Jump to content
Thank you for the great forum, Safe Driving over the weekend. Sincerely Jason
Gerrie

Maximum demand metering

Recommended Posts

Maximum demand where you basically pay  the utility for the highest KVA used during one month cycle. There is some formula mixing KWH x KVA x factor x whatever = $$$

I have a scenario where during lockdown the electricity cost has dropped by about 25% off the normal monthly rate. This drop is about R300/day

This is on a machine that normally runs about 8Hours/day 75KW inverter motor. Unfortunately this machine has to run at least 1/2 an hour to 1hour every day during lockdown. So you would expect a bigger saving but 25% saving is what it is.

Survival mode now has to kick in and trying to reduce this maximum killer charge by comparing diesel cost of generator for 1hour versus Eskom maximum demand cost for 1hour/day.

Plan 1

Switch off Utility whenever the 75KW motor needs to be started for that hour and run on the 200KVA diesel generator, on completion when 75KW load isswitch back to Utility (use utility only for small loads, lights, fridges, essentials)

Obstacle 1

Getting a plan of action into place and convincing operator to stricktly stick to the month long plan of switching off Utility and run 75KW motor on generator only.

Outcome 1

Is this even a option and worth trying or is it going to be wasting a months diesel and still getting that 75% Utility bill? 🧐🙃

 

Share this post


Link to post
Share on other sites
8 minutes ago, Gerrie said:

on completion when 75KW load isswitch back to Utility

Sorry, Afrikaans spelling😝

”on completion when 75KW load is switched off, only than switch small loads back to Utility”

Share this post


Link to post
Share on other sites
11 hours ago, Gerrie said:

This is on a machine that normally runs about 8Hours/day 75KW inverter motor. Unfortunately this machine has to run at least 1/2 an hour to 1hour every day during lockdown. So you would expect a bigger saving but 25% saving is what it is.

Survival mode now has to kick in and trying to reduce this maximum killer charge by comparing diesel cost of generator for 1hour versus Eskom maximum demand cost for 1hour/day.

Plan 1

Switch off Utility whenever the 75KW motor needs to be started for that hour and run on the 200KVA diesel generator, on completion when 75KW load isswitch back to Utility (use utility only for small loads, lights, fridges, essentials)

Good idea!

Have you identified the moment when this machine kVA is peaking? (This is what they are billing you on!)

You've identified this 75kW motor. Since a VSD  is powering it you shouldn't be having the 6x startup current..

Does the utility's bill indicate the time when maximum demand occurred?

Share this post


Link to post
Share on other sites
10 hours ago, Richard Mackay said:

Does the utility's bill indicate the time when maximum demand occurred?

No there is no time on the bill indicating the time for maximum demand, but I know it is normally anywhere from 7h30 to 18h00 as that is the normal times this motor was started, now with lockdown same time frame just very short intervals.

Another thing is this motor could be set to run at about half the normal speed, but the time to complete its cycle will take twice or even three times as long, I think it might be a option to try and bring that amps down, but have not yet tried it.

Share this post


Link to post
Share on other sites
11 hours ago, Richard Mackay said:

Do you have means of recording your maximum amps?

Only normal Amp meters that is installed at the drive panel, these indicate in the 90A range per phase. Unfortunately  nothing with a recording ability. The only recording I think is the council with a Enermax plus demand meter, this thing will have that data if only there was access to it. We will try and get some sort of data from the council but I’m not sure if they would oblige.

The plan is to install a separate circutor meter but this will only be done in the next couple of weeks.

I have installed some of those enermax meters about 10 years ago and it still operated with if I remember correct Windows 7 and a little cable with a magnet on one end that linked to the meter.

Share this post


Link to post
Share on other sites
16 hours ago, Gerrie said:

Another thing is this motor could be set to run at about half the normal speed, but the time to complete its cycle will take twice or even three times as long, I think it might be a option to try and bring that amps down, but have not yet tried it.

Oh that would be the first thing I'd do. I assume there is no downside to doing this?

I heard an amusing story once of how a building brought their bill down substantially by installing a R500 timer device.

They had two air-conditioner units that started simultaneously, causing a very high demand peak. The timer device they installled simply spaced the two units apart by a minute or so, causing a much lower demand and a substantially reduced bill. Remains one of my favourite engineering stories 🙂

 

Share this post


Link to post
Share on other sites
19 minutes ago, plonkster said:

I assume there is no downside to doing this?

As far as I know this motor have no issue to run at lower speed,  the only issue is for the operator who might become impatient as he is used to run the motor at normal speed. I think that will be a bigger problem convincing the operator in having patience as he will have to decide in saving excessive electricity cost now and try to preserve his job, hopefully he will understand the importance of maximum demand cost.

I know some motors when running too slow build up more heat as the self cooling fan cannot push thru enough air, but that can be achieved by adding a normal electric fan blowing directly on the motor.

Share this post


Link to post
Share on other sites
Posted (edited)
3 hours ago, Gerrie said:

I know some motors when running too slow build up more heat as the self cooling fan cannot push thru enough air, but that can be achieved by adding a normal electric fan blowing directly on the motor.

True. The fans aren't linear in their flow vs speed. But if the load is constant it's easy to check if the motor is getting too hot at a reduced speed. What is the reason a VSD has been used? 

Edited by Richard Mackay

Share this post


Link to post
Share on other sites
3 hours ago, Richard Mackay said:

What is the reason a VSD has been used? 

I guess the reason for the VSD is due to a high starting torque that is normally required under normal operating conditions, but is now drastically reduced with the lockdown. Apparently the VSD was installed during a upgrade about 15years ago, it used to have those old resistance starters with the slip ring motor. I think the VSD was also to keep up with technology and is a much more reliable option in terms of motor protection and lifespan.

Share this post


Link to post
Share on other sites

The maximum demand charge is actually based on the highest average demand (KVA or KW) during a 30 minute window period starting at midnight on the beginning of the month.

So the billing period is broken down into 30 minute blocks and then you get charged for the block with the highest demand.

Maximum Demand = kVA = kVAh/T      (Where T = integration period of 30 minutes)

So from this you can see that the starting current of motors will be an insignificant contributor to your maximum demand charge.

So the trick is to reduce your energy consumption for any 30 minute block period.

Share this post


Link to post
Share on other sites
1 hour ago, Carl said:

The maximum demand charge is actually based on the highest average demand (KVA or KW) during a 30 minute window period starting at midnight on the beginning of the month.

So a single "mistake" can ruin the whole month?

Share this post


Link to post
Share on other sites
Posted (edited)
10 hours ago, Carl said:

The maximum demand charge is actually based on the highest average demand (KVA or KW) during a 30 minute window period starting at midnight on the beginning of the month.

The units used for demand charges are kWh (Eskom's Megaflex billing system) Essentially it's the same (average demand over a period) but the units are more universal.

And the demand charges change depending on the demand period: There is a High and Low demand period depending on whether it's summer or winter. Also the day of the week matters as well..

So in order to reduce maximum demand charges all these conditions need to be considered. The control system needs to synchronise with the utilities block metering and to extrapolate the projected demand over the period. If this projection indicates that the electricity consumed will exceed the target then a (non essential) load needs to be disconnected to prevent this happening.  

Edited by Richard Mackay

Share this post


Link to post
Share on other sites
3 hours ago, Richard Mackay said:

kWh

You sure? I am by no means an expert, but as I understand it it is a pretty common thing all over the world. It's a "penalty" of sorts for people with a high peak consumption, and it is usually measured in kVA because they really care about the peak current demand and/or they also penalise for poor power factor by the same token.

Again, I am not an expert, but after I went googling for the same Megaflex tariff, it seems it has both energy costs (it's a TOU tariff, time of use) as well as R/kVA/m charges, where kVA/m is your demand charge.

Share this post


Link to post
Share on other sites
5 hours ago, plonkster said:

You sure? I am by no means an expert, but as I understand it it is a pretty common thing all over the world. It's a "penalty" of sorts for people with a high peak consumption, and it is usually measured in kVA because they really care about the peak current demand and/or they also penalise for poor power factor by the same token.

Again, I am not an expert, but after I went googling for the same Megaflex tariff, it seems it has both energy costs (it's a TOU tariff, time of use) as well as R/kVA/m charges, where kVA/m is your demand charge.

Demand charge is indeed measured in kVA. 
 

Share this post


Link to post
Share on other sites
7 hours ago, francois said:

Demand charge is indeed measured in kVA. 
 

Yeah, the /m bit at the end seems to indicate that it is done on a per-month basis, but also the way it is determined is by the block method as mentioned earlier. I googled into tariffs for US suppliers that have similar figures in $/kVA/m, and where they use the highest 30-minute average.

Share this post


Link to post
Share on other sites
20 hours ago, Richard Mackay said:

The units used for demand charges are kWh

Back in the 90’s when I worked for Eksdom, I use to take monthly readings, those days it was normally 3 x single phase meters with a maximum demand meter that had a dial that would move and got stuck at the highest point for the month, I used to go unlock the meter and reset the dial to zero at month end, The formula did also included KWH readings that was billed at a few cents per KWH together with maximum demand reading and a factor with the size of KVA supply. Those were good old meters, the more biltong you got from the farmer the lower the maximum demand seemed to be😁

Share this post


Link to post
Share on other sites
On 2020/05/12 at 3:07 PM, francois said:

City of Tshwane will also charge you a minimum demand charge based on previous 3 month's use:

This is something I did not know, so it seems those users of MD is stuffed during lockdown😬 

Share this post


Link to post
Share on other sites
Posted (edited)
40 minutes ago, Gerrie said:

This is something I did not know, so it seems those users of MD is stuffed during lockdown😬 

The utilities will not only have one billing system. They will bill for total electricity consumed for starters. Then they will penalise you for a reactive load. Then for large users they will hammer you for erratic demand.. Oh yes, they will also charge you a flat fee just to have the service available..

But if you take an interest in how you are billed there are big savings to be had. Most industrial users in my experience don't bother!

Edited by Richard Mackay

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


Topics

Contact Us - Power Forum South Africa

×
×
  • Create New...