METHOD FOR OPERATING AN INVERTER AND INVERTER FOR CARRYING OUT THE METHOD

Abstract

In a method for operating an inverter for converting DC voltage into AC voltage, having at least one DC/DC step-up converter for converting a DC input voltage applied at the step-up converter DC input into an output voltage higher by a voltage stroke, an intermediate circuit, a DC/AC converter and an AC output for connection to a supply network and/or consumers, a voltage ripple is superimposed on the intermediate circuit voltage and in each step-up converter a switch is switched on/off with a specific switching frequency and a specific duty cycle, for measuring the output voltage of each step-up converter and the intermediate circuit voltage including the voltage ripple. A minimum voltage stroke of each step-up converter is dynamically calculated as a function of the respective measured step-up converter input voltage and the measured intermediate circuit voltage ripple, which minimizes the intermediate circuit voltage setpoint.

Claims

1. A method for operating an inverter (1) for converting a DC voltage (U.sub.DC) into an AC voltage (U.sub.AC), wherein with at least one DC/DC converter formed by a step-up converter (2) an input voltage (U.sub.IN) of a DC source (4) applied at the DC input (3) of the step-up converter (2) is converted into an output voltage (U.sub.OUT) that is higher by a voltage stroke (v=U.sub.OUT/U.sub.IN), with the output voltages (U.sub.OUT) of all the step-up converters (2) an intermediate circuit (5) that has an intermediate circuit capacitor (C.sub.ZK) is supplied with an intermediate circuit voltage (U.sub.ZK), the intermediate circuit voltage (U.sub.ZK) is converted by means of a DC/AC converter (6) into the AC voltage (U.sub.AC) and applied to an AC output (8) connected to a supply network (9) and/or consumers (10), as a result of which a voltage ripple (ΔU.sub.ZK) is superimposed on the intermediate circuit voltage (U.sub.ZK) at the intermediate circuit capacitor (C.sub.ZK), wherein in each step-up converter (2) with at least one choke (17), a switch (18), a diode (19), and an output capacitor (20) the switch (18) is switched on and off with a specific switching frequency (f.sub.S) and a specific duty cycle (D) via a control device (11), so that the output voltage (U.sub.OUT) of each step-up converter (2) corresponds to a setpoint value of the intermediate circuit voltage (U.sub.ZK_soll) between a maximum intermediate circuit voltage (U.sub.ZK_max) and a minimum DC link voltage (U.sub.ZK_min), and wherein the input voltage (U.sub.IN) of each step-up converter (2) and the intermediate circuit voltage (U.sub.ZK) including the voltage ripple (ΔU.sub.ZK) are measured, wherein the voltage stroke (v) of each step-up converter (2) is dynamically calculated and minimized as a function of the measured input voltage (U.sub.IN) of the respective step-up converter (2) and of the measured voltage ripple (ΔU.sub.ZK) of the intermediate circuit voltage (U.sub.ZK), and the switch (18) of each step-up converter (2) is switched on and off with the specified switching frequency (f.sub.S) and the specified duty cycle (D) so that the input voltage (U.sub.IN) is converted according to the calculated voltage stroke (v) into a corresponding output voltage (U.sub.OUT) that corresponds to the setpoint of the intermediate circuit voltage (U.sub.ZK_soll) and therefore the setpoint of the intermediate circuit voltage (U.sub.ZK_soll) is minimized.

2. The method according to claim 1, wherein the switch (18) of each step-up converter (2) is switched on and off via the control device (11), taking into account a specified minimum duty cycle (D.sub.min), with the specified frequency (f.sub.S) and the specified duty cycle (D).

3. The method according to claim 2, wherein the voltage stroke (v) of each step-up converter (2) is dynamically calculated according to the equation
v=1/(1−D.sub.min)+ΔU.sub.ZK/2.Math.U.sub.IN.

4. The method according to claim 1, wherein the calculated voltage stroke (v) of each step-up converter (2) is increased by a defined value (Δv).

5. The method according to claim 2, wherein the input current (I.sub.DC) of each step-up converter (2) is measured and the minimum duty cycle (D.sub.min) of the switch (12) of each step-up converter (2) is changed as a function of the measured input current (I.sub.DC).

6. The method according to claim 1, wherein the input voltage (U.sub.IN) of each step-up converter (2) and the intermediate circuit voltage (U.sub.ZK) including the voltage ripple (ΔU.sub.ZK) and, if applicable, the current (I.sub.DC) through the choke (17), are measured with a sampling frequency (f.sub.A) which corresponds to a multiple of the network frequency (f.sub.N) of the AC voltage (U.sub.AC), and the voltage stroke (v) and, if applicable, the minimum duty cycle (D.sub.min) are calculated from this.

7. The method according to claim 1, wherein each step-up converter (2) is bi-directional and used as a step-down converter for converting the output voltage (U.sub.OUT) into a lower input voltage (U.sub.IN).

8. An inverter (1) for converting a DC voltage (U.sub.DC) into an AC voltage (U.sub.AC), having at least one DC/DC converter formed by a step-up converter (2) for converting an input voltage (U.sub.IN) of a DC source (4) applied at the DC input (3) of the step-up converter (2) into a higher output voltage (U.sub.OUT) with a voltage stroke (v), an intermediate circuit (5) that has an intermediate circuit capacitor (C.sub.ZK) and is supplied with the output voltages (U.sub.OUT) of all the step-up converters (2), a DC/AC converter (6) and an AC output (8) for connection to a supply network (9) and/or consumers (10), as a result of which a voltage ripple (ΔU.sub.ZK) can be superimposed on the intermediate circuit voltage (U.sub.ZK) at the intermediate circuit capacitor (C.sub.ZK), wherein each step-up transformer (2) has at least one choke (17), a switch (18), a diode (19), and an output capacitor (20), and having a control device (11) that is designed to switch the switch (18) of each step-up converter (2) on and off with a specific switching frequency (f.sub.S) and a specific duty cycle (D) so that the output voltage (U.sub.OUT) of each step-up converter (2) corresponds to a setpoint value of the intermediate circuit voltage (U.sub.ZK_soll) between a maximum intermediate circuit voltage (U.sub.ZK_max) and a minimum intermediate circuit voltage (U.sub.ZK_min), wherein a voltage measuring device (22) for measuring the input voltage (U.sub.IN) of each step-up converter (2) and a voltage measuring device (23) for measuring the intermediate circuit voltage (U.sub.ZK) including the voltage ripple (ΔU.sub.ZK) are provided, wherein the control device (11) is designed to dynamically calculate and minimize the minimum voltage stroke (v) of each step-up converter (2) as a function of the measured input voltage (U.sub.IN) of the respective step-up converter (2) and of the measured voltage ripple (ΔU.sub.ZK) of the intermediate circuit voltage (U.sub.ZK) and to switch the switch (18) of each step-up converter (2) on and off with the specified switching frequency (f.sub.S) and the specified duty cycle (D), so that the input voltage (U.sub.IN) can be converted according to the calculated voltage stroke (v) into a corresponding output voltage (U.sub.OUT) that corresponds to the setpoint of the intermediate circuit voltage (U.sub.ZK_soll), as a result of which the setpoint of the intermediate circuit voltage (U.sub.ZK_soll) can be minimized.

9. The inverter (1) according to claim 8, wherein the control device (11) is designed to switch the switch (18) of each step-up converter (2) on and off with the specified switching frequency (f.sub.S) and the specified duty cycle (D), taking into account a specified minimum duty cycle (D.sub.min).

10. The inverter (1) according to claim 9, wherein the control device (11) is designed to dynamically calculate the voltage stroke (v) of each step-up converter (2) according to the equation
v=1/(1−D.sub.min)+ΔU.sub.ZK/2.Math.U.sub.IN.

11. The inverter (1) according to claim 8, wherein the control device (11) is designed to increase the calculated voltage stroke (v) of each step-up converter (2) by a defined value (Δv).

12. The inverter (1) according to claim 9, wherein a current measuring device (24) is provided for measuring the input current (I.sub.DC) of each step-up converter (2) and that the control device (11) is designed to vary the minimum duty cycle (D.sub.min) of the switch (18) of each step-up converter (2) as a function of the measured input current (I.sub.DC).

13. The inverter (1) according to claim 8, wherein the voltage measuring device (22) is designed to measure the input voltage (U.sub.IN) of each step-up converter (2), the voltage measuring device (23) is designed to measure the intermediate circuit voltage (U.sub.ZK) including the voltage ripple (ΔU.sub.ZK) and, if applicable, the current measuring device (24) is designed to measure the input current (I.sub.DC) of each step-up converter (2) for recording measured values with a sampling frequency (f.sub.A) which corresponds to a multiple of the network frequency (f.sub.N) of the AC voltage (U.sub.AC), and from this to calculate the voltage stroke (v) and, if applicable, the minimum duty cycle (D.sub.min).

14. The inverter (1) according to claim 8, wherein the DC source (4) is formed by a photovoltaic module (13), a wind turbine (14), and/or a battery (15).

15. The inverter (1) according to claim 8, wherein each step-up converter (2) is bi-directional.

Description

[0023] The invention will be explained in further detail by reference to the attached drawings. Shown are:

[0024] FIG. 1 a block circuit diagram of an inverter with a plurality of DC/DC converters designed as step-up converters;

[0025] FIG. 2 a simplified circuit diagram of a step-up converter as a DC/DC converter of an inverter;

[0026] FIG. 3 the resulting voltage stroke v of a step-up converter according to the method according to the invention, wherein a voltage ripple ΔU.sub.ZK is superimposed on the intermediate circuit voltage U.sub.ZK;

[0027] FIG. 4 the resulting voltage stroke v of a step-up converter according to the prior art for three different values of the voltage ripple ΔU.sub.ZK of the intermediate circuit voltage U.sub.ZK;

[0028] FIG. 5 the resulting voltage stroke v of a step-up converter according to the method according to the invention for three different values of the voltage ripple ΔU.sub.ZK of the intermediate circuit voltage U.sub.ZK;

[0029] FIG. 6 a schematic diagram of the minimum duty cycle D.sub.min of a step-up converter as a function of the input current I.sub.DC in the prior art; and

[0030] FIG. 7 a schematic diagram showing the minimum duty cycle D.sub.min of a step-up converter as a function of the input current I.sub.DC in the method according to the invention.

[0031] FIG. 1 shows a block circuit diagram of an inverter 1 having a plurality of DC/DC converters designed as step-up converters 2. The inverter 1 contains at least one DC input 3 for connecting to at least one DC source 4. A DC/DC converter, which is often designed as a booster or step-up converter 2 or a buck converter, is arranged at each DC input 3. The step-up converter 2 converts the input voltage U.sub.IN of the respective DC source 4 applied to the DC input 3 into an output voltage U.sub.OUT that is higher by a voltage stroke V=U.sub.OUT/U.sub.IN. A intermediate circuit 5 with an intermediate circuit capacitor C.sub.ZK is supplied by the output voltages U.sub.OUT of all of the step-up converters 2. The intermediate circuit 5 is followed by a DC/AC converter 6 for converting the intermediate circuit voltage U.sub.ZK into a desired AC voltage U.sub.AC. The AC output 8 is connected to a supply network 9 and/or consumers 10. The various components of the inverter 1 are controlled or regulated via a control device 11.

[0032] The inverter 1 is, for example, a photovoltaic inverter of a photovoltaic system for converting the DC voltage U.sub.DC generated by photovoltaic modules 13 as DC sources 4 into a corresponding AC voltage U.sub.AC, which is fed into a supply network 9 or used to supply electrical energy to consumers 10. The DC source 4 can be formed, for example, by wind turbines 14, batteries 15, or other sources.

[0033] At least one energy storage unit 12 can also be connected to the intermediate circuit 5 of the inverter 1, which can be used for the temporary storage of electrical energy. Inverters 1 of this type are referred to as hybrid inverters. Energy storage units 12 are usually connected to the inverter 1 via a battery isolator (not shown) and connected as required.

[0034] Usually, the step-up converters 2 operate with a fixed voltage stroke v. In accordance with the input voltage U.sub.IN and the respective voltage stroke v, an appropriate output voltage U.sub.OUT results, which is within the specified limits of the intermediate circuit voltage U.sub.ZK, hence it must be between the maximum intermediate circuit voltage U.sub.ZK_max and the minimum intermediate circuit voltage U.sub.ZK_min. The power output at the AC output 8 of the inverter 1 causes fluctuations in the intermediate circuit voltage U.sub.ZK in the form of a superimposed voltage ripple ΔU.sub.ZK. This further restricts the operating range of the respective step-up converter 2 with a fixed voltage stroke v in addition to the permissible range for the intermediate circuit voltage U.sub.ZK between the maximum intermediate circuit voltage U.sub.ZK_max and the minimum intermediate circuit voltage U.sub.ZK_min. According to the invention, it is therefore provided that the voltage stroke v is calculated continuously or dynamically, taking into account the input voltage U.sub.IN of the respective step-up converter 2 and the intermediate circuit voltage U.sub.ZK including the voltage ripple ΔU.sub.ZK, and adjusted accordingly. This means that the voltage stroke v can be kept as small as possible in each case (minimum voltage stroke) and thus a correspondingly small or minimized value of the setpoint of the intermediate circuit voltage U.sub.ZK_soll can be achieved. This results in better utilization of the step-up converter 2, which means that the step-up converter 2 can be operated in a larger operating range. As a result, the efficiency of the inverter 1 can be optimized accordingly.

[0035] FIG. 2 shows a simplified circuit diagram of a step-up converter 2 as a DC/DC converter of an inverter 1. The step-up converter 2 has at least one choke 17, a switch 18, a diode 19, and an output capacitor 20. The switch 18 is formed by a semiconductor switch and has a certain parasitic capacitance 21. An input capacitor 16 is also shown in the equivalent circuit diagram of the step-up converter 2. Using the control device 11, the switch 18 is switched on and off with a specified switching frequency f.sub.S and a specified duty cycle D, resulting in a desired output voltage U.sub.OUT. The output voltage U.sub.OUT is greater than the input voltage U.sub.IN by the voltage stroke v. The output voltage U.sub.OUT, which corresponds to the setpoint of the intermediate circuit voltage U.sub.ZK_soll, must be located within certain limits between a maximum intermediate circuit voltage U.sub.ZK_max and a minimum intermediate circuit voltage U.sub.ZK_min. If one of the two limits is reached, the respective step-up converter 2 is deactivated and the DC source 4 connected to the DC input 3 of the deactivated step-up converter 2, for example a photovoltaic module, may not then be able to contribute to the supply of energy into the supply network 9 or to the supply of electrical energy to the consumers 10. Usually, a plurality of step-up converters 2 are wired in parallel and connected to the same intermediate circuit 5. However, only one step-up converter 2 can be provided for converting the input voltage U.sub.IN of a DC source 4.

[0036] According to the invention, a voltage measuring device 22 for measuring the input voltage U.sub.IN of each step-up converter 2 and a voltage measuring device 23 for measuring the intermediate circuit voltage U.sub.ZK including the voltage ripple ΔU.sub.ZK are provided and connected to the control device 11. In the control device 11, the measured values are processed and a dynamic calculation of a minimum voltage stroke v is performed for each step-up converter 2 as a function of the measured input voltage U.sub.IN and of the measured voltage ripple ΔU.sub.ZK to minimize the setpoint of the intermediate circuit voltage U.sub.ZK_soll. This can result in an optimum usage of the operating range of the respective step-up converter 2, even with fluctuating input voltages U.sub.IN. The minimum voltage stroke v of each step-up converter 2 is advantageously calculated according to the equation v=1/(1−D.sub.min)+ΔU.sub.ZK/2.Math.U.sub.IN, where v denotes the voltage stroke, D.sub.min the minimum duty cycle of the step-up converter 2, ΔU.sub.ZK the voltage ripple, and U.sub.IN the input voltage of the step-up converter 2.

[0037] In addition, a current measuring device 24 can be provided for measuring the input current I.sub.DC of the step-up converter 2 and the control device 11 can be designed to vary the minimum duty cycle D.sub.min of the switch 18 of each step-up converter 2 as a function of the measured input current I.sub.DC. This means that the lower limit for the duty cycle of the switch 18 can be increased slightly at lower input currents I.sub.DC, and also in this case, operation of the step-up converter 2 with a slightly higher voltage stroke v can be ensured (see description of FIGS. 6 and 7).

[0038] FIG. 3 shows the resulting voltage stroke v of a step-up converter 2 according to the method according to the invention, wherein a voltage ripple ΔU.sub.ZK is superimposed on the intermediate circuit voltage U.sub.ZK. The figure shows the temporal curve of the essentially constant input voltage U.sub.IN and the intermediate circuit voltage U.sub.ZK with the superimposed voltage ripple ΔU.sub.ZK, with the limits of the permissible range of the intermediate circuit voltage U.sub.ZK in the form of the maximum intermediate circuit voltage U.sub.ZK_max and the minimum intermediate circuit voltage U.sub.ZK_min being drawn as dotted lines. The minimum voltage stroke v is calculated at the measured input voltage U.sub.IN of the step-up converter 2 and the measured voltage ripple ΔU.sub.ZK such that the input voltage U.sub.IN multiplied by the voltage stroke v results in the mean value U.sub.ZK mean of the intermediate circuit voltage U.sub.ZK, which also corresponds to the control setpoint U.sub.ZK_soll of the intermediate circuit voltage U.sub.ZK. The voltage ripple ΔU.sub.ZK is superimposed on the mean value U.sub.ZK_mean of the intermediate circuit voltage U.sub.ZK. The output voltage U.sub.OUT of the step-up converter 2 and the mean value U.sub.ZK_mean of the intermediate circuit voltage U.sub.ZK have the same value.

[0039] FIG. 4 shows the resulting voltage stroke v of a step-up converter according to the prior art for three different values of the voltage ripple ΔU.sub.ZK of the intermediate circuit voltage U.sub.ZK. The limits of the permissible range of the intermediate circuit voltage U.sub.ZK in the form of the maximum intermediate circuit voltage U.sub.ZK_max and the minimum intermediate circuit voltage U.sub.ZK_min (see FIG. 3) are not shown here for the sake of clarity. The left-hand part of the temporal curve of the voltages shows the case in which no voltage ripple ΔU.sub.ZK is superimposed on the intermediate circuit voltage U.sub.ZK. This is the case, for example, when an energy storage unit 12 of the inverter 1 is being charged by the photovoltaic modules 13 as the DC source 4. In the middle part of the figure, a small voltage ripple ΔU.sub.ZK is superimposed on the intermediate circuit voltage U.sub.ZK. This is the case, for example, when a fairly small current flows from the inverter 1 into the supply network 9 or the consumers 10, i.e. at a fairly low power consumption. In the right-hand part of the figure, a large voltage ripple ΔU.sub.ZK is superimposed on the intermediate circuit voltage U.sub.ZK. This situation occurs, for example, with a higher current flow from the inverter 1 into the supply network 9 or the consumers 10, i.e. at a higher power consumption. According to the prior art, the voltage stroke v of the respective step-up converter 2 would be selected so that the size of the setpoint U.sub.ZK_soll of the intermediate circuit voltage is such that for all three cases, i.e. for all three values of the voltage ripple ΔU.sub.ZK, that it lies within the permissible range of the intermediate circuit voltage U.sub.ZK, i.e. between the minimum intermediate circuit voltage U.sub.ZK_min and the maximum intermediate circuit voltage U.sub.ZK_max. The voltage stroke v according to the prior art would be designed for a worst-case operation and would be accepted for the voltage stroke v.sub.1, v.sub.2 and v.sub.3 in all three cases and would therefore be of the same size.

[0040] FIG. 5 shows the resulting voltage stroke v of a step-up converter according to the method according to the invention for the three different values of the voltage ripple ΔU.sub.ZK of the intermediate circuit voltage U.sub.ZK according to FIG. 4. Here also, the limits of the permissible range of the intermediate circuit voltage U.sub.ZK in the form of the maximum intermediate circuit voltage U.sub.ZK_max and the minimum intermediate circuit voltage U.sub.ZK_min (see FIG. 3) are not shown here for the sake of clarity. Here, by taking into account the voltage ripple ΔU.sub.ZK, an optimal calculation of a minimum voltage stroke v can take place. In the left-hand part of the figure, the case in which the intermediate circuit voltage U.sub.ZK has no superimposed voltage ripple ΔU.sub.ZK, the result would lead to a minimum voltage stroke v, which could be increased if necessary by a specified voltage stroke Δv to take a controller reserve into account. Nevertheless, the minimum voltage stroke v.sub.1 in this case is significantly lower than that according to the prior art (see FIG. 4, left-hand part of the figure). In the middle part of the voltage characteristic also, in which a small voltage ripple ΔU.sub.ZK is superimposed on the intermediate circuit voltage U.sub.ZK, the method according to the invention results in a lower value for the voltage stroke v.sub.2 than in the prior art by taking into account the voltage ripple ΔU.sub.ZK (see FIG. 4, middle part of the figure). Only in the right-hand part of the voltage characteristic, where a large voltage ripple ΔU.sub.ZK is superimposed on the intermediate circuit voltage U.sub.ZK, does the method according to the invention result in a voltage stroke v.sub.3 which corresponds to that of the prior art (see FIG. 4, right-hand part of the figure).

[0041] The comparison of FIGS. 4 and 5 clearly shows the effect of the consideration of the voltage ripple ΔU.sub.ZK on the determination of the minimum voltage stroke v of the step-up converter 2. While in the prior art according to FIG. 4, all three cases of different voltage ripples ΔU.sub.ZK result in an equal voltage stroke v and thus an equal output voltage U.sub.OUT of the step-up converter 2, in the method according to the invention the voltage stroke v can be reduced at lower values of the voltage ripple ΔU.sub.ZK, as a result of which the respective output voltage U.sub.OUT of the step-up converter as the setpoint U.sub.ZK_soll of the intermediate circuit voltage U.sub.ZK can also be reduced.

[0042] FIG. 6 shows a schematic diagram of the minimum duty cycle D.sub.min as a function of the input current I.sub.DC of a step-up converter 2. In the prior art, a minimum duty cycle D.sub.min of the switch 18 of the step-up converter 2 is defined independently of the input current I.sub.DC and the region (shaded region) above this limit is used for regulating the step-up converter 2. The minimum duty cycle D.sub.min depends on the parasitic capacitance 21 of the switch 18. Above a certain minimum input current I.sub.DC_min, the step-up converter 2 can no longer be controlled with the minimum duty cycle D.sub.min, which is why operation of the step-up converter 2 is not possible below this minimum input current I.sub.DC_min.

[0043] FIG. 7 shows a schematic diagram of the minimum duty cycle D.sub.min as a function of the input current I.sub.DC of a step-up converter 2 with the method according to the invention. Depending on the size of the input current I.sub.DC of the step-up converter 2, the minimum switching time of the switch 18 is changed due to its parasitic capacitance 21. At a lower input current I.sub.DC, the minimum duty cycle D.sub.min of the step-up converter 2 must be increased, whereas at a higher input current I.sub.DC, the minimum duty cycle D.sub.min must be reduced. This means that in certain cases, for example in the morning hours with a photovoltaic module as the DC source 4 at a lower input current I.sub.DC, the voltage stroke v of the step-up converter 2 can be optimally adjusted by adjusting the minimum duty cycle D.sub.min, and at lower input currents I.sub.DC the step-up converter 2 can be operated and controlled with an increased minimum duty cycle D.sub.min. Accordingly, the operating range (shaded region) of the step-up converter 2 can be increased.