| Summary: | 碩士 === 國立清華大學 === 電機工程研究所 === 83 === The 11.4KV covered conductor fallen on the ground or the 11.4KV
bare conductor fallen on the high resistance ground can cause
the high impedance fault ( HIF ). Because their fault current
is low , the conventional overcurrent ( CO ) and low energy
overcurrent ( LCO ) relays have difficulty in detecting the
fault . This 11.4KV conductor on the ground may cause fire
hazard or human accident. Our research studies the detection
algorithms to detect the HIF. The major contributions of our
research are：（1） We design a relay setting scheme to improve
seven well-known detection algorithms developed by previous
researchers, and test the scheme numerically. The test results
show that our scheme can enhance the detection accuracy
significantly. （2） We propose a new detection algorithm which
detects the HIF on basis of the summed variance of the second,
fourth and sixth harmonics of feeder current. Combined with our
automatic relay setting scheme, this new algorithm is generally
superior, in terms of detection accuracy, to the above well-
known seven algorithms. 3） We conduct the staged HIF three
times, jointly with the Taiwan Power （Taipower） engineers, to
test the HIF detector developed in this research. The field
testing items include the switching of feeder capacitors, the
switching of a load break switch for a normally loaded lateral,
and the HIF.]4） The testing results of the detector are
satisfactory. （5） We further modify the above lower-order
harmonics variance detection algorithm so that the algorithm
can detect not only the HIF but also the very high impedance
fault whose fault current approaches zero.
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