Multi-converter Systems Research Articles

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

This paper focuses on modeling, control and power quality improvement of a microgrid connected system. This last was designed as a multi-converter system with Wind Turbine driven Permanent Magnet Synchronous Generator, and lithium ion Battery Storage Energy System. These sources are connected by a continuous bus to a nonlinear load through a DC /AC converter and three-phases multi-functional voltage source inverter. Where, wind systems are considered as primary power resource, and the grid is used for the effect of consumption of the overage power available from sources, when the battery has been fully charged. Multi Functional Voltage Source Inverter is used to ameliorate the performance of the proposed system, guaranteeing both reactive power and harmonic compensation. Moreover an intelligent control by fuzzy logic control algorithm are managed In order to extract the maximum power from wind turbine, to guarantee an effective storage management, this about the DC side. For the AC side a direct power control strategy with fuzzy logic algorithm have been used. The simulation of the system solution is carried out in Matlab/Simulink. The obtained simulation results show that the technique of fuzzy logic furnish the best solution in term of robustness, optimization performance, low THD and fast dynamic response.

Impact of current limiters and fast voltage boosters in grid-forming VSC-based generators on transient stability

Transient stability is a complex phenomenon presented in multi-machine and multi-converter systems, and it is still considered a key limiting factor for stressed power systems. The increasing integration of non-synchronous generation further emphasises the need to address the challenges of improving the transient stability faced by these power systems. Several studies have focused on developing control strategies for Grid-forming voltage source converter (GFM-VSC) to improve transient stability. These strategies include the use of current limiting algorithms and/or control of active/reactive power injections. This paper investigates the impact of fast voltage boosters (FVBs) and hybrid current limiters (HCLs) on transient stability of power systems with 100% grid-forming VSC-based generators. Short-circuit simulations and critical clearing time analysis are performed to evaluate the effectiveness of HCLs and FVBs in improving transient stability. The simulation results demonstrate the effectiveness of these approaches in avoiding the loss of synchronism. This research contributes to the current studies on transient stability in power systems and provides valuable insights into the potential of HCLs and FVBs as effective approaches to improve system stability.

Saturation-informed current-limiting control for grid-forming converters

In this paper, we investigate the transient stability of a state-of-the-art grid-forming complex-droop control (i.e., dispatchable virtual oscillator control, dVOC) under current saturation. We quantify the saturation level of a converter by introducing the concept of degree of saturation (DoS), and we propose a provably stable current-limiting control with saturation-informed feedback, which feeds the degree of saturation back to the inner voltage-control loop and the outer grid-forming loop. As a result, although the output current is saturated, the voltage phase angle can still be generated from an internal virtual voltage-source node that is governed by an equivalent complex-droop control. We prove that the proposed control achieves transient stability during current saturation under grid faults. We also provide parametric stability conditions for multi-converter systems under grid-connected and islanded scenarios. The stability performance of the current-limiting control is validated with various case studies.

Stability Analysis of Distributed Multi-Converter System

This paper discusses dynamic stability and performance of current control scheme for parallel connection of pulse-width modulated (PWM) voltage source converters (VSCs) fed by distributed energy recourses (DERs) across the distribution level of power grid. The controller has feedback compensation and limits the current harmonics while featuring fast regulation for multi-converter system. The controller enables islanded and grid-connected operations of DERs, dynamically. The load dynamics will be included in the stable control loop to provide perfect regulation of the voltage at the point of common coupling (PCC). The current controller is tested under the considerations of system delays. In the islanded mode, one of the controllable units provides voltage and frequency control as a master controller and the other units operate based on the grid-connected control strategy. The stability assessment of the system is presented based on a detailed simulation. The simulation results show that the controller for the islanded two-parallel controllable DER can ensure the stability of the system when converters are tied together. In the other hand, the weakness of the current control based method is drawn out while mismatch coupling condition.

Admittance-Based Aggregated Modeling of ConverterInterfaced Resources with Heterogeneous Parameters for Transient Analysis of Multi-Converter Systems

Admittance-Based Aggregated Modeling of ConverterInterfaced Resources with Heterogeneous Parameters for Transient Analysis of Multi-Converter Systems

Low-Dimensional Equivalent Models and Multithreading-Based Parallel EMT Simulation Method for Multi-converter Systems

Low-Dimensional Equivalent Models and Multithreading-Based Parallel EMT Simulation Method for Multi-converter Systems

Dynamical Characterization of Multi-Converter System: Simultaneous Measurement of Bus Impedance and Control Loop Gains

Dynamical Characterization of Multi-Converter System: Simultaneous Measurement of Bus Impedance and Control Loop Gains

Aggregation reduced-order modeling of grid-connected multi-converter system for transient stability analysis

Aggregation reduced-order modeling of grid-connected multi-converter system for transient stability analysis

Conditional Droop Adjustment for Reliability-Oriented Power Sharing in Microgrids

Power electronic systems are facing the challenge to function more safely and reliably. Reliability-oriented power sharing is thereby employed in multi-converter systems to reduce the cost of end-of-life maintenance. One typical approach is to adaptively adjust the droop gains of the converters solely based on the accumulated damage to the power devices. However, this does not always hold throughout the entire loading profile, possibly ending up with a contradiction from higher reliability. Hence, this paper generalizes the reliability-oriented power sharing by necessitating the adjustment of active droop gains considering the nominal power. Two operation regions are formalized by the nominal point, where the adjustment goes into opposite directions. The principle is validated by short-term experiments and long-term simulations estimating the B10 lifetime (10% probability of failure), revealing the improvement of system-level reliability especially under fluctuating loads or operator commands.

Stability Analysis and Efficiency Improvement of a Multi-converter System Using Multi-objective Decision Making

Stability Analysis and Efficiency Improvement of a Multi-converter System Using Multi-objective Decision Making

Small-Signal Stability Assessment of Heterogeneous Grid-Following Converter Power Systems Based on Grid Strength Analysis

The increasing penetration of renewable resources into the power network through grid-following converters has increased the risks of small-signal instability resulting from the interaction between the converters and the power network. It is challenging to assess the small-signal stability in a power system with high penetration of renewable resources due to the complex interaction between a large number of grid-following converters and the power network. Moreover, the assessment complexity is further increased in a heterogeneous multi-converter system, where the interconnected converters have different control configurations or parameters from different manufacturers. To tackle such challenges, this paper proposes a method for assessing the small-signal stability in a heterogeneous multi-grid-following-converter system based on grid strength analysis. It is first theoretically proved that the small-signal stability of a heterogeneous multi-grid-following-converter system can be characterized by an equivalent homogeneous one, where all interconnected grid-following converters have the same control configurations and parameters. The equivalent homogeneous system can be decoupled into a set of subsystems for reducing the complexity of the small-signal stability assessment of the original heterogeneous system. On this basis, it is derived that the small-signal stability and stability margin of the heterogeneous system can be estimated based on the generalized short-circuit ratio (gSCR), and the critical gSCR (CgSCR). As a result, a gSCR-based method is proposed for assessing the small-signal stability of heterogeneous multi-converter systems. The efficacy of the proposed method is validated by both modal analysis and electromagnetic transient simulations on two heterogeneous multiple-converter systems with different network topologies and a large-scale practical power system.

Reduced-Order Modeling and Transient Synchronization Stability Analysis of Multiple Heterogeneous Grid-Tied Inverters

This article outlines a generalized reduced-order modeling approach for assessing the transient synchronization stability of multiple grid-tied converter systems operating in grid-feeding mode and having heterogeneous characteristics. The analytical approach utilizes the nonlinear quasi-static dynamics of the phase-locked loop (PLL) for obtaining the reduced-order models (ROM) of multiple heterogeneous grid-feeding converters having same and different point of synchronization (POS). This is a significant contribution in contrast to the prior effort where reduced-order modeling has been carried out only for homogeneous grid-tied multi-converter systems. In addition to this, an innovative methodology has been proposed in this article for analyzing the transient synchronization stability of the multi-converter systems leveraging the Lyapunov's direct method. The Lyapunov functions are constructively synthesized by invoking an assumption that the reduced-order models are conservative in nature. Following this, the well-known Lyapunov's second theorem is applied for deriving the sufficient conditions required for obtaining the transient stability boundary of the heterogeneous grid-feeding converter systems. Numerical simulations are performed in MATLAB 2017a platform to demonstrate that the ROM are highly accurate and computationally efficient. Besides, the efficacy of the proposed methodology for analyzing the transient synchronization stability of heterogeneous grid-feeding multi-converter systems is verified numerically.

An Optimized Nonlinear Droop Control Method Using Load Profile for DC Microgrids

Paralleling of power electronic converters to achieve current sharing is a critical aspect of a dc microgrid. Droop control is a popular technique to parallel converters. This article presents a novel perspective of different practically realizable nonlinear droop characteristics. Using the probability distribution function of the load current, a methodology is proposed to optimize the characteristics of nonlinear droop control. In a stand-alone dc microgrid system, the load current information used in the proposed control method is typically known using historical data or can be estimated using load forecasting methods. The advantages of the proposed nonlinear droop control method over several state-of-the art nonlinear droop control methods are discussed and shown. The effectiveness of the proposed nonlinear droop control method is validated using circuit simulations and hardware-based experiments on a multiconverter system for different load profiles.

Small-Signal Stability Boundary of Heterogeneous Multi-Converter Power Systems Dominated By the Phase-Locked Loops’ Dynamics

While renewable resources are increasingly integrated into the electric power system, the small-signal instability risk may be induced by grid-following converters using phase-locked loops (PLLs) for grid synchronization, especially in low short-circuit strength systems. The stability analysis complexity is further increased in such a heterogeneous multi-converter system (HMCS), where multiple converters may have different PLLs’ control parameters provided by various vendors. To understand how the different PLLs’ dynamics collectively affect the small-signal stability of the HMCS, we transform the HMCS into an equivalent homogeneous multi-converter system for the small-signal stability analysis. Then, we analytically derives the small-signal stability boundary condition of the HMCS dominated by the PLLs’ dynamics (HMCS-DPLL). The derived stability boundary allows us to obtain analytical results about how the small-signal stability of the HMCS-DPLL is collectively affected by network structure, converter operating conditions, and different PLLs’ control parameters. Based on the stability boundary condition, a computationally efficient method is also proposed to identify the design rationality of PLL control parameters as well as the small-signal stability and stability margin of the HMCS-DPLL. The analytical results and proposed method are validated by modal analysis and electromagnetic transient simulation with the full-order models on a 9-converter heterogeneous system.

Assessing Maximal Capacity of Grid-Following Converters With Grid Strength Constraints

The increasing penetration of renewable resources via power-electronic converters is turning the modern power grid into a multi-converter system (MCS). In an MCS, most renewable resources currently use grid-following converters (GFLCs) for grid synchronization. The increasing integration of renewable resources via GFLCs can cause small-signal stability problems, especially under weak grid conditions. One way to prevent the stability issue is limiting the capacity of GFLCs in an MCS. Thus, it is important to assess the maximal capacity of GFLCs in the MCS while considering small signal stability constraints (SSSCs). This assessment is challenging due to 1) the complexity of assessing the small signal stability resulting from the complicated interaction between the power network and a large number of GFLCs, especially for a heterogeneous GFLCs with unknown inner parameters; 2) the difficulty of finding the optimal solution to the relevant nonlinear optimization problem for maximal capacity assessment. To address these challenges, this paper proposes a semi-definite programming (SDP)-based method to assess the maximal capacity of GFLCs with SSSCs. In the proposed method, we first formulate the SSSCs based on the generalized short-circuit ratio (gSCR), which is a grid strength metric that can significantly reduce the complexity of quantifying small signal stability in an MCS. Then, we convert the formulated gSCR-based nonlinear optimization problem into an SDP that can conveniently find the optimal solution to the maximal capacity of GFLCs and their optimal allocations in the MCS. The efficacy of the proposed method is demonstrated on a 39-bus test system and a practical wind power system.

System-level performance and robustness of the grid-forming hybrid angle control

This paper investigates the implementation and application of the multi-variable grid-forming hybrid angle control (HAC) for high-power converters in transmission grids. We explore the system-level performance and robustness of HAC concept in contrast to other grid-forming schemes i.e., power-frequency droop and matching controls. Our findings suggests that, similar to the ac-based droop control, HAC enhances the small-signal frequency stability in low-inertia power grids, and akin to the dc-based matching control, HAC exhibits robustness when accounting for the practical limits of the converter systems. Thus, HAC combines the aforementioned complementary advantageous. Furthermore, we show how retuning certain control parameters of the grid-forming controls improves the frequency performance. Last, as separate contributions, we introduce an alternative control augmentation that enhances the robustness and provide theoretical guidelines on extending the stability certificates of HAC to multi-converter systems.

Analysis of multi-converter network impedance using MIMO stability criterion for multi-loop systems

This work presents a novel method of simplifying multi-converter systems containing complex grid-forming and grid-following controllers for stability analyses. Analysis is conducted via the equivalent converter output impedance. Important points for impedance-based stability analysis of larger networks are considered, including impedance rotations, proper combination of sources and correctly obtaining a minimum system realisation with appropriate tolerances. The proposed method is used to conduct a sensitivity analysis of different grid-forming and grid-following converter control parameters to determine tuning recommendations for the modern electricity network. To achieve this, complex control structures including power/voltage control, negative sequence regulation and synchronous machine emulation are included. Network stability is found to be sensitive to current control, active power control and frequency-based components such as PLLs. However, voltage control exhibits a smaller effect on stability for a range of short circuit ratios. Some recommendations are provided on when the simplified method can be utilised and how to ensure correct application.

Consensus enhanced droop control strategy for islanding mode multi converter system

Under the gradual expansion of the new energy distributed generation, the islanding multi converter microgrid system plays a great role. The stability of voltage and frequency of multi converter systems in islanding mode and the coordination of power allocation among many converter systems have become vital research points. Focusing on the reactive power sharing problem of multi converter system for different line impedances and load disturbances, this paper proposes a consensus enhanced droop control strategy to achieve power sharing and voltage and frequency stability, and the mechanism and steady-state performance of the proposed control strategy are analyzed and studied. In this work, a multi converter system simulation model composed of six inverters is built in the software PLECS, which verifies that the consensus enhanced droop control scheme can make up for the shortcomings of the traditional droop control. Moreover, the hardware experimental setup of three-phase inverter system is further conducted to validate the simulation results. Under load disturbance, the control effects of consensus enhanced droop control proposed in this paper are analyzed and studied, and the good dynamic characteristics of consensus enhanced droop control strategy are verified. Finally, the high robustness of the proposed coordinated control strategy is further illustrated by the jump of unbalanced load. This paper clarifies the effectiveness of the proposed control strategy based on consensus algorithm in solving the reactive power sharing problem of different line impedances and load disturbances around the island mode multi converter system. Furthermore, the work in the paper can ensure effective reactive power sharing under various working conditions with high robustness.

Frequency Synchronization of a High-Order Multiconverter System

We investigate the stability properties of a multi-converter power system model, defined on a high-order manifold. For this, we identify its symmetry (i.e., rotational invariance) generated by a static angle shift and rotation of AC signals. We characterize the steady state set, primarily determined by the steady state angles and DC power input. Based on eigenvalue conditions of its Jacobian matrix, we show asymptotic stability of the multi-converter system in a neighborhood of the synchronous steady state set by applying the center manifold theory. We guarantee the eigenvalue conditions via an explicit approach. Finally, we demonstrate our results based on a numerical example involving a network of identical DC/AC converter systems.

Second-order sliding mode controller design of buck converter with constant power load

Multi-converter systems used in DC microgrid feed supply constant power loads (CPL) as well as classical electrical loads. CPL has a non-linear load nature because of the negative impedance effect. This impact causes instability in converter systems within the micro-grid. This paper proposes a super twisting-based second-order sliding mode control (SOSMC) for regulating the output voltage of a DC/DC buck converter feeding CPL. The goal of this method is to suppress the chattering effect and maintaining the robustness under load variations and parametric uncertainties. With the experimental studies constructed in the laboratory and simulations executed in Matlab environment, the proposed SOSMC was compared with classical first-order SMC for the response of the buck converter feeding CPL under various conditions. The simulation and experimental results show the proposed controller has higher tracking precision and stronger robustness during any disturbance in input voltage or CPL power, as well as reducing the chattering effect.