Transcript Ageing phenomena in cables insulation materials

Condition Monitoring of Electrical Equipment
Spring 2016
Ageing Phenomena in Cables Insulation
Materials
Edit by Ilaria Mele
‘’Ageing’’ is the accumulation of the normal changes of the material. It leads to a performances decreas and increasing of the
failure rate.
It is influenced by :
• Enviromental effects: animals, UV light, dirt
• Operative effects: overvoltages, vibration
• Constructive imperfections: voids, inhomogeneity.
•
TWO PRINCIPALS INSULATIONS
MATERIALS
1.
PAPER AND OIL INSULATION
The oldest insulation system, it
belong to the organic insulation
group. Easy and cheap to use, with
good dielectric proprieties.
THREE TYPES OF
STRESS
2. POLYMER INSULATION
ENVIROMENTAL STRESS
Used since the 1960, the final
use depends by the
manufacturing process. This
categorie concerns the
Thermoplastic and Thermoset
plastic.
MECHANICAL STRESS
ELECTRICAL STRESS
Ageing index:
Weight reduction
Oxidation
Decrease of Maximum Temperature
Performance reduction
Temperature is one of the most studied type of stress.It is, and also, an important parameter that affects the other ageing process.
The chemical reactions are accelerated by the temperature, Arrenhius equation shows the connection between temperature and chemical
reaction acceleration. It is valid only if a single reaction is happening.
life expectancy
[h]
𝐾 𝑇 = AR e−Δ𝐸/𝑘𝑇
2∙10
104
• ΔE : activation energy of the reaction
• T : temperature (K)
• k : Boltzman costant
• AR : material costant
103
Through the Arrhenius
formula is possible to obtain the plot that relates the temperature and
expectancy of life .
102
10120 TI
HIC
0.25
14
00.24
160
0.23
180
0.22
temperature
200
220 T [°C]
-1
0.21 ∙10-2 1/T [K ]
1. Paper and oil insulation degradation
The properties of oil and paper will change during their life time. Cables are affected by difference type of stress at the same time.
Oil reacts with of oxygen, leading to the productions of free radicals.
ELECTROMECHANICAL REACTION
Can be divided in hydrolytic degradation, oxidation depends by the reagents. The hydrolitic degradation is the most dangerous for the cables
because they have a quite well protectection against oxygen and high temperatures.
Different kind of chemical reaction, produce acid compounds which weaken the paper insulation. Paper is formed by cellulose chains and the tensile
strenght, 𝑇𝑠 , is:
1
𝑇𝑠 =
1
1
( + )
𝐹𝑏 𝐹𝑠
𝐹𝑏 is the fiber-fiber boding strenght that increases with the ageing due to the cross linking reactions
𝐹𝑠 is the fiber strenght that decreases with the depolymerization reactions.
Hydrocarbon molecules in oil oxidize
into Hydrogen peroxide that rapidly
disintegrates into free radicals.
A free radical is an atom that contains
an impared electron. They are enstable
and reactive.
Moisture
Hydrolysis and Oxidation
It works in long period, and has dangerous effects only if
the protective lead sheat is mechanically damaged. After
the moisture penetration (it needs months) the process is
characterized by:
Hydrolytic degradation is caused by water and acids. The
result is a reduction in the degree of polymerisation and
weakening of paper mechanical properties.
Oxidation takes place when oxygen attacks the carbon
atoms in the cellulose molecule. Water, acids and carbon
monoxide are produced. Bonds between cellulose are
weakened.
Step
Time
Current
Weeks
10 mA
Sporadic arc
Days
100 mA
Intermittent Arc
Days
Continuos arc
Days
water concentration in
paper [%]
Low level of partial discharge
water in oil [mg/kg]
It leads to a failure
Overloading: It leads to transient
overcurrents and continous overcurrents.
Thermal degradation or Pyrolisis
Is the the most serius phenomena for paper insulated
cables
Can take place without access of oxygen or moisture.
During normal operation when the temperature is
below 140 °C, pyrolysis processes are considered
insignificant.
Electrical Stress
Overvoltages: any voltage level above the rated voltage
In addition,over voltages are dangerous for the insulation
material. Some of them are momentary, whereas others
are sustained.
Partial discharge: is local accumulation of electric field.
PD is a symptom of insulation weakness
DUE TO:
• Small mechanical damage of insulation
• Voids in the insulation due to
manufacturing faults.
Once the void’s withstand voltage is
exceeded, partial discharge occurs.
Inappropriate heat transfer: it affects soil
thermal resistivity and increase the heat
transfer of the cable.
Over heating: it can breakdown polymers
and deforms the insulation layers.
Increase the rate of PD in the insulation.
Increases dielectric losses
Cause erosion and can eventually lead to
breakdown.
Reduction of electrical withstand
strength.
This produces a rapid rate of change of
current
Positive ions and electrons released by
the discharge collide with
the void wall causing slow erosion.
Furthermore…
Different kinds of gases can be produced during the degradation of cables. Gases such as carbon monoxide, carbon dioxide and smaller amounts of hydrogen,
methane and ethane are produced. These gases can displace the oil and fill the areas between paper strips. The electric field strength inside a gas cavity is higher
than in the surrounding insulation. When the withstand level of the cavity is exceeded partial discharge will occur. Partial discharge can produce new gas filled
cavities and conductive particles. The withstand level of oil is reduced because carbon particles tend to dissolve in it. Oil-paper insulation is partly self healing with
respect to partial discharge phenomena. In some cases partial discharges and gas filled cavities can disappear because the oil can move and replace the gas in
cavities.
Mechanical stresses can also promote the inception of partial discharges
Degradation of paper mainly affects its mechanical properties. During degradation the degree of polymerization of the insulating paper is decreased. The
degree of polymerization of new paper is around 1100 to 1500 while a value less than 500 indicates significant thermal degradation . Weakening of
mechanical properties can lead to cable failure during short circuits.
DP
DP, Degree of Polymerization defines the condition
of paper insulation. An high value of DP means
high melting temperature and mechanical strength
relative lifetime
2. Polymer insulation
Thermoset (Irreversibly cured )
Thermoplastic (Can be re-melted and remolded)
Their main proprieties are the mechanical and dielectric strentgh, both are influnced by the temperature.
Mechanical stress:
It is due to the presence of voids , techinical imperfection or inhomogeneous insulation distribution. These features can cause, in the future, internal
mechanical stress.
Chemical degradation:
It is caused by free radicals formed during the oxidation. Their presence depends by the percentage of oxigen, the temperature, and the amount
of radiation. Chemical degradation, is also, connected with the PD (Partial Discharge), because it augments the formation of free radicals .
Electric degradation :
Locally or randomly distributed it does not affect the whole length of the cable.
Partial discharge, Electric trees, Water trees
Partial discharge is a local accumulation of electric field, can lead to erosion and breakdown of the polymeric chains.
It can be schematized in 3 steps:
Gradual erosion and
homogeneous deterioration
of the cavity wall.
Discharge concentrates on a
specific region of the cavity
forming deeper recesses in
the wall.
Conductive channels form
and develop in a branching
pattern , until complete
breakdown occurs .
Electric trees
An electrical tree is a network of thin conductive channels that propagate relatively quickly
through the insulation to cause failure. It can start from an eroded surface of voids or in micro cavieties.
The tree growth rate depends on the applied electrical stress, the temperature, the environmental and mechanical stresses.
The size of the tree is, in the initil phase about 0,1-0,2 pC but when it reaches 5 pC it can lead to thermal runaway. Total breakdown of the insulation can happen
when the branch of an electrical tree bridges the electrodes, sometimes it can occur before this moment.
Water trees
The majority of XLPE insulated cables installed in the ground are exposed to moisture, and as a result are susceptible to degradation due to water trees. This
phenomenon develops mostly in insulation exposed to an alternating electric field and humidity.
Impurities inside the insulation material will increase the risk of water tree initiation.
The water trees starts in microscopic inhomogeneities of the insulation, and start to propagate in the direction of the electric field in a bush or tree form.
Bow-tie water trees:
It reaches a limiting length of some tens of μm, and do not have a
significant effect on degradation at the low electric field stresses.
Vented water trees:
It needs a longer initiation time and can cause more severe
degradation than bow tie water trees
The difference between a electric and a water tree are:
• Water tree, does not necessarily form a permanent and visible track in the insulation Electric trees are
usually visible.
• When electric field and moisture are removed, water trees disappears.
• Formation of the water trees will take years.
• Water trees are formed at lower electric field strengths than electrical trees.
• Water trees can evolve in electric trees.
Thermal degradation:
Insulation temperature should be below 90° C , in fault condition the temperature reaches 120°C. The free radicals
composition happens between 150 and 225 °C, and they cause crosslinking, reduction of thechain strength.
At 350°C there is the thermal craking, and coke formation.
Thermal degradation affects the cable for the whole length.
Water trees
Electric trees
Bow tie
Vented
Partial Discharge
Thermal degradation