The global interest on HVDC power systems has seen a significant growth in the last few years – and has assumed greater importance. HVDC power transmission systems have gone through remarkable developments because of the increasing need for the transmission of bulk power at higher voltages and over longer distances with reduced power loss, least amount of right of way, and better stability and control in the transmission system. HVDC is a proven technology for transmission projects that interconnect asynchronous networks. Today, HVDC land transmission technologies are used to carry electricity over long distances by a) HVDC overhead lines to carry high power (>1,000 MW) over distances above 200 km and b)HVDC underground cables to carry medium and high power (100 MW – 1,000 MW) over distances above 50 km. Few typical lines are: The Itaipu HVDC transmission overhead line in Brazil consisting of two bipolar DC transmission lines of length 785+ 805 km and operating at +/- 600 kV DC is by far the most impressive HVDC transmission.The 350 kV monopolar HVDC link connecting the geothermal plant on the island of Leyte and southern part of main island of Luzon feeding AC grid in the Manila region is of length 430 km. Xiangjiaba-Shanghai +/- 800 V DC, 6400 MW, 1953km commissioned in 2010 is the world’s largest UHV DC transmission system. Some of HVDC lines in India are the Rihand-Delhi HVDC bipolar 500 kV DC transmission link, having rated capacity of 1500 MW is of length 814 km, Chandrapur-Padghe bipolar line of transmission is of length 753 kms with capacity 1500 MW at 500 kV, Talcher-Kolar 500 kV DC transmission line of length 1836 kms has capacity of 2000 MW and many more lines are in progress.
Converter stations are required to connect to the AC system which entail significant investment costs. The new generation of converters (VSC–Voltage Source Converters) use IGBT (Insulated Gate Bipolar Transistors) which allow the power to be transmitted as it is in both directions without requiring polarity reversal. This has allowed re-introducing the use of extruded cables in DC power transmission. The cost factor for the HVDC cable is only to 2 – 3 compared to HVDC overhead line of the same capacity. So far, HVDC cables have mainly been used in submarine applications, either connecting offshore wind farms to land or transmitting electricity over long distance through the sea where overhead lines cannot be used. Now, HVDC cables are also being used for land transmission projects. HVDC undergrounding can safely transport high power loads over long distances with minimal losses. In addition to transport efficiency, only a limited number of cables and joints are required, hence allowing narrow trenches. Globally, long distance HVDC underground transmission cable system projects have been realized, some are in progress and many others may be planned for the near future.
Prospects of HVDC technology
Some of the advantages of HVDC Systems are:
- Can carry more power for a given size of conductor
- Both overhead lines and underground cables can transmit power without any distance limitation.
- Better stability and very fast control of power in the transmission system
- Direction of power flow can be changed very quickly (bi-directionality).
- The need for ROW (Right Of Way) is much smaller for HVDC than for HVAC, for the same transmitted power.
- The environmental impact is reduced with HVDC.
- VSC technology allows controlling active and reactive power independently without any need for extra compensating equipment.
- VSC technology gives a good opportunity to alternative energy sources to be economically and technically efficient.
- HVDC transmissions have a high availability and reliability rate.
HVDC extruded cable technology
High Voltage Direct Current (HV DC) cable technology has features which make it specially attractive for certain transmission systems like long distance, bulk-power and long submarine cable crossings. In case of long distance transmission with HV AC Cables, the capacitive currents which are proportional to the length of the cable, the cable capacitance and the frequency of supply, amount to 10 – 25 A/Km and become large in comparison to the currents that have to be transmitted. For long lengths above 60-80 Kms the capacitive currents become equal to the active current that the cable has to transmit and hence the losses are very much higher and further the current rating of the cable is reduced drastically. With DC Transmission, the frequency of supply is zero and only the conductor resistance plays a major role.
However, the electric field distribution at DC Voltage differs greatly from that for AC Voltages, whereas in AC cables the field distribution within the cable insulation is distributed capacitively in accordance with the permittivity of the cable insulation. The field distribution at DC is determined by the conductivity of the insulation, which is not constant and depends strongly upon the temperature and the electric field. Surface charges and space charges also play an important role. Consequently the fields in DC Cables are space, temperature and time dependent. This makes the determination of the field at DC voltage more complex than in an equivalent case with AC voltage.
History of HVDC cable development
The world’s first operation of HVDC power transmission began in the year 1954 connecting the mainland of Sweden and Island ‘Gotland’. During the period Mass Impregnated (MI) cables (insulating paper impregnated with high viscosity insulating oil) were used for medium voltages while Oil Filled (OF) cables (using insulation paper which was impregnated with low viscosity insulating oil and kept in pressurized condition) were used for higher voltage and larger capacity applications. Since then, MI and OF cables have been the mainstream of DC power transmission cables. Examples of a mass impregnated, paper insulated cable and an oil filled, paper insulated cable are shown in Fig 1 and 2 respectively.
In the year 1999 XLPE extruded cable designed to operate at 80 kV was used in Gotland. Here, a Voltage Source Converter (VSC) was used as an ACDC converter. Polymer insulated cables which are lighter, easier to handle and also offer easier jointing than Paper Insulated Cables can therefore now be used with advantage. With XLPE insulated cables, a higher conductor temperature can be used, and this allows a smaller size of cable to be used with a more compact construction. Additives traditionally used for HV AC Cables such as anti-oxidants, cross linking agents, lubricating additives, those used for increasing cross linking density and scorch resistant additives can be selectively used in various combinations to produce XLPE Insulation for HV DC Cables. Currently HVDC extruded cables based on XLPE insulation are designed to operate at 320 kV and around 1000 MW. The construction of a typical HVDC, XLPE extruded cables is shown in Fig.3. An aluminium or copper conductor is covered with a thin semicondcting layer to have a smooth interface to the following insulation layer. A second semiconducting layer covers the insulation. Depending on the application and design other layers are added to the cable. For exmple in case of damage to the cable, swelling tape absorbs water and expand, blocking the water from moving axially along the cable. The screen wires are grounded. Aluminium laminate provides a diffusion barier against foreign substances, especially humidty. And finally the covering sheath jackets all the layers. An extruded lead sheath as water barrier and armoring wires provide the mechanical protection to the cable. Some of the leading HVDC cable manufacturers are ABB, Sweden; Prysmian Cables, Nexans, France, Germany, Norway; J-Power, Japan; LS Cable, Korea; Europa cables, Brussels; Brugg Cables, Switzerland and many other manufactures.
Development of high voltage DC-XLPE cable system
The history of cable development is as follows
- 1984~1989 : Fundamental research on DC insulation material
- 2007~2010 : Establishment of manufacturing technology of practical DC-XLPE cable
- 2005~2006 : Verification on long-length extruding of DC-XLPE….
- 1988~1995 : Development of DC 250kV cable and factory joint
- 1993~2001 : Development of DC 500kV cable and factory joint
Materials for HVDC extruded cable insulation
At the beginning of the era of extruded HVDC insulation, materials like LDPE, XLPE – which are used for HVAC applications were tried with different thermo-chemical treatments such as cross-linking and or mixed with proper additives. Even currently the materials used for HVDC cables are based on PE are used. Further, development of extruded DC cables has gained attention since 1990s with the use of modified semi conductive and insulating materials for minimizing the space charges under HVDC field. Peroxide cross-linkable polymer systems based on high pressure polyethylene resins or low-density polyethylene compounds with nano-fillers are being explored.
The insulation of HVDC cables should have the following properties:
- Stable Insulation resistivity: i.e., Insensitive to variations of temperature, electric stress, polarity reversals, and constant value during the time of electrification.
- Low thermal resistivity.
- Low space charge retention properties.
- High DC breakdown strength, particularly superimposed impulse conditions and insensitive to temperature and polarity reversals.
Characteristics of polyethylene & XLPE
Polyethylene is the simplest hydrocarbon in polymer and a typical example of synthetic polymer. It is obtained from the polymerization of ethylene. Polyethylene is classified as linear, branched and cross linked polymers. The linear Polyethylene is constituted as long chains, tied up through weak interactions (van der walls bonds) and feature the highest crystallinity degree.
The branched Polyethylene’s are new chains that develop from the intermediate points of the native chain and weak van der walls interactions are established since the chains are less packed; the degree of crystallinity is now lower than the linear polymers. Cross linked Polyethylene are three dimensional structures with primary bonds in all directions. Such materials are hard, mechanically resistant, non-fusible and insoluble in all the reagents and are called thermosets.
Cross linking, which also called vulcanization, is usually realized chemically through peroxides. Such peroxides are activated at elevated temperatures, giving rise to reactions among residual functional groups of different molecules, with the formation of more complex molecules and the definite hardening of the product.
On the contrary, linear and branched Polyethylene can be formed at high temperatures in order to exploit their higher plasticity at such temperatures, without involving further modifications of their molecular structure. The high temperature weakens the van Der Walls bonds, so that the material resembles a paste and can be shaped. Therefore, linear and branched Polyethyleneare denoted as thermoplastic resins and contrary to the thermosetting resins.
Polyethylene vary in molecular weight and degree of branching, with the highest branching occurring in LDPE. Other PE families are unbranched or high density PE (HDPE) and short branched linear low density PE (LLDPE). The polyethylene commonly used of HV insulation process is XLPE, which is LDPE cross linked with an organic peroxide, typically dicumyl peroxide. In the production of XLPE, methane, acetophenone, and cumyl alcohol are among the by-products of the decomposition of dicumyl peroxide.
HVDC extruded cables: challenges
The design of insulation for HVDC extruded cables is one of the most challenging issues, mainly due to the dependence of its dielectric properties on temperature, electric field distortion due to accumulation of space charge in the insulation. Space charge occurs whenever the rate of charge accumulation is different from the rate of charge removal, and may arise due electrons and ions. Space charge occurs due to injected charge carriers at the electrodes, whether mobile or trapped charge (already present in the bulk of the cable insulation) due to three processes in a dielectric under an electric field, namely: a) variation in conductivity, b) ionization of species within the dielectric and c) charge injection from the electrodes driven by a DC field not less than approximately 10 kV/mm and polarization in structures such as water trees. Of these phenomena, the first two contribute most to the space charge accumulation.
Space charges are also formed due to the presence of ionic dissociable additives, impurities, dislocations and chemical or physical defects. Thus, it has become a major challenge to the researchers all over the world to develop a space charge free insulating material with good dielectric properties.
Issues with space charges
It is well established that the electric field distribution over the extruded cable insulation is strongly affected by space charge, which can control the cable system behavior, in particular, its long-term reliability and life expectancy. When a DC voltage is applied across a cable dielectric, space charge accumulates in it at a rate which primarily depends on the voltage level and the properties of the cable insulation and of the conductors. If the space charge density becomes sufficiently high, the local field strength may exceed the breakdown strength of the extruded insulation, leading to insulation failure. This includes distortion of electrical stress distribution due to space charge formation and accumulation, electrical treeing initiation, breakdown as well as insulation aging. Research on the behaviour of space charges in extruded solid cable materials has gained a lot of interest during the last three decades. New materials and application environments set the challenges for improved reliability of extruded insulation and better understanding of the properties of insulation is very essential to achieve this goal.
Mechanisms of formation of space charges
The formation of space charges in extruded cables is associated with the injection of charges from the electrodes involved in the transfer of electrons (and holes) through the electrode-polymer interface. This process is highly dependent on the conditions of the interface, including the electrode material, the surface defects the impurities and the oxidation level. The formation of space charges in extruded cables is associated with ionization of some chemical species. These species can be introduced during manufacturing of the material, such as antioxidants, or cross-linking by products, or other impurities.
For space charge accumulation to take place, the injected charges and / or the charges already present in the polymer must be trapped in the material; thus the phenomenon depends on the availability and nature of traps. The residence time of a charge carrier in a trap depends on the trap depth, the energy required to extract the carrier, the temperature, and the applied electric field. Polymers exhibit both ‘shallow traps’ and ‘deep traps.’ In polyethylene a depth of traps ranging between 0.1 and 1.4 eV has been found. The characteristics of the material have an influence on the trapping process, e.g., the degree of crystallinity, oxidation, impurities, lattice structure defects (both chemical and physical) and by products from the cross linking reactions.
Commercially available PE contains polar impurities such as phananthrene, benzoic acid and benzophenone, which may be oriented by the external field to give anet dipolar polarization. The presence of space charge at the trapping sites such as crystalline-amorphous interfaces, may produce regions of high electrical stress, leading to breakdown of the PE insulation used in HV applications.
Methods for space charge measurement
Space charge measurement methods have been developed and extensively used for laboratory investigation on samples like plates and small length cables, which help to increase the understanding of the intrinsic behaviour of the dielectrics. However, they do not permit to completely take into consideration synergetic effects such as electric fields, byproducts, temperature gradients and semiconductor/ dielectric interfaces on the space charge evolution within the insulation of the cable systems. Further, using thermal step method an industrial facility to take into consideration the synergetic effects on the space charge distribution in the cable insulation is reported.
The space charge measurements are classified into two categories, namely thermal methods and pressure methods.
Thermal methods: Thermal pulse method, Laser Intensity Modulation Method {LIMM} and Thermal Step Method (TSM),
Pressure pulse methods: Some of the pressure methods are a) Pressure Wave Propagation method (PWP), b) Laser Induced Pressure Pulse method (LIPP) and c) Pulse Electro Acoustic method (PEA).
Qualification tests on HVDC cables
The extruded HVDC cable system is qualified according to international standards and recommendations. The latest document governing the qualification of extruded HVDC cables is the CIGRE Technical Brochure (TB) No. 496, which was issued in April 2012. Mechanical testing and other tests not specific to HVDC cables are based on IEC standards –whereas the electrical testing is in TB 496.
Central Power Research Institute (CPRI) is a government of India organization which was set up in 1960 and is the the power house of the Indian electrical industry. It has has full-fledged facilities in the areas of testing andevaluation of power cables upto 400 kV (AC) covering different type of cable insulation. Recently, the laboratory has been augmented with facilities like 600 kV, 4200 kVA ac test system, 600 kV voltage dividers, 2400 kV, 240 kJ Impulse generator, partial discharge detector, dissipation factor bridges, 600 kV standard capacitors etc to conduct long term ageing test ‘Pre-qualification test’ on EHV cables. Some of the available test facilities for EHV cables at CPRI, Bangalore are shown in figure 5, 7 , and 8. Further it is proposed to set up additional test facilities like Polarity reversal test and superimposition tests which are required for qualification of HVDC extruded cables.
Conclusions:
In conclusion, there is a great potential for development of extruded HVDC cables globally. The design of insulation for HVDC extruded cables is one of the most challenging issues, mainly due to the dependence of its dielectric properties on temperature, electric field distortion due to accumulation of space charge in the insulation. Even if the main insulation is XLPE, it is necessary to keep in mind that history of insulation from melting to extrusion and amount of peroxide and anti-oxidant could modify microstructure and so properties of the polymer.
Due to complexity of the phenomena involved (injection trapping /extraction etc) it is not easy to predict the distribution of space charge in the cable and these aspects are great challenges for the cable manufacturers. The Indian cable industry should come forward and network with research organizations in developing indigenous technology to design, develop and manufacture HVDC cables. The space charge issues should be addressed by looking for alternate nono materials and fillers.
The test facilities available at CPRI would be of immense use to Indian cable manufactures and utilitiesfor development of HVDC cables. Active participation of the Indian manufacturers will be highly beneficial to the Indian Power Sector.
