# Dynamic Rating of Three-Core XLPE Submarine Cables for Offshore Wind Farms
## 1 Introduction  
It has not always been as challenging as it is today to match energy generation levels with consumers' power consumption patterns. Energy sources based on fossil fuels are being replaced by renewable power generation, such as wind energy, to reach one of the global climate goals of achieving at least a 27% share of renewable energy by 2030 [1]. Therefore, it is essential to investigate methods to reduce the cost associated with the transmission of renewable power generation. Multiple studies have discussed the application of Dynamic Line Rating (DLR) forecasting to already constructed lines to optimise transmission capacity [2; 3]. The market for renewable power generation is growing fast, and the number and size of offshore wind farms have increased rapidly in the past several years. According to the Global Wind Energy Council, the annual installed global wind energy capacity has increased from 6.5 GW in 2001 to 52.4 GW in 2017 [4]. The disadvantage of such renewable power generation is the uncontrollability, as the power production varies with the speed of the wind. In order to carry massive amounts of power through a submarine cable (shown in Figure 1) connecting an offshore renewable power generation source, such as a wind farm, to an onshore station and simultaneously minimise the levelised energy costs (LEC) [5], it is essential to select the most suitable cable for each specific case. A way to achieve this is to change the design phase and dynamically rate cables on the basis of a worst-case estimation of varying load profiles and surrounding conditions (shown in Figure 1) of different cable environments. Such changing variables include burial depth \(L\), thermal soil resistivity \(\rho\) and ambient temperature \(\theta_{A}\). High voltage alternating current (HVAC) submarine cables are developed to carry a great amount of power across the water. The cable conductors consist of copper (Cu) or aluminium (Al), depending on size and price. Cross-linked polyethene (XLPE), with a maximum operational temperature of 90 \({}^{\circ}\)C, is a commonly used insulation material. A common construction of a three-core XLPE separate lead (SL), sheath-type submarine cable is shown in Figure 2, where the internal part of the cable contains \(l_{1}\): Conductor, \(l_{2}\): Conductor screen, \(l_{3}\): Insulation, \(l_{4}\): Insulation screen, \(l_{5}\): Swelling tape, \(l_{6}\): Metallic sheath/screen, \(l_{7}\): Anti-corrosion sheath and the common covering, containing \(l_{8}\): Bedding, \(l_{9}\): Armour and \(l_{10}\): Outer serving. The element \(f_{1}\) is the optical fibre used for communication and distributed temperature sensing (DTS) [6], and \(f_{2}\) represents the fillers, which are usually filled with water while in operation. Figure 1: Submarine cable installations between onshore grid (Bus 1) and wind turbine generator (WTG) foundations illustrating surrounding conditions: onshore, at the beach landing and offshore. Figure 2: General structure of a three-core cross-linked polyethene (XLPE) separate lead (SL)-type submarine cable with all layers included; diameters \(D\), thickness \(t\) and layers \(l\) are illustrated and labelled. The static rating of HVAC cables can be calculated by using a series of standards from the International Electrotechnical Commission named IEC 60287 [4] and has been historically conservative as it is based on worst-case assumptions and does not take into account real-time changes of surrounding conditions [7]. Renewable power generation such as wind energy delivers an unpredictable production profile that varies from no- to full-load several times within 24 h. In Figure 3, a typical current profile from a wind farm in Denmark is shown. By considering the thermal behaviour caused by the production variation, the cable parameters can be designed according to the actual load, which is the peak value of the actual flowing current \(I_{a}\), instead of using the steady-state rated current \(I_{r}\).