การพัฒนาเคร่ืองวิเคราะห์ผลการตอบสนองทางความถี่ต้นทุนต ่าส าหรับวินิจฉัยหม้อแปลง

Development of Low-cost frequency response analyzer for diagnosis the transformer

รฐนนท์ ศรีเผือก1 ศภุกิตติ ์โชตโิก1 และ ปรัญชลีย์ รัตนสาครชยั2

Ratanon Sriphuek1, Supakit Chotigo1 and Pranchalee Ratanasakornchai2

บทคัดย่อ

การวิเคราะห์ผลการตอบสนองทางความถี่นัน้เป็นเคร่ืองมือที่มีประสทิธิภาพในการประเมิน แกนขดลวด

และขดลวดของหม้อแปลงก าลงั ซึง่ในบทความนีไ้ด้สร้างเคร่ืองวิเคราะห์ผลการตอบสนองทางความถี่ซึง่มีต้นทนุต ่า

โดยประกอบไปด้วยอปุกรณ์ ออสซิลโลสโคปแบบดิจิตอล เคร่ืองก าเนิดสญัญาณ ร่วมกบัซอฟแวร์แลปวิว โดยใน

งานวิจยันีไ้ด้สร้างแบบจ าลองของหม้อแปลงไฟฟ้าโดยใช้วงจรไฟฟ้าพืน้ฐานที่ประกอบไปด้วย ตวัเหนี่ยวน า ตวัเก็บ

ประจแุละตวัต้านทาน โดยได้ท าการวดัการตอบสนองทางความถี่ของแบบจ าลองเปรียบเทียบกบัการตอบสนองทาง

ความถี่ของวงจรไฟฟ้าดงักลา่วท่ีได้จากการจ าลอง โดยซอฟต์แวร์ มลัติซิม ซึง่ผลของการทดสอบแสดงให้เห็นว่าผล

การตอบสนองทางความถี่ของวงจรไฟฟ้าดงักลา่วท่ีวดัได้มีค่าใกล้เคียงกบัผลที่ได้จากการจ าลอง ซึง่วิเคราะห์ได้โดย

ค่าสหสมัพนัธ์ซึง่มีค่าที่ต ่าสดุเท่ากบั 0.95863 ซึง่ยงัถือว่าอยู่ในเกณฑ์ที่ดีและยอมรับได้

Abstract

Frequency response analysis (FRA) is a powerful and sensitive tool to assess the core,

windings and clamping structures within power transformers. In this paper, it presents a Low-Cost

Frequency Response Analyzer (LCFRA). Since the developed LCFRA is composed of digital

oscilloscope, function generator and Labview software. Thus, this experiment creates transformer

model using electrical circuit elements: resistor, inductor and capacitor. Then, the frequency response

of this model is compared with that obtained from simulation using Multisim software. The measurement

results are close to that of the simulation results. This relation is described by the cross correlation

coefficients (CCF). The minimum value of CCF is 0.95863, which shows a good match and acceptable

relation.

Key words: low-cost, frequency response analysis, transformer, labview, cross correlation coefficient e-mail address: pipelife_rv.35@hotmail.com 1ภาควิชาไฟฟ้า คณะวิศวกรรมศาสตร์ มหาวิทยาลยัพระจอมเกล้าธนบุรี กรุงเทพฯ 10140 1 Department of Electrical, Faculty of Engineering, KMUTT, Bangkok, 10140 2ภาควิชาระบบควบคมุและเคร่ืองมือวดั คณะวิศวกรรมศาสตร์ มหาวิทยาลยัพระจอมเกล้าธนบุรี กรุงเทพฯ 10140 2Department of Control System and Instrumentation, Faculty of Engineering, KMUTT, Bangkok, 10140

Introduction

FRA is a powerful diagnostic method in detecting transformer winding deformation, which is generally difficult to get detected by any conventional measurements. It relies on the fact that a transformer winding can be modeled as a network of capacitance, resistance, self-inductance and mutual inductance. When a fault occurs in a winding, the values of these parameters are altered, and hence the frequency response from the winding will also change accordingly.

In this paper a LCFRA is introduced. The system is composed of oscilloscope, function generator and Labview software; it provides a low cost solution for diagnosing a transformer. The result will be compared with the simulated results obtained from simulation by the Multisim software. The results show the FRA results, produced from various basic electrical circuits which can be referred to the structure of core, windings and clamping structure of the transformer.

Construction

A. Hardware

The frequency response of different electrical circuit arrangement containing inductors, capacitors and resistors is investigated. The circuit diagram is shown in Figure 1. The laptop computer is directly connected to a function generator(Grundig FG100) and a digital oscilloscope(TDS1001B) via RS-232 and USB interface, respectively; the input channel of electrical parameter circuit (V1) is connected to a function generator, a digital oscilloscope(CH1) while an output channel of electrical parameter circuit (V2) is connected to digital oscilloscope(CH2) via coaxial cable which had impedance 50 ohms. The Labview software is controller of system. The sinusoidal waveform is generated by function generator and transferred to the digital oscilloscope (CH1) and an input channel of electrical parameter circuit(V1). The response signal(V2) will return to the digital oscilloscope(CH2) and converted into the digital format for processing in Labview software.

Figure 1 Configuration of the circuit diagram

The difference between V2 (output) and V1 (input) are caused by electrical parameter circuit composed of inductive, capacitive (and resistive) characteristics. This means the difference is frequency dependent. The difference between V1 and V2 will be shown in form of amplitude and a phase shift.

The magnitude (in decibel) is calculated as: AdB (1) The phase can be described as: Ø = (2)

B. Software

The developed software is composed of three parts. One is to create the new data, second is shown magnitude plot and the last one is shown phase shift plot as shown in Figure 2. The first part will create a new data such as serial number, manufacturer, location, voltage, etc. and measure a frequency response of the transformer. When the start button is pressed, the software will choose the location for saving files and send the data to a function generator to generate a sinusoidal waveform. At each frequency, a digital oscilloscope receives signal and sends it to the Labview software. Then, this software will measure its voltage magnitude and phase of the signal but there are some limitations on the system. For instance, it spends such a long time for the measurement and dynamic testing in the range of -80dB ~ 20dB because the resolution of digital oscilloscope(8 bits) is not enough.

a)

b) c)

Figure 2 The window of software for (a) create new data and (b) magnitude plot (c) phase shift plot

Experiment and Discussion

Figure 3 shows how to connect the LCFRA system to the electrical parameter circuit which contains the following circuit arrangements:

1. Inductor only 2. Capacitor only 3. Inductors in parallel with capacitors 4. Transformer model

Figure 3 The LCFRA system connection (a) laptop computer (b) function generator (c) digital oscilloscope (d) electrical parameter circuit

The electrical parameter circuit values of the passive components were measured by the BK Precision 889B LCR meter. The frequency response of the electrical parameter circuit was measured over the frequency range from 50 Hz to 1MHz (1000point) and voltage level 10 Vpeak was supplied by LCFRA, and the results are compared with the simulated data obtained from the Multisim software. The results are calculated from the similarity of two curves and they can be described through the cross correlation coefficients (CCF) as shown in equation 3. The CCF can be a useful tool to measure the dependence between two quantities and if they nearly match, they would have a CCF very close to 1.0. Table1 provides an explanation of the CCF values.

(3)

Table1 CCFs Explanation

Prediction CCF

Good match 0.95 - 1.0

Close match 0.90 - 0.94

Poor match <0.89

Very poor match ≤0.0

1. Inductor only The values of tested inductor are 1.08mH, 108.4uH and the circuit is shown in Figure 4

Figure 4 Simulation circuit for measuring inductors frequency response

a) b) Figure 5 Bode plots of the frequency response of inductors (a) magnitude (b) phase shift [simulation results108.4uH (red), 1.08mH (violet) and measurement results108.4uH (green), 1.08mH (blue)]

2. Capacitor only The values of tested capacitor are 8.996uF, 99.75nF and the circuit is shown in Figure 6

Figure 6 Circuit connection of capacitors frequency response

a) b) Figure 7 Bode plots of the frequency response of capacitors (a) magnitude (b) phase shift [simulation results 8.996uF (red), 99.75nF (violet) and measurement results 8.996uF (green), 99.75nF (blue)]

3. Inductors in parallel with capacitors The experiment is fixed value of inductor, L of 1.08mH and capacitors of C (C=10.02nF, C=100.2pF)

Figure 8 Circuit connection of parallel inductors and capacitors frequency response

a) b) Figure 9 Bode plots of the frequency response of parallel LC (a) magnitude (b) phase shift [simulation results10.02nF (red), 100.2pF (violet) and measurement results10.02nF (green), 100.2pF (blue)]

4. Transformer model The model consists of 4 basic elements: R, L and C which values of them shown in Figure 10. The L element represents the magnetic saver element, the R represents watt losses and the C element

represents the component, which keeps the electricity energy.

Figure 10 Circuit connection of transformer model

.

a) b) Figure 11 Bode plots of the frequency response of Transformer model (a) magnitude (b) phase shift [simulation results (red) and measurement results (green)]

Table2 CCF values of each electrical parameter circuit

C (F) L (H) LC (L = 1.08 mH) Transformer Model Values 8.996u 99.75n 1.08m 108.4u 10.02nF 100.2pF

CCF 0.99867 0.99994 0.99945 0.99987 0.95863 0.98642 0.97341

The CCF values of each electrical parameter can be found by Labview software, which the CCF

results is close to 1.0, but the CCF value of LC(1.08mH, 10.02nF) is minimum. However, it is good match of two curves as shown in Table1. Furthermore, other CCF values are still good match.

Conclusion

In experiments, this LCFRA system is effective as shown by the comparison to the simulation results as related through the CCF. From the result of CCF values of each electrical parameter, it shows that the minimum value of the electrical parameter (L = 1.08mH, C = 10.02nF) is 0.95836 but it is still a good match. The cost of LCFRA system very cheap when comparing with commercial FRA; the cost of LCFRA system about 70,000 baht while cost of the commercial FRA about 2.5 million baht, so, the greatly difference cost of them about 97.2%.

Acknowledgements I would like to express my sincere thanks to Dr. Annop Limsimarat who is manager of testing

high voltage department and Mr. Somsak Kungwarnkanok who is general manager of TUSCO COMPANY LIMITED for their kind advice throughout this research.

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