Robust and autonomous dc bus voltage control and stability analysis for a dc microgrid

DC microgrids are composed of loads, renewable sources, and storage devices that require control and protection to operate safely. The flyback converter is an alternative to connect paralleled batteries with nominal voltage DC buses; however, until now, complex controllers have been proposed, making difficult their implementation. On the other hand, when the voltage of a DC microgrid is not properly controlled, the loads may be damaged due to the voltage outside of the safe range. Therefore, proposed in this paper are two adaptive PI-structures to control a battery charger based on a flyback converter to be used in DC microgrids. The first adaptive current controller regulates the magnetizing current for stabilizing the system, and the second adaptive voltage controller regulates the voltage of the DC bus to protect the elements of the microgrid. The methodology to design the adaptive parameters of the PI-structures is developed as follows: first, the power stage of the flyback converter is introduced to derive a control-oriented model. The battery and the DC bus of the microgrid, which are interfaced by the flyback converter, are represented with widely accepted approaches. The second step is focused on modeling the system. The flyback converter, which includes a capacitance to model the DC microgrid, is represented by a dynamic model. The differential equations are averaged, and several transfer functions of the main variables are obtained. In the third step, the transfer functions are used to design the PI adaptive current controller and the PI adaptive voltage controller. In the last step, several recommendations are made to implement the power and control stages in low-cost hardware. An application example with realistic parameters is carried out in PSIM to validate the controller loops design. A battery of 12 V is connected to a DC microgrid of 48 V through a flyback converter with a switching frequency of 50 kHz. The settling time and deviation of the DC microgrid voltage, after a perturbation, are 0.845 ms and 2.04 V respectively, while the maximum values are adjusted to be 1 ms and 2.4 V. The simulation results validate the proposed procedure and the effectiveness of the PI-structures in regulating the magnetizing current and the DC bus voltage.

This paper presents a power-based droop controller for distributed energy resources converters in dc microgrids that are connected to the utility ac grid by a grid-interface converter. Typically, when dc microgrids are interfaced to ac power systems, the microgrid dc bus voltage can be affected by fluctuations caused, for example, by background harmonics present in the ac systems or by fluctuating power absorptions, as is the case, for example, with single-phase rectifiers. By means of notch filters included in the converter controller, the solution proposed herein allows DERs converters to track given power references without being affected by dc bus voltage fluctuations. However, due to the high attenuation and the phase lag induced by notch filters, instability issues may arise. For this reason, a modified notch filter with a smaller phase lag is proposed. Moreover, an output current feedforward method which also enhances the system stability is presented. Experimental results from a dc microgrid prototype composed of four DERs converters and a grid-interface converter, all with rated power of 5kW, are reported.

In order to improve the reliability and the stability of DC microgrids (MG) and performance of DC bus voltage regulator, at the same time, a new concept of DC bus control signal (DBCS) algorithm based on pulse frequency variation is proposed. In conventional power management algorithms such as centralized control, power line signaling (PLS), and DC bus signaling (DBS), the reliability of DC MGs could be degraded. In this paper, the switching ripple carried on the dc-bus voltage is presented as the operating information for communication between interfaced converters in DC MG. By varying the switching frequency of a grid-connected DC-DC converter according to the load conditions, all DC MG components connected to the DC bus can be aware of the operating status. Experimental results verify the practical feasibility and the effectiveness of the proposed strategy through the 3.3 kW prototype dual active bridge converter.

The stability of dc and ac bus voltage is of the most important issues in all microgrids including ac, dc or ac/dc hybrid microgrids. In this paper, a hybrid ac/dc microgrid is proposed to reduce processes of multiple reverse conversions in an ac or dc microgrid and to facilitate the connection of various renewable ac and dc sources and loads to power system. Also, all control schemes used among all converters are developed in order to improve the voltage stability of the dc grid. Results are achieved considering the uncertainty of the generators and loads existed in microgrid verifying the robustness of the controllers to restore and stabilize quickly the voltage of the dc grid.

In this paper, an integral terminal sliding mode controller (ITSMC) based on a modified exponential reaching law (MERL) is developed for providing large-signal DC-bus voltage stability while smoothing power flow in DC microgrids (DCMGs). It is worth mentioning that this control approach is not employed in DCMG applications yet to adjust the DC-bus voltage while preserving power balance. The proposed DCMG is made up of a solar photovoltaic (PV) unit, a battery energy storage system (BESS), and DC loads. A DC-DC boost converter (DDBC) and a bidirectional DC-DC converter (BDDC) are employed to connect the solar PV and BESS, respectively, with the DC-bus, which not only controls the output power of these units but also regulates the DC-bus voltage. First, a detailed dynamical model including external disturbances is developed for each component, i.e., the solar PV and BESS. Then, the proposed control approach is employed on these units to get their corresponding control signals. Afterward, the overall stability of each unit is ensured using the Lyapunov stability theory. Moreover, to ensure the robustness of the proposed controller, external disturbances are also bounded based on the value of user-defined constants. Finally, simulation results are used to evaluate the effectiveness of the proposed control approach in a variety of operational scenarios. Additionally, simulation results of the proposed control strategy are compared to those of existing controllers to demonstrate its superiority.

Grid connected DC Microgrid (DCMG) with wind turbine and solar PV distributed generations (DG), battery storage system and constant power loads (CPL) which are integrated via power electronic converter is presented in this paper. In DCMG interfaced with multiple parallel converters the stability of dc bus can be deteriorated by DG output power intermittency and microsources converter output impedance and load converter input impedance mismatches. Thus to solve these problems, this paper presents an autonomous control using decentralized control scheme based on dc bus voltage as means of communication is proposed in this paper. Droop control strategy for power sharing among the different sources and to stabilize bus voltage is employed. The stability analysis of the system is performed using Lyapunov stability criteria to determine the qualitative behavior of the system based on the simplified equivalent source transmission line. The control method verified via MATLAB/SIMULILK simulation.

Direct current micro-grid (DCMG) based on renewable energy sources (RES) is facing an issue of maintaining standard voltage at DC bus under variable power generation and utilization. This paper proposes the DCMG based on distributed energy sources (DES) and buck-boost converter to maintain standard voltage at DC bus. Photovoltaic panel (PV), wind turbine (WT) and battery are utilized as DES. PV, battery and WT are linked to DC bus through buck-boost converter. The simulation model of PV and WT has been developed using MULTISIM simulation tool. To measure the performance of the designed system, the DC load of 500 watt has been organized for DCMG system. The protection from reverse current flow and over voltage is also provided. The simulation outcomes verified that, at DC bus standard voltage has been maintained for both utilization and generation.

It is known that the constant power load (CPL) property of load converter can degrade the stability of DC bus voltage of cascaded power converters system. An idea by combining virtual capacitor (VC) based active damping control (ADC) method with DC bus voltage conditioner (DBVC) technique for load converter is proposed in this paper. Through implementing common virtual capacitance volume requirement, the DC bus voltage can be stabilized with reduced DC bus capacitor, the impact from DC bus voltage fluctuation to the load converter output voltage can be reduced compared with the case by using ADC method only, and because the VC method shares part of the compensation, the power volume of DBVC can be decreased accordingly, this might result in higher power density and lower power loss potentially compared to the case with DBVC only. Simulation results are provided to demonstrate the validation of the proposed method.

In DC microgrids, distributed energy storage plays a key role in stabilizing the DC bus voltage. The bidirectional DC/DC converter in the distributed energy storage system should be designed according to the voltage level and electromagnetic isolation requirements, and multiple energy storage units should be coordinated for load current distribution according to the state of charge (SOC). This paper proposes a SOC power index droop control strategy by communication lines to coordinate the fast and high-precision distribution of load current among multiple energy storage units, and the SOC between energy storage units quickly converges to a consistent state. Considering that communication lines are susceptible to interference, this paper further proposes an improved SOC power index droop control to overcome the effects of communication line failures. Considering the high cost of the energy storage unit, it should be connected to the DC microgrid in layers to achieve a reasonable allocation of resources in practical applications. In order to provide high-quality power to a large power grid, the quantification standards of the DC bus fluctuation range and the working range of each converter are further discussed to maximize the stability of the DC bus voltage and grid-connected power fluctuation.

Due to the uneven distribution of virtual resistance and line impedance in the DC micro-grid, the traditional droop control has the contradiction between current sharing and voltage regulation. In this paper, the droop control based on the discrete consensus algorithm is adopted. Each power generation unit adopts adjacent communication, and the reference value of the DC bus voltage is corrected according to the mean value after the iterative of the system voltage, thereby compensating for the voltage drop caused by the droop control. The photovoltaic (PV) cell adopts the maximum power tracking (MPPT) control. The battery adopts the constant voltage control with current threshold, which can keep the DC bus voltage stable, and the outlet current is prevented from being too large to shorten the service life of the battery. Stable operation of the DC micro-grid is realized. Finally, the PV/battery DC micro-grid model is built in Simulink. When the environmental conditions and load change, the micro-grid can ensure the DC bus voltage constant and the active power balance, which verifies the effectiveness of the proposed control strategy.

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.

Direct Current (dc) microgrids due to their efficiency and energy savings are being deployed to provide power for servers in Internet of Things (IoT) data centers, in more electric aircrafts (MEA), electric ships and in rail systems round the word. In this paper, Takagi-Sugeno fuzzy inference method is used to establish a Lyapunov stability candidate for a 380 V ring bus dc microgrid modeled with Matlab. To determine suitability of using powerline communication (PLC) to monitor stability condition on the 380 V dc microgrid, impact of distortion caused by microgrid constant power loads (CPL) on signals transmitted over the dc microgrid PLC channel is examined. It is shown in this paper that while Lyapunov asymptotic stability is maintained on the dc bus, increasing CPL on the microgrid causes the dc microgrid PLC channel to experience growing signal distortion.

Due to higher power quality, lower conversion loss, and more DC loads, there has been an increasing awareness on DC microgrid. Previous emphasis has been on equal power sharing among different units in the DC microgrid, while overlooking the coordination of the energy storage units to maintain the State-of-Charge balance. In this paper, a new droop method based on voltage scheduling for State-of-Charge balance is proposed to keep the SoC balance for the energy storage units. The proposed method has the advantage of avoiding the stability problem existed in traditional methods based on droop gain scheduling. Simulation experiment is taken in Matlab on a DC microgrid with two distributed energy storage units. The simulation results show that the proposed method has successfully achieved SoC balance during the load changes while maintaining the DC bus voltage within the allowable range.

Due to the dominance of renewable energy sources and DC loads, modern power distribution systems are undergoing a transformative shift toward DC microgrids. Therefore, this article is structured to present information on the design, optimization, control, and management of DC microgrids, demonstrating that DC systems have superseded AC systems across power production, transmission, and distribution. The core cause of this superiority is the DC microgrid’s scalability, flexibility, and ease of control. This review is focused on the structural analysis, intelligent and management schemes, market employability, and reliability analysis of a DC microgrid. After this work, some methods are presented that ensure the engineered DC microgrid remains robust to various environmental and operational conditions throughout its service life. The article is enriched with methodological flowcharts and block diagrams, from which design insights can be gained to design a reliable, resilient, robust DC microgrid. The article ends with an indication of how the future energy landscape will look, with the realization of modern technologies through DC microgrids.

This paper presents an isolated DC-DC converter with high voltage gain that features an advanced inter-built nanocrystalline-core medium-frequency transformer (NC-MFT). The isolated DC-DC converter with an NC-MFT is specifically designed for applications such as interconnect photovoltaic (PV) systems, DC microgrids, DC loads, and DC buses, where voltage gain is one of the essential issues to consider. The NC-MFT inside the DC-DC converter is designed with a new approach that not only provides isolation but also contributes to achieving high efficiency and a higher step-up ratio. The high efficiency of the converters contributes to the integration of PV systems into DC microgrids. The converter yields a high voltage conversion ratio of 16.17. The experimental results obtained at 41.8 V/676 V and 275 W for the prototype revealed high efficiency (95.63% at full load). The experimental results validate the theoretical analysis and simulation, confirming that the converter achieves the main objective of high voltage conversion and high efficiency. These results will contribute to the interest in the use of this type of energy and its impact on the reduction in CO2 emissions.