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

Electric Power Transmission

Electric power transmission systems are the means of transmitting power from a generating source to various load centers (i.e. where the power is being used). Generating stations generate electrical power. These generating stations are not necessarily situated where the majority of the power is being consumed (i.e. the load center).

Since distance is not the only factor that determines the ideal location for a generating station, the place where the power is generated may be quite far away from where it is used. Land further from the load center (which is generally a high-density central location) can be much cheaper per square meter, and governments may not want such loud and/or polluting stations close to residential areas. Hence power transmission systems are crucial to the supply of power in electrical networks.

Overall, electrical supply systems are the network through which consumers of electricity receive power from a generation source (such as a thermal power station). Power transmission systems – including short transmission lines, medium transmission lines, and long transmission lines – transport the power from the generation source and into a power distribution system. These distribution systems provide electricity to individual consumer premises.

AC vs DC Transmission

Fundamentally there are two systems by which electrical energy can be transmitted:

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

There are some advantages to using DC transmission systems:

  • 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.

Even having these advantages in a DC system, generally, electrical energy is transmitted by a three-phase AC transmission system. The advantages of an AC transmission system include:

  • 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.

But AC transmission system also has some disadvantages, including:

  • 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

During the planning of construction of generating station the following factors to be considered for 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.

There are many other factors also we should consider, but there are beyond the scope of our discussion. All the factors listed above are difficult to be available at load centers. The power station or generating station must be situated where all the facilities are easily available. This place may not be necessary at the load centers. The power generated at the generating station then transmitted to the load center using an electrical power transmission system as we said earlier.

transmission system and network

The power generated at a generating station is at a low voltage level, as low voltage power generation has some economic value. Low voltage power generation is more economical (i.e. lower cost) than high voltage power generation. At a low voltage level, both weight and insulation are less in the alternator; this directly reduces the cost and size of an alternator. But this low voltage level power cannot be transmitted directly to the consumer end because this low voltage power transmission is not at all economical. Hence although low voltage power generation is economical, low voltage electrical power transmission is not economical.

Electrical power is directly proportional to the product of electrical current and voltage of the system. So for transmitting certain electrical power from one place to another, if the voltage of the power is increased then associated current of this power reduces. Reduced current means less I2R loss in the system, less cross-sectional area of the conductor means less capital involvement and decreased current causes improvement in voltage regulation of power transmission system and improved voltage regulation indicates quality power. Because of these three reasons electrical power mainly transmitted at high voltage level.

Again at distribution end for efficient distribution of the transmitted power, it is stepped down to it’s desired low voltage level.

So it can be concluded that first the electrical power is generated at a low voltage level then it stepped up to high voltage for efficient transmission of electrical energy. Lastly, for the distribution of electrical energy or power to different consumers, it is stepped down to the desired low voltage level.

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