HighVoltage4800
Member
- Location
- Detroit
Hey guys I'm a lineman at a utility and having trouble understanding ferroresonance phenomenon.
When we build 3 phase Isolation Wye to Delta (13.2kv to 4.8kv) banks, we put the H1 to the primary and H2 to a floating ground between all 3 H2 bushings. There were instances where people were blowing arresters and or fuses due to ferroresonance. Now our details install a 4th cutout that grounds the floating ground, to be closed, then the 3 cutouts energizing the bank and then open the 4th cutout.
My understanding of ferroresonance is that when the inductive and capacitance is the same that amperage no longer lags voltage by 90* and the transformer acts like resistance load there by canceling out the electromotive force in the coils and causes the arresters and fuses to blow.
This 4th cutout detail only applies to our ISO Wye to Delta banks. Our engineers say it is not required for a Wye primary to Delta secondary (120/240/208) bank. However I'm hearing of people. Losing them on these banks now.
My question is, the guys the other day closed the first transformer and phased across the B1 cutout, they said it was ~13.2kv which makes sense to me as you put 7.6kv into the H1 of the A transformer it goes through the coil onto the floating ground into the H2 on the B1 out the H1 and you get 7.6 x 1.73 to get 13.2kv. They closed the B1 cutout, nothing happened. Phased the B2 cutout and had ~22kv. Now if this is true 21kv our arresters are rated to 10kv, why wouldn't the arresters blow? In my mind you now have 13.2kv going onto the floating ground into the H2 of the B2 and out the H1 and when you phase across the cutout 13.2kv x 1.73 = 22.8kv. They refused to close the cutout. They grounded the floating ground and closed it the. Removed the ground. If the voltage truly is 13.2kv coming out the H1 on the B2 why doesn't it blow the arrester that's only rated to 10kv. Why does it supposedly blow once you close the 3rd cutout?
I closed the same bank 1 pole away with a pogo from the ground and it didn't blow.
Our bulletin says "when there is no neutral" well on Wye Primary to Delta Primary there is no neutral on the Delta side which makes sense so capacitance can build up causing ferroresonance. However when it's Delta secondary (120/240/208) there is a neutral bonded to ground. So in my mind if electrons are building up in the primary coil they should repel the electrons in the secondary coil and they can flow down the neutral to ground. That negating the capacitance build up causing ferroresonance.
Is it because the service of 3 phase 350kcmil wire is hooked up but the breakers are off so that wire is to much of a capacitance and over powers the neutrals ability to bleed off the electrons?
Or is it the electrons will not flow down the neutral because there is no return path to the coil and if you left a single phase transfomer energized long enough will no load eventually the ferroresonance would build up and blow the fuse and arrester?
Thanks in advance guys.
When we build 3 phase Isolation Wye to Delta (13.2kv to 4.8kv) banks, we put the H1 to the primary and H2 to a floating ground between all 3 H2 bushings. There were instances where people were blowing arresters and or fuses due to ferroresonance. Now our details install a 4th cutout that grounds the floating ground, to be closed, then the 3 cutouts energizing the bank and then open the 4th cutout.
My understanding of ferroresonance is that when the inductive and capacitance is the same that amperage no longer lags voltage by 90* and the transformer acts like resistance load there by canceling out the electromotive force in the coils and causes the arresters and fuses to blow.
This 4th cutout detail only applies to our ISO Wye to Delta banks. Our engineers say it is not required for a Wye primary to Delta secondary (120/240/208) bank. However I'm hearing of people. Losing them on these banks now.
My question is, the guys the other day closed the first transformer and phased across the B1 cutout, they said it was ~13.2kv which makes sense to me as you put 7.6kv into the H1 of the A transformer it goes through the coil onto the floating ground into the H2 on the B1 out the H1 and you get 7.6 x 1.73 to get 13.2kv. They closed the B1 cutout, nothing happened. Phased the B2 cutout and had ~22kv. Now if this is true 21kv our arresters are rated to 10kv, why wouldn't the arresters blow? In my mind you now have 13.2kv going onto the floating ground into the H2 of the B2 and out the H1 and when you phase across the cutout 13.2kv x 1.73 = 22.8kv. They refused to close the cutout. They grounded the floating ground and closed it the. Removed the ground. If the voltage truly is 13.2kv coming out the H1 on the B2 why doesn't it blow the arrester that's only rated to 10kv. Why does it supposedly blow once you close the 3rd cutout?
I closed the same bank 1 pole away with a pogo from the ground and it didn't blow.
Our bulletin says "when there is no neutral" well on Wye Primary to Delta Primary there is no neutral on the Delta side which makes sense so capacitance can build up causing ferroresonance. However when it's Delta secondary (120/240/208) there is a neutral bonded to ground. So in my mind if electrons are building up in the primary coil they should repel the electrons in the secondary coil and they can flow down the neutral to ground. That negating the capacitance build up causing ferroresonance.
Is it because the service of 3 phase 350kcmil wire is hooked up but the breakers are off so that wire is to much of a capacitance and over powers the neutrals ability to bleed off the electrons?
Or is it the electrons will not flow down the neutral because there is no return path to the coil and if you left a single phase transfomer energized long enough will no load eventually the ferroresonance would build up and blow the fuse and arrester?
Thanks in advance guys.
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