AC-DC Hybrid Research Articles

Stability and reliability of low voltage hybrid AC-DC microgrids power flow model in islanding operation

The problem of decreasing and increasing power flow in hybrid AC-DC microgrids in island operations is the concern of this research. This condition arises if one sub-microgrid cannot supply load power requirements, either due to a decrease in power at the source or an increase in load on the sub-microgrid network. As a result, in this research used the bidirectional converter as an interlinking converter, the converter can change the power flow from the ac sub microgrid to the dc sub microgrid and vice versa. With this bidirectional converter, load power requirements can be met even though there is a power change in one of the sub microgrid. The simulation in this research support by Simulink/Matlab software. The model is built in a low voltage system, and the power flow analysis is in steady-state condition with two different cases. The simulation results show the stability and reliability of the power flow in both sub microgrid with stable frequency and voltage when power and load change occur.

Study on Power Supply Structure of AC / DC Distribution Network Considering Different Proportion of New Energy Access

Considering the diversification of energy and load, according to the different situation of new energy access to distribution network, three typical AC-DC hybrid distribution network power supply modes are summarized. On the basis of these three power supply modes, the indexes of AC-DC hybrid distribution network are extracted, and the comprehensive evaluation index system is established. A comprehensive evaluation model of AC / DC hybrid distribution network suitable for new energy access is proposed, which aims to maximize economic benefits. The example shows that the optimal power supply mode can be selected through the evaluation model when the proportion of new energy is different, which can lay a foundation for the future distribution network to better serve users on the premise of distribution network profitability.

Virtual Resistor Based Second-Order Ripple Sharing Control for Distributed Bidirectional DC–DC Converters in Hybrid AC–DC Microgrid

The hybrid ac-dc microgrid, consisting of ac subgrid and dc subgrid, is a promising structure to interconnect ac and dc systems to provide high quality and reliable electric energy. However, in single-phase microgrid or unbalanced three-phase microgrid, second-order ripples of the dc subgrid can lead to significant power quality issues. The ripple mitigation function can be added to existing bidirectional dc-dc converters (BDCs) or dc active power filters. Considering these ripple sources can be distributed in the dc subgrid, it is essential to develop ripple mitigation control schemes for distributed BDCs. In this article, a virtual resistor based ripple mitigation strategy is proposed. Autonomous accurate ripple current sharing can be achieved for BDCs installed closely, enabling flexible parallel of ripple mitigation converters. A comprehensive parameter design method is developed to ensure the accurate sharing for closely installed BDCs. For dc subgrid with significant line impedance, the control scheme provides the opportunity for supervisory control to further enhance ripple current sharing and mitigation performance. Both simulation and experimental results validate that the control method and the parameter design method are effective.

Operation and Control of a Hybrid Coupled Interlinking Converter for Hybrid AC/Low Voltage DC Microgrids

The respective advantages of ac and dc microgrids lead to the blooming development of the hybrid ac/dc microgrid, which consists of ac and dc microgrid tied by an interlinking converter. A new structure of hybrid coupled interlinking converter (HCIC), which composed of a converter in series with a static VAR compensator (SVC), is proposed for hybrid ac and low voltage dc (LVdc) microgrids. Moreover, a new droop control method is correspondingly developed for the proposed HCIC. Its fundamental active and reactive power flow in the hybrid ac/dc microgrid is controlled by P-δ and Q-P-V droop control for converter part of the HCIC, and P-Q-α droop control is proposed for SVC part of the HCIC. Furthermore, a fast harmonic control is given based on a decoupled control strategy to improve the filtering characteristics of SVC part of the HCIC, suppress the harmonic power, and/or inject active power. Finally, simulation and experimental results are provided to verify that the proposed HCIC for hybrid ac/LVdc microgrids has the ability of good power flow control and power quality compensation, which not only meets the power flow requirements with low rating system, but also greatly promotes the development of more robust hybrid ac/dc microgrids.

Coordinated Control Strategy Based on MMC and DFIG

The AC-DC hybrid distribution network based on modular multilevel converters (MMC) is beneficial for receiving distributed generation (DG), energy storage devices and DC loads. Therefore, the coordinated control of MMC and DG is very important for its safe operation. In this paper, doubly-fed induction generator (DFIG) are used as DG, and the control strategies of DFIG and MMC systems are analyzed. The system modeling research is carried out on a real time digital simulator (RTDS) platform. Then based on MMC control, an improved adaptive Crowbar circuit is proposed to improve inverter AC system low-voltage ride-through (LVRT) capability. The results show that the coordinated control strategy can better compensate the reactive power of the system to achieve LVRT, and verify the effectiveness of the control strategy adopted by the converter station.

Study on Comprehensive Evaluation System and Method of AC-DC Hybrid System

With the rapid development of renewable energy worldwide, the electricity from large-scale renewable energy bases is transmitted to load-intensive area by DC transmission lines, forming an AC-DC hybrid system with AC and DC coupling obviously and tightly. Measuring the development level of the AC-DC hybrid system is of great significance for grid construction and power transmission. This paper first establishes a kind of comprehensive evaluation system that reflects both the interaction of AC-DC and the overall system capability. Then the evaluation method combining the three methods of AHP-Entropy-TOPSIS is carried out relatively. Finally the evaluation system and method is applied in the actual grid example. The comprehensive evaluation system and method of AC-DC hybrid system constructed in this paper is of reference significance to power grid planning and operation management.

Hybrid AC-DC micro grids with improved power control strategy

Hybrid AC-DC micro grids with improved power control strategy

Stochastic management of hybrid AC/DC microgrids considering electric vehicles charging demands

The high growth of the automotive industry reveals the very bright future of this technology and its high penetration effects on the human society. No doubt that the random and volatile charging demand of these devices would affect the power grid optimal operation and scheduling which may be regarded as a new challenge. Therefore, this paper investigates the stochastic scheduling of hybrid AC/DC microgrids considering the plugin hybrid electric vehicles charging demands, distributed all over the grid. Three different charging patterns, called coordinated, uncoordinated and smart charging models with different characteristics for the charger type, capacity and market share are proposed. Moreover, different types of renewable energy sources including wind turbine, solar panel and fuel cell are modeled and considered in the scheduling process of the hybrid microgrid. In order to mitigate the charging effects of electric vehicles on the hybrid AC–DC microgrid operation, some remotely switches are considered in the system which make it possible for changing the topology and power flow way. In order to model the uncertainty effects, a data-driven framework based on point estimate method and support vector machine is developed. This would make it possible to extract out the standard deviation value of the uncertain parameters and reflect their impacts on the microgrid operation problem through the limited concentration points. A novel evolving solution based on flower pollination algorithm is also proposed to solve the problem optimally. An IEEE standard test system is used as the hybrid AC/DC microgrid case study to assess the performance of proposed model.

Simplified Hybrid AC–DC Microgrid With a Novel Interlinking Converter

Hybrid ac–dc microgrids (HMG) have become more popular recently because of their superior features in comparison with pure ac and dc microgrids (MGs). HMGs have fewer power converters and are more flexible than pure ac and dc MGs, but the number of converters in HMG is still significant, especially when the dc part of HMG has several voltage levels. A new simplified and more flexible architecture for HMGs that features a new multiport interlinking converter (IC) is proposed in this article. Using the proposed IC, the number of power electronic converters in an HMG with several dc bus voltages can be reduced without increasing the number of active switches in the IC or the complexity of its control system. In this article, the new simplified HMG architecture is presented, and the operation of the proposed IC as a single unit and as a part of a simplified HMG are explained, along with its features. Experimental results obtained from a scaled-down prototype are also presented to confirm the feasibility of the multiport interlinking converter. Simulation results that show the effect of load variations in the intermediate bus architecture dc bus on the operation of the HMG are also presented as well.

Applicability Analysis of Directional Components in AC/DC Hybrid System

AC/DC hybrid power grids are more and more widely used in China, and the traditional relay protection directional components have experienced multiple misoperations. The adaptability of traditional directional components in AC / DC hybrid power grids is studied. Traditional direction components are based on fault sequence component network analysis. During the fault, the positive and negative sequence impedance of the AC-DC hybrid power grid is quite different, and the impedance angle of the line impedance and the equivalent impedance of the grid is no longer close to 90°. Misjudgment will occur in the direction component with sudden change, and performance cannot be guaranteed. As a result, directional elements based on the positive and negative sequence current branching coefficients being approximately equal are not suitable for AC/DC hybrid power grids. As for the directional element of the zero sequence principle, since the zero sequence network on the inverter side is relatively stable, its performance is reliable and it can operate correctly.

Research on Improved Droop Control of AC-DC Microgrid Bidirectional Converter Based on Power

The bidirectional converter in AC / DC hybrid microgrid is an important link to balance the power on both sides of AC and DC, Its control method is extremely important. This paper proposes an improved droop control strategy for bidirectional converter using DC-side power amount control. This control can autonomously and smoothly switch in rectifyer, inverter and idle modes. Utilizing DC-side power control avoids normalized calculations. Strategy meets two modes of island and grid connection, eliminating the need for multiple control strategy switches. Simulation results show that the DC bus voltage and AC frequency are near the rated value, which guarantees the stable operation of the AC-DC hybrid microgrid.

Optimization of Hybrid AC DC Microgrid in Grid Connected Mode

The instability of the production of renewable energy resources (RESs) is a major problem for its installation and integration with the utility grid. A hybrid AC/DC microgrid facilitates the good operation of RESs with a storage system in grid-tied mode and the possibilities of smart energy management. The present work is being carried out on smart control strategy for a hybrid AC/DC microgrid (HMG). This paper deals with a real-time Hybrid control system (HCS) for a grid-connected HMG where the supervisory control based totally on fixing an optimization hassle objectives to minimize operating cost with the maximum usage of renewable resources, minimum usage of the utility grid, extending energy storage systems (ESS) lifetime. The power references for renewable energy resources (RESs) are scheduled by the use of CPLEX solver which uses as input strength measurement, archived facts and stored climate forecast statistics previously measured regarding the modern status. The experimental study verifies well the supervision methodology; thus, the proposed algorithm respects the optimization in real-time operation under different constraints.

General form of consensus optimization for distributed OPF in HVAC-VSC-HVDC systems

In this paper a distributed algorithm for ensuring Optimal Power Flows in hybrid AC-DC systems is proposed. While centralized OPF formulations require information of many variables of all the network nodes, the present approach relies on solving an OPF locally in each AC agent and AC-DC agent that only requires to exchange the voltages corresponding to adjacent nodes. Therefore, operation schemes based on this approach would be specially interesting when communications with the central unit fail. This algorithm is applied to a 3 DC - 5 AC bus system, showing that the obtained results coincide with those of the centralized formulation. Additionally, the dynamic behavior of this system operated under the proposed algorithm is analysed in a real-time platform in a realistic communication environment.

Comparative Assessment of Control Strategies Used in a Hybrid AC-DC Micro-Grid Consisting of Composite Energy Sources

In the present paper, comparative assessment of two important available intelligent control strategies namely the Mamdani fuzzy logic controller and the T–S fuzzy controller used for DC-link voltage control in DC micro-grid is carried out, first. Then, a new inverter control technique is proposed to maintain the inverter voltage as well as the DC-link voltage in a hybrid AC-DC micro-grid. The DC micro-grid under study comprises of photovoltaic system, permanent magnet synchronous generator-based wind energy system, battery energy storage system, fuel cell, electrolyzer and DC loads. The complete DC Micro-grid along with its correlated control algorithms are examined and accessed through MATLAB/Simulink environment.

Mitigation of forced oscillations using VSC-HVDC supplementary damping control

Forced oscillation has become one of the main power grid dynamic stability problems, which can cause high amplitude oscillation of the active power in the tie line. The supplementary damping controller (SDC) of the voltage source converter based high voltage direct current (VSCHVDC) has demonstrated its effectiveness in suppressing inter-area modal oscillation, but its role in mitigating forced oscillation has not been studied in the literature. In this paper, an active disturbance rejection control (ADRC) based SDC of the VSCHVDC is designed to mitigate forced oscillations. The mitigation effect of the ADRC based SDC on forced oscillation is compared to the traditional one under different scenarios. Simulation results of four-machine two-area AC-DC hybrid system indicate that the traditional SDC is sensitive to the frequency of the disturbance source, and the reactance between the disturbance source and the VSCHVDC, whereas the ADRC based SDC can effectively mitigate the forced oscillation whatever the frequency of disturbance source is near or far from the inter-area natural frequency, and its mitigating effect is almost not affected by the reactance between the disturbance source and the VSCHVDC.

Impedance Analysis and Stabilization of Point-to-Point HVDC Systems Based on a Hybrid AC–DC Impedance Model

Stability analysis of the point-to-point high-voltage dc (HVdc) transmission systems is crucial for planning and operational purposes. However, due to the protection of trade secret and user privacy, it is difficult to have access to detailed information of the converters of an HVdc system. To avoid detailed information of the converters during system stability assessment, this article adopts a hybrid ac-dc impedance model along with a new network partitioning method to assess stability of the point-to-point HVdc system. The proposed method has the following advantages. First, in contrast to the conventional stability analysis methods, the number of right half-plane poles of the system minor-loop gain is not required. Second, different from the conventional impedance models, the hybrid ac-dc impedance excludes the impacts of the impedances of the ac grid and dc networks, to which the converter is connected. Accordingly, the hybrid ac-dc impedance model solely depends on the converter itself. In addition to the proposed method, this article demonstrates that the power flow direction of a point-to-point HVdc system has no impact on selection of different system minor-loop gains. Accuracy and effectiveness of the proposed method are evaluated and validated by time-domain and frequency-domain simulations in the MATLAB/Simulink environment.

A Vehicle-to-Microgrid Framework With Optimization-Incorporated Distributed EV Coordination for a Commercial Neighborhood

In this paper, an improved vehicle-to-microgrid (V2M) framework is proposed for a commercial locality operating as a hybrid ac–dc microgrid, which optimally coordinates electric-vehicle (EV) storages in a distributed manner. An aggregator model is proposed that solves the economic dispatch problem of parked EV storages in a centralized fashion and generates power references in real time for the designed EV storage controllers. Unlike the conventional EV storage controller, the proposed optimization-incorporated distributed EV storage controller can switch from decentralized to distributed control mode or vice versa based on situations and utilizes a sparse vehicle-to-vehicle communication network. The power flow between the ac and dc subgrids is managed by interlinking converters (IC). The IC control structure is augmented by combining voltage- and power-based droop-control schemes in the power control loop. This modification enables simultaneous ac and dc bus voltage regulations. Case studies are carried out to validate the efficacy of the developed framework with a real commercial network and loads. The results exhibit robust performance of the overall system for generation-demand variability, transitions between islanded and grid-tied conditions, and user-preferred EV disconnections and time delay.

A Single-Stage Fault-Tolerant Three-Phase Bidirectional AC/DC Converter With Symmetric High-Frequency Y-Δ Connected Transformers

This article presents a single-stage isolated three-phase bidirectional ac/dc converter with symmetric Y-Δ connected transformers, which implements ac/dc buck conversion, high-frequency electrical isolation, and high power factor for ac side (PF > 0.99). The converter can be utilized between the ac bus and the dc bus in ac–dc hybrid microgrid and in other applications where three-phase bidirectional ac/dc conversion is needed. With two operating modes available, the converter possesses the fault-tolerant ability and the improved light-load efficiency. When the converter works in the high power application with all the components operating normally, the transformers operate in the symmetric mode and share the power equally. Then, the switch current stress can be reduced. The converter can also operate in dual transformer mode for light-load conditions or with some abnormal components. The Y-Δ connected transformers provide an additional step-down conversion ratio and make the converter suitable for buck conversion. Furthermore, to guarantee the voltage-second balance of the transformers, the corresponding space vector pulse width modulation (SVPWM) algorithm is proposed. Experimental results based on a 2.4-kW prototype are presented in detail, which verify the theoretical analysis and demonstrate the converter's performance well.

Coordinated Power Management of the Subgrids in a Hybrid AC–DC Microgrid with Multiple Renewable Sources

ABSTRACTThis paper presents the assessment of the operation and control of hybrid AC–DC microgrid (HMG) integrated with alternative energy sources. The system is analyzed by incorporating fuzzy logic and adaptive neuro-fuzzy inference system (ANFIS) based controllers in the control scheme of battery. Photovoltaic array and fuel cell system are coupled to the DC bus and an electrolyzer based hydrogen storage system is used for energy storage in the DC subgrid. The output of the wind energy system along with the battery is fed to the AC bus through an inverter. The AC and DC subgrids are integrated through an interlinking converter which is controlled to facilitate power exchange between the subgrids based on the frequency of AC bus and DC bus voltage. Appropriate power management is achieved within the subgrids and between the subgrids by the coordinated control schemes employed in the HMG. The main focus of this article is the analysis of the impact of incorporating conventional proportional–integral controller, fuzzy logic controller and ANFIS based controller in the control scheme of the battery when the subgrids are subjected to power fluctuations. By employing the ANFIS based controller, the transients in the voltage and power output of the battery system are minimized and deviations occurring in the AC bus voltage during load variations are reduced.

Coordinated control of series compensation link and bus interface converter in the AC–DC hybrid microgrid

To improve the fault ride-through capability and power quality of a hybrid microgrid (HMG), a series compensation link (SCL) is inserted between an AC–DC HMG and a utility grid, which forms a new HMG topology. Correspondingly, a decoupling coordinated control strategy for the SCL and a bidirectional AC–DC interlinking converter (BIC) is proposed based on a small signal model of the entire system and the principle of voltage regulation. The interferences between the SCL and the BIC due to the coupling of control factors are eliminated by the series and parallel decoupling methods that adopt additional current closed-loop control and output voltage feedforward control, respectively. The proposed HMG topology and strategy can ensure the realization of the predesigned functions of dual converters and avoid frequent switching between the grid-tied mode and the island mode, which improves the fault ride-through capability and power quality. Finally, the effectiveness of the proposed approach is verified by simulations and experiments.