Complex Power:

[rattus]

Who can tell us the advantages of using complex power and provide an example?

 

[iwire]

Who can tell us the advantages of using complex power and provide an example?

Rattus can.




That is my answer to your question. Do I get a prize? :smile:

 

[rattus]

Not yet:

Not yet:

Rattus can.

That is my answer to your question. Do I get a prize? :smile:

No, because your answer is wrong. I really don't know the advantages. Waiting for the young experts to expound on the subject.

 

[hardworkingstiff]

Ignorant question

Ignorant question

What is complex power?

 

[rattus]

What is complex power?

Stiff, complex power is a method of breaking apparent power into its real and reactive components using complex numbers.

That makes three of us who don't understand the advantages of this method.

There may be an advantage in summing up the total load where the PF varies among the loads.

 

[ron]

rattus,
You don't have a choice. Complex power is everywhere. It has relationship to generation/distributing power, and usage of the power. It is all va and var.
It was surprising to many that the large NE blackout back in 2003 or so, was in part due to how the generating plants reacted to too much or not enough reactive power as it related to real.

 

[rattus]

rattus,
You don't have a choice. Complex power is everywhere. It has relationship to generation/distributing power, and usage of the power. It is all va and var.
It was surprising to many that the large NE blackout back in 2003 or so, was in part due to how the generating plants reacted to too much or not enough reactive power as it related to real.

Ron, are you saying that this method is useful in managing the power grid--especially in maintaining stability?

 

[rattus]

Well, duh!

Well, duh!

I answered one of my own questions:

Why do we use the complex conjugate of the current in the definition for complex power?

It seems that this is done in order to obtain the difference in the voltage and current phase angles. This difference is the PF angle which is needed in order to compute real and reactive power.

 

[mivey]

Who can tell us the advantages of using complex power and provide an example?

It is a useful tool for circuit analysis, load modeling, and power flow analysis. It can be used to evaluate reactive loads to optimize resources or to help calculate charges for poor resource utilization (i.e., power factor calcs).

The complex power quantities are often available from metering equipment (although the use for kVA metering and the resulting accuracy is a discussion unto itself) and simplifies their use. Geometric interpretation is also an advantage. FWIW, the choice of a positive value for inductive quantities is arbitrary but has certainly become the industry standard.

I don't feel like providing an example. I think there have been numerous examples of the use of the power triangle and certainly many are available on the web. I think most of us are well versed in the topic but if someone really needs one, I'll make one.

 

[mivey]

There may be an advantage in summing up the total load where the PF varies among the loads.

Yes. There is a law of conservation of complex power just as there is for conservation of real power. You can sum the kW & kvar (quantities readily available from most modern meters) to calculate the average power factor or average kVA.

One problem is that the accuracy of the kVA for an interval is in question. Suppose for 1/2 of the interval, the load was purely resistive at 10 W. Then suppose the load was purely reactive for the second 1/2 of the interval, say 10 var.

Using the kW & kvar totals for the interval yields 14.14 kVA when the actual kVA for the interval was 20. There are opportunities to under-size equipment based solely on the kW & kvar interval data. I guess that is getting a little off-topic so I might be on a slippery slope.:roll:

 

[mivey]

Ron, are you saying that this method is useful in managing the power grid--especially in maintaining stability?

Absolutely. Do a quick search on power circle diagrams and you should see they use sending-end and receiving-end circles drawn in the complex plane (the P-Q plane) to analyze power flow.

 

[markstg]

Who can tell us the advantages of using complex power and provide an example?

One advantage is that their is some simplification in power calculations if power is considered a complex quantity.

For Example: For a single phase, 1kw motor with a lagging power factor of 0.8, calculate the reactive power needed to improve the power factor to 0.95lagging.

The complex power drawn by the Motor is:

S = The complex power = (1000/0.8)at and angle of the arccosine(0.8)
= 1000 +j750

The complex power needed to be drawn by the load to achieve 0.95 power factor is:

S = (1000/0.95)at and angle of the arccosine(0.95) = 1000 +j329

The complex power need to be drawn by the corrective load for a 0.95 power factor is:

j329 - j750 = -j421

Answer: Add 421 Var of capacitors to the motor.

Complex Power calculations are also used extensively in Load Flow Calculations on Power Transmission and Distribution Systems, where there are many sources and many loads.

 

[jghrist]

I think the main advantage is that you can define a single variable to represent both real and reactive power. Calculations then can be made simpler and easier to document.