DC Switch

Abstract

The invention relates to a DC switch for controllably disconnecting a DC line, which is designed to determine the inductance present on the load side by switching on the DC switch within a determinable time period that is significantly shorter than 1 second and calculating the inductance at least as a function of the current intensity reached, the applied input voltage and the length of the time period.

Description

Technical Field

[0001] The invention relates to a DC switch for disconnecting a DC link, comprising at least one semiconductor switch arranged in a current path of the DC link and a control device for the semiconductor switch. Background Art

[0002] Semiconductor switches are technically useful as switches for DC power grids because they can quickly interrupt DC current without arcing. They are typically positioned directly in the current path. Therefore, in the switched-on state, they carry the entire load current and remain continuously switched on during normal operation. If a fault occurs in the power grid to be protected, the semiconductor switch must be able to interrupt it.

[0003] Two load types are problematic here: capacitive loads with either very low or very high feeder line inductances. In DC grids with capacitive loads and without high feeder line inductances, very high current growth rates occur. If the current growth rate is so high that the maximum controllable current of the semiconductor switch is exceeded within the given switch disconnection delay (caused by dead time in current measurement, disconnection time delay, etc.), the semiconductor switch is no longer able to disconnect the fault. This can lead to damage to the DC switch. Therefore, DC switches are often set with a minimum feeder line inductance specified in their data sheets.

[0004] DC grids with very high feeder line inductances represent the other extreme. Here, the limiting factor is not the switch's turn-off delay, but the energy stored in the feeder line inductance. To reduce this energy, DC switches often have a protective network for overvoltage limitation, also referred to as a discharge network. The energy stored in the feeder line inductance is converted here, for example, into heat (using varistors) or stored (for example, in capacitors). Because the discharge network has a limited load capacity, a maximum feeder line inductance is often set for DC switches.

[0005] In real systems, it is often unclear what the installed feeder line inductance is. Consequently, it is also unclear what load the switch should be subjected to during a switching operation. The load can only be determined through complex on-site measurements or short-circuit tests based on trial and error. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a DC switch in which the disadvantages mentioned at the outset are reduced. In particular, the DC switch should ensure that it can disconnect without damage. This technical problem is solved by a DC switch having the features according to the present invention.

[0007] A DC switch for controllably disconnecting a DC link according to the present invention comprises: at least one power semiconductor switch arranged in a current path of the DC link, a device for measuring an input voltage and an output voltage, a device for measuring a current flowing through the DC link, and a control device for the power semiconductor switch.

[0008] The control device is designed to switch on the DC switch during a first time period, determine the input voltage applied on the supply side, determine the output voltage on the load side present at the end of the first time period, determine the current intensity present at the end of the first time period, and determine the present inductance and / or capacitance from the determined values.

[0009] Here, the switching on of the DC switch means switching from the blocking state of the DC switch to the conducting state. To this end, the power semiconductor switch is switched on, ie, it is switched into the conducting state.

[0010] One of the connections of a DC switch is called the supply side. This refers to the side to which the voltage source is connected, meaning that the voltage level applied on this side is normally only slightly affected by the switching state of the DC switch. The other side is called the load side and is the side to which electrical consumers, such as an electric motor fed by a converter, are connected. The voltage level on the load side can be directly dependent on the switching state of the DC switch and drop to zero when the DC switch is open.

[0011] The distinction between the load side and the supply side is for illustrative purposes. In real DC grids, installations can be more complex. For example, multiple consumers and / or generators may be connected to the load side. On the supply side, there may be a whole network of other DC switches, loads, and generators. Consequently, the installation can even be symmetrical, making it impossible to distinguish between the supply side and the load side. Therefore, the two terms refer to the two connection sides of the DC switch and are interchangeable.

[0012] The present invention has recognized that a suitable control sequence for a DC switch, which includes briefly switching on the DC switch, allows for recording measured values ​​that allow the load-side inductance to be determined. It has also been recognized that the necessary control capabilities and the necessary measuring devices for the brief switching on, namely a voltage measuring device for the load-side voltage and a current measuring device for the current flowing through the DC switch, are typically already present in such switches. Advantageously, therefore, measurements can even be performed without structural changes in a suitably equipped DC switch. If the control device knows the currently effective inductances, i.e., on the load side and the load side, by measuring, the DC switch can advantageously be controlled to account for the value of the inductance and avoid damage.

[0013] The present invention provides an advantageous embodiment of a DC switch. Here, the features of different embodiments of the present invention can be combined with each other. Therefore, the DC switch can also additionally have the following features:

[0014] The power semiconductor switches are typically IGBTs, but other types of power semiconductor switches may also be used.

[0015] A DC switch can include two power semiconductor switches connected in anti-series. This makes the DC switch bidirectional, meaning it can interrupt current flow regardless of the direction of the current flow. In contrast, a DC switch consisting of only one power semiconductor switch can typically only interrupt current in one direction.

[0016] The control device can be designed to use a duration determined before switching on as the first time period. In other words, the DC switch is switched on at the switch-on time and switched off again at the predetermined switch-off time. In this case, the development of current and voltage has no influence on the duration of the switch-on period.

[0017] The control device can also be designed to shorten the first time period when the current intensity reaches a current threshold. In other words, the DC switch is switched on at the switch-on time and switched off again at an unspecified switch-off time. In this case, the switch-off time is derived from the curve of the current flowing through the DC switch. Advantageously, this allows for more precise measurement of the existing inductance, as the current at the switch-off time cannot be arbitrarily low. The current threshold is preferably selected to be sufficiently high so that existing measurement inaccuracies do not dominate.

[0018] However, a prerequisite for operating within a suitable time range is that the inductance present is not too great, as otherwise the first time period could become very long. Therefore, it is also advantageous to use a maximum duration for the first time period, which the first time period does not exceed even if the current intensity does not reach the current threshold. In this case, therefore, not only is a predetermined on-duration used, but a disconnection is also performed when the current threshold is reached. The maximum duration is advantageously less than 1 ms, in particular less than 10 μs.

[0019] The control device can be designed to calculate the existing inductance according to the following formula:

[0020]

[0021] in:

[0022] -L zuleitung is the inductor;

[0023] -U is the driving voltage;

[0024] -t pulsis the first time period, and

[0025] -I peak is the current intensity present at the end of the first time period.

[0026] In this case, the drive voltage is, in a first approximation, the voltage applied on the supply side. In an advantageous refinement of the invention, the control device can be designed to also determine the voltage applied on the load side (also referred to herein as the output voltage) and, to increase accuracy, subtract this from the voltage applied on the supply side to determine the drive voltage.

[0027] The control device can also be designed to determine whether the supply voltage is present and otherwise not perform a measurement. If the supply voltage is not present, the inductance cannot be determined and all results and conclusions drawn from the measurement are invalid.

[0028] The control device can be designed to determine the output voltage present at the beginning of the first time period. This can improve measurement accuracy, as the voltage applied on the load side can vary significantly during the on-time. The output voltage can then be averaged.

[0029] The control device can be designed to compare the determined existing inductance with the maximum and minimum permissible inductances and output a signal indicating whether the determined inductance lies between the permissible inductances. This advantageously allows for direct visual inspection of the device to determine whether the installation is normal, i.e., whether the DC switch is suitable for disconnection. Installation situations in which disconnection could damage the DC switch are thus immediately apparent and can be corrected.

[0030] The control device can be designed to compare the determined current intensity with a threshold value at the time of disconnection, i.e., after the first time period has elapsed, and to redetermine the existing inductance based on the result of the comparison, wherein a second time period, which is longer than the first time period, is used instead of the first time period. This ensures sufficient measurement accuracy because the inductance is determined not using an excessively low current value reached from the first time period, but rather a sufficiently high current value. If the current value reached in the second time period is also too low, the measurement can be continued with another, longer time period.

[0031] The control device can be designed to determine the effective inductance and / or load-side capacitance before each switching operation. This ensures operational reliability even with changes in the installation situation or load. Advantageously, the time period for the measurement lies in the μs to ms range, so that multiple measurements only slightly delay the switching operation.

[0032] To further increase the measurement accuracy, the control device can be designed to perform the determination of the existing inductance multiple times and to determine an average inductance, thereby reducing the noise component of the measurement in the result.

[0033] Furthermore, the control device can be designed to determine the effective capacitance on the load side. This is advantageous because the capacitance must be precharged before the DC switch is switched on. Switches are typically designed for the maximum permissible capacitance. Therefore, the control device performs a test before the actual precharging to determine whether the effective capacitance on the load side exceeds the design limit.

[0034] During the switch-on process for determining the inductance and the switch-on process for determining the capacitance, a pre-charging resistor present in the DC switch can be switched on. In this case, instead of one or more power semiconductor switches arranged in the main current path, a further switch arranged in series with the pre-charging resistor is switched on. Because the pre-charging resistor acts in a current-limiting manner, a longer switch-on duration can be selected in this case than during the switch-on process using power semiconductor switches in the main current path.

[0035] Alternatively, however, the switching-on process can also be carried out using power semiconductor switches in the main current path.

[0036] The control device can be designed to calculate the capacitance present on the load side according to the following formula:

[0037]

[0038] Here

[0039] -C load is the capacitor;

[0040] -Q load1 It is the amount of charge input to the load side in the connected state;

[0041] -Q load2 The amount of charge on the load side after switching to the off state, and

[0042] - ΔU out It is the change in output voltage caused by the amount of input charge.

[0043] The input charge can be determined according to the following formula:

[0044]

[0045]

[0046] Here:

[0047] -t0 is the time of connection;

[0048] -t off is the disconnection time point, and

[0049] t1 is the time at which the current intensity reaches zero again after disconnection.

[0050] For determining the capacitance, the time after the disconnection, during which the current has not yet dropped to 0 A again, is therefore preferably also taken into account.

[0051] In this case, the control device can be designed to determine the input charge quantity by numerical integration of the current intensity.

[0052] The control device can be designed to compare the determined load-side capacitance with the maximum permissible capacitance and output a signal indicating whether the determined capacitance is greater than the maximum permissible capacitance. This ensures that it is immediately detected when the capacitance connected to the load side exceeds the precharging capacity of the DC switch. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] The present invention will be described and explained in more detail below with reference to the accompanying drawings in conjunction with embodiments.

[0054] Figure 1 shows a DC grid having a supply-side section and a load-side section, wherein the sections are connected by a DC switch,

[0055] Figure 2 shows a DC switch with power semiconductor switches,

[0056] Figure 3 A diagram illustrating a measurement cycle performed by a DC switch is shown. DETAILED DESCRIPTION

[0057] Figure 1 A DC grid 10 is shown. The DC grid 10 is divided into a supply-side portion 11 and a load-side portion 12 by a DC switch 20. In this example, the supply-side portion 11 comprises a DC voltage source 14 that provides an exemplary voltage of 650 V. This voltage is applied to the DC switch 20 on the input side.

[0058] Load-side section 12 includes a load in the form of an electric motor 16, upstream of which is connected an inverter 17. Inverter 17 in turn includes a DC voltage intermediate circuit having a capacitor 171 and a half-bridge connected in parallel with capacitor 171, by means of which the three-phase operating voltage for electric motor 16 is generated from the applied DC voltage.

[0059] The load formed by inverter 17 and motor 16 appears as a capacitive load to DC switch 20 via capacitor 171, which forms the DC voltage intermediate circuit. The feed line 18 between DC switch 20 and inverter 17, in turn, has an inductance. As mentioned at the outset, the inductance of feed line 18 must typically be within a predetermined range between a minimum and a maximum inductance, for example, between 4 μH and 80 μH, in order to ensure reliable disconnection by DC switch 20. The capacitance of capacitor 171 must also not exceed a maximum capacitance value, for example, 40 mF, because otherwise the DC switch 20 would not be able to reliably precharge capacitor 171.

[0060] Figure 2 More precisely, a DC switch 20 is shown, which is an exemplary embodiment of the present invention. The DC switch 20 includes two power semiconductor switches 21 and 22 connected in anti-series, such as IGBTs, and a freewheeling diode that is usually already installed in the module together with the power semiconductor switches. The power semiconductor switches 21 and 22 are connected in series to the current path of the DC grid 10. Therefore, they carry the entire load current in the on state of the DC switch 20 and directly cause the current flow to be disconnected and the applied supply voltage to be cut off. The DC switch 20 can use the two power semiconductor switches 21 and 22 to disconnect the current in both directions. In other embodiments, the DC switch 20 can also have only a single power semiconductor switch, but the disconnection capacity is thereby limited to one current direction.

[0061] In parallel with the power semiconductor switches 21 and 22, the DC switch 20 includes a discharge network 23. The exemplary discharge network 23 is connected in parallel with the two power semiconductor switches 21 and 22. The discharge network includes a first series circuit consisting of a capacitor 231 and a varistor 225. A second series circuit comprising a resistor 233 and a second capacitor 234 is connected in parallel with the first series circuit. A second resistor 235 is connected in parallel with the second capacitor 234. The midpoints of the first and second series circuits are connected.

[0062] The DC switch 20 has Figure 2 A pre-charging circuit (not shown) allows charging of a capacitor connected on the load side and limits the flowing current via a series-connected resistor in order to avoid erroneous detection of a short circuit and subsequent undesired disconnection.

[0063] On the input side, DC switch 20 includes a measuring device 28 for measuring the input voltage, i.e., the supply voltage in DC voltage network 10. In this example, the supply voltage is 650V. On the output side, DC switch 20 includes a measuring device 29 for measuring the output voltage. In this embodiment, measuring devices 28, 29 are configured as voltage dividers with sigma-delta converters. In the disconnected state and for a sufficiently long time after the last disconnection, this voltage is typically 0V. However, for a sufficiently long time after the connection, i.e., during continuous operation of the load, this voltage corresponds approximately to the input voltage. The difference with respect to the input voltage is the voltage drop across the two conducting power semiconductor switches 21, 22. DC switch 20 also includes a measuring device 30 for determining the current flowing through DC switch 20. Measuring device 30 is, for example, embodied as a magnetoresistive current sensor.

[0064] The measuring devices 28 ... 30 are connected to a control device 25, which receives and processes the determined signals. Furthermore, the control device 25 also controls the power semiconductor switches 21 , 22 via signals at their respective gate terminals. For this purpose, the control device 25 is connected to a higher-level controller and, based on its signals, switches the power semiconductor switches 21 , 22 on, i.e., conducts, and off. In this document, the DC switch 20 is said to be off or switched off when both power semiconductor switches 21 , 22 are off, i.e., not conducting. When both power semiconductor switches 21 , 22 are on, i.e., conducting, the DC switch 20 is said to be on. Furthermore, the control device 25 is also designed to automatically switch off in certain circumstances. This may be necessary, for example, when a short circuit on the load side is detected.

[0065] Typically, in a real DC grid 10, the value of the load capacitance (here, the capacitance of capacitor 171) and the length and inductance of feeder line 18 are unknown. If the inductance of feeder line 18 exceeds a specific value known for DC switch 20, discharge network 23 can no longer adequately limit the overvoltage that occurs after disconnection. Consequently, damage to power semiconductor switches 21, 22 can no longer be ruled out. Furthermore, DC switch 20 is designed only for precharging the maximum load capacitance, which must also be adhered to. If the values ​​of inductance and capacitance are unknown, operational reliability can only be ensured with additional effort, such as by estimating or manually measuring the required values.

[0066] In order to solve this problem, the control device 25 is further designed to execute the test process according to a trigger, such as a button on the device or an electronically transmitted signal, such as a signal from a tablet computer.

[0067] The test procedure presupposes that DC switch 20 is disconnected and the supply voltage is applied to the input side. If this is the case, control device 25 switches DC switch 20 on for a first time period at the switch-on time and, after the first time period has expired, switches DC switch 20 off again at the switch-off time. The first time period is selected to be very short, for example, 4 μs.

[0068] At least at the disconnection time, the current intensity of the current flowing through the DC switch 20 and the output voltage are determined. In addition, the current curve between the connection time and the disconnection time can also be determined and recorded. The output voltage at the connection time can also be determined.

[0069] Figure 3 An exemplary curve of current intensity 31 in A, supply voltage 32, and output voltage 33 for such a test process is shown. At the switch-on time 34 (here, set to time t=0 for simplicity), DC switch 20 is switched on and switched off again at the switch-off time 35. Supply voltage 32 remains constant for a longer period and is constant in the short time period shown. Due to the inductance, the current intensity does not increase directly to its maximum value, but rather increases. This increase is usually nonlinear. However, since the current only reaches a small fraction of its maximum value in the short first time period, the increase during this time is actually linear. After the switch-off time, the current drops to zero (again at a finite current rate of change). The output voltage also increases linearly during this time, with the value reached at the switch-off time being smaller than the input voltage.

[0070] From the well-known relationship between voltage U, inductance L, current i and time t

[0071]

[0072] The existing inductance L of the feed line 18 can be determined using the variables determined during the test. zuleitung To do this, convert the formula to

[0073]

[0074] Here, t Puls Indicates the on-time duration, i.e. the first time period. peak is the current intensity at the time of disconnection, and U is the drive voltage. In a simplified variant, the magnitude of supply voltage 32 can be used as the drive voltage. This simplification only represents a small error as long as the output voltage obtained during the first time period remains small relative to the input voltage. In this case, the inductance can be determined knowing the input voltage, the current intensity at the time of disconnection, and the length of the first time period.

[0075] A more precise determination can be made by determining the drive voltage according to the following method:

[0076]

[0077] Here, U in is the supply voltage, U out0 is the output voltage present at the switch-on time, and U out1 is the output voltage present at the time of disconnection.

[0078] Assume that Figure 3 The exemplary values ​​shown in FIG. 1 are that the supply voltage is 650 V, the first time period is 4 μs, and U out0 is 0V and U out1 For 3V, the inductance of the feed line is obtained:

[0079]

[0080] The values ​​of the current intensity and output voltage obtained during the measurement depend on the supply voltage, the inductance, and the selected duration. In the case of certain combinations of these values, for example, very high inductances or very short selected durations, the measured values ​​are too small and therefore too inaccurate to reliably determine the inductance.

[0081] In these cases, the control device 25 is designed to compare the measured current intensity with a threshold value. If the current intensity falls below the threshold value, a new measurement is performed, wherein a second time period, which is longer than the first time period, is used as the switch-on duration, i.e., the interval between the switch-on and switch-off times. For example, a time period of 6 μs can be used for the new measurement.

[0082] In particular, the control device 25 can be designed to increase the used on-time in steps of, for example, 2 μs, until the measured current intensity exceeds a threshold value or until a maximum on-time is reached. If the current intensity reached exceeds the threshold value, the measurement result can be used to determine the inductance of the feeder line. Because individual measurements are very short, even a large number of measurements requires less than 1 ms. Therefore, instead of a first time period of 4 μs, a first time period so short as 500 ns can be used that only very low current intensities are achieved in any case. This ensures that the test process is harmless even in unusual installation situations, and that, in subsequent longer measurements, a current intensity that allows the inductance to be determined can be achieved.

[0083] The result can be further processed, for example, by electronically transmitting the determined inductance or directly outputting it to a display. Alternatively, a comparison can be performed with the values ​​of the maximum and minimum inductances of the feeder line 18 stored in the DC switch 20. If the determined inductance lies between the minimum and maximum inductances, a corresponding signal can be output, for example, by lighting a green LED. In another case, a warning signal can be output. Furthermore, the control device 25 can be designed to prevent the DC switch 20 from being switched on for active operation if a negative result is found.

[0084] If the maximum switch-on time is reached, either the effective inductance is too great and therefore not operable, or the input voltage is too low. Consequently, the inductance cannot be determined or can only be determined too inaccurately, and a warning signal can be output again, preventing the DC switch 20 from switching on.

[0085] To achieve further improved measurement accuracy, in certain embodiments of the DC switch 20, the control device 25 can be designed to repeat the measurement without changing the settings, in particular without changing the switch-on duration. The collected and stored results can then be averaged to reduce the influence of fluctuations in the measurement results.

[0086] Typically, the measuring device is designed to determine the input and output voltages and the current intensity as a function of the rated values ​​of the DC switch 20 and therefore provides inaccurate results at relatively small values ​​achieved within the scope of the test process.

[0087] Therefore, in the embodiment described here, control device 25 is also designed to perform the second test procedure when the inductance of feeder line 18 is at least as great as the minimum inductance. Because the inductance of feeder line 18 is sufficiently great, it is no longer necessary to place as much emphasis on the shortest possible on-time in order to avoid excessive currents. Therefore, in the second test procedure, emphasis can be placed on achieving the minimum current intensity, thereby enabling a more accurate determination of the inductance.

[0088] Therefore, in the second test process, in contrast to the first test process, the current intensity is not predefined, but rather the switch-on duration. This predefined current intensity can be selected so that measurement inaccuracies play only a minor role. The current intensity expected during the third time period, given the maximum inductance of the feeder line, can be calculated using the input and output voltages before the test. This value is stored as a reference value in a comparison unit, such as a comparator. This also has the advantage that, in some cases, the reference value can be set more accurately than the current measurement itself. DC switch 20 is switched on for a maximum of the third time period. If the current reaches the calculated value before the end of the third time period, DC switch 20 is immediately disconnected and the elapsed switch-on duration is stored. If the calculated current intensity value is not reached, the current value reached is used as the result.

[0089] In both cases, the formula (2) already introduced can be applied. If the current has reached the calculated value before the end of the third time period, the elapsed switch-on duration is used as the value for time t. Otherwise, the determined maximum current value is used.

[0090] In other possible embodiments of the DC switch 20 , the control device 25 is designed to carry out the second test process without previously carrying out the first test process.

[0091] Furthermore, the control device 25 is designed to check the connected capacitance and compare it with a stored maximum permissible pre-charge capacitance. The maximum permissible pre-charge capacitance is also limited, at least when using a pre-charge resistor. The value of the maximum permissible pre-charge capacitance is stored in the control device 25 for this purpose.

[0092] To do this, use the following relationship:

[0093]

[0094] and

[0095]

[0096] Where Q is the charge and C is the capacitance. Assuming that the supply voltage is significantly greater than the output voltage obtained during the on-time, the curve of the current intensity (as in Figure 3 To simplify the calculation of the integral, the first region (as shown in FIG) can be divided into two regions with an approximately linear profile. The first region is the on-time itself, i.e., for example, the first time period. The second region extends from the off-time to the time when the current intensity reaches zero again. The injected charge can then be calculated using the above formula as follows:

[0097] First area:

[0098]

[0099] Second area:

[0100]

[0101] The resulting total charge is the sum of the two components, which is 20 μAs. As an alternative to calculating using two approximately linear regions, numerical integration methods can also be used in a microcontroller implementation to determine the introduced charge. This eliminates the need for a division into two regions.

[0102] The load capacitance can now be determined from the charge using formula (5):

[0103]

[0104] By comparing the determined value of the connected capacitance with the maximum permissible pre-charge capacitance, the control device 25 determines whether pre-charging is permitted at all. For both the positive and negative cases, the control device 25 can be designed to output a signal electronically or directly display it visually. Furthermore, the control device 25 can be designed to prevent switching if the maximum permissible pre-charge capacitance is exceeded.

[0105] The described DC switch 20 thus losslessly ensures that the installed feeder line inductance is greater than the minimum permissible feeder line inductance and less than the maximum permissible feeder line inductance. Furthermore, it ensures that the installed load capacitance is less than the maximum permissible pre-charge capacitance. This advantageously eliminates the need for complex manual measurements or checks.

[0106] In addition to providing pure feedback on the installation time, the DC switch 20 can also prevent switching during ongoing operation if this would result in an unsafe operating state. For this purpose, it is advantageously also possible to carry out the described test process and determination of the values ​​connected on the load side not only during startup but also during ongoing operation.

[0107] To this end, control device 25 can be designed to determine the connected inductance and / or capacitance before each switching operation, for example, as requested by a higher-level controller. Advantageously, this determination requires only a very short time, in particular less than 1 ms, even when performing multiple test operations with different time periods, and can therefore also be performed before switching on during ongoing operation. If the permissible value range known for DC switch 20 is not adhered to, the switching operation is not carried out, and DC switch 20 outputs an error message.

[0108] Reference Signs List

[0109] 10 DC voltage network

[0110] 11 Power supply side

[0111] 12 Load side section

[0112] 14 Voltage Source

[0113] 16 Electric Motor

[0114] 17 Inverter

[0115] 18 Feeder Lines

[0116] 20 DC switch

[0117] 21, 22 Power semiconductor switches

[0118] 23 Discharge Network

[0119] 231, 234 capacitors

[0120] 225 Varistor

[0121] 233, 235 resistors

[0122] 28, 29, 30 Measuring device

[0123] 25 Control Device

[0124] 31 Current intensity

[0125] 32 Supply voltage

[0126] 33 Output voltage

[0127] 34 Connection time

[0128] 35 Disconnection time point

Claims

1. A DC switch (20) for disconnecting a DC line (18), comprising: at least one power semiconductor switch (21, 22), which is arranged in a current path of the DC line (18), - means (28, 29) for measuring input voltage and output voltage (32, 33), - means (30) for measuring the current (31) flowing through the DC line (18), - a control device (25) for the power semiconductor switches (21, 22), Characterized in that the control device (25) is designed to: - switching on the DC switch (20) during a first time period, - determining the applied input voltage (32), - determining the output voltage (33) existing at the end of said first time period, - determining the current intensity (31) present at the end of said first time period, - determining the inductance and / or capacitance present from the determined values, and - comparing the determined current intensity (31) with a threshold value and, based on the result of the comparison, redetermining the existing inductance, wherein a second time period, which is longer than the first time period, is used instead of the first time period.

2. The DC switch (20) according to claim 1, wherein: The control device (25) is designed to use a time duration determined before switching on as the first time period.

3. The DC switch (20) according to claim 1, wherein: The control device (25) is designed to shorten the first time period and disconnect the DC switch (20) when the current intensity (31) reaches a current threshold value.

4. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to calculate the existing inductance according to the following formula: Among them, L zuleitung is the inductance, U is the driving voltage determined by the input voltage and / or output voltage, t puls is the first time period, and I peak is the current intensity present at the end of the first time period.

5. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to determine whether a supply voltage (32) is present.

6. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to determine the output voltage (33) present at the beginning of the first time period.

7. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to carry out a determination of the existing inductance and / or capacitance before each switching operation.

8. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to compare the determined existing inductance with a maximum permissible inductance and a minimum permissible inductance and to output a signal which indicates whether the determined inductance lies between the permissible inductances.

9. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to carry out the determination of the existing inductance several times and to determine an average inductance.

10. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to determine the time duration for the current intensity (31) to drop to zero after switching to the off state and to take this time duration into account when determining the capacitance present on the load side.

11. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to calculate the capacitance present on the load side according to the following formula: Among them C load is the capacitance, Q load1 is the charge input to the load side when connected, Q load2 is the charge input to the load side after switching to the disconnected state, and ΔU out It is the change in output voltage caused by the amount of input charge.

12. The DC switch (20) according to claim 11, wherein: The control device (25) is designed to calculate the input charge amount according to the following formula: Where t0 is the switch-on time (34), t off is the disconnection time (35), and t1 is the time at which the current intensity reaches zero again after disconnection.

13. The DC switch (20) according to claim 11, wherein: The control device (25) is designed to determine the input charge quantity by numerically integrating the current intensity (31).

14. The DC switch (20) according to any one of claims 1 to 3, wherein: The control device (25) is designed to compare the determined capacitance present on the load side with a maximum permissible capacitance and to output a signal which indicates whether the determined capacitance is greater than the maximum permissible capacitance.

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