Introduction to HVDC.pdf

G it is positive in the example of figure 1 8 the

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is always the same (e.g., it is positive in the example of Figure 1 8 ). The smoothing inductor and dc filter on the dc side of each thyristor three-phase bridge are used to help prevent current flowing through the bridges from becoming discontinuous at low power levels and limit current surges into the thyristor bridges. Figure 1 8 . Basic diagram of a monopolar HVDC transmission system. Operation of HVDC Transmission System s D ISCUSSION O UTLINE D ISCUSSION AC power network 1 Transformer Thyristor bridge 1 Thyristor bridge 2 AC power network 2 Transformer DC filter 1 DC filter 2 Smoothing inductor 1 Smoothing inductor 2 DC transmission line Converter station 1 Converter station 2 I dc E dc E dc 1. 2. 3.
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Basic Operation of HVDC Transmission Systems 23 HVDC Transmission Systems The inductance of the transmission line and the smoothing inductors and dc filters can be represented by a single equivalent inductance, X L . Therefore, the diagram of Figure 1 8 can be simplified, as shown in Figure 19 . a From now on, the thyristor bridges in converter stations 1 and 2 will be referred to as ‘’thyristor bridge 1’’ and ‘thyristor bridge 2’’, respectively. Figure 19 . Simplified diagram of a monopolar HVDC transmission system. As previously mentioned, dc current flow in the transmission line of Figure 20 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 Figure 19 and Figure 20). However, the polarity of the voltage at the dc side of each thyristor bridge can be positive or negative (i.e., each bridge can act either as a rectifier or an inverter), depending on the firing angles of the thyristor bridges. Consequently, power flow on the transmission line can be in either direction (i.e., from station 1 to station 2, or from station 2 to station 1), as explained below. When the firing angles of the thyristor bridges are adjusted to make thyristor bridge 1 operate as a rectifier and thyristor bridge 2 operate as an inverter, the voltage E dc at the dc side of thyristor bridge 1 is of positive polarity. The voltage at the dc side of thyristor bridge 2 is also of positive polarity even if it operates as an inverter because it is reverse connected with respect to bridge 1 (i.e., the common cathode of bridge 2 is connected to the common anode of bridge 1). Therefore, power of positive polarity flows from station 1 to station 2. When the firing angles of the thyristor bridges are adjusted to make thyristor bridge 1 operate as an inverter and thyristor bridge 2 operate as a rectifier, the voltage at the dc side of thyristor bridge 1 is of negative polarity. The voltage at the dc side of thyristor bridge 2 is also of negative polarity even if it operates as an inverter because it is reverse connected with respect to bridge 1 (i.e., the common cathode of bridge 2 is connected to the common anode of bridge 1). Therefore, power of negative polarity flows from station 2 to station 1.
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