Faults And Defects In Power Transformers.pdf

Faults and Defects in Power Transformers – A Case Study Cacilda de Jesus Ribeiro1; André Pereira Marques2,3; Cláudio Henrique Bezerra Azevedo2; Denise Cascão Poli Souza1; Bernardo Pinheiro Alvarenga1; Reinaldo Gonçalves Nogueira1 1

School of Electrical and Computer Engineering, Federal University of Goiás, Goiânia, GO, Brazil 2 CELG Distribuição, Goiânia, GO, Brazil 3 Federal Institute of Education, Science and Technology of Goiás, Goiânia, GO, Brazil

Abstract – Power transformers play a fundamental role in electrical power systems, in addition to representing significant investments involved in the implementation of these systems. To reduce the costs associated with a transformer’s life cycle and to guarantee its reliability and durability, it is essential to monitor its operating conditions, its insulation system, and the working conditions of its accessories and other components. Therefore, the aim of this work is to study the faults and defects that occurred in 34.5 kV, 69 kV, 138 kV, and 230 kV oil-immersed power transformers of the electrical system and the insulation system of CELG, a major electric energy concessionaire in the state of Goiás, Brazil. The results of this study, i.e., the efficacy of the predictive technique for maintenance over the last 28 years (from 1979 to 2007), the characterization of faults and defects during this period, and the presentation of proposals for improvements in the predictive technique, aimed at reducing the number of stoppages in the electric power supply system, are expected to contribute to the body of knowledge in this field I.

A fault is an anomaly in a piece of equipment that inevitably causes stoppage of its operation, forcing its removal from service [2]. As it is used here, the term “stoppage” indicates that the service of a piece of equipment was interrupted, i.e., it was removed from operation due to a defect or fault. The word “transformers” also refers to autotransformers. II.

POWER TRANSFORMERS

The present work was developed based on: • the identification of the main parts of power transformers, which were analyzed and divided into blocks of components, as shown in Fig. 1; and

INTRODUCTION

A power transformer is one of the most important and costly devices in electrical systems. Its importance is attributed directly to the continuity of power supply, since its loss through failure or defect means a supply stoppage. This is a large piece of equipment whose substitution is expensive and involves a lengthy process. Research for new technologies and new predictive maintenance techniques has greatly contributed to reduce supply stoppages, thereby ensuring improved reliability of energy supply. Several studies highlight the importance of optimizing maintenance processes and diagnoses of substation equipment such as transformers [1]. In this context, the purpose of this research was to study faults and defects that occurred in 34.5 kV, 69 kV, 138 kV and 230 kV power transformers immersed in mineral oil for a period of 28 years at the electric power concessionaire CELG, which supplies over two million consumers distributed in 237 municipalities with a population of approximately four million in the state of Goiás, Brazil. A defect is considered an anomaly in a device that can cause it to operate irregularly and/or below its nominal capacity. If not corrected in time, this defect can evolve, leading to failure of the equipment and its removal from service [2].

Fig. 1. Subdivision of a power transformer into blocks

• The characterization and analysis of faults and defects detected in these devices, resulting from stoppages and/or interventions which they underwent. III.

STOPPAGES IN THE ELECTRIC POWER SYSTEM DUE TO TRANSFORMER DEFECTS AND FAULTS

A. Number of Stoppages of the Devices

In this study, 549 service stoppages were recorded from December 1979 to May 2007, involving 255 three-phase

transformers or three-phase transformer banks, and several of these devices showed more than one stoppage. Table I summarizes the number of devices, with their respective ranges of nominal output power and by nominal voltage. TABLE I NUMBER OF DEVICES BY RANGE OF NOMINAL THREE-PHASE OUTPUT POWER AND BY NOMINAL VOLTAGE

Nominal voltage

Total number of devices (3-phase and banks)

3-phase power (MVA) Lowest

Highest

34.5kV

106

0.15

12.0

69kV

79

1.0

20.0

138kV

53

7.0

62.5

230kV

17

36

150

Total

255

Fig. 2. Number of transformer stoppages versus components

Of the transformer service stoppages in the period considered in this work, a certain number were due to faults and other defects, as indicated in Table II, reaching a total of 549 stoppages in this period of 28 years. TABLE II NUMBER OF TRANSFORMER STOPPAGES Stoppages caused by

Number of stoppages

Percent (%)

Faults

413

75.2

Defects

136

24.8

Total

549

100

It should be noted that this study took into account the devices that were removed definitively from operation as well as those that were purchased over the 28-year period of this study. It is estimated that 10% of the devices under study are part of the total number of transformers that belong to the system’s technical reserve over these years. B. Number of Transformer Stoppages versus Damaged Components

Fig. 2 shows the percentage of transformer stoppages versus damaged components in the period of 1979 to 2007, without considering stoppages caused by the protection system and by human error. In this study, it was found that the components most affected were windings (34%), bushings (14%), onload tap changers, OLTC, (10%), and de-energized tap changers, DTC (10%). The item “unidentified component” (11%) refers to components which lack reliable records for several reasons. The insulation system of the transformers in question is composed of mineral oil and solid insulation (cellulose, varnish or polyester), although most of it consists of oilpaper. It was found that the stoppages due solely to problems in the insulation oil accounted for only 4% of the number of stoppages during the 28 years analyzed here. The degradation of a transformer’s insulation system is usually the main parameter that causes electrical faults in these devices.

The aging of oil-paper insulation in a transformer depends on aging of both the paper and the oil. The assessment of the remaining life of a transformer is the desired result of diagnostic procedures. A popular belief is that the life of the insulation paper determines the transformer’s service life [3]. Thus, when factors of transformer insulation degradation such as water, oxygen, the products of decomposition in the oil and temperature are monitored and controlled continuously, there is decrease in the degradation of the insulation system, which means less risk of electrical faults [4]. CELG carries out systematic physicochemical testing and analyses of dissolved gases to control and monitor the insulating oil of its transformers, which is the reason for the low percentage of problems involving insulating oil in its devices (4%). C. Transformer Failure Rates Over Time

As stated above, service stoppages can be caused by both defects and faults. The difference between them is that interventions to correct equipment defects can be programmed, unlike faults, which are generally emergencies in the electrical sector. It is therefore essential to know the individual transformer failure rates. Fig. 3 shows the transformer failure rates in the CELG system per year and class of voltage, without considering the failures resulting from the protection system and from human error. In view of these results, and as can be seen in Figure 3, although failure rates of up to 9% were recorded (1992, 138 kV), the overall rates for the entire 28-year period are quite acceptable. These rates are listed in Table 3, and were calculated using (1).

Tf =

Nf

.100

t

∑N i =1

(1)

e ,i

where: Tf : failure rate in the period under consideration [%] Nf: number of failures in the period under consideration Ne,i: number of devices existing in each year i considered t: number of years of the period considered

Fig. 3. Transformer failure rates over time

Analyzed quantitatively, the slightly higher rates of the 138 kV and 230 kV transformers are justified by the smaller number of devices of these classes of voltage. Table III lists the failure rates of 34.5 kV, 69 kV, 138 kV and 230 kV transformers that occurred in the period under study, without considering the reserve equipment (estimated at 10% of the total number of power transformers). TABLE III TRANSFORMER FAILURE RATES FOR THE PERIOD OF 1979 TO 2007 Voltage

34.5kV

69kV

138kV

230kV

Total failures

93

54

18

5.0

Rate (%)

1.40

2.03

1.36

0.49

As can be seen, the transformers failure rates of CELG’s system are relatively low, which is explained by the use of predictive techniques at this concessionaire. The company’s maintenance engineering sector, which strives to ensure a continuous supply of electric power by reducing the failure rate, has sought new predictive techniques, with emphasis on the detection of partial discharges in transformers by the acoustic method.

IV.

PREDICTIVE TECHNIQUES

The well-known dissolved gas analysis (DGA) technique in insulating oil is sensitive to some types of incipient faults (defects). To quantify the efficiency of this technique in detecting such defects in CELG’s equipment, a comparison was made of the total number of transformer stoppages that

could have been detected by the DGA predictive technique and the stoppages effectively detected by this technique. This comparison revealed that the technique provided an efficiency of approximately 75%. However, sampling of transformer oil for DGA testing is done periodically, according to the chromatography software program CELG uses and to the specificity of each device. Thus, between one sampling and the next, the device may undergo impacts from atmospheric discharges, external short circuits, and adverse operating conditions, which may trigger or accelerate incipient faults and cause the device to fail before the next sampling, masking the efficiency of the chromatography system. It is therefore understood that the efficiency of the DGA technique, per se, is higher than 75%. In addition to DGA, another predictive technique that could be used to increase the monitoring efficiency of the state of transformer insulation is the detection of partial discharges (PDs). The DGA method has only low sensitivity for detecting partial discharges [5]. This may sometimes lead to inaccuracy in analytical methods, which may lead to errors by the person analyzing test results. Furthermore, the DGA technique does not allow for the identification of the site of an incipient fault, making it difficult to locate it, especially if its intensity is low. Particularly interesting is the use of a noninvasive method such as the acoustic PD detection method, which allows for monitoring of the evolution of PDs even while the device is in operation. Throughout its operation, a power transformer has to withstand numerous stresses that generally result in the degradation of the oil-paper insulation system by decomposition of the paper and/or oxidation of the oil.

Degradation reduces the quality of this insulation. Partial discharges can lead to winding breakdowns, and may cause accelerated aging. PDs must be inferred in order to build an early warning system. In this context, PDs serve as an important measuring parameter for on-line monitoring [6]. To illustrate the above, the photograph in Fig. 4 depicts the failure of a 20 MVA power transformer with a nominal voltage of 69 kV/34.5 kV, showing damage sustained by a large extent of the winding.

This paper therefore presents a proposal for improving predictive techniques through the implementation of a set of techniques, highlighting the combination of DGA with the detection of partial discharges by the acoustic emission method [7], which allows PD activity to be pinpointed in the equipment without requiring its shutdown. V.

CONCLUSIONS

Although the failures rates and the number of stoppages that occurred during the period under study were relatively low, it is important to implement other predictive techniques that are sensitive to incipient faults in power transformers – especially in terms of problems involving windings, bushings and tap changers, which, taken together, account for 68% of the events in components of these devices – in order to further improve the performance quality indicators reported here. Among these techniques, this paper highlights the measurement of partial discharges by the acoustic emission method, which could be allied to the DGA method, a technique well-known in the energy sector, thereby increasing the maintenance efficiency and quality of electric power supply. ACKNOWLEDGMENTS Fig. 4. Damaged winding

Systematic equipment monitoring by the DGA technique showed a slight increase in gases, however without providing a warning about the need to remove this device from service, ultimately leading to its damage by short-circuiting between the spirals. As can be seen in Fig. 5, the short circuit caused dislocation of the winding due to an electrodynamic overload.

This work was carried out in collaboration with the Maintenance Engineering Division of CELG Distribuição, CELG D, and the Federal University of Goiás School of Electrical and Computer Engineering (EEEC/UFG) through a partnership in an R&D Project - ANEEL. REFERENCES [1]

[2] [3]

[4]

[5]

Fig. 5. Dislocated winding

This fault could have been avoided by PD detection, preventing the defect from developing into a short circuit between spirals and, hence, failure of equipment. This indicates the need for integrating predictive maintenance techniques in order to improve diagnostic quality and ascertain the state of transformer insulation systems.

[6]

[7]

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