How to Choose Capacitor Bank Supplier?

When the cheapest solution turns out to be the most expensive

Any technician with minimum electrical knowledge can determine or calculate reactive power compensation. The most common practice is using “a single” electricity bill. The emphasis here is on the “single” electricity bill as it is precisely here that a series of errors can start, which can often end up, with higher costs than those involved when a capacitor bank is correctly determined.
The calculation of the reactive power to be compensated using electricity bills provides us with a relatively correct approximation about which order of magnitude we are dealing with; our starting point. In these cases it is important to ensure that these calculations are carried out with the maximum number of invoices, as they may be heavily influenced by seasonality that we may have ignored (Example: offices or hotels with totally different consumptions in summer than in winter).
As we have mentioned before this must be our starting point, but we must bear in mind other factors which are not reflected in the electricity bill, and are of vital importance for correct compensation:

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• Demand fluctuation speed
• System balance
• Harmonic distortion levels

If we focus on the latter, as it is becoming more and more common to find networks with harmonic distortion.

When we carry out inductive reactive power compensation, the incorporation of a parallel capacitor bank is logical to attenuate this demand in order to bring the demanded apparent power (kVA) nearer to the active power (kW) which is really used to carry out the purpose it is designed for. This simple concept can be summarized as a parallel circuit with inductance (L – Transformer and Grid) and capacity (C- Capacitor bank).

If we observe the frequency response of the system we see that for a frequency fR the impedance of the system is much greater than its normal behaviour.

As has been previously stated today’s installations contain loads with demands which are not linear thus provoking greater distortion in harmonic current in the installation, and at the same time in the voltage.

The existence of currents with frequencies higher than the fundamental frequency at 50 or 60 Hz, mean that the resonance conditions previously described are complied with. This would basically cause:

• Amplification of the distortion in voltage for the entire installation (this could affect the equipment and sensitive electrical elements).
• Greater absorption of current by the capacitors, with their consequential overheating, reduction of their capacity and useful life, and in some cases the destruction of the capacitor.

With all these arguments and effects in mind we are going to illustrate a REAL EXAMPLE:

Installation located in Spain, whose activity is set within the metallurgical sector (treatment of metal pieces). This installation comprises a 1 000 kVA transformer, different sub-switchboards with rotary machines (lathes, conveyors belt, elevators, etc.) and services (offices, dispatch warehouse, changing rooms, etc.).

The maintenance technician in charge of this company, having checked that the surcharge level due to reactive energy consumption was significant, calculated, using a single electricity bill, which capacitor bank needed to be installed without taking into consideration any other factors.

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He then opted to purchase a conventional capacitor with 150 kvar switchgear.

After connecting the capacitor, a few weeks later, he observed that the capacitor was smoking; the outcome was that two capacitors were now unusable, in addition to the alarms caused in the nearby work centres. The capacitors were replaced after a few weeks, with the same effect being produced a short time later, together with the tripping of some lesser circuit breakers on smaller switchboards such as changing rooms, auxiliary machines and dispatch warehouse. The broken capacitors were replaced again, this time with capacitors strengthened up to 460 V and a short time later the same thing happened again. Finally they opted to disconnect the capacitor bank, meaning a return to paying the reactive energy surcharge.

The maintenance technician from the company asked CIRCUTOR, leading company in reactive energy compensation, to attempt to find out what had happened with this capacitor battery. Basic measurements were then carried out at the head of the installation. These measurements consist simply of measuring with and without the battery connected (always with the installation on full load).

THD(U)% and THD(I)% schematics indicating the capacitor bank connected and disconnected

Although the system denoted relatively low level current distortion (7-8% THD(I)% with 400 A), on the other hand the voltage level did not go unnoticed ( 3.3%  THD(U)% ). Based on empirical experience, the risk of the system entering into resonance is around 15% of the THD(I)% and 2% of THD(U)% (there is nothing stipulated to this effect).

We manually entered each one of the capacitors and we observed how the increase of the THD(U)% was substantial. This is an evident indicator that parallel resonance is being produced. With the capacitor bank connected, values of 80% of the THD(I)% were reached at full load in the factory and 23% THD(U)% (graphic 1). To get an idea, the limit which the supply quality on voltage establishes (UNE EN-) is 8%.

Without capacitor bank connected

With capacitor bank connected

Finally we can evaluate the expenses generated by this bad choice:

CONCEPTUnitsAMOUNTConventional 150 kvar battery14.400 €400 V Capacitor replacement93.056,50 €460 V Capacitor replacement62.474 €Labour costs (estimated cost 20 €/h) €Production stoppage and expedition ( estimated cost 2,500 €/h)2,56.250 €Surcharge for reactive energy (average monthly cost 958 €/month)21.916 €FR type detuned capacitor bank112.285 €TOTAL FINAL COST30.761,50 €

Here we can see how an apparently cheaper solution turned out to be really more expensive. If a correct technical investment had been carried out with a FR type detuned capacitor bank, the final price would have been reduced by 60%.

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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.

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.

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.

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?

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?

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 Thistles 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?

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.

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.

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.

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.