مقدمه
با تقاضای رو به افزایش برای انرژی الکتریکی، ولتاژهای انتقال نیز رو به افزایشند. انتقال توان زیاد به مسافت های دور، که به علت مبادله قدرت بین کشورها می باشد، نیاز به کابل های فشارقوی موثری دارد تا در مناطق شهری یا برای عبور زیر زمینی یا دریایی استفاده شود. امروزه ولتاژ عملیاتی کابل های فشارقوی الکتریکی تولیدی تا 500 kV افزایش یافته است.
کابل های الکتریکی polymer-insulated یا PE ضرورتا حاوی هادی فلزی با مقاومت پایین که توسط پلیمر عایق سازی شده است هستند. این عایق هادی ها را از یکدیکر و اطرافشان جدا می کند. یک غلاف(sheath or jacket) که بدوا بسته به خواص مکانیکی قالب ریزی شده از کابل مقابل محیط محافظت می کند. محتویات عمده ی دیگر میتوانند شامل لایه های نیمه هادی، screen فلزی، سیم فلزی تقویت کننده، و لایه ی بلوکه کننده ی آب. اگرچه یک تحول محتمل از مواد با خاصیت ابررسانایی ساختار سیستم انتقال نیرو را دگرگون خواهد کرد متخصصان استعمال گسترده ی آن را تا 20-10 سال آینده عملی نمی دانند. در حال حاضر تکنولوژی کابل های فشارقوی توسط گذار از پوشش کاغذی معمول گذشته،کاغذ آغشته به روغن تحت فشار که مشکلاتی از قبیل اتلاف عایقی بالا، مخارج عملکرد بالا و آلودگی و … دارد ،به دای الکتریک اکسترود شده ی مصنوعی (extruded synthetic dielectric) مشخص می شود.
water treeing یکی از مهمترین عیوب در عملکرد کابل های MV و HV است و از این رو طراحی،ساختمان و مواد مورد استفاده به گونه ای که از نفوذ آب،به ویژه در کابل های زیر زمینی و زیر آبی، جلوگیری کنند مهم می باشد.[1-9]
اگرچه ساختمان های بسیار متفاوتی از کابل های فشارقوی در بازار موجود هستند اما تمامی آنها دارای قسمت های ضروری زیر هستند:
* هادی ها
* شیلد های نیمه هادی
* عایق ها
* هادی ها:
هادی ها سیم های مسی با آلومینیمی هستند که می توانند مفتولی (solid) یا افشان (stranded) باشند. هادی های افشان برای بالا بردن انعطاف پذیری کابل استفاده می شوند. به علاوه می توانند maximum electrical stress را تا 20% افزیش دهند. در این هادی ها، آب میتواند در جهت طولی در خلل و فرج ها و فضاهای میان رشته ها به راحتی نفوذ (شارش) کند. جلوگیری از نفوذ طولی آب توسط پر کردن خل و فرج ها با ترکیبی از پلاستیک یا سوار کردن مواد جذب کننده ی آب (نمگیر = hygroscopic) درون رشته های هادی بدست می آید. راه دیگر! استفاده از هادی های مفتولی (solid) است که خلل و فرجی ندارند. برای مس، هادی های مفتولی بالای شماره 1AWG عملی نیستند. در آلومینیم drawn حالت معمول کاملا سخت بودن است. وقتی آلومینیم به جای draw شدن extrude می شود، حالتی نرم پیدا می کند. استانداردهای آمریکایی هادی مفتولی آلومینیمی را نمی شناسند اما این هادی ها در اروپا استاندارد هستند. کابل های فشارقوی می توانند دارای یک یا چند هادی درون کر (core) باشند. در هادی های چند کره (چند هسته ای)، فاصله ی مناسب میان هادی ها باید از فرمول های مرتبط در تنش های الکتریکی محاسبه گردد. شکل دادن به هادی ها فرآیندهایی چون drawing، فشرده کردن، گداخته کردن (annealing)، پوشانیدن (قلع کاریtinning و روکش کاری کردنplating)، باندل کردن (bunching) و افشان کردن را در بر می گیرد. [10]،[11].
شیلد های نیمه هادی:
تحقیق روی شیلد های نیمه هادی در توسعه ی کابل های فشارقوی نقشی اساسی را بازی کرده است. در کابل های فشارقوی، مواد نیمه هادی به منظور جلوگیری از تخلیه ی جزئی در فصل مشترک بین عایق و هادی و بین عایق و لایه ی خارجی شیلد کننده مورد استفاده قرار گرفته اند و به علاوه تنش های الکتریکی را در لایه ی عایقی تعدیل می کند. آنها میدان الکتریکی یکنواختی حول عایق با کاهش دادن گرادیان پتانسیل روی سطح هادی های افشان و درون شیلد فلزی، فراهم می کنند و از تخلیه های جزئی (کرونا) در سطح هادی های افشان و عایق با نگهداشتن تماسی نزدیک بین سطوح داخلی و خارجی عایق جلوگیری می کنند. همچنین آن ها حفاظتی در مقابل آسیب های بوجود آمده از گرم شدن هادی در اتصال کوتاه ها ایجاد می کنند.
مشخص شده است که تحمل دای الکتریکی عایق به مقاومت حجمی (volume resistivity) ماده ی نیمه هادی وابسته است. فاکتورهای دیگر نیز – چون پلاریته، نوع و مقدار کراسلینک کردن ماده ی نیمه هادی – تنها اثری جزئی روی تحمل دای الکتریکی دارند. ناخالصی ها می توانند باعث بیشتر شدن پدیده ی درخت آبی شوند.
در کابل های قدرت، کوپلیمر های(copolymers) اتیلنی پر شده با Carbon Black هادی (CB)، مانند اتیلن ونیل استات و اتیلن اتیل استات، به طور متداول به عنوان لایه ی نیمه هادی استفاده می شوند. فاکتورهایی چون مقدار CB، کیفیت مخلوط کردن و دما (توسعه ی شبکه ی CB را متاثر می کند) تاثیر روی ویژگی های نیمه هادی های پر شده با CB می گذارد. افزایش بارگذاری CB و دمای فرآیند مقاومت حجمی (volume resistivity) را کاهش می دهد که معمولا بین 10 و 100 اهم cmاست و نباید از 4^10 اهم cm تجاوز کند [12]-[15].
* عایق سازی:
الف) XLPE
پلی اتیلن (PE) ترموپلاستی (نرمش پذير دراثر حرارت) پلیمری نیمه بلورین semicrystalline است که دارای ویژگی های الکتریکی خوب می باشد (ضریب دای الکتریک پایین، تلفات دای الکتریکی پایین، استحکام عایقی بالا) به همراه خصوصیات دلخواهی چون تافنسtoughness مکانیکی و انعطاف پذیری،مقاوم در برابر مواد شیمیایی، فرآیند پذیر، و ارزان قیمت بودن. این خصوصیات آن را انتخابی دلخواه برای عایق سازی کابل های قدرت می کند و این در حالی است که عیب عمده ی آن که دمای ذوب پایین آن است تاثیری در تصمیم ما نمی گذارد.این عیب دمای عملیاتی را به C °75 محدود می کند. برای بهبود این خصوصیت، PE کراسلینک می شود (XLPE). کراس لینک کردن دمای ماکزیمم عملیاتی را تا C °90 و دمای اضطراری را تا C °130 و ماکزیمم دمای اتصال کوتاه را (گذرا) تا C °250 بالا می برد. گراس لینک کردن همچنین استحکام ضربه ای، پایداری اندازه، استحکام کششی، خصوصیات حرارتی و مقاومت شیمیایی را بالا می برد و خصوصیات الکتریکی، پیری و مقاومت در برابر حل شدن پلی اتیلن را بهتر می کند.
Polyethylene is commercially crosslinked by three different methods: irradiation-, peroxide-, and silane-crosslinking. In irradiation crosslinking, radiation of accelerated electrons (ß-radiation)
or electromagnetic wave (γ-radiation) abstracts hydrogen from the hydrocarbon chain and produces free radicals; and by combination of these alkyl radicals, C-C crosslinks are formed.
Advantages of this method are higher resistance to heat and chemicals, better mechanical properties, and high productivity Nonuniform crosslink distribution, thickness restrictions (needed
for thick insulation for high voltages), and high cost are disadvantages
of this method. Radiation crosslinked PE has been
commonly used for low-voltage wire and cable. But for highvoltage
cable, this method is not used because of the higher
thickness of insulation.
Silane crosslinking can occur by sioplas (two step) or monosil
(one step) processes in which vinyl silane (normally
vinyltrimethoxy silane) is grafted onto the polymer, using a small
amount of (~0.1%) of peroxide (commonly dicumyl peroxide,
DCP) as initiator. With the addition of a condensation catalyst
(dibutyltin dilaurate), crosslinking occurs via formation of siloxane
linkages. Silane crosslinking is of growing importance;
but in high-voltage cable, peroxide curing is commonly used. In
peroxide curing, radicals, made of decomposition of peroxides,
abstracts hydrogen from the polymer chain; and, like the irradiation
method, C-C bonds are formed between polymer chains.
The most common peroxide for PE is DCP, which gives safe
processing up to 120°C. Today, 2,5-dimethyl-2,5-bis(tbutylperoxy)
hexane-3 (DMTBH) is available to give safe processing
up to 150°C. Extrusion at temperatures below the temperature
of peroxide decomposition is important in order to avoid
problems with precuring. Another limitation of this method is
the need to maintain high pressure (12-20 bar) in the curing step
to avoid formation of voids due to by-products such as acetophenone
and cumyl alcohol.
Although the short-time, intrinsic, dielectric strength of the
base XLPE resin is in exess of 800 kV/mm, the technical dielectric
strength imposed by manufacturing conditions has limited
the design average stress of power cable to approximately 4-8
kV/mm. Extensive work performed over the last 20 years has
served to identify the major defects responsible for relatively
low values of dielectric strength. Defects generally can be classified
into two categories: those that serve to enhance the voltage
stress in small, localized regions of the insulation, and those
that serve to dielectrically weaken small, localized regions within
the insulation. Among the former are such defects as protrusions
from a semiconductiing shield into the insulation and conductive
inclusions within the insulation. Among the latter are ionizable
gaseous or liquid inclusions or voids in the insulation or at
the shield/insulation interface. This may be created by poor
dispersion of crosslinking agent or by improper extrusion or
curing conditions.
A combination of these two categories of defects leads to the
most severe conditions. So, a processing system capable of producing
hv cable that will have acceptable service life must be
designed to reduce or eliminate these defects. In order to meet
requirements that the compound be homogeneous and contain
the minimum size and amount of contaminants, the purification,
mixing, and processing systems are independent, compact and
enclosed. When the PE enters the system, it is never again exMay/
June 2006 — Vol. 22, No. 3 15
posed to unfiltered air. The system screens polyethylene, rather
than crosslinkable polyethylene, because it can be safely heated
to elevated temperatures, at which it can be screened through
mesh opening as small as 0.04 mm. Then the purified PE, in
pellet form, is mixed with DCP at approximately 80°C. The
crosslinkable PE compound is then stored for a short time until it
is transferred to the extruder.
Acceptable extrusion performance is achieved at a processing
temperature between 125°C and 145°C. Screw cooling increases
shear forces and residence time and should be used only
when the compound is not properly mixed. Conical screw tops
and stream-lined flow path can be used to reduce residence time.
Dual- or triple-layer extrusion heads are the principal device
used to reduce the size, number, and effects of protrusions. For
this purpose, incorporation of an emission shield, a thin layer of
high dielectric constant material, at the interface of the semicon
layer and insulation, also could be used.
After extrusion, the insulated conductor immediately enters
the vulcanization zone. Catenary (CCV) and vertical (VCV) continuous
vulcanisation are the well-known curing lines in the HV
cable industry. For good concentricity in the thickest insulation,
VCV must be used because of the sensitivity of the polymer
melt to the force of gravity.
High pressure should be applied in the curing zone to avoid
the formation of voids. Use of silicone oil or eutectic salts that
do not penetrate significantly into the insulation structure as a
heat transfer media, rather than steam or nitrogen that do penetrate,
minimizes the size and number of voids. Another advantage
of such media is that the temperature is not related to pressure,
as for steam curing, and higher temperature can be adopted
independently.
Another technique for curing of medium and HV cables is the
Mitsubishi-Dainichi continuous vulcanization (MDCV) in which
a long lubricated die (about 25 m) is used for direct contact
heating. After curing, the cable is maintained under pressure in
the cooling system and only after the total cable, with shield
applied, is cured and cooled that the cable is exposed to the
factory environment [16]–[25].
B. Tree-Retardant XLPE
The degradation of XLPE insulation due to moisture (water
trees) has been a problem for many years, and extensive studies
have been made to improve the resistance of XLPE to water
treeing. Work in this respect can be categorized into three general
methods.
1. Use of Additive in Conventional XLPE: In this method,
additives, that usually are of low molecular weight organic species
and are in liquid form at room temperature, are used in conventional
XLPE to incorporate water retardancy. Compatibility
with XLPE is very important. Incompatibility causes diffusion
of the additive out of the polymeric matrix and deteriorates the
water retardancy. Dodecanol and silanes are examples of these
additives.
2. Blending XLPE with Polar Polymers: Because PE is a
hydrophobic nonpolar polymer, the electrical condensation of
water occurs easily in voids and contaminant sites. Blending
polar ethylene copolymers with XLPE tends to make it somewhat
hydrophilic and reduces the condensation of water and
thus water treeing. This, however, diminishes the electrical properties
of XLPE.
3. Use of Very Low Density Polyethylene (VLDPE): A newer
and more effective method for increasing the tree resistance of
XLPE is improving the properties of the base PE resin. VLDPE is
a type of linear polyethylene produced by the low/medium pressure
method with the aid of a Ziegler catalyst. The low density is
achieved by introducing a comparatively large amount of aolefin
short chain comonomer into the polymerizing ethylene.
The degree of crystallinity is decreased, thereby obtaining the
so-called VLDPE having a much lower density (0.89-0.91 g/cm3)
than LDPE (0.91-0.93 g/cm3). It has been known that the resistance
of polyolefins to water tree increases with decrease of crystallinity.
Although VLDPE as well as LDPE is a hydrophobic
polymer, the water content of cross-linked VLDPE is higher than
that of conventional XLPE. This could reduce water condensation
and improve water tree retardancy. Extrusion of VLDPE was
found to be more difficult compared to conventional PE, consequently,
it required the modification of extrusion condition and
crosslinking agent [26]–[30].
C. Ethylene propylene rubber
Ethylene propylene rubber (EPR) is one of the extruded dielectrics
used today in medium- and high-voltage cable. Because
ethylene propylene copolymers (EPMs) are fully saturated
and nonpolar, they have excellent resistance to ozone, oxidation,
heat, weathering, water, and polar solvents. EPR can be
cured only with peroxides. Because of a preference for sulfur
curing in the rubber industry, sulfur curable EPR (EPDM) was
developed by insertion of a diene monomer as the third part of
the copolymer composition, but in the cable industry sulfur curing
is not used. Polymers with low ethylene content are amorphous
and easy to process. High-ethylene polymers are semicrystalline
and have better physical properties, but they may give
processing problems. Excellent weathering and water resistance
are key characteristics to the success of EPR in its applications.
Corona resistance, wet electrical stability, water-tree resistance,
high-temperature performance, and flexibility are attractive features
of EPR for electrical insulation.
EPR insulations contain only about 50% of the base polymer
(EPM or EPDM) and are a complex formulation incorporating
more than 10 ingredients. Its formulation may vary from one
manufacturer to another. Fillers are the next large component.
Extrusion of EPR polymer alone typically results in a rough
extrudate surface because of melt fracture, and the mechanical
strength of the extrudate is low, particularly for amorphous EPR.
Filler addition provides the smooth surface and mechanical
strength desired for electrical application, but it compromises
the electrical properties. So, the proper selection of filler type
and amount is very important. Carbon black is not used because
it compromises the electrical properties. Non-black or mineral
fillers such as clay, talc, whiting, silica, and alumina are commonly
used in the electrical industries. For insulation applications,
treated clay and hydrated alumina are preferred. Coupling
agents can be used with non-black fillers for additional reinforcement.
16 IEEE Electrical Insulation Magazine
Although the base EPR has no direct effect on electrical properties,
it is the most important ingredient because it defines the
minimum level of filler for acceptable extrusion and mechanical
properties.
Paraffinic and naphthenic oils generally are used as a plasticizer
or processing aid in EPR. Small amounts (10 phr) of aromatic
oils have been recommended to improve the feeding of
extrusion stock. But larger amounts will kill a peroxide cure and
reduce the state of cure in sulfur curing. Compounds with low
filler levels and no processing oils provide optimum electrical
properties and are preferred.
Although the electrical properties of sulfur (for EPDM) and
peroxide (for EPDM and EPM)-cured EPRs are comparable in
dry service, and initially in water, long-term immersion in hot
water shows the peroxide cure to be decidedly superior. For this
reason, EPR used in electrical insulation is peroxide cured. Other
advantages of peroxide cure includes: best retention of properties
after aging, excellent compression set, fast curing, no discoloring,
and no reversion in high-temperature fast curing. Higher
cost, no curing in the presence of air, slightly lower physical
properties, and poorer tear strength are disadvantages of peroxides.
Peroxide curing involves a chemical reaction by a freeradical
mechanism with the decomposition of peroxide (commonly
DCP) which leads to very stable C-C bonds between polymer
chains. Free-radical vulcanization is enhanced by
polyfunctional coagents such as triallyl cyanurate and
trimethylolpropane trimethacrylate. Because peroxide curing is
more efficient with EPDM than EPM, coagents are not normally
used with EPDM. Acidic compound ingredients promote ionic
breakdown of peroxide rather than free radical initiation and
should be avoided. Zinc oxide, which usually is used with peroxides
and almost all sulfur cures, could be replaced by red lead
in electrical compounds to improve water resistance.
The EPR compounds usually are mixed in internal Banbury
mixers. Internal mixers are fast and efficient with short cycle
time. Semicrystalline EPR can present dispersion problems when
used in compact bale form. Use of either pellet or friable form
solves this problem. Continuous screw mixers also could be used
for mixing. This mixer offers potential control advantages in
dispersion and heat history. But compound ingredients must be
in the form of pellets or powder for feeding purpose. Thus, compounds
based on amorphous EPR are precluded. After mixing,
the EPR preparation is extruded through fine mesh screens (0.1
mm opening) to remove undispersed ingredients and hard contaminants.
This results in increased AC breakdown strength of
the insulation. Even finer mesh screening will further increase
breakdown strengths, but pressure and output rate become limiting
factors. The screened EPR compound is applied for cable
construction by use of conventional extrusion equipment and
double- or triple-crosshead pressure dies. Crosshead temperatures
of 100°C to 120°C soften the polymer to allow good flow
and are below the peroxide decomposition temperature so pressure
or scorch is not a problem. Usually EPR insulated wires or
cables are cured in CV. Pressurized salt curing systems could
also be used to improve concentricity in large diameter cables.
By means of proper compounding, EPR elastomers can show
unusually good mechanical and thermal properties as well as
resistance to environmental stresses. Today, the already outstanding
water resistance of EPR insulations has been so much improved
as to allow a satisfactory solution to the problem of life of
cables laid in wet environment without a metal sheath [11], [31]–
[33].
D. Comparison of XLPE and EPR
The XLPE insulations, which are most commonly peroxide
cured, contain at least 98% polyethylene; EPR insulations are a
complex mixture of only about 50% base polymer and more
than 10 other ingredients. The physical and electrical properties
of XLPE and EPR are compared in Table 1. A range of values is
given for EPR to cover the various formulations commercially
available. The XLPE insulations have higher breakdown strength.
The AC HV tests indicate the ability of the cable to withstand
overvoltage conditions arising from ground faults and system
disturbances. The XLPE cables clearly are superior to EPR cables
when their electrical charateristics are compared. Impulse test
results, which verify the ability of cable to withstand voltage
surges and transient overvoltages due to electrical power systems,
show the superiority of XLPE over EPR. A key difference
between these two materials is in their dissipation factor, which
is a measure of the dielectric loss in the cable. The dissipation
factor of EPR is significantly higher than XLPE, leading to considerable
higher dielectric loss and thus operating cost, especially
at higher voltages. Another disadvantage of EPR is the
increase of dissipation factor (DF) with increasing temperature
and electrical stress, and the variation of DF with the cable length.
With respect to resistance toward environmental stresses such as
moisture, EPR cable shows better performance. Results of the
accelerated cable life tests (ACLT) indicates EPR compounds
last much longer than XLPE and TRXLPE and have better water
treeing resistance. Also EPR compounds are less sensitive to
Table 1: Comparison of XLPE and EPR
Properties XLPE EPR
Density, g/cm3 0.92 1.2–1.4
Tensile strength, MPa 19 9–12
Elongation, % 500 250–350
Modulus of elasticity, MPa 121 5–14
Heat distortion, % 20 5–8
Thermal conductivity W/m°C 0.27 0.27–0.35
Dielectric constant 2.3 2.5–3.0
Dissipation factor, %
at 20°C <0.03 0.16–0.3
at 90°C <0.03 0.3–1.0
Volume resistivity, O. cm at 23°C 1016 1013
Short-term AC breakdown on
miniature cable KV/mm 48 30–40
May/June 2006 — Vol. 22, No. 3 17
voids and contaminant and other related defects. EPR insulations
are more expensive than XLPE. Also the use of EPR involves
the excess cost of higher dielectric loss. It must, however,
be mentioned that EPR insulated cables can be used without
metal sheathing [14], [26]-[29].
Sheathing (Jackets)
To protect against environmental damage, the cable is covered
by one or more metallic or polymeric layers. PVC is commonly
used for jacketing of hv cables. Over the last 25 years,
there has been some replacement of PVC by LLDPE or the harder
MDPE and HDPE compounds, in particular to minimize the risk
of jacket damage during installation. Halogen-free, low fire-hazard
compounds—polyethylene or ethylene vinyl acetate with
60-65% aluminum trihydrate and other additives—have been
used increasingly in the past 10 years. For better mechanical
strength and protection, wire armoring may be used. In order to
keep water out of the extruded dielectric, a metallic water barrier
around the cable could be used. This water barrier can be applied
as a separate metallic laminated tape, incorporated into the
metallic component of the shield as sealed tape, or the cable can
be placed inside a metal tube [1]–[3].
Conclusions
This article has reviewed the different components that make
up extruded, hv cables. The different types of conductors are
described as are the different insulation systems that are available
for medium and HV cables.
There is a comparison of the two main insulations used in
extruded hv cables, EPR, and XLPE. Each material has some
properties that are better than those of the other material.