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1. INTRODUCTION
It is true for both AC and DC transmission systems that in order to deliver bulk power over long distances in an efficient and economic manner, higher and higher voltages are being adopted. This section describes the drivers towards the adoption of high voltage direct current (HVDC) transmission systems such as the monopolar Cabora Bassa, Mozambique to Johannesburg, South Africa (6500 kV DC, 1,420 km, 2,000 MW) link commissioned in 1979 and the Brazilian Itaipu/San Paulo (6600 kV DC, 800 km, 6,300 MW) 23bipolar link (with accompanying parallel 765 kV AC, 60 Hz, series compensated lines) fully commissioned in 1990, and advantages associated with underground and submarine cable HVDC links such as the Murray Link, Australia (6150 kV DC, 180 km, 220 MW). The section goes on to describe the main HVDC system components, AC/DC interaction issues and the emerging trends and technologies.
2. HVAC VS. HVDC TRANSMISSION SOLUTIONS
2.1 Technical Issues
The efficient transmission of AC power over long distances may be limited by system stability limits (see Section 1) and parallel path loop flows. In scenarios where it is difficult to connect two AC networks (due to their asynchronous nature, different nominal frequencies or other such stability reasons) HVDC may be the only feasible solution.
The reactive power flow in long (.50 km, say) AC cable circuits limits the active power transfer. For land cable circuits this effect may be compensated by installing intermediate shunt compensation. However for submarine cables HVDC links may be the only viable alternative.
HVDC systems have the advantage of limiting short circuit levels and fault propagation. They provide negligible contribution to the short circuit levels. Faults and oscillations do not transfer across HVDC interconnected systems, thereby helping to minimize system disturbances.
HVDC power flow controllability may be pre-determined and may be independent of phase relationships. This avoids the inadvertent overloading or under-utilization that may occur in AC systems. An HVDC link may also be employed as part of the support to an AC system so as to improve the stability of interconnected systems by modulating the power in response to minor or major system disturbances.
Transmission line wayleave applications may require environmental impact assessments. It is possible that a DC link may have lower visual impact than an AC equivalent. However other environmental issues such as audible noise, electromagnetic fields, electromagnetic compatibility and especially ground return paths are all issues that will need to be addressed in the planning stage.
A useful checklist to assess the technical applicability of HVDC systems is shown in Table 1.
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TABLE 1 HVDC Transmission Link Applicability Checklist
Parameter
Possible Issue for Consideration (Y/N) in Particular Application
Power transmission range (50 MW UG cable to .1,000 MW OHL)? Need for accurate and fast control? Cable distance ($50 km, say)? OHL distance ($500 km, say)? Wayleave Environmental Impact Assessment (EIA) issues may require a cable link? (Small wayleave footprint, lower visual impact, etc.) Connection between asynchronous networks? Connection between weak AC networks? Risk of dynamic instability leads to preference for DC link? Power quality issues (however note need for harmonic filters with DC converter equipment)? Need for grid 'black start' capability? Technical issues (altitude/weather _ thunderstorms, ice buildup, etc.) arising from the link's geographical route/location lead to preference for an underground (UG) cable system rather than an OHL and thereby possibility of an HVDC cable link? Need for fast voltage and reactive power control to assist overall network security?
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2.2 Financial Issues
On a purely financial basis the capital costs of an HVDC transmission line are less than an equivalent HVAC line for the same transmission medium and capacity. Further it should be noted that HVAC transmission losses per unit distance may increase with distance, whereas HVDC losses are relatively constant. However HVDC transmission systems also require converter stations that are more costly than the sum of their equivalent AC substation components.
This introduces the concept of a 'break-even distance' associated with the cost differentials between technically equivalent AC and DC transmission links. Here the additional HVDC costs associated with the required converter stations are offset by the HVDC transmission line savings when compared to the equivalent overall HVAC link requirements. Transmission link costs as a function of distance are illustrated in FIG. 1.
At present 'break-even distances' in comparison with their HVAC equivalents for HVDC overhead transmission systems are quoted at around 500 km but may be as low as 50 km for the replacement of submarine cable systems. Such capital cost figures will vary on a case-by-case basis depending on such issues as HVAC link series compensation requirements.
Strictly of course these comparisons require not only an assessment of capital costs but also operating costs. A key area for the adoption of such HVDC transmission systems is the power transfer over increasingly long distances from the world's remaining large hydropower resources to the consumer load centers. The reliability of modern solid state converter technology has therefore allowed HVDC systems to compete financially over distances in excess of some 500 km. The overall 'performance of HVDC systems' is covered under IEC 60919 and IEC Working Document TC115 covers 'the reliability and availability of HVDC systems'.
3. HVDC SYSTEM CONFIGURATIONS
3.1 Monopolar Systems
FIG. 3 illustrates a monopolar HVDC link where the two converters (a device which, depending on the flow of power, can convert AC to DC and vice versa _ if the power flows from AC to DC the converter is termed a rectifier and if from DC to AC the converter is termed an inverter) are connected by a single metallic conductor link. The return path is completed either through a ground or sea return or via a second metallic conductor.
The monopole system is adopted where substantial lengths of cable such as in a submarine cable link are required to connect the terminals. The sys tem may also be implemented as the first stage of a bipolar system.
A metallic return conductor may be used in situations where ground resistivity is too high interference with underground/underwater metallic structures is significant _ often because of possible adjacent structure corrosion issues _ or where undesirable DC magnetic field interactions may occur. The conductor forming the metallic return in a monopolar system is earthed so as to be at a low voltage.
FIG. 1 Cost as a function of distance for equivalent capacity HVAC and
HVDC transmission links.
FIG. 2 Comparison between HVDC and HVAC wayleave and tower requirements
for the 1060 km Three Gorges/Shanghai link (233,000 MW, 6500 kV) (ABB).
FIG. 3 Schematic diagram of a monopolar HVDC transmission system: (a)
with ground return and (b) with metallic return.
FIG. 4 Schematic diagram of a bipolar HVDC transmission system.
3.2 Bipolar Systems
A simple bipolar HVDC system is illustrated in schematic form in FIG. 4.
A bipolar system consists of two monopolar systems one with positive polarity and the other with negative polarity (e.g. 6500 kV). Each substation at the end of the DC link has two series connected converters of equal rated voltage.
Under normal conditions current in the two poles is generally equal and there is negligible ground current. A bipolar system is the most commonly adopted for applications where long distance overhead lines (OHLs) are involved and where the expense of the additional converters and pole conductor is offset by the HVDC vs. HVAC OHL savings. They have, however, also been used for shorter cable linked systems. The advantage of the bipolar system is its improved system availability. As long as each system can operate independently with an earth return then one pole may be arranged to continue to transmit power when the other pole is out of service for maintenance or repair.
3.3 Back-to-Back (B2B) Configuration
FIG. 5 shows a schematic diagram of a back-to-back (B2B) HVDC transmission system. This is an arrangement used where the two AC/DC converter stations are physically located in very close proximity (normally on the same site). The two AC systems that are interconnected through the converter stations may be synchronous or asynchronous. B2B systems are particularly used for connecting two AC systems operating at different frequencies as in Japan where, because of historical development, both 50 Hz and 60 Hz systems are used in different parts of the country. Such B2B systems may also be used in order to improve system stability rather than for strict transmission of bulk power. Practical B2B installations therefore often have lower DC link power ratings than other schemes.
FIG. 5 Schematic diagram of a back-to-back (B2B) HVDC transmission system.
3.4 Multi-terminal Configurations
Greater flexibility and lower overall converter losses may be achieved in theory by the implementation of arrangements with more than two terminals (typically three to five terminals -- although only three terminal schemes have been installed to date). Such multi-terminal arrangements may be realized using series or parallel converter arrangements. Parallel multi-terminal systems may be further divided into radial or meshed types. An advantage of meshed systems is that the exchange of power between DC converters may continue even when one of the interconnecting lines is out of service. Series, radial and meshed multi-terminal arrangements are shown in FIG. 6. The power ratings of the intermediate terminal tap(s) is often lower than for the main converters. However, such schemes impose significant control complexity. For example parallel multi-terminal current source HVDC scheme power flow direction reversal in parallel taps will require mechanical switching.
FIG. 6 Schematic diagrams of multi-terminal HVDC configurations: (a)
series connected, (b) radial connected and (c) mesh connected.
4. CONVERTER TOPOLOGIES
HVDC transmission systems may be classified as current source and voltage source schemes.
4.1 Current Source HVDC Converters
A current source HVDC converter is characterized by the fact that DC cur rent flow is always in one direction. Power flow may be modulated by variations in DC voltage and reversed through a reversal of the DC voltage. A stable current on the DC side of the converter is achieved by the connection of a large inductor placed in series between the rectifier and inverter DC link terminals. Since the DC voltage in a current source converter may be at either polarity the converter valves must have both forward and reverse blocking capability. The basic building block of the converter station is the well-known six pulse thyristor bridge. The six pulse bridge has 5th and 7th harmonics as its lowest order. Almost all practical HVDC installations make use of 12 pulse systems (a series connection of two identical six pulse converters along with the use of phase shifting transformers) which allow significant 5th and 7th harmonic attenuation. The largest project to date is the Itaipu system in Brazil with a 6,300 MW, 6600 kV link. Recently several huge orders have been placed, notably with Siemens, Areva and ABB, for installation over the next 10 years in China with ratings of some 6,400 MW at 6800 kV DC voltage using OHLs.
4.1.1 Line Commutated Converters (LCCs)
A typical arrangement of the main components of the widely used 12 pulse line commutated converter (LCC) is shown in FIG. 7.
A limitation of this arrangement stems from the fact that the thyristor valves require a relatively strong AC side voltage to commutate and can only operate with a lagging power factor. Thyristor converters do not have a natural turn-off capability. The AC current going through one valve must naturally cross a zero before another valve can take over conduction. The conversion process therefore requires a source of reactive power. The reactive power demand of the HVDC LCC may be supplied from shunt or series capacitor banks, synchronous condensers, harmonic filters, etc. Any surplus or deficit from these reactive sources must be absorbed or supplied by the AC system. The reactive power consumed by the HVDC LCC can be as much as 60% of the active power capacity of the link, and varies with the active power being transferred. The shunt capacitors and harmonic filters need to be switchable and as such require dedicated circuit breakers with their accompanying cost and footprint penalties.
FIG. 7 Schematic diagram of a basic current source HVDC transmission
system.
FIG. 8 Monopolar capacitor commutated converter (CCC).
FIG. 9 Monopolar controlled series capacitor converter (CSCC) with six
pulse converters.
4.1.2 Capacitor Commutated Converters (CCCs)
The Garabi B2B Brazil/Argentina HVDC link (2,200 MW, four circuit, 500 kV AC, 670 kV DC) uses series capacitors connected between the valves and is referred to as a capacitor commutated converter (CCC). The principal reason for this HVDC link is the interconnection of 50 Hz and 60 Hz AC systems. A monopolar schematic of such a system is shown in FIG. 8.
The capacitor in series with the converter transformer reduces the com mutation impedance of the converter which in turn reduces the reactive power requirement. Increasing the size of the series capacitor can even allow operation at a leading power factor. Other advantages of the CCC over the LCC include the ability to operate on weaker AC systems. However a negative impact of reduced commutation impedance is to impose additional stresses on the valves (rate of rise of current, di/dt, thyristor device limits around gate area and on the converter transformers).
4.1.3 Controlled Series Capacitor converters (CSCCs)
The controlled series capacitor converter (CSCC) is a derivation of the plain CCC scheme. A series connected, thyristor controlled, switched capacitor is located between the AC system and the HVDC converter transformer. This improves the controllability of the interconnected AC systems. A monopolar CSCC with six pulse converters is shown in FIG. 9.
FIG. 10 Schematic diagram of a two level point-to-point HVDC transmission
system.
4.2 Voltage Source HVDC Converters
A voltage source converter (VSC) is characterized by the fact that the DC voltage has a constant polarity. Power reversal takes place with the reversal of DC current. A stable voltage on the DC side is achieved by the connection of large DC capacitors across both inverter and rectifier ends of the HVDC converter link. Since the DC current in a VSC flows in either direction the converter valves have to be bidirectional. Unlike current source converters VSCs utilize more controllable semiconductor devices with inherent turn-off capabilities, such as gate turn-off (GTO) thyristors, insulted gate bipolar transistors (IGBTs), and insulted gate commutated thyristors (IGCTs).
Commercially available voltage source HVDC converters have been exclusively manufactured using modern IGBTs with a turn-off capability which allows operation under both lagging and leading power factor modes for the generation and consumption of reactive power. The VSC does not require an external source of reactive power for reliable commutation and it can there fore be used to feed weak or isolated power systems where little or no generation exists. Voltage controlled converter schemes allow for independent control of the magnitude and phase angle of the AC side voltage which in turn makes it possible to independently control the real and reactive power flows. Commercially installed voltage source HVDC systems are based upon two level, three level and multi-level topologies.
400 MW, 6150 kV VSCs have been realized and manufacturer data sheets indicate development to ratings of some 1,200 MW at 6320 kV. This has not been implemented in practice to date (2010) and the highest VSC HVDC link currently scheduled is the UK/Ireland 500 MW, 6200 kV East-West inter connector scheduled for commissioning in 2012 with no orders for higher ratings currently scheduled up to 2015. Installations have been manufactured by ABB (HVDC light) using two level and neutral point clamped (NPC) three level VSCs. The first VSC manufactured by Siemens (referred to as HVDC plus) using a modular multi-level converter (MMC) is under commissioning in 2010. All VSC HVDC installations to date have utilized IGBTs.
4.2.1 Two Level Voltage Source Converter
A schematic diagram of a two level point-to-point HVDC transmission sys tem which has been commercially manufactured by ABB, is shown in FIG. 10. Such two level voltage converters can switch each output voltage between two possible levels. The advantage is the use of a standardized con figuration and associated manufacturing.
A disadvantage of such a scheme is that there is a need to attenuate the possible electromagnetic interference (EMI) resulting from the frequent and rapid switching of the AC bus voltage between the two levels. In addition care needs to be taken in the equipment design to compensate for the high frequency transient stresses which could cause premature insulation ageing.
4.2.2 Multi-level Voltage Source Converters (Three Level and Higher)
Multi-level converters have been developed to overcome the limitations associated with two level systems such as semiconductor voltage blocking capability, maximum voltage and power transfer. They can vary their output typically between three and nine voltage levels but in some cases much higher. Unlike two level converters which use series connected devices to increase voltage rating, the multi-level converter employs dynamic voltage sharing clamping diodes and/or capacitors built into the converter structure so as to reduce the numbers of series connected devices required. Multi-level configurations allow synthesis of higher voltage levels, improved voltage waveforms, reduced filtering requirements and reduced switching frequency (and consequently reduced switching losses and reduced common mode voltage and EMI emissions).
Multi-level neutral point clamped (NPC) -- ABB 'HVDC light' -- cable installations include Sound Cross (USA, 40 km, 330 MW, 6150 kV DC, 132/ 220 kV AC), Murray Link (Australia, 180 km, 220 MW, 6150 kV DC, 132/220 kV AC) and Eagle Pass (USA, B2B, 36 MW, 615.9 kV DC, 138 kV AC) projects. MMC topologies have also been employed by Siemens in practical commercial VSC HVDC systems such as TransBay ( USA, 88 km, 400 MW, 6200 kV). Other feasible multi-level configurations such as floating symmetrical capacitor (FSC), cascaded H-bridge (CHB), multi-level current reinjection (MLCR) and multi-level voltage reinjection (MLVR) have not yet been employed for practical HVDC links.
5. HVDC SYSTEM COMPONENTS
The schematic diagram of a typical current source line commutated converter (LCC) is shown in FIG. 7. In general the same basic components are used for current and VSCs. Where differences occur these are explained.
5.1 Valves
Thyristor devices are currently the major current source LCC valve components. At present maximum individual thyristor ratings are typically 8 kV/6 kA (12 kV thyristors are available but at a reduced current rating of 2.4 kA). Such modern thyristor devices are light triggered (so as to provide electrical isolation of the gate circuitry and to reduce the complexity of firing circuits). They offer low on-state (conduction) and switching losses. A phase arm of a valve will consist of a series and/or parallel combination of individual thyristor devices.
Each device has its own auxiliary equipment to protect it from over-voltage, excessive rate of rise of voltage (dv/dt) and excessive rate of rise of (inrush or switching, di/dt) currents.
Commercial voltage source HVDC schemes are now using insulated gate bipolar transistor (IGBT) devices which individually have lower voltage and power ratings than thyristors. (IGBTs are presently available with typical ratings of 4.5 kV/2.4 kA. Higher IGBT voltage ratings are available but with lower current ratings.) The advantage of the IGBT over the thyristor is its controlled turn-off capability. The disadvantage is higher switching and conduction losses. As such a VSC has more semiconductor devices in series than an equivalent rated current source LCC. The total losses of a VSC are significantly higher than a current source LCC HVDC scheme of comparable ratings.
FIG. 11 Water cooled valve assemblies associated with the Sayreville,
New Jersey/Long Island 105 km, 750 MW, 500 kV HVDC submarine cable link
(Siemens).
FIG. 12 Optically triggered thyristor in a 'hockey puck' package. Series/parallel
strings of individual devices are used in each valve phase arm so as
to meet the overall converter power rating requirements (Siemens).
FIG. 13 Fully protected 8.5 kV, 125 mm diameter thyristors used in the
valves of the Saudi Arabia, 60 Hz/ Gulf States, 50 Hz, B2B and 1,800
MW HVDC link (Areva).
5.2 Converter Transformers
In addition to providing voltage transformation between the converter AC terminals and the interconnected AC system the converter transformer pro vides the correct phase relationship and phase arms for connection to the converter valve strings. The transformer provides sufficient inductance to assist the commutation process as well as galvanic isolation between the AC and the DC systems. Practically all LCC HVDC transformers are specified to be equipped with on-load tap changers so as to provide supply voltage adjustment. When two units of six pulse converters are connected in series to create a 12 pulse system the transformers may consist of the following arrangements:
_ Six single phase, two winding type.
_ Two three phase, two winding type.
_ Three single phase, three winding type.
Star or delta configurations are used on the DC side for harmonic cancellation. In some cases an additional winding may be used for the connection of harmonic filters. Current source LCC HVDC transformers are placed between the converter and the AC busbar and before the AC harmonic filters. Transformers have to be able to withstand the DC voltage stresses and low order harmonics. Additionally an LCC HVDC converter transformer is subjected to a DC offset voltage depending upon its position with respect to ground. It will also have a higher leakage reactance compared to conventional transformers so as to limit the prospective short circuit currents arising from faulted thyristor (valve arm) devices. In summary LCC transformer windings need to be specified with unique insulation and test level requirements compared to normal AC insulated designs. Tests for such components are specified in the IEC Standards referenced at the end of this section.
Transformers used in connection with voltage source valves are often known as interface transformers. Their primary tasks are as follows:
_ To provide reactance between the VSC and the AC system.
_ To transform the AC system voltage to a value matching the VSC AC output voltage.
_ To allow better utilization of VSC valve ratings (i.e. through the use of tap changer control to supplement VSC voltage control).
_ To prevent zero sequence currents flowing between the AC system and the VSC.
VSC HVDC systems have phase reactors as well as high frequency filters installed between the converter and the interface transformer. In principle the transformers are therefore exposed to a minimum level of high frequency harmonics or DC voltages such that more standard AC transformer designs may be used with the correct design modifications.
5.3 Reactive Power Supplies
An HVDC current source converter LCC scheme may consume as much as 60% of the active power transmitted through the link. Under transient conditions the consumption of reactive power may be even higher. This may be provided by the use of shunt or series connected capacitors, synchronous condensers, static variable capacitors (SVCs), etc. Part of the reactive power demand may be supplied by the required harmonic filters.
An HVDC VSC can generate its own reactive power so long as the apparent power is within the capability curve of the converter. It has a power capability curve similar to that of a synchronous generator. However the VSC characteristic is not limited by the low excitation synchronous generator characteristics. The reactive power generation capability of a VSC drops with an increase in the level of the real power transmitted through the link.
This effect may however be offset by higher component ratings and/or the use of more efficient cooling systems.
5.4 Harmonic Filters
HVDC current source LCCs generate voltage and current harmonics on both AC and DC sides of the link. Harmonic filters are therefore required to mitigate these effects to levels that meet network requirements. The filters are made up of capacitors and reactors with parallel resistances to provide relatively low Q tuned circuits. As previously stated they may also therefore pro vide a contribution to the reactive power requirements of current source HVDC schemes.
The lower order harmonics arising within current source converters may necessitate filter banks up to 20_30% of the converter ratings. The LCC HVDC also generates DC side harmonics, but whether DC filtering is essential will depend, in part, on whether the smoothing reactor, itself, provides sufficient DC harmonic attenuation. The large sizes of harmonic filters as well as reactive power supplies required for an HVDC LCC scheme can result in temporary over-voltages during converter blocking, fault clearing and other system disturbances as discussed in Section 9.
Low energy harmonics are less of a problem in VSCs and the harmonic filters are not required to provide any reactive power. The harmonic filters in these designs are primarily used to soften the voltage wave shape and hence reduce equipment stresses, EMI and minimize the effects of system resonances. Compared to the low order harmonic filters used in current source converter schemes the VSC filters are normally cheaper and more compact.
Dedicated DC filters are usually installed if the DC cables are close to telecommunications lines where other mitigation methods are not feasible.
5.5 DC Reactors
Current source HVDC converters utilize DC reactors to limit the rate of rise of DC current following system disturbances on either side of the converter.
This reduces the risk of commutation failure and limits DC side short circuit currents. A secondary task of the reactor is to filter the DC side harmonics in conjunction with shunt connected capacitors or filters.
For voltage source HVDC schemes and AC side, phase reactor is used to allow control of active and reactive power from the VSC, reduce high frequency harmonic content, and limit AC side short circuit currents. The phase reactor in combination with the reactance of a converter transformer may be used for this purpose or as a high frequency blocking filter. A DC side reactor for limiting DC harmonic currents, if required on a VSC scheme, is generally much smaller compared to its equivalent LCC HVDC counterpart.
5.6 AC Circuit Breakers
AC circuit breakers are used on current and voltage controlled HVDC schemes on both inverter and rectifier sides of the converter so as to isolate the HVDC/HVAC systems when the converter or link is malfunctioning. The breaker must be rated to carry full load current, interrupt fault current and energize the converter transformer.
5.7 Transmission Medium
Point-to-point HVDC links utilize OHLs, underground or submarine cables.
Typical HVDC lines use a bipolar configuration with two independent poles: one insulated at a positive voltage and the other at a negative voltage with respect to ground. Although the structural design of DC OHLs is similar to AC lines there are three main electrical differences:
_ The RMS and peak value of the DC system voltage is the same.
Therefore the maximum insulation requirements of an HVDC line are 1= ??? 2 p lower than the maximum for an HVAC line with the same RMS value. However insulation co-ordination studies to establish the maximum steady state and transient voltage levels to which the equipment will be subjected are crucial so that the correct voltage withstand requirements may be specified. The American Electrical Power Research Institute (EPRI) has embarked upon a research program (Program 162) to further investigate this subject. Articles have appeared from major vendors suggesting increased bushing insulator creepage distances are required and the ways to best achieve this.
_ DC line losses are purely resistive and negligible compared to AC line losses. A bipolar HVDC line with two insulated conductors is comparable in power transfer ability to a double circuit HVAC line with similar insulation requirements but with a fewer number of insulated conductors.
_ DC magnetic fields emitted from OHLs are time invariant, and unlike HVAC lines are comparable with the earth's magnetic field. Additional DC OHL technical issues include corona and radio interference. On the positive side, such lines may have lower environmental visual impact, smaller right-of-way width requirements, and lower audible noise. (FIG. 14) Power cables for DC operation are normally provided as a bipolar bundle or laid close together creating a low residual DC magnetic field and simplifying the installation process. DC cables may be installed in very long lengths as they require no capacitive charging current. Submarine cable lengths up to 700 km have been achieved. There are two dominant types of DC cable available:
_ Mass impregnated (MI) oil type.
_ Cross linked polyethylene (XLPE) polymeric type.
MI cables have been applied to both current source LCC and VSC schemes (e.g. the Moyle Northern Ireland/Scotland, 23250 MW, 23 monopolar 250 kV DC, 63 km, submarine cable interconnector which also incorporates optical communications fibers). DC XLPE cables offer economic advantages with respect to installation and manufacture, environmental considerations and growing in service experience. They have only been applied to VSC schemes to date as insulation is not yet designed to withstand polarity reversals (FIG. 15).
FIG. 14 2,000 MW HVDC overhead line (OHL) in Western United States (ABB).
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FIG. 15 XLPE HVDC cable structure (ABB).
- Conductor-- 1,300 mm^2 (2,570 kcmil) copper conductor
- Conductor screen--Semiconducting polymer
- Insulation HVDC insulation polymer
- Insulation screen--Semiconducting XLPE
- Swelling tape
- Metal Sheath-- Lead alloy 1/2 C
- Protection and bedding--Extruded PE sheath
- Armor--Wires of galvanized steel
- Serving--Layers of bitumen bonded polypropylene yarn
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