Small Signal Frequency-Domain Model of a LCC-HVDC Converter Based on an Infinite Series-Converter Approach

In this paper, an improved analytical small signal model of the line commutated converter (LCC) is developed in direct-quadrature-zero coordinates based on the assumption of infinite six-pulse converters. The dynamics of the commutation inductance and transportation delays during commutation are included in the model, and the sampling of the measurement of firing and extinction angles is added to accommodate actual converters with a finite pulse number (e.g., 12-pulse). The model is validated by comparing the frequency response with one obtained by frequency scanning in an electromagnetic transients (EMT) simulation. The model is shown to be effective in investigating system stability when the LCC is connected to an arbitrary external network. To do this, the frequency response of the combined system is plotted, and the generalized Nyquist stability criterion is applied. This stability result is validated by a time domain simulation on an EMT program using the first CIGRE HVDC benchmark system and the IEEE 14-bus system. Compared to the traditional LCC model, the proposed model is shown to have significantly better accuracy in system stability determination.