Understanding Primary Power Distribution Systems

The electrical power generated at power stations needs to be transmitted and then distributed to consumers. This process involves several stages, including primary transmission, secondary transmission, primary distribution, secondary distribution, and finally, service mains to connect to homes and businesses.

Primary distribution is the part of the power system that takes power from the transmission network and distributes it to the various substations or distribution transformers within a localized area. These substations then further step down the voltage for secondary distribution to the end consumers.

System Used for Primary Distribution

Let's look at the typical systems used in electrical distribution:

• Single Phase Two Wire System: This is commonly used for secondary distribution, connecting power to individual homes and small loads. It provides a single phase voltage.
• Single Phase Three Wire System: This system provides two different voltage levels (e.g., 120V and 240V) from a single-phase transformer. It's used for secondary distribution in some regions, especially for residential areas.
• Two Phase Three Wire System: This system is less common nowadays. It uses two phases with a common return wire. It is generally not the standard for large-scale primary distribution.
• Three Phase Three Wire System: This system consists of three conductors, each carrying one phase of the alternating current. It is widely used for transmitting and distributing large amounts of electrical power over relatively long distances within the distribution network before the final voltage step-down. Three-phase systems are highly efficient for transmitting power to industrial loads and for feeding distribution substations.

Primary distribution networks typically operate at higher voltages compared to secondary distribution (e.g., 11 kV, 33 kV, 66 kV) and are designed to efficiently supply bulk power to distribution substations. A three-phase system is ideal for this purpose due to its ability to transfer power efficiently and support three-phase loads (like large motors) as well as single-phase loads (via transformers).

While three-phase four-wire systems (which include a neutral wire) are also used in distribution, particularly for mixed loads requiring both phase-to-phase and phase-to-neutral voltages, the core of the primary distribution feeder system itself is often represented by the three phase three wire configuration before reaching transformers that might provide a neutral for secondary distribution.

Considering the function and efficiency requirements of carrying bulk power from transmission substations to distribution substations, the three phase three wire system is the standard arrangement for primary distribution feeders in many power grids.

Therefore, primary distribution is typically carried out by the three phase three wire system.