| Wind turbine technology
65 60 55 50 45 40 35
Operating conditions Load: 40%, 60%, 80%, 100% Speed: 1500 rpm
60.3ºC 56.3ºC
Gearbox type Nominal power Gear ratio
Oil lubrication type Volume
Gear type Bearing type mPAO NEW
Above: Figure 3. Result of testing performed with the FZG rig. This shows lubricant temperature after 300 minutes at a circumferential speed of 8.3 m/s
Developing a new formula Since 2018, TotalEnergies Lubrifiants has been investing in the development of energy efficient lubricants for wind turbines. Among the first steps was deployment of what we call the Mini Traction Machine (MTM). This is a tribological testing device used to measure and compare the coefficient of friction of lubricants. Its operation is based on the principle of reciprocating sliding motion and it measures the frictional forces between two surfaces dipped into lubricant. The MTM has been used to identify a new mix of synthetic base oils with a significantly lower coefficient of friction than PAO or mPAO. Typical MTM results are shown in Figure 1. The comparisons shown in Figure 1 are on an equal-viscosity basis to eliminate the effect of different viscosity indexes. Thus, it clearly shows the difference between coefficients of friction deriving from the lubricant chemistry itself. Less friction indicates less wear and also reduced energy losses.
A second key step in development of the new base oil mix involved use of the ‘FVA 345’ gear efficiency test procedure employing a modified FZG* back-to-back gear test rig (Figure 2). The test pinion and the test gear are mounted on two parallel shafts, which are connected to
the slave gear stage. Unlike the standard FZG test rig, two gears identical to the test gears are employed. The two equal stages make up a circulating power loop so the electric motor has only to compensate for frictional losses. Therefore, the test measures energy losses directly and allows different lubricants to be readily compared. In our case we compared the new base oils mix with mPAO, which shares similar viscosity and density.
A two-in-one test was performed using the FZG test rig. The first test consisted of varying the lubricant temperature from 40°C to 120°C, the circumferential speed from 0.5 m/s to 20 m/s and the load applied up to a Hertzian stress of 1720 N/mm². The power loss during the test reached 8.37 MJ in the case of the new chemistry versus 8.75 MJ for the mPAO, a 4 % energy saving. This test gives only a comparison between lubricants, and we should not expect such savings to be achieved in a gearbox operating in an industrial environment. The second test consisted of measuring the lubricant temperature after 300 minutes at a circumferential speed of 8.3 m/s without cooling or heating of the gearboxes. The lubricant temperature is significantly lower with the new chemistry (see Figure 3).
The viscosity index of the new oil is 184, ie similar to mPAO.
Testing at industrial scale Once the new chemistry had proven its capabilities at laboratory scale for decreasing energy consumption, we moved to testing the
lubricant in gearboxes of different sizes and designs. The objective was to reflect the variety of gearboxes we might encounter in the field that we might be called upon to provide lubrication for. Testing included a collaboration with university RWTH Aachen and testing in three different types of gearbox. See table above.
Figure 4 shows gearbox testing at RWTH Aachen. A further programme of testing at RWTH Aachen involved 2 MW wind turbine gearboxes back-to-back in an industrial test rig, to compare different lubricant chemistries at the same viscosity. See Figure 5.
Sensors were used to monitor bearing temperature, oil sump temperature, vibration, power losses and/or energy losses. It was observed that the new formula achieves an efficiency improvement of between 0.65% and 1% relative to a conventional PAO base oil and between 0.25% and 0.5% compared with a metallocene PAO base oil, depending on load. Figure 6 shows energy losses (kWh) recorded for the three oils (PAO, mPAO and new formula) at various gearbox loads.
The table below shows the percentage gain in gearbox efficiency due to the new oil relative to other synthetic base oils, at various loads.
Load
20% (400 kW) 40% (800 kW) 60% (1200 kW) 80% (1600 kW) 100% (2000 kW)
Gain in gearbox efficiency 1.03% 1.18% 0.75% 0.28% 0.27%
Temp. regulation at 55°C Bosch GPV 0.85 MW
1:61.9 Splash
155 litres
1 planetary stage, helicoidal gears + high speed shaft
SRB, CRB, TRB Operating conditions
Load: 0%, 20%, 40%, 60%, 80%, 100% Speed: 1500 rpm/1650 rpm Temp. regulation at 50°C
Bosch GPV442 2 MW
1:113.2
Splash + pressurised 275 litres
2 planetary stages, helicoidal gears + high speed shaft
SRB, CRB, TRB
Pujol ECO 74 1.8 MW 1:94.6
Splash + pressurised 310 litres
2 planetary stages, helicoidal gears + high speed shaft
SRB, CRB, TRB
Above and right: Figures 4a and 4b. Gearbox testing at RWTH Aachen
* Gear Research Center of the Technical University of Munich
www.modernpowersystems.com | October 2023 | 39
Steady state excess temp. (°C)
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