*Article* **Power Flow Control Strategy and Reliable DC-Link Voltage Restoration for DC Microgrid under Grid Fault Conditions**

#### **Thanh Van Nguyen and Kyeong-Hwa Kim \***

Department of Electrical and Information Engineering, Seoul National University of Science and Technology, 232 Gongneung-ro, Nowon-gu, Seoul 01811, Korea

**\*** Correspondence: k2h1@seoultech.ac.kr; Tel.: +82-2-970-6406

Received: 1 June 2019; Accepted: 6 July 2019; Published: 10 July 2019

**Abstract:** In this paper, an e ffective power flow control strategy (PFCS) based on the centralized control method and a reliable DC-link voltage (DCV) restoration algorithm for a DC microgrid (DCMG) under grid fault conditions are proposed. Considering the relationship of supply-demand power and the statuses of system units, thirteen operating modes are presented to ensure the power balance in DCMG under various conditions. In the PFCS, the battery charging/discharging procedure is implemented considering the battery power limit to avoid overheating and damage. Moreover, load shedding and load reconnection algorithms are presented to maintain the system power balance, even in critical cases. To prevent the system power imbalance in DCMG caused by the delay of grid fault detection, a reliable DCV restoration algorithm is also proposed in this paper. In the proposed scheme, as soon as abnormal behavior of the DCV is detected, the battery or wind power generation system instantly enters a local emergency control mode to restore the DCV rapidly to the nominal value, regardless of the control mode assigned from the central controller. Comprehensive simulations and experiments based on the DCMG testbed are carried out to prove the e ffectiveness of the PFCS and the proposed DCV restoration algorithm.

**Keywords:** DC microgrid; experimental evaluation; grid fault conditions; power flow control strategy; reliable DC-link voltage restoration
