Is Corona Discharge a Problem?

[brupp]

My previous work had all been involved with 161 kV and lower so I had little or no knowledge about corona. However we have recently had a substation installed at 345 kV and it sounds like a bacon frying party. Besides the annoyance, is this a problem?

The engineer designed it with a single conductor per phase, approximately 1000 kcmil size, and the corona discharge seems to be significant based upon the snap, crackle sounds. I didn't know enough to make them use two conductors per phase and they apparently didn't know better either.
- The total circuit length is not more than 500 feet so I wouldn't think the losses are any big deal.
- We have tried out our radios below the line and there doesn't appear to be a problem with interference there.
- The worst case current is well below the conductor capacity.
- The plant is in the middle of a corn field and the nearest neighbor can't hear the noise.

So I just want to find out if I should be concerned about a phenomenon which does not appear to be causing any problems. It seems analagous to using a bad paint color choice. It's annoying but no harm done. Comments?

 

[TheBlacksmith]

Corona is a function of voltage, not cable size. It is indicative of the insulation system, even if that system is air, (i.e. conductor separation), being under a high enough voltage stress to ionize the insulation and cause current flow. At best it represents an electrical loss akin to a high resistance ground or short, however it can directly damage insulation, create ozone and nitric acid compounds and lead to tracking and flashover.

Blacksmith

 

[brupp]

Thanks for your reply. The situation I'm describing, however, is uninsulated cable (ACSR). I only recently became aware that corona is in fact a function of cable diameter due to the distribution of the voltage stresses around the perimeter. Larger diameter cable has more surface area and is therefore more immune to corona. But there appears to be a limit to the usefulness of increasing diameter, more or less the law of diminishing returns. That's why EHV lines have multiple conductors per phase.

 

[electricpete]

There is no doubt that smaller conductors are more susceptible to corona for precisely the reasons brupp said. Larger conductor has larger radius of curvature which acts less like a sharp corner (sharp corners are bad).

I'm not sure if I understood your last paragraph right.
The limit on increasing cable size doesn't depend on corona... that is related primarily to current carrying capacity (diminished by skin effect for large conductors), and possibly to series inductance.

345KV can make a lot of noise. Worst time will be very early in the morning when dew is forming.

Improper terminations, loose strands etc can also be a source of corona.

A noticeable increase in corona over time would certainly be cause for concern... might want to visually inspect with binoculars for signs of contamination or schedule washing of insulators.

 

[dydt]

lots of discussion about corona effect in this forum...do advance search and enter "corona effect"....lots of thread to view..

here is a sample...

thread248-13564

 

[jghrist]

It is somewhat unusual to have strain bus in a 345 kV substation instead of rigid tubular bus. On 345 kV transmission lines, bundled conductors are generally used at 345 kV, both to reduce corona and to increase the Surge Impedance Loading (SIL) capability of the line by reducing the inductance. According to the Westinghouse T&D Manual, it would require a conductor diameter of 1.8 inches (2312 kcmil ACSR) to limit the corona loss of a single conductor to the recommended one kW per circuit-mile.

At 345 kV you also need special corona free hardware and/or corona control rings.

While your nearest neighbor is now a corn field, the owner may want to develop the property in the future and the audible and radio noise, particularly during rain, could seriously reduce the value of his property.

 

[SphincterBoy]

One quick and effective way of pinpointing the corona (if you have it) is to mount a 35 mm SLR camera on a tripod, and take a 2-minute timed exposure photograph of the area of the substation where you think the corona may be coming from.

The photograph must be taken during the nightime, under complete darkness.

It (corona) should show up in the photo as a glow surrounding the connection or whatever.

 

[cuky2000]

Brupp,

It is suggested to consider the risk of noise that this installation generates exposing people within and out the property limit including plant worker and occasional visitors such as maintenance personnel.

There is an OSHA regulation that limits the noise level and time of exposure. I understand that there is an old but active regulation for communication interference to protect TV radio and other civilian and military communications.

After evaluate the suggested risk there is possible to implement and evaluate the cost of noise mitigation strategy that should the impact of a potential facility loss of revenue for down time. Keep in mind that not only corona but also wind could cause noise on HV facilities. This is affected mainly related by type of conductor, diameter, surface, configuration, wind, altitude, rain and snowfall.

Aeolian noise occurs when pressure fluctuation around the conductor interacts with the wind. This type of noise could be reduced increasing the conductor roughness and size including the OH ground wire.

Corona noise has two components; one irregular noise caused by high frequency and hums caused by low frequency that is difficult to control. Corona noise are affected heavily by rain and more likely to occur if the conductor surface potential gradient > 14kV/cm. This factor is directly associate with the conductor diameter.

Possible solution to investigate could be the use of elliptical conductor, install spiral rods on existing conductor or use other noise damper devices. One potential manufacturer could be Hitachi Cable.

 

[electricpete]

cuky - interesting stuff. Out of curiosity...
1 - how does elliptical conductor help? I was under impression the radius of curvature was importance (elliptical will have portions with lower radius of curvature than equivalent area circular conductor).
2 - why is there a potential gradient on the surface. I thought the surface would all be at the same potential.

 

[cuky2000]

Dear Electricpete,

I understand that Brupp’s primary concern is to control the audible noise. I believe that should be analysed from two different approaches, corona and aeolian effect.

RESPONSE 1: To mitigate the corona effect, the elliptical conductor should have sufficient radius to achieve voltage gradient in the range 14kV/cm < Eo < 18 kV/cm and simultaneously mitigate the aeolian noise. As alternative elliptical shape conductor could help to reduce noise forcing the air vortice on the opposite site of the wind flow, creating circle with opposite directions cancelling the pressure at the conductor surface.

RESPONSE 2: Potential gradient is the electric field intensity that describe the voltage stress and should not be confused with potential difference. The following simplified relation analytically could describe the potential gradient for single conductor:
(14<Eo<18) = V/ r.Ln(2h/r)
_{WERE:
V = Conductor voltage.
r = Conductor radius.
h = Conductor height.}