power factor correction, help?

[Smudge]

I am after a little help if anyone can, on a problem i've been given at the factory i work in...

We have a limit on our supply capacity we are allowed to use (950kVA) before we get hit with extra charges. Using our own energy monitors, our usage (max) is approximately 880kVA over the last couple of days, and that seems to be the normal sort of load over recent weeks too.

Our problem now is that we want to install a new bit of plant that has a potential demand of around 150kW, pushing us way over our capacity allowance. Upon some quick readings we have taken from the energy meters, they are saying we are achieving a p.f. of around 0.904. After some research, i have come across a general view that the p.f. should be greater than 0.95. We have capacitors on our incoming already, however, these would have been sized at a time before some rapid growth of our plant, and haven't been given any attention since they were put in >10years ago.

How can i check if they are the correct size? Are they easily maintained? And how can i explain to the boss's above that improving p.f. can help achieve greater capacity and lower costs?!?

thanks for reading

 

[plugsandsparks]

Even though they are 10years old, if it were me i would contact the manufacturer of what you have and ask them.

 

[Knobhead]

Look in to automatic PF correction units. Trying to achive 0.95 is a pointless exercise, 0.9 is the usual threshold for PF surcharges. You won't get within you're MD with the new drive fitted and a bit of tweaking of the PF.

 

[Marvo]

If your pf is over 9 even after considerable expansion then your existing correction units might already be automatic. Achieving 9.5 or better is going to be a tall order without large capital expenditure.

 

[desigo]

Whats your voltage like too high no good too low not good

 

[MarkieSparkie]

Trying achieve power factor greater than 0.9 is a pointless, but costly, exercise in diminishing returns ie. the cost of the correction equipment escalates rapidly for every fractional energy saving achieved. Unless the energy usage is large >10 MW, the annual potential savings rarely justify such action.

 

[malcoa]

You need to talk to you DNO and make out an application for a greater authorised supply really, they may have extra capacity on the local network/transformer that they can sell you, obiously your main switch and supply cables may need a look at. you never know until you apply.

 

[Smudge]

thanks for the response. I'm not sure i agree with it being a pointless exercise? Don't take that the wrong way, but we are getting monthly electricity bills of 20-25grand. Even a very slight improvement would save thousands per year. I appreciate that won't help our max capacity, and will get on to Scottish power first thing on Monday.

 

[malcoa]

They may be 20-25K but what is the charge for the reactive element of the supply, it will show on the bills?

 

[Smudge]

thats in a little bit of dispute just now?!? we are waiting on a breakdown of the bill from e-on, as it isn't clear from our statement. It just says £43.87 reactive charge on our latest bill. I'll get back to you on their response, as the person on the phone wasn't sure at all and said they would get back to me.

 

[Knobhead]

So you propose to spend £XXXX to save £43-87? You need you're bumps feeling.

 

[Gage]

Get a new tariff or deal with the charges, if you're still making substantial profit (which I assume you will be) from the new additions + extra charges just leave it. Take the advice given to you here.

 

[Engineer54]

You've been given good advice, your corrected PF of .90 is about as good as your going to get without throwing some serious money at it!!

Now, is the DNO supply coming into your company at MV and an on-site DNO 1 MVA transformer?? If so, it would be worth looking at the cost of providing your own 1200 KVA or larger transformer and any associated switchgear etc, (depending on available DNO power available at MV). It all depends on the viability and profitability of your company at the end of the day, as to whether this is a time, to make a substantial plant investment. As it stands, your dammed if you don't, and dammed if you do. Down to the bosses as to what is the medium and long term plan for the company!! lol!!

Then again, if were only talking of £45 a bill as it stands at the moment...... lol!!

 

[Sparks1962]

Maybe look at calling in a firm who specialise in PF correction gear, I often do this on Industrial and Agricultural installations with significant motor and discharge lighting loads. It'll be easier on you as they will do the necessary calculations, take reading at the intake/metering point and quote you fot a complete package or supply of equipment only. We use a local (ish ) manufacturer and install it ourselves, with them taking care of the maintenance side. Most manufacturers are happy to provide brochures or presentation brochures for your manager/customer with case studies of previous installations.

Hope this helps!

 

[Chr!s]

I am after a little help if anyone can, on a problem i've been given at the factory i work in...

We have a limit on our supply capacity we are allowed to use (950kVA) before we get hit with extra charges. Using our own energy monitors, our usage (max) is approximately 880kVA over the last couple of days, and that seems to be the normal sort of load over recent weeks too.

Our problem now is that we want to install a new bit of plant that has a potential demand of around 150kW, pushing us way over our capacity allowance. Upon some quick readings we have taken from the energy meters, they are saying we are achieving a p.f. of around 0.904. After some research, i have come across a general view that the p.f. should be greater than 0.95. We have capacitors on our incoming already, however, these would have been sized at a time before some rapid growth of our plant, and haven't been given any attention since they were put in >10years ago.

How can i check if they are the correct size? Are they easily maintained? And how can i explain to the boss's above that improving p.f. can help achieve greater capacity and lower costs?!?

thanks for reading

Your active power is 0.9 x 880 = 792 kW

So you would need 168 kvar capacitive compensation to achevive a PF of 0.96

So your new kVA is 825

 

[Smudge]

our supplier phoned back yesterday. She said that as we were on an half hourly bespoke meter, that we would be charged £25 for every half hour that we were over our allowance. This doesn't seem right. I asked if it were £25 every half hour, then how did we get charged £43.87 and logically not a multiple of 25? I then got another phone call later from a guy who said it is a one off payment of £25 per month (which is fine by me) but again he doesn't know why its £43.87. Cracking customer service, shame they don't know what they are meant to be charging us.

All in all, it's been a complete waste of time, thanks for your help and opinions. Scottish power are looking into upping our allowance, however, it is a 1MW transformer so looks like some major investment is needed to expand.

 

[malcoa]

if the £43.87 is a charge for the reactive power then thats the cost for your power factor being below unity and not for over use.

FYI I'm just towards the end of a power upgrade project, the cost of moving from LV 900Kva to a HV 1500Kva supply is £130K but that depends on how you do the job and the London Metal Exchange

 

[Knobhead]

if the £43.87 is a charge for the reactive power then thats the cost for your power factor being below unity and not for over use.

FYI I'm just towards the end of a power upgrade project, the cost of moving from LV 900Kva to a HV 1500Kva supply is £130K but that depends on how you do the job and the London Metal Exchange

How do you work that out? 0.9 is the normal threshold not unity, If you are on a bulk supply tarif. Then the worse I've known was .93 @ 80MW. That also had some very fancy 11/.66 KV transformers for phase load balancing

 

[malcoa]

How do you work that out? 0.9 is the normal threshold not unity, If you are on a bulk supply tarif. Then the worse I've known was .93 @ 80MW. That also had some very fancy 11/.66 KV transformers for phase load balancing

Ok, its simply the charge for Kvars used.....

 

[Engineer54]

our supplier phoned back yesterday. She said that as we were on an half hourly bespoke meter, that we would be charged £25 for every half hour that we were over our allowance. This doesn't seem right. I asked if it were £25 every half hour, then how did we get charged £43.87 and logically not a multiple of 25? I then got another phone call later from a guy who said it is a one off payment of £25 per month (which is fine by me) but again he doesn't know why its £43.87. Cracking customer service, shame they don't know what they are meant to be charging us.

All in all, it's been a complete waste of time, thanks for your help and opinions. Scottish power are looking into upping our allowance, however, it is a 1MW transformer so looks like some major investment is needed to expand.

I asked before, who owns the 1 MVA transformer, the company or the DNO?? If it's the DNO's your being charged a rental fee based on it's size, on top of your power usage. If your company purchases it's own larger transformer say 1200 or 1600 KVA then that's one charge less. The other thing is, how is your company being metered, at MV or LV?? I don't know about now, but it used to be cheaper being metered at MV rather than at LV rates...

 

[Engineer54]

if the £43.87 is a charge for the reactive power then thats the cost for your power factor being below unity and not for over use.

FYI I'm just towards the end of a power upgrade project, the cost of moving from LV 900Kva to a HV 1500Kva supply is £130K but that depends on how you do the job and the London Metal Exchange

Never seen or heard of a 900 KVA transformer, and even if it was, it'll still be supplied at 11 KV!! DNO's don't supply installations at 900 KVA at LV, it'll always be via an on site TX. So are you meaning a 1000 KVA TX?? Not sure, but i thought new transformers these days are sized at the harmonised ratings, where 1500 KVA now becomes 1600 KVA. As i say not totally sure on that one....

 

[malcoa]

Never seen or heard of a 900 KVA transformer, and even if it was, it'll still be supplied at 11 KV!! DNO's don't supply installations at 900 KVA at LV, it'll always be via an on site TX. So are you meaning a 1000 KVA TX?? Not sure, but i thought new transformers these days are sized at the harmonised ratings, where 1500 KVA now becomes 1600 KVA. As i say not totally sure on that one....

900Kva authorised supply from a shared transformer, yes its on a 11Kv 90mm feed and upgraded to an owned TX rated at 1500Kva (so it says in the contract and on the badge) with a feeder pillar supplying the existing switchgear and another 800A switchgear panel, 1500Kva authorised supply metered at HV

 

[Engineer54]

Originally Posted by malcoa ...900Kva authorised supply from a shared transformer, yes its on a 11Kv 90mm feed and upgraded to an owned TX rated at 1500Kva (so it says in the contract and on the badge) with a feeder pillar supplying the existing switchgear and another 800A switchgear panel, 1500Kva authorised supply metered at HV​

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I stated that i wasn't sure on the 1500 rating, just haven't seen one for several years now, they have all been 1600 KVA.

From your last post, i first thought that the new TX was still being shared, but that can't be the case if being metered at MV. So what i don't fully understand now, is this feeder pillar supplying existing switchboard and another 800A switchboard. Where is the main breaker (ACB/GCB) or fuse switch from the TX located?? BTW, was the original shared TX, also rated at 1500 KVA??

 

[malcoa]

There is a transformer mounted MCCB cabinet protecting both supplies, the HV metered intake substation has a HV metered ACB, the original tx was 1Mva and if i recall was shared with the highways lighting (not 100% on that) which has been diverted we need circa 1200Kva, hope this answers your question

 

[bomjac]

OP currently has 880 kVA in use @ pf=0.9 (kindly calculated by chr!s to be 792 kW). To this the OP wants to add an additional 150 kW of load giving a new (potential) loading of 942 kW. The PF correction equipment is most likely working within expected limits, but since your plant load has gone up over the years your PF correction is probably giving as much reactive power as it physically can considering it was sized to an older plant load.

Assuming new load of 942 kW, to achieve unity pf (and hence keep yourself below the 950 kVA of your current supply) would require 457 kVar of pf correction equipment.

There are options to get your KVA down even to the extent of a leading PF, but not cheaply. It may be that your company will may have to approach the DNO with a view to re-negotiating your supply tariff or maybe even additional supply equipment.

ps - be gentle, I never saw the 2nd page of posts before posting lol.

 

[Engineer54]

There is a transformer mounted MCCB cabinet protecting both supplies, the HV metered intake substation has a HV metered ACB, the original tx was 1Mva and if i recall was shared with the highways lighting (not 100% on that) which has been diverted we need circa 1200Kva, hope this answers your question

Is this existing feeder pillar rated at the new KA value of the 1500 KVA TX?? I only ask because you stated it's the existing Feeder pillar, which would have been rated for the 1MVA TX, as will also be the case for the existing switchboard.

One or two further questions, are you sure it's a MCCB mounted on the transformer and not an ACB?? What if any does this new installation include, in the way of protection relays etc?? Any chance of a couple of photo's of this installation??

 

[Knobhead]

There is a transformer mounted MCCB cabinet protecting both supplies, the HV metered intake substation has a HV metered ACB, the original tx was 1Mva and if i recall was shared with the highways lighting (not 100% on that) which has been diverted we need circa 1200Kva, hope this answers your question

What? Are you sure it’s an ACB? It’s a rare beastie if it is.

Definitely want to see a picture of that. Not seen one since I was an apprentice and that had been disconnected years before.

Like E54 I would also like to see this LV MCCB. It would have to be 2000A minimum. That would be cutting things very close (to the bone).

 

[Engineer54]

What? Are you sure it’s an ACB? It’s a rare beastie if it is.

Definitely want to see a picture of that. Not seen one since I was an apprentice and that had been disconnected years before.

Like E54 I would also like to see this LV MCCB. It would have to be 2000A minimum. That would be cutting things very close (to the bone).

It may be, being supplied via a RMU with an ACB rather than switch fuses off to supply the TX.

I'm more concerned with the existing feeder pillar and switchboard being capable of holding the new uprated KA rating, that the new 1500KVA TX will provide, over that of the replaced 1000KVA!! That being around the 20 KA mark and the new around 32 KA. It all depends basically, on what fault rating, these two existing units were built too, or if during the upgrade, these existing units have been uprated as part of the contract. ....Which i very much doubt.

 

[malcoa]

Supplied via RMU as you say, its not and existing feeder pillar i dont recall saying existing feeder pillar but apologies if i did, its a new one with new transformer, the whole installation is new except for an existing switchgear panel, installation was designed & installed by regional DNO
Sorry, unfortunately photos are not possible

 

[Engineer54]

Supplied via RMU as you say, its not and existing feeder pillar i dont recall saying existing feeder pillar but apologies if i did, its a new one with new transformer, the whole installation is new except for an existing switchgear panel, installation was designed & installed by regional DNO
Sorry, unfortunately photos are not possible

I did think as much. Both on my present project and previous projects our RMU's have had ACB's to feed the TX's.

No wonder the installation cost £130,000!! lol!! Why on earth did they contract the DNO to design and install this installation??

OK, it may have been me assuming the feeder pillar was existing. I would suggest you look at the rating plate of the existing switchboard though, and check if it does have a suitable KA rating for the upgraded 1.5MVA TX!!

 

[brookster]

I've seen these power factor correction units capacitor banks catch fire.

 

[Engineer54]

I've seen these power factor correction units capacitor banks catch fire.

Something must have been very wrong with the installation if it actually caught fire!! lol!!

 

[brookster]

Something must have been very wrong with the installation if it actually caught fire!! lol!!

its actually a very common fault LOL!

 

[Engineer54]

Is it indeed??

I've only known of one such case in 30 odd years!! And you would have understood why, if you'd of seen the state of the installation!! lol!!

 

[brookster]

They can catch fire for quite a few reasons and not bad just installation, bad operating temps, over voltage and bad harmonics.(well these things probably could come under the remit of install to be fair.) But there is a wide range of things which can pop them.

 

[Knobhead]

I'll add another 40 years. Only had two fail.

One at 433V, that blew a hole in the cap of a 200A BS88 fuse. Still got the fuse in my black museum.

The other was spectacular to say the least, 11KV goes with a bang.