快速暫態過電壓（Very Fast Transient Overvoltage, VFTO）係為特高壓氣體絕緣開關場（Gas Insulated Substation, GIS）中常見的開關暫態現象，其特性為它的上升波前時間僅在奈秒範圍內。VFTO對設備絕緣之危害，除了它的峰值可能超過絕緣水準外，主要為快速上升的波前會造成設備絕緣內電場的不均勻分佈，而致絕緣材料會有局部的過電壓，進而加速絕緣材料的老化；此外，它的高頻暫態可能造成電磁干擾，導致控制設備的誤動作。對於電力變壓器二次側之設備，因為阻抗匹配的關係，傳遞至變壓器二次側的VFTO一般認為沒有破壞能力。基此，VFTO的相關研究多侷限在GIS本體或GIS週圍與GIS直接相連接的設備，甚少探討傳遞至電力變壓器二次側的VFTO。
然而，少數現場經驗顯示：在特高壓發電廠的中壓系統中，部份設備的故障疑與特高壓GIS之操作有關。此外，作者亦曾於民國91年於台灣電力公司第三核能發電廠之4.16 kV系統測得VFTO之波形，顯示VFTO確實會傳遞至中壓系統。基此，本文遂以中壓系統的VFTO為研究主題，進行量測與模擬分析。
受限於模擬軟體電磁暫態程式的功能限制，實無法以奈秒等級之模擬時距模擬開關動作全程之暫態現象。基此，為分析VFTO的影響，多數研究採用單一次再襲模擬的方式，進行最嚴苛之VFTO的評估，例如以2.0標么之極間崩潰電壓為條件。然而，前述核三廠之現場量測以及本研究後續之模擬顯示，中壓系統的VFTO特性不單與極間崩潰電壓有關，尚與開關動作全程特性相關。基此，本研究發展方法，得以模擬隔離開關之全程動作。本研究獲得之成果摘要如下：
1、藉由元件特性分析，對模擬所需之模型提出注意事項，並提出三相系統之DS動作全程暫態模擬的方法，包括：建議適當的模擬時距，並提出高頻恢復因數俾修正以較大模擬時距所獲得的最大突波峰值之模擬結果；
2、分析中壓系統之VFTO的模擬結果，歸納出中壓系統之VFTO的特性：高匝比變壓器的電容移行、全程多次再襲過程中發生再襲所引起的振盪波疊加於下次再襲波形的波形疊加效應、三相系統的三相互耦效應、再襲電弧之持續時間、DS啟閉特性等；
3、模擬DS操作時可能出現的不同動作角度，據以統計分析中壓系統之VFTO的分佈特徵，包含上述的全程多次再襲、三相互耦等特性；
4、整理多次的現場量測經驗，對中壓系統之VFTO量測提出建議。

The very fast transient overvoltage (VFTO) is a common transient phenomenon caused by the switching operation in the extra-high-voltage (EHV) gas insulated substation (GIS), and is characterized by its fast rising wavefront within nanoseconds. Besides the peak of VFTO higher than the breakdown voltage of insulation material, the main affection of VFTO on the equipments is the fast rising wavefront, causing the non-uniform electric field inside the insulation material, then leading the partial overvoltage inside the insulation material, and resulting in the accelerating aging of the insulation material. Furthermore, the very fast transient may cause electromagnetic interference and result in the malfunction of control instruments. As for the equipments in the secondary side of power transformer, the VFTO is thought to be small and less dangerous due to the mismatch of impedance. Therefore, most researches involving VFTO usually focus on the affection of VFTO on the GIS itself, and the equipments adjacent GIS or directly connected to GIS. Hence, there are few researches dealing with the VFTO in the secondary side of power transformer.
However, a few field experiments showed that some root causes of medium voltage events were suspected to be the switching surge of GIS operation. Moreover, the authors measured the VFTO in 4.16 kV system in the 3rd nuclear power plant of Taipower at 2002, and the VFTO was, hence, confirmed to be able to transmit into medium voltage system. Therefore, this thesis takes the analysis of VFTO in medium voltage system as the major topic by simulating and measuring the VFTO in medium voltage system.
Limited by the capabilities of software, ElectroMagnetic Transient Program (EMTP), it is impossible to simulate the transient phenomena of whole process of switching operation by the 1 ns time step. Therefore, most literatures estimated the affection of VFTO by the single strike simulation with the most series situation, such as 2.0 PU inter-contact breakdown voltage. However, according to the field measurement results in the 3rd nuclear power plant and the study of this thesis, the characteristics of VFTO in medium voltage system are not only affected by the inter-contact breakdown voltage, but also affected by the properties of whole process. Hence, this thesis is devoted to develop simulation method to simulate the whole process phenomena of disconnect switch (DS). This thesis gets the following results:
1. By the analysis of components’ characteristics, the notice is addressed for the simulation, and the simulation method for whole process phenomena of switching operation is proposed, which includes the proposition of an adequate simulation timestep and the high frequency recovery factor (HFRF) to adjust the maximum peak of VFTO simulated at this timestep;
2. By analysis of the simulation results, the properties of the VFTO transmitting into medium voltage system are characterized: the capacitive transfer of high turn-ratio transformer, the superposition effect of the oscillation caused by former restike superimposing into the following restrike, the three phases coupling caused by three-phase system, the duration of arcing time, and the properties of DS opening/closing;
3. By simulating the DS operating at different operation angle, the property of VFTO in medium voltage system is discussed including aforementioned factors;
4. By the several field measurement results, the recommendation is made for the measuring of VFTO in the medium voltage system.

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