DE102006023563B4 - Photovoltaic system - Google Patents

Abstract

Photovoltaic system with several strings each comprising several solar cell modules, each of which is assigned a DC / DC converter, the DC / DC converters being connected on the output side to a common inverter bridge, characterized in that switching elements (S1, S2, S11, S12, S21, S22) affects at least one DC / DC converter (DCW1, DCW2, DCW3) on the input side with the total output voltage of a series connection of the string assigned to it (ST1, ST2, ST3) and at least one further string (ST1, ST2, ST3 ) can be acted upon.

Description

The present invention relates to a photovoltaic system with several strings each comprising several solar cell modules, each of which is assigned a DC / DC converter, the DC / DC converters being connected on the output side to a common inverter bridge.

Photovoltaic systems are used to convert solar energy into electrical energy, which can be provided, for example, for the autonomous supply of a house or for feeding into a public power grid. The output voltage of a photovoltaic system is therefore usually an alternating voltage at the level of the usual line voltage and with the usual line frequency.

The solar cell modules for operating a photovoltaic system are usually connected to one another in series to form several so-called strings. These strings each deliver a direct voltage, the voltage level of which is adjusted by direct voltage / direct voltage converters, hereinafter referred to as DC / DC converters, and converted into alternating voltage by an inverter.

In the German Offenlegungsschrift DE 199 19 766 A1 describes a photovoltaic system in which photovoltaic elements are interconnected to form strings and a DC / DC converter is assigned to each string. The DC / DC converters are connected in parallel to one another and connected to a common inverter bridge.

The advantage of this is that each string can be regulated individually. On the other hand, it is disadvantageous that the voltages at the individual inputs of the DC / DC converter are often too low, depending on the wiring, and sometimes have to be increased significantly. Such an operation is unfavorable for the efficiency.

Another problem is that the voltage generated by the strings depends on solar radiation and ambient temperature and is therefore not constant over time. In addition, the electrical output variables of strings can also differ from one another at a given point in time due to their different arrangement, for example on different sides of a house.

A photovoltaic system that automatically takes into account the varying voltage and power outputs of the strings would therefore be desirable. The individual strings could be connected in series at many times, since the connected strings often deliver currents of roughly the same size. Series connection should not be used if the currents are very different. Another reason for not connecting in series are very high voltages when idling, which can destroy an inverter.

The task is therefore to develop a circuit that adapts flexibly and automatically to the string voltages that are present and enables particularly favorable operation of a photovoltaic system.

The object is achieved in that, influenced by switching elements, at least one DC / DC converter can be supplied on the input side with the total output voltage of a series connection of the string assigned to it and at least one further string.

In detail, the solution is that on the one hand switching elements for coupling are arranged between the various DC / DC converters, and on the other hand switching elements are built into the DC / DC converters that allow the individual DC / DC converters to be connected to the ground Disconnect the intermediate circuit of the inverter.

Such a circuit can significantly increase the efficiency of a photovoltaic system under many operating conditions and reduce the power loss. It is thus advantageously also possible to dimension the heat sinks of the inverters and thus the converter system belonging to the photovoltaic system to be considerably smaller and thus to save considerable costs.

Further advantageous refinements and developments of a photovoltaic system according to the invention are listed in the dependent claims.

The invention is illustrated and explained in more detail below with the aid of a few schematic circuit diagrams.

• 1 ein Prinzipschaltbild einer erfindungsgemäßen Photovoltaik-Anlage,
• 2 einige vorteilhafte Ausführungsvarianten der in der 1 dargestellten Schaltelemente.

Show it

• 1 a schematic diagram of a photovoltaic system according to the invention,
• 2 some advantageous variants of the in the 1 switching elements shown.

The circuit principle of a photovoltaic system according to the invention shows 1 . The photovoltaic system consists of several, here for example three DC / DC converters (DCW1, DCW2, DCW3), whose interconnected output voltages are applied as so-called intermediate circuit voltage (UZK) at the input of an inverter bridge (WR). On the output side, the inverter bridge (WR) preferably generates a multiphase alternating voltage which corresponds in voltage level and frequency to the usual mains alternating voltage and is therefore suitable for feeding into a public power grid.

The DC / DC converters (DCW1, DCW2, DCW3) are each connected on the input side to a string (ST1, ST2, ST3) consisting of several photovoltaic elements connected in series. The individual DC / DC converters (DCW1, DCW2, DCW3) are designed here as simple step-up switching converters that work according to a known functional principle. With a clocked control of a circuit breaker (LS1, LS2, LS2), the input voltage of the switching converter is periodically applied to a storage choke (L1, L2, L3). The electrical energy stored in the storage choke (L1, L2, L3) in the switch-on phases of the circuit breaker (LS1, LS2, LS2) is used in the blocking phases of the circuit breaker (LS1, LS2, LS2) via a diode (D1, D2, D3) an output capacitor (C). The output voltage of the switching converter increases depending on the duty cycle when the power switch (LS1, LS2, LS2) is activated to a value that is higher than the value of the input voltage.

The circuit breakers (LS1, LS2, LS2) are controlled by a control device, not shown here, which controls the circuit breakers as a function of electrical parameters, in particular the input voltages applied to the DC / DC converters (DCW1, DCW2, DCW3) . The control device is also used to record the input currents of the DC / DC converters (DCW1, DCW2, DCW3) and to control the switching elements described below. Such a control device can be designed in a particularly simple and advantageous manner by a circuit arrangement containing a microcontroller.

The mentioned switching elements (S11, S12, S21, S22) that can be actuated by the control device are in the 1 and have the function of the strings (ST1, ST2, ST3) of the three DC / DC converters (DCW1, DCW2, DCW3) depending on the input voltages generated by the strings (ST1, ST2, ST3) either individually or in one To operate in series.

It is initially assumed that the switching elements S11 and S21 are closed and the switching elements S12 and S22 are open. In this case, the three DC / DC converters (DCW1, DCW2, DCW3) work independently of one another, with their output voltages in parallel on the intermediate circuit lines ( ZK + , ZK- ) issue. This operating mode is particularly advantageous if all three strings (ST1, ST2, ST3) supply voltage values that increase with good efficiency through the DC / DC converters (DCW1, DCW2, DCW3) up to the value of the intermediate circuit voltage (UZK) to let.

If this is not the case, it is more advantageous to provide for operation with at least two strings (ST1, ST2, ST3) connected in series. Assuming the first and the second string (ST1, ST2) each deliver a voltage that cannot be converted into the intermediate circuit voltage (UZK) with good efficiency, while the sum of the two voltages would result in a sufficient value in this regard. In this case it is beneficial to connect the two strings (ST1, ST2) in series with one another and to apply their output voltage to one of the DC / DC converters (DCW1) on the input side.

This is achieved in that the control device opens the switching element S11 and closes the switching element S12. As a result, the strings ST1 and ST2 applied to the DC / DC converters DCW1 and DCW2 are connected in series, with their positive potential being connected to the positive line via the positive line of the DC / DC converter DCW1, i.e. the storage choke L1 and the diode D1 ( ZK + ) of the intermediate circuit, while the DC / DC converter DCW2 transfers the common ground potential to the ground line ( ZK- ) of the intermediate circuit.

Since the circuit breaker LS1 of the DC / DC converter DCW1 is also connected to the ground line ( ZK- ) of the intermediate circuit is connected, it now clocks the full total voltage of the strings ST1 and ST2. The positive line of the second DC / DC converter (DCW2) is decoupled by the diode D2.

This operating mode is advantageous if the string voltages applied to the DC / DC converters are low and can therefore only be converted into the intermediate circuit voltage (UZK) with a relatively poor degree of efficiency. With this connection, the voltages of the individual strings do not have to be set as high as without a series connection. The inverter works with significantly better efficiency, generates less heat and therefore regulates less frequently, and can therefore work more economically.

Correspondingly, the strings (ST2, ST3) of the DC / DC converters DCW2 and DCW3 can also be connected in series at times, or all three strings (ST1, ST2, ST3) together.

The switching process can be carried out in such a way that the inverter is connected to the individual DC / DC converters (DCW1, DCW2, DCW3) starts up separately. After the open circuit voltage has been exited and a lower voltage has been reached at each DC / DC converter (DCW1, DCW2, DCW3), you can switch over. When switching over during operation, make sure that initially the "lower" DC / DC converter, ie closer to ground, is no longer clocked and is then switched over quickly.

A useful criterion for switching from individual operation to series connection is that currents of approximately the same size flow through the strings to be switched (ST1, ST2, ST3) and that the sum of the individual voltages is less than the maximum permissible voltage.

It is particularly advantageous if the operating data, such as input voltage, output current or output power, can be scanned individually for each string (ST1, ST2, ST3), especially when the strings are connected in series. For this purpose, the current values can be determined directly, the voltage values by simply subtracting the individual voltage values. This also makes it possible to individually monitor the optimal working point of each string in a series connection. In this way, when connected in series, it can be precisely identified in a simple manner whether individual operation is more sensible and economical. If necessary, you can react quickly. Such situations usually occur when the photovoltaic strings are partially shaded.

The criterion for canceling the series connection is that more energy can be generated for each string in individual operation. This can e.g. B. be determined by suitable algorithms. The circuit described here can significantly increase the profitability of photovoltaic systems.

The ones in the 1 Switching elements shown schematically as simple switches are designed in practice as electronic or electromechanical components.

Possible advantageous configurations of the switching elements are shown in 2 shown schematically. The switching element S1 represents a switching element in the ground line of a DC / DC converter (DCW1, DCW2, DCW3), corresponding for example to the switching element S11 in FIG 1 , while the switching element S2 stands for a switching element which has two DC / DC converters (DCW1, DCW2, DCW3), corresponding for example to the switch S12 in FIG 1 , can connect with each other.

A particularly simple way of realizing the switching elements shows 2a . The first switching element (S1) is designed as a diode, while the second switching element (S2) is a relay contact. When the relay contact (S2) is activated, the diode (S1) blocks and removes the connection between the negative potential of the associated string and the negative line (ZK-) of the intermediate circuit.

With a switching element S2, which has an additional changeover contact ( 2 B) the diode losses that occur when the inputs are individually controlled can be minimized by bridging the diode (S1).

A particularly advantageous embodiment consists in designing the switching elements (S1, S2) as field effect transistors ( 2c ). These allow contactless and particularly fast switching of the inputs.

The exclusive use of relay contacts as switching elements (S1, S2) should also be mentioned ( 2d ). This option is the most cost-effective, but requires particularly good timing for the control.

A combination of a field effect transistor and a relay can also be provided to form the switching elements, the field effect transistor only needing to have a high dielectric strength and only carry the current for a very short time, typically a few milliseconds. Conversely, the relay does not have to be designed for high switching voltages, but only for the corresponding currents, since the switching process itself is effected by the field effect transistor or by a diode. This also greatly simplifies the costly switching of direct voltages via relays.

For switching, it is particularly advantageous if the power switch of the lower DC / DC converter is initially clocked so that the voltage of the string approaches zero. In this case, the switches are only exposed to low voltages during the switching process.

List of reference symbols

C.

Output capacitor

D1, D2, D3

Diodes

L1, L2, L3

Storage chokes

ST1, ST2, ST3

Strings

DCW1, DCW2, DCW3

DC / DC converter

LS1, LS2, LS3

Circuit breaker

WR

Inverter bridge

S1, S11, S21

(first) switching elements

S2, S12, S22

(second) switching elements

UCC

DC link voltage

ZK -

DC link line (ground)

ZK +

DC link line (plus)

Claims (11)

Photovoltaic system with several strings each comprising several solar cell modules, each of which is assigned a DC / DC converter, the DC / DC converters being connected on the output side to a common inverter bridge, characterized in that switching elements (S1, S2, S11, S12, S21, S22) affects at least one DC / DC converter (DCW1, DCW2, DCW3) on the input side with the total output voltage of a series connection of the string assigned to it (ST1, ST2, ST3) and at least one further string (ST1, ST2, ST3 ) can be acted upon. Photovoltaic system after Claim 1 , characterized in that a current and voltage monitor is provided on each DC / DC converter (DCW1, DCW2, DCW3). Photovoltaic system after Claim 2 , characterized in that via switching elements (S1, S2, S11, S12, S21, S22) influenced at least one DC / DC converter (DCW1, DCW2, DCW3) on the input side with the total output voltage of a series connection of the string assigned to it (ST1, ST2 , ST3) and at least one other string (ST1, ST2, ST3) is applied if the detected output currents of the respective DC / DC converters (DCW1, DCW2, DCW3) do not differ by more than a specified maximum amount and at the same time the sum of the Output voltages of the respective DC / DC converters (DCW1, DCW2, DCW3) is below a specified maximum amount. Photovoltaic system after Claim 1 , characterized in that at least some of the switching elements (S1) are designed as diodes. Photovoltaic system after Claim 1 , characterized , characterized in that at least some of the switching elements (S1, S2, S11, S12, S21, S22) are designed as relays. Photovoltaic system after Claim 1 , characterized in that at least some of the switching elements (S1, S2) are designed as controllable semiconductor switches. Photovoltaic system after Claim 6 , characterized in that the controllable semiconductor switches are designed as field effect transistors. Photovoltaic system after Claim 1 , characterized in that the photovoltaic system has at least one microcontroller. Photovoltaic system according to the Claims 2 and 8th , characterized in that the current and / or voltage evaluation takes place directly or indirectly by the microcontroller. Photovoltaic system after Claim 8 , characterized , characterized in that the microcontroller controls at least some of the switching elements (S1, S2, S11, S12, S21, S22). Photovoltaic system after Claim 1 , characterized in that the photovoltaic system is provided for feeding electrical energy into the public power grid.

Images