Photovoltaic grid-connected system and control method therefor

A photovoltaic grid-connected system, comprising:

• a photovoltaic array, at least one photovoltaic inverter, a synchronous motor back-to-back drive system, a transformer and a control system;
• wherein
• the synchronous motor back-to-back drive system comprises a first synchronous motor and a second synchronous motor, and a rotor shaft of the first synchronous motor and a rotor shaft of the second synchronous motor are connected through a mechanical shaft;
• a direct current side of the at least one photovoltaic inverter is connected to a corresponding photovoltaic string in the photovoltaic array;
• an alternating current side of the at least one photovoltaic inverter is connected to a stator winding of the first synchronous motor;
• and
• a stator winding of the second synchronous motor is connected to a power grid through the transformer;
• wherein the control system is configured to execute a control method, the control method comprising:
• controlling the at least one photovoltaic inverter to operate in a maximum power point tracking (MPPT) current source mode after the photovoltaic grid-connected system is started;
• controlling the synchronous motor back-to-back drive system to operate in a generator mode;
• and
• controlling, in response to changing of a stator current of the first synchronous motor an excitation electromotive force of the first synchronous motor, to maintain voltage stability of the alternating current side of the at least one photovoltaic inverter, wherein the voltage stability of the alternating current side of the photovoltaic inverter is related to the stator current of the first synchronous motor and the excitation electromotive force of the first synchronous motor.