High-temperature superconducting（HTS） apparatuses applied in power systems may suffer AC over-current fault to quench from superconducting state to normal state.It is important to detect quench of superconducting apparatus in power systems.However

the inductance voltage induced by AC is much higher than resistance voltage when quench takes place

which results in difficult judgement of quench.In this paper

the over-current faults in power systems are simulated with the amplitude of the over-current changing from a small value to 74 A

which is about 1.5 times of the critical DC current of the superconducting coil

and experimental data are obtained about the quench processes of HTS coil under AC over-current with different amplitudes.The experimental results show that the phase angle formed by the voltage and the current of an HTS coil changes as quench takes place

and the phase angle decreases sharply with the increasing amplitude of AC over-current.Thus

a quench judgement approach of superconducting apparatuses working under AC is proposed by making use of the change of the phase angle.