What is a sub-station?

A sub-station is a place where the voltage/current are stepped up/down so that they can successfully pass through the next step of the journey. This stepping up/down is done with the help of a large transformer. After Power generation(which is present at far off places due to energy access and production considerations and the other various policies), the next step of the journey, the transmission journey, entails the energy produced to travel vast lengths via the help of overhead/ underground cables so that it reaches the respect smaller division for electricity distribution. This travelling of electricity can be only carried out in case that current is relatively lower (or else the wires might get vaporized/burned) and thus, inherently higher voltage(in the ranges of MWs to GWs); thus forming the ideal way to transmit electricity over vast expanses. This is done via the help of a step-up transformer present in a substation located between the power generation and transmission lines. Once the electricity reaches the proximity of a smaller area/household, the voltage is again stepped-down via the help of other distribution sub-stations so that it is viable for the electricity to be fed into the mains. There are various sub-stations which you can find out about in this article.

Electricity transmission lines

Electric power transmission systems are the means of transmitting power from a generating source to various load centers (i.e. where the power is being used). Generating stations generate electrical power.

Since distance is not the only factor that determines the ideal location for a generating station, the place where the power is generated may be quite far away from where it is used. Land further from the load center (which is generally a high-density central location) can be much cheaper per square meter, and governments may not want such loud and/or polluting stations close to residential areas. Power transmission systems – including short transmission lines, medium transmission lines, and long transmission lines – transport the power from the generation source and into a power distribution system. These distribution systems provide electricity to individual consumer premises.

AC vs DC Transmission

Fundamentally there are two systems (as seen in the video above) by which electrical energy can be transmitted:

1. High voltage DC electrical transmission system.

2. High AC electrical transmission system.

There are some advantages to using DC transmission systems:

• Only two conductors are required for DC transmission system. It is further possible to use only one conductor of DC transmission system if the earth is utilized as the return path of the system.

• The potential stress on the insulator of the DC transmission system is about 70% of the equivalent voltage AC transmission system. Hence, DC transmission systems have reduced insulation costs.

• Inductance, capacitance, phase displacement and surge problems can be eliminated in DC system.

Even having these advantages in a DC system, generally, electrical energy is transmitted by a three-phase AC transmission system. The advantages of an AC transmission system include:

• The alternating voltages can easily be stepped up and down, which is not possible in DC transmission system.

• Maintenance of AC substation is quite easy and economical compared to DC.

• The transforming of power in AC electrical substation is much easier than motor-generator sets in a DC system.

But AC transmission system also has some disadvantages, including:

• The volume of conductor required in AC systems is much higher when compared to DC systems.

• The reactance of the line affects the voltage regulation of the electrical power transmission system.

• Problems of skin effects and proximity effects only found in AC systems.

• AC transmission systems are more likely to be affected by corona discharge than a DC transmission system.

• Construction of AC electrical power transmission network is more completed than DC systems.

• Proper synchronizing is required before interconnecting two or more transmission lines together, synchronizing can totally be omitted in DC transmission system.

Primary and secondary transmission

The transmission lines are broadly categorized into two major subcategories: Primary and Secondary transmission.

Primary electric power transmission:

High voltages of the order of 66 kV 132 kV 220 kV and 400 kV are used for transmitting power by 3 phase 3 wire overhead system. This is supplied to substations usually at the out skirts of major distribution center or city.

Secondary electric power transmission:

The primary voltage is reduced to low values of the order of 3.3 kV, 11 kV or 33 kV for secondary transmission.

To look up more into the voltage ranges you can follow this article.

To sum up the above article, the voltage ranges can be broadly categorised as follows based on the above article: