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A novel control scheme to improve the performance of LLC resonant mode controller under light load operation and wide input-output voltage variations is presented in this paper. Poor light load voltage regulation in the LLC resonant converter necessitates the use of burst control mode, which results in performance degradation under light loading conditions. The proposed control scheme uses an adaptive deadband technique that avoids the burst control mode even at extremely light loads resulting in continuous operation and thereby lower output voltage and current ripple, lower noise and higher efficiency. Furthermore, a neural-network-based compensator is proposed in this paper for improving the dynamic performance of the converter under varied operating conditions. Theoretical and simulation results are provided to show the advantages of the proposed control algorithms. A 200W-LLC resonant converter is used for experimental verification of the proposed control scheme. more...

DC network is a desired solution to cater for the increasing penetration of DC-compatible energy sources, storages and loads. This paper proposes a novel topology of DC network in reference to the conventional AC transmission/distribution network, where there are multiple DC subgrids with same or different nominal voltages interconnected to the common bus by DC Transformer (DCT). Different voltage levels have been set in the DC network with considerations of compatibility, power loss, voltage drop, distribution distance, etc. DCT is proposed to substitute the conventional power converter for bus voltage regulation and galvanic isolation. Compared with various topologies and CLLC-type has been recommended due to its capabilities of bidirectional power flow, seamless transition and low switching loss. Different operation scenarios for the DCT of the DC network has been analyzed. DCT is designed in order to maximize the conversion efficiency and minimize output voltage variations in different conditions. The models of DCT and its implementation in the proposed DC network have been developed in Matlab/Simulink. The performances of system under different operation conditions have been confirmed. more...

Hybrid AC/DC microgrid (MG) is a promising electrical power system architecture that enables to harmonize AC and DC energy conversions. Due to the complicated configuration, comprehensive stability analyses for a complete hybrid MG still stand as an unsurmountable challenge. To address this issue, this paper decomposes the MG system into several electrical modules, i.e., an AC source, a DC source, multi-paralleled interlinking converters (ICs) and line impedances. These modules are first individually modeled in the small signal sense. They are then combined to establish a full autonomous mathematical expression for characterizing dynamics of the hybrid MG. To investigate MG system stability, the influences of diversified operating points, line impedances and the selected key control parameters are examined by evaluating eigenvalue loci of the derived mathematical model. Time-domain simulations reveal the perfect matching between the results given by Matlab/Simulink and eigenvalue loci plots. more...

This paper presents an integrated series-parallel microgrid(MG), where a few distributed generation(DG) units in series type are used to form a DG string, then DG strings supply power to the common load in parallel type. With this structure, low-voltage devices could be integrated into MG without DC-DC boost converters. It provides a more flexible and efficient way to form higher voltage level MG. But it is difficult to find a decentralized control method to achieve power balance for this integrated system because of the difference between power transmission characteristics of parallel and series converter system. For this proposed MG in island mode, a unified distributed control strategy is proposed. With low bandwidth communication, accurate power sharing can be realized by exchanging information between the first converters connected to the bus of adjacent DG strings and between adjacent converters inside each DG string. Because the central controller is not used, the communication pressure of the system is reduced and the reliability is guaranteed. Simulation test has verified the feasibility and validity of the proposed control strategy. more...

To enhance the interaction between the thermal and electrical, a hybrid AC/DC microgrid (MG) embedded energy hub (EH) is proposed. In EH, electrical, thermal, i.e. heat and cooling are supplied and managed to increase the operational efficiency while meeting the energy demand. It formulates a two-stage stochastic optimization problem. In the first stage, i.e. day-ahead operation, the hourly generation, conversion, and storage are optimized to minimize the fuel cost. In the second stage, considering the stochastic nature of photovoltaic and demand, the heat and cooling supply are adjusted to realize the power balancing in the electrical system, while meeting the thermal energy demand. The two-stage optimization problem is reformulated to a deterministic optimization problem, using sample average approximation. Simulations have been carried out on the test system. Results have verified the effectiveness of the proposed method, collecting the flexibility of the thermal system to the electrical networks via EH. more...

Hybrid AC/DC microgrid exhibits better compatibility with renewable energy sources, energy storages and various loads. The conventional smart grid system are building based and require customized system design, which is costly and time-consuming. In this paper, smart energy hub, which is the modularized microgrid system design, is proposed to simplify system deployment. In order to realize the three-level hierarchical control to ensure both speed of response and system control accuracy, different sub-systems have been designed and developed. Multi-mode operation of the power electronic converters is enabled to cater for system operation in different scenarios. Resource manager serves as a super smart meter + relay controller to simplify the SCADA system design and enhance the speed of response. Microgrid operating system is designed to coordinate all the function blocks to realize hierarchical control in robust manner. Prototype of the proposed energy hub has been designed and developed to integrate all the sub-systems (both hardware and software). more...

This paper presents the implementation of a hardware-in-the-loop simulation (HIL) workbench for hybrid AC/DC micro grid. The hybrid AC/DC micro grid integrates both distributed sources and loads in AC/DC systems to respective links directly. For DC system, photovoltaic arrays with boost converter (PVBC), battery with bi-directional DC/DC converter and DC loads are covered. Diesel generator (DGs) and conventional AC loads are included in AC grid. Between AC and DC girds, a four-quadrant operating three phase converter is applied which can act as either an inverter or a rectifier to maintain power balance between two systems. The power units of this hybrid micro grid are simulated with Opal-RT real-time simulator while the control units are implemented on TI TMS320F28335 DSPs. This hardware-in-the-loop simulation workbench offers a good platform for system level control algorithm design and verification in micro grid. And it serves as a basis for future power-hardware-in-the-loop simulation (PHIL) with power stage involved. more...

This paper presents a coordination secondary control strategy for autonomous hybrid AC/DC microgrids with global power sharing operation. In the previous work, primary control with droop control method was applied to distributed generators to ensure local power sharing without communication links in either ac or dc microgrid. An extension of local power sharing for individual microgrid is the global power sharing for hybrid microgrids. With global power sharing control, all distributed generators throughout the hybrid system are capable of sharing the entire ac and dc loads without any information exchange. The coexistence of local power sharing and global power sharing in hybrid system is thereafter defined as generalized primary control, which is usually employed for fully decentralized power management throughout the whole hybrid system. To eliminate the inherent voltage/frequency deviations caused by the generalized primary control, the secondary control is usually applied to each distribution generator for voltage/frequency restoration. This however will degrade the performance of global power sharing operation because of its characteristics of voltage/frequency deviation dependence. To achieve voltage/frequency restoration while maintaining the global power sharing operation, a coordination secondary control was proposed and the effectiveness of the proposed method has been verified by the simulation results. more...

A distributed control architecture is proposed in this paper to realize economic operation in a hybrid AC/DC microgrid (MG). The entire system operation cost can be minimized when distributed generators (DGs) that induce costs have the same incremental costs (ICs). The architecture consists of two levels. In the first level, the AC frequency-IC (fac-IC) droop and the DC bus voltage-IC (Vdc-IC) droop are employed in the AC and DC subgrids respectively. With the synchronization offac and Vdc, DG ICs in each subgrid will be equalized. However, the droops will cause deviations of fac and Vdc. To restore fac and Vdc to their nominal values, a novel distributed controller is proposed in the second level to provide a generalized method for fac and Vdc recovery. The controller allows DGs in each subgrid to only communicate with their neighbors. Normally, the variations of fac and Vdc directly indicate the loading conditions of AC and DC subgrids. However, fac and Vdc fluctuations are invisible due to the existence of the distributed control scheme. To solve this problem, a relative loading index (RLI) is proposed to extract the hidden loading condition of each subgrid even though fac and Vdc are clamped as constants. By using RLI, the power reference of the bidirectional interlinking converter (BIC) can be easily defined. All DG ICs in the hybrid AC/DC MG will converge to the same value in the steady state, thus saving the total operation cost. The feasibility and effectiveness of the proposed control architecture are verified by simulations. more...