On-line monitoring system and device for grounding current of transformer iron core

Abstract

The invention relates to a transformer iron core grounding current on-line monitoring system and device. The system comprises a signal acquisition unit, a conditioning unit, a data analysis unit, a storage unit and a control unit. The control unit receives an iron core grounding current measured value and historical data, a clamping piece grounding current measured value and historical data, an amplitude correlation coefficient measured value and historical data and a phase correlation coefficient measured value and historical coefficient, and the control unit comprehensively judges the fault state of the transformer iron core according to the received information. According to the invention, the iron core grounding current and the clamp grounding current are monitored at the same time, and after the corresponding grounding current is amplified and filtered, the correlation degree between the iron core grounding current and the clamp grounding current is analyzed from different dimensions. The principle that the relevance of the two grounding currents is remarkably improved when the iron core and the clamping piece are conducted is used for assisting in judging the early insulation fault of the iron core and the clamping piece, and the detection rate of the early insulation fault with the iron core grounding current lower than 100 mA can be increased.

Description

Technical Field

[0001] The present invention belongs to the technical field of power monitoring, and particularly relates to an on-line monitoring system and device for the grounding current of a transformer core. Background Art

[0002] A transformer is a core device for power transmission, and its operating reliability directly affects the safety of the power grid. As the core component of the magnetic circuit, the transformer core needs to be reliably grounded at a single point during normal operation to eliminate floating potential. However, during long-term operation, multi-point grounding faults of the core may occur due to insulation aging, mechanical vibration, or foreign object intrusion, resulting in abnormal increase in the grounding current, causing serious accidents such as local overheating, insulation deterioration, and even core burnout.

[0003] Traditional detection of core grounding faults mainly relies on manual regular inspections or off-line tests, and the grounding wire current is measured by a clamp-on ammeter. This method has obvious defects: poor real-time performance, inability to capture transient abnormal signals, and is easily affected by human operation errors. With the development of smart grids, some improved solutions use fixed current sensors combined with threshold alarm functions, but there are still the following problems: the thresholds of existing on-line monitoring devices are usually set below 0.1A. When multi-point grounding faults occur in the core at an early stage, the grounding current may be below the threshold, and the existing threshold setting is difficult to effectively identify early faults. If the alarm threshold is reduced, it is easily interfered by noise and triggers false alarms, and the abnormal current caused by external metal foreign objects lapping the downlead may also be misjudged as temporary interference due to current amplitude fluctuations. Therefore, it is necessary to design an on-line monitoring device and system for the grounding current of a transformer core that can balance sensitivity and false judgment rate. Summary of the Invention

[0004] The purpose of the present invention is to provide an on-line monitoring system for the grounding current of a transformer core to solve the problem of difficult identification of early transformer faults in the prior art; the purpose of the present invention also lies in providing an on-line monitoring device for the grounding current of a transformer core using the above monitoring system.

[0005] In order to achieve the above purposes, an on-line monitoring system for the grounding current of a transformer core of the present invention adopts the following technical solutions: An on-line monitoring system for the grounding current of a transformer core, comprising, A signal acquisition unit, which is used to detect the core grounding current flowing through the core grounding lead-out wire and the clamp grounding current flowing through the clamp grounding lead-out wire; A conditioning unit, which is used to process the acquired signals, and at the same time process the core grounding current into a core grounding voltage and process the clamp grounding current into a clamp grounding voltage; A data analysis unit for analyzing the measured value of the amplitude correlation coefficient between the core grounding voltage and the clamping piece grounding voltage and the measured value of the phase correlation coefficient between the core grounding voltage and the clamping piece grounding voltage at a specific frequency component; A storage unit for storing the historical data of the amplitude correlation coefficient between the core grounding voltage and the clamping piece grounding voltage, the historical data of the phase correlation coefficient between the core grounding voltage and the clamping piece grounding voltage, and a set of transient waveform templates corresponding one by one to the switch operation modes under normal operating conditions; A control unit, the signal acquisition unit, the data analysis unit and the storage unit are electrically connected to different input ports of the control unit to transmit signals to it. The control unit comprehensively judges the fault state of the transformer core according to the core grounding current, the clamping piece grounding current, the measured value of the amplitude correlation coefficient, the measured value of the phase correlation coefficient, the historical data of the amplitude correlation coefficient, the historical data of the phase correlation coefficient, the switch operation condition and the operation mode.

[0006] Further, when the core grounding current exceeds the set threshold, judge whether its waveform is consistent with the transient waveform template under the corresponding switch operation mode. If the two are consistent, it is determined that the amplitude fluctuation of the core grounding current is caused by the switch action; if the two are inconsistent, it is determined that the transformer has an insulation fault; When the core grounding current is less than the set threshold, analyze the change of the measured value of the amplitude correlation coefficient relative to the historical data and the change of the measured value of the phase correlation coefficient relative to the historical data. When the increase amplitude of the measured value of the amplitude correlation coefficient relative to the historical data and the increase amplitude of the measured value of the phase correlation coefficient relative to the historical data both exceed the set value, it is determined that the transformer has an early insulation fault.

[0007] Further, the steps for the data analysis unit to perform correlation analysis are as follows: Step 1: Obtain the core grounding voltage and the clamping piece grounding voltage by synchronous sampling; Step 2: Perform discrete Fourier transform on the core grounding voltage and the clamping piece grounding voltage respectively to obtain the frequency domain representations of the core grounding voltage and the clamping piece grounding voltage, and identify the fundamental wave and the main harmonic frequencies; Step 3: Perform frequency division filtering and Hilbert transform on the core grounding voltage and the clamping piece grounding voltage respectively to obtain the instantaneous amplitude and instantaneous phase sequences of the corresponding frequency signals; Step 4: Calculate the amplitude correlation between the core grounding voltage and the clamping piece grounding voltage at the same frequency component and measure it with the amplitude correlation coefficient; Step 5: Calculate the phase correlation of the same frequency components of the core grounding voltage and the clamping piece grounding voltage and measure it with the phase correlation coefficient.

[0008] Further, when the core grounding current is less than the set threshold value, analyze respectively the change of the measured value of the amplitude correlation coefficient under the fundamental wave component relative to the historical data and the change of the measured value of the phase correlation coefficient relative to the historical data, the change of the measured value of the amplitude correlation coefficient under the harmonic component relative to the historical data and the change of the measured value of the phase correlation coefficient relative to the historical data. When the increase amplitudes of the amplitude correlation coefficient and the phase correlation coefficient under both the fundamental wave component and the harmonic component exceed the set value, it is determined that the transformer has an early insulation fault.

[0009] Further, a resistor is connected in series on the core grounding lead-out wire and / or the clamping piece grounding lead-out wire, and control switches for controlling the switching state of the resistor are arranged on both sides of the resistor; when only the increase amplitude of the amplitude correlation coefficient under the fundamental wave component and the increase amplitude of the phase correlation coefficient exceed the set value or only the increase amplitude of the amplitude correlation coefficient under the harmonic component and the increase amplitude of the phase correlation coefficient exceed the set value, the resistor on the core grounding lead-out wire or the clamping piece grounding lead-out wire is switched into the circuit; If after the resistor is switched, the reduction amplitudes of the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the corresponding frequency component both exceed the set value, it is determined that there is an early insulation fault between the core and the clamping piece; if after the resistor is switched, the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the corresponding component do not exceed the set value, it is determined that the transformer is in a normal state.

[0010] Further, the storage unit updates the historical data stored therein in real time.

[0011] Further, in step four, the Pearson correlation coefficient formula is used to calculate the amplitude correlation coefficient of each frequency component; in step five, first directly calculate the phase difference between the instantaneous phase of the core grounding voltage and the instantaneous phase of the clamping piece grounding voltage for each frequency component, then calculate the standard deviation of the phase difference sequence under this frequency component, and then divide the standard deviation by the maximum value of the standard deviation when the phase difference is completely random, and then construct the phase correlation coefficient at each frequency component by subtracting this value from 1.

[0012] Further, the conditioning unit includes a sampling resistor, a filter circuit, an amplification circuit and an A / D converter. The amplification circuit includes a primary amplification circuit and a secondary amplification circuit. The primary amplification circuit is arranged between the sampling resistor and the filter circuit, and the secondary amplification circuit is arranged between the filter circuit and the A / D converter. The filter circuit includes a low-pass filter circuit and a high-pass filter circuit. Both the primary amplification circuit and the secondary amplification circuit adopt an inverting amplification circuit; the signal acquisition unit includes a first Hall sensor and a second Hall sensor, and also includes a bracket. An activity plate that can be guided and moved in the up and down direction is arranged on the bracket, and two grooves for inserting the corresponding Hall sensors to make the two Hall sensors at the same height are arranged on the activity plate.

[0013] Further, during a lightning strike, when the control unit determines that the transformer has an early insulation fault or an insulation fault, the duration of the corresponding criterion needs to be greater than 10 ms.

[0014] The on-line monitoring device for the grounding current of a transformer core according to the present invention adopts the following technical solutions: An on-line monitoring device for the grounding current of a transformer core includes a housing, and the above-mentioned on-line monitoring system for the grounding current of the transformer core is integrated in the housing.

[0015] The beneficial effects of the present invention are as follows: In the present invention, the grounding current of the core and the grounding current of the clamping piece are monitored simultaneously. After amplifying and filtering the grounding current of the core and the grounding current of the clamping piece, the correlation degree between the grounding voltage of the core and the grounding voltage of the clamping piece is analyzed. By using the principle that when the insulation between the core and the clamping piece is good, the correlation of the grounding currents between the two is low, and when the core and the clamping piece are conducting, the correlation in the same loop is significantly improved, the early insulation faults of the core and the clamping piece are assisted in judgment. Even when the grounding current of the core is less than 100 mA, the early insulation faults of the core and the clamping piece can be successfully identified. The present invention determines the specific criteria for identifying the fault risk of the transformer, and can significantly improve the fault identification accuracy.

[0016] Further, the present invention measures the correlation degree between the grounding current of the core and the grounding current of the clamping piece from two different dimensions of the amplitude correlation and the phase correlation under different frequency components, and can filter out the common interference situation of the power grid background harmonics as the interference source. This is because the power grid background harmonics are generally high-frequency harmonics, but the phase responses of the core and the clamping piece to the high-frequency harmonics are different. Therefore, when this interference situation occurs, it may show that the increase amplitude of the amplitude correlation exceeds the set value, but the increase amplitude of the phase correlation does not exceed the set value.

[0017] Further, when the increase amplitude of the amplitude correlation coefficient and the increase amplitude of the phase correlation coefficient exceed the set value only under the fundamental wave component or only under the harmonic component, the present invention determines whether the deviation of the fundamental wave component and the harmonic component following is caused by external interference or early insulation fault through the feedback of the system after the resistor is switched, which can further improve the detection rate of the early insulation fault of the present invention.

[0018] Further, the historical data stored in the storage unit of the present invention changes in real time, that is, after excluding the abnormal data, the data processed by the signal acquisition unit and the data analysis unit replaces the earliest data in the storage unit, so as to compensate for the parameter variation error caused by the aging of the power grid equipment and the transformer equipment. Description of the Drawings

[0019] Figure 1 It is a schematic diagram of an on-line monitoring system for the grounding current of a transformer core according to the present invention; Figure 2Flow chart for correlation analysis of the present invention; Figure 3 Flow chart for judging the fault state of the transformer core; In the figure: 100, transformer; 200, signal acquisition unit; 300, conditioning unit; 400, data analysis unit; 500, control unit; 11, iron core; 12, clamping piece; 13, outer shell; 14, iron core grounding lead-out wire; 15, clamping piece grounding lead-out wire; 16, outer shell grounding lead-out wire; 21, first Hall sensor; 22, second Hall sensor; 23, bracket; 24, movable plate; 25, resistor; 26, control switch; 31, sampling resistor; 32, filter circuit; 34, A / D converter; 321, low-pass filter circuit; 322, high-pass filter circuit; 331, primary amplification circuit; 332, secondary amplification circuit. Specific embodiments

[0020] The present invention will be further described below in conjunction with the embodiments of the present invention and the accompanying drawings.

[0021] Embodiment 1 of an on-line monitoring system for the grounding current of a transformer core according to the present invention: The on-line monitoring system for the grounding current of the transformer core of the present invention is used to monitor the grounding current of the iron core of the transformer 100 in real time. The transformer 100 includes an iron core 11, a clamping piece 12 and an outer shell 13. The primary winding and the secondary winding are both wound around the iron core 11. The clamping piece 12 is used to fix the iron core 11, including an upper clamping piece and a lower clamping piece, which are respectively arranged at the upper and lower ends of the transformer 100. The upper clamping piece and the lower clamping piece are connected by a connecting rod made of a conductive material to make the upper clamping piece and the lower clamping piece conduct. The iron core 11 of the transformer 100 is grounded through the iron core grounding lead-out wire 14, the clamping piece 12 is grounded through the clamping piece grounding lead-out wire 15, and the outer shell 13 is grounded through the outer shell grounding lead-out wire 16.

[0022] The on-line monitoring system for the grounding current of the transformer core includes a signal acquisition unit 200, a conditioning unit 300, a data analysis unit 400, a control unit 500 and a storage unit.

[0023] The signal acquisition unit 200 includes a first Hall sensor 21 and a second Hall sensor 22. The first Hall sensor 21 is used to detect the current flowing through the iron core grounding lead 14, and this current is the iron core grounding current; the second Hall sensor 22 is used to detect the current flowing through the clamping piece grounding lead 15, and this current is the clamping piece grounding current. The distance between the iron core grounding lead 14 and the clamping piece grounding lead 15 is appropriate. The first Hall sensor 21 and the second Hall sensor 22 are supported at the same height by a bracket 23 to balance the influence of the leakage magnetic flux in the transformer 100 on the detection results of the two. The detections of the first Hall sensor 21 and the second Hall sensor 22 use a synchronous clock source to drive a multi-channel ADC for sampling to ensure that the sampling results of the first Hall sensor 21 and the second Hall sensor 22 are completely synchronous.

[0024] The bracket 23 is made of wood or other non-conductive and non-magnetic materials. An activity plate 24 is arranged on the bracket 23. The activity plate 24 can slide up and down along the bracket 23. Two grooves that match the bottom shape of the Hall sensor for it to insert are arranged on the activity plate 24. The first Hall sensor 21 and the second Hall sensor 22 are respectively inserted into the corresponding grooves. The sizes of the two grooves are exactly the same. Therefore, after the first Hall sensor 21 and the second Hall sensor 22 are inserted to the bottom of the grooves, they are at the same height, which can ensure that the influence of the leakage magnetic flux in the transformer 100 on the two is consistent as much as possible. In this embodiment, the two Hall sensors inserted into the grooves are lifted and lowered synchronously. During specific measurement, the activity plate 24 can be moved up and down, and the point with the lowest noise can be selected as the detection point.

[0025] The signal acquisition unit 200 further includes resistors 25 and control switches 26. The two resistors 25 are respectively connected in series on the iron core grounding lead 14 and the clamping piece grounding lead 15, and their switching states are controlled by the corresponding control switches 26.

[0026] The conditioning unit 300 includes a sampling resistor 31, a filtering circuit 32, an amplifying circuit, and an A / D converter 34. The currents collected by the first Hall sensor 21 and the second Hall sensor 22 are converted into analog voltage signals after passing through the sampling resistor 31. The analog voltage signals are filtered by the filtering circuit 32 and then sent into the A / D converter 34. The A / D converter 34 converts the analog voltage signals into digital voltage signals. The amplifying circuit is used to amplify the analog voltage signals and includes a primary amplifying circuit 331 and a secondary amplifying circuit 332. The primary amplifying circuit 331 is placed between the sampling resistor 31 and the filtering circuit 32, and the secondary amplifying circuit 332 is placed between the filtering circuit 32 and the A / D converter 34.

[0027] Both the primary amplification circuit 331 and the secondary amplification circuit 332 are inverting amplification circuits, and their amplification factor is -R2 / R1. The inverting amplification circuit introduces voltage negative feedback, which belongs to the differential-mode signal, has strong anti-interference ability, and is less affected by the external environmental temperature.

[0028] The filtering circuit 32 includes a low-pass filtering circuit 321 and a high-pass filtering circuit 322. When the iron core is grounded at a single point, the detected current value of the first Hall sensor 21 is extremely small. At this time, harmonics will have a very large impact on the detection result. Therefore, in the present invention, the voltage signal is first filtered by the low-pass filtering circuit 321 to remove high-frequency harmonics, and then the measurement accuracy drift caused by low temperature is excluded by the high-pass filtering circuit 322. The conditioning unit 300 converts the iron core grounding current collected by the first Hall sensor 21 into an iron core grounding voltage, and converts the clamping piece grounding current collected by the second Hall sensor 22 into a clamping piece grounding voltage.

[0029] The data analysis unit 400 is used to analyze the correlation degree of the two groups of voltage signals. The specific steps for the data analysis unit 400 to perform the correlation analysis are as follows: Step 1: Obtain the iron core grounding voltage and the clamping piece grounding voltage by means of synchronous sampling.

[0030] Step 2: Perform discrete Fourier transform on the iron core grounding voltage and the clamping piece grounding voltage respectively to obtain the frequency-domain representations of the iron core grounding voltage and the clamping piece grounding voltage, and identify the fundamental wave and the main harmonic frequencies.

[0031] In this step, the main harmonic frequency refers to the harmonics corresponding to the first 90% of the energy.

[0032] Step 3: Perform frequency division filtering and Hilbert transform on the iron core grounding voltage and the clamping piece grounding voltage respectively to obtain the instantaneous amplitude and instantaneous phase sequences of the corresponding frequency signals.

[0033] Frequency division filtering means designing band-pass filters respectively for the fundamental wave frequency and the main harmonic frequency components. For the 50HZ fundamental wave, a 50±2HZ band-pass filter is designed. For the nth harmonic, a 50n±2nHZ band-pass filter is designed, and Hilbert transform is performed on the narrowband signal obtained after filtering.

[0034] Specifically, in this step, calculate the instantaneous amplitude and instantaneous phase of the iron core grounding voltage at the fundamental wave and the main harmonic frequencies, and the instantaneous amplitude and instantaneous phase of the clamping piece grounding voltage at the fundamental wave and the main harmonic frequencies.

[0035] Step 4: Calculate the amplitude correlation between the iron core grounding voltage and the clamping piece grounding voltage at the same frequency components and measure it with the amplitude correlation coefficient. The amplitude correlation coefficients under different frequency components form an amplitude correlation coefficient sequence.

[0036] In this step, first, the instantaneous amplitude sequences at different frequency components are extracted from the core grounding voltage and the clamping piece grounding voltage respectively. After aligning the instantaneous amplitude sequences of the two channels, the Pearson correlation coefficient formula is used to calculate the correlation coefficient of the instantaneous amplitudes of the two channels for each frequency component.

[0037] Step 5: Calculate the phase correlation of the same frequency components of the core grounding voltage and the clamping piece grounding voltage and measure it with the phase correlation coefficient. The phase correlation coefficients under different frequency components form a phase correlation coefficient sequence.

[0038] In this step, first, the instantaneous phase sequences at different frequency components are extracted from the core grounding voltage and the clamping piece grounding voltage respectively. For each frequency component, the phase difference between the instantaneous phase of the core grounding voltage and the instantaneous phase of the clamping piece grounding voltage is directly calculated. Then, the standard deviation of the phase difference sequence is calculated. After that, the phase correlation coefficient at each frequency component is constructed by dividing the standard deviation by the maximum value of the standard deviation when the phase difference is completely random and then subtracting this value from 1. That is, the phase correlation coefficient = , where is the standard deviation of the phase difference sequence, is the theoretical maximum value of the standard deviation of the phase difference sequence.

[0039] It should be emphasized that in Step 4 and Step 5, the amplitude correlation coefficient and the phase correlation coefficient are calculated for at least one complete power frequency cycle. The sampling frequency of the A / D converter 34 is 1024HZ.

[0040] The storage unit stores the historical data of the amplitude correlation coefficient between the core grounding voltage and the clamping piece grounding voltage, the historical data of the phase correlation coefficient between the core grounding voltage and the clamping piece grounding voltage, and a set of transient waveform templates corresponding to the switch operation modes one by one. The control unit 500 judges the operating state of the transformer according to the signals transmitted by the signal acquisition unit 200, the data analysis unit 400, and the storage unit.

[0041] It should be emphasized here that the historical data stored in the storage unit is not constant but is updated in real time. If the waveforms of the core grounding current and the clamping piece grounding current in a certain period are determined to be the currents under normal operating conditions, the data in this period is supplemented into the storage unit, and correspondingly, the earliest piece of data in the historical data is deleted.

[0042] The process of judging the operating state of the transformer is as follows: When the amplitude of the core grounding current is greater than 100 mA, analyze whether the waveform change of the core grounding current is synchronized with the corresponding transient waveform template in the transient waveform template set under the corresponding switch operation. When the two are synchronized, it is determined that the amplitude fluctuation of the core grounding current is caused by the switch action, and the control unit 500 does not issue the corresponding alarm instruction.

[0043] If the waveform change of the iron core grounding current is not synchronized with the switch operation signal, it is determined that the transformer has an insulation fault, and the control unit 500 issues a corresponding alarm trigger command.

[0044] When the amplitude of the iron core grounding current is less than 100 mA, analyze the change of the measured value of the amplitude correlation coefficient under the fundamental wave component and the main harmonic components relative to the historical data, and at the same time analyze the change of the measured value of the phase correlation coefficient under the fundamental wave component and the main harmonic components relative to the historical data. When the increase amplitudes of the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the fundamental wave component and the harmonic components both exceed the set value, it is determined that there is a risk of early insulation fault in the iron core.

[0045] If only the increase amplitudes of the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the fundamental wave component exceed the set value, or only the increase amplitudes of the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the harmonic components exceed the set value, then the control switch 26 is used to switch the resistor 25. After the resistor 25 is switched, if the decrease amplitudes of the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the corresponding frequency components both exceed the set value (10% in this embodiment), it is determined that there is an early insulation fault between the iron core and the clamping piece. After the resistor 25 is switched, if the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the corresponding components do not change significantly or the decrease amplitude does not exceed the set value, it is considered that the increase of the amplitude correlation coefficient and the phase correlation coefficient under the corresponding components is caused by interference, and the transformer state is determined to be normal.

[0046] This is because if an insulation fault occurs, switching the resistor 25 will force the current of the corresponding frequency component to change the conduction path; while the switching of the resistor 25 is insensitive to external interference. Of course, the resistor 25 can be connected in series on the iron core grounding lead 14 or in series on the clamping piece grounding lead 15; the conduction resistance between the iron core and the clamping piece is usually between 1 kΩ and 10 kΩ during the early insulation fault between the iron core and the clamping piece. Therefore, in this embodiment, the resistance value of the resistor 25 is taken as 50 kΩ, which is much larger than the above conduction resistance, so as to achieve the effect of forcing the current to change the conduction path.

[0047] Combined with multiple practical data, it can be seen that when 95% of the early insulation faults occur, the increase amplitudes of the measured values of the amplitude correlation coefficient and the phase correlation coefficient both exceed 20%. Therefore, the set value of the increase amplitude of the measured values of the amplitude correlation coefficient and the phase correlation coefficient is 20%. And when the followability deviation occurs under the fundamental wave component and the harmonic components due to external interference, after switching the 50 kΩ resistor, the situation where the decrease amplitude of the amplitude correlation coefficient and the phase correlation coefficient is less than 10% accounts for 97%. Therefore, it is set that the set value of the decrease amplitude of the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the corresponding frequency components is 10%.

[0048] The core grounding current is mainly caused by eddy currents and leakage magnetic fluxes induced by changes in magnetic fluxes. The main sources of the clamping device grounding current are the induced current generated by structural vibration and the leakage current. The fundamental wave trend of the core grounding current follows the trend of the current applied to the winding. The fundamental wave trend of the clamping device grounding current should be a power frequency sine wave with a low amplitude, superimposed with other frequency components caused by mechanical vibration, and at the same time superimposed with high-frequency noise and occasional interference pulses. The overall waveform is stable without obvious distortion or continuous high-amplitude fluctuations. Therefore, the waveform of the clamping device grounding current is similar to a flat noise band with a very small amplitude, much smaller than the amplitude of the core grounding current; it has the characteristics of irregular random fluctuations and no periodic pulses in the time domain; and it has the characteristics of uniform spectral distribution and no significant harmonic peaks in the frequency domain.

[0049] Therefore, when the insulation between the core and the clamping device is intact, the phase difference between the two follows a random distribution and there is no fixed relationship between them. However, when the insulation between the core and the clamping device is damaged, the two are connected to form a loop. At this time, electromagnetic induction is the dominant factor in the clamping device current, and the phase, frequency domain characteristics, and change trends of the clamping device current and the core grounding current will be synchronized and mainly based on the power frequency. Therefore, through the above criteria, the early insulation fault between the core and the clamping device can be clearly identified, and this early multi-point grounding fault can also be accurately identified when the core grounding current is less than 100 milliamperes.

[0050] Example 2 of an on-line monitoring system and device for the core grounding current of a transformer according to the present invention: The difference between Example 2 and Example 1 lies in: In this embodiment, considering the influence of external lightning strikes, since the duration of the impact caused by lightning strikes is usually 1.2 / 50 microseconds, during lightning strikes, the lightning impulse current makes a major contribution to the core grounding current and the clamping device grounding current. Therefore, the correlation degree between the two will be significantly improved. To avoid misjudgment, when the control unit 500 judges the operating state of the transformer during lightning strikes, a reference basis that the duration of the increase in amplitude correlation and phase correlation exceeding the set value is greater than 10 ms is introduced into the criterion.

[0051] Example of an on-line monitoring device for the core grounding current of a transformer according to the present invention: An on-line monitoring device for the core grounding current of a transformer includes a housing, and integrated in the housing is the on-line monitoring system for the core grounding current of a transformer disclosed in the example of an on-line monitoring system for the core grounding current of a transformer. The specific content will not be elaborated here.

[0052] In actual operation, due to the aging of the clamping insulation pads of a 220 kV transformer, a non-linear contact was formed between the iron core and the clamping parts. After replacing the pads, it returned to normal. However, when the fault occurred, the maximum amplitude of the iron core grounding current was only 43 mA, and this fault could not be detected when using the traditional threshold limit of 100 mA. However, after treatment, it was found that the amplitude correlation coefficient between the iron core grounding voltage and the clamping part grounding voltage at the fundamental frequency changed from 0.34 in historical data to 0.85, and the phase correlation coefficient changed from 0.2 in historical data to 0.6, and was successfully detected by the monitoring system of the present invention.

Claims

1. An on-line monitoring system for the grounding current of a transformer core, characterized in that, including, a signal acquisition unit configured to detect the core grounding current flowing through the core grounding lead-out wire and the clamping piece grounding current flowing through the clamping piece grounding lead-out wire; a conditioning unit configured to process the acquired signals, and at the same time process the core grounding current into a core grounding voltage and the clamping piece grounding current into a clamping piece grounding voltage; a data analysis unit configured to analyze the measured value of the amplitude correlation coefficient and the measured value of the phase correlation coefficient between the core grounding voltage and the clamping piece grounding voltage at a specific frequency component; a storage unit configured to store the historical data of the amplitude correlation coefficient between the core grounding voltage and the clamping piece grounding voltage, the historical data of the phase correlation coefficient between the core grounding voltage and the clamping piece grounding voltage, and a set of transient waveform templates corresponding one-to-one to the switch operation modes under normal operating conditions; a control unit, wherein the signal acquisition unit, the data analysis unit, and the storage unit are electrically connected to different input ports of the control unit to transmit signals thereto, and the control unit comprehensively judges the fault state of the transformer core according to the core grounding current, the clamping piece grounding current, the measured value of the amplitude correlation coefficient, the measured value of the phase correlation coefficient, the historical data of the amplitude correlation coefficient, the historical data of the phase correlation coefficient, the switch operation condition, and the operation mode.

2. An on-line monitoring system for transformer core grounding current according to claim 1, wherein when the core grounding current exceeds the set threshold, it is judged whether its waveform is consistent with the transient waveform template under the corresponding switch operation mode. If the two are consistent, it is determined that the fluctuation of the core grounding current amplitude is caused by the switch action; if the two are inconsistent, it is determined that the transformer has an insulation fault; when the core grounding current is less than the set threshold, the change of the measured value of the amplitude correlation coefficient relative to the historical data and the change of the measured value of the phase correlation coefficient relative to the historical data are analyzed. When both the increase amplitude of the measured value of the amplitude correlation coefficient relative to the historical data and the increase amplitude of the measured value of the phase correlation coefficient relative to the historical data exceed the set value, it is determined that the transformer has an early insulation fault.

3. An on-line monitoring system for transformer core grounding current according to claim 2, wherein the steps of the data analysis unit for performing correlation analysis are as follows: Step 1, obtaining the core grounding voltage and the clamping piece grounding voltage by means of synchronous sampling; Step 2, respectively performing discrete Fourier transform on the core grounding voltage and the clamping piece grounding voltage to obtain the frequency-domain representations of the core grounding voltage and the clamping piece grounding voltage, and identifying the fundamental wave and the main harmonic frequencies; Step 3, respectively performing frequency division filtering and Hilbert transform on the core grounding voltage and the clamping piece grounding voltage to obtain the instantaneous amplitude and instantaneous phase sequences of the corresponding frequency signals; Step 4, calculating the amplitude correlation between the core grounding voltage and the clamping piece grounding voltage at the same frequency component and measuring it with the amplitude correlation coefficient; Step 5, calculating the phase correlation between the core grounding voltage and the clamping piece grounding voltage at the same frequency component and measuring it with the phase correlation coefficient.

4. An on-line monitoring system for transformer core grounding current according to claim 3, wherein When the core grounding current is less than the set threshold value, analyze respectively the change of the measured value of the amplitude correlation coefficient under the fundamental wave component relative to the historical data and the change of the measured value of the phase correlation coefficient relative to the historical data, the change of the measured value of the amplitude correlation coefficient under the harmonic component relative to the historical data and the change of the measured value of the phase correlation coefficient relative to the historical data. When the increase amplitudes of the amplitude correlation coefficient and the phase correlation coefficient under both the fundamental wave component and the harmonic component exceed the set value, it is determined that the transformer has an early insulation fault.

5. An on-line monitoring system for transformer core grounding current according to claim 4, characterized in that a resistor is connected in series on the core grounding lead-out wire and / or the clamping piece grounding lead-out wire, and control switches for controlling the switching state of the resistor are arranged on both sides of the resistor; when only the increase amplitudes of the amplitude correlation coefficient and the phase correlation coefficient under the fundamental wave component exceed the set value or only the increase amplitudes of the amplitude correlation coefficient and the phase correlation coefficient under the harmonic component exceed the set value, the resistor on the core grounding lead-out wire or the clamping piece grounding lead-out wire is switched into the loop; if after the resistor is switched, the decrease amplitudes of the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the corresponding frequency component both exceed the set value, it is determined that there is an early insulation fault between the core and the clamping piece; if after the resistor is switched, the measured values of the amplitude correlation coefficient and the phase correlation coefficient under the corresponding component do not exceed the set value, it is determined that the transformer is in normal state.

6. An on-line monitoring system for transformer core grounding current according to claim 5, characterized in that the storage unit updates the historical data stored therein in real time.

7. An on-line monitoring system for the grounding current of a transformer core according to claim 6, characterized in that, In step four, the Pearson correlation coefficient formula is used to calculate the amplitude correlation coefficient of each frequency component; In step five, first directly calculate the phase difference between the instantaneous phase of the core grounding voltage and the instantaneous phase of the clamping piece grounding voltage for each frequency component, then calculate the standard deviation of the phase difference sequence under this frequency component, and then divide the standard deviation by the maximum value of the standard deviation when the phase difference is completely random, and then construct the phase correlation coefficient at each frequency component by subtracting this value from 1.

8. An on-line monitoring system for the grounding current of a transformer core according to any one of claims 1 to 7, characterized in that: The conditioning unit includes a sampling resistor, a filtering circuit, an amplifying circuit and an A / D converter. The amplifying circuit includes a primary amplifying circuit and a secondary amplifying circuit. The primary amplifying circuit is arranged between the sampling resistor and the filtering circuit, and the secondary amplifying circuit is arranged between the filtering circuit and the A / D converter. The filtering circuit includes a low-pass filtering circuit and a high-pass filtering circuit. Both the primary amplifying circuit and the secondary amplifying circuit adopt inverting amplifying circuits; the signal acquisition unit includes a first Hall sensor and a second Hall sensor, and also includes a bracket. The bracket is provided with a movable plate that can be guided and moved in the up and down direction. Two grooves for inserting the corresponding Hall sensors are formed on the movable plate so that the two Hall sensors are at the same height.

9. An on-line monitoring system for the grounding current of a transformer core according to any one of claims 1 to 7, characterized in that: During lightning strikes, when the control unit determines that the transformer has an early insulation fault or an insulation fault, the duration of the corresponding criterion needs to be greater than 10 ms.

10. An on-line monitoring device for the grounding current of a transformer iron core, characterized in that: It includes a housing, and the housing integrates an on-line monitoring system for transformer core grounding current according to any one of claims 1 to 9.

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