Abstract: The mechanical discharge switch in electric locomotives is affected by factors such as contact sparks, contact oxidation, contact resistance, and working environment, which increases the internal resistance of its discharge circuit. This prevents the discharge switch from completely releasing the residual voltage of the capacitor, thus threatening the personal safety of personnel. An improved self-discharging electronic discharge switch is adopted for the discharge device of the power compensation capacitor in electric locomotives, making the release of residual voltage of the capacitor faster and more reliable. It also serves as a warning to personnel entering the high-voltage room about the presence of electricity, preventing electric shock accidents. Keywords: Electric locomotive; power compensation capacitor; discharge device; self-discharging; electronic discharge switch The residual voltage discharge of the power compensation capacitor in the Shaoshan 7 electric locomotive is achieved by discharging the capacitor through a discharge circuit formed by a mechanical switch and a resistor. The opening and closing of the moving and stationary contacts of the power compensation capacitor discharge switch is achieved by rotating the high-voltage room door interlock, which, when manually rotated, closes the discharge switch, thus achieving the purpose of discharge. Because the rotation speed of the high-voltage compartment door interlock shaft is inconsistent each time, arcing of the contacts occurs when the operation is too slow, leading to contact oxidation and increased contact resistance. This prevents the discharge switch from completely releasing the residual voltage of the capacitor, and in severe cases, it cannot discharge at all, seriously threatening the personal safety of drivers, passengers, and maintenance personnel. To solve this safety problem and prevent electric shock accidents, the power compensation capacitor discharge device needs to be improved. 1. Scheme Selection Scheme 1: Using a high-power bidirectional thyristor and photoelectric and mutual inductance double isolation, with an external signal source controlling the open-type electronic discharge switch. This scheme uses photoelectric and mutual inductance double isolation between the main discharge circuit and the external control signal circuit, ensuring reliable control, strong anti-interference ability, and no discharge dead zone. However, it is costly and has complex internal and external circuits. Scheme 2: Using a high-power bidirectional thyristor and photoelectric and mutual inductance double isolation, with a self-discharging electronic discharge switch. This scheme's self-discharging electronic discharge switch does not require an external control circuit, is simple to connect to the locomotive's original discharge circuit, and is safe and reliable. Considering the installation location and connection method on the locomotive, and the conduction and cutoff characteristics of the thyristor, Scheme 2 is more suitable for the setting requirements of electric locomotives. 2. Circuit Composition The self-discharging electronic discharge switch consists of a main discharge circuit (composed of a high-power bidirectional thyristor and a discharge resistor), a self-storage control circuit, and a display circuit (see Figure 1). 2.1 Main Discharge Circuit The main discharge circuit consists of CK and peripheral components C1, D1, R1, K1, K2, RZ1, and RZ2 (see Figure 2). C1 is a bypass capacitor to reduce the interference of high-frequency current components in the power supply on the control terminal of the power discharge thick-film CK. D1 is a Zener diode at the control terminal of the power discharge thick-film CK, with its input voltage stabilized within the 12V range. R1 is a current-limiting resistor at the control terminal of the power discharge thick-film CK. CK is a high-power discharge thick-film capacitor, which acts as an open/closer for the discharge circuit. K1 and K2 are trip switches, limiting the discharge current to no more than 10A. RZ1 and RZ2 are discharge resistors for the power compensation device capacitor Cg. 2.2 Self-Storage Control Circuit The self-storage control circuit consists of R2, ZN1, R3, D2, D3, C2, and K3 (see Figure 2). R2 and R3 are step-down resistors. The voltage across the power compensation capacitor is divided by R1 and R2 to supply power to the rectifier bridge ZN1. ZN1 is the rectifier bridge, converting the AC voltage divided by R1 and R2 into DC voltage. D1 and D2 are Zener diodes, stabilizing the DC output from the rectifier bridge within the 24V range to charge capacitor C2, which is the energy storage capacitor. K3 is the high-voltage room door interlock switch. 2.3 Display Circuit The display circuit consists of R4, R5, ZN2, and D4 (see Figure 2). R4 and R5 are voltage divider resistors, ZN2 is the commutator bridge, and D4 is an LED indicator. 3. Working Principle 3.1 High-Voltage Chamber Door Closure When the high-voltage chamber door of the locomotive is closed, the door interlock switch K3 is disconnected, the power bidirectional thyristor thick-film CK control terminals 1 and 2 are de-energized, and its output terminals 3 and 4 are in the off state. The power compensation device capacitor Cg has no discharge circuit. 3.2 Lifting the Main Circuit Breaker When the locomotive is in traction operation, when the locomotive electronic cabinet detects that the reactive power of the main transformer reaches 650kvar, the traction winding power compensation device is activated. The potential at both ends of the power compensation capacitor charges the energy storage capacitor C2 of the self-storage circuit of the discharge device through K1, R2, ZN1, and R3 until it reaches 24V. 3.3 The main circuit breaker with reduced power supply has a stored charge in the power compensation device capacitor Cg. The charge is transmitted through the two ends of the power compensation capacitor via R4, ZN2, D4, and R5 to make the LED light up, indicating that the capacitor cabinet capacitor is charged. When the high-voltage room door is opened, the door interlock switch K3 closes. The 24V at the two ends of the self-stored capacitor C2 is discharged through K2 and R1 to the control terminals 1 and 2 of the power bidirectional thyristor thick film CK. Its output terminals 3 and 4 are turned on. The charge of the power compensation capacitor Cg forms a discharge circuit through K1, CK, and RZ until the potential at the two ends of the power compensation capacitor is lower than 1.4V and the bidirectional thyristor is cut off. The LED light goes out to indicate that the discharge is complete. 4. Main Technical Parameters of the Device Operating Temperature: -4℃～85℃; Bidirectional Thyristor: 2 sets; Maximum Discharge Voltage: DC 2000V; Maximum Discharge Current: DC 10A; Turn-on Voltage: DC 12V; Turn-on Current: DC 1001A; Self-Storage Voltage: DC 24V; Self-Storage Discharge Time: (DC: 24V) ≥ 5 s; Main Discharge Time: (Rated Voltage AC: 630V) ≤ 2 s 5. Application Results The electric locomotive power compensation capacitor discharge device was successfully installed and modified in March 2004. On-board power compensation discharge test was conducted in April 2004. In the initial trial period, it was found that the discharge switch would occasionally trip due to excessive discharge current. To address this issue, after multiple on-board investigations, the gate circuit of the bidirectional thyristor in the discharge switch was modified to gradually increase the opening angle. After testing and improvement, the above problem was completely solved. The modified electric locomotive power compensation capacitor discharge device was installed and applied to Shaoshan-type electric locomotives at the Nanning Locomotive Depot in May 2004. It has been operating normally ever since, enabling faster and more reliable release of residual voltage from capacitors. It also serves as a warning to personnel entering the high-voltage compartment about the presence of electricity, thus preventing electric shock accidents and providing reliable safety assurance for staff. The entire device has a simple structure, is easy to install, and is safe and reliable, making it a good safety device.