Power Factor Correction
Power Factor Correction
(OP)
Hello
I ran simulations using CYMCAP to figure out the ampacities for each of our cables. I ran all of my simulations using 1.0 power factor assuming the worst case scenario. But now, my boss wants me to find the ampacities at 0.9 load factor instead! is there a table of multipliers of something of the sort that I can use to get the new pfs at 0.9 from 1.0? or do I have to run all of the simulations again (There are about 60 of them)?
Thanks
RRA
I ran simulations using CYMCAP to figure out the ampacities for each of our cables. I ran all of my simulations using 1.0 power factor assuming the worst case scenario. But now, my boss wants me to find the ampacities at 0.9 load factor instead! is there a table of multipliers of something of the sort that I can use to get the new pfs at 0.9 from 1.0? or do I have to run all of the simulations again (There are about 60 of them)?
Thanks
RRA






RE: Power Factor Correction
For the present issue, make sure that you are clear on the difference between LOAD FACTOR and POWER FACTOR before wasting any more time.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Power Factor Correction
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Power Factor Correction
If your study was of the second type (getting max amps for a given temperature) you only have to check the obtained ampacity against your revised loading at .90 PF.
If you used the first approach, using unity PF, the smaller current value you ran your initial simulations with, naturally gave you lower final temperatures. If you want to save time, try looking at your most stringent case(s), run them at 0.9 PF. This will give you an idea if you still have enough margin left. Then talk to your boss.
Also look at what loading factor your are using. Default value is 100% which might be on the pessimistic side.
You can also call the guys at CYME for help/support if you have a valid maintenance contract for the software.
Desrod
RE: Power Factor Correction
I meant 0.9 Load factor, not power factor! I posted the question here toward the end of my shift.
I ran some simulations in CYMCAP and for a 500 CU cable the difference in ampacity going from a 1.0 to 0.9 LF is 3.33% increase ... then I ran the same simulations but for a 2/0 CU cable and the difference now is 2.7%
Obviously, it is not a linear relation.
Any suggestions on what should I do?
Thanks
RE: Power Factor Correction
Using a lower LF will factor the impact of cooling down the cables to estimate the peak current. A good example is solar power plants. We performed a number of studies for cable runs there and managed to reduce the number of cables by using the proper load factor (lot of cooling at night!).
You should first explain what you are trying to do with your study: check for load increase? optimize design? emergency configuration?
What you will do next depends on that.
If your boss suggested reducing the load factor, I suspect he is trying to save some cables.
Talk to the customer and find out how long last their peak load or even try to obtain a load profile. Use CYMCAP to run the profile. It will produce the best results and more room for optimization.
Hope this helps
RE: Power Factor Correction
I did all of my studies at 1.0 LF, but my boss said we are never at 100% load ... so he suggested a 0.9 LF because it is more reasonable at peak time.
RE: Power Factor Correction
RE: Power Factor Correction
Changing the load factor does not change the ampacity. Using a load factor does change the maximum amount of loads that may be connected to a circuit or feeder on the assumption that not all the various loads will be connected at any given time.
Using a 24 hour base for average current and then using that for cable sizing is a concept that many of us have not considered.
I would have thought that with a steady state current that cables would reach equilibrium temperature in an hour or less.
A commonly used time constant for transformer heating is 3 minutes. Steady state values are often assumed to be reached after 5 time constants or 5 times 3 minutes gives 15 minutes, the time that we assume the temperature will need to stabilize after a step change in current. A more forgiving figure of 30 minutes is occasionally used for transformers. I have to wonder what the soil conditions are that enable you to use 24 hours or 360 minutes.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Power Factor Correction
Figure 9-16 suggests time constants Tau (T=Tf-(Tf-Ti)*exp(-t/Tau)) ranging from 0.5 hours for "small cable in air" to 6 hours for "large cable, direct buried"
=====================================
(2B)+(2B)' ?
RE: Power Factor Correction
I can see where a constant of 6 hours applied to the output of a solar array may allow a reduction in cable size.
What type of soil conditions allow the longest time constants?
I remember some sizing cables for distribution conductors. There was a cutoff point on the ampacity tables and it was strongly suggested that loadings above the cutoff line should only be used for short term emergency situations due to the high I2R losses. The losses are something to consider when pushing the ampacity limits on long cables.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Power Factor Correction
David Castor
www.cvoes.com
RE: Power Factor Correction
General comment: Not all usage will benefit from tighter assets management. In all cases, the key factor is temperature rise. 24 hours Industrial loads are a good counter-example where the margin does not exist.
When I was working for a Canadian utility, we used to plan for transmission transformers overload up to 30-40% based on equipment initial temperature, ambient temperature and winter peak duration (3-4 hours).
Some cables can even be loaded to 130°C for as long as 5000 hr over the life of the cable. We did not implement the practice since it was very hard to reliably keep a running total of the hours when the cable was loaded at such a high level over a period of 30-40 years.
In all case, when not meeting Std, you need a proper study by qualified people.
RE: Power Factor Correction
Once you know the above, then you can use that to match the conductors to the loads, and this is where the LF could be used.
You certainly don't want to get an answer such as, for example, the wire reaching 90C with 462A at a LF of 0.9. Because this really just means the wire reaches 90C at 416A.
RE: Power Factor Correction
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Power Factor Correction
Lionel, that is exactly what I want for my project. We are taking our peak load from our peak day of the year (usually in late july or early august) and we compare average loads from different days to that peak load day. Not a great way to do it, but that's what our resources allows us to do.
Dividing average load to that peak load, we got 0.83. So my boss decided to go with the 0.9