WCA May 2017

The triggering of the signal processing is also essential for the measuring quality and accuracy. 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) just in case the cable fails ( Figure 4 ). 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. AC cable (XLPE, 20 kV) The test configuration consisted of two MV cables connected together in series and of slightly different lengths ( Figure 5 ). ❍ ❍ Figure 4 : 3D models of transient recorders for cable tests (left) and cable monitoring (right)

❍ ❍ Figure 5 : AC cable with spark gap (detail)

The voltage was increased up to 10kVrms until the spark gap was fired. The resulting signals of the travelling waves were recorded. The signals were taken from the HV circuit using a resistive-capacitive broadband divider (for reference measurements) or an undamped capacitive HV AC divider of type WCF [6] ( Figure 6 ).

❍ ❍ Figure 6 : AC source and HV divider

The HV divider output was connected with the transient recorder by a coaxial measuring cable. The reference measurement with the broadband divider is shown in Figure 7 .

Parameters: • Cable 1: • Cable 2:

758m 708m

❍ ❍ Figure 7 : Measurement with broadband divider

• Further parameters: • AC voltage:

unknown

up to 10kV, 50Hz, connected to near end of cable 1 (see Figure 1 , Figure 6 ) transient recorder for fault location, broad- band divider (resistive- capacitive), AC high voltage divider (un- damped capacitive)

• Measurement equipment:

The artificial breakdown was generated by using a spark gap ( Figure 5 ) which was installed either at the far end of the complete cable length or at the connection point between the two cables.

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Wire & Cable ASIA – May/June 2017