Vacuum interrupters performance and tests 2

[DennisPush]

Hi everyone,

(First post here, and please, forgive my English!)

I think on planning some additional tests for vacuum CB's at capacitor switching. (All the VI's are installed in SF6 invironment (1.3bar).) Analysing the data from the event registers I found that some of the CB's have restrikes often (say, every 10 switching-off or more), some don't. Some would perform even a multiple restrike phenomena.

So, I have three questions:

1. Can I assume that the performance (restrike-free) of a vacuum CB depends on vacuum integrity?

2. Is the change from integrity to a broken vacuum quick/stepwise or it may take some significant period of time? How long time? Days? Months? Years?

3. How can the vacuum integrity be tested?
To my knowledge there is a HV test (70-80% of a rated power frequency withstand voltage of a CB). This test helps in case when a VI is in an air environment but is useless (is it?) when the VI is in SF6. This is because SF6 has some 2.5 times higher break-down voltage than air and a VI filled with SF6 will certainly withstand the rated power frequency withstand voltage. Also, there is a possibility to measure the X-rays produced when HV applied across the contacts of a VI. In case of vacuum integrity there will be X-rays and in case of a VI filled with SF6 there will be no X-rays. But how can the results be interpreted if the VI just started being filling with SF6 (again, if this is not a stepwise process)? And finally there is a possibility to analyse the contact movement speed as it is believed that the contacts in VI would be slowed down by gas. But it will be nearly impossible to pick up the vacuum breaking stage and it will be hard to interpret. Does anybody have experience with these tests and how satisfactory were they found?

I would appreciate any advise!!

 

[Gianoli]

***
1. Can I assume that the performance (restrike-free) of a vacuum CB depends on vacuum integrity?

***

No. It is almost impossible to design a vacuum interrupter that will never initiate a restrike when disconnecting a capacitor bank. Much depends on circuit parameters. Proper design dictates that surge arrestors be installed to prevent damaging over voltages. Restrikes will occur, how often depends on the circuit parameters and when the contacts separate.

***
2. Is the change from integrity to a broken vacuum quick/stepwise or it may take some significant period of time? How long time? Days? Months? Years?
***

My experience has been that the change is quick. I must qualify that by saying I have seen very few vacuum interrupters fail, even in extremely demanding applications such as electric arc furnace transformer switching. A mechanical failure that compromises the integrity of the vacuum results in the interrupter being destroyed on the first contact opening. However, if your interrupters are housed in SF6 insulated switchgear they may not be destroyed. I have seen some interrupters slowly fail due to extreme contact erosion but only in the most demanding conditions, high currents and numerous (thousands) operations.

***
3. How can the vacuum integrity be tested?
To my knowledge there is a HV test (70-80% of a rated power frequency withstand voltage of a CB). This test helps in case when a VI is in an air environment but is useless (is it?) when the VI is in SF6. This is because SF6 has some 2.5 times higher break-down voltage than air and a VI filled with SF6 will certainly withstand the rated power frequency withstand voltage. Also, there is a possibility to measure the X-rays produced when HV applied across the contacts of a VI. In case of vacuum integrity there will be X-rays and in case of a VI filled with SF6 there will be no X-rays. But how can the results be interpreted if the VI just started being filling with SF6 (again, if this is not a stepwise process)? And finally there is a possibility to analyze the contact movement speed as it is believed that the contacts in VI would be slowed down by gas. But it will be nearly impossible to pick up the vacuum breaking stage and it will be hard to interpret. Does anybody have experience with these tests and how satisfactory were they found?
***

Just a few comments here for you to consider:

1. My experience has been if a vacuum leak occurs (extremely rare) it fails completely very quickly. I have seen two interrupters fail in 25 years. I currently work for a company who has thousands of vacuum interrupters in service and in 12 years I have not seen one failure due to total or partial loss of vacuum.


2. The quickest and most reliable method to determine if a vacuum interrupter has lost vacuum is to pull open the moving contact open. It will take considerable effort to open an interrupter that has its vacuum intact. The moving contact in an interrupter that has lost all vacuum will be loose and will open with ease, in fact it will probably fall open if you turn it upside down. This means if you can disconnect your vacuum interrupters from the operating mechanism it should be easy to determine the integrity of the vacuum.

 

[ANGIBEW25]

I AGREE WITH GIANOLI, WHEN A VACUUM INTERRUPPTER FAILS, IT USUALLY FAILS INSTANTLY AND THE INTERRUPTER IS USUALLY FUBAR.
IF YOU HAVE A VCB WHICH IS ENCLOSED WITH SF6 HAVE YOU CONFIRMED THE STATUS OF THE SF6? IS THE PRESSURE CORRECT? HAVE YOU CHECKED IT'S MOISTURE CONTENT?
YOU MAY WANT TO CONSIDER TAKING A GAS SAMPLE FOR FIELD OR LAB ANALYSIS.
YOU MAY ALSO CONSIDER A TIMING TEST. IF ONE OR MORE OF THE INTERRUPTERS IS COMPROMISED IT WILL REFLECT IMPROPER TIMING RESULTS.

GOOD LUCK.