Question

c)    A 3-phase load draws an apparent power of 80kVA from a 415V 3-phase supply. The power factor of the load is 0.85. Determine the real power supplied to the load and the rms line current?

D) Explain the importance of energy storage for grids that have large amounts of wind power. Name one type of storage system used within a large electricity grid.

E) Explain why AC loads are often rated in terms of their apparent power (VA), rather than their real power (Watts), even though the real power is the only quantity which indicates the actual work done at the load.

Answer 1

c) power factor = real power / apparent power

    real power = pf × aparent power

                = 0.85 × 80000

               = 68000 watts (real power supplied to the load)

apparent power = 3 V_rms I_rms (For delta connected load )

                    I_rms = 80000/ (3× 415 )

                      = 64.25 Amp

d) Energy storage is very important in grids . generally during the night time supply is greater than the demand .The excess supply can be used to power the storage elements . however during the day time when demand increases and supply decreases thus to create a balance these storage devices can be used to fill the demand and supply difference .

storage system used within a large electricity grid is Pumped hydroelectric storage .

.Pumped hydroelectric storage offers a way to store energy at the grid’s transmission stage, by storing excess generation for later use.

e) since Ac load does not have unity power factor , so its has the ability to deliver/consume both active and reactive power . thus total power is nothing but net resultant of active and reactive power which is called apparent power .

apparent power = [(active power) ^{2 +} (reactive power )²]^0.5

since the unit of apparent power is volt ampere (VA) , Ac loads are generally rated in VA rather than watts .