Electrical Systems_ Instantaneous, Average, Real and Reactive Power

8/10/2019 Electrical Systems_ Instantaneous, Average, Real and Reactive Power

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Instantaneous, Average, Real and Reactive Power

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We discussed and developed some important concepts of transmission linesin last few articles. Last time we

discussed about long transmission lines. Here we discuss a simple but important basic concept Electric Power.

This w ill refresh our knowledge before we move further.

Electric Power has same meaning as mechanical power but here the power or energythat we are concerned is

in Electrical form. We often encounter terms like instantaneous, average, total, real, reactive, apparent and

complex power or simply power. What they mean? how are they related ? That we will discuss here and in next

article.

DC Circuit

As long as our analysis is restricted to Direct Current(DC) circuit the power consumed by the resistance load is

the product of voltage across the resistance and current flowing through the resistance. It is really simple.

P = V . I

The power consumed by the load is the product of voltage across the load and current drawn by the load (Fig-

A). Or the Power supplied by the DC source (battery/cell) is the product of voltage across the cell and current

supplied by the cell. Both are equal in our example figure(considering ideal battery of zero internal resistance).

The law of energy conservation implies power supplied by the source must be same as power consumed by the

circuit. In DC circuit case instantaneous power is same as average power.

AC Circuit

In AC circuit analysis, what is this power that we talk about. The main problem is that the AC voltage andcurrent varies sinusoidallywith time. Moreover the presence of circuit reactive e lements like Inductor and

capacitor shift the current wave with respect to voltage wave (angle of phase difference).

Power is rate at which energy is consumed by load or produced by generator. Whether it is DC circuit or AC

circuit, the value of instantaneous power is obtained by multiplying instantaneous voltage with instantaneous

current. If at any instant of time t the voltage and current values are represented by sine functions as

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Complex Power and

Power Triangle

Instantaneous,

Average, Real and

Reactive Power

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+3 Recommend this on GoogleHOME ABOUT BASIC POWER SYSTEM
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v = Vmsin t

i = Im sin (t-)

Vmand Im are the maximum values of the sinusoidal voltage and current. Here =2 f

f is the frequency and is the angular frequency of rotating voltage or current phasors. It should be clear that

for a power systemf is usually 50 or 60 Hz

is the phase difference between the voltage and current.

As we said the instantaneous power is the product of instantaneous voltage and current, if we name

instantaneous power as p then

p = v.i = Vmsin t . Im sin (t-)

or p = Vm Im sin t sin (t-)

Applying trigonometric formula 2.sin A.sin B = cos(A-B) - cos (A+B) we get

It can be written as

This is the equation of instantaneous power

In the Fig-C is drawn all the three waves corresponding to v, i and p. Graphically also we can get the value of

instantaneous power (p) at any instant of time t by simply multiplying the value of current i and voltage v at

that particular instant t. (You can verify that in the diagram p is negative when either v or i is negative

otherwise p is positive. See the points where p is zero). In the graph we have shown horizontal axis as

angle instead of time t for easy visualization. It should be clear that both way it is correct.

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Clearly the instantaneous power p is composed of two terms. The first term is constant because for a given

load the phase angle is fixed. It does not change unless the load is changed. The second term is varying

with time sinusoidally due to the presence of the term cos (2t-). Look that the instantaneous power

frequency is twice the frequency of voltage or current.

So the instantaneous power in a single phase circuit varies sinusoidally.

The instantaneous power, p = constant term + sinusoidal oscillating term.

In one complete period the average of oscillating term is zero.

Then what is the average power within a given time, say one Time Period of the wave?

It is the constant term.

Here is another way to think about the average power.

Just observe that the instantaneous power is negative for a small time. For any time interval you just find the

total +ve area A+ (above horizontal-axis (blue line) and below p curve) and total -ve area A- (below horizontal

axis and above p curve). The net area is obta ined by subtracting A- from A+. By dividing this net area ( by thetime interval Ti we get the average power(P). You can do this using calculus. What you will ultimately get is

only the first term in the above formula for instantaneous power p.

In still another way it is easier to realize that the formula for instantaneous power p has a constant term

(Vm.Im/ 2) cos and the other sinusoidal term (Vm.Im/ 2) cos (2 wt - ). Actually p is the oscillating power

which oscillates about the average constant term (Vm.Im/ 2) cos .

So the average power is

The above formula can be written as
http://3.bp.blogspot.com/-rKe5JCHz7Us/Tf2OZwCkTAI/AAAAAAAAAgI/Sd36aoLNnu8/s1600/Power_wave.png


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Or,

here,

V and I are the phasor representation of RMS values* of voltage and current sinusoids. The symbols |V| and |I|

are the magnitudes of phasorsV and I. (See at the buttom for definition of RMS value).

This above formula is your favorite formula for useful pow er that we are most concerned about. This average

power formula is used to find the power consumed by the load. The monthly electric energy bill at home is

based on this power. The engineers and technicians in power or electrical industry simply use the term power

instead of average power. So whenever we simply call power it means average power.

Of course the instantaneous power is oscillating in nature. As we already said it does not oscillates about the

horizontal-axis rather about the average power P (cyan color horizontal line).

P will be zero when cos =0 or = 90 degree, that is when the phase angle between voltage and current

waves is 90 degrees. It is only when the load is pure inductive or capacitive. In this case the second term only

remains in the instantaneous pow er formula.

From the above figure for some time the power becomes negative that means the load supply energy to source

for this period. This is due to the presence of reactive element in load.

The above formula for instantaneous power can be written in another form. This form actually is an attempt to

distinguish the oscillating reactive power from the instantaneous power formula. Rearranging the terms in

equation for instantaneous power above we get

p = |V| | I | cos (1-cow2t) - |V| | I | sin sin2t

In this equation the first term |V| | I | cos (1-cow2t) is oscillatory whose average value is |V| | I | cos . We

already talked about this average power.

The second term |V| | I | sin sin2t which is also oscillatory but with zero average value. The maximum value

of this term is |V| | I | sin . This is the so called Reactive power. So Reactive power is the maximum value of a

oscillatory power that is repeatedly drawn from the source and again returned to the source within each cycle.

So the average of this reactive power is zero.

The average power P is called as Real Power. It is also sometimes called active power.

Real power = P = |V| | I | cos

It is usually written as P = VI cos . But it should be remembered that V and I are the rms values of voltage

and current. For example when we say single phase 220 volt AC it means the rms value of voltage is 220 volts

( it is not maximum value of voltage sinusoid)

Reactive power = Q = |V| | I | sin

Real power is measured in Watt and the reactive power is measured in VAR (VoltAmpereReactive). In power

sector these units are too small so real power is measured in Megawatt (MW) and reactive power in Megavar

(MVAR). The letter R at the end denotes reactive power.

Many times students and practicing engineers are confused about the average power (often simply calledpower). They think that what they get by multiplying RMS voltage and RMS current is RMS power. No that is

wrong. There is no RMS power. RMS power has no meaning or not de fined. (Also see definition of RMS value,

below at the end). It is average power or real power or true power.

Power In Three phase Balanced System

Let us consider a three phase balanced system. A three phase balanced system is analysed considering only
http://skm-systems.blogspot.com/2010/05/phasors-in-ac-circuit-analysis.html


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one phase and neutral return. This is called per phase analysis. So the above analysis for single phase is true

for balanced three phase case. Let the total power here is P t. Then we get total three phase power as thrice of

single phase case.

Pt = 3 |V| | I | cos

It should be remembered that |V| and | I | are the per phase values. and is the phase angle of load in per

phase analysis.

The above formula for balanced three phase system can be written as

Pt = 3 |Vl| | Il| cos

In the above formula Vland Ilare line voltage and current (Fig-D). This equation is independent of type of

three phase load connection i.e delta or star connected load. You have to know the line voltage, line current

and phase angle as above. This form is very convenient and used often in power calculation.

There is one main difference between the single phase and total three phase power. The instantaneous single

phase power is pulsating. In the balanced three phase case, each phase instantaneous power is pulsating but

the three pulsating power waves are 120 degrees displaced from each other. At any instant of time the total of

these three instantaneous power waves is a constant which is 3 |V| | I | cos . So the tota l power consumed in

three phase balanced system is not pulsating. Non-pulsating power also imply the desired non-pulsating

torque in case of three phase rotating machines. In large 3-phase motors this is really desired.

*RMS value of AC Sinusoids

The value of AC voltage or current that produces the same heating (or same energy) that is produced if DC

voltage or current numerically equal to RMS value of AC is applied instead of AC. This concept helps make the

formula for power similar for both DC and AC circuits.

You should read the next article about Power Triangle and Complex Power

Labels: Basics

3 comments:

harisaid...

very clear and this topic which i always confused.thank u very much.

August 27, 2011 at 1:02 PM

Dhyeya Shahsaid...

abso lutely amazing description ,very clear idea about this topic...thanks

July 13, 2012 at 7:50 AM

Anonymous said...

I am really happy to read this web site posts which includes tons of

useful facts, thanks for providing such information.
http://skm-systems.blogspot.com/2011/06/complex-power-and-power-triangle.htmlhttp://www.blogger.com/profile/17874007281990382115http://3.bp.blogspot.com/-mw0mZUS_gAI/Tf2aqnYogfI/AAAAAAAAAgM/6gBBk2hKwdE/s1600/Three+phase+power.pnghttp://www.blogger.com/profile/16474480699298379856http://www.skm-eleksys.com/search/label/Basicshttp://www.skm-eleksys.com/2011/06/instantaneous-average-and-total-power.html?showComment=1342146018208#c1802901427660174076http://www.skm-eleksys.com/2011/06/instantaneous-average-and-total-power.html?showComment=1314430353027#c8877470810409880710


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Electrical Systems_ Instantaneous, Average, Real and Reactive Power