# Transformer question (between substation and home)

1. Mar 21, 2015

### blooperkin

1. The problem statement, all variables and given/known data
A power station generates 100kW of power at a voltage of 10kV. Given that the connecting cables in the primary and secondary circuit each have a resistance of 10Ω, find the turn ratio required for an ideal step-down transformer to bring electrical energy to the home at 240V.

2. Relevant equations

Turn ratio:
Ns/Np = Vs/Vp = Ip/Is
(s- secondary coil, p- primary coil)

V = IR
P = V^2/R = IV = I^2R

3. The attempt at a solution
For the primary circuit,
Current = 100000/10000 = 10A
Potential difference across 10Ω resistance = 10*10 = 100V
Potential difference across primary transformer = 10000-100 = 9900V

I know the power across both transformer coils must be equal, so:
Power output at secondary circuit = Power input at primary coil = 9900*10 = 99000W

- What does this 99000W refer to? The power across the secondary coil or the power across the whole secondary circuit?
- Or does the secondary coil act like cell in the secondary circuit? ie. Do I have to consider its resistance?
- Does the home act like a lightbulb in a circuit? So it has a resistance?

I'm stuck here because I can't find the potential difference across the secondary coil to use the turn ratio formula.

Diagram of secondary circuit:

2. Mar 21, 2015

### haruspex

The power is going somewhere, so yes, assume the home load (resistance) is R. What equations can you write down?