02-09-2013, 03:50 PM
Design and Analysis of a 24 Vdc to 48 Vdc Bidirectional DC-DC Converter Specifically for a Distributed Energy Application
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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.
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, super-
capacitor and 48 V batteries. The detailed schematic dia-
gram of grid connected DER at the University of Massa-
chusetts Lowell (UMass Lowell) is presented in Figure 1.
This DER, as it stands now, consists of four roof top mou-
nted 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 microprocessor-
controlled 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.
The Operation
The current sensors, installed on the outputs of inverter 1
and inverter 2 (or 3), signals the micro-controller, and are
responsible for determining the operation of DC-DC con-
verter in either boost or buck mode. The micro-controller
was programmed so that it could control the operation of
the bidirectional DC-DC converter. The control strategies
of this converter that are implemented in the PIC 16F684
are the following:
1) When the micro-controller reads that the current
output in inverter 2 is higher than the current output in
inverter 1, it will enable the PWM1. The microcontroller
will drive MOSFET Q1 and operate as a buck converter.
Steady State Analysis of the Bidirectional
DC-DC Converter
The voltage and current equations of a DC-DC converter
under steady-state conditions can be found by using the
two basic principles namely; the principle of inductor volt-
second balance and the principle of capacitor amp-second
or charge balance. The principle of inductor volt-second
balance states that the average value, or dc component of
voltage applied across an ideal inductor winding must be
zero and the principle of capacitor amp-second or charge
balance, states that the average current that flows through
an ideal capacitor must be zero [3]. Thus, to determine
the voltages and currents of DC-DC converters operating
in periodic steady state, one averages the inductor current
and capacitor voltage waveforms over one switching per-
iod, and equates the results to zero [4]. In Figure 5 the
resistance RL of the inductor in the bidirectional DC-DC
converter power circuit is being considered in order to
model the circuit. Based on the power circuit shown in
Figure 5, the output voltage, and inductor current were
derived. Red arrows are in the direction of current in a
step down mode while the black arrows indicate the cur-
rent direction in a step up mode.
Dynamic Analysis of the Bidirectional
DC-DC Converter
A typical configuration of a power circuit for a DC-DC
converter, when considering voltage and current para-
meters, is shown in Figure 5. When the DC-DC con-
verter operates as a boost or buck converter and in a con-
tinuous conduction mode, the current in the inductor
flows continuously [5]. Red arrows are the direction of
current in a step up mode while the black arrows indicate
the current direction in a step-down operation. Based on
this power circuit, the output and input voltages, and in-
ductor current are being derived.
Conclusion
A bidirectional DC-DC converter for distributed genera-
tion application has been designed, simulated and fabri-
cated. Mathematical models for the buck and boost
modes of the DC-DC converter are derived. Transient
performances of the bidirectional DC-DC converter us-
ing the derived mathematical models are simulated in a
Matlab Simulink environment. It is observed that the cur-
rent change its direction quickly as the mode of operation
changes without any overshoot of the values. Hence, the
converter’s voltage and current are stable in its operation
in both directions. Results show that the simulation is in
good agreement with the physical experiment and they
validate the model. The PIC16F684 micro controller suc-
cessfully works well in the implementation of the de-
signed control strategy design of the bidirectional DC-
DC converter. It was observed that the duty cycle for
buck and boost operation must be greater than 50% in
order to charge or transfer energy from one battery bank
to another. And as the duty cycle increases, the current
flowing through the inductor and load also increases with
a corresponding increase in the amount of energy transfer
from one battery bank to the other.