Despite headline-making growth figures, the wind industry is still a relatively young one and as such, optimization of the technology continues. Indeed, it has been argued that some wind turbines have been pressed into production prematurely and have suffered from design-related failures within their first few years of operation as a result. The full cost of these failures, though often hidden by manufacturers’ warranties, can be extremely high as, in addition to expensive repair costs, owners of such facilities lose revenue every second of downtime.
One of the issues that designers have identified as a potential problem is the production of ‘trenchant’ or stray shaft currents which may occur within the doubly-fed generators commonly used in wind turbines. The existence of these sometimes quite large currents within a generator’s bearings can lead to accelerated component wear and rapid failure. Techniques for addressing these currents are therefore a key area for designers to explore to improve the longevity and reliability of wind turbine generators.
The problem
High-frequency currents, induced in the shaft of a doubly-fed induction generator through parasitic capacitive coupling, can reach levels of 60 amps and 1200 volts or greater. If not diverted, these stray currents discharge through the generator’s bearings, causing pitting and fluting, through the process of electrical discharge machining.
Bearing damage has become the Achilles’ heel of this widely used type of generator, in which the stator is directly connected to the grid, while the rotor is fed by an integrated gate bipolar transistor (IGBT) voltage-source inverter. The rotor-side converter regulates the electromagnetic torque and supplies part of the reactive power to maintain the constant voltage and frequency of the stator output. This arrangement makes operation at varying wind speeds possible while maintaining a constant stator voltage and a constant frequency output to the grid. Because the inverter’s rating can be as low as 25% of the total system power, this design also reduces inverter cost. However, the system’s high-frequency switching introduces troublesome rotor-shaft voltages – exposing bearings, gearboxes, and other critical generator components to high-frequency currents.
There is significant empirical evidence (gathered from studies of large motors) that inadequate generator-shaft grounding significantly increases the possibility of electrical bearing damage. Viewed under a scanning electron microscope, a new bearing race wall is a relatively smooth surface. As the shaft turns, tracks eventually form where ball-bearings contact the race wall. With no electrical discharge, the race wall is marked by nothing but this mechanical wear. Without proper grounding, electrical discharges begin at start-up and grow progressively worse, scarring the race wall with small fusion craters. In a phenomenon called fluting (shown above), the operational frequency causes concentrated pitting at regular intervals, forming washboard-like ridges.
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