Design and Analysis of a 24 Vdc to 48 Vdc Bidirectional DC-DC Converter Specifically for a Distributed Energy Application

Ambrosio B. Cultura II, Ziyad M. Salameh

Mindanao University of Science and Technology, Cagayan de Oro City, Philippines.
University of Massachusetts Lowell, Lowell, USA.

DOI: 10.4236/epe.2012.45041                

Abstract

The design of a bidirectional dc-dc power converter specifically for a distributed energy application is presented. The existing two different DC voltage battery bank of the distributed generation needs to interlink each other using a bi-directional dc-dc converter in order to minimize the unbalance of the output load currents of the three inverters connected to electric grid system. Through this connection, a current can flow from one system to another or vice versa depending on which systems need the current most. Thus, unbalanced currents of the grid line have been minimized and the reliability and performance of the DER grid connected system has been increased. A detailed mathematical analysis of the converter under steady state and transient condition are presented. Mathematical models for boost and buck modes are being derived and the simulink model is constructed in order to simulate the system. Moreover, the model has been validated on the actual operation of the converter, showing that the simulated results in Matlab Simulink are consistent with the experimental ones.

Keywords

Distributed Energy Resources; Bidirectional dc-dc Converter; ELECTRIC Grid

1. Introduction

The distributed energy resources (DER) considered in this study is composed of photovoltaics (PV), wind turbines, a fuel cell, batteries and supercapacitors. The system is divided into two subsystems. Subsystem 1 is composed of PV, a PEM fuel cell, wind turbines and 24 V batteries. Subsystem 2 is composed of PV, a wind turbine, supercapacitor and 48 V batteries. The detailed schematic diagram of grid connected DER at the University of Massachusetts Lowell (UMass Lowell) is presented in Figure 1. This DER, as it stands now, consists of four roof top mounted wind turbines rated for 2.4 kW, 1.5 kW, 500 W, and 300 W; two photovoltaic arrays rated for 2.5 kW and 10.56 kW that are connected through a microprocessorcontrolled maximum power point trackers (MPPT); two battery storage banks rated at 24 V, 45 kWh and 48 V, 30 kWh; a 1.2 kW PEM type fuel cell; four modules of Maxwell Super capacitors, each of which is rated at 48 V, 140 Farads; and three 4 kW sine wave inverters that are connected to the utility grid. A data acquisition system (DAQ) is installed in order to monitor the performance of all energy resources.

In its operation, there are times that the 24 V battery bank supplies less current to Inverter 1 than the 48 V battery bank supplies to Inverters 2 and 3. This causes an imbalance of currents to the 3-phase electric grid. There are also times that Inverter 1 or 2 shuts-down or malfunctions. In other words, the quality, the reliability and the performance of the system are compromised. Hence, integrating the two systems can lessen or resolve this problem. So, there was a need for the design and construction of a bidirectional DC-DC converter. This bidirectional DC-DC converter would connect the two-battery banks of the distributed energy resources. Through this connection, a current would flow from one system to another depending on which system needed the current the most. Imbalanced currents of the grid line would be minimized, and the reliability and performance of the existing DER grid connected system would increase. The author designed the power and control circuit for this bidirectional DCDC converter and it is suited to this use. The mathematical analysis, simulink model and simulation results are presented here and a prototype was fabricated and tested.

2. System Design

Figure 2 illustrates the power circuit configuration of a bidirectional DC-DC converter that was installed between the two banks of batteries at the renewable energy

Conflicts of Interest

The authors declare no conflicts of interest.

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