Additional AC Copper Losses in the Field Winding of Converter-Fed Synchronous Machines for Pumped Storage Hydropower Plants

Converter-fed synchronous machines for variable speed pumped storage hydropower plants are becoming increasingly important as the share of fluctuating renewable energy increases. The inverter connected to the stator terminals causes additional harmonic currents in the stator winding, which induce additional harmonic currents in the field winding. Due to the eddy current effects and the typical form-wound coil design, these additional harmonic currents in the field winding can lead to significant additional losses, even with relatively small amplitudes. The aim of this paper is to calculate these additional losses. Therefore, numerical calculations are used to determine the resistance factor for a frequency band from <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{0}\,\text{Hz}$</tex-math></inline-formula> to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{15}\,\text{kHz}$</tex-math></inline-formula> while also considering saturation. In addition, two measurements are carried out on a research hydro generator at different inverter switching frequencies. Using the measured currents and determined resistance factors, the additional losses due to the switching frequency are calculated and validated using a temperature measurement in the field winding. The results of this work are important for the future design and cooling of large converter-fed synchronous machines with form-wound field coils, as the additional AC losses in the field winding are usually neglected.