Introduction to HVDC.pdf

A common solution for this problem is to keep the

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A common solution for this problem is to keep the firing angle of the inverter bridge near the maximum value (e.g., 165°), and instead reduce the voltage at the ac side of this bridge with a step-down transformer , as Figure 2 3 shows. Figure 2 3 . The voltage at the ac side of the inverter bridge (bridge 2) is reduced by means of a step-down transformer. AC power network 2 Thyristor bridge 1 Thyristor bridge 2 AC power network 1 Step-down transformer Rectifier Power flow Transformer Inverter Inverter Rectifier Smoothing inductors and transmission line X L Converter station 1 Converter station 2
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Basic Operation of HVDC Transmission Systems Discussion 29 HVDC Transmission Systems In Figure 2 4 , for example, the voltage at the ac side of the inverter bridge (bridge 2) is reduced to obtain a 7% decrease in the inverter bridge dc voltage. This lowers the voltage-versus-current relation of the inverter bridge [ α ൌ 165° ሺ‐7%ሻ ]. The voltage-versus-current relations of the rectifier and inverter bridges intersect at a point providing the maximum dc current value (1.0 pu) of the HVDC transmission system with the firing angles of the rectifier and inverter bridges set to 0° and 165°, respectively; this results in a DPF virtually equal to 1 at both bridges. Figure 2 4 . Reducing the voltage at the ac side of the inverter bridge allows an HVDC transmission system to operate at the maximum dc current value (1.0 pu) while keeping the firing angles of the rectifier and inverter bridges close to 0° and 165°, and thus the DPF at both bridges close to 1. Operating point Bridge dc current, I dc (pu) Bridge dc voltage, E dc [1 pu ൌ1.35 ൈ Line voltage ܧ ௅ି௅ ] Inverter bridge (bridge 2) α ൌ 165° ሺെ7%ሻ Inverter bridge (bridge 2) α ൌ 165° I dc 1.0 pu Rectifier bridge (bridge 1) α ൌ 0°
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Basic Operation of HVDC Transmission Systems HVDC Transmission Systems 30 The value of current I dc in Figure 2 4 can be adjusted (i.e., decreased) to the desired value by varying the firing angle of the rectifier bridge from 0° up to approximately 25°. In most cases, this provides a sufficient variation range of current I dc (i.e., from the limit of continuous current flow to the maximum dc line current) while keeping the DPF above 0.9 at the rectifier bridge. Figure 2 5 shows the operating points obtained when the firing angle of the rectifier bridge is set to 0° and when this firing angle is increased to 22°. Figure 2 5 . The value of current I dc can be adjusted (i.e., decreased) to the desired value by varying the firing angle of the rectifier bridge from 0° up to approximately 25°. The operating point obtained when the firing angle of the rectifier bridge is set to 0° provides the maximum dc current (1.0 pu), as Figure 2 5 shows. The operating point obtained when the firing angle of the rectifier bridge is increased Bridge dc current, I dc (pu) Bridge dc voltage, E dc [1 pu ൌ1.35 ൈ Line voltage ܧ ௅ି௅ ] Operating point with α Rectifier Inverter bridge (bridge 2) α ൌ 165° ሺെ7%ሻ Inverter bridge (bridge 2) α ൌ 165° I dc 1.0 pu Rectifier bridge (bridge 1) α ൌ 0° Rectifier bridge (bridge 1) α ൌ 22° Operating point with α Rectifier 22° I dc 0.3 pu
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