London, UK [Renewable Energy World Magazine] 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.
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.
Mitigating Bearing Damage
To guard against electrical damage to bearings, stray currents must be diverted from the bearings by means of mitigation technologies – such as insulation, special current filters, and/or an alternate path to ground. These technologies vary in terms of their cost and effectiveness.
Insulating bearings is a partial solution that more often than not shifts the problem elsewhere. Blocked by insulation (usually an exterior coating of aluminium oxide), stray currents seek other paths to ground. Attached equipment, such as gearboxes, often provide this path, and frequently end up with bearing damage of their own. In addition to being expensive, insulation is subject to contamination. Worse yet, some types of insulation can be totally self-defeating: In certain circumstances, the insulating layer has a capacitive effect on high-frequency induced currents, allowing them to pass right through to the bearings.
Non-conductive ceramic bearings – often called hybrid bearings because the balls are ceramic but the rest of the unit (including the race wall) is metal – can divert damaging currents but leave attached equipment open to damage of its own. Due to very high voltage potential across their surfaces, the ceramic balls can also become pitted and eroded by electrical discharges.
Yet another mitigation attempt, conductive grease, in theory, bleeds off harmful currents by providing a lower impedance path through the bearings. In practice, however, the conductive particles in the grease increase mechanical wear.
Special dv/dt current filters (change in voltage during a change in time – dv/dt is used to refer to high frequency voltage transitions induced by the inverter on the generator shaft) or common mode current filters can mitigate damaging currents in some circumstances, but results in the field have been mixed at best. More effective is a sinusoidal filter back-drafted to the dc-link, but this option reduces the generator’s efficiency.
Conventional spring-loaded carbon block brushes certainly help. They contact the motor shaft to provide an alternate path to ground. Unfortunately, they too have drawbacks. They are notorious for the maintenance they require, due primarily to wear – tiny particles break off from each brush and become stuck in the remaining portion. They also build up on the generator shaft, sometimes diminishing the brush’s effectiveness, only weeks after installation. Carbon block brushes also require optimum humidity – humidity levels that are too low or too high are detrimental to their performance.
As mentioned previously, alternate discharge paths to ground, when properly implemented, are preferable to insulation because they neutralize shaft current. The ideal solution would provide an effective, low-cost, low-resistance path from shaft to frame, affording the greatest degree of bearing protection and maximum return on
investment. One technology designed to achieve these requirements is the AEGIS WTG Bearing Protection Ring™. Its patent-pending Electron Transport Technology™ uses the principles of ionization to boost the electron-transfer rate and promote extremely efficient discharge of the high-frequency shaft currents induced by many wind turbine generators. Harmful currents are steered away from the bearings and channelled safely to ground.
The ring surrounds the generator shaft with millions of conductive microfibres, each with a diameter of less than 10 microns. Strong and stiff, yet flexible, these fibres provide a high density of contact points – parallel paths of least resistance from the motor shaft to ground. Capable of conducting instantaneous currents of many tens of amperes and discharging from tens to thousands of volts with frequencies in the MHz range, the fibres significantly reduce voltage build-up on the generator shaft. The ring is especially suitable for use at high frequencies – because its fibres tend to compensate for variations in the roughness of the shaft surface and/or microscopic misalignment of the ring and shaft.
When the microfibres in one sector lose mechanical contact with the rotating shaft, electric contact is quickly re-established somewhere else along the ring, due to local field emission. Thus the ring fulfils all the functions of conventional spring-loaded carbon brushes with neither the hot-spotting/thermal wear nor the direct frictional wear common to such brushes. Because its multiple microfibres dissipate heat better than single-conductor devices, the ring can also tolerate higher current densities. Furthermore, unlike carbon block brushes, the microfibres of the bearing protection ring are not adversely affected by oil, grease, dust, moisture, or other contaminants.
The protection ring is engineered to safely divert high-frequency shaft current at frequencies as high as 13.5 MHz and discharges of up to 3000 volts (peak). Maintenance-free for a minimum of two years, effective at any RPM, and available for any size generator, the ring is suitable for up-tower retrofits and preventive maintenance programmes as well as for OEM installation. The shaft collar, coated with highly conductive silver paint, enhances the ring’s effectiveness.
Damaged bearings can cause generator failures, which lead to unplanned downtime and very costly repairs. If down for a month, a failed 1.5 MW generator can account for over US$48,000 of lost revenue, and a single month’s wait for parts is unrealistically short considering the worldwide shortage of bearings and other key components. On top of lost revenue, the cost of repairing failed bearings (new bearings, labour, slip rings, additional parts, and so on) can run as high as $50,000. This figure does not include the enormous expense of renting and transporting the large crane needed for many repairs – and all too often there is a long wait for that as well.
The high cost of unexpected maintenance/repairs is of great concern to generator manufacturers and becomes a great concern to facility owners as turbine warranty periods expire. All stakeholders are seeking the increased return on investment that comes with greater turbine reliability, and with its novel current discharge slip ring, Electro Static Technology believes that it has taken a significant step toward achieving that goal.
Trial and error
In a trial of the AEGIS WTG Bearing Protection Ring the current dissipating device was fitted to a wind turbine at a development in Oregon, USA. The generator bearings of the turbine had first failed in May 2006, only 11 months after the machine was brought on-line. The company that owns and operates the wind farm replaced the bearings and slip rings, but the new bearings failed in October 2006, only five months later. Once again, new bearings and slip rings were installed.
The third bearing failure came 11 months later in September 2007. This time, in addition to replacing yet another set of insulated (ceramic-coated) bearings and slip rings on both ends of the generator, the owner decided to try the AEGIS Conductive Microfibre Bearing Protection Ring and shaft collar on the drive end. All components were installed by the regional distributor in mid-September 2007. The generator’s two standard carbon block, spring-loaded brushes, which rub on the slip ring at the non-drive end, were also replaced at the same time.
Measuring shaft voltage on the generator, with and without the new Bearing Protection Ring and collar, shows that the devices had reduced shaft voltage by an average of 84.5%.
Figure 3. Taking readings during experimental comparisons
During full-power operation with a wind speed of 12.1 mph, a baseline voltage of 2.6 volts (peak-to-peak) from the 5.824” (14 cm) shaft of the tower’s doubly-fed, asynchronous 1.5 MW generator was recorded. (Figure 1).
With the Bearing Protection Ring installed, an average shaft voltage of 6.41 volts (peak-to-peak) was recorded, while when using only the spring-loaded carbon block brushes (Figure 2) the average shaft voltage of 41.35 volts (peak-to-peak) was recorded. Furthermore, the voltage wave form with the AEGIS ring and collar was a smooth wave with no detectable discharge to the bearings, while the wave form without the ring and collar showed a bearing-current-discharge pattern with voltage peaks an average of 6.5 times higher.
Similar tests at a facility in Texas, on a 1.5 MW wind turbine generator manufactured by another supplier, yielded similar results. Here the AEGIS Ring was shown to effectively reduce a shaft voltage above 600 volts (continuous) to 30–40 volts (peak-to-peak), safely diverting 50–60 amperes of current.