The High voltage transmission is used to carry electricity over long distances with minimized power loss. When electricity is transmitted at high voltages (e.g 110 kV, 220 kV, or even 765 kV). less energy is lost as heat due to resistance in the wires. In the high voltage transmission, Power stations usually generate an alternating current a.c at a voltage between 11KV and 25KV. The power generated is then stepped up to 132KV-400KV so that it can be transmitted to long distances from the power station. The electrical power is usually transmitted over long distance to substations where the voltage is stepped down to 11KV. From the substations, power is distributed to consumers . This is after being stepped down to the consumable levels according to the needs of each consumer. Consumers can be heavy industry that may need over 30KV. light industry that may need over 10KV or domestic homes that may need only 240V.
Dangers of High Voltage Transmission
- risk of electric shock incase poles collapse or cables hangs too low
- The cables can cause fire on nearby structure and vegetation when cables are too loose
- Because of the high voltage, there is strong electric fields that can interfere with health of people exposed to such fields.
- During thunder and lightening, the cables can conduct excess charges from the lighting causing danger to houses and machines using electricity.
- during strong wind, the cables can come into contact with each other causing fire.
Power losses During High Voltage Transmission
Given that; I=current flowing, V=voltage supplied and R =electrical resistance in the High voltage transmission cables :
Power dissipated in a circuit is given by ; P=VI
From ohm’s law V = IR. Hence, P = (IR)I = I2R
This means that for a given resistance in a circuit, when the current is high , the power loss is large and when the current is low, the power loss is small.
Power loss in High voltage transmission is therefore low when it is transmitted at high voltage and low current.
To attain high voltage and low current, output voltage from power station is stepped up for long distance transmission. This is done to minimize power losses in transmission cables. We do this is ensuring the current is as low as possible with the same power output.
Since long distances are involved, the transmission cables are thick and made from very good conductors of electric current to ensure resistance is kept to minimum.
aluminum cables are preferred in high voltage transmission because :
- It is a good conductor of electric current
- It can be obtained cheaply
- It is light
Example problem
The resistance a of power transmitting cable is 10Ω and is used to transmit 11Kv at 1A current. If this voltage is stepped-up to 16kv by a transformer, determine the power loss.
solution
assuming the transmission is 100% efficient:
power input = power output
VpIp = VsIs
11000 x 1 = 160 000 x Is
= 0.069A
power lost = I2R = (0.069)2 x 10 = 0.048W
If there was no stepping up of the voltage the power loss would be:
power loss = (1.0)2 x 10 = 10W which is actually 200 times wastage compared to power calculated above.
Practice Question on the high voltage transmission
A generator produces 750kW at a voltage of 15kV. For high voltage transmission, This voltage is stepped up to 125kV .It is to be transmitted through cables of resistance 0f 500 Ω to a step-down transformer in a substation. Assuming that both transformers are 100% efficient:
(a) calculate:
(I)The current produced by the generator
(ii) the current that flows through the transmission cables
(iii) the voltage drop across the transmission cables
(iii) power lost during transmission
(iv) power that reaches substation
solution
(I)
power input = Voltage in primary coil x Current in the primary coil
that is: power input = Vp x Ip
Current in primary Ip will therefore be given us:
Ip=PVp
and substituting for the values of power and primary voltage we have:
Ip=750∗1000w15000V=50A
so the current produced originally in the primary coil is 50.0A
(ii)
At the step-up transformer;
power input = power output (no power is lost because the transformer is 100% efficient)
that is: power in primary generator = power after step up
Vp x Ip = Vs x Is
15000V x 50A = 125000xIs
Is=15000V∗50A125∗1000=6A
Power losses during high Voltage transmission
Power losses is common during high voltage transmission . High voltage power transmission involves moving electrical power from the source where it is being generated to consumer premises (sometimes hundreds of kilometers way) where it need to be used.
- High voltage transmission
- Power losses during transmission
- Example Problems
- solution
- Question for practice
- Related Topics
High voltage transmission
In high voltage power transmission, power loss during transmission is minimized by transporting electrical power in high voltage but at a small current. High voltage transmission involves transporting electrical power over long distances through electrical cables. From electricity topics, we learn that conductors have some resistance to the flow of currents. The resistance in a conductor is proportional to the length of the conductor. Power losses during transmission is caused by the heating effect that results from resistance of current in the conductor. Because the distance involved can be hundreds of kilometers, power losses can be significant.
Power losses during transmission
we determines power dissipated in a circuit from the relation:
Power P = VI.
Where V is voltage across the circuit and I is the current flowing in the circuit.
From Ohms law of electric current; V= IR. Where R is the resistance in the circuit.
Therefore, Power = (IR)I = I2R.
From the equations above, you can see that power loss can be high when current is high for a given resistance value in the conductor.
Because power is constant, the idea of transmitting power efficiently is by increasing its voltage. Increasing voltage ensures reduced current for the same power source. The best way to minimize power loss during transmission is to transmit at very high voltage. It should also be done at the smallest current possible. We usually step up the voltage produced at the source to high voltage values. This adjustment brings the current closer to zero as much as possible.
power providers can also reduce electrical resistance in transmission cables by choosing cable with thick diameter but one that is a good conductor of electricity.
Power providers prefers aluminum as thee best choice for transmission cables since it is a good conductor, cheap to obtain and it is not heavy. An alternating current power source is the one preferred in high voltage transmission because it is can easily stepped up and down when needed.
Example Problems
A power company uses a power cable of with overall resistance of 5000Ω to transmit 10Kv at a current of 0.8A. They use a transformer to step up this voltage to 25KV by a transformer, determine:
(i) power loss expected if there was no step-up
(ii) power loss expectations after step-up
(iii) percentage ratio between power power lost before step-up to power loss after step-up
solution
(i) Power = I2R.
hence power loss when voltage is not stepped up=(0.8)R x 5000=3200 watts.
(ii) Assuming that there is no power loss in the transformer; After stepping up the following equation applies:
power before step-up = power after step-up.
power before step-up is the power fed into primary coil of the transformer;
Using the transformer equation; Power in primary coil = power in the secondary coil.
That is; VpIp = VsIs
substituting in the equation to get the current after stepping up we have:
10000v x 0.8A = 25000v x Is
and hence;
Is=10000V∗0.8A25000= 0.32A
stepping up has reduced current flow from 0.8A to 0.32A
power lost due to this new current will be:
Power lost = (0.32)2 * 5000 = 512 watts
(iii) The ratio of power lost before stepping up to power lost after stepping up will be
powerratio=3200watts512watts∗100= 625%
from the above working, one can see that power loss has been reduced by 625% when power is stepped up from 10KV to 25KV. This proves to us that stepping up power to reduce current contributes to reduction of power loss in a big way.
Question for practice
A generator produces 700kW at a voltage of 15kV. The voltage is stepped up to 125KV and the power transmitted through the cables of resistance 300Ω to a step-down transformer in a sub-station. Assuming that both transformers are 100% efficient:
(a) calculate:
(i) The current produced by the generator
(ii) The current that flows through the transmission cables
(iii) The voltage drop across