DE102007041248A1 - Device for detecting a load unbalance in a three-phase network - Google Patents

Abstract

The invention relates to a device (10) for detecting load imbalance in a three - phase system, wherein the rectified time profile of a three - phase electrical current through a three - phase three - phase line (99) is provided as proportional input voltage information (35, 45), characterized in that Input voltage information (35, 45) is supplied to a maximum value module (14, 24), that the auxiliary value information (36, 46) is provided by the maximum value module (14, 24) which an approximately constant held peak value of the input voltage information (35, 45) corresponds to a predetermined period of time, that the first auxiliary voltage information (36, 46) is fed to a proportional module (15, 25) that by the proportional module (15, 25) to the first auxiliary voltage information (36, 46) proportional second auxiliary voltage information (37, 47 ) is output / provided that the input voltage information (35, 45) and the second auxiliary voltage information (37, 47) are respectively fed to a comparison module (16, 26), by which an output voltage information (38, 48) is provided about the load unbalance.

Description

The The invention relates to a device for detecting a load unbalance in a three-phase network, the rectified time course a three-phase electric current through a three-phase three-phase line is provided as a proportional input voltage information.

It is well known that three-phase networks in the power supply area have three different supply lines, in which so-called three-phase loads are connected. Ideally, the burden a power supply network symmetrical, that is, the AC currents through the individual supply lines have the same frequency, are equal in amount and between the individual supply lines in each case by 120 ° against each other postponed. Analogously, the voltage is in a symmetrical Energy supply network characterized, namely that the voltages of the three supply lines at one measuring point opposite one Reference point the same frequency, the same amplitude but a phase shift each of 120 ° to each other exhibit. In this symmetrical case, the currents of the three add up Supply lines through a common return conductor to zero.

Of the Unbalanced state of a power supply network is mostly caused by a load imbalance, that is, the three individual supply lines are different in amount Stream flowed through, which, where appropriate, to undesirable balancing effects leads. The streams through the three supply lines add up in their common return conductor no longer to zero. Such an unbalanced state is in one pure power supply network in any case undesirable, but tolerable to a certain extent, For example, if the currents through the different Supply lines amount not differ by more than 20%.

An asynchronous motor is a three-phase electrical load which, due to the symmetrical mechanical construction of the windings, is considered to be ideally symmetrical. A load imbalance in an asynchronous motor is therefore a criterion for the presence of an error. Typically, asynchronous motors are fed via adapted to them three-phase inverter, which generate three offset by 120 ° to each other sinusoidal voltages which are variable in their frequency and their amplitude depending on the operating condition of the asynchronous motor to be achieved. Asynchronous motors can have rated values of a few kW up to several MW. The rated speeds at which the maximum frequency of the supply voltage to be generated is indirectly oriented are within a speed range of up to 10,000 min ^-1 . Asynchronous motors can also be connected without the use of an inverter to the electrical supply network with, for example, a frequency of 50 Hz or 60 Hz, the control options are then very limited.

to Error detection in asynchronous motors are up to date the art different protection devices common, especially thermal overload relays. These are usually based on the phased measurement of the current through the three-phase supply lines of an asynchronous motor, wherein the Error detection criterion is that the rms value of the Electricity over exceeds a predetermined limit over a certain period of time. This is for example by means of the heating of a bimetallic strip feasible, in which by a current flow, a thermal entry he follows.

adversely in this procedure is in particular that only the crossing a limiting current is detected, so that any errors with a Residual current below the limit current, which for example at a little loaded asynchronous motor can not occur be recognized. In particular, the error detection according to the state The technology with an uncontrolled asynchronous motor proves to be difficult because it requires an increased starting current during startup, the significantly exceed its rated current can. To a possible false triggering to avoid the error detection, usually become an increased limit of the current and / or an extended time constant selected so that the sensitivity of the protection thereby adversely reduced becomes.

outgoing From this prior art, it is an object of the invention, a Device for detecting a load unbalance, in particular in a Asynchronous motor to specify in a three-phase network, which mentioned Disadvantages avoids and a possible Lastunsymmetrie as quickly as possible recognizes.

These The object is achieved by a device for detecting a load unbalance in a three-phase network with the features specified in claim 1.

Accordingly, the device for detecting a load imbalance in a three-phase network of the type mentioned in the fact characterized in that the input voltage information is fed to a maximum value module, that the first module auxiliary voltage information is provided by the maximum value module, which approximately kon constantly held peak value of the input voltage information in a predetermined period of time corresponds to the first auxiliary voltage information is supplied to a proportional module, that is output / provided by the proportional module proportional to the first auxiliary voltage information second auxiliary voltage information that the input voltage information and the second auxiliary voltage information are each supplied to a comparison module, by which an output voltage information contained in the load unbalance is provided.

The Input voltage information is approximate piecewise sinusoidal Course on and sits in a symmetrical load case over a Period, which results from the reciprocal of the signal frequency, at 50 Hz, for example, 20 ms, from the string of 6 Clump together a sine function. The number 6 results from the Phase shift of 120 ° of individual streams and rectifying the respective negative half-wave of the sine function, so that the 6 crests each have a distance of 60 ° to each other exhibit.

One Fall below the input voltage information below the value of 85% of the respective maximum value can be used as a criterion for a possible unbalanced load be valued, as later explained in an example is.

Of the Maximum value of the first voltage information finds its equivalent in the first auxiliary voltage information provided by the maximum value module is formed.

Around a possible underrun of a value of the input voltage information below a fixed percentage of its maximum value, at the most but 85%, the maximum value module is a proportional modulus downstream, with the help of one of the first auxiliary voltage information proportional second auxiliary voltage information is available, For example, 50% of the second auxiliary voltage information.

Out a comparison of the input voltage information with the second Auxiliary voltage information by the comparison module gives you a first voltage information value when the input voltage information the second auxiliary voltage information falls below, this a criterion for the presence of an unbalanced load is. Unless undercut is present, the comparison module is a second voltage information value to disposal posed.

Consequently Is it possible, a possible unbalanced load, in particular with an asynchronous motor, even below its nominal current value comparatively fast too detect.

In Advantageously, the three phase currents are through a three-phase line by means of a measuring module phase by measurement and detected in each case a first, second and third proportional measuring current information convertible. A downstream rectifier module converts these Measuring current information in a fourth measuring current information. at This conversion is the largest amount of measured current information in terms of absolute value first, second and third measuring current information as fourth measuring current information provided. This is by means of a turn downstream converter module into a proportional Voltage information convertible and is the aforementioned Maximum value module or the comparison module as an input variable available.

In can advantageously as voltage information real voltages are used and as stream information real streams.

On this way it is possible the modules mentioned by means of electronic standard components to realize and it is for example a particularly mechanical robust construction of the device according to the invention allows. In addition allows This is a particularly simple production of the modules.

It However, it is also within the spirit of the invention if the measuring current and voltage information exchanged as data values between the modules become. The s is particularly advantageous when programmable electrical circuits or data processing equipment are available, on which the modules at least partially in the form of program modules are feasible. this makes possible In addition, the simple integration of additional functionalities, such as for example, the model-based determination of the warming of a Asynchronous motor based on measured currents.

When it proves advantageous if a proportionality factor of the proportionality module which is adjustable by the ratio of its output voltage information values is formed to its input voltage information values.

Consequently it is possible, the sensitivity of the device according to the invention with respect to Detection of a possible To change unbalanced load.

In a further embodiment, the second input module is preceded by one input of the comparison module to which the input voltage information was originally supplied tet, which has an adjustable proportionality factor.

On this way is realized another way to vary the sensitivity of the device.

In a particularly favorable embodiment is the comparison module still downstream of an evaluation module, which the timing of the output voltage information over a analyzed during certain period of time and when detecting a load unbalance a shutdown of at least one through the supply line supplied load initiated.

Thereby is monitored by the device according to the invention Load before a possible overload protected and the stability of the entire supply network is influenced in a positive way.

In a preferred embodiment One and / or more modules will continue to be used together the device according to the invention meaningful components, such as one or more backups, Switching devices and / or display devices, in a common casing arranged.

On this way is the number of protection and / or monitoring one or more electrical loads necessary overall components reduced and the assembly or maintenance of the device is simplified.

Especially Cheap is the modular design of the device in multiple cases, though for further protection and / or monitoring one or more electrical loads to realize additional functionalities are. These functionalities access a part of the measuring current present in the device and / or voltage information back, for example to the Output of the maximum value module. There is also a device conceivable at the module for current detection arranged in a separate housing is and its measuring current information via a pluggable connection transferable to other modules are, for example, on the one hand an inventively from the described modules existing device and also a in a separate housing arranged and not described in detail for determining device thermal heating a load, such as an asynchronous motor, such devices However, the person skilled in the art are known.

One Such construction reduces the number of individual modules required and / or Components in the overall system.

In a further embodiment are additional functionalities in the same housing integrated, as the inventive device. This is special Cheap, if several functionalities desired together are, for example, unbalanced load protection and protection against thermal warming with an asynchronous motor.

So is cheaper Way further reduces the number of housings used.

In a further embodiment of the subject invention, the Casing, in which individual and / or several modules and / or components arranged are, if possible flexible connection options on, for example, at least one screwable and / or clampable Connectivity. Also a common, pluggable connection bus system for information, Current and / or voltage transmission is conceivable.

So let yourself through the modular design, the handling of one or more housing on to reduce.

Further advantageous design options are the other dependent claims refer to.

Based the embodiments illustrated in the drawings are intended to Invention, further embodiments and other advantages closer to be discribed.

It demonstrate:

1 a first principle circuit for detecting a load unbalance,

2 a further detail circuit of a device according to the invention for detecting a load unbalance based on standard components,

3 an example of the course of an input voltage information over time at a symmetrical load and a frequency of 50 Hz and

4 an example of a time course of the input voltage information, the output voltage information and the second auxiliary voltage information in the event of a phase failure

1 shows a schematic diagram of a device according to the invention 10 , which represents the signaling connection between the individual, also adjacent, modules. A three-phase supply line 99 connects an unillustrated electrical load with a not shown electrical energy source. A module for current detection 11 For example, a current transformer generates according to the through the three phases of the supply line 99 flowing streams a first 31 , a second 32 and a third 33 Measuring current information. In the symmetrical load case, these three measuring current information have a sinusoidal course over time, are equal in magnitude and shifted with respect to their phase angle by 120 ° to each other. In the case of a controlled asynchronous motor, the frequency of the measuring currents represented by the measuring current information can be between less than 10 Hz and more than 10 kHz.

In case of a phase failure, for example due to a defective supply line 99 Accordingly, no current flows through the affected phase of the supply line and a current whose size depends on the supplying network or a feeding inverter, respectively, through the remaining other phases.

In the case of a single-pole short-circuit, the three-phase supply line flows through the affected phase 99 an increased current.

These three measuring current information 31 . 32 . 33 become a rectifier module 12 fed, which at its output in the form of a fourth measuring current information 34 in each case the highest amount of measured current information is available. In the case of real electrical measuring currents, this is done for example via a B6 rectifier.

The fourth measuring current information 34 is in a converter module 13 in a proportional voltage information 35 transformed. In the case of a real electric current, this can be done for example via a measuring resistor, through which a real measuring current 34 flows and generates a proportional voltage drop across the measuring resistor.

Alternatively, for example, in the case where the voltage information 35 digitized values, ie pure data values, are possible, purely mathematical multiplication by means of a programmable integrated circuit, as described below. These data values are then processed, for example, in the form of integrated, programmable circuits, so-called ASICS, in which case the described functionalities of a plurality of individual modules can advantageously be combined.

The voltage information 35 becomes a maximum value module 14 supplied as an output in the form of a first auxiliary voltage information 36 a maximum value of the input voltage information 35 over an immediately preceding period of, for example, a few seconds. This voltage information value is a measure of how high the maximum value of one of the three measurement current information 31 . 32 . 33 over a previously considered period is and serves as a reference value to make any amount deviations of the individual measurement current information to each other recognizable. In the case of real voltage values, this can be achieved, for example, by means of a capacitor, a discharge resistor and a diode. The discharge resistor is used here for the realization of the time window within which the maximum value should be done tion, for example, a few seconds. A further embodiment of a further maximum value module is explained below with reference to an example.

The first auxiliary voltage information 36 is in a proportional module 15 in a proportional second auxiliary voltage information 37 converted and together with the input voltage information 35 a comparison module 16 fed. The ratio of the second auxiliary voltage information 37 to the first auxiliary voltage information 36 is through the proportional module 15 predetermined, for example, lies between the values 50% and 85% and affects the sensitivity of the device according to the invention 10 , The lower this ratio, the less sensitive is the sensitivity to a possible unbalanced load.

The comparison module 16 gives as output an output voltage information 38 from which then a possible unbalance of the load or a unbalanced load can be derived, as shown later by way of example.

2 shows an embodiment 20 the device according to the invention for detecting a consumer asymmetry with a further maximum value module 24 , another proportional module 25 and another comparison module 26 , There is a functional analogy to the modules 14 . 15 and 16 , what a 1 are shown. In an advantageous manner, the individual modules in this example have electronic standard components, for example a resistor, a capacitor, a diode or a differential amplifier. By using such standard components is a relation to environmental influences, such as mechanical vibrations or temperature fluctuations, particularly robust construction of the device according to the invention ensured. The manufacture of such devices is also very easy to perform.

In the example chosen, the input is a real input voltage 45 which the further maximum value module 24 is supplied. This be consists of three resistors R1, R2 and R3, a diode D1, a capacitor C1 and a first differential amplifier N1, wherein the maximum value determination via the capacitor C1 and the diode D1 is realized. The output of the further maximum value module 24 is a real first auxiliary voltage 46 , The real voltages used in this example are, for example, in the range of 0 V to 10 V common for such circuits.

The further proportional module 25 is according to 2 realized as a voltage divider, which consists of the two resistors R4 and R5 and whose output is a real second auxiliary voltage 47 is. The ratio of the resistance R5 to the sum value of R4 and R5 describes the ratio between the second auxiliary voltage 47 and the first auxiliary voltage 46 , To realize a ratio of 50%, the resistors R4 and R5 are identical to choose, for example, each 100 kΩ.

The further comparison module 26 consists in this example of two resistors R6 and R7 and a second differential amplifier N2 in a standard interconnection. The output of the device shown here is a real voltage 48 which can assume a respective first voltage value or a second voltage value and from which the presence of a load imbalance can be derived.

3 shows the time course of the rectified input voltage information 35 in the case of a symmetrical load at a frequency of 50 Hz according to the example 1 , This results in a period of 20 ms for this example, which corresponds to a phase angle of 360 °. The minimum voltage information value over the illustrated period is about 85% of the maximum value over the illustrated period. This corresponds to the cosine function of the angle of 30 °, since the input voltage information 35 is composed over a period length of 6 summits of a sine function, each peak covering a range of 60 °. The angular distance of a summit maximum to the adjacent minimum value thus results in 30 °.

One Falling below the limit of 85% of the maximum value is in principle as criterion for to consider the presence of a load imbalance. Um by measuring errors or stochastic fluctuations caused possible false triggering but should a value lower than 85% be chosen, for example, 75%.

4 schematically shows the time course of the input voltage information 35 , the second auxiliary voltage information 37 and the output voltage information 38 in case of failure of a phase according to the example 1 , A minimum voltage information in this example is approximately 50% of the maximum value, which corresponds to the value of the cosine function of 60 °, where 60 ° is the angular distance from the center of a sine group to the adjacent minimum value. The second auxiliary voltage information is also approximately 50% of the maximum value of the input voltage information by means of the proportional module 35 set. In this case, a load imbalance is detected because the input voltage information 35 assumes partially smaller values than the approximately constant second auxiliary voltage information 37 , Accordingly, the output voltage information decreases 38 in this case, a minimum first value, or in the other possible case, if not underrun, a maximum value. A subordinate and not shown evaluation module also evaluates the output voltage information.

Due to measurement errors or stochastic fluctuations, it is possible that a single slight undershooting of the input voltage information under the second auxiliary voltage information within a period of several seconds is not based on a real unbalanced load. This can be caused, for example, by stochastic fluctuations or measurement errors. Accordingly, to avoid possible false triggering by the possibly downstream evaluation module, it is expedient to regard a load imbalance as detected only in the case when there is at least one repeated undershoot, for example at least ten underflows within a maximum of one second. Alternatively, such a possible false triggering can also be achieved by reducing the sensitivity of the device by means of a suitable choice of the proportionality factor of the proportional modulus ( 15 . 25 ), for example 75% or less.

In the case, that the voltage information values are real voltages move these are preferably within the usual for comparable circuits Voltage band, for example, between 0 V and 10 V.

10

contraption for detecting a load imbalance

11

measurement module

12

Rectifier module

13

converter module

14

Maximum value module

15

proportional module

16

comparison module

20

embodiment a device for detecting a consumer imbalance

24

additional Maximum value module

25

additional proportional module

26

additional comparison module

31

first Measuring current information

32

second Measuring current information

33

third Measuring current information

34

fourth Measuring current information

35

Input voltage information

36

first Auxiliary voltage information

37

second Auxiliary voltage information

38

Output voltage information

45

input voltage

46

first auxiliary voltage

47

second auxiliary voltage

48

output voltage

60

value the voltage information

99

three phase supply line

Claims (12)

Contraption ( 10 ) for detecting a load unbalance in a three-phase network, wherein the rectified time course of a three-phase electrical current through a three-phase three-phase line ( 99 ) as a proportional input voltage information ( 35 . 45 ), characterized in that the input voltage information ( 35 . 45 ) a maximum value module ( 14 . 24 ) is supplied by the maximum value module ( 14 . 24 ) a first auxiliary voltage information ( 36 . 46 ), which has an approximately constant peak value of the input voltage information ( 35 . 45 ) within a predefinable time period corresponds to the first auxiliary voltage information ( 36 . 46 ) a proportional module ( 15 . 25 ) is supplied by the proportional module ( 15 . 25 ) one to the first auxiliary voltage information ( 36 . 46 ) proportional auxiliary auxiliary voltage information ( 37 . 47 ) is output / provided that the input voltage information ( 35 . 45 ) and the second auxiliary voltage information ( 37 . 47 ) each a comparison module ( 16 . 26 ), by which an output information about the load unbalance contained output voltage information ( 38 . 48 ). Apparatus according to claim 1, characterized in that a plurality of phase currents through a three-phase line ( 99 ) by means of a measuring module ( 11 ) can be detected metrologically and in each case a first ( 31 ), second ( 32 ) and third ( 33 ) proportional measuring current information are convertible, that by means of a rectifying module ( 12 ) from the first ( 31 ), second ( 32 ) and third ( 33 ) Measuring current information a fourth measuring current information ( 34 ) which can be generated by a converter module ( 13 ) into a proportional input voltage information ( 35 . 45 ), which is provided as an output. Device according to claim 1 or 2, characterized in that at least one input voltage information ( 35 ) and / or output voltage information ( 38 ) and / or auxiliary voltage information ( 36 . 37 ) a real electrical voltage and / or at least one current information ( 31 - 34 ) a real electrical current and / or at least one voltage and / or current information ( 31 - 38 ) is a digitized value. Device according to one of the preceding claims, characterized in that between the second auxiliary voltage information ( 37 . 47 ) and first auxiliary voltage information ( 36 . 46 ) by means of the proportional module ( 15 . 25 ) adjustable proportionality factor is formed. Device according to one of the preceding claims, characterized in that the comparison module ( 16 . 26 ) to which the input voltage information ( 35 . 45 ) is supplied, on the input side, an additional proportional module is connected upstream, whose input is the input voltage information ( 35 . 45 ) and by the one to the input voltage information ( 35 . 45 ) proportional auxiliary auxiliary voltage information as an input variable for the comparison module ( 16 . 26 ). Device according to one of the preceding claims, characterized in that the comparison module ( 16 . 26 ) an evaluation module is connected downstream, by which a shutdown of at least one connected load can be initiated. Device according to one of the preceding claims, characterized in that at least one of said modules ( 11 - 16 ) has at least one electronic component and / or that at least one of said modules ( 11 - 16 ) has at least one integrated circuit. Device according to one of the preceding claims, characterized in that at least two modules of said modules ( 11 - 16 ) are realized in a single common module. Device according to one of the preceding claims, characterized in that at least one of the aforementioned modules and / or optionally further components arranged in a common housing are. Device according to claim 9, characterized in that that at least connecting element for a detachable connection, in particular clampable or pluggable quick connectors are arranged. Device according to one of the preceding claims, characterized in that for detecting a load imbalance the ratio of the values of the second auxiliary voltage information ( 37 . 47 ) to the values of the first auxiliary voltage information ( 36 ) in the range of 50% to 85%, preferably 75%, of the proportional modulus ( 15 . 25 ) is given. Device according to one of the preceding claims, characterized characterized in that at least one module as a program product a programmable electrical circuit or a data processing system is realized.