DE10253864B4 - Method and arrangement for earth fault monitoring of a stator in star connection - Google Patents

Abstract

Method for monitoring star-connected windings (2a, 2b, 2c) of a stator (1) of an electric machine with non-effectively earthed neutral point (3) for earth faults, in which
The zero voltage is measured at the terminals of the stator (1) to obtain zero voltage measurements,
- the zero current resulting from the currents in the windings (2a, 2b, 2c) of the stator (1) is measured to obtain zero current readings;
Model values are determined in each case from the zero-voltage or zero-current measured values by means of a conversion module, wherein the conversion module determines the model values using a transmission function specific to the stator and wherein the transfer function is designed such that in the case of a stator free of earth leakage (1) the determined model values
In the case of formation from the zero voltage measured values, the respectively associated zero current measured values and
- in the case of formation from the zero-current measured values, correspond to the respectively associated zero-voltage measured values;
- the determined model values are subtracted from the respectively associated zero current or zero voltage measured values with the formation of differential values ...

Description

The The invention relates to a method for monitoring in star connection operated windings of a stator of an electric machine with not properly earthed star point on earth faults.

It is known to detect ground faults within star connected ungraded windings by means of ground fault detection relays operating on the zero or zero voltage criterion. In this case, earth faults can be detected with great certainty up to approx. 90% of the winding length; Errors close to the neutral point or in the star point itself can be detected with the aid of zero-system-forming harmonics. Such is from the German patent application DE 196 29 483 A1 discloses a method for monitoring star-connected windings of the stator of an electrical machine, in which zero-voltage and zero-current measured values are detected at the windings of a stator of an electrical machine. From the acquired measured values, the fundamental and one (for example, the third) harmonic are isolated for further processing with the aid of bandpass filters. The measurement signals thus formed are examined in a so-called evaluation part, for example, on their phase offset and exceeding predetermined threshold values and optionally generates a trigger signal for a current flow in the stator interrupting circuit breaker. With the aid of the measurement signals indicating the harmonics of the zero-current or zero-voltage measured values, furthermore, the setting of variable threshold stages can be made such that no triggering signal for the circuit breaker is produced in the case of a short-term asymmetrical operation of the electric machine or during switch-on and switch-off operations.

The execution a symmetrical winding, e.g. for a long stator section A maglev, is technologically complex and is, for example by cyclic layer exchange of the windings and additional earthing measures reached. A simple low cost mechanical laying of the winding without layer exchange leads against it inevitably to an imbalance of the windings, which in the detection of earth faults after the zero-current or zero-voltage criterion zero currents or Zero voltages caused that are on the order of the thresholds the measured variables is.

task The invention therefore provides a method for monitoring in star connection operated windings of a stator of an electric machine indicate ground fault with not properly earthed star point, with the even with a stator with unbalanced windings a ground fault is reliably detected.

to solution This object is proposed a method of the type mentioned, in which the zero voltage at the terminals of the stator under recovery measured from zero voltage measurements and derived from the currents in the Windings of the stator resulting zero current to obtain zero current readings is measured; from the zero voltage or zero current readings each determined by means of a conversion module model values, wherein the conversion building block model values using a for the stator specific transfer function determined and where the transfer function is designed in such a way that, in the case of an open-circuit-free stator, the determined model values when forming from the zero voltage measured values, the respectively associated zero-current measured values and when forming the zero current readings, the respective zero voltage readings correspond; the determined model values are determined by the respective associated Zero current or zero voltage measured values with formation of difference values subtracted and indicating a ground fault in a winding Error signal is generated when the difference values a predetermined Exceed threshold.

Of the essential advantage of the method according to the invention is that a ground fault detection based on difference values and not based on, for example, the asymmetries of Windings influenced respective zero voltage or zero current measurements he follows. With the help of formed by the described method Model values can the respective zero current or zero voltage measured values in earth fault-free case from the example resulting from asymmetries of the windings Measured value portions are cleaned up. In the earth fault-free case arise So difference values with a value close to zero, so with help a threshold comparison in a simple way the earth fault-free Case can be detected. In case of a ground fault occur on the other hand, difference values are significantly greater than zero, so with the help of Threshold comparisons easily detected even an earth fault can be. This relationship will be explained in more detail later.

Advantageously, a digital filter is used to determine the model values. So that can the inventive method can be realized very easily with a data processing system.

A advantageous embodiment the method according to the invention further provides that the coefficients for adjusting the digital filter during one Calibration phase from the zero voltage or zero current measured values as well the associated zero-sequence current or zero voltage readings. This way you can the digital filter to the transfer function be adapted to the stator without certain parameters of the Stators like inductance, Resistance and capacity must be known from the outset.

advantageously, For determining the coefficients of the digital filter, the filter response of the digital filter to the zero voltage or zero current readings the respective associated Zero current or zero voltage measurements with optimization of the coefficients of the digital filter. This is how the digital filter works in an optimization process set exactly so that the middle Error between the digital voltage from the zero voltage or zero current measured values and the respective associated Zero current or zero voltage measurements is minimal.

advantageously, the determination of the coefficients of the digital filter follows the Steiglitz-McBride algorithm.

alternative is in a further advantageous embodiment of the method according to the invention provided that the coefficients for adjusting the digital filter off the transfer function of Stators specified characteristics determined become. Thus, for the case that the exact parameters of the stator such. B. impedance and capacity already known from the outset, a setting of the digital filter with Help of these characteristics made become. The transfer function of the Stators would be in this case analytically derivable, so that on a calibration procedure can be waived.

A further advantageous embodiment the method according to the invention provides that at model values formed from the zero voltage measurements the differential measurements are generated by taking the zero current readings the respectively associated Model values are subtracted and the error signal is generated, when the difference values exceed a predetermined current threshold. A comparison of model values determined from the zero-voltage measured values with the respective associated Zero current measured values and a subsequent threshold value evaluation from the thus determined current-related difference values is due to the expected transfer function of the conversion module to accomplish more stable than the analog conducted voltage related procedures.

The invention also relates to an electrical device for monitoring star-connected windings of a stator of an electric machine with non-effectively grounded star point on ground faults, comprising voltage detection means for measuring and pre-processing zero-voltage measurements, current detection means for measuring and pre-processing zero-current measurements and an evaluation device, which generates an error signal indicative of a ground fault when values determined from the zero voltage and / or zero current measurements exceed a predetermined threshold. Such an arrangement is also the German Offenlegungsschrift mentioned above DE 196 29 483 A1 refer to.

Based in the known arrangement of the invention is also the task underlying, an electrical device to monitor of star connected windings of a stator electric machine with not properly earthed neutral point Ground faults to propose, with the even in asymmetrically designed stator windings a ground fault in the stator can be reliably detected.

To achieve this object, according to the invention, an electrical device of the abovementioned type is configured in such a way that the voltage detection device is connected on the output side to the input of a conversion module which determines model values from the respective zero-voltage measured values using a transmission function specific to the stator, wherein the transfer function is designed in this way in that, in the case of a stator without earth-fault, the determined model values correspond to the respective zero current measured values associated with the zero-voltage measured values. The conversion module is connected on the output side to an input of a differential former, another input of the differential former is connected to an output of the current detection device and the differential former is connected on the output side to the evaluation device. By converting the zero-voltage measured values into the conversion module, model values are determined with which, in the case of earth-fault-free operation, the portions of the zero current caused, for example, by asymmetries in the windings of the stator can be compensated. In this way, it is very easy to make a decision about the presence of a ground fault by means of a subsequent threshold comparison, since in the case of earth fault the model values no longer correspond to the zero-current measured values and thus can not compensate them nen; Due to the different transfer functions of the stator in the error-free and faulty case even difference values are generated in the faulty case in the differential former with a higher amplitude than that caused by unbalances in error-free case zero-current measurements.

to further explanation the invention is in

1 a schematic representation of the stator of an electric machine, in

2 a schematic representation of an embodiment of the device according to the invention, in

3 a schematic representation for explaining the calibration process and in

4 simplified phasor diagrams of zero current and zero voltage for the zero-earth fault and the faulty stator shown.

In 1 is a stator 1 a not shown in the following electrical machine with the windings 2a . 2 B and 2c shown schematically. The windings shown in the equivalent circuit diagram 2a . 2 B and 2c are in a not properly earthed neutral point 3 electrically connected to each other. Like the representation of the stator 1 can be further seen, have the head of the windings 2a . 2 B and 2c respectively indicated capacities 4a . 4b and 4c against earth. The input terminals 5a . 5b and 5c the windings 2a . 2 B and 2c are via a circuit breaker 6 each with a phase of a busbar 7 electrically connected. On the stator side of the circuit breaker 6 will the zero voltage on the windings 2a . 2 B and 2c of the stator 1 for example with the help of high-voltage resistors 8th detected and with the help of a voltage transformer 9 an entrance 21 a ground fault detection device 10 fed. At another entrance 22 the earth fault detection device 10 becomes the earth fault detection device 10 the determined via a conversion converter zero current with respect to the windings 2a . 2 B and 2c fed. About one in 1 the dashed connection will be the circuit breaker 6 from the ground fault detection device 10 triggers.

Are the windings 2a . 2 B and 2c of the stator 1 executed exactly symmetrical and will be at the input terminals 5a . 5b and 5c fed to a symmetrical current, so there is an ideal zero current reading of zero for the earth fault-free case. Likewise, ideally, the zero voltage readings also have the value zero. In real terms, the zero-voltage and zero-current measured values for symmetrically designed windings are close to zero in zero-earth-fault cases. In the symmetrical case, it is very easy to make a decision about the presence of a ground fault in the stator by comparing the respective measured values with predetermined threshold values 1 and, if necessary, the circuit breaker 6 to be triggered.

To ensure a ground fault safely over the entire length of the stator 1 In order to be able to detect, in addition to the fundamental vibration of the measured values, the harmonics of the nth order are also to be used for measuring and evaluating the zero currents or zero voltages, where n is a multiple of 3 represents. Here, the effect is exploited that the currents of these harmonics in the neutral point do not add up to zero. Harmonics of such orders can be impressed, for example via upstream converter or in the execution of a stator as a long stator, for example, for a maglev, over the vehicle itself.

However, the execution of exactly symmetrical windings is technologically very complex and is achieved for example in a long stator of a maglev train by cyclic position change of the winding and additional grounding measures. In a technically simple installation of the windings without layer exchange caused by coupling impedances between the individual windings 2a . 2 B and 2c different currents exceeding the capacities 4a . 4b and 4c drain to earth. Even at low values of imbalance, the resulting and at the terminals 5a . 5b and 5c of the stator 1 measured zero current above that zero current, which would occur in a ground fault and would thus lead to an unwanted tripping of the circuit breaker. Thus, no reliable detection of a ground fault is possible.

In order to still be able to perform a safe ground fault detection even with a single-ended stator, the ground fault detection device 10 as in 2 to execute shown. At an entrance 21 zero voltage measurements are u by means of a voltage detector 23 and at an entrance 22 Zero current measured values i by means of a current detection device 24 detected. The voltage detection device 23 and the current detection device 24 optionally comprise a pre-filtering and AD conversion of the measured values, wherein the solution according to the invention also includes an embodiment of the voltage and / or current detection device in the form of several electrically interconnected modules, the said tasks separately (pre-filtering, AD conversion, etc.) make. At the output of the voltage detection device 23 stand thus digitized zero voltage measurement values u _n and at the output of the current detection device 24 digitized zero current readings in. The digitized zero voltage readings u _n become a conversion module 25 supplied by means of which are determined from the digitized zero voltage measured values u _n model values i _n *. The digitized zero current measurements in and the model values i _n * become a difference image 26 supplied at its output difference values i _D = | i _n - i _n * | emits. These become a threshold level 27 which performs a comparison of the difference values i _D with a preset threshold value i _s and outputs an error signal F indicative of a ground fault when the difference values i _D exceed the preset threshold value i _s . This is done in the threshold level 27 the effective value of the difference value i _{D is} determined and then compared with the threshold value i _s . If the rms value of the difference value i _D is determined taking into account also the zero-system-forming harmonics, in this way a ground fault can occur over the entire length of the stator 1 (see 1 ) be recognized.

The conversion module 25 becomes in one of the actual operating phase of the earth fault detection device 10 preceded calibration phase to the effect that in the case of earth fault-free the model values i _n * determined from the digitized zero voltage measured values u _n correspond to the respective digitized zero-current measured values i _n as far as possible. This is in 3 shown. For the calibration phase it is assumed that the stator 1 earth fault-free. At the entrances 21 and 22 be during this calibration phase using the voltage detection device 23 and the current detection device 24 Zero voltage measured values u and zero current measured values i are detected and converted into digitized zero-voltage measured values u _n or digitized zero-current measured values i _n . The digitized zero-voltage measured values u _n are the transformation module not yet set 25 then determines model values i _n * from them. These are in the difference pictures 26 deducted from the respective associated digitized zero-current measured values i _n with the formation of differential values i _D.

Since the conversion module 25 is not set correctly at this time, the difference values i _{D will} assume a value not equal to zero. The difference values i _D become an optimization block 31 using the zero-current and zero-voltage measurements, for example according to the Steiglitz-McBride algorithm, the mean error of the difference between the response of the conversion module 25 (This corresponds to the respectively determined model values i _n *) and minimizes the digitized zero-current measured values. This is done according to the regulation

The optimization module 31 also determines in the context of the optimization coefficients K for setting the conversion block 25 , This is done, for example, using the Matlab function (THE MATH WORKS Inc., Natick, Mass., USA). [b, a] = stmcb (i n , i n *, nb, na), where a and b are the coefficient vectors for the conversion device 25 and nb and na indicate the lengths of the respective coefficient vectors a and b. The determined coefficients K become the conversion component 25 fed and this set using the coefficients K.

After this setting of the conversion block 25 In comparison with the model values i _n * determined before the adjustment, the model values i _n * 'are determined and in turn compared with the digitized zero-current measured values i _n to form differential values i _D. Upon successful completion of the optimization procedure and adjustment of the conversion block with correct coefficients, the determined difference values assume a value of zero (or below an acceptable tolerance threshold). The model values i _n * then correspond as far as possible to the digitized zero-current measured values in. In this case, the calibration phase is complete and the conversion module 25 is operated during operation for earth fault detection with the set coefficient K thus set correctly.

During the calibration phase, the algorithm of an iterative algorithm becomes the transfer function of the stator 1 determined, with the help of which the model values i _n * are subsequently determined from the digitized zero voltage measured values u _n . Consequently, characterizing parameters of the stator, such as inductance, capacitance and resistance, need not be known in order to determine the transfer function. Of course, if the complete parameters of the stator are known, it is also possible to determine the coefficients of the conversion module analytically. In this case, the calibration phase can be omitted.

In the 4a and 4b Finally, phasor diagrams for explaining the earth fault detection are shown. In the case of earth leakage, those in the stator 1 due to the asymmetrical design of the windings 2a . 2 B and 2c (see. 1 ) resulting zero currents only on the capacities 4a . 4b and 4c flow away. Consequently, there is a capacitively determined current-voltage system whose pointer representation in FIG 4a is shown. As can be seen, the current leads the voltage with an (ideal) phase shift of 90 °. By the formation of the model values i _n * and the subsequent difference formation between the current i and the model values i _v (in 4a shown in dashed lines) results in a resulting current of approximately zero. In the subsequent threshold comparison can thus be made in a simple manner, the decision that no ground fault exists.

In the case of an existing ground fault in the stator 1 arises from the previously capacitive system of the stator 1 a now inductively determined system, since the current from the windings 2a . 2 B and 2c can drain to earth via an ohmic-inductive zero impedance. As in 4b shown, the current of the voltage (ideal) with a phase shift of 90 ° nachlauf. In the 4a and 4b the same length of the voltage vector is assumed to be the same voltage u in both undisturbed and in the earth-fault case for the sake of simplicity. Since the conversion module 25 Even in the case of earth fault with the coefficients K, which were determined during the calibration phase for the case without earth fault, the model values i _{v are} determined from the voltage values, the current values i are now not compensated, but even increased. As a result of this effect, the difference between the case without earth leakage and the case involving earth fault is intensified in the difference values i _D and thus a decision based on a threshold value comparison is additionally simplified.

Analogous to the case illustrated here in which the model values are determined from the zero-voltage measured values and from the affiliated ones Zero current measured values were subtracted, is within the scope of the method according to the invention also the case of determining the model values from the zero current measured values and a difference between the model values and the zero-voltage measured values possible. The detailed However, the treated case is numerically more stable to handle and will therefore preferred.

Claims (8)

Method for monitoring star-connected windings ( 2a . 2 B . 2c ) of a stator ( 1 ) of an electrical machine with not properly grounded star point ( 3 ) to earth faults, in which - the zero voltage at the terminals of the stator ( 1 ) is measured by obtaining zero-voltage measured values, which is derived from the currents in the windings ( 2a . 2 B . 2c ) of the stator ( 1 ) is measured to give zero current readings; Each of the zero voltage or zero current measured values by means of a conversion module ( 25 ) Model values are determined, the conversion block ( 25 ) the model values using one for the stator ( 1 ) and the transfer function is designed in such a way that in the case of an open-circuit-free stator ( 1 ) the determined model values - when formed from the zero voltage measured values correspond to the respectively associated zero-current measured values and - when formed from the zero-current measured values, correspond to the respectively associated zero-voltage measured values; The model values determined are subtracted from the respective associated zero-sequence or zero-voltage measured values to form differential values, and an error signal indicating a ground fault in a winding is generated if the difference values exceed a predetermined threshold value. Method according to claim 1, characterized in that that - to the Determine the model values using a digital filter. Method according to claim 2, characterized in that that - coefficients to set the digital filter during a calibration phase from the zero voltage or zero current measured values as well as the respectively associated Zero current or zero voltage measurements are determined. Method according to claim 3, characterized that - to the Determining the coefficients of the digital filter, the filter response of the digital filter to the zero voltage or zero current readings respectively associated zero-current or zero voltage measurements with optimization of the coefficients of the digital filter. Method according to one of claims 2 to 4, characterized that - the Determining the coefficients of the digital filter according to the Steiglitz-McBride algorithm he follows. Method according to claim 2, characterized in that that - coefficients to set the digital filter off the transfer function of the stator specified parameters become. Method according to one of the preceding claims, characterized in that - at the model values formed from the zero voltage measured values, the difference measured values are generated by subtracting from the zero-current measured values the respective associated model values, and - generating the error signal if the difference values exceed a predetermined current threshold value. Electrical device for monitoring star-connected windings ( 2a . 2 B . 2c ) of a stator ( 1 ) of an electrical machine with not properly grounded star point ( 3 ) to earth faults, the - a voltage detection device ( 23 ) for measuring and preprocessing zero voltage measured values, - a current detection device ( 24 ) for measuring and preprocessing zero current measured values and - an evaluation device ( 27 ) which generates an error signal indicative of an earth fault when values determined from the zero voltage and / or the zero current measured values exceed a predetermined threshold value, characterized in that - the voltage detection device ( 23 ) on the output side with the input of a conversion block ( 25 ) connected using one of the stator ( 1 ) determined model values from the respective zero-voltage measured values, wherein the transfer function is designed such that in the case of a stator without earth-leakage ( 1 ) the determined model values correspond to the respective zero current measured values associated with the zero voltage measured values, - the conversion module ( 25 ) on the output side with an input of a subtractor ( 26 ), - another input of the subtractor ( 26 ) with an output of the current detection device ( 24 ) and - the subtractor ( 26 ) on the output side with the evaluation device ( 27 ) connected is.