Current Sharing Research Articles

A Distributed Current Sharing Strategy for Islanded AC Microgrids Based on Low-Bandwidth Communication

Bus voltage regulation and current sharing are significant to ensure the normal operation of islanded microgrids. In this paper, a low-bandwidth communication (LBC)-based distributed current sharing strategy for islanded ac resistive microgrids is proposed, especially when the wire impedances of parallel-connected inverters are mismatched. In a general case, all the balanced, unbalanced and harmonic components of load current should be shared accurately. Therefore, in the local controller, those components of inverter output currents are extracted and regulated to follow the component average values generated based on LBC. To guarantee the bus voltage regulation, the influence of current sharing loops on voltage references is eliminated by subtracting the average voltage reference compensation from the current sharing controller outputs. A simplified inverter small-signal model is established, based on which the system stability is analyzed. Besides, the steady-state bus voltage regulation performance is discussed. The simulation in MatLab/Simulink is used to demonstrate the current sharing performance and bus voltage regulation ability. A hardware-in-loop platform of an ac microgrid prototype is built, where the power stage is in a real-time simulator and the controllers are implemented with DSPs. The results validate the effectiveness of the proposed strategy.

Distributed Dynamic Event-Triggered Control for Voltage Restoration and Current Sharing in DC Microgrids

Due to the droop control applied in islanded DC microgrids, DC bus voltage recovery, and current sharing cannot be guaranteed simultaneously. To solve this problem, a distributed dynamic event-triggered control method is proposed in this paper. The presented control scheme can remove DC bus voltage deviation while achieving the current sharing among distributed generation (DG) units. In addition, the proposed method requires no global information, and only the DC bus voltage from the physical network is needed. Moreover, the data of neighboring DGs at only event-triggered times are involved, which greatly reduces the communication burden of the DC microgrid communication network. The stability of the proposed method is demonstrated through the Lyapunov function, and Zeno behavior can be excluded as well. Finally, the experimental results with four case studies are presented to validate the effectiveness of the proposed method.

Consensus-Based Single-Phase Control of Three-Phase Inverter for Current Sharing in Three-Phase Four-Wire Isolated AC Microgrids

This work proposes a novel secondary control system based on the Consensus algorithm for balancing the RMS current between the distributed generation (DG) phases in a four-wire isolated Microgrid (MG) containing unbalanced and non-linear loads. The control strategy proposes using a droop control per phase to regulate the voltage magnitude, whereas the harmonic current sharing is performed using the virtual impedance concept. Additionally, the proposed control scheme guarantees voltage regulation, causing the MG to operate at its nominal voltage value. In addition to sharing the RMS value of the current, the consensus algorithm also provides the equal sharing of the RMS value of the harmonic current between each phase of the DGs that comprise the MG and also ensures that the harmonic distortion of the voltage in the PCC of each DG remains within limits defined by the standards and recommendations. Simulation results demonstrate the functioning and effectiveness of the proposed control strategy.

Simulation-Assisted Design of a Power Stack for Improving Static Current Sharing Among Three IGBT Modules Connected in Parallel

In this study, cosimulation with Q3D and Simplorer were adopted to design a single-phase power stack with equal output current on three paralleled IGBT modules in high power application, therefore, the derating of total current will be reduced, which can improve the current utilization and power density of power stack. The performance of the designed stack was verified using a double-pulse test (DPT). The circuit used in the DPT regarded the power stack output current as static current, which is a critical index to evaluate current sharing. The static current sharing among the IGBT modules was mainly dependent on the stray inductance of the DC busbar, IGBT modules, and phase output bar (POB), which is used to connect the IGBT modules to load, in parallel paths of the power stack. Cosimulation was performed to determine the stray inductance of the IGBT modules, DC busbar and POB, and a constant-current-slope method was used to verify the inductance. Subsequently, design the shapes of DC busbar and POB for attaining maximum ratio of imbalanced current (MRIC) of the three IGBT modules is within 3% under 200% (2400A) rated current. The cosimulation results indicated that the MRIC of the current paths in power stack were 0.8%. Finally, three IGBT modules and a gate driver were used to construct power stack with 1000V DC link. The experimental results obtained by DPT indicated that the MRIC was 0.9% under 200% rated current, demonstrating the effectiveness of the proposed stray inductance design method.

A Dynamic Droop Control for a Dc Microgrid to Enhance Voltage Profile and Proportional Current Sharing

A Dynamic Droop Control for a Dc Microgrid to Enhance Voltage Profile and Proportional Current Sharing

An Iterative Virtual Impedance Regulation Strategy in Islanded Microgrids for Enhanced Balanced, Unbalanced and Harmonic Current Sharing

The balanced, unbalanced and harmonic current sharing inaccuracy caused by the mismatched wire impedances of distributed generation (DG) units influences the normal operation of islanded microgrids. To deal with this issue, an iterative virtual impedance regulation (IVIR) strategy is proposed in this paper, both the resistive and reactive parts in the output impedances of DG units are regulated. The whole process is decomposed into several successive iterations. In each iteration, the fixed virtual impedance and adaptive virtual impedance regulation are combined to generate a virtual resistance increment and uniform virtual reactance increment according to the uniform output current magnitude and phase angle respectively. Based on these increments, the virtual impedance at fundamental-negative and harmonic frequencies also are modified. With the proposed strategy, all the balanced, unbalanced and harmonic current sharing is accurate with a relatively small virtual impedance. Only the local current information is required for the IVIR process, and the communication is merely used to exchange the VIR mode status flags at the beginning and the end of the IVIR process, the dependency on communication is minimized. The feasibility of the proposed strategy is verified with the simulation and experimental results.

Development of Four-Channel Buck-Type LED Driver with Automatic Current Sharing

A buck-type light-emitting diode (LED) driver is proposed herein. The proposed LED driver automatically possesses current sharing and high step-down voltage gain. Without complex control, the proposed LED driver, with a single input and multiple outputs, can achieve automatic current sharing of four-channel LED strings, even under the different number of LEDs of each LED string. Furthermore, as compared with the traditional four-phase interleaved buck converter with a single input and a single output having current sharing required, the proposed circuit has the duty cycle up to 0.5, not 0.25, meaning that under the same input voltage the latter has a wider output voltage range than that of the former. Above all, if the proposed circuit with N outputs, then it still has the duty cycle up to 0.5, not one over N as shown traditionally. Moreover, as compared with the current sharing based on the differential-mode transformer, the proposed circuit has no magnetic resetting loop required. In this paper, the operating principles and design considerations of the proposed converter are discussed. Finally, the theoretical analyses and performances of the proposed LED driver are verified by simulation and experiment.

Partial Series Resonance Pulse Commutated Current-Fed Three-Phase Current Sharing DC–DC Converter: ZCS Analysis, Design, and Experimental Results

A three-inductor current-fed converter leveraging short pulse series resonance to facilitate zero-current switching (ZCS) and voltage clamping of the semiconductor devices is proposed and investigated in detail. The circuit exploits series resonance characteristics during conduction overlap of any two switching signals that naturally reduce the current in outgoing switch to zero accomplishing zero-current turn- off . It succeeds in natural switch voltage clamping eliminating the traditional snubber requirement. Owing to the short resonance period, circulating current through the converter limits the conduction losses particularly at light load, thus improving the light-load efficiency. In addition, current-sharing topology offers reduced current stresses through the primary side devices and simpler gate driving requirements because of common source grounded with power supply. Detailed analysis and design are reported. Experimental results from 1-kW proof-of-concept hardware prototype are established to validate the projected claims and confirm the performance and the efficacy of proposed converter. This concept has been verified in single-phase topologies and first time studied for three-phase current-fed circuit.

Distributed Resilient Optimal Current Sharing Control for an Islanded DC Microgrid Under DoS Attacks

For DC microgrids (MGs), cyber attacks of distributed secondary control system result in communication faults and thus, cause serious stability and cyber-security issues. In this paper, the cyber-security control problems for current sharing and voltage restoration of an islanded direct-current (DC) MG under denial-of-service (DoS) attacks are addressed. To ensure the stable system operation under DoS attacks, a distributed resilient control method is proposed in the secondary control layer. Subject to DoS attacks, the proposed control method does not only achieve bus voltage restoration but also realize the optimal current sharing optimization calculated by the tertiary layer all the time. Besides, different from most existing secondary control methods with a fixed sampling rate, a new resilient sampling mechanism is designed in the secondary layer to improve the security of the whole system against DoS attacks. The theoretical analysis proves that the proposed distributed resilient controller, which is easy for implementation, can still ensure the stability of the overall DC MG system under DoS attacks. Based on our theoretical analysis, a guideline for controller parameter selection can be used for the MG system design in the planning phase, which could enhance the safety of real-time operation. To test the proposed control method, a DC MG test system is built in a controller-hardware-in-the-loop (CHIL) testing platform. The effectiveness and robustness of our proposed control strategy is validated by the CHIL results.

Performance analysis of adaptive smart controllers for islanded microgrid

Abstract This paper presents implementation of control method for dual control of Voltage and Current control and Power sharing coupled with Demand Response for the islanded microgrid. These three controller viz Voltage and current controller, Power sharing controller and smart load controller while acting in unison can able to maintain long layoff from the connection requirement of the main grid. The paper uses state space inverter based generator model with close loop Voltage and Current control and Power sharing controller will enhance microgrid stability and transient response where smart load controller which uses state space model for voltage,current and Phase Locked Loop units to estimate reserve power available from smart load for better voltage and frequency regulation. Voltage current Controller and Power Sharing Controller will also assist the microgrid for smooth disconnection of microgrid from main grid while smart load controller will use its point of load voltage control for smooth islanding of microgrid. This control technique henceforth called tripartite control technique, compared with classical controller connected to microgrid in maintaining frequency, voltage with power sharing of the inverter driven by renewable generator. This paper investigate the operation performance issues of test Microgrid using Tripartite Control Technique. Simulation results indicate close approximation with experimental results

High Gain and High-Efficiency Bidirectional DC–DC Converter With Current Sharing Characteristics Using Coupled Inductor

Application of coupled inductor in designing nonisolated bidirectional dc-dc converter provides flexibility to attain high voltage conversion ratio both in buck and boost mode of operation. In this article, a new coupled inductor bidirectional converter is designed with low winding turns ratio (n = 1). The proposed topology uses two current path inductor structure which improves voltage conversion ratio and shares current in all operating modes. All the active switches in the proposed topology are soft switched utilizing synchronous rectification concept. Higher efficiency operation is possible as no extra circuit elements are required to achieve soft switching. Leakage energy of coupled inductor is successfully stored in a clamped capacitor which is utilized in the circuit topology. The voltage stress and current stress level of main active switches are low. The proposed circuit is simulated and performance parameters are verified through hardware results. Both simulation and experimental results performed on a 250 W prototype confirm to attain a high efficiency of the proposed converter with simple hardware requirement for practical implementation.

Decentralized Adaptive Voltage Control and Equal Current Sharing of Parallel- Connected Buck Converters via Wavelet Neural Network Approximators

A parallel connection of Buck converters improves system reliability and efficiency. However, the open circuit fault, load, and supply voltage uncertainties, and interactions among the converters increase the complexity of output voltage control and balanced current sharing. Thus, in this paper, first, a decentralized backstepping sliding mode control strategy is designed to meet such challenges. However, this controller is quite conservative since the uncertainties and interaction bounds are not known. Moreover, the sliding mode based controllers suffer from chattering phenomena which limits the practical applications. Therefore, a decentralized adaptive backstepping control strategy with wavelet neural network approximators is proposed. This strategy reduces the chattering and approximates the uncertainties and interactions by replacing the switching terms with wavelet neural networks. To show the effectiveness of the proposed controller, different numerical simulations have been performed in the presence of reference voltage changes, load, supply voltage variations, and open circuit faults.

A Robust Control of a High-Power Isolated Battery Charger with Current Sharing Capability Under Transformer Parameters Uncertainty

A Robust Control of a High-Power Isolated Battery Charger with Current Sharing Capability Under Transformer Parameters Uncertainty

Current Sharing Control of an Interleaved Three-Phase Series-Resonant Converter with Phase Shift Modulation

Recently, three-phase series-resonant converters (SRCs) have been proposed for high power applications. Three-phase SRCs can achieve zero-voltage-switching (ZVS) of the primary power switches and regulate the output voltage by pulse-frequency modulation. The interleaving technique is also a conventional method for DC-DC converters to achieve a high power level, reducing the output voltage ripples due to operating out of phase at the same frequency between the two converters. However, an interleaved three-phase SRC cannot easily synchronize switching instants between the two modules due to the component tolerances of circuits. In the proposed control method, phase shift modulation (PSM) is used to solve the output current imbalance caused by component tolerances. The power switches of the converter can also maintain synchronizing switching instants between the two modules. Therefore, the lower output voltage ripple can be achieved. A detailed analysis and design of this new control method for interleaved three-phase SRCs are described. Finally, prototype converters with a 2.4 kW total output were built and successfully tested to verify the feasibility of the current sharing modulation.

Study on Parallel Current Sharing Technology of Power Module in DC Charging Pile for Electric Vehicle

In order to meet the changing requirements of multiple types of electric vehicles, the layout and development of quick charging and DC charging piles are rapid. The modularized DC charging pile becomes the popular research field. Due to the differences in the parameters of each module in the charging pile, the current assumed by each module in parallel may not be the same, leading to the premature aging and damage of some modules and affecting the service life of the charging pile. In this paper, the isolated full-bridge DC/DC converter is taken as the power module topology, and the parallel current-sharing technology is studied. Two control methods are designed, one is the double-closed-loop cascade control of the voltage outer loop and the current inner loop, and the other is the adaptive double-loop based on charging mode. The current-sharing loop is added in the two controllers and the averaging-current method is adopted and the simulation verification of two-module and four-module in parallel is carried out. Through the comparison of load switching, the advantages of the he adaptive double-loop control method in the multimodule parallel connection, such as large stability margin and fast response speed, are validated.

Interleaved High Step-Up Current Sharing Converter with Coupled Inductors

An interleaved high-step-up current sharing DC–DC converter with coupled inductors is proposed in this paper. The operation principle and property of this converter are analyzed. The ripple of the input current in the proposed converter is decreased significantly by using the two-phase parallel interleaved input. The voltage gain is extended and the switch voltage stress is reduced by the coupled inductors. The leakage inductance of the coupled inductors reduces the reverse-recovery problem of the output diode, resulting in the reduction of reverse-recovery losses. As there are two interleaved phases in the proposed converter, the third winding of each coupled inductor is embedded in another phase. With this design, when the leakage inductance or duty cycle is asymmetrical, the current sharing performance is still positive. Consequently, the new topology is very suitable for applications to occasions with low voltage input and high voltage output, such as the fuel cell power system. Finally, the performance of this topological circuit is verified by a prototype with 500 W output.

Optimal Decision on Current Sharing and Voltage Balancing in DC Microgrids With Feedforward Input

This article is concerned with the issue how to achieve proportional current sharing while realizing bus voltage balancing in DC microgrids, or vice versa. In order to address this issue, proportional current sharing is realized first and then the optimal bus voltage is obtained by using the least square method; Second, assuming that the bus voltage balancing is realized, the optimal current sharing is solved by using Lagrange multiplier method. It can be seen that the results of the two cases are identical. Then a global performance index relating to the generation capacities is presented to make an optimal decision on voltage balancing and current sharing. In order to decrease the communication burden, the distributed control schemes are proposed to replace centralized control strategies. It can be proved that the equilibrium points of distributed schemes are completely identical to the optimal solutions without significantly increasing system complexity. Finally, the effectiveness of the proposed distributed schemes is shown via a simulation example.

Current Sharing Based on Incremental Passivity and Unknown Load Finite-Time Estimation for Multilevel Connected DC–DC Converters

The multilevel connected dc–dc converters system is considered a practical technology to increase the capacity and reduce voltage and current stresses on the power devices. In practice, mismatched parameters are inevitable, which increase the challenge of output voltage regulation and current sharing control. In order to conquer these difficulties, this article investigates the current sharing based on incremental passivity control for multilevel connected dc–dc converters. Combined with passivity-based control, a finite-time estimator is given to improve the robustness against the load variations. The proposed method results in a linear control law despite the bilinear nature of this system. Moreover, analysis exhibits that stability and current sharing are independent of inductor parameters. Furthermore, stability of the closed-loop system has been proved that the proposed approach is valid both for a linear and nonlinear load. After introducing the proposed control scheme, simulation and experiments are offered to validate its fast transit response and antidisturbance performance.

Design of Nonlinear Droop Control in DC Microgrid for Desired Voltage Regulation and Current Sharing Accuracy

It is well known that there is a design tradeoff between voltage regulation and load current sharing in conventional linear droop control for several parallel-connected distributed sources in a DC microgrid. To improve both performance areas, this article proposes a design method to identify the nonlinear droop curve coefficient with the desired bus voltage regulation and current sharing accuracy in a specified heavy load range, while a linear droop function with a negative droop resistance is used to tighten the bus voltage regulation in the light load range. The design procedure of load boundary and the effect of negative droop resistances on the system stability are analyzed in detail. Also, a boost-up and curve-fitting method can further improve the voltage regulation and achieve smooth transition between light load and heavy load. In this way, no additional voltage compensation loop is needed. Two parallel-connected DC converters are finally built to verify the theoretical analysis.

Current Sharing Analysis for Novel Paralleled CLLLC Converters

In the field of high-capacity bidirectional converters, paralleled LLC converters are usually used to increase the output power and system power density. However, because of the production tolerances the parameters of the resonant components for each paralleled unit may not be completely identical. The gain characteristic of the LLC resonant converter is sensitive to the parameters of the resonant components. For the paralleled converters, because voltage gains can be slightly different from each other for a same input, system output current may be poorly shared among the parallel units. In this paper, novel paralleled CLLLC converters are proposed. For the proposed converters, the load current sharing performance is not sensitive to component parameter differences or derivations. Equal load sharing can be realized easily. And no additional components or controllers are needed. The gain characteristics are analyzed and they are similar with that of a single CLLLC resonant converter. The mathematical model of the paralleled converters is deduced based on fundamental harmonic approximation (FHA) method. And the load current sharing errors for the conventional and proposed paralleled CLLLC converters are compared or analyzed by introducing variables such as the load current sharing coefficient. It is shown that for the same set of component parameters, when the conventional paralleled converters are used most of the load may be supported by a single converter while the output power of another one is small. On the other hand, for the cases of parameter derivations, the current sharing performance of the novel proposed paralleled converters is obviously much better. And finally the effectiveness of current sharing for the proposed paralleled CLLLC converters is verified by simulations and experiments.