Turboalternator shaft voltage measurements

This paper presents the results of shaft voltages measurement campaign undertaken to investigate the sources of unrecoverable temperature drift observed on the thrust bearing of a small industrial steam turbine of a turbogenerator installed in a cogeneration biomass thermal power station. The bandwidth of the shaft voltage signal recorded and the characteristic time evolution have pointed out the electrostatic charges produced by the turbine steam as the source of the shaft voltage. The study have shown that in this specific plant dangerous shaft voltages can settle in case of a bad shaft to ground connection, originally assured by a grounding brush installed inside the turbine bearing casing only.