Power Transmission
The choice of drivetrain, whether geared or direct drive, very much depends on where it is going to be used. When a single turbine is to be located far from a support network, the requirement is for simplicity of design such as a direct drive turbine because there are no oils, filters and pumps. In the case of a wind farm where there is a support team, then geared systems are practical.
Condition monitoring
Wind farms are being built further offshore, in locations that are harder to reach and more exposed to extreme weather conditions, so reliability is going to be key to success. Turbine protection is vitally important and increasingly relies on condition monitoring, which uses a combination of real time trending data capture and signal analysis, showing time on the horizontal axis and magnitude on the vertical. Applying a Fast Fourier Transform (FFT) provides the
inverse of time or frequency, with the horizontal axis showing the signal broken down into constituent frequency bands with their magnitude along the vertical axis. A component within a turbine will generate a characteristic pattern of vibration when acting normally that can be applied to trending data to alert an operator if the component begins to behave abnormally. The operator may then inspect both the time signal and its frequencies in software. “What they are looking for is frequency patterns that
indicate different types of component failure,” says Ian Pledger, field service engineer at Schaeffler UK. “For example, you can differentiate between components like the inner ring or outer ring of a bearing, or the rolling elements in its cage. “We can differentiate between aspects of gears as well,
whether they are misaligned, if they have a tooth missing or beginning to wear. We can also tell if the gear is running eccentrically or with backlash. All these give characteristic patterns within the frequency spectrum, and we double-check them by looking at the time signal and measuring the distance in seconds between the peaks that we are interested in. “Therefore we can be very specific about what is going
on from the point of view of using vibrations to monitor the condition of various components. Assuming we have two main bearings in a typical gearbox and a typical generator, we can put a vibrations sensor on both main bearings, three sensors on the gearbox, and two on the generator - one at the drive end and one at the other end.” If there is a bearing with ten rolling elements for example and a defective outer ring, every time a rolling element goes over a defect it creates a periodic spike in its vibration, which can be seen along the horizontal axis in a time signal. The periodicity is a function of the distance between one roller and the next, and of the speed at which the shaft is spinning. Another element of the condition monitoring system is
a particle counter for the gearbox oil to detect the presence of unwanted particles like bits of bearing or bits of a gear. Trending values can be set with alarm thresholds in the same way as with vibrations. Pledger adds: “For each of the bearings in our range, and
for everybody else’s for that matter, we can calculate what the defect frequencies are. This is fine for fixed speed but when wind conditions vary, we can take a sensor input from the rotor of a turbine control system back into our software.
“We know the gear ratios between every shaft in a gear train for any given speed to the tail end of the generator, so when wind speed changes, we can take a signal from a speed sensor or a voltage from the control system within a nacelle. This shifts the position of the frequency window to a particular defect frequency according to wind speed.”
Fig. 2. The new 15MW Narec drivetrain test facility will open in Summer 2013.
Pre-insertion resistors
Because of the potentially large changes in loading caused by rapidly increasing wind speed that comes into a wind farm substation, there can be a sustained rise in voltage above that required for generation. Most UK wind farms make their connection to the national network through 33kV overhead lines to grid transformers which are typically 10-30km distant from the wind farm. When these are energised there is a real risk of voltage transients that exceed the three percent limit of Regulation P28 of the Energy Networks Association. The simplest and safest way of mitigating this risk is to make use of three-phase shunt resistors which are inserted in the line for a few seconds
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