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Journal ArticleDOI

Distribution Voltage Control for DC Microgrids Using Fuzzy Control and Gain-Scheduling Technique

01 May 2013-IEEE Transactions on Power Electronics (IEEE)-Vol. 28, Iss: 5, pp 2246-2258
TL;DR: In this paper, a low-voltage bipolar-type dc microgrid with two energy storage units and a dc/dc converter was proposed to achieve both power sharing and energy management.
Abstract: Installation of many distributed generations (DGs) could be detrimental to the power quality of utility grids. Microgrids facilitate effortless installation of DGs in conventional power systems. In recent years, dc microgrids have gained popularity because dc output sources such as photovoltaic systems, fuel cells, and batteries can be interconnected without ac/dc conversion, which contributes to total system efficiency. Moreover, high-quality power can be supplied continuously when voltage sags or blackouts occur in utility grids. We had already proposed a “low-voltage bipolar-type dc microgrid” and described its configuration, operation, and control scheme, through experiments. In the experiments, we used one energy storage unit with a dc/dc converter to maintain the dc-bus voltage under intentional islanding operation. However, dc microgrids should have two or more energy storage units for system redundancy. Therefore, we modified the system by adding another energy storage unit to our experimental system. Several kinds of droop controls have been proposed for parallel operations, some of which were applied for ac or dc microgrids. If a gain-scheduling control scheme is adopted to share the storage unit outputs, the storage energy would become unbalanced. This paper therefore presents a new voltage control that combines fuzzy control with gain-scheduling techniques to accomplish both power sharing and energy management. The experimental results show that the dc distribution voltages were within 340 V ± 5%, and the ratios of the stored energy were approximately equal, which implies that dc voltage regulation and stored energy balancing control can be realized simultaneously.
Citations
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Journal ArticleDOI
TL;DR: In this paper, a review of control strategies, stability analysis, and stabilization techniques for dc microgrids is presented, where overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level.
Abstract: This paper presents a review of control strategies, stability analysis, and stabilization techniques for dc microgrids (MGs). Overall control is systematically classified into local and coordinated control levels according to respective functionalities in each level. As opposed to local control, which relies only on local measurements, some line of communication between units needs to be made available in order to achieve the coordinated control. Depending on the communication method, three basic coordinated control strategies can be distinguished, i.e., decentralized, centralized, and distributed control. Decentralized control can be regarded as an extension of the local control since it is also based exclusively on local measurements. In contrast, centralized and distributed control strategies rely on digital communication technologies. A number of approaches using these three coordinated control strategies to achieve various control objectives are reviewed in this paper. Moreover, properties of dc MG dynamics and stability are discussed. This paper illustrates that tightly regulated point-of-load converters tend to reduce the stability margins of the system since they introduce negative impedances, which can potentially oscillate with lightly damped power supply input filters. It is also demonstrated that how the stability of the whole system is defined by the relationship of the source and load impedances, referred to as the minor loop gain. Several prominent specifications for the minor loop gain are reviewed. Finally, a number of active stabilization techniques are presented.

1,131 citations


Cites background from "Distribution Voltage Control for DC..."

  • ...In [14], fuzzy control-based gain-scheduling droop control is proposed to dy-...

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  • ...Except for setting operating modes and managing secondary/tertiary control, communication technology can also be used to realize advanced functions, such as unit commitment, optimization procedures, or manipulation of internal I–V characteristics by imposing adaptive mechanisms [14], [28]–[30]....

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Journal ArticleDOI
TL;DR: In this article, an overview of the state of the art in dc microgrid protection and grounding is provided, which discusses both design of practical protective devices and their integration into overall protection systems.
Abstract: DC microgrids (MGs) have been gaining a continually increasing interest over the past couple of years both in academia and industry. The advantages of dc distribution when compared to its ac counterpart are well known. The most important ones include higher reliability and efficiency, simpler control and natural interface with renewable energy sources, and electronic loads and energy storage systems. With rapid emergence of these components in modern power systems, the importance of dc in today's society is gradually being brought to a whole new level. A broad class of traditional dc distribution applications, such as traction, telecom, vehicular, and distributed power systems can be classified under dc MG framework and ongoing development, and expansion of the field is largely influenced by concepts used over there. This paper aims first to shed light on the practical design aspects of dc MG technology concerning typical power hardware topologies and their suitability for different emerging smart grid applications. Then, an overview of the state of the art in dc MG protection and grounding is provided. Owing to the fact that there is no zero-current crossing, an arc that appears upon breaking dc current cannot be extinguished naturally, making the protection of dc MGs a challenging problem. In relation with this, a comprehensive overview of protection schemes, which discusses both design of practical protective devices and their integration into overall protection systems, is provided. Closely coupled with protection, conflicting grounding objectives, e.g., minimization of stray current and common-mode voltage, are explained and several practical solutions are presented. Also, standardization efforts for dc systems are addressed. Finally, concluding remarks and important future research directions are pointed out.

964 citations


Cites background from "Distribution Voltage Control for DC..."

  • ...In [20] and [139], the bipolar configuration of...

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Journal ArticleDOI
TL;DR: In this article, a lowbandwidth communication (LBC)-based improved droop control method is proposed to improve the performance of the dc microgrid operation, which does not require a centralized secondary controller.
Abstract: Droop control is the basic control method for load current sharing in dc microgrid applications. The conventional dc droop control method is realized by linearly reducing the dc output voltage as the output current increases. This method has two limitations. First, with the consideration of line resistance in a droop-controlled dc microgrid, since the output voltage of each converter cannot be exactly the same, the output current sharing accuracy is degraded. Second, the dc-bus voltage deviation increases with the load due to the droop action. In this paper, in order to improve the performance of the dc microgrid operation, a low-bandwidth communication (LBC)-based improved droop control method is proposed. In contrast with the conventional approach, the control system does not require a centralized secondary controller. Instead, it uses local controllers and the LBC network to exchange information between converter units. The droop controller is employed to achieve independent operation, and the average voltage and current controllers are used in each converter to simultaneously enhance the current sharing accuracy and restore the dc bus voltage. All of the controllers are realized locally, and the LBC system is only used for changing the values of the dc voltage and current. Hence, a decentralized control scheme is accomplished. The simulation test based on MATLAB/Simulink and the experimental validation based on a 2 × 2.2 kW prototype were implemented to demonstrate the proposed approach.

865 citations


Cites background from "Distribution Voltage Control for DC..."

  • ...In a dc microgrid, no reactive power and harmonics exist in the system, so higher power quality and system efficiency are obtained compared to ac systems [7]–[15]....

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Journal ArticleDOI
TL;DR: In this paper, a cooperative control paradigm is used to establish a distributed secondary/primary control framework for dc microgrids, where the conventional secondary control, that adjusts the voltage set point for the local droop mechanism, is replaced by a voltage regulator and a current regulator.
Abstract: A cooperative control paradigm is used to establish a distributed secondary/primary control framework for dc microgrids. The conventional secondary control, that adjusts the voltage set point for the local droop mechanism, is replaced by a voltage regulator and a current regulator. A noise-resilient voltage observer is introduced that uses neighbors’ data to estimate the average voltage across the microgrid. The voltage regulator processes this estimation and generates a voltage correction term to adjust the local voltage set point. This adjustment maintains the microgrid voltage level as desired by the tertiary control. The current regulator compares the local per-unit current of each converter with the neighbors’ and, accordingly, provides a second voltage correction term to synchronize per-unit currents and, thus, provide proportional load sharing. The proposed controller precisely handles the transmission line impedances. The controller on each converter communicates with only its neighbor converters on a communication graph. The graph is a sparse network of communication links spanned across the microgrid to facilitate data exchange. The global dynamic model of the microgrid is derived, and design guidelines are provided to tune the system's dynamic response. A low-voltage dc microgrid prototype is set up, where the controller performance, noise resiliency, link-failure resiliency, and the plug-and-play capability features are successfully verified.

715 citations


Cites background from "Distribution Voltage Control for DC..."

  • ...ACTIVE distribution systems are moving toward a distributed structure [1]–[3]....

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Journal ArticleDOI
TL;DR: This paper presents an overview of power management strategies for a hybrid ac/dc microgrid system, which includes different system structures, different operation modes, a thorough study of various power management and control schemes in both steady state and transient conditions, and examples of powermanagement and control strategies.
Abstract: Today, conventional power systems are evolving to modern smart grids, where interconnected microgrids may dominate the distribution system with high penetration of renewable energy and energy storage systems. The hybrid ac/dc systems with dc and ac sources/loads are considered to be the most possible future distribution or even transmission structures. For such hybrid ac/dc microgrids, power management strategies are one of the most critical operation aspects. This paper presents an overview of power management strategies for a hybrid ac/dc microgrid system, which includes different system structures (ac-coupled, dc-coupled, and ac–dc-coupled hybrid microgrids), different operation modes, a thorough study of various power management and control schemes in both steady state and transient conditions, and examples of power management and control strategies. Finally, discussion and recommendations of power management strategies for the further research are presented.

707 citations


Cites background from "Distribution Voltage Control for DC..."

  • ...and the DGs can be a part of the droop control or work in MPP [59], [65]–[68]....

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References
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Journal ArticleDOI
01 Nov 2009
TL;DR: The hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to MGs is presented and results are provided to show the feasibility of the proposed approach.
Abstract: DC and AC Microgrids are key elements to integrate renewable and distributed energy resources as well as distributed energy storage systems. In the last years, efforts toward the standardization of these Microgrids have been made. In this sense, this paper present the hierarchical control derived from ISA-95 and electrical dispatching standards to endow smartness and flexibility to microgrids. The hierarchical control proposed consist of three levels: i) the primary control is based on the droop method, including an output impedance virtual loop; ii) the secondary control allows restoring the deviations produced by the primary control; and iii) the tertiary control manage the power flow between the microgrid and the external electrical distribution system. Results from a hierarchical-controlled microgrid are provided to show the feasibility of the proposed approach.

4,145 citations

Journal ArticleDOI
TL;DR: In this paper, a review of electrical energy storage technologies for stationary applications is presented, with particular attention paid to pumped hydroelectric storage, compressed air energy storage, battery, flow battery, fuel cell, solar fuel, superconducting magnetic energy storage and thermal energy storage.
Abstract: Electrical energy storage technologies for stationary applications are reviewed. Particular attention is paid to pumped hydroelectric storage, compressed air energy storage, battery, flow battery, fuel cell, solar fuel, superconducting magnetic energy storage, flywheel, capacitor/supercapacitor, and thermal energy storage. Comparison is made among these technologies in terms of technical characteristics, applications and deployment status.

3,031 citations


"Distribution Voltage Control for DC..." refers background in this paper

  • ...The capital cost per kWh-per cycle for EDLC is 2–4 cents, while that for lead-acid and Li-ion batteries is 20–100 and 15–100 cents, respectively [26]....

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  • ...The capital cost per kilowatthour (kWh) for EDLC is 300–2000 dollars, while that for lead-acid and Li-ion batteries is 200–400 and 600–2500 dollars, respectively [26]....

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Journal ArticleDOI
TL;DR: In this paper, an adaptive decentralized droop controller of paralleled inverter-based distributed generation (DG) units is presented to preserve the power sharing stability, which is based on the static droop characteristics combined with an adaptive transient droop function.
Abstract: This paper addresses the low-frequency relative stability problem in paralleled inverter-based distributed generation (DG) units in microgrids. In the sense of the small-signal dynamics of a microgrid, it can be shown that as the demanded power of each inverter changes, the low-frequency modes of the power sharing dynamics drift to new locations and the relative stability is remarkably affected, and eventually, instability can be yielded. To preserve the power sharing stability, an adaptive decentralized droop controller of paralleled inverter-based DG units is presented in this paper. The proposed power sharing strategy is based on the static droop characteristics combined with an adaptive transient droop function. Unlike conventional droop controllers, which yield 1-DOF tunable controller, the proposed droop controller yields 2-DOF tunable controller. Subsequently, the dynamic performance of the power sharing mechanism can be adjusted, without affecting the static droop gain, to damp the oscillatory modes of the power sharing controller. To account for the power modes immigration at different loading conditions, the transient droop gains are adaptively scheduled via small-signal analysis of the power sharing mechanism along the loading trajectory of each DG unit to yield the desired transient and steady-state response. The gain adaptation scheme utilizes the filtered active and reactive powers as indices; therefore, a stable and smooth power injection performance can be obtained at different loading conditions. The adaptive nature of the proposed controller ensures active damping of power oscillations at different operating conditions, and yields a stable and robust performance of the paralleled inverter system.

1,130 citations

Journal ArticleDOI
TL;DR: In this paper, a low-voltage bipolar-type dc microgrid is proposed to supply super high quality power with three-wire dc distribution line. But, the proposed system is not suitable for large-scale systems.
Abstract: Microgrid is one of the new conceptual power systems for smooth installation of many distributed generations (DGs). While most of the microgrids adopt ac distribution as well as conventional power systems, dc microgrids are proposed and researched for the good connection with dc output type sources such as photovoltaic (PV) system, fuel cell, and secondary battery. Moreover, if loads in the system are supplied with dc power, the conversion losses from sources to loads are reduced compared with ac microgrid. As one of the dc microgrids, we propose “low-voltage bipolar-type dc microgrid,” which can supply super high quality power with three-wire dc distribution line. In this paper, one system for a residential complex is presented as an instance of the dc microgrid. In this system, each house has a cogeneration system (CGS) such as gas engine and fuel cell. The output electric power is shared among the houses, and the total power can be controlled by changing the running number of CGSs. Super capacitors are chosen as main energy storage. To confirm the fundamental characteristics and system operations, we experimented with a laboratory scale system. The results showed that the proposed system could supply high-quality power under several conditions.

880 citations


"Distribution Voltage Control for DC..." refers background or methods in this paper

  • ...We previously proposed a “low-voltage bipolar-type dc microgrid,” and described the configuration, operation, and control scheme, which were demonstrated through experimental results [14], [15]....

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  • ...The dc/dc converters of the storage systems are controlled to maintain the dc distribution voltage within a specified range [15]....

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  • ...In this case, although output power sharing of two energy storage units was achieved, it was found that the storage energy control was necessary for the system operation proposed in [14], [15]....

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  • ...In a three-wire composition, the voltage relative to ground becomes low compared with that in two-wire system, where one of the wires is grounded, and when half bridge inverters are adopted for it, one of the single-phase 100-V output lines can also become a grounded neutral line as long as abiding to the Japanese standard [15]....

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Journal ArticleDOI
TL;DR: In this article, a fuzzy-logic controller for maximum power point tracking of photovoltaic (PV) systems is proposed, which improves the hill-climbing search method by fuzzifying the rules of such techniques and eliminates their drawbacks.
Abstract: A new fuzzy-logic controller for maximum power point tracking of photovoltaic (PV) systems is proposed. PV modeling is discussed. Conventional hill-climbing maximum power-point tracker structures and features are investigated. The new controller improves the hill-climbing search method by fuzzifying the rules of such techniques and eliminates their drawbacks. Fuzzy-logic-based hill climbing offers fast and accurate converging to the maximum operating point during steady-state and varying weather conditions compared to conventional hill climbing. Simulation and experimentation results are provided to demonstrate the validity of the proposed fuzzy-logic-based controller.

578 citations


"Distribution Voltage Control for DC..." refers methods in this paper

  • ...A fuzzy control was applied to maximum power point tracking control of a photovoltaic system in an isolated microgrid to improve the response against rapidly changing weather conditions [24]....

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