Corona Discharges
All the diverse ionization processes involved in producing corona discharges in the highly stressed regions near the conductors of transmission lines, as well as the creation and movement of charged particles in the electric field, require an expenditure of energy.
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| 7 Bad Effects Of Corona On Transmission Lines (photo credit: rovdrone.eu) |
This energy is supplied by the high voltage power source connected to the transmission line, which generates the high electric field near the conductors necessary to sustain corona discharges.
Most of the energy is converted to thermal energy for heating the air in the immediate vicinity of the conductors. A small proportion of the energy is converted to electromagnetic radiation including light emission, to acoustic energy, and to electrochemical energy required to produce gaseous effluents ozone and nitric oxides.
Let’s discuss now seven really bad effects on work of HV transmission lines:
1. Corona Loss
The power loss, defined by the rate at which energy is drawn by corona from the high voltage power source, is known as corona loss. Since the electromagnetic, acoustic, and electrochemical components are only a small part of the overall energy, corona loss is effectively caused by the movement of positive and negative ions in the electric field.
The lifetime of electrons created in the discharge, before they attach to neutral molecules and become negative ions, is very short, and consequently, their movement in the electric field gives rise only to short-duration current pulses, which do not contribute significantly to corona loss.
On an ac transmission line, the sinusoidal voltage applied to the conductors causes a capacitive current to be drawn from the power source.
Before the onset of corona, the power source is called upon to supply mainly the capacitive current. The capacitive current flowing in the conductor gives rise to a small amount of I2R power loss.
At voltages above corona onset, however, the oscillatory movement of the ionic space charge in the alternating electric field near the conductor gives rise to an additional alternating current component.
Unlike the capacitive current, the current produced by the movement of ions is mostly in phase with the voltage and, therefore, gives rise to power loss, known as corona loss.
The corona current also contributes to a small component in phase with the capacitive current, thus causing an apparent increase in the capacitance of the conductor configuration. Analytical treatment of corona loss on ac transmission lines is very complex and requires the solution of time-varying space charge fields.
Information required for design purposes is obtained mainly through experimental studies.
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Credit- Electrical Engineering Portal. Published by Department of EEE, ADBU : tinyurl.com/eee-adbu
