Download - CORONA & Its Reduction

7/31/2019 CORONA & Its Reduction

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CORONA

One of the phenomena associated with all energized electrical devices, including high-voltage

transmission lines, is corona. The localized electric field near a conductor can be sufficiently

concentrated to ionize air close to the conductors. This can result in a partial discharge of

electrical energy called a corona discharge, or corona.CORONA LOSSES

Electric transmission lines can generate a small amount of sound energy as a result of corona.

Corona is a phenomenon associated with all transmission lines. Under certain conditions, the

localized electric field near energized components and conductors can produce a tiny electric

discharge or corona that causes the surrounding air molecules to ionize, or undergo a slight

localized change of electric charge.

Utility companies try to reduce the amount of corona because in addition to the low levels of

noise that result, corona is a power loss, and in extreme cases, it can damage system components

over time.

Corona occurs on all types of transmission lines, but it becomes more noticeable at higher

voltages (345 kV and higher). Under fair weather conditions, the audible noise from corona is

minor and rarely noticed.

During wet and humid conditions, water drops collect on the conductors and increase corona

activity. Under these conditions, a crackling or humming sound may be heard in the immediate

vicinity of the line.

Corona results in a power loss. Power losses like corona result in operating inefficiencies and

increase the cost of service for all ratepayers; a major concern in transmission line design is the

reduction of losses.

Methods to reduce Corona Discharge Effect:1.By minimizing the voltage stress and electric field gradient.: This is accomplished by using

utilizing good high voltage design practices, i.e., maximizing the distance between conductors

that have large voltage differentials, using conductors with large radii, and avoiding parts that

have sharp points or sharp edges.

2. Surface Treatments: Corona inception voltage can sometimes be increased by using a

surface treatment, such as a semiconductor layer, high voltage putty or corona dope.


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3. By increasing the spacing between the conductors: Corona Discharge Effect can be reduced

by increasing the clearance spacing between the phases of the transmission lines. However

increase in the phases results in heavier metal supports. Cost and Space requirement increases.

4. By increasing the diameter of the conductor: Diameter of the conductor can be increased to

reduce the corona discharge effect. By using hollow conductors corona discharge effect can be

improved.

5. Using Bundled Conductors: on our 345 kV lines, we have installed multiple conductors per

phase. This is a common way of increasing the effective diameter of the conductor, which in turn

results in less resistance, which in turn reduces losses.

Advantages of Bundled Conductors:

Bundled conductors are primarily employed to reduce the corona loss and radio interference.

However they have several advantages:

Bundled conductors per phase reduce the voltage gradient

in the vicinity of the line. Thus reduces the possibility of

the corona discharge. (Corona effect will be observed

when the air medium present between the phases charged

up and start to ionize and acts as a conducting medium.

This is avoided by employing bundled conductors)

Bundled conductor lines will have higher capacitance to

neutral in comparison with single lines. Thus they willhave a higher charging current which helps in improving

the power factor.

With increase in self GMD or GMR inductance per phase will be reduced compared to single

conductor line. This result in lesser reactance per phase compared to ordinary single line. Hence

lesser loss due to reactance drops.


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