Adaptive and Robust Control Schemes in Electric Power Systems
(2025)Department of Automatic Control
- Abstract
- This thesis establishes the foundation for improving the control strategy of an active power filter (APF) system connected to a grid with partially or fully unknown and varying impedance. It aims to support the transition from fixed-gain control to an adaptive strategy capable of handling grid impedance variations while meeting key performance targets, including a control bandwidth up to 49th harmonic, strong disturbance rejection, and fast transient response.
First, plant dynamics under varying grid impedances and system delays are analyzed using a simplified single-phase model of the APF, based on a commercial product by Comsys AB and incorporating an LCL filter. Results show grid impedance significantly impacts plant dynamics;... (More) - This thesis establishes the foundation for improving the control strategy of an active power filter (APF) system connected to a grid with partially or fully unknown and varying impedance. It aims to support the transition from fixed-gain control to an adaptive strategy capable of handling grid impedance variations while meeting key performance targets, including a control bandwidth up to 49th harmonic, strong disturbance rejection, and fast transient response.
First, plant dynamics under varying grid impedances and system delays are analyzed using a simplified single-phase model of the APF, based on a commercial product by Comsys AB and incorporating an LCL filter. Results show grid impedance significantly impacts plant dynamics; resonance shifts to lower frequencies, broadens, and intensifies as grid strength weakens. Grid-connected filters—such as power correction, passive, and EMC components—modify resonance characteristics and add resonant behavior. The current system delay notably distorts the phase behavior, while reducing the delay brings it closer to the ideal zero-delay case.
Second, robust controllers designed via loop-shaping are evaluated under ideal zero-delay and current delay conditions with the strongest grid impedance and no grid-connected filters. Controllers counteract resonance by opposing dynamics, increasing noise sensitivity and contributes to sharply reducing normalized coprime stability margins as crossover frequency increases.With the current delay, crossover frequency is limited to 2 kHz, indicating compromised stability and robustness beyond this point, despite a higher resulting bandwidth. Phase margins exceed targets under 90◦, showing robustness demands higher margins at the cost of crossover frequency. Below resonance, controllers resemble PI control, indicating PI may be adequate outside resonant frequencies.
Finally, the suitability of PI controllers tuned by Relay and Lambda methods is evaluated. Both perform poorly under weak grids and disturbances, limiting their practical use for adaptive APF control. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/9207363
- author
- Ahlgren, Sara
- supervisor
- organization
- year
- 2025
- type
- H3 - Professional qualifications (4 Years - )
- subject
- report number
- TFRT-6289
- other publication id
- 0280-5316
- language
- English
- id
- 9207363
- date added to LUP
- 2025-08-08 15:10:48
- date last changed
- 2025-08-08 15:10:48
@misc{9207363,
abstract = {{This thesis establishes the foundation for improving the control strategy of an active power filter (APF) system connected to a grid with partially or fully unknown and varying impedance. It aims to support the transition from fixed-gain control to an adaptive strategy capable of handling grid impedance variations while meeting key performance targets, including a control bandwidth up to 49th harmonic, strong disturbance rejection, and fast transient response.
First, plant dynamics under varying grid impedances and system delays are analyzed using a simplified single-phase model of the APF, based on a commercial product by Comsys AB and incorporating an LCL filter. Results show grid impedance significantly impacts plant dynamics; resonance shifts to lower frequencies, broadens, and intensifies as grid strength weakens. Grid-connected filters—such as power correction, passive, and EMC components—modify resonance characteristics and add resonant behavior. The current system delay notably distorts the phase behavior, while reducing the delay brings it closer to the ideal zero-delay case.
Second, robust controllers designed via loop-shaping are evaluated under ideal zero-delay and current delay conditions with the strongest grid impedance and no grid-connected filters. Controllers counteract resonance by opposing dynamics, increasing noise sensitivity and contributes to sharply reducing normalized coprime stability margins as crossover frequency increases.With the current delay, crossover frequency is limited to 2 kHz, indicating compromised stability and robustness beyond this point, despite a higher resulting bandwidth. Phase margins exceed targets under 90◦, showing robustness demands higher margins at the cost of crossover frequency. Below resonance, controllers resemble PI control, indicating PI may be adequate outside resonant frequencies.
Finally, the suitability of PI controllers tuned by Relay and Lambda methods is evaluated. Both perform poorly under weak grids and disturbances, limiting their practical use for adaptive APF control.}},
author = {{Ahlgren, Sara}},
language = {{eng}},
note = {{Student Paper}},
title = {{Adaptive and Robust Control Schemes in Electric Power Systems}},
year = {{2025}},
}