WHY IS ELECTRICAL POWER TRANSMITTED AT HIGH VOLTAGES?

 

1. Introduction

Electricity generated at power plants must travel long distances to reach industries, offices, commercial establishments, and homes. However, transmitting large amounts of electrical power over long distances presents a major engineering challenge: power losses in transmission lines due to their electrical resistance.

To reduce these losses, electrical power is transmitted at very high voltages using high-voltage transmission lines.

In India, transmission systems commonly operate at 132 kV, 220 kV, 400 kV and 765 kV. High-voltage direct current (HVDC) systems are also used for bulk power transfer over long distances.

The key principle is simple: for the same amount of power, increasing the voltage reduces the current, which significantly reduces resistive power losses.

2. Why Is Electricity Transmitted at High Voltages?

A. To reduce current

For a three-phase AC transmission system, the electrical power transferred is:

P = √3 × V_L × I_L × cosφ

Where:

·         P = active power in watts

·         V_L = line-to-line voltage in volts

·         I_L = line current in amperes

·         cosφ = power factor

For the same transmitted power and power factor, current is inversely proportional to voltage.

I_L = P / (√3 × V_L × cosφ)

Therefore, increasing transmission voltage reduces the current flowing through the conductors.

B. To reduce power losses

Every conductor has electrical resistance. When current flows through it, some electrical energy is converted into heat.

P_loss = I²R

For a balanced three-phase line, the total conductor loss is:

P_loss = 3I²R

Here, R is the resistance of one phase conductor over the line length. Since losses depend on the square of current, even a moderate reduction in current can significantly reduce energy losses.

Practical Example

Suppose a transmission line carries 100 MW of three-phase power at a power factor of 1. Compare transmission at 11 kV and 220 kV, assuming the same conductor resistance per phase.

Parameter	At 11 kV	At 220 kV
Transmitted power	100 MW	100 MW
Line current (approx.)	5,249 A	262 A
Relative resistive losses	100%	0.25%

The voltage increases by 20 times, so the current falls to one-twentieth, assuming the same power and power factor. With identical resistance per phase, resistive losses at 220 kV are 1/400 of those at 11 kV. Actual networks can have different conductor designs, voltages, and operating conditions.

This is why high-voltage transmission is essential for transporting bulk electrical power efficiently.

3. Other Advantages of High-Voltage Transmission

·         Reduced conductor size for a given current limit: lower current can allow a smaller conductor cross-section for a specified thermal limit, although actual sizing depends on mechanical strength, voltage drop, corona, and reliability requirements.

·         Improved transmission efficiency: less electrical energy is wasted as heat in the conductors.

·         Long-distance power transfer: large quantities of electricity can be transported from generating stations to major load centres.

·         Better utilization of transmission corridors: high-capacity lines can transfer more power through a given route, subject to system limits.

·         Lower current-related voltage drop: reduced current generally reduces the resistive component of voltage drop, although reactive effects remain important in AC networks.

However, high voltage also requires greater insulation clearances, more sophisticated substations, protective equipment, and additional safety precautions.

4. What Types of Conductors Are Used for Transmission Lines?

Transmission conductors must offer a suitable combination of electrical conductivity, mechanical strength, low weight, corrosion resistance, and cost effectiveness.

1. ACSR — Aluminium Conductor Steel Reinforced

ACSR is one of the most widely used overhead transmission conductors. Construction: stranded aluminium wires surround a central steel core.

Why it is used: aluminium provides electrical conductivity; the steel core provides mechanical strength; and the design offers a good balance of cost, weight, and tensile strength.

Applications: conventional overhead transmission lines, including many 132 kV, 220 kV and 400 kV networks.

2. AAAC — All Aluminium Alloy Conductor

AAAC is made entirely from stranded aluminium alloy wires. Advantages include a good strength-to-weight ratio, better corrosion resistance than many conventional ACSR designs, and no separate steel core.

Applications: overhead distribution and transmission lines where alloy strength and corrosion resistance are useful.

3. AAC — All Aluminium Conductor

AAC consists of stranded electrical-grade aluminium wires. It offers high electrical conductivity for its material, low weight, and good corrosion resistance. Its tensile strength is lower than that of many steel-reinforced conductors.

Applications: primarily distribution lines and shorter spans where mechanical strength requirements are moderate.

4. ACAR — Aluminium Conductor Alloy Reinforced

ACAR combines electrical-grade aluminium strands with aluminium-alloy reinforcing strands. It offers a useful balance of electrical conductivity and mechanical strength.

Applications: overhead transmission and distribution where the required electrical and mechanical characteristics favour this construction.

5. HTLS — High-Temperature Low-Sag Conductors

HTLS is a category of advanced conductors designed to carry higher current at elevated operating temperatures while limiting sag. Some designs use advanced aluminium alloys and composite or specially engineered cores.

Advantages include higher current-carrying capacity in suitable applications, reduced sag at higher operating temperatures compared with conventional conductors of comparable design, and the potential to increase capacity on existing transmission routes.

Applications: uprating existing transmission lines when acquiring new rights-of-way is difficult or expensive.

Note: Copper has excellent electrical conductivity, but aluminium-based conductors are generally preferred for long overhead transmission lines because they offer a favourable combination of conductivity, weight, cost, and mechanical design.

5. Why Is Aluminium Preferred Over Copper?

Although copper is more conductive per unit cross-sectional area, aluminium has a much lower density and is generally more economical for overhead transmission.

Property	Aluminium	Copper
Electrical conductivity	Lower per unit area	Higher per unit area
Density	About 2.70 g/cm³	About 8.96 g/cm³
Weight for equivalent conductor resistance	Lower	Higher
Material cost per unit mass	Generally lower	Generally higher
Typical overhead transmission use	Very common	Less common

For approximately equal DC resistance at the same length and temperature, aluminium needs around 1.6 times the cross-sectional area of copper. Nevertheless, its mass is only about half that of the copper conductor. This helps reduce the mechanical load on towers and makes aluminium an attractive material for overhead power transmission.

6. Why Are Bundled Conductors Used at 220 kV, 400 kV and 765 kV?

At extra-high voltages, one conductor per phase may produce a strong electric field at its surface. This can cause corona discharge, particularly under adverse weather conditions.

Corona can lead to:

·         Audible noise and radio interference

·         Power loss

·         Localized electrical discharge around conductor surfaces

To reduce these effects, transmission lines may use two, three, four, or more sub-conductors per phase, depending on the voltage level and design. These are called bundled conductors.

Bundling increases the effective electrical radius of each phase, reducing the surface electric-field intensity. It can also reduce line reactance and improve power-transfer characteristics. The exact bundle arrangement depends on the transmission voltage, conductor design, corona performance, and economic considerations.

7. How Does Electricity Travel from the Power Plant to Our Homes?

1.       Generating station: Electricity is generated, commonly at medium voltage.

2.       Step-up transformer: Voltage is increased for efficient bulk transmission.

3.       High-voltage transmission network: Power travels through 132 kV, 220 kV, 400 kV, 765 kV AC or HVDC systems as applicable.

4.       Receiving grid substation: Voltage is stepped down for regional distribution.

5.       Distribution transformer and consumer: Voltage is reduced to suitable utilization levels, such as 230 V single-phase supply in India.

The step-up transformer makes long-distance transmission more efficient, while receiving substations and distribution transformers progressively reduce the voltage to appropriate levels for consumers.

8. Important Engineering Considerations

·         Thermal rating: the conductor must not exceed permissible operating temperatures.

·         Sag and clearance: conductors must maintain safe clearances from the ground, buildings, vegetation, and other infrastructure.

·         Mechanical loading: wind, ice where applicable, conductor tension, and tower loading must be accounted for.

·         Insulation coordination: insulators and clearances must withstand operating voltage and expected overvoltages.

·         Corona and electric fields: conductor diameter and bundle arrangements help control electrical stress.

·         System stability and reactive power: these become particularly important for long AC transmission lines.

·         Safety: high-voltage lines require appropriate protection, earthing arrangements, and safe working distances.