A coordinate strategy of inverter and transformer control modes to improve static voltage stability in AC/DC power systems

In this paper, the impacts of inverter control mode switching and converter transformer control modes on the static voltage stability of AC/DC power systems are analyzed based on the quasi-steady-state model of HVDC systems. Based on the mechanism analysis, a new coordinate strategy of control modes of inverter and transformer is proposed to improve the static voltage stability of AC/DC systems. With load increasing, the coordinate strategy aims at keeping the power factor of the inverter as high as possible to minimize the reactive power demands of the DC system. Simulation results demonstrate the correctness of the voltage stability analysis results and the effectiveness of the proposed coordinate strategy.

I. Introduction

Because conventional LCC-HVDC transmission systems have the advantages of asynchronous AC grid connection and the capacity of bulk power transmission over a long distance, they are widely used in the power transmission of renewable resources, which are usually far from the load centers. But at the same time, conventional LCC-HVDC transmission systems need to absorb a large amount of reactive power from AC systems, which can reach to 40 to 60 percentage of the HVDC active power transmission. Therefore the voltage stability problem of AC/DC power systems is getting more and more attention.