AC microgrids for grid capacity and resilience enhancement

Doktorsavhandling, 2025

Sweden's climate goals drive large-scale electrification across the industrial, transportation, and heat sectors, challenging electricity system operators due to infrastructure overloading. Upgrading this infrastructure is time-consuming and could hinder electrification efforts. Alongside this, grid operators must meet increasing demand while ensuring reliable power supply, especially for critical facilities. Traditionally, these facilities rely on gas or diesel generators for backup, which are environmentally unfriendly.

This thesis investigates the microgrid solution powered by renewable generation and energy storage to facilitate the electrification process. The thesis focuses on the use of microgrid solution to provide two critical grid functions.

The first one being the alleviation of grid capacity bottleneck by reducing local peak net demand. Various microgrid solutions are explored, including separate microgrids based on battery energy storage system (BESS), and interconnected microgrids by back-to-back (B2B) converter or multiport converter (MPC). A linear optimization problem is developed to find the cost-optimal solution without the net demand exceeding microgrids power subscription limit. Results suggest MPC interconnection can reduce investment in energy storage and power electronics when microgrids exhibit a low or intermediate coincidence factor of their net load profiles.

The second microgrid function addresses the unplanned secure island transition of a critical facility, using a hydro-powered microgrid as a case study. A criterion for secure island transition is developed that accounts for frequency and voltage security constraints.

To meet frequency security constraints, a novel BESS frequency controller based on a Proportional-Integral (PI) controller with droop is developed. The controller tuning accounts for the response time of the hydro power plant (HPP) governor to provide coordinated and communication-less frequency support without over-sizing the energy storage. The proposed controller is validated through performance comparison with existing controllers in the literature and laboratory experiments. Results indicate the PI-based droop controller requires less energy reserve from the battery than the commonly used proportional (P) controller.

The voltage stability of the microgrid is challenged by significant reactive power drawn by industry motors during island transition transients. Analysis shows key factors for voltage stability are the online HPP capacity and the point of applicability of the reactive power requirement.