A method for operating a wind turbine having an electrical system, the electrical system comprises a permanent magnet generator (130, 300, 400) having a rotor (310, 410) and a stator (320, 420), a power electronic device (170, 180, 190, 200) electrically connected to the permanent magnet generator, wherein the power electronic device and a circuit breaker are electrically disposed in series between the permanent magnet generator and a grid, and a controller adapted to control the generator (130, 300, 400) and the circuit breaker, wherein the permanent magnet generator has permanent magnets (312, 314, 412, 414) and a generator winding (322, 324, 326, 422, 424, 426) into which a voltage is induced by the permanent magnets when the rotor is rotating, the method comprising: generating a signal for tripping the circuit breaker based on a field modification that is modifying the field of the permanent magnets (312, 314, 412, 414) of the permanent magnet generator and on a rotational dynamic of the rotor of the permanent magnet generator, wherein a temperature of permanent magnets (312, 314, 412, 414) of the generator (130, 300, 400) is determined based on the rotational dynamic of the rotor and the field modification, wherein a magnet wheel voltage (U p) is calculated using the temperature of the permanent magnets (312, 314, 412, 414), wherein the magnet wheel voltage is a voltage induced by the magnetic flux of the permanent magnets, and wherein the signal for tripping the circuit breaker is generated if the magnet wheel voltage (U p) exceeds a predetermined threshold voltage, and characterized by setting a critical over speed limit for the wind turbine adaptively, dependent on the temperature of the permanent magnets. a permanent magnet generator (130, 300, 400) having a rotor (310, 410) and a stator (320, 420), a power electronic device (170, 180, 190, 200) electrically connected to the permanent magnet generator, wherein the power electronic device and a circuit breaker are electrically disposed in series between the permanent magnet generator and a grid, and a controller adapted to control the generator (130, 300, 400) and the circuit breaker, wherein the permanent magnet generator has permanent magnets (312, 314, 412, 414) and a generator winding (322, 324, 326, 422, 424, 426) into which a voltage is induced by the permanent magnets when the rotor is rotating, the method comprising: generating a signal for tripping the circuit breaker based on a field modification that is modifying the field of the permanent magnets (312, 314, 412, 414) of the permanent magnet generator and on a rotational dynamic of the rotor of the permanent magnet generator, wherein a temperature of permanent magnets (312, 314, 412, 414) of the generator (130, 300, 400) is determined based on the rotational dynamic of the rotor and the field modification, wherein a magnet wheel voltage (U p) is calculated using the temperature of the permanent magnets (312, 314, 412, 414), wherein the magnet wheel voltage is a voltage induced by the magnetic flux of the permanent magnets, and wherein the signal for tripping the circuit breaker is generated if the magnet wheel voltage (U p) exceeds a predetermined threshold voltage, and characterized by setting a critical over speed limit for the wind turbine adaptively, dependent on the temperature of the permanent magnets. generating a signal for tripping the circuit breaker based on a field modification that is modifying the field of the permanent magnets (312, 314, 412, 414) of the permanent magnet generator and on a rotational dynamic of the rotor of the permanent magnet generator, wherein a temperature of permanent magnets (312, 314, 412, 414) of the generator (130, 300, 400) is determined based on the rotational dynamic of the rotor and the field modification, wherein a magnet wheel voltage (U p) is calculated using the temperature of the permanent magnets (312, 314, 412, 414), wherein the magnet wheel voltage is a voltage induced by the magnetic flux of the permanent magnets, and wherein a temperature of permanent magnets (312, 314, 412, 414) of the generator (130, 300, 400) is determined based on the rotational dynamic of the rotor and the field modification, wherein a magnet wheel voltage (U p) is calculated using the temperature of the permanent magnets (312, 314, 412, 414), wherein the magnet wheel voltage is a voltage induced by the magnetic flux of the permanent magnets, and wherein the signal for tripping the circuit breaker is generated if the magnet wheel voltage (U p) exceeds a predetermined threshold voltage, and characterized by setting a critical over speed limit for the wind turbine adaptively, dependent on the temperature of the permanent magnets.