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Coordinating control strategies of DC grid include master slave control, voltage margin control and voltage droop control, etc. Droop control could achieve several DC voltage control of DC grid, but the droop coefficients would impact on the dynamic voltage stability of the system. In this paper, modular multilevel converters (MMCs) based DC grid is the main study object. Firstly, the DC side voltage control system model and its transfer functions of the MMCs with droop control are derived. Then, a reasonable value range of droop coefficients that would maintain the system stability is presented by Routh criterion. And the influences of selection of droop coefficients in different ranges on voltage stability are analyzed. Finally, a four-terminal DC grid model is established on PSCAD/EMTDC, and the influence on voltage adjustment and stability of DC grid under different droop coefficients is simulated and analyzed. The simulation results verified the correctness of the theoretical analysis.

With the construction and development of UHVDC transmission system, the problem of insufficient dynamic reactive power reserve of the power grid is becoming increasingly prominent. As a typical UHVDC weak sending side, the short-circuit capacity of Jarud sending-end system is small, which causes high pressure on voltage stability in short-term failures. The introduction of new-type synchronous condensers will effectively improve the power system voltage stability. In this paper, Lugu UHVDC system is investigated. Firstly, the general situation of Jarud sending-end system and the demands for dynamic reactive power compensation are introduced. Then, the technical characteristics of the new-type condenser are analyzed starting from the performance improvement on subtransient, transient and steady state. Finally, simulation models of the condenser and Lugu UHVDC transmission system are built in PSCAD/EMTDC and are used to demonstrate. The effects of the new-type condenser on the system voltage are simulated and verified under the AC-side short circuit, the commutation failure and the bipolar blocking. And the impact of condenser on voltage stability of sending-end system is analyzed.