Protection of LVDC Microgrids in Grid-Connected and Islanded Modes Using Bifurcation Theory

Low-voltage dc (LVdc) microgrids facilitate the integration of renewable energy systems and modern loads. However, they suffer from the lack of a sensitive, selective, reliable, and fast protection strategy. The low fault current of high-resistance faults makes fault detection and faulty zone identification challenging tasks for protection engineers. This article proposes a protection scheme that is effective for both bolted and high-resistance faults at the LVdc feeders. The developed strategy relies on two intelligent electronic devices at both ends of the protected zone, calculating the normalized superimposed component of the line current. By adding this component to the input of the periodically forced harmonic oscillator (PFHO), both fault occurrence and fault direction can be simultaneously determined based on the oscillator state variable with only two possible values during the stable and unstable modes. It eliminates the need for the selection of various protection thresholds for different LVdc microgrid structures/topologies. The proposed protection strategy can classify pole-to-ground and pole-to-pole faults and is effective for both grid-connected and islanded LVdc microgrids. The reliable performance of the developed PFHO-based protection is assessed on the simulation model of a ±750 V meshed LVdc microgrid with the TN-S grounding system.

I. Introduction

Popularity of dc microgrids is increasing due to the proliferation of dc sources and dc loads [1], [2]. Elimination of the most technical challenges of ac microgrids such as synchronization, harmonics, reactive power, and multiple-power conversion stages makes the dc microgrid an effective alternative power supply for industrial parks, commercial buildings, and smart homes [3], [4]. One of the main technical challenges of the low-voltage dc (LVdc) microgrids that remains unsolved is to develop a protection system with acceptable sensitivity, selectivity, reliability, and speed [5], [6]. DC microgrids are prone to both pole-to-ground (PG) and pole-to-pole (PP) faults. The high fault current magnitude and rate of change caused by capacitor discharge make the protection requirements of the dc microgrid different from those of ac microgrids. In addition, the conventional ac circuit breakers are ineffective in dc microgrids due to the need for high-speed current interruption and no zero-crossing point. The limited fault current of the converters and short lines are other protection challenges of the dc microgrids.