AC Transmission and Distribution Systems

Electrical energy is generated, transmitted and distributed in the form of AC. Since, alternating voltage can be changed in magnitude by means of a transformer; it is possible to transmit AC power at high voltage which reduces the current in the conductors hence the line losses.  The conductors system is the means by which electric power is conveyed from a generating station to the consumer’s premises and may generally be divided into two different parts: transmission system and distribution system. Each part can be subdivided into two systems: primary transmission and secondary transmission and similarly, primary distribution and secondary distribution and lastly the individual supply system to the individual consumers.

More often than not, generation and transmission is solely three-phase. 3-phase power transmission requires less copper than either single-phase or 2-phase power transmission. Single-phase power transmission is used only for short distances and for relatively low voltages. The secondary transmission is carried out via 3-phase systems while the distribution to the final consumer may be 3-phase or single phase depending upon the customer’s needs.

A general layout of a generating, transmission and distribution network is illustrated in the figure below:

With reference to the figure above, CS stands for the central station where power is generated by 3-phase alternators at 6.6 or 13.2 or even 32 kV. The generated voltage is stepped up by appropriate 3-phase transformers for transmission purposes. In this case, the 11 kV is stepped up to 132 kV. Primary or high voltage transmission is done at 132 kV. The 3-phase, 3-wire overhead high-voltage transmission line terminates in step-down transformers in a substation referred to as receiving station (R.S). Here the voltage is stepped down to 33 kV. From the receiving station, power is transmitted at 33 kV by either overhead lines or underground cables to different substations (SS) located at various strategic points say in a city. This is known as secondary or low-voltage transmission. From this point onwards we the beginning of primary and secondary distribution.

At the substation (SS) voltage is reduced from 33 kV to 3.3 kV 3-wire for primary distribution. The consumers with demand that exceeds 50 kVA are normally supplied from the SS by special 3.3 kV feeders. The secondary distribution is done at 400/230 V where voltage is reduced from 3.3 kV to 400 V at the distribution substations. The common system for secondary distribution is 400/230 V, 3-phase 4-wire system.

Generally a generation, transmission and distribution systems have:

• Generating voltage: 6.6 kV, 13.2 kV or 33 kV.
• High voltage transmission 220 kV, 132 kV, 66 kV.
• High voltage or primary distribution: 3.3 kV, 6.6 kV.
• Low voltage or secondary distribution: 400/230 V, 3-phase, 4-wire.

Normally, the standard frequency for AC operation is 50 Hz or 60 Hz.

AC Distribution System

The distribution system starts either at the substation where power is delivered by overhead transmission lines and stepped down by transformers or in some instances at the generating station.

The following systems may be used for the distribution:

• Single-phase, 2-wire system
• Single-phase, 3-wire system
• Two-phase, 3-wire system
• Two-phase, 4-wire system
• Three-phase, 3-wire system
• Three-phase, 4-wire system

Due to cost reasons, the three-phase, 3-wire system and the three-phase, 4-wire system are used for primary distribution and secondary distribution respectively.

The distribution system may be divided into feeders, distributors, sub-distributors and service mains. Feeders are the conductors which connect the substation (in some cases the generating station) to the distributors serving a certain apportioned area. From distributors various tappings are taken. The connection link between the distributors and the consumer’s terminals are the service mains.

The AC distribution system is generally classified into:

• Primary distribution system
• Secondary distribution

Primary Distribution System

Primary distribution system operates at voltages higher than general utilization, in other words, it handles large blocks of electrical energy than the average low voltage consumer. The voltage used for primary distribution depends upon the amount of power conveyed and the distance of the substation to be fed. Typically the primary distribution voltages are 11 kV, 6.6 kV and 3.3 kV. Because of economic factors, primary distribution is carried out by 3-phase, 3-wire system.

Secondary Distribution System

The secondary distribution uses the 400/230 V, 3-phase, 4 wire system.

The primary distribution circuit delivers power to various substations referred to as distribution substations. These distribution substations are usually located near the customers localities.

At each distribution substation, the voltage is stepped down to 400V. The power is delivered by 3-phase, 4-wire AC system. The voltage between any two phases is 400 V and between any phase and neutral is 230 V.

The single phase domestic loads are connected between any one phase and the neutral. For the 3-phase 400 V motor loads, they are connected across 3-phase lines directly.

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