DE102014206454B4 - Method and device for checking an arrangement with a number of solar modules - Google Patents

Abstract

A method of inspecting an array (100) having a number of solar modules (110, 112, 114, 116, 118) electrically connected in series between a first terminal (120) and a second terminal (124), at least two voltages are measured from a set of voltages, characterized in that - the set of voltages comprises the following voltages: - a first voltage (U1) between a tap (122), which lies electrically between two solar modules, and the first connection (120), A second voltage (U2) between the tap (122) and the second terminal (124), and a third voltage (U3) between the first terminal (120) and the second terminal (124), and the method further follows Steps comprising: calculating a measured-based voltage ratio between the measured first voltage (U1) and the measured second voltage (U2), or the measured third voltage (U3) and the measured two voltage (U2), or - the measured third voltage (U3) and the measured first voltage (U1), - calculating a voltage (U1, U2, U3) used by the tap (122) and by those used to calculate the measurement-based voltage ratio expected voltage ratio, and - detecting an error condition of the assembly (100) when the measurement-based voltage ratio and the expected voltage ratio differ by more than a predetermined tolerance value.

Description

Field of responsibility and state of the art

The invention relates to a method for checking an arrangement comprising a number of solar modules, which are electrically connected in series between a first terminal and a second terminal. The invention further relates to an associated apparatus for carrying out such a method.

In arrangements of solar modules nowadays often several solar modules are interconnected. This can lead to technical errors or failures of individual solar modules occurring in the course of time even with high manufacturing quality. This leads to yield losses or income reductions, which is often noticed only after a long time, when expected earnings can not be realized. In many cases, such errors remain completely undetected.

It is known that reference data from other independent system components or other systems are used to control such arrangements. This often compares the instantaneous power, the instantaneous current or the output over a certain period of time. However, this requires that the reference system has no similar error. Furthermore, in such a procedure, a relatively large tolerance must be used as a reserve value, in particular if the comparison systems are arranged locally far apart. In this case, defective modules may possibly remain undetected during setup of the arrangement and cause losses for years.

Furthermore, it is known to record a characteristic of solar modules connected in series in order to check their plausibility. However, this leads to lower yields, since this is not the maximum possible power can be fed.

The DE 10 2012 219 690 A1 and the US 2008/0143543 A1 each show methods for verifying an arrangement having a number of solar modules electrically connected in series between a first terminal and a second terminal. In the in the DE 10 2012 219 690 A1 In the case of the method shown, a voltage is repeatedly measured between the first terminal and the second terminal, with the sum of the voltages of different groups being measured for each measurement in order to conclude from the results of these measurements for errors in the solar modules and to assign these to the solar modules.

Task and solution

It is therefore an object of the invention to provide a method for checking an arrangement with a number of solar modules (solar module assembly), which can reliably detect errors without yield losses. It is a further object of the invention to provide a device for carrying out such a method.

This is inventively achieved by a method according to claim 1 and an apparatus according to claim 9. Advantageous embodiments can be taken, for example, the respective subclaims. The wording of the claims is hereby incorporated by explicit reference into the content of the description.

The invention relates to a method for checking an arrangement (solar module arrangement) comprising a number of solar modules, wherein the solar modules are electrically connected in series between a first terminal and a second terminal.

• – Messen von zumindest zwei Spannungen aus einer Menge von Spannungen, welche folgende Spannungen aufweist:
• – eine erste Spannung zwischen einem Abgriff, der elektrisch zwischen zwei Solarmodulen liegt, und dem ersten Anschluss,
• – eine zweite Spannung zwischen dem Abgriff und dem zweiten Anschluss, und
• – eine dritte Spannung zwischen dem ersten Anschluss und dem zweiten Anschluss,
• – Berechnen eines messbasierten Spannungsverhältnisses zwischen
• – der gemessenen ersten Spannung und der gemessenen zweiten Spannung, oder
• – der gemessenen dritten Spannung und der gemessenen zweiten Spannung, oder
• – der gemessenen dritten Spannung und der gemessenen ersten Spannung,
• – Berechnen eines vom Abgriff und von den zum Berechnen des messbasierten Spannungsverhältnisses verwendeten Spannungen abhängigen erwarteten Spannungsverhältnisses, und
• – Feststellen eines Fehlerzustands der Anordnung, wenn das messbasierte Spannungsverhältnis und das erwartete Spannungsverhältnis um mehr als einen vorgegebenen Toleranzwert voneinander abweichen.

The method comprises the following steps:

• Measuring at least two voltages from a set of voltages having the following voltages:
• A first voltage between a tap electrically located between two solar modules and the first terminal,
• A second voltage between the tap and the second terminal, and
• A third voltage between the first terminal and the second terminal,
• - Compute a measurement-based voltage ratio between
• The measured first voltage and the measured second voltage, or
• The measured third voltage and the measured second voltage, or
• The measured third voltage and the measured first voltage,
• Calculating an expected voltage ratio depending on the taps and voltages used to calculate the measurement-based voltage ratio, and
• - Detecting a fault condition of the arrangement when the measured voltage-based relationship and the expected voltage ratio differ by more than a predetermined tolerance value.

By means of the method according to the invention, it is possible to detect a fault condition by comparing typically easily calculated voltages or voltage conditions, without interrupting the operation of the installation or disturbing it in any other way. A loss of earnings can thus be avoided. In the case of a defect, a solar module usually provides a different, in particular lower, voltage than the other solar modules. This can not only be determined that one of the solar modules is defective, it can also be predicted on the basis of the sign of the deviation on which side of the tap is the defective solar module. In addition, in the method according to the invention typically only immediately adjacent solar modules are compared with each other, which increases the reliability of the method.

• – die erste Spannung und die zweite Spannung,
• – die dritte Spannung und die zweite Spannung, oder
• – die dritte Spannung und die erste Spannung.

In particular, the following combinations with respect to the measured voltages are possible:

• The first voltage and the second voltage,
• - the third voltage and the second voltage, or
• - the third voltage and the first voltage.

It should be understood that this does not preclude the measurement of other voltages, but it is preferable to measure just those voltages used to calculate the measurement-based voltage ratio.

Preferably, only the absolute values are considered by the first voltage, the second voltage and the third voltage as well as the expected voltage ratio.

The solar modules can be arranged, for example, in a free-standing system, an in-roof system or an on-roof system. For example, they may be arranged side by side in a row.

The tap is located electrically between two solar modules from the number of electrically series-connected solar modules, so that at the tap relative to the first terminal and the second terminal that voltage is measured, which each between the tap and the first terminal or the generate second connection interconnected modules in total.

• – das erwartete Spannungsverhältnis als Verhältnis der Anzahl von Solarmodulen zwischen dem Abgriff und dem ersten Anschluss zur Anzahl von Solarmodulen zwischen dem Abgriff und dem zweiten Anschluss berechnet werden, wenn das messbasierte Spannungsverhältnis zwischen der gemessenen ersten Spannung und der gemessenen zweiten Spannung berechnet wird;
• – das erwartete Spannungsverhältnis als Verhältnis der Anzahl von Solarmodulen zwischen dem ersten Anschluss und dem zweiten Anschluss zur Anzahl von Solarmodulen zwischen dem Abgriff und dem zweiten Anschluss berechnet werden, wenn das messbasierte Spannungsverhältnis zwischen der gemessenen dritten Spannung und der gemessenen zweiten Spannung berechnet wird;
• – das erwartete Spannungsverhältnis als Verhältnis der Anzahl von Solarmodulen zwischen dem ersten Anschluss und dem zweiten Anschluss zur Anzahl von Solarmodulen zwischen dem Abgriff und dem ersten Anschluss berechnet werden, wenn das messbasierte Spannungsverhältnis zwischen der gemessenen dritten Spannung und der gemessenen ersten Spannung berechnet wird.

The expected stress ratio is often easy to calculate. For example, the solar modules can each have identical characteristics. Especially in this case can

• Calculating the expected voltage ratio as the ratio of the number of solar modules between the tap and the first terminal to the number of solar modules between the tap and the second terminal when calculating the measurement-based voltage ratio between the measured first voltage and the measured second voltage;
• Calculating the expected voltage ratio as the ratio of the number of solar modules between the first terminal and the second terminal to the number of solar modules between the tap and the second terminal when calculating the measurement-based voltage ratio between the measured third voltage and the measured second voltage;
• Calculating the expected voltage ratio as the ratio of the number of solar modules between the first terminal and the second terminal to the number of solar modules between the tap and the first terminal when calculating the measurement-based voltage ratio between the measured third voltage and the measured first voltage.

This is particularly suitable when the solar modules not only have identical characteristics, but are also exposed to an identical solar radiation.

• – wobei die erste Spannungssumme durch Addition von jeweils erwarteten Spannungen der zwischen dem Abgriff und dem ersten Anschluss elektrisch in Reihe geschalteten Solarmodule berechnet wird, und wobei die zweite Spannungssumme durch Addition von jeweils erwarteten Spannungen der zwischen dem Abgriff und dem zweiten Anschluss elektrisch in Reihe geschalteten Solarmodule berechnet wird, wenn das messbasierte Spannungsverhältnis zwischen der gemessenen ersten Spannung und der gemessenen zweiten Spannung berechnet wird;
• – wobei die erste Spannungssumme durch Addition von jeweils erwarteten Spannungen der zwischen dem ersten Anschluss und dem zweiten Anschluss elektrisch in Reihe geschalteten Solarmodule berechnet wird, und wobei die zweite Spannungssumme durch Addition von jeweils erwarteten Spannungen der zwischen dem Abgriff und dem zweiten Anschluss elektrisch in Reihe geschalteten Solarmodule berechnet wird, wenn das messbasierte Spannungsverhältnis zwischen der gemessenen dritten Spannung und der gemessenen zweiten Spannung berechnet wird;
• – wobei die erste Spannungssumme durch Addition von jeweils erwarteten Spannungen der zwischen dem ersten Anschluss und dem zweiten Anschluss elektrisch in Reihe geschalteten Solarmodule berechnet wird, und wobei die zweite Spannungssumme durch Addition von jeweils erwarteten Spannungen der zwischen dem Abgriff und dem ersten Anschluss elektrisch in Reihe geschalteten Solarmodule berechnet wird, wenn das messbasierte Spannungsverhältnis zwischen der gemessenen dritten Spannung und der gemessenen ersten Spannung berechnet wird.

The expected voltage ratio can alternatively be calculated, for example, as the ratio of a first voltage sum to a second voltage sum

• Wherein the first voltage sum is calculated by adding respectively expected voltages of the solar modules electrically connected in series between the tap and the first terminal, and wherein the second voltage sum is electrically connected in series by addition of respective expected voltages between the tap and the second terminal switched solar modules is calculated when the measurement-based voltage ratio between the measured first voltage and the measured second voltage is calculated;
• Wherein the first voltage sum is calculated by adding respectively expected voltages of the solar modules electrically connected in series between the first terminal and the second terminal, and wherein the second voltage sum is electrically connected in series by addition of respective expected voltages between the tap and the second terminal switched solar modules is calculated when the measurement-based voltage ratio between the measured third voltage and the measured second voltage is calculated;
• Wherein the first voltage sum is calculated by adding respective expected voltages of the solar modules electrically connected in series between the first terminal and the second terminal, and wherein the second voltage sum is electrically connected in series by adding respective expected voltages between the tap and the first terminal switched solar modules is calculated when the measurement-based voltage ratio between the measured third voltage and the measured first voltage is calculated.

Preferably, respective amounts of the voltages are added.

Such a procedure allows greater flexibility in the calculation of the expected voltage ratio. For example, the respective expected voltages of the solar modules can be calculated based on technical parameters, shading situations and / or current solar radiation of the solar modules. This allows consideration of the influence of different parameters in the case of interconnection of different solar modules, the influence of shading situations, which can be caused for example by trees or buildings, and the influence of different solar radiation, for example, due to natural phenomena such as clouds.

The tolerance value may preferably be either absolute or relative. It is typically chosen such that if the measured voltage ratio deviates from the expected voltage ratio by more than the predetermined tolerance value, it can be reliably assumed that at least one solar module of the number of solar modules has a defect.

The steps of measuring the first, second and / or third voltage preferably take place at an operating point with maximum power. This achieves a particularly high level of reliability of the measurement. This is due in particular to the flat module characteristics at this point of maximum power, which may for example also be referred to as maximum power point (MPP). Such a point of maximum power is typically uniform for all modules.

According to a development, a detected error condition is discarded if this is detected on at least one determined number of days in a row at a corresponding or equivalent time. It can thus be achieved that a fault condition is not erroneously detected, which is actually due to a shading situation which systematically occurs during a certain time of day. Such a shading situation can, for example, lead to only a certain part of the solar modules having a lower voltage at a certain time, because the corresponding module is completely or partially covered by a shadow.

For example, in a simple implementation, an equivalent time may be the same time, for example. Preferably, the same shading situations exist at the same time over the arrangement. This typically means that the real time changes on the respective days insofar as the shading situation changes due to the different trajectory of the sun.

• – eine Messeinrichtung zur Messung von mindestens zwei Spannungen der ersten Spannung, der zweiten Spannung und/oder der dritten Spannung,
• – eine Auswerteelektronik zum Berechnen des messbasierten Spannungsverhältnisses, Berechnen eines vom Abgriff und von den zum Berechnen des messbasierten Spannungsverhältnisses verwendeten Spannungen abhängigen erwarteten Spannungsverhältnisses und Feststellen eines Fehlerzustands der Anordnung, wenn das messbasierte Spannungsverhältnis und das erwartete Spannungsverhältnis um mehr als einen vorgegebenen Toleranzwert voneinander abweichen, und
• – Ausgabemittel zum Ausgeben eines erkannten Fehlerzustands.

The invention further relates to a device for checking an arrangement with a number of solar modules, which are electrically connected in series between a first terminal and a second terminal, wherein the device is designed for carrying out the method according to the invention. The device has the following:

• A measuring device for measuring at least two voltages of the first voltage, the second voltage and / or the third voltage,
• Evaluation electronics for calculating the measurement-based voltage ratio, calculating an expected voltage ratio dependent on the taps and voltages used to calculate the measurement-based voltage ratio, and determining a device fault condition if the measurement-based voltage ratio and the expected voltage ratio differ by more than a predetermined tolerance value; and
• - output means for outputting a detected error condition.

The device according to the invention enables an advantageous implementation of the method according to the invention and can be integrated, for example, in an inverter. In this case, all embodiments and variants of the method according to the invention mentioned above can be used. Illustrated benefits apply accordingly.

The measuring device may, for example, comprise voltmeters which are designed to measure the voltages. This can be used on known versions. The evaluation electronics can be embodied, for example, as an electronic control device which has processor means and associated memory means, program code being stored in the memory means which controls the behavior of the processor means in a corresponding manner.

The output means may be embodied, for example, by an optical display such as a light-emitting diode or a lamp, a display, a screen or the like directly perceptible optical display means. The output means can however, for example, also be implemented as an interface for a network connection such as a LAN connection or a wireless connection such as WLAN, Bluetooth or a cellular connection. This allows output of a detected fault condition to a remote device such as a central monitoring station.

The invention furthermore relates to a solar module arrangement having a number of solar modules which have been checked by means of a method according to the invention or by means of a device according to the invention. Such a solar module arrangement makes use of the advantages for a solar module arrangement mentioned with reference to the method according to the invention and the device according to the invention. It can be used on all mentioned embodiments of the method according to the invention and the device according to the invention. Illustrated benefits apply accordingly.

In particular, a solar module arrangement according to the invention typically has a particularly low probability of an unrecognized faulty solar module.

The invention further relates to a computer program product with a non-volatile storage medium on which instructions for carrying out a method according to the invention are stored. In this case, all embodiments and variants described with reference to the method according to the invention can be used. Illustrated benefits apply accordingly.

Short description of the drawing

The invention will be described in detail below with reference to the drawings. This shows schematically:

1 a solar module assembly with a device for checking an arrangement of solar modules.

Detailed description of the embodiment

1 shows a solar module assembly 100 which has a number of a total of five solar modules, namely a first solar module 110 , a second solar module 112 , a third solar module 114 , a fourth solar module 116 and a fifth solar module 118 , The five solar modules 110 . 112 . 114 . 116 . 118 are electrically connected in series between a first terminal 120 and a second port 124 , Over the first connection 120 and the second port 124 Thus, the summed total voltage U3, which of the five solar modules 110 . 112 . 114 . 116 . 118 is generated, tapped and converted by means of an inverter not shown in an AC voltage to the grid feed.

In the present case are the five solar modules 110 . 112 . 114 . 116 . 118 executed with technically identical characteristic values. In other words, these are type-identical solar modules. For this reason, it can be expected that with identical irradiation intensity of each of the solar modules 110 . 112 . 114 . 116 . 118 a substantially identical voltage is generated.

Between the third solar module 114 and the fourth solar module 116 there is a tap 122 , This can in particular for checking the solar module assembly 100 can be used as will be further described below.

In 1 is also a device 200 for checking an arrangement with a number of solar modules shown, which may be part of an inverter. The device 200 has a first measuring connection 210 , a second measuring connection 212 and a third measuring port 214 on. The first measuring connection 210 is present with the first connection 120 the solar module assembly 100 connected. The second measuring connection 212 is present with the tap 122 the solar module assembly 100 connected. The third measuring connection 214 is present with the second port 124 the solar module assembly 100 connected.

The device 200 also has a first voltmeter 220 and a second voltmeter 222 on, which together form a measuring device. The first voltmeter 220 is with the first measuring connection 210 and the second measuring port 212 connected. The second voltmeter 222 is with the second measuring connection 212 and the third measurement port 214 connected. The respective voltmeter 220 . 222 each measure a voltage (or magnitude of voltage) between the respective measuring terminals to which they are connected. Thus, in the present case, the first voltmeter measures 220 a voltage U1 between the tap 122 and the first connection 120 whereas the second voltmeter 222 a voltage U2 between the second terminal 124 and the tap 122 measures.

The device 200 also has a processor 230 and a non-volatile memory 232 on, which together form an evaluation. In the non-volatile memory 232 instructions stored by the processor are stored 230 can be executed. The processor 230 then performs a method according to the invention, as will be further described below.

First, the processor gets 230 the first voltage U1 from the first voltmeter 220 and the second voltage U2 from the second voltmeter 222 , in each case in a suitable coding and optionally after an amount formation, ie without signs. These are the voltages measured in each case, as described above. The processor 230 divides the first voltage U1 by the second voltage U2 and thus determines a measurement-based voltage ratio.

With the same solar radiation and perfect function of the five solar modules 110 . 112 . 114 . 116 . 118 It can be expected that the ratio of the first voltage U1 to the second voltage U2 is 3: 2. This is because the first voltage U1 is across three solar modules 110 . 112 . 114 is tapped, whereas the second voltage U2 via two solar modules 116 . 118 is tapped. The expected stress ratio is thus present 3: 2.

If the measurement-based voltage ratio has a value that does not deviate by more than a predetermined maximum deviation from the expected voltage ratio 3: 2, the device gives 200 no error message because expectation and measurement match. Otherwise, an error condition is detected. To output such a detected fault condition, the device 200 Further, an output means in the form of a lamp 240 which lights up when a fault condition is detected. This allows a user to recognize that the device 200 has detected an error condition.

The device 200 Furthermore, it has a mode of operation that can be used if there are different shading situations over the five solar modules during the day 110 . 112 . 114 . 116 . 118 are to be expected.

Then, at a given number of days in succession, at respective equivalent times, that is to say same shading situations, a respective check of the solar modules is carried out 110 . 112 . 114 . 116 . 118 executed. If an error state is always detected at corresponding equivalent times, but otherwise no error state is detected at other times, the error state detected at these times is ignored, since this is apparently based on a temporary occuring shading situation. In this case, then the lamp lights up 240 Not.

Claims (9)

Method for checking an arrangement ( 100 ) with a number of solar modules ( 110 . 112 . 114 . 116 . 118 ) electrically connected between a first terminal ( 120 ) and a second connection ( 124 ) are connected in series, wherein at least two voltages are measured from a set of voltages, characterized in that - the set of voltages comprises the following voltages: - a first voltage (U1) between a tap ( 122 ), which lies electrically between two solar modules, and the first connection ( 120 ), - a second voltage (U2) between the tap ( 122 ) and the second connection ( 124 ), and - a third voltage (U3) between the first terminal (U3) 120 ) and the second connection ( 124 ), and - the method further comprises the steps of: - calculating a measurement-based voltage ratio between - the measured first voltage (U1) and the measured second voltage (U2), or - the measured third voltage (U3) and the measured second voltage (U2 ), or - the measured third voltage (U3) and the measured first voltage (U1), - calculating one of the tap ( 122 ) and expected voltage ratios dependent on the voltages (U1, U2, U3) used to calculate the measurement-based voltage ratio, and - determining a fault condition of the device ( 100 ) when the measurement-based voltage ratio and the expected voltage ratio differ by more than a predetermined tolerance value. Method according to claim 1, characterized in that - the solar modules ( 110 . 112 . 114 . 116 . 118 ) each have identical characteristics. A method according to claim 1 or 2, characterized in that - the expected voltage ratio as the ratio of the number of solar modules ( 110 . 112 . 114 ) between the tap ( 122 ) and the first connection ( 120 ) to the number of solar modules ( 116 . 118 ) between the tap ( 122 ) and the second connection ( 124 ) is calculated when the measurement-based voltage ratio between the measured first voltage (U1) and the measured second voltage (U2) is calculated; The expected stress ratio as the ratio of the number of solar modules ( 110 . 112 . 114 . 116 . 118 ) between the first port ( 120 ) and the second connection ( 124 ) to the number of solar modules ( 116 . 118 ) between the tap ( 122 ) and the second connection ( 124 ) is calculated when the measurement-based voltage ratio between the measured third voltage (U3) and the measured second voltage (U2) is calculated; and the expected stress ratio as the ratio of the number of solar modules ( 110 . 112 . 114 . 116 . 118 ) between the first port ( 120 ) and the second connection ( 124 ) to the number of solar modules ( 110 . 112 . 114 ) between the tap ( 122 ) and the first connection ( 120 ) is calculated when the measurement-based voltage ratio between the measured third voltage (U3) and the measured first voltage (U1) is calculated. Method according to claim 1 or 2, characterized in that - the expected voltage ratio is calculated as the ratio of a first voltage sum to a second voltage sum, - wherein the first voltage sum is calculated by adding in each case expected voltages between the tap ( 122 ) and the first connection ( 120 ) electrically connected in series solar modules ( 110 . 112 . 114 ) and wherein the second voltage sum is calculated by adding in each case expected voltages between the tap ( 122 ) and the second connection ( 124 ) electrically connected in series solar modules ( 116 . 118 ) is calculated when the measurement-based voltage ratio between the measured first voltage (U1) and the measured second voltage (U2) is calculated; - wherein the first voltage sum by adding in each case expected voltages between the first terminal ( 120 ) and the second connection ( 124 ) electrically connected in series solar modules ( 110 . 112 . 114 . 116 . 118 ) and wherein the second voltage sum is calculated by adding in each case expected voltages between the tap ( 122 ) and the second connection ( 124 ) electrically connected in series solar modules ( 116 . 118 ) is calculated when the measurement-based voltage ratio between the measured third voltage (U3) and the measured second voltage (U2) is calculated; and - wherein the first voltage sum by adding in each case expected voltages between the first terminal ( 120 ) and the second connection ( 124 ) electrically connected in series solar modules ( 110 . 112 . 114 . 116 . 118 ) and wherein the second voltage sum is calculated by adding in each case expected voltages between the tap ( 122 ) and the first connection ( 120 ) electrically connected in series solar modules ( 110 . 112 . 114 ) is calculated when the measurement-based voltage ratio between the measured third voltage (U3) and the measured first voltage (U1) is calculated. A method according to claim 4, characterized in that - the respective expected voltages of the solar modules ( 110 . 112 . 114 . 116 . 118 ) based on technical parameters, shading situations and / or current solar radiation of the solar modules ( 110 . 112 . 114 . 116 . 118 ) be calculated. Method according to one of the preceding claims, characterized in that - the steps of measuring the first, second and / or third voltage at an operating point with maximum power take place. Method according to one of the preceding claims, characterized in that - a detected error condition is rejected if it is detected at least a fixed number of days in succession at an equivalent time. A method according to claim 7, characterized in that - at the same time the same shading situations over the arrangement ( 100 ) consist. Contraption ( 200 ) for checking an arrangement ( 100 ) with a number of solar modules ( 110 . 112 . 114 . 116 . 118 ) electrically connected between a first terminal ( 120 ) and a second connection ( 124 ) are connected in series, characterized in that the device ( 200 ) is designed to carry out the method according to one of the preceding claims and comprises: a measuring device ( 220 . 222 ) for measuring at least two voltages from the set of voltages, - evaluation electronics ( 230 . 232 ) for calculating the measurement-based voltage ratio, calculating one from the tap ( 122 ) and expected voltages ratio (U1, U2, U3) used to calculate the measurement-based voltage ratio, and determining an error condition of the device ( 100 ) if the measurement-based stress ratio and the expected stress ratio deviate from each other by more than a predetermined tolerance value, and - output means ( 240 ) for issuing a detected error condition.

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