Bipolar DC Distribution System Research Articles

Modulation of Wide Voltage Range Interface DC-DC Converter in DC Distribution System

Bipolar DC distribution systems rely on various power sources like photovoltaics and distributed energy storage, each with its unique voltage characteristics. To accommodate these fluctuations, interface converters must adjust over a wide voltage range. The bipolar non-isolated DC-DC converter emerges as a promising solution due to its versatile modulation capabilities, reduced switch voltage stress, and cost-effectiveness. This article explores how wide voltage range regulation is achieved in bipolar DC-DC converters interfacing with bipolar DC power grids. It delves into the operational strategies and modulation techniques employed, ensuring stable output despite varying input voltages. Design considerations and challenges associated with implementing such converters are also discussed. An experimental platform was constructed to validate the proposed methodology. Through rigorous testing and analysis, the effectiveness of the topology's operation mode was confirmed. Real-world data from the experimental setup provided insights into the converter's performance under different operating conditions, supporting its applicability for bipolar DC distribution systems. In summary, this article provides a comprehensive examination of wide voltage range regulation in bipolar DC-DC converters, highlighting their potential to enhance efficiency and reliability in bipolar DC power grids. Through theoretical discussions and practical validation, it contributes to the advancement and adoption of these converters in modern energy systems.

Bipolar Semiactive Bridge Converter With Cross-Cycle Modulation for Resilience Enhancement in Bipolar DC Distribution Systems

Bipolar Semiactive Bridge Converter With Cross-Cycle Modulation for Resilience Enhancement in Bipolar DC Distribution Systems

Improved Dual Active Bridge DC–DC Converter With Symmetric Bipolar Output by Utilizing a Switched-Capacitor Circuit for Bipolar DC Distribution System

The bipolar dc distribution system is a promising solution due to its advantages in efficiency and reliability, but there are also challenges of pole voltage imbalance. By using a simple switched-capacitor circuit, this paper presents a semi-dual active bridge converter with symmetrical bipolar output characteristics. The converter is capable of single-stage power conversion, offering the advantages of low cost and high-power conversion efficiency. Moreover, the symmetrical bipolar output voltage can be achieved without an additional voltage balancer and feedback control under unbalanced load conditions, which reduces system cost and control system complexity. In addition, the proposed converter also has a significant advantage over existing solutions in terms of the number of semiconductor devices and magnetic components. A detailed analysis of the proposed converter is presented, including the operation principle, steady-state performance, soft-switching performance, voltage gain, voltage/current stresses, efficiency and loss analysis, and control method. Finally, a 1-kW prototype is developed and tested to verify the performance of the proposed converter.

Improved dynamics of DC bus control with a zig-zag transformer for bipolar DC distribution systems

Improved dynamics of DC bus control with a zig-zag transformer for bipolar DC distribution systems

Voltage-Balancing Dual-Active-Bridge (VB-DAB) Converter for Bipolar DC Distribution System

This article proposes a new voltage-balancing dual-active-bridge (VB-DAB) converter for the bipolar dc distribution system. The proposed converter comprises the same components as those of the conventional DAB converter, and it integrates functions of a VB without additional components. The proposed converter transmits power using the same principle and control as the conventional DAB converter. Furthermore, it regulates both output voltages equally and redistributes the unbalanced load current. Therefore, the proposed converter neither requires additional components nor results in losses by them. Consequently, it can achieve high efficiency and high power density. The effectiveness of the proposed converter is verified by 3-kW prototypes with a 380-V input and ±190-V outputs conditions.

Power and Voltage Control Based on DC Offset Injection for Bipolar Low-voltage DC Distribution System

Power and Voltage Control Based on DC Offset Injection for Bipolar Low-voltage DC Distribution System

Coupled Inductor Based Voltage Balancing in Dual-Output CLL Resonant Converter for Bipolar DC Distribution System

Bipolar dc distribution systems face imbalances in voltages when unbalanced loads are connected at the outputs. To address imbalances in bipolar voltage levels, dedicated voltage balancers are required. However, they add extra active and/or passive components; including, magnetics, capacitors, etc. This increases the cost, reduces the conversion efficiency, and power density of dual-output converters. Therefore, to tackle the imbalances in output voltages without adding any extra active and/or passive components, this article presents coupled inductor ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> ) based voltage balancing in dual-output <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLL</i> resonant converter. The main features of the proposed method are as follows: since <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> is the result of already present resonant inductors of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLL</i> tank circuit, therefore, it does not require extra magnetic components to balance the output voltages; with no additional components, it does not increase the magnetic volume, component count, cost, and complexity of the converter; due to the direct coupling, a half of circuit inductances are integrated to achieve the respective resonant inductance of the tank circuit; and it can also avoid complex control schemes applied to voltage balancing. Besides, with the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> , the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CLL</i> converter maintains all of its good features including; zero-voltage and zero-current-switching (ZVZCS). The detailed analysis of the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> with different load conditions is discussed. Moreover, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LLC</i> or Type-12 resonant tank circuit is presented to validate ZVZCS operation in the reverse power flow condition. To verify the effectiveness of the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CI</i> based voltage balancing, a 3.6-kW hardware prototype has been built and tested.

Enhanced Voltage Control of Bipolar DC Distribution System Based on Modulus Decomposition

There is a coupling between the positive and negative poles of a bipolar dc distribution system, which makes the control of power converters influence mutually. It is difficult to calculate the control parameters of the positive and negative control loop independently. In this article, the modal conversion is introduced into PI control and voltage droop control of the bipolar dc distribution system. Then, the mathematical models of bipolar dc converter, dc lines, and dc loads in modal component space are deduced. Thus, the common-mode and differential-mode decoupling equivalent models are built for the bipolar dc distribution system. The coupling relationship between the common-mode and differential-mode equivalent circuit when the positive and negative loads are balanced and unbalanced is analyzed. The transfer functions from the common-mode and differential-mode duty ratios of the positive and negative power converters to the output voltage are deduced. Finally, the applications of modulus decomposition in the control of parameter design are analyzed based on PI control and voltage droop control. Moreover, a simulation model of the bipolar dc distribution system is built in MATLAB/Simulink and a low-voltage experimental platform is established to verify the validity and accuracy of the proposed algorithm.

Monopolar Fault Reconfiguration of Bipolar Half Bridge Converter for Reliable Load Supply in DC Distribution System

This article proposed the concept of monopolar fault reconfiguration for bipolar dc distribution system realized with Bipolar Half Bridge (BiHB) converter. Monopolar fault reconfiguration oriented from bipolar high voltage direct current (HVdc) transmission grid. For dc distribution, the operation aim is to provide uninterrupted power supply to critical load under monopolar fault. It requires interfacing circuit with fault-tolerant capability as well as control design considering both bipolar and monopolar working mode. The proposed BiHB converter achieves mode switching by reconfiguring the converter switching pattern. Considering both monopolar and bipolar working mode, a bipolar modulation method is proposed for coordinate circuit design. Key influence factors relating to the transient voltage spikes have been identified, including the fault detection delay and the modulation method. Besides, to suppress the voltage spikes caused by monopolar fault, feed-forward compensation is adopted for smooth mode transition. The proposed BiHB converter and control strategy have been comparatively evaluated under fault protection and restoration. Uninterrupted power supply of load has been attained, in validation of the proposed monopolar fault reconfiguration concept.

Modified Three-Port DAB Converter Employing Voltage Balancing Capability for Bipolar DC Distribution System

To construct the bipolar low-voltage dc distribution system, a dual-active-bridge (DAB) converter and a voltage balancer are needed. However, this conventional two-stage circuit is bulky and shows low efficiency. Besides, when a short circuit fault occurs at one of the bipolar dc poles, the conventional topology cannot balance the other normal dc pole voltage due to its single closed circuit, which eventually loses the high reliability of the bipolar system. To mitigate those issues, a modified three-port DAB converter is proposed. The proposed converter is an enhanced topology of the three-port DAB converter, which can balance the bipolar voltage levels without additional voltage balancers. In addition, each bipolar output has an independent closed circuit to balance the normal dc pole under the short circuit fault at one of the dc poles, thereby improving the system reliability. Moreover, constant duty-cycle modulation and magnetizing inductance design are proposed to obtain full-range zero voltage switching (ZVS) of all switches. Since the proposed ZVS modulation uses a constant duty-cycle value, its control can be easily implemented in a digital signal processor. Finally, a 2-kW prototype is developed to validate the effectiveness of the proposed converter.

저압직류 배전계통에서 PV가 전압 불평형에 미치는 영향에 관한 연구

Since the multiple PVs will be connected with bipolar Low-Voltage DC (LVDC) distribution system in the future, several problems caused by PV should be analyzed. In order to speed up to introduction of LVDC distribution system, the effect of PV on the voltage unbalance, which is one of the several issues, should be surely studied because it can lead to the power loss and debase the power quality of LVDC distribution system. This paper has analyzed the voltage unbalance by classifying upstream and downstream of PV. As a result, PV contributes the voltage unbalance to upstream area of Point of Common Coupling (PCC). For downstream area of PCC, however, the voltage unbalance caused by PV is absent theoretically and the voltage unbalance occurred by upstream is just sustained in downstream. Finally, this paper has modeled a test system using ElectroMagnetic Transients Program (EMTP) and the proposed results are verified by the simulation for several cases.

Multiport Converter With Independent Control of AC and DC Power Flows for Bipolar DC Distribution

A multiport ac–dc converter topology is proposed that offers bidirectional power transfer capability between the two dc ports and the ac port. The decoupling of power flows can be exploited as a pole voltage balancing mechanism to accommodate unbalanced dc side loads in bipolar dc distribution systems. The proposed topology gains bipolar operation by using two three-phase two-level voltage-sourced converters connected in a differential fashion with a center-tapped transformer. The analysis shows that no penalties are incurred in terms of increased power ratings for either the grid interface transformer or the semiconductors; only access to the center point of the converter side winding is required. A control scheme for the topology is proposed with the ability to independently control power flows between the different ports. Both the topology and control scheme are validated by extensive simulations and experiments.

Isolated three-port DC–DC converter employing ESS to obtain voltage balancing capability for bipolar LVDC distribution system

A bipolar low-voltage DC (LVDC) distribution system used in residential and building applications requires AC–DC converters and voltage balancers that balance the two DC bus polarities. The bipolar DC bus voltage level only relies on the voltage balancer tied with the AC grid. An isolated DC–DC converter for an energy storage system (ESS) with the voltage balancing capability is proposed to prevent the bipolar voltage level collapse caused by the failure of the grid-tied voltage balancer. The proposed converter topology is an enhanced three-port dual-active-bridge (DAB) converter which can balance the bipolar DC voltage level without complex control. Furthermore, it has less current stress in power switches than that of the conventional three-port DAB converter. The effectiveness of the proposed converter is verified with a 3-kW prototype converter.

Enhanced Dual-Active-Bridge DC–DC Converter for Balancing Bipolar Voltage Level of DC Distribution System

Conventional dual-active-bridge (DAB) dc–dc converters have a limited structure to balance the bipolar voltage level of the bipolar dc distribution system. Therefore, to regulate the bipolar voltage level, additional voltage balancers are needed for the DAB converters, which eventually requires more devices resulting in low power conversion efficiency and low power density. This article proposes an enhanced DAB dc–dc converter that can balance the bipolar voltage level without the additional voltage balancers. The bipolar voltage level can be tightly regulated under entire unbalanced load condition and even when there is no-load at one side of the dc poles. Besides, its zero-voltage-switching capability can be extended to cover the entire load range, including the no-load condition. Furthermore, the proposed converter can be operated by using a simple single phase-shift modulation. Therefore, its control algorithm can be easily implemented in digital controllers. Finally, the theoretical analysis and the effectiveness of the proposed DAB converter are validated by a 3-kW prototype converter.

Analysis of Effective Three-Level Neutral Point Clamped Converter System for the Bipolar LVDC Distribution

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.

Single-Stage Voltage Balancer With High-Frequency Isolation for Bipolar LVDC Distribution System

Conventional voltage balancers for the bipolar low-voltage dc (LVdc) distribution system have been composed of multiple power stages, which degrade their power density and cost effectiveness. To minimize the power stages, this paper presents a single-stage voltage balancer with high-frequency isolation. This voltage balancer is based on the three-level dual-active-bridge converter. Furthermore, for the proposed voltage balancer, an enhanced switching modulation, using the two switching patterns $\alpha$ and $\beta$ , is proposed to fully balance the bipolar voltage level under unbalanced load condition and even for when one of the dc poles has no load. Besides, a closed-loop control system can be implemented by using output voltage information and proportional-integral controllers. Finally, experimental results using a 3-kW laboratory prototype converter verify the effectiveness of the proposed voltage balancer.

Symmetrical Component Decomposition of DC Distribution Systems

Employing bipolar dc distribution systems introduces the possibility of imbalance in the system. To analyze these systems it is important to create novel modelling techniques. Therefore, this paper presents a method to decompose dc distribution systems into symmetrical components. The presented method simplifies the analysis of balanced, unbalanced, and fault conditions of bipolar dc distribution systems. Furthermore, equivalent circuits for several network components in the symmetrical domain are derived and are shown to be independent under symmetrical conditions. Additionally, a dynamic analysis is performed in the symmetrical domain showing that the method simplifies the analysis of dc distribution systems. Finally, the symmetrical domain equivalent circuits of several fault conditions are derived.

Voltage Regulation Method for Voltage Drop Compensation and Unbalance Reduction in Bipolar Low-Voltage DC Distribution System

Owing to the development of highly efficient electric power converters, the number of studies on the construction of low-voltage dc (LVDC) systems is gradually increasing. To use LVDC distribution systems, voltage regulation methods to compensate for the voltage drop and to limit the voltage unbalance are essential. In a bipolar LVDC distribution system, a voltage drop and voltage unbalance could occur because of the load current and the variation in the amount of power supplied to the poles. However, not enough research has been conducted on voltage regulation methods for LVDC distribution systems. To reduce the voltage drop and the voltage unbalance, this paper proposes a voltage regulation method based on the neutral to line drop compensation (NLDC) method, which employs a modified LDC to calculate the sending-end reference voltage. In the NLDC method, the neutral current and neutral line impedance are taken into consideration to compensate for the neutral line potential fluctuation and voltage drop on the pole. The next sending-end voltage is determined by checking the voltage unbalance factor, and it is adjusted to maintain it within the allowable voltage unbalance factor. A voltage regulation algorithm and a bipolar LVDC system model are implemented using the Electromagnetic Transients Program.

An Improved Control and Energy Management Strategy of Three-Level NPC Converter Based DC Distribution Network

To meet the challenge of large-scale renewable energy penetration and take full advantage of existing AC infrastructure, the bipolar DC distribution system is of interest. In this article, the system structure and characteristics of the bipolar DC distribution network are proposed. The three-level Neural Point Clamped Converter (NPC) is used in the proposed system to construct the bipolar DC system. To optimize the DC system performance, an improved cooperative control and energy management strategy is proposed mainly to mitigate DC voltage fluctuation and balance the positive and negative phase voltage. The improved strategy consists of (1) 2-degree of freedom (2DOF) PID controller in traditional voltage control loop; (2) cooperative controller to take full advantage of storage system; (3) voltage equalization controller to balance two-phase voltages; and (4) the energy management system to dispatch the response job to batteries and supercapacitors. Experiments and simulations are performed to validate the effectiveness of the proposed strategy.

Mitigation of voltage unbalance by using static load transfer switch in bipolar low voltage DC distribution system

In this paper, a method is proposed to mitigate voltage unbalance and to reduce power loss due to neutral current in a bipolar Low-Voltage DC (LVDC) distribution system by using a static load transfer switch (SLTS). Furthermore, an algorithm to determine the proper position of loads by using the SLTS is presented; this algorithm is developed by considering the neutral current at each load point. The underlying strategy of the SLTS method is that a local substation generates switching signals by using the proposed algorithm on the basis of data measured by a DC/DC converter; thereafter, SLTSs, which are placed in the DC/DC converter, are operated to reconfigure the structure of loads. To evaluate the performance of the proposed method, the conventional method for calculating the percent voltage unbalance for an AC system is modified and made applicable for a DC system. The modeling of a 1500V bipolar LVDC distribution system carried out using the ElectroMagnetic Transients Program (EMTP) is also presented herein. Finally, a simulation carried out by employing the SLTS method under various load conditions is presented; the results show a decrease in power loss and mitigation of voltage unbalance.