DE102011083330A1 - Inverter arrangement for photovoltaic system, has transformer provided with windings corresponding to number of inverters, and phase-shifted from each other by specific degree - Google Patents

Abstract

The inverter arrangement has inverters (19-22) that are arranged based on the fundamental frequency front end (F3E) topology. A transformer (27) is connected to the inverters. The windings (23-26) of the transformer are provided corresponding to the number of inverters, and phase-shifted from each other by 60[deg] .

Description

The invention relates to a three-phase inverter for use in photovoltaics, in particular with a power of more than 100 kVA

For large photovoltaic (PV) inverters, high efficiency, sufficient reliability, low harmonic content in the AC grid, as well as the ability to output and pick up reactive power for grid voltage control purposes are crucial factors.

• – Spannungsanpassung: Transformation der Ausgangs-Wechselspannung des Wechselrichters auf das Spannungsniveau des Mittelspannungsnetzes
• – Beitrag zur Glättung der AC-Ströme, EMV-Filterung
• – Galvanische Trennung der PV-Anlage vom Netz und damit Vermeidung von höherfrequenten Ableitströmen hervorgerufen durch die Gleichtaktspannung des Wechselrichters am DC-Eingang

Currently, PV inverters for larger power in the range of more than 100 kVA are mostly based on the well-known two-level or three-level AFE-enabled Voltage Source Converters, often with multiple devices housed together in a single container and one device Master and the other devices work as slaves. Obligatory is still at least one transformer for connection to the grid, usually the medium-voltage grid. The transformer has the following tasks:

• - Voltage adaptation: transformation of the AC output voltage of the inverter to the voltage level of the medium-voltage network
• - Contribution to the smoothing of AC currents, EMC filtering
• - Galvanic isolation of the PV system from the grid and thus avoidance of higher-frequency leakage currents caused by the common-mode voltage of the inverter at the DC input

• – Die im Zwei oder Drei-Level Voltage Source Convertern verbauten Elektrolytkondensatoren sind teuer, benötigen einen beträchtlichen Bauraum und sind unzuverlässig
• – Der im AFE-Betrieb pulsende Wechselrichter (typische Frequenz von einigen wenigen kHz) erzeugt beträchtliche Schaltverluste in den IGBTs, was zu Lasten des Wirkungsgrades geht
• – Die DC-Spannung ist ohne weitere Maßnahmen auf max. 820 V beschränkt (typische Spannung: 750 V), wenn man von IGBTs mit einer Spitzensperrspannung von 1200 V ausgeht

The disadvantage of this solution

• - The electrolytic capacitors installed in the two or three-level voltage source converters are expensive, require considerable space and are unreliable
• - The inverter pulsing in AFE mode (typical frequency of a few kHz) generates considerable switching losses in the IGBTs, which at the expense of efficiency
• - The DC voltage is set to max. 820 V limited (typical voltage: 750 V) when starting from IGBTs with a peak reverse voltage of 1200V

Also known, for example from the DE 10 2006 016 502 A1 , is the so-called F3E topology (Fundamental Frequency Frontend) for inverters. The F3E inverter, like the two-level Voltage Source Converter in AFE mode, also has a full bridge with 6 active switches, mostly IGBTs. However, the active switches are only clocked at mains frequency. Each IGBT thus carries 120 ° el. Current and is de-energized the rest of the time. The switching losses are thus negligible, but results from the 120 ° block shape of the current on the AC side, a very high harmonic content.

It is an object of the present invention to provide an improved inverter arrangement, which operates in particular oberschwingungsarm.

This object is achieved by an inverter arrangement having the features of claim 1. The subclaims relate to advantageous embodiments of the inverter arrangement.

The inventive inverter arrangement for the photovoltaic for converting a coming from photovoltaic modules DC voltage in a three-phase AC voltage has a plurality of inverters based on the F3E topology. Furthermore, the inverter arrangement has a transformer connected to the inverters, the transformer on the side of the inverters having a number of inverters corresponding number of mutually entangled by a phase offset windings. It is expedient if the phase offset is 60 ° divided by the number of inverters. For example, in one embodiment variant, the inverter arrangement may have exactly four inverters and the phase offset is 15 °. Suitably, the inverter arrangement is configured such that the inverters are electrically offset by the phase offset offset from each other.

Inverters of the F3E topology have very low switching losses due to the mains-frequency control of the switches, for example IGBTs, in which each of the switches is energized for 120 ° el., But due to the 120 ° block shape of the current with a high harmonic content. For the invention it has been recognized that by an electrically offset control of the plurality of inverters and corresponding design of the transformer a suitable superposition of the currents and thereby a reduction of the harmonic component can be achieved.

This is an architecture for the inverter assembly is given, which works at the same time oberschwingungsarm, but it is simple and inexpensive and has a high efficiency. The construction advantage is achieved in particular by the fact that the inverters of the F3E topology have a cf. have small DC link capacitor and AC side expediently no storage chokes are provided.

In an advantageous embodiment, at least one part of the inverters, in particular all inverters, is preceded by a non-insulating DC / DC converter. The DC / DC converter is preferably designed to perform an MPP tracking. Furthermore, the DC / DC converter may alternatively or additionally be designed to increase the open-circuit voltage to the side facing away from the inverter. The DC / DC converter is advantageously designed to control the power flow to the F3E frontend.

In a further advantageous embodiment, parallel to at least one part of the inverters, a device suitable for reactive power output or reception, such as a so-called static synchronous compensator STATCOM, is provided at the line-side AC output. This advantageously achieves that the inverter arrangement as a whole is able to feed inductive reactive power into the grid or to receive it from the grid, which may be a requirement in the connection conditions for PV inverters. For the connection of the STATCOM, another winding can be provided on the transformer.

A preferred, but by no means limiting embodiment of the invention will now be explained in more detail with reference to the figures of the drawing. The features are shown schematically. Show it

1 a photovoltaic system with four inverters and a transformer,

2 a schematic current flow on the high voltage side of the transformer.

1 shows very schematically a structure of a photovoltaic system 10 , In the present example, this has four separate solar modules 11 ... 14 on. The solar modules 11 ... 14 can themselves be an interconnection of several individual elements. Each of the solar modules 11 ... 14 is with a single DC / DC converter 15 ... 18 connected. So is a first solar module 11 with a first DC / DC converter 15 connected, a second solar module 12 with a second DC / DC converter 16 connected etc.

The DC / DC converter 15 ... 18 are designed, to the solar panels 11 ... 14 to work with an open circuit voltage of 1500 V. On the of the solar panels 11 ... 14 On the other hand, the voltage in this embodiment is 750 V. On the latter side, each of the DC / DC converters 15 ... 18 each with a F3E inverter 19 ... 22 connected. The F3E inverters 19 ... 22 , whose structure is not shown, each have six IGBTs. They are clocked at mains frequency and therefore carry electricity for 120 ° el. And are de-energized the rest of the time.

On the output side are the F3E inverters 19 ... 22 three-phase with one winding system each 23 ... 26 as part of a transformer 27 connected. The winding systems 23 ... 26 are three-phase winding systems, each with 15 ° electrically offset output voltage. In the present example, the phase offset is 15 ° el. On the output side of the transformer 27 For example, there is a medium voltage connection 28 ,

The electrically offset control of the F3E inverters 19 ... 22 and corresponding interconnection of the winding systems 23 ... 26 advantageously provides a form of output current that in 2 is indicated. The phase offset provides a staircase shape that is much closer to the ideal sinusoidal waveform than the pure 120 ° block shape that is a single F3E inverter 19 ... 22 generated. However, this ensures the mains frequency control of the F3E inverters 19 ... 22 at the same time that the switching losses of the IGBTs are very low, so overall a high degree of efficiency is achieved.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102006016502 A1 [0005]

Claims (8)

Inverter arrangement for photovoltaics for converting one of photovoltaic modules ( 11 ... 14 ) coming DC voltage in a three-phase AC voltage, comprising: - a plurality of inverters ( 19 ... 22 ) based on the F3E topology, - a transformer ( 27 ), connected to the inverters ( 19 ... 22 ), the transformer ( 27 ) on the side of the inverters ( 19 ... 22 ) one of the number of inverters ( 19 ... 22 ) corresponding number of mutually entangled by a phase offset windings ( 23 ... 26 ) having. An inverter arrangement according to claim 1, wherein the phase offset is 60 ° divided by the number of windings ( 23 ... 26 ). Inverter arrangement according to claim 1 or 2, configured such that the inverters ( 19 ... 22 ) are electrically offset by the phase offset to each other. An inverter arrangement according to one of the preceding claims, wherein at least part of the inverters ( 19 ... 22 ) a DC / DC converter ( 15 ... 18 ), in particular non-insulating DC / DC converter ( 15 ... 18 ), upstream. An inverter arrangement according to claim 4, wherein the DC / DC converter ( 15 ... 18 ) is configured to perform a MPP tracking. An inverter arrangement according to claim 4 or 5, wherein the DC / DC converter ( 15 ... 18 ), an increase in the open circuit voltage to the inverter ( 19 ... 22 ) to 1500 V on the opposite side. Inverter arrangement according to one of the preceding claims with four inverters ( 19 ... 22 ), wherein the phase offset is 15 °. An inverter arrangement according to one of the preceding claims, in which, parallel to at least a part of the inverters ( 19 ... 22 ) is provided for reactive power input or output device, in particular a so-called. Static Synchronous Compensator STATCOM, is provided.

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