Virtual Synchronous Machine Research Articles

Small‐signal analysis of multiple virtual synchronous machines to enhance frequency stability of grid‐connected high renewables

The virtual synchronous machine technology is considered as an important technique to effectively control the shortcomings of renewable energy-based power electronics interfaces, providing backup inertia and regulating grid stability. Conventionally, the virtual synchronous machine with a large capacity is responsible for controlling the entire grid stability against renewable penetration. It is usually operated as a centralised control system. But what if virtual synchronous machines with small capacities are independently operated by their additional droop control schemes, and will they present better performance than the single virtual synchronous machine? This study proposes the multiple virtual synchronous machine system with different active power-frequency (P-f) droop characteristics to improve inertia support regarding frequency stability improvement. The comprehensive small-signal modelling of the multiple virtual synchronous machine unit is designed to include the additional P-f droop characteristics. Then, the dynamic characteristics (steady-state and transient responses) and static stability of the multiple virtual synchronous machines are compared with the single virtual synchronous machine at the same rated capacity in both eigenvalue/sensitivity-domain and time-domain analysis. The obtained results reveal that the system with the presence of several virtual synchronous machines is more stable than the system with single virtual synchronous machine, maintaining stable and secure system operation during the contingency.

Low-frequency Oscillation Damping Control for Large-scale Power System with Simplified Virtual Synchronous Machine

This study focuses on a virtual synchronous machine (VSM) based on voltage source converters to mimic the behavior of synchronous machines (SMs) and improve the damping ratio of the power system. The VSM model is simplified according to some assumptions (neglecting the speed variation and the stator transients) to allow for the possibility of dealing with low-frequency oscillation in large-scale systems with many VSMs. Furthermore, a virtual power system stabilizer (VPSS) structure is proposed and tuned using a method based on a linearized power system dynamic model. The linear and nonlinear analyses examine the stability of two modified versions of a 16-machine power system in which, in the first case, partial classical machines are replaced by VSMs, and in the second case, all SMs are replaced by VSMs. The simulation results of the case studies validate the efficiency of the proposed control strategy.

New Fault Detection Algorithm for an Improved Dual VSM Control Structure With FRT Capability

Grid forming converters are a promising solution to enhance the stability of electrical networks with a high penetration of renewable generation. Virtual Synchronous Machines (VSM) are a particular type of grid forming converter, which can be implemented without an inner Current Control (CC) loop. This VSM implementation can provide a better inertial response than other grid forming structures with an inner CC. However, it cannot limit the current through the converter during a fault. A standard solution uses a VSM as the primary controller with a conventional inner CC acting as a backup controller during fault conditions, referred to here as a Dual VSM control structure. The switching action between the primary and the backup controllers is dependent on an accurate Fault Detection Algorithm (FDA). The conventional FDA mentioned in the literature has limitations in particular but not uncommon scenarios. The article shows the limitations of the conventional FDA in weak grids and unbalanced conditions and introduces a new FDA with improved performance. Two types of sensitivity analysis are used to study the dynamics of the proposed control approach. The First is the sensitivity of the proposed control structure to different fault locations in strong and weak grids. The Second is the sensitivity of the controller response to changing controller parameters. The second analysis is introduced as a reference for tuning and improvement of the control structure introduced.

Standardized Assessment Framework for Design and Operation of Weak AC Grid-Connected VSC Controllers

Significant research attention is focused on the stability and control of voltage-source converters (VSCs) in weak ac grids. However, the assessment of VSC control strategies has never been standardized, such that novel solutions proposed in the literature are not objectively evaluated against current best-practice. This paper establishes a comprehensive, standardized assessment framework for grid-connected VSC controllers, which allows objective comparison of novel controllers under development and established methods already in operation. The assessment consists of a tuning stage, time-domain analysis and frequency-domain analysis to form a complete evaluation that can be applied in its full aggregated form or as individual assessment steps. Three VSC controllers, namely vector current control, power-synchronization control and a virtual synchronous machine, are compared in this paper using small-signal models, time-domain simulations and control hardware-in-the-loop Real-Time Digital Simulation (RTDS) experiments to demonstrate the versatility and robustness of the proposed framework for controllers with very different structures.

Multi-Rotor Virtual Machine for Grid-Forming Converter to Damp Sub-Synchronous Resonances

Grid-forming power converters (GFMC) have been widely adopted in power systems as an attractive solution against the challenges imposed by the ever-increasing penetration of renewables. Despite its versatility, GFMC is employed only to provide islanded operation, grid regulations, and synthetic inertia. To further extend the use of GFMC in enhancing power system stability, this paper proposes a multi-rotor virtual machine (MRVM) controller to attenuate sub-synchronous oscillations. Driven by the formulation of a virtual synchronous machine (VSM), the proposed MRVM implements a VSM-based GFMC with several virtual rotors whose electromechanical characteristics can be individually adjusted to target specific oscillatory modes in the system. In this work, the MRVM’s working principle is described in detail and tuning guidelines are proposed to simplify the selection of control parameters by using frequency-domain techniques and the eigenvalue locus analyses. To validate the performance of the MRVM, an IEEE benchmark grid model is adopted namely, the three-machine-infinite-bus system. It is evident from the results that the MRVM (i) provides higher degrees of freedom when dealing with sub-synchronous oscillations, and (ii) outperforms conventional GFMC, especially in damping intra-area power oscillations.

A Basic Study on Virtual Synchronous Machine Control Applied to Solid State Transformer Composed of MMC and DAB

A Basic Study on Virtual Synchronous Machine Control Applied to Solid State Transformer Composed of MMC and DAB

Modeling and Initialization of a Virtual Synchronous Machine for Power System Fundamental Frequency Simulations

Modeling and Initialization of a Virtual Synchronous Machine for Power System Fundamental Frequency Simulations

Validating grid‐forming capabilities of hybrid power park technologies in future OFTO networks

In recent years, there has been considerable interest in convertor-based generating solutions which to a greater or lesser extent mimic the behaviour of synchronous machines, thus overcoming many of the disadvantages of the existing technologies which are potentially destabilising at high penetration. Such solutions are frequently referred to as grid-forming convertors (GFCs). This study focuses on the application of GFC technologies in offshore windfarms, where installation, maintenance and/or modification of any offshore equipment is very expensive and carries greater commercial risks, requiring extensive testing and confidence building prior to deployment in real applications. This is time consuming and particularly significant for GB and where there are large quantities of offshore generation. Onshore solutions to stability are therefore desirable for off-shore transmission owners (OFTOs), especially, if they could be applied by retrofitting to existing conventional converter plant. Consequently, this study proposes and investigates the performance of hybrid solutions for offshore networks where the conventional STATCOM onshore unit is replaced by alternative options such as synchronous compensator and virtual synchronous machine converter of similar (or appropriate) rating with the aim of achieving grid-forming capability. A laboratory-scale implementation of the proposed control algorithm is also presented with selected validation test results.

Damping Effect of Virtual Synchronous Machines Provided by a Dynamical Virtual Impedance

Virtual synchronous machines (VSMs) emulate the swing equation for grid synchronization and inertia provision. Usually, a large damping coefficient is necessary in VSMs to provide equivalent damper winding effect, which is, however, not aligned with the fact that the physical swing equation of synchronous generators (SGs) contains only a small mechanical friction factor. This large damping coefficient could result in undesired droop characteristics. In this letter, we use damping torque analysis to reveal the damping mechanism of SGs, and show that the damping effect actually comes from the dynamics of the transient/subtransient reactances aggregated as a dynamical impedance. Based on this finding, we elaborate on how to get rid of the large damping coefficient in VSMs by using a dynamical virtual impedance (DVI) to emulate SG's damping mechanism.

Stability enhancement of DC power systems by VSM control strategy

DC system is characterized by low inertia and weak damping, which leads to a couple of stability issues. This paper proposes a modeling method for analyzing the DC-side oscillations occurred in DC microgrid with taking multi-paralleled converters into account. A new modeling is developed for analysis of dynamic interaction between multiple power converters. Thus, it is found that dynamic interaction between multiple converters will result in the poorly-damped oscillation of DC voltage. Besides, the poorly-damped modes will aggravate the stability of DC system. To cope with this issue, a virtual synchronous machine (VSM) control approach is proposed to suppress the oscillation of DC voltage, strengthen inertia, and improve damping performance of DC microgrid. Besides, the controller tuning of the VSM has a large flexibility that it does not depend on the system parameters. Finally, the proposed conceives are demonstrated by simulations.

Impact of Virtual Synchronous Machines on Low-Frequency Oscillations in Power Systems

The low-frequency oscillations (LFOs) inherent in power systems will be impacted by the increasing penetration of renewable energy sources (RESs). This paper investigates the impact of virtual synchronous machine (VSM) based RESs on the LFOs in power systems. A detailed two-machine test-bed has been developed to analyze the LFOs which exists when VSMs replace synchronous generators. The characteristics of the LFO modes, and the dominant states have been comprehensively analyzed. Furthermore, this study analyzes the LFO modes which exists in an all-VSM grid. The role of the power system stabilizers in the all-VSM grid has been comprehensively evaluated. The IEEE benchmark two-area four-machine system has been employed to corroborate the results of the small-signal analysis, and observe the transient performance. The analysis in this paper have been performed in MATLAB/SIMULINK environment.

Linear and uniform swing dynamics in multimachine converter-based power systems

The increase in RES generation is replacing the classical synchronous generators (SGs), which has resulted in gradually losing the beneficial inherent properties of SGs for power system stability. In order to mimic the inertial behavior of SGs, the concept of Virtual Synchronous Machines (VSMs) has been proposed, but their interactions at different time-scales and their effect on system stability are not yet fully understood. Therefore, a new scientific basis for designing and regulating the system is required. This paper contributes by advancing the idea of Linear and Uniform Swing Dynamics (LSD). This concept can be used to shape the dynamic behavior of these VSMs by controlling the eigenvalues to be constant throughout its operating region. Consequently, the validity of small-signal stability analysis can be extended for larger disturbances. Existing approaches to achieve LSD for a Single-Machine – Infinite Bus (SMIB) system are analyzed and complemented, and extended for multi-machine systems. The stability analysis using system differential equations is presented. Time-domain simulation results in MATLAB/Simulink are presented to validate the concept.

Frequency Stability Using MPC-Based Inverter Power Control in Low-Inertia Power Systems

The electrical grid is evolving from a network consisting of mostly synchronous machines to a mixture of synchronous machines, and inverter-based resources such as wind, solar, and energy storage. This transformation has led to a decrease in mechanical inertia, which necessitate a need for the new resources to provide frequency responses through their inverter interfaces. In this paper we proposed a new strategy based on model predictive control to determine the optimal active-power set-point for inverters in the event of a disturbance in the system. Our framework explicitly takes the hard constraints in power, and energy into account, and we show that it is robust to measurement noise, limited communications, and delay by using an observer to estimate the model mismatches in real-time. We demonstrate the proposed controller significantly outperforms an optimally tuned virtual synchronous machine on a standard 39-bus system under a number of scenarios. In turn, this implies optimized inverter-based resources can provide better frequency responses compared to conventional synchronous machines.

Research on Renewable Energy Access to Power Grid Based on Energy Internet Technology

The Internet of Energy is the key to achieving the goals of China’s energy revolution, and Internet technology will become an important means to promote the transformation and development of China’s energy industry. Based on the analysis of the current research status of virtual synchronous machines, an implementation method of a virtual synchronous machine algorithm based on a synchronous rotating coordinate system is proposed. This algorithm does not require a phase-locked loop, which greatly improves the accessibility of renewable energy and realizes its transformation from auxiliary energy to main energy. Simulation results verify the effectiveness of the proposed algorithm.

Virtual Synchronous Machine/Dual-Droop Controller for Parallel Interlinking Converters in Hybrid AC–DC Microgrids

This paper addresses the operation of parallel interlinking converters (ICs) in hybrid AC–DC microgrids from different perspectives, then proposes a unified mitigation technique. Firstly, the influence of the system nonlinearity on circulating currents between parallel ICs is investigated. Secondly, it is shown that ICs might induce voltage harmonics due to the antiparallel diodes of the power electronic switches which are seen by the AC subgrid as nonlinear diode rectifiers. Lastly, the synchronization of ICs following abnormal operating conditions is discussed and new findings are presented. In order to enhance the operation of ICs and overcome the preceding challenges, a modified virtual synchronous machine (VSM) strategy with a dual-droop (DD) controller is proposed. The positive influence of the proposed VSM-DD controller on the overall system dynamics is clearly justified. Small-signal state-space modeling for the entire system is conducted to verify the results analytically. A nonlinear time-domain simulation model is developed using PSCAD/EMTDC to validate the impact of the VSM-DD controller experimentally.

Electromagnetic Modelling and Operation Analysis of Virtual Synchronous Machine for Frequency Response

Virtual synchronous generators (VSGs) utilizing renewable energy sources (RESs) have been proven to be an appealing solution for overcoming the challenges of renewable energy sources (RESs) consumption in power systems. In order to address these issues, researches have been developed to improve the dynamic characteristics of VSG power systems. However, most of the studies at home and abroad focus on the negative impact of VSGs on the grid voltage, and few on the frequency, safety and efficiency of VSGs. The first part of this paper focuses on the VSG’s electromagnetic detail model with the ability to support the system voltage and frequency deviations, the second part discusses the mathematical investigation on its response as a VSG. Finally, according to the technical requirements of the power grid, mechanical and electromagnetic loads are simulated, the influence of service life is analyzed, and design suggestions are put forward.

Adaptive control of grid connected inverter based on virtual synchronous machine

In recent years, with the rapid development of distributed generation, virtual synchronous generator (VSG) control technology for distributed generation is proposed. On the basis of droop control, VSG further simulates the inertia and damping characteristics of synchronous generator. Compared with droop control, VSG control technology makes distributed energy have the damping and inertia characteristics of synchronous machine, but it can not guarantee the dynamic regulation performance of frequency and power at the same time. To solve this problem, the adaptive control of VSG is realized by establishing the relationship between the moment of inertia and the damping coefficient. Finally, an experimental platform of VSG adaptive control model is built in Matlab / Simulink. The simulation and experimental results verify the effectiveness and correctness of the proposed control strategy.

Parallel Stability Analysis of Generator and Inverter in Independent Micro grid

In the independent micro grid system, the generator set and the new energy grid connected inverter are both important components of the system. To solve the problem of stability of the parallel of the inverter and synchronous generator, this paper studied the droop control and the virtual synchronous machine control principle, analyzed their similarities and differences and essential characteristics; established the small signal model of the parallel system of the inverter generator set, analyzed the stability of different structural systems. It Comed to conclusion that the stability of the virtual synchronous machine control in the parallel system was stronger, and the system simulation model was built by Matlab / Simulink software to verify the theoretical analysis results.

U.s. department of energy funds syndem to advance solar inverter technology [happenings

Using the funds, the burgeoning company will further develop its patented virtual synchronous machine technology to sharply increase the use of solar power.

Dynamic Analysis and Model Order Reduction of Virtual Synchronous Machine Based Microgrid

Dynamic Analysis and Model Order Reduction of Virtual Synchronous Machine Based Microgrid