Switching performance optimization for a hybrid AC/DC microgrid using an improved VSG control strategy

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.

For a VSC-HVDC transmission system based on a hybrid topology converter of full-bridge and half-bridge, a kind of virtual synchronous generator (VSG) control strategy which can be applied to modular multilevel converter (MMC) grid-connected structure was researched and proposed. First, based on the conventional VSG control strategy, the energy stored in the equivalent capacitor of MMC power module was used to imitate the rotor inertia of synchronous generator. The characteristics of generator can be simulated during transient frequency fluctuations and it can help relieve the power fluctuations. Secondly, with respect to the structural characteristics of the direct grid connection through the reactor on the AC side of the MMC, which is unlike the microgrid inverter, there are no additional filter capacitors. So, the existing commonly used VSG control strategy of the microgrid inverter and double-closed-loop structure composed of filter capacitor voltage and AC current cannot be directly applied. For this, a method where the given values of inner current loop are calculated by grid impedance matrix was used. So, a double-closed-loop control structure composed of a power outer loop based on VSG control and a current inner loop is obtained. It can effectively improve the current control capability during the transient process. Finally, a hybrid MMC simulation model was built based on PSCAD to verify the correctness and effectiveness of the proposed methods.

The construction of multiple microgrid clusters (MMGCs) contributes to incorporating increasing distributed energy generations flexibly, supplying local power loads conveniently, and decreasing operating losses appreciatively. To promote the progress of the MMGCs further, research on the transient stability of the MMGCs based on the virtual synchronous generator (VSG) control strategy is done in this paper. Firstly, through introducing the P-ω and Q-V control of the synchronous generator (SG), the theoretical modeling of the VSG control strategy is conducted, which is implemented on the MMGCs. Then, using the Lyapunov direct method, the shrinking admittance matrix model and the rotor motion equation of the MMGCs are established. Therewith, the transient energy function of the MMGCs in the center of the inertia frame is derived. In light of the MATLAB environment, a detailed electromagnetic transient model of the MMGCs with three MGs based on the VSG control is created, and the simulations are carried out to check the suitability of the presented energy function model and the stability analysis method. In the simulations, the effects of the fault clearance time, size of inertia, and voltage droop coefficient on the power angle stability of the MMGCs are quantitatively assessed. The theoretical analysis is well following the simulation findings to determine the efficacy of the proposed approach.

The power storage converter(PSC) using the virtual synchronous generator(VSG) control strategy has the dual droop characteristics of active-frequency, reactive-amplitude. The VSG control strategy of the improved inner loop proposed in this paper can make the grid-connected energy storage converter adopting the control strategy have fast power response characteristics. The power level caused by the frequency step, the amplitude step or the instruction order is completed within 20ms. The voltage loop of the improved inner loop adopts the control mode of dq decoupling, and the current loop adopts the control method of deadbeat. This paper will theoretically derive the control strategy and verify the effectiveness of the control strategy through PSCAD simulation.

In recent years, countries around the world have been vigorously developing new energy and electrifying new energy sources. As a result an increasing number of power electronic devices appear in the grid which makes the power grid develop toward low inertia and low damping. The Virtual Synchronous Generator (VSG) control strategy can simulate the working state of the synchronous generator and provide the grid with inertia and damping, which has become a research hotspot for scholars. Due to the unbalanced load distribution and the random change of power load, the grid voltage may become unbalanced during the actual operation process. The three-phase output current of the VSG would be unbalanced and a fluctuation of output power appears. To solve this problem, a new current and power quality control strategy based on VSG is proposed in this paper, in which the current balancing or constant power is achieved without changing the basic characteristics of the inverters using VSG control 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.

Although the wave power generation system has made gratifying progress, the poor stability of the output of the power generation device is one of the main factors restricting its easy realization of large-scale commercial operation. A single wave power generation system is usually an independent power generation system or directly connected to the offshore power distribution system, the current operation of the wave power is mostly such devices. In this paper, we propose a wave power generator with virtual synchronous generator (VSG) control strategy on the inverter side. The simulation of the oscillating float type wave energy generating device is carried out and the results indicate the control strategy can achieve the effect of smoothing the power generation frequency to suppressing power fluctuation. Finally, combined with the virtual synchronous machine adaptive mode simulation, to prove its use value in the complex conditions of the wave power generation system.

The increasing penetration of renewable energy, such as photovoltaic generation, makes it essential to enhance power system dynamic performance through improved grid-forming control strategies. In the grid-forming control system, the virtual synchronous generator control (VSG) is currently widely used. However, the inertia (J) and damping (D) in the traditional VSG are fixed values, which can result in large overshoots and long adjustment times when dealing with disturbances such as load switching. To address these issues, this paper proposes an adaptive virtual synchronous generator (VSG) control strategy for grid-side inverters, which is accomplished by adaptively adjusting the VSG’s inertia and damping. Firstly, we established a photovoltaic-storage VSG grid-forming system; here, the photovoltaic power is boosted through a DC-DC converter, and the energy storage is connected to the common DC bus through a bidirectional DC-DC converter. We analyzed how J and D shape the system’s output characteristics. Based on the power-angle characteristic curve, the tanh function was introduced to design the control function, and a JD collaborative adaptive control (ACL) strategy was proposed. Finally, simulation experiments were conducted under common working conditions, such as load switching and grid-side voltage disturbance, to verify the results. From the results shown, the proposed strategy can effectively improve the response speed of the system, suppress system overshoot and oscillation, and, to a certain extent, improve the dynamic performance of the system.

To reduce the impact of the imbalance of mixed non-linear loads on an inverter voltage output in the microgrid, we improve the disadvantage of the lack of damping and inertia for traditional droop control. This paper proposes a comprehensive virtual synchronous generator (VSG) control strategy for harmonic suppression and imbalance suppression of a multi-inverter parallel microgrid. On one hand, an improved VSG control strategy is proposed to increase the damping and inertia of distributed generations (DGs) in the microgrid, and secondary control is introduced to improve system stability. On the other hand, the frequency division suppression control strategy is used to eliminate the influence of harmonics, and the negative sequence component is compensated to eliminate the influence of imbalance. Then small-signal analysis is used for analysis of the stability of the strategy. Finally, we verify the comprehensive control strategy proposed in this paper through experiments. The experimental results suggest a significant improvement on the voltage, frequency, power optimization, handling of non-linear load and capacity distribution precision, as well as providing inertia support for the system.

Virtual synchronous generator (VSG) control is a promising communication-less control method in a microgrid for its inertia support feature. However, active power oscillation and improper transient active power sharing are observed when basic VSG control is applied. Moreover, the problem of reactive power sharing error, inherited from conventional droop control, should also be addressed to obtain desirable stable state performance. In this paper, an enhanced VSG control is proposed, with which oscillation damping and proper transient active power sharing are achieved by adjusting the virtual stator reactance based on state-space analyses. Furthermore, communication-less accurate reactive power sharing is achieved based on inversed voltage droop control feature ( V–Q droop control) and common ac bus voltage estimation. Simulation and experimental results verify the improvement introduced by the proposed enhanced VSG control strategy.

Low frequency oscillation (LFO) is dangerous for power systems secure operation. Over the past decades, installations of power system stabilizer (PSS) on synchronous generator (SG) exciter or flexible AC transmission systems (FACTS) are widely-used methods in electrical power industry to suppress power system oscillation and improve the system oscillation stability. But with the rapid increase of renewable energy sources, more and more power electronic converters with seriously different dynamic characteristics from that of SGs’ are connected and operated in the power system. Thus, the future power system is very likely to generate more complex oscillation phenomenon and system stability problems are maybe more serious than ever before. Under these circumstances, adding supplementary control on these existing power electronic converters is becoming an attractive choice. In this paper, inspired by working principle of the PSS, an improved virtual synchronous generator (VSG) control strategy is presented to damp LFO actively via adaptively adjusting its parameter values of virtual inertia and damping coefficient, according to deviation value and changing rate of its output active power. This idea takes full advantage of flexible adjustability of VSG’s parameters. The effectiveness of the proposed VSG strategy is proved by simulation results, which indicates remarkable performance in fast damping of LFO.

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.

With the continuous development of wind power capacity, a large number of wind turbines connected by power electronic devices make the system inertia lower, which leads to the problem of system frequency stability degradation. The virtual synchronous generator (VSG) control can make wind turbines possess inertia and damping. However, the stochastic dynamic behavior of wind generation results in the stochastic changing of operating condition; this paper presents an adaptive subsynchronous oscillation (SSO) damping control method for the wind generation with VSG control. Firstly, the small signal model of the permanent magnet synchronous generator (PMSG) with VSG is built, and the model of state space is derived and built. The active power of PMSG is selected as the variable parameter vector to establish a polytopic linear variable parameter system model. Then, based on the hybrid H2/H∞ control method, each vertex state feedback matrix is solved by linear matrix inequality, and a subsynchronous oscillation adaptive damping controller with polytope is obtained. Finally, the 4-machine 2-area system connected to two PMSGs with VSG control is used as the test system for time domain simulation. The simulation results demonstrate that the LPV based adaptive damping controller could provide enough damping under the circumstances of wider changes of wind power outputs.

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