Introduction to HVDC.pdf

On the other hand when the ac line voltage decreases

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On the other hand, when the ac line voltage decreases slightly at the rectifier station or increases slightly at the inverter station, the dc current flowing through the transmission line decreases significantly, as Figure 3 0 and Figure 3 1 show. In that case, however, it is not possible to compensate for a decrease in ac line voltage at the rectifier bridge below the nominal value or an increase in ac line voltage at the inverter bridge above the nominal value, because the firing angle of the rectifier bridge, which is initially set to 0°, is already at the value which causes the rectifier bridge to produce the maximum possible dc voltage. One
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Basic Operation of HVDC Transmission Systems 35 HVDC Transmission Systems way of solving this problem is to set the initial firing angle of the rectifier bridge to approximately 18° and allow variation of this angle between 0° and 25°. This allows compensation for line voltage fluctuations of up to 4.5% of the nominal value at the rectifier or inverter station, while keeping the DPF above 0.9. Figure 3 0 . Effect of ac line voltage fluctuations at the rectifier bridge on the operating point (dc current) of an HVDC transmission system. Original 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 I dc 0.57 pu Rectifier bridge (bridge 1) α ൌ 18°
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Basic Operation of HVDC Transmission Systems HVDC Transmission Systems 36 Figure 3 1 . Effect of ac line voltage fluctuations at the inverter bridge on the operating point (dc current) of an HVDC transmission system. When the ac line voltage at either of the converter stations fluctuates significantly, its value can vary so far from the nominal voltage value that compensating the effect of this voltage fluctuation on the system operating point (i.e., the dc current flowing through the line) through readjustment of the firing angle at the rectifier bridge while maintaining a good DPF value at the rectifier bridge becomes no longer possible. In this case, the selected tap of the transformer in the converter station where the fluctuation occurred must be changed. Original 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 I dc 0.57 pu Rectifier bridge (bridge 1) α ൌ 18°
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