Non-Contact Measurement and Analysis of Trapped Charge Decay Rates for Cable Line Switching Transients

Stephen Robson, Abderrahmane Haddad, Simon Dennis and Foroozan Ghassemi
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Stephen Robson: School of Engineering, Cardiff University, Cardiff CF24 3AA, UK
Abderrahmane Haddad: School of Engineering, Cardiff University, Cardiff CF24 3AA, UK
Simon Dennis: National Grid, Warwick CV34 6DA, UK
Foroozan Ghassemi: National Grid, Warwick CV34 6DA, UK

Energies, 2020, vol. 13, issue 5, 1-19

Abstract: During reclosure of 275 kV cable circuits used for voltage control, excessive overvoltages were observed on the network. Such events cause onerous and costly failures. Transient simulations have shown that the normal voltage on its own cannot generate such excessive switching overvoltages. Initial investigations by the network operator pointed towards trapped charge on the unearthed as the cause of the failures. Measurement of these trapped charge voltages and their slow decay without interfering with the charge has, to the author’s knowledge, not been done before in an operational substation. This work introduces a technique to measure trapped charge at a 275 kV substation using the Electrostatic Field Mill. Since the electric field is a proxy measurement of surface voltage, field mills can also be used to measure voltage. In this paper, an on-site substation measurement setup using an electrostatic field mill has been developed for the non-contact measurement of trapped charge voltage on a 275 kV underground cable circuit following switching operations at a National Grid substation. Results of field measurements within the substation and laboratory experimentation are discussed. It is demonstrated that with adequate calibration, achieved by using the known pre-switching power frequency steady state voltage, the slowly decaying DC voltage caused by the cable trapped charge can be measured using this non-contact technique. The correlation between the instantaneous time constant and the relative humidity is also analysed.

Keywords: trapped charge; sheath voltage limiter; SVL; cable; field mill; transient; overvoltage (search for similar items in EconPapers)
JEL-codes: Q Q0 Q4 Q40 Q41 Q42 Q43 Q47 Q48 Q49 (search for similar items in EconPapers)
Date: 2020
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