CN104237639A - Online monitoring method of insulation state of medium-voltage power cable based on impact impedance characteristic parameter - Google Patents

Abstract

The invention discloses an on-line monitoring method for the insulation state of medium-voltage power cables based on the characteristic parameters of impulse impedance. The current transformer converts high voltage and large current into low voltage and small current, and collects the impulse voltage and current synchronously through the high-speed data acquisition device, transmits and stores the collected data on the computer, and performs fast Fourier transform at the same time, transforming The obtained voltage in the frequency domain is divided by the current in the frequency domain to calculate the impulse impedance, and then calculate the minimum value of the amplitude frequency and phase frequency of the impulse impedance respectively, and compare them with the respective set values, if one of them If it is lower than the set value, it means that the insulation is lowered, and the online monitoring result of the insulation state is output. Finally, the above steps are repeated for real-time online monitoring. The present invention can detect defects that cannot be exposed under low voltage, and ensure the safe and reliable operation of the power cable.

Description

On-line monitoring method of insulation status of medium voltage power cables based on characteristic parameters of impulse impedance

technical field

The invention relates to an on-line monitoring method for the insulation state of medium-voltage power cables based on characteristic parameters of impulse impedance, and belongs to the technical field of insulation monitoring of electric power systems.

Background technique

Power cables are widely used in urban power grids. Cables age due to the influence of electrical, thermal, mechanical and environmental stresses during use. The task of monitoring cable insulation status and ensuring safe operation of cables is getting heavier and heavier. For 10kV voltage level cable insulation status monitoring, at present, the preventive test is mainly carried out by applying DC high voltage to the cable after power failure. Due to the large number of 10kV cables, the load is too heavy, it is difficult to cut off the power, the test workload is huge, and the cable insulation monitoring task is difficult to complete. The traditional test method for preventing power failure of cables has a long test period and will cause power failure losses. At present, the online monitoring methods mainly include DC component method, DC superposition method, tanδ method, AC superposition method, low frequency superposition method, harmonic component method and partial discharge method, etc. On-line monitoring methods for XLPE cables are more mature and widely used are DC superposition method, tan δ method and partial discharge method. The DC component method and DC superposition method developed by Shanghai Electric Cable Research Institute have been in trial operation for several years. , but the test device has not yet been commercialized. Major universities in China have researched cable online monitoring technology, including DC component current, insulation resistance measurement, dielectric loss tangent measurement and partial discharge, etc., and achieved certain results. There are many online detection methods for XLPE cables that have been developed or researched at home and abroad, but they are still relatively rare to be practically promoted, but they have good application experience in some countries. Singapore is a country that promotes status detection in the world. It has good application experience in partial discharge measurement of cable equipment, but the reliability of online monitoring technology still needs to be further improved.

Chinese patent CN201210199927.7 "XLPE Cable Insulation Monitoring and Life Prediction Method" uses one of the leakage current, insulation resistance and dielectric loss angle of the XLPE cable as the detected quantity, and realizes it by calculating the mutation rate of the detected quantity in real time Monitoring of cable insulation deterioration process. Chinese patent CN200910081348.7 "On-line insulation monitoring method of power cables based on traveling wave electrical quantity measurement" collects in real time the traveling wave electrical quantities measured on all cable lines connected to the same busbar of the substation, and simultaneously monitors whether any occurrence occurs on all cable lines Disturbance, calculate the disturbance time, compare with the preset time, and obtain the cable insulation status monitoring result. Based on the insulation state monitoring under the rated voltage, the voltage is low, and the defects are difficult to expose, and the insulation breakdown of the cable is mainly under the action of overvoltage. Therefore, it is difficult to accurately reflect the real state of the insulation as an insulation parameter under the rated voltage.

Contents of the invention

The object of the present invention is to provide an online monitoring method for the insulation state of medium-voltage power cables based on the characteristic parameters of impulse impedance in view of the deficiencies in the prior art. Voltage) and transient current, calculate the impact impedance of the cable to judge the insulation status of the cable, and realize the online monitoring of the cable insulation status. It can detect defects that cannot be exposed under low voltage, so as to ensure the safe and reliable operation of power cables.

The on-line monitoring method for the insulation state of medium-voltage power cables based on the characteristic parameters of impulse impedance comprises the following steps:

Step 1: switch high-voltage capacitors at the cable joints;

The second step: synchronously collect the impulse voltage and current obtained by the high-frequency voltage divider and high-frequency current transformer through the high-speed data acquisition device;

The third step: transmit and store the data collected by the high-speed data acquisition device on the computer;

Step 4: Perform fast Fourier transform on the data of impulse voltage and current respectively;

Step 5: Divide the voltage in the frequency domain by the current in the frequency domain to obtain the impulse impedance;

Step 6: Find the minimum value of the amplitude frequency and phase frequency of the impulse impedance respectively;

Step 7: Compare the minimum values of amplitude frequency and phase frequency with their respective set values. If one of them is lower than the set value, the cable is defective. Output the online monitoring result of insulation status, otherwise it will not output, will eventually return to the first step.

Features and beneficial effects of the present invention:

By applying overvoltage, defects that cannot be exposed under low voltage can be monitored, and the safe and stable operation of power cables can be improved. The amplitude frequency and phase frequency of the impulse impedance are independently judged, and if one of the minimum values is lower than the set value, the online monitoring result of the insulation state of the power cable can be output, and the reliability is improved.

Instructions attached

Fig. 1 is a flow chart of on-line monitoring of medium voltage power cable insulation state of the present invention

1. Switching high-voltage capacitors, 2. Acquisition of impulse voltage and current, 3. Transmission and storage, 4. Fast Fourier transform, 5. Calculation of impulse impedance, 6. Obtaining the minimum value of amplitude-frequency characteristics of impulse impedance, 7. Amplitude-frequency Compare the minimum value with the set value, 8. Get the minimum value of the phase-frequency characteristic of the impulse impedance, 9. Compare the minimum value of the phase frequency with the set value, 10. Output the online monitoring result of the insulation state.

Fig. 2 is a schematic diagram of the implementation of on-line monitoring of the insulation state of medium-voltage power cables of the present invention

11. High-voltage capacitors, 12. Switching switches, 13. High-frequency voltage dividers, 14. High-frequency current transformers, 15. Power cables, 16. High-speed data acquisition devices, 17. Computers.

Figure 3a is the amplitude-frequency characteristic curve of the impact impedance without defects

Figure 3b is the amplitude-frequency characteristic curve of the impact impedance with defects

Figure 4a is the phase-frequency characteristic curve of the impulse impedance without defects

Figure 4b is the phase-frequency characteristic curve of the impulse impedance with defects

Detailed ways

The present invention is described in detail through the following examples. It is necessary to point out that this example is only used to further illustrate the present invention, but should not be construed as limiting the protection scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments based on the content of the present invention above.

Example

As shown in Figure 1, the switching high-voltage capacitor 1 is connected to the acquisition 2 of the impulse voltage and current, the acquisition 2 of the impulse voltage and current is connected to the transmission and storage 3, the transmission and storage 3 is connected to the fast Fourier transform 4, and the fast Fourier transform 4 Connect with the calculation of the impact impedance 5, calculate the impact impedance 5 and obtain the minimum value of the amplitude-frequency characteristic of the impact impedance 6, obtain the minimum value of the impact impedance amplitude-frequency characteristic 6 and connect the amplitude-frequency minimum value with the set value 7, the amplitude-frequency minimum value Comparing with the set value 7 is connected with the online monitoring result 10 of the output insulation state, and the calculated impulse impedance 5 is connected with the minimum value 8 of the phase-frequency characteristic of the impulse impedance to obtain the minimum value 8 of the phase-frequency characteristic of the impulse impedance and the minimum value of the phase frequency and the setting The fixed value comparison 9 is connected, the minimum phase frequency and set value comparison 9 is connected to the output insulation state online monitoring result 10, and the output insulation state online monitoring result 10 is connected to the switching high voltage capacitor 1.

As shown in Figure 2, the high-voltage end of the high-voltage capacitor 11 is connected to the switching switch 12, the switching switch 12 is connected to the high-voltage end of the high-frequency voltage divider 13, and the input end of the power cable 15, and the high-frequency current transformer 14 is located at the switching switch 12 , the high-voltage end of the high-frequency voltage divider 13 and the connection of the input end of the power cable 15, the output signal end of the high-frequency voltage divider 13 and the high-frequency current transformer 14 is connected with the input end of the high-speed data acquisition device 16, and the high-speed data acquisition device The output terminal of 16 is connected with computer 17 input terminals.

The method for monitoring the insulation state of power cables based on the characteristic parameters of impulse impedance includes the following steps:

The first step: by controlling the closing phase of the switching switch, and then switching the high-voltage capacitor at the cable joint;

The second step: Synchronously collect the impulse voltage and current obtained by the high-frequency voltage divider and high-frequency current transformer through the high-speed data acquisition device. The vertical resolution of the high-speed data acquisition equipment is 12bit, and the sampling rate is 20MHz (the vertical resolution is at least 10bit, The sampling rate is at least 10MHz), the high-frequency voltage divider is selected according to the maximum possible voltage and a margin of 1.2, and the high-frequency current transformer is selected according to the maximum current that may occur in its line and a margin of 1.2;

The third step: transmit and store the data collected by the high-speed data acquisition device on the computer through the data line;

Step 4: Perform fast Fourier transform on the data of impulse voltage u and current i using software, namely FFT(u) and FFT(i), where FFT(u) includes amplitude-frequency Amplitude(u) and phase-frequency Phase(u ), FFT(i) includes amplitude-frequency Amplitude(i) and phase-frequency Phase(i);

Step 5: Divide the voltage in the frequency domain by the current in the frequency domain, that is, Amplitude(u)/Amplitude(i) to obtain the amplitude-frequency characteristic of the impulse impedance Amplitude(f), and Phase(u)-Phase(i) to obtain the impulse impedance The phase-frequency characteristics Phase(f);

Step 6: Between the frequency 500Hz-8.5MHz, calculate the minimum value Amplitude(f) _min and Phase(f) _min of the amplitude-frequency Amplitude(f) and phase-frequency Phase(f) of the impact impedance respectively;

Step 7: Compare the minimum value of amplitude-frequency Amplitude(f) _min with its set value 80, and compare the minimum value of phase frequency Phase(f) _min with its set value 0, as shown in Figure 3 and Figure 4, If one of them is lower than the set value, then the cable is defective, output the insulation state monitoring results, and finally return to the first step.

Claims (1)

1., based on a state of insulation on-line monitoring method for the medium-pressure power cable of impact impedance characteristic parameter, it is characterized in that the method comprises the following steps:

The first step: at cable splice place switching high-voltage capacitor;

Second step: the surge voltage obtained by high-speed data acquiring device synchronous acquisition high frequency voltage divider and HF current transformer and electric current;

3rd step: the data transmission gathered by high-speed data acquiring device also stores on computers;

4th step: the data of surge voltage and electric current are carried out fast fourier transform respectively;

5th step: the electric current of voltage divided by frequency domain utilizing frequency domain, obtains impact impedance;

6th step: ask for the amplitude-frequency of impact impedance and the minimum value of phase frequency respectively;

7th step: the minimum value of amplitude-frequency and phase frequency compared with respective setting value respectively, if one of them is lower than setting value, then this cable defectiveness, exports state of insulation on-line monitoring result, finally all will return the first step.