Download P6506 (a) When a current passes through a wire, a heating effect

P6506
(a)
When a current passes through a wire, a heating effect occurs known as ohmic heating.
The higher the current the greater the loss in energy.
Ploss  i 2 R
Energy is lost by the emission of radio waves due to the oscillating electric fields in the
transmission lines.
Transforms are used to either step-up or step-down voltages. Energy losses occur in
the cores of transformers due to induced eddy currents.
(b)
Electrical energy transmitted at high voltages – low currents.
Laminated transformer cores to reduce eddy currents.
(c)
Pplant = 120 kW = 120103 W = 1.20105 W
L = 10 km = 15103 m R/L = 4.010-5 .m-1
P=VI
R = 0.40 
I=V/R
Current draw from plant I = P / V
P = Pplant
V1 = 240 V
V2 = 24 kW = 24103 W
I1 = Pplant / V1 = (120103) / 240 A = 500 A
I2 = Pplant / V2 = (120103) / (24103) A = 5.0 A
Power losses in transmission lines Ploss = I 2R
Ploss1 = (5002)(0.4) W = 1.0105 W
% Ploss1 = 100  Ploss / Pplant = (100)( 1.0105)/( 1.2105) = 83 %
Ploss2 = (52)(0.4) W = 10 W
% Ploss1 = 100  Ploss / Pplant = (100)( 10)/( 1.2105) = 0.01 %
From these numbers, it clearly shows that it is much more efficient to transfer
electrical energy at high voltages.
(d)
DC electrical energy distribution was initially used.
The DC distribution was supported by Edison. However, an ac system was strongly
supported by Westinghouse to replace the existing DC distribution network.
Finally the ac distribution method was adopted since transformers can be used with ac
to step-up and step-down voltages, enabling transmission lines to transfer electrical
energy at high voltages and low currents to minimize ohmic heating losses.
(e)
Transmission lines consist of a number of conductors in parallel. However, in dry air
sparks can occur between two points when the voltage between them exceeds 10 kV
for every 10 mm of separation. Therefore, for a 330 kV transmission line, sparking
could occur if the spacing was less than 330 mm. To electrical insulate the transmission
lines from their surroundings ceramic insulting stacks are used (ceramics are very good
insulators). The ceramic insulating stacks also protect against lightning strikes.
(f)
Lightning strike protection is given by:
 Shielded conductors – two non-current carrying wires at the very top of the
tower
 Earth cable – runs from top of tower down into metal conductors in the ground
 Insulation chains – saucer shaped stacks of ceramic material (very good
insulators) that attach wires to the tower.
 Metal tower – tower connected to the Earth.
 Distance – distance between towers ~ 150 m – enough to protect each tower
from neighbouring towers.
(g)
Ceramic insulating stacks
(h)
Some claim that living near high voltage power lines is causing a variety of health
problems – cancers – leukemia – however, no conclusive scientific results to link health
risks and close proximity to high voltage power lines. Is there a danger?