BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a schematic representation of a photovoltaic plant with a generator and an inverter of the invention that is connected to a power supply network (state of the art),
FIG. 2 shows a schematic representation of a photovoltaic plant with a generator and an inverter of the invention with an intermediate circuit and a DC-AC converter in the form of a bridge circuit with a transformer connected, downstream thereof, to a power supply network,
FIG. 3 shows a representation of a first variant of the solution of the invention,
FIG. 4 shows a representation of a second variant of the solution of the invention,
FIG. 5 shows a representation of a third variant of the solution of the invention,
FIG. 6 shows a representation of a fourth variant of the solution of the invention,
FIG. 7 shows a representation of a fifth variant of the solution of the invention.
DETAILED DESCRIPTION OF THE INVENTION
In the Figs., like elements are labelled with the same numerals.
FIG. 1 illustrates a photovoltaic plant 1. It comprises a generator 2 that is implemented as a photovoltaic generator. It may consist of several modules. Here however, only one is shown. The photovoltaic generator 2 is connected to an inverter 3 through connection terminals 4, 5. A direct voltage is applied to the terminals 4, 5. The inverter 3, which consists of an intermediate circuit 11 and of a DC-AC converter 12, is connected to a power supply network 6, more specifically to a low-voltage network. The connection terminals 7, 8 serve as an alternating voltage network output. The intermediate circuit 11 and the DC-AC converter 12 are connected together through the connection terminals 9 and 10. The photovoltaic generator 2 has an internal capacitance that is shown as the capacitor 20. The DC-AC converter 12 may for example be implemented as a DC-DC-AC converter.
FIG. 2 shows a photovoltaic plant 1 with an inverter 3 of the invention. The inverter 3 consists of a voltage limiting circuit 13 of the invention that is connected through the connection terminals 4 and 5 to the photovoltaic generator 2 and that is connected upstream of the intermediate circuit 11. The DC-AC converter 12 is implemented here as a full bridge with semiconductor components 15 and a transformer 16, more specifically a 50 Hz-60 Hz transformer, for galvanic isolation of the network 6. In principle, other topologies suited for feeding the network can be utilized for the DC-AC converter. The intermediate circuit serves for temporarily storing electric energy that is converted into alternating current by actuation of the semiconductor switch elements 15. The intermediate circuit may be implemented as a buffer capacitor. There may be one or several buffer capacitors.
In some plants, the buffer capacitor can be replaced by an intermediate circuit choke or be omitted altogether. FIG. 2 also shows an inverter 3 within a photovoltaic plant 1 for connection to a photovoltaic generator 2 with a direct voltage input (terminals 4 and 5) and an alternating voltage network output (terminals 7 and 8) for supplying an energy supply network 6.
In accordance with the invention, a voltage-limiting device 13 is connected downstream of the photovoltaic generator 2, said device consisting in the simplest implementation of a short-circuit switch element that is connected in parallel to the photovoltaic generator 2 and is actuatable in such a manner that, when a voltage threshold value is exceeded, this short-circuit switch element is switched in such a manner that the photovoltaic generator 2 is switched to a short-circuit mode of operation and that, when the voltage falls below the voltage threshold value, it is switched off again so that the photovoltaic generator 2 quits the short-circuit mode of operation.
FIG. 3 shows a variant with a buffer capacitor 14 in the intermediate circuit 11. This buffer capacitor 14 is connected to the photovoltaic generator 2 via a protection diode 17. A transistor, preferably a power transistor, serves as the short-circuit switch element 18. The transistor is located between the protection diode 17 and the photovoltaic generator 2. If the transistor is actuated, the voltage at the generator 2 drops. The voltage at the capacitor 14 is higher than at the generator terminals, so that the diode 17 shuts down. As a result, the buffer capacitor 14 will not be discharged upon actuation of the transistor 18.
In FIG. 4, the power transistor is replaced by a turn-off thyristor (GTO). In principle, a thyristor with a suited quenching circuit may be used. As contrasted to the IGBT, a thyristor or GTO is characterized by maximum energy dissipation while observing the threshold I2t value.
As shown in the variant illustrated in FIG. 5, a load resistor 21 can be arranged in series with the short-circuit switch element 18. This load resistor allows for discharging the inner capacitance 20 of the photovoltaic generator 2 when the semiconductor switch 18 is being switched on. Thus, the energy contained in the capacitance 20 will not be converted, or not converted completely, into heat in the power switch 18. A transistor such as an IGBT or a turn-off thyristor is suited as a power switch 18.
In a particular implementation and as shown in FIG. 5, another switch 19 can be arranged in parallel to the power switch 18 and to the load resistor 21, which is connected upstream thereof. In the first step, the power switch 18 is closed and the capacitor 20 is being discharged. Once the voltage above the capacitor 20 has dropped to a low voltage, the switch 19 is switched on. This way of proceeding offers the advantage that almost the entire energy contained in the capacitance 20 is converted into heat in the resistor 21 without the latter permanently absorbing power during the short circuit.
If the switch 19 is opened, the switch element 18 must be switched off again. The power switch 18 may also be switched off immediately after the switch 19 has been connected. Since the switch 19 needs only dissipate little energy, a MOSFET, more specifically a low-impedance MOSFET, is particularly suited, as shown in FIG. 5.
In an alternative implementation, an inductance 22 can be connected between the photovoltaic generator 2 and the short-circuit switch element 18, as shown in FIG. 6. The inductance 22 is interposed between switch 18 and generator 2. When the switch element 18 is being switched on, the inductance 22 absorbs at least part of the energy contained in the capacitance 20.
In some solar inverters, a boost chopper is mounted upstream of the DC-AC converter 12 in order to achieve a voltage adjustment improving the efficiency. As a result, there is an inductance so that no additional inductances are utilized since the existing inductance can be utilized for the purpose described. If there is no inductance, an additional one can be added between generator 2 and switch element 18.
If a boost chopper circuit is utilized, there is no need for an additional switch element for dropping the idle voltage since the boost chopper is mounted upstream of the DC-AC converter 12 and downstream of the photovoltaic generator 2. The boost chopper is then synchronized in accordance with the method of claim 11, in order to allow for operating the photovoltaic generator 2 in the short-circuit mode of operation.
If there is provided an inductance 22, the switch element 18 can be operated in a current-limited mode of operation, as shown in FIG. 7. For this purpose, the current is measured by the switch element 18 or by the inductance 22 and the switch element 18 is switched off when a certain current intensity, which is clearly exceeding the normal generator current, is reached. At a certain clock frequency, or after the current intensity has dropped below a given, smaller current intensity in the inductance 22, the switch element 18 is switched on again. The advantage of this solution is that the energy content of the input capacitance 20 is transferred through the boost chopper principle into the buffer capacitor 14 and needs not be converted into heat. As a result, the circuit is very tolerant with respect to the magnitude of the input capacitance.
The circuits shown are voltage limiting circuits and protection circuits for solar inverters.
In the circuits, a power switch 18 is arranged in parallel to the photovoltaic generator 2, either directly as shown in the FIGS. 1, 2, 3, 4 and 5 or through an inductance as shown in the FIGS. 6 and 7.
In the variants shown, the inverter 3 contains at least one buffer capacitor 14 and one DC-AC converter 12, the buffer capacitance 14 being preferably connected to the semiconductor switch through a protection diode 17. The power switch 18 is a semiconductor switch like a transistor or a thyristor that may be switched off or quenched.
The voltage is preferably measured at the buffer capacitor 14. The power switch 18 is virtually switched on when the voltage measured is high and switched off when the voltage measured is low.
The voltage threshold value of the not to be exceeded voltage is preferably less than 600 volt so that US regulations can be met.
The FIGS. 2 through 7 show by way of example implementations of protection and regulating circuits for solar plants for mounting into solar inverters. The invention may however also be realized by other protection and regulating circuits. In principle, other turn-off semiconductor switch elements or suited mechanical switches for short-circuiting may also be utilized. Also, the intermediate circuit may consist of a choke rather than of a buffer capacitor.
LIST OF NUMERALS
1 photovoltaic plant
2 generator
3 inverter
4, 5 connection terminals
6 current supply network
7, 8 connection terminals
9, 10 connection terminals
11 intermediate circuit
12 DC-AC converter
13 voltage limitation circuit
14 buffer capacitor
15 semiconductor switch elements
16 transformer
17 protection diode
18, 19 short-circuit switch element
20 internal capacitance of the photovoltaic generator 2
21 load resistor
22 inductance
Patent Application
20080094867
