System and method for measuring broadband characteristic of low-frequency transformer based on pulse injection

Abstract

The invention relates to the technical field of power equipment testing, and discloses a low-frequency transformer broadband characteristic measuring system and method based on pulse injection, the system comprises an impedance shaping pulse generator, a broadband coupling device and a broadband response analysis unit, the impedance shaping pulse generator comprises a monostable master control switch and a frequency-variable impedance network; the frequency-variable impedance network is composed of a first passive branch and a second passive branch which are connected in parallel; the system automatically completes time domain physical switching of output impedance from a transient low-impedance source to a steady-state high-impedance source in a single switching action period by utilizing the time constant difference of two branches, and matches the distributed capacitance charging requirement of a tested transformer in a high frequency band and the iron core magnetic flux establishment requirement of the tested transformer in a low frequency band. The physical contradiction that an existing single excitation source cannot give consideration to broadband energy distribution is solved, high signal-to-noise ratio acquisition of full-band impedance characteristics is achieved through passive topology, and iron core residual magnetism is automatically eliminated through back electromotive force.

Description

Technical Field

[0001] This invention relates to a pulse injection-based wideband characteristic measurement system and method for low-frequency transformers, belonging to the field of power equipment testing technology. Background Technology

[0002] As core equipment in the power grid, the early identification of internal faults such as winding deformation, inter-turn short circuits, and core loosening in power transformers relies on the measurement of the equipment's broadband impedance characteristics. The pulse frequency response analysis method uses an excitation signal injected into the winding and collects port response data. Due to its high testing efficiency and the fact that it does not require changing the equipment's operating wiring status, it is widely used for on-site inspections and transient characteristic analysis of large inductive equipment such as converter transformers. Conventional pulse injection technology usually uses a pulse generator with fixed resistance and capacitance parameters as the excitation source and diagnoses faults by obtaining the fingerprint of the mechanical state transfer function that characterizes the equipment.

[0003] The electrical structure of large converter transformers combines Henry-level main inductance with picofarad-level distributed capacitance. When measuring broadband characteristics, the output impedance of the excitation source is subject to mutually exclusive physical constraints. To excite a high-frequency response at the megahertz level corresponding to the winding's distributed parameters, the excitation pulse must have a steep rise edge at the nanosecond level, requiring the excitation source to exhibit extremely low transient internal resistance to provide high-current charging capability. To excite a low-frequency response at the hertz level corresponding to the core state, the excitation pulse must maintain a long tail at the millisecond level to establish magnetic flux, requiring the excitation source to exhibit high steady-state internal resistance to maintain the voltage decay time constant. Existing single excitation sources based on fixed impedance topologies cannot simultaneously meet the broadband energy distribution requirements in the time domain, resulting in limitations in practical engineering applications. For example, Chinese invention patent CN111460605B discloses a broadband hybrid model of a transformer considering core nonlinearity and its establishment method. It constructs a hybrid mathematical model to simulate and predict the broadband response of the transformer, making up for the lack of nonlinear characterization in the traditional broadband admittance model. However, the accuracy of this post-processing method based on mathematical modeling and simulation depends on the quality of the input measurement data. If there are problems in the initial data acquisition stage, such as poor impedance matching of the excitation source, low signal-to-noise ratio, or incomplete elimination of residual magnetism in the core, no matter how sophisticated the subsequent model is, it cannot avoid the systematic errors of the original data. In particular, the reliability of low-frequency measurements is difficult to guarantee.

[0004] Therefore, the technical problem to be solved by this invention is how to construct a measurement system that automatically adapts to high and low frequency energy requirements during a single injection cycle, isolates source-end interference during the measurement phase, and automatically resets the magnetic state after the test. Summary of the Invention

[0005] To address the problems mentioned in the background art, the technical solution of the present invention is as follows: A low-frequency transformer broadband characteristic measurement system based on pulse injection, comprising an impedance shaping pulse generator, a broadband coupling device, and a broadband response analysis unit:

[0006] An impedance shaping pulse generator is connected between a high-voltage DC source and the transformer under test, forming the source end for injecting excitation energy into the windings of the transformer under test; a wideband coupling device is connected between the output end of the impedance shaping pulse generator and the high-voltage bushing of the transformer under test, forming a signal injection channel; a wideband response analysis unit is connected to the end screen port of the transformer under test, forming a signal acquisition end.

[0007] The impedance shaping pulse generator includes a monostable main control switch and a frequency-varying impedance network. The monostable main control switch is connected in series between the high-voltage DC source and the frequency-varying impedance network to perform a single closing action. The frequency-varying impedance network includes a first passive branch and a second passive branch connected in parallel. The first passive branch has capacitive impedance characteristics, providing a low-impedance path at the moment the monostable main control switch is turned on, establishing a nanosecond-level rising edge of the excitation pulse, and its impedance increases as the capacitor charges. The second passive branch has inductive or resistive impedance characteristics. In the steady-state discharge stage after the rising edge of the excitation pulse is established, its impedance is lower than that of the first passive branch after charging, taking over the discharge circuit and maintaining the millisecond-level decay tail of the excitation pulse. The system utilizes the time constant difference between the first and second passive branches to achieve automatic switching of the source output impedance from low to high within a single operation cycle of the monostable main control switch, so as to match the distributed capacitance charging requirements of the transformer under test in the high-frequency band and the core magnetic flux establishment requirements in the low-frequency band.

[0008] Preferably, the first passive branch includes a sharpening capacitor and a first damping resistor connected in series; the capacitance value of the sharpening capacitor and the resistance value of the first damping resistor are set according to the preset rise time of the excitation pulse to ensure that a dominant high-frequency current injection is provided within the nanosecond time of the monostable main control switch operation.

[0009] Preferably, the second passive branch includes a series-connected energy storage inductor or a second damping resistor; the resistance value of the second damping resistor is set to be greater than the characteristic impedance of the transformer winding under test, so as to form a high internal resistance characteristic during the tailing stage of the excitation pulse and maintain the magnetic field energy decay process in the winding inductance.

[0010] Preferably, the wideband coupling device includes a unidirectional isolation component connected in series between the output of the frequency-variable impedance network and the transformer under test; the unidirectional isolation component is composed of a fast recovery diode stack; the unidirectional isolation component conducts due to the forward voltage difference during the excitation pulse injection, establishing an energy injection channel; and is cut off during the free response period after the excitation pulse ends due to the disappearance of the forward voltage difference or the appearance of a reverse voltage difference, cutting off the electrical connection between the frequency-variable impedance network and the transformer under test, and blocking the interference of residual oscillations on the source side on the measurement signal.

[0011] Preferably, the frequency-variable impedance network is connected in parallel with an automatic demagnetizing branch, which includes a demagnetizing capacitor and a damping resistor connected in series. After the excitation pulse tail ends, the automatic demagnetizing branch is turned on by the inductive back electromotive force generated by the winding of the transformer under test, absorbs the reverse energy and forms an underdamped oscillating circuit with the winding of the transformer under test, injects a demagnetizing current with alternating polarity decay into the transformer under test, and eliminates the residual magnetism of the iron core.

[0012] Preferably, the wideband response analysis unit includes a first measurement channel and a second measurement channel connected in parallel; the first measurement channel is equipped with a high-frequency voltage divider for acquiring the leading edge of the excitation pulse and high-frequency oscillation signals; the second measurement channel is equipped with a low-frequency voltage divider and an integrator circuit for acquiring the tail of the excitation pulse and low-frequency response signals; the wideband response analysis unit splices the data from the first measurement channel and the second measurement channel in the time domain based on a unified clock reference.

[0013] Preferably, the unidirectional isolation component is connected in parallel with a high-frequency compensation capacitor; the capacitance value of the high-frequency compensation capacitor is set such that the equivalent AC impedance of the unidirectional isolation component in the off state is greater than the stray capacitance impedance of the transformer winding under test in the high-frequency band, so as to prevent high-frequency signals from leaking through the diode junction capacitance.

[0014] Preferably, the capacitance value of the demagnetizing capacitor in the automatic demagnetizing branch is... Equivalent main inductance of the transformer under test A specific underdamped oscillation condition must be met to ensure that the generated demagnetizing current has a sufficient number of zero crossings; the underdamped oscillation condition is defined by the following relationship: ,in, It is the sum of the damping resistance and the DC resistance of the winding. This represents the core inductance value of the transformer under test. The capacitance value of the demagnetizing capacitor is given; the system configures the damping parameters according to this relationship to generate a demagnetizing waveform with decreasing amplitude and alternating polarity.

[0015] Preferably, the impedance shaping pulse generator is encapsulated in a grounded metal shielded cavity; the high-voltage DC source, monostable main control switch, and frequency-converting impedance network are arranged in a coaxial structure to reduce the distributed inductance of the circuit; the output terminal is connected to a wideband coupling device through a high-voltage coaxial cable; the system also includes a current sensing transformer connected to the neutral point of the transformer under test; the current sensing transformer is used to monitor the response current flowing through the transformer windings, and together with the voltage signal collected by the wideband response analysis unit, it constitutes the original dataset for calculating the wideband transfer function of the transformer; the original dataset covers the frequency domain range from Hertz to megahertz.

[0016] A method for measuring the broadband characteristics of a low-frequency transformer based on pulse injection includes the following steps:

[0017] A single-closing action is performed by the monostable main control switch in the impedance shaping pulse generator to trigger the energy release of the high-voltage DC source.

[0018] The released energy is time-domain impedance shaped by a frequency-varying impedance network. The first passive branch with capacitive impedance characteristics plays a dominant role at the moment of switch-on to establish the nanosecond-level rising edge of the excitation pulse. The second passive branch with inductive or resistive impedance characteristics plays a steady-state role after the rising edge is established to maintain the millisecond-level decay tail of the excitation pulse. Thus, the source output impedance is automatically switched from low to high within a single pulse cycle.

[0019] The shaped excitation pulse is injected into the high-voltage bushing of the transformer under test via a broadband coupling device, and a unidirectional isolation component is used to conduct during the injection period to establish a channel and to cut off during the free response period to isolate source interference.

[0020] In addition, a wideband signal containing high-frequency distributed parameter response and low-frequency core flux response is collected from the end screen port of the transformer under test using a wideband response analysis unit, and full-spectrum analysis is performed to obtain the wideband impedance characteristics of the transformer.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] 1. In the wideband characteristic measurement of low-frequency transformers, an asymmetric impedance topology of a frequency adaptive shaping network is used. Based on the high-frequency pass-through characteristics of capacitors and the abrupt change in inductor resistance, the output impedance of the excitation source automatically transitions from transient low resistance to steady-state high resistance during a single pulse injection process. This is achieved without relying on the microsecond-level synchronous control logic of multiple high-voltage switches. By utilizing the time constant difference of passive components, nanosecond-level voltage steep leading edge required for winding distributed parameter measurement and millisecond-level voltage tail required for establishing the main inductance flux of the iron core are constructed sequentially within a millisecond-level time window. Based on the energy time-domain distribution method of circuit physical characteristics, this solves the physical contradiction that a single excitation source in the existing wideband characteristic measurement of transformers cannot simultaneously take into account the high-frequency transient burst force and the low-frequency energy persistence. This ensures that the system can obtain impedance characteristics covering the entire frequency band from the iron core response to the winding deformation response through a single injection.

[0023] 2. The system utilizes a transient barrier gating unit set in series at the output end. Based on the cutoff characteristic of the fast recovery diode stack nonlinear component when the forward voltage difference disappears, a unidirectional dynamic connection channel is constructed between the excitation source and the transformer under test. After the excitation pulse injection ends, the unit responds to the voltage drop or reverse zero crossing of the transformer under test port voltage, automatically disconnecting the electrical connection between the shaping network and the transformer winding. This blocks the residual oscillation generated by the energy storage element inside the shaping network from coupling interference to the measurement signal. As a result, the voltage signal collected by the full-spectrum response analysis unit during the free oscillation stage only contains the transformer body impedance response information. This eliminates the masking of high-frequency weak modal fingerprints caused by inter-turn short circuits or minor deformations in the winding by background noise, improving the signal-to-noise ratio and sensitivity of early fault diagnosis.

[0024] 3. By utilizing the reverse flux erasure branch connected in parallel to the shaping network, the inductive back electromotive force generated by the transformer due to magnetic field collapse after the long tail pulse ends is used as the trigger source. This automatically turns on and forms an underdamped oscillating circuit. The branch captures the energy of the reverse magnetic field released by the transformer and converts it into a damped oscillating current with alternating polarity decay, which is then fed back into the winding. This causes the core hysteresis loop to contract and stabilize at the origin. The back electromotive force, which is usually considered a hazard of overvoltage, is transformed into a source of energy for cleaning the residual magnetism of the core. Without the need for an external demagnetizing power supply or additional operating steps, the accumulation of residual magnetism in the core caused by unipolar long pulse injection is automatically eliminated, ensuring the consistency of the measurement benchmark and the comparability of data for low-frequency inductance parameters under multi-phase alternating testing or repetitive retesting conditions. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the transformer broadband measurement system architecture for impedance shaping pulse injection according to the present invention;

[0026] Figure 2 This is a comparison chart showing the impact of the self-triggered magnetic flux reset mechanism of this invention on the stability of low-frequency inductance measurement;

[0027] Figure 3 This is a timing flowchart of the synchronous acquisition and full-spectrum fusion processing of the broadband response signal of the present invention. Detailed Implementation

[0028] The following examples are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0029] This invention provides a low-frequency transformer broadband characteristic measurement system based on pulse injection, comprising an asymmetric impedance shaping pulse generator, a broadband signal coupling device, and a full-spectrum response analysis unit. The asymmetric impedance shaping pulse generator serves as the core excitation source of the system. Its circuit topology design follows the physical mechanism of frequency-domain and time-domain mapping and current splitting. The generator includes a high-voltage energy storage unit, a monostable main control switch, and a frequency adaptive shaping network. The high-voltage energy storage unit is connected to a DC high-voltage source to establish the initial electric field energy. The monostable main control switch is connected in series between the high-voltage energy storage unit and the frequency adaptive shaping network to perform a single closing action. The uncertainty of its closing time needs to be controlled within microseconds to ensure trigger synchronization. The frequency adaptive shaping network is a key component for realizing broadband excitation. It is connected to the output terminal of the monostable main control switch and consists of a first branch connected in parallel as a low-impedance fast path and a second branch connected in parallel as a high-impedance slow path. This parallel topology utilizes the physical characteristics of capacitors passing high frequencies and inductors exhibiting abrupt changes in resistance to automatically complete the transfer of current dominance without active control intervention.

[0030] The first branch has a pulse sharpening capacitor connected in series. and the first damping resistor When the monostable main control switch is turned on At any given moment, because the voltage across the capacitor cannot change abruptly, It exhibits an extremely low transient impedance, approximating a short circuit. At this point, the first branch dominates the discharge circuit, and the total circuit impedance is mainly composed of... In this implementation method, it is decided that... The resistance value is set to be less than To match the characteristic impedance of the transmission cable and provide high current injection capability, this low-impedance path allows current to change at extremely high rates. Injected into the winding of the transformer under test, thereby establishing a rise time at the port of the object under test. Less than The nanosecond-level steep voltage leading edge, containing abundant megahertz-level high-frequency components, is used to excite the response of the winding distributed capacitance and stray parameters, thereby obtaining fingerprint characteristics reflecting winding deformation or displacement; the second branch is connected in series with an energy storage inductor. Or the second damping resistor In the steady-state phase after the pulse leading edge is established, as Once charging is complete, the impedance of the first branch gradually increases and cuts off the DC component, while the inductor... The current gradually builds up or in a purely resistive design. In this embodiment, the second branch exhibits high impedance characteristics, and its equivalent impedance is or The resistance value is set to be greater than This high internal resistance characteristic is used to limit the decay rate of the discharge current during the tail phase of the pulse, maintaining the flux build-up process in the winding inductance, thereby generating a longer duration. Greater than The millisecond-level voltage tail, containing low-frequency components at the Hertz level, is used to excite the response of the main inductor of the iron core, thereby obtaining parameters reflecting the state of the iron core magnetic circuit.

[0031] To address the engineering problem of the excitation source being the noise source during measurement, a transient barrier gating unit is connected in series in the broadband signal coupling device. This unit consists of a diode stack composed of multiple fast recovery diodes connected in series and a non-inductive bias network. This unit utilizes the nonlinear current-voltage characteristics of the PN junction to achieve isolation between the source and load ends. In the injection state, during the excitation pulse injection, the generator output voltage is much higher than the voltage at the transformer port under test, and the diode stack withstands a forward high voltage differential. Greater than the conduction threshold voltage of the diode stack In the isolation state, when the main energy of the excitation pulse is released, the system enters the free response measurement stage. The winding of the transformer under test generates a damped oscillating voltage due to the exchange effect of the internal inductance and capacitance. Since the amplitude of this oscillating voltage decays over time and may reverse polarity, the forward voltage difference across the diode stack disappears or it is subjected to reverse voltage. The diode stack is quickly turned off, exhibiting a high impedance at the megahertz level. At this time, the frequency adaptive shaping network is electrically completely disconnected from the transformer under test, blocking the coupling interference of the residual oscillation generated by the energy storage element inside the generator to the measurement signal. To address the problem of residual magnetism accumulation in the iron core caused by unipolar pulse injection, the frequency adaptive shaping network also has a self-triggered flux reset branch in parallel, including a series-connected reverse energy storage capacitor. Damping resistor And a unidirectional conducting element. The operating logic of this branch is based on an energy feedback mechanism: Energy capture: After the long tail of the excitation pulse ends, the monostable main control switch is opened. The large inductance winding of the transformer under test generates an inductive back electromotive force due to the collapse of the magnetic field. Its polarity is opposite to the injected voltage. This reverse voltage drives the unidirectional conducting element to conduct, transferring the magnetic field energy and storing it in the reverse energy storage capacitor. In the middle; oscillation cleaning: after the peak value of the back electromotive force has passed, As a temporary power source, through Discharge is applied to the transformer winding. At this time, the reset branch and the transformer winding form a second-order RLC oscillating circuit. To achieve the demagnetizing effect, the parameters of this circuit must satisfy the underdamped oscillation condition, namely: In the formula, For including and the total circuit resistance of the DC resistance of the winding, The equivalent main inductance of the transformer under test is given. Under this condition, the circuit generates an oscillating current with an amplitude that decays exponentially and an alternating polarity. When this current flows through the winding, it causes the hysteresis loop of the iron core to gradually contract around the origin, and finally eliminates the residual magnetism when the current decays to zero.

[0032] The full-spectrum response analysis unit is connected to the end-screen port or low-voltage side bushing of the transformer under test to acquire broadband response signals. This unit includes bandwidth coverage... to High-frequency voltage divider and bandwidth coverage to The low-frequency voltage divider is equipped with a dual-channel synchronous acquisition card. Based on a unified clock reference, it records high-frequency transient data at the pulse leading edge and low-frequency slowly varying data at the pulse tail. During the data processing stage, the system executes a time-domain stitching algorithm, using the overlapping area on the time axis as a reference, to fuse the two sets of data into a complete time-domain response curve, and then performs a fast Fourier transform on it to obtain the coverage... to The transfer function curve across the entire frequency band; in specific engineering calibration, the first damping resistor The value is determined based on the surge impedance of the injected coaxial cable. Confirm, Select To eliminate reflections; pulse sharpening capacitor The capacitance value is based on the target rise time. and load equivalent input capacitance Confirmed, satisfied In the demagnetizing branch The capacitance value is determined based on the rated capacity and estimated main inductance of the transformer under test, ensuring that the oscillation frequency falls within a certain range while meeting the underdamped condition. to Within the range, to ensure that the magnetic flux can effectively penetrate the iron core laminations.

[0033] Example 1: In response to In the field operation of preventive testing of converter transformers in ultra-high voltage direct current (UHVDC) transmission projects, the system faces the challenge of extreme impedance loads composed of the main inductance of the Henry stage winding and the stray capacitance at the Nafar stage inlet. This load characteristic makes it impossible for a conventional single excitation source to simultaneously meet the high flow-streaming capability required to excite the megahertz-level winding deformation mode and the high voltage sustaining capability required to excite the Hertz-level core magnetic circuit mode in a single pulse injection. Under this high-voltage, high-interference environment, when the monostable main control switch of the asymmetric impedance shaping pulse generator is closed and triggered, the frequency adaptive shaping network connected in parallel to the output immediately utilizes the pulse sharpening capacitor in the first branch. The physical property that voltage cannot change abruptly results in a low-impedance state, approximating a short circuit, during nanosecond-level transients, causing the resistance to be less than... First damping resistor The main discharge circuit is activated and a large charging current is injected into the load, thereby forcibly establishing a rise time at the large capacitor load end of the transformer under test. Less than The voltage steep leading edge is used to obtain high-frequency distribution parameters. As the capacitor completes charging and the spectral components shift to lower frequencies, the second branch, exhibiting inductive or high-resistivity characteristics, naturally takes over the current dominance, automatically transforming the source end into a circuit with an internal resistance greater than [value missing]. A high-impedance voltage source, thereby limiting the charge discharge rate of the energy storage unit and maintaining the duration of the charge discharge across the winding. Greater than The voltage tailing is used to complete the establishment of the core flux.

[0034] When the main energy of the excitation pulse is released and the free response measurement stage begins, the instantaneous barrier gating unit connected in series in the signal path automatically transitions from a low-resistance conduction state to a high-resistance cutoff state in response to the disappearance of the forward voltage difference across the diode stack or the appearance of the reverse induced voltage. This disconnects the electrical connection between the frequency adaptive shaping network and the transformer under test. This physical isolation action blocks the coupling of residual oscillations of the energy storage element inside the generator to the measurement signal, ensuring that the voltage waveform acquired by the full-spectrum response analysis unit only contains the pure decaying oscillation characteristics of the transformer body in the off-grid state. This improves the signal-to-noise ratio for identifying high-frequency weak fault fingerprints corresponding to inter-turn short circuits or minor deformations. To address the problem of residual magnetism accumulation in the core that is inevitably caused by unipolar long pulse injection, the self-triggered flux reset branch connected in parallel to the shaping network automatically conducts using the reverse induced electromotive force generated by the collapse of the transformer winding magnetic field at the moment the pulse ends as the driving source. This branch captures the inductive reverse charging energy, which was originally considered a hazard of overvoltage, and stores it in the reverse energy storage capacitor. In the middle, it then forms a structure with the transformer winding that satisfies specific underdamped conditions. The oscillation circuit injects a demagnetizing current into the winding with an amplitude that decays exponentially and whose polarity alternates. This process utilizes the system's own waste energy to cause the core hysteresis loop to shrink to zero, ensuring the consistency of the low-frequency inductance parameter measurement benchmark in multi-phase alternating tests or repeated comparison tests.

[0035] Example 2: This example verifies the effectiveness of the pulse injection-based low-frequency transformer broadband characteristic measurement system in obtaining the full-band response under single injection conditions, and compares its performance difference with that of a traditional single fixed impedance source. The test platform consists of an asymmetric impedance shaping pulse generator prototype, a transformer model under test, a broadband signal coupling device, and a data acquisition system. The object under test is a transformer with a rated capacity of [missing information]. Voltage level The power transformer model has windings with typical high inductance and large distributed capacitance characteristics. The key parameters of the generator are set as follows: pulse sharpening capacitor. First damping resistor Second damping resistor Reverse energy storage capacitor The data acquisition system adopts Bit vertical resolution, highest sampling rate High-speed oscilloscope, with bandwidth The high-voltage probe was used to pick up signals. To simulate the electromagnetic interference environment on site, the test was conducted in an unshielded hall, and a power frequency magnetic field generator was placed near the signal circuit to introduce background noise.

[0036] When the test starts, the system control unit triggers the monostable main control switch, and the injected waveform recorded by the oscilloscope is displayed, showing the rise time of the pulse leading edge. Approximately This excites the high-frequency distributed parameter response of the transformer under test, at which point the current flowing through... The transient peak current reached This ensures sufficient signal-to-noise ratio in the high-frequency band, allowing the waveform to smoothly transition to the long tail phase over time, with a voltage half-peak time. Measured as The existence of this millisecond-level trail confirms that the high impedance characteristic of the second branch connects the discharge circuit, allowing the magnetic flux in the iron core inductor to be fully established. To verify the technical advantages of this invention, a comparative test group was set up. In the comparative group, only those containing Traditional low-resistance sources with internal resistance and those containing only A traditional high-resistance source with internal resistance is used to test the same transformer.

[0037] Table 1: Comparison of Response Characteristics under Different Excitation Sources

[0038]

[0039] See Table 1. Although traditional low-resistivity sources... It has a high signal-to-noise ratio (SNR) However, due to the extremely short trailing time ( ), leading to its The signal-to-noise ratio drops sharply in the low-frequency band. It cannot effectively extract core parameters; on the contrary, traditional high-resistivity sources maintain a long trailing shape. ) and low-frequency signal-to-noise ratio ( However, its rise time has degenerated to This results in a signal-to-noise ratio of only [value missing] in the high-frequency band. The key information about winding deformation is lost, while the solution of this invention achieves nanosecond-level leading edge in a single injection. ) and millisecond-level trailing ( ), and maintains excellent signal-to-noise ratio at both high and low frequencies (respectively). and This result strongly demonstrates the effectiveness of the frequency adaptive shaping network in allocating spectral energy in the time domain. Secondly, the effectiveness of the source-load isolation mechanism was verified. After pulse injection, the oscilloscope continued to acquire the free oscillation signal at the transformer port. The data showed that without the instantaneous barrier gating unit, high-frequency spikes were superimposed on the free oscillation waveform, and spectral analysis revealed resonance peaks related to the internal capacitance parameters of the generator. However, after connecting this unit, the free oscillation waveform exhibited a smooth, monotonically decaying curve, and the background noise floor was reduced by approximately [missing information]. This confirms that the high-resistance cutoff characteristic of the diode stack effectively cuts off source-end interference during the low-voltage reverse phase, improving the ability to identify weak fault characteristics. Finally, the flux reset function was tested. After 10 consecutive unipolar pulse injection tests, the low-frequency inductance value of the transformer was measured. With the self-triggered flux reset branch enabled, the deviation of the inductance value measured 10 times was less than [value missing]. When this branch is disabled, the inductance value exhibits a monotonically drifting trend, with the maximum deviation reaching [value missing]. .

[0040] Example 3: This example combines Figures 1 to 3 This document describes a pulse injection-based wideband characteristic measurement system and method for low-frequency transformers, as follows: Figure 1 As shown, the overall architecture of the system includes a high-voltage DC source as the source of injected excitation energy, whose output is connected to an impedance shaping pulse generator, which aims to achieve automatic switching of the source impedance from low to high. Internally, it mainly includes a monostable main control switch that performs a single closed action and a frequency-varying impedance network containing a first passive branch and a second passive branch, which is used to achieve time-domain impedance shaping. The output of the generator forms a signal injection channel through a wideband coupling device containing a unidirectional isolation component, injecting the generated excitation pulse into the high-voltage bushing receiving injection terminal of the transformer under test. The transformer under test outputs a response through the end screen port, and the generated end screen response signal finally enters the wideband response analysis unit, and uses the high-frequency channel and low-frequency channel to collect data to obtain the wideband impedance characteristics.

[0041] like Figure 2 As shown, the horizontal axis of this coordinate graph represents the number of tests, with a scale range from the 1st to the 10th test. The vertical axis represents the low-frequency inductance value, expressed as a percentage (%). The graph uses a legend to distinguish between two sets of data trajectories. The dashed line corresponds to the condition where the self-triggered flux reset branch is disabled, showing that the inductance measurement value increases with the number of tests. The solid line corresponds to the condition where the self-triggered flux reset branch is enabled, showing that the inductance measurement value remains stable near the 100% baseline throughout the process. Figure 3As shown, the processing flow involves five logical columns: the transformer under test, the high-frequency channel, the low-frequency channel, the synchronous acquisition card, and the data processor. The transformer under test outputs high-frequency response signals and low-frequency response signals respectively. The system is configured with a high-frequency channel with a bandwidth covering the MHz level to acquire nanosecond-level leading-edge data, and simultaneously configured with a low-frequency channel with a bandwidth covering the Hz level to acquire millisecond-level trailing-edge data. After the two signals are acquired in parallel, the synchronous acquisition card records and transmits the raw data of the two channels based on a unified clock reference. Finally, the data processor splices the time-domain overlapping area of ​​the data and performs a fast Fourier transform to output a full-band transfer function from Hz to MHz.

[0042] Example 4: This example describes the specific process of calibrating key parameters and performing system performance boundary tests on the above measurement system in a laboratory environment. It establishes the quantitative selection criteria for the first damping resistor and the pulse sharpening capacitor, and verifies the system's response characteristics under extreme conditions. In the constructed standard impedance matching test platform, the core parameters of the asymmetric impedance shaping pulse generator need to be optimized and calibrated. Due to the first damping resistor... The pulse sharpening capacitor directly determines the peak value of the injected current and the steepness of the pulse leading edge. The duration of the transient low-impedance channel is determined, and the coordination between the two directly affects the high-frequency response acquisition efficiency. If it is too large, it cannot provide a sufficiently large instantaneous current to establish a nanosecond-level leading edge; if If the impedance is too small, the low-resistance window closes too quickly, resulting in insufficient high-frequency energy injection. Therefore, a set of [specific parameters] is designed to address this. and Combined parameter scanning experiment.

[0043] In the experiment, the generator output was connected to a standard load with known frequency response characteristics, which was composed of... Non-inductive resistor and A series of high-voltage capacitors connected in parallel are used to simulate the high-frequency input impedance of a transformer winding, and are fixed. for ,adjust The resistance value from to Perform step tests, record the voltage waveform across the load using an oscilloscope, and focus on observing the pulse rise time. With peak voltage Data shows that when from Reduce to hour, from Optimized to ,and promote This indicates that the low-impedance path effectively improves the charging rate for capacitive loads; when Further reduced to At that time, although shortened to However, the waveform exhibits significant overshoot and oscillation, disrupting the signal's monotonicity. Therefore, considering both the leading edge steepness and waveform quality, the following determination is made. The optimal range of values ​​is to ,fixed for ,adjust The capacitance value from to The test results show that when Less than At that time, the pulse began to decay before the leading edge was fully established, resulting in The amplitude of the above spectral components decreased by more than ;when Increase to At the above levels, spectral flatness is improved, and and The corresponding frequency response curves show a difference of less than 1% in the high-frequency range. Considering capacitor size and cost, the following was selected. As The nominal value, this calibration process verifies the engineering rationality of the parameter selection procedure, that is, it must meet the following requirements. The time constant matching relationship.

[0044] and key parameters for the demagnetization branch Boundary condition tests were conducted by continuously injecting pulses into the transformer model to monitor low-frequency inductance drift caused by residual magnetism in the core. The test setup was as follows. They are respectively , and The results showed that when for At that time, the oscillation frequency was too high, greater than The magnetic flux cannot effectively penetrate deep into the iron core laminations, resulting in incomplete demagnetization and a continued drift in inductance value. ;when for At that time, the oscillation frequency was too low, less than Excessive decay time affects testing efficiency, while when for At that time, the oscillation frequency is approximately This ensures both the depth of magnetic flux penetration and... The oscillation decay is completed within the time limit, and the inductor drift is controlled within the range. Within this range, the results establish the selection criteria for the demagnetizing capacitor, namely, it should be made so that... The oscillation frequency of the circuit falls on to The optimal demagnetizing frequency band was determined, and finally, the system's ultimate tolerance was tested under pressure. In a short-circuit fault simulation at the generator output, the monostable main control switch and the first branch component were subjected to full-voltage discharge impact. After 50 consecutive short-circuit tests, the system was retested. The resistance change is less than , The capacitance value does not decay.

[0045] Example 5: This example describes a standardized engineering procedure for eliminating uncertainties in the setting of key parameters in the system of the present invention, ensuring measurement consistency and system stability. It covers the offline calibration of core components in the asymmetric impedance shaping pulse generator, as well as the adaptive calibration process of the system under different measured objects and environments, specifically addressing the first damping resistor in the generator. With pulse sharpening capacitor To address the parameter matching problem, an offline calibration platform based on time-domain response characteristics is constructed, utilizing a set of standard high-voltage capacitors with different nominal capacitance values ​​to simulate the inlet capacitance of the transformer winding. The calibration process connects the system to a specific... ,fixed As the initial value, adjust gradually. The voltage waveform across the load is recorded, and the rise time of the waveform is extracted through numerical analysis. Overshoot amplitude Based on a large amount of experimental data, a system was constructed. , , and A multidimensional mapping table between them, traversing the standard capacitor array For each Nodes Step scan damping resistor ,use A sampling rate digital oscilloscope captures the voltage waveform and extracts the rise time. and peak overshoot Remove For the nonlinear distorted data set, perform least squares surface fitting on the remaining samples to calculate the damping resistance. Regarding load capacitors and the target rising edge bivariate function Discretize the function into a form containing A two-dimensional index matrix of nodes, where each node stores the corresponding optimal... and The parameter pairs, this relationship table, are stored in the non-volatile memory of the system control unit. In practical applications, the operator only needs to input the type of the transformer under test or the estimated input capacitance range, and the system can automatically look up the table to recommend or set the optimal parameters. and combination.

[0046] To address potential strong electromagnetic interference and grounding potential fluctuations during field testing, a preliminary system baseline calibration procedure is established. Before formally connecting to the transformer under test, the system performs a self-test procedure, short-circuiting the measurement ports to collect and calculate the system's background noise level and DC drift. This information is used to set the dynamic trigger threshold and zero-point correction coefficient for the full-spectrum response analysis unit. Next, a low-voltage pre-injection test is performed, injecting an amplitude of only the rated voltage into the tested object. The system analyzes the spectral characteristics and decay rate of the pre-injection response signal in real time to fine-tune the parameters of the second branch in the frequency adaptive shaping network, ensuring a smooth transition and sufficient duration of the pulse tail during high-voltage injection to avoid oscillation instability caused by load impedance mismatch. For verification and optimization of the flux reset effect, a closed-loop feedback adjustment mechanism based on residual magnetism detection is established. After each full-voltage test cycle, the system monitors the residual voltage decay characteristics across the transformer windings. If an unexpected DC component or extremely low-frequency bias is detected, the system determines that residual magnetism exists in the core and automatically activates the self-triggered flux reset branch. During the reset process, the system continuously monitors the amplitude envelope and zero-crossing interval of the oscillation current. If the oscillation decays too quickly or the frequency deviates from the preset range, the system will automatically adjust the resistance value of the variable damping element in the reset branch according to the built-in control algorithm, sample and calculate the logarithmic decay rate of the oscillation waveform. and zero-crossing count like and The system is determined to be in an overdamped state. The FPGA controller outputs a PWM signal to drive the servo motor to reduce the variable damping resistor. Adjust the step size to... ,like And the amplitude at the end The energy dissipation is deemed insufficient, and the energy level increases in the opposite direction. Iterative correction until the system damping ratio is reached. converges to The interval, and satisfying The critical condition is until the oscillation waveform meets the optimal characteristics of underdamped demagnetization.

[0047] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A low-frequency transformer broadband characteristic measurement system based on pulse injection, characterized in that, Includes an impedance shaping pulse generator, a wideband coupling device, and a wideband response analysis unit: The impedance shaping pulse generator is connected between the high-voltage DC source and the transformer under test, forming the source end for injecting excitation energy into the windings of the transformer under test; A wideband coupling device is connected between the output terminal of the impedance shaping pulse generator and the high-voltage bushing of the transformer under test, forming a signal injection channel; The wideband response analysis unit is connected to the end screen port of the transformer under test to form a signal acquisition terminal; The impedance shaping pulse generator includes a monostable main control switch and a frequency-converting impedance network. The monostable main control switch is connected in series between the high-voltage DC source and the frequency-converting impedance network to perform a single closing action. The frequency-converting impedance network includes a first passive branch and a second passive branch connected in parallel. The first passive branch has capacitive impedance characteristics, providing a low impedance path at the moment the monostable main control switch is turned on, establishing a nanosecond-level rising edge of the excitation pulse, and increasing impedance as the capacitor charges. The second passive branch exhibits inductive or resistive impedance characteristics. During the steady-state discharge phase after the rising edge of the excitation pulse is established, its impedance is lower than that of the first passive branch after charging. It takes over the discharge circuit and maintains the millisecond-level decay tail of the excitation pulse. The system utilizes the time constant difference between the first and second passive branches to achieve automatic switching of the source output impedance from low to high within a single operation cycle of the monostable main control switch, so as to match the charging requirements of the distributed capacitance of the transformer under test in the high-frequency band and the core flux establishment requirements in the low-frequency band.

2. The pulse injection-based broadband characteristic measurement system for low-frequency transformers according to claim 1, characterized in that, The first passive branch includes a sharpening capacitor and a first damping resistor connected in series; the capacitance value of the sharpening capacitor and the resistance value of the first damping resistor are set according to the preset rise time of the excitation pulse to ensure that a dominant high-frequency current injection is provided within the nanosecond time of the monostable main control switch operation.

3. The pulse injection-based broadband characteristic measurement system for low-frequency transformers according to claim 1, characterized in that, The second passive branch includes a series-connected energy storage inductor or a second damping resistor; the resistance value of the second damping resistor is set to be greater than the characteristic impedance of the transformer winding under test, so as to form a high internal resistance characteristic during the tailing stage of the excitation pulse and maintain the magnetic field energy decay process in the winding inductance.

4. The pulse injection-based broadband characteristic measurement system for low-frequency transformers according to claim 1, characterized in that, The wideband coupling device includes a unidirectional isolation component connected in series between the output of the frequency-variable impedance network and the transformer under test. The unidirectional isolation component is composed of a fast recovery diode stack. During the excitation pulse injection, the unidirectional isolation component conducts due to the forward voltage difference, establishing an energy injection channel. During the free response period after the excitation pulse ends, it is cut off due to the disappearance of the forward voltage difference or the appearance of a reverse voltage difference, thus cutting off the electrical connection between the frequency-variable impedance network and the transformer under test and blocking the interference of residual oscillations on the source side to the measurement signal.

5. The pulse injection-based broadband characteristic measurement system for low-frequency transformers according to claim 1, characterized in that, The frequency-variable impedance network has an automatic demagnetizing branch connected in parallel. The automatic demagnetizing branch includes a demagnetizing capacitor and a damping resistor connected in series. After the excitation pulse tail ends, the automatic demagnetizing branch is turned on by the inductive back electromotive force generated by the winding of the transformer under test, absorbs the reverse energy and forms an underdamped oscillating circuit with the winding of the transformer under test, injects a demagnetizing current with alternating polarity decay into the transformer under test, and eliminates the residual magnetism of the iron core.

6. The pulse injection-based broadband characteristic measurement system for low-frequency transformers according to claim 1, characterized in that, The wideband response analysis unit includes a first measurement channel and a second measurement channel connected in parallel; the first measurement channel is equipped with a high-frequency voltage divider for acquiring the leading edge of the excitation pulse and high-frequency oscillation signals; the second measurement channel is equipped with a low-frequency voltage divider and an integrator circuit for acquiring the tail of the excitation pulse and low-frequency response signals. The wideband response analysis unit splices the data from the first measurement channel and the second measurement channel in the time domain based on a unified clock reference.

7. The pulse injection-based broadband characteristic measurement system for low-frequency transformers according to claim 4, characterized in that, A high-frequency compensation capacitor is connected in parallel with the unidirectional isolation component; the capacitance value of the high-frequency compensation capacitor is set so that the equivalent AC impedance of the unidirectional isolation component in the off state is greater than the stray capacitance impedance of the transformer winding under test in the high-frequency band.

8. The low-frequency transformer broadband characteristic measurement system based on pulse injection according to claim 5, characterized in that, The capacitance value of the demagnetizing capacitor in the automatic demagnetizing branch Equivalent main inductance of the transformer under test A specific underdamped oscillation condition must be met to ensure that the generated demagnetizing current has a sufficient number of zero crossings; the underdamped oscillation condition is defined by the following relationship: ,in, It is the sum of the damping resistance and the DC resistance of the winding. This represents the core inductance value of the transformer under test. The capacitance value of the demagnetizing capacitor is given; the system configures the damping parameters according to this relationship to generate a demagnetizing waveform with decreasing amplitude and alternating polarity.

9. The pulse injection-based broadband characteristic measurement system for low-frequency transformers according to claim 1, characterized in that, The impedance shaping pulse generator is encapsulated in a grounded metal shielded cavity; the high-voltage DC source, monostable main control switch, and frequency-converting impedance network are arranged in a coaxial structure to reduce the distributed inductance of the circuit; the output is connected to a wideband coupling device via a high-voltage coaxial cable; the system also includes a current sensing transformer connected to the neutral point of the transformer under test; the current sensing transformer is used to monitor the response current flowing through the transformer windings, and together with the voltage signal collected by the wideband response analysis unit, it constitutes the original dataset for calculating the wideband transfer function of the transformer; the original dataset covers the frequency domain range from Hertz to megahertz.

10. A method for measuring the broadband characteristics of a low-frequency transformer based on pulse injection, used to implement the measurement system described in claim 1, characterized in that, Includes the following steps: A single-closing action is performed by the monostable main control switch in the impedance shaping pulse generator to trigger the energy release of the high-voltage DC source. The released energy is time-domain impedance shaped by a frequency-varying impedance network. The first passive branch with capacitive impedance characteristics plays a dominant role at the moment of switch-on to establish the nanosecond-level rising edge of the excitation pulse. The second passive branch with inductive or resistive impedance characteristics plays a steady-state role after the rising edge is established to maintain the millisecond-level decay tail of the excitation pulse. Thus, the source output impedance is automatically switched from low to high within a single pulse cycle. The shaped excitation pulse is injected into the high-voltage bushing of the transformer under test via a broadband coupling device, and a unidirectional isolation component is used to conduct during the injection period to establish a channel and to cut off during the free response period to isolate source interference. In addition, a wideband signal containing high-frequency distributed parameter response and low-frequency core flux response is collected from the end screen port of the transformer under test using a wideband response analysis unit, and full-spectrum analysis is performed to obtain the wideband impedance characteristics of the transformer.

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