Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Reexamination Certificate
2000-08-25
2001-12-11
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
C363S017000
Reexamination Certificate
active
06330170
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a bidirectional power converter and, more particularly, to a cycloconverter which is capable of acting as both an inverter (converting DC power to AC power) and as a rectifier (converting AC power to DC power).
2. Description of the Prior Art
Bi-directional inverters/chargers are increasingly used in line-interactive uninterruptible power systems (UPSs), battery-backup stand-alone inverter systems, and alternative energy systems such as wind power and photovoltaic applications. A simplified block diagram of such a system is shown in FIG.
1
. When voltage the bi-directional inverter/charger
2
functions as an inverter, it converts the dc voltage, V
b
, into an ac output voltage, V
0
, at line frequency to supply loads with various power factors. The dc source can be a low-voltage battery, or an alternative energy source with battery backup. Once the alternative ac source of V
g
, which can be standalone engine-generator set
4
, or a utility line, is available, it will be used to supply the load power
6
with the activation of the transfer switch
8
, S
t
, at the same time the bi-directional inverter/charger switches to charger operation to replenish the battery. An electromagnetic interference (EMI) filter
10
is also included. It is usually preferred for the converter to absorb sinusoidal current from the ac source when it operates as a charger to render better utilization of the available ac capacity.
The functions of such bi-directional inverter charger can be realized with a bi-directional dc/dc converter
12
in cascade with a four-quadrant full-bridge inverter/rectifier
14
as shown in
FIG. 2A. A
high-frequency (HF) transformer line T
r
is usually required to provide electrical isolation and voltage matching between the input dc and output ac voltages. In this kind of two-stage schemes, three HF inverters/rectifiers of either full-bridge, half-bridge or push-pull topology are needed and the power flow in either direction is always processed twice. In addition, extra dc-link filtering components are also a necessity.
In the past decade, single-stage, cycloconverter-based schemes as shown in
FIG. 2B
have constantly been sought. As shown, these comprise a high frequency inverter/rectifier
16
connected to a cycloconverter
18
through transformer T
r
. The cycloconverter-based bi-directional inverter/charger topology was disclosed in U.S. Pat. No. 4,742,441 to Akerson, herein incorporated by reference. Since then, different pulse width modulation (PWM) control methods have been developed to either suppress the transient voltage in the cycloconverter part, achieve reliable four-quadrant operation, or improve the dynamic performance of the converter.
The concern about reliable bi-directional operation has been looming large for the cycloconverted-based single-stage inverters/chargers. It stems from two basic topological traits of the converter, i.e. the lack of self-present current freewheeling paths inside the cycloconverter because all of the switches are bi-directional and need control to activate in both directions, and the intrinsic transient voltage appearing on the cycloconverter switches during boost mode operation when power is transferred from the output (ac side) to the input (dc side). The former can be solved by the application of proper PWM sequence which ensure the existence of the output current freewheeling path while without shortening the transformer secondary winding. The latter is akin to any isolated boost-type of converters, and has to be solved with extra voltage clamping circuitry. One such example is shown in
FIG. 3
using clamping circuitry. However, as discussed herein below, this arrangement has severe limitations which affect its performance and desirability.
SUMMARY OF THE INVENTION
A new self-switched single-phase quasi-single-stage (QSS) bi-directional inverter/charger topology is proposed for converting AC-DC or DC-AC. It features seamless four-quadrant operation in inverter mode, and rectifier operation with unity power factor in charger mode. Simple center-aligned PWM control, single-stage power conversion, standard half-bridge connection of devices, soft-switching (either ZCS or ZVS) for all the power devices, low conduction loss, and high efficiency are among its salient features. The circuit topology is derived from the cycloconverter-based circuit. A seamless center-aligned PWM and control are developed to smoothly control the converter in all of the operation modes. The proposed single-phase QSS inverter/charger topology is also extended to obtain topologies for other applications.
REFERENCES:
patent: 5852558 (1998-12-01), Julian
patent: 5896280 (1999-04-01), Gucyski et al.
Lee Fred C.
Wang Kunrong
Laxton Gary L.
McGuireWoods LLP
Virginia Tech Intellectual Properties Inc.
Wong Peter S.
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