DC Capacitor Research Articles

A Bidirectional Modular Cuk-Based Power Converter for Shore Power Renewable Energy Systems

Supplying shipping vessels with electricity at the ports can improve the air quality of the ports, reduce the greenhouse gas emissions from the shipping industry, contribute to the economic growth, and increase the political dependency of the countries by reducing the dependence on conventional fossil fuels. Several countries promote supplying the vessels when they are docking at ports from renewable energy systems by establishing dedicated funding mechanisms to remove the obstacles facing the shore power systems. In this context, this paper presents a new modular power electronic converter for shore power systems at shipping ports, which can perform three functions. Firstly, it will harvest the energy from a renewable energy source, such as hydroelectric and solar photovoltaic (PV) sources and ensure maximum energy extraction. Secondly, it will control the power flow from these sources to the battery storage. Finally, it will control the power flow from the battery to the vessels and/or the utility grid when necessary. A current-source converter based on isolated Cuk converter is used as the submodule (SM) of the proposed modular converter due to several features. The Cuk SM can provide high efficiency, minimised dc capacitance, and flexible output voltage higher or lower than the input voltage from the PV modules. To verify the mathematical analyses and computer simulations, experimental results are obtained from a small-scale modular prototype controlled by a TMSF28335 DSP.

A New Step-Up Switched-Capacitor Voltage Balancing Converter

The new step-up capacitor grid is connected to the solar photovoltaic (PV) having a voltage balancing converters. It’s suitable for Neutral-Point-clamped (NPC) Multilevel Inverter (MI). To balance the capacitor DC link voltage effectively we've got to use switched capacitors. By using balancing converter, we will enlarge the higher levels and also by boosting input voltage we are able to see the upper levels of voltage without using magnetic components. This attribute allows operating the input and output voltage by self-balancing. The balancing converter for NPC multilevel inverter-based solar PV system is operated by using vector scheme control. PV system incorporates a Maximum power point Tracking (MPPT) and it's executed in Matlab power system. The output of the simulation result topology can successfully balance the DC voltage and excess power from grid. The injected power within the grid can vary the solar light with active performances.

Control of Cascaded Multilevel Converter for Wave Energy Applications

This paper proposes a control scheme for a wave energy conversion system based on a linear generator and a cascaded multilevel converter. The mechanical conversion system is composed of a buoy connected directly to a linear generator. The windings of the generator are individually controlled by a cascaded multilevel power converter using independent maximum power point tracking to improve energy harvesting. The output of the cascaded converter is controlled to keep the DC capacitors balanced and generate a multilevel voltage at the output terminals which reduces the losses in the underwater transmission line. Experimental results show the performance of the proposed control scheme maximizing the power generation by imposing a current with the same waveform of the induced voltage and balancing the DC capacitors.

Design Space Analysis of the Dual-Active-Bridge Converter for More Electric Aircraft

In the literature, different DC/DC power electronic converters (PECs) have been found to interconnect high-voltage DC and low-voltage DC grids in the electric power distribution networks of aircraft. In this scenario, the dual-active-bridge (DAB) converter has been shown to be one of the most promising topologies. The main disadvantages of this PEC are the large output capacitance required to satisfy more electric aircraft (MEA) requirements and the high conduction losses produced in low-voltage power devices of (LV). Therefore, this paper proposes analytical models to determine the voltage ripple and root-mean-square (RMS) current in DC bus capacitors of DABs considering different modulation strategies. Moreover, an analysis of the design space in an MEA case study is performed to evaluate the influence of the design variables in power losses of power devices and peak-to-peak voltage ripple in DC bus capacitors. These models are useful for the design stage of this PEC, as well as to enable multi-objective optimization procedures by reducing the computational cost of these methodologies. Furthermore, the exploration of the switching frequency and limit of the modulation angle aid in reducing the resulting volume of the low-voltage DC capacitor.

Impact of Negative Solar Resistance on DC Microgrid Stability: Virtual Damping Voltage and Current Solar Droop Emulated Controller

The negative solar resistance, is highly dependent on the solar operating point. The system will become sluggish and unstable during low irradiance. It is reported that the impact of negative solar resistance is nullified by parallel impedance emulated condition (PIEC). During low irradiance, the system stability has been moved to an unstable position which is not fully resolved by PIEC. In this paper, a virtual inertia-damping voltage and current solar droop emulated controller is proposed to improve the voltage oscillation profile, inertia problem and damping of the DC microgrid system. In DC microgrids, the inherent inertia from DC capacitors is low, thus affecting the bus voltage oscillation during a reduction of irradiance. The oscillation of the system has been increased due to the equivalent impedance by negative solar resistance at lower irradiance. Oscillation’s issue and voltage instability of the PV system have been enhanced by the inner current controller. In addition, the steady-state error of the PV voltage is minimized by the virtual inertia-damping voltage restoration droop controller with PIEC. Performance analysis and dynamic characteristics are discussed through a pole-zero plot for the various conditions of negative solar resistance using virtual inertia-damping voltage and current Solar Droop Emulated Controller. This paper focuses on stability analysis to strengthen the DC microgrid. The performance of the solar controller with bidirectional converter (BDC) controller has been discussed to verify the power management between the system to manage the power for the unbalanced load.

Nonunit Distance Protection Algorithm for Multiterminal MMC-HVdc Systems Using DC Capacitor Resonance Frequency

High-voltage dc (HVdc) transmission has been largely used to interconnect asynchronous systems and integrate renewable energy resources into electrical grids. However, the high short-circuit currents and low-tolerance of power electronics equipment impose new challenges for these systems’ protection. To address these challenges, a new distance protection algorithm for HVdc grids with modular multilevel converters (MMCs) is proposed in this article. The proposed algorithm identifies the faulty line/cable using the resonance frequency of a dc capacitor installed in each terminal. The technique was tested in a four-terminal HVdc grid with five cables and ten circuit breakers. The algorithm was highly selective for faults in the lines and provided fast identification, in less than 1 ms, without communication amongst terminals. The algorithm was tested in hardware under high-noise conditions and provided reliable results.

Voltage-Fed Isolated Matrix-Type AC/DC Converter for Wind Energy Conversion System

In this article, a voltage-fed isolated matrix-type ac/dc converter (VF-MC) is proposed for wind energy conversion system (WECS). Instead of the line-frequency transformer, the high-frequency transformer (HFT) is used for isolation between the generator and the grid. The direct matrix converter (MC) is employed to connect the wind power generator and the HFT, and the intermediate dc capacitors are saved in the ac/ac conversion. Moreover, the proposed VF-MC can work without use of filter capacitors on generator side, and enable more compact size. The dedicated space-vector-modulation scheme and commutation method are proposed for safe operation of the MC. The voltage spikes due to the leakage inductance of HFT are thus reduced and the additional snubber circuit is avoided in the proposed system. A 1-kW laboratory prototype of the proposed VF-MC-based WECS is built to verify the effectiveness of the system experimentally.

Minimum DC Voltage Ripple Switching Sequence Elicitation for Dual Inverter Through AC Load Error Volt-Sec Computation

In voltage source inverters (VSI), the output voltage is synthesized in an average sense using pulsewidth modulation (PWM) techniques. In space vector PWM techniques, the average realization of the space vector using volt-sec balance results in an instantaneous error that influences the dc bus current. The instantaneous ac load error voltage maps onto the dc bus derive the dc bus current ripple signatures that would be useful to size the dc capacitor. Exploiting concept of instantaneous mapping of ac error voltage onto the dc bus through power factor axis coordinates, in this work, the unprecedented dc ripple voltage computational method is presented. The established synchronous power factor reference frame ensures accurate mapping of ac error voltage onto the dc bus and makes the computational method independent of sensing actual ac load current unlike conventional methods. With the eliminated actual load current dependence, the proposed model can predict the minimum dc voltage ripple yield switching sequence for VSIs at different loading conditions. The efficacy of the proposed model in terms of optimum switching sequence elicitation is demonstrated on the dual inverter configuration experimentally considering the inflated switching stress due to two inverter's switching.

Improving Stability and Power Sharing of Bidirectional Power Converters by Relaxing DC Capacitor Voltage

This article presents a new control structure for bidirectional power electronic converters. Commonly, a proportional-integrating (PI) controller is used in the dc capacitor voltage loop, to generate the set point for the current controller, which tightly regulates this voltage to a given value. This article shows that such an approach causes stability issues that are aggravated during reverse power flow operation and intensify during weak grid conditions and uncertainties. The proposed controller allows small variations of dc capacitor voltage in proportion to the power flowing through the converter. This improves the stability of the converter irrespective of the power direction and in the presence of uncertainties and weak grid conditions. The proposed control structure also simplifies the design process, and it can be applied to dc/dc and dc/ac (single-phase and three-phase) converters. Moreover, the proposed method grants an inherent power-sharing capability when multiple converters share the same dc bus. The article also adopts full state feedback and an optimal control approach, i.e., the linear-quadratic tracker (LQT), for systematic design of the current control loop and enhanced robustness. Detailed analyses, simulations, comparisons, and experimental results are included to demonstrate the effectiveness of the proposed approach.

Research and analysis on suppression and influence of transformer direct current bias

DC bias generates a large DC component in the windings of the transformer in operation, thereby generating harmonics, causing vibration, noise, and overheating. In severe cases, it can cause damage to the transformer, which will affect the safe and stable operation of the AC system. This article takes the DC bias of the main transformer of a 220kV substation as an example, and uses the capacitance isolation method to suppress the DC bias. Due to the addition of capacitor equipment at the neutral point, the zero-sequence impedance of the main transformer and the change of the zero-sequence impedance of the power grid are changed, which has an impact on the zero-sequence (directional) current protection and ground distance protection. Through theoretical analysis and simulation research verification, for general 220kV substations, the capacitor DC isolation method (1.2Ω) can be used to suppress DC bias, which has a small impact on the relay protection and does not need to adjust the related protection settings.

Design and Simulation of Fuzzy Logic Controller for DSTATCOM in Power System

This paper mainly presents the Distribution Static Compensator (DSTATCOM) and control methodology of Direct Current (DC) capacitor voltage. Generally, the DC capacitor voltage is regulated using a Proportional Integral (PI) controller when various control algorithms are used for load compensation. However, during load changes, there is considerable variation in dc capacitor voltage which might affect compensation. In this study, a fuzzy logic based supervisory method is proposed to improve transient performance of the dc link. The fuzzy logic based supervisor varies the proportional and integral gains of the PI controller during the transient period immediately after a load change. A considerable reduction in the error in DC link capacitor voltage during load changes compared to a normal PI controller is obtained. The performance of the proposed strategy is proved using detailed simulation studies.

Nonthermal Plasma Multi-Reactor Scale-Up Using Pulse Capacitive Power Supplies

The scale up of nonthermal plasma (NTP) reactors requires the simultaneous operation in parallel of a large number of units supplied from the same power supply. The present paper aims to demonstrate the feasibility of parallel operation of multiple mini-NTP reactors. In order to demonstrate the parallel operation of a large number of NTP reactors, three different types of power supplies are considered. In addition to the most simple and common solution, which involves the use of individual, independent power supply for each reactor (an ignition coil driven by a pulse generator), two other configurations of supplies (capacitive AC and capacitive DC), simpler and less expensive, are tested. The capacitive pulsed power supplies allow the generation of HV pulses by an AC power supply (usually an AC transformer), as well as by a DC power supply using an R–C circuit. For the DC resistive–capacitive configuration, the frequency can be adjusted. For all configurations, the power of the discharge can be modified by changing the value of capacitors or resistors. The feasibility of the proposed systems was demonstrated by assessing the concentration of hydrogen peroxide induced in water after plasma treatment. The obtained results reveal that the proposed capacitive AC and DC power supplies allow a large number of plasma reactors to operate in parallel independently.

Large signal stability criterion of AC–DC hybrid microgrids with constant power loads considering reference current limitation

Islanded AC–DC hybrid microgrids composed of new energy sources, constant power loads and energy storage system are typical non-linear systems, and guaranteeing large signal stability is a key issue. In this paper, the non-linear model of islanded AC–DC hybrid microgrids is established, and large signal stability criteria are obtained. Considering reference current limitation of the energy DC–AC converter, the derived large signal stability criterion gives important constraints on the constant power loads power, the power of energy storage unit, DC bus voltage, the inductor and equivalent resistance of the AC filter, DC capacitor, the proportional parameter of the inner current controller and the maximum instantaneous power of the DC–AC converter. The paper is summarized as follows. Firstly, the equivalent model of the AC–DC hybrid microgrids in DQ rotating frame is established. Then based on mixed potential theory, the mixed potential model is constructed, and large signal stability criterion is derived and optimized considering the reference current limitation of the DC–AC converter. Finally, the presented criterion is verified by simulation and experimental results.

The Virtual Inertia Control of MMC-HVDC Considering the Effect of Active Power Loss of DC Line

Modular multi-level converter based high-voltage Direct-Current (MMC-HVDC) grid integration is an effective measure to achieve offshore wind power grid integration of large-scale and long-distance. As the offshore wind farms get farther and farther offshore, the active power loss of their submarine DC cables has become a non-negligible part of offshore wind power's participation in frequency regulation. In this paper, on the basis of simulating the inertia of synchronous generator with DC capacitor energy, a virtual inertial control strategy considering the influence of active power loss of DC cable is derived. And, the influence on the frequency stability of the system after considering the active power loss of the DC cable is also analyzed at the same time. Finally, a model of offshore wind power grid-connected through MMC-HVDC is built on MATLAB/Simulink. The simulation results show that the virtual inertia control strategy considering the active power loss of the DC line can effectively improve the frequency stability of the power system.

A Hybrid Multivector Model Predictive Control for an Inner-Interleaved Hybrid Multilevel Converter

A hybrid multivector model predictive control strategy for an inner-interleaved hybrid multilevel converter is brought forward in this article, which can enable the separation of the low- and high-frequency stages. It initiates with the use of sign patterns of the reference vector in the stationary reference frame to determine switching states of the low-frequency stage. Then, the reference vector is converted to the inner virtual space vector diagram, which is further transformed into a 120° oblique coordinate system, where the adjacent vectors are selected. Further, the current tracking is realized through the duty cycle optimization of the chosen vectors. Finally, a symmetric switching sequence (SS), which serves as the best one for dc capacitors voltages balancing and circulating current suppression, is selected among all the SSs that belong to the three chosen vectors with optimal duty cycles. The proposed method reduces both current ripples and computational burden while achieving a constant equivalent switching frequency. Comprehensive experimental results performed on an all silicon-carbide prototype verify the effectiveness of the proposed control.

Comparison of small-signal admittance-based models of doubly-fed induction generators

This paper reviews and compares the main doubly-fed induction generator (DFIG) models for stability assessment in the literature by PSCAD/EMTDC simulation. The main contribution of the paper is a detailed study on small-signal admittance-based modeling of DFIGs for stability analysis. A complete DFIG model characterized from a 2x2 admittance-based transfer matrix is proposed which has frequency coupling between positive and negative sequences resulting from considering the PLL and the DC capacitor. In addition, two simplified DFIG models in the literature are derived from the complete model and discussed. The models are applied to stability analysis using the generalized Nyquist criterion and the positive-mode-damping stability criterion. The predictions are validated for several mechanical speeds and short-circuit ratios by PSCAD/EMTDC time-domain simulation. The study shows the need to apply the complete model to accurately assess subsynchronous stability, as significant differences from simplified models may occur at low frequencies.

Proportional‐resonant based control strategy for grid‐connected packed‐E cell inverters with Lyapunov filter‐based PLL

A proportional-resonant (PR)-based current control strategy for grid-connected packed-E cell (PEC) inverter is presented. Unlike the existing control strategies, which are based on proportional-integral controller, developed for packed-U cell and PEC inverters, the proposed PR-based control strategy achieves zero steady-state error in the grid current. Furthermore, it eliminates the necessity of employing the control loop for regulating the dc capacitor voltages. Also, the grid current synchronization is achieved by using a Lyapunov filter-based phase locked loop (PLL). The consequence of using Lyapunov filter-based PLL is that sinusoidal synchronization signal can be extracted from the distorted grid voltage. The performances of the proposed control strategy under steady-state, load variation and non-ideal grid voltage are investigated on a laboratory-scale experimental prototype. It is reported that the control of both dc- and ac-side variables of the system are accomplished.

Isolated Resonant Switched Capacitor Converter With Voltage Self-Balancing Ability

This article proposes an isolated resonant switched capacitor converter (I-RSCC) for high-voltage applications. The dc capacitors’ voltages achieve self-balancing with the resonant switched capacitor units (RSCUs). The voltage stress of the switching devices is reduced. The isolated rectifier (IR) is used for galvanic isolation and energy transfer. The number of switching devices is reduced by multiplexing parts switching devices functions, which helps the compact design. In this article, the topology and operation principle of I-RSCC are introduced in detail. Then, the output characteristics of I-RSCC are analyzed, including the voltage balance operation, the energy transmission path, the voltage conversion ratio, and the output power. The design example of I-RSCC is also presented. Finally, a 10-kW prototype is built to verify the feasibility of the proposed I-RSCC topology. The results show that soft switching is realized, and the maximum operation efficiency is 96.5% and above 96% at the rated power.

Improved Frequency Control Strategy for Offshore Wind Farm Integration via VSC-HVDC

Voltage source converter based high voltage DC system (VSC-HVDC) has become a very promising solution to integrate offshore wind farm. However, the equivalent inertia of the modern power system with large renewable energy integration becomes small, which will arouse some frequency stability problems. To tackle this problem, this paper proposes an improved frequency regulation strategy for VSC-HVDC integrated offshore wind farm. Firstly, in the frequency decrease stage, the rotor kinetic energy of wind turbines (WTs) is used to suppress the decrease of the frequency, and the control parameters are determined to make full use of the mechanical power and rotor kinetic energy of WTs, the frequency nadir is improved. Secondly, in the rotor speed recovery stage, the DC capacitors of VSC-HVDC are used to release power to compensate the deficiency value of wind farm output power and avoid the secondary frequency drop (SFD) problem. Lastly, the simulation is conducted in PSCAD/EMTDC to validate the effectiveness of the proposed coordinated frequency control strategy.

Improving DFIG performance under fault scenarios through evolutionary reinforcement learning based control

The doubly fed induction generator (DFIG) usually experiences high rotor current and DC capacitor link voltage spikes during system fault events. In this paper, a novel data-driven approach is proposed to enhance DFIG performance under fault scenarios. An advanced reinforcement learning algorithm called guided surrogate-gradient-based evolution strategy (GSES) is used to control the DFIG power and capacitor DC-link voltage by adjusting the optimal reference signals. This controller is able to prevent the DFIG rotor from over-current risk and maintain grid-connected operation. The proposed GSES-based control algorithm was evaluated through simulations on a 3.6-MW DFIG in the PSCAD/EMTDC software. Results have validated the effectiveness of the proposed GSES-based control algorithm in improving DFIG performance under various fault scenarios.