低頻振盪屬於電力系統動態穩定度的範圍，而動態穩定度為在持續微小干擾之情況下電力系統的響應特性。台電系統於民國73年首次出現低頻振盪現象，影響供電品質與系統運轉安全。台電公司於增設電力系統穩定器之後，此類自發性低頻振盪已不復見。目前台電系統所觀測到的皆為大型擾動所引起之低頻振盪，由於大型擾動所引發的低頻振盪波形內含直流偏移成份，傳統之低頻振盪監測方法或振盪參數計算方法係採用加窗型傅立葉轉換法，該法僅適用於自發性低頻振盪，並不適用於直流偏移之低頻振盪波形，遂有必要發展方法監測或分析之。
本論文提出兩套演算法處理可以計算含有直流偏移量的低頻振盪波形參數，第一套演算法係使用離散小波轉換法濾除低頻振盪波形內的直流偏移成份，再利用加窗型傅立葉轉換法計算阻尼常數，如此，得以擴大傳統加窗型傅立葉轉換法的應用範圍；然而該法因須重覆執行小波濾波，以致計算時間頗長且計算程序亦相當複雜。針對這些缺點，本論文提出第二套演算法，稱為四分點演算法，乃係利用低頻振盪波形內四個時點的特性簡化低頻振盪參數的計算流程。四分點演算法算式簡單，對於含有定值型以及衰減型的直流偏移成分的低頻振盪波形，不須經由濾波過程即可在一個振盪週期內分析出低頻振盪參數，大幅縮短計算時間，並簡化計算流程，未來用於線上監測深具潛力。

Low frequency oscillation is a power system dynamic stability problem which describes the response feature of power system under sustained minor disturbances. The low frequency oscillation phenomenon was first observed in the Taipower system in year 1984, which affected power supply quality and system operation safety. This type of self-excited low frequency oscillation has then disappeared since Taipower installed power system stabilizers. Presently the low frequency oscillations observed are all those excited by system’s large disturbances. Because the oscillations excited by large disturbances are contaminated with dc offsets, the conventional low frequency oscillations monitored or oscillation parameter evaluation method, referring to the windows-Furier transform which has well suited the self-excited low frequency oscillation, is however not adapted to the oscillation contaminated with dc.
It is thus highly necessitated to develop monitoring or evaluation method for the dc contaminated oscillation. This thesis presents two approaches to evaluate the damping constant and oscillation frequency of the low frequency oscillation waves which are contaminated with either constant or decaying dc offsets the first approach uses discrete wavelet transform to filt out dc offset, and then calculate the damping constant by the windows-Furier transform so to extend the application of conventional windows-Furier transform. However the repetitive filters by the wavelet lengthens the computational time and increases the calculator complexity. To overcome these difficulties, the research develops the second approach called the four quadrants method, which makes use of the feature at four quadrantal points of oscillation wave to simplify the evaluation. Without the filtering process for dc offsets, the method is simple and straightforward and thus can calculate the damping constant and nature frequency in one oscillation cycle, and thus save considerable amount of computing time. Due to its simplicity and high speed, the method has great potential for future application to the on-line monitoring of low frequency oscillation parameters.

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