active power, reactive power

Dear Vimal singh

Briefly active power is the useful power and reactive power is the wattles component resulting in the heating of the equipment.

For more details i suggest kindly refer to some standard text book.

regards

Industrial load other than incandescent lighting load and furnace with resistance heating elements are inductive load. This means that the power factor is lagging or Cos Φ.is less than unity. As KW ( useful power ) will be KVA x Cos Φ which will be less than KVA drawn resulting in a portion of kVA getting lost. This lost power is called reactive power. To avoid this kVAR loss system power factor is improved with the help of power factor corrector units like static capacitors or synchronous condensers. Please read the following link to get better knowledge on the subject.

http://en.wikipedia.org/wiki/Power_factor

The linked Wikipedia article is generally good, and covers most of the major concepts and gives the appropriate sums. What neither it nor the other postings here cover fully is the possible meanings of so-called reactive power.

First, be aware that reactive power is not power. It is dimensions are those of current times a Voltage, which has the same dimensions as power (V x A, or ML²/T³), but it is not the same under orientational analysis, and the proper term for the unit is kilovar or kVA (as compared to kilowatt).
Clearly, the term "reactive power" originates with AC driven into inductive or capacitive components - but it has been extended to cover the effects of other "non-idealities" in the current waveform. As these non-idealities include peaks in the current drawn that occur mid-cycle it is often not realistic to consider the process as power being delivered to and then extracted from a energy stores.
So: in general usage the value of "reactive power" is derived from the apparent power (using the techniques defined elsewhere), and it includes any effect that causes the RMS value of the current to exceed the minimum that would be required for the real power that is delivered. This will include distortions in the waveform where there is no energy returned to the power system.

The significance of apparent power is that the currrent that the power-distribution system has to handle is equal to Apparent_Power/RMS_Voltage, which determines the size of wiring and transformers that need to be provided, and also the transmission losses in the overall system.

Some Equations:
Real power = average(instantaneous_Voltage x instantaneous_current)
RMS Voltage = sqrt(average(Voltage²))
RMS current = sqrt(average(current²))
Apparent power = (RMS current) x (RMS Voltage)
(Reactive power)² = (Apparent power)² - (Real power)²
Note that the final equation is really a statement about currents, albeit expressed in terms of power.

Vimalsingh,

A graph can explain this simply. Take a piece of paper and draw on it two intersecting lines, one vertical and one horizontal. Label the vertical line "active power" (measured in kW) and the horizontal line "reactive power" (measured in kVAR). Now draw an arrow from the intersection point at an angle upwards and a little to the right. This arrow is the real power, and you can see that it is the vector sum of a vertical arrow and a horizontal one. This represents the real power, which is a vector sum of active and reactive power; and these two are 90° apart. Purely reactive power is found in inductors (arrow to the right) and capacitors (arrow to the left). Purely active power is found in resistors (arrow up). Most of the time, the real power being used has both active and reactive parts; with the active part being the majority.

The active power does the work. However, the reactive power is often necessary to allow the active power to do its work (such as making the magnetic field that then enables the motor shaft to rotate and convert electrical energy into mechanical energy). Usually the reactive power is inductive. If it gets to be too large a percent of the total, then the power line and generator are both being overloaded to supply something more than just work. Under these circumstances, one can add capacitors to the line to reduce the total reactive power. Think of this like an arrow to the right for inductive type of reactive power and a shorter arrow from the end of the first arrow back towards the left for the capacitive type of reactive power; the only part the power line or generator has to worry about is the small difference of these two arrows and its vector sum with the active power.

OK? --JMM

This is not the usual usage of the term "real power". "Real power" is usually just an alternative term for "active power". Watt you are talking about is usually referred to as "apparent power", and is what you would find if you multiplied the RMS current by the RMS Voltage without taking account of any lack of correlation between them.

Correction appreciated--JMM

Appreciate finding a person who is not offended by my clumsiness

Fyz

Dear Sh.Singh,

In power system the power is defined as below.

a) Power generated(available) is denoted in KVA(MVA)

b)Power actually used by loads is denoted in KW(MW)

c)Power cosumed by reactive elements is denoted in KVAR(MVAR)

It is denoted by Right angled triangle, base as KW and altitude as KVAR and hypotenuse as KVA.The angle between base and hypotenuse is Phi (O)

From the total power drawn from the system KVA, it is actually used for real power for the load as KW , with wasted power KVAR in inductive circuits.

The total power drawn is called as Apparant PowerKVA

Apparent power in KVA which is drawn from network is vectoral sum of Real useful power KW and useless(Wattless) reactive power inKVAR.

In ideal case what ever power drawn ie KVA shall be used as Useful power in KW.

In this case efficiency is out put/input is 100%

.However as part of power drawn(KVA) is wasted in inductive circuits which donot contribute useful power.This wasted power is called as Reactive volt ampere(VAR)Hence efficiency of power transfer is always less than 100%

The lesser reactive power consumed , more the use ful power obtained from the same KVA power drawn from source.

Hence power factor is ratio of useful power to actual power drawn from power source

Power factor is cos(Phi).Also it is nothing but efficiency of utilisation of power drawn from the system.Higher pf means cos(phi)=1 when angle Phi=0 .In that case whatever power drawn(KVA) from net works is actually used in real power(KW).Here efficiency of power transfer is 100%.Lesser PF means lesser efficiency and lesser useful powerfor the loads.

Please note we cannot avoid reactiver power (KVAR) in the system.This reactive power is necessarily evil which power system has to live with, since all electric loads except resistance type loads consume/require reactive power for their magnetic circuit operations.(motors.Generators,Transformers,Reactors etc)

Hope the above statement has clarifies your doubts.