The industrial applicability of PEA space charge measurements, for performance optimization of HVDC power cables

Cable manufacturing industries are constantly trying to improve the electrical performance of power cables. During the years, it was found that one of the most relevant degradation factors influencing the cable lifetime is the presence of space charge in the insulation layer. To detect the accumulated charge, the pulsed electro-acoustic (PEA) method is the most used technique. Despite the wide use of the PEA cell, several issues are still present. In particular, the PEA output signal is strongly disturbed by the acoustic waves reflections within the PEA cell. This causes the distortion of the output signal and therefore the misinterpretation of the charge profiles. This, in turn, may result in an incorrect cable characterization from the space charge phenomenon point of view. In 2017, due to the proved degradation effect of the space charge accumulation phenomenon, the IEEE Std 1732 was developed. This standard describes the steps to be followed for the space charge measurement in cables specimens during pre-qualification or type tests. Therefore, cable manufacturing industries started to take a particular interest in these measures. In the light of this, the aim of the present work is to highlight that the enacted standard is not easily applicable since various problems are still present in the PEA method for cables. In particular, in this work, the effect of multiple reflected signals due to the different interfaces involved, but also the effect of the signal attenuation due to cable dielectric thickness, as well as the effect of the PEA cell ground electrode thickness in the output charge profile, are reported. These issues have been demonstrated by means of an experimental test carried out on a full-size cable in the Prysmian Group High Voltage laboratory. To better understand the PEA cell output signal formation, a PEA cell model was developed in a previous work and it has been experimentally validated here. In particular, simulations have been useful to highlight the effect of the reflection phenomena due to the PEA cell ground electrode thickness on the basis of the specimen under test features. Moreover, by analyzing the simulation results, it was possible to separate the main signal from the reflected waves and, in turn, to calculate the suitable ground electrode thickness for the cable specimen under test

A New Numerical Approach to the Calibration and Interpretation of PEA Measurements

In this paper, a new approach for interpreting raw PEA signals is proposed. The theory behind acoustic wave propagation in ideal (no attenuation and no dispersion) materials and in real materials (with attenuation and dispersion) initially free of space charge is described. A simulation model has been developed for the following: (1) the acoustic signal formation in the PEA apparatus, the transmission of the acoustic waves, their attenuation/dispersion and detection; (2) the instrumental effects of having a capacitive piezoelectric sensor driving a 50 Ohm input impedance amplifier. The various layers of a PEA system were considered in the model and the effect of each layer on the acoustic wave propagation is analyzed. Since the model allowed raw PEA data to be simulated, it can be used to identify potential sources of error in interpreting real PEA measurement data, such as acoustic mismatch between PEA layers, electrode material effects, pulse voltage, etc. The results showed good agreement between the simulated and experimentally obtained data in the case of space charge free samples

A Protocol for Space Charge Measurements in Full-size HVDC Extruded Cables

This position paper, prepared by the IEEE DEIS HVDC Cable Systems Technical Committee, illustrates a protocol recommended for the measurement of space charges in full-size HVDC extruded cables during load cycle qualification tests (either prequalification load cycles or type test load cycles). The protocol accounts for the experimental practices of space charge measurements in the thick insulation of coaxial cables in terms of poling time, depolarization time, heating and cooling of specimens, as well as for the experience gained very recently from such kind of measurements performed in the framework of qualification tests relevant to ongoing HVDC cable system projects. The goal of the protocol is not checking the compliance with any maximum acceptable limit of either space charge or electric field. Rather, this protocol aims at assessing the variation of the electric field profile in the cable insulation wall during poling time at the beginning and at the end of load cycle qualification tests for full-size HVDC extruded cables. Indeed, in the design stage the electric field distributions are determined by the cable geometry and by temperature gradient in the insulation. Thus, the design is based on macroscopic parameters conductivity and permittivity and how they depend upon temperature. Any disturbance of the electric field due to space charge accumulation will only be revealed during space charge measurements either in as-manufactured state or in the aged state after load cycle qualification tests

Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

Reliability of Interfaces for HVDC Cable Accessories

HVDC cable systems are widely used in modern transmission networks. This project investigates the dielectric withstand properties of the interfaces existing in the accessories (joints, termination). Interfaces are a weak point as cavities exist at the contact between the cable and the accessory body. Interfaces have been studied as the mating of two elastic surfaces using elastic-plastic mechanics. Their withstand properties are greatly improved by using rubbery material (e.g., silicon rubber) and oiling the surfaces as the number and size of the cavities is reduced. However, thermal aging of the surfaces (with loss of elasticity) and the migration of the lubricant might lead to conditions where the interfaces are again the weak link of the system. The goal of this thesis is to understand breakdown mechanisms of XLPE/XLPE and XLPE/LDPE interfaces through the analysis of partial discharges, space charge accumulation and leakage current measurement. Space charge is measured through the Thermal Step and the Pulsed Electro-Acoustic methods (TSM and PEA). TSM highlights that there exists a critical voltage at which the space charge shifts from homo- to hetero-charge. The conductivity characteristics of the XLPE insulation of HVDC cable are investigated under different applied mechanical stress and the breakdown voltage which could be affected by crosslinking byproducts within the insulation of the material. Partial Discharge is a key tool to ensure the reliability and life extension of High Voltage electrical equipment. For cable interfaces, it is generally assumed that surface partial discharges create carbonized tracks at the contact between the cable insulation and the accessory body leading to breakdown. In this thesis, it was not possible to reproduce this condition. Breakdown occurred through a mechanism that remains to be unraveled. A possible connection with the shifts mentioned above, from homo to hetero charge at the interface, seems to exist

Space charge measurement and analysis in low density polyethylene films

The growing requirement of reliability for an insulation system gives researchers greater responsibility to investigate new techniques for monitoring and diagnosing dielectrics subjected to an electric field. It is well known that the presence of space charge is one of the important factors causing premature failure of polymeric high voltage cables. Space charge surveillance is becoming the most general applied skill to evaluate polymeric materials, particularly high voltage cables. The well-known pulsed electroacoustic method (PEA), a reliable non-destructive method, gives a reasonable resolution to the concentration of the space charge in the insulation material. My work contributes to the measuring of space charge in low density polyethylene using PEA.From the experiments to study space charge formation and distribution at the interface on multi-layer sample under DC and AC applied voltage, electrode materials and frequency are determined as two important factors in measuring the charge injection and distribution; the interface between films acts as a trap for charge carriers, especially for electrons; and positive charge has a high mobility compared to negative charge.Surface potential decay was studied to explain the crossover phenomenon and to find physical mechanism on charge decay of the corona charged film sample. Charge mapping technique (PEA) was successfully introduced to the potential study and it provides an alternative way to investigate charge decay process and allows monitoring charge migration through the bulk of corona charged film. One essential phenomenon, bipolar charge injection, has been first derived from the results of space charge distribution.The advanced PEA measurement system with high rate test and excellent phase resolving capability was designed in the last part of the study. Compared with the old system the new system can provide the enhanced experiment result for fast change situation, which can achieve high-quality diagnosis for the virtual industry situation such as polarity reversal and transient voltage failure

Wave Propagation

A wave is one of the basic physics phenomena observed by mankind since ancient time. The wave is also one of the most-studied physics phenomena that can be well described by mathematics. The study may be the best illustration of what is “science”, which approximates the laws of nature by using human defined symbols, operators, and languages. Having a good understanding of waves and wave propagation can help us to improve the quality of life and provide a pathway for future explorations of the nature and universe. This book introduces some exciting applications and theories to those who have general interests in waves and wave propagations, and provides insights and references to those who are specialized in the areas presented in the book

Feasibility of Space Charge Measurements on HVDC Cable Joints

This review article aims at illustrating the starting of the activities carried out by the Study Group from the viewpoint of the assessment of the state of the art in the measurement of SC in HVDC extruded cable system joints