Power Flow

Question 1

Why is the power factor often described as leading or lagging?

Answer 1

This relates to whether the current leads or lags the voltage.

In inductive loads the current lags the voltage by 90 degrees, so indicates a lagging power factor, with the reactive power Q being positive.

In capacitive loads the current leads the voltages by 90 degrees, so indicates a leading power factor, with the reactive power Q being negative.

Question 2

Why is 3 phase generation often used?

Answer 2

It gives a steady power generation or power input using the theory of the 120 degree separation, which leads to less vibration and torque variation in a machine.

Question 3

Why is it desirable to operate with a power factor close to unity?

Answer 3

The phase difference between the voltage and current is zero and therefore removes the reactive power from the system. All the energy supplied by the source is consumed by the load, which results in a more efficient generation and transmission.

Question 4

What two ways can generators be configured and describe the configuration.

Answer 4

Star Connected (Star): A neutral point where voltage is nominally zero. In star connected loads, if the loads are balanced the return current is zero.

Delta Connected (Triangle); There is no neutral point with the delta connection

Question 5

What are the star connected power relationships?

Answer 5

mag S = 3VphaseIline
mag S = sqroot(3)VlineIline

P = 3Iline^2Rphase

Q = 3Iline^2Xphase

Question 6

What are the delta connected power relationships?

Answer 6

mag S = 3VlineIphase
mag S = sqroot(3)VlineIline

P = 3Iphase^2Rphase

Q = 3Iphase^2Xphase