Electromechanical Oscillation Mode Estimation And Analysis Via Subspace Identification Methods

As the intrinsic nature of inter-connected power systems,electromechanical oscillation limits the power transfer capacity.Blackouts due to electromechanical oscillations worldwide recently show the potential hazards that these kind of lightly damped modes can bring to the whole power grid.As such,stability analysis of electromechanical oscillations is of great importance.Over the past decades,many efforts have been focused on the analysis based on first-principle modeling and some important properties of these modes are presented.This is very informative and helpful in system design and static assessment.However,these methods face great challenges when they are used to model the real time changes of power grids in the context of inter-connected power systems.Owing to the development of measurement technology,measurement-based methods are preferred to estimate the modal properties.Among them,Subspace Identification Methods(SIMs)are widely applied and investigated.However,there are still some problems to be discussed and solved.In this thesis,three problems in the estimation procedure based on SIMs are discussed and solutions are given.They are as follows:(1)By analysis and comparison,two main factors,the sampling rate and the input probing signal,affecting the accuracy of estimates are investigated.And then,methods for improving the accuracy are proposed.Results show that high sampling rate decreases the accuracy of the corresponding estimates of lightly damped electromechanical modes with close poles and that input probing signals can obviously improve the accuracy.Methods proposed for improving accurac'y such as decreasing sampling rate and probing a small amplitude,long-term periodic input signal with more low-frequency energy can magnificiently improve the accuracy of estimates.(2)A mode matching method is proposed for establishing the relation between the estimated modes and the modes from the physical model during the procedure of measurement based methods.By this method based on the mode shape which will be approximately unchanged when the corresponding pole shifts and reflects the physical concepts of an oscillation mode,one can determine the estimates of the mode of interest by matching the groups of mainly against-swinging generator(s).Numerical results illustrate by the proposed method plus the aggregation methods,one can match the corresponding modes accurately and simply.(3)Identifying the dominant path for power transfer of electromechanical oscillation mode.Finding out the power transfer path in the whole power grid can help understand the nature of the oscillation and find out the key transfer lines and subsequently design the damping controller.In this thesis,a method for finding out the dominant path based on the point of view of power transfer is proposed.By establishing the relation between power transfer and modal analysis,one can find out the dominant path starting with the mainly participating generator(s).The brevity and accuracy of the method are illustrated by the numerical results.