EuroWire March 2017

Technical article

The simulation and experiments confirmed that a simple edge triggering is sufficient on DC cable systems. On AC cable systems the operational voltage itself prevents such simple edge trigger. A very fast breakdown detection device is therefore implemented to release the signal processing. The required components of the transient recorder depend on its application for cable testing or monitoring. The recorder for cable testing is a small plug-in device which operates by its own PC or by that of the computerised HV test system. It mainly contains the measurement hardware ( Figure 4 ). The recorder for cable monitoring is a robust and small stand-alone device. Besides the measurement hardware it contains a PC with especially adapted software. This PC runs continuously over years, can be restarted and operated remotely, and must be supplied by an uninterrupted power supply (UPS) in case the cable fails ( Figure 4 ).

with T x as the signal propagation measured from both cable ends. Of course, the calculation by knowledge of the propagation velocity it 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. 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. 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. The triggering of the signal processing is also essential for the measuring quality and accuracy. and T y

HV AC/DC source

HVDivider

▲ ▲ Figure 1 : Principle circuit for online fault location

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.

Theoretical Considerations and Simulation

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

▲ ▲ Figure 4 : 3D models of transient recorders for cable tests (left) and cable monitoring (right)

Experimental Tests Practical measurements on different cable samples were carried out to prove the measuring principle and the simulation results. Thereby the AC or DC cable samples were wound on cable drums or turntables.

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 para- meters L’ and C’ of the cable are effectually 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:

▼ ▼ Figure 2 : Simulated circuit

Capacity to ground

▼ ▼ Figure 3 : Simulation results

Equ. 3

90

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March 2017