As the proportion of new energy generation in the power grid continues to increase, the power grid gradually exhibits weak power grid characteristics, which will affect the quality of grid-connected current and the stability of inverters (Lai et al., IEEE Trans Power Electron 38(4):5266–5282, 2023). A typical feature of a weak power grid is that the grid impedance cannot be ignored (Fan et al. in IEEE Trans Power Syst 38:988–991, 2022), which will cause positive feedback of grid impedance voltage to PCC voltage feedforward, thereby reducing system stability margin (Wang et al. in IEEE J Emerg Select Top Power Electron 11:1191–1202, 2022). There are two types of solutions: one solution is to add a virtual impedance branch to the control loop (Cheng et al. in IEEE Trans Power Electron 37:13743–13754, 2022), and subtract the voltage on the virtual impedance from the PCC voltage obtained by sampling to obtain the grid voltage, in order to solve the problem of instability caused by feedforward of grid impedance voltage. It is not difficult to find that the key to using this strategy is to determine the grid impedance. The paper proposes a strategy for online adaptive tracking of grid impedance based on two kinds of predicted PCC voltage difference in Chap. 4 when studying the improvement of voltage feedforward technology. Another solution is to use controllers (Xu et al. in IEEE Trans Power Electron 38:987–1002, 2022; Silwal et al. in IEEE Trans Power Electron 37:10348–10359, 2022) that can enhance the system stability margin, such as robust H∞ nonlinear controllers. Although such nonlinear controllers have strong robustness and can ensure that the system has good stability margin even when the grid impedance undergoes a large range of changes, they are still very cumbersome to implement. Therefore, this chapter researches how to find easily implementable controllers based on differentiation in order to improve the stability margin of grid-connected inverters in weak power grids, and verifies these controllers under extremely weak power grids with short circuit ratio (SCR) < 2.