Controlled Capacitor Research Articles

Maintain Power Transmission and Efficiency Tracking Using Variable Capacitors for Dynamic WPT Systems

This study introduces a new method for real-time efficiency tracking and stable output power of Dynamic Wireless Power Transfer (DWPT) systems using variable capacitors. A preliminary detailed discussion and an analysis of the DWPT system are carried out to show how the system can optimize power transmission and efficiency when the relative positions of transmitter and receiver change using a dynamic real-time control of the variable capacitors belonging to the compensation networks. This paper shows a detailed model of the DWPT system, including magnetic coupling analysis, circuit dynamics analysis, and efficiency characteristics analysis, in order to modify the control input values as needed. By utilizing a group optimization strategy, the transmission efficiency can be quickly maximized without using a position detection module. Simulation results demonstrate the effectiveness of the proposed method under various dynamic conditions, achieving significant improvements in energy efficiency and transmission reliability of the DWPT system. This research provides a powerful method to increase the overall performances of DWPT systems, which will help the development of future wireless charging technology.

An optimized Closed-loop Precharging Strategy for Modular Multilevel Converters Based on Power Balancing Control under start or fault restart conditions

Modular multilevel converter (MMC)-based high-voltage DC (HVDC) systems have found rapid growth in power grids. To reduce the inrush current, it is necessary to precharge the submodule (SM) capacitors to the rated voltage before normal operation. In this paper, an optimized pre-charging strategy for MMC is proposed based on power balancing . First, the charging power is obtained by a designed closed-loop control of the SM capacitors. Then the total power released by the AC or DC side could be calculated by power conservation and feedback by controlling the charging current. At the same time, the power difference between the upper and lower bridge arm of each phase and the internal voltage balancing of bridge arms are also considered by the proposed bridge arm and SM voltage algorithms. By comparing with conventional closed-loop pre-charging strategies, the pre-charging speed is greatly enhanced as the achievement of synchronous charging of upper and lower bridge arm SMs. Moreover, as the same voltage and current double closed-loop control applied in both MMC pre-charging and normal state, the state transition time could be reduced. Finally, a series of simulations and prototype experiments are conducted to verify the feasibility of the proposed pre-charging strategy.

Nine level switched capacitor inverter with level shifted pulse width modulation approach

This article proposes a nine-level switched capacitor inverter (NLSCI) with a minimum number of switches. In recent years, switching capacitor (SC) multilevel inverters (MLIs) have become one of the most common inverter topologies. These proposed nine level switched capacitor inverter (NLSCI) do not deserve any external control unit for capacitor control. Since, the charging and discharging of the capacitors are controlled by the on and off states of switches. Furthermore, by employing fewer switches and DC voltage sources, the suggested design produces a greater amount of resultant voltage. Additionally, pulse width modulation (PWM) is recommended as a method to enhance output quality and power level quality. The switched-capacitor two-output multilevel inverter (SCMLI) structure's viability and effectiveness have been demonstrated using MATLAB simulation.

A Single DC Source Multilevel Inverter with Extended Linear Modulation Range to the Full Base Speed and Predictive Capacitor Control for Induction Machine Drives

A Single DC Source Multilevel Inverter with Extended Linear Modulation Range to the Full Base Speed and Predictive Capacitor Control for Induction Machine Drives

Discrete Laws of Capacitor Control of Asynchronous Generator Voltage

Discrete Laws of Capacitor Control of Asynchronous Generator Voltage

Frequency regulation method for two-stage PV system based on DC voltage coordination

The current frequency regulation methods for a photovoltaic (PV) system cannot balance frequency support and primary control performances. This paper proposes a frequency regulation method for a two-stage PV system by controlling DC voltage, which is coordinated with the enhanced virtual inertia control (VIC) of the DC capacitor. The power reserve control of the PV arrays is improved considering the characteristics of the VIC, thereby strengthening the frequency regulation ability of the PV system. An active DC voltage recovery control is designed at the inverter to restore the frequency support ability after primary frequency control. With MATLAB/Simulink software, the performance of the proposed method in dealing with different load sizes, load change trends, and DC voltage recovery coefficient is verified by comparing it with other traditional methods in different scenarios. The proposed method can fully use the available power to improve the frequency nadir, rate of change of frequency, and voltage recovery ability.

A novel MIMO SMC for comprehensive unified control of the cascaded DC‐DC and DC‐AC converters in grid connected photovoltaic systems

SummaryIt is well known that DC‐DC boost converters are cascaded with DC‐AC inverters for grid connection of the photovoltaic (PV) systems. In the traditional control approaches, the mentioned DC‐DC and DC‐AC converters are controlled separately to facilitate the controller design problem. However, from the controller design viewpoint, the overall structure of the grid connected PV generator is a multi‐input multi‐output (MIMO) system. The duty cycle of the DC‐DC converter and inverter modulation index are the control inputs and, on the other hand, generated photovoltaic DC power and exported power to the grid are control outputs. Moreover, the inverter DC link voltage should be stabilized by the closed‐loop controller as well as an internal control output. If controllers are designed separately, it means that the interaction between DC‐DC and DC‐AC controllers isn't considered accurately, and since the isolated models of DC‐DC converter and DC‐AC inverter are extracted based on some approximated assumptions, separate controller design cannot guarantee stability and robustness of the whole system in a wide range of operation. To cope with these problems, in this paper, a novel MIMO sliding mode controller (SMC) is developed for comprehensive closed‐loop control of the DC‐DC boost converter cascaded with a single‐phase DC‐AC grid connected photovoltaic inverter. In the proposed approach, the dynamic model of whole system is developed comprehensively at first, and then, a unique MIMO controller is designed to control both DC‐to‐AC and DC‐to‐DC converters together. To cope with the nonlinear characteristic of the system and uncertainty of model parameters in a wide range, a fixed‐frequency SMC is developed using the comprehensive state space model of the closed loop system. In the proposed MIMO‐SMC controller, the AC power (which is exported to the grid) and operating point of the PV source are controlled via inverter modulation index and duty cycle of the DC‐DC boost converter, respectively. Another major advantage of the proposed system is mitigating the non‐minimum phase characteristic of the boost converter through the indirect control of inverter DC link capacitor. To evaluate the performance of the designed control system, simulation results are compared with a standard linear PI controller. It is shown that the developed system has zero steady‐state error and enjoys faster dynamic response during the start‐up and step changes of AC and DC current references. Moreover, it can maintain the stability of closed‐loop systems in a wide range of operations.

Fault Analysis of Power System Transient Stability with Thyristor- Controlled Series Capacitor Controller Model Using Flower Pollination Algorithm for Its Parameters

Fault Analysis of Power System Transient Stability with Thyristor- Controlled Series Capacitor Controller Model Using Flower Pollination Algorithm for Its Parameters

A consensus based adaptive virtual capacitor control strategy for reactive power sharing and voltage restoration in microgrids

In the islanded microgrid, virtual synchronous generator (VSG) control can solve the problems of insufficient damping and low inertia of the system. However, reactive power among distributed generation (DG) units controlled by VSG scannt be shared according to capacity due to the mismatch of line impedance, and the bus voltage can be significantly below the rated value due to the influence of droop control and line impedance. In this paper, an adaptive virtual capacitor control strategy (AVSSC) based on consensus is proposed to solve this problem. The strategy ensures the accurate sharing of reactive power among VSGs and adjusts the estimated bus voltage to the rated value by paralleling virtual capacitors. The voltage estimates used in AVSSC are obtained by the proposed discrete dynamic consensus algorithm (DDCA). The algorithm can obtain the average value of the local voltage estimates of all VSG units to eliminate the limitation of local information and the influence of accidental factors on voltage estimation. The multi-agent consensus theory proves that AVSSC can achieve the expected goal. The simulation results of a microgrid system with four parallel VSGs show that the AVSSC can accurately share reactive power, and the bus voltage error decreases from 0.81% to 0.39%.

Multi-resonant Controller Design for a PV-Fed Multifunctional Grid-Connected Inverter in Presence of Unbalanced and Nonlinear Load

The use of Active Power Filters (APFs) in future power grids with high penetration of nonlinear loads is unavoidable. Voltage Source Inverters (VSIs) interfacing Photovoltaic (PV) generator could play the APF role in addition to power supply. In this paper, the control of a PV-fed multifunctional grid-connected three-phase VSI is addressed with nonlinear and unbalanced load. The control objective is threefold. The first one is to deliver the maximum available power from the PV source to the grid satisfying power quality standards. The second one is the Voltage Balancing Control for DC-link capacitors to guarantee correct operation of the VSI. The last one is shunt APF control to compensate for nonlinear and unbalanced load harmonics, reactive power, and unbalanced sequences. A quasi-Proportional-Resonant (PR) controller with harmonic compensators is proposed for the current control loop. The quasi-PR controller parameters are determined through optimization algorithms such as Particle Swarm Optimization (PSO), Genetic Algorithm (GA), and a combination of both PSO and GA. The aim of the objective function is to improve static and dynamic behavior. The different gains at the fundamental resonant frequency and the selected odd harmonics are obtained for the proposed quasi-PR controller. The dynamic characteristics of the optimized quasi-PR controllers show superiority against conventional ones in terms of gain margin, phase margin, overshoot, and robustness. With the proposed control scheme, the harmonics, reactive power, and unbalanced sequences are appropriately compensated. The performance of the PV-fed VSI shunt APF under irradiance change, load change, and distorted grid voltage conditions is validated through numerical simulations performed on hbox {PSIM}^{copyright } software. The results show that the grid currents Total Harmonic Distortion for irradiance change case study are 4.57%, 4.57%, and 3.22% in phase a, b, and c with the proposed control method, while they are 9.25%, 5.65%, and 10.12% with conventional instantaneous p-q theory.

A hybrid control strategy for dynamic compensation of cross‐coupling effect in two‐load IPT system

AbstractThe cross coupling between receiver coils complicates the control and optimization of “one‐to‐may” multi‐load IPT systems. In this paper, the cross‐coupling effect in a two‐load IPT system is thoroughly discussed. It is noted that the input impedance, the total output power and the power distribution ratio among receivers are all affected. After developing a new leakage‐inductance‐based model considering the cross‐coupling effect, it is suggested that the operating frequency is adjusted according to the cross‐coupling coefficient to ensure a constant power distribution ratio. The total output power and the input impedance are regulated by coordinated adjustment of the inverter's duty cycle and the transmitter‐side compensation capacitance with switched control capacitor (SCC) control. Based on the proposed hybrid control strategy (frequency, duty cycle, and SCC), the power distribution among loads and the total output power are kept constant under variable cross‐coupling coefficient, and the input impedance are shaped to be inductive to achieve zero voltage switching (ZVS) realization for all switches in the inverter. The theoretical analysis is finally verified by experimental results on a 35 W two‐load IPT prototype.

Capacitance balance control strategy for three-phase four-switch open-winding permanent magnet synchronous motor

The open-winding permanent magnet synchronous motor (OWPMSM) system is a novel high-cost system that uses dual-inverters to control both sides of the stator to improve the performance of the motor. To save the system cost, the three-phase four-switch open-winding permanent magnet synchronous motor (TPFS-OWPMSM) system is proposed to replace the general three-phase six-switch OWPMSM system, but it brings the problem of bridge arm capacitor voltage offset. To solve the instability problem caused by capacitance, this paper carries out the state analysis and offset source analysis of the inverter capacitor voltage, and proposes a dual-inverters capacitor voltage balance control strategy. The capacitor balance control converts unstable capacitor offset voltage into current and injects the currents into the stator current loop through closed-loop correction. And this strategy includes the optimal control of different capacitors under dual-inverters with the goal of system stability. The effectiveness of the balancing strategy is verified by the simulation of various motor working conditions.

A Generalized Multilevel Inverter With Extended Linear Modulation Range and Instantaneously Balanced DC-Link Series Capacitors for an Induction Motor Drive

The voltage drift control of series-connected capacitors across a single dc-link is the major challenge for a multineutral point-clamped (NPC) converter. This drifting is caused due to uneven currents drawn from the neutral points (NPs) during the PWM operation. This work presents the working principle of instantaneous voltage control of “ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$n$ </tex-math></inline-formula> ” dc-link series-connected capacitors for an induction machine load. The mechanism of instantaneous voltage balancing is realized by ensuring zero instantaneous currents at each dc-link NP. To get zero instant currents at a dc-link NP branch, the phase terminals tapping on the dc-link are manipulated with the help of cascaded hybrid bridges (CHBs). Charge balancing of CHB capacitors is also taken care of by proper utilization of pole voltage redundancies. The detailed results are presented for a small-scaled inverter (nine-level) with three NPs (four dc-link capacitors). The simulation and experimental results reflect the capability and limitations of such inverters. The results are presented at all modulation indexes operated under low-power-factor (LPF) and high-power-factor (HPF) load conditions. It is found that the linear modulation range (LMR) can be increased to full base speed even for an HPF load.

The Short-Circuit Fault Current Impact Mechanism and Adaptive Control Strategy of an MMC-HVDC

A modular multilevel converter (MMC) is voltage-sourced and can supply fault currents to an AC system. To clarify the fault current impact mechanism of an MMC, this paper examines the control and capacitor discharge processes of an MMC when an AC system has a three-phase short grounding fault. The theoretical analysis shows that the outer loop control of the MMCs changes the fault injection. In different control modes, the nonzero reference value of the outer MMC loop forces the converter to inject and absorb the current into the AC system when a fault occurs. The limiter of the control determines the final injecting current value of the MMC. To help the MMC adjust the AC system’s fault current, adaptive strategies are also proposed, which include an adaptive reference control, an adaptive limiter control, and an adaptive capacitor control. On the basis of the proposed strategies, the fault currents could be increased or decreased within the MMC’s capacity. The simulations verify the theoretical analysis.

Operation Characteristics of Discontinuous Current Mode for a Dual-Active-Bridge AC-DC Converter with an Active Energy Buffer

This paper discusses operation characteristics of discontinuous current mode (DCM) for an ac-dc isolated converter for on-board battery chargers of electric vehicles. The converter discussed in this paper consists of a front-end diode-bridge rectifier, an active energy buffer, and a dual-active-bridge dc-dc converter. The energy buffer circuit absorbs the power pulsation form the single-phase ac line into a small-rated capacitor, and thus, makes it possible to reduce a required capacitance of the dc-link capacitor. This paper explains details of the DCM control method including a technique of the ripple cancel of the inductor current, to realize the power factor correction of the line current and the voltage control of the buffer capacitor at the same time without complex calculations. Then, a reference value of the inductor current and an operation range of the dc-output voltage are discussed. Experimental results obtained by a 220-V, 6.6-kW experimental setup demonstrate a sinusoidal-line current and a controlled buffer capacitor voltage as well as a smoothed dc-output current.

A novel chance constrained joint optimization method under uncertainties in distribution networks

The joint optimization in distribution networks considering the uncertainties in wind power or photovoltaic (PV) outputs is a larger scale stochastic mixed integer nonlinear programming (MINLP) problem. However, how to efficient and accurate solve the problem under uncertainties is still a challenge. To handle the uncertainties, a scenario based chance constrained programming model is established. To improve the accuracy, the network reconfiguration and capacitor control are simultaneously performed by optimizing serious voltage and shunt current sources in the model by using equivalent network transformation. To improve calculation efficiency in dealing with chance constraints, a cluster of scenarios satisfying the confidence level is formed for optimization. To avoid introducing plenty of binary variables for the radial constraint and improving calculation efficiency, a heuristic method opening loops one by one is developed. The numerical simulations on distribution networks show the efficiency and accuracy of the proposed algorithm over the existing methods.

A New MPC Formulation Based on Suboptimal Voltage Vectors for Multilevel Inverters

Different new model predictive control (MPC) formulations have been recently proposed to reduce the real-time computation load, which makes the MPC algorithm promising for multilevel power converters. Unlike the conventional MPC formulation, which searches for the optimal switching state at each sampling time, the existing computationally efficient MPC formulations are to search for the optimal output voltage in the first stage. In the second stage, only the switching state redundancy under this optimal output voltage established in the first stage will be employed to achieve multiobjective. These computationally efficient MPC formulations based on searching an optimal voltage vector are usually validated on the power converter topologies with abundant switching redundancy or without floating capacitors. However, for the emerging topologies with less switching redundancy and floating capacitors, such as the five-level (5L) T-type nested neutral point clamped (T-NNPC) converters topology, the existing computationally efficient formulations based on optimal output voltage vector can lead to potential capacitor control failure due to the sacrificed multiobjective control performance. To address this issue, this article presents a novel MPC formulation based on suboptimal output voltage vectors considering both the system’s multiobjective control performance and computation burden reduction. With the determined suboptimal voltage vectors, a small group of the switching state candidate can be established to improve the system’s multiobjective control performance and efficiency. The proposed MPC formulation is finally validated on a 5L T-NNPC topology.

Development of improved sparrow search-based PI controller for power quality enhancement using UPQC integrated with medical devices

Photovoltaic (PV) panel interfaces with the grid create the issues regarding power quality such as voltage distortion, voltage harmonics, etc. To mitigate the harmonics in PV systems, Active Power Filters (APF) are taken. Unified Power Quality Conditioner (UPQC) is utilized for alleviating the problems correlated with voltage dip/swell in source voltage that has regulated the load voltage absolutely. On the other hand, the existing researches do not solve the power quality problems such as Voltage Stability Index (VSI), UPQC cost and power loss. Power quality is taken as the main demanding one in the analyzing the smart grid. Thus, APFs are primarily selected for compensating the problems occurring in power quality as they has some active compensation and are competent of filtering fast. Moreover, power quality constraint with load current must be addressed, which considers the factors like reduced power factor, huge Total Harmonic Distortion (THD) level, and unbalanced power. To reach the ideal efficiency on UPQC, the voltage control of the DC link capacitor is important. Thus, a new power quality enhancement model with UPQC especially for medical devices is focused. The UPQC-connected medical devices are suggested with a new Adaptively Updating-Sparrow Search Algorithm (AU-SSA)-based Proportional plus Integral (PI) controller, which is integrated with renewable energy like wind turbine Squirrel Cage Induction Generator (SCIG) to eliminate the current and harmonics imperfection accurately. This model implements a new power quality enhancement model by formulating a newly derived objective function. Finally, the experimental verification of the proposed scheme is done.

Research on LADRC of Grid-Connected Inverter Based on LCCL

A grid-connected inverter’s stability is easily influenced by the system’s internal and external characteristics, especially when the grid-connected inverter is connected to the grid by an LCL filter, which complicates the system design and operating conditions. To handle the problem of disturbance in the high-order grid-connected system of the inverter, a linear active disturbance rejection control (LADRC) strategy based on LCCL (which divides the capacitor of the LCL filter into two halves in a specified proportion) is proposed in this paper. First, the system is reduced from third order to first order by using the split capacitor control method, which solves the difficulties of complicated LCL grid-connected inverter controller design and easy resonance. Meanwhile, the internal and external disturbances of the inverter system are treated as generalized disturbances. Then, the LADRC control is used to adjust the closed-loop system’s parameters, which enhances the quality of the grid current and the system’s stability. Finally, the impacts of PI and LADRC control on the power mutation and voltage drop of an LCCL grid-connected inverter are compared by the experiments. The experimental results show the effectiveness of the proposed LADRC control strategy.

What is Impedance and Dielectric Spectroscopy?

Impedance and Dielectric Spectroscopy (IS/DS) is a broadband characterization tool that can provide a wide range of information not often accessible by using single frequency measurements or dc methods. With IS/DS one can detect and often isolate responses due to the bulk of the materials or devices, the presence of internal and external interfaces as well as capture details associated with charge storage and dissipation. Equipment suitable for making measurements over as many as 12 orders of magnitude in frequency is now easily available, and the technique has found niches in the characterization of batteries, fuel cells and solar cells because all of these devices contain many different material layers and a wide variety of electrical responses. Many studies in the literature describe examples of individual components of these energy related devices. IS/DS has also been used as a method of quality control of capacitors and to monitor aging of electrical and structural components. However, the method is not limited to energy related devices only. Any material and/or device regardless of its end application may be characterized to determine the contributions of the different components/interfaces encountered and the frequency dependencies observed. Responses for dielectric type materials are best interpreted in the dielectric formalism while more conducting materials are best described in terms of conductivity as a function of temperature and/or atmosphere.