Monitoring a transformer including a tap changer

Abstract

A method for monitoring a transformer (10) comprising a tap changer (16), wherein the extracted information comprises: a circulating current amplitude (A C ); and / or a circulating current time (t C ) covering at least a part of a tap changing operation represented by at least one current difference waveform; and / or a transition current time (t R ) covering at least a part of a tap changing operation represented by at least one power loss waveform.

Description

Technical Field

[0001] This invention relates to a monitoring device, method, and computer program product for monitoring transformers including tap changers. Background Technology

[0002] Transformers equipped with tap changers are frequently used in various types of power transmission environments, such as at 10kV and above. Transformers including tap changers can alter the turns ratio between windings, thereby changing the voltage level. This capability is present in many systems used to control power delivery.

[0003] Transformers are generally reliable. The probability of them failing is low, such as about 1%. However, of those 1% of transformers that do fail, typically 20% to 40% are due to the failure of the tap changer.

[0004] This is because the tap changer is the only part of the transformer with mechanically moving elements. Therefore, this part of the transformer is more likely to malfunction than the rest.

[0005] When monitoring transformers, the focus is therefore on monitoring tap change operations when the turns ratio changes. Performing such monitoring may allow for the identification of tap changer faults and worn sections at an earlier stage. This makes maintenance planning easier, which is beneficial for both reliability and cost-effectiveness.

[0006] US10473707 (Figure 3) discloses a monitoring device for extracting the operating duration (ODT) (i.e., circulating current time) and amplitude (AMP) associated with tap-changing operations. This monitoring device includes a waveform recorder and a waveform analyzer. The waveform analyzer includes a power loss processing block, a current difference processing block, and a frequency domain processing block.

[0007] Monitoring the transformer involves examining the operation of the tap changer, for example, by analyzing changes in the duration and amplitude of the operation in the tap change "pulses" over time. This information can potentially be used to determine the health of the tap changer and, consequently, the health of the transformer.

[0008] Furthermore, these general-purpose measurements are not always readily available. As can be seen in Figure 3, it is possible to obtain information that can be used for tap changer monitoring purposes, for example, based on power loss. However, Figure 3 shows power loss under low load. If the transformer experiences high load, it is not easy to extract information indicative of tap changer performance from the available measured waveform, as Figure 3 will seem to suggest. There should be side “pulses” in the waveform associated with the load current passing through (multiple) transition impedances. The amplitude of such side “pulses” depends on the load current and can therefore be less than, equal to, or greater than the amplitude of the main “pulse.” Therefore, it is not always possible to extract information related to the main “pulse” and / or side “pulses” using the methods disclosed in the prior art.

[0009] The focus is on a method for monitoring transformers equipped with tap changers, which allows information indicating the performance of the tap changer function to be extracted using available waveforms for condition monitoring purposes and with the same reliability under all loads.

[0010] This invention relates to such improved transformer monitoring. Summary of the Invention

[0011] Therefore, the present invention relates to improved monitoring of transformers in relation to tap changer operation.

[0012] According to a first aspect of this disclosure, this objective is achieved by a method for monitoring a transformer including a tap changer. The transformer has at least two magnetically coupled windings, and the tap changer includes at least one impedance element and a switch configured to allow a load current to gradually pass through the at least one impedance element when switching between two tap changer positions during tap changing operation. The method is performed in a monitoring device and includes the step of acquiring waveforms of the measured current recorded at a first transformer side and a second transformer side. The recorded waveforms are processed to acquire at least one current difference waveform representing the tap changing operation, wherein the processing of the recorded waveforms includes: acquiring a first current deviation waveform and a second current deviation waveform. The method further includes the steps of: refining the current difference waveform by adding the first current deviation waveform to the second current deviation waveform and removing a stable component based on the load current; and extracting information indicating the health of the tap changer from the at least one waveform representing the tap changing operation. The extracted information includes a circulating current time that covers at least a portion of the tap changing operation and is determined based on the circulating current start time and circulating current end time of the tap changing operation. The extracted information may also include the cyclic current amplitude of the at least one waveform representing the tap change operation.

[0013] The circulating current can be represented as a main "pulse" in the waveform. Refining the current difference waveform by summing the first current deviation waveform to the second current deviation waveform and removing the steady-state component based on the load current is used to extract both the circulating current time and the circulating current amplitude. This refined circulating current analysis method is an improvement over existing techniques because it provides only one "pulse" with distinct rising and falling edges associated with the circulating current, independent of the load current. Other methods (such as power loss monitoring) may fail to identify the "pulse" associated with the circulating current during high load periods. The observed amplitude of the circulating current can be compared to the nominal values ​​obtained from impedance values ​​and voltage tap steps. Improved parameters (such as circulating current time and circulating current amplitude) can be used to better plan transformer maintenance and provide more reliable information about specific components of tapped transformers. The improved accuracy of the parameters results in fewer false alarms when monitoring tapped transformers.

[0014] According to a second aspect of this disclosure, this objective is achieved by a method for monitoring a transformer including a tap changer. The transformer has at least two magnetically coupled windings, and the tap changer includes at least one impedance element and a switch configured to allow a load current to gradually pass through the at least one impedance element when switching between two tap changer positions during a tap-changing operation. The method is performed in a protection device and includes the steps of: acquiring waveforms of measured current and voltage recorded at a first transformer side and a second transformer side; and processing the recorded waveforms to acquire at least one current difference waveform and at least one power loss waveform, each representing a tap-changing operation. The processing of the recorded waveforms further includes: extracting the start time and end time of the circulating current from the at least one current difference waveform representing the tap-changing operation; and extracting the tap change start time and tap change end time from the at least one power loss waveform representing the tap-changing operation. The method further includes: determining at least one transition current time and at least one circulating current time using the circulating current start time and circulating current end time, the tap change start time and the tap change end time; and extracting information indicating the health of the tap converter from the at least one current difference waveform and the at least one power loss waveform, each waveform representing a tap change operation. The extracted information includes: circulating current amplitude; and / or circulating current time, which covers at least a portion of the tap change operation represented by the at least one current difference waveform; and / or transition current time, which covers at least a portion of the tap change operation represented by the at least one power loss waveform.

[0015] The second embodiment combines known power loss difference analysis with known current difference analysis. This combination allows for the detection and extraction of the transition current time, which appears as a side "pulse" before and / or after the circulating current "pulse." The transition current time provides timing information about a portion of the tap-changing operation as the at least one impedance element conducts current. For example, in the vacuum-type tap changer example mentioned above, a significantly shorter transition current time may indicate that the main-side vacuum bottle has not yet opened. Arcing can also be detected in the transition current time by studying the acquired waveform.

[0016] Optionally, processing the recorded waveform further includes refining the current difference waveform by adding the first current deviation waveform to the second current deviation waveform and removing the stable component based on the load current.

[0017] In this way, known power loss analysis can be combined with the improved current difference method of the first embodiment as outlined above, thereby enabling the monitoring of previously unavailable properties of the tap changer, such as the health of the impedance elements and the occurrence of arcing during tap changer operation.

[0018] According to a third aspect of the invention, this objective is achieved by a monitoring device for monitoring a transformer including a tap changer. The transformer has at least two magnetically coupled windings arranged on a first transformer side and a second transformer side, and the tap changer includes at least one impedance element and a switch configured to allow load current to gradually pass through the at least one impedance element when changing between two tap changer positions during tap changing operation. The monitoring device includes a waveform analyzer that operates to monitor the transformer according to any of the steps of the method outlined above.

[0019] According to a fourth aspect of this disclosure, this objective is achieved by a computer program product comprising program code for causing the waveform analyzer described above to perform any of the steps of the methods outlined above.

[0020] According to the fifth aspect of this disclosure, this objective is achieved by a computer-readable medium on which the computer program product described above is stored. Attached Figure Description

[0021] The invention will be described below with reference to the accompanying drawings, in which:

[0022] Figure 1 A transformer equipped with a tap changer is schematically shown.

[0023] Figure 2 schematically shown Figure 1The transformer in the middle has a tap changer and a winding.

[0024] Figure 3 illustrates a prior art representation of power losses in a transformer during tap-changing operations under extremely low load or no load conditions.

[0025] Figure 4 The diagram schematically illustrates a waveform recorder along with a waveform analyzer used for monitoring transformers.

[0026] Figure 5 A block diagram of the power loss processing block in a waveform analyzer is shown.

[0027] Figure 6 shows a flowchart of the prior art method steps for monitoring a transformer, performed by a power loss processing block in a waveform analyzer.

[0028] Figure 7 A block diagram of the current difference processing block in the waveform analyzer is shown.

[0029] Figure 8 A flowchart illustrating the method steps in a first embodiment of a method for monitoring a transformer, performed by a current difference processing block in a waveform analyzer, is shown.

[0030] Figure 9 Flowcharts of method steps in a second and optional third embodiment of a method for monitoring a transformer, executed by a power loss processing block and a current difference processing block in a waveform analyzer, are shown.

[0031] Figures 10 to 11 A representation of parameter extraction according to this disclosure is shown, which relates to the power loss and current difference in a transformer occurring during a tap-changing operation including two impedance elements, and

[0032] Figures 12 to 13 A representation of parameter extraction according to this disclosure is shown, the parameter extraction relating to power losses and current differences in a transformer occurring during a tap-changing operation including an impedance element, and

[0033] Figures 14 to 15 A representation of further feature extraction based on embodiments of the present disclosure is shown. Detailed Implementation

[0034] The following provides a detailed description of preferred embodiments of the invention.

[0035] Figure 1A transformer 10 is shown having a first winding 12 and a second winding 14 magnetically coupled to each other. These windings thus form a pair (often denoted as a primary winding and a secondary winding). Furthermore, the first winding 12 is connected to a tap changer 16. Several power transfer properties of the transformer, measurable during operation, are also shown in the figures. An input current In is fed into the first winding 12, and an input voltage Uin is applied to the first winding. An output current Iout is delivered from the second winding 14, and an output voltage Uout is provided by the second winding. The input current In and the input voltage Uin are measurements taken at a first measuring terminal MT1 and a second measuring terminal MT2 of the first winding 12, wherein the first measuring terminal MT1 is located at a first end of the first winding 12, and the second measuring terminal MT2 is located at a second end of the first winding 12. The output current Iout and output voltage Uout are measured quantities at the third measuring terminal MT3 and the fourth measuring terminal MT4 of the second winding 14, where the third measuring terminal MT3 is located at the first end of the second winding 14 and the fourth measuring terminal MT4 is located at the second end of the second winding. Thus, it can be seen that power enters and exits the transformer at these measuring terminals. The above example assumes power transfer from the first winding to the second winding via the transformer. Therefore, the first winding 12 forms the input side of the transformer 10, and the second winding forms the output side of the transformer. However, it should be recognized that power can be transferred in the opposite direction, in which case the input current and input voltage will be measured at the third measuring terminal MT3 and the fourth measuring terminal MT4, while the output current and output voltage will be measured at the first measuring terminal MT1 and the second measuring terminal MT2. Then, the second winding 14 will form the input side, and the first winding 12 will form the output side. Therefore, the measured quantities are also power transfer properties measured at the measuring terminals.

[0036] The transformer shown is schematically represented. It should be understood that it may also include an iron core. It should also be understood that in many cases the transformer is a three-phase transformer. This means there will be three pairs of windings. The transformer may also include more than one secondary winding magnetically coupled to the same primary winding. Alternatively, a tap changer may be connected to a second winding. The transformer can also be a three-phase transformer in a system configured as single-phase transformers, wherein each single-phase transformer is connected to a corresponding tap changer.

[0037] Figure 2The diagram schematically illustrates a first winding 12 together with a tap changer including an adjusting winding 19. The first winding has a first end and a second end, wherein the first end is connected to a first measuring terminal MT1 and the second end is connectable to the adjusting winding 19. The adjusting winding 19 also has a first end and a second end. Furthermore, the tap changer includes a shunt 20 and a selector 18 that selects a number of turns of the adjusting winding 19 to be connected to the primary winding 12.

[0038] Selector 18 has a selector switch 24 for reversing the orientation of the regulating winding 19 and thus having a first end connected to the second end of the first winding 12, and a second end movable between two positions (a first position at the first end of the regulating winding 19 and a second position at the second end of the regulating winding 19). Each winding 12 and 19 includes several turns of electrical conductor. Furthermore, the regulating winding 19 includes several taps, of which six taps 1, 2, 3, 4, 5, and 6 are shown as an example. These taps are used to determine the number of turns of the regulating winding 19 to be connected to the first winding 12 by a first selector arm connected to the first shunt terminal DT1 and a second selector arm connected to the second shunt terminal DT2.

[0039] Shunt 20 further includes a shunt switch 22 having a first end connected to a second measuring terminal MT2 and a second end connectable between four contact positions, wherein a first contact position P1 is connected to a first shunt terminal DT1 via a first shunt arm, a second contact position is connected to the first shunt terminal DT1 via an impedance element in the form of a first resistor R1, a third contact position is connected to the second shunt terminal DT2 via an impedance element in the form of a second resistor R2, and a fourth contact position P4 is directly connected to the second shunt terminal DT2 via a second shunt arm. Shunt 20 is provided for commutating the load between two selector arms.

[0040] It should be recognized here that this is merely one implementation of a selector and shunt tap changer. Several other types of selector and shunt tap changers exist. Other types of tap changers also exist. Another type, for example, is the selector-switch tap changer, which combines selection and commutation in a single movement, but has a contact sequence similar to that of the selector and shunt tap changer. Yet another example is the vacuum tap changer, where the contact sequence is often modified, but still includes the time for power loss in the impedance elements. The tap changer shown also includes resistors as impedance elements. However, the use of other types of impedance elements, such as inductors, is also known. Furthermore, in the tap changer shown, there are two impedance elements. It should be recognized that it is also possible to have fewer impedance elements, such as one or even more (e.g., three or four) impedance elements.

[0041] What all these tap changers have in common is that during tap switching, there is a gradual movement of the switch between two positions. Figure 2 In the case of a tap changer, this gradual movement is the movement of shunt switch 22 between a first position P1 and a fourth position P4. When a tap change is desired, one of the selector arms is initially connected between the tap position and the corresponding shunt terminal. As an example, the first selector arm may be connected to... Figure 2 The second tap position 2 is shown between the first shunt terminal DT1, and the shunt switch 22 is simultaneously in the first contact position P1. Thus, the first selector arm is loaded, meaning the load current therefore passes through the first shunt arm and enters the first selector arm. Then, a tap position is selected for the unloaded selector arm to obtain the desired turns ratio change; in this example, the unloaded selector arm is the second selector arm. Therefore, the second shunt terminal DT2 connects the unloaded second selector arm to the selected tap position. Figure 2 In the example, the selected tap position is the first tap position 1. To commutate the load, i.e., to implement a change in the turns ratio, the shunt switch 22 then gradually moves from the first contact position to the fourth contact position. During this gradual movement, the shunt switch 22 first establishes contact with the first resistor R1 while simultaneously contacting the first contact position P1. Then, the shunt switch 22 disconnects from the first contact position P1 and the first shunt arm. Now, the load current will run only through the first resistor R1 to the first selector arm. Subsequently, the shunt switch connects to the second resistor R2. At this point, the load current will run through both resistors R1 and R2 to both the first and second selector arms. There will also be a circulating current generated by the voltage difference between the shunt terminals DT1 and DT2. After this, the shunt switch 22 disconnects from the first resistor R1, and thus the load current runs only through the second resistor R2 to the second selector arm. Finally, the shunt switch will reach the fourth contact position P4, and then the load current will run through the second shunt arm to the second selector arm. This completes the tap change operation. It can be seen that during such a movement, energy is loaded or deposited into at least one impedance element, and... Figure 2 In the example, energy is loaded or deposited in two resistors R1 and R2. This energy is consumed in the case of resistors, but is only temporarily stored in the case of inductors. Therefore, by moving switch 22, the load current is gradually allowed to pass through the first resistor R1 and the second resistor R2.

[0042] pass Figure 2In the aforementioned operation of the tap changer (i.e., during the gradual change between positions P1 and P4), impedances R1 and R2 will be connected between measuring terminals MT3 and MT4, and thus energy will be deposited in these impedances. In this example, active power is lost because the impedances are resistors. If the impedances were inductive, reactive power would be stored instead.

[0043] Instantaneous power loss can be determined, for example, according to the following equation (1):

[0044] P loss (t)=P in (t)-P out (t)=∑ phases (V in (t)I in (t)-V out (t)I out (t)) (1)

[0045] The equation provided for a three-phase system thus defines the power loss of the three-phase transformer. Therefore, based on the power transfer property measurements obtained at the first measuring terminal MT1, the second measuring terminal MT2, the third measuring terminal MT3, and the fourth measuring terminal MT4, the instantaneous power loss Ploss is calculated as input voltage Uin multiplied by input current In minus output voltage Uout multiplied by output current Iout, where three such differences are obtained (one difference for each phase).

[0046] It is possible to obtain the active power loss as the average value of the instantaneous power loss in the period and the reactive power loss as the oscillation with an average value of zero according to equation (1).

[0047] Figure 3 illustrates the power loss pattern occurring under low load in such a three-phase transformer equipped with a tap changer. The figure shows the power loss for each of the three phases through coordinated tap changer operations. There is a power loss Ploss_A in the first phase, a power loss Ploss_B in the second phase, and a power loss Ploss_C in the third phase. Furthermore, these three power losses are summed to a total power loss Ploss, which has a “pulse” shape. As can be seen in the figure, the tap changer operation is performed between 0.00 and 0.02 seconds. As an example, the “pulse” or interval has a width of 20 ms, and this width is the peak duration of energy deposition, or the peak duration energy deposition time, during which each phase experiences power loss. In the prior art method illustrated in Figure 3, this peak duration is referred to as the operating duration ODT between the rising and falling edges of the “pulse” because the duration corresponds to the duration of at least some steps in the operation of the tap changer. Therefore, it covers at least a portion of the tap changer operation. More specifically, the operating duration ODT corresponds to the time during which the shunt switch 22 is individually connected to resistors R1 and R2. It can also be seen that the power loss during tap changer operation has an amplitude AMP. The amplitude and operating duration ODT are indicators of tap changer performance or health, obtainable via recorded waveforms, and usable for monitoring purposes. In this disclosure, the operating duration ODT corresponds to the circulating current time t. C In this disclosure, the amplitude AMP corresponds to the circulating current amplitude A. C .

[0048] Transformers are generally reliable. The probability of them failing is low, such as about 1% per year. However, of that 1% of transformer failures, typically 20% to 40% are due to tap changer failure.

[0049] As mentioned above, the only moving element of a transformer is contained in the tap changer. Therefore, the tap changer is related to reliability.

[0050] If the operation of a tap changer is monitored to assess the transformer's health, for example by analyzing changes in the circulating current time and amplitude of the tap changer "pulses" over time, it becomes possible to determine the health of the tap changer and, consequently, the transformer. This can be used to better plan transformer maintenance. A commutation time that is too short can be risky because a potential arc is most reliably quenched at the current zero-crossing point. This means the circulating current time should be longer than half a cycle. If an arc survives during operation, a short circuit in the regulating winding will exist, which will gradually generate a large amount of energy and potentially cause transformer breakdown. Therefore, a circulating current time that is too short can correspond to tap changer failure. On the other hand, a commutation time that is too long indicates friction in the system, but not a breakdown. Therefore, a long circulating current time is an indication of the need for maintenance, because for example, for a resistor tap changer, a long commutation time generates more heat, which can affect the transformer's general performance, such as reduced power. It can be an indication that more cooling is needed. By monitoring changes in the circulating current time, it is possible to predict when a failure will occur and when to plan transformer maintenance. The amplitude, in turn, indicates the magnitude of the resistance. If the amplitude changes, the resistance changes. Furthermore, a high amplitude corresponds to a low resistance, and a low amplitude corresponds to a high resistance. If no "pulse" is observed at all, the amplitude may be significant, meaning either the tap changer has not moved or the resistance is close to infinity, i.e., the resistor is broken. Both of these are significant fault conditions, with the latter potentially generating an arc that short-circuits the regulating winding.

[0051] Furthermore, the load current through one or more of the involved transition impedance elements (resistors) generates a power loss proportional to the square of the load current, and manifests as an additional loss "pulse" just before or after the power loss "pulse" associated with the circulating current. These adjacent "pulses" indicate one or more time information segments in the operating sequence. Using existing methods, it may be difficult to identify these transition impedance losses individually under significantly higher loads.

[0052] Another problem is that it is not possible to directly monitor tap changers. Transformers typically do not have any measurements directly related to tap-changing operations. Therefore, it will be necessary to use common transformer measurements (such as input / output current and voltage) to monitor the health of the transformer with respect to the tap changer.

[0053] Furthermore, these general-purpose measurements are not always easy to use. As can be seen in Figure 3, it is possible to obtain information, for example, based on power loss, that can be used for tap changer monitoring purposes.

[0054] The purpose of this disclosure is to improve the prior art and provide a method for extracting more information using a monitoring device based on current difference monitoring and power loss monitoring.

[0055] Figure 4 The diagram illustrates one implementation of monitoring device 25. Monitoring device 25 includes a waveform recorder 26 comprising a waveform recording block WFR 28 that receives measurements of the transformer in the form of the measured power transmission properties In, Uin, Iout, and Oout, as well as a tap changer control signal TC_CTRL, which is a control signal sent to the tap changer to perform or initiate tap changing operations. Such control signals are typically sent from a tap changer control unit, which, for example, may be associated with power control of a power transmission system. Waveform recorder 26 also includes a waveform memory WFM 30 in which the recorded waveforms are stored. When the tap changer control signal TC_CTRL is acquired by waveform recorder 26, waveform recording block 28 records several waveforms of the input and output currents and the input and output voltages (if available), and stores these waveforms in waveform memory 30. Therefore, the acquisition or reception of the tap changer control signal TC_CTRL triggers waveform recording, which then continues for a predetermined time, such as 10 seconds. This has the advantage of providing a waveform that is clearly consistent with the tap changer operation. Furthermore, the length allows for safe waveform prediction. Moreover, the control signal typically precedes the actual tap changer operation, enabling reliable prediction of the steady-state operating waveform.

[0056] The monitoring device 25 also includes a waveform analyzer 34, which comprises a power loss processing block PLH 36, a current difference processing block CDH 38, and an optional frequency domain processing block FDP 40. Each of these blocks provides a separate way to acquire at least one waveform from which information indicating tap changes can be extracted. Finally, the waveform analyzer 34 includes an information analysis block IA 43 and a health data memory HDM 42, in which (timestamped) waveforms and / or (timestamped) tap changer performance indication information are stored for condition monitoring purposes.

[0057] Waveform analyzer 34 can be implemented by a computer or a processor with associated program memory containing computer instructions for implementing the blocks described above. It can also be implemented by one or more dedicated components, such as application-specific integrated circuits (ASICs) or field-programmable gate arrays (FPGAs) implementing the blocks. It should also be recognized that the waveform analyzer may sometimes only need to include one of the power loss processing block 36, the current difference processing block 38, and the frequency domain processing block 40. It should also be recognized that it is possible for information analysis block 43 to be omitted and its functionality provided in another entity, such as a separate monitoring computer.

[0058] The implementation of waveform recorder 26 is known in itself, but it can be implemented in some aspects in the same way as waveform analyzer 34. However, it may also include an analog-to-digital (A / D) converter, and possibly a scaling unit for scaling at least some of the received signal.

[0059] Now will also refer to Figure 5 Figure 6 illustrates one method of operating the waveform analyzer 26 to monitor the transformer 10. Figure 5 A block diagram of the power loss processing block 36 is shown, and Figure 6 illustrates a flowchart of several method steps in a prior art method of monitoring the transformer 10 performed in the power loss processing block 36. The prior art method in Figure 6 is part of the inventive concept of this disclosure, and is only shown in Figure 6 for easier understanding. Figure 9 The improved method of this disclosure shown in the figure is only shown as an isolated method.

[0060] The power loss processing block 36 includes a power loss determination element PLD 44, a power loss prediction element PLP 46, a power loss waveform element PLWF 48, and a waveform analysis element WFA 49.

[0061] As mentioned above, waveform recorder 26 records the waveforms that appear at the measurement terminals when there is a tap change operation triggered by the tap changer control signal TC_CTRL, and these waveforms are stored in waveform memory 30.

[0062] The recorded waveform also has a start point and an end point.

[0063] In order to obtain waveforms indicating the performance of tap transformation, the power loss determination element 44 of the power loss processing block 36 first obtains the recorded waveforms In, Uin, Iout and Uout from the waveform memory 30 of the waveform recorder 26 (step 50). These recorded waveforms are therefore waveforms of power transmission characteristics recorded at the first transformer side and the second transformer side.

[0064] Subsequently, the power loss processing block processes the recorded waveforms to obtain at least one waveform representing the tap change operation.

[0065] The processing here involves using the measured power transfer properties to address the difference between the power transfer properties on both sides of the transformer. The power transfer properties discussed here are derived properties of power, and the difference between the two sides is the power loss through the transformer expressed as a power loss waveform. Furthermore, the processing involves: predicting at least one waveform of the same type as the difference waveform; and obtaining each waveform representing a tap change operation as the difference between the difference waveform and the corresponding predicted waveform, where there is one predicted waveform and one difference waveform.

[0066] Therefore, component 44 uses the waveforms of the input and output currents In, Uin, Iout, and Oout to determine the power loss of the transformer (step 52). In this case, the power loss can be determined in the manner shown in equation (1) above.

[0067] At the same time, the power loss prediction element 46 predicts the power loss (step 54).

[0068] This prediction may include the prediction of the static waveforms of the current and voltage on the first and second sides of transformer 10. Each such signal, Uin, In, Uout, Iout, can be predicted from early observations by generating a spectrum from the oscillating signal and extracting the significant frequency amplitudes. Based on these amplitudes, the waveforms at future times can be predicted. The prediction method is described in more detail in US 8095326, which is incorporated herein by reference.

[0069] Therefore, the predicted waveforms can correspond to the static waveforms of current and voltage at the first and second sides of the transformer, i.e., waveforms without any embedded tap change information. These predicted waveforms can then be used to form the predicted power loss (step 54).

[0070] Subsequently, the power loss waveform forming element 48 determines the difference between the determined power loss and the predicted power loss to obtain a power deviation waveform defining the tap change operation (step 56). This difference is then used as the waveform defining the tap change operation. As mentioned above, the prediction of the power loss will represent the steady-state power loss without tap change operation. Therefore, in such a difference waveform, there should essentially only be the "pulse" previously shown in FIG3, and information indicating the tap change performance can be easily extracted from the waveform. Figure 10 and Figure 12 The diagram also shows the power loss difference waveform derived from the power loss method.

[0071] As an example, the difference can be obtained as follows:

[0072]

[0073] One way to accomplish this is through

[0074] ΔP loss (t)=∑ phases V i (t)ΔI i (t)+ΔV i (t)I i (t)+ΔV i (t)ΔI i (t) (3)

[0075] Equation (3) shows the determination of the input or output power variation, where i is therefore encoded on the input or output side, and ΔI and ΔV are the differences between the predicted current and voltage waveforms and the actual current and voltage waveforms.

[0076] The difference between the actual power loss and the predicted power loss will then be:

[0077] ΔP loss (t)=ΔP in (t)-ΔP out (t) (4)

[0078] Therefore, the steady-state component has been removed, and the result is a waveform that represents the tap-changing operation and essentially includes only the power loss caused by the tap-changing operation. Figure 10 and Figure 12 In the waveform shown, the tap change start time t can be easily determined. TS and tap change end time t TE And thereby determine the total width of the tap change operation (e.g., by identifying the time between the rising and falling edges in the waveform) (step 58), and determine the amplitude A. C (Step 60) These determinations can be performed by the waveform analysis element 49. Then, information indicating the performance of the tap change operation (i.e., amplitude A) can be used. C The waveform of the tap change start time and tap change end time, along with the power loss difference waveform, is stored (possibly timestamped) in health data storage 42 for use in transformer condition monitoring (step 62). Other information, such as initial and final tap positions, load current, etc., may also be stored in this regard.

[0079] Then, health determination and prediction can be performed by the information analysis block 43. Therefore, the information analysis block 43 can analyze the information indicating tap-changing operations stored in the memory 42 via the waveform analysis element 49, estimate transformer wear, and determine when maintenance will be performed, or more precisely, at which maintenance window the transformer will be replaced or repaired. Alternatively, the power loss analysis element 49 may be omitted, and the power loss waveform element 48 may store the power loss difference waveform in the health data memory 42. In this case, the information analysis block 43 can instead retrieve the power loss difference waveform from the health data memory 42 and extract information indicating tap-changing operations from the power loss difference waveform when determining the transformer's health.

[0080] The methods described above for obtaining the start and end times of tap change operations do not inherently allow for the extraction of other parameters of interest (such as transition current time), which could be used to monitor impedance elements. This first embodiment is an improved current difference method that uses the measured current as input and compares the measured current with a predicted current.

[0081] Now refer to Figure 7 and Figure 8 To describe the first embodiment, wherein Figure 7 A block diagram of the current difference processing block 38 is shown, and Figure 8 A flowchart is shown of several method steps in a method for monitoring a transformer and executed by current difference processing block 38.

[0082] The current difference processing block 38 includes a current difference determination element CDD 64, a current difference prediction element CDP 66, a current deviation waveform determination element CDWF 68, and a current difference analysis element 69.

[0083] Similar to the prior art power loss difference described above, in the first embodiment, there is a process for processing the difference between the power transmission characteristics on both sides of the transformer using a measured power transmission characteristic. In this first embodiment, the power transmission characteristic discussed is the measured power transmission characteristic of the current, and the difference between the two sides is the current difference between the two sides. The process involves predicting a waveform of the same type as the difference waveform, i.e., predicting the current difference waveform. The process also includes acquiring each waveform representing a tap change operation as the difference between the difference waveform and the corresponding predicted waveform. In this first embodiment, two waveforms are predicted (one waveform forward in time and one waveform backward in time), and acquiring at least one waveform includes: acquiring two deviation waveforms; one deviation waveform as the difference between the difference waveform and the first of the predicted waveforms, and another deviation waveform as the difference between the difference waveform and the second of the predicted waveforms. According to this first embodiment, the two deviation waveforms are added together, and the stable component based on the load current is removed. See Figure 11 and Figure 13 ,in Figure 11 The current deviation waveforms B and F and the resulting waveform S are shown, while Figure 13 Only the resulting waveform S is shown. Figure 11 In the waveform S, the stable component is approximately 4 amperes. Based on the obtained waveform S, the circulating current time t can be determined. C and amplitude. The stable component can be described as... Load current, where Δn is the ratio change between the two taps, for example, 0.0167.

[0084] In order to obtain the at least one waveform indicating the performance of the tap converter, the current difference determination element 66 of the current difference processing block 38 first obtains the recorded waveforms In and Iout of all phases from the waveform memory 30 of the waveform recorder 26 (step 70), wherein the waveform of the measured power transmission property in this embodiment includes only the current waveforms recorded at the first transformer side and the second transformer side.

[0085] Subsequently, the current difference determining element 64 determines the current difference Idiff between the currents on both sides of the transformer (step 72).

[0086] This current difference can be based on the determination of the difference in partial current for different phases.

[0087] For phase p, the corresponding partial current difference can be obtained as follows:

[0088]

[0089] in It is the effective ratio for a specific phase, which is obtained by using the discrete Fourier transform (DFT) of the current amplitude and depends on the turns ratio and the current sensor calibration error.

[0090] Subsequently, the total current difference across all phases can be determined as follows:

[0091]

[0092] It is possible to use the same prediction technique described above to predict the current difference. However, it should be noted that if the transformer includes, for example, three connected single-phase transformers (each with a tap changer) or three-phase transformers (with more than one tap changer), then expression (6) may not be used to determine the total current difference across all phases. Instead, these current difference waveforms are analyzed separately.

[0093] In this embodiment, the current difference prediction element 66 predicts the first difference current. This prediction can be made based on the same equations (5) and (6) modified above according to equations (2) and (3). Furthermore, in this case, the prediction is made in the positive direction from the start of the waveform toward the end of the recording (step 74).

[0094] Then, the current deviation waveform determination element 68 acquires or determines the first current deviation waveform as the difference between the actual current difference waveform and the first predicted current difference waveform (step 76). In this waveform, the current difference analysis element 69 can then potentially detect the start of a tap change operation (step 78), for example, by detecting the rising edge of the waveform or a high positive time derivative. The current difference analysis element may also detect the amplitude. However, this first embodiment provides an improved method for detecting both the rising edge and the amplitude, as described below.

[0095] However, since the stationary waveform has changed after the turns ratio is changed by the tapping operation, it is not easy to detect the end of the tapping operation using the first current deviation waveform. Therefore, in order to detect the end of the tapping operation, the current difference prediction element 66 predicts the second current difference in the backward direction (i.e., starting at the end of the waveform recording and backward in time) (step 80).

[0096] Following this, the current deviation waveform determining element 68 acquires or determines the second current deviation waveform as the difference between the actual current difference waveform and the second predicted current difference waveform (step 82). In this waveform, the current difference analysis element 69 can then potentially detect the end of the tap change operation (step 84), for example, by detecting the falling edge of the waveform or a large negative time derivative. The amplitude may also be estimated here.

[0097] This first embodiment provides a method for determining the pulse width (i.e., the circulating current time t).C An improved method is proposed. By adding the first current deviation waveform to the second current deviation waveform and removing the steady-state component based on the load current, the current difference analysis element 69 can more accurately determine the "pulse" width of the circulating current (i.e., the circulating current time t). C Furthermore, by removing the stable component based on the load current and dividing by two, the amplitude can be obtained more accurately compared to existing technologies, since only a clear "pulse" remains after the improvement.

[0098] Figure 11 An example of a flag-loop tap converter with two resistors R1 and R2 is shown, illustrating how a first current deviation waveform F and a second current deviation waveform B are added together and the steady-state component based on the load current is removed to produce waveform S, from which the loop current time (t) covering at least a portion of the tap-changing operation can be extracted. C The circulating current time is based on the starting time of the circulating current during the tap change operation (t). CS ) and the end time of the circulating current (t) CE ) Determined, such as Figure 11 As shown in the image. Figure 11 It is evident that neither the first current deviation waveform F nor the second current deviation waveform B provides information about the current amplitude (A). C It is impossible to accurately estimate the current deviation, and it is even impossible to extract the amplitude from the first current deviation waveform F. This is a major improvement over the prior art.

[0099] Figure 13 An example of a flag loop tap converter with a resistor R1 is shown, which is monitored by the current difference method of the first embodiment.

[0100] The current difference analysis element 69 can determine the "pulse" width or circulating current time based on the start and end of the circulating current "pulse" (i.e., rising and falling, respectively) (step 86). The amplitude 88 can be determined or obtained by observing the height of the circulating current "pulse".

[0101] Then, information indicating tap change performance can be extracted and stored (possibly timestamped) in health data memory 42. Other information, such as initial and final tap positions, load current, etc., can also be stored here. Alternatively, current deviation waveform determining element 66 may store a first current deviation waveform and a second current deviation waveform in memory 42.

[0102] Subsequently, the information analysis block 43 can analyze the stored information and / or waveforms for condition monitoring purposes. Furthermore, if this is not yet complete, the information analysis block 43 may extract tap change performance indication information from the sum of the current deviation waveforms.

[0103] Therefore, the first embodiment is an improvement on the existing current difference method, in which more accurate and detailed results are obtained.

[0104] The described embodiments rely on coordinated tap change operations in the three phases. However, for some tap changer installations, it is most noteworthy that when there are three separate tap changers (one for each phase), the tap change operations in different phases may be uncoordinated. Therefore, tap change operations can be performed independently of each other in different phases. Consequently, it may be necessary to monitor the transformer individually for each of the three phases regarding tap change operations. As mentioned above, expression (6) is therefore not applicable to such transformers.

[0105] In the second embodiment, the above-described power loss difference method is combined with a conventional current difference method or with the improved current difference method of the first embodiment described above, thereby producing an improved method for monitoring tap converters. The combined method... Figure 9 The diagram illustrates and involves parallel implementation of power loss difference steps and current difference steps, wherein the results are combined to obtain additional monitoring information, particularly information related to the at least one impedance element R.

[0106] exist Figure 9 In the diagram, the steps of the first embodiment are shown as optional steps, indicated by dashed lines. When using conventional current difference methods, the two current deviation waveforms are not added together, and the stable component of the current is not removed. Instead, the 86-pulse width is determined by combining the two current deviation waveforms, for example, by determining the start time t of the "pulse". CS and end time t CE The amplitude can also be determined or obtained by examining any of the current deviation waveforms (step 88).

[0107] Therefore, the method according to the second embodiment relates to monitoring a transformer 10 including a tap changer 16. The transformer has at least two magnetically coupled windings 12, 14. The tap changer 16 further includes at least one impedance element R and a switch 22 configured to allow load current to gradually pass through the at least one impedance element R when switching between two tap changer positions P1, P4 during a tap-changing operation. The method is performed in a monitoring device 25 and includes acquiring waveforms of the measured current and measured voltage Iin, Uin, Iout, Uout recorded at the first transformer side and the second transformer side at 50, 70; and processing the waveforms recorded at 52, 54, 56; 72, 74, 76, 78 to acquire at least one current difference waveform and at least one power loss waveform, each waveform representing a tap-changing operation.

[0108] The processing of the recorded waveforms further includes: determining the start time (t) of the 86 and 88 circulating currents based on the at least one current difference waveform representing the tap change operation. CS ), the end time of the circulating current (t) CE ) and circulating current amplitude A C Such as the circulating current time; and the extraction of the tap change start time (t) from the at least one power loss waveform representing the tap change operation. TS ) and tap change end time (t) TE ), such as the total tap change duration, and

[0109] Using the cycle current start time (t) CS ) and the end time of the circulating current (t) CE ), tap change start time (t) TS ) and tap change end time (t) TE To determine at least 100 transition current times (t) R ) and at least one cycle current time (t) C ),as well as

[0110] Extract (102) information indicating the health of the tap transformation from the at least one current difference waveform and the at least one power loss waveform. C A C t R Each waveform represents a tap change operation, and the extracted information includes: the amplitude of the circulating current A. C ; and / or circulating current time (t) C ), which covers at least a portion of the tap change operation represented by the at least one current difference waveform; and / or at least one transition current time t R It covers at least a portion of the tap transformation operation represented by the at least one power loss waveform.

[0111] Based on the power loss deviation waveform, waveform analysis element 49 can therefore determine the total tap change operation time 58 by the width of the power loss deviation waveform, for example, by identifying the tap change start time t from the rising edge of the waveform. TS And the tap change end time t is identified from the falling edge of the waveform. TE .

[0112] By combining data extracted from power loss methods and current loss methods, one or more transition current times t can be determined. R The interval is determined by the circulating current time. The transition current time t is presented as a "pulse" to the left of the circulating current "pulse". R The tap change start time t can be determined. TSWith the start time t of the circulating current CS The interval between them. The transition current time, presented as a "pulse" to the right of the circulating current "pulse," can be determined as the circulating current end time t. CE With tap change end time t TE The interval between them. Furthermore, the circulating current time t C The time t is determined as the start time of the circulating current. CS With the end time t of the circulating current CE The interval between them.

[0113] Information indicating the health or performance of the tap transformation can be extracted from the at least one current difference waveform and the at least one power loss waveform. C A C t R .

[0114] Then, information indicating the performance of the tap change operation (i.e., the circulating current amplitude A) can be extracted. C Circulating current time t C and / or at least one transition current time t R This data, along with the power loss difference waveform, is stored (possibly timestamped) in the health data memory 42 for use in transformer condition monitoring. Other information, such as initial and final tap positions, load current, etc., may also be stored in this regard.

[0115] Then, health determination and prediction can be performed by the information analysis block 43. Therefore, the information analysis block 43 can analyze the information indicating tap change operation stored in the memory 42 through the waveform analysis elements 49, 69, estimate the wear of the transformer / tap changer, and determine when maintenance will be performed or, more precisely, at which maintenance window the transformer will be replaced or repaired.

[0116] In this way, it becomes possible to identify tap-changing operations and extract new tap-changing performance indications related to transition current time (from the side "pulses" of the power loss waveform) and circulating current time (from the current difference waveform), regardless of the load size. This can improve maintenance planning.

[0117] In summary, improved monitoring of transformers equipped with tap changers has been achieved, making it possible to evaluate the performance of the tap changers. This allows for more reliable maintenance planning. This can also be accomplished without requiring dedicated tap changer monitoring sensors, instead using only measurements traditionally available for transformers. In this way, improved monitoring is achieved with minimal additional hardware and without introducing additional sensors or modifying waveform recorders.

[0118] The method according to the second embodiment may optionally be refined by the current difference method according to the first embodiment, wherein the processing of the recorded waveform further includes: refining the current difference waveform by adding the first current deviation waveform to the second current deviation waveform and removing the stable component based on the load current by 87.

[0119] Although not detailed in the description of the second embodiment above, the determination of the two current deviation waveforms and the power loss deviation waveform follows the same principle as previously described. Figure 5 The description of the power loss method shown in Figure 6 and Figure 7 and Figure 8 The steps described in the description of the current difference value method of the first embodiment shown are the same as those described in the previous step.

[0120] In addition to the variations already described, several other variations are possible. For example, it is possible that the monitoring device includes only a waveform analyzer configured to communicate with a waveform recorder. It is also possible that the monitoring device is provided as part of an intelligent electronic device (IED) provided for a transformer.

[0121] Figure 10 The diagram illustrates how to determine the starting time t of the circulating current in the current difference method. CS and the end time t of the circulating current CE Tap change start time t of the waveform with power loss difference TS and tap change end time t TE Used together to determine the first transition current time t R1 Second transition current time t R2 The transition current is shown as "pulses" to the left and right of the circulating current "pulse".

[0122] Figure 12 The illustration shows a method used on a pinned cyclic tap converter with a single transition impedance R1.

[0123] Figure 14 and Figure 15 The problematic tap transformation operation is shown in the acquired power loss difference waveform. Figure 14 The diagram illustrates how to detect deviation X in a pinion cyclic tap converter, such as... Figure 12 and Figure 13 The monitoring in [the context]. Figure 14 During this phase, the main vacuum bottle remained closed. C Approaching the nominal expected time. Figure 15 In the middle, the main-side vacuum bottle does not open in any phase, resulting in a transition current time t. R1 The circulating current time t is much shorter than expected and longer than expected. CBecause it starts earlier than normal operation.

[0124] A waveform analyzer can be implemented as discrete components. However, it can also be implemented as a processor with accompanying program memory, which includes computer program code that executes the desired control functions when run on the processor. The computer program product carrying this code can be provided as a data carrier, such as one or more CD-ROM discs, hard disk storage devices, or one or more memory sticks carrying the computer program code, which performs the waveform analyzer functions described above when loaded into the waveform analyzer.

[0125] As will be apparent from the foregoing discussion, the present invention can be varied in numerous ways. Therefore, it should be understood that the invention will be limited only to the following claims.

Claims

1. A method for monitoring a transformer (10) including a tap changer (16), the transformer having at least two magnetically coupled windings, and the tap changer (16) including at least one impedance element (R) and a switch (22) configured to allow a load current to gradually pass through the at least one impedance element (R) when changing between two tap changer positions during tap changing operation, the method being performed in a monitoring device (25) and comprising: - Obtain the waveforms of the measured current recorded at the first transformer side and the second transformer side. - Processing the recorded waveforms to obtain at least one current difference waveform representing a tap change operation, wherein the processing of the recorded waveforms includes: obtaining a first current deviation waveform and a second current deviation waveform, and - The current difference waveform is refined by adding the first current deviation waveform to the second current deviation waveform and removing the stable component based on the load current. - Extract and store information indicating the health of the tap converter from the at least one waveform representing the tap changer operation. The extracted information includes: cyclic current time (t) C The circulating current time covers at least a portion of the tap change operation and is based on the circulating current start time (t) of the tap change operation. CS ) and the end time of the circulating current (t) CE ) determines; and / or represents the cyclic current amplitude (A) of the at least one waveform of the tap change operation. C ).

2. The method according to claim 1, wherein, The acquisition of the first current deviation waveform and the second current deviation waveform further includes: - Based on the measured current, determine a current difference waveform that represents the difference between the currents on both sides of the transformer; predict a first predicted current waveform forward in time and a second predicted current waveform backward in time based on the current difference waveform; and - Obtain the first current deviation waveform as the difference between the difference waveform and the first predicted current waveform, and obtain the second current deviation waveform as the difference between the difference waveform and the second predicted current waveform.

3. A method for monitoring a transformer (10) including a tap changer (16), the transformer having at least two magnetically coupled windings, and the tap changer (16) including at least one impedance element (R) and a switch (22) configured to allow a load current to gradually pass through the at least one impedance element (R) when changing between two tap changer positions during tap changing operation, the method being performed in a monitoring device (25) and comprising: - Acquire the waveforms of the measured current and voltage recorded at the first transformer side and the second transformer side. - The recorded waveforms are processed to obtain at least one current difference waveform and at least one power loss waveform, each representing a tap change operation. The processing of the recorded waveform further includes: - Determine the circulating current start time (t) based on the at least one current difference waveform representing the tap change operation. CS ), the end time of the circulating current (t) CE ) and circulating current amplitude (A C ),as well as - Extract the tap change start time (t) from the at least one power loss waveform representing the tap change operation. TS ) and tap change end time (t) TE ),as well as - Using the cycle current start time (t) CS ) and the end time of the circulating current (t) CE The tap change start time (t) TS ) and tap change end time (t) TE To determine at least one transition current time (t) R ) and at least one cycle current time (t) C ),as well as - Extract health information indicating tap change from the at least one current difference waveform and the at least one power loss waveform, where each waveform represents a tap change operation. The extracted information includes: circulating current amplitude (A) C ); and / or circulating current time (t) C The cyclic current time covers at least a portion of the tap change operation represented by the at least one current difference waveform; and / or the transition current time (t) R The transition current time covers at least a portion of the tap change operation represented by the at least one power loss waveform.

4. The method according to claim 3, wherein, From the at least one current difference waveform, the starting time of the circulating current (t) is included. CS ) and operation end time (t) CE The extraction of information includes processing the recorded waveform, and - Obtain the first current deviation waveform and the second current deviation waveform from the current difference waveform, and Wherein, the cycle current start time (t) CS The first current deviation waveform is extracted as the start time, and the cycle current end time (t) is... CE The current deviation waveform is extracted as the end of the second current deviation waveform.

5. The method according to claim 4, wherein, The processing of the recorded waveform further includes refining the current difference waveform by adding the first current deviation waveform to the second current deviation waveform and removing the stable component based on the load current.

6. The method according to any one of claims 4 to 5, wherein, The acquisition of the first current deviation waveform and the second current deviation waveform further includes: - Determine a current difference waveform that represents the difference between the currents on both sides of the transformer based on the measured current. - Based on the current difference waveform, predict a first predicted current waveform forward in time and a second predicted current waveform backward in time. - Obtain the first current deviation waveform as the difference between the difference waveform and the first predicted current waveform, and obtain the second current deviation waveform as the difference between the difference waveform and the second predicted current waveform.

7. The method according to any one of claims 3 to 5, wherein, Processing the recorded waveform to obtain the at least one power loss waveform representing the tap change operation further includes: - Based on the measured current and measured voltage, determine the power loss difference waveform that represents the power difference between the two sides of the transformer. - Predict the predicted power waveform, - Obtain the power deviation waveform as the difference between the power loss difference waveform and the predicted power waveform.

8. A monitoring device (25) for monitoring a transformer (10) including a tap changer (16), the transformer (10) having at least two magnetically coupled windings arranged at a first transformer side and a second transformer side, and the tap changer (16) including at least one impedance element (R) and a switch (22) configured to allow a load current to gradually pass through the at least one impedance element (R) when changing between two tap changer positions during tap changing operation, the monitoring device including a waveform analyzer (34) operating to monitor the transformer (10) according to any one of claims 1 to 7.

9. A computer program product comprising program code for causing the waveform analyzer (34) according to claim 8 to perform the method according to any one of claims 1 to 7.

10. A computer-readable medium having stored thereon a computer program product according to claim 9.

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