时域反射计METRACBLE TDR精确定位和电缆缺陷故障，文章来源于GMC-I测试仪器。Time Domain Reflectometer METRACBLE TDR pinpoints and faults cable defects, article from GMCI Gomez Instruments.

地下电缆是铺设在地下的电线，其外部具有特别耐用的绝缘层，即电缆护套，可防止其因土壤中的化学影响或生活在地下的小动物（啮齿动物）而损坏。与架空线相比，地下电缆有几个优点。它们受到保护，免受损坏 - 包括风化 - 而且它们不会在视觉上破坏景观。缺点包括更高的维护成本和更难定位故障，例如由建筑工作引起的故障和对地下电缆的意外损坏。与这些因素相关的较高成本在高压范围内的电力工程应用中是不利的。在电信领域，本地连接电缆被加压到 10 mbar 的水平。由于这种过压，在电缆护套发生轻微泄漏的情况下，空气会被吹到任何渗入的湿气上，这样湿气就不会渗入。在压力下降的情况下，传感器会指示空气正在从系统中逸出。然后用一种特殊的气体将电缆抽上来，从而可以在检测设备的帮助下查明泄漏点。Underground cables are electrical wires that are laid underground and have a particularly durable outer layer of insulation, known as the cable sheath, that prevents them from being damaged by chemical influences in the soil or by small animals (rodents) that live underground. Underground cables have several advantages over overhead wires. They are protected from damage - including weathering - and they don't visually disrupt the landscape. Disadvantages include higher maintenance costs and greater difficulty in locating faults, such as those caused by construction work and accidental damage to underground cables. The higher costs associated with these factors are disadvantageous in power engineering applications in the high-voltage range. In telecommunications, local connection cables are pressurized to a level of 10 mbar. Due to this overpressurization, in the event of a slight leakage of the cable sheath, air is blown over any penetrating moisture so that it does not seep through. In the event of a pressure drop, a sensor indicates that air is escaping from the system. The cable is then pumped up with a special gas so that the leak can be identified with the help of detection equipment.

所谓的TDR测量是检查电缆的一种可能选择。TDR代表时域反射计，可用于确定电缆长度，并检测电缆故障、电缆断裂、短路。TDR 测量仪器，例如METRACABLE TDR，会发出一个电脉冲，然后根据脉冲返回变送器之前经过的时间计算电缆长度或故障位置。还可以通过分析各种波形来识别各种各样的故障。通过TDR方法，仪器向电缆发送一个脉冲，该脉冲被电缆故障反射，然后返回仪器。根据特征反射曲线可以确定电缆故障类型。TDR 测量仪器是检查对称线的强大工具。除了确定电缆长度外，显示的反射曲线还可用于识别和查明电缆故障。这可以节省故障排除时间并避免不必要的工作。在阶跃函数发生器的帮助下，在电缆的一端产生一个信号。信号边沿通过介质传播，并在到达另一端时被反射，或者被电缆内的故障反射。在合适的评估电路或示波器的帮助下，然后将传输的信号与其反射进行比较，并确定有关反射的传播时间和幅度的信息，以及它的电容、电阻和电感特性。示波器中反射的简单视图使观察者能够评估它们的特性，即使没有深入的技术知识。So-called TDR measurements are a possible option for inspecting cables.TDR stands for Time Domain Reflectometer and can be used to determine cable lengths and detect cable faults, cable breaks, and short-circuits.TDR measurement instruments, such as the METRACABLE TDR, send out an electrical pulse, and then calculate the length of the cable or the location of the fault based on the amount of time that has elapsed before the pulse returns to the transmitter. A wide variety of faults can also be identified by analyzing various waveforms. With the TDR method, the instrument sends a pulse to the cable which is reflected by the cable fault and then returned to the instrument. Based on the characteristic reflection curve, the type of cable fault can be determined. the TDR measuring instrument is a powerful tool for checking symmetrical lines. In addition to determining the cable length, the displayed reflection curve can be used to recognize and identify cable faults. This saves troubleshooting time and avoids unnecessary work. With the help of a step function generator, a signal is generated at one end of the cable. The signal edge propagates through the medium and is reflected when it reaches the other end, or is reflected by a fault within the cable. With the help of a suitable evaluation circuit or oscilloscope, the transmitted signal is then compared with its reflection and information about the propagation time and amplitude of the reflection is determined, as well as its capacitive, resistive and inductive characteristics. The simple view of reflections in an oscilloscope enables the observer to evaluate their characteristics, even without in-depth technical knowledge.

电缆的各种电气特性会影响长度的测量。为了确保长度测量尽可能准确，必须注意使用正确的传播速度因子（VP 值）。该值越准确，显示的电缆长度就越精确。集成到METRACABLE TDR 等测试仪器中的电缆数据库包括各种电缆类型，以及与长度测量相关的电缆特性。支持对称电缆（参见示例图）。TDR 提供的确切选项在接下来部分中进行了描述。合适的电缆故障检测器可以使用 TDR测试方法检查各种不同的电缆类型，并查明可能的故障位置。METRACBLE TDR 能够检查长达15 公里的电缆。Various electrical characteristics of cables can affect length measurements. To ensure that the length measurement is as accurate as possible, care must be taken to use the correct propagation velocity factor (VP value). The more accurate this value is, the more precise the displayed cable length will be. The cable database integrated into test instruments such as the METRACABLE TDR includes the various cable types as well as the cable characteristics relevant for length measurement. Symmetrical cables are supported (see example diagram). the exact options offered by the TDR are described in the following section. A suitable cable fault detector can use the TDR test method to examine a wide range of different cable types and identify possible fault locations. the METRACBLE TDR is capable of examining cables up to 15 kilometers in length.1. the TDR can be used to examine a wide range of cable types.

测量电力行业中的电缆长度是时域反射仪的首批应用之一。TDR 用于测量发射脉冲在反射后返回仪器所需的时间。如果已知电缆的传播速度（取决于电缆绝缘），则测量时间可用于直接推断电缆长度。有线雷达一词就是从这个应用演变而来的。过去这些测量需要示波器，而现在可以使用现成的测量仪器直接显示长度值。这种方法广泛应用于电信和网络技术领域。建筑物内铺设新线缆时，安装的网线按照时域反射计测量的长度值来核算费用。Measuring cable lengths in the power industry was one of the first applications of Time Domain Reflectometers.TDR is used to measure the time it takes for a transmitted pulse to return to the instrument after reflection. If the propagation speed of the cable is known (depending on the cable insulation), the measured time can be used to directly extrapolate the cable length. The term wireline radar evolved from this application. Whereas in the past these measurements required an oscilloscope, it is now possible to display the length value directly using an off-the-shelf measuring instrument. This method is widely used in telecommunications and network technology. When new cables are laid in buildings, the installed network cables are accounted for according to the length values measured by time-domain reflectometers.

故障源检测的目标是识别损坏或挤压的地下电缆，并查明相应故障的位置。为此，利用了时域反射测量的特性—— 不仅可以检测全反射，还可以检测介质中的每一次变化。全反射仅发生在电缆末端、电缆断裂处或内外导体之间的 短路处。只要脉冲沿未改变的介质传播，电缆中的波阻抗就不会改变。然而，如果脉冲波遇到挤压点，阻抗会发生变化并发生部分反射。然后可以从反射的到达时间及其性质推断挤压的位置和程度。The goal of fault source detection is to identify damaged or crushed underground cables and to locate the corresponding faults. For this purpose, the properties of time-domain reflectometry are utilized - not only total reflection but also every change in the medium can be detected. Total reflection only occurs at cable ends, cable breaks or short circuits between inner and outer conductors. As long as the pulse propagates through the unchanged medium, the wave impedance in the cable remains unchanged. However, if the pulse wave encounters a squeeze point, the impedance changes and a partial reflection occurs. The location and extent of the crush can then be inferred from the arrival time and nature of the reflection.

脉冲宽度或脉冲持续时间也会影响测量。较小的脉冲持续时间具有更好的分辨率，即它们可用于定位靠近在一起的故障。但与此同时也会带来一个缺点，无法再测量更大的长度，因为——由于长电缆导致的衰减——短脉冲的反射太小而无法测量。对于较大的脉冲宽度，这种关系是相反的。可以测量更长的电缆，但分辨率较差，可能不会显示某些故障。这可能需要进行多次测量。Pulse width or pulse duration can also affect the measurement. Smaller pulse durations have a better resolution, i.e. they can be used to localize faults that are close together. But at the same time there is a disadvantage that larger lengths can no longer be measured, because - due to attenuation caused by long cables - the reflection of short pulses is too small to measure. For larger pulse widths, the relationship is reversed. It is possible to measure longer cables, but the resolution is poorer and may not show certain faults. This may require multiple measurements.

 

典型电缆故障及其波形