Introduction to HVDC.pdf

Respect to the ground and one carrying a voltage of

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respect to the ground, and one carrying a voltage of negative polarity with respect to the ground. For this reason, the dc line in this configuration is usually called a two poles, or dual pole, line. One terminal at the dc side of each of the four thyristor converters in the two stations is generally connected to the ground. This provides a return path for the dc line current and allows the dual pole line to be implemented with two only conductors instead of three. During normal system operation, the currents flowing in the two conductors of the dual pole line are equal, and thus no current flows through the ground (i.e., the return path). The amount of power exchanged between ac power networks 1 and 2 is twice that exchanged with one monopolar HVDC transmission system. If one pole of the system stops operating, the other pole of the system can continue to operate using the return path provided by the grounded terminals of the thyristor converters, so that power continues to be exchanged between ac power networks 1 and 2. Figure 8. Bipolar HVDC transmission system. Thyristor converter 3 DC transmission line Thyristor converter 4 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 DC filter DC filter OLTC transformers OLTC transformers AC filters/ capacitor bank AC filters/ capacitor bank
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Introduction – HVDC Transmission Systems HVDC Transmission Systems 11 Back-to-back HVDC transmission system (Figure 9): with this configuration, the two converter stations are generally located in the same building. This configuration is used to link asynchronous ac power systems or independent grids that are subject to different regulations in terms of security, stability, and control strategies. The dc line linking the two stations is very short since there is no transmission of power over long distances. The dc line, in fact, consists of two short conductors, with one conductor being connected to the ground. The dc voltage is also kept at a relatively low value ( 50 kV to 150 kV) to minimize the cost of the thyristor converters. Finally, in certain installations, the smoothing inductors and dc filters are omitted. Figure 9. Back-to-back HVDC transmission system (monopolar configuration). 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 line DC filter AC filters/ capacitor bank AC filters/ capacitor bank
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Introduction – HVDC Transmission Systems 12 HVDC Transmission Systems Figure 10. HVDC transformers are essential components of HVDC converter stations ( © Siemens AG 2012, all rights reserved).
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A HVDC Transmission Systems 13 The Discussion of covers the following points: Voltage-versus-current relations of thyristor three-phase bridges Harmonics and reactive power in thyristor three-phase bridges
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  • One '14
  • High-voltage direct current, Electric power transmission, HVDC transmission systems

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