CN1279365C - Satellite connection earthing method and apparatus for monitoring stators - Google Patents

Abstract

A star connected stator (1) virtual earth (3) connection monitoring (10) procedure measures the stator terminal (5a-c) null voltage and coil (2a-c) currents to give null current for difference calculation of a model value using a digital filter with calibrated and optimized coefficients. An Independent claim is included for equipment using the procedure.

Description

Method and apparatus for monitoring grounding of a star-connected stator

technical field

The invention relates to a method for monitoring the grounding of a star-connected winding of a stator of an electric machine, the winding having a star point which does not function as a ground.

Background technique

It is known that in windings with ungrounded star point, star-connected operation, grounding can be detected by means of ground detection relays operating according to the zero-current or zero-voltage standard. In this case, ground faults up to approximately 90% of the winding length can be detected with high reliability; faults in the vicinity of the star point or in the star point itself can be detected with the aid of harmonics forming a zero-phase-sequence system. A method for monitoring the star-connected windings of a motor stator is therefore disclosed in German patent publication DE 19629483 A1, wherein zero-voltage and zero-current measured values are recorded at the motor stator windings. For further processing, a fundamental oscillation and a (for example third) harmonic are each isolated from the acquired measured values by means of a bandpass filter. The measurement signal formed in this way is checked in a so-called processing section, for example, for its phase change and whether a predetermined threshold value is exceeded, and if necessary a trigger signal is generated for a power switch which interrupts the current in the stator. With the aid of harmonics of the zero-voltage or zero-current measured value, variable threshold levels can also be set for the given measuring signal in such a way that the motor is temporarily asymmetrically operated or is not switched on and off. Generate trigger signal for power switch.

For long stator segments such as magnetic levitation tracks, the implementation of symmetrical windings is technically expensive and is achieved, for example, by periodic winding position replacement and additional grounding measures. On the other hand, a simple and inexpensive mechanical laying of the winding without displacement necessarily leads to an asymmetry of the winding which, when grounded according to the zero current and zero voltage standard, results in zero current and zero voltage in the order of magnitude of the measured value response threshold.

Contents of the invention

Therefore, the technical problem to be solved by the present invention is to provide a method for monitoring the grounding of the windings of the stator star connection of the motor, the winding has a star point that does not function as a grounding, and this method can also be used in asymmetrical Reliable identification of grounding in the stator windings.

In order to solve this technical problem, a method of the type mentioned at the beginning of the text is proposed, in the method of the invention, the zero voltage at the stator terminals is measured under the condition that a zero voltage measurement is obtained, and the zero voltage at the stator terminals is measured under the condition that a zero current measurement is obtained The zero current given by the stator winding, the model value is determined from this zero voltage or zero current measured value, respectively, and the determined model value is subtracted from the respectively associated measured value of zero voltage or zero current to form the difference value, and if the difference exceeds a predetermined threshold, a fault signal is generated indicating that the winding is grounded. In this case, the model value is determined by means of a conversion unit, wherein the conversion unit is configured such that, in the case of an ungrounded stator, the model value given by it corresponds to the respective associated The zero-current measured value corresponds, and when formed from the zero-current measured value, to the respective associated zero-voltage measured value. An important advantage of the method according to the invention is that the ground fault detection is performed on the basis of difference values and not on the basis of individual zero-voltage measured values or zero-current measured values, for example due to winding asymmetry effects. With the aid of the model values formed by the method described, measured value components of the respective zero-current measured value or zero-voltage measured value without grounding due to winding asymmetry, for example, can be eliminated. In other words, the resulting difference has a value close to zero in the absence of grounding, so that the absence of grounding can be detected in a simple manner by means of a threshold value comparison. Conversely, in the event of a ground fault, a difference value significantly greater than zero results, so that a ground fault can also be detected in a simple manner by means of a threshold value comparison. This association will be described in detail later.

Preferably, a digital filter is used for determining the model value. As a result, the method according to the invention can be realized very simply with a data processing device.

Furthermore, in a preferred embodiment according to the invention, the coefficients for setting the digital filter are formed in the calibration phase from the zero-voltage measured value or the zero-current measured value and the respective associated zero-current measured value or zero-voltage measured value to make sure. In this way it is possible to match the digital filter to the transfer function of the stator without prior knowledge of certain parameters of the stator, such as inductance, resistance and capacitance.

In order to determine the coefficients of the digital filter, the filter response of the digital filter to the zero voltage measured value or the zero current measured value is preferably compensated for the respective associated zero current measured value or zero voltage measured value in order to optimize the coefficients of the digital filter . In this way, the digital filter is set precisely during the optimization process in such a way that the comparison value determined by means of the digital filter from the zero voltage measured value or zero current measured value is compared with the respective associated zero current measured value or zero current measured value. The average error between voltage measurements is minimal.

Preferably, the determination of the digital filter parameters is implemented according to the Steiglitz-McBride algorithm.

Alternatively, in another preferred embodiment of the method according to the invention, the coefficients for setting the digital filter are determined as a function of given characteristic quantities of the stator transfer function. Thus, for the case in which exact characteristic quantities of the stator, such as impedance and capacitance, are known in advance, the setting of the digital filter can be carried out by means of these characteristic quantities. In this case, the transfer function of the stator can be derived analytically, so that the calibration process can be dispensed with.

In a further embodiment of the method according to the invention, when the model value formed from the zero voltage measured value produces a difference measured value, the respective associated model value is subtracted from the zero current measured value, when the difference exceeds a predetermined A fault signal is generated when the current threshold is exceeded. Comparison of the model values determined from the zero-voltage measurement with the respective associated zero-current measurement and a threshold value evaluation from the current-dependent difference thus determined, in contrast to similarly performed voltage-dependent methods due to the conversion unit It is more stable to realize the expected transfer function.

The invention also relates to an electrical device for monitoring the star-connected operation of the stator of an electrical machine with grounded windings having a non-grounded star point, the device having: for measuring and preprocessing zero voltage measurements voltage acquisition means, current acquisition means for measuring and preprocessing the zero current measurement, and processing means for generating A fault signal indicating ground. Such a device is also mentioned in the German patent publication DE 19629483 A1 mentioned at the beginning of this text.

On the basis of the known devices, the technical problem to be solved by the present invention is to propose an electrical device for monitoring the grounding of the winding of the motor stator star connection operation, the winding has a star point that does not function as a ground, and the use of this The device can also detect grounding of the stator if the stator winding is made asymmetrically.

In order to solve the above-mentioned technical problems, according to the present invention, the electric equipment of the above-mentioned type is constituted in such a way that the output terminal of the voltage acquisition device is connected to the input of the conversion unit, which determines the model value from each zero-voltage measurement value; The output of the conversion unit is connected to one input of a difference seeker, the other input of the difference seeker is connected to an output of the current acquisition device, and the output of the difference finder is connected to the processing device. By converting the zero-voltage measured value in a conversion unit, a model value is determined with which a component of the zero current, for example due to asymmetry of the stator winding, can be compensated for without grounding. In this way, the presence or absence of a ground fault can be determined very simply by means of a subsequent threshold value comparison, since the model value no longer corresponds to a zero-current measured value if a ground fault is involved and can therefore also not be compensated for; since Different transfer functions of the stator without grounding and with grounding, in the case of grounding the resulting difference in the differenciator even has a higher zero current measurement than in the case of no grounding due to asymmetry amplitude.

Description of drawings

In order to further illustrate the present invention, it is shown in the figure:

Figure 1 shows a schematic diagram of a motor stator,

Figure 2 shows a schematic diagram of an embodiment of the device according to the invention,

Figure 3 shows a schematic diagram for explaining the calibration process,

Figures 4a and 4b show simplified vector diagrams for zero current and zero voltage for an ungrounded and grounded stator.

Detailed ways

FIG. 1 schematically shows a stator 1 in an electrical machine (not further shown), which has windings 2a, 2b and 2c. The windings 2 a , 2 b and 2 c shown in the equivalent circuit are electrically connected to one another at a neutral grounded star point 3 . The representation of the stator 1 also shows that the conductors of the windings 2a, 2b and 2c have shown capacitances 4a, 4b and 4c to ground, respectively. Input terminals 5 a , 5 b and 5 c of windings 2 a , 2 b and 2 c are electrically connected to a phase line of busbar 7 via power switch 6 , respectively. On the stator side of the power switch 6, for example, the zero voltage on the windings 2a, 2b and 2c of the stator 1 is collected by means of a high-voltage resistor 8, and is sent to the input 21 of the grounding collection device 10 by means of a voltage converter 9 . The zero current for windings 2 a , 2 b and 2 c determined by switching converters is supplied to ground fault detection device 10 at a further input 22 of ground fault detection device 10 . The power switch 6 is activated by the ground detection device 10 via the connection indicated by the dashed line in FIG. 1 .

If the windings 2a, 2b and 2c of the stator 1 are made strictly symmetrical and supply them with symmetrical currents at the input terminals 5a, 5b and 5c, the resulting ideal zero current measurement without grounding is zero. Similarly, the zero-voltage measured value ideally also has a value of zero. In fact in symmetrically made windings the zero voltage and zero current measurements are close to zero without grounding. In the case of symmetry, it is extremely simple to determine whether a ground fault has occurred in the stator 1 by comparing the measured values with predetermined threshold values, and to trigger the power switch 6 if necessary.

In order to be able to detect ground faults reliably over the entire length of the stator 1 , for the measurement and evaluation of zero currents or voltages, in addition to fundamental oscillations of the measured values, harmonics of the nth order, where n is a multiple of 3, are also taken into account in each case. The effect that the currents of these harmonics do not sum to zero at the star point is utilized here. These sub-harmonics can be realized, for example, by pre-connected converters, or, for example, in the case of magnetic levitation tracks, the stator can be realized as a long stator, embodied by the train itself.

However, the implementation of strictly symmetrical windings is technically expensive and is obtained in long stators such as magnetic levitation tracks by periodic displacement of the windings and additional grounding measures. In a technically simple winding arrangement without displacement, different currents are formed due to the coupling impedance between the individual windings 2a, 2b and 2c, which flow to ground via the capacitors 4a, 4b and 4c. Even at small asymmetry values, the resulting zero current and measured at the terminals 5a, 5b and 5c of the stator 1 can exceed the respective zero current at the occurrence of grounding and lead to an undesired triggering of the grounding power switch . As a result, reliable identification of grounding is no longer possible.

In order to be able to detect ground faults reliably even in asymmetrical stators, however, a ground fault detection device 10 as shown in FIG. 2 is provided. At input 21 , a zero voltage measured value u is detected by means of a voltage detection device 23 , and at input 22 a zero current measured value i is detected by means of a current detection device 24 . The voltage detection device 23 and the current detection device 24 may include pre-filters and AD converters for the measured values, wherein, according to the solution of the invention, the voltage and/or current data in the form of a plurality of electrically interconnected modules are also included. In the embodiment of the collection device, each module completes the tasks (pre-filter, AD conversion, etc.) independently. A digitized zero-voltage measured value _un is thus formed at the output of the voltage detection device 23 and a digitized zero-current measured value _in is formed at the output of the current detection device 24 . The digitized zero-voltage measured value u _n is sent ^{to a conversion unit 25 , by means of which a model value in*} _is determined from the digitized zero-voltage measured value u _n . The digitized zero current measured value i _n and the modeled value i _n ^* are sent to a differencer 26 which gives at its output the difference value i _D =| _in −i _n ^* |. This difference is sent to a threshold stage 27 which compares the difference i _D with a predetermined threshold i _s and gives a fault signal F indicating ground when the difference i _D exceeds the predetermined threshold i _s . To this end, the effective value of the difference i _D is determined in a threshold level 27 and then compared with the threshold i _s . If the effective value of the difference i _D is determined taking into account also the harmonics of the zero-phase-sequence system, a ground fault can be detected in this way over the entire length of the stator 1 (see FIG. 1 ).

The conversion unit 25 is arranged in a calibration phase prior to the actual operating phase of the grounded detection device 10 in such a way that, in the absence of grounding, the model value ^in* _determined from the digitized zero-voltage measured value u _n corresponds as closely as possible to the corresponding The digitized zero-current measurement value in _of . This is shown in FIG. 3 . A prerequisite for the calibration phase is that the stator 1 is not grounded. During this calibration phase, the zero-voltage measured value u and the zero-current measured value i are acquired on the inputs 21 and 22 by means of the voltage-acquisition device 23 and the current-acquisition device 24 and converted into a digitized zero-voltage measured value u _n and a digitized zero-current Measured value i _n . This digitized zero-voltage measured value _un is sent to a conversion unit 25 , which is not yet provided, which determines the model value ^in* _from it. This model value is subtracted from the associated digitized zero-current measured value in in a difference unit 26 to form _{the difference value i D .}

Since the conversion unit 25 has not yet been set correctly at this time, the difference i _D has a value not equal to zero. This difference i _D is sent to an optimization unit 31 which converts the response of the conversion unit 25 (which corresponds to the respective determined model value i _n ^* ) by means of the zero current and zero voltage measurements according to, for example, the Steiglitz-McBride algorithm and the average error of the difference between the digitized zero current measurement is minimized. This is done as follows

$\mathrm{<span\; class="notranslate">\; min</span>}\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; -</span>{{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}}^{<span\; class="notranslate">\; *</span>}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; min</span>}\underset{\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 0</span>}}{\overset{<span\; class="notranslate">\; \&infin;</span>}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{<span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; D.</span>}}<span\; class="notranslate">\; |</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}$

The optimization unit 31 also determines the coefficient K for setting the conversion unit 25 in the optimization. This is achieved, for example, by using the Matlab function (THE MATH WORKS Inc., Natick, Mass., USA):

[b, a] = stmcb(i _n , i _n ^* , nb, na),

Wherein, a and b are the coefficient vectors for the conversion unit 25, and nb and na give the lengths of the respective coefficient vectors a and b, respectively. The determined coefficient K is sent to the conversion unit 25 , which is set with this coefficient K.

After ^{the conversion unit 25 is set, it is compared with the model value in* determined before setting to determine the optimized model value in*} _{, and} ^it is compared with the digitized zero current measurement value _in again to form a difference value i _D . After a successful optimization process and setting the conversion unit with the correct coefficients, the determined difference is zero (or below an acceptable error threshold). In this way, the model value in _* corresponds as closely ^as possible to the digitized zero-current measured value _in . In this case, the calibration phase ends and the conversion unit 25 will operate during the ground acquisition operation with the set of coefficients K thus correctly set.

Thus, in the calibration phase, an iterative algorithm is used to determine the transfer function of the stator 1 , by means of which transfer function the model value in _* is subsequently determined from the digitized zero-voltage measured value ^u _n . Therefore, the characteristic quantities characterizing the stator, such as inductance, capacitance and resistance, do not have to be known in order to determine the transfer function. If all the characteristic quantities of the stator are known, it is naturally possible to determine the coefficients of the conversion unit analytically. In this case, the calibration phase can be omitted.

Finally, vector diagrams illustrating the detection of grounding are shown in FIGS. 4 a and 4 b. In the absence of grounding, the zero voltage formed in the stator 1 due to the asymmetry of the windings 2a, 2b and 2c (see FIG. 1 ) flows away only via the capacitors 4a, 4b and 4c. Thus, there exists a capacitively determined current-voltage system, the vector representation of which is shown in Figure 4a. It can be seen that the current leads the voltage by an (ideal) 90° phase shift. By forming the model value ^in* _and the subsequently established difference between the current i and the model value _iv (indicated by the dashed line in Figure 4a), the resulting current is given to be close to zero. As a result, the presence of a ground fault can be detected in a simple manner in the subsequent threshold value comparison.

In the case of grounding in the stator 1, the system determined by the previous capacitance of the stator 1 forms a system determined by the inductance at this time, because the current can flow from the windings 2a, 2b and 2c through a resistance induction to the earth with zero impedance. As shown in Figure 4b, the current lags the voltage by an (ideal) 90° phase shift. In FIGS. 4a and 4b, for the sake of simplicity, the same voltage u in the case of no interference and in the case of ground is represented by voltage vectors of the same length. Since the conversion unit 25 also uses the coefficient K determined in the calibration phase without grounding when grounding occurs, the model value _iv is determined from the voltage value, while the current value i is no longer supplemented but even additionally improve. As a result of this effect, the difference between the absence of a grounding and the case of a grounding in the difference i _D is accentuated, so that the determination by threshold value comparison is further simplified.

Similar to the situation shown in which the model value is determined from the zero-voltage measured value and subtracted from the associated zero-current measured value, it is also possible within the scope of the invention to determine the model value from the zero-current measured value, And establish the difference between the model value and the zero voltage value. However, the case discussed in detail is numerically easier to deal with stably and is therefore preferred.

Claims (8)

1. A method for monitoring the grounding of a winding (2a, 2b, 2c) operated in star connection of a stator (1) of an electric machine, which winding has a star point (3) which does not contribute to grounding, wherein, - measurement of zero voltage at the terminals of the stator (1) under conditions where a zero voltage measurement is obtained, - measure the zero current given by the windings (2a, 2b, 2c) of the stator (1) under conditions where a zero current measurement is obtained, - Determination of model values from the zero-voltage or zero-current measured value, respectively, by means of a conversion unit (25), wherein the conversion unit (25) is configured in such a way that the stator (1) is not grounded , the model value given by - when formed from zero-voltage measured values, correspond to the respective associated zero-current measured values, while - when formed from zero-current measured values, corresponding to the respective associated zero-voltage measured value, - subtracting the determined model value from the respective associated measured value of zero voltage or zero current to form a difference, - If the difference exceeds a predetermined threshold, a fault signal is generated indicating that the winding is grounded. 2. The method as claimed in claim 1, characterized in that for determining the model value a digital filter is used. 3. The method according to claim 2, characterized in that the coefficients for setting the digital filter are formed in the calibration phase from a zero voltage measurement or a zero current measurement and the respective associated zero current measurement or zero determined by the voltage measurement. 4. The method according to claim 3, characterized in that in order to determine the coefficients of the digital filter, the filter response of the digital filter to a zero voltage measured value or a zero current measured value is compensated for the respective associated zero current measurement value or zero voltage measurement to optimize the coefficients of this digital filter. 5. The method according to any one of claims 2 to 4, characterized in that the determination of the digital filter coefficients is implemented according to the Steiglitz-McBride algorithm. 6. The method according to claim 2, wherein the coefficients for setting the digital filter are determined according to a given characteristic quantity of the stator transfer function. 7. The method according to any one of the preceding claims, characterized in that, - subtracting the respective associated model value from the zero current measurement value when the model value formed from the zero voltage measurement value produces a difference measurement value, - generating a fault signal when the difference exceeds a predetermined current threshold. 8. An electrical device for monitoring the grounding of windings (2a, 2b, 2c) of a stator (1) operated in star connection of an electric motor, the winding having a star point (3) which does not function as a ground, the device having: - a voltage acquisition device (23) for measuring and preprocessing zero voltage measurements, - current acquisition means (24) for measuring and preprocessing zero current measurements, and - processing means (27) for generating a fault signal indicative of grounding when the value determined by the zero voltage measurement and/or the zero current measurement exceeds a predetermined threshold value, It is characterized in that, - the output of the voltage acquisition device (23) is connected to the input of a conversion unit (25), which determines the model value from the zero voltage measured values, wherein the conversion unit (25) is configured in such a way that That is, in the case of stator (1) without grounding, the model value given by - when formed from zero-voltage measured values, correspond to the respective associated zero-current measured values, while - when formed from zero-current measured values, corresponding to the respective associated zero-voltage measured value, - the output of said conversion unit (25) is connected to an input of a difference seeker (26), - another input of the difference seeker (26) is connected to an output of said current acquisition device (24), - The output of the difference finder (26) is connected to the processing means (27).

Images