Online testing of generating units on their ability to regulate frequency during restoration of an islanded grid

This paper describes a method for testing of frequency control capability in island operation. The method and equipment associated with performing the tests are called SolvSim Power Station or SSPS and allows the generating unit to be tested for island operation capability without being disconnected from the grid. It has been used in Sweden for the last decade at over 50 generating units and many experiments have been made regarding dynamic performance during these tests. Many generating units suffer from unexpected malfunctions or nonlinearities that cause the islanded grid to become unstable and that would not have been identified without tests. It is suggested that PID controllers are used even during droop control and that the gate opening signal is used as feedback for better stability. The extremely nonlinear behavior of limiters is automatically considered using SSPS leading to superior reliability of the test results as opposed to simulations.