2.2.4. Resistivity Test

In the actual operation of the cable, the working temperature is greatly affected by the load. The resistance of the semiconductive layer will increase with the increase in the temperature, showing obvious PTC effect, which will lead to the increase in the interface thermal effect between the semiconducting layer and the insulating layer, and affect the service life of the cable. In this work, the resistivity of the semiconductive layer was measured by the DB-4 wire and the cable semiconductive rubber-resistance tester using the (DC) current-voltage method test principle. The samples, with length 110 mm, width 50 mm, and thickness of 1 mm, were obtained by hot pressing crosslinked. The sample is placed in a drying oven with programmable temperature control, and the resistivity of the sample is recorded at different temperatures. When the instrument is used to measure, the sample does not need surface treatment, and the operation is simple. The resistivity of the sample can be obtained directly without formula derivation and calculation, thus avoiding the error in the calculation process.

### 2.2.5. Pulsed Electroacoustic Measurement (PEA)

The distribution of space charge was tested by PEA. The LDPE insulating sample used for PEA testing had an average thickness of 300 μm and the semiconductive layer had a thickness of 500 μm. The experiment was carried out for 30 min at room temperature under a negative DC electric field of 10 and 40 kV/mm. The PEA test chart is shown in Figure 2.

**Figure 2.** Illustration of the pulsed electroacoustic (PEA) measurement.

### 2.2.6. Thermally Stimulated Current (TSC)

The TSC method includes the thermal stimulation polarization current method (TSPC) and the thermal stimulation depolarization current method (TSDC). The TSDC method is more common in the measurement and characterization of traps in polymer insulation. Thus, the TSC method generally refers to the TSDC method. The TSDC method was used in this experiment. The thickness of the insulating sample and the semiconductive layer used for TSC was 300 and 500 μm, respectively. A negative DC field strength of 10, 30, and 40 kV/mm was applied to both ends of the LDPE for 30 min at room temperature when the semiconductive composites with different LSC contents were used as the semiconductive layer, and then, the sample was rapidly cooled. Next, the temperature was raised from 293 K at a heating rate of 5 K/min to 363 K to measure the value of the thermal stimulation current during the heating process. The schematic diagram of the TSDC test is shown in Figure 3.

**Figure 3.** Schematic diagram of thermal stimulation current method.
