dc/dc converter 3-MW, 4-/40-kV-based system is simulated using MATLAB/Simulink to validate the proposed analytical study. Further, hardware testing is conducted to confirm the dc fault studies with a 500-W 24-/100-V prototype.

Cell capacitor sizing in multilevel converters: cases of the modular multilevel converter and alternate arm converter

Multilevel converters, such as the modular multilevel converter (MMC) or the alternate arm converter (AAC), rely on charged capacitors in their cells to generate their AC voltage waveform. Since the cell capacitors are physically large and occupy approximately half the cell volume, their capacitance must be kept minimal while limiting the voltage fluctuation caused by the current passing periodically through these capacitors. This study proposes a mathematical model which estimates the energy deviation for the stacks of both the MMC and the AAC during steady-state operation under any power factor and for AC voltage magnitude fluctuation of up to ±10%. The analysis is then used to calculate the minimum size for the cell capacitors in order to keep their voltage fluctuation within set boundaries for both topologies. The results show that the MMC requires 39 kJ/MVA of capacitive energy storage under sinusoidal modulation but this reduces with triplen injection modulation. The AAC has a lower requirement for storage in its cells of 11 kJ/MVA but the AAC has a six-pulse DC current ripple which requires a filter estimated to have a further 33% capacitive storage.

Study of flow fields induced by surface dielectric barrier discharge actuator in low-pressure air

Surface dielectric barrier discharge (SDBD) is a promising method for a flow control. Flow fields induced by a SDBD actuator driven by the ac voltage in static air at low pressures varying from 1.0 to 27.7 kPa are measured by the particle image velocimetry method. The influence of the applied ac voltage frequency and magnitude on the induced flow fields is studied. The results show that three different classes of flow fields (wall jet flow field, complex flow field, and vortex-shape flow field) can be induced by the SDBD actuator in the low-pressure air. Among them, the wall jet flow field is the same as the tangential jet at atmospheric pressure, which is, together with the vertical jet, the complex flow field. The vortex-shape flow field is composed of one vertical jet which points towards the wall and two opposite tangential jets. The complex and the vortex-shape flow fields can be transformed to the wall jet flow field when the applied ac voltage frequency and magnitude are changed. It is found that the discharge power consumption increases initially, decreases, and then increases again at the same applied ac voltage magnitude when the air pressure decreases. The tangential velocity of the wall jet flow field increases when the air pressure decreases. It is however opposite for the complex flow field. The variation of the applied ac voltage frequency influences differently three different flow fields. When the applied ac voltage magnitude increases at the same applied ac voltage frequency, the maximal jet velocity increases, while the power efficiency increases only initially and then decreases again. The discharge power shows either linear or exponential dependences on the applied ac voltage magnitude.

고성능 DCM-ZVS 스텝 업-다운 AC-DC 컨버터에 관한 연구

본 논문에서는 고효율과 고역률을 위한 새로운 DCM-ZVS 스텝 업-다운 AC-DC 컨버터에 대해 연구된다. 제안한 DCM-ZVS 컨버터에 사용된 스위칭 소자들은 새로운 유사공진 회로에 의해 소프트 스위칭 기법으로 동작되고, 펄스폭변조(PWM) 방식에 의한 전류불연속모드(DCM)로 구동된다. 제안한 컨버터의 유사공진 회로는 스텝 업-다운 인덕터와 무손실 스너버 커패시터로 구성되고, 또한 DCM에 의해 제어회로 기법과 제어회로 구성이 간단한 장점이 주어진다. 입력 교류 전류파형은 일정 스위칭 주파수에 의한 교류 입력전압의 크기에 비례한 유사 정현파형으로 된다. 그 결과, 제안한 DCM-ZVS 컨버터는 낮은 스위칭 손실과 고효율을 얻을 수 있으며, 컨버터의 입력역률은 거의 단위역률로 된다. 해석적 결과의 타당성은 다양한 컴퓨터 시뮬레이션과 실험 결과를 통해 입증된다. This paper is studied on a new DCM-ZVS step up-down AC-DC converter of high performance, that is, high system efficiency and power factor correction (PFC). The switching devices in the proposed converter are operated by soft switching technique using a new quasi-resonant circuit, and are driven with discontinuous conduction mode (DCM) according to pulse width modulation (PWM). The quasi-resonant circuit uses a step up-down inductor and a loss-less snubber capacitor. The proposed converter with DCM also simplifies the requirement of control circuits and reduces the number of control components. The input AC current waveform in the proposed converter becomes a quasi-sinusoidal waveform proportional to the magnitude of input AC voltage under constant switching frequency. As a result, the proposed converter obtains low switching power loss and high efficiency, and its input power factor is nearly in unity. The validity of the analytical findings is confirmed by some computer simulation results and experimental results.

Bi‐directional flow induced by an AC electroosmotic micropump with DC voltage bias

This paper discusses the principle of biased alternating current electroosmosis (ACEO) and its application to move the bulk fluid in a microchannel, as an alternative to mechanical pumping methods. Previous EO-driven flow research has looked at the effect of electrode asymmetry and transverse traveling wave forms on the performance of electroosmotic pumps. This paper presents an analysis that was conducted to assess the effect of combining an AC signal with a DC (direct current) bias when generating the electric field needed to impart electroosmosis (EO) within a microchannel. The results presented here are numerical and experimental. The numerical results were generated through simulations performed using COMSOL 3.5a. Currently available theoretical models for EO flows were embedded in the software and solved numerically to evaluate the effects of channel geometry, frequency of excitation, electrode array geometry, and AC signal with a DC bias on the flow imparted on an electrically conducting fluid. Simulations of the ACEO flow driven by a constant magnitude of AC voltage over symmetric electrodes did not indicate relevant net flows. However, superimposing a DC signal over the AC signal on the same symmetric electrode array leads to a noticeable net forward flow. Moreover, changing the polarity of electrical signal creates a bi-directional flow on symmetrical electrode array. Experimental flow measurements were performed on several electrode array configurations. The mismatch between the numerical and experimental results revealed the limitations of the currently available models for the biased EO. However, they confirm that using a symmetric electrode array excited by an AC signal with a DC bias leads to a significant improvement in flow rates in comparison to the flow rates obtained in an asymmetric electrode array configuration excited just with an AC signal.