Hydro Generator High Voltage Stator Windings: Part 2 - Design for Reduced Copper Losses and Elimination Of Harmonics

Abstract

Hydro generator stator winding design is one of the key factors when considering machine upgrades and uprates. The hydro generators built 30-50 years ago employed stator insulation systems with lower voltage stresses and correspondingly larger insulation allowances. Modern insulation systems use thinner, more homogenous insulations, which permit higher voltage stresses, while providing much improved thermal conductivity and better heat dissipation. There is therefore quite a scope for the high voltage (HV) stator winding designer to increase overall coil copper content by between 20-40%, which often provides copper losses at increased output not much higher than the losses produced by the existing winding at pre-uprate machine rating. In conjunction with the new insulation improved thermal dissipation, it is possible to design uprated windings with temperature rises similar to the existing ones. The main focus of the HV coil design for hydro generator upgrades and uprates is therefore reduction of copper losses, and optimisation of thermal characteristics for best dissipation of heat losses. The loads connected to the hydro generator terminals (transformers, transmission lines, switchgear, etc.) are all designed to operate with pure sinusoidal EMF (Walker, 1981). The purity limits of the generators’ open circuit wave are prescribed by the standards (AEMC, 2008). Given the salient pole construction of hydro generator rotor with large variation of magnetic permeance in direct and quadrature axes, and the concentrated nature of rotor field pole windings, special measures are implemented on the geometry of rotor pole face. They include pole face shaping for sinusoidal approximation of rotor MMF, and adjustment of damper winding geometry relative to the slotted periphery of stator bore for reduction of the slot ripples (Walker, 1981). Given the relatively slow hydro generator operational speeds and necessarily large number of poles, the stator winding consists of large numbers of pole phase groups having only a few coils (typically 1 to 3). The stator winding MMF will therefore be rather coarse, and special measures must be implemented to achieve the output waveform as close to a pure sinusoid as possible. This second of four papers on hydro generator stator windings (refer to Znidarich, 2008a, Znidarich, 2009b, and Znidarich, 2009c) describes some tools at the hydro generator stator winding designer’s disposal for effective reduction of harmonics and optimisation of output waveform purity. They include skewing of stator core slots, fractional (short) pitching of stator winding coils and fractional slot windings.