Nearest Neighbor and Kernel Survival Analysis: Nonasymptotic Error Bounds and Strong Consistency Rates

We establish the first nonasymptotic error bounds for Kaplan-Meier-based nearest neighbor and kernel survival probability estimators where feature vectors reside in metric spaces. Our bounds imply rates of strong consistency for these nonparametric estimators and, up to a log factor, match an existing lower bound for conditional CDF estimation. Our proof strategy also yields nonasymptotic guarantees for nearest neighbor and kernel variants of the Nelson-Aalen cumulative hazards estimator. We experimentally compare these methods on four datasets. We find that for the kernel survival estimator, a good choice of kernel is one learned using random survival forests.Comment: International Conference on Machine Learning (ICML 2019

Space charge measurement in polymer insulated power cables using flat ground electrode PEA

Data processing methods used to accurately determine the space charge and electric stress distributions in DC power cables using the pulsed electroacoustic (PEA) system are described. Due to the coaxial geometry and the thick-walled insulation of highvoltage cables, factors such as divergence, attenuation and dispersion of the propagated acoustic pressure wave in the PEA can strongly influence the resultant measurements. These factors are taken into account ensuring accurate measurements to be made. Most importantly, a method is presented to determine the electric stress profile across the insulation due to both the divergent applied field and that as a consequence of trapped charge in the bulk of the insulating material. Results of spacecharge measurements and the corresponding derived electric stress distributions in XLPE DC cables are presented

Charge trapping and detrapping in polymeric materials

Space charge formation in polymeric materials can cause some serious concern for design engineers as the electric field may severely be distorted, leading to part of the material being overstressed. At the worst, this may result in material degradation and possibly premature failure. It is therefore important to understand charge generation, trapping, and detrapping processes in the material. In the present paper, the characteristics of charge trapping and detrapping in low density polyethylene under dc electric field have been investigated using the pulsed electroacoustic technique. It has been found that the charge decay shows very different characteristics for the sample with different periods of electric field application. To explain the results a simple trapping and detrapping model based on two trapping levels has been proposed. Qualitative analysis revealed the similar features to those observed experimentally

Electric Field Determination in DC Polymeric Power Cable in the Presence of Space Charge

The pulsed electroacoustic (PEA) technique was used to perform space charge measurements in polymeric power cables. However, for a practical dc power cable the electric field is affected by conductivity of the material, which is a function of both temperature and electric field. The coupled problems inflict difficulties to identify the electric field distribution in high voltage (HV) cables, which therefore poses threat to the reliability in operation of dc power cables. In this paper we proposed a method of determining electric field distribution in XLPE power cable, where under temperature gradient the existence of space charge density of the cable is determined by means of a modified PEA system. Commercial 11 kV ac XLPE power cable is applied and measured under an applied dc voltage of 80 kV. The space charge across the insulation was obtained and COMSOL Multiphysics software package is used to accurately determine the electric field distribution in the dc power cable by considering the influences of both the effects of temperature and electric field on the conductivity of the insulating material. Therefore, the results of the numerical modelling shall give us a clearer representation of the electric field distribution in HVDC cables

Space charge measurement in polymer insulated power cables using flat ground electrode PEA

Data processing methods used to accurately determine the space charge and electric stress distributions in DC power cables using the pulsed electroacoustic (PEA) system are described. Due to the coaxial geometry and the thick-walled insulation of highvoltage cables, factors such as divergence, attenuation and dispersion of the propagated acoustic pressure wave in the PEA can strongly influence the resultant measurements. These factors are taken into account ensuring accurate measurements to be made. Most importantly, a method is presented to determine the electric stress profile across the insulation due to both the divergent applied field and that as a consequence of trapped charge in the bulk of the insulating material. Results of spacecharge measurements and the corresponding derived electric stress distributions in XLPE DC cables are presented