EVALUACION Y RETROALIMENTACION DEL PROCESO

The cable is the most necessary component for electrical connection of the wind farm, whether the power is transmitted as AC or HVDC. The main conductor material of choice is usually aluminium or copper. Copper has 70% higher conductivity than aluminium, but 3.5 times more expensive (based on May 2015 metal prices). The material and thickness of the conductor will determine the ampacity of the cable.

Cable Insulation Material

The material and thickness of the cable insulation will determine the voltage rating of the cable. The most popular cables for underground and sub-sea applications are mass impregnated non-draining (MIND) cables and cross-linked polyethylene (XLPE).

MIND cables have very high voltage and power ratings, up to at least ±500 kV DC and 2000 MW for bipolar operation. They are traditionally used in LCC-based HVDC because the system needs to be large scale and high power to be economic viable.

The XLPE material for insulation is the popular cable insulation material of choice for VSC-based HVDC systems because it is cheaper, lighter, more flexible and easier to manufacture. Despite a limited maximum voltage level of up to ±320 kV the XLPE can withstand and about 1000 MW power rating, it is therefore ideal for VSC-based HVDC systems which are of similar ratings. Their converter stations are already very expensive with limited power rating, therefore it is difficult to justify using expensive MIND cables. Furthermore, VSC does not reverse in polarity as LCC does when reversing the power flow direction. Therefore this reduces the influence of polarity reversal on the space charge accumulation that decreases the insulation performance of polyethylene [47].

Moisture ingression into the XLPE insulation material will increase the forma- tions of ions and therefore increase the conductivity of the material [48]. Impurities, cavities and defects in the insulation also increases the conductivity and shortens the life of the cable. It becomes inclined to water tree breakdown, which is a failure mechanisms in XLPE cables as the breakdown voltage is significantly reduced [49]. Overall, the voltage rating of XLPE is lower than MIND cable insulation for a given thickness.

Other Layers in the Cable

A semi-conducting screening layer between the conductors and the XLPE insulation layer is often necessary to distribute the electrical stress. This helps to prevent electrical discharge that can damage the insulation at certain points of the cable. An armour protection layer is usually needed for sub-sea applications to minimise mechanical damage caused by the likes of fishing trawlers and ship’s anchors.

Circulating currents and eddy current can be induced into the metal sheath layer and armour outside the insulation layer of the conductor, contributing to further losses in the AC cable power transmission, which can be 30% of the total, but sometimes this is required as the metal sheath layer prevents moisture ingression and acts as an electrostatic shield and a current return path [50].

Cable Cross-sectional Formation

A three-phase cable in trefoil formation has reduced magnetic field around the con- ductor because the fields from each phase cancel each other out. Therefore its net cable inductance and sheath armour losses are smaller. This is also why the armour of a three-core cable can be made of steel-wire while it should be non-magnetic for a single-core cable. However, high voltage XLPE cables above 275 kV are not avail- able in the three-core trefoil format because thicker insulation material is required, which can then affects the overall size.

Single-core cables are more flexible, has minimal mutual heating and improved cooling with the wide core separation. As a result, this increases the current rating of the core conductor as shown in Figure 2.8. However, it may not be efficient

2.1. The Grid Connection and Power Electronics 27 overall because the armour losses are higher. The current induced into the armour can be as high as the currents in the conductor [51]. However, the core conductor and insulation material layer can be made thicker per single-core cable, for higher current and voltage ratings and therefore higher power capacity.

DC cables on the other hand, do not have the additional losses associated with the AC frequency, therefore the current carrying capacity of a DC pair with close spacing is comparable to a set of AC cables with wide spacing.

0 500 1000 1500 2000 2500 0 200 400 600 800 1000 1200 1400 1600 Cu rr en t ra tin g (A )

Conductor cross sectional area (mm2₎

DC pair, wide spacing DC pair, close spacing Single core AC wide spacing Single core AC, close spacing Three-core AC

Figure 2.8: The current carrying capacity for various types of copper cables against the conductor cross sectional area, data source: [51].

Capacitance in the Cable

There is significant capacitance in a power cable compared to overhead lines. How much extra capacitance also depends on the separation space between the conductor and outer sheath layer and their surface area. Generally the capacitance decreases with higher voltage ratings because the insulation is thicker, and with smaller current ratings because the conductor diameter is smaller and therefore the circumference surface area is smaller.