Patent Application
20140333141
Disclosed embodiments relate to PV-systems including DC/AC power inverters.
DC/AC micro-inverters and AC photovoltaic (PV) power modules have witnessed recent market success, their use however being limited to small scale, single phase (typically 110 volts AC (VAC) and 220 VAC) residential and commercial PV installations. For large size PV installations, such as solar farms, with a typical power provided from 1 MW to 500 MW, 3-phase power is provided by wiring all the distributed PV panels to a single centralized DC/AC inverter or string inverter.
This Summary is provided to introduce a brief selection of disclosed concepts in a simplified form that are further described below in the Detailed Description including the drawings provided. This Summary is not intended to limit the claimed subject matter's scope.
A photovoltaic (PV)-based alternating current (AC) module includes a PV panel for providing DC source energy from incident sunlight, and a dedicated three-phase micro-inverter system including a DC/AC inverter. The micro-inverter system is typically mechanically attached to the PV panel or further integrated as a part of the PV panel's junction box. The DC/AC inverter includes first, second and third phase circuitry (Phase A, Phase B and Phase C) including semiconductor switches and reactive circuitry.
As used herein, the term “semiconductor power switches” includes field effect transistors (FETs), bipolar junction transistors (BJTs) and Insulated Gate Bipolar Transistor (IGBTs). FETs and IGBTs have gates as their control input, while BJTs have a base as their control input. Thus, although the specific semiconductor switches shown herein are generally Metal Oxide Semiconductor FET (MOSFET) switches, it is understood that the semiconductor power switches can generally be any type of semiconductor power switch.
A solar system includes a plurality of disclosed PV-based AC modules (hereafter “modules”) that are in a distributed arrangement, such as on a rooftop or in an open area as a solar farm, where each module includes a DC/AC inverter stage for converting DC source energy to AC three-phase power. The modules include a control unit that has associated memory which is programmed to implement disclosed communications, such as using a personal area network (PAN), for example ZIGBEE wireless PAN, to exchange data between themselves (adjacent modules) and a central (system) controller.
Disclosed arrangements simplify installation and maintenance-due to plug and play features of the micro-inverter systems and elimination of a high DC voltage hazard, as well as enhancing the solar system's reliability because the failure of any module does not affect the other modules in the solar system. Disclosed embodiments improve the efficiency, reliability, and cost of large size PV installations including Mega-Watt (MW)-class solar farms as well as simplifying system maintenance through the development of relatively low cost, compact, modules that act as “AC bricks”.
Disclosed embodiments are described with reference to the attached figures, wherein like reference numerals, are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate aspects disclosed herein. Several disclosed aspects are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the embodiments disclosed herein.
One having ordinary skill in the relevant art, however, will readily recognize that the disclosed embodiments can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring aspects disclosed herein. Disclosed embodiments are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with this Disclosure.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of this Disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein. For example, a range of “less than 10” can include any and all sub-ranges between (and including) the minimum value of zero and the maximum value of 10, that is, any and all sub-ranges having a minimum value of equal to or greater than zero and a maximum value of equal to or less than 10, e.g., 1 to 5.
The control unit 131 implements stored algorithms to provide maximum power point tracking (MPPT), grid synchronization, protection and communications functionality for the micro-inverter system 130. MPPT is a technique to harvest maximum PV power under varying environments, grid synchronization involves matching the voltage frequency and phase of the micro-inverter system 130 to the voltage frequency and phase of the power grid (grid) 125, and protection is also provided against abnormal grid conditions such as over-voltage and communications functionality problems.
The outputs of each micro-inverter system 130 in the modules 135 is directly connected to the grid 125, such as a 208V, 60 Hz three-phase grid and then through a medium voltage transformer 140 that boosts the low three-phase voltage (e.g., 208V, 60 Hz) to a high voltage (e.g., 33 KV) at the power transmission line 150, where all modules 135 are electrically in parallel. Each module 135 can thus operate independently regardless of the failure of any of the other modules 135 in the solar system 100.
Increasing the switching frequency may be a way to reduce cost of the micro-inverter system 130 by shrinking the size of its reactive components. However, this approach can cause a significant power conversion efficiency drop without employing soft switching techniques.
The micro-inverter systems 130 in the solar system 100 can have a single stage or two stages.
DC/AC inverter stage 130b is shown as an example half-bridge zero voltage switch circuit including phase A circuitry comprising semiconductor switches S1 and S2 and reactive components, phase B circuitry comprising switches S3 and S4 and reactive components, and phase C circuitry comprising switches S5 and S6 and reactive components. The semiconductor switches S1 to S6 are shown as MOSFETs conventionally configured to have their body diodes parallel to the source-to-drain path by shorting the source to the body of the MOSFET.
The control input driver block 332 provides first, second and third control input drivers embodied as gate driver(s) for Phase A circuitry, gate driver(s) for Phase B circuitry and gate driver for Phase C circuitry which couple to the gates of the MOSFETs (Q1 to Q4) in the DC/AC DC/DC converter stage 130a. The control input driver block 333 provides first, second and third control input drivers embodied as gate driver(s) for Phase A circuitry, gate driver(s) for Phase B circuitry and gate driver for Phase C circuitry which couple to the gates of the MOSFET switches (S1 to S6) in the DC/AC inverter stage 130b. As known in the art, each gate driver generally includes both a high side gate driver and a low side gate driver.
The control unit 131 receives sensed voltages and currents from sensing and conditioning integrated circuit (IC) 351 sensing within the DC/DC converter stage 130a and sensed voltages and currents from sensing and conditioning IC 351 which senses within the DC/AC inverter stage 130b. The drivers can be configured to include galvanic isolation as shown in
Each PV-based AC module 135 sends the needed data to manage the plurality of PV panels 120 including its operating voltage, currents, power, frequencies, working status and any faults through a subnet of the modules 135 to the central controller 410, which can comprise a MCU. The central controller 410 includes processor resources to have at least Universal Asynchronous Receiver/Transmitter (UART) or Serial Peripheral Interface (SPI) peripherals to communicate with PAN communications. The central controller 410 sends wireless commands through a subnet of the modules 135 to turn power conversion on/off (e.g., so that a gate drive signal turns off the transistors Q1 to Q4 in the DC/DC converter stage 130a and also the gate drivers of the DC/AC inverter stage 130b) and controls the output reactive power provided by the DC/AC inverter stage 130b.
It is recognized disclosed modules 135 to be advantageous by extending the micro-inverter concept to large size PV plant installations, such as MW-class solar farms where a three-phase AC connection is used. Advantages or benefits of disclosed three-phase micro-inverter-based PV farm systems include significant advantages over traditional PV farm systems that having a single centralized three-phase micro-inverter since they allow MPPT to be implemented on each PV panel 120 to maximize energy harvesting efficiency, and offer a distributed and redundant system architecture. In addition, disclosed modules 135 can significantly simplify system design (including easy modularization and scalability), essentially eliminate safety hazards including making all DC wiring at a relatively low voltage level of a single PV panel, and reducing installation costs. Disclosed micro-inverter systems can be further integrated into PV modules to realize a true plug-and-play solar AC PV generation system.
More specifically, advantages provided by disclosed modules 135 include each PV panel 120 has individualized MPPT. Due to resulting maximum power harnessing from each PV panel 120, solar farm and rooftop system performance degradation due to shading (partial cloudiness) or soiling can be minimized. There is no mismatch losses due to the parallel connection of PV panels 120 to their dedicated DC/AC micro-inverter system 130. Separate micro-inverter systems 130 effectively connect all PV panels 120 in parallel eliminating mismatch losses between PV panels 120. There is ease of installation through a flexible and modular solar farm and rooftop system design.
Conventional electrolytic capacitors can be removed due to the three-phase power balance provided. Unlike single-phase systems, where a bulky power decoupling capacitor is required due to time varying power flow to the grid, three-phase system draws constant power from the micro-inverter systems which allow using the low value, long lifetime film capacitors Disclosed micro inverter systems should significantly reduce installation costs associated with wiring, cabling, DC bus disconnects, and large inverter rooms since each micro inverter system will generate AC power that can be directly coupled to the grid 125. There is also a likely cost reduction due to mass production (economies of scale). Moreover, there will be reduced DC distribution losses because all parallel connected power from each modules 135 is based on AC distribution.
Regarding uses for disclosed modules 135 with the rapid growth of PV power systems in recent years, more and more large-scale PV power plants are being put into use or being built. In 2014 the cumulative power of large PV power plants is more than 3.6 GWp and the PV industry still continues to grow at an unprecedented rate. From this perspective, there has a huge potential large scale deployment for large-scale PV power plants all over world. In 2014, all known large scale PV power plants are based on the centralized inverter technology or string inverter technology, which is recognized to not be able to maximize energy harvest for each PV panel, have high DC voltage hazardous to safety, and are not easy for installation and maintenance. With the advantages described above for disclosed modules 135 the PV power plant architecture based on disclosed three-phase micro-inverter systems at each PV panel can overcome above shortcomings and generally can be applied to any scale three-phase PV power plant, from relatively small scale top-roof applications for commercial building to large scale PV power plants.
While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only, and not as a limitation. Numerous changes to the disclosed embodiments can be made in accordance with the Disclosure herein without departing from the spirit or scope of this Disclosure. Thus, the breadth and scope of this Disclosure should not be limited by any of the above-described embodiments. Rather, the scope of this Disclosure should be defined in accordance with the following claims and their equivalents.
Although disclosed embodiments have been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. While a particular feature may have been disclosed with respect to only one of several implementations, such a feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application.
This application claims the benefit of Provisional Application Ser. No. 61/820,287 entitled “PV-SYSTEM ARCHITECTURE BASED ON THREE-PHASE MICRO-INVERTER FOR PV SOLAR FARM AND COMMERCIAL APPLICATIONS”, filed May 7, 2013, which is herein incorporated by reference in its entirety.
This invention was made with U.S. Government support under Department of Energy (DOE) Award Number: DE-EE0003176 awarded by the DOE. The U.S. Government has certain rights in this invention.
| Number | Date | Country | |
|---|---|---|---|
| 61820287 | May 2013 | US |
