METHOD FOR LOSS-REDUCING OPERATION OF A SYSTEM AND SYSTEM

Abstract

A method for loss-reducing operation of a system (1) in which an Active Front-End AFE (2) supplies a DC bus (3) by power exchange with a connected AC network (7), wherein at least one load (5) is connected to the DC bus (3) via at least one DC / DC converter (4), comprises the following steps: - Acquiring an initial DC voltage value as the operating point of the DC bus (3), a power dissipation value of the AFE (2) and a power dissipation value of the DC / DC converter (4) at a first time point, - Changing the voltage of the DC bus (3) to a second DC voltage value in one direction of change at a second time point in time, - Recording a power dissipation value of the AFE (2) and a power dissipation value of the DC / DC converter (4) at the second time point, and - Maintain the operating point and reverse the direction of change if the sum of the power losses of the AFE (2) and the DC / DC converter (4) at the first time is less than the sum of the power losses of the AFE (2) and the DC / DC converter (4) at the second time, and otherwise set the second DC voltage value as the operating point and maintain the direction of change. A system (1) set up to carry out the procedure is also disclosed.

Description

[0001] The invention relates to a method for loss-reducing operation of a system, and to a system comprising at least one DC load which is connected via a DC / DC converter to a DC bus supplied by an AFE.

[0002] To operate DC loads, such as electrolyzers, industrial robots, or welding equipment, these are typically connected to a DC bus (DC = Direct Current). A DC / DC converter is usually connected between the DC bus and the load to convert the DC bus voltage to the load's operating voltage. To ensure stable power supply conditions for the loads, the DC bus is connected to an AC network (AC = Alternating Current) via an Active Front End (AFE). The AFE handles the conversion between the AC network's AC voltage and the DC bus's DC voltage and is responsible, among other things, for regulating the DC bus voltage to a setpoint within a permissible range. Droop characteristics are typically used for this purpose, where the DC bus voltage is linked to the power to be exchanged with the network.The power exchange between the DC bus and the grid is therefore regulated by the AFE depending on the DC bus voltage, or rather the DC bus voltage results as a function of the total power drawn by all connected loads.

[0003] It is also known that for low-loss operation of the AFE, it is advantageous to select the DC bus voltage only slightly above the peak voltage of the connected AC network. This is only possible to a limited extent when using droop characteristics.

[0004] For low-loss operation of the entire system, which includes not only the AFE but also all loads connected via an associated DC / DC converter, the losses of the DC / DC converters themselves must also be considered. These losses depend, among other things, on the conversion ratio between the DC bus voltage and the operating voltage of the respective load. Even though non-isolated DC / DC converters generally exhibit high and nearly constant conversion efficiency over a wider conversion range, meaning that the aforementioned choice of DC bus voltage already results in relatively low-loss system operation, a different DC bus voltage can further reduce the overall power loss, particularly in systems that also include galvanically isolated DC / DC converters. This is due to a significantly higher level of converter losses and a stronger dependence of these losses on the conversion ratio.However, the optimal operating point with minimal converter losses of the entire system depends heavily on the respective operating power of the loads, which can vary greatly over time, so that the optimal operating point is also variable.

[0005] Accordingly, the object of this invention is to demonstrate an operating method for a system that enables a reduction in the system's power loss, and to provide such a system.

[0006] This problem is solved by an operating method with the features of independent claim 1. Preferred embodiments of the operating method are the subject of the dependent claims. A suitably designed system is described in claim 9.

[0007] In a method according to the invention for the loss-reducing operation of a system in which an active front-end (AFE) supplies a DC bus by power exchange with a connected AC network, wherein at least one load is connected to the DC bus via a DC / DC converter, a first DC voltage value is initially recorded as the operating point of the DC bus, along with a power loss value of the AFE and a power loss value of the DC / DC converter. At a second time point, the voltage of the DC bus is changed to a second DC voltage value in a different direction of change, and a power loss value of the AFE and a power loss value of the DC / DC converter are recorded. If the sum of the power losses of the AFE and the DC / DC converter at the first time point is less than the sum of the power losses of the AFE and the DC / DC converter at the second time point, the operating point is maintained, and the direction of change is reversed.Otherwise, the second DC voltage value is set as the new operating point, and the direction of change is maintained. These steps can be repeated cyclically, allowing the operating point of the DC bus voltage to be shifted continuously to values ​​where the sum of the power losses of the AFE and the DC / DC converter becomes progressively lower compared to the sum of the power losses of the AFE and the DC / DC converter at the initial time point. The operating point can then be specified to the AFE as a setpoint for a control system.

[0008] If multiple loads are supplied via the DC bus using multiple DC / DC converters, the power losses of the DC / DC converters are added together. The power loss of the DC / DC converter(s) can be determined by measuring current values ​​within the converter, such as voltage and current values ​​at the input and output of the DC / DC converter, or it can be estimated based on measured values. The power loss can represent absolute or relative power loss and can preferably be recorded and transmitted together with the power currently being converted to supply the respective load.

[0009] The term "load" here should not be understood restrictively as a mere consumer; rather, the loads can also feed power into the DC bus via the DC / DC converter temporarily or permanently.

[0010] In a preferred embodiment, the measurement of the power loss involves transmitting the power loss value from the DC / DC converter to a controller. The controller can be integrated into the AFE or be a higher-level unit that provides the AFE with specifications for the operating point to be set.

[0011] In a further embodiment, the DC / DC converter also transmits a parameter that is representative of the change in power dissipation when the DC bus voltage changes. This assumes a constant power consumption by the connected load. In this way, it is possible to predict which direction and step size of change in the operating point appear most promising for quickly optimizing power dissipation, and to consider this prediction when setting the second voltage value. Therefore, the second DC voltage value can be determined based on the parameter transmitted at the first time. For example, a larger step size between the first and second DC voltage values ​​can be considered if the parameter indicates that the power dissipation changes significantly with a change in the DC bus voltage.This allows the process to reach operating points with reduced overall power loss more quickly.

[0012] If the converted power at the first time point differs from the converted power at the second time point because the power demand of the loads has changed, it is advantageous to relate the sums of the power losses of the AFE and the DC / DC converter at the first and second time points to the same converted power. This can be done mathematically by appropriate scaling. Alternatively, at a third time point after the second, the first voltage value can be reset, and the power losses of the AFE and the DC / DC converter can be measured again and appropriately averaged between the first and third time points to relate the power losses to the same converted power.

[0013] The method achieves particularly large reductions in power loss through the inventive modification of the DC bus voltage when the at least one DC / DC converter is a galvanically isolated DC / DC converter, and the more DC / DC converters of this type are included in the system. However, it is not necessary for all DC / DC converters in the system to be galvanically isolated DC / DC converters.

[0014] In a further aspect of the invention, a system comprises an AFE, at least one DC / DC converter connected to the AFE via a DC bus and supplying a load, and a controller configured and set up to carry out the method described above. Advantageously, the at least one DC / DC converter is a galvanically isolated DC / DC converter. The controller can be a higher-level controller of the system or be integrated into the AFE.

[0015] The invention is illustrated below with the aid of figures, of which Fig. 1 a system according to the invention and Fig. 2 show a flowchart for a method according to the invention.

[0016] Fig. Figure 1 shows an embodiment of a system 1 according to the invention, in which an AFE 2 is connected to an AC network 7. Depending on the installation conditions, the connection can be made via a transformer or directly, i.e., without a transformer. The AFE 2 is connected on the DC side to a DC bus 3 and regulates the bus voltage by ensuring the necessary power exchange with the network 7 to stabilize the bus voltage. Furthermore, one or more loads 5 are connected to the DC bus 3.

[0017] An adjustment between the operating voltage of the loads 5 and the bus voltage is achieved by a DC / DC converter 4, which is connected to the DC bus 3 on its input side and to the associated load 5 on its output side. The DC / DC converters 4 are shown as galvanically isolated converters, but individual or all converters can also be non-galvanically isolated converters. The individual loads 5 can also feed power into the DC bus 3 continuously or intermittently. Accordingly, depending on the operating situation, the AFE 2 can both draw power from the AC network 7 and feed power into it. The AFE 2 can also perform other grid-supporting functions, such as providing voltage-supporting reactive power or instantaneous reserve, enabling or supporting a black start, and / or providing other grid services.

[0018] A controller 6 has a communicative connection with the AFE 2 for transmitting current operating parameters such as the bus voltage, a power dissipation value of the AFE 2, the converted power, and / or a characteristic value that is representative of a change in the power dissipation value of the AFE 2 when the voltage of the DC bus 3 changes. Simultaneously, the controller 6 can also transmit a setpoint value as an operating point for the bus voltage to the AFE 2. The controller 6 also has a communicative connection with the one or more DC / DC converters 4 for transmitting current operating parameters such as the bus voltage, a power dissipation value of the DC / DC converter 4 or the DC / DC converters 4, the converted power, and / or a characteristic value that is representative of a change in the power dissipation value of the DC / DC converter 5 or the DC / DC converters 4 when the voltage of the DC bus 3 changes.

[0019] Based on the transmitted data, control unit 6 in system 1 is designed and configured to do the following: Fig. 2 procedures shown as a flowchart to be carried out.

[0020] Initially, in a first step S1, a first voltage value of the DC bus is recorded at a first time point as the operating point, along with a power dissipation value of the AFE and a power dissipation value of the DC / DC converter(s). Subsequently, in a second step S2, the voltage of the DC bus is changed to a second voltage value in one direction, and a power dissipation value of the AFE and a power dissipation value of the DC / DC converter(s) are recorded at a second time point.

[0021] In a third step, S3, it is checked whether the sum of the power losses of the AFE and the DC / DC converter(s) at the first time point is less than the sum of the power losses of the AFE and the DC / DC converter(s) at the second time point. If this is the case, the operating point is maintained and the direction of change is reversed. If this is not the case, in a fourth step, S4, the operating point is changed to the second voltage value and the direction of change is maintained.

[0022] In both cases, the procedure can be repeated cyclically by returning to the first step S1, whereby the operating point increasingly approaches the voltage value of the DC bus at which the total power loss of the system, as the sum of the power losses of the converter(s) and the AFE, is minimal. Reference symbol list 1 system 2 Active Front-End (AFE) 3 DC bus 4 DC / DC converters 5 Last 6 Control 7 AC network S1-S4 step

Claims

[1] Method for loss-reducing operation of a system (1) in which an Active Front-End AFE (2) supplies a DC bus (3) by power exchange with a connected AC network (7), wherein at least one load (5) is connected to the DC bus (3) via at least one DC / DC converter (4), comprising the steps: - Acquiring an initial DC voltage value as the operating point of the DC bus (3), a power dissipation value of the AFE (2) and a power dissipation value of the DC / DC converter (4) at a first time point, - Changing the voltage of the DC bus (3) to a second DC voltage value in one direction of change at a second time point in time, - Recording a power dissipation value of the AFE (2) and a power dissipation value of the DC / DC converter (4) at the second time point, and - Maintain the operating point and reverse the direction of change if the sum of the power losses of the AFE (2) and the DC / DC converter (4) at the first time is less than the sum of the power losses of the AFE (2) and the DC / DC converter (4) at the second time, and otherwise set the second DC voltage value as the operating point and maintain the direction of change. [2] Method according to claim 1, wherein the steps are repeated cyclically. [3] Method according to any of the preceding claims, wherein the at least one DC / DC converter (4) is a galvanically isolating DC / DC converter (4). [4] Method according to one of the preceding claims, wherein the detection of the power loss quantity comprises transmitting the power loss quantity through the DC / DC converter (4) to a controller (6). [5] Method according to claim 4, wherein the converted power is transmitted together with the power loss quantity. [6] Method according to claim 4 or 5, wherein the DC / DC converter further transmits a characteristic parameter that is representative of a change in the power loss magnitude when the voltage of the DC bus (3) is changed. [7] Method according to claim 6, wherein the controller (6) determines the second DC voltage value depending on the parameter transmitted at the first time. [8] Method according to any one of claims 4 to 7, wherein the sums of the power losses of the AFE (2) and the DC / DC converter (4) at the first and second time points are related to the same converted power. [9] System (1) comprising an AFE (2), at least one DC / DC converter (4) connected to the AFE (2) via a DC bus (3) and supplying a load (5), wherein a control (6) is provided and set up for carrying out the method according to one of the preceding claims. [10] System (1) according to claim 9, wherein the at least one DC / DC converter (4) is a galvanically isolating DC / DC converter (4). [11] System (1) according to claim 9 or 10, wherein the control (6) is a superior control of the system. [12] System according to claim 9 or 10, wherein the control (6) is integrated into the AFE (2).

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