Uncertainty of Electric Field Estimation in a Full-Sized Cable Based on Space Charge Measurement

Electric fields in cable insulations can be assessed via space charge measurements, but the field distributions estimated are sometimes unstable. In this work, the pulsed electroacoustic (PEA) method was applied to a 275-kV-class extruded cable. The 216 waveforms were obtained under an applied dc voltage of 320 kV without any space charge accumulations, and their field distributions were compared with the Laplace field. Their uncertainty for estimating electric field distribution was ±5.8% with the 95% confidence interval. Similar statistical analyses were conducted again for waveforms measured under various dc voltages. The uncertainty of the field distribution improved, as the applied voltage and SNR increased. If the testing voltage is sufficiently high or if a voltage is applied for a sufficiently long period of time to cause space charge accumulation, a safe side uncertainty is adopted. That is, by using the reference data, the widest error range and safer uncertainty evaluation can be achieved.

I. Introduction

Estimation of the electric fields in the insulating walls of cables is essential to assess their suitability for use in high-voltage dc systems [1]. Space charge accumulation plays a major role in determining the field distribution in the insulation [2], [3], [4], [5], [6]. Therefore, accurate measurement is essential for the assessment baseline. Space charge waveforms acquired via the pulsed electroacoustic (PEA) method [7], [8] require subsequent signal analysis. A method of calibration was proposed in [9], but little attention has been paid for the uncertainty of the analysis.