High voltage DC Transmission system (HVDC)

HVDC stands for High Voltage Direct Current. An HVDC electric power transmission system uses direct current for the bulk transmission of electrical power, in contrast with the more common alternating current systems. 

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For long-distance distribution, HVDC systems are less expensive and suffer lower electrical losses. For shorter distances, the higher cost of DC conversion equipment compared to an AC system may be warranted where other benefits of direct current links are useful.

When HVDC Started?

The transmission and distribution of electrical energy started with 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.
The HVDC (high voltage DC) transmission made the modest beginning in 1954 when a 100 kV, 20 MW DC link was establish between Swedish mainland and the island of Gotland. 
 
Until 1970, the converter stations utilized mercury arc valves for rectification; the successful use of thyristors for power control in industrial devices encouraged its adoption in HVDC converters by development of high power semiconductor devices. 
 
The latest technology in HVDC system is to use LTT’s (Light triggered thyristors) or GTO’s (Gate turn off) or IGBT’s (Insulated gate bipolar transistor).Control is obtained by using VSC (Voltage source converter) using PWM (Pulse width modulation) technique.

Components of HVDC Transmission System

To assist the designers of transmission systems, the components that comprise the HVDC system, and the options available in these components, are presented.

The main elements of an HVDC system are:
  1. Converter unit
  2. Converter transformer
  3. AC filters & Capacitor banks
  4. DC filters
  5. Reactive Power source
  6. Smoothing Reactor
  7. DC Switchgear

HVDC Converter Station

A HVDC converter station uses thyristor valves to perform the conversion from AC to DC and vice versa. The valves are normally arranged as a 12- pulse converter. The valves are connected to the AC system by means of converter transformers. It is  normally placed in a building and the converter transformers are located just outside.
 

The power transmitted over the HVDC transmission is controlled by means of a control system. It adjusts the triggering instants of the thyristor valves to obtain the desired combination of voltage and current in the DC system. Several other apparatus are needed in a converter station, such as circuit breakers, current and voltage transducers, surge arresters, etc.

HVDC Converter Transformer

The converter transformers adapt the AC voltage level to the DC voltage level and they contribute to the commutation reactance. Usually they are of the single phase three winding type, but depending on the transportation requirements and the rated power, they can be arranged in other ways. Almost all of HVDC power converters with thyristor valves are assembled in a converter bridge of twelve-pulse configuration.
 
 The construction of the standard 12- pulse converter transformer system (CTS), it can be obtained with either of the following arrangements. The converter transformers are the heaviest equipment in a HVDC converter station. Single units can often have a total weight of 200 – 550 tons.
 
The converter transformer serves several functions:
  • Supply of AC voltages in two separate circuits with a relative phase shift of 30 electrical degrees for reduction of low order harmonics, especially the 5th and 7th harmonics.
  • Act as a galvanic barrier between the AC and DC systems to prevent the DC potential to enter the AC system.
  • Reactive impedance in the AC supply to reduce short circuit currents and to control the rate of rise in valve current during commutation.
  • Voltage transformation between the AC supply and the HVDC system.
The HVDC converter transformer can be built as three-phase units or as single-phase units depending on voltage and power rating. When built as three phase transformers there is generally one unit with the valve winding arranged for star connection and the other unit for delta connection. In single-phase design the two valve windings are generally built on the same transformer unit.

Design wise the internal voltage distribution from the DC-voltages on the transformer valve terminals will need a different insulation build-up as compared to the insulation system in a conventional transformer.

Converter Valve

The quadruple valve structure is suspended from the ceiling of the valve hall via porcelain insulators. At the top and bottom of the structure, a metallic framework ensures the mechanical stability of the valves. Between the frameworks, the different levels in the valve are mechanically fixed by means of threaded epoxy rods.

 
Thyristor valves are the heart of the HVDC conversion process. Modern valves have an excellent performance record and very small losses.
The thyristor valves do the actual conversion from AC to DC . The basic circuit used is the Graetz bridge consisting of six valve functions.

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