As modern power system toward high renewable energy integration with wind, solar, and storage sources, the increasing share of inverter-based resources leads to stability challenges dominated by multi-loop control with wide-band frequency characteristics. In systems with high penetration of renewable energy, converter-based systems have presented new features, such as large-scale integration and long-distance transmission, causing system faults to exhibit large-signal transient characteristics. The hybrid connection of grid-following (GFL) and grid-forming (GFM) converters has emerged as a potential solution to enhance the stability and efficiency of new energy transmission. However, the high order and strong nonlinearity of these hybrid systems pose challenges to the assessment of their transient stability. Therefore, this study is dedicated to designing an effective method for evaluating the transient stability of GFL/GFM converter hybrid systems.

The research methodology starts with the construction of a detailed fourth-order nonlinear model of the hybrid system, integrating phase-locked loops and virtual synchronous generators, which serves as the basis for the proposed transient stability solution method based on alternating calculation. Further, by calculating the mutation portion at the failure moment, the method derives the computed initial values for each system of the transient process. The essence of the rotation calculations lies in performing energy calculations and resolving the angular velocities in the power angle domain, subsequently mapping them back to the time domain. In the method implementation, energy calculations are first performed for a certain converter system, the dynamics of this system is used to further estimate the motion of the other system in this step, and the order of calculations for the two systems is exchanged to perform the alternating calculations. In this process, the correspondence between the power angles of GFL and GFM control is established, which enables the complex interactive motion patterns of the hybrid system under severe disturbances to be evaluated. During the alternating computation process, for the GFL/GFM system, the equivalent kinetic energy change over the step is computed by integrating the relevant equations that take into account the damped power and kinematic properties, avoiding the uncertainty associated with neglecting damping. During the continuous iterative computation process, the computed values are exchanged and updated between the two systems to ensure accurate transient behavior of the system. Eventually, the computation is stopped after the judgment condition of stability is satisfied.

The experimental and simulation results confirm the feasibility and effectiveness of the proposed method. It accurately depicts the variations in power angles, angular velocities, and GFM converter voltages during the transient processes of the hybrid system. The computational time of this method is significantly reduced compared to existing numerical methods, with at least an order of magnitude improvement. Additionally, the method is applicable to calculating the critical clearing time (CCT), achieving a resolution within 5 ms in the presented examples. It can also accurately characterize the out-of-sync operation of GFL and GFM converters during the fault recovery process.

In conclusion, this study provides a practical solution for evaluating the transient stability of hybrid converter systems. The developed method based on alternating calculation in the discrete domain exhibits clear physical mechanisms and relatively low computational requirements. It has the potential to be incorporated as a subsystem in large-scale simulations to accelerate the simulation speed.