Solid Insulation Breakdown - Internal Discharge

In Greek Mythology, there is a story about Trojan War where the city of Troy is attacked by the Army of Greeks led by the King of Sparta. The war lasted for 10 years until the Greeks used a wooden horse to trick the people of Troy. The Greeks constructed a big wooden horse and place a group of elite soldiers inside it. The horse was left outside the Troy city wall and the Greeks pretend to retreat. 

The Trojan Horse and some Greek soldiers. It is confirmed that the fart's gas is not SF6

The people of Troy thought the war was over and the Greeks were really retreating. They also thought that the wooden horse is a gift from the Greeks and they took the wooden horse inside their city wall without realizing there were a group of elite soldiers inside the wooden horse. At night, the elite soldiers killed the Troy city guards, opened the city gate and let the other Greeks army to enter Troy. The people of Troy is caught off-guard and The city of Troy falls to the hand of the Greeks.

But wait, why there is a boring history lesson inside a high voltage blog?

The baby is also bored and no, this is not the son of Santa Claus

Actually the mechanism and phenomena of Internal Discharge in solid insulation breakdown is closely related to the story of the Trojan Horse in the Trojan War. The explanation below will enlighten this relationship.

Cavities and voids can be formed in insulation materials of high voltage equipment due to mishandling during operation, mechanical force during high voltage fault, insulation material aging, thermal effect or chemical  reaction. Cavities and voids can also be readily exist in insulation material because of the defect in manufacturing processes. These cavities and voids are the Trojan Horses which will lead to the failure of the insulation. It is like you are having an enemy within you, waiting for the right time to destroy you from within just like what the elite soldiers inside the Trojan Horse did to the city of Troy.

11kV XLPE Aluminium Single Core 500mm2 with voids. The white dots in red circle are not your dandruff so please don't wipe your monitor. They are actually cavities/voids.

The picture above shows an 11kV Aluminium Single Core 500mm2 cable which uses Cross Linked Poly Ethylene (XLPE) as its insulation. The cavities / voids inside the XLPE layer (circle in red) is automatically filled with air which has a lower dielectric constant from XLPE. Do take note that the dielectric constant of air is 1 and dielectric constant of XLPE is 2.35. Because of a lower dielectric constant in the cavity / void, higher charges will be available inside the cavity / void which in turn increases the electric field strength inside it. When the high electric field strength exceed its breakdown value, breakdown in the form of discharge (spark) will occur.

Can you see a spark or discharge inside the cavity on the teeth? I can't...because it's a teeth Bro, not a  high voltage equipment

When the discharge occurs, the electric field inside the cavity / void will be zero. However, the same process will be repeated again and again. Charges in the cavity / void will start to accumulate again, electric field strength in the cavity / void will be increasing and when the electric field strength exceed the breakdown value, breakdown in the form of discharge will occur again.

Equivalent circuit of a cavity/void in an insulation. C1 is capacitor representing the cavity /void, C2 representing insulation in series with the cavity/void, C3 representing the rest of the insulation.

The severity of the discharge depends on the operation voltage of the XLPE cable. Operation voltage of 33kV has much more severe discharges compared to 11kV discharges. On the other hand, the frequency of the operation supply will determine the number of the discharge and how frequent the discharge will occur. For example, numbers of discharge in a 50Hz electricity supply is less than in 60Hz electricity supply . The more frequent the discharge occurs, the faster the insulation will deteriorate.

The discharges inside the void will create positive ions and electrons which in turns move and hit the surface of the void because of the availability of electric field. The collision will break the chemical bonding on the void surface thus making it bigger. The frequent numbers of discharges will dissipate heat inside the void. The heat will carbonized the void surface and cause an erosion. This erosion will make the void bigger. If the void keeps getting bigger, catastrophic breakdown can happen as the insulation can no longer withstand the electrical stress. 

So beware of the "Trojan Horses" inside the insulation of your new or existing high voltage equipment. Make sure you always do necessary tests (Insulation Resistance Test, Pressure Test, Transient Earth Voltage, Ultrasound) before commissioning new HV components, after maintenance of HV components or after any repairing work.