Accurate Current Sharing Research Articles

Adaptive Control Approach for Accurate Current Sharing and Voltage Regulation in DC Microgrid Applications

A DC microgrid is an efficient way to combine diverse sources; conventional droop control is unable to achieve both accurate current sharing and required voltage regulation. This paper provides a new adaptive control approach for DC microgrid applications that satisfies both accurate current sharing and appropriate voltage regulation depending on the loading state. As the load increases in parallel, so do the output currents of the distributed generating units, and correct current sharing is necessary under severe load conditions. The suggested control approach raises the equivalent droop gains as the load level increases in parallel and provides accurate current sharing. The droop parameters were checked online and changed using the principal current sharing loops to reduce the variation in load current sharing, and the second loop also transferred the droop lines to eliminate DC microgrid bus voltage fluctuation in the adaptive droop controller, which is different and inventive. The proposed algorithm is tested using a variety of input voltages and load resistances. This work assesses the performance and stability of the suggested method using a linearized model and verifies the results using an acceptable model created in MATLAB/SIMULINK Software Version 9.3 and using Real-Time Simulation Fundamentals and hardware-based experimentation.

A simplified consensus-based distributed secondary control for battery energy storage systems in DC microgrids

DC microgrids have become a promising solution for efficient and reliable integration of renewable energy sources (RESs), battery energy storage systems (BESSs) and loads. To simultaneously achieve average voltage regulation, accurate current-sharing and state-of-charge (SoC) balance, at least two state variables need to be transmitted between neighboring BESS nodes in conventional distributed secondary control. In this paper, a simplified consensus-based distributed secondary control for BESSs in DC microgrids is proposed with only one virtually defined state variable being transmitted, where a cascaded control framework consisting of an SoC controller and a voltage controller is used to regulate DC bus voltages. This virtual state variable combines BESS SoC and its converter output current, there is no need to change the control law when BESSs switch between charging/discharging modes. Once SoCs are balanced, SoC controller works as a current controller. With the proposed control strategy, the number of transmitted state variables and the complexity of controller design are significantly reduced. Stability and steady-state analysis are also conducted to confirm the effectiveness of the proposed control strategy. Finally, Simulink simulation and hardware-in-loop tests are presented to validate the proposed control strategy.

Distributed Data-Driven Secondary Regulation for the Conflict Between Voltage Recovery and Accurate Current Sharing in DC Microgrids

The problem of voltage restoration and current balancing has been a hot topic in the study of DC microgrids (MGs), and how to engineer a secondary controller to solve the conflict between these two is the concern of this paper. With the above consideration, an optimal distributed data-driven secondary control strategy is proposed in this paper, which relies entirely on the input and output (current measurements) data of DC MGs. The proposed framework completes the design of a secondary control strategy for DC MGs based on a data model for the first time, which incorporates the physical coupling relationship between voltage and current into the controller design and is independent of the global circuit parameters and topology information. As a result of the proposed cost function with future current prediction, the designed secondary control strategy not only retains the advantages of consensus control, such as facilitating the plug-and-play of generation units, but also enables DC MGs to achieve better output current quality at steady state, fast dynamic response and improves robustness to external disturbances. Unlike the existing schemes based on average voltage observer, this paper resorts to a combination of limiting the voltage reference value generated by the secondary controller and a current cooperative strategy based on the learned data model to achieve an easy-to-use trade-off between the two objectives of voltage regulation and current sharing in DC MGs. From the design and implementation point of view, the solution proposed in this paper is going to be simpler. Then, an analytical model of a closed-loop DC MG system under the proposed control scheme is developed, and the analysis of stability and consensus of weighted output current is given accordingly. Finally, the proposed control strategy is tested on a hardware DC MG system with solar panels and an MPPT controller to verify its feasibility.

Accurate Current Sharing and Voltage Regulation in Hybrid Wind/Solar Systems: An Adaptive Dynamic Programming Approach

Renewable energy is an advisable choice to reduce fuel consumption and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\rm CO_{2}$ </tex-math></inline-formula> emission. Therein, wind energy and solar energy are the most promising contributors to reach this goal. Although the hybrid wind/solar system has been widely studied, the real-time current sharing based on their maximum capacities is rarely achieved in terms of seconds. Based on this, this paper proposes an accurate current sharing and voltage regulation approach in hybrid wind/solar systems, which is based on distributed adaptive dynamic programming approach. Firstly, the equivalent wind/solar model is built, which is an indispensable preprocessing to achieve the complementary between wind energy and solar energy. Therein, the wind energy and solar energy can output relative current according to their respective capacity ratio, which ensure the maximum utilization ratio of renewable energy source. Furthermore, current sharing and voltage regulation problem is switched into optimal control problem. Under this effect, each source agent aims to obtain the optimal control variable and achieve accurate current sharing/voltage regulation. Moreover, an adaptive dynamic programming approach based on Bellman principle is proposed. It can achieve accurate current sharing and voltage regulation. Finally, the simulation results are provided to illustrate the performance of the proposed adaptive dynamic programming approach.

Nonlinear Controller Design to Enhance Voltage Restoration and Current Sharing Accuracy of Islanded DC Microgrids

This article proposes a feedback linearized control scheme for a single-input two-output control problem of parallel-connected dc&#x2013;dc voltage source converters in islanded dc microgrids. The designed current sharing controller can be reduced the voltage deviation in the islanded dc microgrids. In this control scheme, proportional-integral and proportional-derivative controllers are applied with the design of nonlinear single-input two-output feedback linearized control law to regulate the output voltage and current of each converter. The proposed controller performance is investigated under different case studies. The proposed nonlinear control scheme improves the dc-bus voltage restoration and current-sharing accuracy in the islanded dc microgrids.

Hierarchical energy management system with multiple operation modes for hybrid DC microgrid

This paper presents a developed energy management system (EMS) with a hierarchical three-level distributed control approach proposed for a photovoltaic/wind turbine/diesel generator with energy storage in an islanded hybrid DC microgrid (MG). In the hierarchical three-level control structure, the power to be transferred by the distributed energy sources (DER) is provided by controlling the converters in the primary control layer. The secondary control layer is used to achieve DC bus voltage restoration and increase current-sharing accuracy simultaneously. In the tertiary control layer, it is aimed to minimize the production cost by performing optimum load-sharing. For the operation of this control structure, a strategy is proposed to optimally manage the energy demand of the hybrid system autonomously under variable load conditions. The EMS with three different operating modes (Mode-I, Mode-II, Mode-III) is proposed to share power between DC MG sources optimally. In Mode-I, renewable energy sources (RES) are enabled to operate with MPPT operation. Energy consumption minimization strategy (ECMS)-based linear programming (LP), genetic algorithm (GA) and pattern search (PS) algorithms are used to minimize the fuel consumption and carbon emissions of the diesel generator (DG) in Mode-II operating Mode. In Mode-III operating mode, DG is not needed, and the power flow between DERs is optimized by using the fuzzy logic controller (FLC) and state machine control strategy (SMCS). The amount of fuel consumed, the cost of fuel consumption and ESS's SOC level are used as the EMS's performance criteria for the MG. The minimum fuel consumption of the DG is calculated as 2.328 L for the operation of LP-based ECMS, and the fuel cost, in this case, is found to be $3.26. Considering the charging of the ESS, the highest SOC level is achieved with 64.87% for the PS-based ECMS-SMCS combination. By considering fuel minimization and SOC level together, it can be seen that the best result is achieved with the LP-based ECMS-SMCS combination.

Intelligent and Fast Model-Free Sliding Mode Control for Shipboard DC Microgrids

This article proposes an intelligent and fast controller for the parallel dc-dc converters in the islanded dc microgrids (DCMGs) in the shipboard power system (SPS) applications. The existence of constant power loads and propulsion loads in the SPS makes the voltage regulation an important issue in the islanded DCMG. In this article, the goal is to provide a fast response and proper voltage regulation and optimal current sharing in the stand-alone shipboard DCMG. To achieve this, a nonlinear I-V droop control is first designed in the primary control level to ensure the basic performance. Then, a new model-independent technique, entitled intelligent single-input interval type-2 fuzzy logic controller (iSIT2-FLC) combined with sliding mode control (SMC), is suggested to further improve the voltage regulation and current-sharing accuracy. In this control scheme, an extended state observer (ESO) is employed to estimate the unknown DCMG dynamics, whereas the SMC is adopted to eliminate the ESO estimation error. Moreover, the dynamic consensus algorithm (DCA) is implemented at the secondary control level to achieve coordinated control between DGs in the SPS. The effectiveness and applicability of the new suggested control approach are validated with hardware-in-the-loop (HiL) experimental results.

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.

Control of Distributed Generators and Direct Harmonic Voltage Controlled Active Power Filters for Accurate Current Sharing and Power Quality Improvement in Islanded Microgrids

Harmonics are regarded as one of the main challenges in a microgrid. This issue may even get worse when different distributed generators (DGs) work together to solve the load sharing problems due to mismatched feeder impedances and diversified DG ratings. Even though load sharing can be achieved, the microgrid suffers from voltage unbalance and total harmonic distortion (THD) issues at the output of DG terminals as well as at the point of common coupling (PCC). Thus, in this paper, the power quality improving method is discussed, with a target of load sharing under the hierarchical control of different DG units and an active power filter (APF) in microgrids. To achieve this objective, we propose integrating a direct harmonic voltage controlled APF with DGs to improve their harmonic compensation performance. This proposed control scheme has many advantages over conventional control using a shunt resistive active power filter (R-APF) with voltage controlled DGs. First, based on the existing THD level of the PCC voltage, the proposed scheme provides improved voltage compensation and reduction in THD in the islanded microgrid. Secondly, equal load sharing can be achieved simultaneously. Thus, the proposed scheme provides better performance and a seamless interface as the proposed study mainly contains both the voltage controlled DGs and the local based voltage detection APF.

Distributed Nonlinear Control With Event-Triggered Communication to Achieve Current-Sharing and Voltage Regulation in DC Microgrids

A distributed nonlinear controller is presented to achieve both accurate current-sharing and voltage regulation simultaneously in dc microgrids (MGs) considering different line impedances effects among converters. Then, an improved event-triggered principle for the controller is introduced through combining the state-dependent tolerance with a nonnegative offset. In order to design the event-triggered principle and guarantee the global stability, a generalized dc MG model is proposed and proven to be positive definite, based on which Lyapunov-based approach is applied. Furthermore, considering the effects from constant power loads, the damping performance of proposed controller is further improved which is comparative with the traditional $V\hbox{--}I$ droop controller. The proposed event-triggered-based communication strategy can considerably reduce the communication traffic and significantly relax the requirement for precise real-time information transmission, without sacrificing system performance. Experimental results obtained from a dc MG setup show the robustness of the new proposal under normal, communication failure and communication delay operation conditions. Finally, communication traffic under different communication strategies is compared, showing a drastic traffic reduction when using the proposed approach.

Optimized Settings of Droop Parameters Using Stochastic Load Modeling for Effective DC Microgrids Operation

Droop control is a widely used technique for load sharing in dc microgrids (MGs). However, it has an inherent limitation that leads to voltage deviations at the dc bus. More importantly, its current-sharing performance is degraded due to voltage drops across transmission line impedances. Depending on the amount of these voltage drops over the dc bus, droop control leads to different current-sharing errors and dc bus voltage degradations. Therefore, MG topology and loading conditions (LCs) have a considerable effect on current-sharing accuracy and voltage regulation. In this paper, an optimization procedure accounting all necessary information is introduced to find optimal droop parameters for the droop mechanism. First, a proper optimization problem is defined with required constraints and a cost function formulated as a summation of the current-sharing errors and the voltage degradations occurring at various LCs. Then, an optimization tool named as particle swarm is implemented to converge a satisfactory solution. During the computations, the cost impact of the current-sharing error at each LC is weighted based on the probability of occurrence of that LC, which is obtained from the stochastic load model for the considered MG. The effectiveness of the optimal droop parameters is verified through a simulation performed on MATLAB/Simulink and a dc MG test bench is prototyped for experimental validation.

A GPS-Based Control Framework for Accurate Current Sharing and Power Quality Improvement in Microgrids

This paper proposes a novel hierarchical control strategy for improvement of load sharing and power quality in ac microgrids. This control framework is composed of a droop-based controller at the primary level, and a combination of distributed power sharing and voltage conditioning schemes at the secondary level. The controllers in the primary level use GPS timing technology to synchronize the local reference angles. The voltage reference of each distributed generation is adjusted according to a voltage–current (V–I) droop characteristic to enable proper current and power sharing with a fast dynamic response. The droop coefficient, which acts as a virtual resistance, is adaptively changed as a function of the peak current. This strategy not only simplifies the control design but also improves the current sharing accuracy at high loading conditions. The distributed power sharing scheme uses consensus protocol to ensure proportional sharing of average power. The voltage conditioning scheme produces compensation signals at fundamental and dominant harmonics to improve the voltage quality at a sensitive load bus. Experimental results are presented to validate the efficacy of the proposed method.

Decentralized Current Sharing Accuracy for Multi-terminal High Voltage DC System Based on Droop Control

In this article, a decentralized droop control scheme is proposed based on traditional current-voltage type droop control. This approach not only can eliminate the deviation of DC output voltage, it can also adjust the sharing accuracy of DC output current in multi-terminal high voltage DC systems. The averaging DC output voltage and current of two neighboring converters are employed to reduce the complexity of the communication networks, which include both radial and mesh configurations. In order to demonstrate the stability of the proposed method, the Pi-section line model of DC cable is used to simplify the model of multi-terminal high voltage DC networks, and the impact of varying DC cable impedance parameters and low bandwidth communication delay time are analyzed in detail. Finally, simulation results derived from a multi-terminal high voltage DC system is implemented in MATLAB/SIMULI-NK to verify the proposed approach.

Modeling and Analysis of an Avionic Battery Discharge Regulator

The avionic battery discharge regulator (BDR) plays an important role in a power-conditioning unit. With its merits of high efficiency, stable transfer function, and continuous input and output currents, the non-isolated Weinberg converter (NIWC) is suitable for avionic BDR. An improved peak current control strategy is proposed to achieve high current-sharing accuracy. Current and voltage regulators are designed based on a small signal model of a three-module NIWC system. The system with the designed regulators operates stably under any condition and achieves excellent transient response and current-sharing accuracy.

An Improved Droop Control Method for DC Microgrids Based on Low Bandwidth Communication With DC Bus Voltage Restoration and Enhanced Current Sharing Accuracy

Droop control is the basic control method for load current sharing in dc microgrid applications. The conventional dc droop control method is realized by linearly reducing the dc output voltage as the output current increases. This method has two limitations. First, with the consideration of line resistance in a droop-controlled dc microgrid, since the output voltage of each converter cannot be exactly the same, the output current sharing accuracy is degraded. Second, the dc-bus voltage deviation increases with the load due to the droop action. In this paper, in order to improve the performance of the dc microgrid operation, a low-bandwidth communication (LBC)-based improved droop control method is proposed. In contrast with the conventional approach, the control system does not require a centralized secondary controller. Instead, it uses local controllers and the LBC network to exchange information between converter units. The droop controller is employed to achieve independent operation, and the average voltage and current controllers are used in each converter to simultaneously enhance the current sharing accuracy and restore the dc bus voltage. All of the controllers are realized locally, and the LBC system is only used for changing the values of the dc voltage and current. Hence, a decentralized control scheme is accomplished. The simulation test based on MATLAB/Simulink and the experimental validation based on a 2 × 2.2 kW prototype were implemented to demonstrate the proposed approach.