Modern energy systems are experiencing the transition towards renewable-powered ones. Some conventional thermal units based on synchronous machines are gradually decommissioned and replaced by power electronics interfaced renewables. Thus, the lack of natural inertia and governor damping, which are the features of synchronous machines, raises significant concern about system frequency stability, including the faster rate of change and lower nadir point of frequency. Meanwhile, with the rapid development of communication and Internet of Things technologies, distributed energy resources can be aggregated as a virtual power plant to help balance real-time electricity demand and supply. However, the capability of utilizing the whole virtual power plant to provide adjustable inertia support has not been explored yet. In this paper, we propose a framework of the synchronous virtual power plant based on grid-forming inverter interfaced distributed energy resources. By coordinating the parameter settings of grid-forming inverters, the virtual power plant provides inertia support. Also, we design an online learning-based parameter settings method that makes the inertia of the virtual power plant adjustable. A case study in IEEE 34 nodes system illustrates the effectiveness of the proposed method.

Transforming conventional PV energy system from a grid following one to a grid forming is necessary when PV power generation is dominating the generation mix and replacing traditional synchronous generators. Grid forming PV energy system can provide frequency support functionality which is vital for the stability of the power grid. This paper presents a novel AC coupled solution that transforms an existing grid following PV system to a grid forming one without any hardware and software modification of the PV inverter. The resulting system, the Photovoltaic Synchronous Generator (PVSG), is achieved by an AC coupled supercapacitor-based energy storage system (ESS). The novel control of the PVSG is implemented in the ESS side. The novel control scheme includes a fast and slow instantaneous power controls. The fast-instantaneous power flow control is fulfilled by the DC-link voltage control and AC voltage control. The cascaded voltage source controls enable the fast-instantaneous power balance, while a slow instantaneous power control is used to implement the inertia and grid synchronization. Two important fundamental functions are realized in the PVSG. The first one is the frequency inertia to resist the grid frequency variation where the df/dt-based power control is achieved; the second one is the inertia against the PV intermittent power. The correctness and effectiveness of the proposed PVSG is validated experimentally in a 480V PVSG prototype with TMS320F28379D DSP controller.

For grid-forming inverters, abilities of both disturbance rejection and reference tracking are important for voltage controls, which act as inner loops of power controls. In this article, a complex variable-based dominant pole elimination (DPE) algorithm is applied to the voltage control that includes both positive- and negative-sequence complex variable resonant controllers. The algorithm is effective for both αβ - and dq -frame voltage control. Then, the DPE algorithm is extended for conventional dual-loop proportional resonant control to improve the dynamic response. Impedance-based analysis also indicates that the proposed DPE algorithm results in a suitable impedance to enhance the robustness in grid-connected mode. The effectiveness of the DPE method is verified through an experimental implementation in a three-phase hardware prototype with a TMS320F28379D digital signal processor and simulations in MATLAB/Simulink.

Nowadays, the grid-forming inverter, also referred as voltage-controlled inverter, has gained greater attentions due to its advantages in providing power support to the grid. The power control is central to grid-forming inverters in realizing grid-support functionalities, such as the droop control or virtual inertia emulation. However, in these controls, the dynamic response of the instantaneous power usually suffers from overshoots and oscillations. To improve the dynamic response of the injected instantaneous power, this letter introduces a novel synchronous active power control for the voltage-controlled grid-connected inverter. A novel design method is proposed to reduce the second-order power loop to a first-order model. Consequently, the droop and reference-following controls operate like a first-order system with excellent dynamic response. Moreover, the inertia against frequency change can be designed freely without affecting the performance of the power control. The correctness of the control is verified experimentally.

The inverter control plays a critical role in microgrid (MG) hierarchical control structure. In this paper, a dominant dynamic elimination control is studied for the voltage-controlled MG inverter. A compact control structure is designed to include separated static state feedback and feedforward terms. Benefitting from the designed control structure, the dominant dynamic can be canceled by the static feedforward control via the fundamental zero-pole cancelation principle. Thanks to the cancelation of the dominant dynamic, the load disturbance-rejection performance is improved, and the approximate unity gain from the reference to output is also achieved from the perspective of the application control layer, thereby the performance of power control for the upper application is improved. Furthermore, the strong robustness to parameter variations and low total harmonic distortion are also achieved. Comparative simulations and experiments are adopted to verify the effectiveness and superiority of the designed approach.