CONCEPT OF POWER AND CURRENT

POWER AND CURRENT OF ELECTRICAL EQUIPMENT:

1.Definition of true power (KW) and formula:

True Power: It is the product of current,voltage and power factor (depends upon the nature of load inductive, capacitive, resistive).

OR

The rate of transfer of electrical energy per unit time is known as power.

Its unit is watt.

P = Electrical energy (KWH) / time(t)

Power rating: The maximum power (product of voltage,current and power factor) on which the equipment works normally.

 P = √3V*I*cosθ

V=Line voltage

I=Line current

ex: Let a power transformer (primary side = 66 KV and secondary side = 11 KV) current on secondary side is 500A at power factor 0.95.Thus output power of transformer find by given formula:

P = √3V*I*cosθ

= √3*11*500*0.95*1000

= 9.04 MW

Thus output power of this transformer at load current 500 A, 11 KV at p.f. 0.95 is 9.04 MW.

2.Definition of Apparent power (KVA) and formula:

Apparent power (KVA): It is the product of current and voltage.Its unit is VA.

Apparent power (VA) rating: VA rating of an equipment is that on which it works normally.

From above given example: P = 9.04 MW and p.f. = 0.95

The VA rating of transformer find by given formula:

P/cosθ = VA

9.04 MW/0.95 = VA

= 9.5 MVA

 Thus capacity of transformer = 9.5 MVA

3.Definition of current and formula:

Current: The rate of flow of charge (mainly electron) is known as electric current.

                                 I = Q/t

Q = charge

t = time

Its unit is ampere.

Ampere: If one coulomb charge flow for one second, then it is known as one ampere. 

Current rating: The maximum consumption of current by an equipment on which it works normally.

                              I = P/√3V*cosθ

ex. Let a power transformer (primary side = 66 KV and secondary side = 11 KV) has rating 1 MVA.Then by using above formula we can find the value of current primary side as well as secondary side.

The value of current on primary side is given below:

I = 1 MVA/√3*66 KV*1

=8.75 A

Similarly on secondary side is:

I = 1 MVA/√3*11 KV*1

=52.5 A

The given chart give the value of current of different rating of power transformers:

Transformer current rating