Power Transmission Systems: What Are They? (AC vs DC)

Electric Power Transmission

Electric power transmission systems move bulk electrical energy from generating stations to substations near load centres. A generating station produces electrical power, while the transmission network carries it over distances that local distribution circuits do not serve. Generation and demand are often in different locations.

Plant location depends on the energy source, fuel or water access, land, environmental limits and connection capacity. These constraints can place generation far from demand. Transmission links those sites to load centres and lets several generators and regions share power.

Electrical supply systems connect sources such as a thermal power station to consumers. Engineers model short transmission lines, medium transmission lines and long transmission lines differently because capacitance becomes more important with length and voltage. Substations then hand power to the power distribution system that supplies homes and businesses.

AC vs DC Transmission

Bulk electrical energy can use either high-voltage AC or high-voltage DC transmission:

1. High voltage DC electrical transmission system.
2. High AC electrical transmission system.

High-voltage DC can be useful for long point-to-point routes, submarine cables and links between asynchronous AC systems. The frozen list gives common DC line characteristics:

• Only two conductors are required for DC transmission system. It is further possible to use only one conductor of DC transmission system if the earth is utilized as the return path of the system.
• The potential stress on the insulator of the DC transmission system is about 70% of the equivalent voltage AC transmission system. Hence, DC transmission systems have reduced insulation costs.
• Inductance, capacitance, phase displacement and surge problems can be eliminated in DC system.

Three-phase AC remains common because transformers change voltage efficiently and AC networks support many intermediate substations. HVDC uses power-electronic converter stations at its terminals, so route length and project needs determine whether their cost is justified. The frozen AC list states:

• The alternating voltages can easily be stepped up and down, which is not possible in DC transmission system.
• Maintenance of AC substation is quite easy and economical compared to DC.
• The transforming of power in AC electrical substation is much easier than motor-generator sets in a DC system.

AC lines also have reactive-power, synchronisation and frequency-dependent effects. The following frozen list compares them with DC:

• The volume of conductor required in AC systems is much higher when compared to DC systems.
• The reactance of the line affects the voltage regulation of the electrical power transmission system.
• Problems of skin effects and proximity effects only found in AC systems.
• AC transmission systems are more likely to be affected by corona discharge than a DC transmission system.
• Construction of AC electrical power transmission network is more completed than DC systems.
• Proper synchronizing is required before interconnecting two or more transmission lines together, synchronizing can totally be omitted in DC transmission system.

Constructing a Generating Station

Generation planning weighs the following site factors alongside safety, environmental approval and grid access when estimating economical generation of electrical power.

1. Easy availability of water for thermal power generating station.
2. Easy availability of land for construction of power station including its staff township.
3. For a hydropower station, there must be a dam on the river. So proper place on the river must be chosen in such a way that the construction of the dam can be done in the most optimum way.
4. For a thermal power station, easy availability of fuel is one of the most important factors to be considered.
5. Better communication for goods as well as employees of the power station also to be kept into consideration.
6. For transporting very large spare parts of turbines, alternators, etc., there must be wide roadways, train communication, and the deep and wide river must pass away nearby the power station.
7. For a nuclear power plant, it must be situated in such a distance from a common location so that there may be any effect from the nuclear reaction the heath of common people.

No site offers every advantage in the frozen list. Planners compare fuel or resource access, land, transport, cooling needs, environmental constraints and network capacity for the specific technology. When the selected site is remote from demand, an electrical power transmission system carries its output to the grid.

Large generators usually produce at a medium voltage suited to the alternator design rather than at the final transmission voltage. A step-up transformer connects the generator to the high-voltage network. This separates alternator insulation and winding constraints from the voltage chosen for long-distance transfer. The best values depend on unit size, network design and economics, so low-voltage power generation is not a universal rule.

For a given active-power transfer and power factor, raising voltage reduces the required current. Lower current reduces I²R conductor loss and voltage drop, or permits a smaller conductor for the same thermal limit. Higher voltage also requires more insulation, clearance and substation equipment, so designers balance those costs. Reduced current helps the line hold its receiving-end voltage, while transformer voltage regulation is a related but separate equipment measure.

At the receiving end, substations step the voltage down in stages for subtransmission, distribution and final customer supply.

The usual AC path is generation at the alternator’s design voltage, transformation to a higher transmission voltage and staged reduction near consumers. HVDC projects add converter stations between their AC terminals and the DC line. Both systems use high voltage to transfer bulk power with manageable current and conductor losses.