Transmission considerations of alternating current (AC) and high voltage direct current (HVDC)

by | Jun 21, 2022 | Articles, Engineering

In the world of electrical power transmission, alternating current (AC) and high voltage direct current (HVDC) play pivotal roles, each having unique benefits and drawbacks. While AC is traditionally used for shorter distances and grid interconnectivity due to its ease of voltage transformation and natural zero crossings, HVDC excels in long-distance, point-to-point transmission, submarine cables, and separate grid interconnections, owing to less line loss and no synchronization issues.

Alternating Current (AC) Transmission

AC transmission has been the traditional method of electricity transmission, primarily due to:

  1. Transformer Usage: Alternating current can be easily stepped up or down in voltage using transformers. This is vital in transmission, where electricity is converted to high voltage to reduce line losses, and then stepped down again for use in homes and businesses.
  2. Natural Current Zero Crossing: The alternating nature of AC results in regular points where current crosses zero (zero crossings). This helps with easier interruption of current, facilitating the operation of circuit breakers.

However, AC transmission also has several challenges:

  1. Reactive Power: AC transmission systems have to contend with reactive power, which can cause losses and affect system stability if not properly managed.
  2. Skin Effect: In AC transmission, current tends to flow near the outer skin of the conductor, causing increased resistance and losses.
  3. Electromagnetic Fields: AC lines produce more electromagnetic interference and have larger corona losses than DC lines.

High Voltage Direct Current (HVDC) Transmission

HVDC systems have several advantages:

  1. Long Distance and Submarine Transmission: HVDC systems are more efficient for transmitting electricity over long distances and underwater due to less line loss and the absence of reactive power issues.
  2. No Skin Effect: DC allows usage of the full cross section of a conductor, reducing losses.
  3. No Synchronization Issues: Because DC is not alternating, there are no issues with synchronization between different parts of the grid.
  4. Controllable Power Flows: Power flows in an HVDC link can be precisely controlled, improving the stability of the overall grid.

The main drawbacks of HVDC are:

  1. Cost and Complexity: HVDC systems are generally more expensive and complex than AC systems. The converters needed to switch between AC and DC are costly and require significant maintenance.
  2. No Natural Current Zero Crossing: Unlike AC, DC does not have natural zero crossings. This makes interrupting DC current more challenging, necessitating more complex and expensive circuit breakers.
  3. Converter Losses: While line losses are less with HVDC, the converters that switch between AC and DC introduce their own losses and inefficiencies.

Regarding power generation, traditional methods such as coal, gas, hydro, and nuclear power plants use synchronous generators that produce AC power. On the other hand, newer technologies such as solar photovoltaic cells and batteries naturally output DC power. Even for traditional power sources, the increasing use of power electronics and the demand for grid flexibility make HVDC conversion increasingly feasible, despite the additional steps involved.

The choice between AC and HVDC depends on the specific circumstances and requirements of a power system. AC transmission remains widely used for short distances and areas with high grid interconnectivity, while HVDC is the preferred choice for long-distance point-to-point transmission, submarine cables, and grid interconnections. As technology continues to evolve, the balance between these two methods of transmission may shift.

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