**1.1. Shielded dielectric power cables**

Since the 1970s, shielded dielectric cables have been used overseas, and since the mid-1980s, they have been extensively used in North America as well. Today, because of environmental concerns, shielded solid dielectric cables are used more and more and most new power cables installed are solid dielectric cables. Figure 1 [1] shows the structure of a typical high-voltage shielded dielectric power cable. The conductor core F is the part that delivers power. The conductor shields C and E help smooth the electric field between the conductor and the insulation materials. The dielectric or insulation D is used to insulate high-voltage core F from the ground, i.e. neutral wires B. Encapsulating jacket A prevents water and dust from getting into the cable.

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**Figure 1.** A typical high-voltage extruded solid dielectric cable. A, encapsulating jacket; B, neutral wires; C, ground shield; D, dielectric; E, conductor shield; F, conductor; the conductor shield and ground shield are semiconductive. Usually, they are made by adding carbon black into a polymer and the particle size of the carbon black ranges from 15 to 50 nm. The basic function of this configuration is to confine the electric field within the cable and obtain a symmetri‐ cal radial distribution of the electric within the dielectric [1].

### **1.2. Partial discharge in shielded dielectric power cables**

Most failures occurring in the shielded dielectric cables are related to partial discharge. Partial discharges are localized breakdowns in a small portion of the insulations, which can be solid or fluid electrical insulation. When high voltage is applied to high-voltage equipment, defects introduced during the manufacturing process such as contained insulation cracks, contained insulation surfaces, or voids can all lead to partial discharges. In some cases, even without any defects, aging can cause the degradation of the insulator leading to partial discharges. PD makes damage to the equipment, and equipment with PD occurring within will eventually fail after a certain time depending on the strength of the PD if proper treatments are not applied. It's important to monitor partial discharges in high-voltage systems and if PD is detected, appropriate actions should be taken to prevent sudden failures, which can cause big blackouts.

Figure 2 gives us some basic ideas on how partial discharges occur. The cavity in the insulation normal contains gas which could be ionized when the electrical field exceeds the cutoff strength. When this happens, electromagnetic waves in the radiofrequencies are generated along with light, heat, noise, and possibly gas. With appropriate technologies and by detecting the HF radio signals, the magnitude as well as the location of partial discharges can be identified and used for assessing the health status of the shielded dielectric cable.

**Figure 2.** A "typical" partial discharge mechanism. The cavity in the insulation normally contains gas, which could be ionized when the electrical field exceeds the cutoff strength.
