In view of the case that multi-frequency components exists in the voltage response of the converter under subsynchronous voltage disturbances

the characteristics of the frequency distribution and the magnitude in the response are theoretically derived based on the method of the dynamic response analysis with two-order approximation.More specifically

with the two-order perturbation

the frequency distribution and amplitude characteristics of the response for PLL（Phase-Locked Loop） and current inner-loop controller under the subsynchronous voltage disturbance are firstly derived. Subsequently

the difference and similarity of the proposed method with respect to the traditional small signal analysis are discussed. Simulative results indicate that

under the subsynchronous voltage disturbance

the voltage response of the converter at the terminal of PCC（Point of Common Coupling） that is connected to the external power grid includes not only a pair of complementary sub/supersynchronous frequency components

but also a series of complementary small amplitude frequency components. Besides

the amplitude of the largest complementary sub/supersynchronous frequency components is directly related to integral parameters

while is insensitive to other parameters. Simulative results generally coincide with theoretical results.