About the Corona Ring

Dear sir:

yes ,I agree with you,but under the 275kV(1min power frequency withstand voltage of 145kV class) , the electric field on the upper flange or the bolts is about 70kV/cm, how to ensure the type test pass sucessfully?

the 30kV/cm criteria means under 145/1.732=83.7kV or ohter voltage?

Please note that the 275 kV test is a proofing test and does not mean that the equipment normally operates continuously at this voltage, many other things will go bad if 275 kV were somehow imposed continuously on equipment rated at 145 kV. Corona/grading rings are sized for normally expected voltages.

With all due respect for your modeling, under most circumstances manufacturers do type testing to ensure that the actual equipment is suitable, and passes all required tests, for actual use. Since I don't know your modeling technique, I suggest that you contact the manufacturer and discuss your concerns with them directly.

If you are the manufacturer I suggest that you engage the services of seasoned professionals that have done this type of testing for others. Modeling is used to predict physical behavior, but should never be used as a substitute for full scale proof testing.

Thank you very much for your kind help.

Year 2013 passed quickly and year 2014 is coming, I happy new year to you and I wish you happy and healthy in 2014.

I agree with your idea completely. I am the manufacture . This is the first time I design 145kV bushing, my curious minds still want to know the things below and I hope I can get the engineering answer from you.

1) about the dielectric type test

I did the FEM calculation and I think the 145kV bushing maybe ok in real service(145/1.732=87kV the electric is about 17kV/cm).but I worried that maybe there is something wrong with it in the dielectric type test,(275kV 1min power frequent and 650kV LIWV) so could we add a corona ring in the type test but take it off during the real service? is it agreed with the IEC standards?

2) about the design of 145kV bushing

I asked the lab professinals, their answers are the same as you estimated, 145kV bushing seldom disrupture during the dielectric type test, so is it necessary to design the 145kV bushing carefully such as a good sheild on the top flange to protect the bolts and the sharp edge of the kits?

3) about the criteria of the air

the only criteria about air we have is 30kV/cm under power frequent voltage, Is there any other criteria about air under LIWV and SIWV, Is there some theory to explain such phenomenon(the electirc field is high ,but the type test is still ok)? In the bushing design, the length of the bushing is important or the electirc field is important?

Thanks a lot again

Best regards

Breakdown voltage of dry dust-free air at 25 Celsius is about 30 kV/cm between 2 cm diameter spheres, but about 12 kV/cm between needle points.

It is (approximately, for a given diameter and gap) proportional to pressure and inversely proportional to absolute temperature Kelvin. Eventually, increasing gap gets less increase of breakdown voltage than proportional.

Obviously, outside a laboratory, insulators work in dusty, humid air and necessarily rain or dust storms outdoors. By the sea or on a ship they get salt spray.

In the design of bushings, length is necessary to get required bulk clearance in air but surface length (corrugation shape) and material are vital to avoiding surface breakdown.

You really need to look at existing designs and design rules in your company and published papers on bushing design to be sure you are using proven techniques.

It is is especially necessary to do that if the voltage is much higher than was formerly manufactured.

why I can't reply?

you just waited too long to post, and it timed out.. Go back, hit "copy", then start from the beginning and paste your comment.

Dear Sir:

I met the problem again, the message is :Could not process form: The system was unable to process this form.

Crudely, 275 kV/ (70kV/cm) = 4cm gap.

Clearances in air for 145 kV equipment are far greater than 4 cm.

So the question is - how do you calculate 70 kV/centimetre?

dear sir: thank you very much！ the 70kv/cm means a local electric field. not everywhere.the same principle is like yours 12kv/cm.in the uneven field. best regards

Thank you very much for your kind help.

Year 2013 passed quickly and year 2014 is coming, I happy new year to you and I wish you happy and healthy in 2014.

I agree with your idea completely. I am the manufacture . This is the first time I design 145kV bushing, my curious minds still want to know the things below and I hope I can get the engineering answer from you.

1) about the dielectric type test

I did the FEM calculation and I think the 145kV bushing maybe ok in real service(145/1.732=87kV the electric is about 17kV/cm).but I worried that maybe there is something wrong with it in the dielectric type test,(275kV 1min power frequent and 650kV LIWV) so could we add a corona ring in the type test but take it off during the real service? is it agreed with the IEC standards?

2) about the design of 145kV bushing

I asked the lab professinals, their answers are the same as you estimated, 145kV bushing seldom disrupture during the dielectric type test, so is it necessary to design the 145kV bushing carefully such as a good sheild on the top flange to protect the bolts and the sharp edge of the kits?

3) about the criteria of the air

the only criteria about air we have is 30kV/cm under power frequent voltage, Is there any other criteria about air under LIWV and SIWV, Is there some theory to explain such phenomenon(the electirc field is high ,but the type test is still ok)? In the bushing design, the length of the bushing is important or the electirc field is important?

Thanks a lot again

Best regards

Wei Jun,

Thanks for the feedback, it's nice to see that you are being diligent in your efforts.

1. I would check the current IEC test standards regarding any changes to the EUT (Equipment Under Test) to see if it's allowable to make any change (the addition and removal of the corona ring) before and after the test sequence.

2. I would test first and see if there are any local problems. As you know, sharp edges can become corona hotspots. If there are any problems you could probably fit a spherical cap over the offending bolt and make it part of the installation kit.

3. The properties of the ambient air is a major consideration, along with pollution levels, a unit at sea level in the desert will react differently to one at the sea coast or at high altitude. The SIWV and LIWV (Surge/Lightning Impulse Withstand Voltage) use different wave shapes, repetition rates, and timings, so I suggest that you familiarize yourself with the science and standards for each of the tests.

There is a minimum bushing length, after that the ambient conditions dictate whether additional length and/or creep distance is required, I'll leave the science of shaping the electric field for your further study.

Alan

I can't reply again,why?

Dear Mr Alan:

Thank you for your kind praise.

I want to reply you quickly, but as I said, I don't know why I can't reply, I tried for several times and suceed finally.

The way to test the bushing you suggested is very good, I'll prepare a corona ring, if it is ok without a corona, it is better; if it is not ok, I can fit it immediately, but the cost of the bushing will be high.

For a bushing, it should pass the dielectric type test first. For the dry test, I have ever experienced a flashover of 362kV bushing during 1 min power frequent 510kV, the reason may be that the corona ring may not designed very good.

And I have ever read the standards for bushing. And most the dielectric type test is done under clean condition. So the things I want to know is : Will a polluted bushing not withstand 1 min temporary overvoltage in service, And LIWV and SIWV as well?

For the length and electric field, which is more important, I think if there is a lab for us to test , it may be less difficult to find the answer.

I still have a lot of question about the bushing, the criteria and the standards, such as after so many years development, the standards about bushing changed little. And about the dry arc distance and the creeping distance regulated in the standards, do they all have a respective theory and test explain?

This is the first time for me to register on the CR4, I have received so many help for my first question, I think I will improve a lot with all of your help.

Thanks a lot again.

Best regards

Weijunmei

The insulator type also determines the need for a corona ring, 138kV NCI (EPRI recommendation) are being equipped with corona rings to protect the metal to poly sheath junction on the energized end from corona that destroys the poly covering and the insulator rod.

Agreed the normal for ceramic insulators of 220kV to 345kV is without corona rings, although there are exceptions due to local contamination, salt fog, ...

I have pictures of many 220kV SF6 breakers with corona by the top cap, I will look for them and post. One of the considerations is the direction of the connector / conductor leaving the breaker spade, if the conductor is 'in-line' with the axis of the insulator there is less corona potential. High voltage does not like sharp corners.

I have also seen corona on the grounded end of 500kV dead-end ceramic insulator strings, corona in the center of ceramic insulator strings.

I've been selling and renting corona cameras www.specialcamera.com and providing corona survey services since 2000 and I am Level 2 infrared, and coordinated / assembled the Corona Technology Course www.corona-technology-course.com I have also been on-site with EPRI and ESKOM corona research staff.

I cannot reply, why?

the message is :Could not process form: The system was unable to process this form

Recently (it did not happen in years previous) I have found that if I have been logged on to CR4 for a while then posts are rejected with the message you give.

I have to do following..

1. Click on back arrow to go back to my message.
2. Highlight all my message by wiping over with mouse or placing cursor at beginning, then holding shift key down while using keyboard left/right/up/down arrow keys to select text.
3. Hold Ctrl key down and press C key to copy highlighted text to a buffer memory.
4. Log out of CR4.
5. Log in to CR4.
6. Go back to thread, open new comment box.
7. Click mouse in comment box, cursor appears.
8. Hold Ctrl key, press V key to insert text copied at step 3.
9. Going through the preview and submit stages then works OK.
   

I suppose CR4 have taken steps to avoid folk blocking up access resources when they take a long time over a comment.

67model

Dear Sir: thank you very much,iam looking forward to your photoes！ best regards weijunmei

dear sir: thanks a lot, it seems that i have found the answer, maybe the iexplore. i use my phone to reply. it.is ok now. best regards weijunmei