Virtual Synchronous Generator Strategy Research Articles

A novel strategy to enhance power management in AC/DC hybrid microgrid using virtual synchronous generator based interlinking converters integrated with energy storage system

Hybrid microgrids (HMGs) have a flexible structure, higher reliability, and both AC and DC MG merits. However, in this type of MGs, there are challenges such as accurate power sharing, the voltage control of the DC link, the AC side voltage and frequency stability, the negative impact of telecommunication delay, and so on. Therefore, a power management scheme is presented here that can perform proper and precise power sharing in a hybrid AC/DC MG. This HMG consists of one energy storage system (ESS) along with two interlinking converters (ILC) based on a virtual synchronous generator (VSG). Besides, the physical inertia of a MG is less than that of a power grid, hence, changing the output power of wind turbines and photovoltaic (PV) arrays might make system unstable. To overcome this challenge, virtual inertia is used. As the virtual inertia can be embedded with the VSG method, in this scheme, the VSG strategy is implemented to enhance the system dynamic behavior by imitating the synchronous generator kinetic inertia. The VSG-based ILC1 converter is used to establish interlinking and energy exchange between AC and DC sub-grids and share power. Also, other interlink devices including a bidirectional half-wave DC/DC converter integrated with the ILC2 are considered, which aim to control the voltage of the DC link and exchange part of the power between the AC and DC sub-grids. An ESS is placed between the bidirectional DC/DC converters and the DC link of the ILC2. Such a structure is referred to as a two stage interlinking converter with an ESS (TSILC-ESS). The power management system employed in AC/DC HMG according to the ILC controller is utilized to form the DCMG along with controlling the TSILC-ESS, providing additional and auxiliary services to the power grid. To assess the introduced scheme of both AC and DCMG to investigate VSG-based ILC1 and ILC2 in grid-connected and islanded operational mods, various resource and load profiles are assumed. The simulation study is performed in MATLAB/ Simulink environment to confirm the proposed method.

Improved VSG strategy of grid-forming inverters for supporting inertia and damping

A virtual synchronous generator (VSG) strategy can introduce the rotational inertia and damping characteristics of the synchronous generator to the static inverter, e.g., PV, wind generation, and ESS, which are used to enhance the system frequency support characteristics of the micro-grid. However, under various operations of the VSG, the inertia and damping support capabilities are different, and the conventional VSG strategy with a fixed control coefficient sacrifices a certain degree of dynamic regulation performance with less robustness. This research proposes an improved VSG strategy with adaptive inertia and damping coefficients to increase the flexibility of VSG. To this end, a mathematical model is first established to analyze the impact of various parameters on the characteristics of VSG, and then the root trajectory is used to explore the impacts of various rotational inertia and damping coefficients on the stability of the VSG system. Second, an improved control strategy with adaptive inertia and damping coefficients is proposed based on the characteristics of the second-order system and the system frequency deviations. Finally, a simulation system with the VSG is constructed based on MATLAB/Simulink. The effectiveness of the proposed control strategy is verified by comparing the simulation results to the conventional control strategy with the fixed control coefficients under over-frequency and under-frequency disturbances.

Improved artificial jellyfish search algorithm: virtual synchronous generator control strategy

This study aims to solve the problem of optimal performance parameter selection of the virtual synchronous generator (VSG) control strategy using an improved artificial jellyfish search (IMJS) algorithm. VSG technology helps the inverter to improve the anti-disturbance ability. The VSG control effect depends on the setting of its performance parameter values. The population initialization strategy and dynamic adaptive factor are used to improve the optimization ability of the artificial jellyfish search algorithm. The IMJS algorithm is used to optimize the VSG, and a control strategy for the VSG based on the IMJS (IMJS-VSG) is proposed to improve the control capability. The results show that the active power achieves smooth changes without oscillations under the IMJS-VSG control strategy when the system is running in grid-connected operation. The system voltage drop under the IMJS-VSG strategy is less than 40% of the VSG strategy when the system is in an islanded operation state.

Improvement of Frequency Support for a DFIG Using a Virtual Synchronous Generator Strategy at Large Power Angles

The frequency regulation rate and operation stability of a doubly fed induction generator (DFIG) based on a virtual synchronous generator (VSG) strategy decreases under large-power-angle conditions, which reduces the grid frequency support capacity. This paper proposes the compound adaptive parameter (CAP) and coordinated primary frequency regulation (CPFR) strategies to improve the grid frequency support capacity in terms of multiple dimensions of the transient properties, operation condition range, and regulation duration. Mathematical and small signal models of the DFIG-VSG system are constructed. The effect of large-power-angle conditions on the transient properties under grid frequency perturbations is analyzed based on these models, and the CAP strategy for excitation control and virtual damping is formulated. The constraints of the rotor kinetic energy and the load increase capacity of the grid-side converter are analyzed, and the CPFR strategy is formulated based on this. Finally, the effectiveness of the proposed strategies is verified via simulations of single-machine and wind farm scenarios under grid frequency perturbation.

Coordination of adaptive virtual resistance controller and flux-coupling-type fault current limiter for LVRT fulfillment of virtual synchronous generator

The virtual synchronous generator (VSG) technology equips power inverters with virtual inertia and damping to access generation and storage more friendly. However, the VSG's weak performance to withstand fault current inrush and fulfill low-voltage ride-through (LVRT) operation should be carefully addressed. This paper presents an innovative method to coordinate adaptive virtual resistance controller (AVRC) and flux-coupling-type fault current limiter (FC-FCL). First, the theoretical modeling and transient characteristics of the VSG are illustrated. Then, the implementation of the coordinated AVRC and FC-FCL is done, with an elaboration of the coordination philosophy and the proposed approach's parameter design rules. Further, simulation validations are performed in MATLAB/Simulink. Different severities and types of faults are imitated, and the comparisons of the proposed method against the traditional VSG strategy, the single AVRC or FC-FCL, the inner current control, and the virtual voltage control are conducted. The results prove that: i) the proposed approach offers superior abilities to repress the fault current, alleviate the terminal voltage fluctuation and keep the VSG power balance. ii) the proposed approach can handle different severities and types of faults in the VSG. A satisfactory LVRT is obtained for the VSG with favorable frequency stability and power angle stability.

Virtual synchronous generator of PV generation without energy storage for frequency support in autonomous microgrid

In autonomous microgrids frequency regulation (FR) is a critical issue, especially with a high level of penetration of the photovoltaic (PV) generation. In this study, a novel virtual synchronous generator (VSG) control for PV generation was introduced to provide frequency support without energy storage. PV generation reserve a part of the active power in accordance with the pre-defined power versus voltage curve. Based on the similarities of the synchronous generator power-angle characteristic curve and the PV array characteristic curve, PV voltage Vpv can be analogized to the power angle δ. An emulated governor (droop control) and the swing equation control is designed and applied to the DC-DC converter. PV voltage deviation is subsequently generated and the pre-defined power versus voltage curve is modified to provide the primary frequency and inertia support. A simulation model of an autonomous microgrid with PV, storage, and diesel generator was built. The feasibility and effectiveness of the proposed VSG strategy are examined under different operating conditions.

Impedance Modeling and Stability Analysis of VSG Controlled Grid-Connected Converters with Cascaded Inner Control Loop

This paper develops the impedance models of grid-connected converters under the virtual synchronous generator (VSG) strategy with a cascaded inner control loop and analyzes the system stability of VSG controlled converters with different kinds of weak grid. Different from existing small-signal models with high dimensions, a single-in-single-out (SISO) impedance model with simple mathematical expression is obtained in this paper, which is applied to identify the influence of the cascaded control loop on impedance characteristics and system stability. It is found that the impedance characteristics of VSG controlled converters can become capacitive below the fundamental frequency, and it is mainly caused by the voltage controller in the cascaded control loop of the VSG strategy. Impedance-based stability analysis shows that the capacitive impedance characteristics can benefit the compatibility of converters operated with the series-compensated weak grid, but may deteriorate the system stability with the inductive weak grid, which can be avoided by increasing the proportional coefficients of the cascaded voltage and current controllers or applying a larger virtual resistor to reduce the negative resistance in the capacitive frequency range. Experiments based on the control-hardware-in-loop (CHIL) platform were carried out to verify the developed analytical models and possible system instable cases.

An improved control strategy of virtual synchronous generator under symmetrical grid voltage sag

The virtual synchronous generator (VSG) which mimics the synchronous generator enables the operation of inverter-interfaced distributed generations (DGs) more effectively. However, the insufficiency in the low-voltage ride-through (LVRT) capability of conventional VSG limits its extensive application during the voltage sag condition. In this paper, firstly the performance of conventional VSG under the condition of symmetrical voltage sag is elaborated, and then based on the reference dynamic compensation an improved VSG control strategy is proposed, where the new voltage reference generation algorithm is designated to accommodate to different degrees of voltage sag, avoiding controller switching. Then, a fault current limitation scheme is introduced to guarantee the transient peak current of VSG within an acceptable range, especially during serious system disturbances. When the grid voltage restores to normal, an additional phase angle control loop is implemented to facilitate the smooth and rapid transfer of VSG strategy while the voltage reference is switched back to the rated value. Furthermore, the details of design for the VSG algorithm and the phase angle controller are provided through the formulation of small-signal dynamic model, improving the overall system performance during grid voltage sag. Finally, the validity and effectiveness of the proposed strategy are verified by the hardware-in-the-loop (HIL) simulation results.

Modified virtual inertia control method of VSG strategy with improved transient response and power‐supporting capability

Virtual synchronous generator (VSG) controlled grid-tied converters could increase power overshoot and oscillation if real power reference disturbance (PRD) occurs. Existing VSG methods can mitigate the power oscillations, but they may reduce the supporting capability of grid contingency. To cope with the issue, a modified virtual inertia control method of VSG strategy is proposed. The proposed modified VSG method introduces two frequencies of point of common coupling into the virtual inertia control. The estimated frequency is used to mitigate the power oscillations and the measured frequency aims to provide extra power supporting. The formulated parameters design principles are given according to the transient characteristics. Stability analysis of power and frequency response is carried out to evaluate the proposed method. Besides, the transient power and frequency response of the PRD case and grid-frequency disturbance (GFD) case are studied. The power overshoot of the proposed modified VSG method decreases by 52.5% under the PRD case and the power support peak increases by 34.3% under the GFD case. Experimental results show that the transient performance of the proposed modified VSG controlled grid-tied converters is improved.

Equivalent Modeling and Comprehensive Evaluation of Inertia Emulation Control Strategy for DFIG Wind Turbine Generator

With the increasing penetration of renewable energy in power grid, many inertia emulation control strategies have been proposed to provide extra support to the power grid in system disturbances. To explain the physical resemblance and relations among all kinds of inertia emulation strategies, a proper double-fed induction generator (DFIG) model is proposed based on equivalent electromotive force $\text{E}_{\boldsymbol{r}}^{\mathrm {eq}}$ and equivalent power angle $\delta _{\mathrm {eq}}$ . A co-simulation research platform is also designed based on a real-time digital simulator (RTDS) and GH Bladed simulation software, and the detailed comparisons are made in terms of inertia response performance, weak grid operation stability, secondary frequency drop, and wind tower fatigue load. Research results point out that different control schemes are similar in inertia response while virtual synchronous generator (VSG) control and inertial synchronization control (ISynC) can improve weak grid operation stability, the VSG strategy is also effective in reducing frequency secondary drop and wind tower fatigue load, and the combination of the VSG for RSC and the ISynC for GSC can achieve the best control performance in all evaluation indexes.

Multi‐objective self‐synchronised virtual synchronous generator in unbalanced power grid

This Letter proposes a multi-objective self-synchronised virtual synchronous generator (VSG) strategy in unbalanced power grid. A second-order generalised integrator is applied to the traditional VSG strategy to deal with the double-frequency ripple components. Therefore, the precision of the self-synchronisation process can be confirmed, and the VSG can follow the power references of both DC and double-frequency ripple components. On this basis, the power compensation strategy is combined with the VSG strategy to achieve different control objectives. Comparative experimental studies of the traditional VSG and proposed VSG with different targets are conducted to verify the effectiveness of the proposed strategy.

An improved method of TEF considering the saturation of inverter under VSG strategy

In order to reduce influence of the total inertia of the system on the system stability, the virtual synchronous generator(VSG) technology is becoming a research hotspot for the penetration of distributed power supply with high permeability. Due to the existence of the current limiter in the inverter, the transient energy function which is constructed by first integral (TEF) derivative is not monotonous. It may make using TEF method to obtain the critical clearing time and system stability error. This paper proposes an improved TEF method using the tangent function to replace the saturation one. Firstly, the relationship between the virtual power angle and the power angle of the traditional system is analyzed after the improvement of the inverter current saturation link. Then, the first integral method is used to construct the TEF with improved saturation. Finally, it simulates to a single machine infinite bus system and IEEE three-machine-nine-bus system by MATLAB/Simulink, and the results show that the improved TEF is proposed, it can effectively evaluate the VSG strategy and the transient stability of AC system inverter current limiting.

A Virtual Synchronous Generator Control Strategy for VSC-MTDC Systems

With the rapid increase of renewable energy sources and power electronic converters in the power system, the inertia of the power system is declined and the stability problem is more serious than ever before. In this paper, a virtual synchronous generator (VSG) control strategy is proposed to enable the voltage-source converter-based multiterminal high-voltage direct current (VSC-MTDC) system to damp the low-frequency oscillation modes. Both rotational inertia part and governor part are modeled in the VSG controller. The $V^2-P-\omega$ characteristic is developed based on the $V^2-P$ droop control to emulate the action of the speed governor when the frequency of ac grid deviates from the nominal value. The virtual inertia part is used to emulate the inertia response of the synchronous generator. The communication between VSC stations is unnecessary with the autonomous power allocation ability of the $V^2-P-\omega$ characteristic. Modal analysis is utilized to test the system stability and optimize the controller parameters. Two test systems which include offshore wind farms, VSC-MTDC systems, and onshore power grids are built in the DIgSIENT/PowerFactory. The simulation results verify the effectiveness of the proposed VSG strategy.

Virtual Synchronous Generator Strategy for VSC-MTDC and the Probabilistic Small Signal Stability Analysis

In this paper, a new virtual synchronous generator (VSG) strategy based on the V2-P droop control strategy is proposed for the voltage source converter based multi-terminal high voltage direct current (VSC-MTDC) system. The VSG strategy enables the inverter stations to damp the power oscillation and provide frequency support for the AC system. Virtual inertia coefficient is obtained to couple the DC voltage and the angular frequency. To provide the oscillation damping and frequency support, the inverter stations could adjust the output power by the adjustment of the DC reference voltage. Due to the characteristic of the V2-P droop method, there is no requirement for the communication between inverter stations. The Monte Carlo method is used to test the probabilistic small signal stability of the VSG strategy. The performance of the proposed controller is demonstrated by a hybrid AC/DC system which is modeled in DIgSILENT/PowerFactory.