*Article* **Hierarchical Control with Fast Primary Control for Multiple Single-Phase Electric Springs**

#### **Qingsong Wang 1, Wujian Zuo 1,2, Ming Cheng 1,\*, Fujin Deng 1 and Giuseppe Buja 3**


Received: 22 July 2019; Accepted: 10 September 2019; Published: 12 September 2019

**Abstract:** An electric spring (ES) is a new power compensation device which becomes useful for the large-scale integration of renewable energy sources into the grid. However, when the grid contains two or more ESs connected at di fferent nodes, a voltage drop occurs between the nodes due to the line impedance. Therefore, the voltage of the ES-stabilized nodes cannot be set at the same reference (e.g., 220 V), otherwise one or more ESs break away because of the voltage windup. In this paper, a hierarchical control of multiple ESs tied to a microgrid is proposed to account for the line voltage drop. The primary control relies on the power decoupling control; it is designed for islanded operation of the microgrid, which improves the dynamics of both the voltage and frequency regulation. The secondary control relies on the droop control; it is introduced to coordinate the operation of the ESs by modifying the reference voltage of each ES dynamically. Advantages and disadvantages of the proposed hierarchical control are analyzed together with the explanations of the algorithms developed for its realization. At last, e ffectiveness of the arranged control system is validated by simulations on the MATLAB/Simulink platform.

**Keywords:** Electric spring; hierarchical control; coordinated control; power decoupling control; droop control; microgrid
