A shunt active power filter (SAPF) is coupled with load admittance in weak grid conditions, which poses a challenge to stability analysis. In this article, the admittance model of the SAPF is developed, which reveals the coupling mechanism between SAPF and load concisely and accurately. On top of that, the stability region of the system is investigated. The passivity region of the load admittance to guarantee the system stability is first explored. However, the passivity region has a very narrowband at a specific frequency, which makes it difficult to achieve. The stability region is then studied and it turns out to be an extension of the passivity region, and especially the narrowband is expanded. Therefore, the stability region is much easier to meet. The stability-oriented design is then summarized in a flowchart. In the end, the effectiveness of the newly defined stability region is verified by experimental results.

Nowadays Shunt Active Power Filter (SAPF) system still suffers from stability issue which heavily detect its performance. In this paper, an improved system level model is proposed to investigate the coupling effect that will cause the system level instability. This model describes the coupling effect with an additional output admittance and shows system stability more directly. In order to investigate the correctness of the proposed method, a grid including the SAPF and a non-linear load is modelled with both conventional approach and proposed modeling method. Simulations are carried out with the same parameters. Results show that the instability of the grid current observed in the simulation results is not reflected in the stability analysis based on the conventional model. On the other hand, the stability analysis with the new model matches the simulation results very well. Also, further verification is made by a frequency-sweep result, which proves the accuracy of proposed model.