DC Capacitor Research Articles

Variable Voltage Variable Frequency Modular Multilevel AC/AC Converter With High-Frequency Harmonics Filtering Capability

This article proposes a variable voltage variable frequency modular multilevel ac/ac converter (VVVF-MMC) with a high-frequency isolation link, which can realize voltage and frequency conversion simultaneously with insulated-gate bipolar transistors (IGBTs) of the same rated voltage and current level. The parallel connection of multiple modules is realized by coupling reactors, which can realize large-scale voltage and current transformation and save volume. It combines parallel multiplexing and serial multiplexing technology of submodules (SMs) to increase the equivalent switching frequency, thereby canceling nonzero sequence high-frequency voltage and current harmonics, so it has good power quality in the steady-state. Under the improved carrier phase-shifted pulsewidth modulation (CPS-PWM) strategy, the pulse flip circuit can realize soft switching operation, thereby improving energy conversion efficiency. Furthermore, dc capacitor voltage balancing and parallel SM current balancing control strategies are designed. With the strong filtering ability, the proposed ac/ac converter can be used as the onshore converter station of fractional frequency offshore wind power system. Due to the advantages of small size and high efficiency, it also has broad application prospects in distribution networks and electric drive. Simulation and experimental results validate the effectiveness of the proposed converter topology.

Experimental Investigation on Breakdown Characteristics of Metallized Film Capacitors Under AC and DC Superimposed Voltage

As energy storage elements in modular multilevel converter (MMC) sub-module of a unified power flow controller (UPFC), metallized film capacitors endure ac and dc superimposed voltages, which brings new challenges to insulation performance of the dielectric film. In this article, an experimental setup was created to investigate the breakdown characteristics of the dielectric film for sub-module capacitors. Factors which may affect breakdown strength under ac and dc superimposed voltages were studied experimentally. The results reveal that, as the dc voltage component increases in the range of 0–3200 V, the breakdown strength gradually increases by 16.36%; as the ac voltage component increases in the range of 0–1800 V, the breakdown strength slightly decreases by 3.52%; the breakdown strength is more affected by frequency as the ac voltage component increases and decreases with increasing frequency; as temperature increases from 20 °C to 95 °C, the breakdown strength substantially decreases by 31.12%; as the thickness of the film increases from 7 to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~\mu \text{m}$ </tex-math></inline-formula> , the breakdown strength slightly increases by 6.32%. For capacitors in 500-kV UPFC, the dc voltage component is approximately 1600 V; the ac voltage component, whose peak-to-peak value is within 800 V, mainly contains power frequency and the second harmonic components. According to the experimental results, since the ac voltage component is small and the order of harmonic is low, the breakdown strength of capacitors in UPFC has little difference compared with that of the dc capacitors. This article can provide a reference for insulation design optimization of capacitors in UPFC.

Enhanced Level-Shifted Modulation for a Three-Phase Five-Level Modified Modular Multilevel Converter (MMC)

The rapid growth of the penetration level of renewable energy sources (RESs) in low-voltage (LV) networks increases the research interest in more efficient, compact, reliable, and easily expandable energy conversion systems. The long commutation loops of the existing LV multilevel converters (MLCs), such as active neutral point clamped (ANPC) converters, lead to high-voltage overshoots induced during switching due to the significant parasitic inductance. Modular multilevel converters (MMCs) are gaining an increased interest due to their modularity, flexibility, and short commutation loops also for some LV applications. However, several issues related to the number of components, dc capacitor requirements, and the added complexity need to be considered. This article aims to reduce the five-level modified MMC capacitance requirements where the upper and lower submodules (SMs) are connected in parallel. This reduction has been achieved using an improved modulation technique by minimizing the insertion time of the SMs that require higher capacitance. The proposed modulation is effective in reducing the inner SMs’ capacitor voltage ripple at lower modulation indices. Instead, the circulating current injection of the second harmonics effectively reduces the voltage ripple at higher modulation indices. Thus, maximum reduction in the total capacitance requirement for the entire operating range of modulation index can be achieved by a proper combination of the two solutions. Simulation and experimental results on a laboratory-scaled prototype verify the validity of the proposed solution.

Dynamic Online Grid Impedance Estimation and Its Effects on DC Capacitor Lifetime in Variable Frequency Drive

Grid impedance plays a significant role in the performance and design considerations of nonlinear devices like Variable Frequency Drive (VFD). VFD operates across domestic, industrial, and residential applications under different grid configurations and capacities including renewables. IEEE 519 standard recommends grid short circuit ratio and thereby grid impedance as an index for the individual and overall current harmonic performance of the connected utilities. This research investigation details a comprehensive approach for an online dynamic grid impedance estimation and may also give an insight into the hardware implementation within a VFD to achieve optimum performance and reliability during installation. A VFD of 250 kW is used for real-time measurements and simulation with varying grid configurations to determine grid impedance and lifetime of the DC capacitors. A fuzzy model is developed based on the established knowledge base for the estimation of the grid impedance of a generic grid. The fuzzy model is also developed to estimate the lifetime of the DC capacitor.

Підвищення точності регулювання напруги на конденсаторі активного фільтра

The control scheme of the active filter on the basis of the dq-theory with the block of automatic stabilization of voltage on the condenser containing the system of adaptive control of a delay in time and the filter of low frequencies is considered. The introduction of the delay can be implemented using a microcontroller that will automatically calculate the required delay factor and the output scale factor. The low-pass filter is tuned to the seventh harmonic, which allows you to adjust the delay in the range that provides the minimum value of the allowable error of voltage deviation. A study of the quality of voltage maintenance on the capacitor was also carried out using a set of measures. To improve the compensating capacity of the active filter, the sliding current averaging unit was set to the sixth harmonic in the case of a symmetrical load. The time response of the automatic voltage stabilization unit on the DC capacitor provides a stable charge / discharge process of the capacitor and a high level of compensation of higher harmonics without significant additional generation of active power in the network. It is shown that increasing the accuracy of voltage regulation on the capacitor of the power active filter allows to ensure the achievement of the minimum values of harmonic distortion and stable operation of the system. The transient reaction of the power active filter is determined by the control circuit of the output current of the filter. Therefore, its time response is chosen fast enough to track the current shape of the sinusoidal current. On the other hand, the speed of the control circuit of the capacitor voltage should not be too fast, and it is selected at least 10 times slower than the control circuit of the current filter current. Thus, these two control systems can be divided and designed as two independent systems and adjust each to individual characteristics.

Variable-Inertia Emulation Control Scheme for VSC-HVDC Transmission Systems

This paper proposes a novel variable-inertia emulation control (VIEC) scheme that enables voltage-source-converter based high-voltage DC (VSC-HVDC) transmission systems to flexibly support AC grid frequency stability like synchronous generators. The VIEC scheme allows us to extract the energy from the augmented DC capacitance for inertia emulation by controlling the DC voltage without affecting the stability of the AC system connected on the remote side. In particular, with the proposed VIEC scheme, the inertia time constant can be flexibly emulated and its value can be automatically adjusted according to the rate of change of grid frequency as well as grid frequency deviations. This leads to more effective inertial support across different timescales. Modal analysis is carried out to investigate the impacts of inertia and DC capacitance, and to obtain the optimal control parameters. The effectiveness and advantages of the proposed VIEC scheme are demonstrated on an IEEE benchmark system in the presence of faults and load changes.

Optimal Virtual Inertia Design for VSG-based Motor Starting Systems to Improve Motor Loading Capacity

Virtual synchronous generator (VSG)-based control approaches for AC machine drivers have attracted much attention in recent years. The contradiction between the strong motor loading capacity and the small DC voltage fluctuation is disregarded in previous studies. A larger virtual inertia is required to raise the torque gain and drive the motor with heavier loads, thus resulting in larger DC voltage variations. A systematic method is proposed for the optimal design of the virtual inertia for VSG-based motor starting systems to achieve the maximal motor loading capacity and the low DC voltage fluctuation in this paper. Firstly, the relationship between the virtual inertia and the loading capacity is revealed on the basis of the closed-loop analysis. Considering the DC voltage variation, the constraint on the maximal value of the virtual inertia is proposed from the energy-balance perspective despite the unknown contribution of the DC capacitors to the virtual inertia. To fully exploit the energy storage capacity of the DC capacitors and raise the upper limit of the virtual inertia under the same permitted DC voltage variation, a modified DC voltage reference is introduced. Extensive simulation and experimental results are presented to validate the effectiveness of the proposed methods.

Flatness-SVPWM Control for Three-Phase Shunt Active Power Filter Based on Five-Level NPC Inverter

The increasing use in the industry of nonlinear loads based on power electronic elements has introduced serious perturbation problems in the electric power distribution grids. These nonlinear loads generate non-sinusoidal currents to the mains which degrade the system's performance. Active power filters have been developed over the years to solve these problems and improve power quality. This study focuses on the use of a shunt active power filter to compensate nonlinear load's current harmonics and reactive power. To this end, a regulator based on the flatness control technique combined with an SVPWM strategy for a three-phase shunt active power filter based on a five-level neutral-point-clamped (NPC) inverter is used. The control objectives of this work are twofold: i. compensating the current harmonics and the reactive power produced by the nonlinear loads, and ii. regulating the inverter's DC capacitor voltage. To achieve these objectives, two cascaded loops are designed; an inner loop for the reactive and harmonic currents compensation based on a flatness controller and an outer loop for the DC voltage regulation based on a PI regulator. The simulation results are developed under MATLAB/Simulink environment to prove the satisfactory performance of the proposed controlled system.

Simplified Predictive Direct Power Control of Three-Phase Three-Level Four-Leg Grid Connected NPC Converter

Model Predictive Control (MPC) provides various advantages over traditional controllers in power electronics applications. However, reducing the complexity and computational burden of the Finite-control set-model predictive control (FCS-MPC) in multi-leg multilevel inverters is still a challenging task. In this paper, a simplified model predictive direct power control (MPDPC) for a three-level, four-leg grid-connected converter (4L-GCC) is proposed. The main idea of the proposed strategy is to reduce the computational burden of the control algorithm by using only the switching vectors of the first sector to optimize the cost function. The proposed control method can ensure accurate control of active and reactive powers, control of zero-sequence current, as well as DC capacitor voltages balancing without using any weighting factors. Simulation and experimental results are provided to prove the effectiveness and simplicity of the proposed MPC algorithm compared with most-recently proposed solutions.

Voltage Regulation Enhancement of DC-MG Based on Power Accumulator Battery Test System: MPC-Controlled Virtual Inertia Approach

In a DC-microgrid (DC-MG) composed of a power accumulator battery test system (PABTS), owing to the low inertia of DC capacitance, the charging and discharging of a PABTS can easily cause DC-link voltage fluctuations, which may jeopardize the system stability. Hence, a virtual inertia control (VIC) strategy is proposed to suppress these fluctuations and enhance the stability of the DC-MG. The VIC method is realized in a bidirectional grid-connected converter (BGCC), which combines VIC and model predictive control (MPC). The proposed method can provide inertia support during the transient state and enhance the dynamic characteristics of the DC-link voltage. A prediction model is established that uses the variation range of the DC-link voltage as the constraint, and the output of VIC as well as voltage deviations as optimization objectives. The desired DC-link current increment is calculated using the prediction model to change the input DC current reference of the VIC. To validate the effectiveness of the proposed method, hardware-in-the-loop (HIL) experiments are performed, and the results indicate that MPC-VIC is superior to the existing VIC methods in terms of inertia support and the DC-link voltage variation suppression of PABTS DC-MGs.

Study on Stability of Hand-in-hand Structure of Multi-terminal DC Hydrogen Production System

Multi-terminal DC (MTDC) hydrogen production systems are becoming one of the important forms of power distribution systems with the increasing growth of distributed renewable energy sources (such as PV and wind turbines), energy storage devices, and DC loads. To explore the key factors in stability analysis, the circuit diagram of MTDC hydrogen production system in hand-in-hand structure composed of voltage source converters (VSCs), DC lines, renewable energy and DC hydrogen production load was established in this paper. The overall state space model of the system was put forward, taking the master-slave converter control strategy into consideration. Then, the small-signal stability analysis of the MTDC hydrogen production system was carried out by comparing and analyzing the moving trajectories of the dominant eigenvalues in different system parameters. The key factor affecting the stability of the system such as DC capacitance of the converter and the electrolyzer power in the DC bus are determined. On this basis, a simulation model of the low-voltage MTDC hydrogen production system was built based on MATLAB/Simulink to verify the correctness of the theoretical analysis.

Double–Stage photovoltaic generator augmented with FLL–based synthetic inertia emulator

Distributed virtual inertia (DVI) is a promising method in frequency support provision for power system transition toward ≈100% power electronic–based generation. In this approach, the inertia power is provided by discharging or charging dc capacitors adhered to the grid–tied inverters distributed in the grid. However, the dc–bus voltage drifts with the time from its nominal in the conventional DVI technique. Herein, we suggest a modified DVI–based frequency regulator which addresses the aforesaid issue. The inverter controller comprises two main loops: 1) inner current loop and 2) outer voltage loop augmented with the proposed synthetic inertia emulator, whereby measured grid frequency oscillations are transmuted to the synthetic inertia support. The proposed controller is utilized in a double–stage photovoltaic generator. The synchronization of generator with the host grid is performed using a fast but accurate frequency–locked loop (FLL) implemented in the synchronous reference frame. The small–signal state–space realization illustrates that the generator stability is scarcely sensitive to the dc–link capacitance changes, while the grid strength and the FLL controller gains significantly affect the stability. The efficacy of the proposed control scheme is verified by time–domain simulation results in MATLAB/Simulink. The grid frequency response depicts that the frequency rate of change is improved by 14.6% if the proposed synthetic inertia emulator is employed.

Online Health Monitoring of DC-Link Capacitors in Modular Multilevel Converters for FACTS and HVDC Applications

This article presents an online health monitoring scheme for dc capacitors in modular multilevel converters (MMCs). The health monitoring algorithm is based on detecting changes in the dc capacitance value over time. The proposed algorithm only utilizes measurements that are typically available in flexible alternating current transmission systems and high-voltage direct current applications. Hence, in the proposed estimation method, no additional sensors are used. The estimation scheme considers the presence of noise in voltage and current measurements, and utilizes a recursive least square estimator in conjunction with a special low-pass filter to minimize the estimation errors. Simulation results of a hardware replica, as well as experimental results on a low-power MMC prototype show that the proposed scheme can identify the dc-link capacitance value with a maximum error of 1%.

A Novel Multilevel Converter with Stable Voltage to The Renewable Energy Systems

In this proposed paper, a novel nine-level inverter (NLI) topology is proposed, which uses fewer components to provide nine voltage levels in the output. Ten semiconductor switches, four capacitors, and one dc voltage source are included in this NLI. Through various logical arrangements of the dc source and capacitors and the use of power semiconductor switches, the nine voltage levels are generated. In this case, only two switches can conduct simultaneously in any mode. As a result, the system's efficiency is increased and conduction losses are decreased. The low total standing voltage (TSV) of the power semiconductor switches is another crucial aspect of this design. Gate signals are produced using a suggested modulation approach to lessen the power switches' switching loss. Several Simulink simulations are run in order to validate the NLI. The suggested inverter has a much lower requirement for components and dc sources as compared to conventional topologies. In order to increase the efficiency of the power generating process in renewable energy systems, the proposed NLI structure can be implemented.

Isolated DC–DC Boost Converter for Wide Input Voltage Range and Wide Load Range Applications

A current-fed dual-inductor resonant full-bridge dc–dc isolated boost converter is proposed. The converter is most suitable for applications that demand wide input voltage and load ranges. It can be used as a front-end dc–dc converter that can boost variable low voltage from a power source [battery (home/industrial inverter/industrial UPS application), fuel-cell, or solar-PV] and interface it to a high-voltage dc-link, which typically feeds an inverter. It removes all major drawbacks of conventional current-fed isolated dc–dc converters, namely high-voltage spikes across the switches due to transformer leakage inductance, need for additional start-up circuit, and difficulty in converter tripping in case of faults. Soft-switching of rectifier didoes and clamping power switches is load independent leading to good efficiency over entire load range. Converter can be operated with <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</i> < 0.5, making it suitable for wide input voltage range applications. A detailed comparative analysis with state-of-the-art converters for wide input voltage range applications considering the major important factors, such as power device utilization, transformer utilization, inductive energy requirement, and dc capacitors utilization, confirms the superiority of the proposed converter. Experimental results for a 2.5 kVA inverter application with an input dc voltage range of 90–160 V and output ac voltage of 230 V are provided.

Cascaded Multilevel Rectifier with Common High Voltage Direct Current (HVDC) Bus and Its Control Method

In order to realize the voltage equalization and minimize the total harmonic distortion rate of current of cascaded multilevel rectifier with common high voltage Direct Current (HVDC) bus, the cascaded multilevel rectifier with common high voltage DC bus and its control method are studied. The output side DC voltage of the cascade multilevel rectifier with common high voltage DC bus is sampled, and the output DC current signal amplitude is obtained according to the calculated output side DC voltage signal. On this basis, the AC side voltage corresponding to the applied switching state and the switching state corresponding to the rectifier in the sampling period are solved. The driving signal of the switch device is generated and sent to the corresponding switch device to realize the active power factor correction and sinusoidal input current, so as to realize the balanced control of output DC capacitor voltage of cascaded multilevel rectifier with common High-Voltage Direct Current (HVDC) bus. The test results show that this method can effectively control the three-phase output current of the rectifier and ensure the maximum output of the current. The lowest total harmonic distortion rate of the output phase voltage is about 40%, which can effectively ensure the stability of the output phase voltage after control.

Analysis on the Topology and Control of Power Electronics Converters for Wind Energy Conversion Systems

Power Electronics converters become nowadays the most important part in Wind Energy Conversion Systems (WECS). They are an intermediate between the generator and grid to achieve low cost, high power density and reliability. This paper deals with the analysis on the topology and control of the most power Electronics Converters for generators using in WECS. Design, (dis)advantages, and market penetration are analyzed and discussed. The control includes maximum power point tracking, dc bus voltage control, balancing of the dc capacitor voltages, and reactive power generation are also analyzed. Simulations have been carried out using MATLAB/SIMULINK on the control strategies for the case of back to back converter with Pulse Width Modulation (PWM) demonstrating its good potential to meet the grid connection requirements. Keywords: Wind Energy Conversion Systems (WECS), power electronics converters, back to back converter, maximum power point tracking, Pulse Width Modulation (PWM).

Dielectric Property Improvement of Polypropylene Films Doped with POSS for HVDC Polymer Film Capacitors

This paper focuses on the effects of the nano Polyhedral oligomericsilsesquioxanes (POSS) with three similar molecular structures on the thermal and electrical properties of polypropylene (PP) films, aiming to propose a comprehensive modification method. After doping with micro amount of nano POSS, the crystallinity and regularity of films are elevated. Correspondingly, PP with optimized microstructure has the lowest conductivity, due to the interaction between the nanoparticles and the matrix, as well as the strong binding force of the crystal region to the carriers. The loss of the sample is restricted, especially in the low frequency region, since the combined effect of polarization and conduction. The DC breakdown strength of the modified PP is higher than that of the PP, with an increase of above 20% at 25-85 °C. Based on quantum chemical calculations, the energy level distributions are analyzed. By selecting appropriate additives, deep trap sites can be introduced to catch carriers, preventing the breaks of polymer molecular chains. This modification of PP films shows huge potential in the improvement of dielectric properties in the field of DC capacitors.

Inter-cluster voltage balancing control of a delta connected modular multilevel cascaded converter under unbalanced grid voltage

A new inter-cluster DC capacitor voltage balancing scheme for a delta connected modular multilevel cascaded converter (MMCC)-based static synchronous compensator (STATCOM) is presented. A detailed power flow analysis of applying negative sequence current (NSC) and zero-sequence current (ZSC) injection methods in addressing the issue of inter-cluster DC voltage imbalance under unbalance grid voltage is carried out. A control scheme is proposed which integrates both inter-cluster methods using a quantification factor QF. This is used to achieve the sharing of the inter-cluster active power between the NSC and ZSC injection methods. An accurate method of determining the quantification factor is also presented. The proposed method offers better sub-module DC capacitor voltage balancing and prevents converter overcurrent. The influence of unbalanced grid voltage on the delta connected MMCC-based STATCOM rating using this integrated cluster balancing technique is investigated. The control scheme is verified with a 5 kV 1.2MVA MMCC-STATCOM using 3-level bridge sub-modules, and the results show the advantages of the proposed method over other inter-cluster methods.

Shunt Active DC Filter to Reduce the DC-Link Ripple Current Caused by Power Converters to Improve the Lifetime of Aluminum Electrolytic Capacitors

Electrolytic capacitors are commonly used as a dc-link in power electronics circuits and especially in renewable energy system applications. However, their life expectancy and reliability can be severely affected by the ripple current causing increased thermal stresses. Further, the switching of power electronics and the presence of harmonic components can increase the ripple current that may shorten the lifetime of the electrolytic capacitors. This article proposes the use of a shunt active dc filter to regulate the dc capacitor current to a near-constant value and hence mitigate the events that can reduce the capacitor life expectancy. The proposed filter reduces the ripple in the capacitor dc current irrespective of the frequency spectrum of the ripple. The design and the operation principles of the proposed filter are presented and discussed. The results from the simulation and experimental studies demonstrate the effectiveness of the proposed technique.