Bidirectional Interlinking Converter Research Articles

Enhancing Grid-Forming Converters Control in Hybrid AC/DC Microgrids Using Bidirectional Virtual Inertia Support

This paper presents a new grid-forming strategy for hybrid AC/DC microgrids using bidirectional virtual inertia support designed to address weak grid conditions. The stability of hybrid AC/DC microgrids heavily relies on the AC mains frequency and the DC-link voltage, and deviations in these factors can lead to undesirable outcomes such as load curtailments and power system congestions and blackouts. This paper introduces a unique approach that leverages bidirectional virtual inertia support to enhance the stability and reliability of hybrid AC/DC microgrids under weak grid conditions. The proposed strategy employs virtual inertia as a buffer to mitigate rapid changes in DC-link voltage and AC frequency, thereby enhancing system stability margins. This strategy significantly contributes to a more stable and reliable grid operation by reducing voltage and frequency fluctuations. A standard hybrid AC/DC microgrid configuration is used to implement the bidirectional virtual inertia support, where a bidirectional interlinking converter control is adjusted to deliver inertia support to both the AC and DC subgrids. This converter utilizes the DC grid voltage and AC grid frequency as inputs, effectively managing active power balance and implementing auxiliary functions. Extensive simulations are conducted under weak grid conditions and standalone mode to validate the effectiveness of the proposed strategy. The simulation results demonstrate a remarkable improvement in frequency nadir, rate-of-change-of-frequency (RoCoF), and DC bus voltage deviation in the hybrid AC/DC microgrids. The bidirectional virtual inertia support substantially reduces voltage and frequency fluctuations, enhancing the microgrid stability and resilience. There is an improvement of over 45% and 25% in the frequency deviation and voltage deviation, respectively, achieved through implementing the proposed control strategy.

Distributed Unified Control for Global Economic Operation and Resilience Reinforcement of Hybrid AC–DC Microgrids

The paralleled bidirectional interlinking converters (BILCs) of the hybrid AC/DC microgrid (HMG) provide a flexible and reliable power interaction way between AC and DC subgrids with high power density. A distributed unified control (DUC) is proposed for BILCs to achieve both resilience reinforcement and global economic operation of the HMG. For distributed generators (DGs), the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$f_{ac}-\lambda _{ac}$</tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$v_{dc}-\lambda _{dc}$</tex-math></inline-formula> economic droop controls are employed for AC DGs and DC DGs to decrease the individual subgrid's generation expense by equaling DGs' incremental expenses (IEs). For BILCs, the normalized AC subgrid's frequency and DC subgrid's voltage are coordinated to achieve the economic power interaction (EPI), which further decreases the total generation expense (TGE). Besides economic operation, the proposed DUC is capable of adopting different MG operation modes into a unified control structure without any mode switching. Paralleled BILCs are managed to reinforce the system resilience by supporting the DC voltage or AC voltage once all DC or AC DGs fail. In addition, the proposed DUC enables BILCs with plug-and-play characteristics, resistance to the maximum acceptable communication delay, and the communication disruption robustness. The corresponding hardware-in-the-loop (HIL) results verify the effectiveness of the proposed control strategy.

Enhanced Grid Frequency and DC-Link Voltage Regulation in Hybrid AC/DC Microgrids Through Bidirectional Virtual Inertia Support

AC mains frequency and dc-link voltage are two of the most important parameters that determine the stability of hybrid ac/dc microgrids. Frequency and voltage deviations beyond the acceptable threshold can lead to unscheduled load curtailments, cascading failures, or even power system blackouts. To enhance the stability of hybrid ac/dc microgrids, this paper proposes a hybrid microgrid with bidirectional virtual inertia support, where virtual inertia is introduced to slow down dc voltage and ac frequency changes, thus enhancing system stability. The proposed approach allows significant reductions in voltage and frequency fluctuations. With a standard hybrid microgrid configuration, inertia is delivered to both ac and dc subgrids via the change of bi-directional interlinking converter control. Specifically, the bidirectional interlinking converter uses the dc voltage of the dc grid and ac frequency of the ac grid as inputs through a proportional integral (PI) controller and regulates the active power between two sub grids so that inertia and other supportive functions are implemented. To validate the merits of the proposed approach, experimental results are provided. According to experimental results, the frequency nadir, rate-of-change-of-frequency (RoCoF), and dc bus voltage deviation can be improved by the proposed strategy of bidirectional virtual inertia support in the hybrid ac/dc microgrid.

Adaptive Neural Network Q-Learning-Based Full Recurrent Adaptive NeuroFuzzy Nonlinear Control Paradigms for Bidirectional-Interlinking Converter in a Grid-Connected Hybrid AC-DC Microgrid

The stability of a hybrid AC-DC microgrid depends mainly upon the bidirectional interlinking converter (BIC), which is responsible for power transfer, power balance, voltage solidity, frequency and transients sanity. The varying generation from renewable resources, fluctuating loads, and bidirectional power flow from the utility grid, charging station, super-capacitor, and batteries produce various stability issues on hybrid microgrids, like net active-reactive power flow on the AC-bus, frequency oscillations, total harmonic distortion (THD), and voltage variations. Therefore, the control of BIC between AC and DC buses in grid-connected hybrid microgrid power systems is of great importance for the quality/smooth operation of power flow, power sharing and stability of the whole power system. In literature, various control schemes are suggested, like conventional droop control, communication-based control, model predictive control, etc., each addressing different stability issues of hybrid AC-DC microgrids. However, model dependence, single-point-failure (SPF), communication vulnerability, complex computations, and complicated multilayer structures motivated the authors to develop online adaptive neural network (NN) Q-learning-based full recurrent adaptive neurofuzzy nonlinear control paradigms for BIC in a grid-connected hybrid AC-DC microgrid. The proposed strategies successfully ensure the following: (i) frequency stabilization, (ii) THD reduction, (iii) voltage normalization and (iv) negligible net active-reactive power flow on the AC-bus. Three novel adaptive NN Q-learning-based full recurrent adaptive neurofuzzy nonlinear control paradigms are proposed for PQ-control of BIC in a grid-connected hybrid AC-DC microgrid. The control schemes are based on NN Q-learning and full recurrent adaptive neurofuzzy identifiers. Hybrid adaptive full recurrent Legendre wavelet-based Neural Network Q-learning-based full recurrent adaptive NeuroFuzzy control, Hybrid adaptive full recurrent Mexican hat wavelet-based Neural Network Q-learning-based full recurrent adaptive NeuroFuzzy control, and Hybrid adaptive full recurrent Morlet wavelet-based Neural Network Q-learning-based full recurrent adaptive NeuroFuzzy control are modeled and tested for the control of BIC. The controllers differ from each other, based on variants used in the antecedent part (Gaussian membership function and B-Spline membership function), and consequent part (Legendre wavelet, Mexican hat wavelet, and Morlet wavelet) of the full recurrent adaptive neurofuzzy identifiers. The performance of the proposed control schemes was validated for various quality and stability parameters, using a simulation testbench in MATLAB/Simulink. The simulation results were bench-marked against an aPID controller, and each proposed control scheme, for a simulation time of a complete solar day.

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.

Power Factor Compensation for a Single-Phase AC-DC Hybrid Micro-Grid

A single-phase AC-DC Hybrid Micro-Grid (HMG) power flow control strategy is modeled and simulated in this paper. Two AC and two DC zones are separated by a bidirectional interlinking converter (BIC) in the full bridge IGBT structure, which is a typical H-Bridge inverter/rectifier, in the proposed HMG system topology. Based on the DQ transformation theory, the BIC's switching pattern comes from one voltage loop (Vdc) and two power control loops. The transfer of both active and reactive power between the HMG and the public AC Grid is controlled by this control strategy. Additionally, this can be accomplished in either rectifying or inverting modes by utilizing the bidirectional converter. Matlab-Simulink is used to implement and test the simulation model.

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.

A Nonlinear Adaptive Backstepping Controller for Stabilizing the DC Bus Voltage of an Islanded Hybrid Microgrid

A bidirectional interlinking converter (BIC) interconnects the ac and dc buses of a hybrid microgrid (HMG). Under certain operating conditions, the BIC may appear as constant power load to the dc bus causing low-frequency oscillations in it. This instability is observed during the power flow from dc to ac buses and is attributed to an interaction between the input and output impedances seen at the point of interface. In this article, an active damping scheme based on an adaptive backstepping controller (ABC) is proposed for the BIC of an islanded HMG to address the voltage instability in the dc bus. The proposed scheme can be easily integrated with a conventional backstepping controller and an attractive feature is its ability to disengage itself from the ac voltage controller of the BIC as soon as voltage oscillations in the dc bus die out. Thus, the active damper does not deteriorate the ac voltage quality at steady state. The proposed method uses fewer current sensors as compared with a recently reported active damping technique based on predictive control. Typical of a backstepping controller, large signal stability is ensured while deriving the control and adaptation laws. Experimental validation is included.

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&amp;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.

Multilevel Dynamic Master-Slave Control Strategy for Resilience Enhancement of Networked Microgrids

Conventional power management methods of networked microgrids (NMGs) are limited to the failure of pinned communication terminals and heavy communication burdens. This paper proposes a multilevel dynamic master-slave control strategy via two-level dynamic leaders to realize the resilience enhanced power management of NMGs. The first level dynamic leader with considerations of distributed energy resources (DERs) feature is selected to guide the output of DERs and achieve the power management within individual microgrid (MG). Subsequently, the secondary level leader considering each MG feature is selected among the bidirectional interlinking converters (BICs), whose signals would be shared with other BICs by communication to achieve power management among MGs. Moreover, the local weight selecting method (LWSM) is proposed to automatically select the two-level dynamic leaders according to the real-time system operation state. Compared with conventional methods, the communication among MGs is essentially realized through the dynamic DER leaders instead of pinned ones. Therefore, unreliability issues in the event of pinned terminal outage and converters’ communication failure can be fully addressed and the communication bus within each only needs to transmit one DER’s signals. The proposed strategy can be also extended to NMGs with various topologies and provide the “plug and play” capabilities of DERs or MGs. Finally, the effectiveness and feasibility of the proposed strategy are verified through the PSCAD/EMTDC platform.

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.

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.

Power Management for Islanded Hybrid AC/DC Microgrid With Low-bandwidth Communication

Poor power management of bidirectional interlinking converter (BIC) and sub-grids in a hybrid AC/DC microgrid can lead to a heavy or light load state of some distributed energy resources (DERs), and large deviations of AC bus voltage/frequency and DC bus voltage. This study proposes a distributed power management strategy based on the concept of coordination factors for hybrid microgrids to achieve accurate active power sharing among all AC/DC DERs (instead of the same within one sub-grid), reactive power sharing among AC DERs, and AC voltage/frequency and DC voltage restorations. Power management within a sub-grid is achieved by following the leader DER with the maximum coordination factor. Meanwhile, the BIC can establish a dynamic connection among all DERs by tracking the leaders of each sub-grid to realize power management between sub-grids. Compared to conventional methods, the proposed method can achieve accurate power sharing and voltage/frequency restorations simultaneously. Additionally, communication data and bandwidth can be significantly reduced. Finally, hardware-in-loop experiments are also performed to prove the feasibility of the proposed strategy, even during communication latency or failure.

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.

Model predictive control—Based distributed control algorithm for bidirectional interlinking converter in hybrid microgrids

Model predictive control—Based distributed control algorithm for bidirectional interlinking converter in hybrid microgrids

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.

An Active Damping Technique for PI and Predictive Controllers of an Interlinking Converter in an Islanded Hybrid Microgrid

This article highlights a voltage instability when the bidirectional interlinking converter (BIC) of a hybrid ac-dc microgrid operates as a constant power load (CPL). The instability is attributed to the impedance interactions seen at the dc port of the BIC. The BIC is operated with two different controllers, viz., a PI controller and a model predictive controller (MPC). In order to avert the instability, prohibiting impedance interactions is proposed as a design objective while determining the bandwidth of the PI controller. However, since a flexible control over bandwidth is not available with MPC, an active damping technique is proposed to address the instability. The proposed technique works equally well for the PI controllers with fixed or predetermined bandwidths. An important feature of the proposed technique is that it does not interfere with the original objectives of the controllers, once the instability is addressed. The technique comprises only one tuning factor, which does not depend on the BIC's internal parameters. As a result, it can easily be implemented as a general stabilizing scheme in any BIC. The efficacy of the proposed technique is verified through simulations, frequency-domain impedance analyses as well as experiments on a laboratory prototype of the BIC.

Cyber-Resilient Cooperative Control of Bidirectional Interlinking Converters in Networked AC/DC Microgrids

Bidirectional interlinking converters (BICs) are building blocks of the networked ac/dc microgrids (ac/dc NMGs). Such BICs are controlled using the communication assisted cooperative control thus forming tightly coupled cyber-physical systems (CPSs). One of lethal problems in such CPS is its vulnerability to cyber-attacks. In this article, a cyber-resilient cooperative control for BICs in ac/dc NMGs is proposed to enhance the system resilience under false signal injection attacks. The proposed control can maintain the frequency/voltage regulation and real/reactive power sharing among multiple BICs under both single and multiple node attacks. The suitability of the proposed method is supported by the stability analysis using Lyapunov methods. Efficacy of the proposed method is demonstrated by the controller hardware-in-the-loop (CHIL) experiment. An ac/dc NMG with four BICs is developed in the OPAL-RT-based real-time simulator. The cyber-resilient controllers are implemented on four commercial programmable logic controllers by B&R automation. The CHIL-based experimental results support the effectiveness of the proposed controller under various attack scenarios.

Hybrid Microgrid Energy Management and Control Based on Metaheuristic-Driven Vector-Decoupled Algorithm Considering Intermittent Renewable Sources and Electric Vehicles Charging Lot

Energy management and control of hybrid microgrids is a challenging task due to the varying nature of operation between AC and DC components which leads to voltage and frequency issues. This work utilizes a metaheuristic-based vector-decoupled algorithm to balance the control and operation of hybrid microgrids in the presence of stochastic renewable energy sources and electric vehicles charging structure. The AC and DC parts of the microgrid are coupled via a bidirectional interlinking converter, with the AC side connected to a synchronous generator and portable AC loads, while the DC side is connected to a photovoltaic system and an electric vehicle charging system. To properly ensure safe and efficient exchange of power within allowable voltage and frequency levels, the vector-decoupled control parameters of the bidirectional converter are tuned via hybridization of particle swarm optimization and artificial physics optimization. The proposed control algorithm ensures the stability of both voltage and frequency levels during the severe condition of islanding operation and high pulsed demands conditions as well as the variability of renewable source production. The proposed methodology is verified in a state-of-the-art hardware-in-the-loop testbed. The results show robustness and effectiveness of the proposed algorithm in managing the real and reactive power exchange between the AC and DC parts of the microgrid within safe and acceptable voltage and frequency levels.

A novel flexible power support control with voltage fluctuation suppression for islanded hybrid AC/DC microgrid involving distributed energy storage units

In this paper, a novel flexible power support control with voltage fluctuation suppression is proposed for islanded hybrid AC/DC microgrid involving distributed energy storage units. The virtual inertia and the interactive relationship between DC bus voltage and AC frequency are introduced into energy storage units (ESs) both in AC and DC subgrids. Therefore, mutual power support from opposite subgrid can be realized and the system inertia can be enhanced while the AC frequency and DC bus voltage are managed. Then, the active power reference of bidirectional interlinking converter (BIC) is modified with the proposed control strategy to achieve proportional power sharing in the whole islanded hybrid AC/DC microgrid. In addition, a controller based on linear active disturbance rejection control (LADRC) is designed according to the BIC model to suppress AC bus voltage fluctuation by using the estimates of disturbances obtained from linear extended state observer (LESO). Finally, stability of the system is analyzed and the results of an RT-LAB-based real-time simulation platform verify the correctness and effectiveness of the proposed control strategy.