oil field induction generator

[s22j26]

Hi,
Pumpjacks in the oil fields use 3-phase induction motors. During a certain portion of the pumping cycle, the weight of the falling masses can force the motor to rotate at above its synchronous speed, thereby turning it into a generator. There is a belief that this briefly (~ 1 second out of 5 seconds) generated energy can be passed to another motor in the same field, thereby slightly reducing the demand on the grid. For example, if the first motor generates 4 kW for 1 second and the second motor requires 20 kW during that time, the latter will receive 4 kW (less losses) from the first motor/generator and only need 16 kW from the utility.

What is the likelihood of this scenario?
1. The frequency of the generated energy is hard to predict. Let's say 61-62 Hz rather than 60 Hz. How well will it add with the 60 Hz from the utility?
2. The motors run at 460 V but the distribution to them is 12 kv. How well will the generated energy pass backwards through the step-down transformer at the first motor?
3. What losses should I expect if the 2 motor are 500 meters apart?

Thanks for your help.

 

[Skogsgurra]

You can calculate the generation scenario just like you calculate a load scenario. No difference at all.

But, you need to correct your thinking about frequency. It stays at the grid's frequency (60 Hz) regardless of what your motors/generators do.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[wayne440]

With reference to statement #1, the frequency is easily predicted. It will be equal to that of the local grid.

#2 As far as I know, the X-former won't care. Power will flow either way equally well.

#3 You have not provided enough information for a useful answer, a WAG is losses will be insignificant.

Also don't forget Murphy's law. There is a good chance that at some point the jacks will end up on the upstroke at the same time, so any reduction in "demand" will be negated.

 

[waross]

Are the pumps metered individually or is the revenue meter on the 12 kV feeder?
If the meter is an electronic meter on the individual pump jacks you may be paying for the energy that you regenerate as well as the energy that you use.
Many electronic KWHr meters save KWHr consumption to one register and KWHr regeneration to a second register. The display, (what you pay for) is the sum of the two registers. This combats a popular illegal practice to steal energy.
This is the default setting of many electronic meters.
If there is any chance that you will be regenerating through a KWHr meter, you should discuss the situation with your utility. They may be willing to disable the reverse billing. The Public Utilities Commission may help. I doubt that their tariffs allow the utility to charge for regenerated energy.
Remember that when the weight is falling, the pump is lifting product. A balanced pump jack may be consuming energy to lift the weight and lower the piston and then be consuming energy to lift the piston and the column of product.

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

 

[Compositepro]

As waross said the counterweight on the pump jack is adjusted to balance the weight weight of the pump string and liquid column so it should not really be regenerating, just going from high to low load. Any regeneration during the pump cycle just means that the load is that much higher during the other half of the cycle. The purpose of the counterweight is to prevent that, and lower the peak load. Just like counterweights on elevators.

 

[cranky108]

As noted above, not all the jacks will operate at the same speed, so at some point over the billing period all the jacks will hit a peak demand at the same time.

It's better to balance the jacks to lower the demand on each unit, which in turn will lower the overall metered demand.

Better than that, the lower demand will improve your overall voltage profile, which should lower your losses.

 

[s22j26]

Thanks to everyone for your replies.

Thanks for pointing out my error regarding frequency. I assumed that the frequency of the generated energy would be a function of the speed of the overspeeded motor.

Yes, in this application, it is possible that the pumpjacks are not optimally balanced and are therefore generating during a portion of the stroke.

The meter is on the primary side. It meters several (or several dozen) pumpjacks. If several dozen, then there are always likely to be some consuming the energy that others generate.

Waross, I was intrigued by your description of the kWHr meter. Do you mean that if a pumpjack consumes 20 kWHr and generates 4 kWHr during an interval that I will be billed not for 20 kWHr, or 16 kWHr, but 24 kWHr?

 

[waross]

Yes. That is based on one machine on an electronic meter.
The old electromechanical meters would run backwards on reverse power flow, but the new electronic meters typically sum all power flow, both forward power and reverse power.
I was talking to an old acquaintance recently who connected a small wind generator to the grid without permission. He was billed for the energy that he exported as well as the energy that he consumed.

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

 

[cranky108]

The company I work for for several years, would put a detent on the electromechinical meters if they suspected back generation. That is if there was no application for generation.
With the electronic meters, they just did not read the generation quadrants of the meter. Again if the customer was not on file for generation.

This wasen't a bad policy, as a few customers would try to put cogeneration on, without the state mandated paperwork. There intent was to run the meter backwards, to get a better rate for there cogeneration.

 

[waross]

With any of the electronic meters that I have been aware of, the default setting was to sum the power in both directions.
If co-generation was approved, the defaults would be changed so that regenerated energy could be read separately. The default provided a hefty penalty for unauthorized regeneration. The original reason for the default setting is to combat one method of energy theft.

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