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.

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.

Gas engine generators have been recognized as a set of beneficial generating units for producing electric power owing to their high efficiency, low emissions and appropriate fuel price. However, the major drawback of gas engine generators is the weak transient performance during sudden load change. The reason is that the engine’s mechanical system cannot quickly respond to the demanded torque. Therefore, a change in load causes a large engine speed deviation, especially in stand-alone mode. If the engine speed deviation exceeds the allowable limits, the system operation will be shut down. To fix this drawback, this paper introduces the virtual synchronous generator control method combined with an additional energy storage system to the stand-alone gas engine generator. In this paper, the applied virtual synchronous generator control scheme is presented. Compared to the constant-voltage constant-frequency control method, the benefit of virtual synchronous generator control method for the gas engine generator is evaluated by simulation study in PSCAD software. The energy-variables-based feedback control structure is adopted for the control of energy storage system. The required capacity of energy storage system is estimated. To verify the effectiveness of proposed control structure, simulation is conducted under load connection and load removal cases. Finally, an experimental study is carried out for the same configuration of simulation model using a real gas-engine generator set up.

This paper proposes a new control scheme for the low frequency AC transmission (LFAC) system aiming at extending the point-to-point configuration to form a multi-terminal electrical energy network. The multi-terminal low frequency ac (MT-LFAC) system configuration is based on the use of modular multilevel matrix converters (M3Cs) and virtual synchronous generator (VSG) control. The M3C is the next ac/ac converter generation, which is used as an interface with the conventional AC network and the LFAC electrical energy system. Application of VSG control is proposed to enable proper power sharing, to provide synchronization of each terminal, and frequency stabilization, thus, to offer multiterminal forming capability. Two different operation modes are applied in the system to damp the frequency deviation after a dynamic perturbation, which provides additional stabilization feature to the VSG. Frequency restoration mode and commanded mode of power sharing are applied as dynamic states to validate the robustness of the VSG control system. Besides, to solve the negative impact of low X/R ratio in the LFAC electrical energy system, we enhance the VSG control by proposing a virtual-impedance-based solution, which increases the output total impedance on the low frequency side and prevents the coupling between P and Q. The operation of the proposed system is examined by simulation results with a precise model of M3Cs in the PSCAD/ EMTDC software environment (version 4.2.1, Winnipeg, MB, Canada).

Control, design and operational characteristics of multilevel modular matrix converter (MMxC), which conducts ac/ac direct frequency conversion, are presented in this paper. MMxC consists of a large number of H-bridge cells connected in series and thus is suitable for high voltage and high power applications. After brief explanation about power flow and capacitor voltage balancing control schemes, the relationship between sampling frequency of the control system and switching frequency of switching devices is investigated through numerical simulation of a 9-level MMxC. In addition, the design of the cell capacitor is also described. Moreover, experimental results of 50 Hz / 60 Hz and 60 Hz / 60 Hz conversion operations using the 2-kW 5-level MMxC are presented.