A question related to transformer

[Clara Chung]

Homework Statement

Electricity generated in the power station is stepped up by a transformer X from 25kV to 400kV before transmitted by overhead cable. Then it is stepped down to 200V through another transformer Y before entering domestic circuits. Assume all transformers are ideal.

Given the power transmitted through the cables and the total resistance of the cables is 4x10^3 kW and 1Kohm respectively. Find
(I) the electricity is not stepped up by X (3marks)
(II) the electricity is stepped up by X (2 marks)

Homework Equations

Ip / Is = Ns / Np

The Attempt at a Solution

In (I),I tried to find out the power lost in the cable by finding the current in the transmitting cable by 4 x 10^3 x 1000 /25000 x (25 /400)=10A, Power=10^2 x 1000 = 100000W, I thought it was the electricity NOT stepped up by X, but the answer turned out to be 100000W is the electricity stepped up by X(the answer of (II)). I don't understand what does the question mean.

 

[CWatters]

Have you translated this question into English from another language? The problem statement is ok but the questions don't make sense. Perhaps post the original questions to see if we can make a better translation?

In (I),I tried to find out the power lost in the cable by finding the current in the transmitting cable by 4 x 10^3 x 1000 /25000 x (25 /400)=10A, Power=10^2 x 1000 = 100000W,

The "power transmitted through the cables" is usually the power arriving at the end (domestic circuit) not the power produced by the generator.

 

[Clara Chung]

I'm sorry but the questions are originally in English:(( Can you tell me low efficiency of a transformer causes energy lost in form of decrease in voltage or current? thanks

 

[CWatters]

Can you tell me low efficiency of a transformer causes energy lost in form of decrease in voltage or current? thanks

Low efficiency? The problem says the transformers are ideal which is high efficiency. There is no energy loss in an ideal transformer.

 

[CWatters]

The more I think about it the less sense the questions make. They are just so badly worded as to be almost meaningless. "Electricity" in this context could be the current or the power. The only way to tell would be if they give the units that they expect the answer to have.

This doesn't stop you analysing the system. You can work out all the voltages, currents and powers at all points in the circuit. The answer they want will be in that lot somewhere :-)

I would start with a diagram but I like diagrams.

 

[Clara Chung]

The more I think about it the less sense the questions make. They are just so badly worded as to be almost meaningless. "Electricity" in this context could be the current or the power. The only way to tell would be if they give the units that they expect the answer to have.

This doesn't stop you analysing the system. You can work out all the voltages, currents and powers at all points in the circuit. The answer they want will be in that lot somewhere :-)

I would start with a diagram but I like diagrams.

thanks a lot:) I lost points from this question in my exam and I'm perplexed.
"low efficiency of a transformer causes energy lost in form of decrease in voltage or current?" I wonder how the current and voltage change during energy lost in general cases.

 

[CWatters]

"low efficiency of a transformer causes energy lost in form of decrease in voltage or current?"

I believe this is also a badly worded question. I can't give a one word answer voltage or current...

Consider the energy lost due to the resistance of the copper windings (so called copper losses). This affects the secondary voltage of a transformer. The more current you draw the lower the output voltage. So you could argue that the energy lost in the winding resistance causes a loss of secondary voltage. However you would normally design your transformer to correct for this. You would change the winding ratio to restore the secondary voltage to the required value at it's rated load current. The effect of this correction would be to increase the primary current. So which answer do they want?

 

[rude man]

The idea is this:
2 choices:
1: power genegrated at 25 kV then dropped to 200V at the end-point.
2: power generated at 25 kV, stepped up to 400 kV the reduced to 200V at the end-point.
Seems obvious to me.
Idea is that at 400 kV the current, and therefore i-square-R losses thru the cables are greatly reduced compared to transmitting at 25 kV.

 

[CWatters]

Slaps head. I should have seen that.