Sunday, June 29, 2014

Why Power Transmission is done at Very High Voltages?

After generation of electricity it is important to transform that voltage at user end/ load end.

Reason 1 for Transmission at Higher voltage
During transmission there are losses known as Transmission losses as heat get generated during transmission, Usually this loss is known as heat loss.

Heat loss is = I2R

When Transmission is done at High voltage then Current drawn will get reduced as:-

Transmission power= Voltage X Current X Power factor

Now Current=   Transmission Power
                          Voltage X Power factor

From above you will see that higher the transmission voltage, lower will be the Current so lower will be I2R losses.

Now let’s take an example; For Power transmission at 11 KV and 33 KV for power of 1 MW, in both cases resistance will be same so only variable is Transmission voltage.

Current(11KV) = Transmission power (1 MW) ----------  (i)
                            11 KV X Power factor

Current (33 KV) = Transmission power (1 MW) ---------   (ii)
                                   33 KV X Power factor

Now Heat loss (11 KV)=   (Transmission power (1 MW))2  --------- (iii)
                                              ( 11 KV X Power factor)2

Heat loss (33 KV)=   (Transmission power (1 MW))2 ------------- (iv)
                                              ( 33 KV X Power factor)2

Now divide (iii) by (iv)

Heat loss (11KV)        =  (33 KV)2
Heat Loss (33 KV)           (11 KV) 2
Now Heat loss (11 KV) = 9 times the heat loss at 33 KV ---- (v)

You will see how big difference it will be.

Reason 2 for Transmission at Higher voltage
There is one another reason for transmission at Higher voltage is weight.  When Transmitting voltage at low levels then in order to reduce the transmission losses there is often practice to use conductor of big diameter as Resistance is inversely proportional to conductor diameter, so larger the diameter lower will be the resistance of conductor. So lower will be the transmission losses. This will leads to higher weight of conductor. Now when Transmission is done at higher voltage conductor with smaller diameter will be sufficient as thinner wire will have to transmit lower currents.

As conductor is lower this will lead to smaller snags and leads to more savings. This will also leads to lower number of towers.

Let’s illustrate this with an example; Now from equation (v)
Heat loss at 11 KV=  9 times the heat loss at 33 kV
Now for same current resistance is to be reduced.

Now resistance = Resistivity X Length
 Now Area is directly proportional to square of conductor radius/diameter

So for same heat loss at 11 KV conductor area must be increased to 3 times the conductor for 33 KV.
You can see that how there are advantages of transmitting the power at Higher voltage.