Power System Oscillations - Detection, Estimation and Control

University dissertation from Department of Industrial Electrical Engineering and Automation, Lund Institute of Technology

Abstract: The topic of this thesis is the electro-mechanical oscillations which to some extent always are present in a power system. The demand of electric power is ever increasing. At the same time, the tolerance of disruptions in the power supply is decreasing. The deregulated market together with distributed generation have then pushed the system to operate during circumstances for which it was not designed. To this we can then add that getting concessions for new lines becomes more and more difficult in densly populated areas. All these factors makes the electric power system operate with smaller safety margins. Decreasing these margins limits with sustained availability is achieved by the application of advanced monitoring and control methods. This thesis deals with this in several time-scales. When a large oscillation occurs it is important to detect it as fast as possible as the remedial action depends on if the current operating is due to a fault or an oscillation. In the thesis, a new method to distinguish these incidents from each other is presented. In a slower time-scale it is important to monitor the dynamics of the electro-mechanical modes. This information can be used to verify that simulations correspond to the real world behaviour. Real-time methods can also be used to alarm operators or arm special protection schemes if the power system enters undesired operating conditions. A number of methods are studied and then evaluated on three case studies. Finally, load modulation for damping enhancement is studied. It has previously been shown that modulation of active power at the transmission level increases the damping of the power system when correctly performed. Loads suitable for load modulation at the transmission level are rare and using actuators at the distribution level creates new problems. It is shown that it is possible to detect poor damping at the distribution level, thus reducing the need of communications. It is also shown how the variation of active and reactive power at the transmission level, caused by modulation of active power at the distribution level can be estimated without knowledge of the complete distribution network. This is most important as the active and reactive variations counteract each other. Finally load modulation at distribution level is evaluated on two test systems. It is shown that the damping is increased and that the influence of the reactive variation decreases the performance of load modulation. The degradation of the control scheme is, however, small in the studied cases.