Abstract
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.
Highlights
In recent years, due to resource depletion and environmental problems such as pollution and climate change, renewable energy sources such as photovoltaic (PV) cells and wind energy have been supplied to replace fossil fuels
Since the injected zero-sequence voltage cannot resolve the imbalance of the DC-link voltage at extreme loads or bi-directional operation of individual poles, in order to maintain a stable microgrid for the bipolar low-voltage direct current (LVDC) distribution, the DC/DC converter needs the balancing function of the DC-link voltage
This paper proposes a ±750 V bipolar LVDC distribution system
Summary
Due to resource depletion and environmental problems such as pollution and climate change, renewable energy sources such as photovoltaic (PV) cells and wind energy have been supplied to replace fossil fuels. A 3-level NPC AC/DC converter has an inherent neutral point (NP) voltage balancing problem It limits the performance of the bipolar ±750 Vdc distribution system. The limit of zero-sequence voltage is analyzed in bipolar LVDC distribution To solve this problem, this paper proposes an additional DC/DC converter circuit and a control algorithm that can satisfy the imbalance problem and other conditions to use a 3-level NPC AC/DC converter in a bipolar LVDC distribution. The proposed additional circuit improves NP voltage balancing control for extremely asymmetrical loads It enables the individual bi-directional operation of each pole and maintains the bipolar distribution system in short-circuit conditions. The performance of the proposed system is analyzed and experimentally verified using a 300 kVA prototype 3-level NPC AC/DC converter system
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