Power systems

An electric power system consists of 3 principle sections.

• Power stations: electricity is generated
• Transmission: voltage is stepped up to high voltage
• Distribution: voltage is stepped down to medium voltage for distribution over a relatively small region

Variable load

Load on a power station changes with to uncertain demands of consumers. This is called the variable load.

Load vs time curve is called the load curve. Area under this curve is the total energy requirement.

Power grid

Nation-wide, massive, geographically distributed system for electrical power supply network.

Sri Lankan Scenario

There are 2 major electric utilities in Sri Lanka: CEB and LECO. Public Utilties Commission of Sri Lanka (PUCSL) is the economic, technical and safety regulator of the electricity industry in Sri Lanka.

Voltage Levels

• Generation voltage $13.8\;\text{kV}$
• Transmission: $132\;\text{kV}$ or $220\;\text{kV}$
• Distribution: $11\;\text{kV}$ or $33\;\text{kV}$
• Nominal voltage $230\;\text{V}$
• Nominal line-to-line $400\;\text{V}$

Why high voltage transmission?

Reduction of power losses

$$P_{\text{loss}} = I^2R$$

Power losses in transmission lines are called "$I^2R$" losses. To reduce the power loss, current have to be reduced. $$

Power generation in the generator is constant. So voltage is increased to reduce the current.

Reduce voltage drop

Voltage drop in the transmission lines is proportional to the current flowing through it.

Reduction of power transmission cost

To carry higher currents, the transmission lines must have higher cross sectional area. Reduced current means smaller transmission lines are enough. Which leads to reduced cost.