NOVEL FWS DC-AC GRID CONNECTED INVERTER

Abstract

A new class of DC-AC inverter consists of a buck or two buck converters and two or four low frequency switches, and it achieves ultra-high efficiency, reactive power flow capability, small size and low cost in grid-connected applications.

Claims

1. An DC-AC inverter apparatus comprising: a set of one or more buck converters each having a bidirectional cell, said bidirectional cell comprising two switches not connected in serials, with the second switch conducting freewheeling current while the first switch being turned off; and a set of two low-frequency switches coupled to two respective terminals of an AC source, each low-frequency switch generating a half wave of an AC sinusoidal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The description of the illustrative embodiments can be read in conjunction with the accompanying figures. It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures, unless expressly specified, have not necessarily been drawn to scale. Also, any text and/or any numerical data (numbers) appeared on any drawing figures is provided to illustrate an exemplary embodiment or implementation, and thus is provided for the purpose of illustration and not for the purpose of limitation. For example, the dimensions of some of the elements may be exaggerated relative to other elements. Embodiments incorporating teachings of the present disclosure are shown and described with respect to the figures presented herein, in which:

[0025] FIG. 1A illustrates general form of the presently disclosed DC-AC inverter.

[0026] FIGS. 1B and 1C show exemplary choices of the buck converter shown in FIG. 1A.

[0027] FIG. 1D illustrates an example of the presently disclosed DC-AC Inverter with one buck converter and four low frequency switches.

[0028] FIG. 1E illustrates comparisons among unidirectional and bidirectional cell configurations.

[0029] FIGS. 2A-2C, 2E-2G and 2I-2K illustrate respective conversion topologies in the related art.

[0030] FIG. 3A illustrates an example of the presently disclosed DC-AC Inverter with two buck converters and two low frequency switches.

[0031] FIG. 3B depicts exemplary timing diagrams in connection with the modulation strategy of the presently disclosed DC-AC converter for FIG. 3A.

[0032] FIGS. 3C-D illustrate exemplary timing diagrams in connection with modulating timing for the presently disclosed DC-AC converter for FIG. 3A.

[0033] FIGS. 4A-4D illustrate four respective operation modes for the presently disclosed DC-AC converter for FIG. 3A, including exemplary respective timing diagrams associated with four operation modes.

[0034] FIG. 4E illustrates an example of the presently disclosed DC-AC Inverter receiving reactive power flow from a large inductor series or large capacitor parallel with Vac.

[0035] FIGS. 4F and 4G illustrate exemplary respective timing diagrams in connection with how the presently disclosed DC-AC Inverter handles reactive power flow from an exemplary inductive load and an exemplary capacitive load.

DETAILED DESCRIPTION

[0036] In the following detailed description of exemplary embodiments of the disclosure in this section, specific exemplary embodiments in which the disclosure may be practiced are described in sufficient detail to enable those skilled in the art to practice the disclosed embodiments. However, it is to be understood that the specific details presented need not be utilized to practice embodiments of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims and equivalents thereof.

[0037] References within the specification to one embodiment, an embodiment, embodiments, or one or more embodiments are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. The appearance of such phrases in various places within the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments.

[0038] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms a, an and the are intended to include the plural forms as well, unless the context clearly indicates otherwise. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.

[0039] Those of ordinary skill in the art will appreciate that the components and basic configuration depicted in the following figures may vary. Other similar or equivalent components may be used in addition to or in place of the components depicted. A depicted example is not meant to imply limitations with respect to the presently described one or more embodiments and/or the general disclosure.

[0040] The presently disclosed DC-AC converter is illustrated in general form in FIG. 1A, comprising two buck converters and two low frequency switches. Referring to FIG. 1A, the DC source can be photovoltaic arrays, batteries, fuel cells or others. The AC source can be utility grid, single-phase electric motors or others. Each front converter can be any unidirectional converter that can generate half sinusoidal waveform at the frequency of the connected AC source, including but not limited to a classic unidirectional buck converter (shown FIG. 1B) and a three-level bidirectional buck converter (shown in FIG. 1C) or any other kind of buck converter. The two converters can be identical, or combinations of any buck converters.

[0041] FIG. 3A shows a presently disclosed DC-AC converter with classic buck converter configured with bidirectional cell as an example for description. One possible modulation strategy is depicted in FIGS. 3B-D, together with AC voltage and current waveforms. Four possible operation modes of the presently disclosed inverter in unity power generation with the described modulation strategy are respectively demonstrated in FIGS. 4A-4D.

[0042] For reactive power generation mode in connection with the presently disclosed topologies, FIGS. 4E-4G provide an exemplary configuration diagram and examples of reactive power flow associated with the exemplary configuration, respectively.

[0043] Mode 1

[0044] Referring to FIG. 4A and FIG. 3B, during this mode, T3 operates in PWM in generating an AC waveform using known general configurations that involves one or more suitable inductors and one or more suitable capacitors. T2 works as a low frequency switch. The converter generates half wave AC at point A, while T1 and T4 are off. Accordingly, power transfers from the DC source to inductors L1 to output capacitor C3 and the AC source. As a result, half wave of the AC sinusoidal is generated crosses AC source. As indicated in the timing diagrams, T5, which works simultaneously with T3 and is coupled to L3, conducts the freewheeling current through forward-biased D1 (otherwise reversed-biased during T3 ON periods) during T3 OFF periods.

[0045] Mode 2

[0046] Referring to FIG. 4B and FIG. 3B, during this mode, which can also be appreciated as a specific sub-mode during Mode 1, T2 and D1 are ON, while T1, T3 and T4 are OFF and T5 is ON. Power transfers from the inductors L1, L3, capacitor C3 to the AC source. D1 keeps the freewheeling current flow of the inductor L1 due to T5 being on, with inductor L3, which is coupled to T5, engaging T5 to connect the freewheeling current as well.

[0047] Mode 3

[0048] Referring to FIG. 4C and FIG. 3D, this mode mirrors Mode 1, except for generating the other half wave of the AC sinusoidal. Thus, as a skilled artisan readily appreciate, the operation of Mode 3 corresponds with the operation of Mode 1, except for that the other low-frequency T1 is ON, T4 operates in PWM, while the corresponding switches of T1 and T4, namely, T2 and T3, are off. Thus, power transfers from the DC source to inductor L2 (which corresponds to L1) to output capacitor C4 (which corresponds to C3) and the AC source. As a result, the other half wave of the AC sinusoidal is generated crosses AC source. As indicated in the timing diagrams, T6 (which corresponds with T5), which works simultaneously with T4 and is coupled to L4, conducts the freewheeling current through forward-biased D2 (otherwise reversed-biased during T4 ON periods) during T4 OFF periods.

[0049] Mode 4

[0050] Referring to FIG. 4D and FIG. 3D, this mode mirrors Mode 2, except for generating the other half wave of the AC sinusoidal. Thus, Mode 4 can also be appreciated as a specific sub-mode during Mode 3. Accordingly, during this mode, T1 and D2 are ON, while T2, T3 and T4 are OFF and T6 is ON. Power transfers from the inductors L2, L4, capacitor C4 to the AC source. D2 keeps the freewheeling current flow of the inductor L2 due to T6 being on, with inductor L4, which is coupled to T6, engaging T6 to connect the freewheeling current as well.

[0051] Reactive Power Flow Mode

[0052] Referring to FIGS. 4E, 4F and 4G, during this mode, load (AC source) is reactive, with either inductive load (lagging) or capacitive load (leading). As indicated in FIGS. 4F and 4G, the output current is out of phase with the output voltage, the coupled inductor L1 and L3, L2 and L4 engage the freewheeling current flow during the intervals when T5 and T4 are on. And with the diodes D1 and D2 of the combination conducting the current, the bidirectional current flow is established. See Iac1 and Iac2 in FIGS. 4F and 4G. Those currents travel in two directions, showing the bidirectional current flow.

[0053] In summary, during the positive sinusoidal cycle (v.sub.ac>0), T1 remains off and T2 remains on. T3, D1 and T5 turn on and off in a complementary way to generate required current i.sub.ac1, whereas T4 and D2 remain off. For the negative sinusoidal cycle (v.sub.ac<0), T1 remains on and T2 remains off. T4, D2 and T6 turn on and off in a complementary way to generate required current i.sub.ac2, whereas T3 and D1 remain off.

[0054] While the disclosure has been described with reference to one or more exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular system, device or component thereof to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiments disclosed for carrying out this disclosure.