At present, renewable energy generation mainly use grid-following (GFL) control, which is prone to cause small signal stability problems such as broadband oscillation when connecting to the weak grid system. What’s more, the GFL units have insufficient support capacity for the grid. Grid-forming (GFM) technology construct the grid voltage independently through power synchronization control.When the system is disturbed, GFM units can actively support the grid voltage and frequency to improve the stable operation of the system. Under the situation of rapid development of renewable energy, it’s important to carry out GFM technological transformation and upgrading with renewable energy clusters/stations as the main body. How to reasonably plan the access capacity and location of GFM units of renewable energy grid-integration systemand improve the system stability characteristics, has become a key concern for engineering applications. The paper studies the configuration problem of the GFM unitsin renewable energy grid-integration system, with the focus on the small signal stability constraints.

Firstly, the singlerenewable energy converter grid-integration system was established. Based on the small signal model of the renewable energy converter grid-integration system, a closed-loop power-voltage feedback model was constructed, and the consistency between the former and the current-voltage closed-loop feedback model was verified in analyzing the stability characteristics of the system. Secondly, based on the impedance network circuit model, the system stability characteristic analysis method was extended to the multi-machine system. What’s more, the GFM units access capacity and distribution configuration problem of the multi-machine system was set. The objective of the problem is to minimize the total capacity of GFM units in the system, the constraints are that the system has sufficient small signal stability margin and stability support capability. Finally, the small signal stability margin index, the small signal stability support gain growth rate index, the GFM units access point selection index and the configuration method for GFM units of the system were proposed respectively. An analytical example was constructed based on the real renewable energy cluster grid-integration system, and the effectiveness of the proposed method was verified by time-domain simulation.

The conclusions are as follows: (1) Under the constraint of system small signal stability, the reasonable configuration of the capacity and location of GFM units can ensure that the whole system has sufficient small signal stability margin and stability support capability. Furthermore, reducing the capacity of the GFM units configuration can minimize the economic cost of the system. (2) The proposed configuration methodcan enhance the system's stable operation capability, particularly within a specific range of weak grid strength. It appears that when the capacity proportion of GFM units is constant, the weaker the grid characteristics, the more GFM units needs to be accessed. (3) The higher the percentage of GFL units with poor dynamic characteristics in the system, the larger the proportion of GFM units needs to be accessed. Optimization of system control parameters or control strategies, and explore the configuration method of GFM units in complex system scenarios will be the future research direction.