So I have an engineer selecting some cap's for PF correction. We are putting cap banks on (3) sections of gear. Only problem is the gear is full. Any idea how I can add these cap banks? Would running a tap off the gear and adding a fusible switch work? I've seen this done quite a bit, but I not really a fan. Thanks in advance.
Capacitor banks for PF correction
- Thread starter jes25
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You're going in the right direction. It must be noted that the capacitors maintain a charge after they are de-energized so caution must be used to assure that they are de-energized before accessing the enclosure. The are discharge resistors that are built into the capacitors to bleed off the voltage after a short period of time after they are de-energized but can be quite dangerous before that. A key interlock to access the enclosure may be something that you should consider.
Also, before you select a desco make sure that it rated to disconnect the capacitors as it is not unusual for that to be an issue.
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- Placerville, CA, USA
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- Retired PV System Designer
I would imagine that breaking the connection would not be an issue, but reconnecting while the equipment is energized would be.
Is there also a problem with surge current through the closed contacts when the equipment is energized?
My experience is from providing 5kv load interrupter switches to disconnect 5kv capacitor banks. I designed a series of KK intertlocks the maintenance person had to operate a series of disconnects before he was allowed to operate the disconnect KK inlk and then a KK intlk before entering the enclosure. This is a very serious safety concern regarding the LI switches limitation to open an energized PPCC bank as well as accessing the enclosure. This is a real application that I have experience with, not theory.
That's why I'm raising this point with LV PFCCs. I have never had a requirement to do this with a LV PFCC bank. If I were to do it I would certainly discuss this application with a competent disconnect design engineer advising him of what I intended the switch to be used for and advising what kvar it would be disconnecting. I want to be assure that he understands the application and no give me lip service.
Then I would give some thought to how long the capacitors take to discharge for which there is a standard in doing so. I just can't recall what that time is. You would have to take this into consideration when providing access to the enclosure.
Now, do you have any experience with this application or are you citing your theory?
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
Just theory, which is why I asked the question. Since you raise the point, the extremely low resistance and reactive impedance of the bank could cause problems if the contacts opened too slowly and an arc struck as they were opening. Maybe.
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- Wisconsin
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- PE (Retired) - Power Systems
The only place I remeber having seen 'warnings' concerning LV PFCC has been with contactors/starters.
For example, the Schneider Electric Digest, on page 16-23, says that for repetitive switching applications, sufficient impedance should be employed to limit the capacitor inrush current to about 6X the contactor rating.
- Location
- Placerville, CA, USA
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- Retired PV System Designer
That makes sense for the contacts that disconnect the equipment which includes the correction. I was thinking more of the disconnect between the motor and the cap bank.
Those contacts would stay closed, but would have to carry any inrush current.
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- Wisconsin
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- PE (Retired) - Power Systems
No, the Square D warning has to do with contactors that are just switching a capacitor only, like in an automatic bank.
They have no published literature on applying general mechanical 'safety switches' with LV PFCC.
Information regarding switches and capacitors is very difficult to find. I know that with 5kv LI switches I had to get a switch design engineer to consider the application who was familiar with capacitors. He was very insistent the he in no way would approve the LI switch to open an energized PFCC bank.
I'm somewhat concerned that there are fewer design engineers that truly understand the implications of applications such as this for example. I am becoming more concerned that they don't have enough experience as there was 25 years ago as those guys have since retired. And then there are those in the field that don't know have enough appreciation to ask the question.
As such if I were to mount a bank of PFCCs in an enclosure remotely as the OP suggested I would certainly want a disconnect switch for it. I would also what to verify should there be any concern if the switch opened a live bank.If that isn't an issue now long should someone wait before opening the enclosure.
The OP never included a one line to show the anticipated wiring arrangement either.
The code requires a disconnect for installs at the service. The conductors and switch will be sized at 135% per article 460. My primary concern is that the 480volt switch gear is full. I suppose, I will have to add lugs to the switch gear and employ the tap rule to mount a regular old 480V fusible switch. Are you saying this type of switch won't work?
The code requires a method to drain the cap bank. I assume the caps being proposed will have resistors built in.
My one line is too simple: 480v gear----------fusible switch---------cap bank. I won't know how many kvar until the engineer tells me.
As a former applications and sales engineer I learned to approach providing answers to questions very cautiously. Very often if I provide an answer when I don't know the real application my answer can be applied incorrectly which can be dangerous.
Yes, the caps do have those drawing resistors in them but the is time between them being disconnected until the voltage drops to a safe level.
With your application it appears to be straight forward. You described your arrangement as swgr fused sw caps. As simply described you are coming directly from the swgr bus to the fused switch to the caps. Where does you cable originate from the swgr and what protects the cable or are you considering the cable under the tap rule. I've been concerned with the disconnect being able to disconnect the cap bank while it is energized. This may not be a problem at all but I think that you will agree that this is not a common application for a disconnect switch. How it would be fused could be another question.
Those who have not appreciation may just through a fused disconnect in there with out giving it a second thought but is it a correct application?
The question that should be addressed is if the disconnect switch that you have selected capable of disconnecting the energized capacitors. I would like to think so but I would certainly ask a person who has knowledge of the switch and how they would be applied with as a disconnect for energized fuses.
Then there is the concern with the access to the caps if they have not completely bled down after the switch has been opened. And what prevent access to the cap until they are completely been de-energized.
There may be a very simple answer but you're going outside the box a bit and what you would like to do I believe is quite reasonable. But also CYA.
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- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Absolutely, LV PFCC protection and disconnection is a common application for disconnect switches.
Tap rules, using 240.21(B), off of existing bus are also very common.
Although, one of the most neglected areas, when using tap rules, is the required wire bending space at terminations.
Thanks Jim,
Then the switches are rated for disconnecting capacitors while they are energized with no issues?
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- Wisconsin
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- PE (Retired) - Power Systems
None of the major safety switch manufacturers (i.e. Schneider Electric, Eaton, and Siemens) address LV capacitor switching as an issue, either for or against, in their general catalogs.
The only thing the capacitor manufacturers seem to say, about disconnect switching and sizing, is to follow the NEC.
As previously mentioned, automatic switching will likely require special consideration.
If I still had my connections I would run this application past the safety switch design engineers. It's getting to be more common for the design people to not have a clue as to how the products they design are actually used. They consider motors and other common applications but have not considered capacitors, opening energized capacitors and if there are any limitations. I would anticipate that there are no issues but has that already been determined.
I know that this is a concern for MV LI switches and PFC banks which there are limitations when it comes to kvar.
i
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- Wisconsin
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- PE (Retired) - Power Systems
Yes, switching is a concern for MVPFCC.
LVPFCC switching has never been addressed in any safety switch application manual that I have seen in the past 35 years.
The Eaton website provides an IEEE paper, written by Eaton employees, which also does not mention anything more than needing to comply with the NEC for sizing when switching LVPFCC.
I don't believe that there are the engines out there that there were back in the 50s-80s-. I used to be able to call and talk to engineers that invented and developed the product. There was some tremendous advancements made because of electronics and micro processors. Electronic trip units, ground fault, zone interlocking.
I had access to brilliant transformer design engineers.
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- Wisconsin
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- PE (Retired) - Power Systems
So did I.
We both were both very fortunate as wewll as privilaged to have gotten to know some very fine engineers.
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