Corona and its Effects in Power Transmission Systems

What is Corona?

Corona is the term used to describe the violet glow or ‘brush’ discharge around conductors when the air is stressed beyond the ionization point without flashover developing. When an alternating potential difference is applied across two conductors whose spacing is large as compared to their diameters, there is no apparent change in the condition of air surrounding the wires if the applied voltage is low. But, when the applied voltage exceeds a certain value, referred to as critical disruptive voltage, the conductors are surrounded with this faint violet glow called corona.

In thin films, particularly in spaces between layers of sheet insulation, air can readily become ionized due to electric stress across such spaces exceeding the critical value. This is often due to the fact that, with dielectrics in series, the stress in each section is inversely proportion to its permittivity i. e. the electrical storage ability. When the critical stress in the air or gas is exceeded, discharges occur (often termed to as corona, ionization, glow or brush discharges) and this causes splitting up of the gas molecules. In air this leads to the formation of ozone and nitrogen oxides which in presence of moisture produce nitric acid. The ozone has a strong oxidizing effect. The action of either or both the ozone and the nitrogen oxides on many materials is to cause decomposition and often the formation acids.

Effects of Corona in Power Systems

High voltage gradients surrounding conductors (above about 18kV/cm) will lead to a breakdown of the air in the vicinity of the conductor surface known as corona discharge.

Corona causes a certain amount of energy loss with alternating current especially in high-voltage transmission lines. It produces radio interference and may initiate surface deterioration and breakdown on solid insulation surfaces as explained below. Corona also produces secondary chemical effects.

The action of either ozone or nitrogen oxides or both on a number of materials causes the formation of acids such as oxalic acid that is known to cause brittle fracture in polymeric insulators by the oxidation of cellulose materials, and acetic acid from the decomposition of cellulose acetate.

Related to article: Types of Insulators used in Power Systems

Additional to the chemical effects, discharges in spaces, films or cavities within dielectrics can have serious consequences mainly because of the high energy in some of the individual discharges. Mechanical, electrical and thermal damage can occur and breakdown in service may result after long periods.

Advantages & Disadvantages of Corona

In designing high voltage overhead line, a balance should be found between merits and demerits of Corona.

Advantages

• Corona reduces the effects of transients produced by surges.
• Because of the formation of corona, the air surrounding the conductor becomes conducting and as a result virtual diameter of the conductor is increased. The increased diameter reduces the electrostatic stress between the conductors.

Disadvantages

• Corona leads to loss energy. This affects the efficiency of line transmission.
• The current drawn by the line due to corona is non-sinusoidal and thus non-sinusoidal voltage drop occurs in the line. This may cause inductive interference with neighboring telecommunication lines.
• The ozone produced by corona may cause corrosion of the conductor due to chemical action.

How to Reduce Corona discharges

Typically intense corona effects occurs at voltage of 33 kV or above, thus, a careful design to avoid corona should be considered on the substations or busbars rated for 33 kV and higher voltages otherwise highly ionized air may cause flash-over in the insulators or between phases causing considerable damage to the power equipment.

Two techniques that can employed to reduce corona effects are:

• Increasing the conductor size – by increasing the conductor size, the voltage at which corona occurs is raised; thus corona effects are largely reduced.
• Increasing the conductor spacing – by increasing the spacing between the conductors, the voltage at which corona occurs in the line is raised; thus corona effects are reduced. Nevertheless, space cannot be increased too much to avoid a large increase in the cost of the supporting structure.

Related articles:

• Surge Suppression in Power Systems
• Types of Busbar Arrangements in Grid Stations and Substations
• Switchgear Equipment