Transient stability analysis in Multi-terminal VSC-HVDC grids

A novel approach to transient stability analysis in multi-terminal high voltage direct current (MTDC) grids is presented in this paper. A symmetrical three-phase fault in an ac grid connected to a rectifier terminal of the MTDC grid causes the power injected into the dc grid to decrease, which in turn leads to a lower dc voltage in the MTDC grid. If dc voltage drops below a critical voltage limit before the ac fault is cleared, then the dc grid becomes unstable and its operation is disrupted. An analytical approach is proposed in this paper to calculate the critical clearing time of a fault in an ac grid behind a rectifier terminal beyond which dc voltage collapse occurs. A five-terminal MTDC grid modeled in EMTDC/PSCAD is used to validate the results obtained with the analytical method

Impact on power system frequency dynamics from an HVDC transmission system with converter stations controlled as virtual synchronous machines

This paper evaluates the operation of an HVDC interconnection for providing virtual inertia and the impact on the power system frequency dynamics when one or both the converter terminals are controlled as a virtual synchronous machine (VSM). The analysis aims at assessing the performance of the inertia support from a power system perspective by studying the impact on the frequency nadir and the Rate-of Change-of-Frequency (RoCoF) of two asynchronous power system equivalents. It is demonstrated how the dc voltage control can be integrated with the VSM-based control when both converter terminals should be controlled to provide inertia support, but it is also found that this may require additional capacitance to ensure sufficient stability margins. Simulations in PowerFactory are presented for a case with two asynchronous power system equivalents connected by an HVDC transmission scheme modelled with parameters based on the planned Nordlink connection between Norway and Germany. The results confirm the beneficial effects of the inertia support on the frequency dynamics and demonstrate how the VSM-based control provides a clear improvement of the frequency nadir compared to traditional power-frequency droop control