Optimising HT/LT line length ratio for reducing distribution loss

An analysis by industry experts shows how optimising HT/LT line length ratio will result in reducing distribution loss and how technical loss reduction technique can be implemented by improving ratio of HT to LT lines.

The distribution losses in our country’s power systems are quite high. Large scale expansion of the urban system and rural electrification in the country has resulted in the length of LT lines (circuits) considerably increasing, resulting in high losses in LT lines, excessive voltage drops, and frequent faults on LT network causing higher rate of failure of distribution transformers.

Accepted ration of LT to HT lines
Very large lengths of LT lines have come up as compared to HT lines resulting in high LT/ HT ratios. The ratio of LT to HT lines in our country has crossed 3:1. This results in high losses and low voltages at the consumer end. Increasing HT lines can help in reducing both line losses and voltage drops. Efforts should be made to achieve a ratio of 1:1.2, which would be very beneficial for improving efficiency of power distribution in our country.”
The losses for a given quantum of power supplied by a line are inversely proportional to the square of its operating voltage. SK Khanna, Managing Director, Special Cables says, “For high HT/ LT ratio, the losses will be low. Higher the operating voltage, lower will be the line losses. Therefore, by increasing the HT lines, the losses will be reduced.”

Current distribution system
The presented distribution system involves three-phase 11KV feeders from 33/11 KV substation up to distribution transformers of rating 11KV/440V. From these distribution transformers, 3-phase 4 wire lines come out. Distribution system with low voltage consists of four core cables and long low voltage lines and various loads fed from a bulk power transformer resulting in increase in system losses affecting voltage profile.

Low voltage distribution is done either by 3-phase 4-wire, 3- phase 5-wire, single phase 3-wire and single phase 2-wire low tension lines. This distribution system involves nearly 2:1 ratio of low and high voltage line lengths. This low ratio of HT to LT causes losses. So, it is highly important to optimise this ration in order to reduce such distribution losses. Naman Singhal, Director, Prime Cables says, “We can improve the HT/LT ratio by converting LT distribution network into HT distribution network. Converting LT lines to higher voltage consist high initial cost but after some specified time period this system is beneficial.”

In agricultural sector, if we go for HVDS system, it connects cluster of 2 to 3 AGR customers employed as small distribution transformer of capacity 6.6 KVA, 15KVA and 25 KVA through almost negligible LT distribution lines. In HVDS, there is less distribution losses due to minimum length of distribution line, high quality of power supply with no voltage drop, and less burn out of motor. Installation of additional distribution transformers of low capacity at each consumer’s premises instead of cluster formation will decrease the losses.

We can reduce the losses by installation of single phase transformers to feed residential and commercial customers in rural areas and providing of small distribution transformers with a distribution box attached with its body, having provision for installation of meters.

A ratio of LT to HT lines of 1 to 1.2 would be very beneficial for improving efficiency of power distribution in our country
SK Khanna, Managing Director, Special Cables

Installation of additional distribution transformers of low capacity at each consumer’s premises instead of cluster formation will decrease the losses.
Naman Singhal, Director, Prime Cables