Harmonics’ effect on electrical equipment at hydropower plants : A study on the current situation regarding harmonics at hydropower plants and the challenges with measuring voltage harmonics

This thesis investigates how voltage and current harmonics can affect electrical equipment at hydropower plants. A review was conducted to examine both the impact of voltage and current harmonics on electrical equipment and the challenges with measuring voltage harmonics at higher voltage levels. To further support this investigation, harmonic measurements have been carried out at four hydropower plants. The findings from the analyzed data show that harmonic levels in the grids connecting hydropowerplants are generally low. However, harmonic distortions generated by sources several kilometers away, such as wind farms, were detectable. The study also highlights the limitations of using conventional measurement transformers for measuring voltage harmonics. While inductive voltage transformers may provide acceptable accuracy up to a few kilohertz at lower voltage levels, significant errors can occur at higher voltages and frequencies due to resonance. Capacitive voltage transformers cannot be used for harmonic measurements due to their precise tuning around a specific frequency. Three alternative methods for measuring voltage harmonics are presented in this thesis. A fairly new concern in the electrical grid is the presence of supraharmonics, harmonic components with frequencies between 2 kHz and 150 kHz. These can, for example, be created by frequency converters connected to the grid. To be able to measure these high frequency voltages, special methods with a large frequency range are required. This thesis also investigates potential issues with supraharmonics in hydropower plants. Identified issues include increased losses and failures in cable terminations. However, the risk due to supraharmonics might be small today but may increase if more frequency converters are connected in the proximity ofhydropower plants.