Partial Discharge and Insulation Failure

PARTIAL DISCHARGE

&

INSUALTION FAILURE

1.0 INTRODUCTION

Partial Discharge (PD) is an electrical discharge that does not completely bridge the space between two conducting electrodes. The discharge may be in a gas filled void in a solid insulating material, in a gas bubble in a liquid insulator or around an electrode in a gas. When partial discharge occurs in a gas, it is usually known as corona.

Partial discharge is generally accepted as the predominate cause of long term degradation and eventual failure of electrical insulation. As a result, its measurement is standard as part of the factory testing of most types of high voltage equipment. In addition, partial discharge activity is very often monitored on in-service equipment to warn against pending insulation failure.

Test specifications set a maximum permissible level for partial discharges depending on the type of equipment being tested and the insulating material used. The principle behind such a specification is that discharges below a certain size cause minimal damage to the insulation. As insulation systems have increasingly moved towards polymers, acceptable discharge levels have lowered dramatically as they are less resistant to damage by discharge.

This section will look at the physics behind the phenomenon of partial discharge, the effects partial discharge has on insulating systems and the failure mechanisms these can lead to.

2.0 HISTORY OF DISCHARGE MONITORING

Partial discharge has been observed as a phenomenon occurring in stressed high voltage insulation since the turn of the century. It became of increasing academic interest from the 1930s when its degrading effect on high voltage insulation became increasingly problematic. Early studies used ultrasonic detection techniques to assess discharge activity in oil1. In the 1950's theoretical and practical studies led by John Mason looked at how discharge activity could lead to previously unheard of breakdown processes like electrical treeing2-4_.

From the 1960s to the present time, partial discharge has been studied intensively in terms of the fundamental physics behind it, its effect on insulating systems and how best it can be measured and monitored with time.

3.0 DISCHARGE PROCESS IN VOIDS

Solid insulators are manufactured to give an even distribution of electrical stress between the conducting electrodes. However, in practice this is virtually impossible to achieve. Manufacturing processes invariably give rise to small cavities or voids in the insulation bulk.

These cavities are usually filled with a gas of lower breakdown strength than the surrounding solid. In addition to this the permittivity of the gas is invariably lower than that of the solid insulation, causing the field intensity in the cavity to be higher than that in the dielectric. Therefore under the normal working stress of the insulation, the voltage across the cavity may exceed the breakdown value and initiate electrical breakdown in the void.

Figure 1, Equivalent circuit for cavity in insulator

Assume a solid insulator of thickness d contains a disc shaped cavity of thickness t and area A, as shown in Figure 1. In the equivalent circuit the capacitance Cc corresponds to the cavity, Cb corresponds to the

capacity of the dielectric that is in series with Cc and Ca is the capacitance of the rest of the dielectric.

Given that capacitance C, in Farads/m2_{, is given by;}

Where;

ε0 = permittivity of free space = 8.854 x 10-12 Fm-1

εr = relative permittivity

A = area between electrodes d = separation of electrodes

If we assume that the gas in the cavity (of thickness t) in figure 1 has a relative permittivity of approximately 1, then:

d

A

C

₌

ε

ε

t

A

C

ε

=

and,

where:

εr = relative permittivity of the solid insulator

As Cb and Cc essentially form a capacitive divider, the voltage across the cavity, Vc, can be expressed as;

Substituting into the above equation gives;

Therefore electrical field strength across the cavity (Ec) is given by the equation,

Given that in most circumstances t << d and εr is greater than 1, it can be seen that electrical stress in the

cavity is greater than that in the surrounding insulation. This, coupled with the fact that the breakdown strength of the gas is likely to be significantly lower than that of the insulation, makes the gas in the void liable to breakdown under normal working conditions.

The table below shows relative permittivity's and breakdown strengths of some typical high voltage insulating materials.

From the equations above it can be seen that the voltage across the dielectric at which discharge activity will initiate in the cavity, Vai, is given by;

Where;

Ecb = Breakdown strength of the gas in the cavity

In practice voids in solid insulators are very often approximately spherical. In this case the field in the void is given by;

Where εrc = relative permittivity of gas in void.

t

d

A

C

₋

ε

ε

=

_C

_C

V

C

V

+

=

−

ε

+

=

1

t

d

1

1

V

V

−

ε

+

=

1

t

d

1

1

t

E

V

−

ε

+

=

1

t

d

1

1

t

d

E

E

2

E

3

E

ε

+

ε

ε

=

When εr >> εrc this approximates to:

Material Relative

permittivity strength kVmmBreakdown -1

Air (atmospheric pressure) 1.0006 3

Transformer oil 2.2 28 Polyethylene 2.3 24 Polyurethane 4.0 10 Paper 3.0 9 Mica 6.0 42 Epoxy 4.7 12

Each time a discharge occurs in the cavity, charge is transferred from one side of the cavity to the other until the potential difference across the cavity is too small to maintain the discharge. When the insulator is subject to a sinusoidal alternating voltage, charge builds up within the void as the applied voltage increases or decreases. This causes a series of discharges with charge first moving in one direction, then the other. Figure 2 shows how the voltage and current across a cavity changes with applied voltage.

Figure 2, Voltage and current in discharging cavity.

The dotted curve shows the voltage that would occur across the cavity if the discharges did not equalise the potential difference across the cavity. As the voltage Vc reaches the value V+, a discharge takes place

and the, the voltage Vc collapses and the discharge extinguishes. The voltage across the cavity then starts

again increasing until it reaches V+ _{, when a new discharge occurs. In this way several discharges may}

take place during the rising part of the applied voltage. Similarly, on decreasing the applied voltage the cavity discharges as the voltage across it reaches V-, In this way groups of discharges are generated by a single cavity and give rise to positive and negative current pulses on raising and decreasing the applied voltage respectively. a c

E

2

3

E

=

4.0 DEGRADING EFFECT IN SOLID INSULATION

When the gas in a cavity breaks down, the opposite surfaces of the insulation momentarily become cathode and anode. Some of the electrons hitting the anode are sufficiently energetic to break the chemical bonds of the insulation surface. Similarly, bombardment of the cathode by positive ions may cause damage by increasing the surface temperature and produce local thermal instability. These degrading effects form small channels and pits in the surface that can elongate through the insulation. In addition to the ionic bombardment, chemical degradation may result from active discharge products, like O3 or NO2, formed in

the air by the discharges. The net effect is slow erosion of the insulating material and a very gradual increase in the size of the cavity.

4.1 Electrical trees

Electrical trees were first observed in the early 1920s when the Commonwealth Edison Company began installing the first underground residential cables. Electrical trees are comprised of a series of interconnecting channels or discharge paths with diameters ranging from less than a micron to tens of microns. Discharge activity in voids will eventually become centred at particular sites producing deep cavities in the surface. The cavities grow in length along the discharge axis and the energy of discharges impinging on their tips increases. This combined with electrical stress concentration by virtue of their point like form, produces increasingly intense electrical fields at the tips of the discharging cavities. Eventually the breakdown strength of the material in the immediate vicinity of the tip is exceeded. Breakdown follows with the evaporation, in the space of a few nanoseconds, of a small volume of material. This rapid conversion launches small shock waves into the insulation. These waves create, in time, a structure of fine cracks extending into the insulation. Their name comes from the dendritic patterns they from in the insulation. Figure 3 shows an electrical tree grown from a needle tip in polyester resin.

Figure 3, Electrical tree.

Electrical trees emanate from points of stress enhancement in insulation. This can be a metal inclusion or a protrusion on a conductor but in practice they more usually originate from a void. The exact process by which electrical trees propagate is still not fully understood, however, it is generally accepted as being a combination of mechanical and thermal effects.

There are two clear stages in the development of electrical trees under the application of an alternating voltage, the inception period, which may be considerable and a much shorter formative period. Eventually the tree will bridge the insulation. Discharges continue to occur without breakdown because space charge sets up a reverse field in the channels to counter the field between the electrodes. During this period the channels slowly widen. Eventually the field can no longer be maintained in the widened channels and catastrophic breakdown occurs, creating a very large channel though the insulation.

4.2 Water trees

In the1960s high voltage cables started being made using extruded polyethylene as insulation. By the late 1960s it became apparent that where cables had been laid in wet environments, for instance under rivers, their failure rates dramatically increased. It was found that water was permeating through the outer protective sheaths and being absorbed by the insulation. Up to a few percent (by weight) of water can be held by polyethylene. The discovery of degradation of polyethylene by the combined action of water and electrical stress was first published in 1969 by Miyashita5_{. By the early 1970s this kind of degradation}

became known as water treeing. A water tree is a bush or fan like structure developing like an electrical tree, from points of stress enhancement. Water trees cause a reduction in the insulation's breakdown stress level which encourages breakdown. Electrical trees can, on occasion, be initiated from a water tree, speeding the breakdown process.

Although generally accepted as the major cause of failure in polyethylene insulated cables, there is no general agreement on the morphology of water trees, with two models competing. In one, water trees form continuous paths such as channels while in the other model the tree has a high density of micro-voids that are not connected6.

Figure 4, Vented water tree. Figure 5, Bow-tie water tree.

Water trees are more diffuse than electrical trees and generally grow at lower electrical stresses. Two types of water tree have been recognised according to where the tree initiates, ‘bow-tie’ trees and ‘vented’ trees. Bow-tie trees are initiated in the bulk of the insulating material, often from a void, and grow towards the conducting screens. They clearly derive their name from the pattern they form. Vented trees grow from one of the conducting screens into the insulation bulk.

4.3 Tracking

Tracking is the formation of a permanent conducting path across an insulator surface. Usually the conduction path results from degradation of the insulation. For tracking to occur the insulation must be a carbon based compound.

Most high voltage plant is situated outside. In industrial areas, insulators become contaminated with pollution and dirt from the atmosphere. Where substations are situated near the sea, salt very quickly covers the plant. In the presence of moisture, these contaminating layers gives rise to leakage current over the insulator surface. This heats the surface and through evaporation causes interruption in the moisture film. Large potential differences are generated over the gaps in the moisture film and small sparks can bridge the gaps. Heat from the sparks causes carbonisation of the insulation and leads to the formation of permanent carbon tracks on the surface.

Tracking as a phenomenon severely limits the use of organic insulators in outdoor environments. The rate of tracking depends on the structure of the polymers and can be significantly reduced by adding

5.0 DEGRADING EFFECT IN LIQUID INSULATION

Liquids are extremely useful as insulators because not only can they easily fill any space, they also dissipate heat generated in a high voltage system through convection and sometimes forced circulation The most commonly used liquid insulator is mineral oil. It is used for insulating transformers and switchgear, it's reasonably cheap and has acceptable dielectric strength and heat transfer properties. There are, however, a number of drawbacks to the use of oil, for instance, leaks can pose environmental problems and safety becomes an issue in the event of a catastrophic failure and it is easily contaminated. There is no single universally accepted theory as to how breakdown occurs in mineral oil. There are instead a number of different, occasionally complementary and occasionally conflicting theories. Many factors, such as, oil temperature, applied static pressure, impurities in the oil, electrode area and shape, electrode material and surface conditions, size of the gap, significantly influence the measured properties of transformer oil. Therefore these properties, in particular dielectric strength, cannot be defined simply by their numerical values. Test conditions have to be described in detail if a numerical value is to be meaningful. It is the lack of a single standard and generally accepted test procedure that is partly to blame for conflicting theories on breakdown mechanisms.

There are however three elements that are, in practice, commonly associated with failure processes. These are particles in the oil, water and bubbles.

5.1 Particles

In high voltage systems where oil is circulated for cooling, filters are often used to remove particles and impurities from the oil. It is impossible though to keep the oil completely free from such contamination. Even if the system was particle free when new, periodic inspections involve removing lids and covers allowing dust and particles in. Many systems use pressboard, a cellulose based insulating barrier, or have paper insulated windings and these can release particles into the oil over time.

When an electric field, E, is applied, these particles become polarised. If the particle has a permittivity, ε2,

greater than that of the oil, ε1, (as is generally the case), a force will act on the particle forcing it toward the

area of maximum electric stress between the electrodes. For a spherical particle of radius r the magnitude of the force F is given by;

Because of the high permittivity of water, this force is greatly enhanced if the particle is moist or wet. Other particles will be attracted into the region of highest stress until, eventually, particles will be aligned end to end by the field. In this way a short circuiting bridge can be formed between the electrodes. Current flow

along these bridges will cause localised heating leading to breakdown. If the particles are metallic then ε2

→ ∞, and;

A single metallic spherical particle between the electrodes can increase the electric field at its surface by up to three times which can, in some cases, be enough to initiate breakdown. Many studies have been made on the movement of particles in stressed oil but there is no accurate quantification of how they affect dielectric strength. It is however accepted that particle contamination severely reduces dielectric strength and provides a good indication of the condition of oil.

5.2 Water

Water is invariably present in oil under normal working conditions. It may originate from the atmosphere or be produced by the deterioration of insulating materials and oxidation of the oil. In practice water levels are usually less than about 20ppm. When levels are higher than this, an electric field can cause globules of water suspended in the oil to elongate in the direction of the field and, at a critical field strength, they

2 2 1 1 2 3

_E

2

r

2

1

F

ε

+

ε

ε

−

ε

=

_E

r

2

1

F

=

become unstable. Breakdown channels then propagate from the ends of the ends of the elongated globule to produce total breakdown. A concentration of 50ppm of water may be enough to halve the electric strength of transformer oil.

Occasionally the weather protection of plant can completely fail and large quantities of water will collect at the bottom of the oil tank. Its level rises until the remaining oil insulation is not enough to maintain the voltage. This invariably leads to catastrophic failure.

5.3 Bubbles

Small bubbles may be formed in pits and cracks on the cathode surface by one of two methods: • Disassociation of the liquid molecules to produce gaseous products

• Local liquid vaporisation through electron emission from sharp points on the cathode

Electrostatic forces elongate the bubble as soon as it is created and, as the breakdown strength of gas is much lower than that of oil, the field inside the bubble is likely to exceed the strength of the vapour. This causes a discharge inside the bubble that can chemically degrade the oil, producing in turn more vapour so the bubble grows. Eventually it bridges the whole gap and breakdown follows.

6.0 CORONA

When a gas has a uniform electric field across it, the onset of ionisation usually leads to complete breakdown of the gap. However, in a non-uniform field, discharges can occur long before complete breakdown happens. This activity is called corona.

Most HV transmission and distribution are simply long lengths of air insulated conductors creating non-uniform electric fields. As a result corona is responsible for significant power loss from distribution systems. Corona can also inflict serious long-term damage on insulators through the combined action of ion bombardment of their surfaces and the action of chemical compounds formed by the discharge. The radio frequency electromagnetic energy (RF) given off by corona can also cause interference to communication systems.

Corona is a partial discharge in the sense that gas breakdown begins at a position of high electric field but dies out as the electric field decreases very rapidly as a function of distance from the highly stressed position. The breakdown can die out for two reasons;

1, The region of high field is too small to generate a fully formed breakdown channel.

2, The field falls to such a low value, that even a fully formed breakdown channel cannot propagate.

Corona forms in partially ionised regions adjacent to conductors and causes a change in the electric field between the conductor and the ground. In effect, it can be seen as an extension of the conductor. As such, it will effectively reduce the capacitance between the conductor and ground, as their separation decreases. This causes a drop in the voltage on the conductor, a potential difference between the conductor and the voltage source and, therefore, a current flow from the voltage source to the conductor. The electric field in a corona is sufficiently high that when a free electron occurs, that electron will, on average, generate more than one additional electron (and positive ion). So a corona is full of positive and negative ions (electrons). Thus when the field reduces to the extent that the original current ceases to flow, the electric field does not immediately return to its previously high value. Before that can happen, the positive and negative ions must flow in the field toward the negative and positive electrodes respectively. As the negative charges are in the form of electrons, they can propagate sufficiently fast to contribute to the measured partial discharge signal. However massive positive ions flow so slowly that they typically generate a very small current over a long period of time.

Thus a corona can be thought of as generating a PD signal though three mechanisms. First, the ionisation of a channel, which tends to look like an extension of the conductor and therefore increases the capacitance of the conductor to ground. Second rapid migration of electrons toward the positive electrode in a system where negative charge flows as electrons. And thirdly, flow of positive ions which tends to be too slow to be detected by most PD measuring systems. The time scale for the first two phenomena is nanoseconds to microseconds, while that for the third phenomena can be milliseconds or more.

Corona tends to be repetitive, as once the region is cleared of charge, it returns to the conditions which generated it in the first place. Corona in air is sensitive to air velocity and environmental conditions which affect space charge near the conductor.

In many gases, including air, corona generated by positive and negative voltages differ substantially. This is due to the physical difference between negative charge carriers (electrons) and positive charge carriers (positive ions). Electrons being light and mobile gain kinetic energy very rapidly from an electric field, while positive ions are heavy and much less mobile. The outside surface of molecules is made up of electrons, so violent phenomena, which dislodge charge from a molecule, free an electron and simultaneously create a heavy positive ion. In corona from a negative conductor, electrons propagate away from the conductor in the direction of corona growth. Thus they can create further electrons through molecular collisions. In corona from a positive conductor, the electrons propagate towards the conductor and away from the direction of corona growth. In this case, electrons are generally detached ahead of the corona tip by photons generated within the corona.

7.0 REFERENCES

[1] Kimura, H., Tsumura, T. and Yokosuka, M., "Corona in oil as part of commercial frequency circuit", Electrotechnical Journal of Japan, Vol. 4, 1940, pp. 90-92.

[2] Mason, J.H., "The deterioration and breakdown of dielectrics resulting from internal discharges", Proceedings IEE, Vol. 98 part II, 1951, pp. 44-59.

[3] Mason, J.H., "Breakdown of insulation by discharges", Proceedings IEE, Symposium of Papers on Insulating Materials, Vol. 100 part IIA, 1953, pp. 149-58.

[4] Mason, J.H., "Breakdown of solid dielectrics in divergent fields", Proceedings IEE, Vol. 102 part C, 1955, pp. 254-63.

[5] Miyashita, T., “Deterioration of Water-immersed Polyethylene Coated Wire by Treeing”, Proceedings 1969 IEEE-NEMA Electrical Insulation Conference, Boston, pp. 131-5, 1969.