Introduction to HVDC.pdf

Basic operation of hvdc transmission systems hvdc

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Basic Operation of HVDC Transmission Systems HVDC Transmission Systems to 22° provides a lower dc current of approximately 0.3 pu, which is generally close to the limit of continuous current flow. Operating an HVDC transmission system with a lowered ac voltage at one bridge (bridge 2 in Figure 2 4 and Figure 2 5 ) works well when power flows toward this bridge. However, this provides unsatisfactory operation when the direction of power flow is reversed; i.e., when the bridge operating at a lowered ac voltage (bridge 2 in our example) acts as a rectifier and the bridge operating at a normal ac voltage (bridge 1 in our example) acts as an inverter. Figure 2 6 shows the voltage-versus-current relations of both bridges and the operating points at the maximum dc current (1.0 pu) for both directions of power flow. Figure 2 6 . When the bridge operating at a lowered ac voltage acts as a rectifier, the firing angle required at the inverter bridge results in a rather low DPF at this bridge. Bridge dc current, I dc (pu) Bridge dc voltage, E dc [1 pu ൌ1.35 ൈ Line voltage ܧ ௅ି௅ ] I dc 1.0 pu Operating point B Rectifier bridge (bridge 2) α ൌ 0° ( െ7% ) Rectifier bridge (bridge 2) α ൌ 0° Inverter bridge (bridge 1) α ൌ 145° (Power flows from bridge 2 to bridge 1) (Power flows from bridge 1 to bridge 2) Operating point A Inverter bridge (bridge 2) α ൌ 165° ሺെ7%ሻ Inverter bridge (bridge 2) α ൌ 165° Rectifier bridge (bridge 1) α ൌ 0° 31
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Basic Operation of HVDC Transmission Systems HVDC Transmission Systems 32 When power flows toward the bridge operating at the lowered ac voltage (bridge 2 in our example) and the system operating point (point A in Figure 2 6 ) is at the maximum dc current (1.0 pu), the firing angles of the rectifier and inverter bridges (bridges 1 and 2 in our example) are 0° and 165°, respectively. Consequently, this provides a DPF virtually equal to 1 at each bridge. However, when power flows toward the bridge operating at the normal (nominal) ac voltage (bridge 1 in our example) and the system operating point (point B in Figure 2 6 ) is at the maximum dc current (1.0 pu), the firing angles of the rectifier and inverter bridges (bridges 2 and 1, respectively) are 0° and only 145°, respectively. Consequently, this provides a DPF virtually equal to 1 at the rectifier bridge, but results in a very low DPF (about 0.82) at the inverter bridge. This problem can be solved by using a multiple-tap transformer with the ability to reduce the ac network power voltage in each converter station, as Figure 2 7 shows. The ac voltage feeding the thyristor bridge in the rectifier station is adjusted to 100% (transformer voltage ratio of 1:1), while the ac voltage feeding the thyristor bridge in the inverter station is reduced to the desired level by selecting the appropriate tap on this station’s transformer. Figure 2 7 . Each converter station in an HVDC transmission system is generally equipped with a multiple-tap transformer with the ability to reduce the ac power network voltage applied to the ac side of the thyristor bridge.
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