## Visual Corona

What is corona discharge it is the generated voltage of the visual corona discharge is higher than the critical breakdown voltage. In order to form a visible corona, some degree of ionization must occur and the electrons must be pulled into the excited state. The light produced by the discharge is excited, not ionized, and emits excess energy in the form of light and other electromagnetic waves. To obtain the visible corona formation critical voltage, the critical breakdown voltage must be multiplied by a factor that depends on the air density and conductor radius. In addition, it turns out that the values of the irregular elements are different.

The empirical formula for visual corona formation is

The value of the irregularity coefficient mv of the visual corona is

For smooth conductors, m v = 1.0,

= 0.72 is the local corona on multiple strands (patches)

= 0.82, used for corona measured on twisted lines (all lines)

## Stable corona formation

Considering two conductors, it is only the limit of corona formation. Given a thin layer of ionized air r around each conductor, the effective radius is (r r). The change in electrical stress due to this layer can be determined by differentiation. Thereby

The above expression is negative if loge> 1. That is, d / r> e (= 2.718)

In this case, the effective increase in diameter reduces electrical stress, no further increase in stress is formed and the corona stabilizes. On the other hand, for d / r <e, effective increase in diameter causes electrical stress, which leads to further ionization, further increase in radius, and eventually arc discharge.

In fact, the effective limit of d / r is about 15, not e (= 2.718). For normal transmission lines, the ratio d / r is far above 15, so a stable corona is always produce before the flashover.

For example, on a 132 kV line with a pitch of 4 m and a conductor radius of 16 mm, the value of this ratio is d / r = 4 / 16×10-3 = 250 >> 15.

## Power loss due to corona

At first Corona formation is associate with power loss. This loss has little effect on line efficiency, but its impact on voltage regulation is less important. The more important effect is radio interference.

The power loss due to corona is proportional to the square of the difference between the neutral voltage of the line and the critical breakdown voltage of the line. Given by empirical formula

Where E0, rms = critical breakdown voltage (kV) f = power frequency (Hz) E = phase voltage (line to neutral) (kV)

Under stormy conditions, under clear weather, the critical breakdown voltage is consider to be 80% of the critical breakdown voltage.

## An example – what is corona discharge

Also determining the critical destructive voltage of a 100 km three-phase 132 kV line (including conductors of diameter), in clear and storm conditions, visual voltage of corona onset, line power loss by corona 1.04 cm, equilateral triangle configuration, Distance 3 m. Ambient temperature is 400 ° C and pressure is 750 Torr. The operating frequency is 50 Hz. [The irregular coefficient can be regarded as mo = 0.85 and mv = 0.72]

Air density correction factor is

∴ Critical breakdown voltage = 21.2 0 r loge (d / r) kV to neutral point = 21.2 x 0.925 x 0.85 x 0.52 x loge (3 / 0.0052) = 55.1 kV

Similarly, corona visible voltage = 21.2 x 0.925 x 0.72 x 0.52 x loge (3 / 0.0052) x [1 0.3 / (0.895 x 0.52) 1/2] = 67.2 kV

Power loss in sunny conditions

= 365 kW / phase

Similarly, the power loss in stormy weather is given by:

= 847kW / phase