## 1 Introduction  
In recent years, wind energy has become one of the most important renewable energy sources in the world, and many countries are vigorously developing wind power generation [1]. As the core equipment of wind power generation, the operation and maintenance of wind turbine generators have attracted much attention; however, the life of wind turbine generators in actual operation is much smaller than the design life, which is because the insulation of inverter motors always fails prematurely [2; 3; 4]. The cause of the failure has been shown to be the use of Pulse Width Modulation (PWM) technology after much research and practice [5]. At low frequencies, the length of the cable is so different from the wavelength of the voltage wave in the cable that the effect of the distribution parameters can be neglected. However, in high-frequency motor drive systems, the wavelength of the voltage wave decreases dramatically, and the length of the cable is non-negligible in comparison, so the effect of long cables must be considered. When the high-frequency PWM voltage pulse is transmitted to the motor terminal through a long cable, the PWM pulse generates high-frequency oscillations in the cable due to the characteristic impedance of the cable, resulting in overvoltage [6; 7; 8]. Scholars have focused their primary research attention on the effects of cable lengths. The effect of long cables on overvoltage at the motor end is mainly due to the process of voltage transmission in the cable. As the characteristic impedance of the cable is seriously mismatched with the impedance of the output end of the inverter and the equivalent impedance of the motor, this causes significant reflection of the voltage wave. The superposition of the reflected wave and the incident wave makes the motor end generate a huge overvoltage. Therefore, scholars have researched and analyzed the formation of voltage wave reflections in cables and the process of reflection transmission, explaining the principle of generating overvoltage at the motor end [9]. [PERSON] et al. found that the voltage at the motor terminals approximately doubles when the pulse is transmitted from the inverter to the motor for more than 1/3 of the time of the rising edge [10]. [PERSON] et al. found that the length of the cable under high-frequency pulses causes a delay in the wave propagation time, which further leads to overvoltage generation [11]. [PERSON] et al. found that the overvoltage stops rising when the cable length reaches a critical value and calculated that the critical cable length is proportional to the PWM rising edge time [12]. [PERSON] et al. proposed a reflected wave drive-cable-motor system model, and discussed the effect of distributed vs. transport delay-type models on modeling accuracy [13]. [PERSON] et al. further modeled the correct representation of cable parameters over a wide frequency range up to MHz [14]. Based on these models, [PERSON] constructed a theoretical calculation model of overvoltage at the motor end, and the error between the calculated overvoltage amplitude and the actual measured value is less than 10% [15]. [PERSON] et al. proposed a way to divide the whole system into two parts by establishing the equivalent circuit at the front end of the motor; the inverter and the cable constitute the Davignan equivalent circuit, after which the motor end voltage is finally analyzed by obtaining the equivalent circuit at the motor end [16]. After modeling the connecting cables at the end of the motor, the scholars used software such as ANSYS to perform simulations to study the motor turn-to-turn distribution [17; 18]. It was ultimately determined that the maximum voltage occurs at the first or last turn of the coil, depending on the motor construction. To suppress this overvoltage, scholars have proposed several methods. The first is series-parallel resistance or an RC filter, which suppresses the overvoltage effect obviously but with a large power loss [19].