Local dc distribution systems or dc nanogrids for residential and commercial buildings are growing popular due to the increased focus on energy savings. Using low-voltage dc for the nanogrid provides various advantages such as easier integration with renewable sources and battery banks, increased savings, etc. Future systems would use renewable sources and storage devices to become self-sufficient in generation with bare minimum consumption from the grid. The objective of this paper is to develop an energy management system (EMS) to provide uninterrupted power supply to the dc loads and achieve self-sufficiency by minimizing grid power consumption. This is achieved by developing a control scheme for a cascaded two-stage bidirectional converter interface between the nanogrid and the ac distribution network. In addition, an empirical relation for power flow through the converter is obtained, and an optimal capacitor link voltage range is derived to achieve power transfer between rated limits. The efficacy of the proposed control algorithm for the EMS is analyzed and proved to be well suited for the self-sustained dc nanogrid. A 5-kVA prototype of the converter is designed and developed in hardware, and the system operation is verified with experimental results.

Control Scheme for a Bidirectional Converter in a Self-Sustaining Low-Voltage DC Nanogrid

This paper presents load current sharing of parallel-connected sources in a low-voltage dc distribution grid. Droop control is a common technique for load current sharing in dc distribution grids. The main drawback of the conventional droop method, which has one droop value, is poor current sharing due to the resistance of distribution cables. These resistances create different voltage drops which limits the current sharing for each of the sources. This paper introduces a piece-wise linear droop control to improve current sharing during heavy load demand and decrease current sharing at light load for sources which are far away from the load. This piece-wise linear droop control does not need communications to improve current sharing and is decentralized. The number of segments of the droop curve depend on the size of converters and the requirement to minimize system oscillations. Detailed analysis and design procedures are provided in this paper. Simulations to verify the piece-wise droop concept are presented.

Piece-wise linear droop control for load sharing in low voltage DC distribution grid

To reduce the impact of high penetration of renewable energy on the utility grid and suppress the influence of micro-grid power fluctuation on power quality, a power tracking control strategy for DC micro-grid has been studied. Furthermore, a central energy management method (CEMM) based on this control strategy has been designed. The DC micro-grid, which contains renewable energy and hybrid energy storage unit, is used to build a complete control system model according to the distributed power supply and its control system model. The power tracking control strategy has been analysed and designed based on frequency domain method. With the designed control strategy, the system can realise coordinated operation of the hybrid energy storage unit by absorbing the load power fluctuation within different frequency range. Furthermore, based on the CEMM, the system is controlled to absorb/deliver predefined amount of power from/to the utility grid under grid-connected mode. In this way, smooth switching between different modes and the stable operation of the micro-grid are realised. Finally, the function of the system is verified with an emulated DC micro-grid system.

https://ietresearch.onlinelibrary.wiley.com/doi/epdf/10.1049/iet-rpg.2016.0351

Central energy management method for photovoltaic DC micro-grid system based on power tracking control

Two self-developed control schemes, ON/OFF and supervisory control and data acquisition (SCADA), were applied on a hybrid evaporative and direct expansion (DX)-based model data center cooling system to assess the impact of controls on reliability and energy efficiency. These control schemes can be applied independently or collectively, thereby saving the energy spent on mechanical refrigeration by using airside economization and/or evaporative cooling. Various combinations of system-level controls and component-level controls are compared to a baseline no-controls case. The results show that reliability is consistently met by employing only sophisticated component-level controls. However, the recommended conditions are met approximately 50% of the simulated time by employing system-level controls only (i.e., SCADA) but with a reduction in data center cooling system power usage effectiveness (PUE) values from 3.76 to 1.42. Moreover, the recommended conditions are met at all averaged times with an even lower cooling system PUE of 1.13 by combining system-level controls only (SCADA and ON/OFF controls). Thus, the study introduces a simple method to compare control schemes for reliable and energy-efficient data center operation. The work also highlights a potential source of capital expenses and operating expenses savings for data center owners by switching from expensive built-in component-based controls to inexpensive, yet effective, system-based controls that can easily be imbedded into existing data center infrastructure systems management.

Thermal Control Strategies for Reliable and Energy-Efficient Data Centers

This paper reviews the currently available dc micro-grids and dc grids. Currently, commonly existing and proposed dc microgrids are designed to provide sufficeint energy during system disturbance as in ac grid and, are mainly voltage stiff systems. It also compares the voltage stiff systems to a low energy system, which represents voltage weak dc system, from systems point of view. First, the definition of voltage weak dc systems is provided from a system perspective. Comparisons of both dc systems are reviewed on stability complexity, fault detection and isolation strategies and devices, and power management approaches. Various challenges and opportunities of voltage weak dc systems are briefly addressed. And also several architectures and topologies for voltage weak dc distribution systems are reviewed. Finally, this paper identifies a comparative advantage of such systems from a control, protection and system operation standpoints.

From voltage stiff to voltage weak DC distribution grid: Opportunities and challenges

DC micro-grid, as a self-management grid form of integrating distributed energy systems with utility power systems, will efficiently realize the value and benefit of the distributed energy resources. In this paper,the DC micro-grid mainly consists of five parts: a PV source, a DC/DC converter, a battery energy storage unit, a bi-directional DC/DC, and a bi-directional DC/AC which is connected to the main grid. This DC micro-grid has different control strategies according to different operating modes (grid connected, islanded). Then its control mode is designed and investigated by MATLAB/Simulink models. The simulation results verify the validity of the mathematical model and the feasibility of the proposed control strategies. In the grid connected mode, the transfer energy can flow in bi-directional modes, and DC bus voltage level can be stabilized by DC/AC converter. In the islanded mode, the battery converter serves as a voltage source to ensure a stable DC voltage. At last, DC micro-grid experimental system is used to examine its switching functions among different control modes. The experimental results demonstrate that high quality power can be supplied.

Energy management and control strategy for DC micro-grid in data center

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Energy management and control strategy for DC micro-grid in data center