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May 20, 2013 - 21, No. 2; April 2014. 845. DOI 10.1109/TDEI.2013.004118. Ageing Assessment of Transformer Paper Insulation through Post Mortem Analysis.
IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 21, No. 2; April 2014

845

Ageing Assessment of Transformer Paper Insulation through Post Mortem Analysis N. Azis, Q. Liu and Z. D. Wang School of Electrical and Electronic Engineering The University of Manchester Manchester M13 9PL UK ABSTRACT A post mortem analysis of scrapped transformers provides an opportunity to directly measure the properties of in-service aged paper insulation. Investigations on 10 scrapped 275 kV and 400 kV power transformers were carried out, where Tensile Index (TI) and Low Molecular Weight Acid (LMA) of the paper insulation sampled from various locations were measured. Multiple-layer TI profile demonstrated the effect of oxidized oil on paper ageing which was confirmed by laboratory ageing experiments. It is LMA rather than High Molecular Weight Acid (HMA) that accelerates the paper degradation. LMA acts as the paper ageing accelerator and also as the paper degradation by-product, supporting the view that self-accelerated hydrolysis process is the main paper ageing mechanism. Finally, a generic relationship between LMA in paper and TI of paper was revealed based on both in-service and laboratory ageing data, i.e. the lower the TI of paper, the higher value of LMA in paper is measured. It implies that LMA in oil can be a potential paper ageing indicator providing a link between LMA in oil and paper is found. Index Terms — Power transformer, scrapping, post mortem analysis, paper, oil, tensile strength, tensile index, low molecular weight acid, ageing, end-of-life.

1 INTRODUCTION MANY utilities in the UK and Europe are operating their power system networks with aged transformers, which are rapidly approaching or have exceeded the designed lifetime [1]. In the current economic climate, a replacement strategy simply based on designed lifetime is not an option. This is because the transformer life is associated with design, loading history, geographic location, maintenance strategy and etc. An alternative condition based asset management strategy must be formed in order for asset managers to make the maintenance and replacement decisions. This is based on understanding of insulation ageing and failure mechanisms and mainly involved with health condition assessment and lifetime prediction. Cellulose paper is the major insulation of transformer, and its mechanical strength is commonly used to represent the transformer life. As the paper ages, the performance of a transformer could deteriorate. For example, a reduction of mechanical strength could make the paper fail to withstand the external stresses during short circuit faults or the excessive vibrations originating from the switching operation and ferroresonance. If no remedial action is taken, these events will, directly or eventually after accumulation, lead to a transformer failure. A condition based asset management scheme is therefore helpful to avoid catastrophic and cascaded failures on the power Manuscript received on 20 May 2013, in final form 27 September 2013, accepted 4 November2013.

system network. Such a scheme requires better understanding of the paper ageing mechanisms in a transformer and identifying ageing indicators to determine the ageing state of paper nonintrusively without taking the paper samples. According to the previous studies [2-6], paper ageing involves oxidation, hydrolysis and pyrolysis. Acid catalyzed hydrolysis has been suggested as the main paper ageing mechanism, which is particularly influenced by Low Molecular Weight Acid (LMA) according to laboratory ageing studies [3, 7-9]. This type of acid is mainly generated from the ageing of paper through a hydrolysis process. It tends to stay in the paper and then acts as paper ageing accelerator [7-9]. Due to the complex process of transformer ageing, it is challenging to reliably model the life of a transformer. In fact, the majority of life models are based on laboratory ageing experiments, which are far more simplified than the ageing scenario of a transformer in service; hence it requires verification or calibration from in-service ageing data. Oil samples can be routinely obtained from an in-service transformer but not the paper samples; therefore it is important to obtain in-service paper ageing information through post mortem analysis of scrapped transformers. Overall, the knowledge gained from scrapped transformers could help to reduce the gap between laboratory ageing and in-service ageing, which in turn assists asset management of the aged transformer population. With the growing interests on post mortem investigation, a CIGRE working group, WG A2.45 [10], was set up to develop a

DOI 10.1109/TDEI.2013.004118

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structured procedure from the decision to take a transformer or shunt reactor out of service to careful dismantling. A specific task force is formed to produce a guideline for obtaining representative paper samples as well as the parameters to be investigated from scrapped transformers. In addition, a CIGRE joint working group, JWG A2/D1.46 [11], was set up to compare the existing ageing markers with measurements of Degree of Polymerization (DP) on solid insulation samples taken from scrapped transformers. It is also aimed to recommend the end-oflife criteria of transformer insulation based on ageing markers and improve the ageing/life models. Both working groups were set up recently and are collecting publications/contributions as the references for producing the technical brochures. Published studies on post mortem investigations of scrapped transformers mainly focused on measuring DP of paper and mapping its profile along a winding [12-19]. Paper samples were normally taken at the top, centre and bottom discs in a vertical direction [12, 13, 17]. In several studies, paper samples were taken continuously from top to bottom of a winding with interval of only a few discs [14-16, 18, 19]. Most of the studies showed that the vertical DP profile in a winding follows the expected vertical temperature profile, i.e. the decreasing trend of DP from bottom to top discs corresponds to the increasing temperature from bottom to top discs; it is of importance that the DP profiles obtained from scrapped transformers confirm that the hottest spot is located in the region of top discs [12, 14-16, 19, 20]; and intensive paper sampling would even be able to show that the lowest DP is located in several discs below the top disc, which complied with the latest understanding of the thermal design of a winding where the hottest spot is not necessarily at the topmost disc [14, 20]. The DP profiles show no significant difference among different windings [14], indicating effectively an expected design of balanced oil flows. In addition, water content of paper was measured in [16], which shows slightly high water content of papers at bottom discs. There were also studies aimed to find the relationship between furans and DP based on scrapped transformers [14, 15, 21, 22]. This paper focuses on ageing assessment of transformer paper insulation through post mortem analyses of 10 scrapped power transformers. Alternative to DP, Tensile Index (TI) was measured to indicate the mechanical strength of paper. TI profile along multiple layers of paper wrapped at the same conductor of a scrapped transformer was first revealed, which demonstrated the acid catalyzed paper ageing mechanism. Then effect of acids on paper degradation was verified by laboratory experiments. Through post mortem analyses, correlations between multiple parameters e.g. TI of paper, LMA in paper and transformer age etc. were obtained in order to help understand paper ageing mechanism and support transformer asset management.

Most of the transformers in the UK are of the core type design, and can be further categorized as three limbs or five limbs. Core type transformers have their core limbs concentrically wound by windings as shown in Figure 1. The closest winding to the core is normally a Tertiary Voltage (TV) winding followed by a Low Voltage (LV) winding and a High Voltage (HV) winding. For economic reasons, an autotransformer is usually used for the high voltage transmission system. Generally, the connection is made by letting the LV winding be the common one shared with the HV winding. Through this connection, the LV winding is also called the Common Winding (CW) and the rest of the HV winding becomes the Series Winding (SW). All transformers shown in Table 1 are core type where transformers from 2 to 10 are autotransformers.

2 TEST DESCRIPTIONS

Figure 1. Sketch of paper sampling location.

2.1 PAPER SAMPLING PROCEDURE A total of 10 power transformers were investigated. These transformers were used at transmission voltage levels of 275 kV and 400 kV, where power ratings vary from 120 MVA to 1000 MVA, as seen in Table 1. All of them were scrapped between year 2004 and 2010.

Once a transformer is taken out of service for scrapping, the oil is sampled and properties such as acidity, water content, furanic compounds and etc can be measured first, ideally with extra information being noted such as the time of deenergisation, the time of sampling and temperatures in order to help understanding the partitioning and equilibrium status of

Table 1. Voltage and power ratings of investigated transformers. Transformers

Voltage Rating (kV)

Power Rating (MVA)

Age (years)

1 2 3 4 5 6 7 8 9 10

275/66 275/132 275/132 275/132 275/132 275/132/11 400/132 400/132/11 400/132/13 400/275

180 120 180 240 240 150 240 180 240 1000

37 45 42 41 45 50 11 48 11 35

A phase B phase C phase

Core TV

LV (CW)

HV Tap (SW)

Transformer Phase

Horizontal Inside

Paper sampling

Outside Top

Winding

Inside nth layer

Outside 1st layer In contact with bulk oil

Vertical

Centre

Turn

---

Bottom

Conductor

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these ageing by-products. Next, the oil is drained out and the main tank is then open to reveal the core and windings. At this stage, a visual inspection is carried out to identify any irregularities such as loose clamping bolts, localized conductor erosion and carbonization, corrosive sulphur and etc. Finally parts of the windings are cut off and paper samples are taken from them for further analysis. As seen in Figure 1, paper samples were taken across phases (Phase A, Phase B and Phase C) and windings (TV, CW and SW). For each winding, paper samples at the top, centre and bottom discs in vertical direction and at the outside and inside discs in horizontal direction were normally taken for analysis. In order to analyze the multiple-layer profile, paper samples were taken layer by layer from a single conductor. Normally, multiple layers of paper were wrapped around a single conductor. The exact number of layers varied from 6 to 12 depending on the design of a transformer. Each layer of paper was unwrapped one by one and taken for analysis. Paper in direct contact with bulk oil was assigned as the outside layer while paper closest to the conductor was assigned as the inside layer. 2.2 TENSILE STRENGTH MEASUREMENT 2.2.1 STANDARD MEASUREMENT PROCEDURE All mechanical strength measurements were carried out based on long-span Tensile Strength (TS). According to the previous study [23], the main factor for paper TS reduction is due to the loss of inter-fiber bond strength where long-span TS is known as the best way to represent this parameter. According to standards BS 4415 [24], the gap distance and width of sample required for tests are 180 mm ± 1 mm and 15 mm ± 0.1 mm respectively. The crosshead speed and full scale load range required are 20 mm/min and 0.5 kN respectively. The tests were carried out in a laboratory with a condition of 23 °C and 50% relative humidity. For each paper, 10 tests were carried out using INSTRON-5564 equipment. The test gives the maximum load, and hence the TS, kN/m can be calculated using Equation (1).

TS = F/w

(1)

where F is the maximum load (N) and w is the paper width (mm). By dividing TS (kN/m) with grammage, g, g/m2, the Tensile Index (TI), Nm/g can then be obtained as seen in Equation (2).

TI = (TS/g)  103

(2)

Grammage can be obtained by dividing the weight with the surface area of the paper. 2.2.2 PRACTICAL MEASUREMENT CONSIDERATION Paper samples obtained from various locations and different scrapped transformers can come in different types and hence different grammages. Therefore TI is preferred in practice to TS in order to achieve the cross comparisons between locations and transformers. For example, paper samples taken from TV and CW of transformer 4 have grammage of 64 g/m2 and 128 g/m2 respectively. The TS, TI and DP of the paper samples were determined, as given in Table 2. It was observed

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that the paper from TV has a lower strength than the paper from CW if comparison is made based on TS. However TI results showed that the strength of the paper from TV is slightly higher than that of the paper from CW, which is in agreement with the results of DP. It is then proposed to use TI to represent the paper strength in post mortem analysis of scrapped transformers. In addition, with the constraints of on-site sampling, it was difficult to obtain and test 10 samples with the standard length of 180 mm, so the length of paper samples was reduced to 30 mm. It was confirmed by preliminary tests that such a reduction of length from 180 mm to 30 mm has minor influence (less than 5% difference) on the TI of paper. Table 2. Comparison between tensile strength and tensile index. Paper sample location TV CW

Grammage (g/m2) 64 128

Tensile strength (kN/m) 2.56 4.71

Tensile index (Nm/g) 39.8 36.7

DP 755 674

2.3 LOW MOLECULAR WEIGHT ACID MEASUREMENT According to [7-9], there are several types of acid proposed as representative acid by-products of oil/paper ageing, which are formic, acetic and levulinic acids for LMA and naphthenic and stearic acids for High Molecular Weight Acid (HMA). There is no standardized method of measuring LMA or even acidity in paper. The principle to measure LMA in paper developed in [7] is based on the chemical nature of the acid itself. LMA is known as a polar compound, so by using another polar compound such as water as the solvent, the LMA can be extracted from paper. In this paper, a known weight of paper, ranging from 0.5 g to 1.0 g, was prepared and immersed in 25 ml of water for 3 days without stirring. After the extraction period, 10 ml of the extracting water was titrated according to BS 62021 [7, 25]. The paper sample was then directly dried in an air circulating oven at 70 °C for 2 days before weighting. The value of LMA in paper was calculated by dividing the measured absolute acid value with the 40% of the weight of paper sample since only 10 out of 25 ml was used for titration. LMA measurements for each of the scrapped transformer paper samples were carried out twice, between which the variation can be from 0.4% to 18%. The average value was taken and plotted on the following figures.

3 RESULTS AND DISCUSSIONS 3.1 CASE STUDY ON MULTIPLE-LAYER PROFILE A case study on the TI of different layers of papers from the same conductor was carried out on Transformer 1. This transformer is a 275/66 kV, 180 MVA disc type transformer and with an age of 37 years old. The oil acidity of the transformer exceeded 0.15 mg KOH/g for around 10 years. The transformer failed in service due to severe paper insulation ageing, caused by a combination of poor thermal design at the HV winding bottom and high loading throughout the life of the transformer [26].

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3.1.1 LAYER DISTRIBUTION OF TI The HV winding conductor was wrapped with 8 layers of paper and samples were taken from B phase, top, centre and bottom HV windings. As shown in Figure 2, it can be seen that TI increases gradually from the outside layer (1st layer, in direct contact with bulk oil) to the middle layers and then it levels off till reaching the inside layer (8th layer, in direct contact with conductor) or even decreases slightly as demonstrated by HV top sample [27]. The highest TI value exists at about 5th layer while the lowest TI value is at 1st layer. The mean TI of top, centre and bottom at 1st layer is 32.2 Nm/g, only 47% of mean TI at 5th layer, 68.6 Nm/g. Meanwhile, the mean TI at 8th layer is 58.2 Nm/g, about 85% of that at 5th layer. The visual appearance of paper samples from different layers as shown in Figure 3, provides the additional evidence of the layer distribution. Compared to the middle layer, outside layer papers and inside layer papers gradually show darker colour, especially for the outside 1st layer which is the darkest one.

in direct contact with the copper conductor will suffer higher temperature and thereby has lower tensile strength. However such a thermal impact on the paper layer profile could be mild as shown by centre and bottom samples and hence it is not the focus of this study. The decreasing trend of TI from middle to outside layers is believed to be mainly controlled by chemical degradation, which depends on the condition of the oil. Aged oil contains water and acids which are known to accelerate the degradation of the paper. Since the outer layer paper is in direct contact with the bulk oil, the chemical by-products of oil can easily migrate into this layer and degrade the paper which results in the lowest TI at the outside layer. Moving towards inner layers, the chemical migration becomes more and more difficult. If less acid and water penetrates into the inner layers, the paper ageing of inner layers is less accelerated. In order to demonstrate the existence of chemical degradation impact on the multiple-layer papers, LMA in paper at different layers of the HV bottom sample was measured, as shown in Figure 4. Although there are fluctuations of the LMA values between adjacent layers, it can be seen that LMA at outer layers e.g. 1st and 2nd layers is higher than that at inner layers e.g. from 4th to 8th layers. The trend of LMA distribution among different layers matches reasonably well with the TI distribution i.e. the higher the LMA in paper, the lower the TI of paper tends to be, which confirms the above hypothesis of chemical impact on multiple layer profile.

Figure 2. TI of paper profiles among different layers of Transformer 1 [27].

Figure 3. Visual inspection of different layers of papers according to Figure 2. Paper samples were taken from B phase, HV bottom of Transformer 1 [27].

Figure 4. TI of paper and LMA in paper profiles among different layers of Transformer 1.

3.1.2 MECHANISM BEHIND MULTIPLE-LAYER TI PROFILE The profile of TI along multiple layers could be caused by different mechanisms dominating at the inner and outer layers. Taking the TI profile at HV top as an example, at the region from the middle 6th layer to inner 8th layer, the thermal impact seems dominating. Since the winding conductor is the heat generating source, the inner layer paper

The multiple layer profile due to the chemical degradation might not be always as obvious as the present case study, since Transformer 1 is a special case where the oil acidity exceeded the recommended intervening level of 0.15 mg KOH/g for around 10 years. However such a case study highlights how the oil condition could affect the paper ageing and whether there is a generic relationship between LMA in paper and TI of paper. These questions are going to be addressed in the following sections.

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3.2 VERIFYING EFFECT OF ACID IN OIL ON PAPER AGEING Here we recall the paper ageing process previously proposed in [3]. The process starts with the generation of hydrogen peroxide from the interaction of oxygen and water with metallic cations such as Cu+/Cu2+ or Fe+/Fe2+, as catalysts. Decomposition of this compound will create Hydroxy (HO) radicals which will catalyze the oxidation of oil and paper. Through oxidation, water, carbonyl group and carboxyl group known as carboxylic acids will be generated. There are two types of carboxylic acids normally generated which are HMA and LMA. Dissociation of specific acid such as LMA will generate H+ ions and catalyze the hydrolysis of the paper. Water does help in dissociation process of LMA by increasing the number of H+ ions available in the system. Once the system becomes acidic, the oxidation is reduced and hydrolysis becomes the main mechanism. Due to the auto catalytic process, the hydrolysis will be repeated where the by-products such as LMA and water will act as accelerator to the process [3, 28]. According to the above ageing mechanism, the effect of chemical degradation depends on the type of acids. Therefore two types of acids known as LMA and HMA were studied in the laboratory ageing to see their effects on paper ageing under dry and wet paper conditions. 3.2.1 LABORATORY AGEING DESCRIPTIONS The type of mineral oil used for the laboratory ageing is Nynas made Nytro Gemini X. The oil was first filtered using a nylon membrane filter with a pore size of 0.2 µm and dried in a vacuum oven less than 500 Pa at 85 °C for 48 hours. Next, 200 ml of the oil was mixed with 0.17 g of either LMA or HMA for 3 hours at 50 °C. The LMA and HMA used for the test are formic acid and stearic acid respectively. The range of acidity of initial samples with LMA is between 0.2 mg KOH/g and 0.35 mg KOH/g. On the other hand, the range of acidity of initial samples with HMA is between 0.16 mg KOH/g and 0.24 mg KOH/g. In order to prepare the dry paper sample, the paper was dried in an air circulating oven for 24 hours at 105 °C. After the impregnation procedure under vacuum for 24 hours at 85 °C, the final water content of oil impregnated paper is less than 0.5% by weight. For a wet paper sample, the dried oil impregnated paper was left in a desiccator with a relative humidity of 90% for 7 days. The final water content of paper is around 3% by weight. Each sample contains 2.0 g of paper and 200 ml of oil with either LMA or HMA added. All samples were sealed by a PTFE screw cap with the help of PTFE sealing tape and then aged in an air circulating oven at 110 °C for up to 49 days. All paper samples are measured for long-span TI and the initial TI of paper is around 113 Nm/g. 3.2.2 RESULTS OF LMA AND HMA EFFECT Figure 5 shows the effect of LMA and HMA on paper ageing under dry and wet paper conditions. It is observed that there is no significant effect of HMA on the ageing of

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paper for both under dry and wet conditions as shown in Figure 5a and Figure 5b. The reduction trend of TI for samples with HMA is almost the same as samples without acids. LMA has shown a significant effect on the ageing of paper under dry condition as seen in Figure 5a. This effect is further enhanced under wet condition as seen in Figure 5b.

(a)

(b) Figure 5. Influence of LMA and HMA on TI of paper under (a) dry and (b) wet condition.

For comparison purposes, the relationship between TI and ageing duration are fitted by Weidmann model as given in Equation 3 [29].

TI = TI0 exp(-CTI  t)

(3)

Where TI0 is the initial TI (Nm/g), CTI is the ageing rate (days-1) and t is the ageing duration (days). The ageing rates for each ageing condition are obtained and the values are given in Table 3. It is observed that the ageing rates of samples with and without HMA are close to

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each other. Under dry conditions, the ageing rate for samples with LMA is more than 2 times of that with HMA and without acid. Under wet conditions, the ageing rate of samples with LMA is more than 4 times of that with HMA and without acid. With the presence of water, the ageing rate of each condition is further increased, especially for samples with LMA where the ageing rate is around 2.6 times of that under dry condition. Table 3. Comparison of ageing rate under dry and wet paper conditions. Condition

Without acid With HMA With LMA

Under dry condition Ageing rate (CTI) 8.2×10-3

1.9×10-3

Under wet condition Ageing rate (CTI) 10.1×10-3

6.3×10-3

1.1×10-3

9.7×10-3

0.8×10-3

17.2×10-3

1.9×10-3

44.8×10-3

6.4×10-3

Standard deviation

KOH/g, of fresh oil impregnated new paper in the present study was used as the initial reference point at age 0. The scattering of LMA in paper for each transformer is also due to the difference of sampling location in the transformer, corresponding to those in Figure 6. In general an increasing trend of LMA in paper with transformer age was observed. With the progress of paper ageing, LMA in paper can go up to about 6.5 mg KOH/g at age around 50 years, which is a significant increase compared to the initial value of 0.12 mg KOH/g.

Standard deviation 0.8×10-3

It is clear that LMA and water have multiplicative effect on the paper ageing. Such an effect was also demonstrated with the measurements of DP in [8, 9]. LMA can be easily dissociated since the pKa value is lower than HMA. By adding water, the rate of dissociation for LMA will be increased which in turn generates more H+ ions and accelerates the hydrolysis process of paper ageing. 3.3 CORRELATING TI AND LMA IN PAPER The case study on Transformer 1 demonstrated the effect of oxidized oil on paper ageing and which was then confirmed by a systematic laboratory ageing study. Both of them support that acid (LMA) catalyzed hydrolysis has significant impact on paper ageing. On the other hand, the case study suggested that such an ageing mechanism in in-service aged paper insulation might be traceable through measuring the LMA in paper and linking it to the TI of paper. Therefore an investigation on LMA in paper and TI of paper was carried out for 9 scrapped transformers with different retire/failure ages. 3.3.1 TI AND LMA TESTS FOR SCRAPPED TRANSFORMERS A plot between TI of paper and transformer age is shown in Figure 6. The TI of new paper was obtained from a range of initial TI values published in the literature [3, 23] and the TI of new papers in the present study. For each scrapped transformer, paper samples were taken from various locations, and the outermost two layers of paper were chosen for the TI measurement. Each data point in Figure 6 is an average of around 10 measurements. The variation of TI of each transformer in Figure 6 is due to the difference of sampling locations in the transformer. It is observed that TI gradually reduces with the increase of transformer age. The reduction of TI can go down to about 20 Nm/g at age around 50 years, which is less than 20% of the initial TI value. The relationship of LMA in paper and transformer age is shown in Figure 7. As there are few data of LMA in paper available in the literature, the measured LMA value, 0.12 mg

Figure 6. Relationship between TI of paper and transformer age for 9 transformers (Transformer 2 to Transformer 10).

Figure 7. Relationship between LMA in paper and transformer age for 9 transformers (Transformer 2 to Transformer 10).

3.3.2 RELATIONSHIP BETWEEN TI AND LMA IN PAPER Figure 6 and Figure 7 together suggested that there is indeed a link between TI and LMA in a wide population of inservice aged paper samples. A plot of TI of paper versus the corresponding LMA in paper from the 9 scrapped transformers is shown in Figure 8. It is clear that LMA in paper increases as TI of paper decreases.

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Figure 8. Comparison of relationship between LMA in paper and TI of paper for in-service and laboratory ageing.

Figure 8 also compares the relationship of LMA in paper and TI of paper between in-service ageing and laboratory ageing. Data of laboratory ageing 1 was extracted from our previous ageing studies [30]. Four groups of ageing experiments were included; one was oil/paper with copper and steel added aged at 170 °C; next was oil/paper with copper added only aged at 110 °C and the last two groups were oil/paper only aged at 110 °C and 120 °C respectively. All four groups were conducted under dry (0.5%) paper condition. Data of laboratory ageing 2 was obtained from [31]. The initial water contents of paper used for the experiment were 0.2% and 2% where the ageing temperature for 0.2% condition was 130 °C while for 2% is 110°C. The original data shows the relationship between LMA in paper and DP of paper. The DP of paper was converted to TI of paper based on relationship described in [3]. All the laboratory ageing tests cited were carried out in a sealed condition. It is significant to reveal that the relationship of LMA in paper and TI of paper based on laboratory ageing is in line with that based on in-service ageing. Considering the large variations of ageing conditions at both lab and in-service ageing scenarios, Figure 8 indicates that there is a generic relationship between LMA in paper and TI of paper, which shows itself as being independent to the ageing conditions. Initially, as paper degrades, LMA is generated, which due to its nature mainly stays in the paper. The LMA then further accelerates the degradation of paper that results in further generation of LMA in the paper. As this is a self-acceleration ageing process, the increasing rate of LMA in paper per TI reduction is promoted continuously throughout the life of paper. Therefore such a generic relationship between LMA in paper (LMApaper) and TI of paper (TIpaper) can be represented by an exponential form as given in Equation (4).

LMA paper = 11.6exp(-0.03  TI paper )

(4)

3.4 PRACTICAL IMPLICATION There is no doubt that acid catalyzed hydrolysis process is the main ageing mechanism of paper insulation. The effect of acid in oil, in particular LMA, on the paper degradation was

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demonstrated by both laboratory and in-service ageing. As for transformer asset management, this highlights the importance of maintaining good oil condition. Measurement of LMA in oil might be even more important, besides the conventional oil acidity measurement which gives Total Acid Number (TAN). In addition, the acid catalyzed process could be slowed down if suitable intervention action is carried out. For example, if a transformer is subjected to oil regeneration, the amount of LMA and water in the oil/paper system could be reduced and hence resulting in a slower paper ageing rate [3]. It is promising to have a relationship between LMA in paper and TI of paper, especially when being supported by both laboratory and in-service ageing data. However, inservice ageing data seem more scattered than laboratory ageing data and this could be due to complex in-service ageing conditions, e.g. dynamic temperature, moisture or air ingression, ageing by-products partitioning and etc. If partitioning ratio of LMA in paper and oil is known, LMA in paper can be estimated through measuring LMA in oil. In other words, LMA in oil could be used as an indicator for paper ageing. Further study should focus on investigating LMA partitioning between paper and oil under laboratory and in-service ageing conditions. In addition, better evaluation of moisture in oil/paper insulation system is always crucial for paper degradation assessment. For transformer life modeling, end-of-life criterion is required as well as the estimated paper mechanical strength at different stages of transformer operation. Using DP to represent the paper mechanical strength, DP=200 is widely accepted as the end-of-life criterion. However end-of-life criterion is still unclear when using TI to represent the paper mechanical strength. There are commonly two criteria proposed i.e. 50% retention and 25% retention of TI, as mentioned in IEEE standard, C57.91-1995 [32]. The post mortem analysis shown in Figure 6 confirm that when reaching 50% TI retention, transformer would be around 25 years old, and this indeed underestimates the power transformer’s lifetime. TI retention could reduce to 20-25% when a transformer reaches 45-50 years old. This seems a realistic estimation, which is also supported in [33, 34].

4 CONCLUSIONS Post mortem analyses of 10 scrapped power transformers, with supplement of laboratory ageing study were carried out in this paper. A special case study on a transformer with thermal design defect and long history of high oil acidity was conducted first. Through examining the multiple-layer Tensile Index (TI) profile and measuring corresponding Low Molecular Weight Acid (LMA) in paper, the effect of oxidized oil on paper ageing is clearly demonstrated. Further laboratory ageing study confirmed that LMA rather than High Molecular Weight Acid (HMA) accelerates the paper ageing process, and this can be further enhanced under wet paper conditions. All the results mentioned above support that acid catalyzed hydrolysis process is the main paper ageing mechanism. A relationship between LMA in paper and TI of paper was found during post mortem analyses of the scrapped

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transformers. Paper samples of lower TI contain higher level of LMA. It is significant to see that such a relationship tends to be generic for both laboratory and in-service ageing data and can be described by an exponential equation. It is proposed to measure LMA in oil during post mortem analysis and laboratory ageing. Providing a link between LMA in oil and paper exists, LMA in oil could be used as a potential paper ageing indicator.

ACKNOWLEDGMENT The authors would wish to express their gratitude to National Grid UK, Doble PowerTest UK and Paper Science Research Centre at The University of Manchester for providing technical support and paper samples of scrapped transformers.

[17]

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Norhafiz Azis received the B.Sc. degree in Electrical and Electronic Engineering (2007) from Universiti Putra Malaysia and Ph.D. degree in electrical power engineering (2012) from The University of Manchester, UK. He is a Lecturer at the Department of Electrical and Electronic Engineering, Universiti Putra Malaysia, Malaysia and currently undergoing post-doctoral study as a research associate at the Electrical Energy and Power Systems group of the School of Electrical and Electronic Engineering at The University of Manchester, United Kingdom. His research interests are inservice ageing of transformer insulation, condition monitoring, asset management and alternative insulation materials for transformers.

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 21, No. 2; April 2014

Qiang Liu (S’08-M’12) obtained the B.Eng. degree in electrical engineering (2005) and the M.Eng. degree in high voltage and electrical insulation (2008) from Xi’an Jiaotong University in China, and the Ph.D. degree in electrical power engineering (2011) from The University of Manchester in UK. Currently he is a Lecturer at the Electrical Energy and Power Systems Group in the School of Electrical and Electronic Engineering at The University of Manchester. His research interests are on pre-breakdown and breakdown phenomena in liquids, ester transformer liquids, streaming electrification, ageing of insulating materials, transformer asset management and high voltage testing.

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Zhongdong Wang received the B.Eng. and the M.Eng. degrees in high voltage engineering from Tsinghua University of Beijing in 1991 and 1993, respectively and the Ph.D. degree in electrical engineering from UMIST in 1999. Currently she is a Professor of High Voltage Engineering at the Electrical Energy and Power Systems Group of the School of Electrical and Electronic Engineering at The University of Manchester. Her current research interests include condition monitoring, transformer modeling and FRA & transients’ simulation, insulation ageing and alternative insulation materials for transformers.