WCA May 2017

with T x as the signal propagation measured from both cable ends. Of course, the calculation by knowledge of the propagation velocity is still valid and the measurements can be verified when the right cable length is also known. The test circuit was simulated with OrCAD PSpice and with realistic cable parameters [5] . It allows the simulation of the signal propagation in very long cables and the signal distortion by the measuring circuit on the cable end. and T y

HV AC/DC source

❍ ❍ Figure 1 : Principle circuit for online fault location

Only in the case of tests with a separate HV source repeated measurements can this be done. The applied testing voltage can be increased up to a certain voltage level to enforce the breakdown again. A comparison of the two TDR measurement methods is shown in Table 1 . An advantage of the online method is the absence of reflections from the far end. The breakdown causes a very low impedance at its location and the signals are reflected from here. A simplified circuit for online measurements is shown in Figure 1 . The measurement on both cable ends with two measuring devices improves the fault location accuracy. Of course, this option depends on the configuration of the power cable system and the access to its cable ends. This option is not considered in the experimental tests yet.

Capacity to ground

❍ ❍ Figure 2 : Simulated circuit

The simulation was made with a cable length of 100km and a propagation velocity of 171.25m/µs. The failure was simulated at a distance of 83km from the cable end where the measuring circuit was connected. The simulation results in Figure 3 show a time T = 970 µs and with the aforementioned velocity v the distance to the failure is calculated to l x = 83.06km. The negligible deviation from the reference value is the result of a slightly inaccurate time measurement of the simulation results.

Theoretical Considerations and Simulation

The physics of cables and their behaviour is very complex and has been widely discussed in literature. It shall not be repeated in this paper (example for reference see [4] ). Only two basic equations are needed here:

❍ ❍ Figure 3 : Simulation results

Equ. 1 Equ. 2

When using this kind of TDR the exact knowledge of the propagation velocity v determines the accuracy of the fault location. (It differs to the TDR measurement for partial discharge (PD) fault location where only the time relation of the reflections determines the accuracy.) Therefore this propagation velocity has to be known exactly to be determined in advance. When the parameters L’ and C’ of the cable are effectualy known, the propagation velocity can be calculated by Equation 1 . However, if it is possible, an initial measurement of the propagation velocity should be done for each commissioned cable. The situation changes when the TDR signals are measured on both cable ends. Then the knowledge of the velocity is not necessary (similar to the PD fault location) and the fault location is calculated by:

Measuring Equipment The measuring circuit consists of two main components, the HV divider and the transient recorder. While only one type of transient recorder processes the signals from measurements on AC and DC cables, the HV dividers differ for AC and DC applications. A capacitive HV divider is preferably used for measurements on AC cables. For DC cables a broadband divider with a resistive arm is necessary to achieve the required response characteristic. This response characteristic is also essential when other voltage measuring devices are taken for the online TDR measurements, eg instrument transformers which are installed in power nets. Their ability has still to be approved.

Equ. 3

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Wire & Cable ASIA – May/June 2017