Load Bus Frequency Estimation in Converter-Driven Grids - A Dynamic Phasor Approach

It is well known in the power system community that frequency is not a global variable during transients. The previously proposed frequency divider (FD) is a valid approach to estimate load bus frequencies in large transmission networks wherein the load dynamics are typically ignored. In converter-driven grids, load dynamics may have a significant impact on the local frequency dynamics. This article determines that the FD has low accuracy under such conditions. This paper proposes a centralized dynamic phasor estimator (DPE) to track the frequency of the load buses and additionally estimate the branch current phasors and load bus voltage phasors. The key idea is to formulate the power system state-space model in the dynamic phasor domain such that grid-forming converter (GFC) bus voltage phasors and load power measurements are inputs and the load bus voltage and branch current phasors as states. The dynamic phasor of the GFC bus voltage is calculated by measuring the terminal voltage of the GFC bus and the internal frequency available within the droop-control loop of the GFC. A complex-PLL (C-PLL) is proposed to track the rotation of the load bus voltage phasors to estimate the actual frequency at the load buses. Numerical simulations are carried out on the modified WSCC 9-bus system and the IEEE 39-bus system to demonstrate the effectiveness of the proposed method.