WO2012150933A1

WO2012150933A1 - Topology and control for distributed var generating solar inverters

Info

Publication number: WO2012150933A1
Application number: PCT/US2011/034981
Authority: WO
Inventors: Madhuwanti Joshi; Bruce Modick; Hussam Alatrash; Ronald DECKER; Johan ENSLIN
Original assignee: Petra Solar, Inc.
Priority date: 2011-05-03
Filing date: 2011-05-03
Publication date: 2012-11-08

Abstract

A power system may comprise a direct current (DC) source for generating DC power and a single stage converter for providing a bidirectional power flow between the DC source and an alternating current (AC) grid. The single stage converter may be configured to provide the bidirectional power flow by converting the DC power to AC power by operating in at least one pre-defined mode. The AC power may comprise a reactive power component and an active power component.

Description

TITLE

TOPOLOGY AND CONTROL FOR DISTRIBUTED VAR GENERATING

SOLAR INVERTERS

This application is being filed on 03 May 2011, as a PCT International Patent application in the name of Petra Solar, Inc., a U.S. national corporation, applicant for the designation of all countries except the U.S., and Madhuwanti Joshi, a citizen of India, Bruce Modick, a citizen of the U.S., Hussam Alatrash, a citizen of Jordan, Ronald Decker, a citizen of the U.S., and Johan Enslin, a citizen of the Netherlands, applicants for the designation of the U.S. only.

BACKGROUND

[001] With the growing initiative for clean energy, solar energy is becoming an important element of power generation. In a typical solar power plant, energy from the Sun is converted in to a DC voltage by photovoltaic cells and then converted to grid compatible AC voltage by a DC to AC voltage converter or inverter. This inverter could either be a string inverter that takes input from many panels or a micro-inverter that takes input from a single panel. Most of the microinverters in the market today focus on generating the active power while maintaining highest possible DC to AC power conversion efficiency. However, when they are used in conjunction with the utilities providing the electric power, this feature is not enough. The utilities need reactive power generation in addition to the active power generation. This is because the solar panels are typically part of a big distributed energy generating power plant. In this type of power plant, there are wide fluctuations in the local grid network because of the uncertainties in the weather conditions. Further this power plant could be supplying power to the reactive loads. With the reactive loads, it is desired to have local control of reactive power and improve the grid stability.

The existing solar inverters providing the reactive control capability in addition to active power generation have lower DC to AC power conversion efficiency. SUMMARY

[002] Consistent with embodiments of the present invention, systems and methods are disclosed for a topology and control for active and reactive power generating solar inverters. A power stage circuit may comprise a plurality of semiconductor switches (e.g., MOSFETs or IGBTs): a first switch (Qi), a second switch (Q₂), a third switch (Q₃), a fourth switch (Q₄), a fifth switch (Q₅), a sixth switch (Q ), a seventh switch (Q₇), and an eighth switch ((¾). Switches Q_ls Q₂, Q₃ and Q₄ may be switched on and off at a switching frequency (e.g., much higher than a line frequency). On a secondary side of a transformer T_{1 }} during a positive line voltage cycle, switches Q₆ and Q₈ may be on and switches Q₅ and Q₇ may be switched at the switching frequency. During a negative line voltage cycle, switches Q₅ and Q₇ may be kept on, and switches Q₆ and Q₈ may be switched on and off at the switching frequency.

[003] It is to be understood that both the foregoing general description and the following detailed description are examples and explanatory only, and should not be considered to restrict the invention's scope, as described and claimed.

Further, features and/or variations may be provided in addition to those set forth herein. For example, embodiments of the invention may be directed to various feature combinations and sub-combinations described in the detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[004] The accompanying drawings, which are incorporated in and constitute a part of this disclosure, illustrate various embodiments of the present invention. In the drawings:

[005] FIG. 1 shows the prior art power circuit;

[006] FIG. 2 shows a power circuit;

[007] FIG. 3 shows a power circuit operation in a first mode;

[008] FIG. 4 shows a power circuit operation in a second mode;

[009] FIG. 5 shows a power circuit operation in a third mode;

[010] FIG. 6 shows a power circuit operation in a fourth mode;

[01 1] FIG. 7 shows a power circuit operation in a fifth mode;

[012] FIG. 8 shows a power circuit operation in a sixth mode;

[013] FIG. 9 shows a power circuit operation in a seventh mode; [014] FIG. 10 shows a power circuit operation in an eighth mode;

[015] FIG. 11 is a block diagram showing a control strategy;

[016] FIGs. 12A and 12B illustrate circuit operation in various modes during reactive (FIG. 12 A) and active (FIG. 12B) power generation; and

[017] FIGs. 13A, 13B, and 13C show alternate forms for a power stage topology.

DETAILED DESCRIPTION

[018] The following detailed description refers to the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the following description to refer to the same or similar elements. While embodiments of the invention may be described, modifications, adaptations, and other implementations are possible. For example, substitutions, additions, or modifications may be made to the elements illustrated in the drawings, and the methods described herein may be modified by substituting, reordering, or adding stages to the disclosed methods. Accordingly, the following detailed description does not limit the invention. Instead, the proper scope of the invention is defined by the appended claims.

[019] Embodiments of the invention may provide a method and control system for VAR control capability while maintaining a very high DC to AC power conversion efficiency when used in an active power generation mode. Resonant converters have been studied for different applications since the 1980's. They have low switching losses, low electro magnetic interference (EMI), and have the advantage of sinusoidal voltage and/or current in the circuit. Because of very high efficiency requirement that may be needed for solar inverters, resonant converter seems to be an appropriate choice. Even though embodiments of the present invention may also use resonant converter at a front end, resonant converters may also be applied to all the forward types of topologies using a full bridge converter.

[020] FIG. 1 shows a prior art power circuit. The power circuit of FIG. 1 has a DC to DC converter and a high frequency inverter. The DC to DC converter is a resonant converter that boosts a low voltage DC from a solar panel to a high voltage DC. The high frequency inverter is sine modulated to generate sinusoidal current. Although the circuit shown in FIG. 1 is a resonant DC-DC converter, it can be any DC to DC converter. The prior art circuit of FIG. 1 needs two control loops; one control loop for maintaining the high voltage DC at the input of the high frequency inverter and another control loop for regulating the generated AC source current. The high voltage DC is regulated by varying the frequency of the input switching circuit. The AC grid current is regulated by sine pulse width modulating the high frequency inverter. One of the drawbacks of this prior art circuit shown in FIG. 1 is that it has many components. Another drawback for this circuit is it has less efficiency when operated in active power generation mode. Also, the control of the prior art circuit of FIG. 1 is more complex because it has two switching stages. Furthermore, the need for a separate filter at the output adds cost and additional components to the power circuit of FIG. 1.

[021] Consistent with embodiments of the present invention, Fig. 2 shows a power stage circuit 200 using a series resonant converter topology. As shown in FIG. 2, power stage circuit 200 may comprise a plurality of semiconductor switches (e.g., MOSFETs or IGBTs): a first switch 201 (Q , a second switch 202 (Q₂), a third switch 203 (Q₃), a fourth switch 204 (Q₄), a fifth switch 205 (Q₅), a sixth switch 206 (Q₆), a seventh switch 207 (Q₇), and an eighth switch 208(Q₈). In addition, power stage circuit 200 may include a resonant tank comprising a resonant inductor 210 (Lr) and a resonant capacitor 215 (C_r), a transformer 220 (Ti), a first output capacitor 225 (C₀₁), and a second output capacitor 230 (C₀₂). The resonant tank may comprise any resonant tank topology including parallel resonant (e.g. series L and parallel C), series parallel resonant (e.g. series L, series C and parallel C), or LLC resonant (e.g. series L series C and parallel L), for example. Power stage circuit 200 may connect a solar panel 240 (e.g., a direct current (DC) source for generating DC power) with an AC line 245. An input capacitor 235 ( ) may be connected across solar panel 240.

[022] Transformer 220 (Ti) may have a primary winding 250 and a secondary winding 255. That part of power stage circuit 200 that is connected to primary winding 250 may be considered the primary side of power stage circuit 200 and that part of power stage circuit 200 that is connected to secondary winding 255 may be considered the secondary side of power stage circuit 200. Consistent with embodiments of the invention, any component shown in FIG. 2 may comprise multiple elements. For example, any one or more of the aforementioned switches T/US2011/034981 may comprise one or more switches in series or parallel, for example, and are not limited to one switch. Moreover, while circuit 200 is shown as having one stage in FIG. 2, circuit 200 may comprise one or many stages in series and/or parallel, for example. Furthermore, circuit 200 may include no rectifier.

[023] Consistent with embodiments of the invention, a new topology and control system for active and reactive power generating solar inverters may be provided. As stated above, Fig. 2 shows power stage circuit 200. Circuit 200 may comprise a single stage DC to AC converter. When circuit 200 is connected to an AC source (e.g. AC line 245) or grid, the DC voltage may generate an AC current that has the same frequency as the AC source and is either in phase with the AC source or out of phase with the AC source.

[024] The AC source may act as a sink for the AC current. This may be achieved as follows. First, circuit 200 may take the low voltage DC from solar panel 240 and generate a high frequency AC current (e.g. a first AC current) and a high f equency AC voltage (e.g. a first AC voltage) across primary winding 250 by using switches Qi, Q₂, Q₃ and Q₄, inductor Lr, and capacitor C_r. The first AC voltage may be amplified in to a high amplitude, high frequency AC voltage (e.g. a second AC voltage) across secondary winding 255 by transformer 220. Transformer 220 may also reduce the amplitude of the first AC current in to a secondary side high frequency AC current (e.g. a second AC current). Both the first and the second AC currents may have an AC line frequency current component and a switching frequency component. The high frequency current component may flow through switches Q₅, Q₆, Q₇, Q₈ and capacitors COi and C0₂. The AC line frequency current component may flow through switches Q₅, Q₆, Q₇, and Qg to AC line 245. This current may be referred to as a third AC current. A third AC voltage corresponding to the third AC current may be the same as a voltage on AC line 245.

[025] Consistent with embodiments of the invention, while generating the first AC current, the switches Q_ls Q₂, Q₃ and Q₄ may be switched on and off at a switching frequency (e.g., much higher than a line frequency of AC line 245).

Further, during a positive AC line voltage cycle of AC line 245, switches Q₆ and Q₈ may be on and switches Q₅ and Q₇ may be switched at the switching frequency. During a negative line voltage cycle of AC line 245, switches Q₅ and Q₇ may be kept on, and switches Q₆ and Q₈ may be switched on and off at the switching frequency. A resonant operation may be ensured by inductor Lr and capacitor C_r. The switching frequency can be higher than a resonant frequency of the resonant tank (i.e. comprising inductor Lr and capacitor C_r) or lower than the resonant frequency of the resonant tank. Power stage circuit 200 can be used to support unidirectional or bidirectional current flow with respect to AC line 245.

[026] For reducing the switching losses, consistent with embodiments of the invention, the switches in power stage circuit 200 may be turned on at zero current or zero voltage depending on the mode of operation. When the current flow is from the AC source (e.g., AC line 245) to the DC voltage, energy may be stored in the input capacitor Q. Moreover, circuit 200 may operate in a full bridge or a half bridge mode depending on the input and output voltage. As will be described in greater detail below, based on various switching instants, there may be eight different modes of operation.

Full Bridge Operation

[027] Consistent with embodiments of the invention, power stage circuit 200 may operate in a full bridge operation when the peak input voltage to the resonant tank is equal to the DC voltage from solar panel 240. The third AC current may be regulated by frequency/pulse width/phase shift modulating Qj, Q₂, Q₃, and Q₄.

[028] FIG. 3 shows power stage circuit 200's operation in a first mode (i.e., mode 1). The first mode may be in full bridge operation and when the AC line voltage is positive and the transformer primary voltage is positive. The transformer primary voltage may comprise the voltage measured across the primary winding 250 of transformer (T_x). In this mode, the switches , Q₃, Q₅, Q₆, and Q₈ may be on. The first AC current may flow through the circuit elements Q_1? Lr, C_r, transformer 220, and Q₃. The second AC current may flow through secondary winding 255, Q₅, Q₆, and CO . CO] may be charged to half the AC line voltage. The third AC current may flow through the circuit elements Q₅, Q₆, AC line 245, C0₂, and secondary winding 255.

[029] To reduce the switching losses in power stage circuit 200, the following switching strategy may be used. For the current flowing from solar panel 240 to AC line 245, the switches Qj and Q₃ may be turned on when the voltage across them is zero (e.g. a zero-voltage-switching (ZVS) scheme) and Q₅ may be turned on at the zero crossing instant of the second AC current. For the current flowing from AC line 245 to solar panel 240, the switch Q₅ may be turned on when the voltage across it is zero (ZVS switching scheme) and Qi and Q₃ may be turned on at the zero crossing instant of the first AC current.

[030] FIG. 4 shows power stage circuit 200's operation in a second mode (i.e., mode 2). The second mode may be in a full bridge operation and when the voltage of AC line 245 is positive and the voltage across primary winding 250 is negative. Consistent with embodiments of the invention, the switches Q₂, Q₄, Q6, Q7, and Q8 may be on. The first AC current may flow through the circuit elements Q₂, L_r, C_r, transformer 220, and Q₄. The second AC current may flow through secondary winding 255, C0₂, Q₈, and Q₇. Capacitor C0₂ may be charged to half the line voltage. The third AC current may flow through the circuit elements COi, AC line, Q₇, Q₈, and secondary winding 255.

[031 ] To reduce switching losses in power stage circuit 200, the following switching strategy may be used. For the current flowing from solar panel 240 to AC line 245, the switches Q₂ and Q₄ may be turned on when the voltage across them is zero (ZVS switching scheme) and Q₇ may be turned on at the zero crossing instant of the second AC current. For the current flowing from AC line 245 to solar panel 240, the switch Q₇ may be turned on when the voltage across it is zero (ZVS switching scheme) and Q₂ and Q may be turned on at the zero crossing instant of the first AC current.

[032] FIG. 5 shows power stage circuit 200' s operation in a third mode (i.e., mode 3). The third mode may be in a full bridge operation and when the voltage of AC line 245 is negative and the voltage across primary winding 250 is positive. In this mode, switches Q Q₃, Q₅, Q₇, and Q₈ maybe on. The first AC current may flow through the circuit elements Qj, Lr, C_r, transformer 220, and Q₃. The second AC current may flow through the circuit elements Q₈, Q₇, C0₂ and secondary winding 255. Capacitor C0₂ may be charged to half of the negative line voltage. The third AC current may flow through the circuit elements Q₈, Q₇, AC line 245, CO_l5 and secondary winding 255.

[033] To reduce switching losses in power stage circuit 200, the following switching strategy may be used. For the current flow from solar panel 240 to AC line 245, the switches Qi and Q₃ may be turned on when the voltage across them is zero (ZVS switching scheme) and Q_g may be turned on at the zero crossing instant of the second AC current. For the current flow from AC line 245 to solar panel 240, the switch Q₈ may be turned on when the voltage across it is zero (ZVS switching scheme) and Qj and Q₃ may be turned on at the zero crossing instant of the first AC current.

[034] FIG. 6 shows power stage circuit 200's operation in a fourth mode 4 (i.e., mode 4). The fourth mode may be in a full bridge operation and when the voltage of AC line 245 is negative and the voltage across primary winding 250 is negative. In this mode, the switches Q₂, Q₄, Q₅, Q₆, and Q₇ may be kept on. The first AC current may flow through the circuit elements Q₂, Lr, C_r, primary winding 250, and Q₄. Further, the second AC current may flow through the circuit elements CC , Q₆, Q₅, and secondary winding 255. The capacitor CC may be charged to half of the negative AC line voltage. The third AC current may flow through the circuit elements C0₂, AC line 245, Q₆, Q₅, and secondary winding 255.

[035] To reduce switching losses in power stage circuit 200, the following switching strategy may be used. For the current flow from solar panel 240 to AC line 245, the switches Q₂ and Q₄ may be turned on when the voltage across them is zero (ZVS switching scheme) and Q₆ may be turned on at the zero crossing instant of the second AC current. For the current flow from AC line 245 to solar panel 240, the switch Q₆ may be turned on when the voltage across it is zero (ZVS switching scheme) and Q₂ and Q₄ may be turned on at the zero crossing instant of the first AC current.

Half Bridge Operation

[036] Consistent with embodiments of the invention, power stage circuit 200 may operate in a half bridge mode when the peak input voltage to the resonant tank is equal to the half of the DC source voltage. In the half bridge operation, one of the switching legs connecting to the primary winding of transformer 220 (e.g., comprising either Qi and Q₄ or Q₂ and Q₃) may be operated in such a way that the top switch may be kept off and the bottom one is kept on for the entire duration. The third AC current may be regulated by frequency/pulse width modulating the other switching leg connecting to the primary winding of transformer 220.

[037] FIG. 7 shows power stage circuit 200's operation in a fifth mode (i.e., mode 5). The fifth mode may be in a half bridge operation and when the voltage of AC line 245 is positive and the voltage across primary winding 250 is positive. In this mode, the switches Q], Q₃, Q₅, Q₆, and Q₈ may be kept on. The first AC current may flow through the circuit elements Qi, Lr, C_r, primary winding 250, and Q₃. The second AC current may flow through the switches Q₅, Q₆, C0_ls and secondary winding 255. The capacitor COi may be charged to half of the AC line voltage. The third AC current may flow through the circuit elements Q₅, Q₆, AC line 245, C0₂, and secondary winding 255.

[038] To reduce switching losses in power stage circuit 200, the following strategy may be used. For the current flow from solar panel 240 to AC line 245, switches Qj and Q₃ may be turned on when the voltage across them is zero (ZVS switching scheme) and Q₅ may be turned on at the zero crossing instant of the second AC current. For the current flow from AC line 245 to solar panel 240, Q₅ may be turned on when the voltage across it is zero (ZVS Switching scheme) and Qj and Q₃ may be turned on at the zero crossing instant of the first AC current.

[039] FIG. 8 shows power stage circuit 200's operation in a sixth mode (i.e., mode 6). The sixth mode may be in a half bridge operation and when the voltage of AC line voltage 245 is positive and the voltage across primary winding 250 is negative. In this mode, the switches Q₄, Q₃, Q₆, Q₇, and Q₈ may be on. The first AC current may flow through the circuit elements Lr, Cr, primary winding 250, Q₃, and Q₄. The second AC current may flow through the circuit elements C0₂, Q₇, Qs, and secondary winding 255. The third AC current may flow through the circuit elements COi, AC line 245, Q₇, Q₈, and secondary winding 255. The capacitor C0₂ may be charged to half the line voltage.

[040] To reduce switching losses in power stage circuit 200, the following switching strategy may be used. For the current flow from solar panel 240 to the AC line 245, the switches Q₃ and Q₄ may be turned on when the voltage across them is zero (zero voltage switching scheme) and Q₇ may be turned on at the zero crossing instant of the second AC current. For the current flow from AC line 245 to solar panel 240, Q₇ may be turned on when the voltage across it is zero (ZVS switching scheme) and the switches Q₃ and Q₄ may be turned on at the zero crossing instant of the first AC current.

[041] FIG. 9 shows power stage circuit 200's operation in a seventh mode (i.e., mode 7). The seventh mode may be in a half bridge operation and when the 1

AC voltage of AC line voltage 245 is negative and the voltage across primary winding 250 is positive. In this mode, the switches Q_l5 Q₃, Q₅, Q₇, and Q₈ may be kept on. The first AC current may flow through the circuit elements Q_t, L_r, C_r, primary winding 250, and Q₃. The second AC current may flow through Q₈, Q₇, C0₂, and secondary winding 255. The capacitor C0₂ may be charged to half the negative AC line voltage. The third AC current may flow through the circuit elements Q₇, Q₈, AC line 245, CO_l5 and secondary winding 255.

[042] To reduce switching losses in power stage circuit 200, the following switching strategy may be used. For the current flow from solar panel 240 to the AC line 245, the switches Qi and Q₃ may be turned on when the voltage across them is zero (ZVS switching scheme) and Q₈ may be turned on at the zero crossing of the second AC current. For the current flow from AC line 245 to solar panel 240, Q₈ may be turned on when the voltage across it is zero (ZVS switching scheme) and

and Q₃ may be turned on at the zero current crossing instant of the first AC current.

[043] FIG. 10 shows power stage circuit 200 's operation in an eighth mode (i.e., mode 8). The eighth mode may be in a half bridge operation and when the voltage of AC line 245 is negative and the voltage across primary winding 250 is negative. In this mode, the switches Q₃, Q₄, Q₅, Q₆, and Q₇ may be kept on. The first AC current may flow through the circuit elements L_r, C_r, primary winding 250, Q₃, and Q₄. The second AC current may flow through secondary winding 255, CC , Q₆, and Q₅. The capacitor CO] may be charged to half the negative line voltage. The third AC current may flow through the circuit elements C0₂, AC line 245, Q₆, Q₅, and secondary winding 255.

[044] To reduce switching losses in power stage circuit 200, the following switching strategy may be used. For the current flow from solar module 240 to AC line 245, the switches Q₃ and Q₄ may be turned on when the voltage across them is zero (ZVS switching scheme) and Q₆ may be turned on at the zero crossing instant of the second AC current. For the current flow from AC line 245 to solar module 240, Q₆ may be turned on when the voltage across it is zero (ZVS switching scheme) and the switches Q₃ and Q₄ may be turned on at the zero current crossing instant of the first AC current.

Control Strategy for Active and Reactive Power Control T U 2011/034981

[045] Power stage circuit 200 may be operated, for example, using either frequency modulation (e.g., above or below resonant frequency), pulse width modulation, phase shift modulation, using bursts of high switching frequency modulated over low frequency or pulse skipping. In any case, a control strategy diagram 1100 for active and reactive power generation may be shown in FIG. 11. There are two main loops in diagram 1100, an output current regulator (OCR) loop and an input voltage regulation (IVR) loop.

[046] The AC line current (i.e., the third AC current) may be sensed and may be controlled using the OCR loop. As shown in diagram 1100, OCR loop is the innermost control loop. The OCR loop may be responsible for controlling the instantaneous output current of the inverter (e.g., power stage circuit 200) to follow a sinusoidal reference provided to OCR loop. The reference current waveform for this may be generated, for example, by sensing the grid voltage and a phase locked loop (PLL) generator 1120. PLL generator 1120 may generate two waveforms, one may be in phase with the grid voltage and other one may be phase shifted by 90 degrees. A VAR controller 1125 may decide how much reactive power the converter should generate. During the reactive power generation, power stage circuit 200 may go through modes 1, 2, 3, and 4 as described above. The transition in various modes is shown in FIG. 11. Modes 5, 6, 7, and 8 may also be used for reactive power generation when the load current is small or when the ratio of the instantaneous voltage of AC line 245 to the DC input voltage of solar panel 240 is very small. FIGs. 12A and 12B illustrate power stage circuit 200 operation in various modes during reactive (FIG. 12 A) and active (FIG. 12B) power generation.

[047] The IVR loop may be responsible for matching the DC input voltage of solar panel 240 to a reference point provided by a maximum power point tracking (MPPT) block 1130 as an estimate of the location of a maximum power point (MPP). This may be done by modulating the amplitude of the output current reference signal provided to the OCR. Modulating the amplitude of the output current varies the amount of average power injected to the grid, and the average power drawn from a PV source (e.g., solar panel 240). The output of the IVR loop may be a slowly-varying (e.g., DC in steady state) signal that may be multiplied by the PLL output sine wave to produce a clean output current reference signal synchronized with the grid voltage. m^v^yjj PCT/US2011/034981

[048] Consistent with embodiments of the invention, FIGs. 13 A, 13B, and 13C show other embodiments of power stage circuit 200 where the front end is a parallel resonant circuit (FIG. 13 A), a series parallel resonant circuit (FIG. 13B), and an LLC resonant circuit (FIG. 13C). In other words, FIG. 13A shows a single stage DC to AC converter with parallel resonant circuit at the primary side, FIG. 13B shows a single stage DC to AC converter with series parallel resonant circuit at the primary side, and FIG. 13C shows a single stage DC to AC converter with LLC resonant circuit at the primary side.

[049] Embodiments of the invention may be practiced in an electrical circuit comprising discrete electronic elements, packaged or integrated electronic chips containing logic gates, a circuit utilizing a microprocessor, or on a single chip containing electronic elements or microprocessors. Embodiments of the invention may also be practiced using other technologies capable of performing logical operations such as, for example, AND, OR, and NOT, including but not limited to mechanical, optical, fluidic, and quantum technologies. In addition, embodiments of the invention may be practiced within a general purpose computer or in any other circuits or systems.

[050] Embodiments of the invention, for example, may be implemented as a computer process (method), a computing system, or as an article of manufacture, such as a computer program product or computer readable media. The computer program product may be a computer storage media readable by a computer system and encoding a computer program of instructions for executing a computer process. The computer program product may also be a propagated signal on a carrier readable by a computing system and encoding a computer program of instructions for executing a computer process. Accordingly, the present invention may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.). In other words, embodiments of the present invention may take the form of a computer program product on a computer-usable or computer-readable storage medium having computer-usable or computer-readable program code embodied in the medium for use by or in connection with an instruction execution system. A computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. [051] The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific computer-readable medium examples (a non-exhaustive list), the computer- readable medium may include the following: an electrical connection having one or more wires, a portable computer diskette, a random access memory (RAM), a readonly memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, and a portable compact disc read-only memory (CD-ROM). Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory.

[052] Embodiments of the present invention, for example, are described above with reference to block diagrams and/or operational illustrations of methods, systems, and computer program products according to embodiments of the invention. The functions/acts noted in the blocks may occur out of the order as shown in any flowchart. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved.

[053] While the specification includes examples, the invention's scope is indicated by the following claims. Furthermore, while the specification has been described in language specific to structural features and/or methodological acts, the claims are not limited to the features or acts described above. Rather, the specific features and acts described above are disclosed as example for embodiments of the invention.

Claims

WHAT IS CLAIMED IS:

1. A power system comprising:

a direct current (DC) source for generating DC power; and

a single stage converter for providing a bidirectional power flow between the DC source and an alternating current (AC) grid, the single stage converter being configured to provide the bidirectional power flow by converting the DC power to AC power by operating in at least one pre-defined mode, wherein the AC power comprises a reactive power component and an active power component.

2. The system of claim 1 , wherein the single stage converter further comprises:

a transformer;

a plurality of primary side switches being configured to connect to the DC source having a DC source voltage, the plurality of primary side switches being connected to a primary side of the transformer, the plurality of primary side switches comprising a first switch, a second switch, a third switch, and a fourth switch; and a plurality of secondary side switches being configured to connect to a line in the AC grid, the plurality of secondary side switches being connected to a secondary side of the transformer, the plurality of secondary side switches comprising a fifth switch, a sixth switch, a seventh switch, and an eighth switch wherein the system includes no rectifier.

3. The system of claim 2, wherein the single stage converter further comprises a resonant tank connected between the plurality of primary side switches and the primary side of the transformer, the resonant tank comprising a resonant inductor and a resonant capacitor connected in series.

4. The system of claim 2, wherein the single stage converter further comprises a resonant tank connected between the plurality of primary side switches and the primary side of the transformer, the resonant tank comprising one of the following: parallel resonant including a series L and a parallel C; series parallel resonant including a series L, a series C, and a parallel C; and LLC resonant including a series L, a series C, and a parallel L.

5. The system of claim 1, further comprising an output current regulator (OCR) loop configured to control the bidirectional power flow.

6. The system of claim 1 , further comprising an input voltage regulation (IVR) loop configured to control the bidirectional power flow.

7. The system of claim 6, wherein the IVR loop utilizes maximum power point tracking (MPPT).

8. A method for controlling a power stage circuit, the method comprising:

determining a polarity of a line voltage of a line connected to the power stage circuit, the power stage circuit comprising:

a plurality of primary side switches connected to a direct current (DC) source having a DC source voltage, the plurality of primary side switches being connected to a primary side of a transformer, the plurality of primary side switches comprising a first switch, a second switch, a third switch, and a fourth switch, and

a plurality of secondary side switches connected to the line, the plurality of secondary side switches being connected to a secondary side of the transformer, the plurality of secondary side switches comprising a fifth switch, a sixth switch, a seventh switch, and an eighth switch;

switching on the sixth switch and the eighth switch when the determined polarity of the line voltage is positive;

operating the plurality of primary side switches consistent with a given operation mode; and

switching on one of the fifth switch and the seventh switch based on the given operation mode.

9. The method of claim 8, wherein, operating the plurality of primary side switches consistent with the given operation mode comprising a first mode comprises turning on the first switch and the third switch at a zero-voltage-switching (ZVS);

switching on one of the fifth switch and the seventh switch based on the given operation mode comprising the first mode comprises turning on the fifth switch at a zero crossing of a current in the secondary of the transformer; and

wherein a peak voltage on the primary side of the transformer is substantially equal to the DC source voltage.

10. The method of claim 8, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a first mode comprises turning on the first switch and the third switch at a zero crossing instant of a current in the primary of the transformer;

switching on one of the fifth switch and the seventh switch based on the given operation mode comprising the first mode comprises turning on the fifth switch at a zero voltage of the line voltage; and

11. The method of claim 8, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a second mode comprises turning on the second switch and the fourth switch at a zero-voltage-switching (ZVS);

switching on one of the fifth switch and the seventh switch based on the given operation mode comprising the second mode comprises turning on the seventh switch at a zero crossing of a current in the secondary of the transformer; and

12. The method of claim 8, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a second mode comprises turning on the second switch and the fourth switch at a zero crossing instant of a current in the primary of the transformer;

switching on one of the fifth switch and the seventh switch based on the given operation mode comprising the second mode comprises turning on the seventh switch at a zero voltage of the line voltage; and

13. The method of claim 8, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a fifth mode comprises turning on the first switch and the third switch at a zero-voltage-switching (ZVS);

switching on one of the fifth switch and the seventh switch based on the given operation mode comprising the fifth mode comprises turning on the fifth switch at a zero crossing of a current in the secondary of the transformer; and

wherein a peak voltage on the primary side of the transformer is substantially equal to half the DC source voltage.

14. The method of claim 8, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a fifth mode comprises turning on the first switch and the third switch at a zero crossing instant of a current in the primary of the transformer;

switching on one of the fifth switch and the seventh switch based on the given operation mode comprising the fifth mode comprises turning on the fifth switch at a zero voltage of the line voltage; and

15. The method of claim 8, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a sixth mode comprises turning on the third switch and the fourth switch at a zero-voltage-switching (ZVS); switching on one of the fifth switch and the seventh switch based on the given operation mode comprising the sixth mode comprises turning on the seventh switch at a zero crossing of a current in the secondary of the transformer; and

16. The method of claim 8, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a sixth mode comprises turning on the third switch and the fourth switch at a zero crossing instant of a current in the primary of the transformer;

switching on one of the fifth switch and the seventh switch based on the given operation mode comprising the sixth mode comprises turning on the seventh switch at a zero voltage of the line voltage; and

17. A method for controlling a power stage circuit, the method comprising:

determining the polarity of a line voltage of a line connected to the power stage circuit, the power stage circuit comprising:

switching on the fifth switch and the seventh switch when the determined polarity of the line voltage is negative; operating the plurality of primary side switches consistent with a given operation mode; and

switching on one of the eighth switch and the sixth switch based on the given operation mode.

18. The method of claim 17, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a third mode comprises turning on the first switch and the third switch at a zero-voltage-switching (ZVS);

switching on one of the eighth switch and the sixth switch based on the given operation mode comprising the third mode comprises turning on the eighth switch at a zero crossing of a current in the secondary of the transformer; and

19. The method of claim 17, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a third mode comprises turning on the first switch and the third switch at a zero crossing instant of a current in the primary of the transformer;

switching on one of the eighth switch and the sixth switch based on the given operation mode comprising the third mode comprises turning on the eighth switch at a zero voltage of the line voltage; and

20. The method of claim 17, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a fourth mode comprises turning on the second switch and the fourth switch at a zero-voltage-switching (ZVS);

switching on one of the eighth switch and the sixth switch based on the given operation mode comprising the fourth mode comprises turning on the sixth switch at a zero crossing of a current in the secondary of the transformer; and wherein a peak voltage on the primary side of the transformer is substantially equal to the DC source voltage.

21. The method of claim 17, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a fourth mode comprises turning on the second switch and the fourth switch at a zero crossing instant of a current in the primary of the transformer;

switching on one of the eighth switch and the sixth switch based on the given operation mode comprising the fourth mode comprises turning on the sixth switch at a zero voltage of the line voltage; and

22. The method of claim 17, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a seventh mode comprises turning on the first switch and the third switch at a zero-voltage-switching (ZVS);

switching on one of the eighth switch and the sixth switch based on the given operation mode comprising the seventh mode comprises turning on the eighth switch at a zero crossing of a current in the secondary of the transformer; and

23. The method of claim 17, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a seventh mode comprises turning on the first switch and the third switch at a zero crossing instant of a current in the primary of the transformer;

switching on one of the eighth switch and the sixth switch based on the given operation mode comprising the seventh mode comprises turning on the eighth switch at a zero voltage of the line voltage; and wherein a peak voltage on the primary side of the transformer is substantially equal to half the DC source voltage.

24. The method of claim 17, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a eighth mode comprises turning on the third switch and the fourth switch at a zero-voltage-switching (ZVS);

switching on one of the eighth switch and the sixth switch based on the given operation mode comprising the eighth mode comprises turning on the sixth switch at a zero crossing of a current in the secondary of the transformer; and

25. The method of claim 17, wherein,

operating the plurality of primary side switches consistent with the given operation mode comprising a eighth mode comprises turning on the third switch and the fourth switch at a zero crossing instant of a current in the primary of the transformer;

switching on one of the eighth switch and the sixth switch based on the given operation mode comprising the eighth mode comprises turning on the sixth switch at a zero voltage of the line voltage; and