Current Sharing Control Strategy Research Articles

A Fast State-of-Charge (SOC) Balancing and Current Sharing Control Strategy for Distributed Energy Storage Units in a DC Microgrid

In isolated operation, DC microgrids require multiple distributed energy storage units (DESUs) to accommodate the variability of distributed generation (DG). The traditional control strategy has the problem of uneven allocation of load current when the line impedance is not matched. As the state-of-charge (SOC) balancing proceeds, the SOC difference gradually decreases, leading to a gradual decrease in the balancing rate. Thus, an improved SOC droop control strategy is introduced in this paper, which uses a combination of power and exponential functions to improve the virtual impedance responsiveness to SOC changes and introduces an adaptive acceleration factor to improve the slow SOC balancing problem. We construct a sparse communication network to achieve information exchange between DESU neighboring units. A global optimization controller employing the consistency algorithm is designed to mitigate the impact of line impedance mismatch on SOC balancing and current allocation. This approach uses a single controller to restore DC bus voltage, effectively reducing control connections and alleviating the communication burden on the system. Lastly, a simulation model of the DC microgrid is developed using MATLAB/Simulink R2021b. The results confirm that the proposed control strategy achieves rapid SOC balancing and the precise allocation of load currents in various complex operational scenarios.

Autonomous Load Current Sharing Control Strategy for Distributed DC Micro-sources Based on Active Frequency Injection and Line Impedance Compensation Control

Autonomous Load Current Sharing Control Strategy for Distributed DC Micro-sources Based on Active Frequency Injection and Line Impedance Compensation Control

Current sharing control strategy with uncertainties and network attacks for electric vehicle charging station

AbstractAlthough the electric vehicle supplied through distributed generators (DGs) is one of the most promising methods to reduce carbon emission and has been widely studied, the accurate current sharing regarding multi‐bus DC charging stations considering uncertainties and network attacks is rarely studied. Based on this, a fully distributed current sharing control strategy is presented, which can improve the reliability of system under denial of service (DoS). Firstly, the DC charging station system (DCCSS) with uncertainties is modelled. Primary control is designed to provide stable voltage and inaccurate current sharing. Furthermore, the state‐space function considering power coupling among different DC buses is built, which lays the foundation for the design of the following control. Then, the model of DoS attacks is proposed. Based on this, the fully distributed consensus control is proposed to achieve the accurate current sharing for DCCSS under DoS. Meanwhile, a method for solving the control gain without global information is given and proved. Moreover, it can be solved by LMI toolbox. As a comparison, a common control strategy is introduced that only considers the uncertainties. Finally, the feasibility of the proposed method is verified through comparison of simulation results.

Fully Distributed Dynamic Edge-Event-Triggered Current Sharing Control Strategy for Multibus DC Microgrids With Power Coupling

Although the current sharing control of dc microgrids has been widely studied, the high communication bandwidth and global communication network structure information demands hander the renewable energy consumption. Thus, this article proposes a fully distributed dynamic edge-event-triggered current sharing control strategy for multibus dc microgrids with power coupling. First, the system model with power coupling is built, which is further switched to the linear heterogeneous multiagent systems with unknown disturbance. It is an indispensable preprocessing for controller design. Furthermore, the fully distributed current sharing control strategy is proposed through adaptive coupling weights. Note that the global communication network structure information demand is eliminated. Moreover, the fully distributed dynamic edge-event-triggered mechanism is proposed to reduce communication bandwidth. Compared with the previous dynamic event-triggered mechanisms applied into dc microgrids, the continuous communication between neighboring agents is avoided, and controller updating frequency is reduced. Finally, the simulation and experimental results verify the proposed control performance.

Battery-Supercapacitor Current Control Using Fuzzy Supervisory with PI Controller for Electric Vehicle Application

The configuration of the battery-supercapacitor as a hybrid power source (HPS) is one solution to overcome the limitations of battery life, such as in an electric vehicle. To achieve battery energy savings when a supercapacitor is added, a control scheme is needed. This paper proposes a battery-supercapacitor current sharing control scheme using Fuzzy Supervisory Control (FSC). The FSC is designed to determine how much current of the battery and supercapacitor need to discharge based on the current requirement of the load. The output of the FSC will produce a reference current which is then controlled in the main closed-loop control using a Proportional-Integral (PI) controller. The control signal in the form of a duty cycle is then used to regulate the battery-supercapacitor current through a DC/DC converter. This control scheme is tested using a resistive load change scenario both simulation and experimental. The results show that the battery-supercapacitor current can be controlled according to the loading conditions.

A Multi-Functional De-Icing Equipment Using Modular Parallel PWM Current Source Converters

For transmission line de-icing, a five level specific harmonic elimination (5l-she) modulation method for high-power parallel current source is proposed, which realizes the current balance of DC bridge arm and the suppression of common mode voltage, and ensures the power quality of grid connected current at low switching frequency. In addition the reactive power of the low voltage load can be compensated at the same time. Firstly, the switching states are classified according to the different mode lengths of PWM current, and the common mode voltage corresponding to each switching state is calculated. Secondly, the current sharing control strategy is established based on the analysis of the influence of redundant switching state on the current sharing of DC bridge arm. Then, the 5l-she waveform is constructed based on the switching state with lower common mode voltage, and the redundant switching states is optimized according to the current sharing strategy. Finally, the effectiveness of the proposed method is verified by simulation.

A Voltage-Quadrupler Interleaved Bidirectional DC–DC Converter With Intrinsic Equal Current Sharing Characteristic for Electric Vehicles

A voltage-quadrupler interleaved bidirectional dc-dc converter with intrinsic equal current sharing is presented in this article. The proposed converter offers a wide voltage conversion ratio, which makes it suitable for interconnecting the energy storage unit with dc bus for electric vehicle applications. A high voltage gain and low voltage stress across switches are achieved by adopting the capacitive voltage-divider stage, which enables the designer to employ a low-voltage switch with a small on-resistance RDS(ON). As a result, it allows the converter to run more efficiently and cooler. An interleaved connection at the low-voltage side paves the way for reducing the current ripple, which reduces the filter size and increases the power-density. Moreover, a built-in equal current sharing is achieved for two interleaved phases without using a current-sharing control scheme. The operating principle, steady-state characteristics including the current and voltage stress of the switches, and comparison with other state-of-the-art bidirectional converters are carried out in detail. Finally, to verify theoretical analysis, a 2-kW scaled-down laboratory prototype of the proposed converter using Gallium-Nitride switches is implemented.

A Delay-Tolerable Master–Slave Current-Sharing Control Scheme for Parallel-Operated Interfacing Inverters With Low-Bandwidth Communication

Parallel-operated inverters with common dc and ac bus can be used as an interface for dc systems connecting to an ac network. High cost and complicated wiring of communication are the main drawbacks of communication-based current-sharing methods for the parallel systems. Many new communication tools, e.g., power line communication (PLC), Bluetooth, and Zigbee modules, are cheap and require no extra wire connection, but have low communication bandwidth and low data rate, which result in a large communication delay and zero-order-hold period that could cause stability issues if used to transmit control signals. This article presents a method to mitigate the instability issue caused by communication effects, in parallel grid-tied inverter system with master-slave current-sharing control. First, the master-slave current-sharing control scheme is introduced. Then, the influence of communication delay and zero-order-hold on the system is analyzed. And design criterion to reduce the influence of low-bandwidth communication (LBCom) is studied and determined. New approaches, by designing a current reference low-pass filter and a voltage feedforward loop in slave modules, are introduced. The performance of the proposed control structure and design criteria are experimentally verified. With the proposed method, LBCom tools can be applied to systems with master-slave control architecture while keeping the robustness and transient performance, with improved modularity and operation flexibility.

A Novel Fault-Tolerant Control Method for Interleaved DC–DC Converters Under Switch Fault Condition

This article proposes an adaptive democratic current sharing control scheme for the interleaved dc–dc converters under faulty conditions. The proposed fault-tolerant control approach can achieve the desired output-voltage generation and equal current sharing among the remaining healthy modules as more and more modules fail. The main idea is to adaptively change the reference currents and the inner loop current regulator gains once the fault is detected in the modules. The proposed control scheme is compared to the democratic current sharing control scheme under the fault condition. The results show that it achieves improved output regulation and accurate current sharing among the modules. The presented fault-tolerant control method has been analyzed, tested, and validated for the interleaved boost converter.

Novel Piecewise Linear Formation of Droop Strategy for DC Microgrid

This paper proposes a new load current sharing control strategy for dc microgrid (MG). It adopts the piecewise linear formation to adaptively assign droop gains to balance the load sharing accuracy with the deviation of bus voltage. Communication among the distributed units is unnecessary for the proposed operation. The approach is considered as a comprehensive solution to deal with the unbalanced performance in dc MG for different load levels. The balancing performance is shown by comparing with the traditional linear droop control and the latest nonlinear droop gain solution. The study is proceeded by the in-depth analysis of both steady state and system dynamics with simulation support. The claimed advantage is finally verified by both simulation and experimental evaluations.

Circulation suppression of synchronous/interleaving paralleled 400 Hz three‐phase four‐leg inverter based on third harmonics injection

In this study, the third harmonic injection method based on analogue circuit is applied in the interleaving parallel three-phase four-leg (3P4L) inverter, the adverse effects of injected signals on the circulating current are found, and the corresponding control strategy solution is proposed. At the beginning of this study, by using the large-signal model of leg, it is proved that four legs can be decoupled, on the basis of this conclusion, the current sharing control strategy based on dual-loop scheme and third harmonic injection can be applied in paralleled 3P4L inverter, however, peak value of circulating current is increased by zero-sequence component which is caused by injected signals. Therefore, output impedance analysis method is applied, and the control strategy including zero-sequence component feedforward loop is proposed, which can be implemented with analogue circuit, and is suitable for the 400 Hz synchronous/interleaving paralleled inverter systems. Finally, the effectiveness of the control strategy is verified by simulation and experiment.

A Novel Autonomous Current-Sharing Control Strategy for Multiple Paralleled DC–DC Converters in Islanded DC Microgrid

Due to the existence of line impedances and low-bandwidth communication, the traditional peer-to-peer control method based on droop control has difficult meeting the requirements of current sharing and voltage stability in islanded DC microgrids at the same time. In this paper, a novel current-sharing control strategy based on injected small ac voltage with low frequency and low amplitude is proposed for multiple paralleled DC–DC converters. The small ac voltage is superimposed onto the output voltage of each converter. Then, the reactive circulating power is generated and used to regulate the output DC voltage of each converter. Under the droop characteristic between the injected frequency and output DC current, a feedback mechanism is generated to realize the accurate current sharing. On this basis, a reactive power-voltage limiter link and virtual negative impedance are added. Under the interaction of the two links, the bus voltage drop caused by line impedances can be almost completely eliminated. This method does not need any communication or to change the hardware structure. The controller design process is presented in detail along with a system stability analysis. Finally, the feasibility and effectiveness of the proposed control strategy are validated by the results obtained from simulations and experiments.

Communication-Free Current Sharing Control Strategy for DC Microgrids and Its Application for AC/DC Hybrid Microgrids

The load sharing in the DC microgrids is affected by the practical factors such as the cable resistances and changes in the system topology. This paper presents a new control algorithm for the application in the DC microgrids to achieve accurate load distribution between droop controlled converters. An identical steady state voltage feedback is used for all droop controlled converters to ensure accurate current sharing. The information about the common voltage is obtained by means of the proposed communication-less control strategy. The proposed strategy does not require prior information about the grid topology and parameters. Therefore, it is applicable to both single- and multi-bus microgrids. The conducted stability analysis demonstrates that the proposed approach and the conventional droop control deomstrate similiar dynamic performance. Furthermore, the proposed algorithm has a potential to enhance the operation of the Interlinking Converters in the hybrid AC/DC microgrids. A comprehensive nonlinear simulation study is conducted in the Matlab/Simulink environment. The simulation results verify the effectiveness of the proposed communication-less control scheme to enable proper load distribution between droop controlled converters as well as enhance the load sharing between AC and DC microgrids. Finally, the proposed control approach is verified using an OPAL-RT setup.

Ultra-Wide Output Voltage Range Power Supply Based on Modular Switched-Converter Principle

A systematic method for developing ultra-wide output voltage range power supply based on modular switched-converter principle is proposed in this paper. The basic symmetric switched-converter (SSC) module is composed of two dc–dc modules, two diodes, and one active switch. Multi-stage SSC modules are generated by symmetric cascaded connection method. By controlling the active switches in multi-stage SSC modules, the connections of all the dc–dc modules’ output ports can be changed flexibly among series, parallel, and partial–parallel–partial-series connections. Then segmented regulation can be realized and system output voltage range can be widely extended. Meanwhile, the voltage range of each dc–dc module can be narrowed and the voltage/current stress can be reduced to simplify the design, increase device utilization ratio, and improve overall efficiency. A full-bridge isolated buck–boost converter with a semiactive rectifier, which features soft-switching and wide voltage adjustment range, is adopted as the dc–dc module to realize the proposed system. Input current sharing control strategy is adopted to ensure proper power balancing in all possible operating modes. The stability of the proposed system is also explored. Simulation and experimental results are provided to verify the effectiveness and advantages of the proposed system configuration, the switched-converter principle, and the control strategies.

Analysis and Control of Input-Parallel Output-Series Based Combined DC/DC Converter With Modified Connection in Output Filter Circuit

Input-parallel output-series (IPOS)-based combined converter is usually used in the situation with low input voltage and high output voltage to satisfy the requirements in switch voltage stress reduction and power expansion. In such a system, the voltage balance of filter capacitor in the output side or current sharing in the input side usually used to guarantee the control performance of the conversion system. In this paper, a phase-shifted full bridge (PSFB)-based IPOS converter with modified connection of output filter circuit used for the wide variation of input voltage is proposed, which is beneficial to reduce ripple current of filter inductor significantly. A control scheme using the same phase shifting for the two different modules of this IPOS converter is detailedly investigated with both steady state and small signal model-based analysis, it manifests that special voltage balance and current sharing control scheme are not necessary for IPOS system under the common phase shifting control method. And the small signal transfer functions of this topology used for dual loop control system design are also developed. The simulation and experiment results are presented to validate the analysis and control scheme.

Parallel Control Method Based on the Consensus Algorithm for the Non-Isolated AC/DC Charging Module

A high-power charging station for electric vehicles usually adopts a parallel structure of multiple power modules. However, due to the parameter differences among power modules, a parallel system always has circulating current issues. This paper takes a non-isolated AC/DC charging module as the research object and proposes a current sharing control strategy for multiple power modules based on the consensus algorithm. By constructing a sparse communication network with the CAN (Controller Area Network) protocol and exchanging current information, accurate current sharing among power modules is realized. Firstly, the zero-sequence circulating current issue is analyzed through a parallel model of the three-phase rectifier, with an improved circulating current restraint strategy proposed based on the zero-sequence voltage compensation. Then, the principle of the consensus algorithm is explained, which is applied to the current sharing control of multiple power modules. Finally, the proposal is tested by the designed simulation and experimental cases. From the obtained results, it can be seen that the proposed control strategy can effectively realize accurate current sharing among multiple power modules and well restrain the zero-sequence circulating current at the input side.

A New Power Management Scheme for Parallel-Connected PV Systems in Microgrids

The problem of control of parallel-connected photovoltaic (PV) based distributed generation units in an ac distribution system is studied in this paper. First, a new robust backstepping sliding mode based maximum power point tracking strategy is designed for each PV system. Then, a new control scheme is presented for power management of parallel-connected inverters of PV systems. The proposed control technique is an improvement to the instantaneous average current sharing control scheme (IACS) and uses adaptive fuzzy controllers to adjust the gains of the controllers of each inverter module. The fuzzy inference system is optimized using the $H_{\infty}$ optimal theory to reduce the error in the fuzzy system. This adaptive fuzzy gain scheduling is then introduced to the IACS controller to improve the current and power sharing for a condition in which the system parameters change. Simulation results are presented to show the effectiveness of the proposed control strategy in improving the load sharing among parallel-connected PV systems.

Fault Tolerant Operation of Parallel-Connected 3L-Neutral-Point Clamped Back-to-Back Converters Serving to Large Hydro-Generating Units

In recent years, doubly fed induction machine (DFIM) is increasingly being preferred in variable speed pumped storage power plants due to its merits like partial-rated excitation system (cost factor), unity power factor, real and reactive power controls. In large-rated DFIM (250 MW), parallel-connected three-level neutral-point clamped (3L-NPC) back-to-back converter systems are employed as rotor excitation circuit to handle slip power requirements (–10% to +8.33% in Tehri PSPP, India). The power semiconductor device failure in the rotor excitation circuit may cause serious concerns in grid stability and system reliability. To ensure the continuous operation of a generating unit, this paper aims to design a redundant parallel leg configuration with six antiparallel IGBTs for a 3L-NPC back-to-back converter structure to acquire fault tolerant operation under power semiconductor device failure. Furthermore, an efficient active current sharing control strategy with fault tolerant operation is implemented for each converter to obtain better performance of the drive system. In fault tolerant operation, fault identification is computed based on a normalized phase currents average method with less computational effort and timely identification of open switch faults in both grid side converter and rotor side converter. The circuit reconfiguration under postfault operation is performed by antiparallel IGBTs with the help of fault tolerant control scheme. The effectiveness of a designed fault tolerant control scheme is validated through MATLAB/Simulink environment for a 250-MW DFIM system with 25 MW (5-channel) power converters. To ensure the trustworthiness of the simulation, an experimental demonstration is performed through a 2.2-kW DFIM laboratory prototype with a 2-channel 3L-NPC back-to-back converter structure.

A Dynamic Consensus Algorithm to Adjust Virtual Impedance Loops for Discharge Rate Balancing of AC Microgrid Energy Storage Units

A dynamic consensus algorithm (DCA)-based coordinated secondary control with an autonomous currentsharing control strategy is proposed in this paper for balancing the discharge rate of energy storage systems (ESSs) in an islanded ac microgrid. The DCA is applied for information sharing between distributed generation (DG) units to regulate the output power of DGs according to the ESS capacities and state-of-charge (SoC). Power regulation is achieved by adjusting the virtual resistances of voltage-controlled inverters with an autonomous current-sharing controller. Compared with existing methods, the proposed approach can provide higher system reliability, expandability, and flexibility due to its distributed control architecture. The proposed controller can effectively prevent operation failure caused by over-current and unintentional outage of DGs by means of balanced discharge rate control. It can also provide fast response and accurate current sharing performance. A generalizable linearized state-space model for n-DG network in the z-domain is also derived and proposed in this paper; the model includes electrical, controller, and communication parts. The system stability and parameter sensitivity have been analyzed based on this model. To verify the effectiveness of the proposed control approach, this paper presents simulation results from a ten-node network and a comparison between experimental results obtained from the conventional power sharing control and the DCA-based SoC coordinated control in a setup with three 2.2 kW DG units.

A Decentralized Current-Sharing Controller Endows Fast Transient Response to Parallel DC–DC Converters

This paper proposes a decentralized current-sharing control strategy to endow fast transient response to paralleled dc–dc converters systems, such as dc microgrids or distributed power systems. The proposed controller consist of two main control loops: an external voltage droop control for current-sharing proposes and an internal current loop. The external droop control loop is designed as a voltage loop with embedded virtual impedance, which avoids the use of a slow voltage loop and a separate extra virtual impedance loop that may limit the system bandwidth. The internal current loop, thanks to the external control loop simplification, plays a major role in the system bandwidth, so that an adaptive proportional-integral (PI) controller is proposed for this matter. In the paper, two different droop control methods have been modeling, designed, simulated, and tested: The conventional virtual-impedance-loop based V–I droop and the proposed embedded-virtual-impedance based I–V droop. In order to compare the dynamic response performances between two droop controllers, their state-space models have been developed and analyzed in this paper. The results show that the dynamic response of the I–V droop control is faster than that of the conventional V–I droop control. Furthermore, by analyzing the effects from I–V droop control parameters, the errors can be reduced faster by enlarging the proportional terms, but with no fluctuations, and then completely eliminated by restoring back to small proportional values. Meanwhile, there exists a tradeoff phenomenon between the fast dynamic response and good steady-state performance; thus, an adaptive PI controller is proposed to both improve dynamic response and guarantee good steady-state performance simultaneously. Experimental results are shown to verify the accuracy of the models and the effectiveness of the proposed control framework.