Descripion of The Power Distribution System

You are familiar with the power supply system. You know that electricity is generated at 11 kV by electrical generators which utilise the energy from thermal, hydro, nuclear, and renewable energy resources. To transmit electricity over long distances, the supply voltage is stepped up to 132/220/400/800 kV, as required. Electricity is carried through a transmission network of high voltage lines. Usually, these lines run into hundreds of kilometres and deliver the power into a common power pool called the grid.The grid is connected to load centres (cities) through a sub-transmission network of usually 33 kV (or sometimes 66 kV) lines. These lines terminate into a 33 kV (or 66 kV) substation, where the voltage is stepped-down to 11 kV for power distribution to load points through a distribution network of lines at 11 kV and lower.

The power network of concern to the end-user is the distribution network of 11 kV lines or feeders downstream of the 33 kV substations. Each 11 kV feeder which emanates from the 33 kV substation branches further into several subsidiary 11 kV feeders to carry power close to the load points (localities, industrial areas, villages, etc.). At these load points, a transformer further reduces the voltage from 11 kV to 415 V to provide the last-mile connection through 415 V feeders (also called Low Tension (LT) feeders) to individual customers, either at 240 V (as single-phase supply) or at 415 V (as three-phase supply). The utility voltage of 415 V, 3-phase is used for running the motors for industry and agricultural pump sets and 240 V, single phase is used for lighting in houses, schools, hospitals and for running industries,commercial establishments, etc.

A feeder could be either an overhead line or an underground cable. In urban areas, owing to the density of customers, the length of an 11 kV feeder is generally up to 3 km. On the other hand, in rural areas, the feeder length ismuch larger (up to 20 km). A 415 V feeder should normally be restricted to about 0.5 −1.0 km. Unduly long feeders lead to low voltage at the consumer end. The power supply system, including the distribution network, is depicted in Fig.

Typical Electric Power Supply System with Distribution Network

The main components of the power distribution system and their brief descriptions are given in Table .

Components of the Power Distribution System

Voltage Levels

You have just learnt that the voltage range varies widely in various parts of the power supply system. We give these voltages in Table .

Typical High Voltage Distribution System

Higher voltages are used for 3-phase, 3-wire supply to large consumers. Low voltage distribution of generally 415 V, 3-phase 4-wire system and 240 V single phase, two wire, phase to neutral system is used for small and medium consumers. The size and, hence, voltage of supply to a consumer is decided by the load of the consumer.

Conductors

The 11 kV feeders carry comparatively bulk power from secondary substation (33/11 kV) to distribution substation transformers (DTRs). Distributors (or secondary network) carry power from DTRs through service lines (or LT feeders) which deliver power from the supplier’s nearest support to consumer’s premises up to the energy meter, through a weather-proof service wire.

All lines have inherent resistances, inductances and capacitances, resulting in a voltage drop in the line. Thus, to minimise voltage drop in a line, the values of these parameters should be carefully selected. For LT supply, the declared voltages at the consumer premises are 415/240 V. All appliances and motors give good performance for long duration if this voltage is maintained.

The following factors should be considered for the proper selection of conductor size:

• current carrying capacity; and

• tensile strength of the conductor.

The size of conductor for a distributor is determined in the following manner:

• The current that the distributor has to carry is calculated on the basis of the load incident on the conductor (including anticipated load growth).

• The conductor size capable of carrying this current at the ambient temperature of the area is selected from standard tables.•

The voltage drop is calculated taking products of loads and their distances.

The following types of conductors are available:

• All Aluminium (Standard) Conductor (AAC);

• Aluminium Conductor Steel Reinforced (ACSR Conductors);

• All Aluminium Alloy Conductors (AAAC).

ACSR and AAAC conductors are used for secondary distribution systems.ACSR conductors are preferred to AAC conductors for long spans owing to their greater tensile strength. The current carrying capacity of ACSR conductors is as follows:

Squirrel (7/2.11) 115 A

Weasel (7/2.59) 150 A

Rabbit (7/3.35) 208 A

The numbers in bracket indicate the number of strands/diameter in mm.

High Voltage Distribution System (HVDS)

You have learnt in previous unit that significantly high losses take place in the secondary distribution system. This is due to higher current densities and ease of pilferage at low voltages. One of the latest innovations in efforts to reduce technical and commercial losses is the use of High Voltage Distribution System (HVDS) or LT-less system.

Voltages at Different Segments in the Power Distribution System

Typical High Voltage Distribution System

In this system, the secondary distribution system with long LT feeders running up to consumer premises from the distribution substation is totally absent.The primary distribution system at HT level (11 or 33 kV) is used to reach the nearest point for a group of small number of consumers. The consumers are then connected to the HT Distribution System at these points through small pole mounted transformers used for supplying power to them through LT service lines.

We now describe the advantages of HT distribution compared to conventional LT distribution system.Low Losses and Improved Voltage Profile The comparison of current, losses and voltage drop for the distribution of the same power through HT and LT systems is presented in Table.

Comparison of Current, Voltage Drop and Power Losses for Power
Distribution through HT and LT Distribution Systems

We have considered 100 as the base value for LT system. From the table,you can see that for the distribution of the same power, technical losses and voltage drop are much less in HT distribution system when compared to LT distribution systems.


LT distribution systems are easily accessible and prone to pilferage and the use of HVDS reduces the chances of theft of electricity to a very low level. Now-a-days, utilities are installing meters at the HT transformer itself to ascertain commercial losses on that particular transformer. In sum, the HT distribution system has the following advantages:

• use of small size ACSR or aluminium alloy conductor or high conductivity steel wire;

• better voltage profile;

• reduced line losses; and

• reduced commercial losses.

Improved Reliability and Security of Supply The use of HT distribution leads to improved reliability and security of supply for the following reasons:

• The faults on HT lines are far less compared to those of LT lines.

• In order to avoid theft in LT lines from transformer to consumer premises, usually Aerial Bunched Cables (AB Cables) are used to
supply power at LT to consumer from the distribution transformer.

•With AB Cables, the faults on LT lines are eliminated. This, in turn,reduces the failure of distribution transformers and enhances reliability of supply.

Since the number of small distribution transformers is high in HVDS,the failure of one transformer does not affect supply to other
consumers connected to other transformers. In the event of failure of distribution transformers, only a small number of consumers (2 to 3 power consumers or 10 to 15 domestic consumers) would be affected. On the other hand, a large distribution transformer supplies power through LV distribution lines to even remotely located consumers in LVDS. Hence, the failure of an existing large size distribution transformer would affect a group of 40 to 50 power consumers and/or 100 to 200 domestic consumers.You may like to consolidate these ideas before studying further.