Course EE-1:General overview of Generation, transmission & distribution stages of electricity

fig (1) : generation, transmission and distribution stages

There are three stages of electric power supply (see fig.1) ; generation, transmission and distribution. Each stage must be understood generally to any electrical engineer and here we will give a general overview of these stages as follows:

Generation stage:
Electricity generation is the process of generating electric energy from other forms of energy.

fig (2) : Steam Turbine

Electrical power starts at the power plant, In almost all cases the power plant consists of a spinning electrical generator. A generator is a machine that transforms mechanical energy into electric power.

Something has to spin that generator; it might be a water wheel in a hydroelectric dam, a large diesel engine or a gas turbine. But in most cases the thing spinning the generator is a steam turbine (see fig.2)

The steam might be created by burning coal, oil, natural gas or the fission of nuclear fuel. And some generating stations use renewable energy sources like sun & wind (see fig.3)

fig (3) : some generation stage stations

Sometime, another stages of power generation is provided in transmission and distribution stages (Embedded generation) to meet additional power requirements in some load areas (see fig.4)

 fig (1) : Embedded Generation in transmission and distribution stages

Transmission stage:
Electric-power transmission is the bulk transfer of electrical energy; from generating power plants to substations located near population centers (see fig.5)

fig (1) : transmission stage 

The 3-phase power leaves the generator and enters a transmission substation at the power plant (see fig.6). This substation uses large transformers to convert the generator's voltage (which is at the thousands of volts level) up to extremely high voltages for long-distance transmission on the transmission grid. Typical voltages for long distance transmission are in the 155,000 to 765,000 volt range in order to reduce line losses.

fig (1) : transmission Substation 

Transmission stage may include sub-transmission stages (secondary transmission) to supply intermediate voltage Levels. Sub-transmission stages are used to enable a more practical or economical transition between transmission and distribution systems.

The Distribution stage:

Electricity distribution is the final stage in the delivery of electricity to end users. A distribution system's network carries electricity from the transmission system and delivers it to consumers (see fig.7)

fig (1) : distribution stage

For power to be useful in a home or business it comes off the transmission grid and is stepped down to the distribution grid in a power distribution substation (see fig.8), and this may happen in several phases as follows:

fig (1) : distribution substation

Primary distribution system (HV distribution):
It is that portion of the network between the Sub-transmission substations and secondary distribution system. The primary system consists of step-down transformers and sometimes embedded generation can be used at voltage levels which range from 33 KV to 6.6 KV.


The secondary distribution system (LV distribution):
It is that portion of the network between the primary feeders and utilization equipment. The secondary system consists of step-down transformers and secondary circuits at utilization voltage levels which range from 480V to 120V.

Note :Residential secondary systems are predominantly single-phase, but commercial and industrial systems generally use three-phase power.



Example for a complete power system Grid: see fig.9

fig.9: Electrical Power Grid Example

• The generator produces 20,000 volts. 
• This, however, is raised to 138,000 volts for the long transmission journey. 
• This power is conducted over 138,000-volt (138 kV) transmission lines to switching stations located in the important load area served. 
• When the power reaches the switching stations, it is stepped down to 34,500 volts (34.5 kV) for transmission in smaller quantities to the substations in the local load areas and industrial consumers can utilize electrical power at this stage. 
• Then it is stepped down to 13,800 volts [13.8 kV] for direct distribution to local areas and industrial, commercial and residential consumers can utilize electrical power at this stage by using appropriate step down transformer to their grid’s voltage level. 

In the next topic, iI will explain some important and Basic electrical definitions & terminology.