Modified norm type stability criterion for cascade AC system

In cascade AC system, the interaction between converters and non-ideal source may cause instability issues, especially when the converters behave as constant power load. From the view of small signal model, the stability issues can be evaluated based on the relationship between the terminal impedance and admittance of sub-modules. Therefore, the stability criterion based on terminal impedance and admittance is effective way to estimate the stability of total system. The impedance/admittance-based stability criterions have been widely accepted in DC system. However, the stability criterion for DC system can´t be applied to AC system directly due to the difference in complexity of system behavior. In this paper, a norm type stability criterion is proposed to analyze the stability issues in AC system. G norm and sum norm are employed and a modified method to reduce the conservatism is also proposed. Finally, the conservatism of different norm type criterion are evaluated and compared. The results show the proposed criterion is better than other criterions in practical application.