Question

Which of the following describes the change in current from the primary circuit to the secondary circuit of a step-up transformer? a. The current in the secondary circuit is less than the current in the primary circuit. b. The current in the secondary circuit is equal to the current in the primary circuit. c. The current in the secondary circuit is greater than the current in the primary circuit. d. none of the above

A step-up transformer is used to change the voltage produced by a generator from 30 kA to 300 kA. The amount of power produced after the voltage increase is 600 MW and the energy is then sent through a transmission line with a resistance of 0.2 W. How much power is lost in transmission? a. 1 MW b. 0.8 MW c. 0.4 MW d. 0.6 MW

What could happen during power transmission if the voltage is too high? a. There will not be enough current to travel through the wires. b. The magnetic field that is induced could interfere with satellite communications. c. It would be impossible to step down to a level for home usage. d. The energy could discharge through the air and to the ground.

thank you so much.

Answer 1

1. Option a).The current in the secondary circuit is less than the current in the primary circuit is correct here because ,

In case of change in current from the primary circuit to the secondary circuit of a step-up transformer we have the relation as,

Primary power = SECONDARY power

So that here,

V1/V2 = I2/I1

Means with increase in voltage current will be reduced in step up transformer . So that Option a).The current in the secondary circuit is less than the current in the primary circuit , will be correct here.

2. Option b.) 0.8 MW

Here we have given that,A step-up transformer voltage regulation as 30 kA to 300 kA.

The amount of power produced after the voltage increase is 600 MW

transmission line resistance of 0.2 ohm

For the power is lost in transmission we have ,

Current in the transmission line will be,

I = power/ SECONDARY voltage = 2000 A

So that power lost in transmission line will be

Pt = I^2 × R = 2000^2 × 0.2 = 0.8 MW

hence option  b. 0.8 MW is correct here

3. Option a). Is correct here.

If during power transmission if the voltage is too high then a. There will not be enough current to travel through the wires. Is the right option because usually

current not flows through transmission lines, electrical power flows and Power is the product of current flowing through a line and voltage difference across it. P=VI.

In a transmission line to minimise loss, we want to make sure that a small amount of current passes through the wires. Hence the power is delivered at high voltage levels and for that the amount of current flowing through transmission lines is actually quite small.