Introduction to HVDC.pdf

P is the active power w q is the reactive power var

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P is the active power (W). Q is the reactive power (var). As shown in the equation, the DPF is a pure number with a value between 0.0 and 1.0. For instance, the higher the DPF, the lower the amount of reactive power exchanged between a thyristor three-phase bridge and the ac power source to which it is connected, and thus the more efficient the transfer of active power between the source and bridge. Note that when the waveform of the line current is sinusoidal (i.e., no harmonics), the DPF has the same value as the power factor (PF). When harmonics are present in the line current, the DPF is not equal to the power factor and the reactive power ( S ) is not equal to ඥሺܲ ൅ ܳ . Static var compensators (SVCs) are commonly used in ac power systems to achieve reactive power compensation and operate at unity DPF and high efficiency. For detailed information on SVCs, refer to the Lab-Volt manual Static Var Compensator , part number 86370. Relation between the DPF and the firing angle of a thyristor three-phase bridge Figure 16 shows the relation between the DPF and the firing angle of a thyristor three-phase bridge, for a given current flowing through the thyristor bridge. As the figure shows, the DPF is close to the maximum value (1) when the firing angle is 0° or 170°, and it is minimum (zero) when the firing angle is 90°. The value of the DPF decreases rapidly when the firing angle α passes from 0° to 90° or from 170° to 90°.
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Voltage Regulation and Displacement Power Factor (DPF) in Thyristor Three-Phase Bridges Discussion HVDC Transmission Systems 21 Figure 16. Displacement power factor (DPF) versus the firing angle α , for a given current flowing through a thyristor three-phase bridge. Figure 17. Thyristor three-phase bridges are used in converter stations at the ends of HVDC power transmission lines (photo courtesy of ABB). 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 Firing angle α ሺ °) Displacement power factor (DPF)
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HVDC Transmission Systems 22 The Discussion covers the following points: Fundamentals of HVDC transmission systems Adjustment of the system operating point Effects of ac line voltage fluctuations at the converter stations on the operating point of an HVDC transmission system Fundamentals of HVDC transmission systems Figure 1 8 shows a basic diagram of an HVDC transmission system. The system consists of two converter stations interconnected by a transmission line to allow dc current flow between the stations. Each converter station consists of a thyristor three-phase bridge whose ac side is connected to the ac power network through a transformer, and whose dc side is connected to the transmission line via a smoothing inductor and a dc filter. The thyristor three-phase bridges in converter stations 1 and 2 are connected in opposite directions to permit dc current flow in the direction indicated by the arrowheads in the thyristor bridge symbols. Therefore, dc current flow on the transmission line is always from station 1 to station 2, and the polarity of the line current I Line is always the same (e.g., it is positive in the example of
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