System dynamics of asynchronous generators at islanded grid operation

Abstract

It is well known that the phenomenon of self-excitation allows the asynchronous generator to operate as a standalone unit. The robustness and low cost of the asynchronous generator makes it beneficial for small hydro power plants below 1 MW. Investigation of the self-excitation process shows that significant overvoltages can occur if a generator with sufficient capacitors is suddenly disconnected from the utility grid. The precondition for a successive voltage build-up is that the generator is left with enough capacitive power and a low load after the disconnection. The Lønnestad radial in Seljord, Norway, is a distribution radial with both asynchronous and synchronous generators connected. In order to investigate the system dynamics in the radial after it is disconnected from the rest of the 22 kV distribution grid, the radial was modeled and simulated in Dymola. From the simulations it was seen that the cables in the grid represent enough capacitive power to initiate self-excitation of the seven generators in Sagbekken 1, and Sagbekken 2 and 3 momentarily after the radial is brought into islanded operation. Since the amount of load connected to the radial is low, the self-excitation leads to a successive voltage build-up, resulting in a harmful overvoltage. For simulations with a load connected to the radial, the overvoltage reached its peak value of circa 50 kV only 0.4 seconds after the asynchronous generators are left in standalone operation. To avoid voltage build-ups caused by self-excitation it is essential with proper parameters in the protection relays. It is recommended to have monetarily disconnection when the voltage exceeds a given limit slightly above the nominal voltage. In addition to the protection relays, a damping load can be installed in Seljord substation to momentarily connect when the radial is disconnected from the rest of the distribution grid.