Designing a HVDC Insulation System to Endure Electrical and Thermal Stresses Under Operation. Part I: Partial Discharge Magnitude and Repetition Rate During Transients and in DC Steady State

Abstract

This paper has the purpose to investigate HVDC insulation design considering real operating conditions, when DC steady-state is affected by frequent voltage transients or load variations that may be present during all life. Electrical field distribution in insulation, and in insulation defects, may change significantly from DC steady-state when voltage and load vary with time, which can cause partial discharge activity often not been properly accounted for at the design stage. The Part I of this paper is dedicated to prove, through experiments, models and simulations, that electrical and thermal transients may incept partial discharges in defective insulations during cable energization, voltage polarity inversion at a constant nominal load, as well as during load variations at a constant nominal voltage. This can cause accelerated aging and premature breakdown even if the insulation system is designed properly to withstand DC electrothermal stress, without partial discharges in steady state, for all life, as it will be shown in Part II. Focus is on cables, but the approach described here is general for any DC insulation system. INDEX TERMS Cable insulation, DC, voltage and load transients, design methodology, finite element analysis, partial discharges. Hadi Naderiallaf was born in Mashhad, Iran, on April 16th, 1986. He received the M.Sc. degree in electrical engineering in 2012 from the Leibniz Universität Hannover, Germany. He did his master thesis at the Schering Institute of High Voltage Techniques and Engineering in the Leibniz Universität Hannover. He has working experience for five years as a transformer fluid specialist. From November 2017, he has been a PhD student in the University of Bologna in Italy working on the EU project GRIDABLE. His main research interests are liquid and nanostructured solid electrical insulating materials, HVDC cables design, space charge measurement and analysis, AC and DC partial discharge detection and modelling, insulation systems for electrical machines, condition monitoring techniques, HV transformer asset management, DGA and transformer oil reclamation. Paolo Seri (M'17) was born in Macerata, Italy, on June 4th 1986. He received the M.Sc. degree in energy engineering in 2012 and PhD in electrical engineering in 2016, both from the University of Bologna. From 2017 he is part of the Laboratory of Innovative Materials for Electrical Systems (LIMES) of the University of Bologna as a research fellow, currently working on the topics of HVDC cables design, partial discharge detection and modelling, and characterization of dielectric materials. He is also

Simulation and Modelling of Transient Electric Fields in HVDC Insulation Systems Based on Polarization Current Measurements

“…In particular, the time behavior of the electric field profile and magnitude can change noticeably when referring to the dynamic model illustrated here, in comparison to the static approach (1) generally used [4,[23][24][25]. Knowing the real value of the electric field during the transient following, e.g., a voltage polarity inversion, is functional to carry out reliable insulation system design that is partially discharge free, and address the specified life and failure probability [2,3]. However, while the dynamic model illustrated here, based on the Debye model, worked well for oil-impregnated paper, it may not properly describe polymeric materials.…”

“…A basic need is the availability of accurate models for electric field calculation in insulation systems, which will help in gaining insights on the risk of triggering extrinsic ageing processes, such as partial discharges, especially during voltage transients [3]. Since conductivity drives the electric field under a DC steady state, and it contributes to determine the field behavior during transients (together with permittivity), measurement and modelling of conductivity behavior as a function of electric field and temperature constitutes the basis of building up accurate models.…”

An innovative approach to partial discharge measurement and analysis in DC insulation systems during voltage transient and in steady state

“…As an example, Figure 4 shows the PD phenomenology for the same defect under DC steady-state voltage and AC sinusoidal voltage, just above the relevant PDIV (note that, according to (6) (7) and Figure 3, PDIV DC is generally much higher than PDIV AC at room temperature, for typical DC insulating dielectrics [12,13]). An expert, upon denoising (the patterns can be denoised by the innovative algorithm described in the next section) can recognise from the phase-resolved PD pattern that the PD source is an internal defect.…”

“…Figure13 and Figure14show the results of PD monitoring on a DC defective cable from the beginning of energisation to DC steady state. The nominal voltage was chosen to exceed both PDIV AC and PDIV DC .The global TRPD pattern is reported in Figure13a.…”

Investigating Optimal Approaches for Energizations of DC Systems: The Stepwise Technique

“…Electric field transients, consequence, e.g., of voltage steps, may last long times, from minutes to hours [2,3]. Hence related phenomena (as field magnification along insulation thickness, [4], and partial discharges, [5][6][7][8]) could affect insulation with accelerated intrinsic and extrinsic aging, possibly in non-negligible way, considering that a DC insulation system can be subjected to many thousands voltage transients during operation life [9].…”