Virtual Synchronous Machine Control Research Articles

A data‐driven identification method for impedance stability analysis of inverter‐based resources

AbstractObtaining inverter controller information may be a premise for seeking its dynamic behaviour. But accurate knowledge of such information would be unrealistic for real functioning inverter‐interfaced generators (IIGs), which hinders the stability analysis of the IIG. A new data‐driven impedance identification method is proposed for stability analysis, which involves an improved sparse identification algorithm as an ancillary function within the system identification framework. It contains mainly two design stages. First, the transform basis matrix (TBM) is devised systematically as a prior knowledge library to contain the possibly existing control structures. In the second stage, a sparse identification algorithm is reformulated in order to extract the relevant structures in TBM while obtaining controller parameters. The authors demonstrate that the sparse vector between the TBM and output signal is closely related to the controller structure. The effectiveness of the proposed method is verified on grid‐connected inverters based on droop control and virtual synchronous machine control.

Virtual Synchronous Machine Control for Doubly Fed Induction Machine-Based Wind Energy Conversion Systems

Virtual Synchronous Machine Control for Doubly Fed Induction Machine-Based Wind Energy Conversion Systems

Parameter Optimization of Energy Storage Virtual Synchronous Machine Based on Particle Swarm Optimization Algorithm

Energy transformation is a severe challenge and major demand faced by China’s sustainable development, and new energy development has become a key driving force for energy transformation. The issue of system stability is brought to light by the steadily rising share of renewable energy sources like wind and solar, which in turn places greater demands on the inertia support capacity of power plants. In order to replicate the dynamic equations of a conventional synchronous machine and add inertia and damping to the system, a new technology known as virtual synchronous machine (VSM) control has arisen. Therefore, improving the system’s absorption capacity of new energy generation and ensuring its safe and stable operation are both greatly aided by the study of energy storage VSM control technology’s grid stability and parameter design methods. Although VSM control adds inertia to a system, which is good for stability, it also complicates the system’s ability to withstand tiny disturbances. This paper investigates the small disturbance stability of VSM-controlled power generating and storage systems. The stability analysis method of storage VSM when subjected to random power sources is investigated further, and a method for optimising the system’s parameters is proposed. VSM with optimised parameters can provide sufficient inertia and damping under complex operating conditions, as shown by simulation results in real-world systems. Finally, the proposed method’s efficacy was assessed with the help of an enhanced binary particle swarm optimisation (EBPSO) algorithm, and the outcomes confirmed its superiority.

Transient Stability Enhancement Strategy for Islanded Microgrids Based on Energy Storage–Virtual Synchronous Machine Control

In a high percentage of new energy-islanded microgrids, the overall inertia of the system gradually decreases, and the transient stability requirements of the microgrid frequency and voltage become more and more demanding under low-inertia conditions. To improve the transient stability of low-inertia islanded microgrid frequencies and voltages, this paper proposes a transient stability enhancement strategy for islanded microgrids based on energy storage system (ESS)–virtual synchronous generator (VSG) control. Model predictive control (MPC) is added within the active control loop of the VSG to achieve dynamic correction of the active power reference value of the VSG; PI control link is added within the reactive control loop to achieve a fast dynamic response of the reactive power command value. The ESS achieves fast and accurate regulation of frequency and voltage according to the power reference value of the VSG active control loop and the power command value of the reactive control loop simultaneously. Considering the need to ensure the ability of VSG to operate stably during transients, a comprehensive current-limiting technique combining virtual impedance and phase limiting is used to limit the fault current of VSG and maintain its synchronization and stability. Finally, the simulation results verify the strategy’s effectiveness and the superiority of the transient stability enhancement effect.

Design of Energy Recovery Control for General Virtual Synchronous Machines Based on Various Forms of Energy Storage

The reduced inertia in the power system due to renewable energy integration introduces operation challenges in frequency stability and control. The current options for virtual inertia and frequency support are limited by the energy resources and the power electronic interface. Considering the demand on response speed and energy capacity, a general virtual synchronous machine (VSM) control based on various forms of energy storage systems (ESS) is proposed. The steady-state energy variation of energy storage is found to be proportional to the virtual damping or governor gain, while inversely proportional to the integral gain of system frequency control. It is found that the size of energy storage can be at the second time scale (for example, 6.8 p.u.·s) for VSM implementation, which is significantly smaller than the conventional hour-scale energy storage in the power system. Based on energy dynamic analysis, stability requirement, and bandwidth separation rules, an energy recovery control is designed to maintain constant state of charge (for example, 50%) while avoiding conflicts with frequency regulation. The time scale of the designed energy recovery control loop (for example, hundreds of seconds) is longer than the secondary frequency control. The effectiveness of the proposed control is verified through comprehensive case studies.

Virtual Inertia Response and Frequency Control Ancillary Services From Hydrogen Electrolyzers

This paper presents the modeling foundations to study the capabilities of hydrogen electrolyzers (HEs) to provide frequency control ancillary services (FCAS), including virtual inertia and primary and secondary frequency response. To do so, we propose a general, unified HE dynamic model for the electrolyzer stack circuit, power-electronics interface (PEI), and relevant converter-level control loops. The equivalent circuit of the stack is derived from its current transfer function, with poles and zero obtained from its step response characteristics (e.g., rise/settling time). The stack model also considers relevant physical nonlinearities and downstream hydrogen buffer/process operational constraints. The PEI control loops account for stack model parameters, hydrogen production operational constraints, stack temperature dynamics, and active power reference generation strategy for contingency and regulation frequency support services. Further, we propose a virtual synchronous machine (VSM) control approach to study the VSM HE capabilities to also provide virtual inertia response. We apply the modeling to both alkaline and proton exchange membrane (PEM) technologies, including design of appropriate control schemes. Finally, we assess the FCAS performance of alkaline and PEM HEs via dynamic simulation of the Australian south-east interconnection in a 50%-renewable scenario, also discussing comparison and cooperation with battery energy storage systems.

Grid-forming VSM control for black-start applications with experimental PHiL validation

The rising penetration of power converters interfaced generation into the electrical grid is leading to a paradigm shift where distributed resources are expected to provide ancillary services. The voltage-source behavior of grid-forming converter (GFC) makes it an attractive black-start provision alternative. This paper thus proposes a modified grid-forming virtual synchronous machine (VSM) control that is tailored for black-start applications. Voltage and power loops in the proposed controller are modified to provide soft energization to mitigate transformers inrush current, in addition to improved voltage support for the restored AC network and a smooth synchronization with neighboring islands or the main grid after a black-start event. The VSM control performance is first validated through simulations. Then, Power Hardware-in-the-Loop (PHiL) technique is used to validate the ability of a hardware GFC equipped with the modified VSM controller to restore a simulated network in a digital real-time simulator (DRTS) platform. Current-type Ideal Transformer Model (I-ITM) interface technique is successfully used in the experiment with time-delay impact compensation in the synchronous dq0 frame. The novel presented PHiL demonstration in this paper paves the way for similar testing of industrial GFCs for black-start and ancillary services provision under flexible network restoration conditions and complexity.

Modeling and Analysis of Multiple Inverters With Dual-Loop-Based Virtual Oscillator Control

Virtual oscillator control (VOC) is an emerging decentralized control technique for grid-forming inverter applications. In contrast to conventional phasor-based droop control or virtual synchronous machine control, VOC is a time-domain controller designed to emulate the dynamics of a nonlinear oscillator. VOC is a current-controlled voltage source, lacking the ability to regulate the inverter terminal voltage, which will result in significant voltage deviations caused by dead-time effects, nonideal semiconductor devices, and output filter voltage drops. In this article, a voltage and current dual-loop control structure augments the VOC to compensate for these voltage deviations and regulate the inverter output variables directly. A complete small-signal model for a multiple inverter-based microgrids with the proposed control structure is presented in order to assess system stability using eigenvalue and participation factor analysis. Analytical results show that the parameter related to the frequency regulation and the integral gain of the voltage controller affect the location of the system’s dominant modes significantly. The stability margin is determined by modifying these control parameters. Experimental results on a laboratory test microgrid verify the predication from the small-signal analysis and time-domain simulations.

Virtual Synchronous Machine Control of RES Plants in Isolated Power Systems

Because of the increase in renewable energy sources (RESs) share, new control strategies of isolated power systems have been developed to improve the frequency and voltage stability of inverter-interfaced RESs. A voltage source converter (VSC) with a virtual synchronous machine (VSM) is among the most promising control schemes. This paper demonstrates how VSM control of inverter-interfaced RES can be efficiently used to improve the dynamic stability in small isolated power systems. In the proposed analysis, the RESs of a Mediterranean island are assumed interfaced to the grid by VSCs with a swing controller and a vector-current controller (VCC) with two different options for the reference current (RC) to regulate the voltage at the point of common coupling (PCC) and the real power output. The system is modelled in a PSCAD environment, and the behavior of the control is tested in the case of a phase-to-phase fault. The results of the simulations for different scenarios and values for the control parameters show the effectiveness of the control in small isolated grids. Finally, the level of grid power quality is verified via harmonic analysis of the PCC voltage.

Review on grid-forming converter control methods in high-proportion renewable energy power systems

Grid-forming (GFM) converters can provide inertia support for power grids through control technology, stabilize voltage and frequency, and improve system stability, unlike traditional grid-following (GFL) converters. Therefore, in future “double high” power systems, research on the control technology of GFM converters will become an urgent demand. In this paper, we first introduce the basic principle of GFM control and then present five currently used control strategies for GFM converters: droop control, power synchronization control (PSC), virtual synchronous machine control (VSM), direct power control (DPC), and virtual oscillator control (VOC). These five strategies can independently establish voltage phasors to provide inertia to the system. Among these, droop control is the most widely used strategy. PSC and VSM are strategies that simulate the mechanical characteristics of synchronous generators; thus, they are more accurate than droop control. DPC regulates the active power and reactive power directly, with no inner current controller, and VOC is a novel method under study using an oscillator circuit to realize synchronization. Finally, we highlight key technologies and research directions to be addressed in the future.

Design and Analysis of Parallel-Connected Grid-Forming Virtual Synchronous Machines for Island and Grid-Connected Applications

Transmission system operators (TSOs) are increasingly requesting frequency support, reactive power compensation, and grid-forming capability to power plants interfaced by power converters. In these plants, a large number of converters is operated in parallel and all of them should be adequately coordinated to provide these services. For that purpose, the virtual synchronous machine (VSM) control technique is commonly adopted. However, the dynamic characteristics of the whole plant depend on the number of converters connected, their size, control parameters, and electrical interconnections. This makes it difficult to know the aggregated dynamic characteristic of the plant and how each VSM contributes to the plant response. In this article, the operation of VSMs in parallel configuration for island and grid-connected applications is studied. The developments make it possible to adapt the VSMs parameters so that the grid sees the power plant as an equivalent generator whose low-frequency characteristics do not vary with the number of converters in operation. The developments were validated in a laboratory consisting of four 15 kVA converters operating as VSMs, and a grid emulator.

Droop Control-Based Grid Forming Inverters with an Auxiliary Grid Support Control for Frequency Regulation in a Low Voltage Microgrid

Aim: Improving the frequency regulation in a Low Voltage (LV) microgrid during grid mode transition using Grid Forming Grid Support (GFGS) converters with inertia emulation. Background: The ever-increasing demand for energy and the depleting fossil fuels has increased the proliferation of converter-fed Renewable Sources (RES) in the conventional grid. The reduced system inertia results in frequency regulation issues which are further aggravated by the unpredictable nature of RES and dynamic switching of loads. This scenario results in blackouts, unwanted load shedding, power quality issues, and discrepancies in protection schemes. Objective: A control scheme is presented on the principle of swing equation-based synchronization to regulate the active power output of a grid-tied voltage-source converter operating in a grid forming grid supporting mode in a low voltage PV-based microgrid. The Virtual Synchronous Machine control (VSM) is implemented for controlling the active power output, which follows the grid frequency deviations, ensuring the safe State of Charge (SoC) limits of the Battery Energy Storage System (BESS). Methods: The stability issues due to the absence of inertia in an inverter-fed LV microgrid are staved off by giving auxiliary grid support by implementing VSM control in a BESS inverter in the grid forming mode. The LV microgrid model with all the converters in the GFGS converter is developed in the MATLAB/Simulink environment. Particle Swarm Optimization (PSO) is used to fine- tune the control parameters. The effectiveness of the control strategy has been investigated for unintentional islanding events. Results: The droop control-based GFGS grid-tied converters with power balance equation-based VSM control gave frequency regulation within the range stipulated by the grid codes during the islanding events. Conclusion: The work demonstrates the effectiveness of VSM control in providing the auxiliary grid support for droop-based grid forming converters in an LV microgrid.

Design of Hybrid-Storage-Based Virtual Synchronous Machine With Energy Recovery Control Considering Energy Consumed in Inertial and Damping Support

The reduced inertia in power system introduces more operation risks and challenges due to the degraded frequency performance. The existing virtual inertia control and fast frequency response to tackle this issue are restricted by the energy resource behind the power converter. In this article, an improved virtual synchronous machine control is proposed, considering the limitation of energy storage in response speed and energy capacity. The fast-acting energy storage system is used to emulate inertia and damping. The energy consumed by both services in dynamic process and steady-state state is also investigated. An energy recovery control is designed to restore the energy consumed in inertia emulation and damping provision, thereby ensuring constant energy reserve. Besides, the slow-acting energy storage with larger energy capacity, is controlled like the governor regulator of synchronous generator and capable of providing long-term energy service. A design procedure considering storage limitation, control stability, and bandwidth separation is proposed. Effectiveness of the proposed control and design is validated by comprehensive simulation and hardware test results.

Multi-VSM based fuzzy adaptive cooperative control strategy for MVDC traction power supply system

This paper studies the control of a medium-voltage DC (MVDC) traction power supply system for rail transit (RT). In order to optimize the ability of DC voltage fluctuation and frequency regulation of rail trains, a fuzzy adaptive cooperative secondary control strategy based on improved virtual synchronous machine (VSM) is proposed. Firstly, the improved VSM control strategy is adopted in the traction substations (TSSs), which makes the RT trains have the external characteristics of synchronous generator and have certain inertia and damping support capacity, so as to improve the ability of the RT traction system to cope with traction network transient changes. Secondly, the virtual inertia and virtual damping can be dynamically adjusted by the system frequency deviating from the nominal steady-state frequency, and the dynamic fuzzy adjustment can be carried out by the fuzzy logic system to slow down the frequency fluctuation. Then, due to the different power supply distances and parameter settings between multiple TSSs, cooperative control is used to coordinate local TSS perceptions of information between adjacent TSS to achieve consistency of frequency response of multiple TSSs under different parameters. Besides, a time-varying switching topology handoff method is considered to select the optimal communication topology between adjacent base stations. Finally, simulation results verify the effectiveness of the proposed control strategy.

Control of inverters in standalone and grid-connected microgrid using different control strategies

PurposeThe share of renewable energy sources (RESs) in the power system is increasing day by day. The RESs are intermittent, therefore maintaining the grid stability and power balance is very difficult. The purpose of this paper is to control the inverters in microgrid using different control strategies to maintain the system stability and power balance.Design/methodology/approachIn this paper, different control strategies are implemented to the voltage source converter (VSC) to get the desired performance. The DQ control is a basic control strategy that is inherently present in the droop and virtual synchronous machine (VSM) control strategies. The droop and VSM control strategies are inspired by the conventional synchronous machine (SM). The main objective of this work is to design and implement the three aforementioned control strategies in microgrid.FindingsThe significant contributions of this work are: the detailed implementation of DQ control, droop control and VSM control strategies for VSC in both grid-connected mode and standalone mode is presented; the MATLAB/Simulink simulation results and comparative studies of the three aforementioned controllers are introduced first time in the proposed work; and the opal-RT digital real-time simulation results of the proposed VSM control show the superiority in transient response compared to the droop control strategy.Research limitations/implicationsIn the power system, the power electronic-based power allowed by VSM is dominated by the conventional power which is generated from the traditional SM, and then the issues related to stability still need advance study. There are some differences between the SM and VSM characteristics, so the integration of VSM with the existing system still needs further study. Economical operation of VSM with hybrid storage is also one of the future scopes of this work.Originality/valueThe significant contributions of this work are: the detailed implementation of DQ control, droop control and VSM control strategies for VSC in both grid-connected mode and standalone mode is presented; the MATLAB/Simulink simulation results and comparative studies of the three aforementioned controllers are introduced first time in the proposed work; and the opal-RT digital real-time simulation results of the proposed VSM control show the superiority in transient response compared to the droop control strategy.

Fuzzy‐based frequency security evaluation of wind‐integrated power systems

The transition to renewable energy-based power systems is fast progressing. One of the main challenges in keeping a power system with high operational reliability is to maintain the system frequency. As synchronous generator units are being replaced with power-electronic converters, the rotating mass and the system inertia are decreasing. Virtual synchronous machine (VSM) control is a modern control technique that aims to compensate for the reduction in inertia. The usage of power electronic-based converter units equipped with VSM control has to be managed and scheduled by system operators. An assessment of the operational frequency reliability is used to evaluate different service usages. A method is proposed that allows the comparison of different frequency management strategies. The proposed method uses fuzzy logic to evaluate the system risk for abnormal frequency and the system effort in the form of frequency control usage. This allows to quickly compare different frequency management strategies whilst keeping in mind many different reliability indices. The proposed method is validated with a modified IEEE Reliability Test System with integrated wind power capacity.

A Brief Review of Smart Control of Renewable Energy Power Generation

The smart control strategy in the operation of renewable energy station is of great significance to alleviating the shortage of fossil energy, improving the efficiency of renewable energy utilization and the safe operation of power system. Firstly, this paper took the renewable energy stations represented by wind and solar energy as the research object, summarized the smart control strategy of single unit power generation, and elaborate introduced the mechanical structure control, maximum power point tracking control and virtual synchronous machine control in electrical converter level. Secondly, the self-protection control of renewable energy power generation station in the case of power system faults were analysed. The key issues involved in the low voltage ride-through, high voltage ride-through, protection circuit of rotor side, stator side and converters were summarized. Thus, a brief overview of smart control of renewable energy station operation is presented in this paper, hoping to provide technical guide for prosperous development of renewable energy in the future.

Robust Secondary Frequency Control for Virtual Synchronous Machine-Based Microgrid Cluster Using Equivalent Modeling

The technology of virtual synchronous machine (VSM) is attracting interest of researchers as it controls converters mimicking the synchronous machine's response so as to provide inertial support for power electronics dominated smart grids. For the VSM-based microgrid, its slow dynamics are dominated by synchronous generators (SGs) and the VSM control loops, which makes it possible to model this microgrid into an equivalent SG (EqSG) model. This paper proposes a robust secondary frequency control design method for the VSM-based low voltage (LV) microgrid cluster (MGC) using equivalent modeling. The EqSG model is used to construct the MGC model so as to reduce the model order and the complexity of controller synthesis. Modeling errors caused by the EqSG model and different operating conditions are integrated into the MGC model as unstructured uncertainties. The proposed secondary frequency control strategy is based on the distributed-centralized hybrid control structure to coordinate frequency restoration among LV microgrids. Structured μ-synthesis is applied for tuning control parameters realizing H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> robust performance against unstructured uncertainties. To reduce the communication resource consumption, an event-triggered mechanism considering communication delay is introduced in the robust secondary frequency control strategy. The triggering condition is analyzed using a Lyapunov function to guarantee H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">∞</sub> robust stability. Simulation and real-time experiment results on a MGC composed of four CIGRÉ benchmark LV microgrids are presented to demonstrate the effectiveness of the proposed control strategy.

Implementation and comparison of droop control, virtual synchronous machine, and virtual oscillator control for parallel inverters in standalone microgrid

In recent times, renewable energy sources (RESs) have been potential alternatives over the conventional generation systems connected to the grid. The power electronic inverters are the principal media of interface for connecting the RESs to the utility grid system. This work is primarily focused on the comparative analysis of Droop, virtual synchronous machine (VSM), and virtual oscillator control (VOC) techniques for the parallel operated inverters in a standalone Microgrid (MG). Droop control emulates only the droop characteristics of synchronous machines such that the transient response of this controller is not significant. VSM imitates not only the droop characteristics of synchronous machines but also the swing equation. Therefore, VSM has a remarkable difference in the dynamic performance of the system. In droop and VSM, the feedback signals such as voltage and current are measured to calculate the averaged real and reactive powers. However, the VOC works on instantaneous feedback signals such that it achieves much faster synchronization and good power-sharing. The philosophy of the proposed VOC is to control an inverter such that it emulates the behavior of a nonlinear dead-zone oscillator. The performance analysis of the system with the aforementioned controllers has been studied based on MATLAB/Simulink and Opal-RT digital simulation. From the comparative analysis, it is observed that the VOC gives a better performance compared to droop and VSM control. The experimental results show the efficacy of the proposed VOC control method.

Fuzzy Controller-Based Self-Adaptive Virtual Synchronous Machine for Microgrid Application

Low-inertia microgrids are prone to large frequency fluctuations due to high penetration of inverter-fed renewable sources. A virtual synchronous machine (VSM) is capable of handling these frequency fluctuations. VSM imparts inertial behaviour in grid-connected inverters with the help of an electric storage device. Most of the reported VSM control schemes use a constant inertia parameter that leads to an inadequate dynamic frequency response. In this paper, a fuzzy controller-based self-adaptive VSM control scheme is proposed. Here, the value of the inertia parameter is decided using fuzzy rules which take the penetration index of renewable energy, frequency deviation and rate-of-change-of-frequency as the input parameters. These rules are formulated with the help of the sample data obtained from simulations done in MATLAB/Simulink, where the inertial parameter is optimized using a genetic algorithm for various cases. The proposed method is validated for solar photovoltaic-fed microgrid test systems using power-hardware-in-the loop and the results indicate a reduction in maximum frequency deviation compared to the constant inertia parameter based VSM schemes. The introduction of renewable energy penetration level and fuzzy expert system in modifying the inertia parameter are the novel contributions in this paper.