Pre‐charging strategy with damping control of isolated station connected to the modular multilevel converter based DC grid

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Abstract
It is vital of practical engineering significance to connect the isolated station into the DC grid smoothly without affecting other stations or triggering protective systems. To this end, an active charging method with damping control of the isolated station is proposed and will be used in a 500 kV‐level DC grid in China. First, the analytical expression of starting current is calculated. The key factors influencing the starting current are analysed to provide theoretic evidence for simplifying the calculation model. Second, the selecting expression of start‐up resistor is derived by simplifying the main circuit parameters, DC line parameters, control processes and so on. Third, the principle of active charging method is clarified, the selecting principle of decreasing slope and parameters of damping controller are presented, and the required time of charging process is calculated. This method does not need the start‐up resistor any more but still ensure no oscillation and impulse during the active charging process. Meanwhile, it is also faster than the traditional method with a better economy. Finally, electromagnetic transient simulation models are built to verify the correctness of the resistor selecting expression and the effectiveness of the active charging strategy with damping control.

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