Transmission and Distribution of Electric Power

Geographic Distribution: Electric power is generated at power generating stations, which are typically situated in locations very far from the urban and industrial areas where the electricity is actually consumed.
Choice of Generation Voltage: At these generating stations, power is produced at a specific voltage of $11\,kV$ . The choice of this voltage is based on two primary engineering constraints:
- Upper Constraint: Generating electricity at a voltage higher than $11\,kV$ is not feasible because it leads to significant insulation difficulties within the generator itself.
- Lower Constraint: Generating electricity at a voltage lower than $11\,kV$ is avoided because it would involve a very high current for a given amount of power, leading to inefficiencies.
Alternating Current (AC) Generation: The power generated is always alternating voltage rather than direct current (DC). This is because DC voltage cannot be easily stepped up or stepped down for efficient transmission.
Temporal Frequency Metrics:
- The generating station produces an alternating voltage with a frequency of $50\,Hz$ .
- This frequency implies that the polarity of the terminals changes $100$ times every second.
- Specifically, it undergoes $50$ positive cycles ( $+)$ ) and $50$ negative cycles ( $-$ ) per second.

Transmission Strategy: Power is never directly transmitted to consumers at the initial $11\,kV$ generation level. Instead, the voltage is significantly raised before transmission to minimize energy losses occurring in the transmission line wires.
The Relationship between Voltage and Current:
- The relationship between Power ( $P$ ), Voltage ( $V$ ), and Current ( $I$ ) is given by the formula: $P = VI$
- For a fixed quantity of power ( $P$ ), the current ( $I$ ) can be expressed as: $I = \frac{P}{V}$
- This indicates an inverse relationship: the higher the voltage at which power is supplied, the lower the resulting current.
Minimization of Heat Energy Loss:
- Energy loss in the form of heat in the transmission lines is determined by the formula: $\text{Heat Loss} = I^2Rt$
- Where $I$ is the current, $R$ is the resistance of the wires, and $t$ is the time duration.
- By increasing the voltage to a very high level, the current $I$ becomes very low. Consequently, the loss of energy due to heating ( $I^2Rt$ ) in the line wires is drastically reduced.

Stage 1: Grid Sub-station (The Generating Station):
- The initial generated alternating voltage of $11\,kV$ is stepped up to $132\,kV$ using a step-up transformer.
- After being stepped up, the power is transmitted to the main sub-station.
Stage 2: Main Sub-station:
- At this station, the voltage is stepped down from $132\,kV$ to $33\,kV$ using a step-down transformer.
- Supply to Heavy Industry: A portion of the power at this $33\,kV$ level is transmitted directly to heavy industries.
Stage 3: Intermediate Sub-station:
- The remaining power undergoes another reduction, stepping down from $33\,kV$ to $11\,kV$ via a step-down transformer.
- Supply to Light Industry: Power at the $11\,kV$ level is transmitted to light industries.
Stage 4: City Sub-station:
- For residential and general city use, the voltage is further reduced from $11\,kV$ down to $220\,V$ using a step-down transformer.
- This $220\,V$ supply is then delivered to consumers for domestic and commercial purposes.

Cable Infrastructure: Power is supplied from the city sub-station to the consumer via cables that may be run underground or overhead on poles.
Three-Wire System Configuration: The distribution cable consists of three distinct types of wires:
- Live Wire (L): Also known as the Phase wire, it carries the alternating current to the consumer.
- Neutral Wire (N): This wire provides the necessary return path for the electrical current.
- Earth Wire (E): This wire is used for safety and grounding purposes.
Potential Levels and Safety:
- The Neutral wire ( $N$ ) and the Earth wire ( $E$ ) are connected together at the local sub-station.
- This connection ensures that both the Neutral and Earth wires are maintained at zero potential ( $0\,V$ ).
- The Live wire is the one that carries the high alternating potential relative to the earth.
Connection to Consumer Meters:
- Before the power enters the consumer's internal wiring, it passes through an electricity meter (often housed in a meter box).
- The system is designed to maintain consistent power delivery, though there is usually a preference/allocation of different phases to different sections of a city or building to balance the load.