Dielectric Strength

Introduction

Dielectric strength is a property of insulating materials. Electricity is the utility power that drives the entire world's equipment in the way that we see it today. The transmission and distribution of electric power for industrial and household purposes are through insulated cables. Dielectric strength materials available intrinsically in cable insulations. The dielectric strength prevents electricity leakage from the transmission strands (usually copper) inside the insulation. This tutorial covers the definition, methods of ascertaining the dielectric strength of a material, and other related concepts core to dielectric strength.

Dielectric Materials

• Some insulating materials have the functionality of storing electrical energy as capacitors do.

• Some can prevent electrical shock that comes in contact with the human body. In the first case, the insulator helps build electrical charge between two conducting plates without the plates coming in direct contact. Such materials are called dielectric materials.

• In the second case, insulators protect from electrical shock through insulation properties, and such materials are called insulators.

• If a material’s primary function is to provide electrical insulation, then it is an insulator. If a material’s prime function is to hold and build a charge in a context, then it is a dielectric.

Fundamental Definitions in Dielectric

There are many scientific terms defined in connection with the word dielectric. In the context of insulators, the tutorial focuses on dielectric strength, and hence the definitions of few terms are detailed here. The list, however, is worthy and is as follows.

• Dielectric effect

• Dielectric breakdown

• Dielectric strength

• Dielectric materials

• Dielectric constant

• Dielectric polarisation

• Dielectric permittivity

• Dielectric heating

• Dielectric loss factor

Dielectric Constants

A dielectric constant is a number without units and shows the ability to build electrical energy. It is denoted by the symbol k. By definition, the dielectric constant is the relative permittivity of the material. Relative permittivity is the ratio of the permittivity of material to that of vacuum. This parameter is important in the characterization of capacitors.

Dielectric Strength

• The highest dielectric strength is of a perfect vacuum. Dielectric strength is measured in Volts per meter (V/m) or Mega Volts per meter (MV/m).

• The value of DBS for a perfect vacuum is 1×10¹² MV / m .

• The higher the DBS, the better the insulating material's ability to prevent electrical conductivity.

• Dielectric strength is the threshold voltage with which the insulating property of the material breaks down and starts allowing electricity to flow like a conductor.

• The list below shows the dielectric strength of different insulation materials used in transmission and distribution applications. The figure shows how breakdown strength is tested.

  • Perfect vacuum - 1×10¹² MV / m

  • Mica - 118 MV / m

  • Transformer Oil - 24 MV / m

  • Teflon - 60 MV / m

  • Porcelain - 12 MV / m

  • Air - 3 MV / m

Factors Affecting Dielectric Strength

Dielectric strength increases when the thickness of the material increases initially, but you cannot expect the dielectric strength to double when the thickness doubles. Other factors that affect dielectric strength are temperature, moisture, content, shape, and time rate of raising the voltage.

Conclusion

The tutorial describes dielectric strength, which is a property of insulators. After a brief introduction and differentiating dielectric from an insulator, the fundamental definitions of dielectric are explained. The tutorial includes an illustrative figure of the dielectric testing procedure. Then the explanation of dielectric constant and dielectric strength is provided in the context of capacitor and insulator, respectively. Finally, we present the factors affecting the dielectric strength of a material.

FAQ’s

1. What are the method strategies used to measure dielectric strength?

Identifying scope is the first step where the what and why of the test are studied. The test procedure is then identified according to the requirement and perfected with instructions. The sample specimen size is ascertained and collected as per the test procedure. The final test results are recorded, like the cause and location of the failure, insulator thickness, and applied voltage, for further analysis. The inference from the strategy results is the quality of the insulation material. Higher dielectric strength means a better-quality insulator.

2. How are the multilayer poly-electrolyte films helping the high-voltage electronics segment with their high dielectric breakdown strength?

The main features of poly-electrolyte films that provide high DBS (dielectric breakdown strength) are molecular weight, polymer architecture (backbone), and thermal cross-linking. Increasing the molecular weight of polymer ions increased the DBS. From the backbone architecture point of view, a linear polymer architecture was found to increase the dielectric strength compared to branched polymer architecture. And with thermal cross-linking, DBS value increased due to its effect on chemical and microstructure. Multi-layer polymer films exhibit DBS of 300 to 400 kV / mm. High-voltage electronics segment gained by the usage of such 200-400 mm layer thick polymer films.

3. How is the dielectric strength of air connected with lightning?

The context of lightning is the cloud, the atmospheric air, and the electrically neutral ground. The clouds build up a charge, and when it crosses a threshold of dielectric strength of atmospheric air (that is 3kV / mm), then the charge grounds to earth which we see as lightning. Ionized air is a good conductor, and the charge can flow from the cloud to the ground through this conducting path.

4. What are the four types of testing principles?

Before testing, let the minimum requirement or the test threshold be made available in clear terms. The four tests are

• Threshold time tested − It is an endurance test. The threshold level is applied, and the withstanding time of this threshold is measured.

• Test to break down − It is a short test. The threshold level is reached from zero at a uniform rate and tested until the breakdown happens. The rising rate is selected based on the material’s time characteristic on any parameter and overall test time.

• Test to threshold level − It is a step-by-step test. The threshold level is reached starting from half of the threshold value. The increase in level happens step by step, and for each step, the parameter (voltage in case of dielectric strength) is applied for a specific time.

• Test to threshold value to break down − It is a slow-rate test. This test also starts from half the threshold value and at a uniform rate till the breakdown level is reached. The rate is selected for equal exposure of the test specimen. The size of the test specimen varies.

5. Why is the oil used in transformers that deliver electrical distribution?

The oil in transformers provides advantages as listed below.

• It acts as an insulator between the live electrical parts and prevents flash-overs.

• The oil impregnates the fibrous materials and contributes to arc quenching.

• It dissipates the heat developed in the copper winding.

• Transformer oil has a good dielectric strength of 24 Mega Volts per meter.