What is Partial Discharge?

The Partial Discharge (PD) is an electrical discharge that, as the name suggests, does not completely “bridge” the insulation between electrodes or conductor materials. It is basically a flashover on small parts of the insulating system where the dielectric's capability is lower than the localized electric field. 

This is mainly due to imperfections or defects of the dielectric, which cause a no-uniform dielectric strength and in the parts where it becomes lower than the local field applied, the discharge can occur. It’s partial, because it does not involve the entire dielectric material. 

Figure 1 shows how it’s possible to model the presence of defects or imperfections in a dielectric. As you can see, we can associate a separate capacitance to each imperfection within the dielectric. If the electric field across a given capacitor creates a voltage higher than that the capacitor can withstand, partial discharge occurs leading to  a current pulse that travels along the capacitor. 

 

Figure 1. Scheme of electrical model of defects present inside a dielectric

 

Usually, this effect can occur in MV and HV electrical application as it depends strongly on the voltage level, the higher the voltage, the bigger the probability that this effect will appear. 

Partial Discharge can occur everywhere, independently of the kind of insulation used to separate electrical components: it can occur in a liquid insulation because of the presence of gas bubbles or in a solid insulation for example inside the dielectric of a cable. The effects of the Partial Discharges are different depending on the location where the discharge occurs. It is possible to classify them in the following categories: 

1. Surface Discharge: occurring across the insulation surface
2. Internal Discharge: inside voids or defects of the solid insulation
3. Arcing Discharge: electrical breakdown of a gas producing a plasma discharge
4. Corona Discharge: ionization of the fluid or air surrounding a conductor

The theory to describe the Partial Discharge effect is basically the same for all the categories, so the above classification is just a sorting according to the “location” where the discharge occurs and the different ways how it manifests. 

The main causes of partial discharge can be related to: defects on the material, wrong installation with material damages, surface contamination, wrong application and ageing of material. 

 

Understanding Corona Discharge

All these causes can be easily associated with the first three categories mentioned above. Corona discharge is worth a more detailed description, due to its nature. 

The Corona effect is quite common in HV overhead transmission lines or in presence of sharp edges of HV components. In this case the dielectric material is the air, which has a dielectric strength equal to 3 kV/mm measured under standard conditions (at a certain pressure, with a defined humidity level and so on). 

When the environmental conditions differ from the standard ones, the value of the dielectric strength of the air changes, for example it decreases if the humidity content increases. So, there could be some moments in which the electrical field of the conductor exceeds the dielectric strength of the air and the discharge occurs. 

When the electric field exceeds this value, the air surrounding the conductors start to be ionized, due to the continuous electrical discharges and this creates a “circle” around the conductor with a visible blue/violet color that looks like a ring around the conductor or the sharp edge. 

Usually this phenomenon is visible in proximity of a HV Substation or in presence of HV overhead lines. 

In case a Corona discharge occurs between two close conductors, the lowest voltage at which a continuous corona of specified pulse amplitude occurs is called Corona Inception Voltage (CIV) and it’s defined according to Peek’s law (from 1929):

e_v=g_v r l_n(S/r)

Where 

r is the radius of the wires in cm.

S is the distance between the center of the wires.

g_v is the "visual critical" electric field, and is given by:

$$g_{\nu } = g_{0}\delta \left ( 1+\frac{c}{\sqrt{\delta r}} \right )$$

where respectively δ is the air density factor with respect to SATP (25°C and 76 cmHg), g₀ is the "disruptive electric field" and c is an empirical dimensional constant.

The Corona discharge has basically the same effects of the other type of Partial discharges, which are: 

• Power loss (due to leak current that would flow through the capacitance represented in Figure 1)
• Possible interferences with radio, due to the high frequency current that it generates
• High frequency noise (like a buzzing)
• Production of Ozone gas

Figure 2. Corona discharges on an insulator string of HV lines. Image courtesy of Nitromethane (CC BY-SA 3.0)

 

Usually to minimize those effects, corrective choices are implemented already in the design phase, like avoiding sharp edges, or installing specific rings to reduce the sharp effect, or sizing the conductors in numbers, section and distances in order to limit possible corona discharges.

 

Partial Discharge: Evidence of a Future Major Issue

In most of the cases the Partial Discharge is the first evidence of a future major failure that can occur, if no corrective actions and proper maintenance are put in place. 

Indeed each discharge reduces the dielectric properties along the time up to the point that a final very big discharge occurs leading to complete destruction, damage to the nearby components and serious safety risks.  

It is very important to check the quality of the material during manufacturing of components such as cables and switchgears. In addition to monitoring the manufacturing process, it is necessary to conduct similar tests on the installation site to ensure that installation activities did not cause any damages that can lead to partial discharges.