TEXT 13. Insulators and Accessories

Electric insulation is a vital part of an electrical power system. Insulation failure may cause permanent equipment damage and long-term outages. As an example, a short circuit in a 500-kV system may result in a loss of power to a large area for several hours. The insulation of an electric system is divided into two broad categories: internal insulation and external insulation.

The external insulation (transmission line or substation) is exposed to electrical, mechanical, and environmental stresses. The weather and the surroundings (industry, rural dust, oceans, etc.) produce additional environmental stresses. The insulator must withstand these stresses for long periods of time. It is anticipated that a line or substation will operate for more than 20–30 years without changing the insulators.

Transmission line and substation insulation integrity is one of the most dominant factors in power system reliability. The major components of the line are: conductors, insulators, support structure tower. The insulators are attached to the tower and support the conductors. In a suspension tower, the insulators are in vertical position or in a V-arrangement. The typical transmission line is divided into sections and two dead-end towers terminate each section. This sectionalizing prevents the propagation of a catastrophic mechanical fault beyond each section. As an example, a tornado caused collapse of one or two towers could create a domino effect, resulting in the collapse of many miles of towers, if there are no dead ends.

       The electrical stresses on insulation are created by: continuous power, temporary overvoltages, switching overvoltages, lightning overvoltages. The insulation has to withstand normal operating voltage fluctuations from changing load. The line-to-ground voltage causes the voltage stress on the insulators. Temporary overvoltages include ground faults, switching, load rejection, line energization and resonance, cause power frequency, or close-to-power frequency, and relatively long duration overvoltages. The duration is from 5 to several minutes. The most frequent causes of switching overvoltages are fault or ground fault clearing, line energization, load interruption, interruption of inductive current, and switching of capacitors. Lightning overvoltages are caused by lightning strikes: to the phase conductors, to the shield conductor, to the ground close to the line. Lighting strikes cause a fast-rising, short-duration, unidirectional voltage pulse. The time-to-crest is between 0.1–20 µsec. The time-to-half value is 20–200 µsec.

       Most environmental stress is caused by weather and by the surrounding environment, such as industry, sea, or dust in rural areas. The environmental stresses affect both mechanical and electrical performance of the line. The temperature in an outdoor station or line may fluctuate between -50°C and +50°C, depending upon the climate. The temperature change has no effect on the electrical performance of outdoor insulation. It is believed that high temperatures may accelerate aging. Temperature fluctuation causes an increase of mechanical stresses, however it is negligible when well-designed insulators are used. Wind drives contaminant particles into insulators. Insulators produce turbulence in airflow, which results in the deposition of particles on their surfaces. The continuous depositing of the particles increases the thickness of these deposits. After a long time (months, years), the deposits are stabilized and a thin layer of solid deposit will cover the insulator. Because of the cleaning effects of rain, deposits are lighter on the top of the insulators and heavier on the bottom. Moisture, fog, and dew wet the pollution layer, dissolve the salt, and produce a conducting layer, which in turn reduces the flashover voltage. The pollution can reduce the flashover voltage of a standard insulator string by about 20-25%. Heavy industrial pollution occurs in the industrialized areas and near large highways. In northern areas and in higher elevations, insulators and lines are frequently covered by ice in the winter. The ice produces significant mechanical loads on the conductor and on the insulators. The transmission line insulators need to support the conductor’s weight and the weight of the ice in the adjacent spans. This may increase the mechanical load by 20–50%.

       Porcelain is the most frequently used material for insulators. Toughened glass is also often used. Several insulators are connected together to form an insulator string. Insulator strings are used for high-voltage transmission lines and substations. They are arranged vertically on support towers and horizontally on dead-end towers. Nonceramic insulators use polymers instead of porcelain. High-voltage composite insulators are built with mechanical load-bearing fiber-class rods, which are covered by polymer weather sheds to assure high electrical strength. The major components of a composite insulator are: end fittings, corona ring(s), fiberglass-reinforced plastic rod, interface between shed and sleeve, weather shed. Electrical field distribution along a nonceramic insulator is nonlinear and produces very high electric fields near the end of the insulator. High fields generate corona and surface discharges, which are the source of insulator aging. Above 230 kV, each manufacturer recommends aluminum corona rings be installed at the line end of the insulator. All high-voltage insulators use rubber weather sheds installed on fiberglass rods. The interface between the weather shed, fiberglass rod, and the end fittings are carefully sealed to prevent water penetration. The most serious insulator failure is caused by water penetration to the interface.

1. Read and translate the text.

2. Read the text again and make a plan in the form of questions.

3. Read and decide if the following statements are true (T) or false (F).

1). External insulation is exposed to the environment.

2). The sectionalizing is needed for the propagation of fault beyond every section.

3). In a dead-end tower, the insulators are in a horizontal position.

4). Typical external insulation is the porcelain insulator supporting transmission line conductors.

5). Corona rings are used at low or medium voltages.

4. Match the words from the text with their corresponding definitions.

5. Complete the sentences using the words from the box below.

1). However, regular … is needed to minimize the number of faults per year.

2). A(n) … caused by flashover produces only a temporary fault.

3). Suspension insulators need to carry the weight of the conductors and the weight of occasional ice and wind ….

4). These insulators are not used in the US because vandals may break them and cause ….

5). Lightning-caused … produce short circuits.

6. Give a short summary of the text.