阅读说明：本技术 电力电缆金属护层交流电阻测定方法 (Method for measuring alternating current resistance of power cable metal sheath ) 是由 刘�英 张何燕 张博剑 陈佳美 于 2021-08-17 设计创作，主要内容包括：本发明公开了一种电力电缆金属护层交流电阻测定方法,包括如下步骤：步骤1：定制测试夹具；步骤2：处理电缆端头；步骤3：设备预热及校正；步骤4：安装测试夹具,连接测试线路；步骤5：测量并处理数据。本发明测定方法可直接在成盘的电缆成品上使用,无需截取电缆样段；测量设备为恒流源、电流测试设备和电压测试设备,操作过程简单,测试数据准确；本发明设计的测试夹具可用于不同结构形式金属护层的电阻测量,且可根据被测对象进行尺寸调整,适配性高；本发明提出将金属护层与外部铠装层连接,利用铠装层作为电流的回流通路的方法,以抵消电磁干扰对金属护层交流电阻测量的影响,具有良好的可操作性,同时兼具准确性、实用性和灵活性。(The invention discloses a method for measuring alternating current resistance of a power cable metal sheath, which comprises the following steps: step 1: customizing a test fixture; step 2: processing the cable end; and step 3: preheating and correcting equipment; and 4, step 4: mounting a test fixture and connecting a test circuit; and 5: the data is measured and processed. The measuring method can be directly used on coiled cable finished products without cutting out cable sample sections; the measuring equipment is a constant current source, current testing equipment and voltage testing equipment, the operation process is simple, and the test data is accurate; the test fixture designed by the invention can be used for measuring the resistance of the metal protective layers with different structural forms, can be used for carrying out size adjustment according to a measured object, and has high adaptability; the invention provides a method for connecting the metal sheath with the external armor layer and using the armor layer as a current return path so as to offset the influence of electromagnetic interference on the measurement of the alternating current resistance of the metal sheath, and the method has good operability, and simultaneously has accuracy, practicability and flexibility.)

1. A method for measuring the alternating current resistance of a power cable metal sheath is characterized by comprising the following steps: the method comprises the following steps:

step 1: customizing a test fixture:

the main body part of the test fixture is a metal ring and is made of two copper plates with preset width and thickness and different lengths; the reserved lengths of the two ends of the long copper plate are kept in a straight state, and the middle part of the long copper plate is processed into an arc with a central angle of 300 degrees and a radius of R, so that the assembly is called as a first assembly; the right end of the short copper plate is processed into a U-shaped structure, and the rest part of the short copper plate is processed into an arc with the radius of R and the central angle of 100 degrees, and the arc is called as a component II; clamping the right side straight part of the first component into the U-shaped structure on the right side of the second component for assembly, and drilling through holes at the same horizontal position of the right side U-shaped part of the second component and the two end straight parts of the first component; a screw rod with the length not less than 2R and the outer diameter matched with the inner diameter of the penetrating hole is selected to penetrate through the through holes on the first component and the second component, and nuts matched with the screw rod are selected to fix the two ends of the screw rod to form a test clamp;

by adjusting the position of the left nut, the arc section of the second assembly slides on the inner side of the arc of the first assembly, so that the inner diameter of the test fixture is correspondingly changed; the test fixture can be arranged on a columnar body with the outer radius of 0.85R-1.1R;

the method comprises the following steps of customizing a plurality of clamps with different arc sizes in advance according to the structure size of a common cable, and selecting an adaptive test clamp according to the actual cable size when testing the alternating current resistance of the metal sheath;

step 2: and (3) processing a cable end:

stripping off the parts except the metal sheaths at the two ends of the cable to ensure that the distance between the metal sheath at one end of the cable and the end of the exposed part of the armor layer is not less than 50cm, naming the end as a first end, and naming the distance between the metal sheath at the other end of the cable and the end of the exposed part of the armor layer as a second end;

and step 3: preheating and correcting test equipment:

selecting a test device comprising: the frequency of the constant current source is adjustable, the range of the constant current source covers the frequency to be measured, and the precision is +/-0.1 mA; the current testing equipment requires that the measurement precision reaches +/-0.01 mA, and the phase angle measurement precision reaches +/-0.05 degrees; voltage testing equipment requires that the measurement precision reaches +/-0.01 mV and the phase angle measurement precision reaches +/-0.05 degrees;

preheating the test equipment and completing corresponding correction operation before measurement;

and 4, step 4: installing a test fixture, and connecting a test circuit:

selecting an adaptive test fixture according to the size of the cable, mounting a first test fixture on a metal sheath at a first end of the cable at a position close to the head, mounting a second test fixture at a position close to the tail, and mounting a third test fixture on an armor layer; mounting a No. four test fixture on the metal protective layer of the second end; adjusting the sizes of the first test fixture, the second test fixture, the third test fixture and the fourth test fixture by adjusting the position of the left screw cap, and enabling the sizes to be tightly attached to the outer surface of the metal sheath or the armor layer;

the short-circuit wire is a copper braided flat wire and is arranged between the second end metal protective layer and the armor layer, and two ends of the short-circuit wire are firmly welded on the head part of the second end exposed out of the metal protective layer and the head part of the armor layer;

the high-voltage end of the constant current source is connected with the first test fixture, the low-voltage end of the constant current source is connected with the low-voltage end of the current test equipment, and the high-voltage end of the current test equipment is connected with the third test fixture; the high-voltage end of the voltage test equipment is connected with the second test fixture, and the low-voltage end of the voltage test equipment is connected with the fourth test fixture;

the output current of the constant current source flows into the metal sheath at the first end through the first test fixture, flows out of the metal sheath at the second end, flows into the armor layer at the second end through the short connecting wire, flows out of the armor layer at the first end and flows back to the constant current source;

the current testing equipment is connected in series in the current loop and measures the current passing through the metal protective layer; the voltage testing equipment is connected in parallel to the two ends of the metal protective layer and is used for measuring the voltage on the metal protective layer;

and 5: measuring and processing data:

adjusting the output current and frequency of the constant current source, and reading the current amplitude I and the phase angle theta from the ammeter after the number is stable_AReading the amplitude U and phase angle theta of the voltage from the voltmeter_VBy the formula

Calculating a resistance value; and repeating the measurement for three times, and taking the average value of the three measurements as the alternating current resistance value of the metal protective layer at the test frequency.

2. The method for measuring AC resistance of a power cable metal sheath according to claim 1, wherein: the hemispherical protrusions are machined on the inner side of the arc-shaped copper plate at preset intervals, so that the clamping force is increased, and errors caused by non-uniform distribution of current are avoided.

Technical Field

The invention relates to the field of power cable metal sheath parameter measurement, in particular to a method for measuring power cable metal sheath alternating current resistance by using a special test fixture and a small current input method.

Background

With the acceleration of industrial development and urbanization process, underground cables gradually replace overhead lines and become main power transmission and distribution lines of power systems in China. Compared with overhead transmission lines, the underground cable has the advantages of not affecting city appearance, saving corridor space, not being easily affected by weather environment and the like.

The metal protective layer is used as one of necessary composition structures of the power cable, is positioned outside the insulating layer and the insulating shielding layer, and mainly has the following functions: (1) the capacitance current flowing through the power cable under normal conditions passes through the short-circuit current when the cable has short-circuit fault; (2) the electromagnetic field generated when the cable is electrified is shielded in the insulating wire core so as to reduce the electromagnetic interference generated to the outside; (3) and the cable provides mechanical protection and a waterproof barrier for the cable insulation core. The metal sheath structure of the common power cable at present comprises: wire/tape wrapping, coaxial corrugated sheaths, coaxial smooth sheaths, etc. The material of the common metal passivation layer is typically copper, aluminum or lead.

The resistance of the metal sheath is an important electrical parameter of the power cable, and is required in the calculation of steady-state current-carrying capacity, short-circuit capacity, characteristic impedance and the like. In addition, the resistance value can be used as a parameter for characterizing the integrity of the metal sheath. For example, when the metal sheath material is corroded or oxidized, or is broken in the length direction, the resistance value will be increased significantly. Therefore, the method has important significance in accurately measuring the resistance value of the metal protective layer.

The metal passivation layer is usually a thin layer structure and made of metal with good conductivity, so the resistance value is small, the resistance value per unit length is usually only in the order of μ Ω/m, and it is very difficult to accurately measure the low resistance value. Especially, when the metal protective layer adopts a metal wire or metal belt spiral wrapping structure, accurate measurement is more difficult.

For the measurement of the direct current resistance of the metal sheath layer, an electric bridge method can be adopted; however, no special measuring instrument or reference technical method is available for measuring the metal sheath alternating current resistance, especially for producing coiled cables.

Disclosure of Invention

The invention aims to provide a method for measuring the alternating current resistance of a power cable metal sheath by using a special test fixture and a low-current input method, which can provide accurate and reliable metal sheath resistance values for the structural design of the cable metal sheath, the determination of cable transmission parameters, the design of rated carrying capacity and the like. The testing method can be directly used on coiled cable finished products without cutting out cable sample sections; the measuring equipment is a constant current source, current testing equipment and voltage testing equipment, the operation process is simple, and the test data is accurate; the test fixture designed by the invention can be used for measuring the resistance of the metal protective layers with different structural forms, can be used for carrying out size adjustment according to a measured object, and has high adaptability; the invention provides a method for connecting the metal sheath with the external armor layer and using the armor layer as a current return path so as to offset the influence of electromagnetic interference on the measurement of the alternating current resistance of the metal sheath, and the method has good operability, and simultaneously has accuracy, practicability and flexibility.

In order to achieve the purpose, the invention adopts the technical scheme that:

a method for measuring the alternating current resistance of a power cable metal sheath comprises the following steps:

step 1: customizing a test fixture:

the main body part of the test fixture is a metal ring and is made of two copper plates with preset width and thickness and different lengths; the reserved lengths of the two ends of the long copper plate are kept in a straight state, and the middle part of the long copper plate is processed into an arc with a central angle of 300 degrees and a radius of R, so that the assembly is called as a first assembly; the right end of the short copper plate is processed into a U-shaped structure, and the rest part of the short copper plate is processed into an arc with the radius of R and the central angle of 100 degrees, and the arc is called as a component II; clamping the right side straight part of the first component into the U-shaped structure on the right side of the second component for assembly, and drilling through holes at the same horizontal position of the right side U-shaped part of the second component and the two end straight parts of the first component; a screw rod with the length not less than 2R and the outer diameter matched with the inner diameter of the penetrating hole is selected to penetrate through the through holes on the first component and the second component, and nuts matched with the screw rod are selected to fix the two ends of the screw rod to form a test clamp;

by adjusting the position of the left nut, the arc section of the second assembly slides on the inner side of the arc of the first assembly, so that the inner diameter of the test fixture is correspondingly changed; the test fixture can be arranged on a columnar body with the outer radius of 0.85R-1.1R;

the method comprises the following steps of customizing a plurality of clamps with different arc sizes in advance according to the structure size of a common cable, and selecting an adaptive test clamp according to the actual cable size when testing the alternating current resistance of the metal sheath;

step 2: and (3) processing a cable end:

stripping off the parts except the metal sheaths at the two ends of the cable to ensure that the distance between the metal sheath at one end of the cable and the end of the exposed part of the armor layer is not less than 50cm, naming the end as a first end, and naming the distance between the metal sheath at the other end of the cable and the end of the exposed part of the armor layer as a second end;

and step 3: preheating and correcting test equipment:

selecting a test device comprising: the frequency of the constant current source is adjustable, the range of the constant current source covers the frequency to be measured, and the precision is +/-0.1 mA; the current testing equipment requires that the measurement precision reaches +/-0.01 mA, and the phase angle measurement precision reaches +/-0.05 degrees; voltage testing equipment requires that the measurement precision reaches +/-0.01 mV and the phase angle measurement precision reaches +/-0.05 degrees;

preheating the test equipment and completing corresponding correction operation before measurement;

and 4, step 4: installing a test fixture, and connecting a test circuit:

selecting an adaptive test fixture according to the size of the cable, mounting a first test fixture on a metal sheath at a first end of the cable at a position close to the head, mounting a second test fixture at a position close to the tail, and mounting a third test fixture on an armor layer; mounting a No. four test fixture on the metal protective layer of the second end; adjusting the sizes of the first test fixture, the second test fixture, the third test fixture and the fourth test fixture by adjusting the position of the left screw cap, and enabling the sizes to be tightly attached to the outer surface of the metal sheath or the armor layer;

the short-circuit wire is a copper braided flat wire and is arranged between the second end metal protective layer and the armor layer, and two ends of the short-circuit wire are firmly welded on the head part of the second end exposed out of the metal protective layer and the head part of the armor layer;

the high-voltage end of the constant current source is connected with the first test fixture, the low-voltage end of the constant current source is connected with the low-voltage end of the current test equipment, and the high-voltage end of the current test equipment is connected with the third test fixture; the high-voltage end of the voltage test equipment is connected with the second test fixture, and the low-voltage end of the voltage test equipment is connected with the fourth test fixture;

the output current of the constant current source flows into the metal sheath at the first end through the first test fixture, flows out of the metal sheath at the second end, flows into the armor layer at the second end through the short connecting wire, flows out of the armor layer at the first end and flows back to the constant current source;

the current testing equipment is connected in series in the current loop and measures the current passing through the metal protective layer; the voltage testing equipment is connected in parallel to the two ends of the metal protective layer and is used for measuring the voltage on the metal protective layer;

and 5: measuring and processing data:

regulating the output current (recommended value is 60-80mA) and frequency (for example 50Hz) of constant current source, after the number is stabilized, reading the current amplitude I and phase angle theta from ammeter_AReading the amplitude U and phase angle theta of the voltage from the voltmeter_VBy the formula

Calculating a resistance value; and repeating the measurement for three times, and taking the average value of the three measurements as the alternating current resistance value of the metal protective layer at the test frequency.

Preferably, the hemispherical protrusions are machined on the inner side of the arc-shaped copper plate at preset intervals, so that the clamping force is increased, and meanwhile, errors caused by non-uniform distribution of current are avoided.

The invention provides a method for measuring the alternating current resistance of a power cable metal sheath by using a special test fixture and a low current method, which has the following advantages:

1) the measuring method can directly measure the alternating current resistance of the metal sheath of the coiled finished cable without cutting a cable sample section, has simple operation process and accurate test process, can realize the measurement of the resistance of the metal sheath in different structural forms, and has strong engineering practicability.

2) The constant current source used by the invention provides the cable metal sheath with the current amplitude of mA magnitude in the test process, and the test process is short in use time, thereby avoiding the condition that the temperature of the metal sheath is increased due to the long-time action of the test current, so that the test resistance value is larger.

3) The invention provides a method for connecting the metal sheath and the armor layer in the test process and using the external armor layer as a return current path of current in the internal metal sheath, thereby effectively avoiding the adverse effect of induced voltage generated by an external electromagnetic field on the metal sheath on the measurement result, reducing the measurement error and ensuring the measurement precision.

4) The spherical bulge is designed on the inner side of the test fixture, so that on one hand, the clamping force between the fixture and a tested object can be increased, and good contact is facilitated; on the other hand, the test current is uniformly injected into the circumferential surface of the metal protective layer through the bulge, so that errors caused by non-uniform distribution of the current are avoided, and accurate measurement is facilitated.

5) The test fixture designed by the invention has adjustable size so as to realize resistance measurement on the metal protective layers of cables with different sizes, and has strong adaptability and wide application range.

6) The invention is also suitable for measuring the direct current resistance of the metal sheath of the power cable.

Drawings

Fig. 1 is a schematic structural diagram of a circular ring-shaped clamp. By adjusting the position of the left nut, the short copper plate arc section can slide in the long copper plate arc, so that the inner diameter of the clamp is changed.

Fig. 2 is a schematic view of the cable termination processing in preparation for testing. When the first end is processed, the distance between the metal sheath and the end of the exposed part of the armor layer is not less than 50cm, and the distance between the second end metal sheath and the end of the exposed part of the armor layer can be properly reduced to about 5-10 cm.

FIG. 3 is a schematic diagram of the test circuit connection.

Detailed Description

The present invention will be described in more detail below with reference to the accompanying drawings and specific embodiments.

The method provided by the invention is used for measuring YJV62-26/35kV-1 x 120mm manufactured by a certain cable factory²The direct current resistance value and the power frequency of cable are exchange resistance value down, and this cable metal sheath external diameter is 36.96mm, and the armor external diameter is 43.08mm, and test cable length is 85m, and the implementation step is as follows:

step 1: customizing a test fixture:

the test fixture was customized according to the circular ring test fixture schematic shown in fig. 1. The main part of the clamp is a metal ring and is made of two copper plates with the width of 10mm and the thickness of 2 mm. Two ends of the long copper plate are reserved with 20mm and kept in a straight state, the middle part of the long copper plate is processed into an arc with a central angle of 300 degrees and a radius of R, and the arc is called as a first assembly. The right end of the short copper plate is reserved with 40mm and processed into a U-shaped structure, and the other part of the short copper plate is processed into an arc with the radius of R and the central angle of 100 degrees, and the assembly is called as a second assembly. Clamping the preset straight part at the right side of the first component into the U-shaped structure at the right side of the second component for assembly, and drilling at the same horizontal position of the U-shaped part at the right side of the second component and the straight parts at two ends of the first componentOf (3) a through-hole. And a screw rod with the length not less than 2R and the outer diameter the same as or slightly smaller than the inner diameter of the through hole is selected to penetrate through the through holes on the first component and the second component, and nuts matched with the screw rod are selected to fix the screw rod at two ends.

And processing hemispherical protrusions with the diameter of 2.0mm on the inner side of the arc copper plate at intervals of 10 mm.

By adjusting the position of the left nut, the short copper plate arc section can slide on the inner side of the long copper plate arc, so that the inner diameter of the test fixture is correspondingly changed. The test fixture can be mounted on a cylindrical body with an outer radius of 0.85R-1.1R.

A plurality of clamps with different circular arc sizes (the design circular arc diameter of the clamp can be 35mm, 45mm, 60mm, 80mm and 100mm) are customized according to the common cable structure size, and each size is 4. When the metal sheath alternating current resistance is tested, the adaptive test fixture is selected according to the actual cable size.

Step 2: and (3) processing a cable end:

according to fig. 2, the metal sheaths at the two ends of the cable are partially stripped, and the cable comprises an outer sheath, an armor layer, an inner sheath, a wrapping tape and other structures. As an embodiment, the structure of the cable includes: the cable comprises a conductor, a semi-conductive conductor shielding layer, an insulating layer, a semi-conductive insulating shielding layer, a copper strip shielding layer, a non-woven fabric wrapping layer, a PVC isolation sleeve, a stainless steel strip armor layer and a PVC outer sheath. First end processing: stripping a PVC outer sheath with the length of 60-70cm by using a tool to expose an armor layer, stripping a stainless steel strip armor with the length of 50-60cm, and leaving a bare stainless steel strip armor with the length of about 10 cm; and stripping the PVC isolation sleeve and the non-woven fabric wrapping layer to expose the copper strip shielding layer by 50-60 cm. Second end processing: stripping off the PVC outer sheath with the length of about 20cm by using a tool to expose the armor layer, stripping off the stainless steel strip armor with the length of about 10cm, and leaving the exposed stainless steel strip armor with the length of about 10 cm; and stripping the PVC isolation sleeve and the non-woven fabric wrapping layer to expose the copper strip shielding layer by about 10 cm.

And step 3: preheating and correcting equipment:

as an example, the test equipment adopts a HIOKI 3522-50LCR impedance analyzer, and the measurement frequency range is as follows: 1mHz-100kHz, the device can provide the current required by the test, simultaneously can directly measure and display 14 electrical parameters including voltage, current and resistance values, and the test precision of the current, the voltage amplitude and the phase angle can meet the requirement. The power supply is integrated in the device, no extra device is needed to provide current input for the test system, and the device only reserves a voltage test port (H)_POT、L_POT) And a current test port (H)_CUR、L_CUR). And connecting the test wire of the impedance analyzer, connecting the power supply of the impedance analyzer and starting the impedance analyzer. Preheating impedanceThe analyzer is used for 15-30 minutes. And adjusting the impedance analyzer to an open-circuit compensation mode and a short-circuit compensation mode, and performing open-circuit compensation and short-circuit compensation on the impedance analyzer to finish the correction of the impedance tester.

And 4, step 4: installing a test fixture, and connecting a test circuit:

no. one test fixture, No. two test fixture, No. four test fixture select for use the anchor clamps that the circular arc diameter is 35mm, and No. three test fixture select for use the anchor clamps that the diameter is 45 mm. As shown in fig. 3, a first test fixture is mounted on the metal sheath at the first end near the head, a second test fixture is mounted on the metal sheath near the tail, and a third test fixture is mounted on the armor; and a fourth test fixture is arranged on the metal protective layer of the second end. The sizes of the first test fixture, the second test fixture, the third test fixture and the fourth test fixture are adjusted by adjusting the left screw cap, and the first test fixture, the second test fixture, the third test fixture and the fourth test fixture are tightly attached to the outer surface of the metal sheath layer or the armor layer.

The short-circuit wire is a copper braided flat wire with the length of 10-20cm, is arranged between the second end metal sheath layer and the armor layer, and two ends of the short-circuit wire are firmly welded at the head part of the second end exposed metal sheath layer and the head part of the armor layer.

Current testing high-voltage terminal (H) of impedance analyzer_CUR) The low-voltage end (L) is tested by connecting the test wire to a test fixture I_CUR) The test fixture is connected to the third test fixture through a test wire; voltage testing high-voltage terminal (H)_POT) Is connected to the second test fixture through a test wire, and tests the low-voltage end (L)_POT) And the test fixture is connected to the test fixture No. four through a test wire.

The output current of the impedance analyzer flows into the metal sheath of the first end through the first test fixture, flows out of the metal sheath of the second end, flows into the armor of the second end through the short-circuit wire, flows out of the armor of the first end, and flows back to the impedance analyzer.

The current test end is connected in series in the current loop and measures the current passing through the metal protective layer. The voltage test end is connected in parallel on the two ends of the metal protective layer to measure the voltage on the metal protective layer.

And 5: measuring and processing data:

before each resistance test, the impedance analyzer is subjected to open circuit compensation and short circuit compensation to eliminate the influence of the test wire and the clamp on the test result. After the test system is corrected, the impedance analyzer is adjusted to a constant current source (CC) mode, the amplitude of output current is set to be 80mA, and the output current is kept constant in the test process; the frequency values are respectively set as 'DC' and '50 Hz', and the resistance value is read after the reading is stable, namely the direct current resistance value and the alternating current resistance value of the metal protective layer under the power frequency. The measurement was repeated 3 times, and the test results are shown in table 1.

TABLE 1

The theoretically calculated dc resistance was 127.786m Ω, and the relative error between the measured value and the theoretically calculated value was 2.17%.

The ac/dc resistance ratio is 1.025.

The invention selects the constant current source, the current testing device and the voltage testing device as the basic testing devices, has simple operation process and accurate testing data, can be directly used on the coiled cable finished product, does not need to intercept a cable sample section, can realize the resistance measurement of the metal sheath layers with different structural forms, can adjust the size according to the measured object, enhances the engineering practicability and saves the measuring cost. The invention provides a method for connecting a metal sheath with an external armor layer and using the armor layer as a current return path, so as to counteract the influence of electromagnetic interference on the measurement of the alternating current resistance of the metal sheath and improve the accuracy of the measurement result. The relative error between the DC resistance value tested by the invention and the theoretical calculated value is about 2%, and the test result is accurate and reliable. The method for measuring the alternating current resistance of the metal sheath of the power cable by using the special test fixture and the low-current input method provided by the invention makes up the defects that no test method and theoretical calculation method for the alternating current resistance of the metal sheath are clear in engineering.