Automatic under-frequency load shedding mal-operation in power systems with high wind power penetration

Abstract

Countries with a limited interconnection capacity suffer substantial frequency variations after large incidents so they use automatic under-frequency load shedding schemes to arrest the frequency decay. Some of these countries such as Portugal, Spain and Ireland also have very high wind penetrations. This can cause additional frequency excursions due to generation time variability but also to the fact that variable speed wind turbines do not add directly their inertia to the power system. Thus several transmission system operators have announced new grid codes requiring wind turbines to provide frequency response. In some scenarios, however, wind energy support may be detrimental to frequency control because it generates an extra energy that reduces decay and derivative but that cannot be maintained over time. These lower values of frequency decay and derivative are currently expected after a reduced incident or when conventional generation, which can maintain the extra generation, provides frequency support, so lead to low or no load shedding. This paper has studied, in particular, the effect of wind generation emulating inertia. A reduction of frequency derivative is achieved, which looks positive at first, but in some cases leads to initial smaller load shedding than the incident requires. A reduced frequency derivative triggers less under-frequency relays as if there were a significant amount of conventional generation that is online. However, this generation has been substituted by wind generation emulating inertia, and as it can maintain extra generation over time, the frequency continues to decay until the shedding of the next load step. As a result there is an excessive frequency deviation and an incorrect load shedding for the magnitude of the initial disturbance. In order to prevent this problem, automatic under frequency load shedding settings may need readjustment when a large amount of wind generation provides frequency support.