To prevent the auxiliary power unit from over-speed,a study was done in autonomous over-speed protec-tion.The protection overall scheme,protection strategy and built-in-test(BIT)were presented.The selection of speed signal and the protection speed were determined.If either speed 1 or speed 2 is larger than protection speed,the protection shall be performed.When an over-speed condition was detected,APU shall be shutdown.The pro-tection shall not be performed when the APU speed was less than 10％.A built-in-test(BIT)technique was devel-oped to meet the airworthiness requirement.The integrity of speed measuring circuitry test carried out by injecting known frequencies.Through the ground test of APU,the effectiveness of APU autonomous over-speed protection was verified.
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