Autonomous Synchronizing and Frequency Response Control of Multi-terminal DC Systems With Wind Farm Integration

Abstract

Recent analyses have shown that the grid-integration of offshore wind farms through MTDC systems has brought low inertia and small-signal stability issues, in which the dynamics of phase-locked-loop (PLL) play a crucial role. To address this issue, this paper proposes a control strategy for the multi-terminal VSCs aiming at PLL-less synchronization and autonomous frequency response of the MTDC system. One of the significant features of the proposed control is that the deviation of the grid frequency can be instantaneously reflected on the deviation of the DC voltage without ancillary control. Based on this feature, a fast inertia response and primary frequency regulation among wind farms and AC systems interconnected by the MTDC system can be achieved. A small-signal model is established to evaluate the overall system stability using the proposed control. Finally, comparative studies of this proposed control with the conventional PLL-based vector control are conducted in PSCAD/EMTDC based on a practical MTDC system in China, the Zhangbei fourterminal HVDC transmission system. The analysis shows that the proposed control exhibits advantages in weak grid operation and autonomous frequency response.Index Terms-frequency response, weak grid, wind farms, inertia, MMC MTDC, small signal stability analysis NOMENCLATURE Ceq Equivalent DC capacitance Udc DC voltage Udc_nom Nominal DC voltage Pdc DC side active power of receiving end converter (REC) Pac AC side active power of the REC Qac AC side reactive power of the REC Urec AC voltage of REC ωrec AC frequency of REC ωnom Nominal frequency J Moment of inertia ωm Rotor speed of synchronous generator (SG) Pm Mechanical power input of SG ωe AC frequency of SG Pe Active power output of SG

Resilient DC Voltage Control for Islanded Wind Farms Integration Using Cascaded Hybrid HVDC System

“…The existing large-scale onshore wind farms integration schemes mainly include LCC-HVDC with SC or STATCOM parallelled at the rectifier side [14] [15] and MMC-HVDC [26][27], as depicted in Fig. 2.…”

A Fault Clearance and Restoration Approach for MMC-Based MTDC Grid

“…One of the essential benefits of the MTDC grid is to transmit power to isolated, remote communities or different load centers while providing voltage stability and operational reliability to the entire system under AC transients. Several weak grids, renewable energy sources, or various asynchronous grids can be easily connected to an MTDC grid [1,2]. Unlike a two-terminal DC grid, an MTDC grid flexibly manages heavily loaded AC systems, configuring some terminals as an inverter to inject more power into the grid.…”

Novel cascading scheme of VSC‐HVDC with DC voltage synchronisation control for system frequency support

“…Above VSG-based schemes [20][21][22][23][24][25][26][27][28] of VSC-HVDC stand out by the grid-friendly property and the robustness for supporting the weak AC grid. However, they may inevitably introduce control impacts [30][31][32] to the prime power source for rendering system frequency support, impairing the stability of SEACS and sacrificing captured energy [30][31][32] of WTs, which is not cost-effective for commercial use.…”

“…However, communication may be required to coordinate two-end VSC stations in [24]. Another kind of VSG control, namely, DC voltage synchronisation control (DVSC), was recently developed in [25][26][27][28][29], seeking frequency regulation of VSC-HVDC through its inherent dynamics of capacitor without embedding additional control loops. In above schemes [25][26][27][28], both energy from SEACS and HVDC capacitor of VSC-HVDC is utilised for frequency support.…”