Method and computing unit for controlling a DC voltage system

Abstract

The invention relates to a method for controlling a DC voltage system (1000), in particular in a vehicle, comprising a multiplicity of voltage sources (21a, 22a), a multiplicity of loads (31a, 32a) and a link circuit (10), wherein each of the multiplicity of voltage sources (21a, 22a) is connected to the link circuit (10) via a source power electronics unit (21b, 22b) and each of the multiplicity of loads (31a, 32a) is connected to the link circuit (10) via a load power electronics unit (32a, 32b). In the method, each of the multiplicity of source power electronics units (21b, 22b) sets (S100) a respective output voltage (Ui, U21b, U22b) into the link circuit (10) on the basis of a link circuit reference voltage (USoll) and of a power that is output into the link circuit (10) by the respective source power electronics unit (21b, 22b), and determines (S110) a voltage average value (UMittel) as the average value of the voltage (Ui, U21a, U22a) that is output at the multiplicity of voltage sources (21a, 22a). A voltage difference (UDiff) is then determined (S120) as the difference between a voltage target value (UZiel), which indicates a desired voltage level of the link circuit (10), and the voltage average value (UMittel), and a link circuit reference voltage(USoll) is specified (S130) on the basis of the voltage difference (UDiff).

Description

[0001] Method and computing unit for controlling a DC voltage system

[0002] Description

[0003] The present invention relates to a method for controlling a direct current system as well as a computing unit and a computer program for carrying out the method.

[0004] Background of the invention

[0005] DC voltage systems can be used, for example, to supply power to multiple electrical machines, for example in a vehicle's on-board power system. Particularly in off-highway vehicles or commercial vehicles such as wheel loaders, excavators, tractors, or similar, electrical machines operated as motors are used to drive the vehicle or various vehicle components, such as an excavator bucket, a boom, a superstructure drive, etc. Additional power consumers can also be connected to the DC voltage system and supplied with DC voltage by the DC voltage system. The electrical machines and power consumers are generally referred to as loads below.

[0006] Traditionally, a so-called droop control is used to control parallel voltage sources in a DC voltage system. The voltage supplied by a voltage source depends not only on the nominal voltage or target voltage of the DC voltage system, but also on the output current or power supplied by the voltage source. The primary goal of drooping is to distribute the generators' power evenly. Furthermore, drooping can prevent a rapid voltage drop or rise and the associated subsequent voltage oscillation, thus stabilizing the system. However, by lowering the output voltage, the DC voltage in the DC voltage system is simultaneously reduced. Disclosure of the Invention

[0007] According to the invention, a method for controlling a DC voltage system, as well as a computing unit and a computer program for implementing the method, are proposed, having the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

[0008] The invention is based on a DC voltage system, in particular for a vehicle, for example an off-highway vehicle or a commercial vehicle such as an excavator, a tractor, or the like. The DC voltage system comprises a plurality of voltage sources, a plurality of loads, and an intermediate circuit.

[0009] The DC voltage system may have an increased nominal voltage, i.e. be a so-called high-voltage network, which has voltages of over 60 V up to several hundred volts, for example up to 1,500 V.

[0010] Each of the plurality of voltage sources is connected to the intermediate circuit via a source power electronics unit, and each of the plurality of loads is connected to the intermediate circuit via a load power electronics unit. The plurality of voltage sources are, in particular, electrical machines that are primarily operated as generators and are driven, for example, by an internal combustion engine. The loads are, in particular, electrical machines that are primarily operated as motors and are used to drive the vehicle or components of the vehicle, such as an excavator bucket, a boom, a superstructure drive, etc. The source power electronics units are designed to convert the voltages output by the voltage sources into a direct voltage, if necessary, and to set this voltage to a predetermined value.The load power electronics are designed to convert the DC voltage into an AC voltage with a specified amplitude. Furthermore, additional components, particularly high-voltage DC components, can be connected to the intermediate circuit as loads.

[0011] When power is demanded by the loads, the voltage delivered to the DC link by the source power electronics is reduced (e.g., due to droop control), so that the DC link voltage can drop, especially below a DC link target voltage. In the case of short-term high power demands from the loads and the associated high currents, a voltage reduction of up to 25% due to droop control can occur for long-term stable operation, depending on the quality of the voltage sources. Typically, the voltage reduction is up to 10%.Due to the reduction in the voltage supplied by the source power electronics and the associated reduction in the intermediate circuit voltage, it may happen that the power electronics and electrical machines are not operated with optimum efficiency or that the power, in particular the current generated, of the electrical machines operated as generators is not sufficient to cover the peak power of the electrical machines operated as motors.

[0012] Furthermore, for the functionally unrestricted operation of other components connected to the DC link as loads, a minimum or maximum voltage level is required, which may no longer be achieved by lowering the DC link voltage, causing these components to no longer function.

[0013] To avoid these problems, the invention uses the measure of compensating for the reduction in the DC link voltage caused by the droop control by adjusting, in particular increasing, the DC link target voltage. To this end, the DC link target voltage is redetermined during operation depending on any deviation of the voltage delivered by the source power electronics from a target voltage value.

[0014] More specifically, the invention relates to a method for controlling a DC voltage system, in which each of the plurality of source power electronics sets a voltage delivered to the intermediate circuit as a function of a desired intermediate circuit voltage and a power delivered to the intermediate circuit by the respective source power electronics. A mean voltage is then determined as the mean value of the voltage delivered by the plurality of source power electronics. The mean value is determined, in particular, by dividing the sum of all detected voltages by the number of voltage sources, i.e., as an arithmetic mean.

[0015] Expressed in equations, this procedure can be represented as follows: with the mean voltage U Mittei , the output voltage U t the voltage source i and the total number of voltage sources n. Subsequently, a voltage difference UDiff as the difference between a voltage target value U zie i, which indicates a desired voltage level of the intermediate circuit, and the average voltage U Mittei determined and depending on the voltage difference U Diff a DC link voltage U So u is specified. The voltage difference U Diff can be expressed as

[0016] As a result, if the intermediate circuit voltage falls below the target voltage value, it is regulated upwards so that the intermediate circuit voltage does not drop or only drops slightly, thus ensuring that the electrical machines and power electronics continue to operate at an optimal voltage level.

[0017] In one embodiment, the DC link voltage is specified as a function of the voltage difference such that the voltage difference assumes a specified setpoint, in particular zero. When the voltage difference assumes the specified setpoint, it is ensured that the voltage in the DC link has the desired value at which the electrical machines and power electronics can continue to operate with optimal efficiency.

[0018] In one embodiment, the voltage difference is regulated by specifying the DC link setpoint voltage to the setpoint, wherein in particular a controller with a proportional and integral component, e.g. a so-called PI or PID controller, is used to regulate the voltage difference. To regulate the voltage difference to the setpoint, in particular an integral of the voltage difference is determined for a specified period of time and the DC link setpoint voltage is specified as a function of the voltage difference and the integral of the voltage difference. The voltages output by the source power electronics and therefore also the voltage difference change or can change continuously over time. By using the integral of the voltage difference, this change can be taken into account when specifying the DC link setpoint voltage.

[0019] This procedure can be represented by an equation as follows: with the DC link voltage U Soll (t), the voltage difference U Di ff(t) and the proportional and integral controller gains k p and ki. The intermediate circuit voltage is thereby adjusted in such a way that the voltage difference is minimized, ie the mean voltage value is increasingly brought closer to the voltage setpoint, which indicates the desired voltage level, or, if the setpoint is zero, reaches it.

[0020] This allows the DC link target voltage to be specified in a simple manner and with little computational effort.

[0021] In this case, the specification of the DC link target voltage can in particular further comprise a feedforward control, in particular taking into account the load power requested by the loads or the source power supplied by the voltage sources. From this, the expected power to be supplied can be determined for at least one, in particular each, source power electronics, from which an expected value of the voltage supplied to the DC link can then be determined. When determining the expected value, in particular the droop function is taken into account. Furthermore, a droop voltage can be determined as a function of the at least one expected value, which represents the expected voltage drop due to the droop function. Finally, the DC link target voltage is also specified as a function of the droop voltage, whereby the droop voltage is in particular added to the DC link target voltage.

[0022] The DC link voltage is then specified as shown by the following equation: with the droop voltage U Droop t).

[0023] By adding the droop voltage, the quality of the control loop can be increased.

[0024] In one embodiment, determining the average voltage comprises filtering the average voltage, in particular with a PT1 filter or low-pass filter. This limits the bandwidth of the feedback variable, i.e., the average voltage, so that large jumps in the DC link target voltage are prevented and the electrical components of the DC voltage system are thus protected.

[0025] A computing unit according to the invention, e.g. a control unit of a motor vehicle, is configured, in particular in terms of programming, to carry out a method according to the invention.

[0026] Implementing a method according to the invention in the form of a computer program or computer program product with program code for performing all method steps is also advantageous, as this entails particularly low costs, especially if an executing control unit is also used for additional tasks and is therefore already present. Suitable data storage devices for providing the computer program include, in particular, magnetic, optical, and electrical storage devices, such as hard disks, flash memories, EEPROMs, DVDs, and others. Downloading a program via computer networks (Internet, intranet, etc.) is also possible.

[0027] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0028] The invention is illustrated schematically in the drawing using exemplary embodiments and is described below with reference to the drawing.

[0029] Short description of the drawings

[0030] Figure 1 shows a block diagram of a DC voltage system designed to carry out an embodiment of the method,

[0031] Figure 2 shows a block diagram of the interconnection of a computing unit with the source power electronics,

[0032] Figure 3 shows a flowchart of an embodiment of the method according to the invention, and

[0033] Figure 4 shows a block diagram of a computing unit with elements for implementing an embodiment of the method. Embodiments of the invention

[0034] Figure 1 shows a block diagram of a DC voltage system 1000 designed to carry out an embodiment of the method. For this purpose, the DC voltage system 1000 has an intermediate circuit 10. Two voltage sources 21a, 22a are each connected to the intermediate circuit 10 via a source power electronics 21b, 22b. The voltage sources 21a, 22a are, in particular, electrical machines operated as generators. To drive the electrical machines, the DC voltage system 1000 further has an internal combustion engine 40, for example, a diesel internal combustion engine, which drives the voltage sources 21a, 22a via a transmission 41.

[0035] The intermediate circuit 10 is further connected to two loads 31a, 31b via two load power electronics 31b, 32b. The loads 31a, 31b are, for example, electrical machines, but operated as motors and used, for example, to drive a vehicle in which the DC voltage system 1000 is installed. It is also possible to supply other types of loads, such as any electrical components installed on the vehicle, or to recuperate kinetic energy, i.e., operate the loads 31a, 31b as generators.

[0036] Furthermore, further loads (not shown), in particular high-voltage DC loads that require a voltage of more than 60 V for operation, can be connected to the intermediate circuit 10.

[0037] In the example shown, each of the voltage sources 21a, 22a generates a three-phase alternating voltage, which is converted into a direct voltage by the source power electronics 21b, 22b and fed to the intermediate circuit 10. The intermediate circuit 10 then outputs the direct voltage to the load power electronics 31b, 32b, which in turn convert it into a three-phase alternating voltage and output it to the loads 31a, 32a.

[0038] Figure 2 shows a block diagram of the interconnection of a computing unit 100 with the source power electronics 21b, 22b. The source power electronics 21b, 22b each detect the voltage U 21a , U 22a the voltage source 21a, 22a connected to the source power electronics 21b, 22b and output them to the computing unit 100. The computing unit 100 carries out an embodiment of the method according to the invention and thus determines the intermediate circuit target voltage U Sou and outputs it to the source power electronics 21b, 22b. The source power electronics 21b, 22b receive the intermediate circuit voltage U So u and, in a multiple generator control system 21 ba, 22 ba (particularly with a droop function), determine the setpoint of the voltage to be delivered to the intermediate circuit. The setpoint is determined as a function of the intermediate circuit setpoint voltage as the higher-level nominal value and the power delivered by the source power electronics. In particular, the setpoint can be calculated as the product of the intermediate circuit setpoint voltage with a power-dependent factor (a so-called droop factor). The factor decreases with increasing power, i.e., the greater the delivered power, the lower the setpoint.

[0039] The setpoint for the voltage is then passed on to the voltage control 21 bb, 22bb of the source power electronics 21b, 22b, which adjusts the output voltage to the setpoint.

[0040] Figure 3 shows a flowchart of an embodiment of the method according to the invention. Figure 4 shows a block diagram of a computing unit 100 with elements for implementing an embodiment of the method. Both figures will be described together below.

[0041] In the method, in step S100, each of the plurality of source power electronics 21b, 22b outputs the voltage UU 21a , ^22a depending on the DC link voltage U Sou and a power delivered by the respective source power electronics 21b, 22b into the intermediate circuit 10. Using the delivered power enables the so-called droop function, which prevents voltage transients that would occur during a short-term increase in the delivered power. The values ​​of the set delivered voltage continue to be output to the computing unit 100. For this purpose, they are measured, in particular.

[0042] Subsequently, in step S110, an average voltage value U Mittei as the mean value of the voltage UU supplied to the plurality of voltage sources 21a, 21b 21a , U 22a determined, for example, by measuring all output voltages UU 21a , U 22a to a voltage sum U Sum are summed and then the voltage sum U Sumby the number n of voltage sources 21a, 22a, in the case of Figure 1 there are two (equation (1)). The average voltage U Mittei , which is in particular an average value U mean^ over time, is fed to a PT1 filter 110 in step S111, whereby a filtered average voltage value is obtained. The PT1 filter prevents the average voltage value U used as a feedback variable from Mittei has unwanted jumps. As an alternative to the PT1 filter 110, a low-pass filter could also be used.

[0043] In the following step S120, a voltage difference U Diff as the difference between a voltage target value U ziei , which indicates a desired voltage level of the intermediate circuit 10, and the average voltage U Mittei or the filtered mean voltage U Mittelifilt certainly.

[0044] Depending on the voltage difference U DiffIn step S130, the intermediate circuit voltage U So u specified. The DC link voltage U So u is specified so that the voltage difference U Diff assumes a predetermined setpoint, which is zero. For this purpose, the voltage difference U Diff a controller 120, for example a controller 120 with proportional and integral parts, and in step S131 by specifying the intermediate circuit voltage U So u is regulated to the specified setpoint. For this purpose, the controller 120 calculates the integral of the voltage difference U Diff for a given period of time and then the sum of the voltage difference U Diff and the integral of the voltage difference U Diff In particular, equation (2) can be used for this purpose.

[0045] Furthermore, the load power P delivered by each plurality of load power electronics 31b, 32b to the loads 31a, 32a Lastdetermined and depending on the delivered load power P Last for at least one of the plurality of source power electronics 21a, 21b, an expected value of the voltage Ui, U delivered into the intermediate circuit 10 21b , U 22b Subsequently, a droop voltage U Droop determined and the intermediate circuit voltage U So u depending on the voltage difference U Diff and the droop voltage U Droop In particular, the determined droop voltage U Droop with the sum of the voltage difference U Diff and the integral of the voltage difference U Diff is added (see equation (3)), so that the droop voltage is reduced to the DC link voltage U So u is pre-controlled.

[0046] In embodiments of the invention, an expected value for each source power can be determined as an average from the sum of the delivered load powers if all source power electronics and voltage sources are to deliver the same power. From the expected value for the source power, the expected value for the output voltage can be determined according to the droop function, and the droop voltage can be determined as the magnitude of this voltage reduction.

Claims

Claims 1. A method for controlling a DC voltage system (1000), in particular in a vehicle, comprising a plurality of voltage sources (21a, 22a), a plurality of loads (31a, 32a) and an intermediate circuit (10), wherein each of the plurality of voltage sources (21a, 22a) is connected to the intermediate circuit (10) via a respective source power electronics unit (21b, 22b) and each of the plurality of loads (31a, 32a) is connected to the intermediate circuit (10) via a respective load power electronics unit (32a, 32b), the method comprising: Setting (S100) a voltage ([ / ;, U 21b , U 22 b) depending on a DC link voltage (t / Soii ) and a power delivered by the respective source power electronics (21b, 22b) into the intermediate circuit (10); Determining (S110) an average voltage value as the mean value of the voltage t, U output by the plurality of source power electronics (21b, 22b) 21b , U 22b ), Determining (S120) a voltage difference as the difference between a voltage target value {U zie i), which indicates a desired voltage level of the intermediate circuit (10), and the voltage average value Specifying (S130) a DC link target voltage (t / Soii ) depending on the voltage difference 2. Method according to claim 1, wherein the intermediate circuit voltage (t / Soii ) is specified so that the voltage difference assumes a predetermined setpoint, in particular zero.

3. Method according to the preceding claim, wherein the specification (S130) of the intermediate circuit target voltage (U So u) depending on the voltage difference so that the voltage difference assumes a given setpoint, includes: Control (S131) ​​of the voltage difference to the setpoint by specifying the intermediate circuit voltage (U So u).

4. Method according to the preceding claim, wherein for regulating (S131) ​​the voltage difference a controller with proportional and integral components is used.

5. The method according to claim 4, wherein the regulation (S131) ​​of the voltage difference to the target value includes: Determine (S131a) an integral of the voltage difference for a specified period of time, Specify (S131b) the DC link voltage (t / Soii ) depending on the voltage difference and the integral of the voltage difference 6. Method according to the preceding claim, wherein the specification (S131 b) of the intermediate circuit target voltage (USo u) depending on the voltage difference and the integral of the voltage difference also includes: Determining a load power (P Last ) and / or a source power output by each of the plurality of source power electronics (21b, 22b), Determining, at least for one of the plurality of source power electronics (21 b, 22 b), an expected value of the voltage ([ / ;, U 21b , U 22 b) depending on the number of load powers P Last ) and / or the multitude of source services; Determining a droop voltage ([ / Droop ) depending on at least one expected value; Specifying the DC link voltage (t / Soii ) depending on the voltage difference and the droop voltage (U Droop ).

7. Method according to one of the preceding claims, wherein the determination (S110) of the mean voltage value (U Mitte i) further includes: Filtering (S111) of the mean voltage value (U Mitte i), in particular with a PT1 filter or low-pass filter (110).

8. Method according to one of the preceding claims, wherein the plurality of voltage sources (21a, 22a) are electrical generators, which are driven in particular by an internal combustion engine, and / or wherein the plurality of loads (31a, 32a) are electric motors, which are used in particular to drive a vehicle.

9. A computing unit comprising a processor configured to carry out the method according to any one of the preceding claims.

10. A computer program comprising instructions which, when executed by a computer, cause the computer to carry out the method according to claims 1 to 8.

11. A computer-readable data carrier on which the computer program according to claim 10 is stored.

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