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The electrons, ions, atoms, radicals and excited molecular species produced in a partial discharge move under the influence of the following forces variously:

• thermal excitation

• the electric field

• electrostatic forces

• the electric wind, generated by the collision of the ionic species, moving under the influence of the electric field, with the molecules of the surrounding gas.

The distribution of the reactive species within the gas discharge, and their resulting impact at the discharge surfaces, will be complex. The following sections discuss this complete interaction by considering the different stress mechanisms likely to be prevalent.

4.2.1 Particle impact stress

As has been explained earlier in this book, a gas discharge consists generally of electrons, positive and negative ions and photons. In relation to partial discharges, when these particles impact on a surface at the ends of the discharging channel, they may cause degradation at that surface. Any of these particle types may con-tain sufficient energy to cause bond scission, often with an associated electron release.

An impacting ion at an insulating surface may result in local molecular changes as a result of either an electronic interaction between the incoming charged particle and the shell electrons of the molecules of the insulating material or through a tight interaction between the ionising ion and one (or more) ions of the surface lattice.

As detailed by Hepburn [1], the interaction which occurs when an electron collides with a molecular surface will depend upon the structure and energy state of the impacted species and upon the electron energy.

An energetic electron impacting upon an uncharged molecular species can interact in four ways:

(i) an electron, with velocity v₁, colliding with a molecule, M, can lose part of its kinetic energy to the molecule without becoming attached:

e⁻(v₁)+ M → e⁻(v₂)+ M^∗

the electron continues on with a lower velocity, v₂, and the excited molecule M^∗either emits the extra energy as a photon:

M^∗→ hν + M

or loses the energy by collision with a second molecule:

M^∗+ M1→ M + M^∗₁

(ii) an electron can collide with a molecule and become attached:

e⁻+ M → M⁻ forming a negative ion

(iii) an electron can be energetic enough to detach an electron from a molecule:

e⁻+ M → e⁻+ e⁻+ M⁺

increasing the number of free electrons in the system and creating a positive ion

(iv) an electron can become attached to a molecule and cause a division into charged and neutral subspecies:

e⁻+ M → M1+ M⁻₂

In a similar manner to the possible reactions between electrons and molecules, there are a variety of interactions possible between an electron and an ion:

(i) an electron colliding with a negative molecular ion can cause a reaction similar to that given above [3] but the resultant molecule is neutral:

e⁻+ M⁻→ e⁻+ e⁻+ M

(ii) for a number of molecular species, it is possible for the molecule to become doubly negatively charged:

e⁻+ M⁻→ M²⁻

(iii) when an electron interacts with a positive ion the electron can become attached to the molecule and any excess energy may be released as a photon:

e⁻+ M⁺→ M + hν

As with electron impact, the transfer of energy from a photon to a molecule will cause changes in a number of ways:

(i) the energy transfer can cause photoionisation: where the ionisation energy is less than that of the photon, the excess can be released as a less energetic photon or as increased kinetic energy in the electron:

hν₁+ M → M⁺+ e⁻(v₁)+ hν2

hν₁+ M → M⁺+ e⁻(v₂)

(ii) a molecule can be split into ionic subspecies:

hν+ M → M⁺₁ + M⁻₂

(iii) a molecule can be split into ionic and neutral species and an electron:

hν+ M → M1+ M⁺₂ + e⁻

(iv) a molecule can be divided into free radical species, which are highly reactive hν+ M → M^·₁+ M^·₂

In relation to photon impact with an ion, the energy transferred from a photon to an ionic molecular species can cause changes to occur in the following manner:

(i) release of an electron from a negative ion:

hν+ M⁻→ M + e⁻

(ii) release of an electron from a positive ion:

hν+ M⁺→ M²⁺+ e⁻

(iii) splitting a negative ion into neutral and charged species:

hν+ M⁻→ M⁺₁ + M2+ 2e⁻

(iv) splitting a positive ion into neutral and charged species:

hν+ M⁺→ M1+ M⁺₂

The photon energy and the energy state of the molecular or ionic species involved in the reactions given above will determine which of the reactions will occur.

Particle impact has been attributed variously as the mechanism of partial discharge degradation [2, 3]. Interestingly, in polyethylene, Mayoux [4] has shown that degra-dation due to ion bombardment occurs at a significant rate only if the charge density of ions exceeds≈1.5 × 10²cm⁻². Further, he has demonstrated that although, in theory, electrons may produce degradation, electron energies in excess of 500 eV are required to cause substantial damage. However, once again, the synergetic nature of discharge degradation mechanisms cannot be overstressed; that one form of stressing does not in itself result in degradation does not mean that it can be dismissed as a contributory mechanism to degradation.

In respect of the potential for particle impact damage at a given insulating surface, it is useful to consult one of the many excellent texts on gas breakdown, e.g. Reference 5, to determine, for a given gaseous situation, the nature of the par-ticles involved and for a given set of conditions (partial pressure etc.) the statistical distribution of energies associated with those particles. Having obtained this infor-mation, consultation of materials texts will provide details on molecular structure and bond energies. Comparison of the two sets of information should provide some guidance of the likelihood of bond scission etc. and where it might occur. An example of the latter form of data is provided in Table 4.1.

Table 4.1 Chemical bond energies associ-ated with epoxy resin

Bond type Group structure Energy (kJ/mol)

C–H aromatic 435

C–H methyl 410

C–H methylene 400

C–O ether 331

C=O ketone 729

C–C aliphatic–aliphatic 335 C–C aromatic–aliphatic 347

4.2.2 Thermal stress

The energy injected into the gaseous environment by the discharge will increase the temperature of the gas in the local vicinity. In turn, this thermal differential will cause the gas molecules in the hotter region to migrate to the cooler regions. Although attempts have been made to ascertain the gas temperature under partial discharging [6], in general these results must be treated with caution. In addition, the combinations of particle input energy transfer, chemical bond restructuring and other potentially exothermic reactions will result in temperature increases at a discharging surface.

The author’s personal experience has suggested that the thermal stress created by a partial discharge may be sufficient to cause damage to polymeric materials but not to other forms of solid insulating material. Even in the case of polymers, the degradation sustained due to thermal stressing alone is likely to be insignificant compared with other stresses present. However, since the degradation is due to a synergetic interaction of a number of stresses, it must still be considered where partial discharges are present.

The reaction of polymeric materials to a purely thermal stress is dependent on the structure of the material [1].

For example, simple structures, such as polyethylenes, degrade by random chain scission:

–CH₂–CH₂–CH₂–CH₂–CH₂–CH₂–→ –CH2–CH₂–CH₂–CH₂–CH^·₂+ ^·CH₂– the left-hand side of the reaction continues:

→ –CH^·–CH₂–CH₂–CH₂–CH₃→ –CH^·+ CH2=CH2–CH₂–CH₃

thus producing subgroups of length determined by the fold back length of the chain transferring the radical element.

Other simple polymers, e.g. polyvinyl chloride, degrade not by chain scission but by loss of side constituents:

–CH₂–CHCl–CH₂–CHCl–CH₂–CHCl– → –CH2–CH=CH–CH2–CHCl–

+ HCl

thus although the polymer chain remains the same length it loses stability by developing unsaturated sites in the chain.

Depolymerisation will occur in situations where the polymer molecule contains no easily abstracted atoms or groups. Polymethylmethacrylate is an example of this type of reaction:

–[CH₂–C(CH₃)(COOCH₃)]–[CH2–C(CH₃)(COOCH₃)] –[CH₂–C(CH₃)(COOCH3)]

→ –CH2–C(CH₃)(COOCH₃)–CH₂–C^·(CH₃)(COOCH₃)

→ –CH2–C(CH₃)(COOCH₃)+ CH2=C(CH3)(COOCH₃)

here, it can be seen that the polymer chain unzips, i.e. each segment of the polymer will return to the prepolymer state.

The presence of aromatic rings, i.e. benzene-type structures, in a polymer stiffens the chain and raises the glass transition temperature, i.e. the temperature at which the structure changes from a glassy to a plastic state. This can be seen in the work by Black [7] who has determined that the glass transition temperature of polymer (1) below, which has aliphatic rings in the chain, is 80^◦C whereas polymer (2), which has aromatic rings, has a glass transition temperature of 380^◦C:

[–NH–(CH₂)₆–NH(C=O)–(CH₂)₆–(C=O)–]n (1)

[–NH–(CH)6–NH(C=O)–(CH)₆–(C=O)–]n (2)

The reaction of a polymer to thermal stress in air will also result in thermal oxidation of the material. Main chain scission or side group removal will create a radical species which reacts readily with oxygen to form peroxy radical species:

–CH₂–CH₂–CH₂–→ –CH2–CH₂+ O2

–CH₂–CH₂O₂

The peroxy radical can abstract hydrogen from a polymer group in the vicinity to form a hydroperoxide and a second radical species:

–CH₂–CH₂O₂+ R–H → –CH2–CH₂O₂H+ R

The hydroperoxide species can also decompose due to the application of heat to form radical species:

ROOH→ RO + OH

From the preceding discussion, it can be seen that the application of heat to a poly-meric insulating system will produce a number of reactive sites and species given the application of sufficient heat.

These are just some of the reactions which can occur due to the thermal effects.

However, as indicated earlier, all effects must be considered to occur in a synergetic manner.

4.2.3 Mechanical stress

A vibrational mechanical stress will be set up in a solid insulating material subject to partial discharge stressing under normalAC operating conditions due to the interaction of trapped charge in the solid matrix from the discharge interacting with the applied AC electric stress field. In addition, trapped charged particles of similar polarity will be repelled from each other and dissimilar charges attracted to each other, again resulting in a local mechanical stress within the solid matrix. There will also be a mechanical stress resulting from the impact of larger particles at the discharging surface depending on the mass number and collision velocities of these particles. It is unlikely that this will have sufficient energy to cause fracture, as the shock wave is likely only to have energies of the order of 10⁻¹² J [8]. However, once again the synergetic effects of such a process cannot be dismissed.

Particle impact from a partial discharge can also result in bond breakage and the production of ionic and radical species, as indicated earlier. These species may react with the gas but may also react with the solid to produce, for example, in the case of polymers, extra crosslinks. These extra crosslinks may produce a stiffer section in the polymer making it less resistant to shear, tensile and compressive forces induced in the polymer by the electric stress/trapped charge effects. Arbab, Auckland and Varlow [9] have been long proponents of mechanical stress damage to materials via partial discharge/AC electric field stressing, and their various papers on this subject are extremely illuminating.

4.2.4 Chemical stress

As indicated earlier, particle impact, thermal stressing and mechanical stressing can all result in changes to the chemical structure of a solid insulating system subject to partial discharge stressing. In addition, the species generated in the gaseous environment of the discharge may also interact chemically with the solid material when they impinge at its surface. Given the range of potential interactions (on the basis of the range of gases, liquids and solids plus contaminants) involved, it is impossible within the confines of this chapter, to detail all possible effects. However, to give the reader some sense of the issues involved, some examples are presented.

Air is the most common atmospheric medium through which partial discharges propagate and, as such, it is worthy of consideration from a chemical viewpoint.

Air is a complex mixture of gases, of which the major components are nitrogen and oxygen with minor concentrations of argon, water vapour and oxides of carbon. The molecular species, ions, etc., generated by discharges in air [10–14] are, therefore, most likely to be combinations of nitrogen, oxygen, carbon oxides and hydrogen (from breakdown in atmospheric moisture). The gaseous species produced will be, for an AC stress situation, of both positive and negative polarity e.g. O⁻and O⁻₂ , N⁺ and O⁺₂. In addition, in a discharge atmosphere the polar nature of water molecules causes them to be attracted to charged species in the discharge.

The ionic species formed in positive and negative DC corona in air are shown in Table 4.2 to illustrate these differences.

Column A shows the principal species formed during discharges where the high voltage point electrode is negatively charged. The species listed are all hydrated, i.e. had attached (H₂O)ngroups. The principal species generated when the point is positively charged are found in columns B and C; the species listed in column B are not hydrated, those in column C are hydrated.

The difference in the ionic species generated in the discharges is significant in that the character of the chemical reactions which will occur on the material surface, due to the impact of the species produced in discharges from positive and negative points, will be different. In the case of an AC field stress condition, in which both potential discharge surfaces may act to form discharges, all species may be present.

The production of reactive oxygen species and oxides of nitrogen in the dis-charge atmosphere is particularly important when considering degradation processes.

The triplet form of oxygen, ozone (O₃), is a strong oxidising agent and oxides of

Table 4.2 Ionic species formed in DC coronas

Column A Column B Column C

CO⁻₂ O⁺ H⁺

CO⁻₃ O⁺₂ N⁺

O⁻₂ N⁺ NO⁺

O⁻₃ NO⁺ N⁺₂NO

NO⁻₃ NO⁺₂

NO⁺NO

nitrogen are known to react in air to form nitric acid, also strongly degrading. This phenomenon and, indeed, the importance of chemical degradation in partial dis-charge stressing in general is exemplified by the work of Shields and the present author [15, 16] in comparing discharge degradation of mica in an air and a nitrogen environment. Under similar experimental conditions, including discharge repetition rate and magnitude, it was found that degradation was much more severe over a given time period in an air environment. Given the physical similarities between the two gases, it would be expected that the only differences between degradation in the two atmospheres would be attributable to their chemical differences. In this respect, the formation of nitric acid (HNO₃)at the mica surface in air appears to offer the most likely explanation of the variation in degradation. Although surface reactions are pos-sible with active nitrogen, no nitric acid will be produced in a nitrogen atmosphere.

The active nitrogen will transfer energy to the mica structure in order to return to the ground state. In air, however, where oxygen and water are also present the following reactions are likely to occur:

3O_2(g)+ hν ⇒ 2O3(g)

and

N2(g)+ O2(g)+ hν ⇒ 2NO(g) NO(g)+¹₂O2(g)⇒ NO2

and finally

2NO_2(g)+ H2O_(g,l)⇒ HNO3(g,l)+ HNO2

It is therefore suggested that the increased erosion of mica in air can best be explained by nitric acid, formed in the discharge environment, causing surface erosion by an acid reaction mechanism on the mica. This mechanism would also account for the presence of metallic elements from the electrode at the degraded surface, found during the experimental programme, as the electrode too would suffer similar erosion.

As damage was observed in both gaseous environments, however, a second degra-dation mechanism had to be postulated to account for degradegra-dation in nitrogen. Given

the stress conditions prevalent, chemical, bulk thermal and surface/bulk field effects could be rejected, and, on this basis, the most likely source of degradation was consid-ered to be energetic particle bombardment, as discussed earlier. Bearing in mind that mica consists of a lattice of SiO₄units, this mechanism would involve the energetic particles within the gas transferring their energy to the mica surface causing either direct bond scission:

M^∗+ –Si–O–Si– ⇒ –Si–O + Si– + M

where M^∗is the energetic particle, or bond scission by cumulative localised heating:

NM^∗⇒ T + NM

where N is the number of energetic particles and T is the increase in temperature, then:

–Si–O–Si–+  ⇒ –Si–O + Si–

where  is the heat applied.

As air contains a large proportion of nitrogen, and the presence of other molecules does not preclude this mechanism, it was assumed that a similar reaction was occurring in air, concomitantly with, but secondary to, the acid degradation mechanism.

Another example of the importance of chemical stressing under partial discharge stressing is exemplified by the work of Hepburn et al. [17] but this time in an organic polymeric material, i.e. epoxy resin, as opposed to the inorganic, crystalline mica structure.

Examination of the epoxy resin degraded surfaces following partial discharge stressing in air indicated the presence of various nitrogen compounds, carbonaceous anhydrides, acids and peracids and led to the following reactions being proposed for the resin degradation.

Nitric acid is known to react with organic compounds as follows:

nitric acid breaks down into a nitrous oxide and a hydroxyl radical HNO₃→ OH + NO2

an organic radical is formed when hydrogen is extracted by the hydroxyl radical

R–H+ OH → R + H2O

the organic radical then reacts with nitrogen oxide to form nitrate R+ NO2→ R–NO2

nitrite rather than nitrate may be formed R+ NO2→ R–O–N–O

The reactions described would account for the presence of nitrogen compounds but do not explain the other reactions taking place.

It is known [10, 11, 18] that reactive carbonaceous compounds are present in air discharges. Formation of reactive carbon species and possible routes to production of anhydrides are thought to rely on either:

a activated oxygen attack on the methyl group:

R–CH₃+ O2→ R–CH2+ OOH or

b hydroxyl radical attack on the methyl group:

R–CH₃+ OH → R–CH2+ H2O

both of these initiating reactions produce a methylene radical on the resin chain.

The radical reacting with oxygen produces an aldehyde:

R–CH₂+ O2→ R–CH2O₂→ R–(C=O)–H + OH the aldehyde reacts with oxygen as follows:

R–(C=O)–H + O2→ R–C=O + OOH Carbonyl and hydroxyl radicals interact as follows:

R–C=O + OH → R–(C=O)–OH

A second interaction with a hydroxyl radical produces another radical as shown:

R–(C=O)–OH + OH → R–(C=O)–O + H2O

Interaction of the two radicals highlighted will produce a linear anhydride, as detected on the epoxy resin surface after electrical stressing:

R–(C=O)–O + R–C=O → R–(C=O)–O–(C=O)–R

Given that activated oxygen species are less prevalent in a moist atmosphere [11]

and that anhydrides are widespread following stress in a moist atmosphere but less so in a dry atmosphere, reaction b was considered the more probable initiating step.

The production of nitrated species following normal air discharges is explained by the higher levels of nitrogen oxides in a moist atmosphere [11].

Interactions between the radical species involved in development of anhydrides can also be used to postulate reactions for the production of acids and peracids detected on the stressed resin surface.

The production of radical species, R, on the bisphenol chain (by removal of the methyl group) was postulated earlier. Interaction with oxygen and carbon species allows the following reaction:

R+ O2+ R–H → RO2+ R–H → RO2H+ R Peroxides can thus be formed on the resin surface.

Where the radical species, RO₂and R, are formed in close proximity, additional crosslinks can be formed in the resin matrix by the oxygen molecule:

R+ RO2→ R–O–O–R + hν

Once again the importance of chemical stressing is exemplified.

A careful trawl through the literature (e.g. Goldman, Mayoux, Bartnikas, Wertheimer and the present author plus appropriate general chemical texts, variously) will provide much information and data from which potential reactions can be hypothesised based on the specific materials and gaseous environment in use.

4.2.5 Electrical stress

The superposition of an electric field due to charge deposition from a partial discharge at a solid insulating surface will result in both local microscopic and macroscopic effects which may cause degradation. Electric fields can be responsible for

The superposition of an electric field due to charge deposition from a partial discharge at a solid insulating surface will result in both local microscopic and macroscopic effects which may cause degradation. Electric fields can be responsible for

In document JOAN MIRÓ: EL COMPROMISO DE UN ARTISTA, 1968-1983 (página 57-63)