Download In order to solve the problem of high voltage conversion

A NOVEL PWM HIGH VOLTAGE CONVERSION RATIO BI-DIRECTIONAL
THREE-PHASE DC/DC CONVERTER WITH Y-Δ CONNECTED TRANSFORMER
ABSTRACT:
In order to solve the problem of high voltage conversion ratio between DC bus and energy
storage unit, a novel bi-directional three-phase DC/DC converter applicable to high power
condition is proposed in this paper. With the new PWM strategy and inherent ability of
increasing voltage conversion ratio of Y-Δ connected transformer, the voltage conversion ratio
is doubled. With the new PWM strategy, the effective duty cycle range of three-phase DC/DC
converter is expanded, which results in halved filter inductance. Three-phase full-bridge
structure is connected to the DC bus, while current-Tripler unit is connected to the battery. As
three-phase interleaved structure is adopted, not only current stress of switches decreases, but
also the frequency of input and output current ripple increases to three times of switching
frequency, which leads to smaller filters and extended lives cycle of batteries. The operating
principle and characteristics of the converter are analyzed in detail. A prototype with the 360V
DC bus converted to 48V/42A batteries was built and the results are given based on the
experiments.
INTRODUCTION:
Bi-directional DC-DC converter (BDC) is usually applied between DC bus and energy storage
unit in DC micro-grid. The DC bus voltage level is about 360-400V, while the voltage of
battery cell is as low as 12V or 48V. Thus there is a big disparity between the two power
sources in which the BDC needs high voltage conversion ratio (HVCR).
A high voltage level can be acquired with batteries in series; however, extra charging and
discharging balance circuits are needed in order to avoid the problems of overcharge and over
discharge which will influence the lifespan of batteries.
In high power conditions, the voltage of battery cell is 12V or 48V that the power
semiconductors and magnetic components face severe current stress. Paralleling of power
switches becomes popular solution to increase the current rating; however, it may incur several
problems, such as current sharing and complicated control strategy.
Therefore, it’s necessary and meaningful to research BDC with HVCR in high power ratings
application. In high power conditions, the voltage of battery cell is 12V or 48V that the
power semiconductors and magnetic components face severe current stress.
Paralleling of power switches becomes popular solution to increase the current rating;
however, it may incur several problems, such as current sharing and complicated control
strategy.
Therefore, it’s necessary and meaningful to research BDC with HVCR in high power ratings
application. For traditional isolated BDC, adopting large turn ratio of transformer can achieve
HVCR. However, large leakage inductance and parasitic capacitance owing to large turn ratio
cause the problems such as: high voltage stress of power switches, serve electromagnetic
interference, lower efficiency.
summarizes the strategies of achieving HVCR of non-isolated DC/DC converter, including
introducing coupled inductor cell or switching capacitor cell, utilizing cascaded converters et
al. For isolated DC/DC converter, there are also several methods to achieve HVCR. introduces
series connected multi-windings to improve converter voltage gain resulting in complicated
manufacture and larger parasitic parameters of transformer.
Besides, the multi-windings need extra voltage-sharing circuit. proposed a HVCR DC/DC
converter with voltage-doubler rectifier cell but voltage-sharing problem of voltage dividing
capacitors is also under consideration for BDC.
Two-stage converters are proposed o improve voltage gain despite low efficiency and the
stability problem of cascading converters. Three-phase DC/DC converter (TPDC) is defined as
the converter in which three-phase transformer is applied and chopper cell and rectifier cell is
constituted of three bridge legs
EXISTING SYSTEM:
The TPDAB is composed of three interleaving single-phase dual active bridge current source
bidirectional DC/DC converters (DAB), and each phase output is 120 degree apart from each
other.
In this mode, the input current rating is increased by interleaving three phases, not by
paralleling components, and the ripple frequency will be increased to three times the switching
frequency and current ripple will be greatly reduced due to cancelling effects between each
phase.
The power flow of the TPDAB is controlled by the shifting phase between active bridges at
low voltage side (LVS) and high voltage side (HVS). When energy flows from the 12V net
side to the high voltage DC (HVDC) net side, the TPDAB works in boost mode, in which the
driving pulses at LVS are ahead of that at HVS. The leakage inductances La~Lc of the high
frequency transformers are acting as intermediate elements to store and transfer the power.
The 12V net, the coupling three-phase input inductors L1~L3, and the switches S2, S4, S6
compose boost circuits. The duty cycles of the switches S1, S3, S5 are set at D, so the voltage
V1 of capacitor C1 is boosted to 1/D times of VIN. The capacitors C1, C2 are used to handle
high switching current ripple and maintain a nearly constant dc bus voltage. The stress of each
switch of TPDAB converter is about one third of the single-phase topology,
although the total stress is about the same for both converters since the number of switches is
increased three times in TPDAB converter. The current stress of capacitors C1 and C2 of threephase topology is much lower than that of the single-phase circuit therefore smaller capacitor
could be used for three-phase topology and power density will be improved.
PROPOSED SYSTEM:
The topology of proposed three-phase BDC with Y-Δ connected transformer. The high voltage
side (HVS) connected to DC bus is three-phase full-bridge chopper unit composed of power
MOSFETs QH1-QH6 paralleled with body diodes DH1 DH6. The low voltage side (LVS) is a
current-Tripler unit composed of power MOSFETs QL1-QL3 paralleled with body diodes
DL1-DL3, filter inductors Lf1-Lf3 and filter capacitor CfL. If the energy transfers from HVS
to LVS, the converter works in Buck mode, otherwise in Boost mode.
ADVANTAGES:
The current stress of power switches decreases, and the converter is preferred in high power
applications.
The voltage conversion ratio is doubled, thus the turn ratio of transformer is halved in high
voltage conversion ratio conditions.
The primary voltage of transformer is 2/3Vin, therefore the insulation requirement of
transformer reduces.
The ripple frequency of input and output current is three times of switching frequency, so the
input and output filter capacitors can be smaller to improve dynamic performance of the
converter.
Compared with traditional asymmetrical PWM control, the equivalent voltage ratio is doubled,
thus the rms current of primary switches and transformers decreases
BLOCK DIAGRAM:
TOOLS AND SOFTWARE USED:
MPLAB – microcontroller programming.
ORCAD – circuit layout.
MATLAB/Simulink – Simulation
APPLICATIONS:
High step-up or high step-down application.