State monitoring for converter

Abstract

By means of a converter controller (6) of a converter (4), operating data (B) of the converter (4) are repeatedly captured at a capture frequency (f1). Values (B') derived from the captured operating data (B) are ascertained and are filtered temporally and on the basis of the respective switching state of the converter (4). The derived values (B') are stored for a period of time predetermined by a completeness criterion (V). Furthermore, internal values (I) are assigned to the stored derived values (B'). Characteristic variables (K) are ascertained for the temporarily-stored derived values (B') of the time period predetermined by the completeness criterion (V) and for the assigned internal values (I), said characteristic variables characterising portions of the temporarily-stored derived values (B') at predetermined frequencies (f). Filtered characteristic variables (K') are ascertained for a number of ascertained characteristic variables (K). The filtered characteristic variables (K') are combined at an operating point (BP) of the converter (4). Furthermore, captured measurement variables (M) originating from the surroundings of the converter (4) are assigned to the operating point (BP). At least one operating point (BP) and the assigned measurement variables (M) are transmitted to a computing device (16) at a respective transmission time.

Description

[0001] Description

[0002] Condition monitoring for inverters

[0003] The present invention relates to a monitoring method for a frequency converter,

[0004] - whereby operating data of the inverter is repeatedly recorded by means of an inverter control system with a recording frequency,

[0005] - whereby values ​​are derived from the recorded operating data of the inverter,

[0006] - whereby operating points and associated measured variables are transmitted to a computing facility at the respective transmission times.

[0007] The processing unit allows for long-term data storage and further analysis. This long-term storage and further analysis are not part of the present invention, but rather build upon it.

[0008] The sensing frequency – this applies both to the prior art and to the present invention – is generally greater than 1 kHz. For example, the sensing frequency can be 8 kHz or 16 kHz. The sensing frequency usually corresponds to the clock frequency at which the inverter control system drives the inverter. The operating data can include, for example, voltages, currents, semiconductor temperatures (especially their junction temperature), ambient temperature, setpoints, the modulation level of the inverter's semiconductors, and other parameters. This also applies both to the prior art and to the present invention.

[0009] The present invention further relates to a control program, wherein the control program comprises commands which, when executed by the control system, cause the control system to perform the steps of such a monitoring procedure.

[0010] The present invention further relates to a control system, wherein the control system is programmed with such a control program, so that the control system performs such a monitoring procedure during operation.

[0011] The aforementioned items are known. The monitoring (more precisely, condition monitoring) of power electronic systems aims to determine the gradual deterioration of the inverter's system components based on the collected data. For this purpose, monitoring and data acquisition over a long period (10,000 operating hours and more) are necessary. Furthermore, the inverter's operating data is generated at very short intervals of less than 1 ms. In some cases, the interval is even less than 100 ps. Therefore, considering only the inverter's operating data, a very large amount of data can accumulate over the inverter's lifetime.

[0012] It is theoretically possible to store the entire data set. However, such storage is generally not feasible due to a lack of sufficient storage capacity in the inverter control unit and also in higher-level control system components. The data would therefore have to be transmitted to a computing unit. This computing unit, which could, for example, be part of the so-called cloud, can store the corresponding data volumes.

[0013] However, it is not enough to simply store the data. If the entire data set is to be stored, it must also be transmitted to the computer at the data rate at which it is generated on the control side. Due to a lack of sufficient data transmission capacity, this proves impossible in practice. Therefore, data compression must be performed along the signal processing chain from the inverter controller to the computer.

[0014] The inverter's operating data is acquired by the inverter controller. In the current state of the art, part of the data compression is typically achieved by the inverter controller transmitting only one of n1 values ​​to a higher-level control unit. The higher-level control unit implements a further part of the data compression by transmitting only one of n2 values ​​to a plant controller. The plant controller, in turn, transmits only one of n3 values ​​to the computing unit. n1, n2, and n3 are natural numbers greater than 1, usually significantly greater than 1.Alternatively, the inverter control could perform a statistical evaluation of n1 values ​​and transmit the resulting statistical parameters, such as mean and standard deviation, to the higher-level control unit. The higher-level control unit and the plant control system could then implement analogous procedures. However, such procedures do not guarantee that the corresponding data compression will preserve all relevant information about the inverter and its operation. In fact, there is a risk that relevant information will be lost.

[0015] The object of the present invention is to create possibilities by which, despite a significant reduction in the amount of data along the signal processing chain from the inverter control to the computing unit, it is ensured that the relevant information about the inverter is retained.

[0016] The problem is solved by a monitoring method with the features of claim 1. Advantageous embodiments of the monitoring method are the subject of dependent claims 2 to 6.

[0017] According to the invention, a monitoring method is created in which it is provided that

[0018] - that operating data of the inverter is repeatedly recorded by means of an inverter control system with a recording frequency,

[0019] - that values ​​derived from the recorded operating data of the inverter are determined, which are related to the respective switching state of the inverter and filtered over time,

[0020] - that the derived values ​​are stored for a period of time predetermined by a completeness criterion,

[0021] - that internal values ​​are additionally assigned to the stored derived values,

[0022] - that for the temporarily stored derived values ​​of the time period predetermined by the completeness criterion and the associated internal values, key figures are determined that characterize the proportions of the temporarily stored derived values ​​at predetermined frequencies,

[0023] - that filtered parameters are determined for a number of determined parameters,

[0024] - that the respective filtered parameters received are summarized for an operating point of the converter and that measured variables originating from the environment of the converter are further assigned to the operating point and

[0025] - that at least one operating point and the associated measured variables are transmitted to a computing device at each transmission time.

[0026] The derived values ​​are typically determined by the inverter controller. However, their storage for the predetermined time period is usually performed by a control unit superior to the inverter controller. The same applies to the assignment of internal values, the determination of the key performance indicators (KPIs), and the subsequent determination of the filtered KPIs. Therefore, the inverter controller typically transmits the derived values ​​to the superior control unit, which then receives them.

[0027] The aggregation of the filtered parameters to an operating point of the inverter is typically performed by a plant control system that is superior to the higher-level control unit. The same applies to the assignment of the measured values ​​originating from the inverter's environment. The higher-level control unit therefore transmits the filtered parameters to a plant control system at a specified frequency. The plant control system receives the filtered parameters. The transmission of the operating points and the associated measured values ​​to the computer is then also performed by the plant control system.

[0028] Referencing the switching state implies that the control chip "knows" the current switching state of the inverter. This is automatically the case because the inverter controller controls the inverter. Furthermore, referring to the switching state implies that the switching state changes at most with the detection frequency; for example, with a detection frequency of 16 kHz, it would change at 2 kHz, 4 kHz, or even 16 kHz, but not at, say, 32 kHz.

[0029] The time interval predetermined by the completeness criterion can be defined as needed. In the common case where the inverter supplies electrical energy to a rotary electric machine with a rotor and a stator, the predetermined time interval can, for example, correspond to a predetermined number of complete rotor revolutions. In this case, the number of complete revolutions is at least 1, but it can also be 2, 3, or more. Typically, the number of complete rotor revolutions corresponds to a time frame of a few milliseconds to approximately 1 second. When connected to an AC power grid, the predetermined time interval can correspond to a predetermined number of AC power grid cycles.However, the predetermined time period can also be defined differently, especially in the case of a connection to a DC source or a DC load, for example a battery or a photovoltaic system.

[0030] As part of determining the parameters that characterize the proportions of the buffered derived values ​​at predetermined frequencies, the amplitude of a single oscillation or the ratio of two oscillations relative to each other (e.g., their amplitude ratio or their phase angle) can be determined for a specific frequency – for example, a frequency of 300 Hz. This determination is carried out by the implementing system in addition to its actual control engineering task.

[0031] The filtered parameters are transmitted to the plant control system at a transmission frequency. This frequency is usually between 0.1 Hz and 10 Hz, often around 1 Hz. However, higher or lower frequencies are possible in individual cases.

[0032] As part of the assignment to an operating point, the filtered parameters are classified and summarized.

[0033] The measured parameters can be particularly relevant for the extent of cooling of the inverter. These can include, for example, the ambient temperature, the humidity, the intensity of active cooling of the inverter (e.g., by means of a fan), and the like.

[0034] The time interval between transmission times is usually between 30 seconds and 5 minutes.

[0035] Preferably, the inverter's operating data is acquired using a microchip in the inverter controller, which generates control commands for the inverter's semiconductor switches. This design is particularly simple and cost-effective. The microchip can be, in particular, an FPGA or an ASIC.

[0036] Preferably, if the operating data is recorded using the microchip, the derived values ​​are also determined using the microchip.

[0037] Preferably, the characteristic values ​​are determined by means of a Fourier analysis followed by the extraction of specific values ​​from a spectrum determined by the Fourier analysis, or by determining only Fourier coefficients at the predetermined frequencies, or by applying the Goertzel algorithm to the predetermined frequencies. These methods are simple, reliable, and efficient. Furthermore, these algorithms are already widely implemented, making them easy to use. Preferably, it is checked whether the operating points and associated measured values ​​transmitted at the respective transmission times meet a basic stability criterion. If the basic stability criterion is not met, the time interval between the transmission times is reduced. If the basic stability criterion is met, the time interval between the transmission times is maintained or increased.In particular, this adjustment of the time interval ensures that the relevant information is retained while minimizing the amount of data to be transmitted and stored.

[0038] Preferably, if the operating points and associated measured values ​​transmitted at the respective transmission times meet the basic stability criterion, it is checked whether they also meet an additional stability criterion. If the additional stability criterion is not met, the time interval between transmission times is maintained. If the additional stability criterion is met, the time interval between transmission times is increased. This optimizes the data rate of the data transmission to the computer.

[0039] The stability criteria can be defined as needed. In particular, the stability criteria can be met if and as long as more or less smooth curves are observed. If the expected behavior of the inverter is known, the stability criteria can alternatively or additionally be designed to check whether the transmitted operating points and measured values ​​correspond exactly, or at least approximately, to values ​​predicted based on the expected behavior of the inverter. It is even possible to implement a learning function. For example, if a change occurs for the first time, this would be recognized as "problematic," and the basic stability criterion would therefore not be met. However, if similar changes occur repeatedly thereafter, the system control can learn this and adjust or update the stability criteria accordingly.The basic stability criterion and the additional stability criterion must, of course, always be coordinated in such a way that the additional stability criterion is stricter than the basic stability criterion.

[0040] The problem is further solved by a control program with the features of claim 7. According to the invention, when executed by a control system, the commands cause the control system to execute a monitoring method according to the invention. The control system comprises at least the inverter control, often additionally a higher-level control unit and / or a plant control unit, and optionally further devices within the signal transmission chain. The effects of the control program correspond to those of the monitoring method according to the invention.

[0041] The problem is further solved by a control system with the features of claim 8. According to the invention, in a control system of the type mentioned at the outset, the control system is programmed with a control program according to the invention, such that the control system performs a monitoring method according to the invention when in operation.

[0042] The properties, features, and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more readily understandable in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawings. These show, in schematic representation:

[0043] FIG 1 an overview image and

[0044] FIGS. 2 to 8 Flowcharts.

[0045] According to FIG. 1, a load 1 is designed as a rotary electric machine 1, which has a rotor 2 and a stator 3. The load 1 is supplied with electrical energy from a power source 5 via a converter 4. In FIG. 1, the power source 5 is shown as a three-phase AC network. The power source 5 could also be of a different design. The load 1 could also be a load other than a rotary electric machine. However, the case shown in FIG. 1 is the most common.

[0046] The inverter 4 is controlled by an inverter controller 6. The operation of the inverter controller 6 is determined by first commands 7 of a control program 8. The execution of the first commands 7 by the inverter controller 6 causes the inverter controller 6 to execute a method, which is explained below in conjunction with FIG. 2. The method executed by the inverter controller 6 comprises, firstly, a control method for the inverter 4, i.e., the determination of control commands C with which the inverter 4 is controlled and with which, in particular, semiconductor switches 9 of the inverter 4 are controlled. Secondly, the method executed by the inverter controller 6 comprises part of a monitoring method. This monitoring method is the subject matter of the present invention. The control method as such is not the subject matter of the present invention.However, in many cases it is integrated into the monitoring process, more precisely into the part of the monitoring process executed by the inverter controller 6. According to FIG. 2, the inverter controller 6 receives setpoints G* for electrical quantities G, which the inverter 4 is to generate, in step S1. The electrical quantities G can be, for example, currents or voltages. Based on the setpoints G*, the inverter controller 6 determines the control commands C in step S2 and outputs the determined control commands C to the inverter 4 or the semiconductor switches 9 in step S3. The semiconductor switches 9 of the inverter 4 are controlled according to the control commands C. The semiconductor switches 9 can be configured as required. The illustration in FIG. 1, according to which the semiconductor switches 9 are configured as IGBTs, is purely exemplary.

[0047] Steps S1 to S3 correspond to the control procedure for the inverter 4. The further steps S4 to S8 of FIG 2 correspond to the part of the monitoring procedure executed by the inverter control 6.

[0048] In step S4, the inverter control 6 acquires operating data B from the inverter 4. This operating data can include, for example, electrical quantities G and other data, such as the modulation level of the semiconductor switches 9. In step S5, the inverter control 6 stores the acquired operating data B. This storage is performed by adding the operating data B to previously stored operating data B. The previously stored operating data B is therefore not overwritten. This applies at least to those operating data B whose (previous) acquisition time is less than a memory interval away from the current acquisition time.

[0049] In step S6, the inverter control 6 checks whether a data acquisition cycle has ended. If not, the inverter control 6 returns directly to step S1. If, however, it has ended, the inverter control 6 determines derived values ​​B' in step S7. The derived values ​​B' are related to the respective switching state of the inverter 4 and are time-filtered. Thus, on the one hand, the operating data B related to the same switching state of the inverter 4 are grouped together. On the other hand, the operating data B within each group are filtered. This filtering can, in particular, be a low-pass filter.

[0050] In step S8, the inverter controller 6 transmits the derived values ​​B' to a control unit 10 (see FIG. 1). The control unit 10 is superior to the inverter controller 6. Therefore, the control unit 10 is subsequently referred to as the superior control unit. If the separation into inverter controller 6 and the superior control unit 10 does not exist, step S8 can be omitted.

[0051] The inverter control 6 repeatedly executes steps S1 to S6 with a cycle time T1. The reciprocal of the cycle time T1 corresponds to a detection frequency f1. The detection frequency is typically several kHz, for example, 8 kHz or 16 kHz. Steps S7 and, if applicable, S8, on the other hand, are usually executed less frequently, for example, only after every fourth or eighth execution of steps S1 to S6.

[0052] The operation of the higher-level control unit 10 (provided it exists as a separate unit from the inverter control 6) is determined by the second commands 11 of the control program 8. The execution of the second commands 11 by the higher-level control unit 10 causes it to execute a procedure, which is explained below in conjunction with FIG. 3. The procedure executed by the higher-level control unit 10 forms a further part of the monitoring procedure.

[0053] According to FIG. 3, the higher-level control unit 10 receives the derived values ​​B' from the inverter control 6 in step S11. Step S11 can be omitted, analogous to step S8 in FIG. 2.

[0054] In step S12, the higher-level control unit 10 stores the derived values ​​B' received in step S11 (or already known to it). In step S13, the higher-level control unit 10 assigns additional internal values ​​I to the derived values ​​B' stored in step S12. These internal values ​​I can depend, for example, on a previous or current state of a system into which the load 1 is integrated. In the case of a previous state, this could be an initial state. The internal values ​​I could also be stored values ​​from another, not shown, converter. Another example of internal values ​​I is application information, such as process parameters or production sequences. A further example of internal values ​​I is values ​​from the environment of a controlled system, such as temperature or humidity.

[0055] In step S14, the higher-level control unit 10 checks whether a completeness criterion V is met. The completeness criterion V can be met, for example, if the rotor 2 of the electric machine 1 has completed a certain number of full revolutions since the last execution of steps S15 to S17. The number of revolutions can be 1, but it can also be greater than 1. In other cases, a predetermined number of network periods may have occurred, or a predetermined time period may have elapsed. If the completeness criterion V is not met, the higher-level control unit 10 returns to step S11. Otherwise, the higher-level control unit 10 executes steps S15 to S17.

[0056] In step S15, the higher-level control unit 10 determines characteristic values ​​K for the buffered derived values ​​B', which have been stored since the last execution of steps S15 to S17, and the associated internal values ​​I. The determined characteristic values ​​characterize the proportions of the buffered derived values ​​B' at predetermined frequencies f. In step S16, the higher-level control unit 10 determines filtered characteristic values ​​K' for a number of determined characteristic values ​​K. The filtering can, in particular, be low-pass filtering. In step S17, the higher-level control unit 10 transmits the filtered characteristic values ​​K' to a plant controller 12 (see FIG. 1). The plant controller 12 is superior to the higher-level control unit 10. It can be specifically assigned to the higher-level control unit 10 as required, or it can be superior to other controllers in addition to the higher-level control unit 10.If the division into the higher-level control unit 10 and the plant control unit 12 is not given, step S17 can be omitted.

[0057] The operation of the plant control unit 12 is determined by third commands 13 of the control program 8. The execution of these third commands 13 by the plant control unit 12 causes it to execute a procedure, which is explained below in conjunction with FIG. 4. The procedure executed by the plant control unit 12 constitutes the remaining part of the monitoring procedure.

[0058] According to FIG 4, the plant control 12 sets a cycle time T2 to an initial value T0 in a step S21.

[0059] In step S22, the plant control unit 12 receives the filtered parameters K' from the higher-level control unit 10. Step S22 can be omitted, analogous to step S17 in FIG. 3.

[0060] In step S23, the system controller 12 combines the filtered characteristic values ​​K' received in step S22 (or already known to it) into an operating point BP of the inverter 4. In step S24, the system controller 12 assigns measured values ​​M to the operating point BP. The measured values ​​M can be acquired, for example, by means of corresponding sensors 14 (see FIG. 1). The sensors 14 are located in the vicinity of the inverter 4. However, they can also be acquired by the system controller 12 in other ways. The measured values ​​M originate from the environment of the inverter 4. They can, for example, be characteristic of temperature, humidity, or other thermal quantities.

[0061] In step S25, the plant control system 12 checks whether the cycle time T2 has elapsed since the last execution of step S26. If the cycle time T2 has not elapsed, the plant control system 12 returns to step S22. Otherwise, the plant control system 12 executes step S26 and only then returns to step S22. In step S26, the plant control system 12 transmits at least one operating point BP and the associated measured variables M – preferably via the internet 15 (see FIG. 1) – to a computer 16. During step S26, a timer that records the elapsed time is also reset.

[0062] FIG 1 shows not only the preferred embodiment of a control system. In this embodiment, the control system comprises the inverter controller 6, the higher-level control unit 10, and the plant controller 12. However, it is also possible for the inverter controller 6 and the higher-level control unit 10, or the higher-level control unit 10 and the plant controller 12, to be combined into a single unit. It is even conceivable to combine all three components into one unit.

[0063] FIG 1 also shows an advantageous embodiment of the inverter control 6. Specifically, according to FIG 1, the operating data B of the inverter 4 is acquired by means of a microchip 17 of the inverter control 6. The microchip 17 is a microchip that also generates the control commands C for the semiconductor switches 9 of the inverter 4. Preferably, as can be seen from the illustration in FIG 1, the derived values ​​B' are also determined by means of the microchip 17. The microchip 17 is typically designed as an FPGA or as an ASIC.

[0064] There are several ways to implement step S15 of FIG. 3. For example, as shown in FIG. 5, a Fourier analysis of the temporarily stored derived values ​​B' can first be performed in step S31 to determine a spectrum Fou. In this case, the parameters K can then be determined in step S32 by extracting specific values ​​from the spectrum Fou. Alternatively, as shown in FIG. 6, only Fourier coefficients at the predetermined frequencies f can be determined as parameters K in step S41. Another alternative, as shown in FIG. 7, is to determine the parameters K in step S51 by applying the Goertzel algorithm to the predetermined frequencies f.The configurations shown in FIGS. 5 to 7 are implemented here by the higher-level control unit 10. However, depending on the design of the control system, they can also be implemented by another unit.

[0065] Furthermore, the procedure shown in FIG. 4 can also be advantageously modified. This is explained below in conjunction with FIG. 8.

[0066] FIG 8 is based on FIG 4. However, steps S61 and S62, and preferably also steps S63 and S64, are additionally present.

[0067] In step S61, the plant control system 12 checks whether the operating points BP and the associated measured variables M transmitted at the respective transmission times meet a basic stability criterion K1. For example, the plant control system 12 can determine (fit) parameters of a family of curves based on the operating points BP and the associated measured variables M and check whether the approximation of the operating points BP and the associated measured variables M by the curve defined in this way is possible with sufficient accuracy. For example, the plant control system 12 can check whether the deviations of the operating points BP and the associated measured variables M from the determined curve comply with a predetermined standard of variation.

[0068] If the basic stability criterion K1 is not met, the plant control system 12 proceeds to step S62. In step S62, the plant control system 12 reduces the cycle time T2 and thus the time interval between transmission points. öT is a suitably chosen step size. The plant control system 12 then proceeds to step S25. If, however, the basic stability criterion K1 is met, the plant control system 12 maintains or increases the time interval T2 between transmission points.

[0069] It is possible that steps S63 and S64 are not present. In this case, if the basic stability criterion K1 is met, the plant control 12 proceeds directly to step S25. However, as shown in FIG. 8, it is particularly preferred that the plant control 12 checks in step S63 whether the operating points BP and the associated measured variables M transmitted at the respective transmission times meet an additional stability criterion K2. For example, the plant control 12 can check whether the deviations of the operating points BP and the associated measured variables M from the determined curve comply with a further predetermined variation, whereby the further predetermined variation is smaller than the variation of the basic stability criterion K1.

[0070] If the additional stability criterion K2 is not met, the plant control system 12 proceeds directly to step S25. If, however, the additional stability criterion K2 is met, the plant control system 12 increases the cycle time T2 in step S64, and thus the time interval T2 between transmission times. Only then does the plant control system 12 proceed to step S25.

[0071] The present invention has many advantages. The greatest advantage is that, despite significant data reduction, it ensures that the relevant information is retained up to and including its transmission to the computer 16. Nevertheless, the effort required by the inverter control 6, the higher-level control unit 10, and the plant control unit 12 is kept to a minimum. Furthermore, by adjusting the cycle time T2, it can be ensured that the amount of data to be transmitted to the computer 16 can be optimized.

[0072] Although the invention has been further illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples and other variations can be derived from them by the person skilled in the art without leaving the scope of protection of the invention.

Claims

Patent claims 1. Monitoring procedure for a converter (4), - wherein operating data (B) of the inverter (4) are repeatedly recorded by means of an inverter control (6) of the inverter (4) with a detection frequency (f1), - wherein values ​​(B') derived from the recorded operating data (B) of the converter (4) are determined, which are related to the respective switching state of the converter (4) and filtered over time, - where the derived values ​​(B') are stored for a time period determined by a completeness criterion (V), - where internal values ​​(I) are additionally assigned to the stored derived values ​​(B'), - wherein, for the temporarily stored derived values ​​(B') of the time period predetermined by the completeness criterion (V) and the associated internal values ​​(I), characteristic values ​​(K) are determined that characterize the proportions of the temporarily stored derived values ​​(B') at predetermined frequencies (f), - wherein filtered parameters (K') are determined for a number of determined parameters (K), - wherein the filtered parameters (K') are summarized to an operating point (BP) of the converter (4) and further measured variables (M) originating from the environment of the converter (4) are assigned to the operating point (BP) and - wherein at least one operating point (BP) and the associated measured variables (M) are transmitted to a computing device (16) at each transmission time.

2. Monitoring method according to claim 1, characterized in that the acquisition of the operating data (B) of the converter (4) is carried out by means of a microchip (17) of the converter control (6), by means of which control commands (C) for semiconductor switches (9) of the converter (4) are generated.

3. Monitoring method according to claim 2, characterized in that the determination of the derived values ​​(B') is carried out by means of the microchip (17).

4. Monitoring method according to claim 1, 2 or 3, characterized by , that the parameters (K) are determined by means of a Fourier analysis and subsequent selection of specific values ​​from a spectrum (Fou) determined by means of the Fourier analysis, or by determining only Fourier coefficients at the predetermined frequencies (f), or by applying the Goertzel algorithm to the predetermined frequencies (f).

5. Monitoring method according to one of the above claims, characterized by , - that it is checked whether the operating points (BP) and associated measured variables (M) transmitted at the respective transmission times meet a basic stability criterion (K1), - that if the basic stability criterion (K1) is not met, the time interval between transmission times will be reduced and - that if the basic stability criterion (K1) is met, the time interval between transmission times is maintained or increased.

6. Monitoring method according to claim 5, characterized by , - that if the operating points (BP) and associated measured variables (M) transmitted at the respective transmission times meet the basic stability criterion (K1), it is checked whether the operating points (BP) and associated measured variables (M) transmitted at the respective transmission times meet an additional stability criterion (K2), - that if the additional stability criterion (K2) is not met, the time interval between transmission times will be maintained and - that if the additional stability criterion (K2) is met, the time interval between the transmission times will be increased.

7. Control program for a control system, wherein the control program comprises commands (7, 11, 13) which, when executed by the control system, cause the control system to perform the steps of a monitoring procedure according to any of the above claims.

8. Control system, wherein the control system is programmed with a control program (8) according to claim 7, such that the control system performs a monitoring method according to one of claims 1 to 6 during operation.

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