Method for monitoring thermal load

Abstract

Method for monitoring the thermal load of at least one damping resistor (8) of an electrical conversion circuit, wherein: the at least one damping resistor (8) is part of at least one filter (5) comprising capacitors (7), preferably at least one LCL filter; the filter (5) has been connected between a transformer (2) and a power converter (1) along connecting lines (3) which connect a first side of the transformer (2) and an AC side of the power converter (1); a calculation parameter is determined which is characteristic of the thermal load of the damping resistor, characterised in that the calculation parameter (18, 19) is calculated from the voltages of the connecting lines (3).

Description

[0001] Methods for monitoring thermal stress

[0002] The invention relates to a method for monitoring the thermal load of at least one damping resistor of an electrical conversion circuit, wherein the at least one damping resistor is part of at least one filter comprising capacitors, preferably at least one LCL filter, wherein the filter is connected between the transformer and the power converter along connecting lines connecting a first side of a transformer and an AC side of a power converter, wherein a calculation parameter is determined which is characteristic of the thermal load of the damping resistor.

[0003] It also relates to a method for controlling an electronic conversion circuit.

[0004] It also relates to an electrical conversion circuit for converting electrical current, comprising at least one power converter and at least one transformer, wherein a first side of the transformer and an AC side of the power converter are connected via connecting lines and wherein at least one filter comprising capacitors and at least one damping resistor, preferably at least one LCL filter, is connected along the connecting lines between the transformer and the power converter, and that at least one control unit is configured to control the conversion circuit, preferably at least the power converter.

[0005] Electrical circuits with transformers and converters are used to convert electrical current. The conversion can involve changing at least one frequency, voltage, current, waveform, type of oscillation, and / or at least one other property. Such circuits often serve to convert current between an electrical machine or a power grid powered by alternating current and an energy storage system such as a battery powered by direct current. They have recently gained particular importance in the field of electromobility, where they convert alternating current into the appropriate direct current.

[0006] The described filters, particularly LCL filters, which are arranged between the transformer and the power converter, also called the main rectifier (MR), are preferably used to attenuate harmonics around the switching frequency. To ensure filter stability at the resonant frequency, where the impedance is close to zero, active and passive methods can be used. The active method does not use dissipative elements, but merely modifies the control system. This avoids attenuation losses. In contrast to the passive method, the control design is more complex with this approach, and additional sensors are required. One such system is disclosed, for example, in US 2004 / 0095784 A1.

[0007] Passive methods use damping resistors added to the systems' LCL filters. It has recently been recognized that these damping resistors can burn out and damage the MR power stacks. This is because a high amplitude ripple of the current demand at the system's DC output, where the ripple frequency is close to the resonant frequency, can cause this problem. The cause can be found in the feedforward regulation of Vdc within the MR control system attempting to meet this high dynamic demand. As a result, high currents flow through the damping resistors. If the power dissipation of the damping resistor exceeds the maximum rated value for an extended period, its thermal destruction occurs. After this time, the LCL filter is no longer stable at the resonant frequency. This can lead to damage to the MR power stack.

[0008] It can be provided to disable the feedforward control, eliminating these high currents and reducing the load on the damping resistor. However, this can impair normal control performance. For example, the intermediate DC voltage rises to zero at full positive power and rises significantly above the defined limit at negative power, and the error is received.

[0009] Typically, the passive damping resistors are chosen based on an estimate of the power losses to achieve a compromise between the stability and the effectiveness of the filter.

[0010] US 2018 / 0100889 A1 proposes a passive system. The filter circuit includes an inductor connected between a filter input terminal and a filter output terminal. The damping resistor is connected in series with a blocking capacitor across the inductor. The detection circuit is connected to the capacitor to detect the capacitor voltage and indicate a resistor fault condition. If the DC voltage achieved after rectification of the blocking capacitor is lower than a predetermined low value, a fault in the damping resistor is detected. This requires additional voltage transformers to monitor the voltages across the capacitors. Due to space limitations and higher costs, the installation of additional power converters is disadvantageous.

[0011] The object of the invention is therefore to provide a method for monitoring the thermal load of at least one damping resistor and an electrical conversion circuit for converting electrical current, which enables the conversion circuit to operate as safely as possible, but which also has the simplest possible structure and is as cost-effective as possible.

[0012] This object is achieved according to the invention in that the calculation parameters are calculated from the voltages of the connecting lines.

[0013] This calculation parameter is preferably used to monitor the thermal load of at least one damping resistor.

[0014] It is also achieved in that the control unit is designed to calculate at least one calculation parameter from the voltages of the connecting lines, which is characteristic of the thermal load on the damping resistor, and in that the control unit is designed to control the electronic conversion circuit on the basis of the calculation parameter.

[0015] Calculating the calculation parameter from the voltages of the connecting lines means that the calculation parameter is calculated based on the voltages of the connecting lines or by including the voltages of the connecting lines. In other words, the voltages of the connecting lines are used as the initial parameters for calculating the calculation parameter. In addition to the voltages, other output parameters can be included in the calculation. These additional output parameters can be, for example, measured values, characteristic values, constants, predefined values, or input values.

[0016] Since thermal stress changes the ohmic resistance of the damping resistor, thermal stress influences the voltages in the connecting lines. Using appropriate calculations, it is possible to calculate such a parameter from the voltages in the connecting lines and infer the thermal stress.

[0017] "Characteristic of the thermal load on the damping resistor" means that the calculation parameter provides information about the thermal load, preferably directly. The calculation parameter does not necessarily have to represent the temperature of the damping resistor. It can also be a parameter that provides information about the thermal load on the damping resistor through simple conversion, for example, by multiplying by a multiplier, another simple calculation method such as quadrature, and / or by relating it to a temporal value. This can be an instantaneous parameter, such as power or current, that provides information about the thermal load occurring on the damping resistor when this parameter remains at a certain value over a certain period of time.

[0018] Such a calculation parameter may, for example, include: the current flowing through the damping resistor, preferably the effective value of this current, the power dropped across the damping resistor and / or the energy consumed by the damping resistor.

[0019] The risk of overheating can be determined by calculating this calculation parameter, usually comprising at least a current flowing through the damping resistor and / or at least a power loss at the damping resistor, whereby the invention uses the voltages of the connecting lines. This has the particular advantage that the electronic structure between the transformer and the power converter is not further complicated. Furthermore, derivatives for measuring or calculating the voltages of the connecting lines or other methods for measuring this voltage are usually already available and can be used for the invention. Thus, through the targeted use of available resources, the invention enables the risk of overheating to be estimated without having to arrange additional parts or components in the electrical circuit. This enables a particularly cost- and space-saving embodiment.

[0020] The connecting lines are preferably designed to transmit current between the first side of the transformer, preferably the secondary side, and an AC side of the power converter. The connecting lines preferably comprise at least one three-phase line set, typically at least three phase lines. The connection can also be single-phase, for example, originating from one or two half-bridges in the power converter. The connecting lines can therefore also comprise at least one or at least two phase lines.

[0021] Preferably, at least one coil is connected between the transformer and the power converter along at least one, preferably all, connecting lines, particularly preferably in series and / or parallel. Preferably, at least two connecting lines are connected to one another via at least one capacitor. Particularly preferably, each connecting line is connected to every other connecting line via at least one capacitor.

[0022] Preferably, at least one damping resistor is connected between at least one connecting line and at least one capacitor.

[0023] The damping resistor preferably comprises at least one power resistor. It preferably comprises a defined ohmic resistance and particularly preferably negligible capacitance or inductance.

[0024] Preferably, the calculation parameter correlates with the decreasing power or the energy loss at the at least one damping resistor.

[0025] The calculation parameter is a parameter or value that provides information about the declining power or energy loss, preferably through its magnitude. Alternatively, it can also contain this information through another property, for example, its frequency, phase, and the like.

[0026] The calculation parameter can essentially represent an instantaneous value or can also represent the declining power or energy loss over a given time.

[0027] Preferably, the calculation parameter includes the power dissipated across the damping resistor. This is a direct indicator of the heat dissipation.

[0028] Preferably, the calculation of the calculation parameter includes calculating at least one current flowing through the at least one damping resistor based on the voltages of the connecting lines. The current flowing through the damping resistor provides information about the power drop across the damping resistor and thus the heat generated, since the power represents the square of the current multiplied by the resistance of the damping resistor.

[0029] It is preferably provided that the calculation of the calculation parameter comprises that, based on the voltages of the connecting lines, at least one voltage is calculated which drops across the at least one damping resistor. Similar to the current, the square of the voltage divided by the resistance indicates the power and can thus be used to estimate the heat production. It is preferably provided that the calculation of the calculation parameter comprises that the voltages of the connecting lines are differentiated against time and preferably multiplied by a multiplier, wherein the multiplier depends on the capacitance of the capacitors. In this way, the current flowing between connecting lines can be calculated in a simple manner. Preferably, the multiplier is calculated based on the capacitance of the capacitors, in particular when the capacitors are connected between the connecting lines.It can be provided that the calculation of the multiplier includes calculating the total capacitance of the capacitors. It can be provided that the calculation of the multiplier includes adding together the capacitances of the capacitors. If the connecting lines comprise a three-phase line, it can be provided that the multiplier includes the sum of the capacitances of the capacitors connected between the connecting lines. Accordingly, it can also be advantageous if it is provided that the control unit has at least one differentiating unit for differentiating the voltages of the connecting lines over time and at least one first multiplying unit connected downstream of the differentiating unit for multiplying by a multiplier.

[0030] Differentiation means forming the derivative of the signal with respect to time.

[0031] The differentiation unit preferably has at least one time delay unit, at least one subtraction unit, and at least one division unit. Preferably, an input of the time delay unit and an input of the subtraction unit are connected to an input line, wherein the input line is configured to deliver a signal containing the voltages of the connecting lines. The output of the time delay unit is preferably connected to another input of the subtraction unit, and the output of the subtraction unit is connected to the input of the division unit.

[0032] The division unit is configured for division by a divisor, which preferably includes the time interval of the time delay of the time delay unit. It can also be implemented as a multiplication unit, since division and multiplication by the reciprocal of the divisor are equivalent.

[0033] It can be provided that the differentiation unit and the first multiplication unit are at least partially implemented together. For example, it can be provided that the division unit and the first multiplication unit are implemented together. It can be advantageous for the calculation of the calculation parameter to include squaring a calculated current and then calculating an average value over a predetermined period of time, and preferably taking the root of the average value. In this way, the square of the effective value can be determined, which provides information about the analog DC current quantity of the damping resistor.Accordingly, it may be advantageous for the control unit to have at least one squaring unit for squaring, which is preferably connected downstream of the differentiation unit and particularly preferably of the first multiplication unit and / or for the control unit to have at least one averaging unit for forming an average, which is preferably connected downstream of the squaring unit.

[0034] It is preferably provided that the calculation of the calculation parameter comprises calculating the effective value from a calculated current and / or a power drop across the damping resistor, taking into account the resistance of the damping resistor. The power drop can be used to directly determine the heat development. Preferably, the temporal progression of the power drop is used. In parallel, the effective value of the current can also provide information about the power drop. Preferably, the calculation of the effective value comprises extracting the square root of a mean value, preferably the root mean square. Accordingly, it is also advantageous if the control unit has at least one root extraction unit for extracting the root and / or at least one second multiplier unit, which is preferably connected downstream of the squaring unit.

[0035] It may also be advantageous if the calculation of the calculation parameter involves multiplying the calculated current, preferably a root mean square of the current over a specified period of time, by the resistance of the damping resistor. Multiplying by the resistance allows for the direct calculation of power. This allows the power drop across a damping resistor to be calculated directly.

[0036] It is particularly advantageous if the calculation of the calculation parameter comprises at least one voltage of the connecting lines being measured or calculated, and preferably that the voltages of the connecting lines are measured or calculated. Since these voltages are usually measured or calculated anyway, existing measured or calculated values ​​can be used for the calculation. In this sense, it can also be advantageous if the connecting lines are connected to inputs of the control unit via measuring lines. In this case, in the sense of measuring the voltages of the connecting lines, it can also be provided that at least one of the voltages is measured indirectly. This means that at least one parameter other than this at least one voltage is measured and the at least one voltage is calculated from this at least one parameter.For example, other voltages can be measured and at least one voltage can be calculated from these.

[0037] It is also advantageous to use the voltages of the connecting lines to the ground potential. The ground potential is assigned the potential zero.

[0038] The calculation of the calculation parameter can be provided by at least partially measuring the voltage between the connecting lines and preferably calculating the voltages of the connecting lines relative to ground potential from these measured voltages. Measuring the voltages between the connecting lines is often very simple or already provided, and therefore requires little or no additional measures.

[0039] For example, if there are three connecting lines that conduct three different phases and two phase-to-phase voltages are measured, the voltages of the individual connecting lines to the ground potential can be calculated using the following formulas:

[0040] V a = l / 3*(2*Vpp(l)+Vpp(2))

[0041] V b =l / 3*(V P p(2)-Vpp(l))

[0042] V c =-l / 3*(2*Vpp(2)+Vpp(l))

[0043] Vabc = [Va^ Vb Vc]

[0044] Where V a , Vb and V c the voltages of the individual connecting lines to ground potential and V pp (l) and V pp (2) the two measured phase-to-phase voltages.

[0045] The invention also relates to a method for controlling an electronic conversion circuit with a filter comprising at least one damping resistor, in particular an electrical supply line of an electric motor, wherein the method according to the invention is carried out for monitoring the thermal load of at least one damping resistor and the electronic circuit is controlled based on the calculation parameter. It has been found that monitoring the currents and power losses in the damping resistors within the electric storage devices and triggering shutdown at high currents is a good solution for preventing the damping resistors from overheating. Preferably, the control of the electronic circuit includes controlling the power output or power consumption of the power converter depending on the calculation parameter.By controlling the power output or power consumption of the converter, the power drop across the damping resistor is directly changed. This makes it possible to prevent or delay overheating of a damping resistor.

[0046] Preferably, the control of the electronic circuit includes reducing or interrupting the power output or power consumption of the power converter depending on the calculation parameter. By reducing or interrupting the power output or power consumption, the power drop across the damping resistor is reduced in the event of imminent overheating, thus lowering the risk.

[0047] Furthermore, the control of the electronic circuit can be provided by monitoring the temporal development of the calculation parameter over a specific period of time and incorporating this temporal development into the control. This is particularly advantageous because it allows the heating of the damping resistor over time to be taken into account.

[0048] It is particularly advantageous if the control system includes comparing the calculation parameter with at least one predefined threshold value and incorporating the result of the comparison into the control system. By comparing with a threshold value, the converter circuit can be operated normally until the threshold value is reached. This enables operation with as little disruption as possible. The calculation parameter can be compared directly with a threshold value, or a comparison value calculated from the calculation parameter can be compared with the threshold value. This comparison value can, for example, include the temporal development of the calculation parameter.

[0049] It is particularly advantageous if the power output or power consumption is reduced or interrupted if the calculation parameter lies above or below the threshold value for a predetermined period of time and / or for a predetermined number of sampling steps. In this way, heat development at the damping resistor can be determined as accurately as possible, and control can be carried out accordingly. It can be provided that measurements are taken over a predetermined period of time to determine how often or for how long the threshold value was exceeded, and control can be carried out accordingly. It can also be provided that the power output or power consumption is reduced or interrupted if the threshold value is exceeded once. In this case, the number of sampling steps is selected as 1.It can be provided that the value of the calculation parameter is evaluated at predetermined times, and the evaluation of the calculation parameter, in particular the evaluation of the calculation parameter over a predetermined time interval, is included in the control of the electronic circuit. Preferably, it is provided that the power output or power consumption is reduced or interrupted if the evaluation of the calculation parameter or a sum of the evaluations of a calculation parameter over a predetermined time interval exceeds a threshold value.

[0050] Several threshold values ​​can also be provided, whereby the calculation parameters are evaluated or weighted differently depending on whether the respective threshold values ​​are exceeded.

[0051] Preferably, the electrical conversion circuit may include at least one, preferably at least two, voltmeters for measuring voltages in the connecting lines. The voltmeters may be implemented as part of the control unit.

[0052] Preferably, the voltmeters are arranged with the connecting lines between the filter and the first side of the transformer.

[0053] The invention will be explained in more detail below using non-limiting embodiments in the figures. They show:

[0054] Fig. 1 is a circuit diagram of a first embodiment of an electrical conversion circuit according to the invention;

[0055] Fig. 2 is a block diagram of part of a first embodiment of a control unit according to the invention.

[0056] The embodiment of the invention shown in Fig. 1 uses the method according to the invention and comprises two power converters 1 and a transformer 2. A first side of the transformer 2 has two coils, each of which is connected to a power converter 1 via connecting lines 3. The conversion circuit thus has two parallel strands, both of which flow into the transformer.

[0057] The connecting lines 3 are connected to a control unit 4 via measuring lines 3a. The control unit 4 is connected to the power converters 1 via at least one data transmission line 1a and thus controls the power converters 1.

[0058] Furthermore, two voltmeters 9 are provided, each measuring the voltage of a connecting line 3 with respect to another connecting line 3. These are connected to the control unit 1 via measuring lines 3a. The voltmeters 9 can also be part of the control unit 1 and can—as shown in Fig. 1—be provided in addition to or instead of the measuring lines 3a that connect the connecting lines 3 to a control unit 4. The voltmeters 9 are arranged between the coils 6 and the transformer 2.

[0059] Using the measurement results of the voltmeter 9, the phase-to-phase voltages between the connecting lines 3 can be measured or calculated. From these, the voltages of the individual connecting lines 3 to ground potential can subsequently be calculated, as given by the above formulas.

[0060] Both strands have the following structure:

[0061] A power converter 1 has a direct current side connected to a battery system (not shown). The alternating current side of the power converter 1 is connected to the connecting lines 3. Both the alternating current side of the power converter 1 and the first side of the transformer are designed as three-phase sides. Accordingly, three connecting lines 3 connect the transformer 2 to the power converter 1. The second side of the transformer is connected to an electrical power grid (not shown).

[0062] An LCL filter 5 is arranged along the connecting lines 3 between the power converter 1 and the transformer 2. The LCL filter 5 comprises three coils 6, each of which is connected along a connecting line 3 between the power converter 1 and the transformer 2. Three capacitors 7 are interconnected in a delta configuration, with each of the three contact corners connected via a damping resistor 8 to a connecting line 3 between the coils 6 and the transformer 2. Another induction part of the LCL filter 5 is represented by the coils of the transformer 2.

[0063] An ohmic resistor R_L_f_A, R_L_f_B, R_L_f_C, R_C_f_A, R_C_f_B, R_C_f_C, etc., is shown in series with the coils 6 and the capacitors 7. These symbolically represent the ohmic resistances of the respective coil 6 or the respective capacitor 7.

[0064] Fig. 2 shows a control unit 4 of an embodiment according to the invention or a part of a control unit 4 in detail as a block diagram, as it could be used in the embodiment of Fig. 1.

[0065] The input variables are the voltages V a t>c of the connecting lines 3 of a string to the ground potential and introduced via an input line 10. These can be measured as explained in Fig. 1 or derived from the phase-to-phase voltages V pp (l) and V pp (2). Preferably, the control unit 1 comprises at least one voltage calculation unit configured to convert phase-to-phase voltages into voltages relative to ground potential. Particularly preferably, the voltage calculation unit is connected to the differentiation unit 13a, as shown in Fig. 1.

[0066] A time delay unit 11 is connected at its output side to the input line 10, is supplied with the voltages V a t>c supplies and delays the signals. The input line 10 and the time delay unit 11 are connected on their output side to a subtraction unit 12, which subtracts the time-delayed signal from the signal. The subtraction unit 12 is connected to a first multiplication unit 13b, which performs a derivative of the voltage against time by dividing it by the time difference generated by the time delay unit 11. In addition, the first multiplication unit 13b multiplies the signal by the sum of the capacitances of the capacitors 7 as a multiplier. The time delay unit 11, as well as the subtraction unit 12 and the first multiplication unit 13b, are part of the differentiation unit 13a.

[0067] The first multiplication unit 13b is connected at its output side to a squaring unit 14 which squares the signal.

[0068] The squaring unit 14 is connected at its output side to an averaging unit 15 which calculates an average value over the input values ​​over a predetermined time period.

[0069] The averaging unit 15 is connected at its output to a root extraction unit 16, which extracts the square root of the signal. This yields the effective value of the current flowing through the damping resistors 8 as the first calculation parameter 18.

[0070] The averaging unit 15 is connected at its output to a second multiplication unit 17, which multiplies the calculated mean value by a second multiplier, preferably the resistance of the respective damping resistor 8. Thus, the power drop across the respective damping resistor 8 is obtained as the second calculation parameter 19.

Claims

PATENT CLAIMS 1. A method for monitoring the thermal load of at least one damping resistor (8) of an electrical conversion circuit, wherein the at least one damping resistor (8) is part of at least one filter (5) comprising capacitors (7), preferably at least one LCL filter, wherein the filter (5) has been connected between the transformer (2) and the power converter (1) along connecting lines (3) which connect a first side of a transformer (2) and an AC side of a power converter (1), wherein a calculation parameter is determined which is characteristic of the thermal load of the damping resistor, characterized in that the calculation parameter (18, 19) is calculated from the voltages of the connecting lines (3).

2. Method according to claim 1, characterized in that the calculation of the calculation parameter (18, 19) comprises calculating at least one current flowing through the at least one damping resistor (8) on the basis of the voltages of the connecting lines (3).

3. Method according to claim 1 or 2, characterized in that the calculation of the calculation parameter (18, 19) comprises differentiating the voltages of the connecting lines (3) against time and preferably multiplying them by a multiplier, the multiplier depending on the capacitance of the capacitors (7).

4. Method according to one of claims 1 to 3, characterized in that the calculation of the calculation parameter (18, 19) comprises calculating the effective value from a calculated current and / or a power loss at the damping resistor (8) is calculated taking into account the resistance of the damping resistor (8).

5. Method according to one of claims 2 to 4, characterized in that the calculation of the calculation parameter (18, 19) comprises multiplying, on the basis of the calculated current, preferably a root mean square value of the current over a predetermined period of time, by the resistance of the damping resistor (8).

6. Method according to one of claims 1 to 5, characterized in that the calculation of the calculation parameter (18, 19) comprises measuring or calculating the voltages of the connecting lines (3).

7. Method according to claim 6, characterized in that the voltages of the connecting lines (3) are used to ground potential.

8. Method according to one of claims 6 or 7, characterized in that the calculation of the calculation parameter (18, 19) comprises that the voltage between the connecting lines (3) to each other is at least partially measured and preferably from these measured voltages the voltages of the connecting lines (3) to the ground potential are calculated.

9. Method for controlling an electronic conversion circuit, in particular an electrical supply line of an electric motor, with a filter (5) comprising at least one damping resistor (8), wherein the method for monitoring the thermal load of at least one damping resistor (8) is carried out according to one of the preceding claims and the electronic conversion circuit is controlled on the basis of the calculation parameter (18, 19).

10. Method according to claim 9, characterized in that the control of the electronic conversion circuit comprises that the power output or power consumption of the power converter (1) is controlled depending on the calculation parameter (18, 19).

11. Method according to claim 9 or 10, characterized in that the control of the electronic conversion circuit comprises monitoring the temporal development of the calculation parameter (18, 19) over a certain period of time and including the temporal development in the control.

12. Method according to one of claims 9 to 11, characterized in that the control comprises that the calculation parameter (18, 19) is compared with at least one predetermined threshold value and that the result of the comparison is included in the control and particularly preferably that the power output or power consumption is reduced or interrupted if the calculation parameter (18, 19) is above or below the threshold value for a predetermined period of time and / or for a predetermined number of sampling steps.

13. Electrical conversion circuit for converting electrical current, comprising at least one power converter (1) and at least one transformer (2), wherein a first side of the transformer and an AC side of the power converter are connected via connecting lines (3) and wherein along the connecting lines (3) between the transformer (2) and at least one filter (5) comprising capacitors (7) and at least one damping resistor (8), preferably at least one LCL filter (5), is connected to the power converter (1), and at least one control unit (4) is designed to control the conversion circuit, preferably at least the power converter (1), characterized in that the control unit (4) is designed to calculate at least one calculation parameter (18, 19) from the voltages of the connecting lines (3), which is characteristic of the thermal load on the damping resistor, and in that the control unit (4) is designed to control the electronic conversion circuit on the basis of the calculation parameter (18, 19).

14. Electrical conversion circuit according to claim 13, characterized in that the electrical conversion circuit has at least one, preferably at least two voltmeters (9) for measuring voltages of the connecting lines (3).

15. Electrical conversion circuit according to claim 14, characterized in that the voltmeters with the connecting lines (3) are arranged between the filter (5) and the first side of the transformer (2).

16. Electrical conversion circuit according to one of claims 13 to 15, characterized in that the connecting lines (3) are connected to inputs of the control unit (4) via measuring lines.

17. Electrical conversion circuit according to one of claims 13 to 16, characterized in that the control unit (4) has at least one differentiating unit (13a) for differentiating the voltages of the connecting lines (3) over time and at least one first multiplying unit (13b) connected downstream of the differentiating unit (13a) for multiplying by a multiplier.

18. Electrical conversion circuit according to one of claims 13 to 17, characterized in that the control unit (4) has at least one squaring unit (14) for squaring, which is preferably connected downstream of the differentiation unit (13a) and particularly preferably of the first multiplication unit (13b).

19. Electrical conversion circuit according to one of claims 13 to 18, characterized in that the control unit (4) has at least one averaging unit (15) for forming an average value, which is preferably connected downstream of the squaring unit (14).

20. Electrical conversion circuit according to one of claims 14 to 19, characterized in that the control unit (4) has at least one root extraction unit (16) for extracting the root and / or at least one second multiplier unit (17), which is preferably connected downstream of the squaring unit (14).

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