CN103941161A - On-line monitoring system for current and carrying capacity of cable sheath - Google Patents

Abstract

The invention relates to an on-line monitoring system for cable sheath current and carrying capacity, and belongs to the technical field of on-line monitoring of power cables. The technical solution is: ① Set up multiple different measurement points, set a clamp current sensor on the incoming line of the cross-connected grounding box under the condition of live cable operation, and collect the sheath current signal; through the clamp current sensor set on the cable body, Obtain cable ampacity data; ②The data acquisition card collects the sheath current signal and ampacity data synchronously; ③Uploads the collected sheath current signal and ampacity data to the host computer for theoretical calculation; ④Through the real-time collected sheath The current value is compared with the expected sheath current value under non-fault conditions, and the sheath current values at different measurement points are compared at the same time to locate the fault and give an early warning. The invention can monitor the current flowing in the metal sheath of the power cable on-line, and provides a reliable basis for diagnosing faults in the cable.

Description

An online monitoring system for cable sheath current and ampacity

technical field

The invention relates to an on-line monitoring system for cable sheath current and ampacity, in particular to a method for synchronously measuring power cable sheath current and ampacity and simulating and calculating the expected value of sheath current at each measurement point under fault and non-fault states The invention belongs to the technical field of power cable on-line monitoring.

Background technique

When both ends of the medium and high voltage cables are grounded at the same time or through cross interconnection, a closed loop is formed between the cable sheath and the earth. The sum of the induced current generated by the induced voltage in this circuit and the leakage current generated in the cable insulation is called the sheath current. At present, in the calculation method of the cable sheath current in the background technology, the current induced by the induced voltage in the cable metal sheath is regarded as the sheath current of the cable, and the leakage current generated in the cable insulation is small due to its small amplitude. Ignored [1-3]. Ignoring the leakage current will increase the error between the calculated results and the actual results, and it is easy to misdiagnose the cable operation when performing fault diagnosis under the condition of high loop impedance.

Contents of the invention

The purpose of the present invention is to provide an online monitoring system for cable sheath current and ampacity. By establishing a reasonable mathematical model, the simulation of the sheath current in the case of common cable joint faults provides a reliable basis for subsequent cable fault diagnosis and location. To solve the problems in the background technology of insufficient consideration of the influence of the sheath current and insufficient analysis of the change of the sheath current under fault conditions.

Technical scheme of the present invention is:

An online monitoring system for cable sheath current and ampacity, comprising the following steps:

① Set up multiple different measurement points, set clamp current sensors on the incoming line of the cross-connected grounding box under the condition of live cable operation, and collect the sheath current signal; through the clamp current sensor set on the cable body, obtain the current carrying capacity of the cable data;

②The data acquisition card synchronously collects the sheath current signal and current carrying capacity data;

③Upload the collected sheath current signal and ampacity data to the host computer for theoretical calculation;

④By comparing the sheath current value collected in real time with the expected sheath current value under non-fault conditions, and at the same time comparing the sheath current values at different measurement points, the fault in the cable can be automatically identified under abnormal conditions According to the type of fault, locate the fault and give an early warning prompt.

When the cable line adopts the cross-connection method with both ends directly grounded, install clamp current sensors at the inlets of the two cross-connection grounding boxes (JX1 and JX2), and each cross-connection grounding box has 3 wire inlets, ( As shown in Figure 1, the three inlets of the cross-interconnection grounding box JX1 are A1 (A2), B1 (B2), and C1 (C2)), and a total of six measurement points are set; each measurement point is collected synchronously through the data acquisition card A total of six sets of current waveforms are obtained each time; at the same time, the current carrying capacity in the cable is collected synchronously by three current sensors respectively installed on the three-phase cable body; the coaxial cable is used as a cross-connection connection to connect the cable connector and In the cross interconnection grounding box, the current value measured by the current sensor is the vector sum of the current flowing through the inner and outer conductors of the coaxial cable. This is also not considered in the existing cable sheath current diagnosis technology, and is also an important feature of the present invention.

The present invention adopts the method of computer simulation, according to the original data and algorithm of the power cable, calculates the expected sheath current value under the non-fault situation, that is, the theoretical value of the sheath current; the sheath current under the non-fault situation includes the induced voltage in the cable The induced current and the leakage current generated by the insulation resistance; the influencing factors of the induced current include the current carrying capacity of the cable, the length of the cable segment, the cable laying method and the design parameters of the cable body; the leakage current is mainly caused by the capacitance flowing through the cable insulation The current composition is affected by the cable operating voltage and the length of the cable segment; in fault and non-fault conditions, it is another main feature of the present invention to consider the influence of leakage current on the current amplitude of the sheath.

The present invention can carry out on-line monitoring and fault diagnosis on the cables with and without return lines. The value affects the magnitude of the current in the sheath, which can vary several times depending on how the return wire is designed for the line.

The invention includes a current sensor, a front-end computer, a communication system, a host computer, a power supply unit and an alarm unit. The characteristics of the system are: (1) automatically and synchronously collect the cable sheath current and ampacity of each measurement point in each large cycle section of a cable line; (2) through the program built in the system, the sheath at each measurement point can be The expected values under current fault and non-fault conditions are simulated and stored in the database, which provides an important reference for further fault diagnosis and location. The invention provides great support for real-time monitoring of cable running status and timely response to abnormal situations.

The beneficial effect of the present invention is that the current flowing in the metal sheath of the power cable can be monitored online, and the expected current value of each sheath current measurement point under fault and non-fault conditions can be simulated by theoretical calculation, so as to diagnose the fault in the cable provided a reliable basis.

Description of drawings

Fig. 1 is the main wiring diagram of the embodiment of the present invention;

Among them, 1 refers to the three-phase transmission line, 2 refers to the cable terminal joint, 3 refers to the direct grounding box (J1 refers to the No. 1 direct grounding box, J2 refers to the No. 2 direct grounding box), and 4 refers to the clamp 5 refers to the lead wire of the metal sheath, 6 refers to the intermediate connector, 7 refers to the small section of the cross-connection cable, 8 refers to the cross-connection grounding box, and JX1 refers to the No. 1 cross-connection grounding box , JX2 refers to the No. 2 cross interconnection grounding box, #1—#9 respectively represent nine small sections of cables;

Fig. 2 is a schematic diagram of the theoretical calculation method of the induced current due to the induced voltage in the present invention;

Fig. 3 is the equivalent circuit diagram of Fig. 2;

Fig. 4 is the calculation method schematic diagram of the cross interconnection cable sheath current under the non-fault situation of the present invention;

Fig. 5 is the current flowing in 6 measuring points of the cross interconnection grounding box of the present invention;

Fig. 6 is an equivalent circuit diagram when the cross interconnection box is flooded;

Fig. 7 is an equivalent circuit diagram when the insulation barrier of the C1-C2 joint at the JX1 joint breaks down.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing.

An online monitoring system for cable sheath current and ampacity, comprising the following steps:

①Clamp current sensor is installed on the incoming line of the cross-connection grounding box under the condition of live cable operation to collect the current signal of the sheath; through the clamp current sensor on the cable body, the current carrying capacity data of the cable is collected; ②Through the data acquisition card The current signal is collected synchronously; ③Upload the collected sheath current and ampacity data to the host computer for theoretical calculation, providing a reliable basis for further diagnosis of cable faults.

A more specific implementation:

Referring to attached drawing 1, the cable line adopts the cross interconnection method with both ends directly grounded, and the metal sheath connection method is as follows: A1-B2, B1-C2, C1-A2; B3-C4, C3-A4, A3-B4. The induced current flowing in the metal sheath is shown in Figure 2, respectively represent the induced currents in the three sheath loops, Respectively represent the current carrying capacity in the three-phase cable, Indicates the cable segment length, ,..., ,..., Respectively represent the respective induced voltage and impedance in the corresponding nine cable segments, representing the ground resistance. Through Figure 3, the sum of the induced currents in the three loops can be obtained, as shown in formula (1):

The induced voltage in formula (1) can be calculated by formula (2):

in Indicates the self-inductance coefficient of the three-phase cable, Indicates the mutual inductance coefficient between phase A, phase B, phase B, phase C and phase A, phase C, and can be obtained by formula (3):

in, represents the diameter of the cable core, Represents the spacing between A-phase B-phase, B-phase C-phase and A-phase C-phase cables respectively.

The line impedance in equation (1) can be derived from the following equation:

in, Indicates the unit impedance of the metal sheath, Indicates the unit inductive reactance of the metal sheath. They can be calculated by formulas (5) (6):

in formula (6) stands for the conductor resistivity, is the cross-sectional area of the metal sheath, is the conductor temperature coefficient, Indicates the ambient temperature. Substituting formulas (2)-(6) into formula (1), the amplitudes of the sum of induced currents in the three sheath circuits can be calculated respectively.

The sheath current includes not only the induced current generated by the induced voltage, but also the capacitive current flowing through the insulation. As shown in Figure 4, represents the capacitive current flowing in both directions generated in cable #1, , represents the capacitive current flowing in both directions generated in cable #2, and so on, Represents capacitive currents in both directions generated in cable #9. Total capacitive current generated in each cable section It can be calculated with the following formula:

in, Indicates the operating voltage of the cable. Indicates the unit capacitance of the cable insulation, and its magnitude can be calculated by formula (8):

The capacitive current generated in each small circulation section of the cable will be shunted in two directions under the action of the resistance, and the size of the shunted capacitive current is determined by the impedance in the path. Take cable #1 as an example:

By analogy, the shunted capacitive currents generated in all cable small loop sections can be calculated.

The power cable sheath current data acquisition system is shown in Figure 5. Clamp-on current sensors are respectively placed at the inlets of the two cross-connected grounding boxes, and a total of 6 measurement points are set, of which Indicates the current measured by the current transformers at the three inlets of the JX1 cross-connection grounding box, Indicates the current measured by the current transformers at the three inlets of the JX2 cross-connection grounding box. Since each cross interconnection connection line is connected to two circulating current loops, the current signal measured by the clamp current transformer is the vector sum of the currents of the two circulating current loops.

For the cross interconnection grounding box JX1, the currents measured by the current transformers at the three measurement points are , then the formula (10) can be obtained, the formula is as follows:

For the cross interconnection grounding box JX2, the currents measured by the current transformers at the three measurement points are respectively , then the formula (11) can be obtained, the formula is as follows:

The current flowing in each cable small loop section should be the sum of the induced current and the capacitive current. Their magnitudes can be calculated by the following formula:

Substituting formula (12) into formula (11) can calculate the theoretical calculation value of the sheath current measured at the six measurement points under non-fault state.

The current change of the cable sheath caused by the breakage of the cable metal sheath, the water ingress of the cross-connection grounding box and the breakdown of the joint insulation partition can be diagnosed by the cable online monitoring system and the location of the fault point can be located to the sheath circuit or fault. connector. When the cable has an open circuit fault caused by sheath rupture, the current in the fault loop becomes zero. The sheath current at the measuring point containing the fault loop will drop compared to normal conditions.

When the cross-interconnection joint JX1 enters water, because the water body inside and outside the interconnection box is connected, the area of the water body is much larger than the area of the interconnection box, so the resistance of the water body is ignored. Its equivalent circuit diagram is shown in Figure 6. When a fault occurs, the original three sheath circuits become six fault circuits due to fault grounding. These six fault currents are defined as , then according to the voltage and impedance in the loop, it can be deduced that:

The induced current at the six measurement points can be expressed by Equation (14), with the capacitive current kept constant:

The breakdown fault of the insulation partition of the joint takes the joint at C1-C2 as an example, and the equivalent circuit diagram is shown in Figure 7. When connector C1-C2 fails, it will not affect The current situation in the loop, so in Figure 7, this loop is omitted. in the picture Represents the fault current flowing in the four fault branches respectively, and the value of the fault current in the four branches can be obtained by using the loop current method. The three loop currents are defined as , you can launch:

fault current It can be expressed by formula (18):

According to the installation position of the sensor, the induced current value of the six measurement points can be expressed by formula (19), and the capacitive current value remains unchanged:

The above analyzes the theoretical values of the sheath currents that can be measured at the measurement points under the three fault conditions, by comparing the real-time collected sheath current with the expected sheath current value under non-fault conditions, and the sheath current values of different measurement points The comparison between the currents can evaluate the health status of the cable line. The ratio between the expected value of the sheath current of the power cable under the condition of non-fault and the sheath current of different measurement points is stored in the database in the master station after simulation calculation. When the data returned by the cable sheath current monitoring data acquisition terminal is calculated and reaches the fault identification level, the system will alarm and give an early warning prompt. In addition to the method of simulation calculation, the expected value of the sheath current in the non-fault state of the sheath current can also be obtained by collecting actual data over a period of time to obtain the average value.

In addition to the diagnosis of the cross interconnection cable system installed with the return line, the present invention can also perform online monitoring on the cross interconnection cable system without the return line installed. The main difference between the two is the size of the ground resistance resistance. Since the return line is usually a metal wire, the sheath loop forms a path through the return line, and the impedance in the entire loop is very small. If no return line is installed, the sheath loop passes through the earth to form a loop, and the total impedance in the loop is affected by the ground resistance. According to different geographical conditions, the ground resistance in each cable system is also different. Under normal circumstances, the resistance range is floating between 4Ω-10Ω. This needs to make the simulation result close to the actual result by measuring the ground resistance before the simulation calculation. By setting the size of the ground resistance, the three-phase cable system with two states of whether the return line is installed or not can be monitored online.

The advantage of the on-line monitoring system of the power cable sheath current in the present invention is that the current and carrying capacity of the cable sheath can be measured online in real time through the current transformer installed at the cross interconnection box and set on the cable body. When the cable grounding system fails, the power cable sheath current on-line monitoring system can work together with the cable fault diagnosis system to detect the fault in time at the early stage of the fault and locate the fault to the fault point, which provides important support for the maintenance and repair work of the cable management personnel. Information support will play an important role in ensuring the stable operation of the power system.

Claims (3)

1. A cable sheath current and ampacity online monitoring system, is characterized in that, comprises the steps: ① Set up multiple different measurement points, set clamp current sensors on the incoming line of the cross-connected grounding box under the condition of live cable operation, and collect the sheath current signal; through the clamp current sensor set on the cable body, obtain the current carrying capacity of the cable data; ②The data acquisition card synchronously collects the sheath current signal and current carrying capacity data; ③Upload the collected sheath current signal and ampacity data to the host computer for theoretical calculation; ④By comparing the sheath current value collected in real time with the expected sheath current value under non-fault conditions, and at the same time comparing the sheath current values at different measurement points, the fault in the cable can be automatically identified under abnormal conditions According to the type of fault, locate the fault and give an early warning prompt. 2. A kind of cable sheath current and ampacity online monitoring system according to claim 1, it is characterized in that when the cable line adopts the cross-connection method of direct grounding at both ends, the two cross-connection grounding boxes (JX1 and JX2 ) to install clamp-on current sensors at the line inlets, and each cross-interconnection grounding box has three line inlets, and a total of six measurement points are set; the current waveforms of each measurement point are collected synchronously through the data acquisition card, and a total of six sets of currents are obtained each time Waveform; at the same time, the ampacity in the cable is collected synchronously by three current sensors respectively installed on the three-phase cable body; use the coaxial cable as the cross interconnection wiring to connect the cable joint and the cross interconnection grounding box, and the current measured by the current sensor The value is the vector sum of the currents flowing through the inner and outer conductors of the coaxial cable. 3. A kind of cable sheath current and ampacity online monitoring system according to claim 1 or 2, it is characterized in that adopting the mode of computer simulation, according to power cable original data and algorithm, calculate the expected sheath current under non-fault situation value, that is, the theoretical value of the sheath current; the sheath current under non-fault conditions includes the induced current generated by the induced voltage in the cable and the leakage current generated by the insulation resistance; the influencing factors of the induced current include the current carrying capacity of the cable , cable segment length, cable laying method and cable body design parameters; the leakage current is mainly composed of capacitive current flowing through the cable insulation, which is affected by the cable operating voltage and the cable segment length; in both fault and non-fault conditions, consider Influence of leakage current on sheath current amplitude.