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farm, which will lead to the mutual influence between two systems. So this
integration method is used in offshore wind farm system of small capacity and
short transmission distance.
2.2 VSC-HVDC Integration Mode
VSC-HVDC technology is a new type of transmission technology in recent years,
its core was the use of insulated gate bipolar transistor (IGBT) of VSC. Con-
verter stations based on the power supply type DC transmission system structure
shown in Figure 2, voltage source inverter used in both sides of VSC-HVDC, ca-
pacitance paralleled on DC side can provide voltage support, buffer cliffs impact
of the shutdown current, reducing the role of the DC side harmonics.
Fig. 2.
VSC-HVDC transmission system structure
Advantages of offshore wind farms using VSC-HVDC transmission:
(1) VSC can both absorb reactive power generated by wind farm and can
provide reactive power to the wind turbine to provide dynamic reactive power
compensation. Wind turbine without load generally supplies reactive power di-
rectly by the power factor correction capacitors in parallel with the wind turbine
side. On the opposite, in the case of wind turbine with load, corresponding re-
active power required in accordance with the size of the active power output of
wind turbines, this dynamic reactive power can be provided by the port to the
bus side of VSC.
(2) Frequency control to achieve maximum wind energy capture. By the VSC
control of the bus side when the wind turbine with variable speed control, in
according with the wind speed to adjust to the wind turbine speed, it can achieve
maximum wind energy capture.
(3) Good flexible power characteristics. VSC-HVDC can control the power
exchange of sending end and receiving end, any change of the wind energy can
be detected through the control system, so that the input and output power of
the generator would soon reach equilibrium.
(4) Capability of black start. VSC technology of HVDC lines with induction
motor reverse so that the wind turbine start-up and connected with a black
network.
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