Insulators 101: Electrical Stresses

There are several aspects that need to be considered carefully when designing and planning the transmission line network. This is very crucial because in the long term it will greatly affect the reliability of the system while reducing costs incurred during the construction process. On the technical side, of course, we want to design a robust system using the best materials or the latest technology. However, a good engineer in addition to considering technical factors also needs to be aware of the limitations of existing costs.

This is the challenge of how and what methods should be applied when designing and planning an optimal system while keeping the cost as efficient as possible.

An interesting case occurred when I joined a zoom meeting discussing about the pollutant areas that SUTET Paiton-Banyuwangi will be passed through and the consideration of applying polymer insulator on SUTET/GITET.

Because the terrain that will be traversed by SUTET consists of dusty coastal and hilly areas so the risk of corrosion, salt, and dust will be high it needs more frequent maintenance. So the engineer considering applying polymer insulators.

The problem is, polymer insulator is more expensive compared to porcelain and glass insulator.

So, how exactly are the steps needed to consider what kind of material of insulator would be applied and why polymer insulator will become a trend in insulation technology. I\\\’ll cover up in the next few posts talking about the insulator and its related theories.

For this chapter, I\\\’d be talking about the electrical stress on the insulator. Electrical stress is the crucial consideration in term of insulator requirements because it directly related to how proper the insulator withstand the electrical properties of the system.

Continuous Power Frequency Voltage

The insulator must withstand the nominal voltage of the system. The value of nominal voltage will fluctuate during load changing during operation. In that case, PLN stated the standard for voltage fluctuation is around +-10%. Specifically, the phase-ground voltage will generate stresses on the insulator so for example in a 500KV system the requirement insulator needed shall be,

This value, later on, becomes the minimum standard for determining how many insulator disks are needed in addition to creepage distance requirements.

Temporary Overvoltage

Phase to ground fault, switching, transmission line energizing and resonant which generate potential surge with varying periods of the time related to the amplitude.

In most cases, the capability of the insulator to endure the temporary overvoltage will be stated by the manufacturer during the compliance test. For example, NGK porcelain fog insulator 120 kN has a \\\”dry power frequency withstand voltage\\\” 85 kV per disk and around 45 kV per disk in wet conditions.

Lightning Overvoltage

The overvoltage surge caused by lightning that strikes to,

1. Phase conductor
2. GSW (induction current that not entirely neutralized by grounding system and lead to backflash to insulator and phase ground)

A lightning strike will generate a high rising and short periods of overvoltage. The peak magnitude in a lightning surge that is generated by the direct strike to the phase conductor is so high that it can cause flashover to the insulator. The level of stresses felt by the insulator is equal to the lightning impulse on the structure.

With that consideration, the BIL of an insulator is set to apply lightning impulse standard value with peak time (T1) 1.2 us and (T2) 50 us for half time. That standard is used to run a simulation and measure the withstand voltage of the insulator during impulse strike. When the insulator is being tested at a high voltage laboratory, peak overvoltage will be raised step by step until the insulator show how much the flash occurred in every level voltage applied. Which, well represented in distribution statistics of how much the insulator endure the overvoltage.

That is the electrical stresses that take place on the insulator. For the next post, I\\\’ll be talking about the environmental stresses that are important also because it will affect the durability and the lifetime of the insulator. So, see ya.