DC Subgrids Research Articles

A novel stability index-based virtual synchronous machine droop scheme for performance improvement of hybrid AC/DC microgrid under volatile loading conditions

The large-scale electric vehicle penetration into the grid and the volatile nature of the electric vehicle loading adversely affect grid stability. This paper presents a novel index-based droop scheme for improving the performance of the hybrid AC/DC microgrid under volatile loading conditions. The novelty of the proposed control scheme is that the adaptiveness in selecting active droop gain based on the volatile stability index generates an accurate power reference rather than a constant power reference as in conventional droop gain methods. The state–space modelling of the proposed controller and index-based approach is used to determine the theoretical stability margin. The simulation experiment results agree with the theoretical stability margin limit. The efficacy of the control scheme is validated using real-time experiments on a hardware-in-the-loop real-time simulator, OPAL-RT. The results show that the proposed control scheme improves the performance of the hybrid AC/DC microgrid by maintaining the frequency/voltage fluctuations in the AC and DC sub-grids within the acceptable operating limits of ± 2% and ± 10% as per IEEE 1547 standards. As a result, the percentage improvement in voltage profile is 2.68% at the AC sub-grid and 5.55% at the DC sub-grid. • A novel stability index to reflect volatile loading conditions of HMG. • HMG stability enhancement using HVI-based VSM-V droop controller. • Mathematical modelling and analysis of VSM-V droop scheme. • Real-time experimental verification using OPAL-RT.

Control of ILC in an Autonomous AC–DC Hybrid Microgrid With Unbalanced Nonlinear AC Loads

This article presents a multiobjective control scheme for a bidirectional interlinking converter (ILC) of renewable energy based ac–dc hybrid microgrid. The prime focus of the power management algorithm is to feed the ac loads, and dc loads from ac source, and photovoltaic battery energy storage (BES), respectively. The second objective is to utilize the solar power to maximum capacity, by feeding it to dc load, BES, and ac load, in the given order of priority. The ac and dc subgrids operate independently, while supporting each other in case of overloading/underloading, thus reducing the amount of power reserve required in each, and eliminating the need of dump loads. The ANFIS-MN-based power quality control is integrated in order to ensure the total harmonics distortion of ac source current remains below 5% as per IEEE 519 standard, irrespective of the nature of load connected. In addition, the ILC control prevents any negative sequence currents from entering the ac source, even if there is any open circuit fault leading to unbalanced loading on the three phases. The performance of ANFIS-MN-based ILC control is demonstrated to be superior when compared with the conventional and contemporary techniques under different scenarios.

Application of Artificial Intelligent Techniques for Power Quality Improvement in Hybrid Microgrid System

The hybrid AC-DC microgrid (MG) has gained popularity recently as it offers the benefits of AC and DC systems. Interconnecting AC-DC converters are necessary since the MG has both DC and AC sub-grids. Adding an extra harmonic adjustment mechanism to the interlinking converters is promising because non-linear AC loads can worsen the quality of the voltage on the AC bus. The interlinking converters’ primary function is to interchange real and reactive power between DC and AC sub-grids, so the typical harmonic controlling approach implemented for active power filters (APFs) might not be appropriate for them. When the MG’s capacity is high, it is desirable that the switching frequency be lesser than the APFs. The performance of harmonic correction or even system stability may suffer at low switching frequencies. In this study, a harmonic compensating technique appropriate for hybrid AC-DC interlinking converters with lower switching frequencies is planned. The suggested strategy, modeling techniques, stability analysis, and a thorough virtual impedance design are discussed in this work.

Autonomous power balance in hybrid AC/DC microgrids

A control paradigm is proposed in this paper for decentralized power balance in hybrid AC/DC Microgrids (MGs). In this technique, the AC and DC sub-grids can transact energy from or towards the other side based on their loading conditions. Hybrid MGs use an interlink converter (ILC) as a back-to-back bi-directional converter at the Point of Common Coupling (PCC) between the sub-grids. The main duty of the ILC unit is to maintain the loading balance between the sub-grids. For this purpose, a superimposed AC voltage on the network’s nominal DC voltage is utilized in the DC sub-grid. Also, the AC sub-grid is equipped with the conventional f/P droop control. The frequency of the superimposed AC voltage in the DC sub-grid and the AC network frequency in the AC sub-grid are used for estimating the loading percentage of each side. As a result, the ILC unit acts as a load for one side and concurrently as a source for the other side. The ILC controller adjusts the participation of this unit in a fully decentralized manner. Simulation studies via PLECS test the efficacy of the suggested control system.

Bidirectional Virtual Inertia Control Strategy for Hybrid Distributed Generations Integrated Distribution Systems

The bidirectional ac/dc converter between the ac and dc subgrids plays a crucial role in enhancing the inertia support ability and ensuring the stable operation of the hybrid ac/dc distribution systems. In this paper, the bidirectional virtual inertia control strategy of hybrid distributed generations (DGs) integrated distribution systems is proposed for desired inertia support ability of both ac frequency and dc voltage. Firstly, the active power droop characteristics of the bidirectional ac/dc converter are analyzed, which reveals the steady power-sharing relationship between the ac and dc subgrids. Then, based on the dynamic power balance of the bidirectional ac/dc converter, the bidirectional virtual inertia control is proposed by introducing the virtual capacitor and moment of inertia, which can enhance the interactive inertia support ability of the hybrid distribution system. Finally, the effectiveness of the proposed control scheme is verified with a hybrid DGs integrated distribution simulation system in the PSCAD platform.

Multisource Grid-Connected DC/AC Hybrid Microgrid Decentralized Control Under Input Sources Variation

This article proposes a multisource distributed generation-based dc/ac grid-interactive microgrid providing quality power to a distribution network featured with a constant power load (CPL). The hybrid dc/ac system is a consolidation of a Permanent Magnet Synchronous Generator (PMSG) based wind turbine, Photovoltaic array, and Battery Energy Storage System (BESS) at a common dc-bus. The BESS system ensures continuous power support and takes care of the infrequent nature of photovoltaic and wind. The decentralized control is applied in the proposed grid-interactive configuration during variable wind speed and solar irradiance source disturbance to achieve three goals, namely, regulating the dc-bus voltage, extracting maximum power from distributed generation, and supplying energy-efficient power to the grid. The control strategy combines Maximum Power Point Tracking (MPPT), phase-locked loop (PLL) for the grid-linked synchronization, reduction of harmonic output current control, active/reactive power (P/Q) control, and virtual droop control used simultaneously. Applying virtual dc bus voltage droop control and MPPT can regulate the proper power-sharing among sources in a dc sub-grid. During the switching of the interlinking converter, high-frequency harmonics are created, which are eventually mitigated by LCL filters employed in the ac bus. Simulations are developed in Matlab Simulink, and it is verified using real-time digital simulator OPAL-RT to ratify the effectiveness of the suggested control strategy.

An improved model-based interlink converter control design in hybrid AC/DC microgrids

In hybrid AC/DC microgrids, a bidirectional interlink converter (IC) connecting the AC subgrid and DC subgrid is controlled to manage proper power sharing between both subgrids. This study proposes the design scheme of a model-based IC controller for frequency and voltage regulation of connected subgrids. System identification is used to estimate the state-space model representing the hybrid AC/DC microgrid dynamics. A step response is applied to observe the hybrid microgrid dynamic where the IC power is considered as the input while the output is the deviation of voltage or frequency in each subgrid. As the system characteristics in grid-connected mode and islanded mode might be different, the active power and reactive power transfer schemes are designed specifically for each operating mode to improve the performance of IC controller. According to simulation results, it is verified that an efficient gain scheduling by model-based gain tuning can deal with various operating conditions.

Resiliency Augmented Hybrid AC and DC Distribution Systems With Inverter-Dominated Dynamic Microgrids

In this paper, the implementation of inverter-dominated dynamic microgrids (MGs) in hybrid AC/DC distribution systems is discussed. Compared to conventional networked MGs structure, dynamic MGs operate with varying electric boundaries, which enables more flexible system operating topologies and power flow options. A control framework is developed for hybrid AC/DC distribution systems. The entire feeder is sectionalized into multiple dynamic MGs that interact at varying points of interconnection (POIs). A set of distributed controllers is proposed to coordinate multiple inverter-interfaced distributed generators (DGs) and interlinking converters (ICs) across the hybrid AC/DC feeders under autonomous operation. Various control modes are developed for the system to operate under either static topology or reconfigure as requested. Secondary regulations over the system operating frequency and voltage are enabled in both AC and DC subgrids, along with proportional power sharing among DGs. Dedicated control efforts are developed to achieve seamless system topology transitions, and the operational stability of the proposed controllers is analyzed. The performance of the proposed work in augmenting the operation resiliency of a hybrid AC/DC test feeder is validated using comprehensive case studies.

An adaptive energy management strategy for supercapacitor supported solar-powered electric vehicle charging station

Abstract Large-scale electric vehicle integration into the grid is expected in the imminent future. Congestion caused by these integrations can be addressed through a hybrid AC/DC microgrid. The DC sub-grid is exclusively dedicated to incorporating electric vehicles. Solar-powered charging station supported with a supercapacitor energy storage system in the DC sub-grid reduces the stress on the AC sub-grid. This paper presents a hybrid AC/DC microgrid voltage/frequency stability improvement strategy based on adaptive DC-link voltage regulation assisted by energy storage. The proposed volatility-based approach is an efficient strategy to enhance the microgrid stability during generation intermittency and load fluctuations. The combined control scheme of the interlinking converter and supercapacitor energy storage in the independent and hybrid operation modes achieve energy balance with the variations in solar irradiation and the addition/disconnection of electric vehicles. The proposed energy management strategy is tested in real-time under various volatile conditions to validate the performance. The hybrid microgrid with proposed energy management strategy provided a frequency improvement of 0.55% and voltage improvement of 5.4%.

Hybrid AC/DC-coupled charging station architecture with comprehensive hierarchical control for optimal economical operation and load variance minimization

This paper introduced a novel hybrid AC/DC-coupled charging station architecture and corresponding hierarchical control strategy. The core element of the architecture is an integrated interlinking converter (ILC) named the energy routing unit (ERU). The ERU features multiple conversion stages, dedicated plug-and-play interfaces, and varied operation functionalities. The dual inputs of utility sources enhance the reliability of the power supply. The contained scalable charging module facilitates the DC fast charging via DC grids directly, and the traditional AC slow charging is also accommodated in this architecture. Regarding the architecture, a comprehensive hierarchical control strategy containing the local-level short-timescale transient control and upper-level long-timescale optimal power scheduling is proposed. The multi-objective optimization concerning the optimal economical operation and minimum load variance is incorporated into the upper-level power flow control. The detailed simulation cases demonstrate that the stable operation of the proposed architecture could be realized, and the desirable economical operation and low peak-to-valley load variance are to be implemented simultaneously. The enabled vehicle-to-vehicle (V2V) power coordination achieves mutual power support between AC and DC subgrids and increases the local consumption of renewable energy sources effectively.

Distributed Dynamic Event-Triggered Power Management Strategy for Global Economic Operation in High-Power Hybrid AC/DC Microgrids

With the increasing integration of distributed generators (DGs), renewable energy sources (RESs), and energy storage systems (ESSs) in the hybrid AC/DC microgrid (HMG), the paralleled bidirectional interlinking converters (BILCs) play a significant role in the economic power interaction (EPI) between AC and DC subgrids. This paper proposes a distributed dynamic event-triggered control (ETC) strategy for the HMG to realize the global economic operation. In each subgrid, economic power sharing (EPS) among DGs can be achieved by economic droop controllers with no global information. The proposed distributed dynamic ETC includes a continuous-time part and a discrete-time part, where the former part realizes the EPI between two subgrids, while the latter part ensures the proportional power sharing between BILCs. Thus, a particular over-stressed BILC can be avoided, and system reliability and scalability are significantly improved. Additionally, only data from neighboring BILCs at the event-triggered times are involved, which greatly minimizes the communication burden and costamong BILCs. Furthermore, the asymptotic stability of the closed-loop system dynamics and the communication time delay stability are proven. Finally, the hardware-in-loop experimental results are conducted to demonstrate the effectiveness of the proposed control strategy.

Optimal Power Flow in AC/DC Microgrids With Enhanced Interlinking Converter Modeling

In this article, we propose an optimal power flow (OPF) paradigm for hybrid ac/dc microgrids. A meticulous model of the interlinking converter (IC) is developed and integrated into the OPF problem formulation. The resulting formulation is capable of solving the OPF of ac and dc subgrids in their standalone operations. A computationally efficient parabolic relaxation method transforms the nonconvex OPF model into a convex quadratic-constrained quadratic programming form. A sequential penalization method is applied to the relaxed OPF to achieve feasible solutions for the original formulation. The developed OPF formulation is tested on multiple modified 5-, 14-, 30-, 24-, 118-bus test systems, and a significant reduction in computational time is achieved in comparison with semidefinite programming relaxation. For a sequential penalized ac/dc OPF, the optimal values of both relaxations have a maximum difference of 0.025% for a 5-bus system. Finally, the modified IEEE 14-bus system is emulated in a real-time controller/hardware-in-the-loop setup to validate the proposed framework with continuously varying loads and IC switching status.

Implementation of Multi-Level Bidirectional Inter Allied Converter Community for Global Power Sharing in Hybrid AC/DC Microgrids

Bidirectional Inter-Allied Converters (BIACs) plays a major role in hybrid ac-dc microgrids (HM) forming as a bridging unit for power exchange between ac and dc subgrids. Overcoming the stress faced by single BIAC multiple converter structure i.e BIAC community came into existence. Considering advantages over two-level converters, multi-level converters sustain its position in today and future applications. Implementing the multi-level converter topology for distributed power management enhances the system efficiency with less amount of harmonic content. This paper presents implementation of Multi Level Bidirectional Inter Allied Converter (MLBIAC) community along with the Localized Distributed Proportional Integral Controller (LDPIC) located at each converter in HM for distributed power management. The localized distribution controller includes PI controller for system stability and which allows exchanging the information in more flexible way. To achieve the global power sharing by implementing MLBIAC in HM, the concepts like balanced power sharing, leading role transition, bidirectional power flow, system stability were analyzed and simulated using MATLAB/Simulink software.

Design of Decentralized Hybrid Microgrid Integrating Multiple Renewable Energy Sources with Power Quality Improvement

Due to the energy crisis and exhaustion in the amount of fossil fuels left, there is an urge to increase the penetration of renewables in the grid. This paper deals with the design and control of a hybrid microgrid (HMG) in the presence of variable renewable energy sources. The DC sub-grid consists of a permanent magnet synchronous generator (PMSG) wind turbine, solar PV array with a perturb-and-observe (P&O) MPPT algorithm, boost converter, and battery energy storage system (BESS) with DC loads. The AC sub-grid consists of a PMSG wind turbine and a fuel cell with an inverter circuit synchronized to the grid to meet its load demand. A bidirectional interlinking converter (IC) connects the AC sub-grid and DC sub-grid, which facilitates power exchange between them. The decentralized control of converters allows all the renewables to operate in coordination independently without communication between them. The proposed control algorithm of the IC enables it to act as an active power filter in addition to the power exchange operation. The active power filtering feature of the IC helps to retain the power quality of the microgrid as per IEEE 519 standards by providing reactive power support and reducing the harmonic levels to less than 5%. The HMG with the proposed algorithm can operate in both grid-connected and islanded modes. While operating in grid-connected mode, power exchange between DC and AC sub-grids takes place and all the load demands are met. If it is in islanded mode, a diesel generator supports the AC sub-grid to meet the critical load demands and the BESS supports the DC microgrid. The proposed model is designed and simulated using MATLAB-SIMULINK and its results are analyzed. The efficacy of the proposed control is highlighted by comparing it with the existing controls and testing the HMG for load variations.

A Novel Hybrid AC/DC Microgrid Architecture With a Central Energy Storage System

This paper proposes a novel hybrid AC/DC microgrid architecture incorporating a central energy storage system (ESS) for both the AC and the DC sub-grids. To ensure effective operation of the proposed hybrid AC/DC microgrid architecture, a coordinated control strategy between the central ESS and the inter-linking converter (ILC) is proposed in the paper. The proposed central ESS provides voltage and frequency control to the AC sub-grid by measuring voltage and frequency at the point of common coupling (PCC) of the AC sub-grid, while providing DC voltage support to the DC sub-grid by measuring DC sub-grid’s PCC voltage. Furthermore, per-unit frequency and the DC sub-grid PCC voltage are utilized to transfer power between the two sub-grids via the ILC. Therefore, two control strategies are coordinating with each other and maintain stable operation. The central ESS eliminates the use of multiple ILCs and complex control schemes for charge and discharge control to maintain same state of charge levels at each ESS. Effectiveness of the proposed coordinated controller is verified by applying various load disturbances, faults, and under different modes of operation.

An autonomous flexible power management for hybrid AC/DC microgrid with multiple subgrids under the asymmetric AC side faults

In this paper, an autonomous flexible power management is proposed for hybrid AC/DC microgrid with multiple subgrids under the asymmetric AC side faults. First, based on the double-frequency disturbance of the DC common bus (DCB) voltage caused by asymmetric AC side faults, this paper analyzes the characteristics of the transmission power of bi-directional DC/DC converters (BDCs) and bi-directional AC/DC converters (BACs) adopt the normalized droop control (NDC). Then, interactive relationships between DCB voltage and DC subgrids bus voltage, AC subgrids bus frequency are derived. According the interactive relationships, suppression links combined with virtual component adjustment coefficients are designed for BDCs and BACs to eliminated the double-frequency disturbance of the transmission power. Meanwhile, the support links is added to the power control of BACs to maintain the frequency automatically once the energy storage (ES) in faulty AC subgrid becomes unavailable due to the line protection. Furthermore, with the autonomous flexible power management, the total load demand proportionally shared among the remaining subgrids can be accordingly attained when faulty AC subgrid or its local ES quit the operation. Finally, the system stability analysis is given and the results of Hardware-In-Loop tests verify the correctness and effectiveness of the proposed autonomous flexible power management.

A Fuzzy-Based Coordinated Power Management Strategy for Voltage Regulation and State-of-Charge Balancing in Multiple Subgrid-Based DC Microgrid

Multiple DC subgrids in close proximity can be connected to form a DC microgrid. The interconnected DC subgrids enhance reliability and efficient utilization of resources. In this paper, a novel coordinated power management strategy is proposed for interconnected subgrids of a DC microgrid. Each DC subgrid has PV and battery units. The proposed strategy is implemented on interlinking bidirectional converters (IBCs). The strategy considers the state-of-charge (SoC) and charging/discharging current of the batteries for SoC balancing-based power sharing through IBCs. A fuzzy logic controller is implemented to decide the amount of power flow through the IBCs from SoC and the charging/discharging current of battery units. Moreover, with the coordinated strategy, a high SoC battery takes more loads compared to a lower SoC battery. The proposed strategy supports fast charging and fast discharging of batteries depending on the SoC level. In addition, the coordinated strategy can seamlessly transfer the control of IBC from the power regulating mode to the voltage regulating mode without any additional control scheme during an outage of the battery unit. The feasibility of the proposed strategy is validated through simulation in MATLAB/Simulink and OPAL-RT real-time digital simulator.

Stability improvement of MMC‐based hybrid AC/DC grids through the application of a decentralized optimal controller

Interconnection and expansion of AC networks through high-voltage direct current grids based on modular multilevel converters to form a multiterminal hybrid AC/DC grid can pose stability issues. These challenges can arise from dynamic interactions between/within AC and DC subgrids due to poorly damped modes that are potential sources of persistent and disruptive oscillations. This paper aims to ensure the stability of multiterminal hybrid AC/DC grids via a decentralized optimal controller. The proposed methodology analytically and simultaneously identifies both the decentralized optimal controller and the worst-case perturbation scenario under the grid control inputs' and state variables' constraints, without the need for detailed and time-consuming dynamic simulations of all possible scenarios. Eigenvalue stability analysis and time-domain simulations show that the proposed controller can efficiently enhance the test grid stability margins and reduce the oscillations, not only under the worst-case perturbation scenario (increasing damping ratios of the two pairs of least damped modes by 2.34 and 4.05 times) but also under other critical fault conditions. Furthermore, the controller's superior performance is validated through comparison with the power system stabilizer and modular multilevel converter droop controller under small and large disturbances, and its robustness is assessed against parametric uncertainties.

A novel communication-less control method to improve proportional power-sharing and SOCs balancing in a geographically dispersed hybrid AC/DC microgrid

Batteries’ SOC balancing, proportional power-sharing, and keeping voltage and frequency in permissible operation areas along with ensuring system stability are of the most important challenges of the microgrids control algorithm. In this paper, a novel communication link-less control method for hybrid microgrids is proposed. In the AC subgrid, the droop gain of the energy storage units (ESUs) is defined as a function of their SOC. Next, a novel control method based on the AC signal injection for SOCs balancing in the DC side is suggested. After that, a new droop curve for the interlinking converter (IC) is defined. The IC droop coefficient is adjusted based on the DC subgrid SOC such that not only all SOCs in the AC and DC sides are converged, but also proportional power-sharing is attained. The proposed method benefits of modularity and expandability, hence it can be implemented to a multi-subgrid microgrid with various DC buses voltage and different frequencies. The small-signal stability analysis proves that the proposed method is stable. Simulation results of the nonlinear model of the system confirm that the proposed method can balance batteries SOC, dispatch power properly, and maintain voltage and frequency deviations within the statutory range.

Modeling and robust structural control design for hybrid AC/DC microgrids with general topology

Hybrid AC/DC microgrids are designed to utilize different types of renewable energy sources, meeting the requirements of numerous kinds of loads. A hybrid microgrid consists of a DC sub-grid, an AC sub-grid, and an interfaced sub-grid. In this paper, it is assumed that the DC and AC sub-grids contain several distributed generation (DG) units with arbitrary topology, and the interfaced region consists of several bidirectional interlink converter (IC) units. In the islanded mode of hybrid microgrid operation, one of the key control objectives is to stabilize and regulate the point of common coupling (PCC) voltages of both DC and AC sub-grids and also manage the proportional power-sharing between the sub-grids. This paper proposes a modeling framework and an optimal decentralized output-feedback-based control strategy for the voltage regulation of both DC and AC sub-grids as well as the current adjustment of IC units in a hybrid AC/DC microgrid with general topology. The proposed scheme guarantees robust stability and performance of hybrid microgrids subject to different uncertainty sources. In our proposed modeling setting, the uncertainty sources including plug-and-play (PnP) functionality of DG and IC units, microgrid topology changes, loads variations, and uncertain filter parameters of IC units are modeled as a convex polytope. The control design problem for the polytopic model is converted into a multi-objective convex optimization problem with structural constraints on decision variables. Several scenarios are performed in MATLAB/Simulink to validate the effectiveness of the proposed control mechanism for hybrid AC/DC microgrids.