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The use of rechargeable batteries for electrical energy storage requires a voltage balancing system. The voltage balancing system requires monitoring in the use of rechargeable batteries so that they can be utilized properly and can improve the electrical energy storage system. In this research, a monitored lead acid battery voltage balancing system was designed so that the management of the battery voltage balance level and the storage of electrical energy in rechargeable batteries can be stored and used optimally. In this research a series of lead acid battery voltage detection and power dissipation circuits were designed. The power dissipation circuit uses the controlled shunt resistor method which is used when the voltage of the lead acid battery being charged exceeds the maximum voltage. This method is easy to implement and can display the value of the lead acid battery voltage and other parameters, so that it can be monitored by the user. The results obtained show that the average voltage error for batteries 1 and 2 is 0.09% and 0.3% respectively. The power dissipation circuit can dissipate lead acid battery power above the reference voltage VREF=±7 V, with a balanced voltage of 6.8 V at 140 minutes and 160 minutes. [ABSTRACT FROM AUTHOR]

The development of power electronics technology and the growth of distributed energy resources have brought attract to bipolar DC microgrid. However, voltage unbalances between positive and negative poles occur in bipolar DC microgrid due to irregular load. To resolve this problem, voltage balancers are being used. Voltage balancers are classified into two types: centralized and distributed. The centralized voltage balancers have the drawback of not resolving voltage unbalances over long-distance lines or to the end-load. The distributed voltage balancers increase the cost due to the construction of additional facilities. To resolve for these disadvantages, the energy storage system is installed in a bipolar DC microgrid. However, energy storage system has the disadvantage of having to be installed individually at each voltage pole. This paper proposes a voltage balancing method that applies the functions of bidirectional converters and voltage balancers to a single energy storage system. By using the proposed method, power is stored and supplied through battery charging/discharging control while simultaneously reducing voltage unbalances between positive and negative poles. Additionally, the proposed method reduces the number of power conversion stages and enhances the reliability of bipolar DC microgrid. The proposed method was validated through simulation analysis and experimental validation using a 2.5 kW capacity prototype.

The increasing demand for electrical energy must be balanced with using renewable energy. However, the use of renewable energy requires a system to maximize the conversion of renewable energy into electrical energy. Various topologies have been studied to achieve maximum energy conversion. Two types of inverters, namely step up-down, have been widely used but have their respective limitations. Step-down inverters can only be used in lower output voltage than input voltage conditions with simple control, while step-up inverters can only operate in higher output voltage than input voltage conditions. This paper aims to combine these two converters to have both step-up-down voltage functions, with the goal of expanding the operating range and maintaining the advantages of each type. Thus, a topology called a step-up-down five-level inverter with a simple voltage-controlled capacitor balancing system is proposed in this paper. Finally, the simulation and laboratory tests were done. This inverter operates in step-up-down voltage, and the voltage in capacitors is always balanced to reach the desired level. The voltage control was done to get the constant voltage event the load was changed.

DC microgrids are now starting to attract huge attention from researches and engineers. Among the several DC microgrid architectures, bipolar DC microgrids are advantageous to accommodate a wider range of DC loads. However, due to mismatched loads connected to the two poles, bipolar DC microgrids may show voltage balancing issues. In this context, this article presents a new high input-to-output voltage gain, non-isolated DC–DC converter specifically designed for bipolar DC Microgrids. The proposed converter supports the balance of voltages at both bipolar DC microgrid poles, as the converter can transfer energy between the two poles in an unbalanced way. The description, theoretical support, and operation of the proposed converter will be presented. The theoretical assumptions will be complemented with several results obtained from computer simulation tests. The simulation tests will be compared to laboratory results from an experimental prototype. The obtained results confirm the theoretical properties of the proposed converter. [ABSTRACT FROM AUTHOR]

The unbalanced loads and distributed generations (DGs) will lead to an unbalanced current in the neutral line and further result in a voltage imbalance which will increase circuit power consumption and affect the stable operation of the bipolar DC microgrid. The application of a voltage balancer (VB) can address the issue of unbalanced voltage and current. In this paper, the proposed coordinated control strategy can realize VB to provide a reference voltage to voltage source converter (VSC), battery energy management system (BEMS), and photovoltaic (PV) power generation. The essence of the proposed coordinated control strategy is to adaptively adjust the voltage droop coefficient, which can realize the suppression of unbalanced voltage, power sharing, and regulation of battery state of charge (SOC). In this research, the coordinated control structure of VB and the control scheme of BEMS are investigated. In addition, the small-signal model of the bipolar DC microgrid is also presented to show the system involved is stable. Subsequently, the effectiveness of the proposed coordinated control strategies is verified in MATLAB/Simulink software.

Bipolar DC microgrids (BDCMGs) have several issues related to the voltage and require numerous converters to supply power to both poles. To solve these issues, a bidirectional dual-input dual-output (DIDO) converter is proposed for the voltage balancer in BDCMG. The DIDO converter has dual-input sources and a dual-output port connected to the grid. Additionally, the DIDO converter simultaneously performs independent bidirectional power control and voltage balancing control. Based on the input voltages, this paper proposes modulation methods for three cases. The modulation method of the second case has a wide operating range and low balancing current ripple without increasing the switching frequency. Moreover, only voltage balancer mode without active input sources is proposed, considering the intermittent source. Therefore, it can operate as a voltage balancer under all conditions. The voltage balancing performance of the three cases was analyzed. Finally, the proposed modulation and control method of the DIDO converter were verified through experimental results. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents an Multi-Input Multi-Output (MIMO) analysis to investigate the mutual interactions and small-signal stability of bipolar-type dc microgrids. Since bipolar dc microgrid is replete with power-electronic converters, its dynamics can not be understood unless the interactions among control systems of converters are properly investigated. To tackle the challenge, each converter in microgrid is modeled via an MIMO transfer matrix. Then, the MIMO models are combined together based on the interactions among the control systems of source and load converters. From this integrative MIMO model, the mutual interactions between various input-output pairs are quantified using Gershgorin Band theorem. Also, Singular Value Decomposition (SVD) analysis is carried out to estimate the frequency of unstable poles. Test results not only successfully validate the effectiveness of the MIMO method but also show that the control system of voltage balancer has a major impact on the overall stability of bipolar dc microgrid, making it a suitable location for applying damping systems.

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. [ABSTRACT FROM AUTHOR]

This paper proposes a multi-loop voltage balancing strategy for half-bridge voltage balancers. The proposed multi-loop scheme consists of a voltage controller and a current variation controller without current sensors. The proposed current variation controller (CVC) aims to regulate the derivative of the average inductor current to be zero at the steady-states as well as to increase the damping of the entire control loop. Furthermore, the CVC is implemented based on a current sensorless scheme. For this purpose, a simple method for detecting current variation by using dc-link voltages and a control model is also proposed. To secure the stability of the proposed CVC with the current sensorless method, a well-known type II regulator is adopted, and its design procedure is detailed. The proposed multi-loop strategy is applied to a 2.4-kW half-bridge voltage balancer. Both simulation and experimental results show good agreements with the theoretical analyses. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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.

In this paper, a hierarchical control strategy is introduced to control a new three-port multidirectional DC-DC converter for integrating an energy storage system (ESS) to a bipolar DC microgrid (BPDCMG). The proposed converter provides a voltage-balancing function for the BPDCMG and adjusts the states of charge (SoC) of the ESS. Previous studies tend to balance the voltage of the BPDCMG buses with active sources or by transferring power from one bus to another. Furthermore, the batteries available in BPDCMGs were charged equally by both buses. However, this power sharing method does not guarantee efficient operation of the whole system. In order to achieve a higher efficiency and lower energy losses, a triple-layer hierarchical control strategy, including a primary droop controller, a secondary voltage restoration controller and a tertiary optimization controller are proposed. Thanks to the multi-functional operation of the proposed converter, its conversion stages are reduced. Furthermore, the efficiency and weight of the system are both improved. Therefore, this converter has a significant capability to be used in portable BPDCMGs such as electric DC ships. The converter modes are analyzed and small-signal models of the converter are extracted. Comprehensive simulation studies are carried out and a BPDCMG laboratory setup is implemented in order to validate the effectiveness of the proposed converter and its hierarchical control strategy. Simulation and experimental results show that using the proposed converter mitigates voltage imbalances. As a result, the system efficiency is improved by using the hierarchical optimal power flow control. [ABSTRACT FROM AUTHOR]

In this paper, a new three-port multidirectional DC-DC converter is proposed for integrating an energy storage system (ESS) to a bipolar DC microgrid (BPDCMG). The proposed converter provides a voltage-balancing function for the BPDCMG and adjusts the charge of the ESS. Thanks to the multi-functional operation of the proposed converter, the conversion stages of the system are reduced. In addition, the efficiency and weight of the system are improved. Therefore, this converter has a significant capability when it comes to use in portable BPDCMGs such as electric DC ships. The converter modes are analyzed and small-signal models of the converter in each of the independent modes are extracted. Finally, comprehensive simulation studies are carried out and a BPDCMG laboratory prototype is implemented in order to verify the performance of the proposed voltage balancer using the burst mode control scheme. [ABSTRACT FROM AUTHOR]

This paper proposes an optimal power flow (OPF) formulation for bipolar DC microgrids. The bipolar DC microgrid can integrate distributed generations (DGs) and supply power to end-users with high effectiveness and reliability. In comparison with the existing power flow methods for DC grids, which are based on power injection, the current injection power flow method should be adopted for bipolar DC microgrids to entirely consider the bipolar structure. In this regard, the OPF established herein is based on the current injection power flow method. The output current of DGs as well as pole voltages should be adopted as decision variables to utilize the linearity of the power flow equation. The optimal set point of the output power and balanced voltage level of the DGs are determined with respect to three objective functions: generation cost, voltage unbalance, and network loss. Depending on the existence of the constant power load and the generation cost function that cause bilinear equality constraints, the proposed OPF model is classified as a convex quadratic programming (QP) or a nonconvex quadratically constrained QP problem, which can be solved using commercial solvers. The case study validates that the proposed OPF formulation can find the optimal operating point with low computational burden at various grid conditions and operational priority. • Optimal power flow formulation for bipolar DC microgrids is proposed. • Proposed OPF utilizes current injection power flow method to entirely consider bipolar DC structure. • Proposed OPF determines optimal output power and balanced voltage level of distributed generations. • Developed method can be solved by off-the-shelf solvers with low computational burden. [ABSTRACT FROM AUTHOR]

This paper proposes a generic power flow algorithm for bipolar DC microgrids based on the Newton–Raphson (NR) method. The bipolar DC microgrid has gained considerable attention for its effectiveness and reliability with regard to power supply. To date, however, power flow algorithms for bipolar DC networks have not been fully investigated and generalized, although power flow analysis is an essential tool, with diverse applications ranging from network design to real-time control. In this study, a current injection-based power flow algorithm is established that considers the grounding scheme and voltage control method of energy source. Six bus types are defined, depending on the grounding scheme and voltage control combination. The unknown pole voltages and equations to be iteratively solved are identified. Afterwards, the unknown pole voltages are updated via a single NR method, which makes the proposed method straightforward and efficient. The case study validates that the proposed power flow algorithm can find the bus voltages of each pole sufficiently close to the simulation results of PSCAD/EMTDC, while having a low computational burden. • Generic power flow algorithm for bipolar DC microgrids is proposed. • Proposed algorithm considers grounding schemes and control modes of energy sources. • Proposed algorithm aids various power system applications for bipolar DC microgrids. • Developed method has high accuracy and low computation time with single NR method. [ABSTRACT FROM AUTHOR]

This paper analyzes the load unbalance problem and voltage fluctuation problem in a 3-wire DC distribution system. It also analyzes a solution to these problems; a positive Buck-Boost voltage balancer is proposed and explored in order to fulfill the requirements of high quality power supply for the loads on its load side. Compared with the conventional balancer, a positive Buck-Boost converter is added to solve the voltage fluctuation problem, and the theories and methods of the voltage balancer are extended to analyze the working principle, derive the design equations, explore the stability, and calculate the efficiency. Both simulations and small power experiments are carried out to verify the validity of the working principle, the topology, and the control strategy. [ABSTRACT FROM AUTHOR]

Micro-dc grid is a novel power system focused on the development of renewable resources. However, two-wire transmitting power mode is generally accepted in a micro-dc grid, which is usually not suitable for the requirements of the input voltage levels of different power converters and loads. In order to meet the requirements, a half-bridge voltage balancer was introduced in a micro-dc grid, which can convert a two-wire mode into a three-wire mode in a micro-dc grid via a neutral line. However, the shoot-through problem existing in bridge-type converters degrades the reliability of the voltage balancer. In this paper, a dual-buck half-bridge voltage balancer and a control strategy are proposed, which can avoid the shoot-through problem. The small-signal model of the voltage balancer is derived for designing the control parameters and the current relationships of the inductors; the capacitors and the unbalanced loads are analyzed particularly. Finally, a prototype, which can deal with 2-kW unbalance ability, is built to verify that the proposed voltage balancer may have a good ability of balancing the voltage by building a neutral line. [ABSTRACT FROM PUBLISHER]

In a bipolar DC distribution network, the unbalanced load resistance, line resistance and renewable energy source will cause an unbalanced current for each node of the neutral line and lead to its unbalanced voltage. This is a unique power quality problem of bipolar DC distribution networks, which will increase the power loss in the network and lead to overcurrent protection of the neutral line in serious cases. A voltage balancer can be adopted to suppress the unbalanced voltage and current. However, the existing literature does not consider the consistent application of multiple voltage balancers in a multi-node bipolar DC distribution network. This paper creatively proposes a consensus control topology combining primary control and secondary control in a radial multi-node bipolar DC distribution network with voltage balancers. In this paper, the formulas for the positive and negative current and duty cycle of a bipolar DC distribution network with voltage balancers are derived, and improved voltage balancer modeling based on a consensus algorithm is built. The radial multi-node bipolar DC distribution network is established in MATLAB/Simulink. The simulation results compare the consensus control with the traditional droop control and verify the effectiveness of the new control structure with voltage balancers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]