Design, Modeling, and Validation of Grid-Forming Inverters for Frequency Synchronization and Restoration

Abstract

This paper focuses on the modeling, analysis, and design of grid-forming (GFM) inverter-based microgrids (MGs). It starts with the development of a mathematical model for three-phase voltage source inverters (VSI). The voltage and current controllers consist of two feedback loops: an outer feedback loop of the capacitance-voltage and an inner feedback loop of the output inductance current. The outer voltage loop is employed to enhance the controller’s response time. The inner current loop is used to provide active damping for the resonance created by the LCL filter. A two-layer control scheme is adopted for the GFM inverter control. The primary decentralized control uses droop control and virtual impedance loops to share active and reactive power. Simultaneously, the centralized secondary control addresses frequency and amplitude deviations induced by the droop control. Additionally, a synchronization loop is proposed for seamless reconnection of GFM inverters to the MG and to connect the GFM-controlled MG to the main grid. It has the advantage that the inverter operates in GFM mode even after the synchronization has occurred. The simulation results have shown that the voltage controller ensures a 0.005 s settling time and maintains the steady-state error at its minimum value of 0.1 V. Similarly, the current controller ensures a 0.006 s settling time with a 10−5 steady-state error. The system with the designed controller has a low total harmonic distortion (THD) of 1.46% and improved power quality of the output voltage. Furthermore, a quick restoration time is observed during load steps and tripping events, with a restoration time of 1 s with 10−10 steady-state error. Synchronization is achieved within 0.8 s for the incoming inverters and requires 3 s to synchronize the MG with the main grid, maintaining a steady-state error of 10−9.

A Grid-Connected Inverter with Grid-Voltage-Weighted Feedforward Control Based on the Quasi-Proportional Resonance Controller for Suppressing Grid Voltage Disturbances

“…This is referred to as the active damping grid capacitor current dual feedback control strategy, and the resulting diagram Figure 9, where Hi1 is the capacitor current feedback coefficient and Hi2 is the feedback system. In the active damping system, Zc is the sum of the capac parallel resistance [31,32], so Replacing G LCL with the transfer function G LCL−d after introducing damping and properly designing δ such that its open-loop transfer function is G OPd , the resulting Bode plot is shown in Figure 7. It can be observed that, by appropriately designing the value of δ, it is possible to ensure that the amplitude at the resonance frequency point ω r does not pass through the 0 dB point, thereby having no effect on the stability of the system [30].…”

“…This is referred to as the active damping grid current and capacitor current dual feedback control strategy, and the resulting diagram is shown in Figure 9, where Hi1 is the capacitor current feedback coefficient and Hi2 is the grid current feedback system. In the active damping system, Zc is the sum of the capacitor and the parallel resistance [31,32], so…”

Power Sharing Control Trends, Challenges, and Solutions in Multi-Terminal HVDC Systems: A Comprehensive Survey

“…Controlling several converters in a multi-terminal HVDC system can be difficult. Synchronizing the control systems of each converter is critical for balanced power distribution and grid stability [102].…”

“…This alignment is instrumental in maintaining stability, facilitating efficient power flow control, and enabling effective fault management. Similarly, in AC systems, synchronization ensures that generators, transformers, and loads operate at a consistent frequency and phase, vital for grid stability and power quality [102]. When MTDC and AC systems intersect, synchronization becomes even more critical.…”