DC microgrids are gaining popularity due to high efficiency, high reliability, and easy interconnection of renewable sources as compared to the ac system. Control objectives of dc microgrid are: 1) to ensure equal load sharing (in per unit) among sources; and 2) to maintain low-voltage regulation of the system. Conventional droop controllers are not effective in achieving both the aforementioned objectives simultaneously. Reasons for this are identified to be the error in nominal voltages and load distribution. Though centralized controller achieves these objectives, it requires high-speed communication and offers less reliability due to single point of failure. To address these limitations, this paper proposes a new decentralized controller for dc microgrid. Key advantages are high reliability, low-voltage regulation, and equal load sharing, utilizing low-bandwidth communication. To evaluate the dynamic performance, mathematical model of the scheme is derived. Stability of the system is evaluated by eigenvalue analysis. The effectiveness of the scheme is verified through a detailed simulation study. To confirm the viability of the scheme, experimental studies are carried out on a laboratory prototype developed for this purpose. Controller area network protocol is utilized to achieve communication between the sources.

/pdf/distributed-control-to-ensure-proportional-load-sharing-and-4kwfhqh1ww.pdf

Distributed Control to Ensure Proportional Load Sharing and Improve Voltage Regulation in Low-Voltage DC Microgrids

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.

Distributed Cooperative Control of DC Microgrids

Solid-state switch mode AC-DC converters having high-frequency transformer isolation are developed in buck, boost, and buck-boost configurations with improved power quality in terms of reduced total harmonic distortion (THD) of input current, power-factor correction (PFC) at AC mains and precisely regulated and isolated DC output voltage feeding to loads from few Watts to several kW. This paper presents a comprehensive study on state of art of power factor corrected single-phase AC-DC converters configurations, control strategies, selection of components and design considerations, performance evaluation, power quality considerations, selection criteria and potential applications, latest trends, and future developments. Simulation results as well as comparative performance are presented and discussed for most of the proposed topologies.

Comprehensive Study of Single-Phase AC-DC Power Factor Corrected Converters With High-Frequency Isolation

Due to the variable characteristics of renewable generation, batteries used in renewable-power systems can undergo many irregular, partial charge/discharge cycles. In turn, this can also have a detrimental effect on battery lifetime and can increase project costs. This study presents a method of improving battery lifetime in a small-scale remote-area wind-power system by the use of a battery/supercapacitor hybrid energy storage system. The supervisory control algorithm and hardware implementation are described and projected long-term benefits of the proposed system are assessed by simulation. A representative dynamic model of the overall system, incorporating realistic wind-speed and load-power variations has been developed. An analysis is presented of the potential improvement in battery lifetime that is achievable by diverting short-term charge/discharge cycles to a supercapacitor energy-storage system. This study introduces a method by which supercapacitor energy storage systems and control algorithms can be evaluated and implemented in the application area considered. The composition of a prototype test system is described and experimental results are presented to demonstrate system feasibility.

Analysis of Battery Lifetime Extension in a Small-Scale Wind-Energy System Using Supercapacitors

This paper presents power loss comparison of single- and two-stage grid-connected photovoltaic (PV) systems based on the loss factors of double line-frequency voltage ripple (DLFVR), fast irradiance variation + DLFVR, fast dc load variation + DLFVR, limited operating voltage range + DLFVR, and overall loss factor combination. These loss factors will result in power deviation from the maximum power points. In this paper, both single-stage and two-stage grid-connected PV systems are considered. All of the effects on a two-stage system are insignificant due to an additional maximum power point tracker, but the tracker will reduce the system efficiency typically about 2.5%. The power loss caused by these loss factors in a single-stage grid-connected PV system is also around 2.5%; that is, a single-stage system has the merits of saving components and reducing cost, and does not penalize overall system efficiency under certain operating voltage ranges. Simulation results with the MATLAB software package and experimental results have confirmed the analysis.

Power Loss Comparison of Single- and Two-Stage Grid-Connected Photovoltaic Systems

The establishment and control for a switched-reluctance generator (SRG) are presented in this paper. First, the switching behavior and the DC-link ripple characteristics of a SRG are studied. Accordingly, its circuit and control models are established. And the power circuit components are properly designed to minimize the DC-link voltage ripples caused by commutation and pulsewidth modulation switching. Second, the quantitative voltage feedback control of the SRG system considering the effects of voltage ripples is made. In this issue, a unified approach is developed to estimate the plant dynamic model parameters from measurements. Then conversely, the feedback controller is designed according to the prescribed regulation voltage control specifications. Third, a dynamic commutation shift controller (DSC) is devised to enhance the voltage control performance of a SRG. As the voltage feedback control is failed when winding current becomes single-pulse mode, the proposed DSC is actuated automatically. The turn-on and turn-off angles are dynamically and automatically shifted to assist voltage feedback regulation control. More stable and better regulation control performance can be obtained. Finally, the energy conversion efficiency improvement via static commutation shift under static operation is evaluated experimentally. Theoretical bases of the proposed control approaches are derived, and their effectiveness is demonstrated by some simulation and measured results.

On the Design of Power Circuit and Control Scheme for Switched Reluctance Generator

This paper presents a dc microgrid including a switched-reluctance generator (SRG) with isolated boost dc-dc converter, a battery energy storage system, and a single-phase three-wire (1P3W) load inverter for generating ac 220/110 V 60-Hz outputs. The 400-V dc grid is established by the SRG with dc 48-V output followed by a current-fed push-pull (CFPP) dc-dc converter. The robust commutation and dynamic control schemes are developed for the SRG to have excellent output performances. For providing the domestic load power sources, a 1P3W transformerless 220/110 V inverter is established. A master and slave robust control schemes are proposed to yield balanced sinusoidal output voltage waveforms in both voltage outputs under varying load conditions. To improve the dc grid power supporting reliability, a battery energy storage system with bidirectional buck-boost interfacing converter is established. It can support the common dc bus voltage immediately when the main power source fault occurs. Conversely, the battery bank can be charged from the common dc bus through the same converter. Particularly, an auxiliary charger formed by a flyback switch-mode rectifier is equipped to allow the battery to be charged from the plug-in utility power as the long duration of microgrid fault occurs. Normal operation and good operating performance of the established microgrid are verified experimentally.

Establishment of a Switched-Reluctance Generator-Based Common DC Microgrid System

The speed control of the surface-mounted permanent-magnet synchronous motor (SPMSM) based on Takagi–Sugeno (T–S) fuzzy models is designed and implemented in this paper. In motor drive speed control, proportional-integral controller is generally applied. However, the tracking performance and load regulation capability should be compromised in the designing process of the controller. This study develops a speed controller based on the T–S fuzzy model with good tracking ability and load variation regulation. First, the dynamic model of an SPMSM is demonstrated and the load torque variation is treated as disturbances. Accordingly, the speed control strategy is developed based on the T–S fuzzy model. The defuzzification process and parallel distributed compensation are introduced. The ability of disturbance suppression and the Lyapunov stability of this system are established and analyzed. The controller gains are obtained via the linear matrix inequality toolbox. The speed control and all essential procedures are realized by the microcontroller Renesas RX62T. The tracking performance and load regulation capability are verified by the experimental results.

Speed Control of the Surface-Mounted Permanent-Magnet Synchronous Motor Based on Takagi–Sugeno Fuzzy Models

This paper develops the operational control of two maximum power point trackers (MPPTs) for two-string photovoltaic (PV) panels in dc distribution systems. This dc distribution system is connected to ac grid via a bidirectional inverter. Two PV strings and two MPPTs are implemented in this system. The proposed MPPT topology consists of buck and boost converters to deal with wide output voltage range of PV panels. To accurately determine the input current of MPPTs, the PV-string configuration check is accomplished online. The perturbation and observation method are applied for maximum power point tracking. Moreover, the current balancing of two MPPT modules in parallel is achieved. In this paper, the system configuration and the operational principle of the proposed MPPT are first introduced. Afterward, the perturbation and observation method and the mode transition are demonstrated. Flowcharts of the online PV-string configuration check and current balancing are explained. The validity of configuration check and current balancing is verified via the experimental results. Maximum power tracking performance and power conversion efficiency are also obtained.

Development and Operational Control of Two-String Maximum Power Point Trackers in DC Distribution Systems

This paper presents two-phase modulated digital control for a three-phase bidirectional inverter with wide inductance variation in dc distribution systems. The bidirectional inverter can fulfill both grid connection and rectification with power factor correction. With the proposed control, the inverter can track its sinusoidal reference currents, and it is allowed to have wide inductance variation, reducing core size significantly. The control laws based on space-vector pulsewidth modulation are first derived with either an accurate approach or an approximated one, and the gate signals were then derived based on two-phase modulation for lower switching loss and switching noise spectra. Determination of control parameters and stability analysis are also presented. In the design and implementation, the inductances corresponding to various inductor currents were measured and tabulated into a single-chip microcontroller for tuning loop gain cycle by cycle, ensuring system stability. Measured results from a 10 kVA 3φ bidirectional inverter have been presented to confirm the feasibility of the discussed control approaches.

Yuan-Chih Chang

Papers

On the Design of Power Circuit and Control Scheme for Switched Reluctance Generator

Establishment of a Switched-Reluctance Generator-Based Common DC Microgrid System

Speed Control of the Surface-Mounted Permanent-Magnet Synchronous Motor Based on Takagi–Sugeno Fuzzy Models

Development and Operational Control of Two-String Maximum Power Point Trackers in DC Distribution Systems

Two-Phase Modulated Digital Control for Three-Phase Bidirectional Inverter With Wide Inductance Variation