Voltages in Power Transmission Lines or Transmission Voltages

Large generators commonly produce electricity at medium voltage, often in the range of about 11 kV to 25 kV. The exact generator voltage depends on the plant and grid.

A step-up transformer raises this voltage for transmission. Other transformers change the voltage again as power moves from the transmission network into local distribution. The nominal values and number of stages vary by country, utility and network design.

• The alternator supplies power at its designed terminal voltage. Values from about 11 kV to 25 kV are common for large generating units, but they are not universal.
• A generating-station step up transformer raises the voltage to a level selected for the connected network. The first step is not limited to 33 kV.
• Substations transform and switch power between network voltage levels. Examples include 66 kV and 132 kV, depending on the regional standard and system plan.
• Bulk transmission can use hundreds of kilovolts. Systems at 400 kV, 765 kV and higher exist where their power-transfer and distance requirements justify them.
• Near a load centre, a transmission substation uses step down transformers to feed subtransmission or distribution circuits. A 132 kV stage is one possible design, not a required boundary.
• A Power transformer may reduce a subtransmission voltage to a primary distribution voltage such as 33 kV or 11 kV. Other grids use different nominal values.
• Distribution transformers supply the local low-voltage system. A three-phase nominal voltage near 400 V or 415 V is common in many 50 Hz systems, while other countries use different service voltages.

Types of Power Lines

From power generation to the customer, transmission lines and distribution circuits can be grouped by their role and nominal voltage. Labels such as low, medium, high, extra-high and ultra-high voltage depend on the standard or utility convention being used.

Why Use High Voltage for Long Transmission Lines?

Long-distance lines use high voltage so they can transfer a given amount of power at lower current. Lower current reduces resistive heating in the conductors.

A long line usually has more total resistance than a comparable short transmission line because it contains more conductor length. For a specified power transfer, raising voltage reduces the required current in each conductor. This lowers resistive loss during power transmission.
For a balanced three-phase AC system, real power is P = √3 V_L I_L cos φ.

The total conductor loss is P_Loss = 3I_L²R when R is the resistance per phase.
Here R is the resistance in ohms per phase of the transmission line.
Substituting current from the power equation gives the voltage relationship.

For fixed transmitted power, power factor and resistance, loss varies inversely with the square of line voltage.

For a simple two-conductor DC link, transmitted power is P = VI and conductor loss is proportional to I squared times the total loop resistance.

Equations (2) and (3) show the same principle: for fixed power and resistance, a higher line voltage gives lower current and lower resistive loss. Conductor size is then selected from loss, temperature, voltage drop, mechanical strength, corona and economic requirements rather than voltage alone.

Why Use HVAC for Power Transmission?

Most interconnected power networks use AC. HVAC remains a practical choice for many overhead transmission routes and compatible AC grid connections because:

1. Transformers can change AC voltage efficiently at generating stations, substations and distribution points.
2. AC substations use mature equipment and established operating practices. Project cost and maintenance still depend on the voltage, route and equipment.
3. Connecting two compatible AC networks avoids the converter stations required at HVDC terminals. The wider electrical power system can therefore remain AC from generation through distribution.

Why Use HVDC for Power Transmission?

HVDC is considered for long overhead routes, long submarine or underground cables, controlled bulk-power transfer and connections between asynchronous AC systems. Converter stations change AC to DC at one terminal and DC back to AC at the other. Their cost means that HVAC or HVDC must be selected through a project-specific technical and economic study.

1. Bipolar HVDC links commonly use two pole conductors. Monopolar and other return-path arrangements also exist, so the conductor count depends on the design.
2. HVDC lines still have inductance and capacitance, but they do not carry steady-state AC reactive current. This makes long cable links practical and gives operators direct control of real-power flow.
3. HVDC systems can experience switching surges, lightning overvoltages and faults. Their insulation, surge arresters and protection systems are designed for these events.
4. DC conductors have no power-frequency skin effect in steady state, although current distribution during transients can still be non-uniform.
5. For comparable transfer requirements, HVDC can need a narrower right-of-way or fewer conductors. Insulation design still depends on voltage, polarity, environment and overvoltage requirements.
6. HVDC lines can reduce some corona discharge effects for a chosen design, but corona and its losses are not eliminated.
7. Power-electronic controls regulate power flow and can connect AC systems that are not synchronized. Grid support depends on the converter technology and its control design.

Why Use Medium and Low Voltage for Distribution?

Primary distribution commonly operates at medium voltage so it can move power around a local area with manageable current and loss. Values such as 11 kV and 33 kV are used in some networks. Local distribution transformers then reduce the voltage to the service level required by customers. These transformers are placed near loads because low-voltage circuits carry higher current for the same power and therefore become uneconomic over long distances. The actual service voltage depends on the country’s supply standard.

Disadvantages of AC or HVAC Transmission

HVAC transmission has several limitations that become more important as voltage, distance or cable length increases:

1. A three-phase AC circuit normally needs three phase conductors, while some HVDC arrangements can transfer comparable power with fewer conductors.
2. Long HVAC lines can require reactive-power compensation and additional equipment to control voltage and stability.
3. Skin effect increases effective AC resistance and contributes to conductor loss.
4. Line and cable capacitance draws charging current. This limits the practical length of high-voltage AC cable circuits.

Disadvantages of DC or HVDC Transmission

HVDC also has costs and operating constraints:

1. Bulk Electric power systems usually generate and distribute AC, so an HVDC link needs costly converter stations and filters at its terminals.
2. A conventional transformer cannot directly step DC voltage up or down. Changing DC voltage requires power-electronic conversion.
3. DC fault interruption is difficult because current has no natural zero crossing. High-voltage DC switches and circuit breakers are specialised and can be costly.