Inrush currents on large power factor cap banks? 1

[bdn2004]

I’ve read power factor correction capacitor banks have large inrush current when first energized. Does anyone have experience with this and has seen this? What is value of inrush like 8x FLA?

 

[thermionic1]

I've seen on medium voltage systems where there is a reactor in series with the cap banks.

 

[jraef]

Yes, there is inrush current. Capacitors charge instantaneously at a rate commensurate with the available current in the system, but it also depends on a number of other factors. At the moment a cap is connected, the current will flow to try to equalize the the voltage between the source and the cap. Assuming that at the exact moment when the switch is closed the cap is at zero voltage and the line is at peak, the peak value of the inrush current will depend on the capacitance and inductance of the entire circuit all the way back to the source. So if you look at it from the standpoint of the available fault current in the system at the point of connection, the peak magnitude can be very very high, as in up to 15x the steady state current rating of the capacitors for a single bank. Multiple cap banks can show multiples of THAT value, i.e. up to 250x the steady state under the right conditions. That's why cap banks generally come with some form of inrush current limiter system. Without that they could destroy themselves just by being energized.




" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[bdn2004]

the one I'm looking at has three stages. Is that the purpose of that? This is a brand new substation. When they bring this online is there a sequence of operations - like the cap bank gets brought online first / last ?

 

[bacon4life]

Back to back capacitor banks have extremely high inrush. Without inrush inductors/resisitors, the only think controlling energy discharging from the energized cap bank to the just closed in cap bank is the impedance of the cables connecting the banks together.

Switching a capacitor bank changes the voltage. There may be multiple sections to keep voltage from change more than ~1% in a single step. There may also be multiple sections to keep power factor within a narrow range even as loads fluxuate.

 

[crshears]

The one I'm looking at has three stages.

A picture [or in this case, an SLD] is worth a thousand words!

My utility has some stations with three distinct cap banks, meaning each has its own SF6 CB. Only two of the three cap banks have a current limiting reactor in series with them; an operating restriction states that the last capacitor to be removed from service must be one with a series reactor.

One of our stations has a two stage cap bank connected like this, with no reactors at all:

Bus
/\/
cap breaker
/\/
bus <> "cap bank part 1"
/\/
cap breaker
/\/
"cap bank part 2"

How is yours configured?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[bdn2004]

My real question is .... do we have to consider this inrush current for the overcurrent protection on this capacitor like we do a transformer?

 

[crshears]

Hmmm...

Hello bdn2004, be forewarned: I'm going to attempt to do a little sharpening here...

You haven't posted an SLD, which would be the preferred thing to do.

Neither have you alternatively stated the operating voltage, or provided the MVA rating of the supply trafo's or the MX rating of the cap bank, or its configuration, or stated one way or the other if or where there will be current limiting reactors installed.

You seem to be ignoring this:

jraef said: Cap banks generally come with some form of inrush current limiter system. Without that they could destroy themselves just by being energized.

The point both jraef and I are trying to make is that it's much more important right out of the gate to ensure the charging inrush is limited so the cap bank doesn't self-destruct with continued switching; tweaking the relay settings around that is child's play in comparison.

Then again, I suppose you could feel free to not worry about that and just set the cap bank protection settings enough over the cap's TCC that it doesn't trip from cap protection the first time you energize it - which means not only that the protection scheme won't be needed again until all the destroyed cans - and maybe even the cap breaker - are replaced, but the cap protection scheme itself will never wear out because the bus back-up protection will end up clearing the entire zone instead each and every time, maybe even, with a little luck, collapsing the local grid. Repair and repeat as many times as need to finally realize it might be a good idea to install current limiting reactors.

Please don't put on blinders/get tunnel vision about cap protection relay settings; there's a bigger world out there...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[bdn2004]

ok correction heeded. I do appreciate everyone's comments on here.

Attached is the three line diagram. It does indeed have reactors. This thing is the size of a small bus. They've others of similar size at this plant site, and it didn't look like there was anything exceptional with the coordination. The power software does show an inrush current of 4X FLA at 6 cycles.

After talking with the plant maintenance electrician, he said they don't normally have any trouble starting (as in tripping the breaker) the other capacitors at the site - except early in the morning when there's condensate built up?

 

[crshears]

Hey there bdn2004, thanks much for the SLD; helps hugely.

My real question is .... do we have to consider this inrush current for the overcurrent protection on this capacitor like we do a transformer?

Short answer is Yes.

Elaborating a little, how exactly that is done depends...

The most cost-effective place for the prot is likely using the CTs in the main cap "feeder breaker," and provided appropriate time displacement limits are in place between the switching of the various cap legs in and out of service the circulating current will occur between them and the system connection point protections need only be tuned to allow for the highest charging current encountered in service, as so ably spelled out by jraef.

Being one whose knowledge on this topic derives principally from operational history, I now yield the floor to others who have actual design experience with these things...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[crshears]

Hey all,

The following redacted e-mail describes an issue we encountered within out utility when switching LV [28 kV & 44kV] capacitors:

<begins>

Hello!
Having witnessed arcing at cap enclosure padlocks and other equipment as well as CT failure, I can offer the following.

Anecdotal:
(1979) - a vague pattern of switching related failures developed when removing caps from service. A small group of ’78 OITs [operators in training] were requested to be at a specific transformer station when the next such operation was performed to observe any failure. When removing one of the ‘back-to-back’ capacitors, a CT exploded sending shrapnel across the switchyard. Over time, capacitor failures during ‘back-to-back’ switching had become such a concern that field staff would request any caps they were working close to not be in service.

The problem was thought to be caused when the removal of a 2nd cap from service would result in a surge from the 1st cap, spiking the voltage & causing untoward effects. This effect is of very short duration, viz., until the bus voltage settles to its new level. Adding the limiting reactor to the 1st Cap would reduce this spike to safer levels.

There is no similar problem when switching caps I/S, as switching the 2nd cap I/S would cause an MX spike into the 1st cap – not stressing all of the surrounding equipment.

Rule of thumb is: Reactor Equipped Cap is First In and Last Out - It’s the Last Out that is important.

Analogy: Looks like a spring to absorb the shock of switching the other cap...

<ends>

Incidentally, I don't think I've ever encountered a "large power factor cap bank" - they've all operated at almost zero, viz., very close to 90° leading...; they've only varied in voltage from, say 13.8 kV up to 240 kV, with ratings of between 10 and 410 MX; the higher voltage ones typically employ independent pole closing circuit breakers to mitigate that otherwise intensely nasty inrush current. Some of the 14 , 28 and 44 kV caps have current limiting reactors, and some don't, depending on local system configuration; our 115 and 230 kV caps have them almost without exception.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[bacon4life]

bdn2004-Inrush of 4x at 6 cycles seems kind of weird. Typically capacitor inrush decays to a negligible amount with 1 or 2 cycles. This paper

https://www.eaton.com/content/dam/e...p-capacitor-bank-installations-td230004en.pdf

shows how to calculate inrush currents.

crshears- Typically turn a capacitor on causes a bigger transient than turning it off. Perhaps your capacitor switching devices were having issues with restrikes during current interuption?

 

[bdn2004]

"Field experience indicates that the inrush current for multi-step banks is usually between 20 and 250 times the steady-state capacitor
current.
The transient current usually decays to some insignificant value in less than one cycle on the system frequency basis (50 or 60 Hz) and often
will have decayed to a low value within one-half cycle on the system frequency basis."

Thanks for the article....Looks like a bit of work to come up with a magnitude of current.
Is the above the best there is on how long such currents will be on the system? From this it doesn't seem to be a big concern?

 

[bdn2004]

In modeling this staged reactor - capacitor circuit and providing primary overcurrent protection like I’m trying to do...

it seems you could use the same logic you do when protecting a transformer on the primary side with a relay: set the instantaneous trip to a value slightly higher than the fault current of the primary circuit when the secondary is shorted. As any current higher than this would represent a fault on the line side of the reactor.

Is there a flaw with this reasoning?

 

[bdn2004]

Attached is my attempt at drawing this cap bank into our power model. The second page is the same system shorted. And the third page is the nameplate of the cap bank. Hopefully I've converted this right from mH to kVA and ohms.

I'm showing the max kA when the system is shorted - to be about 2kA on one of the cap circuits. How could the inrush be more than that ?

I plan to talk to the Vendor about this. I'll post what I find out.

 

[jghrist]

What you have are tuned banks that provide harmonic filtering as well as power factor correction. If they are designed properly, the inrush will not be a problem. If you want to analyze them fully, you should use calculations that consider higher frequencies. Inrush current has a high frequency with the frequency and magnitude dependent on the capacitance and inductance of the circuit. For fundamental frequency overcurrent protection, I believe you can ignore the inrush which is of a very short duration.

 

[bdn2004]

As a follow up on this and talking to the Vendor... The reactors do cut down the short circuit current. The long term setting on the feeder breaker needs to be set at 135% of the of the FLA...in this case: 370A x 1.35 =499.5A

They use 4X that rating as the short circuit value. When asked if this is at 6 cycles...they said that's safe.

 

[cuky2000]

Below is an excerpt with suggested margins for the cap bank protective devices and recommended sites for TRV awareness, Inrush & outrush online calculator and recommended breaker sizing criteria.
Engineers should be aware that damping reactors should not be freely added without a good understanding of the transient during switching of cap bank.

1) TRV Awareness:

https://w3.usa.siemens.com/us/inter...ocs_MV/TechTopics/ANSI_MV_TechTopics35_EN.pdf

2) Inrush Current Calculator:

http://nepsi.com/resources/calculators/peak-inrush-current-from-capacitor-bank-switching.htm

3) Breaker Sizing per IEC Std:

https://www.schneider-electric.com/...412.679805147.1560874689-242829503.1560874689