1. Overview The Gongzui Hydropower Plant is located on the Dadu River in Sichuan Province. It consists of two power stations, Gongzui and Tongjiezi, forming a cascade hydropower system. Gongzui Power Station has an installed capacity of 7×100 MW, and Tongjiezi Power Station has an installed capacity of 4×150 MW, for a total installed capacity of 1300 MW. The Gongzui Hydropower Station has above-ground and underground powerhouses. The above-ground powerhouse is a dam-type open-air powerhouse with an installed capacity of 4×100 MW, while the underground powerhouse is a tunnel-type powerhouse with an installed capacity of 3×100 MW. The Tongjiezi Power Station is located approximately 33 km downstream of the Gongzui Power Station and is also a dam-type open-air powerhouse. The drainage system of Gongzui Hydropower Station is divided into a unit maintenance drainage system, a powerhouse seepage drainage system, a dam seepage drainage system, and an underground powerhouse area seepage drainage system, with a total of 6 collection wells and 19 drainage pump motors; the drainage system of Tongjiezi Hydropower Station is also divided into a unit maintenance drainage system, a powerhouse seepage drainage system, a dam foundation seepage drainage system, and a dam seepage drainage system, with a total of 4 collection wells and 13 drainage pump motors. [b]2 Problem Statement[/b] Traditional motor starting methods generally adopt full-voltage, star-delta, or reduced-voltage starting. Currently, the AC motor reduced-voltage starters used in large motors in China are mainly in the form of autotransformers and resistors. The Gongzui Dam collection well has 2 225 kW motors and 2 75 kW motors, the Tongjiezi Dam collection well has 3 132 kW motors, and the Tongjiezi maintenance collection well has 3 225 kW motors. Their starting method is reduced-voltage starting, and the reduced-voltage device is an autotransformer. During operation, issues such as contactor contactor burnout or autotransformer burnout frequently occur. This is particularly true for the two 225 kW motors in the Gongzhan Dam's sump pump motors. These motors have two power sources: one from the plant's 400 V system and the other from the dam's 400 V system (backup power). When using the backup power, the load on the dam's 400 V system easily approaches the rated load of the dam's 400 V transformer. Therefore, during motor startup, the large inrush current causes voltage fluctuations, resulting in large voltage drops and high current peaks, high torque, and prolonged startup time, creating a vicious cycle. This can easily lead to contactor contactor overheating and damage, or autotransformer overheating and burnout, seriously affecting the power plant's safe operation. Furthermore, the autotransformer equipment is bulky and complex, incurring high maintenance costs and requiring significant repair work. Therefore, our plant has modified the starting method for the power plant's sump pump motors, adopting a new motor starting method: soft start. [b]3 Application of Soft Starters[/b] Based on the actual conditions and operating characteristics of the power plant, after a long period of research and selection, the Aitstart46 soft starter manufactured by Schneider Electric was finally selected. This model of soft starter is a series product with a rated current of 12 A to 1200 A, and can control motors with a power supply voltage between 208 and 500 V and a rated power between 2.2 and 800 kW. 3.1 Working Principle of Aitstart46 Soft Starter The working principle of the soft starter is that the thyristor is connected to the three-phase power supply line of the motor. Utilizing the electronic switching characteristics of the thyristor, the microcomputer controls the size of its firing angle to change the opening degree of the thyristor, thereby changing the magnitude of the motor input voltage to control the starting characteristics of the motor. After the motor starting process is completed, the controller controls the vacuum contactor to engage, shorting all thyristors, allowing the motor to directly connect to the grid and run, thus avoiding unnecessary energy loss. 3.2 Control Modes of the Aitstart46 Soft Starter: Current-Limited Soft Start Control Mode: During soft starting, the output voltage rapidly increases from zero until the output current reaches the set current limit value I. Then, while ensuring the output current does not exceed this value, the voltage gradually increases, and the motor gradually accelerates to its rated speed. In this control mode, the motor starts with a set current limit value, which can be adjusted within the range of 2 to 5 times the motor's rated current. The starting characteristics of the soft starter are shown in Figure 1. The Aitstart46 consists of a control module, a power supply component, and the starter unit itself. The microprocessor in the control module can be used to control the torque during motor start-up and shutdown without requiring a tachometer. See Figure 2. 3.3 Functions and Features: During motor operation, the Aitstart46 soft starter can monitor and control its status, monitor the motor's rotation direction, and prevent phase reversal. When the motor is overheated, overloaded, overvoltaged, undervoltaged, or experiencing a phase loss, it can automatically provide timely protection. Its overcurrent and overload values ​​are adjustable. The Aitstart46 soft starter also features underload protection, reducing malfunctions caused by water pump drainage. It also has remote control functionality. [b]The soft starter has the following characteristics:[/b] ① Smooth motor starting, reducing starting current surges and avoiding grid impact. ② Adjustable starting voltage ensures minimum starting torque for the motor, preventing overheating and energy waste. ③ Adjustable starting current according to load conditions, reducing starting losses and generating optimal torque with minimal current. ④ Adjustable starting time; the Aitstart46 soft starter's starting time can be adjusted between 0 and 999 seconds, within which the motor speed gradually increases, avoiding speed surges. ⑤ Adjustable soft stop speed. ⑤ Reduces mechanical stress, protecting production equipment and extending its lifespan. ⑥ Reliable performance, simple and convenient operation, and intuitive display. The Aittart46 soft starter comes with a display module. This display can be used to adjust the soft starter parameters, show the motor operating current during operation, and display the fault name in case of a fault, facilitating operator troubleshooting. ⑦ It adapts to the special environmental requirements of hydropower plants, other harsh environments, and heavy-load starting requirements. 3.4 Operating Mode In the sump drainage system, our plant has modified the control system, changing from the original relay control unit to PLC control. The start and stop of the soft starter are controlled by the PLC. The PLC provides a pair of trigger contacts (normally open contacts) and a pair of stop contacts (normally closed contacts) to complete the start and stop of the soft starter. However, the soft starter itself is self-contained and can operate independently; that is, a PLC failure does not affect the operation of the soft starter. The wiring diagram of the soft starter is shown in Figure 3: The soft starter has 6 power supply terminals. A1, B1, and C1 are three-phase power input terminals, and A2, B2, and C2 are output terminals. RNN and PL are the soft starter's operation control terminals. [b]4 Conclusion[/b] Our plant's first soft starter was successfully modified on the drainage pump motor of the dam's sump at the Gongzui Hydropower Station. Based on the successful commissioning of this soft starter, we promoted its use throughout the plant, successfully applying it in the maintenance of the Tongzhan Hydropower Station, the dam foundation sump pump motor, the sump pumps at the upper and lower plants of the Gongzhan Hydropower Station, and the oil pump motor of the spillway gate at the Tongzhan Hydropower Station. After commissioning, the soft starter controller operated well, demonstrating stability and safety during operation, smooth motor starting, effectively protecting the motor, extending its service life, and providing a guarantee for the plant's safe production and economic operation. It also met the technical requirements of the hydropower station for "unmanned operation and minimal staffing." Practice has proven that the soft starter has advantages such as simple installation, convenient operation, complete protection functions, reliable operation, and convenient maintenance, and its price is moderate, making it a new type of contactless starting equipment worthy of promotion and application.