measuring real-time measured case temperature by measuring apparatus;

calculating a thermal resistance and a thermal capacitance corresponding to each layer inside the power device, and establishing a corresponding Cauer thermal network model;

calculating power consumption at n future moments according to state variables at the n future moments corresponding to a minimized cost function, to obtain a variation trajectory of a heat flux parameter, comprising:

calculating an equivalent thermal impedance of the power device by using thermal resistance values and thermal capacitance values of the all layers of the Cauer thermal network model; and

calculating a variation trajectory of the junction temperature of the power device by using the real-time measured case temperature, the calculated equivalent thermal impedance, and the calculated variation trajectory of the heat flux parameter;

calculating a variation trajectory of a junction temperature of the power device according to the Cauer thermal network model and the variation trajectory of the heat flux parameter; and

comparing the calculated variation trajectory of the junction temperature with a warning temperature, and determining a current operating state of the power device, comprising:

if the variation trajectory of the junction temperature does not exceed the warning temperature, applying an optimal state corresponding to a minimized cost function to the offshore wind power converter to proceed with normal operation;

if the variation trajectory of the junction temperature exceeds the warning temperature beyond a warning period, performing warning in advance for a moment at which the variation trajectory of the junction temperature exceeds the warning temperature, and still applying the optimal state corresponding to the minimized cost function to the offshore wind power converter; and

if the variation trajectory of the junction temperature exceeds the warning temperature within the warning period, blocking a pulse signal, and controlling a switch state, a voltage, or a current of the power device, to protect the power device.