## 1 Introduction  
A wind turbine drivetrain transfers mechanical energy from the rotor to the generator. It consists of main shaft, main bearing, a gearbox, and other components. Wind turbine gearboxes (WTGBs) that transfer input power (low speed and high torque) from a rotor to a generator (high speed and low torque) are expensive and generally recognized as a component with the highest failure rate among all other components in the drivetrain. Additionally, because WTGBs are installed in the narrow space of a nacelle, repair in the case of failure is costly and time-consuming. The design of a drivetrain influence not only the performance of the wind turbine but also durability. The service life of a wind turbine can be maintained at or extend beyond 20 years by reducing the failure rate of its drivetrain. The first step to reducing the failure rate is to investigate the performance of the WTGB (i.e., efficiency, noise,vibration, and durability). The reliability of a WTGB, measured in terms of performance and durability, can be evaluated through laboratory tests [1; 2]. The test rig for WTGBs can be categorized into electrical and mechanical power circulation types, as shown in Figure 1[3; 4]. The major difference between the two test rigs is the type of power that is circulated through the system. In the case of electrical power circulation, an electric motor drives the input shaft of the test gearbox. Its output shaft is coupled to a generator that feeds the power back to the grid. In this way, the total energy consumption is significantly reduced [4]. However, in this type, when components such as motor, generator, and inverter are being selected, the total power required for the test, including the gearbox driving power, and the losses that occur at the gear/bearing must be considered. Because large and expensive motors, generators, and inverters are required to test large gearboxes, the initial cost of the test is high. Nevertheless, the advantages of this type include high accuracy and ease of torque control, and the test rig configuration is simple compared to the mechanical power circulation test rig. In the case of mechanical power circulation type, input and output shafts of two gearboxes with identical gear ratios are connected to each other by intermediate shafts and gearboxes, building a closed-loop. A torque-applying device can be included in the power loop. The applied torque loads both gearboxes. An external motor is used to rotate the system, which, in steady-state, must compensate only for the total power loss of the system. Therefore, its power rating is much lower than that which actually loads the gears. The degree of power circulation is controlled by the torque, which is applied by the torque-applying device and the motor speed. The advantage of this test rig is the significantly reduced installation cost because it does not require a generator and the driving motor only needs to be rated accordingly to the total power loss [4]. The disadvantage, however, is that the test rig becomes complex due to the addition of auxiliary devices (e.g., shaft, gearbox) to form the mechanical closed loop. Furthermore, the conventional test rig for a WTGB that uses mechanical power circulation uses an actuator installed in the torque arm of the gearbox to apply torque to its closed loop. In other words, because the torque-applying device is installed on the test gearbox, if the size or shape of the test gearbox is changed, the test rig should also be changed. Each mechanical power circulation test rig is  
Figure 1: Power circulation test rig for a wind turbine gearbox(WTGB). (**a**) Electrical type. (**b**) Mechanical type. designed to test WTGB of a certain specification. That is, there is no universal test rig that can test all WTGBs. [PERSON] et al. surveyed published papers on the construction and procedure of a test rig for noise, vibration, lubrication, cleanliness, and efficiency tests, using various test rigs of the two types. In addition, [PERSON] et al. introduced a new compact test rig developed.