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This paper presents a novel hierarchical cooperative control strategy to solve the problems of unbalanced State of Charge (SoC), unreasonable load current sharing, and unstable bus voltage for battery storage system (BSS) in islanded DC microgrid. In the communication layer, a neighbor-to-neighbor communication network architecture is constructed in which each battery storage unit (BSU) only communicates information with neighboring nodes, and a multi-agent consensus algorithm is used to iteratively estimate the relevant average global variables. In the main control layer, the droop coefficient is dynamically adjusted by constructing a fast SoC convergence function to ensure that the load current is reasonably distributed based on the rated capacity of each BSU. In the secondary control layer, a virtual voltage drop equalization controller and an average bus voltage compensation controller are designed to eliminate the impact of line impedance mismatch and compensate the average bus voltage. Furthermore, an optimized factor is designed to accelerate the later stage of SoC convergence. Finally, a DC microgrid experimental platform based on StarSim HIL is built, and the results demonstrate that the proposed strategy can achieve the control objectives of fast SoC balance, proportional load current sharing, and bus voltage stability.

This paper proposes an improved distributed cooperative control strategy for the energy storage system (ESS) in islanded DC microgrid. To meet the requirements of State of Charge (SoC) balance, accurate current sharing, and voltage regulation, a hierarchical control structure is established. In the primary control layer, the droop coefficient is adaptively adjusted through the SoC dynamic changes of each energy storage unit (ESU) to achieve SoC balance, and two balance adjustment factors are introduced to improve SoC convergence speed and accuracy. In the secondary control layer, an information state factor ζ is designed to eliminate the influence of the inconsistent line resistance on the SoC balance and realize the DC bus voltage recovery. In the communication layer, each ESU only exchanges information with neighboring nodes through a sparse communication network, in which a dynamic consensus algorithm is used to achieve stable convergence of global average information. Finally, a DC microgrid simulation model and StarSim HIL experimental platform is built. The results show that the proposed control strategy can achieve SoC balance and accurate load current sharing under a variety of complex operating conditions, and have excellent plug-and-play performance.

The key to the sustainable development of family farms and ranches is to solve the problem of financing difficulty, and the construction of the credit rating system of family farms and ranches is the basis of solving the financing difficulty. Therefore, the construction of the credit rating system of family farms and ranches is not only a theoretical problem but also has strong practical significance. On the basis of dichotomous method and improved DEA technology, this paper sets up a credit rating model based on dynamic cycle dichotomous DEA method with the goal of maximizing the efficiency value of credit scoring of family farm and ranch and the constraint of increasing default loss rate. The first feature of this paper is that it constructs a direct mapping from the credit index system to the credit score through the three-stage DEA model, which avoids the disadvantages of the classical linear weighting formula that relies heavily on the measurement results of different weights, and obtains more accurate and stable credit score results of family farms and pastures. Second, the three-stage DEA efficiency value is used as the credit score value of the family farm and pasture, and whether it is located in the optimal preset interval is taken as the basis for efficiency dichotomy. The DEA credit score of the family farm and pasture is calculated by dynamic cycle, and the samples belonging to each level of the family farm and pasture are calculated successively according to the order of the credit grade from high to low. Finally, the result of reasonable credit rating of family farm and ranch with high default loss rate and low credit score was obtained. The pyramid test results of matching credit rating and default loss rate show that this research method is effective and has a good pyramid shape.

The power electronic converter that supplies power to the unmanned ship's servers can cause self-excited oscillation of the LC input filter under heavy load. To suppress voltage oscillation, an input impedance shaping method based on a sliding mode disturbance observer is presented. The self-excited oscillation mechanism of the LC filter caused by the buck converter's positive feedback is analyzed by using the small-signal model. The sliding mode disturbance observer calculates the input voltage according to the output voltage, inductance current and duty cycle of the buck converter. The feedforward compensation of the observed voltage reshapes the input admittance of the buck converter, making the system not meet the conditions of self-excited oscillation and suppressing the voltage oscillation of the LC filter. The phase shaping of input admittance is compared with the traditional amplitude shaping by the Bode diagram and Nyquist diagram. The experimental results show that the phase shaping method could better suppress the LC filter's self-excited oscillation and the sliding mode disturbance observer reduced the hardware cost.

As an easily used and powerful heuristic search technique based on population, differential evolution (DE) algorithm has been widely used for many optimization and real engineering projects. Similar to other evolutionary algorithms (EA), DE could not avoid from premature convergence due to over concentrated population, which could be called losing population diversity. To improve the performance, a neutral mutation (NM) operator for DE algorithm is proposed. The proposed operator is inspired by neutral theory of molecular evolution, which claims that most mutations are neutral at the level of molecular. The NM operator maintains slightly deleterious trial vectors with a certain probability in the conventional selection operator of DE. At the same time, two control parameters of Neutral Mutation operator are investigated and a dynamic neutral mutation rate tuning strategy is designed. Besides, some of these trial vectors have a chance to be mutated neutrally within the search domain randomly. As a result, the population is diversified with costing negligible function evaluations. Comprehensive experimental results demonstrate that the presented NM operator could improve population diversity to some extent, especially when the population is not divergent at all. Moreover, a real word problem is used to further evaluate NM operator. Also, this operator can be easily used in other EAs to keep population diversity.

It is a very challenging and important task to adaptively adjust the scale factor F and the crossover rate Cr for Differential Evolutionary (DE) algorithms. Most recent adaptive techniques were designed to generate parameters randomly based on successful trial values during the previous evolving process, lacking explicit guidelines to generate appropriate values. This paper proposes a novel parameter adaption strategy, which could incorporate promising F and Cr pairs extracted by using Association Rule Mining (ARM) into DE algorithms. First, all successful F and Cr values generated by their original methods are recorded during the whole evolution, resulting in an increasing dataset. Second, we discretize the dataset and extract the most frequent itemset of parameters by using a modified version of the widely used Apriori algorithm. Third, a greedy operator is developed to generate new parameters in the next generation by comparing the presented ARM-based and original-method-based fitness values. The presented technique provides an additional pair of F and Cr values to be evaluated, without replacing existing strategies for the control parameters. The main contribution of this paper is that we propose a novel way, which utilizes information generated during the evolutionary process, to enhance exploration capabilities by adjusting control parameters. Experimental results demonstrate that the proposed ARM-based parameter adaptive strategy is able to enhance performances of some state-of-the-art DE variants. Further, this methodology might be helpful for other control parameters of Evolutionary Algorithms (EA).

In this paper, a distributed secondary control strategy is proposed to achieve accurate load current sharing and DC bus voltage recovery for battery energy storage system (BESS) in DC microgrid. Firstly, each battery energy storage unit (BESU) is regarded as a multi-agent, and a neighbor-to-neighbor sparse communication network architecture is constructed. Each BESU is based on distributed consensus protocol, and global consensus can be achieved only by communication between adjacent BESUs. On this basis, a state of charge (SoC) equalization algorithm is proposed to achieve accurate load current sharing, and a DC bus voltage controller is designed to achieve secondary regulation of the bus voltage. Finally, the simulation results show that the proposed strategy reaches the expected control objectives under various operating conditions of the microgrid.

During the navigation of all-electric ships, a hybrid energy storage system (HESS) is required to compensate power imbalance and maintain bus voltage stability. For a HESS composed of multiple energy storage (ES) devices, an unreasonable power distribution causes the ES devices with a low state of charge (SoC) to draw from power supply early, which deepens the operating pressure of the other ES devices. This in turn, affects the stable operation of the entire system. To achieve power distribution based on the SoC of different ES devices, a novel power distribution strategy for use in all-electric ships was proposed. In the proposed strategy, the virtual impedance of an ES device is connected with the SoC through exponential functions. As a result, the output power can be dynamically changed according to changes of the SoC. On the premise of obtaining a proper dynamic power distribution among ES devices with complementary characteristics, the SoC balance among ES devices with the same characteristics can be realized. Meanwhile, the bus voltage deviation induced by the virtual resistor is eliminated via an added compensation voltage. The effectiveness of proposed method is verified by both simulations and a StarSim hardware in loop (HIL) experimental platform.

State of charge (SOC) estimation is a significant task for lithium-ion batteries. However, the accuracy of SOC estimation is closely related to parameters of battery and system non-linearity. To identify the parameters of lithium-ion battery better, this proposed a self-adaptive particle swarm optimization differential evolution (SaPSODE) algorithm. First, to describe the dynamic behaviors of battery, we presented a first-order RC equivalent circuit model (ECM). Second, to calculate open-circuit voltage (OCV) versus time during the dynamic test procedure, an optimizing objective function was built to minimize errors between the true and optimized terminal voltages. Further, control parameters of F and Cr were also self-adapted according to previous successful records. Third, by using OCV-SOC mapping curves, this work obtained estimated SOC curve which was compared with true SOC curve. Comprehensive experimental results demonstrated effectiveness of the proposed framework and methodology, compared with several highly-cited DE variants.

In this paper, a State-of-Charge (SoC) dynamic balancing control strategy considering system communication failure and energy storage capacity difference is proposed to reach the SoC balancing and proper current sharing for distributed energy storage units (DESUs) in DC shipboard microgrid. Firstly, the distributed equalization algorithm is applied in the DESU’s controller when the system communication is normal, each DESU exchanges their SoC information through low bandwidth communication (LBC) and dynamically adjusts the droop coefficient according to the change of their SoC, so as to realize the SoC dynamic balance of the energy storage system. Secondly, the DESU’s controller is switched to the decentralized equalization algorithm with communication failure, each DESU can reach SoC balancing and load current reasonable distribution only according to the local information. After that, a DC bus voltage compensation method is introduced to solve the problem of bus voltage deviation caused by the proposed SoC equalization. Finally, the simulation model comprised of three DESUs is implemented and the results show that the proposed control strategy can make each DESU reach SoC equilibrium state quickly and effectively reduce the bus voltage deviation under the normal and failure communication conditions.

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.

Accurate current sharing between micro-sources is an important control objective in DC microgrid. The traditional droop control based on virtual resistance is difficult to meet the requirements of sharing accuracy and stability at the same time because of the differential line impedances and low-speed communication. A new current sharing approach for paralleled micro-sources based on superimposed frequency droop control is proposed in this paper. A small ac voltage with low frequency and low amplitude is superimposed onto the output dc voltage of each converter. Therefore, dc converters can be coordinated together with the frequency of the small ac voltage, without any communication. Utilizing the droop characteristic between superimposed frequency and output DC current, reactive circulating power would be generated between converters and then be used to regulate the output DC voltage of each converter. Then, the accurate current sharing can be achieved. On this basis, a reactive power limiter is proposed to compensate the voltage drop, so as to make the bus voltage operating within the specified range. Small signal analysis is used to verify the system stability and select proper parameters. Finally, the feasibility and effectiveness of the proposed control strategy are validated by the results obtained from simulations.

With the photovoltaic (PV) penetration increases in dc microgrid, the traditional PV converters with maximum power point tracking (MPPT) control, which are equivalent to current sources, can hardly meet the needs of coordinated operation. The PV converter should operate in both MPPT and constant voltage control (CVC) modes. When the operation mode changes, the PV converter needs to switch between current and voltage sources. Inevitably, the bus voltage instability would be caused. This paper presents a novel switching strategy for the smooth and seamless operation of the PV converter between MPPT and CVC modes. The PV converter under these two modes is always controlled as a voltage source, and its control variables are both the output voltage of PV arrays. When its operation mode changes from MPPT to CVC, a small offset signal based on DC bus voltage would be introduced to the reference voltage of inner loop of the PV converter, so as to shift the maximum power point (MPP) voltage. Meanwhile in the voltage inner loop, a double integral sliding mode controller (DISMC) is used to improve the performance and anti-interference ability of the PV converter. This method unifies MPPT and bus voltage adjustment strategy without any communication and changing any hardware structure and control parameters. When mode switching occurs, only the reference value of voltage inner loop would be slightly adjusted near the maximum photovoltaic power point, so the seamless transition for PV converter between the two modes can be ensured. Finally, the effectiveness of the mode seamless switching control strategy is demonstrated by the simulation and experimental tests.

Multi-mode operation and transient stability are two problems that significantly affect flexible microgrid (MG). This paper proposes a multi-mode operation control strategy for flexible MG based on a three-layer hierarchical structure. The proposed structure is composed of autonomous, cooperative, and scheduling controllers. Autonomous controller is utilized to control the performance of the single micro-source inverter. An adaptive sliding-mode direct voltage loop and an improved droop power loop based on virtual negative impedance are presented respectively to enhance the system disturbance-rejection performance and the power sharing accuracy. Cooperative controller, which is composed of secondary voltage/frequency control and phase synchronization control, is designed to eliminate the voltage/frequency deviations produced by the autonomous controller and prepare for grid connection. Scheduling controller manages the power flow between the MG and the grid. The MG with the improved hierarchical control scheme can achieve seamless transitions from islanded to grid-connected mode and have a good transient performance. In addition the presented work can also optimize the power quality issues and improve the load power sharing accuracy between parallel VSIs. Finally, the transient performance and effectiveness of the proposed control scheme are evaluated by theoretical analysis and simulation results.

In the DC microgrid with traditional droop control, the inconsistency of circuit parameters and line impedance results in the diverse droop characteristics of DC-DC converters, which reduces the current sharing accuracy of converters. Meanwhile, droop control inevitably leads to DC-bus voltage deviation and the secondary control has the problem of poor communication fault tolerance. To improve the control performance of DC microgrid, a droop control method based on virtual frequency is proposed by simulating the frequency droop control mechanism in AC microgrid. A virtual voltage with a frequency proportional to the output current is constructed and shared among the converters through low bandwidth communication (LBC). The phase difference among virtual voltages generates virtual reactive power, which is calculated by Fourier transform to coordinate the output current of each converter. The algorithm can keep high performance under some communication faults and can guarantee that the converter with serious communication faults can automatically exit the current sharing control without affecting other converters. Moreover, the actual reactive power and harmonics wouldn’t be introduced, and the power quality of the microgrid can be ensured. The stability of the algorithm is demonstrated by small-signal analysis and mathematical calculation. Finally, simulation and experiment show that the algorithm has high current sharing accuracy, excellent communication fault tolerance and plug-play performance.

This study proposes a new solution for the parallel operation of microgrid inverters in terms of circuit topology and control structure. A combined three-phase four-wire inverter composed of three single-phase full-bridge circuits is adopted. Moreover, the control structure is based on adaptive three-order sliding-mode control and wireless load-sharing control. The significant contributions are as follows. 1) Adaptive sliding-mode control performance in inner voltage loop can effectively reject both voltage and load disturbances. 2) Virtual resistive-output-impedance loop is applied in intermediate loop to achieve excellent power-sharing accuracy, and load power can be shared proportionally to the power rating of the inverter when loads are unbalanced or nonlinear. 3) Transient droop terms are added to the conventional power outer loop to improve dynamic response and disturbance rejection performance. Finally, theoretical analysis and test results are presented to validate the effectiveness of the proposed control scheme.

Robust control and seamless formation are the two crucial problems that affect smart microgrids. This paper proposes a new solution for microgrid inverters in terms of circuit topology and control structure. The combined three-phase four-wire inverter, which is composed of three single-phase full-bridge circuits, is adopted in this study. The control structure is based on the inner adaptive three-order sliding-mode closed-loop, the immediate virtual output-impedance loop, and the outer power control loop. Three significant contributions are obtained: (1) the microgrid inverters effectively reject both voltage and load disturbances with the adaptive sliding-mode controllers regardless of whether the inverters are operating in the grid-connected mode, islanding mode, or transition from the grid-connected mode to the islanding mode; (2) the virtual output impedance loop is applied to make a resistive equivalent output impedance of the inverters and to meet the requirements of the inverter parallel operation in the islanding mode; (3) the proposed droop method reduces the line inductive impedance effects and improves the power sharing accuracy by optimizing the droop coefficients. The theoretical analysis and test results validate the effectiveness of the proposed control scheme.

In this paper, the reason of high-frequency leakage current in neutral point clamped photovoltaic grid-connected inverters that adopts traditional modulations is analysed. In order to solve the problem, this article puts forward two improved methods based on the three-level space vector pulse width modulation (SVPWM), which can reduce the leakage current by applying three medium vectors or using only two medium vectors and one specific zero vector to compose the reference vector. In addition, the system control method adopts the coordination control of the current inner loop and the DC voltage outer loop, which can reduce voltage fluctuation at the DC side. Thereby, the suppression of leakage current can be realised without requiring any modification on the multilevel inverter. Compared with the traditional three-level sinusoidal pulse width modulation (SPWM) and SVPWM, the two new methods are advantageous in having a stable common-mode voltage, low leakage current, and low harmonic component of grid curr...

The traditional grid-connected inverter (GCI) has the disadvantage of high total harmonic distortion under the condition of unbalanced and distorted grid voltage. On this basis, a novel GCI control strategy is proposed in this paper. Firstly, a second order generalized integrator is built in the αβ coordinate to realize the grid voltage phase shift 90° and separate the positive-and negative -sequence. Then, the grid current reference command of the GCI is derived. Secondly, a proportional + multi-resonant (PMR) controller, which does not need positive-and negative - sequence separation of the grid current, is employed to achieve zero current error control and improve the grid current power quality. Finally, simulation results demonstrate the effectiveness of the proposed scheme under the condition of unbalanced and distorted grid voltage.

By applying WSN technology in the field of industrial monitoring, the single CSMA or TDMA protocol can not meet the requirement of dealing with the urgent data without delay when some accidents occur. In order to solve this problem, this paper puts forward a hybrid CSMA-TDMA MAC protocol for wireless sensor network, and makes a test in a star network which is made up of one center node and several sensor nodes. The test result indicates that the protocol is feasible and effective.