Introduction to HVDC.pdf

# When the firing angle is 90 the bridge dc voltage is

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When the firing angle is 90°, the bridge dc voltage is approximately null (zero). In this particular example, the bridge dc voltage passes from -0.01 pu to -0.06 pu as the bridge dc current passes from the minimum value to the nominal value. When the firing angle is 170°, the bridge dc voltage is approximately the nominal negative value (-1.0 pu). In this particular example, the bridge dc voltage passes from -0.99 pu to -1.04 pu as the bridge dc current passes from the minimum value to the nominal value. The nominal value of a pa- rameter is equal to 1.0 per unit (1 pu) . For example, when the value of a parame- ter is twice the nominal value, the parameter value is equal to 2.0 pu. When the value of a parameter is half the nominal value, the parameter value is equal to 0.5 pu. Bridge dc current, I dc (A) Bridge dc voltage, E dc (V) α ൌ 30° Voltage Regulation and Displacement Power Factor (DPF) in Thyristor Three-Phase Bridges -Discussion

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HVDC Transmission Systems 15 Figure 12. Voltage-versus-current relations at the dc side of a thyristor bridge, for different firing angles (smoothing inductor of infinite inductance value). As previously discussed, the voltage-versus-current relations in Figure 12 are valid for a smoothing inductor of infinite inductance value. Since real smoothing inductors have a finite inductance value, they cannot keep the current flow continuous at low bridge dc current values. Consequently, the voltage-versus- current relations actually look more like those shown in Figure 13. Near the origin (i.e., for low dc current values), the relations have a curved section indicating that the bridge dc voltage does not decrease linearly with current because current flow is discontinuous . The curved section of each relation ends when the bridge dc current is high enough to ensure continuous current flow in the bridge. Past that point (i.e., for higher dc current values), each relation follows a linear downward slope since current flow remains continuous. α ൌ 30° α ൌ 15° α ൌ 0° α ൌ 45° α ൌ 60° α ൌ 75° α ൌ 90° α ൌ 105° α ൌ 120° α ൌ 135° α ൌ 150° α ൌ 165° Bridge dc current, I dc (pu) Bridge dc voltage, E dc [1 pu ൌ1.35 ൈ Line voltage ܧ ௅ି௅ ] α ൌ 170° Voltage Regulation and Displacement Power Factor (DPF) in Thyristor Three-Phase Bridges -Discussion
16 HVDC Transmission Systems A Figure 13. Voltage-versus-current relations at the dc side of a thyristor three-phase bridge, for different firing angles (smoothing inductor of finite inductance value). As the dashed line in Figure 13 shows, the curved section at the beginning of the relations increases in length as the firing angle passes from 0° to 90°. The length of the curved section is maximum at firing angle 90°. As the firing angle passes from 90° to 170°, the curved section of the relations shortens, reaching the minimum length at firing angle 170°. To ensure continuous current flow over the entire firing angle range, the current must be kept above the minimum current ensuring continuous current flow at firing angle 90°. In Figure 13, for example,

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• High-voltage direct current, Electric power transmission, HVDC transmission systems

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