Comparative Study of Power Trasformer Internal Faults

(1)

International Journal of Electrical, Electronics and Computer Systems (IJEECS)

________________________________________________________________________________________________

ISSN (Online): 2347-2820, Volume -2, Issue-7, 2014 40

Comparative Study of Power Trasformer Internal Faults

1Vijay. S. Chavan, ²G. S.Changan, ³B.B.Kadam, ⁴Sachin. A. Jalit Department of Electrical Engineering

1,2,3SRE’s K. B. P. Polytechnic, Koparagaon., Dist- Ahmednagar, Maharashtra, India

4P. R. Pote(Patil) College of Engineering & Mgt, Amravati. Maharashtra, India Email: ¹[email protected], ⁴[email protected] Abstract — Dissolved Gas Analysis (DGA) is a

widely used technique to estimate the condition of power transformer.- The measurement of the level and the change of combustible gases in the insulating oil is a trustworthy diagnostic tool which can be used as indicator of undesirable events occurring inside the transformer, such as hot spots, electrical arcing or partial discharge. The objective of this paper is mainly to analyze available data from DGA, and investigate data that may be useful in quantitative modeling of the transformer’s reliability. There are standards available for this purpose the DGA interpretation should also be based on other information about the reliable particular transformer. This paper describes a realistic method for power transformers using readily available data.

I. INTRODUCTION

Power transformers play an important role in both the transmission and distribution of electrical power.

In service, transformers are subject to electrical and thermal stresses, which can cause the degradation of the insulating materials. The degradation products are gases, which entirely or partially dissolve in the oil where they are easily detected at the ppm level by dissolved gas analysis. Faults can be differentiated for their energy, localization and occurrence period. Along with a fault, there are increased oil temperatures and generation of certain oxidation products such as acids and soluble gases. These gases, hydrogen (H2), methane (CH4), ethane (HC62), ethylene (HC42), acetylene (HC22), propane (HC83), propene (HC63), together with carbon monoxide ( CO ) and carbon dioxide (CO2) are considered as fault indicators and can be generated in certain patterns and amounts depending on the characteristics of the fault Hence, qualitative and quantitative determination of dissolved gases in transformer oil are of great importance in order to assess fault condition and further operating reliability of power transformers. Although there are various methods based are DGA to diagnose the condition of transformer, such as IEC, IEEE, CIGRE,

MSZ National Standard’s ratio codes and graphical techniques, they sometimes fail to determine the faults.

This is generally due to the existence of more than one fault in a transformer.

In multiple fault conditions, gases from different faults are mixed up resulting in confusing ratio between different gas components.

II. FAULT CONDITION OF POWER TRANSFORMER

Fault conditions occur primarily from the thermal and electrical deterioration of oil and electrical insulation.

Each combustible gas level will vary depending upon the fault process .

Arcing:- Large amounts of hydrogen and acetylene are produced, with minor quantities of methane and ethylene. Arcing occurs through high current and high temperature conditions. Carbon dioxide and carbon monoxide may also be formed if the fault involved cellulose. In some instances, the oil may become carbonized.

Corona:- Corona is a low-energy electrical fault. Low- energy electrical discharges produce hydrogen and methane, with small quantities of ethane and ethylene.

Comparable amounts of carbon monoxide and dioxide may result from discharge in cellulose.

Sparking:- Sparking occurs as an intermittent high voltage flashover without high current. Increased levels of methane and ethane are detected without concurrent increases in acetylene, ethylene or hydrogen.

Overheating: - Decomposition products include ethylene and methane, together with smaller quantities of hydrogen and ethane Traces of acetylene may be formed if the fault is severe or involves electrical contacts.

(2)

________________________________________________________________________________________________

Overheated Cellulose:- Large quantities of carbon dioxide and carbon monoxide are evolved from overheated cellulose. Hydrocarbon gases, such as methane and ethylene, will be formed if the fault involved an oil-impregnated structure.

Partial Discharge:- The temperature plays a less important role in the chemical reaction occurring in the partial discharges since the vapor temperature in the discharge zone is not higher than 60-150°C.

Hydrocarbon cracking in the partial discharges occurs as a result of excitation of molecules and their subsequent dissociation by collision with high energy electrons, ions, atomic hydrogen and also free radicals.

III. DISSOLVED GAS ANALYSIS (DGA)

Dissolved gas analysis is probably the most widely used preventative maintenance technique in use today to monitor the operation of transformers. Properly used it can be a powerful tool in a well disciplined maintenance program. Depending on the location of a transformer and the nature of its usage, an appropriate dissolved gas analysis schedule can be set up. The more critical the unit is the more frequently it should be sampled. When an adverse situation is detected the sampling frequency should also be increased. This latter philosophy allows one to determine how rapidly the gases are being generated and thus how serious the problem might be so that proper action can be taken before the unit suffers additional damage. It is also quite important to maintain a history of each unit so that one can determine if any gases are residual ones from a previous fault or are they due to a newly developing situation.

If a fault is detected in a transformer there are other tests that might be recommended that can help to locate the site of the fault within the unit. The more information that can be obtained before repairing a unit, the less down time will be required. Working with laboratory personnel that are familiar with the various methods of interpreting dissolved gas analysis in conjunction with other tests and the history of the unit are essential to properly utilize this technique in a preventative maintenance program.

The oil inside a transformer has two important properties; the first being that it is a good insulator and the second is as a coolant. It is one accessible portion of the transformer since most components are contained within the main tank. Internal inspections are not necessary when a transformer is functioning properly.

And an inspection would do more harm than good, as it would allow air and moisture to be absorbed by the insulation system. Air and moisture are two enemies of transformers.

IV. DIFFERENT METHODS OF DGA

Different methods of DGA for preventive maintenance for power transformer are as follows

1) Roger Ratio Method:

The Roger’s method utilizes four gas ratios: CH4/H₂, C₂H₆/CH₄, C₂H₄/C₂H₆ and C₂H₂/C₂H₄. Diagnosis of faults is accomplished via a simple coding scheme based on ranges of the ratios as follows

Gas ratios Ratios codes

CH4/H2 I

C2H6/CH4 J

C2H4/C2H6 K

C2H2/C2H4 L

Table 1- Rogers gas ratios

Ratio code Range code

I

<=0.1

>0.1,<1.0

>=1.0,<3.0

>=3.0

5 0 1 2

J <1.0

>=1.0

0 1 K

<1.0

>=1.0,<3.0

>=3.0

0 1 2 L

<0.5

>=0.5,<3.0

>=3.0

0 1 2 Table 2:- Rogers ratio code 2)IEC 60599 Ratio Method:

The IEC 60599 standard establishes a classification by which five different types of faults can be detected by means of DGA. These faults are classified in the following way:

Partial discharge (PD): cold plasma (corona) type fault, resulting in possible by-products of degradation (X-wax deposition).

Discharges of low energy (D1): in oil or/and paper, evidenced by larger perforations in paper, tracking, or carbon particles in the oil.

Discharges of high energy (D2): in oil or/and paper, evidenced by extensive carbonization of paper, metal fusion and possible tripping of the equipment.

Thermal faults: below 300 ºC if the paper has turned brownish (T1) and above 300 ºC if it has carbonized (T2).

(3)

________________________________________________________________________________________________

Thermal faults above 700 ºC (T3): evidenced by the carbonization of the oil, metal coloration or metal fusion.

The three basic gas ratios used by IEC are:

r1=C2H2/C2H4 (= acetylene/ethylene), r2 = CH4/H2 (= methane/hydrogen) and

r3= C2H4/C2H6 (=ethylene/ethane). Another ratio is used r4= CO2/CO (= carbon dioxide/carbon monoxide) can be used, and if r4 < 3 indicates the possible participation of paper insulation

in the fault, but not always. For this reason, this ratio must be used with caution. Other typical gas limits are shown below.

Faults Ratio

PD r2<0.1, r3<0.2

D1 r1>1, 0.1<r2<0.5, r3>1 D2 0.5<r1<2.5, 0.1<r2<1, r3>2 T1 r1<0.1, r2>1, r3<1

T2 r1<0.1, r2>1, r3<4 T3 r1<0.2, r2>1, r3>4

Table 3-IEC ratio with faults 3) Dornenberg Ratio Method :

In this method the gas concentration ratio of CH₄/H₂, C2H2/CH4, C2H4/C2H6 and C2H2/C2H4 The value of the gases at first must exceed the concentration to ascertain whether there is really a problem with the unit and then whether there is sufficient generation of each gas for the ratio analysis to be applicable Table shows the key gases and their concentration.

Key gas Concentration in ppm

Hydrogen H2 100

Methane CH4 120

Carbon monoxide CO 350 Acetylene C2H2 35

Ethylene C2H4 50

ethane C2H6 65

Table 4- Dornenberg ratio methods

As per IEEE Standard C57.104-1991, the step-by-step procedure to diagnose faults using Dornenberg ratio method is:

Step1. Gas concentrations are obtained by extracting the gases and separating them by gas chromatograph Step2. If at least one of the gas concentrations (in ppm) for H₂, CH₄, C₂H₂, and C₂H₄ exceeds twice the values for limit (see table) and one of the other three gases exceeds the values for limit, the unit is considered faulty; proceed to Step 3.

Step3. Determining validity of ratio procedure: If at least one of the gases in each ratio CH₄/H₂, C₂H₂/CH₄, C₂H₂/CH₄ and C₂H₆/C₂H₂ exceeds limit, the ratio procedure is valid. Otherwise, the ratios are not significant, and the unit should be re-sample and investigated by alternative procedures.

Step4. Assuming that the ratio analysis is valid, each successive ratio is compared to the values obtained in the order of ratio CH₄/H₂, C₂H₂/CH₄, C₂H₂/CH₄and C2H6/C2H2

Step5. If all succeeding ratios for a specific fault type fall within the values the suggested diagnosis is valid.

4) Key Gas Method:

The principle of the Key Gas method [15], is based on the quantity of fault gases released from the insulating oil when a fault occurs which in turn increase the temperature in the power transformer. The presence of the fault gases depends on the temperature or energy that will break the link or relation of the insulating oil chemical structure. This method uses the individual gas rather than the calculation of gas ratios for detecting fault. The significant and proportion of the gases are called “key gases”. Figure 4 indicate these “key gases”

and relative proportions for the four general fault types .

(4)

________________________________________________________________________________________________

Fig 1-Principle Gas- Carbon mono-oxide

Fig.2 Principle gas- acetylene 5) Nomograph Method :

The logarithmic nomograph method [15], was developed by J. O. Church. This method combines the fault gas ratio concept with the Key Gas threshold value in order to improve the accuracy of fault diagnosis. It was intended to provide both a graphic presentation of fault- gas data and the means to interpret its significance. The Nomograph consists of a series of vertical logarithmic scales representing the concentrations of the individual gases as shown in Fig 2.18.

With this method, straight lines are drawn between adjacent scales to connect the points representing the values of the individual gas concentration. The slopes of these lines are the diagnostic criteria for determining the type of fault. The key at the bottom of the chart between the two axes indicates the fault type for the two axes. A visual comparison of the slopes of the line segments with the keys given at the bottom of the Nomograph is all needed to identify the type of fault. The position of the lines relative to the concentration scales provides a means of assessing the severity of the fault.

Figure 3: The Logarithmic Nomograph

REFERENCES

[1]. R. R. Rogers, “IEEE and IEC codes to interpret incipient faults in transformers, using gas in oil analysis,” IEEE Trans. on Electrical Insulation, vol. 13, no. 5, pp. 349-354, 1978.

[2]. M. Duval, "New techniques for dissolved gas-in- oil analysis, Electrical Insulation Magazine, IEEE, vol. 19, pp. 6-15, 2003.

[3]. Muhammad Arshad and Syed M. Islam “Power Transformer Critical Diagnostics for Reliability and Life Extension” CCECE 2004- CCGEI 2004, Niagara Falls, May imai 2004 0-7803-8253-6/04 02004 IEEE.

[4]. Tony McGrail “Transformer Frequency Response Analysis: An Introduction”, Spring 2005.

[5]. Diego R. Morais, Juliano R. da Silva, Jacqueline G. Rolim, “A Fuzzy System For Detection Of Incipient Faults In Transformers Based On The Dissolved Gas Analysis Of Insulating Oil”

SDEMPED 2005 - International Symposium on Diagnostics for Electric Machines, Power Electronics and Drives Vienna, Austria, 7-9 September 2005

[6]. M. N. Bandyopadhyay, “Transformer Diagnostics in the Practical Field”, GMSARN International Conference on Sustainable Development: Challenges and Opportunities for GMS 12-14 Dec. 2007

