Detection of islanding state in electricity network

A system for detecting a three-phase islanding state in a three-phase electricity network, the system comprising:

• a power supply assembly for supplying power with an output frequency into a three-phase electricity network;
• and
• control means configured to:
• form a frequency reference signal;
• control the output frequency of the power supply assembly with the frequency reference signal, the frequency reference signal being adapted to deviate the output frequency of the power supply assembly from a grid frequency representing a frequency of a common electricity network whose part the three-phase electricity network is in normal operating conditions;
• monitor a magnitude of a frequency quantity obtainable from the output frequency of the power supply assembly;
• and
• detect a three-phase islanding state in the electricity network if the magnitude of the frequency quantity is outside an allowable value range,
• wherein the control means is configured to form a stimulus signal obtainable by multiplying a difference between the output frequency of the power supply assembly and the grid frequency by a stimulus coefficient,
• wherein forming the frequency reference signal includes addition of the output frequency of the power supply assembly and the stimulus signal, and during a normal operating state of the electricity network the stimulus coefficient is a function of an active output current of the power supply assembly.