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malby
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Join Date: Jun 2011
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Why do Power Engineers Specify PF and Either Power or Reactive Power for Loads?

01/27/2012 4:20 AM

Hey, i am a control engineer working on modelling a Low Voltage distribution network and am not too familiar with the power industry. One thing that is really confusing me is why do power engineers specify power and power factor, or reative power and power factor, or power and reactive power with nominal voltage when describing constant impedance, constant current or constant power loads?

It seems to me that for constant current and impedance loads it would be much easier to describe them as there actual impedance or constant current value within a specific voltage range....... I must be missing something here, does anyone know what it is?
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PWSlack
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#1

Re: Why do power engineers specify pf and either power or reactive power for loads?

01/27/2012 6:18 AM

It helps with cable sizing.

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malby
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#2
In reply to #1

Re: Why do power engineers specify pf and either power or reactive power for loads?

01/27/2012 6:50 AM

I would have thought cable sizing was dictated by current rating, the IEC ampacity?
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PWSlack
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#3
In reply to #2

Re: Why do power engineers specify pf and either power or reactive power for loads?

01/27/2012 7:40 AM

...which is a function of load and power factor....

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malby
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#4
In reply to #3

Re: Why do power engineers specify pf and either power or reactive power for loads?

01/27/2012 7:51 AM

so if the reason for specifying power parameters for loads is cable sizing, which actually requires a max current to be computed, why not specify loads as variable current sinks and negate the need for derivation of current from pf and power?
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PWSlack
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#5
In reply to #4

Re: Why do power engineers specify pf and either power or reactive power for loads?

01/27/2012 8:00 AM

Because something that is intrinsically variable cannot be specified. All that can be tied down is its maximum and minimum.

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malby
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#6
In reply to #5

Re: Why do power engineers specify pf and either power or reactive power for loads?

01/27/2012 8:23 AM

OK, so power, pf and nominal voltage are specified..... In the case of a constant impedance load, the calculation of maximum current can be made but requires more computation than if you have the value of constant impedance and max node voltage.... If the max is all you are interested in then it would be easier to specify max node voltage and impedance to get max current, as less computation is required.....

I am struggling to see the practical reason for specifying load types as a function of power and pf, this convention must make something computationally more efficient.....
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PWSlack
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#7
In reply to #6

Re: Why do power engineers specify pf and either power or reactive power for loads?

01/27/2012 8:33 AM

How about adding loads onto a network, then?

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malby
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#8
In reply to #7

Re: Why do power engineers specify pf and either power or reactive power for loads?

01/27/2012 8:53 AM

OK, I think i am slowly converging on the answer thanks to your help..... so if you specify every load as a function of power and pf it means you can specify worst case for all loads using common units regardless of thier type. This has the added benifit of providing a better overview of your whole network, making it very easy to see if generation meets worst case demand, does that sound about right?
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