Interview questions about Active,reactive and apparent power?

Question:Alternative names of active power?

Ans:The following are the alternative names normally used for active power (real power,true power,useful power,wattful power etc)

Question:Why we have simple "P=VI" real power without any phase in DC circuits?

Ans:In dc circuits real power is simply the product of voltage present across load and the specific amount of current flowing into that load.The reason for simple "P=VI" formula having no phase difference or phase angle is due to absence of power factor in dc circuits or we can power factor does not exist in dc circuits.

Question:What is real power in AC circuits?

Ans: In AC circuits phase angle or phase difference exists between voltage and current so in the AC circuits real or active power is as follows "P=VI COS𝛉" where angle theta is in between voltage and current.

 Question:What is real power in case of resistors?

Ans:There is interesting fact about real or active power in resistors that for purely resistive circuits voltage and current are in phase so there is no phase difference between voltage and current so the theta=0 and  cos(0)=1,so the real or reactive power in case of purely resistive circuits is simple product of "V and I" just.

                                              P = VI

Question:Write down some formulas of real power?

P = VI
P = VI COS𝜃
P = √(S^2 - Q^2)
P = √(VA^2 - VAR^2

Question:Define reactive power?

Ans;Reactive power is defined as the power that continue to sweep or exchange between source and load is called reactive power. Reactive power is termed as reactive power due to its reactive properties and simple absorption and returning of power in the load is the obvious demonstration of reactive properties.

Special note about reactive power:

The name reactive power indicates that power first is absorbed and then released in the form of electric field in capacitor and in the form of magnetic field in inductor.

Question: Formulae about reactive power?

P = VI SIN(θ)
Q = √(S^2 - P^2)
VAR = √( VA^2 - W^2)

Question:Different definitions of apparent power?

The product of voltage and current in such a way that phase difference between voltage and current is neglected.
The sum of active and reactive power is called apparent power.
The product of current and voltage with zero phase angle.
If circuit is ac then product of r.m.s current and r.m.s voltage is apparent power.
Note:
Apparent power is equal to real power in case of purely resistive circuit.
Apparent power is greater than real power if circuit is inductive plus capacitive.

Question:Power in resistor,capacitor and inductor?

Real power is absorbed by resistor and is dissipated in the form of light and heat.
Reactive power is absorbed inductor and dissipated in the form of magnetic field.
Reactive power is absorbed by capacitor and dissipated in the form of electric or electrostatic field.

Question:Why AC cannot be stored in batteries?

Ans: AC current storage in batteries in totally impossible due to the problem that during positive half cycle charging takes place but during negative half cycle discharging takes place charging and discharging cancel effect of each other and net current or voltage remains zero.
Furthermore AC continue to change its polarity 50 or 60 times per second so polarity creates problem in storage of AC currents.

Active, reactive, and apparent power

In a simple alternating current (AC) circuit consisting of a source and a linear load, both the current and voltage are sinusoidal. If the load is purely resistive, the two quantities reverse their polarity at the same time. At every instant the product of voltage and current is positive or zero, with the result that the direction of energy flow does not reverse. In this case, only active power is transferred.

If the load is purely reactive, then the voltage and current are 90 degrees out of phase. For two quarters of each cycle, the product of voltage and current is positive, but on the other two quarters, the product is negative, indicating that on average, exactly as much energy flows toward the load as flows back. There is no net energy flow over one half cycle. In this case, only reactive power flows—there is no net transfer of energy to the load.

Practical loads have resistance, inductance, and capacitance, so both active and reactive power will flow to real loads. Power engineers measure apparent power as the magnitude of the vector sum of active and reactive power. Apparent power is the product of the root-mean-square of voltage and current. Electrical engineers take apparent power into account when designing and operating power systems, because though the current associated with reactive power does no work at the load, it heats the conductors and wastes energy. Conductors, transformers and generators must be sized to carry the total current, not just the current that does useful work. Failure to provide for the supply of sufficient reactive power in electrical grids can lead to lowered voltage levels and under certain operating conditions to the complete collapse of the network or blackout. Another consequence is that adding the apparent power for two loads will not accurately give the total apparent power unless they have the same displacement between current and voltage (the same power factor).

Conventionally, capacitors are considered to generate reactive power and inductors to consume it. If a capacitor and an inductor are placed in parallel, then the currents flowing through the capacitor and the inductor tend to cancel rather than add. This is the fundamental mechanism for controlling the power factor in electric power transmission; capacitors (or inductors) are inserted in a circuit to partially compensate reactive power 'consumed' by the load. Purely capacitive circuits supply reactive power with the current waveform leading the voltage waveform by 90 degrees, while purely inductive circuits absorb reactive power with the current waveform lagging the voltage waveform by 90 degrees. The result of this is that capacitive and inductive circuit elements tend to cancel each other out.

The Power Triangle

The complex power is the vector sum of active and reactive power. The apparent power is the magnitude of the complex power.
  Active power, P
  Reactive power, Q
  Complex power, S
  Apparent power, |S|
  Phase of current, φ
Engineers use the following terms to describe energy flow in a system (and assign each of them a different unit to differentiate between them):

Active power, P, or real power: watt (W)
Reactive power, Q: volt-ampere reactive (var)
Complex power, S: volt-ampere (VA)
Apparent power, |S|: the magnitude of complex power S: volt-ampere (VA)
Phase of voltage relative to current, φ: the angle of difference (in degrees) between current and voltage; current lagging voltage (quadrant I vector), current leading voltage (quadrant IV vector)
These are all denoted in the diagram to the right (called a Power Triangle).

In the diagram, P is the active power, Q is the reactive power (in this case positive), S is the complex power and the length of S is the apparent power. Reactive power does not do any work, so it is represented as the imaginary axis of the vector diagram. Active power does do work, so it is the real axis.

The unit for all forms of power is the watt (symbol: W), but this unit is generally reserved for active power. Apparent power is conventionally expressed in volt-amperes (VA) since it is the product of rms voltage and rms current. The unit for reactive power is expressed as var, which stands for volt-ampere reactive. Since reactive power transfers no net energy to the load, it is sometimes called "wattless" power. It does, however, serve an important function in electrical grids and its lack has been cited as a significant factor in the Northeast Blackout of 2003.[4] Understanding the relationship among these three quantities lies at the heart of understanding power engineering. The mathematical relationship among them can be represented by vectors or expressed using complex numbers, S = P + jQ (where j is the imaginary unit).Courtesy of wikipedia....