Multiple mode operation and control for VSG interfacing DC distribution system

• The paper focuses on optimising Virtual Synchronous Generator (VSG) control in Modular Multilevel Converter (MMC)-based HVDC systems to enhance power system stability and oscillation damping. • It introduces virtual inertia and damping to mimic synchronous generator behaviour, improving frequency and voltage regulation. • The proposed control strategy ensures smooth transitions between grid-connected and islanded modes, suppresses harmonics, and addresses challenges like low inertia and weak grid conditions. • Simulation results validate the effectiveness of the VSG-controlled MMC in maintaining stability during dynamic and transient events, making it suitable for future power systems with high renewable energy integration. This paper discusses the development of a Virtual Synchronous Generator utilizing a Modular Multilevel Converter. The control system employs an internal simulation of the electromechanical swing equation to replicate the inertia and damping characteristics of a synchronous generator, enabling the generation of the necessary frequency and phase angle for MMC control as a VSG. The ac-side management involves cascaded voltage and current controllers operating within a synchronous reference frame defined by the virtual swing equation. Simulation findings validate the performance of the VSG-controlled MMC when connected to an external AC grid and in islanded mode with a resistive load. This paper validates, through experimentation, the effectiveness of controlling a Modular Multilevel Converter (MMC) as a Virtual Synchronous Generator (VSG) designed to deliver virtual inertia and robust frequency stabilization in power systems with low physical inertia. The proposed control structure incorporates a virtual swing equation, cascaded voltage and current regulation loops, reactive power control, and emulated impedance. A 60 kVA MMC model was employed to demonstrate the system’s capabilities. During frequency disturbances such as a drop from 50 Hz to 49.8 Hz, the controller facilitated dynamic power injection up to approximately 35 kW while capping peak currents at 100 A. Frequency deviations were mitigated within 300 milliseconds, and the converter exhibited seamless transitions to islanded operation along with reliable voltage control during load variations. These findings confirm the proposed controller’s effectiveness in maintaining grid stability under diverse operating conditions.

Virtual synchronous generator (VSG) is the concept of controlling the grid-connected inverters to mimic conventional synchronous generators (SGs), and it is considered to be a promising method to enhance the grid inertia and power regulation ability when the renewable energy has a high penetration rate in the grid. In this paper, the power frequency characteristic and the synchronization mechanism of the VSG are analyzed in detail, and a modified pre-synchronization method is proposed. Compared to the existing methods, the proposed method in this paper regulates the VSG frequency by resetting its active power reference according to the local load and eliminates the phase deviation by feedback control of the virtual power, thus the phase angle of the VSG in standalone mode is in accordance with the steady operation point in grid-connected mode and the dynamic power adjustment can be avoided when the VSG connect to the grid. A 30kV A VSG prototype was developed and tested in the laboratory, and the effectiveness of the proposed method is verified with the simulation results based on Matlab/Simulink results based on the VSG prototype.

Inverters controlled in virtual synchronous generator(VSG) method is able to operate in both grid-connected mode and islanding mode. Due to this characteristic, this paper put forwards a low voltage ride-through(LVRT) strategy which is based on VSG control. If the grid voltage sags, VSG can limit the output current amplitude and meanwhile inject reactive power to the grid. Once the voltage drop time exceeds the maximum ride-through time, VSG would smoothly switch to islanding mode and continues to carry loads independently. The proposed strategy achieves a control algorithm unity between normal operating and grid voltage sag status in grid-connected mode. Simulations can support the validity of the strategy.

Aiming at the problem that the power supply quality of the microgrid needs to be improved and the large impact of grid connection, a new type of virtual synchronous generator (VSG) control strategy is proposed to control the microgrid inverter. The VSG model based on stator voltage model and rotor motion model is established by deformation and vector decomposition of the hidden-pole synchronous generator second-order equation. The virtual frequency regulation module and the virtual voltage regulation module are also obtained by improving the principle of generator frequency and voltage regulation, and the grid-connected control module with voltage phase angle double feedback is also designed. By constructing relevant models, compare the performance of voltage, frequency and power before and after grid connection. The simulation results show that the VSG control strategy can better adjust load changes under the premise of less impact on the grid, and verify the effectiveness of the proposed control strategy.

Small-signal synchronization stability of grid-forming converter influenced by multi time-scale control interaction

Integrating renewable resources in microgrids (MGs) poses a substantial challenge: ensuring stability with low inertia. This paper provides control techniques for the AC frequency and the DC voltage for an isolated/islanded hybrid AC/DC MG using intelligent virtual synchronous generators (VSGs) and intelligent virtual capacitors (VCs). The suggested approaches take advantage of VSG's adaptive damping coefficient and VC's adaptive virtual resistance as defined by intelligent controls. The controllers operate independently, relying solely on local DC voltage and AC frequency measurements without additional communication infrastructure. Compared to the traditional VSG and VC, the hybrid MG with intelligent VSG and VC controls performs better in suppressing AC frequency and DC voltage deviations, regardless of operating mode (rectifier or inverter). Simulation studies in MATLAB/Simulink validate these findings, showing that the artificial neural network (ANN) approach reduces DC voltage deviation by 34.4% and AC frequency deviation by 26.3%, while the fuzzy logic control (FLC) approach further reduces these deviations, particularly DC voltage deviation by 41.9% and AC frequency deviation by 23.2%. ANN excels in reducing deviations for AC frequency, while FLC is more effective in lowering deviations for DC voltage. The proposed controllers' positive impact on system stability is assessed using small‐signal stability analysis.

Virtual synchronous generator (VSG) can simulate synchronous generator turning and damping characteristics, which can further improve the penetration rate of renewable energy in the grid. However, the output impedance of virtual synchronous generator in medium and low voltage microgrids is dominated by resistance, which leads to the coupling of active/reactive power and affects the grid integration control effect, and the phase of VSG and grid voltage will be biased in the absence of a pre-synchronization link. For this purpose, a strategy of grid-connected control of VSG with virtual impedance is proposed. Firstly, the VSG mathematical model is established and virtual impedance is introduced into the VSG electrical portion to improve the grid-connected inverter output characteristics. Secondly, a secondary frequency and voltage regulation strategy to improve VSG control is proposed to restore the frequency and voltage of the VSG, and the SOGI phase-locked loop collects the grid voltage phase to complete the grid-connected pre-synchronization design. Finally, the effectiveness and correctness of the proposed strategy were verified by the simulation results based on MATLAB/ Simulink.

The technology of Virtual Synchronous Generator (VSG), which can make the inverter connected to distributed generation work like traditional synchronous generator, plays an important role in improving the stability of the power system because of its good voltage and frequency supporting capability. In this paper, an improved VSG control method is proposed, which can ensure seamless transfer between grid-connected and stand-alone modes, and achieve accurate tracking control of active and reactive power. The control topology and mathematical model of VSG are constructed firstly based on the rotation equation and electrical equation of synchronous generator; then, the corresponding VSG active-frequency droop control, reactive-voltage droop control and inner loop voltage-current control are designed, and the influences of key parameters are analyzed through frequency domain method. Finally, two simulations with different operation modes and loads are conducted to verify the effectiveness in Matlab/Simulink environment, and the results show that the performance is well.

An integration and operation of single phase bidirectional inverter with two buck/boost maximum power point trackers (MMPTs) is provided for dc distribution system. In a dc distribution system a bidirectional inverter is required to control the power flow between dc bus and ac grid, and to regulate the dc bus to the certain range of voltage. A droop regulation mechanism is followed to reduce the capacitor size and to balance the power flow between the dc bus and ac grid. The photovoltaic (PV) array voltage can be vary from 0 to 600V, especially with thin-film PV panels, the MPPT topology is formed with buck and boost converters to operate at the dc-bus voltage around 380V, reducing the voltage stress of its followed inverter. In this paper the fuzzy logic technique is used to control the bidirectional inverter for improve overall efficiency of the system and it is designed by using MATLAB/SIMULINK software.

This paper proposes a novel model for the analysis and design of a virtual synchronous generator (VSGs). To enhance the stability in power systems with distributed generators, the concept of VSG has been proposed. The VSG is designed based on the swing equation of a synchronous generator ignoring the dynamics of current flowing through an interconnection reactor. However, responses obtained with actual system are different from those calculated from the model. It implies that the VSG cannot be correctly analyzed and designed by using the conventional model. This paper constructs an accurate VSG model considering the dynamics of current. Moreover, we address the stability analysis of the VSG and reveal a relation between the parameters of VSG controller and the rated power/energy capacity of energy storage systems equipped with the VSG based on the proposed model. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

The trend of future power systems is rapidly changing towards renewable energy resources (RESs) due to the depletion of fossil fuels. Meanwhile, the integration of RESs leads to the instability of the power system due to the lack of inertia and frequency instability. In this paper, the virtual synchronous generator (VSG) model is originated from imitating the synchronous generator characteristics. The VSG control approach is systematically researched and studied for different inverter-interfaced systems that include RESs integrated with the power grid. Parallel multi VSG based Photovoltaic (PV) systems are analyzed to validate the frequency response and power stability under high penetration of PV system. An advanced master-slave droop control approach is proposed for frequency regulation. Additionally, three case studies are analyzed under various effects of irradiance and temperature on the PV-VSG based system based on a specific area during a year. Meanwhile, an advanced particle swarm optimization (PSO) technique is utilized which effectively optimized the optimal parameters of droop control of VSG and improved the frequency regulation.

Renewable power systems dynamic security using a new coordination of frequency control strategy based on virtual synchronous generator and digital frequency protection

This article presents an improved virtual synchronous generator (VSG) control strategy of doubly-fed induction generator (DFIG) during symmetrical voltage fault. Based on VSG control, DFIG can behave like a conventional synchronous generator (SG), which can support frequency and voltage in grid-connected mode. Moreover, when microgrid disconnects from grid, DFIG can provide voltage and power for local loads. However, during symmetrical voltage fault, the transient components in DFIG cannot be controlled properly by VSG. To solve this problem, the air-gap flux feedback is designed in VSG control to accelerate the decay of transient components, and rotor current limiting method is also introduced in the article. In the proposed method, there is no need to switch the control structure or control targets. DFIG always works in the VSG control. Therefore, DFIG can operate in the standalone mode when the connection to grid is cut off. The decay time constant, the voltage control, current limiting control and parameter variation of the improved VSG control are analyzed in detail. Finally, the simulation and experimental results in both normal and fault grid condition are given to validate the effectiveness of proposed VSG control strategy.

In this paper, frequency control of the islanded mode photovoltaic (PV) / Battery storage (BS) / Diesel generator (DG) power system is achieved when system subjected to excess load and suffering fault. The frequency control combination in this hybrid system can achieve by using a virtual synchronous generator (VSG), the role of VSG is used as the interface between the AC bus and DC bus. Solar PV-Battery working with an inverter it mimics the property of conventional generator, it called VSG. If PV-Battery is not able to supply the required power to the load such condition the diesel generator help to fulfill this demand, and such a combination called PV/Battery/DG-VSG. In practically here we control the output power of the load which helps to control the stability of microgrid. Simulations based on MATLAB Simulink VSG control model developed for the proposed to improve the dynamic frequency characteristic of the microgrid

In this paper, the stability performance is analyzed in detail for an energy storage converter with the virtual synchronous generator (VSG) control strategy in weak grid. By applying the VSG strategy, the mathematical model of the grid-connected converter is established, including the active-frequency and the reactive-voltage controller. Moreover, in order to analyze the stability of the energy storage converter using VSG in weak grid, a sequence impedance model of the system is derived to achieve stability analysis by the harmonic linearization method. Meanwhile, the impact of the grid-side filter is introduced into the impedance model. Based on which, combined with the small signal model of the energy storage converter, the stability of the converter in weak grid is studied through the Nyquist stability criterion. The stability analysis indicates that the converter with VSG can still operate stably under weak grid condition. Finally, the simulation verified the feasibility and correctness of the above analysis.