Transformer Winding Deformation Diagnostic System ... - IEEE Xplore

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C. Yao et al.: Transformer Winding Deformation Diagnostic System Using Online High Frequency Signal Injection

Transformer Winding Deformation Diagnostic System Using Online High Frequency Signal Injection by Capacitive Coupling Chenguo Yao, Zhongyong Zhao, Yu Chen, Xiaozhen Zhao, Zhaojiong Li State Key Laboratory of Power Transmission Equipment & System Security and New Technology College of Electrical Engineering, Chongqing University Chongqing 400030, China

Yong Wang, Zehong Zhou and Gang Wei State Grid Chongqing Electric Power Company Chongqing 400014, China

ABSTRACT Transformer winding deformation is common among all sorts of transformer failures. Cumulative deformation can eventually burgeon into catastrophic faults and result in entire network outage. It is possible to detect the early signs of faults with continuous online monitoring transformer. However, frequency response analysis (FRA) is considered to be a useful and accurate tool for sufficient detection. This paper aims at proposing a novel diagnostic system for online monitoring power transformer winding deformation based on FRA. In order to realize online monitoring transformer winding, the system uses the capacitive coupling method to inject controllable nanosecond pulses, which function as the excitation signal of winding, and to obtain the response signal. This proposed method may extend frequency range for analysis and perhaps could early detect minor winding movement and looseness. Transformer experiments show great prospect in the application of the system. Index Terms - Transformer, winding deformation, frequency response analysis, online monitoring, controllable pulse, capacitive coupling.

1 INTRODUCTION AS the heart of power equipment in substation, power transformer assumes the important role in converting voltage. It will cause power outage and bring unexpected losses in the event of catastrophic failure. Since a transformer is very complicated, the restoration time is often long and the repair cost is high after the occurrence of the serious accidents, which will further reduce the effectiveness of the transformer. According to abundant literature reviews, mechanical failure is the most common among all the faults. Winding is the key component of the transformer, however, deformation may occur during the transportation, operation and earthquake, especially in the occurrence of external short circuit. The short circuit current form the huge electromagnetic force to emerge the cumulative deformation failure [1-2]. In general, the type of transformer winding deformation could be divided into radial forces, axial forces and combined forces [3-4]. Recently, many researchers have carried out lots of work to detect transformer winding deformation. Some research

Manuscript received on 28 August 2013, in final form 15 January 2014, accepted 21 January 2014.

results, including methods and standards have been applied to routine test items in lots of Power Company, where the frequency response analysis (FRA) is an accurate, economical, reliable, fast and non-destructive method [4], favored by many researchers. According to the nature of input signal, two methods exist: sweep frequency response analysis (SFRA) and impulse frequency response analysis (IFRA) [5-6]. In SFRA, sinuous signal with changing frequency is used as the excitation signal of transformer winding, and the test results are converted to the frequency domain to analyze; the first measured frequency response curve was treated as “fingerprint”, and subsequent measurements are requested to compare with the “fingerprint” to determine whether there is a fault and the degree of the fault. SFRA developed over the years; there have been a variety of standards and commercial products [7-8]. In IFRA, the transformer also needs outage from the network; the controllable pulses are used as excitation signal and the response signal is measured, the Fourier transforms of both excitation signal and response signal are mathematically processed to construct the frequency response curve, fingerprint comparison is also used for fault diagnosis [9]. As for IFRA, Wang [1, 10] believes uncontrollable signal such as power system transient overvoltage could be used to detect winding deformation.

DOI 10.1109/TDEI.2014.004283

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 21, No. 4; August 2014

At present, online monitoring requires assessing the state of power equipment in real time with few resources. It is necessary to develop online monitoring technique for transformer winding deformation detection as this kind of failure may induce the outage of transformer. Recently, Tom De Rybel et al [11] developed an initiatory online monitoring system, in which high-frequency signal is injected to a 650 kV transformer winding through the capacitive bushing tap. In work presented in [12], Vahid Behjat et al proposed noninvasive capacitive sensors (NICs), which are installed on the surface of the bushings to inject output alternating voltage of network analyzer. Rogowski coils are also used to measure phase currents. Both NICs and Rogowski coils could handle a wide bandwidth signal. Mehdi Bagheri et al [13] have studied the impacts of bushing characteristic on online FRA of transformer winding, bushing test tap was used to inject signal into the transformer winding without requiring a direct connection to the main feeder, a paralleled impedance with bushing test tap was used to reduce the voltage when bushing test tap is not grounded throughout on-line measurement. In this paper, an independent diagnostic system is proposed, in which a homemade nanosecond pulse signal generator connecting to the bushing capacitive coupling sensor has been used to produce controllable pulses as the excitation signal of transformer winding, and the bushing capacitive coupling sensor has also been used for measurement of response signal. Firstly, the basic principle of the system is introduced in chapter 2, and then chapter 3 presents the configuration of the system parts, followed by validation experiments in chapter 4, finally conclusion and future work.

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generally longer. However, nanosecond pulses with up to MHz spectrum could be used as excitation signal of transformer winding to construct TF. In our scenario, HV bushing capacitive coupling sensor (CCS) is used as an interface through which controllable high frequency pulses with high amplitude are coupled into transformer winding, and CCS is also used to measure the response signal in the output end to construct TF. The use of excitation signal with enough high amplitude could eliminate the influence of poor SNR. The principle and installation of CCS will be described in detail in chapter 3. In addition, the detection time is reduced because the nanosecond high frequency pulses have rich spectrum component. Meanwhile, the upper limits frequency of most existing commercial FRA test equipment is 1 MHz or 2 MHz. Compared with the conventional SFRA, the proposed system may extends the frequency range to several MHz for analysis, and resonance points in the high frequency band (>3 MHz) probably detect minor winding deformation [16].

a Wye connection with neutral point ungrounded [3], taken and modified

2 THE BASIC PRINCIPLE According to the literature [2, 14-16], power transformer winding could be treated as a distributed parameter model which consists of inductors, capacitors and resistors in the high frequency band. When an excitation voltage signal is applied to one end of the winding, the response signal which changes with the frequency of the applied signal could be recorded in the other end, then it is easy to get the transfer function (TF), that is the frequency response curve, represented by the flowing equation (1). TF  20 log

R out ( f ) V in ( f )

(1)

where Rout could be the response voltage or the response current. The inductors, capacitors and resistors which constitute the equivalent model depend on the physical dimension and materials of the winding, deformation or damage of a portion of the winding will lead to the change in TF. Therefore, by comparing the measured TF and reference TF, it is possible to judge the deformation and damage. Currently, the commercial network analyzer used for SFRA has low output voltage (2-25 V). Then, if it is used for online monitoring of transformer winding deformation, the response signal will suffer from external interferences and there will be poor Signal to Noise Ratio (SNR) [11]. In addition, the network analyzer is based on the sinuous signal output with changing frequency, in order to get TF; the detection time is

b Delta connection [3], taken and modified

c Wye connection with neutral point grounded [18, 20], taken and modified Figure 1. Installation schematic representation of online monitoring diagnostic system.

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The online monitoring circuits of winding for Wye connection with neutral point ungrounded and Delta connection are shown in Figure 1a and 1b, respectively. The controllable high frequency signal generator produces nanosecond pulses, which are delivered to the signal injection and protection circuit. The CCS then couples this excitation signal into transformer winding. At the end of transformer winding, the response signal is leaded by CCS and measured by capacitive voltage divider. Both excitation signal and response signal are recorded by data acquisition device; finally the data are processed by PC. The online monitoring circuit of winding for Wye connection with neutral point grounded is shown in Figure 1c, all the setup is the same as those in Figure 1a except that response signal is measured by Rogowski coil.

3 THE CONFIGURATION OF THE SYSTEM 3.1 CONTROLLABLE NANOSECOND PULSE GENERATOR The generation of high frequency signal is the crucial part in diagnostic system. In order to produce repeatable, high-precision, low loss and adjustable high-voltage nanosecond pulses, a controllable all-solid-state highvoltage nanosecond pulse generator based on the Marx generator concept was developed [17]. The basic principle of the generator is shown in figure 3.

Figure 3. The basic principle of controllable nanosecond pulse generator.

Figure 2. Equivalent injection circuit.

The equivalent lumped injection circuit of nanosecond pulse connecting to the power grid and transformer winding through CCS is shown in Figure 2. Where Cc is the equivalent coupling capacitor of the CCS, which is about 30pF in 110 kV bushing. Zgrid and Ztw are characteristic impedance of highvoltage bus and transformer winding, respectively. According to reference [11], Zgrid is about 300 Ω, while Ztw has a range of 300 Ω to 3 Ω between the frequencies of 200 kHz and 2.5 MHz, Cbt is the main capacitor of the 110 kV bushing tap, which is about 250 pF. The bushing tap is grounded when transformer is at work. In Figure 2, Ue is the output voltage of the pulse generator, and Uc is the coupled voltage of the inner conductor of the bushing.

Uc 

Z gird // Z tw // Z cbt Z cc  Z grid // Z tw // Z cbt

(2)

Uc can be calculated in 200 kHz and 2.5 MHz according to those values.

 0.0054U e , f  200kHz Uc   0.0014U e , f  2.5MHz

The generator consists of a dc charging power, a solidstate Marx circuit using metal-oxide-semiconductor field effect transistors (MOSFETs), a control circuit using a field-programmable gate array (FPGA), and the load. In Marx circuit, MOSFETs are used as control switches instead of spark gap, and diodes instead of resistors. FPGA produces synchronous trigger pulses which are isolated by optical fibers to function as original control signal of MOSFETs. The charging voltage, pulse width and frequency are also controlled by FPGA. Besides, the function of overcurrent and short protection is designed. This generator has capability of producing controlled high-

a The waveforms with same pulse width and different amplitude.

(3)

It can be concluded that Ue should be as high as possible, otherwise the coupled voltage is too small to detect winding deformation when transformer is in service. The amplitude of the nanosecond pulse must be at least a few hundred volts to avoid poor SNR.

b The waveforms with same amplitude and different pulse width. Figure 4. The basic principle of controllable nanosecond pulse generator.



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voltage repetitive pulses with voltage up to 8 kV, pulse width of 200 to 1000 ns, repetition rate from 1 to 1000 Hz and rise time of 35 ns with various resistive loads (50 Ω1000 Ω) and 1 kV dc input voltage. The typical output waveforms are shown in Figure 4. They are the waveforms with same pulse width and different amplitude (600ns pulse width, 3-8 kV pulse amplitude), and waveforms with same amplitude and different pulse width (8 kV pulse amplitude, 200-1000 ns). 3.2 SIGNAL INJECTION AND PROTECTION CIRCUIT Nanosecond pulses are injected into the internal winding via CCS mounted outside the high-voltage bushing. A signal injection and protection circuit is developed to function as the interface of pulse generator and CCS; it also protects the generator from 50 Hz power frequency voltage and overvoltage in power system. It requires that the circuit is all-pass when it sees from pulse generator to CCS (called forward direction), to ensure the lossless injection; it also requires that the circuit function as the filter and protection circuit when is sees from CCS to pulse generator (called reverse direction), to ensure 50 Hz power frequency voltage is filtered and overvoltage is eliminated [11, 12]. Actually, to prevent the distortion of the transient waveform, the pulse generator output signal is transferred to signal injection and protection circuit via coaxial cable, it is necessary to match the cable impedance in this circuit, too. The designed signal injection and protection circuit is shown in Figure 5.

a, The forward direction.

b, The reverse direction. Figure 6. The frequency response characteristic of the circuit.

Figure 7. Real product of signal injection and protection circuit.

Figure 5. The signal injection and protection circuit.

In Figure 5, R and C are components of the filter circuit and R is also the match impedance for the cable, G is a gas discharge tube, K is a varistor. R and C function as a filter; G and K function as overvoltage protection components. The frequency response characteristics of the forward direction and reverse direction are shown in Figure 6a and 6b, respectively. It concludes that circuit works as the allpass circuit in forward direction and it filters 50 Hz, low frequency, some medium frequency overvoltage signals in reverse direction. The circuit is placed in an aluminum box to shield from external interference and Bayonet Nut Connector (BNC) connectors are used. The real product of the signal injection and protection circuit is shown in Figure 7.

Figure 8. Installation schematic representation of CCS.

3.3 CAPACITIVE COUPLING SENSOR AND CAPACITIVE VOLTAGE DIVIDER Power transformers work under complex condition with large interferences. In order to inject nanosecond high frequency pulses without loss and measure response signal

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accurately, a capacitive coupling sensor (CCS) is developed, which does not affect the normal operation of power equipment. The mounting position of CCS is shown in Figure 8. CCS is essentially a metal strip wrapped around the bushing insulating layer near the grounded flange, configured with the interface. Its basic principle is that metal strip, inner conductor and bushing layers form the stable capacitor [18-20]. The installation of CCS does not change the configuration of power system. To measure response signal, capacitors are placed along with CCS to form the capacitive voltage divider [21]. The configuration of capacitors is shown in Figure 9a, Rs is a highvoltage low inductance damping resistor with resistance of 100 Ω; Cs is a high-voltage ceramic capacitor with capacitance of 200 pF, and four capacitors are arranged in parallel to reduce the inductance; Rp is a matched resistor with resistance of 50 Ω to match the characteristic impedance of the coaxial cable. These components are also placed in an aluminum box with BNC connectors to shield from external interface. The real product of the capacitive voltage divider is shown in Figure 9b.

operating band of capacitive voltage divider, Figure 12 shows the result. The divider ratio of the capacitive voltage divider is 82 and operating frequency band is 140 Hz to 15 MHz, which provides support to improve the detection frequency band to 10 MHz when using FRA method. In further work, an overvoltage protection circuit is needed in data acquisition device to prevent from power system transient overvoltage, a band stop filter or a high-pass filter is also needed to eliminate the 50 Hz power frequency voltage. The lossless response signal could be obtained by digital filtering techniques, too.

Figure 10. The setup for step response test and operating band test.

a The circuit diagram.

Figure 11. The step response of capacitive voltage divider. 300 250 200 150 100 50

b The real product. Figure 9. The configuration of capacitive voltage divider.

For the sake of obtaining the response characteristic and operating frequency band of the capacitive voltage divider, an experimental platform was built in an electromagnetic shielding laboratory. The basic setup of the test is shown in Figure 10, where CCS was mounted on a 10 kV ceramic bushing. Tektronix DPO4054 Oscilloscope was used to record waveforms, the controllable nanosecond pulse generator worked as the signal source of step response test and a typical waveform is shown in Figure 11. In Figure 11, the blue waveform with amplitude of 1000 V, pulse width of 400 ns is the output of the pulse generator, measured by Tektronix P5100A Probe, while the green one is measured by homemade capacitive voltage divider. Tektronix AFG3102 Arbitrary Waveform Generator was used to determine the

0 0.000001 0.00001

0.0001

0.001

0.01

0.1

1

10

100

Figure 12. The frequency response characteristic of capacitive voltage divider.

3.4 DATA PROCESSING Denoising is a key step to obtain useful information, since the existence of noise not only reduces sensitivity but also reduces upper limit of the frequency band for analysis. The signal acquired by data acquisition device can be expressed as

c (t )  s (t )  n(t )

(4)

In the equation (4), s(t) is the useful signal and n(t) is the noise signal. The mathematical expectation of the random noise signal is generally considered to be zero. Therefore, using the average of multiple measurements under the same



condition is an effective denoising measure. In our scenario, the average of 100 measurements under the same condition is used to analyze.

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-40 -50

A-phase C-phase

-60 B-phase

-80 -90 -100 A-phase winding B-phase winding C-phase winding

-110 -120 -130 0

1

2

3

4

5 MHz

6

7

8

9

10

Figure 14. The experimental result of winding Wye connection with neutral point ungrounded. -60 A-phase

-70

C-phase

-80 -90 DB

In order to verify the feasibility of proposed winding deformation diagnostic system, a test platform was built in a real transformer, the operation frequency of transformer is 50 Hz, the rated capacity is 31.5 MVA and the rated transformation ratio is 110 kV/10.5 kV. This transformer was overhauled, windings of the same side were exactly healthy and similar, and there was minor difference between those windings. Figure 13 shows the tested transformer and part of experiment configuration. The HV side windings of transformer are Wye connection and the LV side windings are Delta connection.

DB

-70

4 THE VALIDATION EXPERIMENTS FOR DIAGNOSTIC SYSTEM

B-phase

-100 -110 A-phase winding B-phase winding C-phase winding

-120 -130 -140 0

1

2

3

4

5 MHz

6

7

8

9

10

Figure 15. The experimental result of winding Wye connection with neutral point grounded.

b Part of the configuration.

Figure 13. The experimental scene.

As for HV side windings Wye connection with neutral point ungrounded, the test connection is shown in Figure 1a, a resistor connecting neutral point bushing and “ground” was used to simulate neutral point ungrounded. The output voltage of controllable nanosecond pulse generator was used as input signal, and neutral point voltage measured by homemade capacitive voltage divider was used as response signal to construct frequency response curve. Three-phase winding were tested and analyzed, respectively. Figure 14 shows the result of data analysis; it concludes that the test results of three windings are quite similar and there is only small difference between phases, which might be caused by minor difference between windings. As for HV side windings Wye connection with neutral point grounded, the test connection is shown in Figure 1c. The output voltage of pulse generator was used as input signal, and the current of neutral point bushing lead measured by Pearson coil was used as response signal. Three-phase winding were also tested, respectively. The result in Figure 15 shows the same conclusion with that in Figure 14. However, the differences between phases are more obvious in high frequency band than the result in Figure 14, which indicate that current response signal might be more useful when used to be analyzed.

-30 b-phase

-40

a-phase

-50 -60 DB

a The tested transformer.

As for LV side windings Delta connection, the test connection is shown in Figure 1b. For instance, when testing a-phase winding, the input signal was injected into a-phase bushing, and the response signal measured by homemade capacitive voltage divider was recorded from c-phase bushing. Configurations of the other two-phase were the same with aphase. Figure 16 shows the overlay of three-phase test results and it also concludes that differences between FRA curves are more obvious in high frequency band. This result and previous results indicate minor difference between windings is not easy to detect in low frequency band. The conclusion in reference [16] “FRA curves are more sensitive to changes in the transformer at high frequency band” could exactly interpret the test phenomenon.

c-phase

-70 -80 -90

b-phase winding c-phase winding a-phase winding

-100 -110 0

1

2

3

4

5 MHz

6

7

8

9

10

Figure 16. The experimental result of winding Delta connection.

5 CONCLUSION AND FUTURE WORK This paper proposed a newly developed diagnostic system for online monitoring transformer winding deformation. The system is based on capacitive coupling, including injection coupling and measuring coupling. The

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basic principle and devices of the system were introduced in detail. A real 110 kV transformer was used to verify the diagnostic system; the experimental results show that the proposed system is likely to monitor winding deformation when transformer is in service. Further work needs more typical fault winding and transformer experiments and data analysis.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 51377175) and Science and technology fund of State Grid Chongqing Electric Power Company (SGCQ0000YJJS1300187).

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[17] C. G. Yao, X. M. Zhang, F. Guo, S. L. Dong, Y. Mi, and C. X. Sun, “FPGA-Controlled All-Solid-State Nanosecond Pulse Generator for Biological Applications”, IEEE Trans. Plasma Sci., Vol. 40, pp. 2366-2372, 2012. [18] A. Setayeshmehr, A. Akbari, H. Borsi, and E. Gockenbach, “On-line monitoring and diagnoses of power transformer bushings”, IEEE Trans. Dielectr. Electr. Insul., Vol. 13, pp. 608-615, 2006. [19] A. Akbari, H. Borsi, E. Gockenbach and M. Hadighi, “Experiences with a Non-invasive Capacitive Sensor for On-line partial Discharge Detection in power Transformers”, ETG-Fachtagung “Diagnostik elektrischer Betriebsmittel”, Köln, Germany, pp. 301-304, 2004. [20] A. Setayeshmehr, H. Borsi, E. Gockenbach and I. Fofana, “On-line monitoring of transformer via transfer function”, IEEE Electr. Insul. Conf. (EIC), pp. 278-282, 2009. [21] G. M. Ma, C. R. Li, J. T. Quan, and J. Jiang, “Measurement of VFTO Based on the Transformer Bushing Sensor”, IEEE Trans. Power Del., Vol. 26, pp. 684-692, 2011. Chenguo Yao (M’08) was born in Nanchong, Sichuan, China, on 1 February 1975. He received the B.S., M.S., and Ph.D. degrees in electrical engineering from Chongqing University, Chongqing, China, in 1997, 2000, and 2003, respectively. He became a Professor with the College of Electrical Engineering, Chongqing University in 2007. His current works include pulse power technology and its application in biomedical engineering, online monitoring of insulation condition and insulation fault diagnosis for HV apparatus. Zhongyong Zhao was born in Guangyuan, Sichuan, China, on 1 October 1988. He received the B.S. degree in electrical engineering from Chongqing University, Chongqing, China, in 2011, where he is currently working toward the Ph.D. degree with the combined Master–Ph.D. Program in electrical engineering. His areas of research include pulse power technology, online monitoring of insulation condition and insulation fault diagnosis for HV apparatus. Yu Chen was born in Zhuhai, Guangdong, China, on 4 June 1988. He received the B.S. degree in electric engineering and automation from Huazhong University of science and technology, Wuhan, China, in 2011, where he is currently working toward the M.S. degree in electrical engineering in Chongqing University. His current works include online monitoring of insulation condition and insulation fault diagnosis for HV apparatus. Xiaozhen Zhao was born in Taian, Shandong, China, on April 20, 1989. He received the B.S. degree in electric engineering and automation from China University of Mining and Technology, Xuzhou, China, in 2012, where he is currently working toward the M.S. degree in electrical engineering in Chongqing University. His current works include the fault diagnosis of transformer and its state evaluation. Zhaojiong Li was born in Liangshan, Sichuan, China, on 16 February 1991. He received the B.S. degree in applied physics from Chongqing University, Chongqing, China, in 2012, where he is currently working toward the M.S. degree in electrical engineering. His areas of research include the fault diagnosis of transformer and its state evaluation.