EP2839490B1 - Method for monitoring a step switch - Google Patents

Description

The invention relates to a method for monitoring a tap changer, which is used for uninterrupted switching between taps of a tapped transformer.

Tap changers have been used worldwide for many years to continuously switch between different winding taps of high-speed tap-changers. Such tap changer according to the present invention consist of a selector for powerless selection of the respective winding tap of the tapped transformer to be switched to, and a diverter switch for actual switching from the wired to the new, preselected winding tap. The jump-like switching, also called Lastumschaltersprung, is usually carried out with the aid of a force accumulator, when its triggering a switching shaft is rotated quickly. The diverter switch also usually has switching contacts and resistor contacts. The switch contacts are used for direct connection of the respective winding tap with the load dissipation, the resistor contacts for short-term wiring, d. H. Bridging by means of one or more switching resistors.

Such a method is from the DE 197 44 465 C1 or DE 10 2010 033195 B3 known, which forms the preamble of claim 1 of the present invention. In this known method, the torque is detected on the drive motor during operation of the tap changer, at the same time there is a position detection of the respective current positions of the tap changer. Subsequently, the values of the torque curve determined over time are stored, with the torque curve being broken down into typical time ranges, in each of which a separate setpoint / actual value comparison is made.

In the known method, therefore, there is an assignment of the corresponding torques over time, which in turn corresponds to the angle of rotation traveled during the load switching.

Subsequently, a synchronization by means of a synchronizing pulse, which is generated when reaching a characteristic state in the load switching to a specific, defined time. With the aid of synchronization, the torque curve is normalized and then subdivided into typical time periods, the monitoring windows, the specific, switch-specific parts of the Switch sequence correspond. This is followed by a comparison of the torque values of the individual monitoring windows with previously stored characteristic nominal values. This distribution of the torque curve in individual windows is already the subject of said DE 197 44 465 C1 , For the described synchronization and thus the generation of the sync pulse in the described known method preferably the triggering time of the energy storage, which in turn triggers the sudden movement of the diverter switch used. This triggering of the force accumulator as well as the subsequent diverter switch jump represent a typical, fast-running and thus easily detectable and a short time associated result at each actuation of the tap changer.

In the known method, it is therefore necessary to monitor a tap changer by means of the "window technique" to determine the triggering time of the energy storage and thus the Lastumschaltersprung as accurately as possible in order to derive the explained synchronization can. This usually serves a known switching monitoring in tap changer or in the associated motor drive. However, if this switching monitoring fails, synchronization is no longer possible. Furthermore, there are numerous tap-changers, which do not have a switch monitoring system by default.

However, a lack of synchronization leads to the fact that, due to temperature fluctuations, different directions of rotation dependent Losmomente the gear train between the motor drive and tap changer and other external influences, it to incorrect torque calculations, based on the respective time or the corresponding window, and as a result to wrong or unjustified warning messages or even shutdown of the motor drive can come without actual error of the tap changer.

From the DE 10 2010 033 195 B3 Furthermore, a method is known which determines the Lastumschaltersprung via a differentiated torque in a limited evaluation window. In this case, the detected torque curve is differentiated at a changeover, subsequently determined the minimum of the differentiated torque curve and the time of the determined in this way minimum as the time of Lastumschaltersprungs evaluated, thus forming the sync pulse.

However, this advanced method has several disadvantages. For some tap changers, the (too short) time frame ranges from the diverter switch jump to the Standstill of the motor drive is not sufficient, so that the position of the diverter switch jump can not be reliably detected. In addition, it must be taken into consideration that supply voltages of the motor drive which are strongly affected by harmonics lead to a noisy torque curve, whereby misinterpretations of the differentiation of the torque curve and thus deviating positions of the diverter switch jump are not excluded. This results in a significant dispersion, which prevents reliable synchronization.

The object of the invention is to provide an advanced method for monitoring a tap changer, which allows a simple and reliable way to determine the triggering time of the energy storage and thus the Lastumschaltersprunges and thus allows reliable synchronization.

This object is achieved by a method for monitoring a tap changer having the features of the first claim. The subclaims relate to particularly advantageous developments of the invention.

The general inventive idea is to use the change in the load current, ie the current at the tap changer, during a load switching for the determination of a synchronizing pulse. Within a load switching, the tap changer current changes, due to the short-term electrical contact on the two adjacent winding taps of the transformer and the subsequent completed switching to the new winding tapping, ie the next stage. According to the invention, this change in current is reproduced by means of a sliding effective value formation and subsequent differentiation of the effective value. Subsequently, the maximum value of the differentiated effective value of the current is determined; the time of its occurrence, ie the maximum amount, the time of Lastumschaltersprungs, ie the triggering time of the energy accumulator, assigned and used as a synchronization time for the sync pulse to order a normalization of the monitoring process on the predefined, characteristic event, namely the Lastumschaltersprung during to achieve the changeover of the tap changer. By thus according to the invention has defined the Lastumschaltersprung as the synchronization time, you can then in knowledge of its triggering time according to the so-called windowing, the detailed in the DE 197 46 574 C1 is described, determine the location of each Fester and thus conclusions about the function of the individual modules of the tap changer, such as preselection or Turns, fine selectors or diverter switch, which are operated sequentially with each load switching in a specific sequence.

It is particularly advantageous, in order to compensate for any network disturbances and to avoid false synchronization, to additionally consider the current before and after the determined maximum of the differentiated effective value. If the current does not differ before and after the detected load switching, no load switching has taken place; rather, it is a network failure. In this case, the detected value is discarded and not used for synchronization.

Furthermore, it may be advantageous to avoid a misdetection in that the current is monitored only in a narrow temporal evaluation window and its effective value is differentiated, in which the Lastumschaltersprung is to be expected in the correct operation of the tap changer.

A particular advantage of the invention is that from the continuous recording of the current load switching when the transformer is switched directly from the changing current can be determined during a switch and thus is independent of mechanical influences. Furthermore, it is advantageous that even with a manual operation of the tap changer, i. when cranking operation without electrically moving motor drive, the inventive method can be applied. This is not possible in the prior art.

Below, the inventive method for monitoring a tap changer example by means of drawings will be explained in more detail.

• Fig. 1: Den schematischen Ablaufplan eines erfindungsgemäßen Verfahrens
• Fig. 2: Den typischen Stromverlauf sowie die entsprechenden Verläufe nach der Differenzierung gemäß dem erfindungsgemäßen Verfahren bei einer Lastumschaltung des Stufenschalters
• Fig. 3: Eine vorteilhafte Weiterbildung des in Fig. 1 dargestellten erfindungsgemäßen Verfahrens.

Show it:

• Fig. 1 : The schematic flow chart of a method according to the invention
• Fig. 2 : The typical current profile and the corresponding gradients after the differentiation according to the inventive method in a load switching the tap changer
• Fig. 3 : An advantageous development of in Fig. 1 represented inventive method.

In the following description of the method for monitoring a tap changer, the method steps according to the invention for determining the triggering time of the energy accumulator will be explained in detail. Although the remaining process steps are mentioned, they are assumed to be known to the person skilled in the art, since they are already described in German patents, which are also based on the Applicant DE 197 46 574 C1 . DE 197 44 465 C1 as well as the DE 10 2010 033 195 B3 explained in detail.

In Fig. 1 the method according to the invention is shown schematically. Upon actuation of the tap changer, ie the initiation of a switch from a winding tap to an adjacent other, an occurring current I is first determined at a diverter switch of the tap changer. For this purpose, various means are available in the prior art.

Next, in a known manner, the position detection of the tap changer, i. its relative position over time t during complete switching. From this it can be deduced in which current position the individual subassemblies, such as preselector, selector, and diverter switch, are located within the overall switching sequence to be traversed. This position detection is particularly advantageous by means of a resolver, which allows continuous detection. In addition, the torque of the tap changer associated drive motor is detected during the operation. This can be determined in a particularly simple manner, for example by detecting the rms value of the current and the voltage of the drive motor assigned to the tap changer, in order to determine the active power in a manner known per se in order to calculate the corresponding torque in turn. Subsequently, a storage of the values of the current I at the diverter switch determined over the time t takes place.

Again, according to the invention, the effective value leff of the current I at the diverter switch is formed. This is done continuously.

Wiederum nachfolgend wird das Maximum bzw. das Minimum des differenzierten Verlaufs $\frac{\mathrm{dIeff}}{\mathrm{dt}}$

gesucht und dem entsprechenden Zeitpunkt t₂, an dem es auftritt, zugeordnet. Die Begründung hierfür ist, dass, je nachdem, ob mit dem Lastumschaltersprung ein steigender oder ein sinkender Stromverlauf verbunden ist, sich im differenzierten Verlauf ein Maximum oder ein Minimum ergibt. Mit anderen Worten: Es wird erfindungsgemäß das (vorzeichenlose) Maximum des Betrags erfasst. Dieser Zeitpunkt des Auftretens des Maximums bzw. des Minimums t₂ wird als Zeitpunkt des Lastumschaltersprunges t_LU, also dem Auslösezeitpunkt des Kraftspeichers, definiert. Damit ist ein eindeutiger Synchronisationszeitpunkt ermittelt. Die Synchronisation wird vorgenommen.Subsequently, the respective effective value leff of the current at the diverter switch is differentiated; it turns out $\frac{\mathrm{dIeff}}{\mathrm{dt}}\mathrm{,}$

Again the following becomes the maximum or the minimum of the differentiated course $\frac{\mathrm{dIeff}}{\mathrm{dt}}$

searched and associated with the corresponding time t ₂ , where it occurs. The reasoning for this is that, depending on whether with the Ladumschaltersprung an increasing or decreasing current profile is connected, resulting in the differentiated course, a maximum or a minimum. In other words, according to the invention, the (unsigned) maximum of the amount is detected. This time of occurrence of the maximum or the minimum t ₂ is defined as the time of the diverter switch jump t _LU , ie the triggering time of the energy accumulator. This determines a unique synchronization time. The synchronization is done.

This is followed - after successful synchronization - in a known manner, the decomposition of the torque curve of the drive motor in typical time periods, ie "window". Each window corresponds to a characteristic part of the respectively running switching sequence. Such windows can z. B. include the period of operation of the preselector, the fine selector or the diverter switch. Each window is limited in each case by two characteristic times, the time at which the beginning and end of the window are defined: t ₀ -t ₁ , t ₁ ,..., T _syn -t _n . Each of these windows is compared with pre-stored characteristic setpoints. By means of the selective comparison method, it is not only possible to detect a deviation of the actual and the setpoint values of the torque and thus an error, but it can also be used to allocate an occurring error of a specific module which caused it, in order to draw conclusions about the function the individual modules of the tap changer, such as selection or turning, fine or diverter switch to make, which are operated sequentially with each load switching in a particular sequence.

Ebenfalls gefettet ist das Maximum dargestellt, dass sich während der Lastumschaltung kurzfristig ergibt. Fig. 2 shows a schematic representation of the corresponding courses during a changeover. Shown is first the respective current I at the diverter switch, continue to its upper limit I _g . This current I is first subjected to RMS; the resulting effective value leff is greased. At a time t ₁ , the diverter switch LU is actuated, the actual load switching begins. After a certain period of time then begins the actual electrical switching between the winding taps. There is, as already explained, a differentiation of the RMS value $\frac{\mathrm{dIeff}}{\mathrm{dt}}\mathrm{,}$

Also greased, the maximum is shown that results during the load switching at short notice.

Der Zeitpunkt des Auftretens des dargestellten Maximums von $\frac{\mathrm{dIeff}}{\mathrm{dt}}$

wird einem Zeitpunkt t₂ zugeordnet und erfindungsgemäß als Zeitpunkt des Lastumschaltersprunges t_LU gewertet, der Grundlage der nachfolgenden Synchronisation ist.Background is that during the actual load switching briefly the current I at the tap changer and thus its rms value leff, due to the short-term electrical contact of the diverter switch contacts to two Winding taps, namely the previous tap and the new tap, which is switched to increased. This is known in all tap changers according to the principle of the resistance quick switch known in the art and the expert. This short-term, rapid current increase, which, as explained, is functional, leads to a maximum of the differentiated value $\frac{\mathrm{dIeff}}{\mathrm{dt}}\mathrm{,}$

The time of occurrence of the displayed maximum of $\frac{\mathrm{dIeff}}{\mathrm{dt}}$

is assigned to a time t ₂ and evaluated according to the invention as the time of Lastumschaltersprunges t _LU , which is the basis of the subsequent synchronization.

It can be seen that, before the actual load switching, the rms value of the current leff1 is higher or lower than the rms value of the current leff2 after the load switching. Whether it is higher or lower depends on the direction in which the tap changer is actuated, i. whether a voltage increase or a voltage reduction takes place.

This effect can usefully be used for a further development of the method according to the invention, which in Fig. 3 is shown. It is between the in Fig. 1 with a and b designated times of the process sequence according to the invention still in Fig. 3 inserted sub-procedure inserted. In this case, the rms value of the current leff1 is compared with the rms value of the current leff2 after the time t ₂ before the time t ₂ . When the two rms values leff1 and leff2 significantly different from each other, which is considered as an indication for a proper load transfer and the detected point of time t ₂ is defined as the time of the diverter switch jump t _LU accepted and used for synchronization. If this is not the case, it is assumed that no load switching may have taken place. This suggests possible network disturbances; in such a case, no synchronization takes place, since the underlying time t ₂ in such a case is inaccurate and does not represent a time of load switching.

It is also possible within the scope of the invention to carry out the monitoring of the current I only in a temporal (step-switch-specific) evaluation window in which - with proper function of the tap changer - the diverter switch jump is expected.

From the continuous determination of the current I and the subsequent effective value formation and differentiation according to the invention, the time of a load changeover can thus be determined very accurately when the transformer is switched on; He is also independent of mechanical influences.

A further advantage of the method according to the invention is that it can also be used in manual cranking, i. without electrically moving motor drive, is applicable.

Claims (3)

1. Method of monitoring a tap changer,
   wherein a torque at a drive motor is detected during actuation of the tap changer,
   wherein a positional detection of the respective instantaneous position of the tap changer is carried out at the same time,
   wherein thereafter storage of the values of the torque plot ascertained over time is carried out,
   wherein synchronisation by a synchronising pulse is subsequently carried out, and wherein the torque plot is broken down into typical time ranges, in each of which a separate comparison of target value and actual value is undertaken,
   characterised in that
   a current I present at a load changeover switch is continuously determined,
   subsequently the effective value leff of the current I at the load changeover switch is similarly continuously determined,
   subsequently the respective effective value leff of the current at the load changeover switch is differentiated in such a manner that $$\frac{\mathrm{dIeff}}{\mathrm{dt}}$$

   

   is given,
   subsequently the maximum of the amount of the differentiated plot $$\frac{\mathrm{dIeff}}{\mathrm{dt}}$$

   

   is in turn determined and assigned to the corresponding time instant t₂ at which it occurs and
   the time instant t₂ of the occurrence of the maximum of the amount is judged as time instant of the load changeover switch leap t_LU in order to utilise this as synchronisation time instant for the synchronising pulse.
2. Method according to claim 1, characterised in that
   after the maximum of the differentiated plot $$\frac{\mathrm{dIeff}}{\mathrm{dt}}$$

   

   has been determined and assigned the corresponding time instant t₂ at which it has occurred a comparison of the effective value of the current leff1 before the time instant t₂ with the effective value of a current leff2 after the time instant t₂ is carried out and t₂ is assumed to be the time instant of the load changeover switch leap t_LU and serves for the synchronisation only if the two effective values leff1 and leff2 significantly differ from one another.
3. Method according to claim 1, characterised in the determination of the current I is undertaken only in a time evaluation window in which the load changeover switch leap is expected, the window being specific to the tap changer.

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