Patent Publication Number: US-2021184593-A1

Title: Inverter System and Method for Operating an Inverter System

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a US 371 application from PCT/EP2018/070163 entitled “Inverter System and Method for Operating an Inverter System” filed on Jul. 25, 2018 and published as WO 2020/020452 A1 on Jan. 30, 2020. The technical disclosures of every application and publication listed in this paragraph are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an inverter system comprising a plurality of inverters, a solar power system comprising such an inverter system, and a method for operating an inverter system comprising a plurality of inverters. 
     BACKGROUND 
     Solar power systems are of growing importance. Solar power systems are used for example to generate electric power or heat by using a large number of solar panels. In general, a solar power system comprises an inverter system having a plurality of inverters to transform the DC power generated by the solar panels into controlled AC power using e.g. pulse width modulation (PWM) switching. Because of the switching, the AC current supplied from the inverters to a grid has switching ripples which cause a distortion on the grid. This total harmonic distortion (THD) is limited by standards. Accordingly, there is a need for reducing THD. 
     For reducing THD, conventional inverter systems use filters with passive circuit elements at the output for filtering the switching ripples. For better filtering of the switching ripples, the passive circuit elements such as inductances (L) and capacitances (C) have to be large. This means that the passive circuit elements need more space and are more expensive. 
     SUMMARY 
     According to a first aspect disclosed herein, there is provided an inverter system, comprising a plurality of inverters each having an input connectable to at least one DC source, an inverter circuit for converting a DC current into an AC current, and an output connected to a bus which is connectable to a grid, and a controller for controlling the plurality of inverters which is configured to control the switching processes of the inverter circuits of the plurality of inverters such that the switching processes of at least two inverters of the plurality of inverters are phase-shifted to each other. 
     In an example, the switching processes of all inverters or all active converters are phase-shifted to each other. The inverters of the inverter system may be single-phase or multiple-phase inverters. The grid may be a public grid or an isolated grid. 
     By adding a phase angle to the switching processes of the inverter circuits of the inverters, the current ripples of the AC currents generated by the inverters also have a phase angle. As a result, the summation of the AC currents generated by the inverters at the bus will eliminate or at least reduce the current ripples mutually so that the AC current provided by the inverter system has an improved THD level. The inverter system does not need special filtering having additional circuit elements so that it has a simple and cheap configuration. 
     In an example of the first aspect, one inverter of the plurality of inverters serves as the controller. In other words, the inverter system includes a master-slave system with one inverter being the master-inverter and the other inverters being the slave-inverters, wherein the switching processes of the slave-inverters are controlled by the master-inverter. Alternatively, the inverter system may have a separate master controller for controlling all inverters of the plurality of inverters. 
     In an example of the first aspect, the plurality of the inverters and the controller are connected to each other via a communication line. In an example, each of the inverters comprises a controller, and the controllers of the plurality of inverters are connected to each other via the communication line. 
     In another example of the first aspect, the switching processes of the plurality of inverters are each controlled by a respective carrier wave signal having a modulation frequency to generate a PWM output signal, and the controller is configured to phase-shift the carrier wave signals of at least two inverters, in an example all or all active inverters of said plurality of inverters, relative to each other. The carrier wave signal may have for example a triangular or saw tooth waveform. 
     In yet another example of the first aspect, a phase difference between the phase-shifted switching processes of two inverters of the plurality of inverters is δ=360°/m with m being the total number of inverters or active inverters. 
     According to a second aspect disclosed herein, a solar power system comprises an above-described inverter system according to any of the first aspect and examples of the first aspect, and a plurality of solar energy devices connected to the inputs of the plurality of inverters of the inverter system. 
     According to a third aspect disclosed herein, in a method for operating an inverter system comprising a plurality of inverters each having an input connectable to at least one DC source, an inverter circuit for converting a DC current into an AC current, and an output connected to a bus which is connectable to a grid, the switching processes of the inverter circuits of the plurality of inverters are controlled such that the switching processes of at least two inverters of the plurality of inverters are phase-shifted relative to each other. 
     In an example, the switching processes of all inverters or all active converters are phase-shifted to each other. The inverters of the inverter system may be single-phase or multiple-phase inverters. By adding a phase angle to the switching processes of the inverter circuits of the inverters, the current ripples of the AC currents generated by the inverters also have a phase angle. As a result, the summation of the AC currents generated by the inverters at the bus will eliminate or at least reduce the current ripples mutually so that the AC current provided by the inverter system has an improved THD level. The inverter system does not need special filtering having additional circuit elements so that it has a simple and inexpensive configuration. 
     In an example of the third aspect, the switching processes of the inverter circuits of the plurality of inverters are controlled by one inverter of the plurality of inverters, which acts as a master-inverter. This means the inverter system includes a master-slave system with one inverter being the master-inverter and the other inverters being the slave-inverters, wherein the switching processes of the slave-inverters are controlled by the master-inverter. Alternatively, all inverters of the plurality of inverters may be controlled by a separate master controller. 
     In an example of the third aspect, the switching processes of the plurality of inverters are each controlled by a carrier wave signal having a modulation frequency to generate a PWM output signal, wherein the carrier wave signals of at least two of the plurality of inverters, in an example all or all active inverters of the plurality of inverters, are phase-shifted relative to each other. The carrier wave signal may have for example a triangular or saw tooth waveform. 
     In another example of the third aspect, a phase difference between the phase-shifted switching processes of two inverters of the plurality of inverters is δ=360°/m with m being the total number of inverters or active inverters. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       To assist understanding of the present disclosure and to show how embodiments may be put into effect, reference is made by way of example to the accompanying drawings in which: 
         FIG. 1  shows schematically the configuration of an example of an inverter system according to an embodiment of the present disclosure; 
         FIG. 2  shows schematically diagrams for explaining the PWM structure of the inverter circuits of the inverters according to an example of the present disclosure; 
         FIG. 3  shows schematically diagrams for explaining the switching ripples of one inverter of the inverter system according to an example of the present disclosure; 
         FIG. 4  shows schematically a diagram for explaining the phase-shifted carrier wave signals according to an example of the present disclosure; 
         FIG. 5  shows schematically diagrams for comparing the output currents of an inverter system according to an example of the present disclosure and a conventional inverter system; and 
         FIG. 6  shows schematically zoomed details of the diagrams of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows schematically an embodiment of an inverter system for a solar power system according to an example of the present disclosure. 
     The inverter system comprises a plurality of inverters  10   x  and  10   a . . . n , wherein one inverter  10   x  serves as a master-inverter and the other inverters  10   a . . . n  serve as slave-inverters. 
     Each of the inverters  10   x ,  10   a . . . n  comprises an input  12  which can be connected to a at least one solar energy device serving as a DC source. The solar energy devices convert incident solar energy into electrical energy. The solar energy devices may be in the form of for example a solar panel, which has a number of solar cells, which generate electrical power from incident solar energy. A solar cell is an electrical device that converts the energy of light into electricity. A solar cell may be for example a photovoltaic device which is a semiconductor device that converts light energy directly into electricity by the photovoltaic effect. As an alternative, the solar energy devices may be in the form of “concentrators”, which concentrate the solar energy into a small area. 
     Further, each inverter  10   x ,  10   a . . . n  comprises an inverter circuit  13  for converting a DC current provided by the solar energy devices connected to the input  12  into an AC current. The inverter circuits  13  may be configured as single-phase or multi-phase inverter circuits. The inverter circuits  13  have for example half-bridges comprising two switching elements connected in series to each other, the switching elements being power devices, such as for example MOSFETs or IGBTs. Furthermore, each inverter  10   x ,  10   a . . . n  comprises an output  14  which is connected via transmission lines  16  to a common bus  18 . The total AC current of all inverters  10   x ,  10   a . . . n  is supplied from the bus  18  via transmission lines  20  to a grid  22 . The grid  22  may be a public grid or an isolated grid. 
     In addition, each inverter  10   x ,  10   a . . . n  comprises a controller  15  formed by e.g. a processor or microcontroller, etc. The inverters  10   x ,  10   a . . . n , more specifically the controllers  15  of the inverters  10   x ,  10   a . . . n , are connected to each other via a communication line  24 . The master-inverter  10   x  controls the switching processes of the inverter circuits  13  of the master-inverter  10   x  and the slave-inverters  10   a . . . n . More precisely, the controller  15  of the master-inverter  10   x  controls the switching processes of the inverter circuit  13  of the master-inverter  10   x  as well as, via the respective controllers  15  of the slave-inverters  10   a . . . n , the switching processes of the inverter circuits  13  of all slave-inverters  10   a . . . n.    
     In the example of  FIG. 1 , the inverter system is configured as a master-slave system of inverters. In alternative examples of the present disclosure, there can be a separate master controller to control all inverters of the inverter system, in particular the controllers of all inverters of the inverter system. 
     Next, with reference to  FIGS. 2 to 6 , an example of operating such an inverter system as shown in  FIG. 1  according to the present disclosure will be explained. 
       FIG. 2  shows how the AC output of an inverter  10  is generated by its inverter circuit using pulse width modulation (PWM) switching. The PWM output signal c shown in the lower diagram of  FIG. 2  is generated by comparing a carrier wave signal a and a reference wave signal b both shown in the upper diagram of  FIG. 2 . In the example shown in  FIG. 2 , the carrier wave signal a has a triangular waveform, but in other examples, the carrier wave signal a may have a saw tooth waveform or some other waveform. The reference wave signal a typically has a sinusoidal waveform. When the reference wave signal b is higher than the carrier wave signal a, the one switching element of a half-bridge of the inverter circuit  13  is triggered on and positive DC voltage is applied to the inverter output  14 . In the other case, when the reference wave signal b is lower than the carrier wave signal a, the other switching element of a half-bridge of the inverter circuit  13  is triggered on and negative DC voltage is applied to the inverter output  14 . The magnitude and frequency of the reference wave signal b determine the amplitude and the frequency of the output voltage, and the frequency of the carrier wave signal a is called the modulation frequency. 
     Because of the PWM modulation, there is a switching ripple in the current output of an inverter  10 . Especially, the switching ripple in current supplied by an inverter is a result of the square waveform of the PWM output signal c of the inverter.  FIG. 3  shows the AC current ripple d 1  of an inverter  10  for a small switching frequency, wherein waveform d 2  shows the average of the switching ripple. 
     As shown in  FIG. 4 , the carrier wave signals a of the inverter circuits  13  of all inverters  10  are phase-shifted relative to each other. If some of the inverters  10  are not active, because for example the solar energy devices connected to their inputs  12  are not generating electric current at present, in an example only carrier wave signals a of the inverter circuits  13  of the active inverters  10  are phase-shifted relative to each other. The phase difference δ between the carrier wave signals a of the inverters depends on the total number of inverters  10  or active inverters  10 . When m is the total number of (active) inverters  10 , the phase difference δ is determined by δ=360°/m. 
     In the present example of a master-slave system of inverters  10 , the master inverter  10   x  determines the phase shifts of the carrier wave signals a of the inverter circuits  13  of the slave-inverters  10   a . . . n . For example, the carrier wave signal a of the master-inverter  10   x  will be the reference having a phase shift of 0°, whereas the phase shifts of the n slave-inverters  10   a . . . n  will be equal to ((360°/(n+1)*number of the slave inverter). In detail, the carrier wave signal a of the master-inverter  10   x  has a phase shift of 0°, the carrier wave signal a of the first slave-inverter  10   a  has a phase shift of 0°+1δ, the carrier wave signal a of the second slave-inverter  10   b  has a phase shift of 0°+2δ, and the carrier wave signal a of the n-th slave-inverter  10   n  has a phase shift of 0°+nδ. In an example of an inverter system comprising four inverters  10 , there will be one master-inverter  10   x  and three slave-inverters  10   a ,  10   b ,  10   c , resulting in a phase difference δ of 360°/4=90° and phase shifts of 0°, 90°, 180° and 270°, respectively. 
     The phase differences between the carrier wave signals a of the inverters  10   x ,  10   a . . . n  shift the switching processes of the inverter circuits  13  of these inverters. As a result, also the switching ripples of the current outputs of the inverters  10  are phase-shifted relative to each other. Thus, the summation of the current outputs of all (active) inverters  10  at the bus  18  results in an at least partially mutually elimination of the switching ripples, as it is exemplarily shown in  FIGS. 5 and 6 . 
     The upper diagrams of  FIGS. 5 and 6  shows the AC current output of an inverter system, i.e. the summation of the AC current outputs of the plurality of inverters  10 , according to a conventional solution. The lower diagrams of  FIGS. 5 and 6  show the AC current output of an inverter system, i.e. the summation of the AC current outputs of the plurality of inverters  10 , according to the present disclosure. As shown in the zoomed details of  FIG. 6 , the AC current output of the conventional inverter system has a large THD, whereas the switching ripples in the AC current output of the disclosed inverter system are decreased significantly. In an exemplary software simulated inverter system, the magnitude of the switching ripples could be decreased by about 80% for example. 
     As explained above with reference to  FIGS. 1 to 6 , the inverter system of the present disclosure does not need bigger or additional circuit elements for filtering the switching ripples at the outputs of the inverters to achieve a better THD and lower switching ripple levels. Instead, there is just added a phase-shifting of the switching processes of the inverters, especially a phase shifting of the carrier wave signals for the switching processes of the inverter circuits of the inverters. Because of its cost effectiveness and simple configuration, the inverter system of the present disclosure is advantageous in particular in solar power systems. The inverter system of the present disclosure can be used in any type of solar farms with a plurality of inverters. 
     It will be understood that the processor or processing system or circuitry referred to herein may in practice be provided by a single chip or integrated circuit or plural chips or integrated circuits, optionally provided as a chipset, an application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), digital signal processor (DSP), graphics processing units (GPUs), etc. The chip or chips may comprise circuitry (as well as possibly firmware) for embodying at least one or more of a data processor or processors, a digital signal processor or processors, baseband circuitry and radio frequency circuitry, which are configurable so as to operate in accordance with the exemplary embodiments. In this regard, the exemplary embodiments may be implemented at least in part by computer software stored in (non-transitory) memory and executable by the processor, or by hardware, or by a combination of tangibly stored software and hardware (and tangibly stored firmware). 
     The examples described herein are to be understood as illustrative examples of embodiments of the invention. Further embodiments and examples are envisaged. Any feature described in relation to any one example or embodiment may be used alone or in combination with other features. In addition, any feature described in relation to any one example or embodiment may also be used in combination with one or more features of any other of the examples or embodiments, or any combination of any other of the examples or embodiments. Furthermore, equivalents and modifications not described herein may also be employed within the scope of the invention, which is defined in the claims.