## 2 Partial Discharge Data Acquisition  
The vehicle cable termination undergoes stress due to electrical, magnetic, thermal, mechanical, and flow effects during operation, leading to aging of the insulation material and the formation of gaps between different insulation materials. This can result in partial discharge under high electric field, leading to insulation failure and faults in the cable termination. The termination structure of vehicle-mounted cables mainly consists of cable cores, inner semiconducting layers, main insulation, carbon traces, air gaps, insulation pipes, stress control pipes, insulation glue, and umbrella skirts. In the laboratory environment, to simulate on-site air-gap discharge faults, the manufacturer was commissioned to manufacture cable terminals with air-gap defects. Cable termination is cut at the interface of the stress layer and the main insulation to create a 50 mm long and 2 mm wide gap, and the integrity of the defect was maintained by introducing a steel needle into the gap, which was subsequently removed for testing purposes. The structure of the artificial defect cable termination can be observed in Figure 1, and the physical image is provided in Figure 2.  
The high-frequency current technique (HFCT) is employed for the detection of the pulse current signal emanating from partial discharge sources. The coil-type sensor is installed on the cable termination grounding wire. The partial discharge testing platform is mainly composed of a power supply, a corona-free transformer, a protective resistor, and a coupling capacitor. Connecting the AC power supply to the isolation transformer can ensure that its power supply has no interference. The input of the voltage regulator is 380 V AC power frequency, and the output is continuously adjustable from 0 to 500 V. The voltage level is improved using a non-partial discharge transformer with a transformation ratio of 1:400 and a capacity of 200 kVA. A 5 k\(\Omega\) current-limiting resistor is connected in series to the test circuit to prevent large currents from occurring during breakdown. The transformation ratio of the capacitor voltage divider is 1000:1.  
When there is an air gap between the inner layers of the cable termination, partial discharge occurs within the gap and is coupled to the cable-shielding layer through cable distributed capacitance, generating pulsed current at the shielding layer ground connection. High-frequency current sensors (Rogowski coils) are installed on the ground wire for the detection of ultrahigh-frequency pulsed current signals [71]. The laboratory constructed a circuit for partial discharge testing using the HFCT method, as shown in Figure 3. The cable termination with a gap defect in the above test circuit is replaced with a defect-free cable termination, and a probe is inserted at the cable end to detect and record corona discharge without partial discharge. The cable termination with a defect is probed at the end to identify and document mixed signals of partial discharge and corona discharge. The entire experiment is conducted with a sampling frequency of 100 MHz. Figures 4 and 5 show waveform diagrams of partial discharge, corona discharge, and a mixture of corona and partial discharge obtained from three types of experiments.  
Figure 1: Structure of vehicle-mounted cable termination.  
Figure 2: Physical image of vehicle-mounted cable termination.