| Summary: | 博士 === 國立清華大學 === 電機工程學系 === 99 === 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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