maximum demand charges 1

[Tarl]

Hello everyone, this is my first post and I hope i outline my problem properly! Any help would be very much appreciated.

I have a client who is using 5 large motors to run mixers and extruders and they are seeing very high demand both at the startup and during the middle of their processes. During conversations with the client it seems that the majority of the current being drawn is when their largest extruder is at peak capacity whilst the other processes are running.

My first thought was that something like a soft starter or VSD would help reduce the startup current but is there anything I can recommend which can help reduce the current spikes in the middle of the running cycles?

Thanks again and I look forward to any help offered

Tarl

 

[roydm]

As you probably know demand charges can be a significant part of the energy bill, one peak in the month is all you need.
If it's possible to shed some load at the peak period they could save them a bundle and you will be the new hero.
I installed a PLC system to monitor the peak in a casting operation where they turned 6 induction furnaces on high at the start of the week to melt Aluminium then turned them down once up to temperature using one at a time for the casting. The system would turn several furnaces down to low power if it saw a peak approaching just keeping the one they were currently using on high. They saved the $20,000 it cost to install the system in the first 6 months.
It took me ages to convince them the benifit until I obtained a printout from the Maximum Demand meter and ploted the data using dollars as the vertical axis.
Welcome to the forum & Good Luck
Roy

 

[itsmoked]

Dollars as a vertical axis always works - one way or another.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[itsmoked]

While reaching for a VFD might be a first choice it would probably be much better to actually study the plant and the meter readout in detail. Slight operational changes might do the trick.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[hesselberth]

You may want to also loook into Peak Shaving generators, I know Baldor/Reliance is saving a lot of $ by switching during peak periods.

 

[jraef]

Chances are about 95% that the issue is not going to be solved by limiting starting current. Demand Meters in most of the world operate based on a sliding demand window. This means that any individual peak within that window is not really seen, only the total current demand for that time period. There are a very small number of Instantaneous Demand Meters in use, primarily in third-world countries where the supply system has severe limitations. If you have one of those, then a soft starter will help. Your utility should be able to explain it to you.

As for limiting running peaks, that is somewhat impractical on something like an extruder. The current is demanded by the load, in this case usually a hydraulic press. At some point in teh cycle the hydraulic pressure required will demand that peak current, if you limit the current, the work does not get done (or done properly).

But you can effectively save overall energy by using VFDs on the extruder motor, because you can run the press slower whenever you can and reduce the amount of recirculation in the hydraulics. It's a specialized field and there are numerous companies that specialize in retrofitting extruders with VFDs that have well documented favorable ROI values. Once you save energy overall, you reduce demand as well. Worth lokking into from that standpoint.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[jraef]

My speel chekker is'nt werkin so wel todat...

 

[waross]

A little purple start any weigh J.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Tarl]

Hi guys, thanks for all of your help, I think I know the right direction to go in. I've spoken to the client again and got some more information, again any help would be invaluable. I am only just starting out in this industry so I hope I will be able to learn this all for myself soon!

This is what they have in their factory.

1x extruder of about 130kW – This has a VSD on it already as it is an application where the speed needs to be changed.
1x mixer 135kW – This has a VSD on too
2x mixer 30kW – These are started DOL and are where the problem is coming from. When they start the lights on site flicker. The mixers are actually fixed speed but have 2 speeds, slow and fast. Below is the typical running cycle of the mixers.

Start on slow speed for 1 minute then switch to high speed for 15 to 20 minutes, (a reaction takes place during this time and the motor amps rise accordingly, when they begin to drop the reaction is complete) then to slow for 2 to 3 minutes then high for 30 seconds after which the mixed load is transferred to the cooler for 10 to 15 minutes. The mixer is stopped and any residual mixed resin is manually cleared to the cooler. The mixer is then started empty and run on high speed to cool the chamber prior to the next batch for 20 minutes or so. The mixer is stopped prior to recharging the chamber and carrying out the next mix.

Again thank you all for your time and help, I hope with your help I can find the root of this problem

Tarl

 

[LionelHutz]

Correcting a problem with the lights flickering when you start a motor is not the same as correcting the demand charges on your utility bill. The peak motor current will last for a few seconds and your demand is likely metered over 15 minutes meaning the few seconds of peak current will not make a significant difference on the metered demand.

There are typically 2 ways to reduce demand charges.

1. Correct the power factor so you are billed for the kW demand instead of the kVA demand.

2. Lower the amount of power drawn during the highest 15 minute load. The only way to do this is to spread the loading out.


The power factor correction savings can be substantial if the site power factor is low. I recently sold a capacitor bank to a company with hydro bills in the $10k a month range and the capacitor bank will save them $12k a year in demand charges.

To impliment #2 means you can not have as many large loads running at one time. This may not be possible if production relies on all motors and the company does not wany to change the work hours.

 

[waross]

First how many dollars are involved in high demand charges?
The best bang for the buck will come from an overall survey and controls, (written procedures or hardware such as interlocking relays or PLC control) to avoid having motors reach peak load simultaneously. Can the operation of the 30 kW mixers be staggered? May the 130 kW mixer be slowed when one of the 30 kW mixers is peaking?

In support of jraef's comments on demand meters, a historical perspective may be helpful.

As you probably know, overheating shortens the life of a transformer.
Demand charges were based on the heating effect on transformers.
An assumption was made that the temperature rise time (the original definition of "rise time") was about 3 minutes and the temperature would reach the terminal value after 5 rise times or 15 minutes.
The original demand meters used a thermal element designed to have the same thermal characteristics. (I have seen literature indicating that demand meters were available with a 30minute window but I have never seen one in service).
Extrapolating back down the inverse exponential heating curve:
In 1/5 of 3 minutes or 36 seconds, the temperature will have risen to 63% x 63% of the final value.
So in 36 seconds the demand will only register 40%
In 1/5 of 36 seconds or 7.2 seconds the demand will register 63% x 40% or 25%.
Getting into motor starting times:
In 1/5 of 7.2 seconds or 1.44 seconds, the demand will register about 16% of the total.
It looks as if the demand for a 1.44 second motor start will register only 16% of the instantaneous demand but we're not through yet.
Demand is measured in kW and a starting motor has a poor power factor. The result is that the demand in kW is quite a bit less than the KVA demand as indicated by an ammeter.
The voltage dip during starting will also lessen the KVA demand although it may extend the start time and negate part of the saving. It is my understanding that the algorithms in an electronic demand meter mimic the response curve of the old thermal demand meters.
For a 2.88 second start, the indicated demand will be 22%
What is this in dollar$?
If we assume that a 30 kW motor draws 200 KVA during starting, the kW during starting may be only 100 kW or less due to the very poor power factor. That's a 22kW indicated demand to start a 30 kW motor. If the sliding window is taking an average of a few seconds starting and 15 minutes of running lightly loaded the indicated demand may even be less.
You may not save a lot of money by reducing starting demands!!
Look for heaters of fans or other similar that may be shut down or curtailed during peak mixing operations without a negative impact on production.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Bill;

The new electronic meters will measure the kVA demand as well as the kW demand. You pay the demand charges based on the higher of the 2 - the kVA demand with an allowance for a minimum power factor or the kW demand.

As an example, say you have 500kW demand and the utility allows a 90% power factor. You will get billed based on 500kW demand when your power factor is greater than 90%. However, if your power factor is <90% or the kVA demand is greater than 556kVA then you get billed based on the kVA demand.

 

[Tarl]

Guys, thanks very much for all your help. It seems that some thinking outside of the box with potentially some new control gear might be the way forward.

Thank you again,
Tarl

 

[waross]

Thanks for the information, LionelHutz. Given the reasons for demand charges, KVA demand makes more sense.
Back in the day, phase shifting transformers were used with conventional meters and you could measure kW demand or KVAR demand directly but you had no way to measure KVA demand with standard equipment. Hence kW demand charges in lieu of KVA demand charges.
Has anyone seen an arrangement to measure KVA demand with old school equipment?

Bill
--------------------
"Why not the best?"
Jimmy Carter