Study on the Power Transmission Capacity of Wind-Photovoltaic-Thermal Power Bundled MMC-HVDC Transmission System

The different supply ratio of wind, photovoltaic and thermal power in the wind-photovoltaic-thermal power bundled MMC-HVDC transmission system can affect the power transmission capacity. In this paper, the steady-state mathematical model and small-signal model of the wind-photovoltaic-thermal power bundled MMC-HVDC transmission system are developed, and the correctness of the small-signal model is verified by the electromagnetic transient simulation. On this basis, the method for determining the power transmission capacity of the system, considering both steady-state operating constraints and small-signal stability constraint, is proposed. And the impact of different active power outputs from thermal power, wind power, and photovoltaic power on the system power transmission capacity is studied. The results show that, as the active power output of thermal power plant decreases and the imbalance in the active power output ratio of wind and photovoltaic power increases, the maximum active power the system can transmit decreases. In the case of this paper, when the thermal power output decreases below 0.305p.u. and active power outputs of wind farm and photovoltaic plant are equal, the MMC-HVDC under vector current control is no longer able to transmit the rated active power.