jurnal 2sd

2013 International Conference on Power, Energy and Control (ICPEC)

197 978-1-4673-6030-2/13/$31.00 ©2013 IEEE

Study and Diagnosis the Failure of Power Transformers by Sweep Frequency Response Analysis.

Amit Kumar Mehta R.N.Sharma Sushil Chauhan S.D.Agnihotri EED,NIT,Hamirpur EED,NIT,Hamirpur EED,NIT,Hamirpur HPSEB Hamirpur H.P.India H.P.India H.P.India H.P.India [email protected]

Abstract-Sweep Frequency Response Analysis (SFRA) testing has become a valuable tool for verifying the geometric integrity of electrical apparatus, specially transformers. The SFRA technique provides internal diagnostic information using non-intrusive procedure. Power Transformers are specified to withstand the mechanical forces arising from both shipping and subsequent in- service events, such as faults and lightning. Transportation damage may lead to core and winding movement. This research is undertaken to study and diagnosis the failure of power transformer by investigation of transformer mechanical integrity using the Doble’s M5200. This paper presents case studies related to the SFRA testing, & their result interpretation through the statistical indicator and the standard interpretations available. Keywords - Insulation system, power transformer, Bushing, power factor, capacitance.

I. INTRODUCTION

Sweep Frequency Response Analysis (SFRA) is a tool that can give an indication of core or winding movement in transformers. This is done by performing a measurement, albeit a simple one, looking at how well a transformer winding transmits a low voltage signal that varies in frequency. Just how well a transformer does this is related to its impedance, the capacitive and inductive elements of which are intimately related to the physical construction of the transformer. Changes in frequency response as measured by SFRA techniques may indicate a physical change inside the transformer, the cause of which then needs to be identified and investigated. This behavior becomes apparent when we model impedance as a function of frequency [1, 2].The result is a transfer function representation of the RLC network in the frequency domain. Frequency response analysis is generally applied to a complex network of passive elements. For practical purposes, we will consider only resistors, inductors, and capacitors as passive circuit elements, and they are assumed to be ideal. These three fundamental elements are the building blocks for various physical devices, such as transformers, motors, generators, and other electrical apparatus.

It is important to understand the difference between the physical device and the mathematical model we intend to use. When large and complex systems are electrically analyzed, we are often faced with a poorly defined distributed network. A distributed network contains an infinite number of infinitely small RLC elements. For example, transmission lines are generally distributed in nature. It is practical to model such distributed systems by lumping the basic RLC components together, resulting in a lumped network. Lumping elements together for a single frequency is a trivial task, but when system modeling requires spanning a significant frequency interval, producing a suitable lumped model becomes difficult[3,4].

II. THEORY AND FUNDAMENTAL SFRA is based on analysis of a windings transfer function. A signal generator is connected together with a voltage reference to a phase outlet on the transformer and the response is measured on the neutral outlet of the same winding (see figure 1).

Figure 1

The signal generator produces a sweep of signals (sine waves) with increasing frequency. The reference and response voltages are logged and processed so that a response curve can be plotted. The response curve shows the relationship between the two voltages (attenuation) as a function of the frequency. The phase shift between reference and response are also measured and can be plotted as o function of frequency. With the SFRA method input and output signals are measured at one frequency a time, within a frequency range. How the input signal (x) is affected by the specimen’s characteristics will depend upon, what is mathematically described as, the transfer function H(s) = Y(s) ⁄ X(s)

(where s is a frequency dependant parameter, which for continuous sinusoids equals to jω). The transfer function is physically the combination of inductance, capacitance and resistance within the specimen (i.e. the admittance or, if inverted, the impedance) [5]. When the sinusoidal input signal is applied there will be a dynamic response, dependant on the transfer function. After a while (duration depending on the

transfer function) the signal will stabilize into a steady state. Now the response will (ideally) be a sine wave (y), but the amplitude and phase might be different from the input sine wave. Again the transfer function will affect how the response will differ from the input.


198

III. HOW SFRA WORKS

The Double’s M5200 sends an excitation signal into the transformer and measures the returning signals across a broad frequency range. By comparing this response to baseline and other results (such as from similar units), we can identify deviations and confirm internal mechanical problems. The general rule areas below [7]. i) The transformer under test should be completely de-energized and isolated from the power system before performing any tests using an M5000-series SFRA instrument. ii) The method of testing a high-voltage apparatus (transformer) involves exciting the apparatus with the SFRA instrument. Take care to avoid contact with the apparatus being tested, its associated bushings and conductors, and the SFRA instrument’s cables and connectors. iii) The test crew must make a visual check to ensure that the apparatus terminals are isolated from the power system. Because the apparatus under test may fail, take precautions (such as barriers or entrance restrictions to the test area) to avoid harm in case of violent failure. iv) All of your company rules for safe practice in testing must be strictly conformed to, including all practices for tagging and isolating apparatus during testing and maintenance work. State, local, and federal regulations, e.g., OSHA, may also apply[8].

A. Software The M5000 instruments come with intuitive, Windows-based SFRA software runs on a standard PC supplied by the user (for the M5200 or M5400) or on the M5300 itself. The software allows you to make and compare SFRA measurements. The test is easy to perform, but recording all relevant details for future reference is important; otherwise, it becomes difficult to reproduce test results. The software requires a minimum set of details before taking a measurement: • Test location • Testing organization • M5000 instrument serial number • Transformer manufacturer • Transformer serial number • Red lead location • Black lead location

B. Short-Circuit Lead Response

Perform a short-circuit measurement. The short-circuit lead response test verifies proper condition of the test specimen

cable. Since there is no attenuation, signal loss between the Source/Reference and Measure, the resulting data graph plots along the 0 dB horizontal line as frequency increases, until an inductive roll off occurs in figure 2. This roll off is a feature of the cables, because of the 12 ft / 3.7 m ground connections. This roll off is consistent for all tests and reduces the variability in response arising from variations in ground lead length. It is expected and acceptable.

Figure 2 C. Open-Circuit Lead Response

Open-circuit behavior is around –90 to –100 dB but is clearly affected by noise and shows a lot of “hashing” compared to the short-circuit lead response. It is relatively easy to identify in figure 3.

Figure 3

IV. RESULTS AND DISCUSSION Generally, an SFRA measurement is made from one terminal on the transformer (e.g., H1 or A) to another terminal (e.g., H2 or N). It is important to record all relevant information, which includes tap position, oil level, and terminals grounded or shorted. Where previous test results exist, the best testing procedure is to repeat those tests, taking note of tap position, shorted or grounded bushings, and any details for specific tests. Doble is a key member of international bodies such as CIGRE and IEEE, which are pursuing FRA test standards. As standards


199

develop, recommended tests may be changed with input from experienced users around the world. Doble will reflect those changes [6].

A. Measurement Types

i) Open Circuit An open-circuit measurement is made from one end of a winding to another, with all other terminals floating. For a delta winding, connections would be H1 to H3, for example. For a star (wye) winding, measurements are taken from HV terminals to neutral, such as X1 to X0.

ii) Short Circuit A short-circuit measurement is made with the same SFRA test lead connections as an open-circuit measurement, but with the difference that another winding is short circuited. To ensure repeatability, Doble recommends that the three voltage terminals on the shorted winding be shorted together. This would mean, for example, shorting X1 to X2, X2 to X3, and X3 to X1. This ensures that all three phases are similarly shorted, to give a consistent impedance as shown in table 1. Any neutral connections available for the shorted winding should not be included in the shorting process[9].

Test Type Test 3ф 1ф Series winding (OC) All other Terminals Floating

Test1 H1-X1 H1-X1 Test2 H2-X2 Test3 H3-X3

Common winding (OC) All other Terminals Floating

Test4 X1-H0X0 H1-H0X0 Test5 X2-H0X0 Test6 X3-H0X0

Short Circuit(SC) High(H) to Low(L) Short[X1-X2-X3]*

Test7 H1-H0X0 H1-H0X0 Short [X1-H0X0]*

Test8 H2-H0X0 Test9 H3-H0X0

Table 1

* Indicates short-circuit tests where the terminals are shorted together with three sets of jumpers, to provide symmetry (X1-X2, X2-X3, X3-X1) OR (Y1-Y2, Y2-Y3, Y3-Y1). The neutral is not included for 3φ wye connections but may be included for 1φ test connections [6].

B. Case Studies

Case No. I

Figure 4

Figure 5

Table 2. S-86069 ECE Industries Ltd. 3-Ph 2 Wind Y-Y 132/33 Red Lead

Black Lead

LTC

DETC Terminal Grounded

Terminal Shorted

Measurement Type

H1

H0

5

As-found-make note

none

none

Open Ckt.

H2

H0

5

As-found-make note

none

none

Open Ckt.

H3

H0

5

As-found-make note

none

none

Open Ckt.

Table 3.S-86069 ECE Industries Ltd. 3-Ph 2 Wind Y-Y 132/33 Red Lead

Black Lead

LTC

DETC Terminal Grounded

Terminal Shorted

Measurement Type

H3

H0

5

As-found-make note

none

a-b-c-a

Short Ckt.

H2

H0

5

As-found-make note

none

a-b-c-a Short Ckt.

H1

H0

5

As-found-make note

none

a-b-c-a Short Ckt.


200

Figure 6

Observation:-The SFRA test conducted on 132/33kV, 25/31.5 MVA Transformer Sr. No. S-86069 on 09-03-2009 and results of figure 4,5 and 6 are compared with the signature obtained on the sister unit installed at 132/33 kV sub-station Anu Hamirpur. The results obtained do not have any deviation. This implies that there is no loss of geometric integrity of the transformer during transportation.

Case No. II

Figure 7

Figure 8

Table 4. S-86069 ECE Industries Ltd. 3-Ph 2 Wind Y-Y 132/33 Red Lead

Black Lead

LTC

DETC

Terminal Grounded

Terminal Shorted

Measurement Type

X1

X0

5

As-found-make note

none

none

Open Ckt.

X2

X0

5

As-found-make note

none

none

Open Ckt.

X3

X0

5

As-found-make note

none

none

Open Ckt.

Table 5.S-25897 General Electric. 3-Ph 2 Wind Y-Y 132/33 Red Lead

Black Lead

LTC

DETC

Terminal Grounded

Terminal Shorted

Measurement Type

H1

H0

8

As-found-make note

none

none

Open Ckt.

H2

H0

8

As-found-make note

none

none

Open Ckt.

H3

H0

8

As-found-make note

none

none

Open Ckt.


Black Lead

LTC

DETC

Terminal Grounded

Terminal Shorted

Measurement Type

H1

H0

8

As-found-make note

none

a-b-c-a

Short Ckt.

H2

H0

8

As-found-make note

none

a-b-c-a

Short Ckt.

H3

H0

8

As-found-make note

none

a-b-c-a

Short Ckt.


201

Figure 9

Observation: - The SFRA test conducted on 132/33kV, 16 MVA Transformer Sr. No. B-25897 dated 17-06-2009. From figure 7, 8 and 9 signatures it is found that there are some abnormalities in the operation of this transformer. This test is not sufficient to detect the exact cause for such abnormal deviation in the signatures. It is recommended to conduct leakage reactance test and Insulation Resistance (IR) Test for winding to identify the actual cause of such deviations.

V. CONCLUSION

SFRA is an effective tool, which considers that part of transformer for diagnostics, which cannot be detected by other methods.The above case studies on the two different transformers showed how the M5200 detects mechanical failure or movement of windings due to short circuits, mechanical stresses or transportation. It is used to ensure transformer performance, reduce maintenance cost, and increase the service life of transformers.

ACKNOWLEDGEMENT

The authors are thankful to Technology Information Forecasting & Assessment Council and Centre of Relevance & Excellence (TIFAC-CORE) on “Power Transformer Diagnostic” at NIT

Hamirpur H.P. INDIA for providing necessary infrastructural facilities for caring out the research work.

REFERENCES [1] Tobias Stirl, Raimund Skrzypek, Stefan Tenbohlen,Rummiy Vilaithong,

“Online condition monitoring of Power Transformers.” AREVA research and development center, Germany, 1973.

[2] Brian Richardson, “Diagnostics And Condition Monitoring Of Power Transformers” IEE, ABB Power Transformer Research And Development Ltd, 1997.

[3] Luwendran Moodley, Brian de Klerk “SweepFrequency Response Analysis as A Diagnostic tool to Detect Transformer Mechanical Integrity”, eThekwini Electricity pp.1-9, 1978

[4] Dick, E. P. and Erven, C. C, "Transformer Diagnostic Testing by Frequency Response Analysis,"IEEE/PAS-97, No. 6, pp.2144-2153, 1978.

[5] P.T.M. Vaessen, N.V. KEMA, Arnhem E.Hanique. “A New Frequency Response Analysis Method for Power Transformers.” IEEE 384 Transactions on Power Delivery, Vol. 7 No.1, January 1992.

[6] Doble Engineering Company “Reference Book on Insulating Liquids and Gases” RBILG-391.

[7] Double digital library,www.double.com and http://www.double.com / product/m5400_sfra.html

[8] Tony Mograil,”SFRA Basic Analysis,Vol.1, Version1.0”,2003 Double Engineering Co.,pp4-13.

[9] Jashandeep Singh,Yog Raj Sood,Piush Verma and Raj Kumar Jarial, ”Novel method for detection of transformer winding fault using Sweep Frequency Response Analysis ,”IEEE Transactions Vol.1 , PESGM 2007-001023,2007.


Black Lead

LTC

DETC

Terminal Grounded

Terminal Shorted

Measurement Type

X1

X0

8

As-found-make note

none

none

Open Ckt.

X2

X0

8

As-found-make note

none

none

Open Ckt.

X3

X0

8

As-found-make note

none

none

Open Ckt.

Mr. Amit kumar Mehta was born on March 25, 1969. He obtained his bachelor degree in electrical engineering from Bangalore University in the year 1993 and master in power engineering from Punjab technical university in year 2008. Presently he is pursuing his Ph.D. from NIT Hamirpur H.P.

Dr R. Naresh was born in Himachal Pradesh INDIA in 1965. He received BE in electrical engineering from Thapar Institute of Engineering and Technology, Patiala, India in 1987, ME in Power Systems from Punjab Engineering College, Chandigarh in 1990 and Ph D from the University of Roorkee, Roorkee (now IIT Roorkee), India in 1999. Presently he is working as Head in the Electrical Engineering Department, National Institute of Technology, Hamirpur, HP,

Prof. Sushil Chauhan was born on August 22, 1963. He obtained his bachelor degree in electrical engineering from Madan Mohan Malviya Engineering College Gorakhpur in the year 1986 and master in Power System Engineering from IIT Roorkee in the year 1988. He obtained his Ph.D. in ANN based Power System Security Assessment in the year 1999 from IIT, Roorkee Presently he is Dean Academics at NIT

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