50 years in the development of insulating liquids

September/October Vol. 29, No. 5 13

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50 Years in the Development of Insulating LiquidsKey words: dielectric coolant, mineral oil, vegetable oil, ester, silicone liquid, liquid-filled equipment

I. FofanaCanada Research Chair on Insulating Liquids and Mixed Dielectrics for Electrotechnology (ISOLIME), Universit du Qubec Chicoutimi, Qubec, Canada

The importance of liquids in the field of dielectrics and electrical insu-lation is highlighted. Engineering problems in using these liquids in electrical equipment are discussed. Their applications and perspectives are addressed.

IntroductionThe role of electrical insulation is critical for the proper op-

eration of electrical equipment. Power equipment cannot oper-ate without energy losses, which lead to rises in temperature. It is therefore essential to dissipate the heat generated by the energy losses, especially under high load conditions. Failing to do so results in premature aging, and ultimately to failure of the equipment. Heat dissipation can be achieved by circulating cer-tain liquids, which also ensure electrical insulation of energized conductors. The insulating-fluids market is therefore likely to be dominated by liquids, leaving to gases (such as compressed air and SF6) limited applications in power equipment such as circuit breakers and switchgear [1][3]. Several billion liters of insulating liquids are used worldwide in power equipment such as transformers (power, rectifier, distribution, traction, furnace, potential, current) [4], resistors [5], reactors [6], capacitors [7], cables [8], bushings [9], circuit breakers [10], tap changers [11], thyristor cooling in power electronics, etc. [12].

In addition to their main functions of protecting solid insu-lation, quenching arc discharges, and dissipating heat, insulat-ing liquids can also act as acoustic dampening media in power equipment such as transformers. More importantly, they provide a convenient means of routine evaluation of the condition of electrical equipment over its service life. Indeed, liquids play a vital role in maintaining the equipment in good condition (like blood in the human body). In particular they are responsible for the functional serviceability of the dielectric (insulation) system, the condition of which can be a decisive factor in determining the life span of the equipment [13]. Testing the physicochemical and electrical properties of the liquids can provide information on incipient electrical and mechanical failures. In some equip-ment, liquid samples can be obtained without service interrup-tion.

Petroleum-based oil, so-called mineral oil, has been the main insulating liquid in industrial power systems since the 1900s [14][16] because of its good aging behavior, low viscosity, ready availability, and low cost [14]. Because of the necessity of operating distribution transformers in locations where high fire security standards were required, polychlorinated biphenyl (PCB)-based insulating liquids were introduced in the early 1930s. Until the 1960s they did not raise environmental concern. However, since the 1970s the public has been increasingly criti-cal of the use of PCB-based transformer oils [1][4]. While most PCB oils have now been replaced with PCB-free liquids, the lat-ter have not been widely accepted as alternatives to mineral oils in power equipment. Driven by the desire for a safer nonflamma-ble and environmentally acceptable insulating liquid for use in power equipment, researchers and engineers have investigated countless alternatives to mineral oil, and significant advances have been achieved during the last four decades [17][25].

The sixth of a series of invited reviews to be published during 2013 to mark

the 50th anniversary of DEIS.

14 IEEE Electrical Insulation Magazine

In this review article some of the developments in the field over the past 50 years are described.

Classification of Insulating LiquidsVarious liquids are being used as lubricating agents. How-

ever, they will not be considered in this article. Insulating liquids can be subdivided into different categories based on their chemi-cal structures or their fire points. In Table 1 (updated version of a table published in [22]) 14 categories of insulating liquids used during the last 50 years are listed.

Mineral OilsMineral oils are complex mixtures of hundreds of different

organic compounds, consisting mainly of carbon and hydro-gen in molecules with different structures [22][24]. They are made by refining a fraction of the hydrocarbons collected during the distillation of petroleum crude stock. The physicochemical properties of an oil may vary significantly from one batch to another, even from the same supplier.

There are three categories of crude oils, namely paraffinic, naphthenic, and mixed crudes [14][16], [22], [24]:

(a) Paraffinic crudes contain a small amount of naphthenic hydrocarbons and can be subdivided into normal par-affins (straight chain wax-type molecules) and isopar-affins (branched paraffins). Isoparaffins are preferred over normal paraffins because of their lower pour points.

(b) Naphthenic crudes have higher naphthenic compound content than do paraffinic crudes.

(c) Mixed crudes are intermediate between paraffinic and naphthenic crudes.

The early mineral oils were paraffin based, but after 1925 they were replaced with naphthenic oils because of the high pour points of paraffinic oils [14]. In addition, paraffinic crudes con-tain waxes, whereas naphthenic crudes contain very little wax. Since naphthenic oils have lower viscosities than paraffinic oils, the former become thinner and less viscous at elevated operating temperatures and therefore provide, at least theoretically, better heat exchange.

Modern petroleum refining has changed dramatically over the last 30 to 40 years. Improved technology, specifically cata-lytic hydroprocessing, has brought many benefits and efficien-cies. Reviews of various refining technologies exist [14], [15], [26], [27]. Refined oils are very complex blends and may consist of more than 3,000 different hydrocarbons, principally paraf-finic (4060%), naphthenic (3050%), and aromatic (520%) chains of carbon atoms [14][16], [19], [24][27]. Aromatic hy-drocarbons contain unsaturated ring molecules and have many names, e.g., polynuclear aromatics, polycyclic aromatics, and polyaromatic hydrocarbons. Aromatic content is the main factor that determines the difference in the water solubilities of differ-ent oils [13]. Polyaromatic hydrocarbons may present a health concern; recent studies suggest that naphthenic oils with more than 2% polyaromatic hydrocarbon content are potentially car-cinogenic [13], [28].

Naphthenic oils are typically manufactured by solvent refin-ing processes with hydroprocessing/hydrotreating or mild hy-drofinishing. Such processes leave residual substances in oil, including sulfur compounds and aromatic nitrogen [26], [27], [29]. Naphthenic isoparaffinic liquids are highly refined using hydrocracking and hydroisomerization, which eliminate almost all contaminants from the liquid, leaving it almost free of sulfur [26], [27]. Isoparaffinic oils have better heat transfer capabilities than do naphthenics [30].

Synthetic Insulating LiquidsAlthough mineral oils play a very important role in the power

industry, synthetic liquids are used when special properties are sought, e.g., fire resistance, partial discharge resistance, negative gassing tendency (gas absorption). A large number of synthetic insulating liquids are available, e.g., halogenated hydrocarbons, aromatic hydrocarbons, high molecular weight hydrocarbons, polybutenes and phthalates.

Halogenated Hydrocarbons (Nonflammable Liquids)

Nearly all nonflammable liquids are classified as halogenat-ed hydrocarbons, typically including chlorine or fluorine [31] which react with hydrogen atoms to form HCl and HF.

Because of the desire for nonflammable liquids, PCBs were used for insulation purposes between 1929 and 1977 [32]. These liquids (generic name askarels = fire resistant) were mixed with varying quantities of trichlorobenzene or tetrachloroben-zene [19], [22], [24] and marketed as insulating fluids under the name Aroclors (one of the most commonly known trade names for PCB mixtures). Technical specifications of askarels can be found in CEI 60588-3 [33]. They were used primarily in trans-formers (for their fire resistance), capacitors (for their resistance to partial discharges), and hydraulic machines requiring stable, fire-retardant materials [19], [22], [24]. When it became clear that PCBs may cause adverse health effects, their manufacture was banned after 1978 [32]. In the European Union, equipment containing more than 50 ppm of PCBs was to be destroyed by incineration by the end of 2009 [24].

Researchers have tried countless combinations of chemicals to remove PCBs from older equipment and to find other insulat-ing liquids with which to fill new equipment. Various chlorinated fluids, e.g., benzyltoluene, perchlorethylene, trichlorobenzene, and dichlorotoluene were developed as replacements for PCBs in applications where nonflammability was important [34][36]. The most popular include polychloro-diphenyl-methanes and chlorinated diphenyl (or benzyltoluene) substitutes. Polychloro-diphenyl-methanes are readily biodegradable and do not form dioxins in case of fire [22], [24].

PCB-free halogenated hydrocarbons have had limited indus-trial success, mainly because of strong resistance to chlorination [22], [24]. Fluorinated products have been investigated in the United States, but their high cost has inhibited their develop-ment [24]. Perfluorinated polyethers were proposed in 2000 for capacitors in which resistance to fire is of great importance [1], [24].


Tabl

e 1.

App

licat

ion

of In

sula

ting

Liqu

ids

in E

lect

rical

Pow

er E

quip

men

t (Up

date

d Ve

rsio

n of

a T

able

Pub

lishe

d in

[22]

).

Insu

latin

g liq

uid1

Pow

er

trans

form

ers

and

reac

tors

Dist

ribut

ion

trans

form

ers

Trac

tion

tra

nsfo

rmer

sIn

stru

men

t tra

nsfo

rmer

sSp

ecia

l tra

nsfo

rmer

sBu

shin

gsTa

p

chan

gers

Term

inal

bo

xes

Circ

uit

brea

kers

Capa

cito

rsCa

bles

Load

ing

re

sist

ance

Min

eral

oils

XX

XX

XX

XX

OO

O

Poly

chlo

rinat

ed b

iphe

nyls

(P

CB)

Othe

r hal

ogen

ated

hy

droc

arbo

ns

O

Silic

one

oils

X

X

X

High

-mol

ecul

ar-w

eigh

t hy

droc

arbo

ns

X

X

Tetra

este

r of p

enta

eryt

hrito

l

XX

X

X

Alky

lben

zene

s

X

X

XX

Arom

atic

hyd

roca

rbon

s

(M/D

BT, P

XE, M

IPB,

etc

.)

X

X

Vege

tabl

e oi

ls

X

X

Phth

alat

es (D

OP, D

NP)

X

Poly

bute

nes

X

Liqu

efie

d ga

ses

or c

ryog

enic

liq

uids

X

X

Nano

fluid

s

X

Mix

ed li

quid

s

X

XX

X : I

n us

e in

the

equi

pmen

t.O:

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long

er re

com

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ded

for t

he e

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t, bu

t stil

l in

use

here

and

ther

e.

: Ban

ned,

and

ther

efor

e sh

ould

not

be

in u

se a

nyw

here

.1 M

/DBT

= m

ono/

dibe

nzyl

tolu

ene;

PXT

= p

heny

l-xyl

yl-e

than

e; M

IPB

= m

ono-

isop

ropy

l-bip

heny

l; DO

P =

dioc

tyl p

htha

late

; DNP

= d

i-iso

nony

l pht

hala

te.


Aromatic HydrocarbonsThese liquids are mainly used in capacitors and cables [19].

Their technical specifications are defined in IEC 60867 [37].Alkylbenzenes

In the 1960s a new family of insulating liquids known as al-kylbenzenes became available. Their stability under partial dis-charge is their most important property for todays applications. They have considerably greater ability (relative to mineral oil) to absorb gas produced by partial discharges [38] and are therefore often used in hermetically sealed equipment. Their low viscos-ity is another important property, especially for oil circulating in cables. Heavier alkylbenzenes are used for filling capacitive dividers [22], [24]. The addition of about 30% of alkylbenzene liquid (by volume) to mineral oil used for impregnation of cable systems greatly improves the gas absorbency of the oil [22], [24], [38].Polyarylalcanes

Polyarylalcanes were primarily developed to replace PCBs as impregnating liquids for capacitors [22], [24]. Among the first generation developed in the 1970s were phenyl-xylyl-eth-ane, mono-isopropyl-biphenyl, and di-isopropyl-naphthalene. Mono-isopropyl-biphenyl is no longer used in North America, but phenyl-xylyl-ethane and di-isopropyl-naphthalene are still used in Asia [24]. In the 1980s phenyl-xylyl-ethane and mono-isopropyl-biphenyl were replaced by diphenyl-methane deriva-tives, specially developed for capacitors, e.g., mono/dibenzyl-toluene 75/25 and benzyl-toluene/diphenyl-ethane 60/40. (The numbers indicate the mass percentages in the mixtures.) Mono/dibenzyltoluenes have been used in power capacitors for more than 25 years. They have also been approved for other electrical equipment such as capacitive voltage transformers and bushings [19]. Polyarylalkanes are also used for filling capacitive divid-ers. Their dielectric properties, much better than those of alkyl-benzenes and mineral oils, improve their competitiveness [24].

Polyarylalcanes have kinematic viscosities less than 8 mm2/s at 40C [22], [24], so that removing moisture from them under vacuum is much easier than from mineral oils. Their thermal sta-bility is much better than that of mineral oils and alkylbenzenes. However, they are more polar, and their dielectric properties are more sensitive to contaminants. Because of their high aromatic content they generate very little gas under partial discharge and have high hydrogen absorbance capacity. Their low solubility in water limits their biodegradability. Their toxicity is very low, as is the rate at which they accumulate in the environment.

PolyolefinsPolyolefins are unsaturated hydrocarbons with double bonds.

The polybutenes, especially polyisobutenes, are the most widely known and used. Polybutenes are nontoxic and environmentally friendly and are used as insulants in cables and LV metalized capacitors because of their high viscosities. Their technical specifications are given in CEI 60465 [39]. Polyolefins recently emerged as alternatives to mineral oils in transformers. They are nontoxic and biodegradable, and their flashpoints are in the range 240 to 250C [24]. However, their use has been limited by their high cost.

PhthalatesThe phthalates include two liquids, namely di-isononyl

phthalate and dioctyl phthalate. They were developed as al-ternatives to PCBs in LV and medium-voltage capacitors, but their use has declined significantly over the last 30 years, as a result of the development of dry-type metalized polypropylene film capacitors. However, dioctyl phthalates are still used for LV capacitors because their good dielectric properties facilitate the manufacture of capacitors that are more reliable than dry-type metalized polypropylene units. Their most important property is their high relative permittivity (5.2 at 20C), which leads to higher capacitances than those achievable using polybutenes or silicone oils. The main properties of dioctyl phthalate are listed in IEC 61099 [40].

High-Fire-Point LiquidsHigh-flash-point liquids, also known as less flammable liq-

uids, were developed as replacement impregnants in transform-ers formerly filled with PCBs. Qualifying liquids must have a minimum open-cup fire point of 300C [31], [35]. Because of this high degree of resistance to ignition, they are specified for transformers in locations with significant fire risks. At present, most high-flash-point liquids are produced from four different chemical bases, namely high-molecular-weight hydrocarbons (HMWHs), synthetic esters, dimethyl silicone, and vegetable oils [22], [24], [25].

High-Temperature or High-Molecular-Weight Hydrocarbons (HMWHs)

Other alternatives to PCB liquids are the HMWHs or high-temperature hydrocarbons. These liquids are chemically similar to regular petroleum-based mineral oils used in transformers. However, they have higher boiling points and higher molecu-lar weights, and therefore much higher fire points. HMWHs are classified as paraffinic, consisting mainly of saturated com-pounds with long, straight-chain structures. They have good di-electric and lubricating properties. They also have a higher vis-cosity, which reduces their heat-transfer capabilities.

HMWH-based liquids have been used in transformers fitted with load break devices such as load tap changers [41] and in unit substations, pad-mounted transformers, and oil retrofills [31]. Combined with Aramid insulation materials, they are also used in high-temperature transformers, i.e., those with 175 to 185C hot spots [30] found in mobile or double-ended substa-tions.

Synthetic EstersEsters are a broad class of organic compounds synthesized

from organic acids and alcohols. They do not generate dioxins or other toxic products in the presence of fire, and have good bio-degradability [42], [43], forming only carbon dioxide and water. Ester liquids have been developed to resist oxidation and can ab-sorb considerably more moisture than mineral oils before their performance as insulants deteriorates significantly. Previous work [35] has shown that ester liquids can be used for retrofill-ing mineral oilfilled transformers. Mixing ester liquid with up to 3% mineral oil does not degrade the electrical and dielectric


properties of the insulating system. Several types of esters are used in electro-technology:

(1) Tetraesters (or pentaerythritol esters) are environmen-tally friendly liquids, but their high costs compared to those of other less-flammable fluids limit their use to traction and mobile transformers and other special applications. Their properties are listed in IEC 61099 [40], and a guide for maintenance of ester-filled trans-formers is available [44]. In hermetically sealed trans-formers operating at normal temperatures, tetraesters can absorb moisture produced by thermal degradation of cellulose (paper) present in the windings, and are unlikely to require maintenance during the life of the transformer. In breathing transformers the need for liquid-insulator servicing depends on operating con-ditions and on the performance of breathing devices. Tetraesters can absorb much larger quantities of water than mineral oil, because of their carbonyl structure [35], [41], [42]. Since the viscosity of ester liquids is higher than that of mineral oil, a more efficient heat-transfer system is required with the former [35]. The higher moisture content results in some hydrolysis of the liquid, forming the mild free fatty acids typical of ester-based liquids.

Tetraesters have been used as alternatives to PCBs in compact railroad traction transformers since 1984, and in klystron modulators where their high lubricity and low pour points (close to that of mineral oils) jus-tify their higher cost [31]. Following the replacement of askarels with polyol esters, failure rates of traction transformers have significantly decreased.

(2) Phosphoric esters have been proposed for various ap-plications (capacitors, transformers) because of their high fire point and permittivity. However, their use is

very limited [19], [22], because of the environmental risks that they pose [45]. Dibutyl sebacate, dioctyl-seb-acate, and benzyl-neocaprate were used as impregnat-ing fluid for capacitors in the past, but not today be-cause of health hazards [19].

Silicone OilsSilicone oils are known chemically as poly-dimethyl silox-

anes or PDMS [22], [24]. They were introduced in the 1970s as substitutes for PCBs and have proven popular in retrofilling transformers [40], [42]. However, their use has been limited to situations where fire could pose a risk to personnel and property. Their chemical structure has been described [25], [46], [47]. Silicones are environmentally friendly and flame retardant, age well, and are strongly resistant to oxidation and sludge forma-tion [48]. As far as their cooling and insulating properties are concerned, they compare well with mineral oils. Silicone oils are colorless, and have very low pour points compared with mineral oils, even though their viscosities at 20C are much higher. In order to avoid large temperature rises during operation, silicone-filled transformers must be de-rated (up to 10%) or provided with additional cooling capacity.

Vegetable OilsVegetable oils are readily available natural products, and

therefore should be considered as ideal raw materials for fully biodegradable insulating liquids [49][52]. They consist essen-tially of triglycerides, which are naturally synthesized by esterifi-cation of the tri-alcohol glycerol with three fatty acids. The fatty acid composition of some vegetable oils is shown in Table 2 [17].

Experimental investigation of vegetable oils as dielectric coolants began around the early 1900s, concurrently with min-eral-oil trials [48]. Their poor dissipation factor and oxidation stability, and higher pour point, relative permittivity, and vis-cosity [31] have been their main disadvantages as dielectric flu-ids. A literature survey [17], [22], [24] indicates that for many

Table 2. Typical Fatty Acid Composition of Some Vegetable Oils [17].

Vegetable oil Saturated fatty acids, %

Unsaturated fatty acids, %

Mono- Di- Tri-

Canola oil1 7.9 55.9 22.1 11.1

Corn oil 12.7 24.2 58 0.7

Cottonseed oil 25.8 17.8 51.8 0.2

Peanut oil 13.6 17.8 51.8 0.2

Olive oil 13.2 73.3 7.9 0.6

Safflower oil 8.5 12.1 74.1 0.4

Safflower oil, high oleic content 6.1 75.3 14.2

Soybean oil 14.2 22.5 51 6.8

Sunflower oil 10.5 19.6 65.7

Sunflower oil, high oleic content 9.2 80.8 8.4 0.2

1Low erucic acid variety of rapeseed oil. Recently canola oil with more than 75% monounsaturated content has been developed.


years they were considered suitable only for capacitor use. Their unsaturation confers good gas-absorbing properties, which are desirable in capacitors, cables, and instrument transformers. However, the unsaturated parts of the chain are vulnerable to oxidation, resulting in poor oxidation stability. Castor oils have been widely used in capacitors (with cellulose insulation) since 1962 [17], [53], [54]. Unlike other vegetable oils, which are fatty acid esters, castor oil is 80% hydroxy-acid ester, the acidic part being ricin-oleic acid.

In the 1990s, mainly because of environmental concerns, utilities became interested in fully biodegradable insulating liq-uids, particularly for use in transformers located in coastal areas where oil spills would contaminate water [31]. Many vegetable oils, currently available, therefore contain additive chemical packages that reduce the pour points and enhance oxidation sta-bility. Typically a 10C lowering of the pour point, with negligi-ble change in electrical conductivity, can be achieved by adding a polymethyl-acrylate derivative at concentrations below 1%. In some cases the packages contain an antimicrobial agent or cop-per deactivator [17], [48][53]. Recently, a new vegetable-based insulating oil for transformers, called palm fatty acid ester, has been proposed. Relative to mineral oil its dynamic viscosity is 60% lower and its relative permittivity is 30% higher [55], [56]. Using chemical treatment of filtered samples, Abderrazzaq et al. [57] significantly improved the acidity of olive oil.

NanoliquidsNanotechnology is now being used or considered for use

in many engineering applications, with the aim of improving equipment efficiency. Nanofluids are finding applications in a wide variety of industries, from transportation to power engi-neering, in microprocessors and in micro-electro-mechanical systems (MEMS), and in biotechnology [58], [59]. A review of the most commonly used nanoliquid production methods can be found in the literature [58][62]. They consist of a base liquid in which nanosized particles (1100 nm) are suspended. The addition of nanoparticles can greatly improve the thermal and dielectric properties of the liquid, more specifically extending transformer lifetime and increasing loading/cooling capacity.

The most commonly used nanoadditives include metals and metal oxides. Yue-Fan et al. [2] developed a nanoliquid by dop-ing mineral transformer oil with TiO2 nanoparticles, in order to enhance its dielectric performance. Choi et al. [62] evaluated dispersions of nanosized Al2O3 and AlN powders in transformer oil, with small amounts of oleic acid as a dispersant. Nano-dia-mond particles were found to increase the dielectric strength and life of transformer oil [59], [63]. It has been suggested that the addition of magnetic nanoparticles may also increase the dielec-tric strength of transformer oil [64], [65]; although the dielectric

strength of the magnetic nanoliquids may be up to 13% higher, their increased loss factors may cause thermal problems under operational conditions. Recently, nanoliquids containing a new type of semi-conductive nanoparticles were investigated [66]. It was found that the semi-conductive nanoparticles improve the insulating and anti-aging properties of mineral oil, but have little effect on other electrical parameters, e.g., conductivity/resistiv-ity and dissipation factor.

Cryogenic Liquids and Liquefied GasThe discovery of superconductivity by Onnes in 1911 was

followed by the development of high temperature superconduc-tivity in 1986 [67]. The latter has found many applications in power engineering, e.g., in generators, magnetic energy storage systems, power transmission lines, transformers, and fault cur-rent limiters.

One of the critical components for superconducting devices is the liquid used to achieve cryogenic temperatures. In cables and transformers the cooling liquids must act simultaneously as insulating liquids under the relevant voltage stress. These liq-uids are condensed from atmospheric gases. The boiling points (at a pressure of 1 atmosphere) of some common cryogenic liq-uids are listed in Table 3. Carbon dioxide and nitrous oxide have slightly higher boiling points.

Liquid helium is an established cooling and insulating agent. Since the development of high-temperature-superconductivity materials, liquid nitrogen, with its superior dielectric breakdown strength, has become the preferred cooling and insulating liquid. A comparison of the dielectric properties of liquid helium and liquid nitrogen is available [67].

The feasibility of substituting CF4 gas for SF6 gas as insulant for the bushings of high-temperature-superconductivity materi-als was recently explored [68]. Mixtures such as liquid oxygen/nitrogen may exhibit better cooling performance than liquid ni-trogen [69]; however serious risk associated with highly chemi-cally reactive oxygen may arise under partial discharge or arc-ing.

Superconductors cooled by a cryogenic liquid have consider-able industrial and research potential because they facilitate high current densities without Joule heating [70][76]. However, much work is needed to exploit this potential to the full.

Mixed Insulating LiquidsIn order to obtain stable insulating liquids with specific di-

electric, flash point, and thermal properties, various mixtures have been investigated over the last 50 years [22], [24], [77][81]. Some examples are listed in Table 4. Generally, the physi-cochemical properties of the mixture are intermediate between those of the constituents, depending on the mixture ratio.

Physicochemical and Dielectric Properties of Insulating Liquids

Despite great progress in power-equipment design in recent years, the weak link in the chain still remains the insulation sys-tem. A low breakdown voltage compromises operational safe-ty, and the irreversible aging process shortens life expectancy.

Table 3. Boiling Points of Common Cryogenic Liquids.

Ar He H2 N2 O2 Ne

Boiling point (1 atm), C

186 269 253 196 183 246


When electrical equipment fails, the fault can usually be traced to defective insulation [13].Dielectric Behavior

During the last 50 years much work has been done with the aim of improving our basic knowledge of insulating liquid per-formance. Understanding the fundamental causes of insulation breakdown, i.e., the conditions necessary for electron avalanche formation, is essential to ensure reliable design of liquid-filled equipment. Earlier studies were concerned with the physical phenomena involved in electro-hydrodynamic processes [82][84], and with streamer initiation and development [85][94]. Reviews are available [23], [88]. Mathematical models have been developed to simulate the fundamental processes govern-ing discharges in oil [95][98]. Pioneering work by Forster has clarified the mechanisms by which high-voltage fields interact with insulating oils [99]. Static electrification is another impor-tant factor threatening the safety of power transformers [100][106]. Using modern laboratory testing techniques, researchers have improved our understanding of the physical mechanism by which discharges are initiated in dielectric liquids. However, much work is still required.

Insulating Liquid GassingFundamental investigations have also been carried out on

the gassing of oils [100][106]. Knowledge of the resistance of insulating fluids to gassing under high electrical stress is of utmost importance to electrical equipment designers and opera-tors [107]. While in service, insulating liquids undergo a slow but steady decay process under the impact of electrical, thermal, mechanical, and environmental stresses. Incipient failures such as hot spots and partial discharges are responsible for the gassing of oil. Since the resulting fault gases dissolve in the oil, the dis-solved gas analysis technique was developed in order to detect incipient failures at an early stage, and is now probably the most frequently used in-service tool for detecting faults in liquid-filled electrical equipment [108]. About 20 dissolved gas analy-sis interpretation techniques have been developed so far [109],

e.g., IEC 60599, IEEE C57.104, Duvals Triangle and the Key Gas method [110][113]. Since all these methods are heuristic in nature, i.e., not based on scientific formulation, combining several of them may reduce the risk of mistaken diagnostics and enhance accuracy [114]. Dissolved gas analysis techniques are being developed for liquids other than mineral oil [115][117].

A Powerful Tool for Life ManagementIn addition to gases, insulating liquid decay generates aging

by-products, which promote further degradation [118][120]. Aged or moistened liquids may be treated by drying out, de-gassing, reclamation, re-refining. and reconditioning [121]. A review of the present state of knowledge of liquid treatment pro-cedures is presented in [122].

The presence of moisture (considered the main enemy of in-sulation) in solid and liquid transformer insulation is known to play a critical role in transformer life [123][125]. The mois-ture content of the oil can change quickly within an operational transformer. Direct measurement of moisture content in paper insulation (cellulose) is complex; moisture partitioning curves between oil and paper under equilibrium conditions have been published by several authors [126], [127], so that, the moisture content of the oil having been measured, the moisture content in the paper can be quickly estimated and the probability of failure predicted. A comprehensive review and comparison of various partitioning curve sets has been published [126]. Complications due to fast dynamic diffusion processes arise. Another problem with the partitioning diagrams is that they are based on new oil and do not take into account the effects of aging by-products found in aged transformer oil. Diagnostic techniques, based on dielectric spectroscopy, for assessing the condition of the insula-tion in aged transformers are reviewed in [108].

In free breathing units such as transformers, the insulation system ages under the influence of electrical stress, moisture, dissolved oxygen, and excessive heat. The chemical aggressive-ness of oxygen facilitates the formation of soluble oxidation products and insoluble sludge, which are detrimental to solid

Table 4. Examples of Mixed Insulating Liquids.

Mixture Application

Mineral oil + alkylbenzene Oil-impregnated-paper capacitor (to improve the gassing tendency)

Mineral oil + perchloroethylene (C2Cl4) Distribution transformers (C2Cl4 oil blends have been classified as nonflammable)

Polychlorinated biphenyl (PCB) + trichloro-benzene (TCB) Distribution transformers (to improve viscosity)

Tetracholorobenzyltoluene (TCBT) + TCB Distribution transformers (to improve viscosity and biodegradability)

Alkylbiphenyl + alkyldiarylalcane Capacitors (biodegradability)

Ester + TCB Capacitors (PCB substitute)

Ester phosphate + alkyldiarylalcane Capacitors (PCB substitute)

Ester phosphate + aromatic hydrocarbon Capacitors (PCB substitute)

Tetracholorodifluoroethane + perchloroethylene (C2Cl4) Traction transformer

Synthetic ester + mineral oil Distribution transformers (to improve fire point, hygroscopicity, and biodegradability)


insulation. Use of antioxidant additives, nitrogen cushions, and elastic rubber or plastic bags is advantageous because it limits access of oxygen to the liquid insulant. Recently, an environ-mentally friendly on-line innovative maintenance procedure was found to remove a large fraction of the oxygen and water dis-solved in the oil of freely breathing transformers [128].

The easiest and most convenient way to diagnose the state of the insulation in liquid-filled equipment is to use the liquid as a diagnostic medium. The development of several new laboratory testing procedures for insulating liquids over the past 50 years has resulted from cooperation between refiners, manufacturers, and users of insulating oils. Mutually acceptable standards and test requirements have been written, e.g., [33], [37], [39], [43], [44], [47], [129][134].

Use of AdditivesVarious additives expected to improve the dielectric or physi-

cochemical properties of oil have been investigated during the last 50 years. They include various chemicals that act as inhibi-tors, passivators, electron scavengers, or pour-point depressants [15], [23]. Small amounts of these additives improve oxidation stability, optimize gas absorption and/or gas evolution, increase dielectric strength or partial discharge inception voltage, protect against catalytic reactions, and reduce electrostatic charging. In-hibitors/additives are blended into the oil during the manufactur-ing process or introduced during routine servicing.

Two primary phenolic antioxidants are approved for use in electrically insulating oils, namely 2,6-ditertiary-butyl para-cresol and 2,6-ditertiary-butyl phenol. ASTM D3487 [133] and IEC 60296 [134] specify 0.08% (by mass) of 2,6-ditertiary-butyl para-cresol, butylated hydroxytoluene, or 2,6-ditertiary-butyl phenol for Type I oils; ASTM D3487 specifies 0.3% and IEC 60296 specifies up to 0.4% for Type II oils. These addi-tives enhance the resistance of the oil to oxidation and therefore increase the expected lifetime of the insulation. Severe hydro-processing, which effectively removes natural contaminants and pro-oxidants such as sulfur, nitrogen, and oxygen compounds, and some aromatics, enhances the effect of added synthetic anti-oxidants [135]. The antioxidants perform better in these cleaner oils since they do not have to counteract the negative aspects of contaminants [135]. The cleaner the oil, the better is its antioxi-dant function and the longer is the life of the transformer.

Aromatic compounds influence the oxidation resistance, gas-sing properties, and impulse strength of oil. However the poly-cyclic aromatic hydrocarbons are environment pollutants, and some of them are recognized as cancer initiators. The 16 main polycyclic-aromatic-hydrocarbon pollutants are listed in [136]. The polycyclic-aromatic-hydrocarbon content of oil must be less than 0.1% (by volume) [13], [136], [137]. In order to avoid labeling as carcinogenic, mineral oils are hydrotreated, which results in the removal of most aromatic hydrocarbons, including those that are beneficial [136]. Mono/dibenzyl-toluene, a new type of impregnant for all-film power capacitors, has therefore been added to mineral oil or to in-service oil-filled power trans-formers since 2000 in order to increase the aromatic content of the oil [24]. The addition of a few percent of mono/dibenzyl-tol-uene can transform a gas-generating oil into a gas-adsorbing oil.

In recent years, several failures of transformers and reactors because of copper sulfide formation in the cellulose insulation have been reported worldwide [138][140]. The concentration of sulfur in mineral oil depends on the parent crude oil and the degree and method of refinement. Under high electrical stress, high temperature, and dissolved oxygen, sulfur can become cor-rosive and react chemically with copper. Some transformer and oil manufacturing companies recommend the use of metal pas-sivators (Irgamet 39) in at-risk transformers. A concentration of 100 ppm can be added to the oil during hot oil filtration or refurbishment. Metal passivators react chemically with the sur-face of a metal, forming a microscopic protective coating against catalytic reaction.

Conclusions and PerspectivesInsulating liquids are a vital part of the electrical insulation

system in many types of electrical power equipment, includ-ing transformers, bushings, cables, and capacitors. Each ap-plication requires an insulating liquid with specific electrical, chemical, and physical characteristics. However, research and development on oil-filled circuit breakers and cables have al-most stopped; extruded insulated cables (mainly polypropylene or cross-linked polypropylene) have replaced oil-filled cables, and circuit breaker technology has evolved toward the use of SF6 at high voltage and vacuum at medium voltage.

Mineral oils have been used in electrical apparatus for over a century, and they have a long and proven track record. Qual-ity and stability requirements have become more stringent over time; the last 50 years have seen much improvement in quality as refining technology has advanced through the use of catalytic hydro-processing [26]. Because of their excellent performance, availability, and low cost, mineral oils have met with little com-petition. Concern over fire safety has prompted the development of high-temperature mineral oils for critical applications.

PCBs, once promoted for their excellent fire-safety proper-ties, have fallen out of favor since the mid-1970s because of the health hazards that they present and their environmental per-sistence. Their banning led to the development of several other nonflammable halogenated liquids such as perchloroethylene, which are however no longer marketed [65], mainly because of strong public resistance to chlorination.

Growing demands for improved fire safety, material sustain-ability, environmental friendliness and extended asset service lifetimes have driven the development of alternative insulat-ing liquids. Since the end of the 1970s, natural/synthetic esters, which are less-flammable than mineral oils, have been developed mainly for fire-safety applications. Hydrocarbon-based fluids are only approximately 30% biodegradable, silicone oils have very low biodegradability, poly--olefins have approximately 70% biodegradability, and pentaerythritol-based ester and veg-etable oils are fully biodegradable. Nevertheless, mineral insu-lating oil is still used extensively. Concomitant development of advanced new materials (for high-temperature insulation) will ensure a significant upgrade in the reliability of liquid-filled power equipment. Reliable long-term performance of a biode-gradable insulating liquid is crucial in any power equipment [141]. It must have a sufficiently high withstand voltage over


the equipment lifetime, say at least 30 years, in order to ensure return on investment.

No liquid is superior to all the others. Each has its advantages and disadvantages, and must be used in specific applications. Table 5 summarizes the main features and applications of sev-eral types of insulating liquid.

What Does the Future Hold?The inherent properties of mineral oils ensured their use as

electrical insulants over the last century, and will ensure their continued use for decades to come. However, there are two rea-sons why we should be seeking alternative natural insulating liquids. These are the poor biodegradability of mineral oil, and

Table 5. Properties of Insulating Liquids Used in the Last 50 Years (Updated Version of a Table Published in [19]).

Category Type of liquid Applications Particular properties

Mineral oils Naphthenic, paraffinic Liquid-filled power equipment (transformers, circuit breakers, load tap changer, etc.)

Good resistance to oxidation Good viscosity index Relatively low fire point Low moisture tolerance Possible sulfur corrosion

High-molecular-weight hydrocarbons

Paraffinic Transformer, load tap changer High flash point

Vegetable oils Castor, soybean, cotton, palm, etc. Capacitors, transformers Low dielectric losses at frequency higher that 1 kHz Readily biodegradable Low oxidation stability

Synthetic liquids

Synthetic hydrocarbons Polybutenes Cables Low dielectric losses Adjustable viscosity

Alkylbenzenes Bushing, cables, capacitive dividers Gas absorbing under partial discharges Good lightning impulse breakdown strength

Alkylated hydrocarbons with condensed aromatic rings (DIPN)

Capacitors Good lightning impulse breakdown strength

Alkylbiphenyls (MIPB) Capacitors Readily biodegradable Gas absorbing under partial discharges

Alkyldiarylalcanes (BT, DBT) Capacitors Gas absorbing under partial discharges

Halogenated hydrocarbons Askarels (PCB) Capacitors and distribution transformers

Nonflammable Thermal stability

Polychoro-diphenyl methanes Transformers Nonflammable Biodegradable Thermal stabilityPolychoro-alcanes Transformers

Silicone oils Poly-dimethyl siloxanes or PDMS, poly-methylphenyl siloxanes

Traction and distribution transformers

Good viscosity index High flash point Gas absorbing under partial discharges High oxidation stability Low biodegradability

Organic esters Simple esters Capacitors Relative permittivity, at 20C, higher than 5

Phtalates PCB substitute Used to increase flash points of some liquids

Complex esters, tetraester of pentaerythritol

Traction and distribution transformers

High flash point High moisture tolerance Readily biodegradable High oxidation stability

Other liquids Ethers (alkyl-diphenyl ether, ditolyl-ether)

Capacitors Relative permittivity, higher than 3 at 20C Adaptability

Nanofluids Transformers Adaptability

Mixed liquids Capacitors, transformers Adaptability

Cryogenic dielectric liquid (nitrogen) Superconductivity and cryogenic applications

Reduction or suppression of Joule heating


the growing demand for petroleum products, which could lead to serious shortages as soon as the mid-21st century.

Given growing environmental concerns, fully biodegradable oils, improved with suitable additives, will be important in the future. Natural and synthetic esters are generally limited to dis-tribution, traction, and mobile transformers, and other special-ized applications. It is not yet known whether the use of natu-ral esters in power transformers over lengthy periods will have any adverse outcomes. Collection of track records for in-service equipment, and continuing fundamental investigations, would be expected to increase the use of natural esters in power trans-formers [115][117], [142][145].

Nanotechnology is poised to affect the insulating-liquid in-dustry dramatically [58]. The dielectric properties of nanoliq-uids have not been fully explored. Although nanoparticles such as metal oxides are already widely used, research in this area is still at an early stage. It has been demonstrated that the heat transfer properties of oils can be significantly improved by us-ing nanoparticle additives [58], [67]. Ongoing research will in-crease our knowledge of the fundamental mechanisms through which nanoparticles interact with liquid matrices. The ever-growing demand for electrical power will lead to a demand for more highly rated oil-filled apparatus. A potential alternative in many cases is the replacement of mineral oil with appropriately modified nanoparticle liquids, with considerable cost savings. Another important potential benefit of nanofluids is an increase in breakdown voltage, which should allow more compact elec-trical apparatus design.

Smart fluids, whose flow properties can be changed through application of a low-power control signal, have also emerged during the last decade [146], [147]. Two main classes of smart fluid are available, namely electrorheological and magnetorheo-logical. Electrorheological fluids generally consist of semi-con-ducting particles suspended in a dielectric oil, whereas magneto-rheological fluids use magnetizable particles suspended in a non-magnetizable carrier liquid. In both cases the flow mechanism is the same; excitation of the fluid by the appropriate field (electric or magnetic) causes polarization and subsequent alignment of the particles suspended within the liquid. It is believed that smart liquids containing multifunctional nanoparticles could be cus-tomized with specific properties, e.g., reduced dielectric loss, for application in liquid-filled power equipment.

Superconductivity is also going to affect the future of insulat-ing liquids. The critical temperatures of several high-tempera-ture superconductors are around 135 K [148]. Around the world, many research projects to develop commercial superconducting devices are underway, seeking new materials with higher critical temperatures [48][51], [58][76]. Cryogenic liquids are there-fore going to play an important role in the future.

AcknowledgmentsThe author is much indebted to Dr. R. J. Fleming, co-editor-

in-chief EIM, for his help in improving the English expression and quality of the manuscript.

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Issouf Fofana (M 05, SM 09) received the electro-mechanical engineering degree in 1991 from the University of Abidjan (Cte dIvoire) and the masters and PhD degrees from the Ecole Centrale de LyonFrance in 1993 and 1996, re-spectively. He was a postdoctoral re-searcher in Lyon in 1997, and, from 1998 to 2000, at the Schering Insti-tute of High Voltage Engineering Techniques, University of Hanover, Germany. He was a Fellow of the Al-exander von Humboldt Stiftung from

November 1997 to August 1999. He joined the Universit du Qubec Chicoutimi (UQAC), Quebec, Canada, as an associate researcher in 2000, and is now a professor there. Dr. Fofana has held the Canada Research Chair, tier 2, of insulating liquids and mixed dielectrics for electrotechnology (ISOLIME), since Sep-tember 2005. He is registered as a professional engineer in the province of Quebec, and is currently a member of the Executive and Technical Committees of the IEEE CEIDP and the Interna-tional Advisory Committee of the IEEE ICDL. He is a member of the IEEE Task Force on atmospheric icing performance of line insulators, and member of the ASTM D27 Task Group to develop a test method for partial discharge inception voltage. He has authored or coauthored more than 200 scientific publications and holds 3 patents.

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