kW Demand Control Using PLC 1

[OhioAviator]

Hello All, please forgive me if this isn't posted in the right forum.

We need to implement kW Demand control at one of our industrial plants that is controlled by a SLC 5/05 & RSView32 HMI. Our electric utility will supply us with KYZ energy pulses (Form C contacts), estimated to be approximately 770 pulses per 15 minute demand interval at maximum load. Our desired application is relatively simple... all we want to do is activate/deactivate digital output(s) in a PLC based on the number of kWh pulses counted during each 15 minute demand interval. When the number of pulses counted reaches a pre-determined setpoint, or when the projected kW demand looks like it is going to exceed the setpoint, we want to activate/deactivate one or more digital outputs in the PLC. We are thinking that this probably could be accomplished using a Micrologix 1500 or maybe a spare SLC 5/03 that we have lying around. Anyone have any suggestions on how to write the ladder logic for this? If this has been done before, can someone point me in the right direction to beg, borrow, or purchase the ladder logic code?

Many Thanks!

 

[OldPfart]

Depends what you require. Have done kW demand in Omron PLC's on a regular basis. It is easy using a counter. Move data every 15 minutes then reset counter to zero.

If you require anticipation logic, have not been there but compares every minute with a data table should do the trick.

 

[OhioAviator]

Thanks OldPfart (interesting handle there),

Doing the pulse counting and comparing that register to a fixed setpoint register is easy and straightforward. It's the anticipation logic that's got me scratching my head. Any suggestions anyone?

Thanks,
--John R.

 

[djs]

Hello;
You probably do not need to have an additional PLC. Depending on the max frequency of the pulses, you may need to add a pulse counter card to one of the SLC racks. You can then read the number of pulses directly from that card.
On of the easiest ways to limit demenad is limit the number of large loads (motors) at one time. For example, if a 1000 Hp motor has just started, lock out all other motors from start for a period of time. This time should coorespond to the start time of the 100o Hp motor. This type of programming is very common in plants when operating on emergency generators.
Another way of limiting your demand is to have thresholds on the demand pulses. If the number of pulses exceeds a certain value lock out certain loads for the next 15 minutes. Can be many different levels and many different loads and load combinations.

 

[OhioAviator]

Thanks djs. Thank you. You're probably right about just adding a pulse counter card to the existing SLC.

Fortunately or unfortunately, our total connected load is approximately 4,000 HP of which 3,500 HP is in one single motor. And that single 3,500 HP motor deals with large impact loads, i.e. loaded at 700 HP at one minute and then 4,000 HP the next minute. Needless to say, our load factor is horrible at around 20%.

I know that COTS demand controllers are available but they either (a) require CT'd & PT's which are expensive to purchase and install at 15kV, or (b) they work with the utility's kWH energy pulses but their user interface and communications capabilities are severely lacking and very cryptic and unfriendly. I was hoping to "roll my own" basic demand controller out of spare PLC components.

Again, suggestions welcomed!

 

[Macbeth]

Ohio Aviator,

Sound like you are trying to mimic a KWh Sliding window demand controller this is some what simple but you do need to understand how the power company is billing you for demand.

I generally setup two things from the KYZ first actual KW from the pulse rate. Time the pulses see how many fit in an hour multiply that by your KHW per pulse. Use this for any predicted KWH needs. Lock out any more motors from starting or inhibit loading of running equipment.

Next KW demand is the total KWH over a period you must keep a running total over the entire length of the window (15min generally.) Keep a running count on going pulses for every min, Then add up the last 15 min. every min drop off the oldest min. Use this for your load shedding. Sliding KW.

Also use the TOD clock to adjust for shoulder Peak and Off-Peak times, don’t forget Weekends.

You will also require a way to override demand control such as allow a high temp or lagging production to override control.

A good way is to offset system set points.

 

[dpc]

I second Macbeth's comment that you need to verify how the utility is computing demand and how they bill for it. There are variety of methods used, depending on local custom and the type of metering.

As mentioned, if a "sliding window" is used, you are billed for the highest 15 min (or 30 min, or 60 min) period that occurs, regardless of when it is. Other older systems simply look at each 15 min increment and reset after each block of time. Some utilties adjust the demand charge (up, never down) based on power factor.

 

[eriond]

Any of these measures will require you to have one of two conditions satisfied. All of your heavy load equipment will have to be routed through contacts controlled by your power monitoring PLC or all of the PLC's that control those loads must be networked. For a small facility, that would be workable. For one like mine (over 40 PLC's of various makes and models), the task is daunting! I was also unaware that exceeding the "demand limit" for less than 15 minutes would save you anything. The explanation I recieved led me to believe that exceeding that limit, for any amount of time, would subject you to the higher charge for the entire month.
My facility is beginning to examine our options for avoiding those demand charges, so I am very interested in any information that contradicts or agrees with what I have set forth. Good luck in your efforts!

 

[Macbeth]

Eriond,
The "Demand Charge" is generaly the peak average KW over a time period usualy 15 min. This is referd to as the "Sliding KW Window". Usualy this "Peak Demand Charge" Resets evey time the power company reads the meter (30 days). Bolth of these vary from place to place. These are referd to as Tariffs. I know of some "Sliding KW Window" as short as 5min with a "Peak Demand Charge" of 6 months! That's insane!

Also you may have a Power Factor Charge, per KVAR.

If you have not done so yet get your local power company involved. They often have programs that will split cost or have low intrest loans.

If you are unable to Load Shed equipment, another way to KW avoid is Generator Power.

 

[darrenjm]

as a sparky's point of view..to expand on macbeth

The utility does determin your max kw..first check if you have an old meter with an adjustable pointer..If you have this then ANY peak is what you are billed on..if you have a digital meter then things get more interesting.You then need to ask when the look for peak.(As macbeth said it is USUALY every 15 mins but they can change that.
macbeth also brings up a good point of KVAR and power factor...The average plant can save a ton by installing a cap bank

Bottom line..talk to your provider find out as much info as you can..policies..when they were updated,if there is any changes in the works

Its a tough one but do-able with the right info..