A method for operating a wind turbine generator connected to a power grid using direct-quadrature (d-q) control technology, the method comprising: measuring an alternating-current (a-c) quantity of the power grid ; converting the a-c quantity to a d-q quantity in a controller of the wind turbine generator ; characterized by : providing the d-q quantity to a d-q control loop within the controller , said d-q control loop comprising a first control path for determining a q-axis current reference I q * for regulating voltage of the wind turbine generator and a second control path for determining a d-axis current reference I d * for regulating torque of the wind turbine generator ; altering , with respective first and second symmetric control components , a respective transfer function of the first and second control paths of the d-q control loop such that the resulting transfer functions are similar to each other in the sub-synchronous frequency range; generating the q-axis current reference I q * and the d-axis current reference I d * for the wind turbine generator based on the altered transfer functions so as to achieve symmetric control of the wind turbine generator ; generating a control signal for the wind turbine generator based on the q-axis current reference I q * and the d-axis current reference I d *; and operating the wind turbine generator based on the control signal so as to stabilize sub-synchronous interaction of the wind turbine generator . measuring an alternating-current (a-c) quantity of the power grid ; converting the a-c quantity to a d-q quantity in a controller of the wind turbine generator ; characterized by : providing the d-q quantity to a d-q control loop within the controller , said d-q control loop comprising a first control path for determining a q-axis current reference I q * for regulating voltage of the wind turbine generator and a second control path for determining a d-axis current reference I d * for regulating torque of the wind turbine generator ; altering , with respective first and second symmetric control components , a respective transfer function of the first and second control paths of the d-q control loop such that the resulting transfer functions are similar to each other in the sub-synchronous frequency range; generating the q-axis current reference I q * and the d-axis current reference I d * for the wind turbine generator based on the altered transfer functions so as to achieve symmetric control of the wind turbine generator ; generating a control signal for the wind turbine generator based on the q-axis current reference I q * and the d-axis current reference I d *; and operating the wind turbine generator based on the control signal so as to stabilize sub-synchronous interaction of the wind turbine generator . providing the d-q quantity to a d-q control loop within the controller , said d-q control loop comprising a first control path for determining a q-axis current reference I q * for regulating voltage of the wind turbine generator and a second control path for determining a d-axis current reference I d * for regulating torque of the wind turbine generator ; altering , with respective first and second symmetric control components , a respective transfer function of the first and second control paths of the d-q control loop such that the resulting transfer functions are similar to each other in the sub-synchronous frequency range; generating the q-axis current reference I q * and the d-axis current reference I d * for the wind turbine generator based on the altered transfer functions so as to achieve symmetric control of the wind turbine generator ; generating a control signal for the wind turbine generator based on the q-axis current reference I q * and the d-axis current reference I d *; and operating the wind turbine generator based on the control signal so as to stabilize sub-synchronous interaction of the wind turbine generator . <EOS>