Search Results (3)

A voltage balancer (VB) can be used to balance voltages under load unbalance in either a bipolar DC microgrid or LVDC (Low voltage DC) distribution system. An interleaved buck-type VB has advantages over other voltage balance topologies for reduction in output current ripple by an aspect of configuration of a physically symmetrical structure. Similarly, magnetic coupling such as winding two or more magnetic components into a single magnetic component can be selected to enhance the power density and dynamic response. In order to achieve these advantages in a VB, this paper proposes a VB with a coupled inductor (CI) as a substitute for inductors in a two-stage interleaved buck-type VB circuit. Based on patterns of switch poles under load variation, the variation in inductor currents under four switching patterns is induced. The proposed CI is derived from self-inductance based on the configuration structure that has a two-stage interleaved buck type and mathematical design results based on the coupling coefficient, where the coupling coefficient is a key factor in the determination of the dynamic response of the proposed VB in load variation. According to the results, a prototype scale is implemented to confirm the feasibility and effectiveness of the proposed VB. Full article

The low voltage direct current (LVDC) distribution networks are connected with too many kinds of loads and sources, which makes them prone to failure. Due to the small damping value in the DC lines, the fault signal propagates so fast that the impact current with the wave front of millisecond and the transient voltage pose great challenges for fault detection. Even worse, some faults with small currents are difficult to detect and the communication is out of sync, resulting in protection misoperation. These problems have severely affected the new energy utilization. In view of this, a DC fault current limiter (FCL) composed of inductance, resistance, and power electronic switch was designed in this paper. The rising speed of fault current can be decreased by the series inductance and the peak value of the fault current can be limited by series impedance, thus in this way the running time can be gained for fault detection and protection. For distributed energy access, by deducing the short circuit fault characteristic expression of LVDC distribution network, the feasibility of FCL was verified. Based on the structure of the bridge-type alternating current (AC) current limiter, the structure and parameters of the DC FCL were determined according to the fault ride-through target. Then, a low voltage ride-through strategy based on DC FCL was proposed for the bipolar short-circuit fault of LVDC distribution network. Finally, MATLAB/Simulink simulation was used to verify the rationality of the proposed FCL and its ride-through strategy. Full article

Low-voltage direct current (LVDC) distribution has attracted attention due to increased DC loads, the popularization of electric vehicles, energy storage systems (ESS), and renewable energy sources such as photovoltaic (PV). This paper studies a ±750 V bipolar DC distribution system and applies a 3-level neutral-point clamped (NPC) AC/DC converter for LVDC distribution. However, the 3-level NPC converter is fundamental in the neutral-point (NP) imbalance problem. This paper discusses the NP balance control method using zero-sequence voltage among various solutions to solve NP imbalance. However, since the zero-sequence voltage for NP balance control is limited, the NP voltage cannot be controlled to be balanced when extreme load differences occur. To maintain microgrid stability with bipolar LVDC distribution, it is necessary to control the NP voltage balance, even in an imbalance of extreme load. In addition, due to the bipolar LVDC distribution, the pole where a short-circuit condition occurs limits the short current until the circuit breaker operates, and a pole without a short-circuit condition must supply a stable voltage. Since the conventional 3-level NPC AC/DC converter alone cannot satisfy both functions, an additional DC/DC converter is proposed, analyzed, and verified. This paper is about a 3-level NPC AC/DC converter system for LVDC distribution. It can be used for the imbalance and short-circuit condition in bipolar LVDC distribution through the prototype of the 300 kW 3-level NPC AC/DC converter system and experimented and verified in various conditions. Full article