Corona discharge

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8. Corona discharge

8.1 Introduction
Corona on transmission lines causes power loss, radio and television interference, and audible noise near the transmission line. At extra-high-voltage (EHV) levels (i.e.; at 345kV and higher), the conductor itself is the major source of audio noise, radio interference (RI), television interference (TVI), and corona loss. RI is a noise type that occurs in the AM radio reception, including the standard broadcast band from 0.5 to 1.6 MHz. It does not take place in the FM band. Radio noise (RI or TVI) is usually expressed in mV/m or in dB above 1 µV/m. The effects of corona in EHV transmission lines depend on a number of parameters that may not remain constant over a period, and the contributions of each add to the effects in a complex manner. The determination of the disruptive critical voltage requires the assignment of average values for the conductor irregularity factor that may vary considerably with the weathering effects on the conductor.
The radio and television interference levels depend upon the variations in radial distance from the conductor to the antenna and the line height. Some parameters are known with good accuracy and may be taken as constant, whereas there are others, which are affected by errors of evaluation and may vary with time .The usual interest consists in evaluating the disruptive critical voltage and the radio noise due to corona with respect to these variable parameters.

The term corona is used to describe the partial discharge that develops in zones of highly concentrated electric fields. A number of observable effects, such as visible light, audible noise, electric current, energy loss, radio interference, mechanical vibrations and chemical reactions, accompanies the corona discharge. The chemical reactions that accompany corona in air produce the smell of ozone and nitrogen oxides.

Corona has long been a main concern for power transfer engineers because of the power loss it causes on the lines and the noise it causes in radio and TV reception. Corona has several beneficial applications as in Van De Graff generators, electrostatic precipitations, electrostatic printing, electrostatic deposition, ozone production.
“Corona is a luminous discharge due to ionization of the air surrounding a conductor around which exists a voltage gradient exceeding a certain critical value.”
8.2. Corona Discharge:

If the voltage is high, the surface stress may reach a value at which the air breaks down, and becomes a conductor. The conducting layer of air forms part of the conductor so that r increases and the maximum stress decreases. If the spacing is small enough, the corona may bridge the conductors and cause flashover. Generally, the spark is large enough for the corona to cease spreading long before it bridges the conductors; values of r and E are reached such that the stress is insufficient to ionize any more air.
The phenomenon of corona is accompanied by a faint glow and a hissing noise. There is also energy loss.
8.3. Disruptive critical voltage:

A transmission line should operate just below the disruptive critical voltage in fair weather so that corona only takes place during adverse atmospheric conditions. Therefore, the calculated disruptive critical voltage is an indicator of the corona performance of the line. However, a high value of the disruptive critical voltage is not the only criterion of satisfactory corona performance. The sensitivity of the conductor to foul weather should also be considered, and the fact that corona increases more slowly on stranded conductors than on smooth conductors. According to Peek2, after making allowance for surface condition of the conductor by using the irregularity factor, the expression for the disruptive critical voltage,

P = barometric pressure in centimeters of mercury, P0=76cm
t = ambient temperature in degrees Celsius, t0=25°C
The breakdown strength of air at 76 cm pressure and 25° C is 30 kV/cm or 21.1 kV (rms)/cm. This value is called E. At a barometric pressure of P cm of mercury and t° C, the breakdown strength is E.
V0 is the disruptive critical voltage. Bad atmospheric conditions such as fog, rain or sleet may reduce V0 to 0.8 of the value given above.

Figure 8 Corona discharge on a 160 kV transmission line ceramic insulator. The humidity is 80%.
8.4. Visual Critical Voltage

When the voltage of the line is the disruptive critical value, there is no visible corona. This is because the charges ions in the air must be able to receive a finite energy before they can cause further ionization by collision, which is necessary for the corona discharge. Peek states that the disruptive critical voltage must be so exceeded that the stress is greater than the breakdown value up to a distance of 0.3√δr cm from the conductor. Thus, visual corona will occur when the breakdown value is attained at a distance r + 0.3√δr from the axis, instead of a distance r.
This requires that the voltage to neutral be (1+0.3/√δr) times the disruptive critical voltage.

The expression for the visual critical voltage, Vv, given by Peek is:

Note that the voltage equations (1) and (2) are for fair weather. For wet weather voltage values, multiply the resulting fair weather voltage values by 0.80. For a three-phase horizontal conductor configuration, the factors 0.96 and 1.06 multiply the calculated disruptive critical voltage for the middle conductor and for the two outer conductors, respectively.
8.5. Corona Loss

If the visual corona voltage is exceeded, the power loss due to corona is given by Peek's formulae. The expression for the fair weather corona loss per phase or conductor, Pc is:

Corona loss occurs on T.L conductors when the voltage gradient in the immediate vicinity of the conductor surface exceeds the breakdown strength of air. The breakdown of air in this region generates heat, light, audible noise and radio interference. Corona loss on an EHV line can fluctuate from a few kilowatts per mile in fair weather to several hundred kilowatts per mile in rain or snow.
The magnitude of fair weather corona loss is significant in comparison with foul weather loss. Fair- weather loss occurs for a large percentage of time and the consideration will affect the value of the total energy consumed by the line.
The resistance losses depend on line loading, while the corona losses depend on the weather conditions.
8.6. Avoidance of corona:

• The critical voltage can be raised either by increasing the spacing or the diameter of the conductors. The spacing cannot be increased greatly or the cost of the supports will be very high. The diameter of the conductor can be increased by using hollow conductors.
• Steel-cored Al conductors have a large diameter for a given conductivity and weight, and are thus good from the point of view of corona.
• For very high voltages 275 kV, it has been found economical to use the hollow conductors.
• A line is usually designed to work at a voltage just below the disruptive critical voltage for fair weather (= 1). It is economical to have a small corona loss in bad weather, rather than have larger conductors to avoid corona entirely. Moreover, corona acts as a safety value for surges.

8.7. Current effects of corona:

Corona forms when the voltage of a conductor passes the disruptive critical voltage, and disappears when the voltage descends through the same value. This occurs on each conductor every half cycle and contributes a triple harmonic to the charging current, since the effective capacitance of the conductor increases when the corona is present. The triple harmonic currents pass through neutral to earth in an earthed system, in a non-earthed system the neutral has a voltage to earth of triple frequency.
8.8. Radio and Television Interference:

One of the possible consequences of T.L corona discharges is radio noise. Radio noise (RN) refers to any unwanted disturbance within the radio frequency band, such as undesired electric waves in any transmission channel or device. The radio frequency band extends from 3 kHz to 30000 MHz. The corona discharge process is pulsatory in nature producing pulses of current and voltage on the T.L. conductors. These pulses are characterized by rise and decay time constants, which may be in the order of hundreds of microseconds, and repetition rates, which may be in the MHz range. The frequency spectra of those pulses can cover a considerable portion of the radio frequency band. The electromagnetic fields resulting from the corona discharges may create unwanted disturbances in the operation of a transmission channel over a wide range of frequencies. T.L corona may be a source of a radio noise.

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