A potential energy source that most Britons are never thankful for is wind. Boris Sedacca delves into the intricacies of mechanical to electrical energy conversion.
Test: Wind energy is one of the cheapest of the renewable energy technologies. Because of its geographic location within the Gulfstream in the North Atlantic, the UK is believed to have 40 per cent of the whole collection of possibilities for wind farms in Europe. However, taming this energy and harvesting it effectively occupies the minds of transmission engineers throughout Europe.
It all depends on factors like turbine size, use, site conditions and whether a single turbine is to be used or several turbines in a wind farm. Turbine size can be anything from a kilowatt to several megawatts and this has a bearing on drivetrain design.
For today’s utility scale wind turbines there are two main electrical drivetrain concepts according to ABB – doubly fed and full converter. The primary difference in these concepts is the type and size of generator used and the function of the converter. The full converter can be further categorised into low-speed (also known as direct-drive because it does not require a gearbox), medium-speed, and high-speed.
While the market has largely been dominated by the doubly fed concept, the full converter concept is gaining market share. There are several factors driving this shift, including grid code compliance and optimised power generation at slower wind speeds.
Doubly fed uses a wound rotor asynchronous generator where the rotor windings are connected to a small converter using slip-rings and brushes, while the full converter decouples the generator and the mechanical drivetrain from the grid with all the generated power flowing through the converter to the grid.
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. [Page Break]
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. [Page Break]
“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. [Page Break]
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.” [Page Break]
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 during energisation. Pre-insertion resistors (PIRs) from Cressall are used as part of the safety control system on most UK and Irish wind farms.
“PIR has not normally been associated with wind farm generation collection systems at 33kV in the past, but is now fairly commonly used since some of our customers have pioneered it,” claims Martin Nicholls, sales director at Cressall Resistors.
The wind farm is energised through the PIR which remains in circuit for a very short period of time. The PIR limits the magnetising inrush current produced by the turbine transformers and as a consequence the resultant voltage drop at the point of connection with the utility. It is not in constant use all the time but is ready to protect the generating equipment when needed.
Nicholls expands: “The turbines normally generate at 690V DC and the collection system on a wind farm goes to the substation through a transformer that raises the voltage to 33kV AC. The collection system terminates at the substation switchgear at which point it connects to the grid through our resistors.”
The design of the Cressall PIRs has evolved over the last eight years using oval edge-wound coil elements such those as used in neutral earthing resistors in distribution substations in the UK and throughout the world. They are assembled in three-phase sets in single large stainless steel outdoor cabinets, sealed and equipped with IP54 cable boxes and anti-condensation heaters for the tough climatic conditions inevitably found at many exposed wind-farm sites.[Page Break]
Drivetrain testing
Summer 2013 will see the opening of a £35m 15MW capacity wind turbine drivetrain testing facility at Narec in Blyth, Northumberland. The main test hall will be completed by the end of the year and test rig components will then begin to be assembled onsite.
The Energy Technologies Institute (ETI) is investing £25m in the Narec facility, by providing funding to GE Energy Power Conversion and MTS Systems Corporation for the design, development and commissioning of the test rig.
Steve Abbott, corporate affairs manager at Narec, says: “We have already built and are now operating a 3MW test rig for tidal turbines. Our flagship 15MW facility is under construction and the test hall is nearly finished with the rig being assembled inside. We hope to commission the rig in April or May and to have the first test turbine in the summer.”
The ETI and Narec have specified the rig with sufficient capability to accommodate the projected growth in turbine size over its designed 20 year life. Narec expects to use it initially to test prototype and early development models currently available in the 4-7MW class, and then for the next generation of 8-10MW machines. [Page Break]
Drivetrain commissioning
HGV Class 1 driver Paul Anderson writes: At 60 metres up from the ground the crane lifts the huge 12 ton gearbox and disk brake assembly into position during construction of the Scout Moor Wind Farm, the largest onshore wind farm in England.
It produces electricity from 26 Nordex N80 wind turbines, each with a generating capacity of 2.5MW, producing a total capacity of 65MW - enough power to supply the average needs of 40,000 homes. The engineer inside the nacelle is in contact by radio with the crane operator below.
Construction of the wind turbines began back in November 2007 at Turbine No 16. On 6th April 2008, Turbine No 25 became the very first turbine to begin generating. Early on Sunday morning technicians from the turbine manufacturer Nordex connected a laptop as part of the commissioning process which monitors more than 300 different parameters like temperature sensors, hydraulic sensors, pitch parameters, wind sensor set and power output.
By June 2008 engineers had constructed 26 wind turbine towers and installed 78 turbine blades 450 metres above sea level on Scout Moor amid adverse weather conditions of thick fog, gale force winds, torrential rain, snow and ice during the winter months of 2008.
Commissioning and testing of the turbines was the last stage, in which the electrical and mechanical systems were set-up, calibrated and fine tuned. The cabling works included the installation of many kilometres of fibre-optic cable, and linked from each individual turbine to a central supervisory control and data acquisition (SCADA) computer system which collects real time data from each turbine. Data from the turbines is logged and any faults that may occur with the turbines are recorded.
Fig. 1. A Schaeffler wind turbine gearbox.
Fig. 2. The new 15MW Narec drivetrain test facility will open in Summer 2013.
Fig. 3. Schaeffler’s FAG WiPro s condition monitoring system.
Fig. 4. Schaeffler’s Ian Pledger: “We know the gear ratios between every shaft in a gear train.”
Fig. 5. Drive train commissioning. Photo credit: Paul Anderson
