Method for operating a battery converter, battery converter and system

Abstract

A method for operating a battery converter (4) in a system (1), wherein, in addition to the battery converter (4), an inverter (2) connected to a power grid (7) and a DC load (6) are connected via a DC bus (8) to a common intermediate circuit (3), the method comprising the following steps: - adjusting the conversion power of the battery converter (4) in relation to an inverter characteristic curve (10) depending on the voltage of the intermediate circuit (3), - identifying a drop in the intermediate circuit voltage below a rectified value (12) of the permissible AC voltage of the power grid (7) connected to the inverter (2), and - if said drop is identified, temporarily shifting the inverter characteristic curve so that the maximum discharge power of the battery converter (4) is achieved at a value of the intermediate circuit voltage which is above the rectified value (12) or which is said rectified value. Furthermore, a battery converter (4) which is set up to carry out the method and a system having the battery converter (4) are described.

Description

Technical Field

[0001] The present invention relates to a method for operating a battery converter, a battery converter configured for implementing the method, and a system having the battery converter. Background Technology

[0002] To operate DC loads in systems, such as industrial production equipment, these loads are powered via a common DC bus. This DC bus is typically connected to the AC grid via an inverter to ensure the power conversion required to operate the DC loads. The inverter ensures that the DC bus voltage, as the intermediate circuit voltage connecting the inverter to the DC bus, is maintained within an allowable voltage range around a predetermined DC rating. Additionally, the system typically includes a battery converter that provides a short-term regulation margin for voltage fluctuations exceeding the inverter's capacity, or that this margin is induced by the inverter achieving additional regulation purposes beyond regulating the DC bus voltage, particularly providing grid services, such as frequency or voltage stabilization of the connected grid. The battery converter typically determines the conversion power with the DC bus using predetermined converter characteristic curves, which predetermine the rated conversion power as a function of the intermediate circuit voltage. The battery converter's control unit then adjusts the conversion power accordingly. Converter characteristic curves typically have a dead zone around the DC bus voltage at the DC rating, during which the battery converter does not operate. Within the framework of the aforementioned disclosure, the concept of a DC load is not only understood as a purely electrical appliance, but more precisely, also includes components that temporarily or continuously transfer power to the DC bus.

[0003] To achieve efficient system operation, it is desirable that the DC bus voltage's DC rating be only slightly higher than the rectified value of the AC grid connected to the inverter, as this reduces the inverter's converter losses.

[0004] Meanwhile, there is an increasing demand for the aforementioned systems to provide grid support services for the power grid. As part of this grid support function, the inverter needs to attempt, at least for a predetermined time, to support the grid by providing a relatively high current with a phase-corrected input when the grid voltage collapses, restoring the grid voltage to its grid-configured values ​​(e.g., voltage amplitude, frequency). This behavior is called FRT (Fault-Ride-Through). In the case of the inverter, this is achieved by also regulating the DC bus voltage on the DC side. According to the prior art, this results in significant discharge of the intermediate circuits, and the DC bus voltage drops below the rectified value of the grid voltage rating. When the DC rating is only slightly above the rectified value, the battery converter, which is controlled by conventional converter characteristic curves, cannot adequately resist this drop in DC bus voltage.

[0005] When the DC bus voltage drops below the rectified value, the sudden and completely reversed grid voltage will cause a large current to flow uncontrollably through the idle diodes of the inverter's bridge circuit into the intermediate circuit of the DC bus discharge, thereby damaging them. Summary of the Invention

[0006] Therefore, the object of the present invention is to adjust the regulation of the battery converter in a system comprising an inverter that regulates voltage, a DC load, and a battery converter with a connected battery, so that a sudden reverse of the grid voltage in the case of FRT does not damage the system.

[0007] This task is solved by the method according to the invention and the battery converter according to the invention, or the system according to the invention. Preferred embodiments are also described.

[0008] In one aspect, a method for a battery converter in an operating system according to the invention is proposed, wherein, in addition to the battery converter, an inverter connected to the power grid and a DC load are also connected to a common intermediate circuit via a DC bus, the method comprising the following steps:

[0009] - The conversion power of the battery converter and the battery connected to the battery converter is adjusted according to the voltage of the intermediate circuit, based on the converter characteristic curve.

[0010] - Identify the drop in intermediate circuit voltage below the permissible rectified AC voltage value of the grid connected to the inverter, and

[0011] - If the drop is detected, the converter characteristic curve is temporarily shifted to achieve the maximum discharge power of the battery converter at the following values ​​of the intermediate circuit voltage, which are above or equal to the rectified value.

[0012] By identifying the drop, the battery converter can independently and without explicit signal from the outside that the system may be facing a fault-tolerant reversal (FRT) condition. In this condition, the battery converter's regulation behavior is altered so that it attempts to maintain the DC bus voltage within a predetermined time period that is above the rectified value of the allowable AC voltage of the connected grid. This overcomes the risk of inverter damage caused by a sudden reversal of the allowable grid voltage after the FRT condition ends. In normal operation, i.e., directly before the drop is identified, the inverter's task is to maintain the DC bus voltage within the allowable voltage range by providing the corresponding conversion power between the DC bus and the grid. The battery converter supports this regulation using its converter characteristic curve.

[0013] In a preferred embodiment, upon recognizing the drop, the converter characteristic curve is shifted such that the battery discharges at maximum power for an intermediate circuit voltage that is less than or equal to a critical voltage at least 10% above the rectified value. This provides an additional safety margin, for example, preventing a brief voltage amplitude exceeding permissible values ​​when the grid voltage reverses. It also better compensates for power consumption losses due to brief spikes in the DC bus caused by DC loads, inverters, or other components.

[0014] To further improve the battery converter's regulation margin during its response to a drop in bus voltage, the converter characteristic curve can be shifted such that the maximum discharge power of the shifted converter characteristic curve is at least 20%, preferably at least 50%, above the battery converter's maximum permissible continuous discharge power. Because the maximum discharge power is only provided briefly, the battery converter can withstand the overload without damage.

[0015] Preferably, the converter characteristic curve has a dead zone around the DC rating of the intermediate circuit voltage before shifting, and the shifted converter characteristic curve is dead zone-free. In this case, a dead zone in the shifted characteristic curve only worsens the converter's regulation characteristics or unnecessarily widens the voltage range within which the battery converter regulates based on the shifted characteristic curve, whereas a dead zone during normal operation results in significant unloading of the battery converter and the battery connected to it.

[0016] Advantageously, the converter characteristic curve shifts during a first duration, which is selected based on a second duration of FRT (Power Transfer Rate) for the system, specifically selected to be greater than or equal to the second duration. This ensures that the battery converter provides DC bus support functionality throughout the FRT period. The second duration of the FRT period includes not only the phase where the inverter actively attempts to support the grid and appropriately exchanges power with it, but also, if necessary, the phase where the inverter passively maintains connection with the grid, for example, without a bridging switch actively operating at a clockwise speed. This passive phase can also reflect the FRT situation without a previous active support phase and last for several minutes. When the grid reverses during this duration, the inverter can be reactivated without delay through the necessary grid connection process again, providing appropriately regulated power conversion with the grid.

[0017] After the second duration, the FRT condition is successfully eliminated, or the inverter is disconnected from the connected grid. In this manner, it is possible to continue operating the system for at least a certain period of time by means of a battery converter. This continued operation can, in principle, be supported by the battery converter using the shifted characteristic curves, but particularly by the original converter characteristic curves after the inverter is disconnected from the grid.

[0018] In another aspect of the invention, a battery converter is proposed, which, in addition to an input terminal for connection to a battery, includes a control device configured to provide converted power to an output terminal based on the voltage applied to the output terminal according to a converter characteristic curve. The control device is further configured to detect a drop in the voltage applied to the output terminal below a rectified value of the permissible AC voltage of the power grid connected to the inverter, and if such a drop is detected, to temporarily shift the converter characteristic curve so that the maximum discharge power of the battery converter is achieved at a voltage applied to the output terminal that is above or equal to the rectified value. This achieves the advantages described within the framework of the foregoing method.

[0019] The present invention also proposes a system comprising a DC load, an inverter, and the aforementioned battery converter. The DC load, inverter, and battery converter are connected to a common intermediate circuit via a DC bus. In the system according to the invention, the battery converter is connected to the common intermediate circuit via the DC bus at its output terminal. Furthermore, the DC load and inverter are connected to the intermediate circuit. The inverter is specifically configured here to be connected to the power grid on its output side, and the intermediate circuit or DC bus is supplied with conversion power through the power grid in such a way that the DC bus operates within a permissible voltage range. The battery converter here specifically supports the inverter in that it provides additional conversion power to the connected battery according to a predetermined converter characteristic curve. Direct communication between the inverter and the battery converter to achieve a common regulation target is not required here. Attached Figure Description

[0020] The invention is described below with reference to the accompanying drawings, in which:

[0021] Figure 1 A system for implementing the method according to the invention is shown.

[0022] Figure 2 A flowchart of the method according to the present invention is shown, and

[0023] Figure 3 The converter characteristic curves and their shifts are shown within the framework of an embodiment of the method according to the invention. Detailed Implementation

[0024] Figure 1 An embodiment of a system 1 according to the invention, having an inverter 2, is shown. The inverter is connected to the power grid 7 on the AC side and to a DC bus 8 on the DC side via an intermediate circuit 3. In the illustrated example, the intermediate circuit 3 is part of the inverter; however, it could also be a separate component of the system 1. Furthermore, a DC load 6 is connected to the DC bus 8, through which electrical power is supplied. To compensate for fluctuations in the power consumption of the load and the associated voltage fluctuations on the DC bus 8, a battery 5 is also connected to the DC bus 8 via a battery converter 4. The battery converter 4 is controlled via a converter characteristic curve that predetermines a rated power conversion value for the battery 5 for each value of the DC bus voltage. The conversion power of the battery converter 4 is adjusted by a control device corresponding to the rated value. The converter characteristic curve need not be constant in time, but can be adjusted, for example, as a function of the state of charge of the battery 5, to achieve a desired state of charge for the battery 5.

[0025] Furthermore, the control device is configured to detect when the voltage applied to the DC bus 8 drops below the rectified value of the permissible AC voltage of the grid 7 connected to the inverter 2. If such a drop is detected, the converter characteristic curve is temporarily shifted so that the maximum discharge power of the battery converter 4 has been reached at a voltage applied to the output terminal that is above or equal to the rectified value 12. This drop is triggered by an FRT condition, a specific operating condition of system 1. The duration during which the characteristic curve of the battery converter is used to control the conversion power between the battery converter and the DC bus after the shift can preferably be selected corresponding to a duration during which the inverter 2 attempts to implement FRT, for example based on a predetermined value from the grid provider, to compensate for the voltage drop in the grid. The battery converter 4 supports the implementation of FRT in this manner without requiring direct communication between the inverter 2 and the battery converter 4.

[0026] exist Figure 2 The flowchart of the method according to the present invention is shown. In the first step S1, the conversion power of the battery converter and the battery connected to the battery converter is adjusted according to the voltage of the intermediate circuit, corresponding to the converter characteristic curve. This corresponds to the normal operating mode of the battery converter. In the second step S2, the battery converter checks the voltage at its output terminal connected to the intermediate circuit to see if there is a drop in the intermediate circuit voltage below the rectified value of the permissible AC voltage of the power grid connected to the inverter. If no such drop is identified, the method returns to the first step S1.

[0027] If the drop is detected, in the third step S3, the converter characteristic curve is shifted by a predetermined voltage value, thereby achieving the conversion power according to the characteristic curve while the intermediate circuit voltage increases by the predetermined voltage value. In the fourth step S4, before the converter characteristic curve returns to its original characteristic curve in normal operating mode and the method returns to the first step S1, the battery converter operates for a predetermined time using the shifted characteristic curve. The predetermined time can be selected corresponding to the maximum duration, which the voltage-regulated inverter operates in FRT mode for the maximum duration, so that the battery converter supports the intermediate circuit voltage by means of the shifted converter characteristic curve throughout the FRT condition. If other reasons also cause a temporary drop in the intermediate circuit voltage, a temporary shift of the characteristic curve is implemented.

[0028] exist Figure 3The converter characteristic curve 10 is shown, where the rated conversion power P is expressed as a function of the voltage U at the output terminal of the battery converter to be connected to the DC bus. The converter characteristic curve 10 has a dead zone around the DC rated value 14 with constant power (i.e., zero in this case), bordered by regions where the conversion power P increases with increasing voltage in the direction of increasing charging power (or decreases with decreasing discharging power). These regions terminate at the maximum permissible continuous charging power or, for decreasing voltage, at the maximum permissible continuous discharging power, which are not exceeded. This yields a voltage range with varying conversion power of the battery converter, where the rectified value 12 of the permissible AC voltage of the connected power grid is within this voltage range in the illustrated case.

[0029] When the battery converter detects a drop in voltage at its output terminals, it temporarily replaces converter characteristic curve 10 with the shifted characteristic curve 11. The shifted characteristic curve 11, relative to converter characteristic curve 10, is characterized in that, for each value of the conversion power P between the maximum permissible continuous charging power and the maximum permissible continuous discharging power, the configured value of the DC bus voltage U of the shifted characteristic curve 11 is greater than, or in optimal cases equal to, the value of converter characteristic curve 10. Simultaneously, the maximum discharge power of the battery converter has been reached at a DC bus voltage value that is above or equal to the rectified value 12. The battery converter operates in this manner within its framework against the possibility of the DC bus voltage dropping below the rectified value 12.

[0030] Unlike converter characteristic curve 10, the shifted characteristic curve 11 does not have a dead zone. Optionally, the shifted characteristic curve may have a maximum discharge power, which is increased relative to the maximum permissible continuous discharge power by overload 13. Because the shifted characteristic curve 11 is only used for a predetermined duration, the DC bus voltage can be additionally supported by overload 13 without the risk of damage to the battery converter. Preferably, the voltage at which the shifted characteristic curve 11 achieves maximum discharge power is within the permissible voltage range of the DC bus.

[0031] Figure 3The characteristic curves shown are merely examples of variations of the converter characteristic curve and the shifted characteristic curve. The variation curves are not necessarily required to have linearly extended segments, and the slope of the shifted characteristic curve does not necessarily correspond to the slope of the converter characteristic curve. For each value of the conversion power P between the maximum permissible continuous charging power and the maximum permissible continuous discharging power, the configured voltage U of the shifted characteristic curve is above the configured voltage U of the converter characteristic curve. This is indicated by the arrow at shift 15. Alternatively, for each voltage value U of the two characteristic curves, the configured power value P of the shifted characteristic curve has a power value that shifts relative to the converter characteristic curve along the direction of enhanced discharge or reduced charging, or, optimally, the two characteristic curves have the same power value when the power value P corresponds to the maximum permissible continuous charging power or the maximum permissible continuous discharging power. This is indicated by the arrow at shift 16.

[0032] List of reference numerals

[0033]

Claims

1. A method for operating a battery converter (4) in a system (1), wherein, In addition to the battery converter (4), the inverter (2) and the DC load (6) connected to the power grid (7) are also connected to a common intermediate circuit (3) via a DC bus (8), wherein the method includes the following steps: - Corresponding to the converter characteristic curve (10), the conversion power of the battery converter (4) and the battery (5) connected to the battery converter (4) is adjusted according to the voltage of the intermediate circuit (3). - Identify the drop in intermediate circuit voltage to the allowable AC voltage (12) below the rectified value of the grid (7) connected to the inverter (2), and - If the drop is detected, the converter characteristic curve is temporarily shifted so that the maximum discharge power of the battery converter (4) is achieved at the following values ​​of the intermediate circuit voltage, which are above or equal to the rectified value (12).

2. The method according to claim 1, wherein, The converter characteristic curve (10) is shifted such that the battery is discharged at maximum discharge power for the intermediate circuit voltage, which is less than or equal to a critical voltage at least 10% above the rectified value (12).

3. The method according to claim 1 or 2, wherein, The converter characteristic curve (10) has a dead zone around the DC rating (14) of the intermediate circuit voltage before the shift, and the converter characteristic curve after the shift is dead zone-free.

4. The method according to any one of the preceding claims, wherein, The converter characteristic curve (10) is shifted so that the maximum discharge power of the shifted converter characteristic curve (11) is at least 20% above the maximum allowable continuous discharge power of the battery converter (4).

5. The method according to claim 4, wherein, The maximum discharge power of the converter characteristic curve (11) after the movement is at least 50% above the maximum allowable continuous discharge power of the battery converter (4).

6. The method according to any one of the preceding claims, wherein, The converter characteristic curve (10) moves within a first duration, which is selected based on a second duration for fault ride-through of the system (1).

7. The method according to claim 6, wherein, The first duration is selected to be greater than or equal to the second duration.

8. A battery converter (4) having an input terminal for connection to a battery (5) and a control device configured to provide conversion power to an output terminal based on a voltage applied to the output terminal according to a converter characteristic curve (10), wherein, The control device is further configured to identify a drop in the voltage applied to the output terminal below the rectified value (12) of the allowable AC voltage of the grid (7) connected to the inverter (2), and if the drop is identified, to temporarily shift the converter characteristic curve so as to achieve the maximum discharge power of the battery converter (4) at the following values ​​of the voltage applied to the output terminal, which are above or equal to the rectified value (12).

9. A system (1) comprising: - DC load (6) - Inverter (2) and - The battery converter (4) according to claim 8. The DC load (6), the inverter (2) and the battery converter (4) are connected to a common intermediate circuit (3) via a DC bus (8).

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