Energy storage for frequency support in weak electrical grids

Jayamaha, Prasanthi Charmalee (2015) Energy storage for frequency support in weak electrical grids. PhD thesis, University of Nottingham.

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INTEGRATING local renewable resources into the low voltage distribution system can create weak micro-grids with limited power capabilities and challenging control requirements. The low inertia in these micro-grids can result in significant frequency excursions when large load or generation changes are seen. Micro-grids may also be more susceptible to high harmonic distortion in their voltage due to non-negligible supply impedance. This calls for advance control techniques to regulate frequency and maintain voltage at the standards required of the utility grid. Failure to do so may incur disconnection of the weak grid from the main supply leading to cascading power disruptions to the consumers. This thesis explores one approach to improving the frequency control in weak grids in the presence of poor voltage quality by exploiting energy storage to independently and optimally dispatch active power to constrain frequency fluctuations within acceptable limits.

Power system frequency - a direct indication of generation-demand imbalance -is selected as the main control signal in the energy storage control system. The energy storage injects active power when the frequency falls below the lower threshold and absorbs active power when the frequency rises above the upper threshold. Imposing thresholds close to but not equal to the nominal frequency allows a sufficient speed error for the conventional speed governor of the prime movers of the power system to operate unhindered and acquire the full control of the load at the equilibrium. Frequency control by the energy storage support combined with the continuous speed governing acts to restrict the frequency excursions within the thresholds well within the primary frequency time requirements.

The power system frequency is detected directly from the measured supply voltage in order to eliminate the need for any communication between system elements and make the energy storage device plug and-play. This has meant the investigation of a suitable frequency tracker which can respond quickly, and at the same time operate correctly in the presence of high supply voltage distortion. Frequency detection introduces a transient delay and a steady state ripple to the estimated frequency. Hence, a set of frequency detection requirements are defined that must be satisfied by any candidate frequency detection method. In identifying the best technique(s), this thesis presents a comparative analysis of candidate frequency detection methods when applied specifically in a weak grid. The analysis revealed that the Double Second Order Generalised Integrator Frequency Locked Loop (DSOGI-FLL) displays optimum transient and steady state characteristics for the energy storage control application requirements, closely followed by a Discrete Fourier Transformation (DFT) technique generalised in the study.

The proposed frequency trackers and energy storage control techniques were validated experimentally and tuning of the key parameters to match user requirements was presented for both single and multiple energy storage systems.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Sumner, M.
Williams, A.A.
Keywords: Energy Storage, Frequency Control, Weak Grids, DSOGI-FLL, DFT, Power System Frequency Detection, Primary Frequency Response
Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 29104
Depositing User: Jayamaha, J
Date Deposited: 15 Dec 2015 09:50
Last Modified: 19 Dec 2017 09:39

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