Introduction to HVDC.pdf

Operating at different frequencies without the need

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operating at different frequencies, without the need for phase-angle controllers as in FACTS. This is advantageous in certain situations, as the ac power systems linked by HVDC transmission systems can keep their own controllers and be developed independently. HVDC transmission systems are often used together with flexible AC transmission systems (FACTS) to improve the control of the flow of electrical power in large interconnected grids. The use of HVDC transmission systems in conjunction with FACTS can help to stabilize ac power grids, increase the power- carrying capacity of existing transmission lines, limit the effects of disturbances in ac power systems and provide more accurate and faster frequency control compared to conventional frequency controllers. Configurations of HVDC transmission systems There are three common configurations of HVDC transmission systems: Monopolar HVDC transmission system (Figure 6): with this configuration, the two converter stations are interconnected by a dc transmission line (usually a cable) consisting of a single conductor with the ground (or sea) being used as a return path for the dc line current. For this purpose, one terminal at the dc side of each of the two thyristor converters is connected to the dc transmission line conductor (via the smoothing inductor), while the other terminal at the dc side of each of the two thyristor converters is connected to the ground. The dc voltage on the dc transmission line is either positive or negative, with respect to the ground. For this reason, the dc transmission line in this configuration is usually called a single pole line. The monopolar HVDC transmission system is often used in underwater applications (i.e., when the dc line is a submarine cable). This configuration minimizes cable costs since it requires only one conductor. However, in certain cases such as when the resistance of the ground is too high or the underground or underwater metallic components cause interference, it can be necessary to use two conductors to interconnect the thyristor converters in the two stations.
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Introduction – HVDC Transmission Systems HVDC Transmission Systems 9 Figure 6. Monopolar HVDC transmission system. Figure 7. Converter station at one end of an HVDC transmission system linking the states of New Jersey and New York in the United States of America ( © Siemens AG 2012, all rights reserved). AC power network 1 AC circuit breaker Thyristor converter 1 DC filter Converter station 1 AC power network 2 OLTC transformers OLTC transformers Thyristor converter 2 Converter station 2 AC circuit breaker DC transmission line DC filter AC filters/ capacitor bank AC filters/ capacitor bank
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Introduction – HVDC Transmission Systems 10 HVDC Transmission Systems Bipolar HVDC transmission system (Figure 8): this configuration is the most commonly used for overhead dc transmission lines. As Figure 8 shows, the bipolar HVDC transmission system consists of two monopolar HVDC transmission systems: one carrying a voltage of positive polarity with
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  • One '14
  • High-voltage direct current, Electric power transmission, HVDC transmission systems

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