Introduction to HVDC.pdf

The minimum current value required to ensure

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the minimum current value required to ensure continuous current flow at firing angle 90° is approximately 0.2 pu. Generally, the operating point of a thyristor bridge is set in order to keep the dc current above 0.25-0.3 pu to ensure continuous current flow. α ൌ 30° α ൌ 15° α ൌ 0° α ൌ 45° α ൌ 60° α ൌ 75° α ൌ 90° α ൌ 105° α ൌ 120° α ൌ 135° α ൌ 150° α ൌ 165° Bridge dc current, I dc (pu) Bridge dc voltage, E dc [1 pu ൌ1.35 ൈ Line voltage ܧ ௅ି௅ ] α ൌ 170° Voltage Regulation and Displacement Power Factor (DPF) in Thyristor Three-Phase Bridges -Discussion
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Voltage Regulation and Displacement Power Factor (DPF) in Thyristor Three-Phase Bridges Discussion HVDC Transmission Systems 17 Harmonics and reactive power in thyristor three-phase bridges Figure 14 shows the waveform of one of the line currents ( I 1 ) at the ac side of a thyristor three-phase bridge, when the firing angle α is 0°. It is assumed that the current at the dc side of the thyristor bridge is smoothened perfectly by an inductor of infinite inductance value. The waveform of line current I 1 is composed of two rectangular pulses of equal duration per cycle of phase voltage E 1‐ N , separated by an interval during which the current is null (zero). Thus, line current I 1 consists of a pulse of positive polarity having a width of 120°, followed by a 60° interval during which the current is null, followed by a pulse of negative polarity having a width of 120°, followed by another 60° interval during which the current is null. Since the conduction of the thyristors is not delayed (i.e., the firing angle is 0°), the rising edge of the positive pulse in line current I 1 occurs 30° after the beginning of the positive half of phase voltage E 1‐ N . Figure 14. Line current I 1 at the ac side of a thyristor three-phase bridge (firing angle α set to 0°). Since the waveform of line current I 1 is rectangular (i.e., non-sinusoidal), it contains harmonics (i.e., sinusoidal current components at frequencies equal to integer multiples of the fundamental frequency, which is the ac power network frequency). These harmonics produce undesirable effects in the ac power network. As long as the waveform of line current I 1 remains the same, the harmonics produced keep constant amplitudes and frequencies, so that it is possible to eliminate them almost entirely using carefully selected filters. Harmonics filters and the underlying theory, however, are beyond the scope of this manual. Phase voltage E 1 ‐N Phase angle E 1‐ N (°) Line current I 1 E (V) I (A) 120° 120° 60° 60° Waveform obtained when the current at the dc side of the thyristor bridge is not perfectly smoothened (shown for comparison) Phase angle E 1‐ N (°)
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Voltage Regulation and Displacement Power Factor (DPF) in Thyristor Three-Phase Bridges Discussion 18 HVDC Transmission Systems In addition to being non-sinusoidal, the waveform of line current I 1 is not aligned (in phase or phase shifted 180°) with phase voltage E 1‐ N when the firing angle α is not 0° or 180°. For example, Figure 15 shows the waveform of line current
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