Grid-forming (GFM) converters have received much attention in renewable energy applications because of their voltage support capability. Existing studies generally assume constant dc-link voltage for cascaded GFM converters, which may lead to the stability challenges under some occasions with dc-link dynamics. Thus, this article first analyzes the small-signal stability of GFM converters considering dc-link dynamics. It is found that dc-link exhibits inherent instability risk due to the nonminimum phase characteristics of the dc-link capacitor, which may lead to subsynchronous oscillations on the ac side. To solve this issue, dc-link dynamics are introduced into both active and reactive power loops through proportional controllers. This method does not increase the order of the controller, and it is easy to implement digitally. Compared with the conventional damping method that only introduces dc-link dynamics into the active power loop, the proposed method can dampen the subsynchronous oscillations caused by dc voltage fluctuations, and it can enhance the transient response performances of ac side. The experimental results validate the correctness of the small-signal model and the effectiveness of the proposed damping method.

The electric vehicle (EV) charging station is a critical part of the infrastructure for the wide adoption of EVs. Real-time simulation of an EV station plays an essential role in testing its operation under different operating modes. However, the large numbers of high-frequency power electronic switches contained in EV chargers pose great challenges for real-time simulation. This paper proposes a compact electromagnetic transient program (C-EMTP) algorithm for FPGA-based real-time simulation of an EV station with multiple high-frequency chargers. The C-EMTP algorithm transforms the traditional EMTP algorithm into two parallel sub-tasks only consisting of simple matrix operations, to fully utilize the high parallelism of FPGA. The simulation time step can be greatly reduced compared with that of the traditional EMTP algorithm, and so the simulation accuracy for high-frequency power electronics is improved. The EV chargers can be decoupled with each other and simulated in parallel. A CPU-FPGA-based real-time simulation platform is developed and the proposed simulation of the EV station is implemented. The control strategy is simulated in a CPU with 100 μs time-step, while the EV station circuit topology is simulated in a single FPGA with a 250 ns time-step. In the case studies, the EV station consists of a two-level rectifier and five dual-active bridge (DAB) EV chargers. It is tested under different scenarios, and the real-time simulation results are validated using PSCAD/EMTDC.

Efficient electromagnetic transient (EMT) simulation is crucial for addressing the challenges associated with the modularity, cascading, and complex topologies of power electronics (PE) systems. This paper introduces a novel portal analysis (PA) approach that adopts a unique “port-component” view, leveraging the characteristics of “ports”. Different from the classic nodal analysis (NA) method, the networks and the components are described based on the portal voltage equations with lower orders. Furthermore, a port tearing method is introduced to partition the circuit, resulting in a block-bordered-diagonal (BBD) matrix and a small number of boundary variables in the extended port equation. The parallel processing of the network solution is implemented by utilizing the BBD forms and the band features. To validate the effectiveness of the proposed PA approach in capturing dynamics, a down-scale dual active bridge (DAB) prototype is constructed. Additionally, a simulated microgrid comprising cascaded H-bridge-type-DAB (CHB-DAB), solar photovoltaic (PV), and energy storage system (ESS) is used to furtherly verify the accuracy and efficiency. The results demonstrate that compared with the NA-based methods, the proposed models exhibit high precision under various transient conditions, with maximum relative errors of less than 2%. Moreover, they achieve a remarkable acceleration of one to two orders of magnitude.

The decoupling method based on the natural or inserted artificial delays, when applied to the simulation of power electronic (PE) systems, may encounter challenges such as inadequate length of delay lines or numerical instability and precision issues due to the high-frequency voltage/current variations at interfaces. To deal with the challenge, a delay-free decoupling method is proposed in this article. The method reduces both the dimension of matrix multiplication and the number of switch state combinations without sacrificing numerical stability and compresses the calculation progress by representing the decoupled system with the discrete state-space equation. The PE system is decoupled at the series/parallel interface of submodules, treating currents as boundary variables linked to each submodule's extended port. A preliminary formula for these variables is derived by simultaneously solving nodal voltage equations and KVL equations. Submodule solutions are compacted and parallelized based on the discrete state-space equations. These equations are then substituted back and decomposing boundary variables into independent segments, to achieve parallelization of boundary variable solutions. The proposed method is validated through real-time simulation of cascaded PE systems on an FPGA platform with a 250ns time step. Results show that it achieves high precision compared to non-decoupled systems in PSCAD across diverse transient conditions. Additionally, it boosts the simulation scale by roughly 2-7 times on the FPGA-based platform compared to non-decoupled nodal analysis.