Method for controlling doubly-fed machine

A method in connection with a doubly-fed machine, the machine comprising a stator, which is connected to a power network, and a rotor, which is connected to the power network through an inverter, the method comprising the steps of:

• determining a rotational speed (nact) of the machine,
• forming a rotational speed reference (nref) for the machine,
• measuring network voltage,
• measuring network current, and
• calculating network active power (Pact) and network reactive power (Qact) from the network voltage and current,
• calculating a shaft torque (T) of the machine on the basis of the active power (Pact) and the rotating speed-(nact),
• forming a frequency reference (Fref) for the inverter with a control circuit on the basis of the determined machine rotating speed (nact), rotating speed reference (nref) and shaft torque (T), a pole pair number of the machine and a network frequency,
• forming a reactive power reference (Qref) for the machine,
• forming an Ir compensation reference (IRref) for the inverter with the control circuit on the basis of the reactive reference (Qref) and the reactive power (Qact), and
• controlling the inverter to produce voltage for the rotor of the machine on the basis of the formed frequency reference (Fref) and the IR compensation reference (IRref).