Introduction to HVDC.pdf

For example figure 15 shows the waveform of line

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not 0° or 180°. For example, Figure 15 shows the waveform of line current I 1 for different firing angles. When the firing angle is 0° (Figure 15a), current I 1 is in phase with voltage E 1‐ N . Therefore, no reactive power is drawn by the thyristor bridge and the power factor is maximum. Similarly, when the firing angle is 180° (Figure 15b), current I 1 and voltage E 1‐ N are 180° out of phase. Therefore, no reactive power is drawn by the thyristor bridge and the power factor is maximum. When the firing angle is set to 45° (Figure 15c), current I 1 lags behind voltage E 1‐ N . This causes the thyristor bridge to draw reactive power from the ac power network (i.e., to operate like an inductor, which lowers the power factor). Increasing the firing angle to 90° (Figure 15d) further increases the lag of current I 1 relative to voltage E 1‐ N , causing more reactive power to be drawn and a further drop in power factor. Whenever the firing angle α is not 0° or 180°, shunt capacitors or a static var compensator (SVC) generally need to be used to compensate for the reactive power drawn and correct (increase) the power factor.
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Voltage Regulation and Displacement Power Factor (DPF) in Thyristor Three-Phase Bridges Discussion HVDC Transmission Systems 19 Figure 15. When the firing angle is not 0° or 180°, line current I 1 lags behind phase voltage E 1 ‐N and reactive power is drawn from the ac power network, which lowers the power factor. Phase angle E 1‐ N (°) Phase angle E 1‐ N (°) Line current I 1 I (A) I (A) Phase voltage E 1 ‐N (b) Firing angle α = 180° (c) Firing angle α = 45° (d) Firing angle α = 90° Line current I 1 Phase voltage E 1 ‐N Line current I 1 I (A) Phase voltage E 1 ‐N Phase angle E 1‐ N (°) Phase angle E 1‐ N (°) Line current I 1 I (A) Phase voltage E 1 ‐N (a) Firing angle α = 0°
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Voltage Regulation and Displacement Power Factor (DPF) in Thyristor Three-Phase Bridges Discussion 20 HVDC Transmission Systems Displacement power factor (DPF) The displacement power factor (DPF) is similar to the power factor (PF). The DPF indicates the amount of reactive power resulting from the phase shift between the fundamental-frequency component of the line current and the phase voltage, which is generally sinusoidal. The value of the DPF is equal to the cosine of the angle (phase shift) between the phase voltage and the fundamental-frequency component of the line current: ܦܲܨ ൌ cos φ (2) where: ܦܲܨ is the displacement power factor (pure number between 0.0 and 1.0). φ is the angle (phase shift) between the phase voltage and the fundamental-frequency component of the line current (°). The DPF can also be calculated using the following formula: ܦܲܨ ൌ ܲ ඥሺܲ ൅ ܳ (3) where: ܦܲܨ is the displacement power factor (pure number between 0.0 and 1.0).
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