ELECTRICAL INSULATION MATERIALS - Lec3...ELEC712: Electrical Insulation Materials and HV Testing page 3/64 In simple configurations such as cables and overhead lines, the electric

ELEC712: Electrical Insulation Materials and HV Testing page 1/64

ELEC9712 High Voltage Systems

ELECTRICAL INSULATION MATERIALS

Power system function: to generate, transport and distribute electrical energy over large geographical areas in an economical and reliable manner.

Power transmission is best accomplished at high voltage (to reduce losses). Thus HV equipment is the backbone of modern power systems. For such equipment, electrical insulation of the HV conductors is critically important.

For proper design, we need to know physical and chemical phenomena which determine dielectric properties of the insulation materials.

Need to know processes which lead to degradation and failure of such materials. Also, what appropriate diagnostic techniques are available to assess the state of the materials?

This lecture reviews the various insulants utilized in the power system and describes the types of measurements available for assessment of the basic materials and how particular techniques are applied in the power industry.

It must be emphasized that much of the value of diagnostic testing would be lost unless several different techniques are employed and, also, the measurements are carried out as part of long-term routine maintenance programs.


1. Electric stress and temperature effects

High voltage → high electric stress → cause insulation failure → short circuit.

High temperature → gradual insulation deterioration or thermal breakdown.

Note also other effects, e.g. mechanical stress, environmental→ multi-factor ageing

1.1 Electric field stress

The electric field intensity E at any location in an electrostatic field is related to the force F experienced by a charge q as F=qE. Moreover, the electric flux density D associated with E is given as D=εE. If the medium is free of any space charge, the electric field is obtained from the solution of the Laplace equation:

2 0φ∇ =

where φ is the potential which is related to E and path l through which the charge is moved by:

.E dφ = −∫ l

The electric field in the insulation material must be known under all possible conditions of operation, so that the insulation can be designed and chosen to have a dielectric strength greater than the applied field levels. This design must be such that this is the case under normal operating electric field stress and also under high-voltage transient (impulse voltage) conditions.


In simple configurations such as cables and overhead lines, the electric field can be calculated easily. However, in more complicated field geometries such as those in bushings and transformers, the electric fields have to be computed using computer software which performs finite element analysis solutions of the Laplace equation.

Equipment

Operating stress

(kVrms/cm)

Design stress (kVpeak/cm)

Insulation complexity (relative)

Generators Transformers SF6 equipment Capacitors

25 15 40

600-1000

130 115 180

2000-3000

1.0 0.9 0.3 0.2

Dielectric design parameters of typical HV power equipment. 1.2 Temperature consideration

The conductor is at high temperature due to Ohmic heating. Thus, the insulation temperature is primarily determined by the temperature of the conductor that is in contact with the insulation. Furthermore, at HV there can be other contributions to heating from the electric field in the insulation which will cause dielectric losses: (1) Ohmic heating:

This is due to conduction current through the insulation. The insulation material is not ideal and hence its resistance is not infinite.


(2) Polarization:

Most electrons in insulating materials are bound and not free to move. Under influence of applied electric field, resulting electrostatic forces cause polarization and form dipoles. It is this electronic polarization which results in

1rε > for most dielectrics.

In some crystalline dielectrics, relative displacement occurs between positive and negative ions, producing polarization.

In some organic substances including many polymers, permanent molecular dipoles are reoriented in electric field.

Interfacial polarization can occur in heterogeneous materials whereby mobile conduction charges are held up at some boundary within the dielectric, e.g. in electrolytic capacitors.

(3) Partial discharges:

Electrical partial discharges can occur locally, e.g. voids within the solid insulation structure. Such breakdowns can also generate heat.

All the above-mentioned dielectric losses are lumped together as the dielectric dissipation factor (DDF or tanδ). These losses increase the temperature of the dielectric and are themselves temperature dependent. In regions in which the dielectric losses increase steeply with temperature, there is a danger of overheating, and this will eventually leads to thermal breakdown.


2. Examples 2.1 High-voltage cables

(i) Electric field

V

r=br=a

a brE(b)

E(a)

r( ) ( ).ln /VE r

r b a=

( )maxE E a=

( )minE E b=

V

r=br=a

a brE(b)

E(a)

r( ) ( ).ln /VE r

r b a=

( )maxE E a=

( )minE E b=

i.e. highest stress occurs at surface of inner conductor. In HV cables, this is taken as the operating stress, which is the determining factor in the design. Usually, choose Emax to be about 30-40% of insulation dielectric strength. Typically, design electric field levels in HV cables are about 10-15kV/mm (peak) for 50Hz polymeric insulated cables (XLPE breakdown strength is about 40kV/mm). (ii) Heating effects

1. Ohmic heating in the conductor 2. Eddy currents in outer sheath and armour 3. Dielectric losses in the insulation


2.2 Overhead lines

(i) Electric field

V

x=rx=d/2

VN

E(x)E

( ) ( )1 1

2ln /VE xd r x d x

⎡ ⎤= +⎢ ⎥−⎣ ⎦

d r

( ) ( )2 .ln /VE x

x d r=

x=0 x=dx=d-r

VN

For

( )max 2 .ln /VE

r d r= V/m

V

x=rx=d/2

VN

E(x)E

( ) ( )1 1

2ln /VE xd r x d x

⎡ ⎤= +⎢ ⎥−⎣ ⎦

d r

( ) ( )2 .ln /VE x

x d r=

x=0 x=dx=d-r

VN

For

( )max 2 .ln /VE

r d r= V/m

The major problem is the generation of corona discharge which can occur when the electric field at the bare conductor surface is high enough to cause some local ionization (discharge) of the air in the vicinity of the conductor. Corona discharge will start at an electric field


level of about 3E kV/mm in air at normal levels of temperature, humidity and pressure.

There are no permanent effects of breakdown in air insulation. Recombination of ions and electrons occurs to re-generate air molecules. There is no chemical change, apart from a small ozone generation and thus there is no permanent insulation damage. It is infinitely renewable in open-air insulated installations.

Although corona is not a major problem (does not stop operation of transmission system), it represents a power loss and also produces significant electromagnetic interference problems and so must be avoided in normal operation. (ii) Heating effects

The DDF of air is negligible and so heating is not a problem. The only factors to be considered in the design are ambient conditions. Rain or high moisture levels in air will cause a reduction of dielectric strength, as will a decrease in the air density, such as occurs in elevated or mountainous areas.

Similarly, increase in temperature can cause reduction in air density and thus also reduce dielectric strength. This is particularly important when bushfires are burning under HV lines. Fires can also inject conducting particles (soot) which changes electric field distribution and result in reduction of breakdown voltage of line to ground.


3. Electrical properties of dielectrics The four electrical properties of practical importance are resistivity, dielectric constant, dielectric dissipation factor, and dielectric strength. 3.1 Resistivity

The volume resistivity ρ :

Insulators: 1019 to 106 Ω.cm Semiconductors: 106 to 10-3 Ω.cm Conductors: 10-3 to 10-6 Ω.cm ρ varies with temperature T:

( ) ToT e αρ ρ −=

The decrease may be by a factor of 10 for about 100oC rise. There is also a dependence on the electric stress E but it is fairly weak. 3.2 Dielectric permittivity

Dielectric permittivity, also called relative permittivity or dielectric constant, rε , is defined as:

/r oC Cε =

where C is capacitance between two parallel plates having space between them filled with the particular insulating material, and Co is capacitance for same parallel plates when these are separated by vacuum.


Generally, rε is not a constant but varies with temperature, frequency and molecular structure of insulating material. 3.3 Dielectric dissipation factor

The insulation is taken as a lossy capacitor and modeled as as parallel RC network. Here R represents all dielectric losses.

δ is called the loss angle. Typically, it is very small and if so, the (open-circuit) power factor of the dielectric is:

cos tanφ δ δ≈ ≈

tanδ or simply δ is the commonly-used factor to describe the quality of the insulation. The correct terminology is the dielectric dissipation factor (DDF) or loss tangent. The unit is radian but typical level of a good insulation material is a few milli-radians. Note that in the above phasor diagram, R insulationI V R≠ . The insulation resistance is very high. The resistance used to model the dielectric losses is only a notational, not a real resistance.

CI CVω= tan tanR CI I CVδ ω δ= =

C R

IC IR

I

V

IR

IC I

δ φ V


Power loss = 2 tanRVI CVω δ= watts

Also: tanR CI I jI CV j CVω δ ω= + = + ( )tano r r o rj C V j j C Vω ε ε δ ω ε= − =

where rε is the complex relative permittivity. 3.4 Dielectric strength (breakdown strength)

Defined as maximum value of applied electric field at which a dielectric material, stressed in a homogeneous field electrode system, breaks down. It is given as V/m.

In many practical applications, the breakdown strength under inhomogeneous field conditions needs to be defined and is sometimes referred to as the non-uniform field dielectric strength.

Dielectric strength of insulation is dependent on frequency of applied electric field. In general, high frequencies reduce dielectric strength. This is important when considering effects of very fast transient impulse voltages often present in power systems. These voltages contain frequencies up to 100kHz or higher. Note that dielectric strength to DC is usually greater than that for AC voltage stress.


4. Insulating materials

Hundreds of insulating materials are used in electrical power industries. These can be broadly classified into different categories: gases, liquids, solids, vacuum and composites. kV/mm εr

oC W/(moC) kg/m3 Dry air Hydrogen Nitrogen Oxygen Sulphur hexafluoride SF6 Helium

3 2.7 3.5 3

30 400kPa

1.5

1 1 1 1 1 1

2000 - - - - -

0.024 0.17

0.024 0.025 0.014

-

1.29 0.09 1.25 1.43 6.6

-

Solid asbestos Asbestos wool Askarel Epoxy Glass Magnesium oxide

1 1

12 20 100 3

- -

4.5 3.3

5 - 7 4

1600 1600 120 130 600 1400

0.4 0.1 -

0.3 1.0 2.4

2000 400

1560 ~1800 2500

- Mica Mineral oil Mylar Nylon Paper (treated)

40-240 10 400 16 14

7 2.2 3

4.1 4 - 7

500-1000110 150 150 120

0.36 0.16

- 0.3

0.17

2800 860

1380 1140 1100

Polyamide Polycarbonate Polyethylene Polyimide Polyurethane

40 25 40 200 35

3.7 3.0 2.3 3.8 3.6

100-180 130 90

180 90

0.3 0.2 0.4 0.3

0.35

1100 1200 930

1100 1210

Polyvinylchloride (PVC) Porcelain Rubber Silicon Teflon

50 4

12 – 20 10 20

3.7 6 4 - 2

70 1300 65

250 260

0.18 1.0

0.14 0.3

0.24

1390 2400 950

~2300 2200

Material properties - breakdown strength, relative permittivity, thermal stability limit, thermal conductivity, and density.


4.1 Gases In normal states, most gases are good insulators. Overhead lines and some circuit breakers use air insulation. In addition to air, a few gases are used as insulants. These include sulphur hexafluoride (SF6), nitrogen (N2) for applications in equipment such as switchgear, cables and transformers and hydrogen (H2) in large turbo-generators. Atmospheric air is the most abundant dielectric material which has played a vital role in providing a basic insulating function in almost all electrical components and equipment. The electrical properties of air are well documented. As with all materials, the electric strength of air is relatively stronger for smaller spacings and is also a function of pressure. This relationship is referred as the Paschen’s law. In some cases the predictable breakdown strength and self restoring property of air are used in protective devices, e.g., rod gaps and gap type surge arresters. At very high voltages, and therefore long gaps, it is found that switching surges (e.g. 200/2000μs) will cause flashovers at relatively low values if the more highly stressed electrode is at positive potential. This factor is of importance in the design of transmission lines and substations having system voltages of the order of 500kV and above. Sulphur hexafluoride (SF6) gas is non-toxic and non-flammable. It has a relatively high dielectric strength compared to other gaseous dielectrics. It possesses excellent cooling capability and arc-quenching property. As such, SF6 has been one of the key gaseous insulants widely used in HV


power equipment such as switchgear, transformers and cables for many years. The use of SF6 eliminates the problem of fire hazards (associated with oil insulation) and also resulted in considerable reduction in weight and size of the equipment. There are some drawbacks associated with the use of SF6. It is one of the most potent man-made greenhouse gases, about 25 thousand times worse than CO2 gas in terms of global warming potential. It has a very long lifetime in the atmosphere, estimated to be a thousand years or longer. Nearly all the SF6 which has been released to date is still in the atmosphere. Another concern is the decomposition of SF6 under electrical discharges as the decomposition process will result in the formation of lower fluorides of sulphur which are toxic and corrosive. Consequently, much recent research interest has been on the possible use of SF6/N2 gas mixtures as the alternative, in particular mixtures of low concentration SF6 (<30% of SF6 by volume in the mixture). Small amounts of electron attaching gases such as SF6 in N2 can substantially increase the dielectric strength of the mixture. Although SF6/N2 gas mixtures do not have the full desirable properties of pure SF6, it can be used to substantially reduce the amount of SF6 used in the power industry. Both air and SF6 exhibit high electric breakdown gradients at small spacings and in near uniform fields. Nitrogen gas is used at pressures up to 1.0 MPa in standard capacitors and in some forms of cable: for large power transfers the use of SF6 may become economic. SF6 is widely used in many forms of


switchgear and busbar configurations - giving a great reduction in volume compared with conventional systems. Hydrogen exhibits a breakdown strength of about half that of air at the same pressure. Little published data is available on the freon gases but they have been used successfully for the insulation of distribution transformers. 4.2 Vacuum Has excellent insulating and arc quenching properties. True vacuum is very difficult to achieve and vacuum insulated equipment may have residual gas pressure of ~10-9 to 10-12 bar. In such equipment, material, shape and surface finish of electrodes, residual gas pressure and contaminating particles are important factors. Examples are medium voltage switches and circuit breakers.

Note: 1atm = 1013mbar ; 1bar = 100kPa ; 1torr=133.32Pa 4.3 Liquids Insulating liquids are used in electrical equipment such as transformers, switchgear, cables, bushings, capacitors, mainly as insulants but also in some cases (transformers, cables) as cooling media. In certain cases, these liquids act as arc extinguishers (switchgear) and even as lubricants where moving parts are present (switchgear, tap changer). Different types of equipment require dielectric fluids with special properties. Transformers and most other types of electrical equipment need liquids with high dielectric


strength, impulse strength and resistivity but low dielectric dissipation factor. It must also possess high specific heat and thermal conductivity along with low viscosity and pour point in order to keep equipment cool. Good thermal and chemical stability and gas absorption properties are also desired. Capacitors also require dielectric fluids with high discharge resistance and switchgear needs fluids with arc quenching properties. Other desirable properties are high flash points, if possible liquids should be non-flammable and for ease of handling and for ecological purposes they should be non-toxic. It is also important to determine not only whether the fluid possesses good electrical properties but also whether these properties can be maintained during the life of the equipment with or without processing or small additions to the liquid of other materials (eg. oxidation inhibitors, passivators, acid scavengers) to help the dielectric fluid to maintain its original electrical properties. Examples of liquid insulants include petroleum (mineral) oils, esters, chlorinated liquids, silicones, synthetic hydrocarbons, fluorinated hydrocarbons, liquefied gases, electronegative fluids. Petroleum oil is the most widely used. It is classified according to proportion of 3 types of naturally occurring chemical structures originating from crude oil used in their manufacture: paraffinic oil, naphthenic oil and aromatic oil.


Synthetic oils were introduced for use in distribution transformers many years ago in order to overcome the fire hazard associated with hydrocarbons. The particular oil, askarel (predominantly polychlorinated biphenyl - PCB) is now unacceptable in the majority of cases because of health reasons. An advantage of PCB oils is that they have a high permittivity which is of value in power capacitor applications. Alternatives have now been developed and are replacing PCB as practicable: one difficulty is the disposal of the existing PCB oils. Replacement oils include silicone liquid, the price of which is now more acceptable. Other liquids have been developed and are being used in mixed and all-film capacitors. A well established synthetic oil is dodecylbenzene (DDB) developed for use in high voltage cables. It is claimed that the liquid has better ageing and gas absorption characteristics than natural oils. If low temperature cables prove to be commercially viable, it appears that liquid nitrogen and/or helium will be considered as impregnants for lapped plastic dielectrics. The technology exists if such an application becomes economically viable. 4.4 Solids The use of a "solid" material is essential in any system or equipment as parts at different potentials must be held apart physically. This is achieved by providing puncture strength through the insulating material and a surface of sufficient length around the external surfaces. The relative values of thickness and creep will depend on the material, the ambient


medium and the electric field (magnitude and distribution). The necessary existence of creep surfaces requires much careful design of terminations and support structures. The correct choice of materials is essential, especially where no electric stress control is possible or economic and environmental conditions are poor. In some cases the insulation system is designed to flashover before puncture of the solid, as for the components in an overhead transmission line where air is the ambient medium.

Synthetic polymers

Organic

Inorganic

Thermoplastic

Thermosetting

amber paper pressboard rubber wood resins

ceramics glass mica fiber glass enamel

perspex polyethylene polypropylene polystyrene polyvinyl chloridepolyamid polycarbonate

epoxy resins phenolics melamine urea formaldehyde crosslinked polyethylene elastomers

Classification of some commercial solid dielectrics. Ideal solid dielectric must have some of properties mentioned above for gases and liquids. In addition, it should have good mechanical and bonding properties. Solid dielectrics have high breakdown strength as compared to liquids and gases. The most common classification method is by chemical composition, such as organic, inorganic and synthetic polymers. From application’s point of view, they can be classified as: (1) thermoplastic compounds, (2)


thermosetting compounds and (3) embedding and jacketing compounds. Organic materials are derived from either vegetable or animal matter. They deteriorate rapidly if operating temperature exceeds 100oC. They are mostly employed after treatment with varnishes or impregnation in oil, e.g. paper and pressboard. Inorganic solids are difficult to fabricate but they are good dielectrics and can operate at higher temperature. The most important inorganic materials are ceramic and glasses which are used to manufacture insulators, bushings and other HV components. Synthetic polymers are divided into 2 groups: thermoplastic and thermosetting. Thermoplastic polymers have low melting temperatures (100-120oC). Thermosetting polymers are moldable when first heated, but after they cool, they will no longer soften when heated. The most prominent materials are thermosetting epoxy resins or thermoplastic materials such as poly-vinylchloride (PVC), polyethylene (PE) or cross linked polyethylene (XLPE). Thermoplastic materials are mainly used for manufacture of extruded dielectric power cables. 4.4.1 Wood Wood is one of the oldest insulations used by electrical engineers and despite limitations in the natural form it is still widely utilized. Outstanding application is in transmission


lines where its relative cheapness and insulating properties are attractive - particularly in areas with high lightning levels. 4.4.2 Glass Glass is a thermoplastic inorganic material comprising a complex system of oxides (SiO2). Glass is defined as a liquid that has cooled to a rigid solid without crystallization. Most common application is in the form of fiber glass, which is used (1) in bandaging core packets of transformers, (2) as resin-impregnated fiberglass cores for composite insulators, (3) as resin-impregnated fiberglass mats and insulating plates, and (4) as fiberglass reinforced plastics in the form of tapes in electrical machines. Glass in the form of paper is also used which is composed of glass microfibers. Outstanding feature is thermal stability up to 538oC. Other attributes include high thermal conductivity, low moisture absorption and good chemical resistance. 4.4.3 Ceramics Ceramics are inorganic materials produced by consolidating minerals into monolithic bodies by high-temperature heat treatment. Most common are porcelain (4K2O-Al2O3-3SiO2) and alumina (Al2O3). For many years porcelain, and to a lesser degree glass, had no competitor as an insulation for overhead transmission lines. It weathers well, even under moderate pollution, has good


flashover characteristics and methods have been developed to meet the stringent mechanical requirements Also, radio interference from the insulators has been reduced to an acceptable level. The design concepts are such that rain alone will not cause breakdown. The interaction of electric field, leakage current and environment is very complex and many investigations have been carried out including tests for UHV applications. Although having a relatively low puncture strength, porcelain is used very extensively in bushings, current transformers, stand-off insulators and similar components. In most of these applications the resistance of the material to atmospheric conditions and possible flashover without catastrophic failure are the insulating properties being invoked. 4.4.4 Paper, boards and laminates Paper and boards are produced from a variety of materials, including wood, cotton, organic fiber, glass, ceramics and mica. Paper is generally <0.8mm thick whereas boards are thicker. Boards are also referred to as pressboard, transformer board or fuller board. For >6mm, boards are laminated with adhesive. The paper used for insulation purposes is a special variety known as Kraft paper. Paper is hygroscopic (attract moisture) and thus it has to be dried and impregnated with oil.


The laminates include the paper and cloth based resin glued boards, special plywoods either fully or partially impregnated with resin, layered pressboard for oil impregnation and the high quality materials of glass fibre lays impregnated with silicone, epoxy, polyester and other resins. For external use, and in some dry type equipment, the choice of material will often depend on its tracking properties and resistance to deterioration due to moisture and dirt. For internal applications where good mechanical strength or support is required in high electric fields, the presence of voids between layers of the laminate can result in partial discharges; also the losses in the resins can produce excessive local dielectric heating and any impurities in layers parallel to the field may give a low breakdown strength. Production of high quality boards (and tubes) to meet most requirements is possible but it is essential that the material selected should have properties matched to the electrical test and operating conditions. Meeting mechanical and thermal specifications without detailed consideration of the electrical conditions is often insufficient - especially in the long term. 4.4.5 Cast epoxy resins The use of casting resins in power engineering is extensive: manufacturers now offer a wide range of components in which designs incorporating traditional materials have been modified to exploit the advantages of cast epoxy resins.


Apart from thermal difficulties due to the differences in expansion coefficients between the resin and conductors, a major development problem was the elimination of partial discharges in voids as it was found that the resins were susceptible to PDs of low value. Initially this knowledge was not available in some production units where items such as current transformers were cast satisfactorily but contained voids which discharged in service with subsequent failure. Such events are now rare following improved production techniques, introduction of routine PD testing and much service experience, although continuous quality control is still essential. One major restriction was the limitation of cast resins for use only indoors. After much R&D effort, the cycloaliphatic epoxy resins (better UV resistant) with appropriate fillers were introduced for outdoor applications. A number of current transformers have been in service for many years and long term tests have been carried out on line insulators including one piece units for operation at very high voltages. Such insulators with fibre glass reinforcement have been in operation for about the same period. 4.4.6 Mica-based resin systems Mica-based resin systems in formed or tape configurations are widely used in high voltage rotating machines - both motors and generators. The techniques are very specialised but are of considerable interest in power station operations. 4.4.7 Polyethylene (including XLPE)


The use of polyethylene as an insulating material is attractive as it has a low loss and high electric strength. Unfortunately its thermal stability was unacceptable at the temperatures required in power engineering until crosslinked polyethylene (XLPE) became available. The material can be extruded and was found suitable for cable manufacture once a number of problems were solved. These included developments of methods for curing and cooling long lengths, at the same time eliminating voids in which PDs might develop: XLPE is also very susceptible to discharges. Lapped polyethylene tape in conjunction with SF6 has been proposed for high voltage high power cables. 4.4.8 Elastomers From the range of natural and synthetic "rubbers" which may be classed as elastomers probably only two have the properties suitable for electrical insulation; butyl and ethylene-propylene (EPR). The latter has been well developed for HV applications and is used, in extruded form, for some 33kV cables. It has good environmental resistance and one type is applied as sheds in combination with a fibre glass rod to form a lightweight line insulator. 4.4.9 Heat-shrinkable materials An important development was the introduction of heat shrinkable polymeric materials. This led to changes in the techniques adopted for 11kV cable terminations at switchgear and similar locations. Much testing has been completed in the


laboratory and at outdoor test sites. Assessment included the determination of the behaviour of the shrunken material when subjected to thermal cycling, as in a cable. Air gaps must not appear between the sleeve and the cable insulation (plastic or oil impregnated) as this could result in partial discharges with subsequent failure 4.5 Composites More than one class of insulating materials used together. Examples of solid/gas or solid/vacuum composites are in transmission line and gas insulated switchgear (GIS). Here, the interface is the weak link and has to be carefully designed. It is important to ensure both components of the composite should be chemically stable and not react with each other and should have nearly equal dielectric constants. At the higher voltages, it seems that liquid impregnated insulation systems (solid/liquid composites) will continue to retain their superiority in EHV cable, bushing, and transformer technology, and in power capacitors. Despite its apparent disadvantages, oil impregnated paper (O.I.P) has proved a reliable and economic insulant in many applications. The achievement of the efficient utilisation of natural materials has resulted from R & D effort over many years: in particular, the establishment of conditions necessary to withstand high electric stresses for the expected life times of 20 to 30 years. Of special interest to the design and operating engineers are: (i) moisture content, (ii) gas content of the impregnant, (iii) losses at operating stresses ( )2 tanV Cω δ ,


(iv) the partial discharge inception stress and (v) the location of any PDs. The losses may be related to (i) as well as to the quality of the material: a low loss paper for cable manufacture is used. It is essential that low moisture contents are maintained in the practical systems if thermal runaway (and PDs) is to be avoided. This is particularly important with cables where the conductor losses all pass through the O.I.P. and add to the dielectric losses. In HV cables, arrangements are made to seal the system against the atmosphere whilst in transformers the situation is less critical, the stresses being lower and the insulation structure not so compact. The latter fact also means that gas absorbent oils are not usually required in transformers. However, in cables and power capacitors the existence of a gas bubble can be very significant as it may lead to PDs which are confined, eventually leading to deterioration of the tape or sheet material. The drying and impregnation conditions necessary to avoid failure are well established and form part of the production "know how". Vacuum ovens capable of pressures down to 0.01 torr and temperatures of 100 – 130oC are common. The processing times of 2 - 3 weeks for a large transformer may be shortened by installation of vapour phase heating equipment. This involves the use of, for example, a low boiling point (50oC at 50 torr) kerosene for heat transfer in the early stages of dry out. Extensive developments in power capacitor insulation systems included the introduction of polypropylene film in


conjunction with the more traditional paper, the latter acting as an impregnating interface. However, for a number of years all-film capacitors, in which a "hazy" polypropylene film is used to enable impregnation to take place, have been in service. Capacitor insulation is very susceptible to impurities because of the thicknesses of only tens of micrometers(μm) and the very high stresses (tens of V/μm) used. The dielectric losses in the new designs are very low and no longer a thermal limitation in design, giving DDF values of 0.05% or less.


5. Insulation deterioration from high T The great majority of electrical insulation is organic in its chemical nature and is subjected to chemical change which is caused by the chemical reactions which are continually occurring.

These chemical reactions cause gradual deterioration and reduce the effectiveness of the insulation. The reaction rate is very sensitive to temperature, i.e. exponentially dependent on temperature (following Arrhenius’ law).

Thus, the operating temperature of insulation is of primary importance in determining effective lifetime of insulation and of equipment. A higher operating temperature will result in a shorter effective life.

Lifetime: ATL B e= ×

where A and B are constants which depend on the insulation class and T is the absolute temperature.

Hence: log loge eAL BT

= +

Plot of logeL against 1/T is straight line and slope is generally such that an increase of 100K causes a 50% reduction in L. Consequently, it is necessary to specify very precisely the operating temperature of electrical equipment.


In most cases, it is the conductor temperature where it is contact with the insulation that determines the insulation operating temperature.

Class Examples 105oC

A Materials or combinations of materials such as cotton, silk, and paper when suitably impregnated or coated or when immersed in a dielectric liquid such as oil.

130oC B

Materials or combinations of materials such as mica, glass fibre, asbestos, etc with suitable bonding substances.

155oC F

Materials or combinations of materials such as mica, glass fibre, asbestos, etc with suitable bonding substances.

180oC H

Materials or combinations of materials such as silicone elastomer, mica, glass fibre, asbestos, etc with suitable bonding such as silicone resins.

200oC N

Materials or combinations of materials which by experience or accepted tests can be shown to have required thermal life at 200oC.

220oC R


240oC S


> 240oC C

Materials consisting entirely of mica, porcelain, glass, quartz, and similar inorganic materials. Other materials or combination of materials may be included if can be shown to have required thermal life at above 240oC.

Classes of insulation systems. Above classes indicate normal life expectancy of 20,000 to 40,000 hours at stated temperature (approx. 2-5 years).


6. Applications of insulating materials 6.1 Transformers

HV power transformers use enameled conductors, paper, glass or thermoplastic insulating tape, pressboard, glass fabric, porcelain and mineral or silicone oil. Windings are insulated by tape, held in place over iron core by pieces of pressboard, glass fabric or porcelain, and impregnated with an insulating liquid which also acts as cooling medium. Small power transformers and also instrument transformers (VT, CT) use thermosetting resins, insulating tapes, SF6 gas, etc. 6.2 Circuit breakers

HV breakers use SF6 gas, air, vacuum or mineral oil as the main insulation and arc quenching medium. Ceramic or epoxy resin parts are used for mechanical support, bus bar insulation and arc chamber segments. In low-voltage breakers, synthetic resin moldings are used to carry the metallic parts. 6.3 Power cables

Use paper or plastic tape, thermoplastic materials (such as PE, XLPE or PVC), silicon rubber, EPR, thermosetting resins, SF6 gas and mineral oil. In oil-filled cables, conductor is insulated by lapped paper tape and impregnated with mineral oil. In polymeric insulated cables, conductor and insulating materials are extruded jointly and then insulation is cured and crosslinked. In gas-insulated cables, inner conductor is held concentrically in a metallic tube by


insulating spacers made of thermosetting resins and tube is filled with pressurized SF6. Low-voltage cables employ PVC, PE or XLPE insulations without the outer screen. 6.4 Bushings

Made of porcelain, glass, thermosetting cast resin, air, SF6 gas, paper tape and oil. Typically, feed-through conductor is insulated by paper tape and oil and is housed in a porcelain tube that enters the HV equipment enclosure. Usually, condenser-graded bushing types are used for rated voltages >50kV and non-condenser bushings for lower voltages. The paper tape is typically resin bonded paper, oil impregnated paper or resin impregnated paper. 6.5 Overhead lines

Use porcelain, glass, thermosetting resin and air. Conductors are suspended via insulator chains from towers. Insulators are made out of porcelain or hard glass, or recently of fibre glass and cast resins. Room-temperature vulcanized rubber (RTV) is also used for coating the ceramic insulators as protection against polluted environment. 6.6 Gas insulated switchgear (or GIS)

Use SF6 gas, thermosetting resins and porcelain. Different components such as bus bars, interrupters and earthing switches are located in adjacent cylindrical compartments which are air-tight, and contain compressed SF6 gas. Inner live conductors are separated, at regular intervals, from grounded enclosure by epoxy resin spacers.


6.7 Surge arrester and protective gaps

Used for limiting transient overvoltages. Consist mainly of air, SF6, porcelain and metal oxide resistors. In its simplest form, air-insulated rod-rod chopping gaps are used. Alternatively, non-linear resistors made of metal oxide (eg. zinc oxide) with or without series spark gaps are used. Ceramic or porcelain housing is used for mechanical support and for protection against environment. 6.8 Power capacitors

Modern power capacitors consist of metallized polypropylene film, aluminum foil and polypropylene film, or metallized paper electrodes and polypropylene, and the impregnation fluid. The fluid (eg. isopropylbiphenyl, silicone liquid) minimizes the voids and increases the dielectric strength. 6.9 Rotating machines

Generators use mica tape system on conductors impregnated with either an epoxy or polyester resin. Other materials include polyvinyl acetal, polyester enamel or bonded fiber glass for inter-turn insulation; bakelized fabric, epoxy fiber glass, mica glass sheet, epoxy impregnated mica paper and varnished glass for inter-coil or phase-to-earth insulation; and bakelized fabric or epoxy fiber glass strips for slot closure. The impregnation treatment normally consists of alkyd phenolic estermide or epoxy based varnishes.


7. Assessment of insulation materials No two pieces of insulation are identical. This can be due to any number of reasons, some of which are: (i) tolerance in manufacture, (ii) presence of impurities, (iii) presence of voids, (iv) variation in external factors such as temperature, pressure, radiation and humidity. Any or all of these may alter the performance of a given insulation. Therefore, any method for the assessment of a particular insulation system or the comparison of different insulation materials must be statistical in nature and based upon an accumulation of large amounts of data from both controlled laboratory conditions and from field experience. In addition to normal overvoltage tests, the condition of new and aged insulation as used in power system equipment may be assessed by application of various diagnostic procedures. The preferred methods are non-destructive although some of the tests are designed to check the ultimate strength, or end-point, of samples utilized in quality control or long term stability measurements. The more important techniques adopted for assessment of the materials, and some structures, are outlined below, together with references to relevant standard specifications.


7.1 Assessment techniques A complete review of all the diagnostic techniques available for assessing insulating materials is not practical in the present lecture. However, the following list is a useful guide: (A) Electrical measurements

1. Insulation resistance and Resistivity: IEC 60167 and 60247

2. Loss tangent/dielectric dissipation factor: IEC 60894, 60247, 60250, 61620

3. Capacitance and Permittivity: IEC 60247, 61620 4. Polarisation Index (machine insulation) - IR

changes. 5. Partial discharge characteristics: AS 60270-2001 6. Electrical Endurance: e.g. AS 2897:1986 App.C 7. Resistance to surface tracking under polluted

conditions: IEC 60112 and 60587 8. Electrostatic charging tendency (ECT) of oil 9. Radio interference measuring apparatus: e.g.

AS/NZS 1052:1992 (B) Physical/Chemical measurements on liquids

1. Gas chromatography for analysis of dissolved gases in oil (DGA): IEC 60599

2. Moisture content of liquids: AS 1767 3. Chemical characteristics of oils: AS 1767 and AS

1883:1992


4. High performance liquid chromatography (HPLC) for detection of heavy molecules dissolved in oil: CIGRE Paper 15-08, 1988

5. Particle concentration (C) Mechanical/Physical/Chemical measurements on solids

1. Thermal endurance: AS 2768-1985, IEC 60216 and 60610

2. Moisture content of solids 3. Water absorption: e.g. AS 1795-1979 Pt.1 4. Degree of polymerisation e.g. for ageing of

cellulosic materials 5. Mechanical strength, e.g. under compression,

bending, tension, shear, torsion and vibration according to application

6. Multi factor assessment methods: CIGRE papers 15-01 and 15-11, 1986

(D) Measurements on gases

1. Moisture content (dew point) of hydrogen and SF6 2. Types of particulate in hydrogen (as monitor of

machine insulation condition) 3. Air content of SF6 samples: Electra 113, 1987 4. Contaminant and particulate levels of SF6

(E) Special Measurements

1. X-ray examination 2. Ultrasonics for partial discharge location


7.2 Sample measurements in the laboratory In the power industry laboratories, many of the tests listed above would not be required as samples of solid materials are rarely available - except, perhaps, after a failure on test or in service. The major effort is normally aimed at assessing condition of the insulating liquids from such plant as power transformers, instrument transformers, bushings, cables and switchgear. In most cases the oil condition is used as an indication of the state of the solid materials impregnated or immersed in the liquid. The most common tests are those specified and described in AS1767 and AS1883 (for oils in service). A brief outline is given here of three measurements often performed on a routine basis. This is followed by summaries of more recent techniques which may become of considerable value in assessing long term effects. 7.2.1 Dielectric dissipation factor (DDF) of oil The measurement is carried out using a special test cell with a suitable guarding system. A 50Hz bridge, e.g. a transformer ratio arm type, is used for determining DDF values of a few milli-radians. A sensitivity of tanδ<10-4 is required. Facilities should be available for testing over a range of temperature up to 100oC. The method of cleaning the cell and the temperature of measurement are very critical and many investigations have been carried out in this respect.


Resistivity measurements are made with similar test cells but the repeatability (even within the same laboratory) can prove erratic and the method is no longer specified in the IEC document. Measurements on solid materials use identical principles and bridges but the guarding often has to be adopted to the type and form of sample. 7.2.2 Determination of moisture content in liquids The well established method for determination of moisture content in oil (and some cellulosic materials) is the application of the Karl Fischer technique. With care it is claimed that moisture contents of a few ppm (10-6) can be quantified in oil and perhaps 0.1% in O.I.P. The development of the new automated instruments has removed much of the 'art' involved in these measurements and many hundreds of such tests are now carried out on a routine basis. As with such techniques, good house keeping is required with special attention being given to calibration and the method of obtaining and presenting the oil sample. Each supply authority would be expected to issue special instructions to field personnel in this respect. A supplementary method is sometimes practised where a measurement is made of the dew point of the moisture in the oil. This enables the Relative Humidity (RH) to be related to the moisture content of the O.I.P. with which the oil sample is assumed to have been in equilibrium. The method was developed in an attempt to overcome the problem of relating oil moisture content, which varies with type and condition of the oil, to the paper moisture content.


7.2.3 Analysis of dissolved gas in oil (DGA) For twenty years it has been possible to detect, separate and quantify gases such as hydrogen, methane, ethane, ethylene and acetylene produced by thermal or electrical degradation of oil and the carbon oxides created when associated materials deteriorate. Oil samples are carefully obtained from the power equipment - predominately power transformers and instrument transformers at the present time - and a gas chromatograph used for analysis in the laboratory. The earlier manual instruments have been replaced by automated units enabling DGA in oil measurements to become a routine procedure. However when comparing results between laboratories, as might be required following transformer heat runs in the factory, it is important to have confidence in the reproducibility of results, especially for the lower values of gas content (few ppm). Australian Panel AP15 of CIGRE recently conducted a series of round-robins to determine the order of statistical variations to be expected under Australian conditions- this includes the effects of sampling techniques and transport methods as well as the different laboratory procedures practised. 7.2.4 High performance liquid chromatography (HPLC) A complementary analytical technique to DGA which has been developed overseas and is being investigated within Australia is the application of high performance liquid chromatography (HPLC). The methods allow the heavier molecules dissolved in oil to be separated and identified. For


example furfuraldehyde is produced by the mild overheating of paper and the phenol and cresols similarly produced from synthetic resin-impregnated paper board. Also it is possible to detect cumyl alcohol and -methyl styrene from deteriorated XLPE in cables. The basic instrumentation for liquid chromatographic studies has long been available and its commercial adaptation for analysis of the specific compounds produced by insulation deterioration is now well established. 7.2.5 Electrostatic charging tendency (ECT) of oil In the early 80s it was considered that oil being circulated at high velocity in directed flow ducts in transformer windings might produce hazardous electrification effects. As part of the many investigations carried out in a number of countries - notably Japan and America - a technique was developed for measuring the electrostatic charging tendency of different oils. Although the effect is still not fully understood the test arrangement is being utilised by some manufacturers and users in order to allow comparisons to be made between different types and between new and aged oils. 7.2.6 Degree of polymerization (DP) Cellulosic materials decompose into alcohol, aldehyde, acid and finally into carbon dioxide based on the degree of ageing. The number of cellulose molecules indicates the degree of polymerization (DP). For many years it has been known that


one measure of the ageing of cellulosic insulating materials is the DP compared with the value for the new material. Although strictly a destructive test the amount of material required is small and could possibly be removed from transformer windings being overhauled, in addition to damaged units under investigation. The procedures for determination of the DP are well known and can be completed by most chemical laboratories without undue difficulty. Values of DP for Kraft paper vary from about 1000 for new material to 500 or so for insulation known to be near the end of its life when the paper becomes extremely brittle and cracks upon bending. 7.2.7 Moisture determination in XLPE insulation An important area of investigation is ability to quantify the small amounts of moisture, probably in the form of water trees, which might be present in aged XLPE insulated cables. The subject has been under study for many years thus reflecting its importance. No simple technique has evolved but a CIGRE paper describes some of the sophisticated methods now being developed, particularly with respect to localised changes in the XLPE. This work was carried out on behalf of EPRI in the U.S.A. 7.2.8 Determination of air pollution in SF6 gas A simple technique has been developed within CIGRE for determining the amount of air pollution allowable in gas of G1S equipment before the breakdown strength of the SF6


becomes unacceptable. By applying a breakdown test on a sample, it is possible to estimate the condition of the insulating gas. The concept is similar to that often used by field personnel for checking the condition of oil with the AS1797 standard cell.


8. Diagnostic measurement on power equipment The insulation condition is the primary factor which determines the life of major HV equipment plant. During its life the insulation deterioration can be accelerated by excessive operating temperature, by moisture or other contamination, by mechanical stress or by environmental factors such as UV radiation. Because of this multiplicity of potential degrading factors, insulation condition monitoring is perhaps the most important feature of asset management in large HV power systems. Thus the tests methods used for insulation assessment are of primary importance. The most generally used tests for insulation condition assessment in high voltage electrical power equipment are:

Dielectric Dissipation Factor (DDF) measurement. Dissolved gas-in-oil analysis (DGA) for oil-impregnated paper insulation

Insulation Resistance (IR) measurement (and associated quantities).

Overvoltage Tests on equipment o Power Frequency. o Impulse (lightning) Tests. o Switching overvoltage Tests.

Partial Discharge Tests. Other diagnostics

Of the above diagnostics, the DDF, DGA and the IR measurement can be done easily on-site and usually off-line (although the actual DGA tests have to be performed later in


a chemical laboratory). DDF measurement usually requires a separate HV test source, but some tests today can be performed on-line using sophisticated test techniques to determine and record the phase angle difference between the operating voltage and current for the item of equipment under test. The DGA tests are perhaps the simplest to perform in that only a small sample of oil is required for chemical analysis and this sample removal can be done while equipment is on-line. Similarly, partial discharge tests are usually done off-line and require a separate HV source, but modem techniques now allow on-line monitoring of PDs, but electromagnetic interference is a major problem that must be contended with. The overvoltage tests must be performed as separate source tests. Normally only power frequency overvoltage tests can be performed on-site: the impulse and switching tests are normally done only in the laboratory or in the factory. 8.1 DDF (or tanδ or loss angle) tests The DDF (tanδ) and the capacitance value C of the dielectric are normally measured by a high voltage AC (4-terminal) bridge technique. The measurement must be done with the insulation at rated voltage (or higher) to determine its efficiency at rated operational voltage, because the DDF is a voltage-dependent parameter. Also, because the variation of DDF with voltage is an important parameter, the test is usually done over a range of voltage above and below the normal operating voltage. Usually insulation will exhibit a


"turn-up" in the DDF vs. V curve just above operating voltage. If this insulation is degraded the "turn-up" may occur at a lower voltage level and this will be an indication of potential insulation problems. The most commonly used bridge arrangements for DDF measurements are the Schering Bridge and the Transformer Ratio Arm Bridge. These tests are performed with the test object held at full rated voltage (or higher), but the variable balance impedance components which have to be manually adjusted for balance are effectively at earth potential [modern bridges are automatically balancing, however, and do not require manual manipulation of the dials]. Both of the above types are 4-terminal bridges and thus require both amplitude and phase balance. The typical circuit used with the Schering bridge for DDF measurement at power frequency is shown below:

C

Cx

Rx

R1

R2C1

Z1

Z3

Test object

Earth

H.V.

Vδ

Z2

Z4

}

Note that:


a) C1 and R1 are the variable impedances (they are high precision components: R2 may sometimes also be variable).

b) R2 is a standard non-inductive resistor, also of high precision.

c) C is a standard HV precision capacitor with negligible losses.

d) Rx and Cx is the test object (represented by a series equivalent combination)

The bridge is energised at the rated voltage of the equipment item, but there are only a few hundred volts at most on R1, C1 and R2, the components which are manually operated to achieve balance.

At balance: 2 1

4 3

Z ZZ Z

= or 2 4

1 3

Z ZZ Z

=

From the circuit diagram, we can write the following (ω is the angular frequency):

11 1

1 1 j CZ R

ω= +

2 2Z R=

3

1 j CZ

ω=

41

xx

Z Rj Cω

= +


and thus, for balance of the bridge, we have the following relationship:

( )2 11

1 1x

x

R j C R j CR j C

ω ωω

⎛ ⎞⎛ ⎞+ = +⎜ ⎟⎜ ⎟

⎝ ⎠ ⎝ ⎠

or: 21 2

1x

x

R Cj C R j R CR C

ω ω+ = +

Equating real and imaginary parts, we get:

(a) 2

1 x

R CR C

= ⇒ 1

2x

RC CR

= ×

(b) 2 1 xR C R Cω ω= ⇒ 2 1x

R CRC

=

tanδ is the required quantity of the DDF test measurement, and is given by the following:

2 1 11 1

2

tan xx x

x

R R C R CR C R CX C R

δ ω ω ω= = = =

Typical values of tanδ are millradians or less at 50Hz. Tanδ (and the capacitance) values are important characteristics of HV insulation because of the following features:

a) It gives a measure of impurities (e.g. moisture) in the insulation.

b) It gives information on partial discharge activity in the insulation.


c) ωCV2tanδ gives the dielectric heat loss and this will determine, to a considerable degree, the operating temperature at high voltage.

d) Plots of tanδ vs. Voltage below and above rated voltage will give useful information on the insulation condition.

As the Schering Bridge uses a series equivalent circuit for Rx-Cx, the value of Cx may vary from that for a parallel circuit, but this only occurs for high levels of DDF 8.2 Dissolved Gas Analysis (DGA) The majority of large power transformers, many older cables and many switchgear components are insulated by oil and by oil-impregnated paper. Whenever any faults occur that may cause deterioration of the oil-based insulation, the generation of gases by discharges in the oil is almost always a concurrent event. These gases are then dissolved in the oil and an analysis of the gas constituents and their quantities (and particularly their relative quantities) can provide very useful information about the fault. In many cases it is possible to identify the type of fault from the relative quantities of the gas components. The gases which are generated in such oil based insulation include:

Carbon monoxide CO Carbon dioxide CO2 Hydrogen H2 Methane CH4


Ethane C2H6 Ethylene C2H4 Acetylene C2H2

The above are not the full list of gases but they are the ones which are used to determine any deterioration effects of the oil and to identify any particular fault types that may be generating the gases. Also, O2 and N2 are also often monitored to give indication of possible oxidation. The measurement is done by gas chromatograph. In general, CO and CO2 are generated by hot spots such as may arise from hot metal shield surfaces or core surfaces or winding hot spots in the transformer. Hydrogen, methane and to a less extent ethane may also be generated by such effects. Ethylene and acetylene are not generated significantly by hot spots. Partial discharge (low level sparking) will generate hydrogen, methane, ethane and ethylene. High power arcing will generate hydrogen, ethylene and particularly acetylene. The key gas method identifies faults by association as follows:

Hydrogen: Partial discharge Ethylene: Overheating in oil Acetylene: Arcing in oil CO & CO2: Solid insulation deterioration (e.g paper)

In addition there are the various ratio methods which take the ratios of the quantities of specific pairs of gases and then use these ratios to predict the type of fault. The ratio methods currently in use include:


1. Rogers ratio method 2. IEC (International Electrotechnical Committee) ratio

method 3. Dornenberg ratio method 4. Duval triangle method (this analyses data using groups

of three gases) In addition to the above there are also many "home-grown" methods of analysis used by various utilities. In recent years the fuzzy logic technique has also become popular for analysis of dissolved gas data. DGA is a valuable diagnostic technique and is probably one of the most widely practiced techniques in current use. It is simple to do, does not require disconnection of equipment from supply and most utilities have the chemical analysis tools available. The problems with the technique are that there may be a delay in getting results and as it is often used as a routine maintenance technique, the data is generally not analysed as thoroughly as is warranted. The other problem is that DGA gives only a measure of the integrated effects of the fault. It does not give any information as to whether the fault has been in existence for a short or long time and thus it is not easy to identify the magnitude of the fault. It is also not able to give continuous on-line monitoring. There are some continuous on-line DGA monitors (HYDRAN monitors) available but they are restricted in the range of gases that can be identified and not so wide-ranging in identifying fault types as the full DGA laboratory tests.


In addition to normal DGA, which essentially identifies mainly problems with oil, there are more sophisticated test techniques of a similar type which are used to monitor more accurately the degradation of the paper insulation. These tests look at gases generated by the chemical decomposition of the cellulosic structure of the paper as it ages. The gases generated in this case are much more complicated in structure and thus more difficult to extract and analyse than the relatively simple hydrocarbons analysed by DGA techniques. The gases are the so-called furans or furfuraldehyde group of gases. Their measurement requires the use of HPLC or high performance liquid chromatography, a technique which is not available in-house to many utilities. [It should be noted that the “paper” referred to above is an inclusive term which also includes pressboard and wood etc. which are cellulosic in their chemical structure]. 8.3 Insulation Resistance (IR) Insulation resistance is a very simple test to apply and can be done very quickly and easily, but must be done off-line. The IR must be measured using DC voltage (usually between 500V DC to 10kV DC, depending on the application). However although it is a simple test, the insulation resistance value in itself is not a particularly useful parameter except when the insulation is extremely poor and near to failure. However the time variation of the insulation resistance is a much more useful property because the IR-t characteristic depends on the dielectric polarization properties of the insulation and thus on its condition. A number of useful


parameters for insulation assessment can thus be derived from the IR vs time variation. The transient current that flows in a dielectric material when it is subjected to a DC voltage step is made up of a number of current components, all of which are time varying. The total current (I) is composed of three components:

(i) a conduction (Ohmic leakage) current (Ie), (ii) a displacement (true capacitive) current part (Ic) and (iii) a so-called absorption current component (Ia). It is Ia

which is dielectric-dependent and thus useful for insulation condition analysis.

The various transient current components are shown below:

The Ohmic leakage current is that (constant) component which is left after a long period (1 minute, say) of DC voltage


application. It is affected by moisture, and contamination etc., as well as the intrinsic material insulation resistivity and geometry. Because of the effect of contaminants leakage current is important for insulation condition monitoring.

The absorption current is affected by the nature and condition of the insulation dielectric and is thus an important component for insulation condition monitoring.

The capacitive current is determined only by the insulation capacitance. It is not important for condition monitoring. From above, the leakage current and absorption current are the two quantities which are indicative of insulation quality and their comparative values can give useful information about the condition of the dielectric. The insulation resistance (IR)-derived parameters which are used for assessment are: (a) Short-time Test

Here, the insulation resistance is measured just once, after 60 seconds of DC voltage application. This is the simplest but least useful form of IR test.

(b) Resistance-time variation Time variation of insulation is recorded for a period up to 10 minutes. For good insulation, resistance increases with time. This is not a specific test in itself, but the data obtained is used to derive the following quantities:

(c) Polarisation Index (PI)


This quantity is ratio of insulation resistance measured at 10 minutes to resistance measured at 1 minute. A value of 1-2 is characteristic of poor insulation, a value greater than 3 is representative of good insulation.

(d) Step Voltage Test This test uses different DC voltage levels applied in steps: eg. 500 V followed by 2500 V over 60 seconds.

(e) Dielectric Absorption Ratio This is a variation of the Polarisation Index parameter. Usually the absorption ratio is obtained from IR value at 60 seconds divided by that at 30 seconds. A value less than 1.3 is characteristic of poor insulation, and a value greater than 1.5 is characteristic of good insulation.

Insulation resistance is measured simply by using a "MEGGER" type tester. The tester is normally battery-operated and should be capable of application of any voltage in the range up to 10,000 volts DC, with an accuracy of about 1-2%. In the basic form, the tester provides a reading of insulation resistance only and the values must be recorded over time and then plotted to determine the required IR-derived parameters. Modem instruments however are microprocessor-controlled to apply the chosen voltage (including the step variations if necessary), record the data over time and then calculate the parameters with the test result then printed out. Typical insulation resistances of HV equipment are in the hundreds of MΩ to GΩ levels and at these levels the accuracy of measured resistances are only about 10-15%


when performed in the field. Care must be taken to ensure that no surface leakage current paths are involved in the measurement as these will obscure the test result. Normally guard electrodes should be used if possible, although this is not always possible. The instrument accuracy should be checked regularly against standard high value resistors. The advantage of using parameters which involve calculation of a ratio of resistances is that any measurement errors are minimised (assuming the errors are systematic). 8.4 Overvoltage Tests Overvoltage tests are essentially pass or fail tests and are usually performed at the final stage of manufacture in the factory for power frequency and impulse tests and again after installation in the case of the power frequency overvoltage test. These tests may also be performed on suspect equipment or on refurbished equipment. The power frequency test can be done on-site, but the impulse and switching tests are normally done in the test laboratory or factory only. Overvoltage tests can cause damage to the insulation if the tests and test voltage levels are not properly controlled. The table below shows the range of standard test voltages for power frequency and impulse voltage tests for transformers. Note that the power frequency overvoltage test requirement is about twice system voltage for one minute: the criteria of success is simply that no insulation breakdown failure occurs. Normally only one such test is performed. Voltage is monitored during the test to aid in identifying breakdown as the breakdown current is limited to prevent damage.


The impulse test level varies greatly with rated voltage of the equipment: a number of tests (perhaps about 5 may be required) and different polarities may also be required. The impulse voltage waveform is recorded and will indicate if any failure occurs during the test. The equipment buyer may also specify other insulation tests that must be done before the item of equipment is accepted from the manufacturer. For example an induced overvoltage test at a higher frequency may be performed on transformers. 8.4.1 Power Frequency Overvoltage Test These are relatively simple to achieve if the necessary power supplies are available. For on-site tests this will require use of a mobile HV transformer, which may not be easily available for the full transmission voltage range of a transmission utility's operation. Generally only items with low capacitance can be tested on-site because of the problems of supplying large quantities of reactive power with low capacity mobile test transformers. Thus, only items such as transformers, instrument transformers, circuit breakers and similar equipment items are able to be tested at full voltage on site. Cables cannot be tested in this way because of reactive power requirements. For HV power cable overvoltage tests, HV resonant test sets using tunable inductors to achieve resonant conditions with the cable capacitance are used. At resonance, the HV transformer only needs to supply the pure ohmic losses, which are generally quite small as they arise only from leakage currents in the cable.


It can be seen from the table that a typical power frequency test voltage is about 2 per unit or 2Uo. There are some reservations about imposing such a high test voltage on new equipment but the accepted standard tests require these levels.

Line Voltage

(kV)

Phase Voltage

(kV)

AC test Voltage kV rms

1 minute

Imp. test Voltage

kV 1.2/50μs

Power Ratings MVA

Current Ratings

kA

11 18 (2)

22/23 (2)

33 66

132 275 330

400

500 750 1100 1200

1500 (4)

6.35 10.4

12.7/13.3

19.05 38.1 76.2 158.8 190.5

230.9

228.7 433

635.1 693 866

23

45 66 132 264 460 570 510 630

950

1400

100/75

220 380 550 1050 1300 1175 1425 1000

1400 ? 2200

2900 ? 3200

(600)

(1200) 500

600 1443

600 6000

20 40 16

0.866

0.314 3.14

Typical test voltage levels for overvoltage tests on power Tx. 8.4.2 Impulse testing There are two types of transient overvoltages that must be tested for in high voltage equipment. These are:


lightning overvoltages switching overvoltages

Power system equipment items must be tested for their ability to withstand the effects of such transient voltages, of magnitudes which are typical to their voltage rating, without suffering dielectric breakdown. 8.4.2.1 Lightning impulse testing

Lightning impulses can be up to 1000kV or more in amplitude and the associated current may be up to 100 kA in each stroke, although 10-20 kA is typical. If the actual strike is to an overhead line, a travelling wave results which then moves along the transmission line (unless it causes breakdown of the air insulation between lines). This HV propagating surge may test the electrical insulation of any equipment connected to the line and thus the equipment must be tested for its withstand ability to such transients. Even if the strike is not to a line, but occurs close to it, induced overvoltages may be coupled into the line inductively or capacitively. The test waveform used for equipment to test against lightning impulse voltage is the generally agreed shape of 1.2/50μs. The voltage rises to a peak in 1.2μs and decays to half peak in 50μs. There are some tolerances allowed for the rise time and decay time when testing (typically about +/- 20%). In some cases chopped wave tests are performed where the voltage is driven to zero very quickly. This test can be used to provide tests of, for example, inter-turn insulation


where the capacitive voltage coupling is enhanced by the fast chopping. If failure occurs under impulse testing, the disruptive discharge will produce something akin to a chopped wave shape.

The figures above show typical lightning impulse voltage waveshapes for (a) full wave, (b) chopped wave on decaying side and (c) chopped wave on the rise. Typical standard test voltage amplitudes are listed in the previous table.


8.4.2.2 Switching impulse testing Switching impulses occur as a result of operation of circuit breakers, switches etc. in the power system. Their shape is very dependent on the system parameters and there are thus very substantial variations in magnitudes and in shape of switching impulses. However their risetime is generally much slower than that of lightning impulses and their duration is also generally much longer. The amplitude of typical switching impulses is about 2-3 pu and is thus a little lower than lightning impulse amplitudes. However the longer duration may stress the insulation equally as much as lighting effects or even more in some cases. A standard switching impulse waveform has a risetime of about 250μs and a decay to half peak of about 2500μs. This is designated as the 250/2500μs switching waveform. There are tolerances of about 20% on the rise and fall time values in testing. Because of the variability in shape for different systems, there are other standard waveshapes which can be used if required by the test situation: these include the 100/2500μs and 500/2500μs. The amplitudes of switching impulses are generally a little lower than lightning impulses, with the amplitude depending on the application.


The figure above shows a typical waveshape of a switching transient test voltage. Note that switching transients are not applied as chopped waves. 8.4.3 Impulse voltages for LV and communications systems With the increasing susceptibility of modem electronics to impulse voltages and with the increasing use of power electronics and such hard switching elements as IGBTs, there is now a need to impulse test low voltage equipment. There is also particular need to test information technology equipment for impulse voltages and in the case of IT equipment there are very many different impulse voltage waveshapes that are used for these tests. 8.5 Partial Discharge Tests Partial discharges (PD) in insulation can cause substantial degradation of the insulation because of the very high energies of ionized particles which are produced in the partial discharge ionization processes. These ions and electrons can then change the chemical structure and composition of the insulation, thereby degrading it over time. The damage due to


PDs is primarily to organic insulation: mica-based insulation such as is found in large generators is able to withstand very high levels of partial discharge activity without any significant deleterious effect on insulation properties. Because of this direct damage to insulation and because PDs can be monitored directly, PD testing and monitoring is perhaps the best insulation assessment technique available. Modern developments in computer-based data acquisition systems have allowed the development of PD testing to a stage where it is able to give a very sensitive measure of the insulation integrity. To this end there are very substantial programs of PD research which are aimed at:

Developing continuous on-line PD monitors Development of signal processing techniques to remove interference

Using PD data to determine fault type and location Using PD data to estimate the remnant life of insulation

These are very ambitious programs and may not be fully realisable, but PD monitoring is the area where most development is occurring at present. The attached sheets give some basic information about PD monitoring methods. PD monitoring covers the widest spectrum of voltages and equipment. It is used for transformers, for cables, for switchgear, for bushings, for insulators, for busbars systems, for SF6 gas insulated systems, for instrument transformers and for motors and generators. The PDs can be monitored electrically by resistive, inductive or capacitive sensors or by EHF aerial-coupling units at frequencies up to many GHz.


PDs can also be monitored by using piezoelectric detectors sensitive to the ultrasonic acoustic pressure waves generated by the PDs. The major problems with PDs are interference from corona discharge for example, and the difficulties in some cases of gaining access to the appropriate location for the sensor in or on the equipment item. 8.6 Other diagnostic tests The present situation in the electrical industry is aimed at extending the life of large items of capital equipment and thus asset management and condition monitoring are now very important aspects of operation. There is a very substantial program of development of new monitoring techniques. These include, for example:

Frequency response analysis (FRA) of windings Recovery voltage (RV) tests of insulation. Isothermal relaxation testing of cables Very low frequency testing of cables On-line DDF monitoring systems On-line PD monitoring of cables Fibre-optic acoustic sensors for PDs Dielectric spectroscopy Polymerisation testing of composite insulators On-line leakage current monitors for insulators Oscillating voltage tests of cable joints EHF (up to a few GHz) PD monitoring systems for transformers




AS1931.1-1996 High-voltage test techniques – General definitions and test requirements AS1931.2-1996 High-voltage test techniques – Measuring systems

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ELECTRICAL INSULATION MATERIALS - Lec3...ELEC712: Electrical Insulation Materials and HV Testing page 3/64 In simple configurations such as cables and overhead lines, the electric