SOLAR MODULE, PHOTOVOLTAIC SYSTEM AND VOLTAGE LIMITING METHOD

Description

[0001] The invention relates to a solar module, a photovoltaic system (PV system) with a series connection of solar modules, and a method for limiting the voltage on a solar module.

[0002] To generate electricity cost-effectively using PV modules, these modules are connected to form strings of ever-increasing length. To manage the rising string voltage resulting from the growing string length, safety circuits are required to protect a connected inverter from overvoltages or to eliminate the need to disconnect the inverter from the strings for overvoltage protection.

[0003] As an alternative to disconnecting the strings from the inverter, German patent application DE102010009120 A1 proposes short-circuiting a portion of the string via a switch when the string voltage exceeds a critical value. This allows the remaining string to stay connected to the inverter and the photovoltaic system to continue producing energy. However, this solution results in increased complexity due to the need to provide and control the additional short-circuit path.

[0004] Voltage-reducing short-circuiting devices can also be integrated into PV modules according to the WO2015 / 095414 standard, thus avoiding additional wiring effort. Each short-circuiting device can short-circuit a sub-module in case of overvoltage, thereby reducing the voltage at the solar module's terminals to, for example, 75% of the full voltage.

[0005] Alternatively, according to document EP1914857 A1, an inverter can also be operated by means of a short-circuit switching element arranged in parallel to the entire PV generator in such a way that the generator is temporarily short-circuited when a predetermined maximum voltage is exceeded, and the short circuit is lifted when the maximum voltage is undershot.

[0006] It would be desirable to design the voltage limitation of a series connection of solar modules to be continuous, so that in the event of overvoltage, the voltage is only reduced as much as necessary and the energy generation of the series connection is restricted only as little as necessary.

[0007] The object of this invention is therefore to provide solar modules for strings which, with the help of a protective circuit, autonomously ensure compliance with voltage limits with minimal loss of energy generation, without requiring additional wiring effort for the string.

[0008] This problem is solved by a photovoltaic system of independent claim 1 and by a method of independent claim 8. Preferred embodiments are described in the dependent claims.

[0009] In one aspect, the invention relates to a solar module comprising a series connection of solar cells, a switch arranged in parallel to a section of the series connection, and a control circuit for controlling the switch. The control circuit is configured for pulsed control of the switch with a duty cycle, the duty cycle being determined as a function of a voltage drop across the series connection or a section thereof, as determined by the control circuit (13) when the switch (12) is open. The pulsed control reduces the voltage of the solar module in a first time interval, and in a second time interval, the solar module supplies an unlimited voltage. The duty cycle thus determines the ratio between the lengths of the first and second time intervals and, consequently, the time-averaged value of the voltage supplied by the solar module.Even if the voltage of the solar module fluctuates between two values, using many identical solar modules according to the invention in a string results in only a slight variation in the string voltage over time by superimposing the voltage fluctuations. For this purpose, the control circuits of the solar modules are preferably operated asynchronously to distribute the switching times of the switches as evenly as possible over time. This can be achieved, for example, by configuring the control circuit to vary the frequency of the pulsed switching, in particular to vary it statistically. This can be achieved by using a random number generator within the control circuit to generate the frequency.

[0010] Advantageously, the series connection section comprises the entire series connection of the solar cells, thus enabling the maximum possible voltage limitation. However, only a portion of the series connection of the solar cells can constitute the section, for example, one or two of the three sub-modules of the solar module.

[0011] Because the control circuit is designed to determine the voltage across the part of the series circuit when the switch is open, the measured voltage is not affected by the voltage drop caused by closing the switch, and the measurement result is independent of the duty cycle used.

[0012] In a preferred embodiment of the invention, the solar module comprises a disconnect switch which, when opened, electrically isolates the section of the series connection of the solar module's terminals. This disconnect switch is preferably arranged at one end of the section of the series connection bridged by the switch. In particular, one terminal of the disconnect switch is connected to both a terminal of the switch and a terminal of the solar module. The other end of the disconnect switch is connected to the series connection of the solar cells. The control circuit can be configured to open the disconnect switch in a pulsed manner, with the disconnect switch being opened before the solar module's circuit is closed and closed after the circuit is closed. This prevents the discharge of any capacitance associated with the series connection of solar cells when the switch is closed.Such a discharge leads to undesirable thermal stress on the switch, especially at high switching frequencies.

[0013] The switch and the disconnect switch are preferably designed as semiconductor switches, for example as IGBTs or MOSFETs, and can have an intrinsic or a separate freewheeling diode. The freewheeling diode of the switch is aligned parallel to the bypass diode with respect to the terminals of the solar module, while the freewheeling diode of the disconnect switch is aligned antiparallel.

[0014] The control circuit, switch, and disconnect switch can preferably be housed in a junction box of the solar module. The control circuit is preferably powered by the series connection of the solar cells or a portion thereof.

[0015] In another aspect of the invention, a photovoltaic system comprises a plurality of solar modules connected in series and / or parallel, at least some of which are solar modules according to the invention. A photovoltaic system constructed exclusively with solar modules according to the invention is also conceivable. If only some of the solar modules are solar modules according to the invention, and the photovoltaic system has parallel strings, the proportion of solar modules according to the invention should be chosen to be equal in each string in order to avoid reverse currents in individual strings due to unequal voltage reduction.

[0016] A photovoltaic system according to the invention can further comprise an inverter which is configured to detect the pulsed activation of switches of the solar modules according to the invention as a frequency signal on a connecting line for the majority of solar modules. Such a frequency signal indicates a detected overvoltage in parts of the photovoltaic system. In particular, in response to the detection of the frequency signal, the inverter can be configured to shift an operating point of the majority of solar modules, for example, to an operating point with a higher current or lower voltage on the connecting line, in order to counteract an overvoltage detected by the solar modules. In particular, the reduction of the voltage at the operating point can be carried out gradually or continuously until the frequency signal disappears.

[0017] An additional aspect of the invention relates to a method for limiting the voltage across a solar module comprising a series connection of solar cells, wherein a switch is arranged in parallel to a section of the series connection. The method comprises determining a voltage across a portion of the series connection, wherein the voltage is determined with the switch (12) open, and determining a duty cycle as a function of the voltage, as well as pulse-controlled operation of the switch with the determined duty cycle. Preferably, the voltage is determined across the entire section of the series connection to which the switch is connected in parallel. To avoid any influence of the closing of the switch on the voltage determination, the voltage is determined with the switch open.

[0018] It is advantageous to further provide a disconnect switch arranged such that, when opened, it electrically isolates the section of the series circuit from one of the solar module's terminals. The disconnect switch is opened in a pulsed manner, coinciding with the pulsed activation of the switch, and is opened before the circuit closes and closed after the circuit closes. As previously described, this prevents the switch from being stressed by the repeated discharge of the series circuit's capacitance.

[0019] In an advantageous embodiment, the duty cycle is zero at a specific voltage below a predetermined voltage threshold, so that the switch is not operated in pulsed mode. Above the voltage threshold, the duty cycle has a minimum value of at least 5%, preferably at least 10%. As the voltage increases, the duty cycle can then increase further up to a maximum value, which can be 100%, but can also be lower, for example, 95%. However, if 100% is chosen as the maximum value, it should still be ensured and included that the switch is repeatedly opened briefly, at least to determine the voltage across the section of the series circuit.

[0020] To achieve low voltage ripple within the photovoltaic system, a high switching frequency is advantageous. It has been shown that, for example, a switching frequency of 2 kHz is sufficient to achieve a low residual fluctuation in the inverter's input voltage, even with commonly used DC link capacitances. On the other hand, a low switching frequency results in lower switching losses, since when the switch in one solar module closes, the operating points of the other solar modules in the string change according to the voltage reduction in that module. This change in the operating points of the other solar modules leads to a recharging of the generator capacitance associated with their solar cells, and thus to additional power losses.A compromise between these losses and the remaining voltage ripple is typically achieved in a clock frequency range between 200 Hz and 2 kHz.

[0021] To minimize the simultaneity of switching operations of the switch with other switches in other solar modules according to the invention and instead achieve the most uniform distribution of switching times possible, the frequency of the clocked control can be varied, in particular statistically varied. Statistical variation of the frequency can be achieved by a random number generator as part of the control circuit.

[0022] The invention is illustrated below with the aid of figures, of which Fig. 1 shows an embodiment of a solar module according to the invention, Fig. 2 shows another embodiment of a solar module according to the invention, Fig. 3 shows a time course of switching operations of switches used in the solar module according to the invention, Fig. 4 shows a diagram illustrating the determination of a duty cycle as a function of a voltage, and Fig. 5 shows a photovoltaic system using the solar modules according to the invention.

[0023] Fig. 1 Figure 1 shows a solar module 1 according to the invention, comprising a first terminal 15 and a second terminal 16. A series connection of solar cells 10 is arranged between the terminals 15 and 16, with a bypass diode 11 connected in parallel to each section of the solar cells 10. A switch 12 is also connected in parallel to a section of the solar cells 10, for example, one of the three sub-modules from which the solar module 1 is formed. This switch bypasses the section of the solar cells 10 when a control unit 13 closes the switch 12, i.e., makes it conductive. The control circuit 13 is configured to control the switch 12 with a pulsed duty cycle, the duty cycle being determined as a function of a voltage applied to inputs 17 of the control circuit 13. The inputs 17 are connected to the series connection of solar cells 10 in such a way that they detect the voltage across a portion of the solar cells 10.As shown here, this section can be identical to the section of solar cells 10, but it can also measure the voltage across another part of the series connection of solar cells 10 or even across the entire series connection. The voltage measurement can be timed to coincide with the actuation of switch 12, in particular, it can occur when switch 12 is open.

[0024] Advantageously, the control circuit 13 also obtains the electrical energy required for operation via the inputs 17, i.e., from the part of the series circuit to which the inputs 17 are connected. This eliminates the need for a separate energy source for the control circuit.

[0025] The in Fig. 2 The embodiment shown of a solar module 1 according to the invention differs initially from the solar module 1 made of Fig. 1 This is achieved by connecting switch 12 in parallel to the entire series connection of solar cells 10, thus bridging the entire solar module 1. Furthermore, the series connection of solar cells 10 was supplemented by a disconnect switch 14, which is also controlled by the control circuit 13. The disconnect switch 14 is generally located at any position within the section of the series connection of solar cells 10 bridged by switch 12; here, it is directly connected to terminal 16 of the solar module 1.

[0026] In a preferred embodiment, the disconnect switch 14 is operated together with the switch 12 by the control circuit 13, wherein the disconnect switch 14 is opened before the switch 12 closes and is closed again after the switch 12 opens. The disconnect switch 14 is therefore open for the entire time that the switch 12 bridges the section of the series connection of the solar cells 10. This fact is reflected in the switching sequence in Fig. 3 The diagram above shows the time course of the switching states of disconnect switch 14, the diagram below shows the time course of the switching states of switch 12.

[0027] The use of the disconnect switch 14 is particularly advantageous when the series connection of solar cells 10 has a high capacity, which would be discharged through the switch 12 when it is closed and would lead to a significant thermal load on the switch, which is avoided by opening the disconnect switch 14.

[0028] In Fig. 4 The diagram illustrates how the control circuit 13 determines the duty cycle to be used on the ordinate from the voltage across the portion of the series connection of the solar cells 10 on the abscissa of the diagram that is connected to the inputs 17 of the control circuit 13. Below a first voltage threshold U1, the duty cycle is zero, meaning that the switch 12 remains permanently open and the disconnect switch 14, if present, remains closed. Upon reaching the first threshold U1, the switch 12 or the disconnect switch 14 begins to operate in a pulsed manner with a duty cycle greater than or equal to a minimum duty cycle TGmin. Above the first threshold U1, the duty cycle is selected according to a predefined characteristic curve 30 and can increase up to a maximum duty cycle TGmax, for example, 95%. Fig. 4 A linear characteristic curve 30 is shown, but another characteristic curve, in particular a monotonically increasing characteristic curve, can also be used. The characteristic curve 30 reaches the minimum duty cycle TG min, for example 5%, at a second threshold value U 2. The second threshold value U 2 is less than or equal to the first threshold value U 1. If the voltage at the inputs 17 of the control circuit 13 falls below the second threshold value U 2, a duty cycle of zero is applied again, thus ending the clocked operation of the switch 12. It is also possible to set the minimum duty cycle to zero or to choose it according to the minimum possible pulse width of the switch 12.

[0029] Fig. 5Figure 1 shows the use of the solar modules according to the invention as components of a photovoltaic system 2. The solar modules 1 are connected in series as a string 41, with several strings 41 being connected in parallel to each other between connection lines to an inverter 40. The inverter 40 converts the direct current power generated by the strings 41 into alternating current power for feeding into a grid connected to a grid connection point 42. The maximum string voltage is automatically limited by the solar modules 1 according to the invention by means of the pulsed operation of the switches 12 or disconnect switches 14, which automatically reduce their voltage in the event of a voltage exceeding the first threshold value U1.This makes it possible to extend the length of the string(s) 41 such that the maximum theoretical open-circuit voltage, i.e., the open-circuit voltage without pulsed operation of the switches, of the string(s) 41 exceeds the maximum input voltage of the inverter 40. This is possible because the solar modules 1, through the use of the invention, prevent the maximum open-circuit voltage from being reached. The increased string length enables a more cost-effective photovoltaic system 2. Reference symbol list

[0030] 1 Solar module 2 Photovoltaic system 10 Solar cell 11 Bypass diode 12 Switch 13 Control circuit 14 Switch 15, 16 Connection 17 Input 30 Characteristic curve 40 Inverter 41 String 42 Grid connection point

Claims

1. A photovoltaic installation (2) comprising a plurality of solar modules (1) interconnected in series and / or in parallel, wherein each of the plurality of solar modules (1) comprises: - a series circuit of solar cells (10), - a switch (12), being arranged in parallel with a section of the series circuit, and - an actuation circuit (13) of the switch (12), characterized in that the actuation circuit (13) is configured for clocked actuation of the switch (12) with a duty cycle, wherein the duty cycle is determined depending on a voltage dropped at the series circuit or a portion of the series circuit by the actuation circuit (13) with opened switch (12).

2. The photovoltaic installation (2) as claimed in claim 1, wherein the section of the series circuit is the entire series circuit.

3. The photovoltaic installation (2) as claimed in claim 1 or 2, wherein the section of the series circuit is identical to the portion of the series circuit.

4. The photovoltaic installation (2) as claimed in one of the preceding claims, further comprising a circuit breaker (14) arranged in such a way that, when opened, the circuit breaker (14) electrically isolates the section of the series circuit from one of the terminals (15, 16) of the solar module (1).

5. The photovoltaic installation (2) as claimed in claim 4, wherein the actuation circuit is configured to open the circuit breaker (14) in a clocked manner together with the clocked actuation of the switch (12), wherein the circuit breaker (14) is opened before the switch (12) is closed and is closed after the switch (12) is opened.

6. The photovoltaic installation (2) as claimed in one of the preceding claims, wherein the actuation circuit (13) is configured to vary the frequency of the clocked closure, in particular to vary said frequency in a statistical manner.

7. The photovoltaic installation (2) as claimed in one of the preceding claims, further comprising an inverter (40) configured to detect the clocked actuation of switches (12) of the solar modules (1) as a frequency signal on a connection line for the plurality of solar modules (1), wherein the inverter (40) is configured to shift an operating point of the plurality of solar modules (1), in particular to shift the operating point to a higher current on the connection line, in response to detection of the frequency signal.

8. A method for voltage limitation in a photovoltaic installation (2) as claimed in one of the preceding claims, comprising the steps of: - determining a voltage across a portion of the series circuit, wherein the determining of the voltage is performed at opened switch (12), - determining a duty cycle depending on the voltage, - closing the switch (12) in a clocked manner with the duty cycle.

9. The method as claimed in claim 8, wherein the voltage across the section of the series circuit is determined.

10. The method as claimed in one of claims 8 to 9, wherein a circuit breaker (14) is furthermore provided, arranged in such a way that, when opened, the circuit breaker (14) electrically isolates the section of the series circuit from a terminal (15, 16) of the solar module (1), wherein the circuit breaker (14) is opened in a clocked manner together with the clocked closure of the switch (12) and the circuit breaker (14) is opened before the switch (12) is closed and is closed after the switch (12) is opened.

11. The method as claimed in one of claims 8 to 10, wherein, at a prescribed voltage below a prescribed voltage limit value, the determined duty cycle is zero and, above the voltage limit value, has a minimum value of at least 5%, preferably at least 10%.

12. The method as claimed in one of claims 8 to 11, wherein the clocked closure takes place at a frequency of more than 200 Hz and preferably less than 2 kHz.

13. The method as claimed in one of claims 8 to 12, wherein the frequency of the clocked closure is varied, in particular is varied in a statistical manner.