Extra Long Power Transmission

[elkangorito]

Voltage drops are not constant and are determined by the load.

You are all assuming a constant load on the opposing end of the line to the transformer.

No load (assuming infinate resistance on the insulators) = no voltage drop.

Voltage drop is a factor of resistance, voltage & load based on these formula.

P=VxI - Power (Watts) = Volts x Amps

R=V/A - Resistance (Ohms) = Volts / Amps

Lets assume a 1.5km stretch of 50mm2 cable has a resistance of 1 ohm.

Lets assume the load per phase is 5kw.

Lets calculate a voltage drop for one 220V phase.

Using P=VxI we can calculate a required amperage of I=5000/220 - 22.7 Amps. (Well within current carrying capacity of 50mm2 aluminium cable.

We can now determine the resistance of the load using R=V/A - R=220/22.7 - 9.69 Ohms.

Voltage drop is now a calculation of the resistance of the supply line & the resistance of the load.

Voltage = 220-((R1/R2)x220) where R1 = the resistance of the supply line & R2 = the resitance of the load. = 197V or a voltage drop of 23V at 5kw.

Lets rework these formula for a load of 500W.

I=500/220 - 2.3 amps R=220/2.3 97 Ohms 220-((1/97)x220) = 217.7 Volts or a drop of 2.26V at 500W.

Double the size of the supply wire to 100mm2 & the voltage drop will half. These calculations will work with any wire provided the saturation point of conductivity is not exceeded.

(These caculations are assuming a three phase suppy with load balanced over three pahses, where neutral resistances are negligable.)

Cheers,

Soundman.

Although Soundmans calculations are generally correct, they do not take into account the following;

1] prospective fault current.

2] method of cable installation.

3] type of cable insulation (if any).

These 3 points are critical in determining the appropriate cable size for a given length. It is simply not good enough to do a voltage drop calculation as this is invariably, very incorrect.

To all those who wish to do this, "make hay while the sun shines" because it may be in the near future that you will have some strange occurences or sudden (& expensive) failure.