Interlinking Converter Research Articles

Equilibrium mechanism between dc voltage and ac frequency for ac-dc interlinking converters

The equilibrium between dc bus voltage and ac bus frequency (Udc-f equilibrium) is the algorithm core of unified control strategies for ac-dc interlinking converters (ILCs), because the equilibrium implements certain mechanism. However, what the mechanism is has not been explicitly explored, which hinders further studies on unified control. This paper reveals that the state-space model of a Udc-f equilibrium controlled ILC is highly similar to that of a shaft-to-shaft machines system. Hence a detailed mechanism is discovered and named “virtual shaft-to-shaft machine (VSSM)” mechanism. A significant feature of VSSM mechanism is self-synchronization without current sampling or ac voltage sampling.

Implementation of hardware‐in‐loop for DC‐link voltage balancing in hybrid ac/dc microgrid using interlinking converter

AbstractBipolar hybrid ac/dc microgrid (MG) controls a DC and AC bus at the same time, supplies local loads with local resources, and offers multiple levels of dc voltages to resources and services. The applications are getting more popular in power systems. Due to its vital tasks in numerous applications, the interlinking converter (ILC) is an utmost crucial portion of the MG system. From this study, a 10‐switch converter is provided as an ILC for a hybrid MG, which gains from both two‐level (2L) and three‐level (3L) converters at the same time, and it can be used to a wider range of power levels. This work provides a space vector modulation (SVM) for a 10‐switch converter to maximize dc‐link voltage consumption and minimize total harmonic distortion (THD) compared with other modulation techniques. The behaviour of the suggested SVM is compared with that of sinusoidal PWM. The suggested SVM was evaluated using OPAL‐RT to show that it used DC bus voltage more reliably and produced significantly less THD than existing techniques. Furthermore, grid‐tied ILC's architecture is improved to reduce low‐order harmonics and used for dc‐link voltage balancing. A real‐time (RT) digital simulator (OP‐5700) was used to test the inverter control technique. In MG, this hardware‐in‐the‐loop simulation provides an excellent environment for designing and verifying system‐level control algorithms. Simulations were run in a MATLAB/Simulink environment and confirmed using an RT simulator to validate the feasibility of the suggested topology.

A novel solid state transformer based control topology for interconnected MV and LV hybrid microgrids

Hybrid microgrid continues to be an attraction for researchers as it brings the advantages of both ac and dc systems under one roof. The interlinking converter (ILC) of this system regulates the ac frequency and dc voltage while maintaining bidirectional power. This paper proposes a novel solid-state transformer (SST) based topology for interconnected medium voltage (MV) and low voltage (LV) hybrid microgrids. Based on the topology, various control modes are identified and correspondingly, an autonomous power sharing scheme is presented with dc voltage and ac frequency regulation. A linear voltage-frequency control is discussed for ac/dc and dc/ac converters of SST which provides a secondary control mechanism. The dual active bridge (DAB) of the SST operates at voltage control mode while providing electrical isolation and stable system operation. The proposed topology and control scheme is significant in the sense that it allows integration of ac and dc grids with different voltage and frequency levels. In addition, the secondary dc voltage or ac frequency regulation control at any hybrid microgrid automatically regulates the ac frequency and dc voltage in the system which reduces system cost. Various simulated scenarios in PSCAD with load changes highlight the optimal performance of the proposed scheme.

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.

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&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 algorithm for contingency analysis of the AC-DC networks using modified Z-bus technique

The steady state Contingency Analysis (CA) utilizes non-iterative distribution-factors (DFs) based linear estimation method of current-redistribution, to deliver approximate solutions in a shorter timeframe, which is favored over recurrent usage of iterative power flow-based non-linear techniques. However, the established principles of conventional DF-based CA cannot be directly implemented on burgeoning AC-DC networks. The DFs obtained from single unified Z-bus constructed for the entire AC-DC network leads to erroneous estimation of current redistribution post any contingency event. The reason being unified Z-bus built overlooked the characteristics of AC and DC lines. As a paradigm change toward a co-existing AC-DC network emerges, an adequate framework for understanding contingencies within such a network becomes critical. We present a quick non-iterative DF-based approach for the CA of any AC-DC network in this research by merging the use of separate AC and DC impedance matrices while accounting for the intertwined implications of the AC and DC networks. The interlinking converters (ILCs) are simulated using a reactive current bypassing approach. The proposed CA method is capable of handling contingencies occurring within ILCs, AC and the DC networks. The ETAP simulations and the Newton-Raphson (NR) technique for the AC-DC network have been utilized to corroborate the results.

Priority-Driven Self-Optimizing Power Control Scheme for Interlinking Converters of Hybrid AC/DC Microgrid Clusters in Decentralized Manner

Hybrid ac/dc microgrid clusters are key building blocks of smart grid to support sustainable and resilient urban power systems. Practically, in networked microgrid clusters, the subgrid load-priorities and power quality requirements for different areas vary significantly. In order to realize optimal power exchanges among microgrid clusters, this article proposes a decentralized self-optimizing power control scheme for interlinking converters (ILCs). First, a priority-driven optimal power exchange model of ILCs is built that fully considers the priorities and capacities in subgrids. The whole optimization objective is to minimize the total dc-voltage/ac-frequency state deviations of subgrids. Second, to realize the decentralized power flow control, an optimal-oriented quasi-droop control strategy of ILCs is introduced. Consequently, as each of ILCs only monitors the local ac-side frequency and dc-side voltage signals, the whole optimal power control of the wide-area microgrid clusters is achieved in a decentralized manner without any communication link. Thus, the proposed control algorithm has the features of decreased cost, increased scalability, reduced geographic restrictions and high resilience and robustness in terms of communication faults. Finally, the proposed method is validated by three cases in hardware-in-loop environment.

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.

Control Strategy of Interlinking Converter in Hybrid Microgrid Based on Line Impedance Estimation

Due to the voltage drop of the line impedance, power-transfer deviation occurs in the interlinking converter (IC) of hybrid microgrids. In this paper, an IC control strategy in hybrid microgrids is proposed, which can estimate the line resistance precisely. Moreover, the proposed method can easily realize accurate power transmission between sub-grids and maintain the power balance of hybrid microgrid. Firstly, one specified small signal is injected into the IC at the AC bus of the distributed generation so as to estimate the line impedance. The frequency of the injected small signal is highly relevant to the AC bus voltage. As a result, the IC can accurately detect its line impedance by extracting the injected small signal frequency with a filter and phase-lock-loop (PLL) without communication. Secondly, with the line impedance estimation, the voltage drop on the line can be compensated with local information of the IC, and consequently, accurate power transfer can be realized. Finally, the validity of the proposed method is verified by simulation and experiment results.

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.

Decentralized control of islanding/grid-connected hybrid DC/AC microgrid using interlinking converters

This paper presents decentralized control of an islanding/grid-connected DC/AC hybrid microgrid (μG) using battery energy storage system (BESS) and distributed energy resource (DER). In DC/AC μG unified with different DERs and BESS, the intermittent output nature of DERs due to fluctuating input characteristics and strong interaction of sources, loads, and primary grid, causes variations in frequency and bus voltage. To solve these issues, the paper proposes a decentralized control method which incorporates active/reactive power (P/Q) control, DC virtual droop control, voltage control, frequency droop control, current control, maximum power point tracking, and a phase-locked loop (PLL). The coordination and integration of various control parameters will ensure a smooth transition as well as the overall stability of the system. The BESS and DER’s interfacing converters are autonomously controlled in this control scheme without communication using the frequency droop and bus voltage control. Correspondingly, the control techniques comprehend different operating modes depending on the state of charge of BESS, distributed generation, and grid. During switching the two interlinking converters (IC), high-frequency harmonics are created, which are eventually mitigated by the inductor-capacitor-inductor (LCL) filters employed in the AC bus. Virtual droop control will ensure active damping of the harmonics in the LCL network. The maximum power transfer in interlinking converters (ICs) is improved using the proposed approach for input-source variations. Simulations were performed using MATLAB/Simulink and verified using the hardware in the loop (HIL) to prove the effectiveness of the control scheme proposed.

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.

Modeling and Design of Parallel LCC-VSC Interlinking Converters’ Unified Controller in AC/DC Network

While both line-commutated converters (LCCs) and voltage-source converters (VSCs) have been adopted in ac/dc grids, they are mostly used individually despite some point-to-point dc transmission. The combination of both technologies as a single interlinking converter (IC) unit in an ac/dc grid can bring advantages, such as capacity expansion, bidirectional power flow, reactive power support, and harmonics compensation. For such a parallel LCC-VSC unit, a suitable control scheme and careful control parameters’ design are important to ensure that the system can maintain the above advantages in different operation modes. In this article, the control scheme and parameter design for a parallel LCC-VSC unit for use in an ac/dc grid are conducted with a focus on capacity expansion scenario, where a unified control scheme that reserves most of the original LCC terminal’s control while also smoothly incorporates the VSC operation is considered. The operation states of a hybrid ac/dc network are modeled. The proposed unified control scheme is considered for each state for the stability study. Then, a set of control parameters for the unified control scheme suitable for all operation states with good dynamics and stability is designed. Real-time simulations and experiments are conducted to verify the design and study.

A Novel Distributed Control Method for Interlinking Converters in an Islanded Hybrid AC/DC Microgrid

In this paper, a novel distributed active power control method for interlinking converters (ICs) in an islanded hybrid ac/dc microgrid (IHMG) is proposed. The goal is to implement power sharing among all distributed generators (DGs) in an IHMG and among the ICs in a distributed manner, without the need for additional proportional-integral controllers. A mathematical formula for the active power references of the ICs is derived to achieve these objectives. Then, an estimator providing the data required for the reference calculation is proposed based on the modified dynamic consensus algorithm. In addition, a unified state-space model is derived for the entire IHMG, including the cyber-physical interactions, and a design method is presented for the parameters of the estimator. Finally, the effectiveness of the proposed method is verified by performing small-signal analysis and controller-hardware-in-the-loop verification. Compared to a previous distributed control method, the proposed method significantly improves the robustness in terms of communication delay and variation in the status of DGs while achieving the objectives.

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

Robust Control of Interlinking Converter Using PSO and ABC Algorithms

Objective: In this paper, an optimal control scheme for the Interlinking Converter (IC) system is achieved by the proper regulation of its gate switching functions through appropriate optimal feedback controller design. Methods: Proportional-Integral-Derivative (PID), Fractional Order Proportional-Integral-Derivative (FOPID) and H-infinity loop shaping controller have been designed for the two-fold control objective of simultaneous improvement in system robustness and reduced tracking error using parameter tuning via Particle Swarm Optimization (PSO) and Artificial Bee Colony (ABC) optimization algorithms. Results: The controller parameters are obtained by optimization algorithms. The comparative analysis of the controller performance is carried out through simulation in the MATLAB platform to validate the effectiveness of the controller design under various changing situations. Conclusion: The optimized controller parameters obtained through ABC algorithm are better than that obtained through PSO algorithm in terms of both objective function values and execution time. The resultant robust control strategy for IC system obtained through the H-infinity loop shaping controller provides reduced tracking error and improved stability as compared to PID and FOPID controller, as proved by the simulation results.