High Voltage Direct Current Transmission
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High Voltage Direct Current Transmission

High Voltage Direct Current Transmission – Proven Technology for Power Exchange | PDF Free Download.

High Voltage Contents

  • Why High Voltage Direct Current? 
  • Main Types of HVDC Schemes 
  • Converter Theory
  • Principle Arrangement of an HVDC Transmission Project
  • 5 Main Components
  • System Studies, Digital Models, Design Specifications
  •  Project Management

Preface to High Voltage Direct Current Transmission

The transmission and distribution of electrical energy started with a direct current. In 1882, a 50-km-long 2-kV DC transmission line was built between Miesbach and Munich in Germany.

At that time, conversion between reasonable consumer voltages and higher DC transmission voltages could only be realized by means of rotating DC machines. In an AC system, voltage conversion is simple.

An AC transformer allows high power levels and high insulation levels within one unit and has low losses. It is a relatively simple device, which requires little maintenance. Further, a three-phase synchronous generator is superior to a DC generator in every respect.

For these reasons, AC technology was introduced at a very early stage in the development of electrical power systems. It was soon accepted as the only feasible technology for the generation, transmission, and distribution of electrical energy.

However, high-voltage AC transmission links have disadvantages, which may compel a change to DC technology: 

  • Inductive and capacitive elements of overhead lines and cables put limits on the transmission capacity and the transmission distance of AC transmission links. 
  • This limitation is of particular significance for cables. Depending on the required transmission capacity, the system frequency, and the loss evaluation, the achievable transmission distance for an AC cable will be in the range of 40 to 100 km. It will mainly be limited by the charging current. 
  • The direct connection between two AC systems with different frequencies is not possible. 
  • The direct connection between two AC systems with the same frequency or a new connection within a meshed grid may be impossible because of system instability, too high short-circuit levels, or undesirable power flow scenarios.

Engineers were therefore engaged over generations in the development of a technology for DC transmissions as a supplement to the AC transmissions.

The invention of mercury arc rectifiers in the nineteen-thirties made the design of line-commutated current sourced converters possible.

In 1941, the first contract for a commercial HVDC system was signed in Germany: 60 MW were to be supplied to the city of Berlin via an underground cable of 115 km length. The system with ±200 kV and 150 A was ready for energizing in 1945. It was never put into operation.

Since then, several large HVDC systems have been realized with mercury-arc valves. The replacement of mercury arc valves with thyristor valves was the next major development. The first thyristor valves were put into operation in the late nineteen-seventies.

The outdoor valves for Cahora Bassa were designed with oil-immersed thyristors with parallel/series connection of thyristors and an electromagnetic firing system. Further development went via air-insulated air-cooled valves to the air-insulated water-cooled design, which is still state of the art in HVDC valve design.

The development of thyristors with higher current and voltage ratings has eliminated the need for parallel connection and reduced the number of series-connected thyristors per valve.

The development of light-triggered thyristors has further reduced the overall number of components and thus contributed to increased reliability.

Innovations in almost every other area of HVDC have been constantly adding to the reliability of this technology with economic benefits for users throughout the world.

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