Note for High Voltage DC Transmission - HVDC By Shataroopa Mohapatra

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HVDC TRANSMISSION MODULE-1

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CHAPTER:1 HVDC TRANSMISSION SYSTEM INTRODUCTION Electric power transmission was originally developed with direct current. The availability of transformers and the development and improvement of induction motors at the beginning of the 20th century, led to the use of AC transmission. DC Transmission now became practical when long distances were to be covered or where cables were required. Thyristors were applied to DC transmission and solid-state valves became a reality. With the fast development of converters (rectifiers and inverters) at higher voltages and larger currents, DC transmission has become a major factor in the planning of the power transmission. In the beginning all HVDC schemes used mercury arc valves, invariably single phase in construction, in contrast to the low voltage poly-phase units used for industrial application. About 1960 control electrodes were added to silicon diodes, giving silicon controlled- rectifiers (SCRs or Thyristors). Today, the highest functional DC voltage for DC transmission is +/- 600kV. D.C transmission is now an integral part of the delivery of electricity in many countries throughout the world. In the last fifty years DC power transmission is considered for  Power transfer over long distances from point to point  Underwater transmission  Interconnection of two AC system with two different frequencies  Better power system stability as parallel link with existing AC system CHALLENGES WITH AC POWER LINES:     AC lines become loaded closer to their thermal capacity with increasing losses. Reduced power quality causes the risk of declining network stability. Overhead lines change the landscape, causes public resentment. Limit the distance of traditional AC underground cables to around 80km. SOLUTION-HVDC TECHNOLOGY:  HVDC technology has the potential to play an important role in achieving this solution.  It provides improved power quality and power flow control as well.  Introducing extruded DC-cables which have no technical limit to distance which can be installed.  It can provide an alternative to overhead lines particularly when the total capital and environmental costs are considered. [Shataroopa Mohapatra] HVDC Transmission

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WHY DC INSTEAD OF AC? To reduce electrical losses and costs, sometimes because it is the only possible solution. AC cable transmission links have a maximum distance of 50 to 100 km. For longer distances, HVDC is the only solution. Furthermore, with HVDC it is also possible to connect grids that could not otherwise be connected, e.g. with different frequencies – 50 Hertz and 60 Hertz. 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. Fig.1 Comparison of AC and DC system High Voltage Direct Current Transmission: The system which uses the direct current for the transmission of the power such type of system is called HVDC (High Voltage Direct Current) system. The HVDC system is less expensive and has minimum losses. It transmits the power between the unsynchronized AC systems. HVDC transmission is an efficient technology designed to deliver large amounts of electricity over long distances with low losses. It can also interconnect incompatible AC networks and stabilize the surrounding grid. Each transmission link has its own set of requirements justifying the choice of HVDC, but the most common points in favor include: − Asynchronous interconnections − Long distance water crossing − Lower losses − Controllability − Environmental concerns − Limit short-circuit currents − Lower investment cost [Shataroopa Mohapatra] HVDC Transmission

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Why use high voltage instead of low voltage? To reduce electrical losses when electric power is transmitted over long distances, electrical losses are lower if the current is low. This is obtained by using high voltage. What is an HVDC transmission link? An HVDC transmission link consists of three main components: a station to convert the alternating current of the grid to direct current, the transmission equipment itself in the form of cables and overhead lines, and another station that converts DC back into AC so that it can be used by consumers. How does an HVDC system work?  HVDC transmission utilizes a converter station at either end of the system.  A mercury arc valve or solid-state valve (thyrister) is used for the conversion of AC and DC current.  The valve at the beginning of the system converts alternating current to HVDC, the HVDC travels to the next location through a cable.  The valve at the end of the system converts the HVDC back to alternating current. HVDC GRIDS: Energy infrastructure is an essential building block of our society. With the increased integration of energy markets, the need to enhance security of supply and to integrate renewable energy sources, a European HVDC grid could evolve. Over time, the point-to-point connections and the regional multi-terminal projects could be connected with each other to more extensive grids. The first small regional systems can be operated without DC breakers, but as the size and complexity increase, breakers will be gradually introduced. What is an HVDC grid? An HVDC electricity grid that can operate: − Independently of one or several disturbances (isolate a failure) − In different operation modes in the connected AC and DC systems Technology gaps for the full realization include: − Power flow control − Automatic network restoration − High voltage DC/DC converters Global rules and regulations for operation are required for market acceptance. [Shataroopa Mohapatra] HVDC Transmission