More insight into the effects of load cycles and electrothermal stress on HVDC extruded cable reliability in the prequalification test

Abstract

The long-term prequalification (PQ) test of HVDC extruded cable systems according to CIGRE Technical Brochure (TB) 496 consists of a one year long series of "24 hours" load cycles. The duration of this load cycle test relies on a design life of 40 years at constant rated voltage and on an IPM voltage-time characteristic, without any explicit consideration of thermal and synergistic electro-thermal aging effects related to load cycles. Here, as a sequel of a former investigation, a previously proposed method for estimating failure time percentiles of HVDC extruded cables that accounts for the time-varying temperature during the PQ test load cycles is refined. Moreover, the application of the method is broadened for assessing the sensitivity of a typical HVDC cable design to the electrothermal stress levels within insulation thickness, as well as to the variation of insulation resistivity dependence on temperature and electric field. Index Terms--HVDC Extruded Cables, Prequalification Tests, Electrothermal Life Model. I. INTRODUCTIONS far as long-distance, high-power underground or underwater transmission is concerned, High Voltage Direct Current (HVDC) cable links are in general either the best or the only option. Thanks to recent R&D efforts, the voltage rating of extruded HVDC cable systems has now raised up to 300 kV and above, and commercial HVDC extruded cable system projects at the 320 kV level are being undertaken [1], [2]. In this perspective, the new CIGRE Technical Brochure (TB) 496 [3] has extended up to the 500 kV level the "Recommendations for testing dc extruded cable systems" that in former TB 219 were holding up to a rated voltage of 250 kV.According to CIGRE TB 496, the long-term prequalification (PQ) test of HVDC extruded cable systems has to be carried out applying to a proper cable loop, under constant DC voltage, a sequence of 360 daily load cycles, since the longterm PQ test "…is intended to indicate the long-term performance of the complete cable system" [3]. The sequence differs between cable systems for Current Source Line Commutated Converters (LCC) and for Voltage Source Converters (VSC), and includes full load, zero-load and heating-cooling cycles.The duration of the long-term load cycles PQ (LCPQ) test G. Mazzanti is with ). M. Marzinotto is with TERNA Rete Italia S.p.A.,

Life Estimation of HVDC Cables Under the Time-Varying Electrothermal Stress Associated With Load Cycles

“…Indeed: 1) a two-step temperature profile was set up throughout the LC period of the PQ test; thus, the transient electric fields and the loss-of-life fractions were strongly inaccurate; 2) an approximated version of the electrothermal life model (18) was employed; 3) one single cable different from those treated here was analyzed; 4) no sensitivity analysis was performed like that included below. For all of these reasons, the results in Table II differ quantitatively from those in [23], with larger differences in the loss-of-life fractions and lives between the inner and the outer insulation. Thus, it can be concluded that the new procedure proposed and applied here is essential for a sound life estimation of HVDC cables in the presence of load cycles.…”

“…An analysis similar to the present one was carried out in [23], but the approach was far less rigorous than here. Indeed: 1) a two-step temperature profile was set up throughout the LC period of the PQ test; thus, the transient electric fields and the loss-of-life fractions were strongly inaccurate; 2) an approximated version of the electrothermal life model (18) was employed; 3) one single cable different from those treated here was analyzed; 4) no sensitivity analysis was performed like that included below.…”

Dynamic Thermal Analysis of High-Voltage Power Cable Insulation for Cable Dynamic Thermal Rating

“…To increase the power flow of a cable while still maintain a satisfied safety margin, dynamic thermal rating methods are proposed [8]- [12]. These methods calculate the maximum conductor operating temperature on the basis of instantaneous cable loading and environment conditions [13]- [15].…”

Introducing Mutual Heating Effects in the Ladder-Type Soil Model for the Dynamic Thermal Rating of Underground Cables

“…Better modeling, more advanced grid infrastructure, and faster computation capabilities in the context of smart grid operations lead to the necessity of determining the real-time ratings of power cables [8], [12]. Also, current research shows that accurate real temperature transients can be used to determine the useful life of cables in service with dedicated life models [15], [16]. This is very important for electric utilities because it directly impacts their asset management strategy.…”