Transmission of electric energy

Transmission of electric energy 

The main objective of the power system is to provide electrical energy from power source to the consumers in a safe and reliable way at the lowest possible cost  . The bulk movement of electrical energy from a generating site, usually in remote areas, to an electrical substation near cities is called the electric power transmission. This is possible with the help of interconnected lines facilitating this movement known as a transmission network. The electrical energy received in HV substation is then transferred to customers through local wiring known as electric power distribution. The combined transmission and distribution network is known as the « power grid ».

The electrical distribution systems were somewhat fully developed during the twentieth century by connecting the consumers and generators using national and international grids. Despite the development, the current electrical transmission network needs to be strengthened to transmit huge amounts of power long distances across continents. In North America, the power grid is highly integrated as there are over 35 electric transmission interconnections between the Canadian and US power systems. This integration is set to continue expanding, with multiple cross-border transmission projects currently being developed.

Throughout the grid, electricity is being transmitted at high voltages (>115 kV) to reduce the energy loss. The two main means of HV transmission are overhead and underground power transmission lines. HV overhead transmission lines are a reliable, low-cost, easily maintained and established method to transport bulk electricity across long distances. Their conductors (aluminum or copper) are not covered by insulation and are, therefore, exposed and vulnerable to adverse weather conditions.

Throughout the grid, electricity is being transmitted at high voltages (>115 kV) to reduce the energy loss. The two main means of HV transmission are overhead and underground power transmission lines. HV overhead transmission lines are a reliable, low-cost, easily maintained and established method to transport bulk electricity across long distances. Their conductors (aluminum or copper) are not covered by insulation and are, therefore, exposed and vulnerable to adverse weather conditions.

The use of HV power cable is increasing in recent years. Increasing of population of urban areas in industrialized countries has led to the increasing of energy consumption where the use of power cable is the only viable option. Power cables eliminate the environmental problems that are associated with the overhead transmission lines. Many developing countries have changed their power system network to meet the increasing of demand by using power cables. Also, parts of the existing power cable networks have reached the end of their lifetime and need to be replaced.

Extruded HV power cables

A cable includes a conductor and insulation, and is suitable for being run underground or underwater. High voltage power cable has a common design, independent of its operating voltage and frequency. Basically it consists of the conductor, the insulation, the inner and outer semi-conductive screens, earthed metallic screen and protection sheath that form long concentric cylinder.

The insulation is the most critical part in cable structure due its crucial task to withstand a long term electrical stress during the service life of the cable. The use of extruded synthetic insulation in single layer construction is increasing due to its advantages in relatively easy processing and handling of this insulation. This insulation can be selected to have 10% lower dielectric losses than cellulosic paper, higher intrinsic breakdown strength four times as high as impregnated paper insulation (Ryan 2001). The disadvantage is that a single defect can produce large influence on the whole insulation due to its homogeneity of this type of insulation (Ryan 2001).

HVAC versus HVDC systems 

The first high voltage transmission line goes back to 1882, thanks to Thomas Edison, when a 45-km High-voltage direct-current (HVDC) link was constructed to connect Miesbach and Munich using rotating DC machines at each end. Later on, alternating-current generation, transmission, and utilization started to be dominant (Long and Nilsson 2007). They were realized to be more favorable because of benefitting from efficient and easy-to-manufacture transformers instead of high cost convertors that are necessary in DC lines. Voltage conversion in AC systems is simply via AC transformers achieved with low losses and little maintenance that allows high power and insulation levels within one single unit. Therefore, shortly after AC technology was introduced, it was accepted as the only feasible technology for generation, transmission, and distribution of electrical energy (Siemens 2011).

However, the inductive and capacitive elements of cables limit the transmission distance of AC transmission links. There are induced loss in all parts of AC cables. Also direct connection between two AC systems with different frequencies is not possible. Whereas HVDC transmission lines have no range limit, can be directly connected, and their only loss is the ohmic loss in conductor.

Insulating materials for HV cables 

Apart from mechanical stability and extrudability, a dielectric to be chosen for realizing the insulation of both HVAC and HVDC cables should have high breakdown strength and lowest possible thermal resistivity. HVAC cable insulation must show low losses, while in HVDC systems low space charge retention properties are important.

Most of the cable insulation materials for both AC and DC applications are based on polyethylene (PE). PE is a semicrystalline polymer that has good electrical properties (low dielectric constant, low dielectric loss, and high breakdown strength) together with other desirable properties such as mechanical toughness and flexibility, good resistance to chemicals, easy processing, and low cost. Its main drawback is the low melting temperature. This restricts the maximum operation temperature to 75°C. To improve this property, PE is cross-linked (XLPE). Crosslinking increases maximum operation temperature to 90°C, the emergency temperature to 130°C, and the short circuit maximum temperature to 250°C. Crosslinking also increases impact strength, dimensional stability, tensile strength, thermal properties, chemical resistance, and it improves electrical properties, aging, and solvent resistance of polyethylene. However, crosslinking makes XLPE a thermoset polymer, therefore non-recyclable. This is a drawback that cannot be easily tolerated nowadays as the trend is that the environmental issues must be addressed. In addition, the cross-linking by-products within XLPE can create an irregular distribution of the dielectric stress and often cause the formation and growth of storage centers of space charge that remains trapped within the dielectric.