What is a Corona Ring? What about Discharging?

its function is to prevent corona phenomena, which causes sizzling sounds on conductor, some discharge without flashs, and some dust and powders to accumulate on the isolator. İts real cause is ionozation of the the nitrogen in the air.

Corona deteriorates insulation in the vicinity of the discharge, reduces distribution efficiency, and disassociates some gases creating noxious gases and odors

yes it is connected to the live conductor (bushing exit here )

i dunno why but i think it can be because of some induction on the line or ring.

The ring, which surrounds the energized end of the transformer bushing serves two functions. It is a corona ring that is intended to electrically shield the bushing terminal and connections. It does so by reducing the voltage gradient to a level well below the ionizing gradient of the surrounding air at the maximum tranformer output voltage. It's also a grading ring, which helps electrically grade the external voltage on the bushing from line to ground (at the bushing flange). The bushing is likely a condenser bushing, which contains a capacitance-graded core to grade the voltage radially from 100% at the central conductor to ground at the flange and, axially from ground to the top and bottom ends of the core.

Grounding the test transformer following a circuit breaker test is necessary for safety but you are grounding the entire test circuit; not just the corona ring. I suspect the corona ring just happens to be a convenient attachment point for the hook on your ground stick.

Thanks for the replies.

Bluestone,

Yes, you are right. We are grounding the test transformer following a test and the corona ring is an obvious point for the hook of the ground rod.

I believe the grounding achieves two purpose:

1. when not in test, the transformer remains earthed.

2. after the test, the static charges on the HV path are removed to the earth. Could you throw some light on the risks/hazards due to the static charge? Can the static charge magnitude be estimated?

First, you should not even consider not grounding the test transformer following a test. In many high voltage labs, grounding is done automatically when the test voltage is removed and before the safety gate into the area is even opened.

I don't know what other equipment is part of the test but I presume the test includes RIV which means you also have a coupling capacitor in the circuit. Grounding the test transformer HV bushing means you also simultaneously ground everything else that is attached to that bus.

If there is a coupling capacitor in the circuit there could be a significant static charge. It would be possible to estimate the static charge, based on the coupled capacitance of the circuit but why bother? You can also get a pretty good estimate by observing the gap between the hook on the ground stick and corona ring as you ground the test setup following a test. Make the connection slowly, observing the air gap as the hook approaches the corona ring. Watch and listen for a gap discharge. Note the distance between the hook and ring when it drew the arc. The dry power frequency dielectric strength of air at sea level and 25C is about 10 kV rms per inch.

The voltage as well as the electrostatic field strength can also be directly measured.

Hi,

Can I ask a question about corona ring. Is the chemical composition of corona ring important or not for function. A356, A380 like this. Best regards.