In recent researches on inverter-based distributed generators, disadvantages of traditional grid-connected current control, such as no grid-forming ability and lack of inertia, have been pointed out. As a result, novel control methods like droop control and virtual synchronous generator (VSG) have been proposed. In both methods, droop characteristics are used to control active and reactive power, and the only difference between them is that VSG has virtual inertia with the emulation of swing equation, whereas droop control has no inertia. In this paper, dynamic characteristics of both control methods are studied, in both stand-alone mode and synchronous-generator-connected mode, to understand the differences caused by swing equation. Small-signal models are built to compare transient responses of frequency during a small loading transition, and state-space models are built to analyze oscillation of output active power. Effects of delays in both controls are also studied, and an inertial droop control method is proposed based on the comparison. The results are verified by simulations and experiments. It is suggested that VSG control and proposed inertial droop control inherits the advantages of droop control, and in addition, provides 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.

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.

Virtual synchronous generator (VSG) control is a promising control method for grid-tied inverters, as it can provide inertia support for the grid. However, VSG-controlled grid-tied inverters are subjected to intrinsic low-frequency oscillation, because the emulation of swing equation of a conventional synchronous generator (SG) also introduces an oscillatory mode. In this paper, a damping term produced from state feedback control is added to VSG control to solve this issue. This method is then further developed by applying a low pass filter to the measured active power, to improve its ripple attenuation ability, at the cost of increased design complexity. Both the proposed state feedback damping method without and with low pass filter have one more design degree of freedom than previous methods, which is used to improve dynamic response to active power reference change. The small-signal analyses and experimental results show that both methods can well damp the intrinsic low-frequency oscillation with better dynamic responses than previous fixed-parameter damping methods, whereas the method with a low pass filter has the best overall performance thanks to its improved ripple attenuation ability.

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.