The fault model of the inter-turn short circuit of the dry-type air-core reactor is firstly analyzed in this paper

by which an on-line monitoring method of the reactor inter-turn insulation fault based on the changed power angle is proposed. The accuracy of power angle measurement is vital for diagnosing the reactor inter-turn insulation fault. The influence of the frequency fluctuation and harmonic interference on measuring power angle is analyzed by comparing the errors of fast Fourier transformation(FFT)

Hanning windowed interpolation FFT and fundamental harmonic separation algorithm improved by Hanning windowed interpolation. An on-line monitoring system of the reactor inter-turn insulation fault is also designed

which consists of the monitoring device

the intelligent electronic device(IED)of dry-type air-core reactor and the expert software. The experiment results show that the fundamental harmonic separation algorithm improved by Hanning windowed interpolation is superior to FFT and Hanning windowed interpolation FFT

and the inter-turn short circuit of the dry-type air-core reactor can be detected precisely by the on-line monitoring system.