Connection mode of transformer in Xiamen flexible DC transmission project

The main wiring of flexible DC transmission system adopts two-level and three-level converters. Usually, flexible DC transmission system adopts DC neutral grounding, while modular multi-level flexible DC transmission system generally adopts AC grounding.

Whether it is a two-level or three-level converter with DC neutral grounding or a modular multilevel converter with AC grounding, the flexible DC transmission system is a unipolar symmetrical system. During normal operation, there will be no working current flowing through the grounding, and there is no need to set up a special grounding electrode. When the DC line or converter fails, the whole system will not be able to continue running. In addition, a monopole asymmetric structure can be formed through the earth or metal loop, which is similar to one pole of the traditional HVDC transmission system.

Under the same system parameters, compared with the unipolar symmetric system, the voltage level of the converter valve in the unipolar asymmetric system is twice as high as that in the unipolar symmetric system, and the asymmetry on the DC side will also increase the voltage level on the AC side of the converter.

In order to improve the power capacity and voltage level of flexible DC transmission system and meet the requirements of UHV and long-distance high-power transmission, the converter in unipolar converter station can also be composed of several small-capacity converter units in series and parallel. As shown in figure 1, two unipolar asymmetric systems connected in series can also form a bipolar symmetric system similar to traditional HVDC transmission.

The transformer of bipolar system needs to bear DC bias voltage caused by DC voltage asymmetry. Different from the conventional DC transformer, the transformer does not need to bear the harmonic components generated by the converter station at this time. At present, the main reason why flexible DC transmission system adopts unipolar structure is to reduce the incidence of DC side faults, and most flexible DC transmission projects use cables as transmission loops. In this way, the reliability of using a single converter is relatively higher and the engineering cost is reduced. For multi-terminal flexible DC transmission system, the connection mode of the system is generally parallel to ensure that the converters work at the same DC voltage level. Parallel multi-terminal flexible DC network can be divided into two basic structures: star and ring. Other complex structures can be regarded as extensions and combinations of these two structures. Figure 2 shows four topologies respectively.

Fig. 2 Typical connection mode of multi-terminal DC transmission system Parallel converter stations run at the same level of DC voltage, and power distribution is realized by changing the current of each converter station; Series converter stations operate at the same DC current level, and power distribution is realized by changing DC voltage. The combination of parallel connection and series connection increases the flexibility of multi-terminal DC connection. Compared with series connection, parallel connection has smaller line loss, wider regulation range, easier insulation coordination, more flexible expansion mode and outstanding economy, so all the multi-terminal DC transmission projects that have been operated at present adopt parallel connection mode.