Investigation of temperature dependence of dielectric processes in thermally aged PVC insulation

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Abstract

The temperature dependence of dielectric processes was investigated of PVC cable insulation. For the investigation voltage response measurement was used, since this method ensures the independent investigation of conductive and polarisation processes. From the results of the measurement activation energies have been calculated to conductivity and polarisation conductivity of the material, the results are in 105.4 kJ/mol…133.8 kJ/mol and 32.1 kJ/mol…51.8 kJ/mol ranges, respectively.

These results suggest that different charge carrier distribution mechanisms act main role in dc conductivity and slow polarisation processes in PVC cable insulating material.

Introduction

Strong temperature dependence of insulation resistance of polyvinyl chloride (PVC) is well known. This phenomenon must be taken into account at evaluation of resistance measurement, because this parameter is very important in the field of the electrostatic hazards. Since, in cases of on-site measurements the same temperature cannot be ensured for all cases, therefore several correction tables have been recommended by manufacturers to normalise the measured value to a given temperature. These normal temperatures, which are used for comparison, are 15 °C or 20 °C, usually.

Other important issue is that the insulation resistance of an arrangement is continuously increasing after energizing due to the very slow dielectric polarisation processes taking place in insulating material. Since, the time for a measurement is usually limited, hence the insulation resistance is read in an agreed time after energization. In practice, 60 s reading time is generally accepted. However, this measurement time is not enough to develop all of slow polarisation processes hence the resulted resistance is given from a summarisation of dc conductive current and polarisation current. The Fig. 1 shows the components of current in a given time after the connection of voltage source and the currents in the electrical model of insulation can be seen in Fig. 2. The current has three components i.e. Icap (capacitive), Ipol (polarisation) and Icond (conductive) components, nevertheless the separation of these components from the result of a simple insulation resistance measurement is not solved easily therefore conductive and polarisation processes cannot be investigated independently. This is the reason why voltage response measurement has been used for investigation of dielectric processes.

Section snippets

The voltage response method

The conductive and slow polarisation processes can be investigated separately by the voltage response measurement. This method measures of decay and return voltages. The timing diagram of the measurement can be seen in Fig. 3. The decay voltage (Vd(t)) is measured after long duration charging (tch) of insulation after disconnecting the voltage source from the insulation. The decay voltage curve shows self-discharging process of insulation in case of lossless voltmeter usage. The return voltage (

Samples and measurement

The laboratory tests have been carried out on cable specimens were prepared from service aged low voltage (0.6/1 kV) PVC insulated four-core cables. Four solid copper conductors of cable are covered by core insulations, which are prepared from coloured PVC material. Blue, red and black colourants have been mixed in the compound and the cable has one red, one blue and two black coloured cores. The four insulated cores are covered by PVC tape belting layer on which steel tape armour is wound.

Estimation of activation energies

Temperature dependence of conductivity of polymers follows the well-known Arrhenius formula:σ(T)=σ0ⅇ−EackTwhere σ(T) the conductivity at absolute temperature (T), σ0 the conductivity extrapolated to 0 K, Eac the activation energy of conductivity and k the Boltzmann constant.

A similar equation can be used to describe the temperature dependence of polarisation conductivity (β(T)):β(T)=β0ⅇ−EapkTwhere β(T) the conductivity at absolute temperature (T), β0 the polarisation conductivity extrapolated

Conclusion

Temperature dependence of dielectric processes has been investigated on aged PVC cable samples. The dielectric processes have been carried out by voltage response measurement, which method is a very effective tool to investigate conduction and slow polarisation processes, separately. From the results of measurements i.e. initial slope of decay and return voltages, the activation energies have been estimated for conductivity and polarisation conductivity. These results mean however the

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