DE19919766A1 - Inverter for a photovoltaic unit for use in a solar generator to maximize the power/performance output, the so-called maximum power point tracking - Google Patents

Abstract

An inverter has a common inverter bridge with several parallel switched DC/DC converters/transducers added to convert the DC from one voltage level to a higher voltage level. The inverter has several parallel-arranged slots to accommodate the converters/transducers, with a computer interface unit for central control, while the inverter can be formed without galvanic separation and with no transformer on the AC side.

Description

The invention relates to an inverter for a photovoltaic system.

In the case of grid-connected photovoltaic systems, it was the case until a few years ago Time the inverter depending on the installed power of the Solar generator were selected. So it became small for small plants Other inverters are used, single inverters for medium-sized systems or sometimes also inverters in master / slave configuration for the corresponding performance, and usually came with large plants central inverters up to the 500 KW range.  

The construction of central photovoltaic systems (PV systems) turns out to be a parallel connection of individual strings. Under strings the Understanding several PV modules in series. To increase performance the strings were in a separate and very expensive DC distribution connected in parallel and a common one Inverter fed into the grid.

The disadvantages of such a conventional PV system technology with strings connected in parallel on the DC side are as follows:

• - elaborate earth and short-circuit proof DC Cable laying,
• - expensive direct current distribution,
• - High expenditure in the DC wiring as well as switching and Protection technology,
• - Efficiency reduction through mutual influencing of modules and Partial shading,
• - no system expansion possible in small output levels,
• - no standardization of plant designs possible,
• - Monitoring for disturbances of individual strings is very complex.

In order to remedy these disadvantages, such were now built up Modular PV systems. That is, individual strings, i.e. H. in row switched PV modules, each assigned a modular inverter.

By assigning one inverter to a string achieved that the PV direct voltage is regulated individually for each string can be, whereby the disadvantages of the previous technology at the same early increase in system efficiency can be avoided. The String inverters are therefore only according to this state of the art still designed for a series connection of PV modules.  

The previously necessary expensive direct current distribution for parallel connection of Strings and the installation of a DC main line are therefore superfluous become. Such string inverters enable performance maximization, a so-called maximum powerpoint tracking (MPP- Tracking), each for a small group of modules - consisting of several in Series of connected PV modules.

The independent grid integrated in each string inverter wachung is making the connection in the area of smaller systems for the first time at any point (single-phase connection) of the 230 V house distribution, and for large ones. Attach a simple collection of performance on the AC side ("Alternating Current" = alternating current) possible.

The advantages of this system technology are as follows:

• - Simplification and significant cost reduction of PV systems because no DC distribution is required;
• - Reduction of the DC wiring to the series connection with low current;
• - Row diodes are necessary according to the previous system superfluous;
• - The coupling of the module groups takes place on the AC voltage level with simpler and cheaper Installation technology;
• - Easy subsequent expansion of PV systems;
• - Cost-effective provision of even small plant sizes in Range of 300-500 W;
• - The strings are not connected in parallel, so there are no losses Mismatching on;
• - MPP tracking takes place individually for each module group;
• - Easy monitoring of every string possible.

It has now been found, however, that with all the advantages that this new, so-called string technology are inherent, the specific Price (DM / KW) of the inverter is relatively high. The share of Cost of the inverter is due to the cost of the entire PV system between 10% and 20%. Especially considering the The fact that PV modules are becoming cheaper and cheaper comes at the price of the inverter a growing importance in terms of the cost of Overall system too.

The performance of string inverters cannot be as large as since the module voltages add up when connected in series and thus there are limits due to the dielectric strength of the components. However, a significant price reduction of the inverters is only at Realization of a higher performance per device possible.

The invention is therefore based on the object of an inverter to create a PV system that is much cheaper to manufacture is, but still the advantages of the so-called string technology maintains.

The object is achieved in that the inverter has a common inverter bridge that has several in parallel switched DC / DC converters are connected upstream. Such a DC / DC converter ("Direct Current" = converts DC) converts the DC current from one certain voltage level to another, mostly higher level. From this it is clear that the so-called inverter bridge is only simple must be present, whereas the individual DC / DC converters have multiple can be present, with each DC / DC converter a string assigned.  

That means you can have all the advantages of string technology with one such inverters can be realized, but such Inverters can be manufactured inexpensively insofar as only one so-called inverter bridge including the necessary Control, operational management and network monitoring units (micro processor) is required. In particular, it is now possible due to of the individual DC / DC converters with respect to each string an MPP tracking perform.

Such an inverter bridge can be used in a single-phase system Power between 3 KW and 5 KW show, which means that from In principle, up to six individual inverters compared to the previous string methods can be saved. With multi-phase Systems, the power range can be up to several hundred KW.

In detail, in order to enable the expandability, it is provided that the Inverters several slots arranged in parallel for removable Includes recording of the DC / DC converter. It can be provided that such a DC / DC converter a computer connection for a central Controller, or such a DC / DC converter has its own MPP Tracking. The inverter can also be used without galvanic isolation, d. H. without a transformer on the AC side be trained. There is also the possibility of galvanic Bring about separation via the DC / DC converter.

The single-phase or three-phase inverter either feeds into a supply network (network-connected) or supplies network-independent ones Consumer (island operation).  

The invention also relates to the use of a bill of exchange richters according to one or more of the above Requirements for a PV system with multiple PV strings Modules, each string being assigned a separate DC / DC converter.

The invention is illustrated below by way of example with reference to the drawing explained.

The individual strings 1 show several PV modules 1 a connected in series. Each string 1 with corresponding PV modules 1 a connected in series is connected to a DC / DC converter 2 . These DC / DC converters 2 are connected in parallel and connected to the inverter bridge 3 . The inverter bridge 3 and the several DC / DC converters connected in parallel form the inverter 4 . The entry point into the AC network is designated 5 .

Claims (7)

1. Inverters for a PV system, characterized by a common inverter bridge with several DC / DC converters connected in parallel. 2. Inverter according to claim 1, characterized, that the inverter bridge for parallel slots Detachable recording of the DC / DC converter has. 3. Inverter according to claim 1, characterized, that the DC / DC converter can be connected to a computer. 4. Inverter according to claim 1, characterized, that the DC / DC converter has MPP tracking. 5. Inverter according to claim 1, characterized, that the supply on the AC side single phase or can be three-phase. 6. Inverter according to claim 1, characterized, that feeding on the AC side into a supply network or can be done on off-grid consumers.   7. Use of an inverter according to one or more of the Claims 1 to 6 in a PV system with several strings switched PV modules, with each string a separate DC / DC converter is assigned.