Long Term Oscillation of Ultra-Low Frequency Oscillations in the Nordic Power System
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Sähkötekniikan korkeakoulu |
Master's thesis
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Authors
Date
2024-03-11
Department
Major/Subject
Electrical Power and Energy Engineering
Mcode
ELEC21
Degree programme
AEE - Master’s Programme in Automation and Electrical Engineering (TS2013)
Language
en
Pages
118
Series
Abstract
In the Nordic power system, ultra-low frequency oscillations (ULFO) cause the system frequency to oscillate around the nominal value of 50 Hz, usually with an amplitude of 20–40 mHz and a time period of 40–90 seconds. They occur primarily due to the significant presence of hydropower. During an ULFO phenomenon, synchronous generators oscillate coherently. The water hammer effect and the quick operation of the hydraulic governors reduce the damping of oscillations. ULFOs lead to unnecessary changes in governors’ setpoints and wear and tear of electrical equipment. ULFOs are characterised by frequency, amplitude and damping ratio. This thesis looks into ULFOs in the Nordic power system and how they depend on various power system variables – inertia, electricity production, electricity consumption, hydroelectricity production, wind power production and transfer of electricity between Finland and neighbouring countries. To carry out the analyses, modal analysis with multivariate autoregressive (MAR) method is used to estimate the frequency, amplitude and damping ratio of ULFOs. The correlation coefficients between these parameters of ULFOs and power system variables are then calculated. According to the results, the frequency and amplitude of ULFOs are higher during the summers and lower during the winters. In the recent years, the ULFO amplitude has increased, while its damping has decreased. During summers, the electricity supply and demand as well as the power system inertia is low. High inertia during winters, when production and consumption of electricity is high, stabilizes the power system. The power system inertia has decreased over the past few years, but the inertia varies. ULFOs show high negative correlation with inertia, electricity production and consumption – when one parameter increases the other decreases and vice versa. Hydroelectricity produced in Norway has a higher influence on ULFOs, compared to Sweden and Finland. Wind power integration reduces system inertia. Wind power, however, does not have a clear impact on ULFOs. On a similar note, the import or export of electricity to or from Finland does not affect ULFOs. The correlations calculated by using the daily averages of the power system variables are stronger than the correlations obtained by using the hourly averages.Description
Supervisor
Seppänen, JanneThesis advisor
Seppänen, JanneKeywords
ultra-low frequency oscillations, Nordic power system, MAR method, correlation coefficients, frequency