DC Subgrids Research Articles

Design and analysis of closed loop control of power-converters for forming droop-controlled AC–DC subgrids of an islanded hybrid AC–DC microgrid

Abstract Hybrid AC–DC microgrid (HMG) is formed by connecting separate AC and DC microgrids with an interlinking converter (IC). In islanded mode of operation of the HMG its AC and DC subgrids are droop controlled to achieve power-sharing. The power flow between the subgrids happens through the IC, which regulates the amount of power being transferred based on the values of the frequency of AC bus voltage and the voltage at the DC bus. This paper presents the utilization of power-electronic converters to form the droop-controlled AC–DC subgrids of an islanded HMG which will be beneficial for further studies of IC and HMG. The detailed design and analysis of the closed-loop control system parameters of both the power converters is presented. The results obtained from simulations carried out in MATLAB/Simulink, and the real-time simulation results obtained by implementation using OPAL-RT OP4500 real-time digital simulator hardware are presented for the verification.

A novel strategy for improving power quality of islanded hybrid AC/DC microgrid using parallel-operated interlinking converters

The hybrid AC/DC microgrid (HM), which combines the advantages of both AC and DC subgrids, is a promising structure. However, the penetration of a large number of non-linear loads and single-phase/unbalanced loads in the HM can degrade the power quality of the HM. Therefore, this paper proposes a novel control of parallel-operated interlinking converters (ILCs) to integrate power quality compensation, which is able to improve the power quality effectively. In the proposed strategy, an ILC with the largest capacity is selected to be controlled into the voltage source mode, which is referred to as the master ILC, to achieve a balanced and harmonic-free AC bus voltage. Moreover, by introducing virtual impedance to regulate the equivalent impedance, the master ILC is capable of operating in different compensation modes according to the different power quality requirements of HM. Other ILCs are called slave ILCs and are controlled in current source mode to eliminate DC bus voltage ripple by controlling the pulsating power transmitted by multiple ILCs counteracting each other. The proposed control strategy makes full use of the remaining capacity of the ILCs to integrate the power quality compensation in HM without adding additional hardware, thus reducing the power quality control cost of the system and achieving balanced and harmonic-free AC bus voltage as well as DC bus voltage ripples-free simultaneously. Finally, the simulation results verify the effectiveness and feasibility of the proposed ILC power quality control method.

Optimised coordinated control of hybrid AC/DC microgrids along PV-wind-battery: a hybrid based model

This work presents a novel coordinated control approach for an MG with hybrid AC/DC loads and energy resources. As a result, power interactions between the DC and AC subgrids are appropriately enabled by distributing power fluctuations in a coordinated manner. Initially, a coordinated control scheme of distributed converters was presented. Then, a model predictive power and voltage control (MPPVC) technique is used for DC/AC interlinking converter that offers high-quality voltages and guarantees the optimal power transfer amongst ac and dc sub-grids. This paper aims to enhance the control structure of the energy storage system (ESS), where the optimised PI controller is insisted. Particularly, the gain of PI controller is optimally tuned by a new hybrid algorithm named as index-based fly-rider combined model (IF-RM) that incorporates both the concepts of rider optimisation algorithm (ROA) and firefly algorithm (FF). Finally, the betterment of the proposed work was validated over other existing models.

A Newton–Raphson-Based Sequential Power Flow Algorithm for Hybrid AC/DC Microgrids

A hybrid ac/dc microgrid (MG) has the advantages of both ac and dc subgrids, and the operator should perform an accurate power flow analysis to ensure the MG's operational economy and reliability. This article proposes a Newton—Raphson (NR)-based sequential power flow algorithm for the hybrid ac/dc MG. First, the MG's islanded operation mode and the three-phase unbalance feature of ac subgrids are considered, and the droop control for distributed generations participating in the voltage and frequency regulation is modeled. Then, a sequential algorithmic framework is proposed to handle the coupling between ac and dc subgrids. Within this framework, the power flow of ac and dc subgrids is solved separately and iteratively with modified NR subalgorithms. The accuracy of the proposed sequential algorithm is validated in comparison with the results from MATLAB/Simulink, and the advantage of employing this algorithm is confirmed compared with an equivalence approach. The case studies demonstrated that the proposed algorithm can significantly improve the result accuracy with acceptable computational time. Finally, the convergence performance of the proposed algorithm is verified in the cases of varying ac and dc subgrid loading.

Distributed Uniform Control for Parallel Bidirectional Interlinking Converters for Resilient Operation of Hybrid AC/DC Microgrid

This paper proposed a distributed uniform controlstrategy for parallel bidirectional interlinking converters(BICs) to realize resilient operation of hybrid ac/dc microgrid (HMG). The proposed control strategy can coordinate the control objectives of BICs in different HMG operation modes with a uniform control structure, thus, to eliminate any mode transition detections. In normal condition, the power coordination between ac and dc subgrids can be achieved, while ac or dc voltage regulation can be maintained when main voltage source(MVS) fails in ac or dc subgrid. The coordination between parallel BICs relies on low-bandwidth neighbourhood communication. With the communication planned, accurate power sharing can be achieved. Communication fault ride-through capability is enabled with less power sharing accuracy. The system reliability and resilience of HMG under various fault conditions can be improved by the proposed control strategy. To verify the effectiveness of the proposed control strategy, the controller hardware-in-the-loop experiment results have been provide in this paper.

A Power Control Method for Hybrid Electrical Accommodation Systems

This paper presents a power control method for a hybrid electrical system, which enhances a conventional main AC grid by adding a DC sub-grid in parallel. In this system, each prosumer connects to the sub-grid via a couple of half-bridge inverters of a device called a power router. All of the power routers connected to the sub-grid are controlled such that they maintain the voltage of the sub-grid equally, which is an essential feature of peer-to-peer power accommodation without synchronization. This paper proposes an adaptive hysteresis current-control technique for controlling sub-grid-connected inverters that maintains the voltage of the sub-grid while accommodating a given power. The proposed control method also balances currents between positive and negative lines of the sub-grid simultaneously. The proposed control method is implemented on a high-speed FPGA-based digital controller and is evaluated on three prototypes of the power router. The experimental results show that the proposed method yields fast and stable responses and enables the power router to accommodate power with a high accuracy.

Feedback Linearization-Based Control Strategy for Interlinking Inverters of Hybrid AC/DC Microgrids with Seamless Operation Mode Transition

This study proposes an advanced control scheme for the interlinking inverters of the hybrid AC/DC microgrids, which facilitates a seamless transition between grid-connected and stand-alone/islanding modes for the microgrid. Due to a nonlinear relationship between the terminal voltages of the voltage-source inverter (VSI) interfacing through inductor–capacitor (LC) filters with the grid voltages and currents, a feedback linearization technique (FLT) is employed to control the interlinking VSI under both grid-connected and islanding operations. The FLT-based current controllers are applied in the grid-connected mode, in which they adjust the power exchange between the DC and AC subgrids and mitigate the distortion of the grid currents produced by nonlinear loads. Under the stand-alone operation, the AC bus voltages are directly regulated by the FLT-voltage controllers of the interlinking VSI. In order to reduce the inrush currents and voltage overshot at the instant of mode switching, the FLT-based controllers are performed all the time regardless of the operating modes, where the voltage references for the VSI are not changed abruptly. The control performance of the VSI is highly satisfactory with low-transient overshoot values of the voltages and currents and low total harmonic distortion (THD) values of the grid currents and AC bus voltages are about 3.5% and 2.7%, respectively, under the nonlinear load condition. The validity of the new control strategy is verified by the simulation work, which investigates the operation of a hybrid AC/DC microgrid.

A scalable and reconfigurable hybrid AC/DC microgrid clustering architecture with decentralized control for coordinated operation

Microgrid clusters are effective to increase utilization of renewable energy resources (RESs), and improve reliability and stability of power systems. Facilitating flexible configurations, grid networking and coordinated operation among multiple microgrids are essential for the microgrid cluster. In view of this, this paper presents a scalable and reconfigurable hybrid microgrid clustering architecture and a corresponding decentralized control method. Initially, a new interlinking converter referred as energy networking unit (ENU) is proposed. The ENU is used to interface the AC and DC subgrid in single hybrid microgrid, and also facilitates the connection with the external power grid. Then the ENU-based hybrid microgrid clustering architecture is established, which is characterized by scalability, reconfigurability and modularity. Thanks to the reconfigurable topology of ENUs, the architecture could realize flexible AC/DC interconnection between microgrids by the same converter modules with less power conversion units. Finally, a proposed decentralized control strategy for this architecture could realize autonomous power exchange between subgrids in single microgrid with considering energy storage restrictions and smooth mode switch. And an adaptive and autonomous power interaction among neighboring microgrids is also achieved with full utilization of the compensation capacity of clusters, thus improving the system reliability, as well as the utilization and local consumption of RESs. The effectiveness of the proposed clustering architecture and decentralized control strategy is verified by comprehensive cases.

Performance analysis of hybrid AC / DC microgrid under influence of battery energy storage location

In microgrids, energy storage is generally located based on power management criteria and economic objectives that lean on the system's steady-state operation. However, the grid configuration, dynamics of primary energy sources, and storage type (AC or DC) impact its transient operation. This paper conducts a comparative study of the dynamic performance of Hybrid AC/DC Microgrids (HMGs) under the influence of Battery Energy Storage System (BESS) location, which has been neglected in the literature. The HMG performance is compared for one single BESS unit located either in AC or DC sub-grid during various grid operational cases, considering interlinking converter (ILC) control topology that alters depending on the type of BESS. Two types of microgrid case studies with different structures have been considered, that is, a high inertia HMG and a converter-interfaced low inertia HMG. Simulation results reveal that the HMGs with both BESS locations perform robustly in handling grid disruptions. However, the BESS located in the DC sub-grid of a high inertia microgrid case presents an average of 40% higher quality of performance in key performance indicators compared to that of the AC sub-grid BESS. Conversely, the performance of the low inertia HMG with the BESS connected to the AC sub-grid is superior to that of the DC sub-grid BESS, by an average of 65% and 54% in key performance indicators.

Performance Enhancement of a Hybrid AC-DC Microgrid Operating with Alternative Energy Sources Using Supercapacitor

Microgrids with non-conventional energy sources have become popular recently. Hybrid AC-DC microgrid (HMG) architecture is effectual as it avoids several power conversions for the consumers. Therefore, this article presents a comprehensive study on grid-tied HMG with PV array and wind energy conversion system (WECS) as principal sources. Fuel cell (FC) acts as the auxiliary source in the DC subgrid and the supercapacitor (SC) is used for instantaneous energy management. The hydrogen storage system is used to store surplus power produced by the PV array. The power flow between the subgrids is regulated using the interlinking converter (ILC) by a PQ controller. The main contribution of this article is the comparative investigation of system operation in the HMG configuration in the presence and absence of a supercapacitor bank on the DC bus. The maximum DC bus voltage fluctuation during load variations in the absence and presence of SC bank is found to be 6.6 V and 3.9 V respectively. Similarly, the maximum transient fluctuation in the power supplied to the DC load is found to be 830 W in the absence of SC bank and 340 W in the presence of SC bank

Unified load flow calculation for flexible interconnected AC-MTDC distribution system considering control strategy switching with high penetration of DGs

A new unified algorithm is proposed in this paper to solve the power flow problem of AC distribution systems incorporating voltage-sourced converter (VSC) based multi-terminal direct current (MTDC) grids. In this method, the VSC modulation indices, mode changes, power losses as well as MTDC control strategy switching are all considered. Active power mismatch equations of VSC boundary buses in AC side have been modified to eliminate VSC power injection items. Various VSC control strategies are demonstrated in both active power function of DC boundary buses and reactive power function of AC boundary buses. In this way, only few modifications are required for existing AC load flow to incorporate the MTDC models. Moreover, the change of VSC operation mode and the switch of MTDC control strategies can be easily realized. The power flow calculation of AC and DC subgrids is solved simultaneously by Newton-Raphson (NR) method using a modified Jacobian matrix. Two case studies have been presented to demonstrate that the proposed algorithm is fast, flexible and effective in solving power flow of AC-MTDC distribution systems with various control strategies and can easily accommodate any changes in VSC control modes with high convergence performance.

Distributed control strategy for global economic operation and bus restorations in a hybrid AC/DC microgrid with interconnected subgrids

With the increasing penetration of distributed generators (DGs) in microgrid (MG), the economic power dispatch (EPD) is a fundamental optimization problem of MG systems. This paper investigates a distributed control strategy, which is consists of local economic control (LEC) and global economic control (GEC), for a hybrid MG with multiple interconnected ac and dc subgrids. The incremental cost (IC) of each DG is selected as a discrete consensus variable and the mismatch of bus parameters, such as dc bus voltage and ac bus frequency, between measured and reference values is assigned as a feedback variable. Thus, both the economic power allocation demand for all DGs and the bus restorations within each subgrid can be simultaneously realized by the proposed LEC. This reduces implementation cost, enhances the operation reliability, and accelerates the converging speed of algorithm. Furthermore, the interconnected subgrids are connected by a set of bidirectional interlinking converters (BILCs) that play a critical role in the power flow performance. Hence, the distributed GEC makes the whole MG system is in global economic operation in accordance with the equal IC principle. The impacts of communication time delays and communication link failures are also analyzed. Finally, the effectiveness of the proposed control strategy is verified by the real-time hardware-in-loop (HIL) tests.

An improved coordination control for a novel hybrid AC/DC microgrid architecture with combined energy storage system

This paper proposes a novel hybrid ac/dc microgrid (MG) architecture that integrates a combined energy storage system (ESS) for both ac and dc subgrids, which avoids the problems associated with the distributed ESSs with different state-of-charges (SoCs). To realize effective operation of the hybrid MG, this paper has also proposed an improved coordination control strategy between the combined ESS and the paralleled bidirectional interlinking converters (BILCs). First, a novel SoC-based adaptive droop control is designed for the combined ESS to provide dc bus voltage support and ac bus frequency support, and the lifetime of the combined ESS is extended because the overcharge and overdischarge can be avoided. Second, ac bus frequency and dc bus voltage are regarded as global parameters to enforce the power interaction between ac and dc subgrids. The power interactions between ac and dc subgrids can be proportionally allocated with the coordination coefficients to avoid an over-used BILC. Furthermore, the proposed distributed coordinated power control (DCPC) for BILCs network has plug-and-play capacity and allows an acceptable communication time delay and communication link failures. Finally, the effectiveness of the proposed coordination control strategy applied into the novel hybrid ac/dc MG system is verified by the real-time hardware-in-loop (HIL) tests.

Integrated Nonlinear Hierarchical Control and Management of Hybrid AC/DC Microgrids

Increasing the usage of distributed generations (DGs) leads the microgrids (MGs) to develop. Considering the existence of alternating current (ac) loads and sources and concerning increasing direct current (dc) loads and sources, hybrid ac/dc MGs have been used to have the advantages of both ac and dc MGs and reduce the drawbacks of them. By integrating the DG units in hybrid ac/dc MGs. Hence, a proper control method is required in such MGs to achieve precise voltage regulation and power-sharing, desired power quality, high efficiency, and high reliability. This article focused on nonlinear exponential control and distributed secondary control schemes to properly control the MG and form an integrated nonlinear hierarchical control and management for hybrid ac/dc MGs. Finally, to evaluate the proposed nonlinear control strategy's performance, offline digital time-domain simulation studies are carried out on a test MG system in MATLAB/Simulink environment. The results are also compared with previously reported methods. The simulation results and the comparisons showed that the proposed methods could properly share the power among subgrids and DGs in both ac and dc subgrids. In contrast, the proposed control scheme can prove its effectiveness and superiority over conventional controllers.

Virtual Oscillator Control of Distributed Power Filters for Selective Ripple Attenuation in DC Systems

In this article, a decentralized controller, based on the current mode virtual oscillator, is proposed to control the distributed power filters in dc systems. This communication-free solution only requires local dc bus voltage measurements as its control input. With the proposed control algorithm, the power filters can automatically share the target ripples based on their storage capacities, realizing modular and fault tolerant design. One typical application is that they can be placed on the dc subgrid of a hybrid ac/dc microgrid to attenuate the second-order harmonics from ac side. A frequency adaption mechanism is further embedded within the controller; hence, it can process any ripples at an unknown frequency for more general applications. To eliminate the circulating current caused by the power line dynamics, virtual impedance is employed to modify the feedback voltage measurement of the virtual oscillator. The proposed control algorithm has been verified by both simulation and down-scaled experiments.

A Decentralized Control Strategy Based on V-I Droop for Enhancing Dynamics of Autonomous Hybrid AC/DC Microgrids

This article presents a GPS-based decentralized control strategy for hybrid ac/dc microgrids. The proposed method uses V-I droop characteristics for the distributed energy resources (DERs) in each of ac and dc subgrids. In particular, the output voltage of each DER is adjusted as a piecewise linear function of the per-unit (pu) output current. In addition, a droop control method is proposed for the interlink converter (IC) to attain global power sharing among the dc and ac DERs. The IC controller changes the deviation of the ac bus voltage as a function of the deviation of the dc bus voltage. This way, the power flow between ac and dc subgrids is adjusted such that the ac and dc DERs experience the same pu voltage deviation and hence deliver the same pu current and active power. In contrast with the existing control approaches, the proposed method does not require frequency or power calculations but directly uses voltage and current measurements in the control loops. So the delay associated with the feedback signals is minimized, which avails accurate load sharing during transients. Experimental results show the proposed method eliminates the transient overshoots of DERs' output powers by offering a fast and overdamped dynamic response.

Power sharing in hybrid microgrids with multiple DC subgrids

Control strategies are prominent for the efficient and stable operation of emerging hybrid microgrids (HMGs), where bidirectional dc/dc converters (BDCs) are used to connect multiple dc subgrids to a common dc bus, and bidirectional interlinking converters (BICs) are used to exchange power between the dc and ac sides. Conventional droop controls may not be able to provide accurate power sharing in such sophisticated HMGs. In this paper, a framework is proposed for accurate power sharing among BDCs and BICs using existing harmonics at dc networks and a multi-dimensional droop control. In particular, we employ the frequency of available harmonics as global variables at the dc subgrids and the common dc bus to design a droop control approach for accurate power sharing among BDCs. It outperforms the existing voltage-based droop approaches when practical situations of HMGs, such as different cable resistance of BDCs, are considered. For the BICs, we propose a multi-dimensional droop as an integrated control strategy to adjust the power exchanged between dc and ac sides as well as the power of the BIC’s energy storage system. As a result, not only the converters are operated in a more economical and flexible manner, but also their overstressing and spurious loading are avoided resulting in better maintenance and more robust operation. An HMG with different types of converters and subgrids is used to evaluate the proposed features.

Distributed Energy Management of the Hybrid AC/DC Microgrid with High Penetration of Distributed Energy Resources Based on ADMM

This paper aims to investigate energy management of the hybrid AC/DC microgrid with the high penetration of distributed energy resources (DERs), such as electrical vehicles, heat pumps, and photovoltaics. In the previous studies, energy management of the hybrid microgrid is usually carried out by the system operator in a centralized manner, which suffers from the compromise of privacy information protection and the risk of single‐point failure. Therefore, this paper proposes a distributed energy management scheme of the hybrid microgrid using the projection function‐based alternating direction method of multipliers (P‐ADMM), which allows each subgrid, i.e., AC subgrid and DC subgrid, to make day‐ahead schedules independently with information exchanges while obtaining the optimal energy management solution. The energy management problem of the hybrid microgrid is formulated as a mixed‐integer quadratic programming (MIQP) model, considering DER and energy storage system operation constraints, system operation constraints, and converter operation constraints. Then, the MIQP model is decomposed and distributed into smaller‐scale QP models between subgrids using the P‐ADMM algorithm, which can handle binary variables through projection functions. The numerical results conducted on the hybrid microgrid demonstrate that the proposed distributed scheme can effectively achieve optimal energy management for the hybrid AC/DC microgrid in a distributed manner.

Bi-directional power flow control with improved power quality using intelligent controller for AC-DC coupled hybrid micro-grid system

Grid connected renewable sources are affected by harmonic distortion due to non-linear power electronics interfaces. Depending upon the nature of the load, many conversion stages are incorporated in the micro-grid which leads to increase in the losses. Therefore, in this paper, the AC-DC coupled hybrid micro-grid system is presented in which AC renewable sources and AC loads are connected to AC sub-grid and DC renewable sources and DC loads are connected to DC sub-grid. The shunt active power filter based 3φ 4-leg bi-directional intermediate converter using d-q load current technique is proposed to achieve the power flow between AC and DC sub-grids. The MATLAB simulation is carried out, and the performance of the proposed system is analysed using the d-q load current technique based fuzzy logic and PI controller. The obtained results demonstrate that the overall system performance is improved with the d-q load current technique based fuzzy logic controller.

Stability Evaluation of AC/DC Hybrid Microgrids Considering Bidirectional Power Flow Through the Interlinking Converters

The bidirectional power flow through the interlinking converter (IC), in ac/dc hybrid microgrids (HMGs) consisting of distributed generators (DGs) with droop controllers, plays an important role on the stability of such systems during islanding. This paper investigates the impact of the power flow direction on the small-signal stability of islanded droop-based HMGs. Firstly, a linearized state-space model of an HMG is developed. Secondly, eigenvalue analysis is carried out to realize the dominant modes, which are the rightmost eigenvalues. Thirdly, participation factor analysis is performed to identify the system and control parameters that effect stability the most. Lastly, sensitivity analysis is conducted to determine the critical droop gains and stability margin. It is observed from the eigenvalue and sensitivity analysis that the dominant modes of HMGs become more stable as more power flows from dc to ac subgrid. On the contrary, an increase in the power flow from ac to dc subgrid degrades the HMG stability. Additionally, the sensitivity of the dominant modes to changes in ac and dc droop gains is studied under bidirectional power flow through the IC to ascertain their impact on the stability margins. Finally, time-domain simulations, in MATLAB/Simulink, suggest that more generation on the dc subgrid would enhance the overall HMG stability margin during islanding.