A technology evolution from the conventional to the non conventional energy sources, the traditional design of the grid is to be modernised. One such is  High Voltage Direct Current transmission  .The acute increase in awareness about the need for renewable energy, growth in the market and the support provided by the government made us think for the efficient promotion of the generated power. HVDC provides a gateway to transmit the electrical energy efficiently over long distances at cheaper cost.

Need for HVDC

A question may arise that all the generating units generates and delivers AC power(except solar, etc.,) to the consumers, then where comes the need for developing DC grid? Ofcourse It is impossible to connect the AC networks of two different frequencies. But it  can be made possible by providing HVDC links. With the emerging era in the smart power systems, development of HVDC is expected to go beyond the traditional position as supplement to AC power system. Electricity is usually transmitted using three-phase AC systems. In DC systems, only two conductors are necessary to transmit electricity, and with lower losses than AC systems of similar scope. DC long distance transmissions require only a narrow power corridor.

 

Possibility of HVDC

Technology gaps for the full realization include Power flow control,  Automatic network restoration and High voltage DC/DC converters.  An efficient HVDC breaker is available. It can sectionalize multi terminal HVDC systems into several protection zones to facilitate fault clearance with continuous transmission in the non-affected areas.

Types of HVDC schemes

  1. DC circuit
  2. Back – back converters
  3. Monopolar long distance transmission
  4. Bipolar long distance transmissions

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Converter transformer

The chief component in the HVDC transmission is the converter transformer that converts the available AC to the DC of high voltages. Normally in earlier periods of 1997, the power transfer upto 1200MW for short and medium transmissions were carried out. Due to development in the power electronic devices and their capability to withstand high voltages, it is now possible to transfer 800kV in the transmission lines.

The project sanctioned by the Power  Grid Corporation of India Ltd to the Alstom is worth about 400 million pounds where HVDC is to be laid between Champa (State of Chhattisgarh), Central India and Khurukshetra (State of Haryana) in Northern India, using ±800 kV 3,000 MW Ultra High Voltage Direct Current (UHVDC) technology. This advanced UHVDC system will meet bulk power transfer requirement from Chhattisgarh region – a hub of Independent Power Producers of thermal power to the load centre located in the northern region of the country, through a 1,365 km transmission line, creating an “energy highway” of clean, efficient power.

The HVDC system uses a special type of transformer rather than a conventional type of transformer we use for AC systems. Its functions are as follows: The converter transformer transforms the voltage of the AC busbar to the required entry voltage of the converter. The 12-pulse converter requires two 3-phase systems which are spaced apart from each other by 30 or 150 electrical degrees. At the same time, they ensure the voltage insulation necessary in order to make it possible to connect converter bridges in series on the DC side, as it is necessary for HVDC technology. The transformer main insulation, therefore, is stressed by both the AC voltage and the direct voltage potential between valve-side winding and ground. The converter transformers are equipped with on-load tap-changers in order to provide the correct valve voltage.

Main components of the converter transformer

  • Core
  • Windings
  • Tanks
  • Bushings

Special test for Transformer

In order to check the durability of the transformer, the tests are carried out at the applicable international standards that are subjected to further development. Tests with DC voltages, switching and impulse voltages with different voltage load. The transformers are tested at 2MV DC voltage generator. Partial discharge is tested and maximum of 10 discharges over 2000pC during the last 10 minutes of the test is allowed.

Applications of  HVDC

  • Connecting remote generation
  • Interconnection of grids
  • Connecting offshore wind
  • Power from shore
  • DC links in AC grids
  • Connecting remote loads
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