High gain - high power (HGHP) DC-DC converter for DC microgrid applications: design and testing

The use of green energy sources to feed DC microgrids is gaining prominence over traditional centralised AC systems. DC microgrids are characterised by the use of intermediate DC-DC converter which acts as power conditioning units. Hence, the choice of an appropriate DC-DC converter becomes significant as the overall system efficiency is strongly dependent on the converter’s performance. This paper proposes a novel high gain high power (HGHP) DC-DC converter for DC microgrid, which is of one of the significant step forward in the development of DC microgrids. The suitability of the proposed HGHP DC-DC converter is demonstrated by experimental tests of the 60V/1.1kV, 3kW converter; test results validate the converter’s suitability for DC distribution. A significant number of performance parameters of the proposed converter is compared with state of the art converter topologies demonstrating the superior capabilities of the proposed converter. This paper also portrays the potential benefits that could be reaped by trending towards DC instead of existing AC system. The advantages and challenges to be confronted in the foreseeable future while implementing sustainable DC microgrids are also highlighted. Finally, this paper encapsulates renewable energy fed DC microgrid system as an appropriate, technically feasible, economically viable and competent solution for efficiently utilising the sustainable energy sources

Source-load-variable voltage regulated cascaded DC/DC converter for a DC microgrid system

Solar energy is available abundantly, the utilization of solar energy is developing rapidly and the photovoltaic based direct current (DC) microgrid system design is under demand but the stability of the DC voltage is of most important issue, as the variation of the output DC voltage is a common problem when the load or source voltage varies, hence a regulated DC output voltage converter is proposed. This paper presents source-load-variable (SLV) voltage regulated cascaded DC/DC converter which is used to obtain regulated output voltage of 203.1 V DC at 0.4 duty ratio with ±2% voltage fluctuations for the variation in the input source voltage and ±1.5% voltage fluctuations for the variation in load resistance of the nominal value with lower output voltage ripple and without use of sub circuits. A simulation model of SLV voltage regulated cascaded DC/DC converter in LTspice XVII software environment for the assessment of converter performance at different input source voltages and load resistances are verified

Interconnection of islanded DC microgrids

In this paper, the interconnection of islanded DC microgrids are discussed. The interconnection increases the power supply reliability since a power shortfall in one of the microgrids can be supplied by the others. The islanded DC microgrids are interconnected through a bi-directional DC-DC converter. A high frequency transformer isolation is used in the converter. For interconnecting more than two microgrids, a multiple winding transformer is used. The interconnected system is studied for both equal and unequal capacity DC microgrids. A detail description of the interconnection technique is provided with simulation results and illustrations

Transition from Islanded to grid-connected mode of microgrids with voltage-based droop control

Microgrids are able to provide a coordinated integration of the increasing share of distributed generation (DG) units in the network. The primary control of the DG units is generally performed by droop-based control algorithms that avoid communication. The voltage-based droop (VBD) control is developed for islanded low-voltage microgrids with a high share of renewable energy sources. With VBD control, both dispatchable and less-dispatchable units will contribute in the power sharing and balancing. The priority for power changes is automatically set dependent on the terminal voltages. In this way, the renewables change their output power in more extreme voltage conditions compared to the dispatchable units, hence, only when necessary for the reliability of the network. This facilitates the integration of renewable units and improves the reliability of the network. This paper focusses on modifying the VBD control strategy to enable a smooth transition between the islanded and the grid-connected mode of the microgrid. The VBD control can operate in both modes. Therefore, for islanding, no specific measures are required. To reconnect the microgrid to the utility network, the modified VBD control synchronizes the voltage of a specified DG unit with the utility voltage. It is shown that this synchronization procedure significantly limits the switching transient and enables a smooth mode transfer

DC Microgrid Modeling and Control in Islanded Mode

Microgrid is new emerging power distribution infrastructures, in smart grid architectures that has the potential to solve major problems arising from distributed generation. Microgrid is defined as the cluster of multiple distributed generators (DGs) that supply electrical energy to consumers with lower power loss. The realization of demand response, efficient energy management, high capability of Distributed Energy Resources (DERs), and high-reliability of electricity delivery leads to a successful Microgrid. In this thesis, DC Microgrid in islanded mode was modelled and controlled and its performance is tested for 24 hours period. The different distributed energy generation systems used are photovoltaic (PV) system, battery energy storage (BESS) system and fuel cell (FC). PV system is modelled by calculating series and shunt resistances of the real life equivalent circuit of the Solar Cell. Four experiments were performed in the Smart Energy lab, RIT Dubai, for the PV systems, to calculate open circuit voltage and short circuit current, to plot the IV characteristics of the PV system, and to track the maximum power point at different irradiances and calculating the daily irradiances. FC modeling was performed in Simulink, the fuel flow was controlled by the output current of the FC to reach the nominal current of 133.3 A and nominal voltage of 45 V. Lithium Ion batteries were used for storing energy generated by the PV system when the supply power exceeds the demand power. Demand power was estimated as the usual daily demand for 24 hours. Controlling these generation systems is performed using converters. Boost Converter used for the PV system was controlled by Maximum Power Point Tracking (MPPT) incremental conductance algorithm to maintain a constant voltage of 300 V at the DC bus despite daily change of the solar power in a day. Boost Buck converter is used to control the charging and discharging processes of the BESS to maintain a constant voltage at the input terminals of the battery to charge it at 130 V and a constant voltage at the DC bus. Boost Converter used for the FC maintained a constant voltage of 100 V

Integration of air-cooled multi-stack polymer electrolyte fuel cell systems into renewable microgrids

Currently, there is a growing interest in increasing the power range of air-cooled fuel cells (ACFCs), as they are cheaper, easier to use and maintain than water-cooled fuel cells (WCFCs). However, air-cooled stacks are only available up to medium power (<10 kW). Therefore, a good solution may be the development of ACFCs consisting of several stacks until the required power output is reached. This is the concept of air-cooled multi-stack fuel cell (AC-MSFC). The objective of this work is to develop a turnkey solution for the integration of AC-MSFCs in renewable microgrids, specifically those with high-voltage DC (HVDC) bus. This is challenging because the AC-MSFCs must operate in the microgrid as a single ACFC with adjustable power, depending on the number of stacks in operation. To achieve this, the necessary power converter (ACFCs operate at low voltages, so high conversion rates are required) and control loops must be developed. Unlike most designs in the literature, the proposed solution is compact, forming a system (AC-MSFCS) with a single input (hydrogen) and a single output (high voltage regulated power or voltage) that can be easily integrated into any microgrid and easily scalable depending on the power required. The developed AC-MSFCS integrates stacks, balance of plant, data acquisition and instrumentation, power converters and local controllers. In addition, a virtual instrument (VI) has been developed which, connected to the energy management system (EMS) of the microgrid, allows monitoring of the entire AC-MSFCS (operating temperature, purging, cell voltage monitoring for degradation evaluation, stacks operating point control and alarm and event management), as well as serving as a user interface. This allows the EMS to know the degradation of each stack and to carry out energy distribution strategies or specific maintenance actions, which improves efficiency, lifespan and, of course, saves costs. The experimental results have been excellent in terms of the correct operation of the developed AC-MSFCS. Likewise, the accumulated degradation of the stacks was quantified, showing cells with a degradation of >80%. The excellent electrical and thermal performance of the developed power converter was also validated, which allowed the correct and efficient supply of regulated power (average efficiency above 90%) to the HVDC bus, according to the power setpoint defined by the EMS of the microgrid.This research was funded by “H2Integration&Control. Integration and Control of a hydrogen-based pilot plant in residential applications for energy supply” Spanish Government, grant Ref: PID2020-116616RBC31,”; and “SALTES: Smartgrid with reconfigurable Architecture for testing controL Techniques and Energy Storage priority” by Andalusian Regional Program of R+D+, grant Ref: P20-00730