Abstract:
An inverter for feeding a grid-compatible AC voltage into a grid is described, wherein the inverter includes an inverter bridge for converting a DC voltage to a first AC voltage and a grid interface between the inverter bridge and the grid for converting the first AC voltage to the grid-compatible AC voltage for feeding into the grid. An AC interface via which an AC module for feeding into the grid can be connected, is arranged between the inverter bridge and the grid interface.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of international application number PCT/EP2011/064840 filed on Aug. 29, 2011, which claims priority to German patent application number 10 201 0 036 033.3 filed on Aug. 31, 2010. 
     
    
     FIELD 
       [0002]    The disclosure relates to an inverter. 
       BACKGROUND 
       [0003]    Conventional photovoltaic (PV) inverters such as central or string inverters or inverters having a multiplicity of parallel strings carry out a number of additional functionalities in addition to their main task of converting the direct current produced by the PV generator to grid-compatible alternating current. Amongst others, these additional functionalities may be: communication with the user via an HMI (human-machine interface) or via other communication channels, grid monitoring functions, grid support functions, and safety functions. 
         [0004]    Furthermore, the PV generator is kept at the maximum-power point by maximum-power point tracking (MPP tracking). 
         [0005]    At the same time, the size of the PV generator is generally governed by the nominal power of the inverter, thus restricting the scalability of the overall photovoltaic system. 
         [0006]    Since the current in a string is governed by the characteristics of its weakest PV module, only identical modules, or modules that are as similar as possible, of the same technology should be used within the string. 
         [0007]    If the incident radiation on the PV generator is not homogeneous, for example because of partial shadowing, the maximum possible power cannot be extracted from the PV generator, because the PV modules have different optimum operating points (MPPs), which cannot be set individually in a series or parallel circuit. 
         [0008]    The disadvantages of conventional PV inverters, like for example the restricted scalability, restrictions when modules of different types are used together and high sensitivity to incident radiation that is not homogeneous, are almost completely avoided when using inverters that are referred to as being close to the module, module-oriented or module-integrated (that is to say a dedicated inverter with an AC output and its own MPP tracking for each module, which is referred to in the following text as an AC module). However, if the intention is to integrate the additional functions mentioned at the beginning in an AC module, the specific price (costs related to the power) is considerably higher than for conventional PV inverters. Furthermore, the efficiency of the AC modules can, in principle, not reach the efficiency of conventional PV inverters. Until now, this has led to it not being possible to introduce AC modules to the market successfully. 
       SUMMARY 
       [0009]    A system would therefore be desirable that combines the advantages of both technologies with one another. One aspect of the present disclosure is therefore an AC interface that is integrated in an inverter and via which the power from decentralized AC modules connected thereto is fed into a grid. An inverter such as this is also referred to in the following text as a basic inverter. This results in minor additional costs, while the costs of the decentralized AC modules can be drastically reduced, since their functionality can be reduced to the basic functions such as inversion and MPP tracking, with the additional functions mentioned above being provided by the basic inverter also for the proportion of the power that is produced by the AC modules. 
         [0010]    This allows almost all building locations to be used better for PV installations. In particular, a system such as this represents a considerably more cost-effective variant than DC/DC controllers close to the module, so-called power optimizers. 
         [0011]    The disclosure relates to an inverter for a photovoltaic system that partially or completely satisfies the following requirements and provides the following functionalities: conversion of the direct current provided by the PV generator in the photovoltaic system to grid-compatible alternating current, module-by-module optimized MPP tracking, high energy efficiency, low specific price, communication of the system with the user via a standard HMI, grid monitoring functions, grid support functions, adjustable reactive power (supply, reference, compensation), safety functions, simple and flexible scalability of the PV generator, use of different PV generator types and technologies (for example thin-film cells and monocrystalline cells) in one photovoltaic system, energy-optimized use of a PV generator that is not illuminated homogeneously (shadowing, different module alignment, etc.), and capability to use the PV inverter with or without galvanic isolation, in which case both types can also be used within the same photovoltaic system. 
         [0012]    According to one embodiment an inverter comprises an inverter bridge for converting a DC voltage to a first AC voltage and a grid interface between the inverter bridge and the grid for converting the first AC voltage to the grid-compatible AC voltage for feeding into the grid. An AC interface via which one or more AC modules for feeding into the grid can be connected, is arranged between the inverter bridge and the grid interface. 
         [0013]    It has been found that many installations have a part of the generator area that is freely illuminated homogeneously, without being impeded by obstructions that throw shadows, throughout the majority of the time of the year. For rural installations, this part normally represents the entire area. This part decreases as the area affected by shadowing obstructions increases. One typical example are roof areas with dormers that throw a shadow onto PV generators installed in the vicinity at some times over the course of the day. However, only in rare cases more than 50% of the generator area is affected. It is therefore worthwhile combining areas of the generator that are illuminated homogeneously and freely to form a unit, and to allow the power to be converted centrally in one inverter. This is the best solution in terms of energy and investment costs. 
         [0014]    Decentralized power conditioning as close to the module as possible is the energetically best solution for parts that are not illuminated homogeneously at some times. AC modules, in particular, can be used for this purpose, since they can carry out MPP tracking independently of one another. In order to allow them to be designed as cost-effectively and energy-efficiently as possible, it is desirable to reduce the design of these devices to the necessary basic functions. This allows considerably better utilization of existing roof areas, since area parts for which the use was not economic when using previous system architectures become economic in this way. 
         [0015]    According to the disclosure, the inverter is extended such that one or more AC modules can be connected to it by means of an AC interface. This allows the basic functions of the AC modules to be reduced, such that inclusion in the photovoltaic overall system is possible, even though direct connection of the AC modules for feeding power into a power grid would not be permissible. 
         [0016]    It is apparent that the conversion of the DC voltage into the first AC voltage by the inverter bridge and the conversion of the first AC voltage into the grid-compatible AC voltage is accompanied with a conversion of a DC current into a first AC current and further into a grid-compatible AC current. The use of the term “voltage” in the claims is not limiting in this sense. 
         [0017]    In one embodiment of the inverter, the grid interface has switch disconnectors for disconnecting the inverter from the grid and/or for connecting the inverter to the grid. In one embodiment the switch disconnectors are operated dependent on a state of the grid. Further, the state of the grid concerns a voltage and/or a frequency of an electrical current within the grid and/or an islanding condition. Switch disconnectors are means for controlling that power is fed into the grid in a grid-compatible manner. 
         [0018]    In a further embodiment of the inverter, the grid interface has a filter device. The filter device is used to form a sine-like AC-voltage for feeding power into the grid. The filter is a further means to ensure that power is fed into the grid in a grid-compatible manner. 
         [0019]    In a further embodiment of the inverter, the grid interface has a safety device. In yet a further embodiment of the inverter, the inverter has measurement points via which a current value via the inverter bridge and a current value via the AC interface can be recorded. The safety device as well as the measurement points allow to control that power is fed into the grid in a grid-compatible manner. 
         [0020]    In a further embodiment of the inverter, the inverter has a communication unit for exchanging data with AC modules that are connected to the AC interface. It is advantageous in one embodiment that the communication unit is designed to exchange data via lines of the AC interface or is designed for wireless exchange of data. Further, in one embodiment the communication unit is designed to exchange data for one of the following purposes: remote control or remote diagnostics of connected AC modules; storage or transmission of measured values, breakdown or failures of connected AC modules; transmission of control signals to connected AC modules; and display of data of connected AC modules on a display unit of the inverter. 
         [0021]    The communication unit allows for an integral controlling and monitoring of all modules of a photovoltaic system, i.e. also of the AC modules connected to the inverter via the AC interface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will be described in the following text using example embodiments and with reference to the drawings. The illustrated figures shall be understood in an illustrative and non-restrictive manner and are intended to make it easier to understand the invention. In this case: 
           [0023]      FIG. 1  shows a schematic illustration of a conventional inverter; 
           [0024]      FIG. 2  shows a schematic illustration of a conventional inverter with a grid interface; 
           [0025]      FIG. 3  shows a first embodiment of an inverter; 
           [0026]      FIG. 4  shows a second embodiment of an inverter; 
           [0027]      FIGS. 5 to 7  show further embodiments of an inverter with various arrangements of measurement points within the inverter; and 
           [0028]      FIGS. 8 and 9  show further embodiments of an inverter. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]      FIG. 1  shows a photovoltaic system  100  with a conventional inverter  19 . The inverter  19  has an inverter bridge  21 , by means of which a direct current from a connected PV generator  10  can be converted to an alternating current. The inverter bridge  21  is connected to a grid  40  for feeding the power that is produced from the PV generator in the form of real power P inv  and reactive power Q. The feeding can be monitored, synchronized and controlled via a central processor (CPU)  22  by means of the measurement points  24 , which are designed to measure current and/or voltage values. In this case, the PV generator  10  comprises a number of series-connected PV modules, which form a string. Frequently, a number of strings are connected in parallel, and are connected to the inverter. 
         [0030]    Further optional elements of a conventional inverter  19  are shown in the photovoltaic system  100  in  FIG. 2 . This inverter  19  also has an HMI  23 , with the aid of which operating values of the inverter  19  can be displayed and the operation of the system  100  can be influenced, for example, by presetting nominal values  28  for the real power P ref  and/or reactive power Q ref  to be provided by the system. These nominal values  28  can alternatively also be transmitted via a communication unit  26 , for example, as a value preset by an operator of the grid  40 . The inverter  19  also has a grid interface  30  via which the abovementioned additional functionalities can be provided by the inverter. In one embodiment the grid interface  30  contains electromechanical or electronic switch disconnectors  32 , in order to be able to disconnect the basic inverter from the grid  40 . Safety functions, such as detection of voltage, frequency and isolation faults as well as detection of undesirable islanding or fault current monitoring that is sensitive to all types of current, are carried out by the safety device  31 . The individual components of the grid interface  30  may in this case be connected centrally to the CPU  22 , and may be controlled by it, or may be connected directly to one another. 
         [0031]      FIG. 3  illustrates one embodiment of a basic inverter  20  according to the invention. The basic inverter  20  is connected to the AC modules  50  via a connecting area  65  by an AC interface  60  that is integrated in the basic inverter  20  and that is arranged between the inverter bridge  21  and the grid interface  30 . The illustrated basic inverter  20  furthermore has a measurement point  61  that can be used to selectively determine the current fed in by the AC modules  50 . A PV system configuration can thus be implemented that is appropriate for the requirements mentioned above. The connecting area  65  may have one or more connections for AC modules  50 , in which case one or more AC modules  50  can be connected to each connection. The connected AC modules  50  are in this way likewise protected by the grid interface  30  without themselves having to carry out the protection functions provided by the grid interface  30 . The switch disconnectors  32  in the grid interface  30  are designed for the total power of the system  100 , as a result of which the AC modules  50  do not require their own grid interface or their own switch disconnectors. The basic inverter  20  may either be designed as an inverter without a transformer, or as an inverter with a high-frequency transformer, or as an inverter with a grid transformer. 
         [0032]    The basic inverter  20  in  FIG. 4  has a communication unit  26  that is designed for wireless communication, for example radio transmission. In this case, the communication unit  26  may communicate in a simple manner with the AC modules  50  via their communication units  51 , in order to transmit data in one or both directions, or else in order to control or to monitor the AC modules  50 . Alternatively, the communication may, of course, also be provided via dedicated signal lines or by modulation onto the AC lines. The HMI  23  can also perform control and display functions for the AC modules  50  via the control and display elements for the basic inverter. Remote evaluation, remote diagnostics and/or remote control can be provided for the AC modules  50  via the communication unit  26  of the basic inverter  20 . Measured values or records of events of the AC modules  50 , such as breakdown or failures, can likewise be stored or transmitted. It is also feasible to transmit control signals for the connected AC modules  50 . 
         [0033]    In order to allow the feeding parameters of the inverter bridge  21  and of the connected AC modules  50  to be determined separately from one another, further measurement points are provided as compared to the conventional inverters  19  in  FIGS. 1 and 2 . In  FIGS. 3 and 5 , a further measurement point  61  is provided between the AC interface  60  and the grid interface  30  in order to additionally allow to record the sum current of the inverter bridge  21  and the AC interface  60 . The current fraction of the AC interface  60  can therefore be determined easily by subtraction. 
         [0034]    Alternatively,  FIG. 6  shows an additional measurement point  62  in the current path of the AC interface  60  for determining the different current fractions. A further, alternative arrangement of the measurement points, which is not shown, can be used to measure the total current of the connected AC modules  50  and the sum current of the overall system  100 , and to determine the current fraction of the inverter bridge  21  from this. 
         [0035]    As shown in  FIG. 7 , each connection of the connecting point  65  can also be equipped with a measurement point  63  in order to allow a selective recording of the current fractions from the AC modules  50  connected to the AC interface  60 , and, for example, to use this for installation monitoring. 
         [0036]    Furthermore,  FIG. 5  shows a grid interface  30  that has a filter device  33 . The filter device  33  is used to remove or to sufficiently damp AC voltage components from the AC voltage to be fed into the grid  40  that are not at the grid frequency. The filter device  33  may, for example, be in the form of an LC bandpass filter and may be designed such that it can provide this function both for the inverter bridge  21  and for the connected AC modules  50 . Alternatively, as is shown in  FIG. 7 , a further filter device  34  can also be arranged between the connecting area  65  and the AC interface  60 , providing the filter function selectively for the connected AC modules  50 . 
         [0037]    As additional elements,  FIG. 6  also shows a disconnecting device  67  for each connection of the connecting area  65 . This can be controlled by the basic converter  20  such that the inverter or inverters that is or are connected to the respective connection can be selectively disconnected from the AC interface  60  by means of switches. In an operating variant, it is possible this way to record feeding parameters or to perform diagnostics of the connected AC modules  50  separately for the respective AC modules  50  or module groups connected to the various connections, such that the measured values determined by the further measurement points  61 ,  62  can be associated with the AC modules  50  or with a group of AC modules. Alternatively, one single disconnecting device  67  can also be provided in order to disconnect all of the connected AC modules  50 . The disconnecting device can be integrated in all of the illustrated embodiments of the inverter according to the invention. 
         [0038]    One connection of the connecting area  65  may be used as a reference input, and its current or feeding power can be recorded separately. It is then worthwhile installing a PV module, which is connected to an AC module  50 , at a point in the PV installation that is never shadowed by obstructions, and to connect the corresponding AC module to the reference input. Discrepancies between the specific power of the reference module and of the remaining installation, or parts of the remaining installation, can thus be assessed. 
         [0039]    A further option for the use of the basic inverter  20  according to the invention is for power factor correction for a load. For this purpose, at least one load is connected to the AC interface  60  in addition to (optional) AC modules  50 . The basic inverter  20  can produce the reactive power demanded by the load itself via the measurement points at the input and at the grid connection, such that the overall photovoltaic system  100  behaves in a neutral form on the grid. 
         [0040]    Nominal values  28  for the real power P ref  and for the reactive power Q ref  can be received via the communication unit  26  of the basic inverter  20  and transferred to the CPU  22 , in order to allow the output real power and reactive power of the system  100  to be regulated as required. This allows grid services to be implemented, although the AC modules  50  need not necessarily themselves be capable of providing reactive power. It is sufficient for the basic inverter  20  to be designed as a feeding unit capable of providing reactive power. 
         [0041]    The above statements apply to three-phase systems analogously; the basic inverter according to the invention with an AC interface can be used in this way for single-phase or polyphase grids. In the case of a polyphase grid, power distribution can be carried out via the connected single-phase AC modules  50  by splitting the AC modules  50  between the various phases via the AC interface  30 . This power distribution can be controlled for the connected grid  40  by the basic inverter  20  in accordance with the requirements. In particular, it is considered to configure the assignment of the AC modules  50  to the phases they are feeding into to be variable in accordance with the requirements of the three-phase grid, for example in order to counteract grid unbalances. 
         [0042]    By way of example,  FIG. 8  shows an arrangement of the AC interface  60  in which the single-phase AC modules that are connected to the three connections of the connecting area  65  each feed into one of the three phases of the basic inverter  20 . All of the AC modules are in this case connected to a neutral conductor  66  of the basic inverter. A further measurement point  62  is in each case provided for each feeding point. Alternatively,  FIG. 9  shows an arrangement in which one three-phase AC module can in each case be connected to each of the three connections of the connecting area  65 . Instead of the illustrated connection arrangement of the respective AC modules between a neutral conductor and a selected phase (star arrangement), it is likewise feasible to connect the AC modules between two selected phases (delta arrangement). 
         [0043]    The operation of the external AC modules  50  is monitored with the aid of the measurement points  24 ,  61 ,  62 ,  63  and an algorithm that is implemented in the CPU  22 , which means that in one operating variant of the invention there is no need for communication between the AC modules  50  and the basic inverter  20 . The AC modules  50  can therefore be designed very cost-efficiently, since their functionality can be reduced to inversion and MPP tracking. Monitoring of compliance with predetermined feeding parameters for the overall system  100 , such as nominal values of the reactive power and real power, can be ensured despite the unregulated feeding by the AC modules, since the basic inverter  20  can control the feeding from the inverter bridge  21  such that, overall, the feeding from the inverter bridge  21  and the AC modules  50  corresponds to the nominal values. This results in a cost-effective, efficient overall photovoltaic system. It is likewise possible to monitor the feeding operation of the AC modules  50  using the basic inverter  20 , and to produce an appropriate warning in the event of discrepancies that lead to the deduction that the AC modules  50  have failed, and to send or to indicate this warning by means of the communication unit  26  or the HMI  23 . There is no need for communication with the AC modules for this purpose; the discrepancies are detected on the basis of the feeding parameters of the AC modules connected to the AC interface  60 , which can be recorded selectively via the further measurement points  61 ,  62 ,  63 , as described above, even if it may in some cases not be possible to unambiguously identify which of the connected AC modules has failed. 
         [0044]    The basic inverter  20  can be equipped with a residual-current circuit breaker that is sensitive to all types of current. This allows the AC modules  50  to be configured as transformer-less inverters, and without their own safety components sensitive to all types of current, while still complying with existing safety regulations. 
         [0045]    In one advantageous configuration, the photovoltaic system  100  is designed such that the part of the generator area that is at any time unshadowed feeds into the connected grid  40  via the inverter bridge  21 . The part of the generator area that is shadowed at least at some times is included in the system  100  via the AC interface  60  by means of the AC modules  50 . In general, that part of the area that is always unshadowed will make up the majority of the total generator area, such that the basic inverter  20  is advantageously configured such that the maximum power to be fed via the inverter bridge  21  is at least as high as the maximum power to be fed via the AC interface  60 . This results in the components of the basic inverter  20  being used particularly efficiently, while at the same time reducing the complexity for the overall photovoltaic system  100 , by means of the described joint use of certain components.