DC Voltage Conditions Research Articles

Modified control approach for MPP tracking and DC bus voltage regulation in a hybrid standalone microgrid

This paper proposes modified MPPT control technique for hybrid standalone microgrid. Standalone PV based microgrid has emerged as potential solutions to electricity challenges in the region where grid is not available. The battery energy storage system (BESS) is integrated into the system to facilitate synchronized load control and power flow. Variation in solar irradiation, load demand and fluctuation in battery SOC results in DC voltage fluctuation and maximum power point tracking challenges. Hybrid PV-Battery systems therefore must require control algorithms that can both flexibly adjust power flows as well as stabilize bus voltages. Proposed control technique is successful in regulating DC bus voltage and balancing the power flow in system under various operating conditions. A bidirectional converter is employed to maintain the nominal DC voltage conditions by charging/discharging the battery due to intermittent PV generation. It regulates the voltage of the DC bus to the maximum power point of the PV array. The main goal of this research article is to implement modified MPPT technique which includes Incremental conductance algorithm with double closed loop controller to track maximum power and to regulate the DC bus voltage of the system under different type of dynamic and atmospheric conditions.

An improved double‐vector model predictive control strategy for four‐switch three‐phase inverter‐fed permanent magnet synchronous motor based on visualisation analysis considering DC voltage pulsation

AbstractTo address the problem of considerable current distortions in traditional single‐vector model predictive control (MPC) method for four‐switch three‐phase inverter (FSTPI)‐fed permanent magnet synchronous motor, a double‐vector MPC scheme has been studied. However, it still lacks a sufficient theoretical foundation, causing doubts about its effectiveness. To perfect this deficiency, a novel visualisation analysis scheme considering DC voltage pulsation is presented to demonstrate the validity of the traditional double‐vector MPC method for FSTPI. The analysis results reveal for the first time that the traditional double‐vector MPC method is not always superior to the single‐vector method as there is no zero voltage vector for FSTPI. So, a new virtual voltage vector is defined to compensate the deteriorated control performance caused by the absence of zero voltage vectors, based on which an improved double‐vector MPC method is further put forward to reduce the current ripples. Then, the presented visualisation analysis is adopted to demonstrate the validity of the improved MPC strategy under pulsating DC voltage conditions. Finally, a new voltage sector division technology is further put forward to simplify the implementation steps of the presented method. Contrastive experimental studies demonstrate the availability of the improved MPC strategy.

A Novel Boost Marx Pulse Generator Based on Single-Driver Series-Connected SiC MOSFETs

This paper presents a new type of all solid state boost Marx pulse generator (BMPG). On the basis of the classic pulse forming Marx circuit, without changing the voltage level of each voltage module, a new switch is added for the isolated inductor energy storage and recovery, so as to realize the output of high-voltage nanosecond pulses. At the same time, in order to improve the voltage level of the semiconductor switch and break the limitation of the voltage of the single-stage pulse forming circuit, a series-connected SiC MOSFETs with a single external driver is designed. The series switch has a simple structure with few components and is extremely easy to use in a modular high voltage pulse generator. Its theoretical feasibility is verified by mathematical model and circuit simulation, and the 10-stage prototype is built in this paper. Under the condition of input DC voltage of 500 V, it can output 18 kV high voltage pulses of 200 ns-1200 ns, and the gain multiple has reached 36 times.

Grid-Connected Solar PV System with Maximum Power Point Tracking and Battery Energy Storage Integrated with Sophisticated Three-Level NPC Inverter

In this research, a solar photovoltaic system with maximum power point tracking (MPPT) and battery storage is integrated into a grid-connected system using an improved three-level neutral-point-clamped (NPC) inverter. An NPC inverter with adjustable neutral-point clamping may achieve this result. To achieve this result, a modernized NPC inverter is used. Using the three-level vector modulation approach, the correct AC voltage may be generated when DC voltage conditions are present in an unbalanced situation involving an NPC inverter. A higher degree of precision is made possible by this. In-depth information on the many possible NPC inverter designs and modulation strategies was provided. By incorporating the necessary sophisticated algorithms into the NPC inverter, the necessary solar PV-MPPT functionality is made available, allowing for the regulation of power transfer among the solar PV system, the battery, and the grid. Modified INC method is proposed which has a tracking efficiency of 99.5% for varying irradiance. The use of sufficiently sophisticated algorithms makes this a reality. Using simulation with MATLAB/Simulink, we were able to examine an NPC inverter’s performance in several setups. Several amounts of solar irradiation were used to test the battery’s charging and discharging capabilities. The probe turned out to be fruitful. Laboratory testing ensures that the proposed method works by simulating real-world conditions.

Development of Current-Fed DAB Converter Applying Triangular Current Mode for Wide Voltage Applications

This paper proposes a control and modulation method for a Current-Fed Dual Active Bridge (DAB) converter to improve the conversion efficiency for wide voltage range applications. The current-fed DAB converter is preferred in battery applications owing to its high-power density and low current ripple. On the other hand, the high-conversion efficiency range is limited to close to the nominal DC voltage condition as the DAB converter. The proposed method provides Zero-Voltage-Switching (ZVS) in a wide DC voltage range by Triangular Current Mode (TCM). Furthermore, the proposed control reduces the inductor RMS current compared with the DAB converter to minimize the conduction loss. The validity of the proposed method is established by experimental results. According to the experiment using a 2-kW prototype, the conversion efficiency reached 97.2% when the input voltage was varied by 50% from the nominal DC voltage, then the conversion loss was reduced by 52.5% owing to the proposed control. Finally, the maximum power density of the current-fed DAB converter was improved by 38.1% compared to that of the DAB converter.

An Expanded Lyapunov-Function Based Control Strategy for Cascaded H-Bridge Multilevel Active Front-End Converters

Cascaded H-bridge multilevel (CHBML) active front-end (AFE) converters exhibit some enticing benefits, comprising high adaptability for numerous applications, such as tractions, solid-state transformers, electric vehicle charging stations, and medium and high power electric drives. Yet, when the CHBML- AFE is operating under an unbalanced load condition, it is crucial to utilize an advanced control technique to maintain system stability. In this research, a Lyapunov Function (LF)-based control approach is utilized for regulating a single-phase CHBML- AFE with LCL filter to achieve global asymptotic stability. A capacitor voltage feedback is introduced and added to the traditional LF-based control strategy to reduce the resonance of the LCL filter. Furthermore, the proportional-resonant (PR) control procedure is utilized to derive the grid current reference, offering improvement to the robustness of the current control design. A balanced DC voltage control method is also employed to suppress the unbalanced DC voltage conditions among CHBML- AFE cells. In addition, the transfer function of the reference grid current and the actual grid current is evaluated for the CHBML- AFE with LCL filter parameters and their eventual variations in the employed control technique. The effectiveness of this control strategy is validated utilizing simulation and experimental studies.

Fluorescent fiber-based identification of incipient discharges in liquid nitrogen

Identification of incipient discharges is a major challenge in liquid nitrogenbased super-conducting power apparatus. In the present study, a variety of fluorescent fibers with different absorption/emission characteristics were used for the identification of corona and surface discharges, and we find that a particular type of fluorescent fiber is more sensitive to identify discharges in liquid nitrogen insulation than the UHF technique. Enhancement in the magnitude of fluorescent signal/UHF signal is observed for an increase in applied voltage beyond the corona inception voltage (CIV). Also, it is observed that while reducing the applied voltage, the corona activity quenches at a much lower voltage than the inception voltage. No characteristic variation in rise time or the fall time of the fluorescent signal is observed on corona inception and due to discharges at higher voltages. The corona inception voltage is least for AC voltage conditions, and slightly higher inception voltage is observed for positive DC and negative DC voltage conditions. Different weight percentages of MgO doping in GFRP nanocomposites are prepared and used in the surface discharge studies. It is observed that the 3 wt% sample exhibits enhanced surface discharge inception voltage (SDIV) compared to samples with other doping concentrations and that the inception voltage is lower for AC voltage when compared with the DC voltage conditions. The characteristic spectral content of the fluorescent signal due to corona and surface discharge activity is observed up to 3 MHz. A phase-resolved partial discharge (PRPD) analysis was also carried out to identify corona and surface discharge activity using fluorescent fiber sensor and with a UHF sensor. From the PRPD studies, it is observed that the corona discharge occurs near the peak magnitude whereas surface discharge is observed at the rising part of AC voltage.

Distributed Reactive Power Compensation Method in DC Traction Power Systems With Reversible Substations

This paper proposes a potential distributed reactive power compensation method in dc traction power systems with reversible substations. It utilizes the entire capacity of reversible substations during night hours and the reminding capacity except for regenerative braking energy feedback during day hours. A unified dispatch scheme is developed to allocate reactive power compensation capacity for each reversible substation. During night hours the entire capacity is used to accomplish the optimal power flow objective. During day hours the excess capacity after active power production is used to improve the system reactive power factor dynamically. The different objectives throughout the day are implemented in a unified formula which avoids manually change. In addition the outage of reversible substations and the low dc voltage conditions are considered and an over-modulation suppression strategy is included in the unified dispatch scheme to adjust the reactive power allocation among reversible substations. This novel utilization method of reversible substations is able to provide significant improvements in the power factor of dc traction power systems which would otherwise require expensive additional equipment such as static synchronous compensator. Finally the effectiveness of the proposed method is verified with MATLAB/SIMULINK.

Effect of Nonlinear Conduction on Needle Tip-Plane Breakdown Fields under DC Conditions

In this paper, electric field and space charge accumulation have been investigated for needle-plane system (defect) under DC voltage conditions. Prolate spheroidal electrode system, believed to be closer to needle-plane system, has been used for solving the governing differential equations, numerically, for space charge and electric field distributions. Unlike past works in which space charge at needle tip was assumed either qualitatively or quantitatively, in this work, space charge formation is estimated using nonlinearity of material properties alone. A comparison with previous methods based on concentric spherical electrode approximations reveals that the results are different for prolate spheroidal system at different nonlinearities. Interesting results on the role of nonlinear conductivity on the DC electric field and space charge accumulation at needle-tip are presented. Furthermore, needle tip-plane breakdown experiments are conducted and the results suggest a reasonable and realistic estimate of tip-field at breakdown, using the proposed model.

Physical and chemical properties of a magnetic-assisted DC superimposed nanosecond-pulsed streamer discharge plasma

In the present work, a magnetic-assisted DC superimposed nanosecond-pulsed streamer discharge (DC-NPSD) with a 0.4 T parallel magnetic field is developed that achieves good performance for ozone production and toluene degradation. The influence of the assisted parallel magnetic field on the electrical characteristics, streamer propagation behavior, reactive species generation and plasma chemical properties of the DC-NPSD are systematically investigated. The experimental results indicate that better impedance matching of a nanosecond pulsed power supply and a discharge reactor can be realized by superimposing DC voltage (U DC), which facilitates reactive species production and toluene degradation. The discharge current, input energy and reactive species production can be further enhanced by the application of a parallel magnetic field under different pulse and DC voltage conditions. There are two distinct streamer phases in the DC-NPSD: a primary streamer (PS) with longer propagation distance and higher propagation velocity and a secondary streamer (SS) with shorter propagation distance and lower propagation velocity. The propagation velocities of both the PS and the SS increase with increasing U DC. Only PS propagation velocity is accelerated by a parallel magnetic field; however, that of the SS remains almost constant with or without a magnetic field. Both ozone generation and toluene degradation performance are improved by a magnetic field, which is attributed to the lengthened electron motion path under the action of Lorentz force and the constraint effect on energetic electrons in the presence of a parallel magnetic field.

Operational Principles of Three-Phase Single Active Bridge DC/DC Converters Under Duty Cycle Control

Single active bridge (SAB) dc/dc converters are attractive options for unidirectional dc/dc conversion in future medium power generation applications, offering galvanic isolation and a diode-based output side. SAB dc/dc converters must be controlled by adjusting the active bridge switching duty cycle, unlike dual active bridge converters where the phase-shift angle control is normally used. This article presents a comprehensive analysis of the operational principles of three-phase SAB dc/dc converters that arise from different duty cycle operation ranges. Moreover, the converter performances such as transformer harmonic currents are analyzed and compared between different input and output dc voltage conditions. Finally, an experimental validation using a small-scale prototype is presented.

Analysis and Design of a Double Fuzzy PI Controller of a Voltage Outer Loop in a Reversible Three-Phase PWM Converter

Aiming at the application of a reversible three-phase pulse width modulation (PWM) converter with a wide range of AC side voltage and DC side voltage, a double fuzzy proportional integral (PI) controller for voltage outer loop was proposed. The structures of the proposed controller were motivated by the problems that either the traditional PI controller or single fuzzy PI controller cannot achieve high performance in a wide range of AC and DC voltage conditions. The presented double fuzzy controller studied in this paper is a sub fuzzy controller in addition to the traditional fuzzy PI controller; in particular, the sub fuzzy controller can get the auxiliary correction of PI control parameters according to the AC side voltage and the DC side given voltage variation of PWM converter after the reasoning of the sub fuzzy controller, while the traditional fuzzy PI controller outputs the correction of PI control parameters according to the DC voltage error and its error change rate. In this paper, the traditional fuzzy PI controller can be called the main fuzzy controller, and the adaptive adjustment of PI control parameters of the voltage outer loop is the sum of the PI parameter correction output by the main fuzzy controller and the auxiliary PI parameter correction output by the sub fuzzy controller. Finally, the experimental results show that the reversible three-phase PWM converter can achieve excellent dynamic and static performance in a wide range of AC voltage and DC voltage applications by using the proposed double fuzzy PI controller.

DC and AC electric field analysis and experimental verification of a silicone rubber insulator

This paper presents the electric field analysis as well as the experimental verifications of a silicone rubber insulator by considering DC and AC voltage conditions. As a first step, a 3D electric field simulation of the insulator was performed by using a commercially available software based on finite element method. DC voltage and 50 Hz power–frequency voltage simulations were conducted for the given insulator. Low frequency field distribution along the insulator has been investigated to find threshold frequency values for resistive and capacitive field regions. The effect of the conductivity of air surrounding the insulating system on the electric field was evaluated. For each condition, electric potential and field distribution along the creepage path of the insulator was obtained. In addition to the simulation studies, voltage measurements under AC and DC conditions utilizing a different approach for the silicone rubber insulators were conducted on the insulator by using a non-contact type electrostatic voltmeter to validate the simulation results.

Optimum Injection of Second Harmonic Circulating Currents for Balancing Capacitor Voltages in Hybrid MMC during Reduced DC Voltage Conditions

Hybrid modular multilevel converter (Hyb-MMC), which consists of both half-bridge (HB) and full-bridge (FB) submodules (SMs), has the capability to withstand wide variations in dc-link voltage ranging from nominal to dc short circuit conditions. With around 50% FBSMs, the Hyb-MMC can block the dc short-circuit current while exchanging reactive power with its ac grid. Other than faults, often, the high voltage direct current system may have to operate at reduced dc voltage levels with rated dc-link current, which results in arm currents becoming unipolar. This phenomenon affects the cyclic charge/discharge of the HBSM capacitors. To overcome this problem, it is proposed to inject second harmonic circulating current (SHCC) for making the arm currents bipolar and, hence, ensuring capacitor voltage balance. However, the choice of suitable amplitude and phase angle of the SHCCs is crucial to avoid increase of losses in the converter. Hence, a method to optimize the amplitude and phase angle of the SHCCs is also presented. The proposed method is verified through simulations performed on PSCAD/EMTDC platform followed by experiments on laboratory scale prototype.

Assist Gate Driver Circuit on Crosstalk Suppression for SiC MOSFET Bridge Configuration

Crosstalk in a phase-leg configuration significantly limits the high switching speed performance of silicon carbide (SiC) MOSFET due to their lower threshold voltage and higher allowable negative gate voltage as compared with the traditional silicon (Si). To fulfill the potential fast switching speed quality of SiC MOSFET, an assist gate driver (AGD) circuit for crosstalk suppression based on the negative-biased turn-off voltage was proposed, which contains an auxiliary capacitor and two passive transistors without additional control signals. First, a recommended gate driver by several SiC device manufacturers was introduced to analyze the mechanism of crosstalk. Then, the operating principle of the proposed AGD circuit was elaborated, and the parameter design criteria of the main elements were given. Finally, a simulation using LTspice and an experiment based on Wolfspeed 1200-V SiC MOSFET tests were used to verify the effectiveness of the proposed gate driver. Simulation and experimental results show that the AGD circuit has a superior characteristic of crosstalk suppression under varying operating conditions. Thus, the proposed circuit could meet the fastidious requirement of fast-speed SiC MOSFET under high dc voltage condition with less complexity.

Selective harmonics elimination in multilevel inverter by a derivative-free iterative method under varying voltage condition

Selective Harmonics Elimination Pulse width modulation (SHE PWM) presents an alternative modulation technique to operate medium voltage medium and high power converters as it reduces the switching losses. SHE PWM generates high-quality output waveform at the low switching frequency. Its application to multilevel converters furthers leads to improvement in output waveforms with lower switching losses, lesser dv/dt and lowers switching devices ratings. In this Paper SHE PWM for an asymmetrical cascaded H-bridge multilevel inverter under unequal DC voltage condition for fundamental and multiple switching cases are investigated. More harmonics can be eliminated by considering multiple switching’s. The Solution of the Nonlinear transcendental SHE equation is obtained using an advanced derivative free numerical technique which. The method is simple in implementation with fast calculations per iteration as it avoids the evaluation of Jacobian and its matrix. Also the need for precise initial guess in the proximity of actual solution is not needed as an identity matrix of the size of designed variable serves the purpose. Different inequality in the dc voltages is taken to eliminate the lower order harmonics from the output. Only exact solutions for the switching angles which ensure the complete elimination of targeted harmonics are reported. Simulation and Hardware results are presented to confirm the validity of the proposed technique.

Imbalance Mechanism and Balanced Control of Capacitor Voltage for a Hybrid Modular Multilevel Converter

Due to different charging and discharging characteristics of full-bridge submodules and half-bridge submodules in hybrid modular multilevel converters (MMC), capacitor voltage imbalance will occur under boosted ac voltage or reduced dc voltage conditions. To address this issue, the mechanism of capacitor voltage imbalance is carefully studied, with three main factors—modulation index, power angle, and hybridization ratio—summarized and their effect on capacitor voltage imbalance analyzed. Further, a control strategy based on fundamental frequency reactive circulating current injection is proposed to keep the capacitor voltage balanced in the hybrid MMC. The amplitude and phase angle of the injected circulating current are calculated and their influence on the energy fluctuation in the submodules’ capacitors and the semiconductors’ current stress is explored. Experimental results under boosted ac voltage and reduced dc voltage conditions demonstrate the feasibility and validity of the proposed scheme.

A Gate Driver of SiC MOSFET for Suppressing the Negative Voltage Spikes in a Bridge Circuit

SiC mosfet has low on-state resistance and can work on high switching frequency, high voltage, and some other tough conditions with less temperature drift, which could provide the significant improvement of power density in power converters. However, for the bridge circuit in an actual converter, high dv/dt during fast switching transient of one mosfet will amplify the negative influence of parasitic components and produce the significant negative voltage spikes on the complementary mosfet , which will threaten its safe operation. This paper proposes a new gate driver circuit for SiC mosfet to attenuate the negative voltage spikes in a bridge circuit. The proposed gate driver adopts a simple voltage dividing circuit to generate a negative gate-source voltage as traditional and a passive triggered transistor with a series-connected capacitor to suppress the negative voltage spikes, which could satisfy the stringent requirements of fast switching SiC mosfet s under the high dc voltage condition with low cost and less complexity. An analysis is presented in this paper based on the simulation and experimental results with the performance comparison evaluated.

Comparative study of AC and DC inclined plane tests on silicone rubber (SiR) insulation

Currently there are no international standards for evaluating the tracking and erosion resistance of DC polymeric insulation under contaminated conditions. Researchers often modify the existing AC inclined plane test standards such as the IEC-60587 to accommodate DC voltage conditions but this has been reported to give various inconsistences. This paper presents comprehensive experimental results on inclined plane tests of silicone rubber (SiR) insulation at 3.5 and 4.5 kV AC and positive DC using intravenous (IV) system as the pollutant supply. The leakage currents (LC) were recorded throughout the entire tests. In addition, various physiochemical tests namely, Fourier Transform Infrared analysis, thermo-gravimetric analysis, scanning electron microscopy and energy dispersive spectroscopy were performed on the aged and unaged samples. Results show that DC LC is bigger (about three times) than that under AC for the same equivalent voltages. Furthermore, DC LC variations are less random and the average magnitudes increase with duration of voltage application compared with AC. The physiochemical analyses show that 3.5 kVrms AC and 3.5 kV DC aged samples have comparable chemical characteristics albeit with electrode corrosion elements detected on the DC aged samples. Under 4.5 kV DC the degradation becomes significantly more severe and unrepeatable. It is therefore concluded that at 0.3 ml/min, pollutant flow rate, 3.5 kV positive DC and 3.5 kVrms AC are comparable as test voltages for inclined plane accelerated ageing of SiR insulation.

Advanced Field Weakening Control for Squirrel-Cage Induction Motor in Wide Range of DC-Link Voltage Conditions

This paper proposes a field weakening control method for operating an induction motor with a variable DC input voltage condition. In the variable DC voltage condition such as a battery, the field weakening method are required for the maximum output power. The conventional field weakening control methods can be used for operating the induction motor over the rated speed in a constant DC-link voltage condition. However, the conventional methods for operating the motor with the variable DC voltage is not suitable for the maximum output power. To overcome this problem, this paper proposes the optimized field weakening control method to extend the operating range of the induction motor with a rated power in a limited thermal and a wide DC input voltage conditions. The optimized d-axis and q-axis current equations are derived according to the field weakening region I and II to extend the operating region. The experimental results are presented to verify the effectiveness of the proposed method.