Module 1 : Introduction
Lecture 1 : Modern Power Systems
   Structure
   Present day power systems are characterised by:
AC generation (3 phase synchronous machines using hydro, fossil and nuclear as primary energy sources)
AC transmission (a limited number of DC transmission links are present in some systems)
3 phase AC utilization (some loads are single phase, but they are usually distributed equally in all phases to obtain a balanced system)

The transmission network consists of an extra high voltage (220 kV and above) lines interconnecting large generators to major load centers. These lines are capable of transmitting large amounts of power over large distances (1000 km is not unusual!).

For example, several 400 kV lines may be required to transmit, say, 2000 MW from a large thermal power plant to a distant load center.

Note : high voltage dc lines are suitable if distances are very large (we shall see why later in the course). Generators, typically having terminal voltages of around 15 kV and are connected to these EHV lines via transformers. Interconnecting transformers connect EHV lines at different voltage levels (they are usually auto-transformers - why?).

 

(click on figure to enlarge)

A sub-transmission network, consisting of transmission lines at 66 kV and 132 kV are used to deliver power received from EHV network to various distribution substations. Finally, a distribution network consisting of lines at less than 66 kV are used to transfer power to each consumer.

A large power system may also be connected to neighbouring power systems via ac transmission lines or dc links. The power exchange between such neighbours is usually regulated (you will learn how this is done later).
1