Device for testing influence of terminating load on cable induced voltage

Abstract

The invention discloses a device for testing the influence of a terminating load on cable induced voltage, and relates to the field of electromagnetic compatibility. Based on a steel structure metal test platform, an impact current generator is used in an impact chamber to generate simulated lightning current, and the influence of cable end connection load resistance on cable induced voltage is researched. The method specifically comprises the steps of 25 ohm resistor grounding, 50 ohm resistor grounding, 100 ohm resistor grounding and core wire non-grounding at a core wire end. When a core wire end is connected with a load, for a multi-core cable, the unoccupied core wire is grounded to serve as a shielding wire, a lightning electromagnetic pulse field around the cable is reduced by using an electromagnetic field generated by opposite current diffusion paths of core wire current and lightning current, and cable coupling is inhibited.

Description

Technical Field

[0001] The present invention belongs to the technical field of electromagnetic compatibility, and relates to a test device for the influence of a terminating load on the induced voltage of a cable. Background Art

[0002] Cables are widely used in electrical systems, microwave communication systems, and electronic information systems. The complex interconnection of cables increases the electromagnetic sensitivity of the system, and correspondingly reduces the reliability and safety of the system in a complex and changeable strong electromagnetic environment such as lightning. When the cable is within the effective influence range of a strong pulsed electromagnetic field, through the coupling effect between the field and the cable, a large amount of electromagnetic energy will be conducted by the cable to the working circuit, resulting in the malfunction of the working circuit and even damage.

[0003] Lightning, also known as thunderbolt, is a strong discharge phenomenon that occurs between a charged cloud and another cloud with opposite charge or between the cloud and the ground. The lightning discharge time is short, the discharge voltage is high, the generated current is large, and at the same time, there is a transient strong electromagnetic radiation phenomenon, also known as lightning electromagnetic pulse. The lightning current has a pulse rising edge of microseconds or even nanoseconds, and its current waveform is characterized by a short duration and a large current amplitude. In engineering, a surge current is usually used to simulate the lightning current generated during the lightning discharge process under natural conditions, and to test and evaluate electrical and electronic equipment.

[0004] Lightning mainly damages electronic and electrical equipment through the way of inductive coupling, and its discharge process will generate a lightning current with an amplitude of the order of kA. According to Faraday's law of electromagnetic induction, when the lightning strike point is near the cable, the instantaneous changing lightning current will generate a strong lightning pulse electromagnetic field near the cable. Under the action of this electromagnetic field, the cable generates an induced voltage and current, causing the voltage and current signals in the working circuit connected to the cable to change, and then resulting in the failure of the corresponding working system. This phenomenon is often referred to as the lightning transient electromagnetic effect.

[0005] At present, electrical cables are mainly applied in the power industry, construction industry, communication industry, transportation industry, etc. With the rapid development of these industries, the connection networks of various cable systems are becoming increasingly complex. Parallel arrangement is one of the most common arrangement forms in cable systems. When a lightning pulse electromagnetic field acts on parallel cables, according to the different cable arrangements and connection conditions, induced voltages and currents will be coupled in the cables, affecting the normal operation of internal electrical and electronic equipment in the system. In severe cases, it may lead to the paralysis of the system function. Therefore, the electromagnetic protection of cable systems is particularly important. Studying and mastering the parallel inductive coupling response characteristics of electrical cables is the prerequisite for carrying out reasonable electromagnetic protection design, and can provide guidance for the reasonable layout of cables, electromagnetic shielding of equipment, filtering of incoming waves, etc. Building an inductive coupling simulation model of parallel cables and combining with the corresponding test results to carry out the calculation of cable response characteristics under different conditions can provide reference for the lightning electromagnetic pulse protection design and related simulation verification research of cable systems. Summary of the Invention

[0006] To overcome the above defects of the prior art, the present invention provides a test device for the influence of a terminated load on the induced voltage of a cable. To achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] The present invention provides a test device for the influence of a terminated load on the induced voltage of a cable, including a lightning tower, a steel structure experimental platform, test cables, a ground grid, a load matching box and an impact chamber;

[0008] The steel structure experimental platform includes a metal platform, support steel pipes for supporting the metal platform, and measurement chambers at the bottoms of both ends of the metal platform. The ground grid is a metal flat steel arranged in a criss-cross pattern and buried underground, and is connected to the measurement chamber, the load matching box, the impact chamber and the lightning tower through vertical wires. The test cables are buried under the edge of the metal platform. The leading end of the test cable is connected to the measurement chamber and the core wire is grounded through a terminated load. The trailing end of the test cable is connected to the load matching box and the core wire is grounded through a terminated load. The shielding layers at the leading end and the trailing end of the test cable are not grounded. The impact chamber is connected to the lightning tower and contains an impact current generator for generating a simulated lightning current. An oscilloscope is installed in the measurement chamber. One channel of the oscilloscope is connected to the core wire of the test cable through a voltage probe, and the other channel of the oscilloscope is connected to the shielding layer through a current sensor.

[0009] The test device of the present invention synchronously measures the cable voltage and the shielding layer current with a voltage probe and a current sensor and records them with an oscilloscope. The entire device is located in the measurement chamber at the bottom of the platform. There is also an AC uninterruptible power supply in the measurement chamber to supply power to the oscilloscope, and a set of resistors is symmetric with the resistors in the load matching box.

[0010] The present invention also provides an analysis method for the influence of various terminated loads on the induced voltage of a cable by using the test device described in the present invention, including the analysis of the lightning coupling effect of the cable when the core wire is terminated with a load.

[0011] The test cable is a four-core armored buried cable laid in a straight line. Tests are carried out under four working conditions: "25Ω resistor grounded", "50Ω resistor grounded", "100Ω resistor grounded", and "core wire not grounded" at both ends of the test cable. During the test, the simulated lightning current always occurs through the impact chamber and is injected from the lightning attracting tower, and the shielding layer of the test cable remains ungrounded. The corresponding voltage waveforms under different working conditions are observed and analyzed.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0013] The device of the present invention uses an impact chamber and a lightning attracting tower to simulate a real lightning environment, more accurately simulating the induced voltage situation in the actual environment, so as to better evaluate the influence of the cable induced voltage. The present invention uses an oscilloscope to synchronously measure the induced voltage and shielding layer current of the test cable, which is beneficial to real-time monitoring and recording of the data of the cable induced voltage and shielding layer current, and improves the accuracy and reliability of the test data.

[0014] The present invention provides an analysis method for the influence of various terminated load characteristics on the induced voltage of a cable, which can quantitatively analyze the lightning coupling effect of the cable under different working conditions. Through the comparison and analysis of experimental data, the influence degree of various termination situations on the cable induced voltage can be more clearly understood. Brief Description of the Drawings

[0015] Figure 1 It is a schematic diagram of the test device structure.

[0016] Figure 2 It is a schematic diagram of the voltage connection in the measurement room.

[0017] Figure 3 It is a flow chart of the lightning coupling analysis of the shielded cable when the core wire is terminated with a load.

[0018] Figure 4 It is a schematic diagram of the test layout for the influence of the core wire terminated with a load

[0019] Figure 5 It is a schematic diagram of different core wire terminated load conditions.

[0020] Figure 6 It is the injected lightning current waveform and the core wire voltage waveform of the buried cable under different core wire terminated load conditions.

[0021] Figure 7 It is a bar chart of the core wire voltage peak value and the first peak steepness under different core wire terminated load conditions of the buried cable.

[0022] 1 - Lightning tower, 2 - Power line, 3 - Metal platform, 4 - Impact chamber, 5 - Measurement chamber, 6 - Buried cable, 7 - Ground grid, 8 - Load matching box, 9 - Vertical wire, 10 - Voltage probe, 11 - Oscilloscope, 12 - Current sensor, 13 - AC uninterruptible power supply, 14 - 50Ω resistor, 15 - Outer shielding layer, 16 - Inner shielding layer. Specific embodiments

[0023] The present invention will be further described below in conjunction with embodiments. These embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. The processes, conditions, experimental methods, etc. for implementing the present invention, except for the specifically mentioned content below, are all common knowledge and well-known common sense in the art, and the present invention has no special limiting content.

[0024] The present invention provides an analysis device and method for the influence of various termination load characteristics on the induced voltage of a cable.

[0025] The steel structure metal test platform is as Figure 1 shown.

[0026] The present invention provides a test device for the influence of termination load characteristics on the induced voltage of a cable, including a lightning tower, a steel structure experimental platform, a test cable, a ground grid, a load matching box, and an impact chamber.

[0027] Further defined, the steel structure experimental platform includes a metal platform, support steel pipes for supporting the metal platform, and measurement chambers at the bottoms of both ends of the metal platform. The entire platform is made of steel, with a length of 24.7 meters, a width of 6 meters, and a height of 2.7 meters.

[0028] Further defined, the test cable, according to the layout position, is also called a buried cable.

[0029] Further defined, the ground grid is a metal flat steel arranged in a crisscross pattern and buried about 0.65 meters deep, and is connected to the measurement chamber, the load matching box, the impact chamber, and the lightning tower through vertical wires.

[0030] Further defined, the buried cable is laid under the edge of the metal platform and buried about 0.65 meters deep; the head end of the cable is connected to the measurement chamber, the core wire is grounded through the termination load, the end core wire is terminated with a resistor in the load matching box and then grounded, and the shielding layer is not grounded.

[0031] Further defined, the impact chamber contains an impact current generator for generating simulated lightning current.

[0032] The connection of the voltage probe / coil in the measurement chamber is as Figure 2 shown.

[0033] When conducting the core wire termination load experiment, the flow chart of the lightning coupling analysis of the shielded cable is as Figure 3As shown, the test setup is as Figure 4 shown. For multi-core cables, the unused core wires are grounded as shield wires. By utilizing the characteristic that the overall current dissipation paths of the currents in the core wires and the lightning currents are opposite, the generated electromagnetic fields are used to reduce the lightning electromagnetic pulse fields around the cables, thereby achieving the effect of suppressing cable coupling. This is also a commonly used means of lightning protection for cables. Some literature has found that the shielding effect of using the spare core wires in multi-core cables as shield wires is even better than that of the external shield wires of the cables because the core wires in the same cable are more closely related. In addition to grounding the spare core wires, directly changing the termination loads at both ends of the core wires will also affect the lightning transient response of the cables. Taking a four-core armored cable laid in a straight line and buried underground as the analysis object, by adjusting the grounding resistance of the core wires at both ends in the matching load box and the measurement room, the influence of the termination load conditions of the core wires on the shielding effect is explored. An impulse current generator is used in the impulse chamber to generate simulated lightning currents, which are injected through the power line at the bottom of the lightning attracting tower and then return through the ground grid. In the measurement room, a type voltage probe and a type oscilloscope are used to measure the core wire coupling voltage waveforms. The four core wire termination load conditions analyzed in this section are respectively "grounded with a 25Ω resistor", "grounded with a 50Ω resistor", "grounded with a 100Ω resistor", and "core wire not grounded", as Figure 5 shown.

[0034] To unify the variables, in the present invention, the simulated lightning current is always repeatedly injected from the lightning attracting tower, and the cable shielding layer remains ungrounded. It should be noted that except for the slightly lower peak value of the injected lightning current in the "core wire not grounded" condition, the peak values of the other three times are all maintained at 7.6 kA, as Figure 6 shown.

[0035] Under a similar lightning electromagnetic pulse environment, the core wire voltage waveforms corresponding to each condition are observed by the voltage probe in the measurement room as Figure 7 shown. The results show that changing the grounding resistance of the core wire will not affect the change trend of the voltage waveform, and as the grounding resistance increases, the peak value of the core wire voltage increases accordingly. Among them, in the "core wire not grounded" condition, although the peak value of the injected lightning current is slightly lower, the peak value of the core wire voltage is still significantly the largest among several conditions, indicating that the influence of the change in the core wire grounding condition on the voltage peak value is higher than the influence caused by the peak value of the lightning current at this time.

[0036] The specific statistics of the core wire voltage waveform parameters under different conditions are shown in Table 1.

[0037] Table 1 Statistics of voltage parameters of buried 1Z core wires under different termination load conditions

[0038]

[0039] In order to eliminate the influence of slightly smaller peak injection current when "not grounded", combined with the conclusion that the peak core wire voltage is positively correlated with the peak injected lightning current, linearly amplify the measured peak core wire voltage under this working condition, that is, unify the peak injected lightning current under the four working conditions to 7.6 kA, and the calculation results are shown in the row numbered 5 in the table. As Figure 7 shown, as the grounding resistance of the core wire decreases (the resistance is regarded as infinite when the core wire is not grounded), the voltage peak and the initial peak steepness also decrease, and the effect is significant. For example, comparing the two working conditions of "grounded with a 50 Ω resistor" and "core wire not grounded", the voltage peak after the core wire is grounded is suppressed to less than half of the original peak, while the initial peak steepness is reduced to about one-fourth of the original steepness.

[0040] In summary, the successful use of the present invention verifies that the load condition at the core wire end of the cable has a significant impact on its lightning electromagnetic pulse transient response characteristics, and it is found that for buried cables in this experiment, the lower the grounding resistance of the core wire, the lower the voltage peak and the initial peak steepness, and the weaker the interference power generated by the lightning electromagnetic pulse coupling. By deeply understanding the influence of the cable induced voltage on the equipment, the present invention can optimize the design and deployment methods of the equipment, reduce the interference caused by the cable induced voltage, and improve the reliability and stability of the equipment.

Claims

1. A test device for the influence of the termination load on the cable induced voltage, characterized in that: Including lightning tower, steel structure test platform, test cables, ground grid, load matching box and impact chamber; The steel structure experimental platform includes a metal platform, a supporting steel pipe supporting the metal platform, and a measuring room at the bottom of both ends of the metal platform. The ground grid is a metal flat steel arranged in a vertical and horizontal manner and buried underground, and is connected to the measuring room, load matching box, impact room, and lightning tower through vertical wires. The test cable is buried under the edge of the metal platform, with its head end connected to the measuring room and the core wire grounded through a terminated load, and its tail end connected to the load matching box and the core wire grounded through a terminated load. The shielding layers of the head end and the tail end of the test cable are not grounded. The impact room is connected to the lightning conductor and contains a surge current generator for generating simulated lightning current. An oscilloscope is installed in the measuring room, one channel of the oscilloscope is connected to the core wire of the test cable through a voltage probe, and the other channel of the oscilloscope is connected to the shielding layer through a current sensor.

2. The test device according to claim 1, characterized in that: The measuring room also has an AC uninterruptible power supply for supplying power to the oscilloscope, and a set of resistors symmetrical to the resistors in the load matching box.

3. A method for analyzing the effect of a terminated load on a cable induced voltage using the device of claim 1, characterized in that: The test cable adopts a four-core armored buried cable laid in a straight line. The test is carried out under four working conditions: 25Ω resistance grounding, 50Ω resistance grounding, 100Ω resistance grounding and core wire ungrounded at both ends of the test cable. During the test, the simulated lightning current always occurs through the impact chamber and is injected from the lightning tower. The shielding layer of the test cable remains ungrounded, and the corresponding voltage waveforms under different working conditions are observed and analyzed.

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