What are the High Tension and Low Tension line and how are they different from each other?

A transmission line is used to transfer electric power over a long distance, transmission lines can connect generating stations and substations. The transmission lines are classified according to their operating voltage. The word ‘tension’ stands for voltage.

Low tension

A low tension line can be considered as the line that has an operating voltage till 1000 volt which is 1Kv. This is the 230V and 440V in our household so it is the local distribution line. Through the low tension or low voltage line we are receiving power to our home. Low tension lines are more susceptible to faults because it is connected to the loads.

HT line

The high-tension line has an operating voltage of 33KV, so the operating voltage of this line will range from 11Kv to 33KV and if it is above 33KV then it is called an extra high tension line. We can’t see many faults in high tension lines because it is not directly connected to the load. The higher voltage transmission would need larger structures with longer insulator chains

How high voltage AC is generated?

The high voltage transmission is done by using several transformers, cascade arrangement of the transformer is done. The low voltage winding will be connected to the transformer’s primary and this will be connected to the transformer tank, the above figure shows how cascade-connected transformers can generate high voltage by using the low voltage.

What are the factors that affect the insulation of the overhead transmission line?

The operating voltage could influence the leakage distance, other factors are switching surge and lightning overvoltage and they would influence the required insulator chain length and striking distance.

Why do we need high voltage transmission lines?

We need high voltage transmission lines to transport energy over a long distance, power transmission is done in high voltage because while transmitting the power in high voltages less current is used. So because of the less amount of current we only need small conductors.

What is the difference between HT and LT lines?

The major difference between these lines are their operating voltage, we can see lots of faults in the low voltage transmission side while comparing to the high voltage transmission and it is because the LT sides are connected to the load. So on the LT side, we can see a lot of variations and lots of trouble, while in the high tension line we won’t able to see this much fault and it is because that the HT lines are not directly connected to the load.

The insulators that are used in low tension lines are small pin type insulators and shackle type insulators, while for underground transmission this will be different. The insulators that are used in high tension lines will be pin type insulators and disc type insulators, the cables that are used in HT lines up to 11KV will be rabbit ACSR cables and these cables are capable to hold 157-ampere current. In the case of HT lines, it is more than 11KV which could be up to 33KV then dog-type cables are used. The cables that are used on the low tension side will be a squirrel type. In low tension power transmission, we would use four wires which would be R, Y, B, and neutral, and the low tension wires will always be star connected. In the case of high tension line, there will be only three wires and we can’t see neutral in this section and these lines will be delta connected.

The transmission line from the substation to the pole-mounted substation will be a high tension line and from the pole mounted substation our house or office is a low tension line and mostly there won’t be any transformers between low tension lines but we can see transformers in high tension lines. So basically we can say that a high tension line and low tension line is for power transmission but to do the operation of our equipment we need low voltage.


post-graduate in Electronics & communication.

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