To provide inertia support for the grid, virtual synchronous generator (VSG) control of inverter-interfaced distributed generators (IIDGs) becomes a focus of worldwide attention. However, a VSG-based IIDG behaves differently in the grid-connected mode, the islanded-single-DG mode, and the islanded-multi-DG mode, whereas the mathematical and physical interpretations of this phenomenon are not well studied. In this paper, we propose a unified modeling method of VSG-based IIDG to analyze its different dynamic performance in each operation mode. The proposed unified formulas can obtain the state-space models of islanded-single-DG mode and islanded-multi-DG mode from that of grid-connected mode for any VSG control method. With the obtained models, for several different types of VSG control in different operation modes, we analyze the distribution and sensitivity of the closed-loop poles and investigate the step responses both analytically and experimentally. These analyses reveal the intrinsic differences and correlations of the dynamics of VSG-based IIDG between each operation mode. These intrinsic features are valid independent from the applied VSG control scheme, thus a test method to evaluate the parameters and performance of an unknown IIDG is derived. The findings of this paper provide important instructions for engineers to model, design and test multi-operation-mode distributed generators.

Distributed generations (DG) are expected to be penetrated widely and largely in the future power grid. Virtual synchronous generator (VSG) control can play a vital role to support the frequency stability of such power grid thanks to its virtual inertia feature. However, power quality and current control are still challenging aspects of the VSG-based DGs operation under transient disturbances such as voltage sag. In this paper, a novel finite-set model predictive control (FS-MPC) based method for VSG was studied. The proposed scheme allows multiple inputs multiple outputs (MIMO) system to control voltage and current simultaneously. Under these constantly placed voltage and current constraints, the controller displayed the ability to prevent the system from overcurrent condition and to ride through fault, whereas the injection of non-sinusoidal current under conditions of unbalanced voltage sag was suppressed. Several simulation and experimental studies were conducted, which validate the superior current-limiting ability and FRT capability of the proposed strategy in comparison with existing methods.

Power quality of inverter-based microgrids is a challenging issue due to nonlinearities of inverters, multiple resonance modes of network impedance and unbalanced and nonlinear loading condition. The ideal solution is to assure the best power quality at the common bus to which loads are connected, and share the negative and harmonic sequence currents between the inverters without any communication. However, it is difficult to achieve this objective with existing virtual-impedance-based methods when nonlinearities of inverters, e.g. dead time, are not negligible. In this paper, a novel cost-function-based method is proposed to solve this problem. The proposed cost functions are optimized using continuous-control-set model predictive control (CCS-MPC). As unknown nonlinearities of inverters can be observed by introducing disturbance models, even when they are not negligible, the presented method can compensate bus voltage unbalance and harmonics and share the compensated current between inverters autonomously, with neither communication nor upper-level controller. Parameter tuning of the proposed method is the key to achieve the balance between bus voltage quality and current sharing, and between stability and fast response. Experimental results demonstrate that the proposed control scheme is effective even under conditions of large inverter nonlinearities.

The virtual synchronous generator (VSG) control, which enables inverter-interfaced distributed generators (DGs) to possess inertia like synchronous generators (SGs), provides a promising solution to the lack of inertia of a future power grid. The energy buffer emulating the kinetic energy variation is essential to this technique. In this study, we propose a new control scheme for a smart motor, i.e., a back-to-back motor drive system which temporarily utilizes the kinetic energy stored in rotating loads. With this system, significant frequency regulation support can be achieved through merely a control mechanism update, and without any dedicated energy storage system. To achieve it, the VSG control is applied to the grid side converter of the back-to-back motor drive system, and a coordinated control between the VSG control and the motor speed control is proposed. The proposed coordinated control has a faster frequency support response and a lower sensitivity to grid voltage unbalance and distortion than previous control schemes based on frequency measurement. The parameter tuning of the proposed control is discussed based on stability analyses and verified by simulation studies. The effectiveness of suppression of grid frequency fluctuation by the proposed system is verified through experimental results obtained with a commercial blower.

This paper presents a design-oriented modeling approach to analyze the Q-V control loop of grid-forming distributed generators (DGs). Unlike previous modeling methods, the proposed approach covers all typical operation modes of grid-forming DGs. No matter which type of inner control scheme is applied, once its state-space model in grid-connected mode is established, the models in islanded single-DG mode and islanded multi-DG mode can be obtained through the proposed formulas. With this novel framework, we analyze and compare two major Q-V control schemes of grid-forming DGs, i.e. the voltage/current double loop-based control, and the reactive power loop-based control, through respective reduced-order models. Closed-loop pole analyses of both control schemes in all three operation modes are investigated to highlight the different features between control schemes and operation modes. These analyses are verified by step-response analyses, which well match corresponding experimental results. The findings of this paper help us to better understand the Q-V control loop of grid-forming DGs, e.g., the different features and relations in each operation mode, the pros/cons of the two major control schemes, and the design concerns of inner-loop parameters considering all operation modes.

To analyze the grid integration effect of a virtual synchronous generator (VSG), this paper proposes a simple power system modeling method. It is shown that for a specific disturbance, by simplifying the original network to a radical n generators single-load model, its state-space small-signal model can be obtained using a unified formula, as long as the single generator infinite bus model of each generator is given. Case studies based on the IEEE 9-bus test system as well as the New York New England test system show high efficiency of the proposed modeling to represent slow power and frequency deviations. Modal analyses give insight into how system poles are synthesized by individual generator poles, and how different designs of VSGs affect the system oscillation modes. Potential errors due to the topology simplification and the measures to avoid them are discussed. The time-domain results and modal analyses highlight some important features of VSG, which are overlooked in previous studies.