CN1834673A - Insulating state on-line monitoring method of cross-linked PE cable - Google Patents

Abstract

The invention discloses an on-line monitoring method for the insulation state of cross-linked polyethylene insulated power cables. A DC superposition method is used to measure the aging degree of insulation water branches and insulation resistance of three-phase XLPE cables on-line. The transceiver unit GPRS sends to the host computer, and the host computer analyzes the measurement parameters through the set data acquisition and analysis software, gives the judgment of the cable insulation state according to the given evaluation standard, and gives the comprehensive diagnosis result. This method can effectively evaluate the cable breakdown accident prevention and the remaining life of the cable insulation, and realizes the unattended online monitoring and wireless remote control measurement by using the unique wireless data transmission mode of the public mobile communication network.

Description

On-line Monitoring Method of Insulation State of XLPE Insulated Power Cable

Technical field

The invention relates to an on-line detection method and a device thereof, in particular to an on-line detection method for the insulation state of a cross-linked polyethylene (XLPE) insulated power cable.

Background technique

Cross-linked polyethylene (XLPE) insulated power cables are widely used in power systems due to their good electrical and thermal properties and many other advantages. Paper) The main cable varieties of high voltage cables. Relevant departments in Japan have investigated and analyzed the accident causes of 6.6kV XLPE cables put into operation from 1963 to 1979 and found that more accidents occurred in cables laid before 1970. In terms of the number of years of use, the number of cable accidents and replacements increased sharply after 8 years of operation. From the perspective of the types of accidents, water trees, natural aging, and water immersion account for 50% of the total accidents. When considering the life of XLPE cables, early failures such as poor terminals and joints and external damage accidents are excluded, and the site insulation can be considered Diagnosis should be based on the aging state of water branches. Timely and accurate detection of hidden dangers of cable insulation degradation can take necessary countermeasures in time to reduce the huge economic losses caused by power outages and improve the safety and reliability of power supply and use.

At present, the research and development hotspots of the main online monitoring methods for XLPE cable insulation are: DC component method, DC superposition method, leakage current method, online loss factor (tanδ) method, partial discharge method, distributed optical fiber temperature measurement method, etc. Typical studies such as the on-line monitoring technology of cable and joint partial discharge conducted by German and British scholars, and the on-line monitoring technology of cable DC component and DC superposition and loss factor (tanδ) conducted by Japanese scholars have proposed more than 100 online monitoring methods. Among them, the more mature and widely used ones are the DC component method, the DC superposition method, and the tanδ measurement method. The former is sensitive to the deterioration state of insulating water branches, especially to individual water branches with dangerous lengths, while the latter two are more sensitive to the overall aging and damp of the cable. The combination of the two can obtain better insulation diagnosis results. At present, the monitoring device combined with the two has been put into operation, and certain measurement data have been accumulated. There are also examples in China where superimposed voltage method and DC component method have been tried for monitoring. For example, the DC component method and DC superposition method developed by Shanghai Electric Cable Research Institute have been used to monitor cable insulation on-line, and have been in trial operation in Baosteel for several years, but no such method has been retrieved. Reports on the commercial operation of the equipment.

At present, there are nearly 10 online detection methods for XLPE cables that have been developed or researched at home and abroad, but almost none of them have successfully entered into practical promotion. There are three reasons: 1) The chemical potential Es and the sheath insulation resistance Rs drop caused by the corrosion of the cable sheath metal is an important reason that restricts most online detection methods from entering the practical stage, especially the impact on the "DC component method" is particularly serious . This is because the series equivalent branch composed of the grounding chemical potential Es of the cable shield and the sheath insulation resistance Rs is actually in parallel with the detection impedance of the on-line detection equipment, and their interference or shunt to the detected weak real signal is also very small. It is unavoidable; 2) Trace monitoring is a characteristic of insulation on-line detection, electrostatic induction is the enemy of DC micro-current detection, power frequency strong electric field and corona multi-spectrum interference will cause strong interference to all online detection weak signals; 3) Safety The problem is the primary problem that all online testing institutes must solve, and the hazards to personnel and equipment must be considered in case of cable insulation breakdown.

Contents of the invention

In view of the defects or deficiencies in the above technologies, the purpose of the present invention is to provide an on-line monitoring method for the insulation state of cross-linked polyethylene insulated power cables.

In order to achieve the above tasks, the technical solution adopted by the present invention is: an on-line monitoring method for the insulation state of cross-linked polyethylene insulated power cables, which is characterized in that the method of superimposing direct current is used for measurement, and specifically includes the following steps:

1) Disconnect the grounding wire of the cable shielding layer, set the current sensor on the cable grounding wire, and connect the measuring instrument between the shielding layer and the earth; the measuring instrument includes the superimposed power supply E _N connected to the three-phase power supply, Layer-connected safety capacitors, grounding chemical electromotive force E _S and sheath insulation resistance R _S , as well as filtering and protection units, lower hosts, upper computer systems, lower hosts, and upper computer systems communicate through the data transceiver unit GPRS;

2) Measurement of E _S and R _S

First measure and calculate the influencing factors grounding chemical potential E _S and sheath insulation resistance R _S by superimposing the potential method, and then correct the measurement results; the measurement method is as follows:

The electromotive force E _n1 of about 1V is superimposed on the two input terminals of the measuring instrument, and the current I ₁ ⁺ flowing through the sheath insulation resistance R _S is measured, and the polarity of E _n1 is reversed to obtain E _n2 , and then the above current is reversed by measurement. The size of the current is I ₁ ^- ; while ignoring the influence of the cable insulation resistance RI with a large resistance in parallel, the I ₁ ⁺ and I ₁ ^- values obtained through two measurements are obtained through the following formulas (1) and (2 ) calculates the size of Es and Rs;

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; \&ap;</span>\frac{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; -</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="I"\; filenames="A20061004188800191.png,A20061004188800193.png"\; state="\{\{state\}\}">I</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; +</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; \&ap;</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; -</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; +</span>}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; -</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="I"\; filenames="A20061004188800191.png,A20061004188800193.png"\; state="\{\{state\}\}">I</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; +</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

3) DC component measurement

Set the superimposed power supply E _n = 0V, then measure the current I ₁ flowing on the sampling resistor, and substitute the two values of the grounding chemical potential ES and the sheath insulation resistance _RS into the equivalent circuit of the compensation potential method to obtain the value of I _d , which is also It is the DC component current, and the calculation formula of I _d is;

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}}<span\; class="notranslate">\; \&CenterDot;</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

4) DC superposition method measurement

By controlling and opening the switch K ₁ of the superimposed power supply, a voltage E _N is superimposed on the cable conductor core, and the automatic control switch is closed and opened to measure I ⁺ , then reverse the polarity of the superimposed source, and then measure the current I ^- ;

Correct as follows:

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}}{\mathrm{<span\; class="notranslate">\; I</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

In the formula, _R1 is the measured insulation resistance. According to the measured cable length, the insulation resistance of the cable per unit length is obtained, and the positive and negative polarities of the superimposed source are exchanged twice for measurement to reduce measurement errors;

5) After the above parameters are measured, the lower host sends it to the upper computer through the data transceiver unit GPRS, and the upper computer analyzes the measurement parameters through the set data acquisition and analysis software, and gives the judgment of the cable insulation state according to the given evaluation standard. Comprehensive diagnosis results.

The method of the invention can measure the aging degree and insulation resistance of three-phase XLPE cable insulation water branches on-line. Discriminate the operating reliability of the cable main insulation, prevent cable breakdown accidents and effectively evaluate the remaining life of the cable insulation.

Description of drawings

Fig. 1 is a schematic diagram of the system structure;

Figure 2 is a schematic diagram of internal superposition;

Fig. 3 is the equivalent circuit schematic diagram of compensation potential method;

Fig. 4 is a schematic diagram of the generation of the DC component current component (the rectification action model of the water branch);

Fig. 5 is a schematic diagram of the system protection part;

Figure 6 is the shape and electrical parameters of the ceramic gas discharge tube;

Fig. 7 is an automatic protection switch circuit;

Fig. 8 is a composition and working block diagram of the amplifier;

Figure 9 is a schematic diagram of the installation of the transformer and the XLPE online detection host;

Fig. 10 is a measurement flow chart;

Figure 11 is the result display interface;

Fig. 12 is a data acquisition process waveform;

Figure 13 is the single-core cable simulation test result interface;

Figure 14 is the three-core cable simulation test result interface;

Figure 15 is the online measurement result interface of the three-core cable.

The present invention will be further described in detail below in conjunction with the accompanying drawings and principles.

Detailed ways

One of the principles on which the present invention is based is that water tree branches are the primary form of insulation aging of cross-linked polyethylene cables, and water tree branches are a phenomenon that occurs in polymer-insulated power cables under long-term operating voltages in the presence of trace moisture. A dendritic insulation aging defect. The generation and continuous growth of water tree branches under the long-term operating voltage of the cable is the main reason for shortening the insulation life of XLPE cables and finally causing electric trees at the tip of water tree branches and causing main insulation breakdown accidents. The water tree branch will produce a rectification effect under the AC voltage. The nA-level weak DC current and operation trend generated by the rectification effect in the main insulation of the cable can be detected on the cable grounding wire, and the aging status of the cable insulation water tree can be known. When the DC component current exceeds the limit (over 100nA) or increases sharply, it indicates that the cable insulation is on the eve of breakdown and should be replaced.

The second principle is that the insulation resistance detected online by the DC superposition method has a good correspondence with the offline detection data.

The third principle is that the cable grounding full current contains a wealth of information on the deterioration of the main cable insulation. When the aging of the cable main insulation water tree branches is serious, high-order harmonics will be generated in the AC leakage current on the cable grounding wire. The full current size and the degree of waveform distortion can be used as one of the criteria for the cable water tree aging degree.

In summary, the present invention uses the DC superposition method to measure the size and running trend of the cable insulation resistance; uses the DC component method to obtain the aging state and change trend of the insulating water branches, and uses the comprehensive method of measuring the AC leakage current to obtain the cross-linked polyethylene cable. Aging condition of insulating water branches; online measurement of shielding grounding chemical potential Es and sheath insulation resistance Rs by using compensation potential method.

The specific technical measures are as follows:

1. Principle and method

When the cable is in the non-measurement state, the shielding layer is grounded through the grounding wire to ensure safety. During the measurement process, the cable grounding wire needs to be disconnected and connected to the measuring instrument in series. In this way, the potential of the shielding layer to the ground and the shielding The series circuit composed of the resistance to ground (that is, the insulation resistance of the cable outer sheath) forms a parallel connection with the measuring device, which affects the current passing through the measuring device, which deviates from the ideal measurement and causes measurement errors. Here we first measure and calculate this influencing factor by superimposing the potential, and then make corrections on the measurement results.

As shown in Figure 3, the online detection of E _S and R _S by the device is carried out using the "compensation potential method". The measurement method is as follows: superimpose E _n1 (about 1V) on the two input terminals of the measuring instrument and measure the current I ₁ flowing through the sampling resistor ⁺ , exchange the polarity of E _n1 to obtain E _n2 , and then measure to obtain the above current, and obtain the reverse current with the magnitude of I ₁ ^- . At the same time ignoring the influence of the cable insulation resistance R ₁ with a large resistance in parallel, the values of I ₁ ⁺ and I ₁ ^- obtained by two measurements can be used to calculate the size of Es and Rs through formulas 1 and 2.

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; \&ap;</span>\frac{{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; -</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; +</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; l</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

${\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; \&ap;</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; -</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; +</span>}}{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; -</span>}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; +</span>}}{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; no</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}<span\; class="notranslate">\; )</span>$

1) DC component measurement

Substituting these two values into the circuit diagram in Figure 3, you can get the size of Id, which is the DC component.

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 3</span>}<span\; class="notranslate">\; )</span>$

2) DC superposition method measurement

By controlling and opening the switch of the superposition power module, superimposing a voltage E _N on the cable conductor core, closing and opening the automatic control switch to start measuring to obtain I ⁺ , then changing the polarity of the superposition source, and then measuring to obtain the current I ^- .

fix:

${\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; I</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; E.</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}}{\mathrm{<span\; class="notranslate">\; I</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 6</span>}<span\; class="notranslate">\; )</span>$

R _I is the measured insulation resistance, and the insulation resistance of the unit length cable can be obtained according to the measured cable length. The measurement error can be effectively reduced by exchanging the positive and negative polarities of the superimposed source for two measurements.

After these parameters are measured, they will be sent to the host computer through GPRS, and the host computer will calculate and analyze through software, give the judgment of the cable insulation state according to certain evaluation standards, and make suggestions for the power system.

2. Detailed description of the hardware

1) Protective measures

Multiple protections are carried out through grounding capacitors, fast-fuse ceramic discharge tube automatic protection switches, single-phase grounding protection, etc. The advantages and disadvantages of these protections complement each other, which can play a good protective role and ensure the safety of personnel and equipment, as shown in Figure 5.

(1) Grounding capacitance: The grounding capacitance is to ensure that the cable has a reliable AC grounding path under any circumstances, which is also the requirement of the cable operation regulations. The size of this grounding capacitor is generally tens of microfarads. The specific requirements for the grounding capacitor are good voltage resistance (AC 1000V), large insulation resistance (R>1000MΩ), and small grounding impedance.

(2) Fast fuse: Fast fuse is used to ensure the absolute safety of personal and equipment. When an insulation breakdown accident occurs in the cable, the fuse will be broken quickly and the test circuit will be automatically disconnected.

(3) Ceramic discharge tube: The system adopts the ceramic discharge tube of model B8G070H or B8G090H, which is a mature product mass-produced by a certain factory. Under normal conditions, the equivalent resistance of the tube is close to infinity, which protects the safety of measuring instruments and operators. The appearance is shown in Figure 6, and its parameters are as follows: model Nominal DC breakdown voltage DC breakdown voltage range Impulse breakdown voltage Insulation resistance breakdown current Resistance to common frequency current capacitance V % V GΩ DC KA A PF B8G070H 70 25 ≤600 ≥1 25V 20 20 ≤2 B8G090H 90 25 ≤700 ≥1 50V 20 20 ≤2

(4) Automatic protection switch: It can control the grounding or the on-off of the measurement circuit. During the measurement process, if an accident occurs, the measurement circuit will be automatically disconnected, and the input will be short-circuited quickly to protect the safety of the instrument and personnel. When overvoltage occurs, the protection circuit automatically controls the main circuit to quickly realize "zero resistance" hard grounding to ensure the safety of personnel and instruments, as shown in Figure 7.

2) Single-phase grounding protection

In order to prevent possible threats to personnel and equipment in the event of an unpredictable single-phase cable insulation breakdown grounding accident, a large current (above 100A) grounding protector (not shown on the way) has been specially developed to ensure the continuous flow of large grounding currents hours or more. The protector is a one-time device, once it breaks down, it needs to be replaced with a new protector.

The device uses the thin film material spaced between two metal sharp parallel electrodes, and uses the basic principle that the electric field strength of the tip is relatively concentrated, so that the thin film can be broken down and short-circuited at a lower voltage. Experiments have proved that by changing the thin film material, thickness and electrode shape, It can effectively reduce the breakdown voltage. After the breakdown, the electrodes are in a short circuit state. Due to the large area of the parallel electrodes, the current flow capacity is strong, and the protector will not burn out and lose its function. This protection is for one-time use and needs to be replaced after one action, which can effectively protect the safety of people and equipment. This protection is for one-time use and needs to be replaced after one action. But in general, due to the various protective measures in front, the protector is only the last protection, so it is generally unlikely to break down and need to be replaced.

3) Signal acquisition and amplification module

The signal acquisition and amplification module is mainly composed of a self-designed multi-stage differential amplifier (see Figure 8), which minimizes the influence of external interference on the system on the measurement results. In order to ensure the dynamic range of the measuring instrument, according to the magnitude of the measured current Select different ranges and use different sampling resistors. Considering the large resistance value of the sampling resistor, it is necessary to use an ultra-high input impedance amplifier and use multi-stage amplification to complete it. Table 1 shows the nominal value of the detection impedance of each gear of the amplifier module.

Table 1 The sampling resistor R _i and the corresponding current gear of each gear of the signal amplification module

4) DSP control module

The DSP control module adopts the 2400 series DSP digital signal processing chip, which is integrated on the circuit board and fixedly installed in the lower computer. Main functions: collect the signal processed by the amplification module, and perform mathematical operations on it to obtain the measurement display value; realize the control of the protection filter module, signal amplification and acquisition module, GPRS communication module, and display module. It is the key link to realize system automation.

5) GPRS communication module

The GPRS communication module is a data transmission and remote monitoring terminal device based on the GSM/GPRS communication network. This module is based on the latest GPRS digital mobile communication network, which overcomes the shortcomings of short communication distance and unstable performance. Main function: to complete the data communication between the upper and lower computers, and the tariff is charged according to the telecommunication operation regulations. The lower computer connects with the dynamic host domain name server through the GPRS module, and the upper computer logs in the dynamic host domain name server through the Internet. GPRS detects the user who logs in to the server through the dynamic host server and transmits the data to the user, that is, the upper computer system.

6) Display module

The display module adopts an industrial-grade special liquid crystal display device with temperature compensation function produced by Jingdian Pengyuan Display Technology Co., Ltd., and the number of dot matrix is 240×128. Main functions: It can display ground current and DC micro-current measurement values for on-site debugging and maintenance. Installed on the outside of the lower chassis, it is convenient for the operator to observe the displayed content. The displayed content is consistent with the content on the host LCD.

7) Leakage current sensor

The current sensor uses a Rogowski coil set on the ground wire of the cable to measure the ground full current signal. The sensor is installed in the body of the lower computer. When the frequency of the sensor is 50Hz, the signal amplification gain is the largest, which can effectively suppress the interference of other frequencies.

8) DC working power supply

The power module adopts the MW-T-40A switching power supply with a power of 40W, which can provide +5V and +12V output voltages, and a large enough current to meet the requirements of the system. The volume is relatively small (129×98×38mm). The main function is to provide power for the measurement unit (lower computer).

9) DC superposition power module

Provide +24V, -24V DC voltage, which is added between the neutral point of the three-phase voltage transformer and the earth, and the superimposed power supply disconnection and voltage polarity conversion are automatically controlled by the host computer (as shown in Figure 9). There are high-voltage fuses, switch metal connectors, XLPE cable heads, and XLPE cables, which are connected to the XLPE online detection host through the cable grounding wire.

3. Software

The application program is the application software of the cable online monitoring program. It is compiled based on the graphical programming software LabVIEW (Laboratory Virtual Instrument Engineering Workbench) of American NI Instrument Company. This software can also realize the automatic monitoring of the partial discharge measurement process, simplifying the operation of the staff, and the user can also realize more functions such as query and analysis through manual operation.

This software mainly consists of the following modules:

●Data acquisition software

database software

A) Acquisition and analysis software

This partial discharge online monitoring and control software is compiled by using LabVIEW graphic editing software. The software is powerful in writing control panels.

This module is mainly used to complete the program of hardware interface control. It can perform network acquisition control, network data transmission, anti-interference and other functions. Realize acquisition, output waveform, and set acquisition time, acquisition parameters and alarm display settings, etc. Refer to Figure 10 for the measurement flow chart, and Figures 11 to 15 for specific pages.

B) Database software

The partial discharge data database built by Microsoft SQL Server can facilitate the realization of data server/client structure and upgrade to data network transmission. This module connects the modules of partial discharge acquisition signal, diagnosis and query, and hides it from general users. Only users with advanced management rights can perform data operations on it. Three parts of data are stored in the database:

1) Store real-time data and preprocessed data, which can generate waveform diagrams of various parameters.

2) Store historical partial discharge data information, according to this data can generate a trend graph of each parameter, and realize the query of historical partial discharge related parameters

The first functional feature of the cable using the superimposed potential method is that it can be used for on-line monitoring of the aging degree of three-phase XLPE cable insulation water branches (DC component method) and insulation resistance (DC superposition method). The former is used to judge the operating reliability of the cable main insulation and prevent cable breakdown accidents; the latter operation trend diagram can be used to evaluate the remaining life of the cable insulation.

The second feature is the on-line measurement of the sheath insulation resistance R _s and the grounding chemical potential E _s , and the use of the measured values of R _s and E _s to correct the online measurement values of the micro-current and main insulation resistance of the DC component method, ensuring the accuracy of the measurement credibility.

The third feature is that it can measure the magnitude and waveform of the sheath grounding full current online, and use the change trend of the full current value and the distortion degree of the current waveform to assist in the judgment of the aging degree of the cable insulation water tree.

The fourth original feature is the system's uniquely designed quadruple protection technology and special high-current protection components, which can ensure the absolute safety of online detection.

The fifth function is to use the unique wireless data transmission method of the public mobile communication network to realize unattended online monitoring and wireless remote control measurement.

Claims (5)

1. an insulating state on-line monitoring method of cross-linked PE cable is characterized in that, adopts DC stacked method to measure, and specifically comprises the following steps:

1) the cable shielding layer earthing line is disconnected, current sensor is sleeved on the cable grounding line, inserts surveying instrument between screen layer and the earth; Safe electric capacity, ground connection chemical potential ES and jacket insulation resistance R S that surveying instrument is included in the stack power supply that inserts on the three-phase supply, inserts at cable shield, and filtering and protected location, the next main frame, host computer system, the next main frame, host computer system are by data transmit-receive unit GPRS communication;

2) E _SAnd R _SMeasurement

Come first measurements and calculations to go out this influence factor ground connection chemical potential E by the method for stack electromotive force _SWith the jacket insulation resistance R _S, and then on measurement result, revise; Measuring method is as follows:

Electromotive force E about two input end stacks of surveying instrument 1V _N1, measure and flow through the jacket insulation resistance R _SElectric current I ₁ ⁺, with E _N1The polarity transposing obtains E _N2, measuring above-mentioned electric current again, to obtain reverse size be I ₁ ^-Electric current; Omit the influence of the very big insulating resistance of cable RI of resistance in parallel simultaneously, by the I that measures for twice ₁ ⁺And I ₁ ^-Value calculates E by following formula (1) and (2) _SAnd R _SSize;

$R_S\&ap;\frac{{2E}_n}{I_1^{-}+I_1^{+}}-(R_i+R_l)---\left(1\right)$

$E_S\&ap;\frac{I_1^{-}-I_1^{+}}{I_1^{-}+I_1^{+}}{E}_n---\left(2\right)$

3) flip-flop is measured

Put stack power supply E _n=0V measures the electric current I that flows through on the sampling resistor again ₁, ground connection chemical potential E _SWith the jacket insulation resistance R _STwo value substitution compensating potential method equivalent electrical circuit can obtain I _dSize, flip-flop electric current just, I _dComputing formula be;

$I_d=\frac{R_S+R_l}{R_S}\&CenterDot;I_1---\left(3\right)$

4) dc superposition method is measured

Open the K switch of stack power supply by control ₁, be superimposed upon voltage E on the cable conductor core _N, close and open automatic control switch and begin to measure I ⁺, the source polarity that will superpose then transposing measures electric current I again ^-

Revise by following formula:

$R_I=\frac{R_N}{I}---\left(6\right)$

In the formula, R _IBe the insulation resistance that measures, obtain the insulation resistance of unit length cable according to measured cable length, the positive-negative polarity exchange of stack source is measured for twice, to reduce measuring error;

5) measure after the above-mentioned parameter, the next main frame sends to host computer by data transmit-receive unit GPRS, host computer is analyzed measurement parameter by the data collection and analysis software that is provided with, and provides the judge of cable insulation state by given evaluation criterion, provides the comprehensive diagnos result.

2. the method for claim 1, it is characterized in that, described data transmit-receive unit GPRS has the wireless data transceiving function, slave computer is connected with the DynamicHost name server by data transmit-receive unit GPRS, host computer lands the DynamicHost name server by the internet, data transmit-receive unit GPRS passes through the user that the DynamicHost name server detects game server, and data are passed to master system.

3. the method for claim 1 is characterized in that, described current sensor is positioned on the host computer, and its centre frequency is 50Hz.

4. the method for claim 1 is characterized in that, described stack power supply is added between threephase potential transformer neutral point and the earth.

5. method as claimed in claim 3 is characterized in that, described current sensor adopts Rogovski coil.

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