Optimization of Heat Dissipation and Control Scheme for Wind Turbine Gearbox Based on FloMaster-Simulink co-Simulation

Due to favorable wind conditions and steadily rising temperatures, the demand for heat dissipation for the gearbox of wind turbines rises every spring and summer, and the energy consumption of the heat dissipation system rises as well. In cases where the gearbox oil temperature is excessively high due to poor heat dissipation, the wind turbine unit may only operate at a limited load or shut down. This article suggests a restoration strategy that uses seawater to cool the gearbox lubrication system, based on the operation of the gearbox cooling system of wind turbines. The original wind turbine gearbox cooling system and the enhanced seawater cooling system simulation models are built at the same time, and dynamic working conditions simulation research is done by utilizing the FloMaster simulation platform. To achieve automatic management of the gearbox temperature, we alter the fan or pump’s speed by the oil temperature at the gearbox’s outlet. We design a control scheme based on the working principle and use the FloMaster-Simulink co-simulation to calculate the operating characteristics of the system under dynamic working conditions. The results show that by simultaneously controlling the speed of the pump and the speed of the fan, the cooling system has the best control effect and the lowest energy consumption, providing a reference for the development of the gearbox cooling system of wind turbines.


Introduction
Countries are expanding their investments in wind turbines as global awareness of new energy sources rises.The topic of gearbox overheating still poses a challenge despite advancements in wind energyproducing technologies.The gearbox overheating is due to the inability to design a matching heat dissipation system while the power generation of wind turbines is increased.In addition to affecting the lubrication of the gearbox's gears, accelerating the mechanical wear of the gears, shortening the gearbox's service life, and diminishing its economic benefits, a gearbox's oil temperature that is too high will also impair the power output of the wind turbine.
Therefore, many scholars have proposed a series of solutions to solve the problem of gearbox heat dissipation.In [1], the reasons for the increase in gearbox oil temperature are analyzed and proposed to change the structure of the heat dissipation fins and solve the heat dissipation problem.In [2], it is proposed to connect a set of heat exchangers in series within the original cooling system to increase the heat exchange area and improve the efficiency of the gearbox cooling system.In [3], a one-way valve is used instead of a temperature valve to prevent gearbox oil temperature from effectively dissipating heat due to the failure of the temperature valve.In [4], a thermal network model is constructed for the oil temperature exceeding the limit fault of the wind turbine gearbox to accurately analyze the oil temperature of the gearbox.In [5], a low-temperature helium circulation pump is utilized to cool the high-temperature lubricating oil in the radiator.In [6], the benefits, drawbacks, and potential application situations of matching solutions are evaluated based on the principles of various wind turbine gearbox heat dissipation systems.The practice has shown that the above schemes can improve the reliability of gearbox heat dissipation, but they have not proposed a cooling treatment for the coolant after cooling high-temperature lubricating oil.This article analyzes the principle of a gearbox radiator system for a 3 MW offshore wind turbine and proposes to use the constant temperature characteristics of the bottom seawater [7] to cool the high-temperature coolant.The dynamic analysis of this scheme is carried out through the co-simulation of FloMaster-Simulink [8].We develop innovative solutions for the gearbox cooling system of wind turbines to enable the construction of powerful offshore wind farms.

Principle of the cooling system
The traditional gearbox cooling and lubrication system mainly consists of a gearbox, lubricating oil pump, filter element, temperature control valve, air-cooled radiator, and pipelines.The lubricating oil pump provides the power for system circulation, extracts lubricating oil from the gearbox, filters it through the filter element, and determines the flow direction of the lubricating oil by the temperature control valve based on the temperature of the lubricating oil.When the temperature of gear oil is not more than 45℃, the lubricating oil will flow directly to the gearbox oil distributor without passing through the radiator; When the temperature of the lubricating oil is greater than 45℃, all the lubricating oil is cooled by the radiator before flowing to the gearbox oil distributor.Finally, the lubricating oil passes through the gearbox oil distributor and then flows back to the gearbox, completing a cycle.The improved cooling system has added a seawater cooling system, which mainly consists of a plate heat exchanger, coolant tank, filter element, coolant pump, throwing tube heat exchanger, and pipelines.The coolant is transported to the coolant tank through a coolant pump, then to the plate heat exchanger, and finally to the throwing tube heat exchanger for seawater cooling, completing a cycle.To provide equal pressure on both sides of the pipeline in this system, coolant is put in the pipeline to a level that matches the inlet and outlet heights of the coolant water tank.This can help to lower the power needed for coolant circulation, as shown in Figure 1.

Model of the cooling system
The FloMaster software can simulate and analyze one-dimensional thermal fluid systems, which have been widely used in fields, such as automobiles, ships, aerospace, and supply and drainage [9].This article selects corresponding modules in FloMaster software, inputs the parameters of each module, and establishes simulation models based on the actual usage requirements of offshore 3 MW wind turbines, as shown in Figure 2. According to the design requirements, the initial temperature of the gearbox lubricating oil and environment is 30℃, and the temperature of the bottom seawater is 20℃.The rated flow rate of the seawater pump is 12.6 m 3 /h, the rated speed is 1, 480 r/min, and the head is 110 m.In addition, the coolant used in the seawater cooling system is ethylene glycol solution.The coolant pipeline uses nickel brass pipes with an inner diameter of 80 mm and a wall thickness of 3.5 mm, which is made of B10.Plate heat exchangers and throw tube heat exchangers are selected with Table 1 and Table 2 as their main parameters.

Control scheme
By using FloMaster-Simulink co-simulation technology for dynamic simulation of the cooling system, complex control strategies are combined with fluid simulation.Firstly, based on the Flomaster and Simulink platforms, we establish corresponding load models and control systems.Then, through the module library of Simulink provided by FloMaster and the setting of FloMaster communication components, the invocation of Simulink to the load model in FloMaster is achieved [10].The simulation principle is shown in Figure 3.To achieve optimal control of gearbox temperature, this article improves the cooling system of the original wind turbine gearbox and compares the original control scheme with the improved seawater cooling control scheme.The details are as follows:

Control scheme for the cooling system of the original wind turbine generator
The PID control element regulates the speed of the fan to control the temperature of the gearbox lubricating oil. Figure 4 shows the control logic diagram of the original wind turbine cooling system, with a target temperature set at 60℃.When the temperature of the lubricating oil at the gearbox outlet changes, the PID control element adjusts the heat dissipation efficiency of the radiator by outputting the fan speed.Finally, the temperature of the lubricating oil at the gearbox outlet is controlled to approach the target temperature.The temperature measurement element monitors the lubricating oil temperature at the gearbox outlet in real-time and outputs it to the PID control element, forming a closed-loop control. is obtained, which is the actual temperature after the lubricating oil absorbs heat.We directly control the fan speed based on the steady-state temperature difference.

Control scheme for seawater cooling system
Open-loop and closed-loop synchronous control of the speed of the seawater pump is conducted.The control logic diagram is shown in Figure 6, and the control flowchart diagram is shown in Figure 7. Firstly, based on data from multiple steady-state operating points of the system, we fit the relationship between the centrifugal pump speed and the lubricating oil temperature at the gearbox outlet: where n is the speed of the centrifugal pump; 1 , 2 , 3 and 4 are the undetermined coefficient; T is the temperature of the lubricating oil after cooling.The formula can be used as a preset working relationship for seawater centrifugal pumps.The control scheme for controlling the fan speed is the same as the original scheme, which inputs the outlet temperature of the lubricating oil 0 T and outputs the speed 1 n of some seawater pumps according to the preset formula.In the closed-loop control of the seawater pump, the PID control element takes the cooled lubricating oil temperature as the input and the steady-state temperature difference n .This control scheme uses two control cycles to control the lubricating oil temperature.When  is greater than 0, 2 n is greater than 0, and when  is less than 0, 2 n is equal to 0. We adjust k based on engineering requirements to shorten feedback adjustment time.But this scheme is relatively complex and requires a lot of hardware facilities.Figure 8 shows the dynamic simulation diagram of the gearbox lubricating oil outlet oil temperature of the original wind turbine cooling system.When the temperature signal undergoes a sudden change at 150 seconds, the temperature valve is fully open, and the flow of lubricating oil through the heat exchanger increases.When the temperature reaches 60.52℃, the fan works at full power.After significant temperature fluctuations, it is adjusted to the target temperature for 49 s. Figure 9 shows the dynamic response results of the control scheme for the seawater cooling system.When the temperature signal undergoes a sudden change at 183 s and 719 s, in the open loop system, the speed 1 n of the seawater pump is output through the gearbox outlet lubricating oil temperature.In a closed-loop system, the fan speed is adjusted based on the gearbox outlet oil temperature 0 T , and the output speed 2 n is adjusted through closed-loop control based on the cooled oil temperature to quickly adjust the temperature to a stable state.At this point, the maximum temperature is 53.94℃, and the adjustment time is 30 seconds.

Conclusion
The seawater cooling system added in this article can effectively reduce the maximum temperature of the gearbox oil temperature.The scheme uses joint open-loop and closed-loop control, which not only meets the equipment's cooling needs but also precisely regulates the flow rate of the seawater pump and lowers the use of system energy.

Figure 1 .
Figure 1.Schematic diagram of the cooling system

Figure 2 .
Figure 2. Model diagram of the cooling system

Figure 4 .Figure 5 .
Figure 4. Control logic diagram of the original wind turbine cooling system

Figure 6 .Figure 7 .
Figure 6.Control logic diagram of seawater cooling system for the control scheme as the objective function to output the speed 2 n of another part of the seawater pump.Finally, through open-loop and closed-loop control of the seawater pump speed, the seawater pump speed 2 1 kn n  is input to achieve the target temperature of the gearbox lubricating oil, where k is the weight of 2

Figure 8 .Figure 9 .
Figure 8. Temperature changes of oil at the outlet of the original wind turbine cooling system

Table 1 .
Parameters of plate heat exchanger

Table 2 .
Parameters of throw tube heat exchanger