The Severity-Lifespan Model (SL) provides a means of evaluating
particular transformer risks to enable effective plans to be put in place for
condition recovery. Present theory supports the notion that the transformer
health is directly related to the condition of the insulation [Cigre WG12.18].
Insulation deterioration can be rapid due to high energy and mechanical forces
or slow due to chemical degradation mechanisms. Ageing models relate thermal
characteristics as the prime mechanism of insulation ageing yet a significant
number of transformers that suddenly fail have not exhibited high elevated
temperatures [McNutt1]. The SL Model was created with a specific focus on the
holistic conditions that contributed to the transformer health.
The model was based on three levels of impacts. These
are long term, medium term and short term impact and the details are captured
in figure 1.
Long term ageing factors of figure 3-1 are those that are present in the
transformer but do not significantly affect the transformer life immediately.
These may however be catalysts and supporting mechanisms for other more severe
degradation processes. These ageing mechanisms are usually contained and
originate in the oil medium.
Oil Quality
Oil Quality
Oil quality is a major aspect of transformer health [Lapworth1]. For oil to carry out the intended
functions of dielectric and as a heat transfer medium it needs to be within
specification. The following parameters are utilised for oil quality
monitoring; dielectric strength, water, acidity and interfacial tension.
Dielectric Strength
All insulating material has some resistance to breakdown. This is
measured as dielectric strength and in terms of transformer mineral oil; it is
the ability of the oil to withstand electrical stress caused by high voltage
gradients [Wang2]. This measure is applied predominantly to the oil giving an
indication of its condition due to the dielectric strength being highly
affected by contaminants such as water and free particles in the oil. The
dielectric strength is measured in kV/2.5mm gap.
Water (H2O)
Water is one of the major contaminants in a power transformer
[Lundgaard1]. It is usually found in the paper insulation or dissolved and as
free water in the oil [McNutt1]. Water is always present in an oil-immersed
transformer and originates either from the atmosphere or internally from
chemical reactions during the degradation process of oil and paper insulation [Garcia1]. Water content in the oil is dynamic
as its solubility is highly temperature dependent and migrates between the oil
and paper to reach an equilibrium state [Sheiretov1].
Water forms a significant catalyst in the breakdown process of the oil and paper insulation. When left uninfluenced, it creates a sustained cycle of cellulose breakdown and increase in water levels [McNutt1]. Water is measured in parts per million.
Water forms a significant catalyst in the breakdown process of the oil and paper insulation. When left uninfluenced, it creates a sustained cycle of cellulose breakdown and increase in water levels [McNutt1]. Water is measured in parts per million.
Acids
The acidity of the oil
originates from chemical reactions due to the degradation of the oil, which is
usually as a result of oxidation of the oil and particles and this process is
highly sensitive to temperature and levels of oxygen in the oil [Lundgaard1].
With the presence of water, the acidity can affect metal parts like the tank
and supporting structures causing corrosion. If the acidity of the oil reaches
high proportions, it results in the production of sludge [Gossling1]. Acidity is measured in mg.KOH/g
oil.
Interfacial Tension
Interfacial Tension is the
measure of tension at the interface between oil and water [Shaban1]. The measuring unit is dynes/cm with
good oil having a reading between 40-50 dynes/cm. Both water and by-products of
oil oxidation have the effect of reducing the interfacial tension measure
[Eklund1].
Hydrolysis
Hydrolysis is the
decomposition of a chemical compound by reaction with water. [Cigre WG12.18] highlighted
that the energy of activation of pyrolysis is 1.4-2.0 times as great as the
energy of activation of hydrolysis with hydrolysis being the dominant process
for temperatures between 110-120 °C. The hydrolytic process requires an acid catalyst to
provide a hydrogen atom to initiate the process [Emsley3].
Oxidation of Oil
Oxygen has a strong affinity
for electrons and the process of bonding where oxygen consumes electrons from
other atoms is oxidation [Sabau1]. This
is evident in power transformers especially on the oil and paper insulating
material. The oxidation of transformer oil produces acids, aldehydes, ketones,
esters and sludge. A mixture of hydrocarbon molecules and particles form sludge
[Sabau1].
Medium Term Impact
Medium term impact factors of
figure 3-1 are composed of degradation processes, which have already been
initiated and reaching levels that may reduce the transformer lifespan
drastically. These consist of pyrolysis, oxidation of cellulose and the effects
of corrosive sulphur. This section identifies methods of how these processes
can be effectively measured.
Pyrolysis
Pyrolysis may be defined as
the chemical decomposition of organic material when heated at high
temperatures, usually greater than 300 °C and within 700 °C. One of the key aspects of pyrolysis is that it does
not require oxygen or any other reagents [Demirbas1].
The process predominantly yields hydrogen and methane but for extreme cases
yields carbon.
DiGiorgio identified relationships of
typical fault gas distributions for pyrolysis in oil and of cellulose as
highlighted in table 3.1 [DiGiorgio1].
Table 3.1: Typical fault gas
distribution for pyrolysis in oil and cellulose [DiGiorgio1]
Oil
|
Cellulose
|
|
Chemical
Reaction
|
||
H2
|
16 %
|
9%
|
CO2
|
Trace
|
25%
|
CO
|
Trace
|
50%
|
CH4
|
16 %
|
8%
|
C2H6
|
6%
|
|
C2H4
|
41%
|
4%
|
C2H2
|
Trace
|
0.3%
|
Experiments conducted noted
that at 700 °C
the oil is completely decomposed to form methane, ethane and propane with no
traces of acetylene [Buenomak1].
Oxidation of Cellulose
Due to cellulose having oxygen
as one of its molecular components, it degrades at a faster rate than oil. The
oxidation process produces water, carbon dioxide and furfurals [Liland1]. High temperatures and higher levels
of oxygen and acidity significantly affect this process, which causes rapid
breakdown of the glucose molecules into shorter chains decreasing the overall
strength of the paper. Aging of the paper also increases the acidity of the oil
[Lundgaard1].
Corrosive Sulphur
The formation of copper
sulphides in transformers became a problem of note in the late 1990’s to
present date. This was due to the changes in the methods of refining and oil
purification with new suppliers in the transformer oil industry introducing new
sources of crude oil having different profiles of naturally occurring sulphurous
compounds [Sundin1].
The two mechanisms of copper
sulphide are the corrosion of copper and the depositing of copper sulphide on
conductors and paper insulation. The effect of this is either a reduction in
the dielectric strength of the solid insulation or providing a conductive path
[Lewand1]. Significant copper sulphide build-up results in the flaking of these
conductive particles in the oil, which reduces the dielectric, and cause
related failures [Griffin2].
The corrosive sulphur
destructive process is highly temperature dependent. During conditions of low temperature,
the process is one of either predominantly the formation of conductive
sulphides, which then suddenly results in failure at elevated temperatures, or
when the amount of copper sulphide is enough to cause conduction.
This process does not usually result
in the production of fault gases and thus failure is unexpected and virtually
undetectable. Copper sulphide production has been found to be a localised
phenomenon.
Figure 3‑2:
Copper Rods and Kraft paper insulation after testing [Lewand1]
The Cigre WG A2-32 (Copper sulphide in transformer insulation) has identified the following key aspects of
corrosive sulphur in power transformers [Cigre WGA2-32, Dahlund1]:
- Sulphur must be present in the
transformer oil. This includes oil types such as naphthenic and paraffinic or
intermediate oils.
- Temperature plays an important role
in the process but is not the sole factor for occurrence. Copper sulphide
formation at temperatures from 80 °C to 150 °C was demonstrated in a laboratory.
- Most failures recorded to date have
been found in GSU transformers, shunt reactors and HVDC
transformers, which were attributed to the high loading tendency of the
equipment.
- Failures recorded were also mainly
found in closed units, which may indicate that the presence of oxygen slows
down the copper sulphide production process.
- The DGA and the usual oil monitoring
parameters like tan δ are
not susceptible to the detection of copper sulphide production.
- It is generally found that the
deposition of copper sulphide closely relates to the thermal profile in the
transformer with typical location at the top of windings.
Short Term Impact
Short term impacts of figure
3-1 are conditions that when experienced will result in failure or initiation
of failure of the transformer. These are further broken up into three
categories which are controlled defective events, major component failure and defective
events.
Short Term Impact are conditions within the
transformer that are changing in the current environment with visible effects
on the parts of the transformer. These are usually evident in the oil medium
where measurements can be easily taken from oil samples. These parameters
consist of the water content, dielectric
strength, interfacial tension and top
oil temperature of the transformer.
By combining these parameters,
an effective indicator can be established on the immediate condition of the
transformer insulation medium and control further deterioration to limit medium
and long-term impacts.
Controlled Defective Events
These events are controllable
by the engineers and consist of design, operating conditions and manufacturing
of the transformer. Operating conditions cover events such as overloading
beyond the capability or close to limits. Transformers in the modern era are
pushed to limits especially when utilities are faced with under supply of
power. Short time overloading can affect transformer life and the process may be
irreversible. Transformer designs can also contribute to the lifespan of the
transformer. The transformer must be effectively designed to handle a certain
level of short circuits and over voltages. The design of the cooling of the
active parts is also another important parameter to be considered. The
manufacturing process is another risk area where poor workmanship of important
components like the core and windings can result in premature failures.
Major Component Failure
Major component failures such
as bushings and tap changers can be catastrophic to the life of the
transformer. These events usually end the life of the transformer no matter how
well the main transformer windings and core is designed and manufactured.
Defective Events
Defective events are system
and environmental conditions that can severely affect the transformer. Defective
system conditions consist of through faults, line over voltages and inrush
currents. Environmental conditions consist of lightning and geomagnetic induced
currents caused by solar storms. The nature of these events is such that they
pose high levels of mechanical and thermal stress on the transformer electric
and magnetic components and may be the one event that either causes failure or
initiates failure mechanisms.
Partial Discharge and Corona
Partial discharge usually
occurs within insulation systems that have voids, cracks, and along the
boundary between different insulating materials or in bubbles within liquid
dielectrics [McNutt2]. Discharges predominantly occur within localised areas
with the discharges only partially bridging opposite electrodes [Mathes1].
Partial discharges within the winding paper insulating material are
usually initiated within oil-filled voids within the dielectric. This causes
the dielectric constant of the void to be significantly lower than the
surrounding dielectric, which results in the electric field across the void
being considerably higher than across an equivalent distance of dielectric. PD
activity usually starts when the voltage stress across the void is increased
above the corona
inception voltage. The
mechanism of deterioration caused by PD is slow and persistent until electrical
breakdown occurs within the area [Mathes1].
Table 3.2 below summarises the
chemical breakdown and fault gas distribution for typical corona in oil.
Table 3.2 Typical fault gas
distributions for corona in oil [DiGiorgio1]
Oil
|
|
Chemical
Reaction
|
|
H2
|
88%
|
CO2
|
1%
|
CO
|
1%
|
CH4
|
6%
|
C2H6
|
1
|
C2H4
|
0.1%
|
C2H2
|
0.2%
|
Arcing
An electric arc is formed from
an electric breakdown of a gas which is able to sustain long enough to cause a
current flow through the non-conductive medium [Perkins1]. In
the case of a power transformer arcing can occur in the oil (non-conductive
medium) between areas of high potential differences. An electric arc is
associated with high temperatures that are capable of degrading or destroying
the area of the medium around which is forms. The key difference between a
partial discharge and an electric arc is that the electric arc results from a
continuous discharge of electrons until permanent breakdown occurs [Perkins1]. Typical fault gas
distribution for arcing is provided in table 3.3 below.
Table 3.3 Typical fault gas
distributions for arcing in oil [DiGiorgio1]
Oil
|
|
Chemical
Reaction
|
|
H2
|
39%
|
CO2
|
2%
|
CO
|
4%
|
CH4
|
10%
|
C2H4
|
6%
|
C2H2
|
35%
|
Relevance of Severity-Lifespan Model
The severity–lifespan model is
significant in this study as it organises the transformer ageing risks into an
integrated chart in relation to the impact and time. The key information from
this model is the identification of the insulation degradation mechanisms and
how these propagate over time to become catastrophic mechanisms to the life of
the transformer. This model also provides a high-level summary of the
degradation parameters that would be most effective in identifying the impact
of the particular degradation processes.