CN104714155B - A kind of detection to direct current XLPE cable shelf depreciation and apparatus for evaluating and method - Google Patents

Abstract

The invention discloses a device and method for detecting and evaluating the partial discharge of a DC XLPE cable, comprising a cable to be tested and a DC partial discharge detection device for obtaining partial discharge signals; two ends of the cable to be tested are connected with a DC high voltage output device and current output device; the tested cable is equipped with a temperature detection device. According to the difference between direct current partial discharge and alternating current partial discharge, the present invention adopts a means suitable for the detection and analysis of alternating current partial discharge, and fills the blank of the direct current partial discharge detection and analysis method. The present invention considers the influence of temperature on partial discharge, and adopts the method of comparing the measurement results of partial discharge measurement carried out under the no-load or offline state of the simulated cable at normal temperature with the measurement result of the partial discharge measurement carried out by using the current output device to simulate the cable under load. Combined with the comprehensive analysis method, a more comprehensive and comprehensive evaluation of the cable insulation condition is carried out.

Description

A device and method for detecting and evaluating partial discharge of DC XLPE cables

technical field

The invention relates to a detection and evaluation device and method for partial discharge of a DC XLPE cable.

Background technique

Cross-linked polyethylene (XLPE) power cables are widely used in power systems due to their simple structure, strong load capacity, easy laying, convenient operation and maintenance, excellent insulation performance, and simple production process. With the wide application of XLPE cable lines, the use of intermediate connectors as an important part of power cables has also increased. However, the cable intermediate joint is also a frequent site of transmission line accidents, and its reliability directly affects the safety of the power supply system. Due to various factors such as inferior materials, rough manufacturing process, irregular installation on site, and electrical, thermal, and mechanical stresses, the intermediate joints of cables will form latent insulation defects, which will bring potential safety hazards to the operation of the power grid. Both operating experience and research have shown that most of the typical defects in the design stage of cable joints will generate partial discharges. Therefore, partial discharge detection of cable joints is of great significance to the normal operation and fault prediction of high-voltage cables.

At present, with the rapid increase of offshore wind power installed capacity and the continuous development of DC transmission technology, the demand for DC cables is rapidly increasing due to their advantages such as low loss and easy power adjustment. Especially in recent years, the development of VSC (voltage source converter) technology based on voltage source converter has greatly promoted the development of flexible DC cables with cross-linked polyethylene (XLPE) as the main insulating material. Compared with oil-paper cables, XLPE The DC cable has the advantages of being more environmentally friendly and simple to manufacture and lay. Usually, the DC cable line is long, the voltage level is high, the project is huge, and the importance is extremely high. Therefore, it is very necessary to detect its working state to ensure that it is in a normal working state. Considering the different operation modes of DC cables and AC cables, it may not be appropriate to directly apply AC cable detection methods. At present, the detection and analysis methods for DC cables are still very scarce.

As an important means of AC cable detection, partial discharge detection can be used as an important means to reflect the insulation level. For XLPE DC cables, partial discharge may also occur when the main insulation or accessories of the cable are defective and under a certain voltage, so the partial discharge detection of the DC cable system can also find possible defects. For partial discharge detection, the characteristics of DC partial discharge and AC partial discharge are significantly different. Compared with AC partial discharge, the repetition rate of DC partial discharge is lower, and there is only discharge amplitude and discharge interval, and there is no concept of discharge phase angle. Therefore, Commonly used in AC partial discharge analysis -qn and other analysis methods related to the phase angle cannot be applied to DC partial discharge. The analysis of DC partial discharge requires special means, which is still relatively blank.

For high-voltage AC cables, the distribution of the electric field in the insulating layer mainly depends on the geometric distribution of the electric field of the cable capacitance, and it can be easily obtained when the dielectric constant is assumed to be a constant that does not change with temperature. It is always constant at the operating temperature. Although the actual dielectric constant will change to a certain extent, its influence is almost negligible.

The electric field in a high voltage AC cable insulation can generally be expressed as:

Where: E _AC (r) = AC electric field distribution at radius r (kV/mm or MV/m)

U ₀ = the design voltage of the cable, that is, the voltage on the dielectric (kV); in the AC field, it means the phase voltage

r = radius distance (mm)

r _i = inner radius of cable insulation (mm)

r ₀ = outer radius of cable insulation (mm)

For high-voltage DC cables, the electric field distribution changes with temperature and time. This is because the electric field distribution of the DC cable is determined by the resistance at the steady state, that is, the resistivity of the insulation, and the resistivity of the dielectric. Depending on the temperature and the electric field strength, the electric field distribution of the DC cable is basically the same as that of the AC only when there is no temperature gradient inside the unloaded cable.

The electric field in HVDC cable insulation can usually be expressed as:

in

in:

a = temperature coefficient of conductivity (1/K or 1/°C)

b = Stress coefficient of conductivity (mm/kV or m/MV)

in:

W _C = Joule heat loss in conductor (w/m)

λ _T = thermal conductivity of the insulation

The change of the local field strength at the discharge location will mainly be reflected in the change of the discharge repetition rate, and due to the thermal effect of the electric field, the change of the electric field will in turn cause the change of the temperature, resulting in the coupling relationship between the electric field and temperature, which is complicated for the discharge. Impact.

Contents of the invention

The purpose of the present invention is to solve the above-mentioned problems in the prior art, and provide a detection and evaluation device and method for partial discharge of DC XLPE cables.

In order to achieve the above object, the present invention adopts the following technical solutions to achieve:

A DC partial discharge measurement device, including a cable under test and a DC partial discharge detection device for obtaining partial discharge signals; both ends of the cable under test are connected with a DC high-voltage output device and a current output device; the cable under test is equipped with a temperature detection device.

The DC high-voltage output device uses a power frequency test transformer to raise the power frequency AC 220V to power frequency high voltage U _t , conducts half-wave rectification through the silicon stack D, and then connects the current limiting resistor R in series to charge the filter capacitor C, and finally outputs to on the cable under test.

The temperature detection device adopts PT100 thermocouples arranged on the surface of the tested cable and the cable accessories.

The current output device adopts a feed-through transformer, and a voltage equalizing cover for preventing corona is installed at the joint between the feed-through transformer and the tested cable; a voltage regulator is also connected to the feed-through transformer.

The DC partial discharge detection device includes a detection impedance measurement circuit, a high-frequency current transformer and an ultrasonic sensor; the detection impedance measurement circuit adopts the regulations in the IEC60270 standard, and the high-frequency current transformer is clamped on the ground wire of the cable under test, and the ultrasonic sensor The sensor is tightly attached to the cable connector under test.

A method for detecting and evaluating partial discharge of DC XLPE cables, comprising the following steps:

1) When the current output device is closed, use the DC high-voltage output device to pressurize the tested cable, boost the voltage to the set test voltage, and record the temperature displayed by the temperature monitoring device;

2) The DC partial discharge detection device couples the partial discharge signal;

3) denoising the coupled partial discharge signal;

4) Extract the discharge amplitude Q in the partial discharge signal in the time domain, expand it in the form of a time function t, and form a Q-t diagram for expressing the variation of the discharge amplitude with time for DC partial discharge analysis;

5) Establish a three-dimensional spectrogram suitable for DC partial discharge analysis;

6) By performing DC partial discharge on multiple tested cables with different defects, and saving the obtained three-dimensional spectra of different H(Q, Δt) and corresponding discharge types, a fingerprint library is established; the obtained H(Q, Δt) ) image is compared with the discharge image to realize the judgment of the defect type of DC partial discharge;

7) Perform fast Fourier transformation on the partial discharge signal obtained in step 3), analyze the distribution of the partial discharge signal in different frequency ranges in the frequency domain, and analyze the peak frequency for actual discharge or noise, internal discharge or corona discharge And judge the type of defect that produces discharge;

8) Adjust the DC high-voltage output device, reduce the voltage to 0kV, turn on the current output device through the core transformer, adjust the current to simulate the load status of the cable, observe the surface temperature of the cable and its accessories in the temperature monitoring device, and raise the surface temperature to 70 ℃ or above, in order to simulate the situation under the high-load operating conditions of the cable;

9) After the temperature reaches a stable level, use the DC high-voltage output device to pressurize the tested cable sample, and after boosting the voltage to the set test voltage, record the temperature displayed by the temperature monitoring device;

10) Repeat step 2) to step 7) for the detection and analysis process to complete the measurement. After the measurement is completed, turn off the current output device and the DC high voltage output device, let the tested sample stand still to lower the temperature and perform effective discharge for 24 hours.

In the step 2), the partial discharge signal coupled to the sensor is amplified and filtered, and then the step 3) is executed.

In the step 3), the denoising process is to capture the pulse signal and filter the signal opposite to the polarity of the applied DC voltage.

In said step 5), the specific method for establishing a three-dimensional spectrogram suitable for DC partial discharge analysis is as follows:

The phase angle is replaced by the time interval Δt between the discharge pulse and its preceding discharge pulse in the DC partial discharge process , to establish the percentage of different discharge times in the total discharge times, that is, the relationship H(Q, Δt) between the discharge densities H, where the discharge amplitude Q has been extracted in the drawing of the Qt diagram; for the extraction of the adjacent pulse time interval Δt, according to The setting of the discharge threshold Q _min takes the pulse exceeding the discharge threshold as the effective discharge pulse, and records the coordinate parameters (Q _i , t _i ) corresponding to each discharge pulse at Qt, where Q _i >Q _min , and Δt=t _i -t _{i -1} to get Δt.

Compared with the prior art, the present invention has the following beneficial effects:

According to the difference between direct current partial discharge and alternating current partial discharge, the present invention adopts a means suitable for the detection and analysis of alternating current partial discharge, and fills the blank of the direct current partial discharge detection and analysis method. According to the low repetition rate of DC partial discharge, a long-time partial discharge measurement of 30-60 minutes is proposed, and according to the concept of phase angle corresponding to no pulse of DC partial discharge, the analysis method related to phase angle in the analysis method of AC partial discharge is not applicable For the analysis of DC partial discharge, the Q-t diagram is used as the main research method, and the N-t and N-Q diagrams are assisted. At the same time, the time interval Δt between two adjacent discharge pulses is used instead of the phase angle to form a three-dimensional spectrogram suitable for DC partial discharge analysis. At the same time, according to the particularity of the unipolarity of the DC partial discharge signal pulse, the pulse of the opposite polarity is removed as interference in the denoising process to ensure the accuracy of the measurement. The present invention considers the influence of temperature on partial discharge, because temperature has a significant influence on the electric field distribution under direct current and directly responds to the change of discharge repetition rate, so the partial discharge measurement will be carried out under normal temperature simulation cable no-load or offline state The measurement results are combined with the measurement results of the partial discharge measurement using the current output device to simulate the cable under load, and a more comprehensive and comprehensive evaluation of the cable insulation status is carried out.

Description of drawings

Fig. 1 is a test circuit diagram of the present invention;

Fig. 2 is test flowchart of the present invention;

Fig. 3 is the flow chart of DC partial discharge analysis method of the present invention;

Fig. 4 is the background noise waveform of the embodiment of the present invention;

Fig. 5 is the corresponding frequency domain distribution diagram of the background noise waveform of the embodiment of the present invention;

Fig. 6 is the pulse waveform of the pulse source B of the embodiment of the present invention;

Fig. 7 is the corresponding frequency domain distribution diagram of the pulse waveform of the pulse source B of the embodiment of the present invention;

Fig. 8 is the Q-t spectrogram of the embodiment of the present invention;

Fig. 9 is the N-t of the embodiment of the present invention, where N is set as the number of discharges within 1 min;

Fig. 10 is the N-Q spectrogram of the present invention, wherein N is set as the number of discharges within 1 min;

Fig. 11 is a Q-Δt spectrum diagram of the present invention.

detailed description

Below in conjunction with accompanying drawing, the present invention is described in further detail:

Referring to Fig. 1, the present invention provides a detection and analysis method for the combined partial discharge temperature of an XLPE DC cable. Compared with the AC partial discharge signal, the DC partial discharge signal has significant differences such as low discharge repetition rate and no phase angle corresponding to the discharge pulse. , so the present invention adopts 30-60min long-term DC partial discharge measurement, and adopts Q-t diagram as the main combined with the new three-dimensional ordinary H(Q, Δt) suitable for DC partial discharge analysis for pattern recognition. Since the distribution of the electric field in the insulation depends on the distribution of the resistivity under DC, the resistance will change significantly with the change of temperature, thereby affecting the distribution of the electric field at the place, and the change of the local field strength at the discharge position will be mainly reflected in the change of the discharge repetition rate . Due to the thermal effect of the electric field, changes in the electric field will in turn cause changes in temperature, resulting in a coupling relationship between the electric field and temperature, which has a complex impact on the discharge. The present invention considers the influence of temperature change on discharge, and includes temperature into the scope of comprehensive evaluation.

Circuit arrangement of the present invention:

(1) DC high-voltage output device: The DC high-voltage generator cannot be used in the DC pressurization system, because it will introduce obvious pulse-shaped interference, and it is close to the partial discharge pulse frequency range, so the signal-to-noise ratio of the measurement results cannot reach 2:1. Standard even submerged discharge pulses. The present invention adopts half-wave rectification to generate DC high voltage. First, the power frequency AC 220V is raised to the power frequency high voltage U _T by using a power frequency test transformer _. ) silicon stack for half-wave rectification, the current limiting resistor R is connected in series to charge the filter capacitor C, and the withstand voltage of the filter capacitor C is at least The flow capacity of the silicon stack is determined by the type of the silicon stack, and 3A is recommended in the present invention. The current limiting resistor and filter capacitor value are determined by the ripple requirements, test voltage and current according to the relevant methods in the high voltage test technology. According to the relevant content of the national standard GB/T: 16927.1, the ripple coefficient of the DC high voltage test equipment should be less than 3%. A resistor divider is connected in parallel behind the high-voltage DC pressurization unit, and the resistor divider displays the actual value of the applied DC voltage.

(2) Current output device: The feed-through transformer mainly provides high current for cables and joints to withstand high voltage, so as to simulate the operating load of cables. During the loading process of the cable, the temperature rises due to the thermal effect of the current, and the influence of temperature is simulated. A voltage equalizing cover is installed at the connection between the feedthrough transformer and the cable to prevent corona.

(3) Temperature monitoring device: Arrange thermocouple PT100 on the cable surface and cable accessories, and monitor the temperature of the cable surface and its accessories.

(4) DC partial discharge measuring device: The method of DC partial discharge detection and acquisition of partial discharge signals is divided into the electrical measurement method for collecting electrical signals generated during the partial discharge process and the non-electrical method for collecting non-electrical signals such as sound and light during the partial discharge process. Electrometry. The present invention is suitable for electrical measuring method and acoustic measuring method. For the electrical measurement method, it is applicable to the pulse current method recommended by IEC60270 (that is, to detect the impedance for partial discharge signal detection), and the high-frequency current transformer (HFCT) to collect the electrical signal obtained during the partial discharge process, while the acoustic measurement method is suitable for Using piezoelectric material as a sensor, the mechanical vibration signal is converted into an electrical signal. The detection impedance measurement circuit refers to the provisions of the IEC60270 standard. The installation position of the high-frequency current transformer is clamped on the grounding wire, and the ultrasonic sensor is closely attached to the cable connector under test (if the cable under test is equipped with accessories such as cable connectors or terminals, it is best to attach on the attachment).

The specific detection and evaluation method of the present invention comprises the following steps:

(1) First, when the feedthrough transformer of the current output device is turned off, that is, no current is output, the DC high-voltage output device is used to pressurize the tested cable sample, boost the voltage to the agreed test voltage, and record the temperature displayed by the temperature monitoring device.

(2) The DC partial discharge measuring device couples the partial discharge signal. For partial discharge under DC voltage, due to the low repetition rate, the measurement time needs to reach 30-60 minutes.

(3) In order to improve the measurement accuracy, a signal processing link can be added after the sensor, and the signal coupled to the sensor can be amplified and filtered as necessary. If the electrical measurement method is used, in an environment with strong environmental electromagnetic interference, a filter can be connected to filter the signal coupled to the sensor, and the filter parameters should be selected according to the actual noise environment. If the discharge signal is weak, add an amplifier after filtering to amplify the signal. For the field test amplifier, the frequency band can be selected to be relatively narrow. The recommended frequency band is 1MHz-500MHz, and a broadband amplifier is used in the laboratory. For measurement by ultrasonic method, a preamplifier is connected behind the sensor, the amplification factor is above 20dB, and the recommended working frequency is 10kHz-2000kHz.

(4) For on-site measurement, noise interference is an important factor limiting the use of partial discharge detection, and denoising processing is very critical. In DC partial discharge measurement, the applied DC voltage is unipolar, so the partial discharge signal is also unipolar. For DC The partial discharge signal is based on its unipolar characteristics. By capturing the pulse signal, the signal with the opposite polarity to the applied DC voltage is regarded as a noise signal and filtered, thereby reducing the influence of noise.

(5) The signal coupled to the sensor is subjected to signal processing and denoising processing to obtain an effective discharge signal. Due to the lack of phase angle of DC PD compared to AC PD The concept of the AC partial discharge analysis, so the phase angle The relevant analysis methods are not applicable in DC, and it is necessary to re-establish the analysis methods suitable for DC partial discharge. For the analysis method of DC partial discharge, the present invention extracts the discharge amplitude Q in the partial discharge signal in the time domain, expands it in the form of a time function t, forms a Qt diagram as the main analysis means, and expresses the discharge amplitude Variations over time. In the actual measurement, the threshold should be set first, set the threshold above the background noise level, and at the same time ensure that it does not exceed 1/2 of the discharge amplitude to ensure that the signal-to-noise ratio reaches 2:1, and if it exceeds the set threshold, it is judged as an effective discharge pulse. In addition, the relationship between the discharge repetition rate N within one minute and the test time t is established, and an Nt diagram is formed to show the change rule of the discharge repetition rate with time. In the detection of DC partial discharge, the average discharge repetition rate per minute is not less than once, that is, N≥1, and an effective partial discharge phenomenon occurs. The voltage that just makes the sample generate N=1 is regarded as the DC partial discharge of the sample. starting voltage. For the relationship between Qt graphs, for example, at a certain test voltage, the discharge capacity Q is large and remains stable within the test time, or the discharge repetition rate N is large and remains stable within the time, or increases with time, and Q and N show an obvious upward trend , attention should be paid to the insulation condition of the tested sample.

(6) Due to the DC cable system, there may be many reasons for discharge, such as main insulation scratches, stress cone dislocation and many other factors. Since the discharge characteristics of different discharge types are significantly different, and their development trends are also different, the corresponding countermeasures should be Refer to different defect types. The PRPD spectrum commonly used in AC partial discharge detection for pattern recognition of different discharge types due to the lack of phase angle at DC concept and cannot be used. The invention establishes a three-dimensional spectrogram applicable to the analysis of DC partial discharge. The phase angle is replaced by the time interval Δt between the discharge pulse and its preceding discharge pulse in the DC partial discharge process , established on the basis of the percentage of different discharge times in the total discharge times, that is, the relationship H(Q, Δt) between the discharge density H, where the discharge amplitude Q has been extracted in the drawing of the Qt diagram in step (5), and for the adjacent pulse time For the extraction of the interval Δt, according to the setting of the discharge threshold Q _min , the pulse exceeding the discharge threshold is regarded as the effective discharge pulse, and the coordinate parameters (Q _i , t _i ) corresponding to each discharge pulse at Qt are recorded, where Q _i >Q _min , and Δt= t _i -t _i-1 yields Δt.

(7) By performing DC partial discharge on a large number of samples with different defects, and saving the obtained three-dimensional spectra of different H(Q, Δt) and corresponding discharge types, a fingerprint library is established. For the obtained H(Q, Δt) image, by comparing with the discharge image, the defect type can be judged, that is, pattern recognition.

(8) In addition, the present invention uses fast Fourier transform FFT for the signal-processed and denoised discharge signal obtained in step (5), and analyzes the distribution of the discharge signal in different frequency ranges in the frequency domain. Record different types of discharge waveforms and have different waveform components at different frequencies, so the actual discharge or noise, internal discharge or corona discharge, and the type of defect that generated the discharge can be judged by analyzing the peak frequency.

(9) Adjust the DC high-voltage output device, reduce the voltage to 0kV, turn on the current output device through the core transformer, adjust the current to simulate the load status of the cable, observe the surface temperature of the cable and its accessories in the temperature monitoring device, and make the surface temperature rise to 70°C or above to simulate the conditions of high-load operation of the cable.

(10) After the temperature has stabilized, use the DC high-voltage output device to pressurize the tested cable sample, boost the voltage to the agreed test voltage, and record the temperature displayed by the temperature monitoring device. Finally, repeat the detection and analysis process of steps (2)-step (8) to complete the measurement. After the measurement is completed, the current output device and the DC high-voltage output device are turned off, and the test sample is left to cool down and effectively discharged for 24 hours.

The above content is only to illustrate the technical ideas of the present invention, and cannot limit the protection scope of the present invention. Any changes made on the basis of the technical solutions according to the technical ideas proposed in the present invention shall fall within the scope of the claims of the present invention. within the scope of protection.

Claims (7)

1. A DC partial discharge measuring device, characterized in that: it comprises a cable under test and a DC partial discharge detection device for obtaining a partial discharge signal; the two ends of the cable under test are connected with a DC high voltage output device and a current output device; A temperature detection device is arranged on the measuring cable; the DC high voltage output device uses a power frequency test transformer to raise the power frequency AC 220V to power frequency high voltage U _t , conducts half-wave rectification through the silicon stack D, and then connects the current limiting resistor R in series to The filter capacitor C is charged, and finally output to the cable under test; the DC partial discharge detection device includes a detection impedance measurement circuit, a high-frequency current transformer and an ultrasonic sensor; the detection impedance measurement circuit adopts the regulations in the IEC60270 standard, and the high-frequency current The transformer is clamped on the ground wire of the tested cable, and the ultrasonic sensor is attached to the tested cable joint. 2. The DC partial discharge measurement device according to claim 1, characterized in that: the temperature detection device adopts PT100 thermocouples arranged on the surface of the cable to be tested and on the cable accessories. 3. The DC partial discharge measuring device according to claim 1, characterized in that: the current output device adopts a feed-through transformer, and a pressure equalizing cover for preventing corona is installed at the connection between the feed-through transformer and the tested cable ; The feedthrough transformer is also connected with a voltage regulator. 4. A method for detecting and assessing the partial discharge of DC XLPE cables using any one of the devices of claims 1-3, characterized in that, comprising the following steps: 1) When the current output device is closed, use the DC high-voltage output device to pressurize the tested cable, boost the voltage to the set test voltage, and record the temperature displayed by the temperature monitoring device; 2) The DC partial discharge detection device couples the partial discharge signal; 3) denoising the coupled partial discharge signal; 4) Extract the discharge amplitude Q in the partial discharge signal in the time domain, expand it in the form of a time function t, and form a Q-t diagram for expressing the variation of the discharge amplitude with time for DC partial discharge analysis; 5) Establish a three-dimensional spectrogram suitable for DC partial discharge analysis; 6) By performing DC partial discharge on multiple tested cables with different defects, and saving the obtained three-dimensional spectra of different H(Q, Δt) and corresponding discharge types, a fingerprint library is established; the obtained H(Q, Δt) ) image is compared with the discharge image to realize the judgment of the defect type of DC partial discharge; 7) Perform fast Fourier transformation on the partial discharge signal obtained in step 3), analyze the distribution of the partial discharge signal in different frequency ranges in the frequency domain, and analyze the peak frequency for actual discharge or noise, internal discharge or corona discharge And judge the type of defect that produces discharge; 8) Adjust the DC high-voltage output device, reduce the voltage to 0kV, turn on the current output device through the core transformer, adjust the current to simulate the load status of the cable, observe the surface temperature of the cable and its accessories in the temperature monitoring device, and raise the surface temperature to 70 ℃ or above, in order to simulate the situation under the condition of high-load operation of the cable; 9) After the temperature reaches a stable level, use the DC high-voltage output device to pressurize the tested cable sample, and after boosting the voltage to the set test voltage, record the temperature displayed by the temperature monitoring device; 10) Repeat step 2) to step 7) for the detection and analysis process to complete the measurement. After the measurement is completed, turn off the current output device and the DC high voltage output device, let the tested sample stand still to lower the temperature and perform effective discharge for 24 hours. 5. The method for detecting and evaluating the partial discharge of DC XLPE cables according to claim 4, characterized in that: in the step 2), the partial discharge signal coupled to the sensor is amplified and filtered, and then step 3 is performed ). 6. according to claim 4 or 5 described detection and evaluation method to the partial discharge of DC XLPE cable, it is characterized in that: in described step 3), denoising process is by capturing pulse signal, and applying DC voltage Filtering of signals of opposite polarity. 7. the detection and evaluation method to direct current XLPE cable partial discharge according to claim 4, it is characterized in that, described step 5) in, set up the specific method that is applicable to the three-dimensional spectrogram under the direct current partial discharge analysis situation as follows: The phase angle is replaced by the time interval Δt between the discharge pulse and its preceding discharge pulse in the DC partial discharge process Establish the percentage of different discharge times to the total discharge times, that is, the relationship H(Q, Δt) between the discharge densities H, where the discharge amplitude Q has been extracted in the drawing of the Qt diagram; for the extraction of the adjacent pulse time interval Δt, according to the discharge The setting of the threshold Q _min takes the pulse exceeding the discharge threshold as the effective discharge pulse, and records the corresponding coordinate parameters (Q _i , t _i ) of each discharge pulse at Qt, where Q _i >Q _min , and Δt=t _i -t _{i- 1} to get Δt.