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Old Aug 11th 2008, 11:01 AM   #1
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power

a power station which generates 100mw of power at 25kv. the voltage is stepped up to 400kv and then the power is passed over transmissions lines to a step down transformer in order to supply a towns housing.
  1. calculate the current flowing in the transmission line
  2. if the total resistance of the transmission line is 100 ohms calculate the volt drop across the length of the line
  3. calculate the power that is dissipated along the transmission line
  4. if the step up transformer hass 500 turns on its primary coil, how many turns will there be on the secondary coil?
  5. what will the current supplied by by the power station to the step up transformer?
  6. explain why power is passed over transmission lines at high voltage and low current rather than low voltage and high current
thanks alot for any help
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Old Aug 11th 2008, 06:58 PM   #2
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I'm going to put this comment in a separate post to highlight the need for correction.

The power station puts out "100 mw."

I presume this is measured in Watts. Note the capital letter here. There is also a capital on Volts and Ohms. The capital letters are there because the units are in honor of the men whose name they take.

Another problem is the "m." Do you mean to say that the power station is putting out 100 mW = 0.1 W? I suspect it is supposed to be $\displaystyle 100~MW = 1 \times 10^8~W$.

Please watch the capital letters in the future!

-Dan
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Old Aug 11th 2008, 07:15 PM   #3
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Originally Posted by pete View Post
a power station which generates 100mw of power at 25kv. the voltage is stepped up to 400kv and then the power is passed over transmissions lines to a step down transformer in order to supply a towns housing.
  1. calculate the current flowing in the transmission line
  2. if the total resistance of the transmission line is 100 ohms calculate the volt drop across the length of the line
  3. calculate the power that is dissipated along the transmission line
  4. if the step up transformer hass 500 turns on its primary coil, how many turns will there be on the secondary coil?
  5. what will the current supplied by by the power station to the step up transformer?
  6. explain why power is passed over transmission lines at high voltage and low current rather than low voltage and high current
thanks alot for any help
Step by step. Power is not lost in step up or step down processes. Thus
the power in the line has not changed by the step up to 400 kV. So use P = IV to determine the current, where V = 400 kV. Now you know the current so if you know the resistance you can get the potential drop using Ohm's law. You have the potential drop and the current in the line so you can calculate the power lost via P = IV.

For the number of turns:
$\displaystyle \frac{V_s}{V_p} = \frac{N_s}{N_p}$

There's a typo in question 5. I assume they are asking for the current the power station supplies to the step up transformer. This can be found by P = IV.

As far as the last question, consider the three forms of the power equation:
$\displaystyle P = IV = \frac{V}{R^2} = I^2R$
The current in the line is constant, so the only things to affect the power loss is the potential drop and the resistance. So under what conditions do we have the smallest power drop? High V and low I or low V and high I?

-Dan
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