## 2 Overall Design of Centralized Lubrication and Waste Oil Recovery System for Wind Turbines
### Optimization Design of Centralized Lubrication System  
The internal structure and maintenance environment of the wind turbine determines that centralized lubrication is generally selected for lubrication. Manual lubrication is performed by regularly adding a large quantity of grease. In the early stages of addition, the grease is used in excess, but the actual use in the later stages is severely inadequate. At the same time, due to the remote location of the fan, manual lubrication could not be added in time to meet the demand for lubrication of the fine parts. A centralized lubrication system can regularly and quantitatively supply lubricating grease according to the needs of each lubricating component. The system has the function of automatically completing the timing and quantitative supply of grease to multiple lubricating parts [19; 20]. The centralized lubrication system avoids the uneven lubrication and waste of manual lubrication, saves grease, maximizes the service life of components and considerably reduces maintenance costs. A complete lubrication system generally consists of a lubrication pump station, a grease distributor, a controller, corresponding sensors and plumbing components. The generator bearing is a high-speed rotating part, and the sealing mode of the whole generator is a mechanical labyrinth seal [21; 22]. Therefore, the bearing will maintain an elevated temperature state during operation. In order to avoid failure of the lubrication system caused by failure of the grease distributor, the grease distributor is removed based on the construction described above, and a double plunger lubrication pump is used to directly lubricate the front and rear bearings of the generator. In control mode, the lubrication system can automatically adjust the oil injection cycle and oil injection rate of the generator bearing based on the speed of the generator, which can effectively save grease and make lubrication reasonable. For the method of oil discharge, the active waste oil collection system is adopted. It allows the grease to flow and avoids deposition of waste grease in the bearing cavity. The spindle bearing is a low-speed rotating part, and the operating condition is heavy load [23]. The lubrication system adopts a single wire lubrication system with a fully parallel structure to lubricate the front and rear bearings with grease, which can avoid lubrication system blockage caused by structural defects of the progressive distributor and improve the lubrication reliability of the spindle. The pitch bearing and yaw bearing are sealed by a large-diameter bearing, which is a micro-motion lubrication operation [24; 25]. The three pitch bearings and one yaw bearing are lubricated by a single-wire lubrication system with a fully parallel structure that avoids lubrication system blockage caused by structural defects in the progressive distributor and improves the reliability of the variable o  
Figure 1: The structure principle of centralized lubrication system. A high-pressure lubrication pumping station is used in the lubricating pumping station, which can help to avoid deposition, hardening and deterioration of grease over a long period of time. The physical structure of the high-pressure lubrication pump station is shown in Figure 2a. The progressive distributor mostly adopts the single-line oil supply mode [26]. However, this form makes it possible for a single outlet blockage to affect the work of other oil outlets. In view of the shortcomings of a progressive distributor, a fully parallel single-line distributor is developed that adopts a new type of grouping structure and a single-point independent oil supply mode. Therefore, when a single branch fails, other branches can normally supply oil without interference. The system is equipped with an indicator rod to visually display the operation status of the oil distributor. The distributor structure with an indicator rod is shown in Figure 2b.