Introduction to HVDC.pdf

Notice that in the example of figure 2 1 the point

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Notice that in the example of Figure 2 1 , the point where the relations of the rectifier and inverter bridges intersect is considered to be the operating point of the HVDC transmission system. This is true when the system is back-to-back configured (rectifier and inverter bridges in the same station) because the voltage drop across the smoothing inductors at the dc side of the bridges is negligible compared to the operating dc voltage of the bridges, making the bridges operate at virtually the same dc voltage. However, when a dc transmission line is connected between the rectifier and inverter bridges, a non-negligible voltage drop occurs across the transmission line. Therefore, the inverter bridge operates at a dc voltage lower than that of the rectifier bridge, causing the operating point to move toward the left in Figure 2 1 , thereby decreasing the value of the current I dc flowing through the bridges.
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Basic Operation of HVDC Transmission Systems Discussion HVDC Transmission Systems 26 Figure 2 1 . The voltages-versus-current relations of the rectifier and inverter bridges intersect at a point that determines the dc current in the HVDC transmission system. Operating point Inverter bridge (bridge 2) α ൌ 165° Bridge dc current, I dc (pu) Bridge dc voltage, E dc [1 pu ൌ1.35 ൈ Line voltage ܧ ௅ି௅ ] I dc 0.3 pu Rectifier bridge (bridge 1) α ൌ 0°
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Basic Operation of HVDC Transmission Syste ms 27 HVDC Transmission Systems The dc current flowing through the HVDC transmission system can be varied by adjusting (mainly decreasing) the firing angle of the inverter bridge. For example, decreasing the firing angle of the inverter bridge of Figure 2 1 to 152° sets the dc current I dc to 1.0 pu, as Figure 2 2 shows. Figure 2 2 . Adjusting the firing angle of the inverter bridge modifies the point at which the voltage-versus-current relations of the rectifier and inverter bridges intersect, and thus, the dc current flowing through the HVDC transmission system. Operating point Bridge dc current, I dc (pu) Bridge dc voltage, E dc [1 pu ൌ1.35 ൈ Line voltage ܧ ௅ି௅ ] I dc 1.0 pu Inverter bridge (bridge 2) α ൌ 152° Rectifier bridge (bridge 1) α ൌ 0°
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Basic Operation of HVDC Transmission Systems HVDC Transmission Systems 28 Varying (i.e., mainly decreasing) the firing angle of the inverter bridge as shown in Figure 2 2 allows the dc current in an HVDC transmission system (and thus the amount of power transferred) to be adjusted. However, this method is not normally used because decreasing the firing angle of the inverter bridge also significantly decreases the DPF of this bridge. For instance, the DPF of a thyristor bridge passes from approximately 1 to approximately 0.88 when the firing angle passes from 165° to 152°, as in the example of Figure 2 2 (generally, a DPF of 0.88 is considered to be a bit too low and unacceptable). The DPF continues to decrease rapidly as the firing angle of the inverter bridge continues to decrease (e.g., DPF 0.76 at a firing angle of 140°).
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  • One '14
  • High-voltage direct current, Electric power transmission, HVDC transmission systems

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