Virtual Synchronous Machine Concept Research Articles

General Method to Foresee the Behavior of Virtual Synchronous Machines Working With Distorted and Unbalanced Voltage Conditions

To make photovoltaic and wind power plants able to provide grid services (i.e., inertial behavior, grid support and harmonic compensation), several control algorithms have been proposed in the literature during the last years. The most promising ones make the power electronic converters behave as conventional alternators, using the concept of Virtual Synchronous Machine (VSM). Several VSM models are available in the literature, some of which can improve the voltage quality at the point of connection with the grid behaving as harmonic and unbalance sinks under non–ideal grid voltage conditions. However, the literature lacks a general method to foresee the behavior of a generic VSM configuration in such conditions along with a well–established definition of the needed features to make VSMs able to work as harmonic or unbalance sinks. Therefore, this paper proposes a simple and general method to foresee the behavior of different VSM configurations under non–ideal grid voltage conditions before any experimental verification. The proposed method accurately foresees the VSMs behavior, as experimentally demonstrated on five VSM models available in the literature, working with fifth harmonic and inverse sequence voltage distortions. Moreover, the method identifies which VSM configuration can feature a beneficial harmonic and unbalance compensation.

Dead–Time Effect on Two–Level Grid–Forming Virtual Synchronous Machines

The concept of Virtual Synchronous Machine (VSM) is a promising solution to integrate renewable energy sources into the grid. Thanks to this approach, renewable energy sources can provide grid services (e.g., inertial behavior), grid support during faults and island operation. Moreover, VSMs can enhance the voltage quality at the point of connection to the grid by behaving as harmonic and unbalance sinks under non–ideal conditions (i.e., grid harmonic distortion and unbalance). However, the inverter dead–time affects the harmonic and unbalance sink capability of grid–forming VSMs with no closed loop current control. Although the literature contains several papers concerning the dead–time issues, the negative influence of the switching dead– time on the harmonic and unbalance sink capability of VSMs has not been analyzed. Therefore, this paper demonstrates through experiments that: (1) the inverter dead–time effect limits the harmonic and unbalance sink capability of grid–forming VSMs under non–ideal grid voltage conditions and (2) a dead–time compensation is mandatory to make grid–forming VSMs behave according to the theoretical analysis. A first experimental test shows that both the dead–time and its compensation do not influ- ence the VSM response under normal operating conditions (i.e., symmetrical and sinusoidal grid voltage). Next, two experimental tests under a 5% grid voltage unbalance and a 10% grid voltage fifth harmonic distortion validate the negative influence of the dead–time and the beneficial effect of its compensation.

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

The concept of virtual synchronous machine (VSM) was proposed to deal with the shortcomings of low inertia and damping of traditional control strategies for power electronic converters. But what if all distributed energy resources and controllable loads in a microgrid adopt the VSM control strategy, and will it present better performance than conventional droop control-based microgrid (DMG)? In this paper, the VSM-based microgrid (VSMG) is analyzed. The small-signal modeling of the VSMG is studied at first. Then static stability and dynamic characteristics of the VSMG are analyzed and compared with the DMG in both frequency-domain and time-domain. With the growing scale of microgrids, their modeling and simulation are becoming significant computational burdens. Inspired by the participation factor analysis of the VSMG and the concept of coherency in power systems, the VSMG small-signal model is equivalent to a modified third-order synchronous generator (SG) model in this paper. The equivalencing involves gray-box system identification and is realized by estimating equivalent electrical parameters alternately and iteratively. The equivalent SG (EqSG) model is compared with three representative model order reduction methods to verify its effectiveness. Simulation results confirm the accuracy of the EqSG model substituting detailed VSMG model in time-domain simulations.

Centralized VSM control of an AC meshed microgrid for ancillary services provision

This paper proposes a centralized control strategy for distributed generation (DG) sources connected to an AC meshed microgrid (MG). Through the proposed strategy, power production of distributed generators is managed such that the injected active and reactive powers at the point of common coupling are equal to predefined set-points. Thus, the MG is able to provide the main ancillary services including frequency and voltage control. An extended load flow model is used to identify active and reactive power production of each DG unit such that the total power injected to the utility grid satisfies the required setpoints. Hence, from the point of view of the transmission system operator (TSO), the whole MG can be seen as a single entity that behaves as a «PQ» generator. This control is intended to facilitate the management of power systems including a large number of microgrids and DG sources in terms of stability analysis, optimal reactive power dispatch, optimal power flow, etc. Furthermore, since the centralized control allows to set «PQ» power production to any predefined value, it was proposed to operate the whole MG using the concept of Virtual Synchronous Machine (VSM). It has been shown that, through the proposed control, the MG can reproduce the behavior of conventional synchronous generators during normal conditions and during faults.

Improved VSM control of PMSG-based wind farms for transient stability enhancement

This paper analyzes the optimal control strategy of PMSG-based wind farms during faults in order to improve the transient stability of a power system with a high penetration rate of wind power. The investigated control strategies are the unity power factor (UPF), reactive power control (RPC), and particularly the concept of virtual synchronous machine (VSM). The transient stability level is assessed using the critical clearing time index, which was calculated based on trajectory sensitivity analysis. In light of the obtained results, it was found that RPC gives the best transient stability level, followed by the classical VSM control. Hence, an improved VSM control was proposed based on the transient model of a synchronous generator. The improved VSM has shown a behavior similar to a real synchronous generator, which results in better transient stability performances.

Phasor Control of Converter Output Voltage for Frequency Regulation

The synchronizing torque of a power system may be weakened by increasing installation of static power converters accompanied by renewable energy resources because they used to trade their favorable active power by synchronizing their output voltage with the one at the point of common coupling. In the circumstances, a concept of Virtual Synchronous Machine (VSM) is proposed, where the self-commutated power converters are emulating synchronous generators. This paper describes a converter control to contribute to enhancing the synchronizing torque. The proposed control is similar to the VSM but it simply realizes active power trades among power generation units including converter-based generators by modulating phase angles of their output voltages. Therefore, it can provide an effective support to regulate the system frequency where the total rated power of the converter-based generators increases as much as the one of conventional rotating generators like a microgrid. This paper especially focuses on its robustness where the number of converter-based generators is increased or they are dispersed in the power network. The effectiveness is verified by simulation study based on instantaneous values.

系統連系自励式変換器の運転位相制御方式の検討

As increasing installation of renewable energies which are accompanied by self-commutated power converters, installed capacity of the converters cannot be negligible as compared to the total generation capacity. Those self-commutated converters trade active power with adjusting their output voltage phase angle according to the voltage at the point of commoncoupling (pcc). Therefore, increasing installation of the self-commutated power converters may result in reducing a synchronizing torque of the power system. The tendency is remarkable in a microgrid where the rate of converter-based generation is high. The synchronizing torque is supplied by traditional synchronous generators inherently because, at some disturbance, their inertias make phase differences among their terminal voltages and they can trade active power by the phase differences. In order to keep the synchronizing torque under high penetration of renewables, a concept of Virtual Synchronous Machine was proposed. The VSM emulates a conventional synchronous generator and realizes the power trade among them. This paper deals with a controller of self-commutated converter like the VSM. Instead of applying the same characteristics of the conventional synchronous generator, the phase angle of the output voltage is adjusted as lead or lagged to the pcc voltage in order to trade the active power. The effectiveness of the control is verified in a microgrid system model where the rotating generator and self-commutated converter have almost same rated capacity. The effectiveness of the controller is verified from the viewpoint of frequency control through some simulation studies.