WO2017072593A1

WO2017072593A1 - System and method for improved performance and synchronization of a grid tie inverter

Info

Publication number: WO2017072593A1
Application number: PCT/IB2016/050119
Authority: WO
Inventors: Rakesh Goel; Ankur Goel
Original assignee: Rakesh Goel; Ankur Goel
Priority date: 2015-10-26
Filing date: 2016-01-12
Publication date: 2017-05-04

Abstract

The present disclosure generally relates to AC power control. More particularly, the present disclosure relates to a system and method for a Grid Tie Inverter (GTI) that provide for improved synchronization with the grid and better anti-islanding protection. In an aspect, the present disclosure provides for a system and method for improved synchronization of a GTI using a modulation method to turn OFF all switches near zero crossing that simplifies grid presence, voltage, frequency, phase detection and estimation, and also improves Phase Locked Loop (PLL) to synchronize the generated voltage with the grid. The system and method of the present disclosure also provides for a modulation method of using 3 switches out of 4 to avoid any reverse current from the grid to the GTI.

Description

SYSTEM AND METHOD FOR IMPROVED PERFORMANCE AND

SYNCHRONIZATION OF A GRID TIE INVERTER

FIELD OF DISCLOSURE

[0001] The present disclosure generally relates to AC power control. More particularly, the present disclosure relates to a system and method for a Grid Tie Inverter that provide for improved synchronization with the grid.

BACKGROUND OF THE DISCLOSURE

[0002] The background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

[0003] Increasing use of localized power sources in residential and industrial settings coincides with the increasing desire to exploit potentially cheaper, cleaner, or more reliable alternative energy sources. Historically, individuals and businesses relied on central generation stations with power transmission and distribution networks, i.e., the traditional "grid," almost exclusively but the developing trend is toward a number of smaller generating units, distributed geographically close to the loads. Local power generation using power sources such as photovoltaic systems is becoming a viable option for businesses and homeowners. These local power generating systems can be configured as stand-alone systems that operate as the exclusive source of power for their local loads or they can be interconnected with the grid.

[0004] More commonly, these local power generating systems are configured as grid-connected systems that provide an opportunity for co-generation wherein power can be taken from the grid or provided to the grid. A grid-connected system allows a local load to remain connected both to the local system and to the power grid. In a typical setup, a local load is connected in parallel to an ac power inverter and to a utility grid. The load can be powered solely by the inverter, by the grid, or can receive a portion of its power from both.

[0005] Synchronization of the inverter to the grid is a prerequisite for changing from stand-alone mode to grid -connected mode. That is, the inverter's voltage must be matched in amplitude, frequency and phase to the voltage of the grid before electrically connecting the inverter to the grid. [0006] A grid tie inverter (also called a GTI hereinafter) is a means of operating an alternate power source like renewable energy system, fuel cell, etc. in parallel with an electric grid. The power processing circuits of a GTI are similar to that of a conventional portable DC-AC converter that operates as a stand-alone device. The main differences are in their control algorithm and safety features. A GTI basically takes a variable voltage from a DC source, such as solar panels array or a wind system, and inverts it to AC synchronized with the mains. It can provide power to different loads and feed an excess of the electricity into the grid. Depending on power and voltage levels, GTIs circuits normally have from one to three stages.

[0007] Any grid tie power source has to synchronize its frequency, phase and amplitude with the utility and feed a sinewave current into the load. If the inverter output (Vout) is higher than utility voltage, the GTI will be overloaded. On the other hand, if Vout is lower than the utility voltage, the GTI may sink current rather than source it. In order to allow a limited current flow into the loads as well as back into the line, "Vout" has to be just slightly higher than the utility voltage. Usually there is an additional coupling inductor (Lgrid) between GTI and the mains that "absorbs" the extra AC voltage and also reduces the current harmonics generated by Pulse Width Modulation circuits being used in the GTI. Because the grid acts as a source with a very low impedance, normally, a GTI is designed to work as a current controlled source, rather than a voltage source.

[0008] In solar applications, to maximize the system efficiency, a GTI also has to meet certain requirements defined by the photovoltaic (PV) panels. Solar panels provide different power in different points of their volt-ampere (V-I) characteristic. The point in the V-I curve where output power is maximum is called maximum power point (MPP). The solar inverter must assure that the PV modules are operated near their MPP. This is accomplished with a special control circuit in the first conversion stage called MPP tracker (MPPT).

[0009] As known in the art, FIGs. 1A and IB illustrate standard block diagrams of

Isolated 100 and Non-isolated GTI 150, wherein many applications demand isolated GTI 100, while non-isolated GTI 150 is less expensive to use. Typically, a GTI 100/150 includes a solar photovoltaic panel (SPV) 102/152 to power the inverter 100/150, an input filter 104/154, Maximum Power Point Tracking (MPPT) 106/156, H-bridge based power stage 108/158, output filter 110/160, and grid interface inductor named Lgrid 112/162. Optionally, a relay can also be used to connect or fully disconnect the grid. [00010] FIGs. 2A and 2B illustrate known representations of a GTI showing standard balanced sine modulation. FIG. 2C illustrates expanded TTL signals of balanced sine modulator and FIG. 3 illustrates scheme/typical construction of H-bridge in which lower and higher switches are operated with inserted dead time. FIG. 4A and FIG. 4B illustrate photographs of a GTI, wherein FIGs. 1-4 have been incorporated to explain a standard GTI operation/construction.

[00011] A GTI also has to provide so-called anti-islanding protection. When mains fails or when its voltage level or frequency goes outside of acceptable limits, the SPV system output should quickly disconnect from the line in order to prevent any threat to workers who may be working on the line. The maximum time within which to achieve this is specified in regulations such UL 1741. In the worse cases, when utility voltage drops below 0.5 of nominal, or its frequency deviates by +0.5 or -0.7 Hz from the rated value, GTI should cease to export power back to the grid in less than 100 milliseconds. An anti- islanding can be accomplished for example via AC under voltage or output over current detection functions.

[00012] Generally, GTI uses a sine wave balanced modulator that operates all 4 switches of its H-bridge considering dead time in a complementary mode using a complex transformation based PLL and associated digital switches and relays etc. Such systems have considerable limitations. Some of them reverse power from the power grid back to the GTI that has a high risk of damaging the GTI and/or loads connected to it. Others cannot operate with poor grid parameters such as voltage and/or frequency variations. Still others cannot support the voltage and/or frequency variations of Diesel Generators, and hence cannot be used along with such equipment.

[00013] There is therefore a need in the art for a system and method to enable a more efficient synchronization of output from a GTI to that of a grid or line (line or grid being used interchangeably hereon) that can be used in circumstances of poor grid parameters such as fluctuating voltage, current and frequency and is also able to transfer power to the grid even under poor grid parameters as one objective of the GTI is to reduce power consumption from the grid. Likewise, such GTIs should also be able to work along household / industrial inverters or with Diesel Generators or any other source that are generating power with large voltage and/or frequency variations.

[00014] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[00015] In some embodiments, the numbers expressing quantities or dimensions of items, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term "about." Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[00016] As used in the description herein and throughout the claims that follow, the meaning of "a," "an," and "the" includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of "in" includes "in" and "on" unless the context clearly dictates otherwise.

[00017] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. "such as") provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[00018] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims.

OBJECTS OF THE INVENTION

[00019] It is an object of the present disclosure to provide for a system and method for a Grid Tie Inverter that enables a more efficient synchronization of output from a GTI to that of a grid or line that can also be used in circumstances of poor grid parameters such as fluctuating voltage, current and frequency, while improving grid presence detection and anti-island protection.

[00020] It is an object of the present disclosure to provide for a synchronizing system and method for a Grid Tie Inverter that enables a GTI to transfer power to the grid even under poor grid parameters.

[00021] It is another object of the present disclosure to provide for a synchronizing system and method for a Grid Tie Inverter that enables a GTI to work along household inverters or with Diesel Generators or any other power source that are generating power with large voltage and/or frequency variations.

SUMMARY

[00022] Embodiments of the present disclosure generally relate to improvements in systems and methods for synchronizing a Grid Tie Inverter (GTI). In an aspect, the proposed system and method includes grid parameters detection, blocking modulation, sensing line or grid, and tracking the grid parameters to significantly improve performance of the PLL, and thereby that of the GTI. The present disclosure further discloses modulation technique(s) to avoid reverse current from grid to inverter.

[00023] In an aspect, the synchronization system for a Grid Tie inverter as disclosed in the present disclosure can include a zero crossing detection module configured to detect when waveform of at least one grid parameter being monitored is near its zero crossing point and a PWM module configured to switch OFF one or more switches of H bridge of the GTI when the waveform of the at least one grid parameter being monitoring is near its zero crossing point.

[00024] In an aspect the synchronization system can be further configured to switch ON one or more switches of the H bridge of the GTI such that frequency and phase of the output of the GTI is in synchronization with the grid. In another aspect the synchronization system can be further configured to monitor grid presence for better anti-islanding protection.

[00025] In an embodiment of the present disclosure, the synchronization system can operate in a 3-switch mode or in a 4-switch mode to control the one or more switches of the H bridge of the GTI.

[00026] In another embodiment of the present disclosure, the synchronization system can automatically shift from the 3-switch mode to the 4-switch mode or vice versa so as to reduce transient waveforms in the system while ensuring that no reverse power flows from the grid to the GTI.

[00027] In another embodiment of the present disclosure the synchronization system can switch ON or OFF one or more switches of the H bridge of the GTI so to prevent anti- islanding.

[00028] In another embodiment of the present disclosure the synchronization system can further comprise a PLL module configured to be operatively coupled with a control unit and further configured to generate one or more control signals to synchronize the frequency and phase of the output of the H Bridge inverter with the grid voltage.

[00029] In an embodiment of the present disclosure, the synchronization system can further comprise a PID module configured to control current to grid based on Iref * Sin(theta), wherein Iref is reference current amplitude and theta is instant electrical angle, and wherein computation of Iref is based on available power on SPV to operate it at MPPT.

[00030] In an embodiment of the present disclosure, the synchronization system can comprise an SFDFT module configured to apply at least one algorithm of Single Frequency Discrete Fourier Transform (SFDFT) for one complete electrical cycle on output voltage and current signal to compute complex components of these signals that are further used to compute vector of the output voltage and current, wherein the voltage vector information is used to initially synchronize the grid when the grid is present but GTI is OFF and the current vector information is used during GTI run to compute and control precise error and distortion.

[00031] In another embodiment of the present disclosure, the synchronization system can use different electrical cut angles near zero crossing based on stability of the grid to improve PF.

[00032] In yet another embodiment of the present disclosure, the synchronization system can switch OFF one or more switches of the GTI based on grid stability. [00033] Various objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like features.

BRIEF DESCRIPTION OF DRAWINGS

[00034] The diagrams are for illustration only, which thus is not a limitation of the present disclosure, and wherein:

[00035] In the Figures, similar components and/or features may have the same reference label. Further, various components of the same type may be distinguished by following the reference label with a second label that distinguishes among the similar components. If only the first reference label is used in the specification, the description is applicable to any one of the similar components having the same first reference label irrespective of the second reference label.

[00036] FIGs. 1A and IB illustrate typical block diagrams of Isolated and Non- Isolated GTIs.

[00037] FIGs. 2A and 2B illustrate known representations of a GTI showing standard balanced sine modulation. FIG. 2C illustrates expanded TTL signals of balanced sine modulator.

[00038] FIG. 3 illustrates a typical scheme of H-Bridge modulator.

[00039] FIGs. 4A and 4B illustrate exemplary photographs of typical GTIs.

[00040] FIGs. 5 A and 5B illustrate exemplary waveforms generated based on the proposed modulation technique near zero crossing in accordance with an embodiment of the present disclosure.

[00041] FIGs. 6A and 6B illustrate exemplary gate driving TTL signal of the H bridge in accordance with an embodiment of the present disclosure.

[00042] FIGs. 7A, 7B and 7C illustrate exemplary TTL signal and output voltage signal in normal run with different cut angle near zero crossing in accordance with an embodiment of the present disclosure.

[00043] FIGs. 8A, 8B and 8C illustrate exemplary representations showing grid connect and disconnect transient response in accordance with an embodiment of the present disclosure. [00044] FIGs. 9A and 9B illustrate exemplary representations showing gate driving TTL signal of modified modulation from 4 switches to 3 switches in accordance with an embodiment of the present disclosure.

[00045] FIGs. 10A and 10B illustrate modified modulation with output voltage and current in accordance with an embodiment of the present disclosure.

[00046] FIGs. 11A and 1 IB illustrate simulated phase error and voltage error between Grid and GTI.

[00047] FIG. 12 illustrates a block diagram of the hardware of an exemplary embodiment of the present disclosure explaining in more detail the control and feedback signals.

[00048] FIG. 13A and 13B illustrate exemplary flow diagrams illustrating the operation of an exemplary embodiment of the present disclosure.

[00049] FIG. 14 illustrates the various modules of the synchronizing system of an exemplary GTI in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

[00050] The description hereinafter, of the specific embodiment will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify or adapt or perform both for various applications such specific embodiment without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

[00051] Various terms as used herein are defined below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

Definitions :

[00052] The expression 'phase lock loop' of 'PLL' in the instant disclosure refers to a control system that generates an output signal whose phase is related to the phase of an input signal. [00053] The expression Ή Bridge' in the instant disclosure refers to an electronic circuit that enables a voltage to be applied across a load in either direction. Most DC to AC converters use H Bridges. The H bridge with a DC supply will generate a square waveform across the load, while for a purely inductive load the waveform shall be a triangle with its peak depending on the inductance, switching frequency and input voltage. The term H bridge is derived from the typical graphical representation of such a circuit. An H bridge is built with four solid-state switches that are switched on or off by various control signals to generate a square waveform across the load.

[00054] The expression 'dead time' in the instant disclosure refers to delay that has to be provided to the turn-on side of the low-side switches of a GTI. It should be at least as much as the turn-off time of the high side switches of the GTI. The same goes for the other transition, when switching is from the low-side to the high-side. The purpose is to prevent a short-circuit on the input voltage source.

[00055] The expression 'Transistor-transistor logic' or 'TTL' in the instant disclosure refers to a class of digital circuits built from bipolar junction transistors (BJT) and resistors. It is called transistor-transistor logic because both the logic gating function (e.g., AND) and the amplifying function are performed by transistors.

[00056] The expression 'phase error' in the instant disclosure refers to difference in phase, after adjustment for the effect of frequency error between two AC voltage sources.

[00057] The expression 'Maximum Power Point Tracking' or 'MPPT' in the instant disclosure refers to a system to sample the output of the SPV generator and apply the proper resistance to obtain maximum power for any given environmental conditions. MPPT devices are typically integrated into an electric power converter system that provides voltage or current conversion, filtering, and regulation for driving various loads, including power grids, batteries, or motors.

[00058] The expression 'pulse width modulation' or 'PWM' in the instant disclosure refers to a process of modifying the width of the pulses in a pulse train in direct proportion to a small control signal; the greater the control voltage, the wider the resulting pulses become. By using a sinusoid of the desired frequency as the control voltage for a PWM circuit, it is possible to produce a high-power waveform whose average voltage varies sinusoidally.

[00059] The expression 'proportional-integral-derivative controller' or 'PID controller' in the instant disclosure refers to a control loop feedback mechanism (controller) commonly used in industrial control systems. A PID controller continuously calculates an error value as the difference between a measured process variable and a desired set point and attempts to minimize the error over time by adjustment of a control variable.

[00060] In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details.

[00061] Embodiments of the present invention include various steps, which will be described below. The steps may be performed by hardware components or may be embodied in machine -executable instructions, which may be used to cause a general- purpose or special-purpose processor programmed with the instructions to perform the steps. Alternatively, steps may be performed by a combination of hardware, software, and firmware and/or by human operators.

[00062] Embodiments of the present invention may be provided as a computer program product, which may include a machine-readable storage medium tangibly embodying thereon instructions, which may be used to program a computer (or other electronic devices) to perform a process. The machine-readable medium may include, but is not limited to, fixed (hard) drives, magnetic tape, floppy diskettes, optical disks, compact disc read-only memories (CD-ROMs), and magneto-optical disks, semiconductor memories, such as ROMs, PROMs, random access memories (RAMs), programmable read-only memories (PROMs), erasable PROMs (EPROMs), electrically erasable PROMs (EEPROMs), flash memory, magnetic or optical cards, or other type of media/machine- readable medium suitable for storing electronic instructions (e.g., computer programming code, such as software or firmware).

[00063] Various methods described herein may be practiced by combining one or more machine -readable storage media containing the code according to the present invention with appropriate standard computer hardware to execute the code contained therein. An apparatus for practicing various embodiments of the present invention may involve one or more computers (or one or more processors within a single computer) and storage systems containing or having network access to computer program(s) coded in accordance with various methods described herein, and the method steps of the invention could be accomplished by modules, routines, subroutines, or subparts of a computer program product. [00064] If the specification states a component or feature "may", "can", "could", or "might" be included or have a characteristic, that particular component or feature is not required to be included or have the characteristic.

[00065] Exemplary embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. These embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those of ordinary skill in the art. Moreover, all statements herein reciting embodiments of the invention, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future (i.e., any elements developed that perform the same function, regardless of structure).

[00066] Thus, for example, it will be appreciated by those of ordinary skill in the art that the diagrams, schematics, illustrations, and the like represent conceptual views or processes illustrating systems and methods embodying this invention. The functions of the various elements shown in the figures may be provided through the use of dedicated hardware as well as hardware capable of executing associated software. Similarly, any switches shown in the figures are conceptual only. Their function may be carried out through the operation of program logic, through dedicated logic, through the interaction of program control and dedicated logic, or even manually, the particular technique being selectable by the entity implementing this invention. Those of ordinary skill in the art further understand that the exemplary hardware, software, processes, methods, and/or operating systems described herein are for illustrative purposes and, thus, are not intended to be limited to any particular named element.

[00067] Embodiments of the present disclosure generally relate to improvements in systems and methods for synchronizing a Grid Tie Inverter (GTI). In an aspect, the proposed system and method can include grid parameters detection, blocking modulation, sensing line or grid and tracking the grid parameters that significantly improves performance of the PLL and thereby that of the GTI. The present disclosure further discloses modulation technique(s) to avoid reverse current from grid to inverter.

[00068] Although the present disclosure has been described with the purpose of implementing and improving the synchronizing system of a GTI, it should be appreciated that the same has been done merely to illustrate the invention in an exemplary manner and any other purpose or function for which the explained structure or configuration can be used, is covered within the scope of the present disclosure.

[00069] In an aspect, the synchronization system for a Grid Tie inverter as disclosed in the present disclosure can include a zero crossing detection module configured to detect when waveform of at least one grid parameter being monitored is near its zero crossing point and a PWM module configured to switch OFF one or more switches of H bridge of the GTI when the waveform of the at least one grid parameter being monitoring is near its zero crossing point.

[00070] In an aspect the synchronization system can be further configured to switch ON one or more switches of the H bridge of the GTI such that frequency and phase of the output of the GTI is in synchronization with the grid. In another aspect the synchronization system can be further configured to monitor grid presence for better anti-islanding protection.

[00071] In an embodiment of the present disclosure, the synchronization system can operate in a 3-switch mode or in a 4-switch mode to control the one or more switches of the H bridge of the GTI.

[00072] In another embodiment of the present disclosure, the synchronization system can automatically shift from the 3-switch mode to the 4-switch mode or vice versa so as to reduce transient waveforms in the system while ensuring that no reverse power flows from the grid to the GTI.

[00073] In another embodiment of the present disclosure the synchronization system can switch ON or OFF one or more switches of the H bridge of the GTI so to prevent anti- islanding.

[00074] In another embodiment of the present disclosure the synchronization system can further comprise a PLL module configured to be operatively coupled with a control unit and further configured to generate one or more control signals to synchronize the frequency and phase of the output of the H Bridge inverter with the grid voltage.

[00075] In an embodiment of the present disclosure, the synchronization system can further comprise a PID module configured to control current to grid based on Iref * Sin(theta), wherein Iref is reference current amplitude and theta is instant electrical angle, and wherein computation of Iref is based on available power on SPV to operate it at MPPT.

[00076] In an embodiment of the present disclosure, the synchronization system can comprise an SFDFT module configured to apply at least one algorithm of Single Frequency Discrete Fourier Transform (SFDFT) for one complete electrical cycle on output voltage and current signal to compute complex components of these signals that are further used to compute vector of the output voltage and current, wherein the voltage vector information is used to initially synchronize the grid when the grid is present but GTI is OFF and the current vector information is used during GTI run to compute and control precise error and distortion.

[00077] In another embodiment of the present disclosure, the synchronization system can use different electrical cut angles near zero crossing based on stability of the grid to improve PF.

[00078] In yet another embodiment of the present disclosure, the synchronization system can switch OFF one or more switches of the GTI based on grid stability.

[00079] Various objects, features, aspects and advantages of the present disclosure will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like features.

[00080] FIG. 5A and 5B illustrate exemplary waveforms generated by the proposed modulation techniques near zero crossing. In an aspect, the present disclosure does not require any hardware change as the improvements are implemented by change in the embedded software. FIG. 5 A illustrates output current at channel CI, and output voltages at channel C3 and C4 at 502 near the zero cutoff points of the respective waveforms. FIG. 5B illustrates another zero cut-off region for C3 at 504 with math function of C3-C4, which is net output or grid voltage. In the GTI of the present disclosure, the PWM frequency can be 14.4 KHz that can generate double frequency 28.8 KHz ripple due to balanced modulation. This ripple can be significantly filtered but can be still present on output lines that are visible on these waveforms. It is to be appreciated that it is clear from the waveforms 504 that all switches are OFF near zero crossing while the grid signal is present near zero crossing at 506 as no PWM ripple is present near zero crossing.

[00081] FIGs. 6A and 6B illustrate exemplary gate driving TTL signal of the H bridge of the GTI in accordance with an embodiment of the present disclosure. In an exemplary embodiment, FIG. 6A illustrates, at 602, output current at CI, TTL signal to drive H bridge switches AH and BH at C3 and C4 respectively, whereas with respect to FIG. 6B, expanded signals 604 also illustrate balanced sine modulation. The PWM width during modulation changes as per amplitude of the modulating sinewave. The high side width of these PWM signals are illustrated, which shows symmetrical gap on both sides due to balanced modulation. [00082] In an aspect, two control loops are required in this particular example of GTI, wherein the first control is to synchronize the frequency and phase that is conducted by PLL, and the second control is current control loop from GTI to grid. In another aspect, the number of control loops can be more than 2 also.

[00083] In an aspect, Grid signal is captured near zero crossing with respect to electrical angle when all the switches are OFF. At this particular time instance, the packet of signal can be used to detect grid presence, voltage, current, frequency and phase error or any combination of those. If the grid is present, this signal is almost in straight line else it does not follow straight line. A simple mathematical assumption is Sin(Theta)=Theta for Theta tends to zero. It explains that sinusoidal waveform near zero crossing within +/- 10 deg will be almost straight line. Therefore, the deviation from straight line is used to calculate the presence of grid. Its peak-to-peak magnitude (vl) is directly proportional to grid voltage. Many methods are applicable to compute phase error, one of them being to use voltage at zero crossing (v2), based on which phase error can be computed by v2/vl, and position of electrical angle at 0 or 180 deg can also be computed. Phase error computation is most important to run PLL.

[00084] In another aspect, current to grid can be controlled by high speed Proportional Integral Differential (PID) loop, that can regulate the current based on Iref * Sin(theta), where Iref is reference current amplitude, and theta is instant electrical angle. The Iref can be computed based on available power on SPV to operate it at MPPT.

[00085] In an aspect, one algorithm of single frequency Discrete Fourier Transform (SFDFT) for one complete electrical cycle can be applied on output voltage and current signal to compute complex components of these signals that can be further used to compute vector (magnitude and phase) of the output voltage and current. In another aspect, the voltage vector information can be used to initially synchronize the grid when the grid is present but GTI is OFF while the current vector can be used during GTI run to compute and control precise error and distortion.

[00086] FIGs. 7A, 7B and 7C illustrate exemplary TTL signal and output voltage signal in normal run with different cut angle near zero crossing in accordance with an embodiment of the present disclosure.

[00087] The figures illustrate output voltage waveforms with switches OFF near zero crossing at 702, whereas waveforms 704 and 706 are captured at +/- 10 deg and +/- 20 deg cut angle and show different widths near zero crossing to explain that different electrical cut angles can be used near zero crossing to switch OFF the devices in the present disclosure. [00088] FIGs. 8A, 8B and 8C illustrate exemplary representations showing grid connect and disconnect transient response in accordance with an embodiment of the present disclosure.

[00089] The performance of GTI start in presence of grid is illustrated at 800. Signal at 802 which is before the point of 804 shows that grid is present but PWMs are OFF. Signal after this point 804 shows that GTI is started as the PWMs and output voltage ripples are present. Grid fail detection is illustrated at 806 and its expanded waveforms at 808, wherein it is clear that grid failure detection is easier by analyzing voltage samples near zero crossing. It shows a different shape in case the grid fails based on grid load and/or its power factor (PF), which explains that GTI will be turned OFF without detecting under voltage and over current while the grid fails and better anti-islanding protection within 100 ms.

[00090] In an exemplary embodiment, PF (power factor) is above 0.95 at zero crossing cut of +/- 10 deg, which is good and acceptable. Furthermore, most of the load will also have similar power factor, and therefore feeding power at this 0.95 PF is not a disadvantage. In an exemplary embodiment, software of the present disclosure can also dynamically reduce this cut angle based on stability of the grid to improve PF.

[00091] FIGs. 9A and 9B illustrate exemplary representations showing gate driving TTL signal of modified modulation from 4 switches to 3 switches in accordance with an embodiment of the present disclosure.

[00092] GTI using modulation on 3 switches is illustrated, wherein waveforms at 902 show all 4 TTL drive signals CI, C2, C3 and C4 to H Bridge of the GTI, making it clear that C2 is OFF during one half of the modulating sinewave, and C3 is OFF in the other half of the sinewave. All signals are OFF near zero crossing. Waveform 904, at FIG. 9B, shows expanded signal of 902, which shows C 1 switching as per modulation, C2 OFF, and C3 and C4 operating with dead time.

[00093] In an aspect, whenever the lower and higher switches are operated with dead time, GTI of the present disclosure behaves as low impedance voltage source. Therefore, the 4 switches modulation of the H bridge operates as a low impedance voltage source that is connected to grid using inductor Lgrid. If the GTI output voltage is higher than grid voltage, the power is delivered from GTI to grid. However, reverse current or reverse power can flow from grid to GTI in case the GTI voltage is lower than Grid voltage. The 3-switch modulation of the present disclosure ensures that one bridge leg will not be operating as low impedance voltage source, and it ensures that reverse power cannot flow from Grid to GTI. [00094] FIGs. 10A and 10B illustrate modified modulation with output voltage and current in accordance with an embodiment of the present disclosure.

[00095] Output current, voltage, and math waveform of the 3 switches modulation is illustrated at 1002, wherein the next waveform 1004 is captured using PC based tool, which also captures electrical angle, voltage, and current. This technique also works efficiently and its key advantage is that reverse current from grid to GTI cannot flow, enabling the GTI to be tuned with this technique to run at very low power from SPV. However, it has higher ripple content at the output, and therefore both techniques can be implemented in the GTI. In an aspect, the 3 switch modulation can be used to start the GTI in presence of the grid, and then GTI changes its operation mode of 4 switches modulation with all switches OFF near zero crossing.

[00096] In an aspect, Grid Tie Inverter of the present disclosure can automatically shift from a 3-switch mode operation to a 4-switch mode or vice versa so as to reduce transient waveforms in the system, while ensuring that no reverse power can flow from Grid to GTI.

[00097] In an aspect of the present disclosure, it is not necessary to apply all switches OFF near zero crossing at every 180 deg electrical angle. It can be applied at two time out of n, where n can be any even number between 2 to 32000. This number can be chosen based on grid stability. It is preferred that GTI starts at n=2, and then value of n is increased in even numbers depending on the stability of the grid.

[00098] FIGs. 11A and 1 IB illustrate simulated phase error and voltage error between Grid and GTI. The simulated waveform of phase error and its effect is illustrated in FIG. 11 A, which shows that simulated 10 deg phase error can generate incorrect currents due to instantaneous difference in voltage between GTI and grid as the impedance of inductor Lgrid is reasonably low. Therefore, in an aspect of the present disclosure, PLL can be operated to correct the frequency and phase at every half cycle. The simulated voltage error is illustrated in FIG. 11B. Some error in this loop can either generate over current trip or reverse current can flow from Grid to GTI. Therefore high-speed PID loop is operated to maintain estimated current.

[00099] FIG. 12 illustrates a block diagram of the hardware of an exemplary embodiment of the present disclosure explaining in more detail the control and feedback signals.

[000100] As shown, in an aspect, the various control and feedback signals can include SPV voltage feedback 1202 for solar panel voltage and to operate at MPPT, Bus voltage feedback 1204 for regulating DC bus, Ii Feedback 1206 for current measurement from first bridge leg of H bridge and protection, I₂Feedback 1208 for current measurement from second bridge leg of H bridge and protection, I_out Feedback 1210 for output bipolar current measurement and control, Line feedback 1212 for grid line voltage measurement w.r.t. floating common, and Neutral feedback 1214 for grid neutral voltage measurement w.r.t. floating common.

[000101] In another aspect, the control signals can also include UP converter PWM 1216 for UP converter stage. A typical example is that SPV delivers 340V DC open circuit and 288V DC at MPPT while the DC bus is maintained at 360V to generate AC power up to 250V. Therefore UP converter stage boosts the DC voltage and maintains at desired level.

[000102] In yet another aspect, control signals can include H Bridge PWM (4 lines) 1218 for H-Bridge power stage that are used to generate balanced sine modulation and also for ON-OFF control of switches that is required in this invention and RS485 communication 1220 signals that are applied to isolated interface for Host / PC communication

[000103] FIGs. 13A and 13B illustrate exemplary flow diagrams illustrating the operation of an exemplary embodiment of the present disclosure. As illustrated in FIG. 13 A, the main routine flow can include initializing peripherals, variables and memory at 1302, shutting down GTI at power up at 1304, and initializing interrupt vector and enabling interrupt service routine at 1306. At 1308, the process to synchronize at fixed speed can start. At 1310, various control communications can be handled. At 1312, captured signals can be processed that can be used to compute grid parameters at step 1314, and at 1316, the system can determine whether the GTI is shut down or not. If no, the PLL can be operated to control frequency and phase as shown at 1318 and if yes, the PLL can be operated based on captured signals near zero crossing as shown at 1320. At 1322, various protections can be processed and at 1324 shutdown can be controlled or run based on errors and PLL. At 1326, the MPPT loop can be operated and then the control loop repeated from 1310.

[000104] FIG. 13B illustrates the flow of the interrupt service routine. At 1352, ADC signal can be captured in synchronization and next conversion can be initiated. At 1354, grid voltage, SPV voltage and currents etc. can be computed, and at 1356 grid frequency and phase can be computed. At 1358, sinusoidal signal can be generated. At 1360, PWM signals can be generated and at 1362 PWM override control can be handled for selective switches OFF. At 1364, PWM signals can be outputted. At 1366,UP converter PWM signal can be generated, and at 1368 high speed current control loop can be operated. At 1370, control can return from interrupt service routine and proceed further. [000105] FIG. 14 illustrates the various modules of the synchronizing system of an exemplary GTI in accordance with an embodiment of the present disclosure. As shown in FIG. 14, the proposed disclosure can include zero crossing detection module 1402, grid parameters detection and tracking module 1404, PWM module 1406, PLL module 1408, PID Module 1410 and SFDFT module 1412.

[000106] In an aspect zero crossing detection module 1402 can detect when the waveforms of grid parameters being monitored are near to their zero crossing point. These parameters can include voltage, current, frequency, phase error or any combination of those. In another aspect, zero crossing detection module 1402 can provide appropriate signals to grid parameters detection and tracking module 1404 when these grid parameters are approaching near to their zero crossing point.

[000107] In another aspect, grid parameters detection and tracking module 1404 can detect the various grid parameters near their zero crossing point. In another aspect, grid parameters detection and tracking module 1404 can detect the various grid parameters each time they are near their crossing point. In yet another aspect, grid parameters detection and tracking module 1404 can detect the various grid parameters when so commanded by the control circuit of the present disclosure.

[000108] In an aspect, PWM module 1406 can switch OFF or switch ON the various switches of the H Bridge of the GTI when so commanded by the control circuit of the present disclosure. In another aspect, PWM module 1406 can switch OFF or switch ON the various switches of the H bridge of the GTI whenever the grid parameters being tracked by grid parameters detection and tracking module 1404 are near to their zero crossing point. In yet another aspect, PWM module 1406 can provide various switch ON or switch OFF signals to the switches of the H bridge of the GTI in such a fashion that the output of the H Bridge inverter achieves synchronization with the line or grid voltage. In yet another aspect, PWM module 1406 can provide various switch ON or switch OFF signals to the switches of the H bridge of the GTI in such a fashion that the H bridge can operate in 3 switches mode or 4 switches mode as required. In yet another aspect PWM module 1406 can provide various switch ON or switch OFF signals to the switches of the H bridge of the GTI in such a fashion so to prevent anti -islanding.

[000109] In an aspect, module PLL module 1408 can generate appropriate control signals to synchronize the frequency and phase of the output of the H Bridge inverter with the grid voltage. [000110] In an aspect, PID Module 1410 can control current to grid based on Iref * Sin(theta), where Iref is reference current amplitude, and theta is instant electrical angle, computing Iref based on available power on SPV to operate it at MPPT.

[000111] In an aspect, SFDFT module 1412 can apply one algorithm of single frequency Discrete Fourier Transform (SFDFT) for one complete electrical cycle on output voltage and current signal to compute complex components of these signals that can be further used to compute vector (magnitude and phase) of the output voltage and current. In another aspect, the voltage vector information can be used to initially synchronize the grid when the grid is present but GTI is OFF while the current vector information can be used during GTI run to compute and control precise error and distortion.

[000112] While embodiments of the present invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the invention, as described in the claim.

[000113] As used herein, and unless the context dictates otherwise, the term "coupled to" is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms "coupled to" and "coupled with" are used synonymously. Within the context of this document terms "coupled to" and "coupled with" are also used euphemistically to mean "communicatively coupled with" over a network, where two or more devices are able to exchange data with each other over the network, possibly via one or more intermediary device.

[000114] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms "comprises" and "comprising" should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C ... .and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc. The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.

[000115] While embodiments of the present disclosure have been illustrated and described, it will be clear that the disclosure is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions, and equivalents will be apparent to those skilled in the art, without departing from the spirit and scope of the disclosure, as described in the claims.

ADVANTAGES OF THE INVENTION

[000116] The present disclosure provides for a synchronization system and method for a Grid Tie Inverter (GTI) using a grid parameter detection technique that includes blocking modulation, sensing line or grid presence and tracking the line or grid parameters to significantly improve performance of the PLL and thereby that of the GTI.

[000117] The present disclosure provides for a synchronization system and method for a Grid Tie Inverter that does not require any hardware change as the improvements are implemented by change in the embedded software.

[000118] The present disclosure provides for a synchronization system and method for a Grid Tie Inverter that can enable the GTI to be synchronized with a grid with poor grid parameters such as voltage and frequency and still transfer power to the grid while improving grid presence detection and anti-island protection.

[000119] The present disclosure provides for a synchronization system and method for a Grid Tie Inverter that can enable the GTI to be synchronized with power sources having wide variations in frequency and voltage such as Diesel generators and so, can be used with such machineries as well. [000120] The present disclosure provides for a synchronization system and method for a Grid Tie Inverter that can enable the GTI to be synchronized with the output of a normal residential inverter as well.

[000121] The present disclosure provides for a synchronization system and method for a Grid Tie Inverter that can enable the GTI to operate in a 3 switch mode as well that can ensure that one bridge leg will not be operating as low impedance voltage source and so reverse power cannot flow from Grid to GTI.

[000122] The present disclosure provides for a synchronization system and method for a Grid Tie Inverter that can enable the GTI to automatically shift from a 3 switch mode operation to a 4 switch mode or vice versa so as to reduce transient waveforms in the system, while ensuring that no reverse power can flow from Grid to GTI.

Claims

We Claim:

1. A method of operating a grid-tie inverter (GTI), said method comprising the steps of:

detecting when a grid signal is near zero-crossing;

switching OFF one or more switches of H-bridge of the GTI when grid signal near zero-crossing is detected; and

switching ON the one or more previously switched OFF switches so as to synchronize frequency and phase of the GTI and the grid.

2. The method of claim 1, wherein the grid signal is detected based on one or a combination of parameters selected from voltage, current, frequency, and phase.

3. The method of claim 1, wherein the steps of detecting, switching ON, and switching OFF are performed by a control circuit of the GTI.

4. The method of claim 1, wherein the step of switching ON the one or more previously switched OFF switches is performed when the grid signal is near zero-crossing.

5. A synchronization system for a Grid Tie Inverter (GTI), said system comprising:

a zero crossing detection module configured to detect when waveform of at least one grid parameter being monitored is near its zero crossing point; and

a PWM module configured to switch OFF one or more switches of H bridge of the GTI when the waveform of the at least one grid parameter being monitoring is near its zero crossing point.

6. The system of claim 5, wherein the system is further configured to switch ON the one or more switches of the H bridge of the GTI such that frequency and phase of the output of the GTI is in synchronization with the grid.

7. The system of claim 5, wherein the system operates in a 3-switch mode or in a 4- switch mode to control the one or more switches of the H bridge of the GTI.

8. The system of claim 7, wherein the system automatically shifts from the 3-switch mode to the 4-switch mode or vice versa so as to reduce transient waveforms in the system while ensuring that no reverse power flows from the grid to the GTI.

9. The system of claim 5, wherein the one or more switches of the H bridge of the GTI are switched ON or OFF so to prevent anti-islanding.

10. The system of claim 5, wherein the system further comprises a PLL module configured to be operatively coupled with a control unit and further configured to generate one or more control signals to synchronize the frequency and phase of the output of the H Bridge inverter with the grid voltage.

11. The system of claim 5, wherein the system further comprises a PID module configured to control current to grid based on Iref * Sin(theta), wherein Iref is reference current amplitude and theta is instant electrical angle, and wherein computation of Iref is based on available power on SPV to operate it at MPPT.

12. The system of claim 5, wherein the system further comprises an SFDFT module configured to apply at least one algorithm of Single Frequency Discrete Fourier Transform (SFDFT) for one complete electrical cycle on output voltage and current signal to compute complex components of these signals that are further used to compute vector of the output voltage and current, wherein the voltage vector information is used to initially synchronize the grid when the grid is present but GTI is OFF and the current vector information is used during GTI run to compute and control precise error and distortion.

13. The system of claim 5, wherein different electrical cut angles are used near zero crossing based on stability of the grid to improve PF.

14. The system of claim 5, wherein the one or more switches of the GTI are switched OFF based on grid stability.