Synchronous Machine Emulation of Vsc for Interconnection of Renewable Energy Sources through Hvdc Transmission

The majority of the energy demand over the past years has been fulfilled by centralized generating stations. However, with a continuously increasing energy demand, the integration of decentralized renewable energy sources (RES) into the power system network becomes inevitable even though these sources affect the stability of the grid due to their intermittency and use of various power converters. The transmission of power over long distances from RES is usually accomplished either by AC or DC transmission. High voltage DC transmission (HVDC) is preferred over high voltage AC transmission (HVAC) due to numerous and complex reasons, such as its lower investment cost for long transmission cables, lower losses, controllability, and limited short circuit currents. Several control methods for grid-connected voltage source converters (VSCs), such as power-angle and vector-current controls, are being adopted in RES interconnections. However, these methods face several issues when used for a weak grid interconnection. This thesis develops a control strategy for a VSC–HVDC transmission system by referring to the synchronverter concept. In the proposed method, the sending-end rectifier controls emulate a synchronous motor (SM), whereas the receiving end inverter emulates a synchronous generator (SG) to transmit power from one grid to another. The two converters connected by a DC line provide a synchronverter HVDC (SHVDC) link. Given the high demand for sustainable energy, integrating RES—which can be extended to wind-based resources—into the long-haul HVDC link becomes essential. Therefore, in this thesis, a windfarm with a type 4 permanent magnet SG is integrated into the HVDC link through a rectifier. Depending on the wind speed, the proposed control strategy automatically shares and manages the wind generator power on the DC side by using a battery energy storage system (BESS) connected to the HVDC link to stabilize the power fluctuations generated by the intermittency of the wind farm. The performance of the synchronverter-based HVDC transmission was verified by using a MATLAB Simulink model. Results show that the controller can effectively control the power flow from one grid to another and that the effect of wind fluctuation on the grid can be mitigated by introducing a BESS at the DC link. Therefore, by properly controlling the SHVDC, BESS, and RES connected to the HVDC system, the power from remote RES can be connected to a weak AC grid in a stable manner