Average-Value Modeling of Hybrid LCC-VSC HVDC Systems with Direct Interfacing in PSCAD/EMTDC

Line-commutated converters (LCCs) and voltage-source converters (VSCs) are widely deployed in HVDC systems. Average-value models (AVMs) of such converters have proven to be numerically efficient for real-time/offline simulation studies where detailed switching models are computationally expensive. However, with a non-iterative solution, the conventional so-called indirectly-interfaced AVMs (IDI-AVMs) may lead to inaccurate or unstable results at large simulation time steps due to the one-time-step delay between the AVM input values and the interfacing variables. In this paper, the AVMs of LCCs and VSCs are directly interfaced with the external system to eliminate the one-time-step interfacing delay. The new directly-interfaced AVMs (DI-AVMs) of LCCs and VSCs are formulated in nodal form, and their resultant matrices are merged into the external system nodal equations. The effectiveness of the presented method is investigated on a hybrid LCC-VSC HVDC system in PSCAD/EMTDC. It is verified that the proposed DI-AVMs are numerically superior and more accurate than the conventional IDI-AVMs and allow simulations with much larger time-step sizes.