*Article* **Regulation Performance of Multiple DC Electric Springs Controlled by Distributed Cooperative System**

#### **Daojun Zha 1,**†**, Qingsong Wang 1,**†**, Ming Cheng 1,\*, Fujin Deng 1 and Giuseppe Buja 2**


Received: 4 August 2019; Accepted: 3 September 2019; Published: 5 September 2019

**Abstract:** DC electric springs (DCESs) have been recently developed to improve the voltage stability of a DC microgrid. A lately proposed DCES topology is comprised of a DC/DC three port converter (TPC), a bi-directional buck-boost converter (BBC) and a battery, and is arranged as follows: The TPC input port is fed by a renewable energy source (RES) whilst the two TPC output ports supply a non-critical load (NCL) and a critical load (CL) separately; in turn, BBC together with the battery constitutes the DCES energy storage unit (ESU) and is connected in parallel to CL. In this paper, a set of DCESs with such a topology and with their CLs connected to a common DC bus is considered. The control of the DCESs is built up around a distributed cooperative system having two control levels, namely primary and secondary, each of them endowed with algorithms committed to specific tasks. The structure of the control levels is explicated and their parameters are designed. The control system is applied to a DCES set taken as a study-case and tested by simulation. The results of the tests show the excellent performance of the control system in both regulating the CL DC bus voltage and keeping the state-of-charge of the battery within predefined limits.

**Keywords:** DC microgrid; DC electric spring; distributed cooperative control; adaptive droop control; consensus algorithm
