Ageing and reliability of electrical insulation: the risk of hybrid AC/DC grids

Abstract

With an impetuous growth of DC links and future grids that will be often hybrid, providing AC and DC supply to nearby or the same customer, an important element is often neglected, that is, electrical insulation. There are no doubts that overhead lines will step back compared to insulated cables, for known reasons of environmental impact, right of way and reliability, especially in an urban or harsh environment. On the other hand, while AC insulation systems have been investigated and used for decades, there is no analogue experience for DC polymeric insulation, nor for insulation systems subjected to power electronics supply. This study aims at enhancing the attention on this topic, focusing on insulation ageing mechanisms and models of relevance for hybrid AC/DC assets and grids. Ageing and life models for AC, DC and power electronics supply are discussed and compared to experimental data, focusing on partial discharge and space charge. It is shown how these two phenomena affect lifetime for different AC, DC and transient operating conditions. The main drivers for insulation design and material selection are discussed, showing that insulation reliability can be achieved only if operating stresses, ageing mechanisms and life models are known and accounted for properly by insulation designers.

A Study on the Life Estimation and Cavity Surface Degradation Due to Partial Discharges in Spherical Cavities within Solid Polymeric Dielectrics Using a Simulation Based Approach

“…The values between parentheses in Table 3 correspond to the 95% confidence intervals. As the damage in the cavity surface accumulates along the ageing phases, the time-to-breakdown under the last ageing phase can be calculated using the Miner law as follows [20]:…”

“…On the other hand, nowadays applications such as hybrid AC and DC transmission schemes and nonlinear loads impose stresses conditions different to those considered at rated conditions. In hybrid AC and DC systems, dynamic ageing and life modeling is required because ageing factors and mechanisms are changing depending on power planning and dispatch constrains [20]. Under DC voltage applied, the field distribution is different from the case of AC voltage, which causes that the inception of PDs activity at internal cavities is variable depending on their location and the type of applied voltage, AC or DC.…”

“…Under DC voltage applied, the field distribution is different from the case of AC voltage, which causes that the inception of PDs activity at internal cavities is variable depending on their location and the type of applied voltage, AC or DC. In addition, at DC, the PD inception voltage can become lower than under AC because the conductivity increases due to the temperature rising by load increments [20]. Harmonic components are also able to increase the thermal and electrical stresses due to superposition with the fundamental component causing increments in the PD rate, the PD pulse magnitude and, consequently, the degradation rate [21].…”

New Method to Estimation Electrical Stresses on 300kV OHTL/Cable Joints Due to Switching Operation Using ANN to Determine the Insulation Lifetime

“…Different graphs and equations are produced to estimate the lifetime of the insulation material that are operated under repeated TOV condition. These methods need TOV amplitude, frequency and number of pulses as input [12] [21] [22]. In case of TOV generated by switching an OHTL has an interface point with cable, usually there is no metering system to capture the required input at the OHTL/cable joints.…”

Charging and Discharging Current Measurements and Impact of Polarization Dynamics on Electric Field Modeling in HVDC Paper-Insulated Cables

“…There are two major factors causing extrinsic, accelerated electrical insulation ageing under DC supply [1], that is, space charge (SC) and partial discharges (PD). The former is responsible for the distortion of the Laplacian electric field that may lead to considerable local field magnification, increasing unpredictably electrical stress and, thus, reducing life [2,3,4].…”