DEVELOPMENT OF A COMPUTER-BASED DATA LOGGER. FOR RECOVERY VOLTAGE MEASUREMENTS. Y.H. Lu, C.K. Tan, B.T. Phung, R.H. Khawaja, T.R. ...
DEVELOPMENT OF A COMPUTER-BASED DATA LOGGER FOR RECOVERY VOLTAGE MEASUREMENTS Y.H. Lu, C.K. Tan, B.T. Phung, R.H. Khawaja, T.R. Blackburn School of Electrical Engineering and Telecommunications University of New South Wales, Australia Abstract: Recovery voltage measurement (RVM) is a relatively new insulation diagnostic technique, which has the ability to detect the moisture content present in oil/paper insulation systems. It can be applied as an indicator of insulation quality and future insulation ageing problem based on the analysis of polarization spectrum. This paper describes the development of a computer-based virtual instrument using LabVIEW for automating the full-cycle of RVM process: charging, discharging, measurement and relaxation. The system measures the maximum recovery voltage, central time constant and displays the polarization spectrum. Tests were carried out on a laboratory test rig which was constructed to simulate a transformer winding section. 1. INTRODUCTION The main function of a power system is to deliver electrical energy reliably, economically and effectively. The reliability of a power system depends on trouble free transformers operation. Unfortunately, a notable number of power transformers around the world are now drawing near to the end of their design life. Factor such as economics and evironmental concern prevent us to replace them at will. The same situation is evident in Australia. Insulation diagnosis is therefore expected to become increasingly important as it can be used to determine the integrity of the insulation, to estimate the remaining life of the insulation and to provide the methods to improve the present insulation. A number of insulation diagnostic techniques are available and can be categorized into electrical, mechanical and chemical methods. Electrical insulation diagnostic technique includes measurement of insulation resistance (IR), dielectric loss factor (DLF), partial discharge (PD) and recovery voltage measurement (RVM). Mechanical method includes tensile strength of paper and pressboard. Analytical chemical techniques such as Fourier Transform Infra Red (FTIR) absorption and nuclear magnetic resonance (NMR) are mainly used for studying of materials. The insulation system of a typical power transformer consists of paper, pressboard and mineral oil. The function of the paper is to provide insulation for the winding and connections while the oil is used as an insulant and a cooling medium. The remnant life of an aged transformer is mostly influenced by the condition
of insulation system. The aging and life expectancy of the insulation, in turn, are greatly dependent on environmental conditions, which involves hydrolytic, oxidative and thermal degradation [1]. Both the thermal and hydrolytic degradation produce a common end product – water. As such, the level of moisture content in the oil/paper is one of the decisive factors used in determining the total breakdown strength of an insulation system [2,3,4]. High moisture content in the oil/paper due to ageing can cause failure to the transformer. To assess the extent of degradation of an insulation system, it is therefore necessary to know the moisture content of the oil/paper. RVM is a technique for determining the level of moisture content and hence the degradation of the oil/paper insulation. In this paper, the RVM process is implemented using the computer and with the aid of a software known as LabVIEW (Laboratory Virtual Instrument Engineering Workbench). This is a graphical programming language that has been widely adopted throughout industry and academia as the standard for data acquisition and instrument control software. 2.
RECOVERY VOLTAGE MEASUREMENT
The first piece of work on RVM was proposed by the Hungarian utilities and subsequently tests were carried out by the Budapest Technical University in the mid70s. The main idea behind this was to review the conventional diagnostics used on oil immersed systems in large power transformers and hence to develop new insulation diagnostic methods for assessing large transformers in service [5].
RVM is known to be a non-intrusive and nondestructive test that can be performed on-site with minimal down time. It relies on the dielectric properties of the oil/paper insulation and gives an indication of the moisture content in the insulation system. It also provides a method for monitoring the condition of insulation over time. This method is based on the determination of the polarization time constant in the region of 10ms to 1000s of the spectrum of dielectric dissipation factor (tan δ). This time constant, in turn, is due to the interfacial and boundary polarization [6]. Interfacial polarization occurs whenever there is an accumulation of charges at the interface between two materials, e.g. in the interfacing of the paper and oil in the power transformers. It is strongly influenced by the moisture content and some other ageing products, resulting in a reduction of the time constant. V Vc dV/dt Next cycle
V max
tc
Phase 1
td
tpe ak
relaxation
Phase 2
Phase 3
Phase 4
Time
Fig.1: Test phases in one RVM measurement cycle. Each RVM test cycle consists of 4 phases: (1) charging, (2) discharging, (3) measurement and (4) relaxation (see Figure 1). The sample is to be charged with a DC voltage; molecules are polarized and align in the direction of the electric field. The sample is then short-circuited for a predetermined period of time (usually half of the charging time); causing the molecules to be partially depolarized. Upon opening the short circuit, a voltage due to the remaining charge will build up between the terminals of the insulation sample. This residual polarization results in a voltage is known as recovery voltage. Two measurements are taken in each test cycle and they are the maximum recovery voltage (Vmax) and central time constant (time to peak). The charging/discharging procedure is repeated using a sequence of increasing charging times range from fractions of a second to thousands of seconds. A curve of maximum recovery voltage (Vmax) against charging time (tc) is then plotted. This curve is known as polarization spectrum, as shown in Figure 2.
V
tc Dominant time constant τcd
Fig.2: Typical RVM polarization spectrum A significant characteristic of the polarization spectrum is the time at which the peak occurs - known as dominant time constant. This value is dependent on the properties of the insulating material. More precisely, this value directly reflects the moisture content of the oil/paper insulation system. The displacement of the peak of the curve towards a small time constant indicates the degradation of the insulation system. The insulation system with higher moisture content has a relatively fast polarization response in which the polarization capacitance charges and discharges faster, resulting in maximum recovery voltage attains at shorter charging time. Conversely, the insulation system with lower moisture content has a relatively slow polarization response in which the polarization capacitance charges and discharges slowly. Hence, the value of maximum recovery voltage can only appear at longer charging time. The polarization spectra can be divided into two basic groups: (1) standard (2) non-standard. The standard polarization spectra have only one global maximum, which reasonably provide an accurate estimate of the actual moisture content in the oil/paper insulation. Curves with multiple peaks, flat curves and curves with discontinuities are considered non-standard. More information on the test objects and experiences are required to interpret this kind of graph. 3.
Measurement system
Figure 3 shows the diagram of the RVM system developed by the authors. The computer is used as a controller to automate the measurement process. Interfacing is through a National Instruments data acquisition card (PCI-6025E). Three digital output channels are used for controlling the relays in the switching unit. One analog input channel is used for recording the recovered voltage signal from a Keithley electrometer. The use of the electrometer for voltage measurement is necessary as this instrument has a very high input impedance.
Computer + LabVIEW
DAC
HV DC supply
Switching Unit
Test object
Fig.4: Front panel of the RVM system.
ADC
Electrometer
Fig.3: RVM System Interfacing Diagram
The key advantage of using LabVIEW is that automated measurement systems that leverage lowcost and flexible PC technology can be quickly developed. More importantly, it is fully integrated for communications with a large range of plug-in DAQs, GPIB, RS232, TCP/IP for remote monitoring.
The relays in the switching box are controlled by the computer to switch on and off at predetermined charging, discharging and measurement times. These time interval values are keyed in by the user into the LabVIEW program. At the end of each RVM cycle, the test object will be discharged for a certain period of time to remove the residual charge. The charging voltage is provided by a variable high-voltage DC supply which can be set in the range 0V-3000V. LabVIEW acts as the “brain” of the system which controls the relays, captures the signal and calculates the required parameters. LabVIEW is a program development environment, much like C or BASIC. The significant difference is that it uses graphical approach whereby programs are constructed in block diagram forms instead of textual language [7]. Each LabVIEW application is known as a Virtual Instrument (VI). Each VI consists of 2 windows. One window is known as the front panel (Figure 4), and the other is known as the block diagram. The front panel is what the user sees and sometime it is known as Graphical User Interface (GUI). It enables the user to key in the desired parameters and execute the program. Every front panel has at most one block diagram. The block diagram is the heart of the program and contains the graphical source codes of the VI. Codes are added by using graphical representation of functions such as “while loop” and “sequence structure”.
Fig.5(a): A noisy recovery voltage.
Fig.5(b): Filtered signal. The recovery voltage obtained from the electrometer can be somewhat noisy as shown in Fig.5(a). A filtering circuit was added to clean up the signal, see Fig.5(b). Features such as data storage, plotting of the recovery voltage of individual measurement, plotting of the polarization spectrum and concurrent temperature sensing were also incorporated into the system developed.
EXPERIMENTAL RESULTS
A laboratory test rig was constructed to simulate a transformer winding section. The test configuration consists of a lower, earthed, electrode of copper conductors and a “plane” HV brass electrode for applying stresses equivalent to those across axial ducts at the inside of a winding. The duct between the electrodes may be oil only or pressboard spacers may be included.
Fig.5: The LV electrode. The LV electrode is an array of five 100mm long conductors, laid out side-by-side in parallel but electrically connected in series (Figure 5). Since each conductor is 10mm wide, the electrode is equivalent to a 50x100mm rectangular area. The individual conductors are wrapped in three layers of 0.06mm thick kraft paper. In addition, seven sheets of kraft paper are placed on the LV electrode surface. The HV electrode is a 96x58mm plate with smooth edges (6.35mm radius) and insulated with two layers of 0.25mm thick crepe paper.
cylindrical glass “bushing” on top for HV connection (Figure 6). Two small glass windows are available on opposite sides of the tank for viewing. Also there are various inlet/outlet ports for oil filling, circulation and thermocouple connections. The volume capacity of the tank is 12 litres. The pad under the base, which is controlled thermostatically, is used to heat the test chamber. The copper electrodes, in which the heating current can be circulated through, are used to raise the temperature of the conductor (for simulating overloading conditions). The duct between the HV and LV electrodes contains oil and pressboard spacer. The thickness of the pressboard spacer is 3mm. The temperatures of the conductor, paper and oil are monitored separately using fibre optic sensor and conventional thermocouples. Moisture (water) may be injected into the tank as required. Oil sample extracted from the tank was sent to an industrial laboratory for measuring its moisture content. The experiments were done at three different temperatures: 20°C (room temperature), 80°C (normal operating temperature) and 140°C (overloading condition). The charging voltage was set to 500V and 1000V. A relaxation time of 10 minutes was applied in all the tests. In addition to recovery voltage measurement, partial discharge measurements were also carried out using the computerised discharge analyser (CDA3) system. Max. recovery volt. (V)
4.
1
500V
0.8
1000V
0.6 0.4 0.2 0 0
10
20 30 Charging time (s)
40
50
Max. recovery volt. (V)
Fig.7: Polarization spectrum (80oC, 135ppm).
Fig.6: The test rig The complete setup is housed inside a steel tank (280mm wide x 330mm long x 210mm high) with a
7
500V
6 5
1000V
4 3 2 1 0 0
10
20
30 40 50 60 70 Charging time (s)
80
90 100
Fig.8: Polarization spectrum (80oC, 265ppm).
voltage attains in a shorter period, as the time constant is directly proportional to resistance. Max. recovery volt. (V)
Many tests were carried out under different conditions in terms of moisture content, temperature and charging voltage. Figure 7 shows the polarization spectrum for the tests conducted at temperature of 800C with moisture content of 135ppm. It can be seen increasing the charging voltage results in an increase in the recovery voltage and that the scaling effect of the charging voltage on the polarization spectrum is reasonably constant. The dominant time constant is ~10 seconds.
1000V
6 4 2 0 10
20 30 40 50 60 70 80 Charging time (s)
Max. recovery volt. (V)
5
500V
4
1000V
3 2 1 0 0
135ppm
10
20
265ppm
4
90 100
Fig.10: Polarization spectrum (20oC, 265ppm).
6 5
500V
8
0
30 40 50 60 70 Charging time (s)
80
90 100
Fig.11: Polarization spectrum (140oC, 265ppm).
3 2
Max. recovery volt. (V)
Max. recovery volt. (V)
Figure 8 shows the polarization spectrum for the tests conducted at the same temperature of 800C but with a higher moisture content of 265ppm. Note the smaller dominant time constant of 5 seconds. Insulation system with higher moisture content has a relatively fast polarization response, therefore the value of the maximum recovery voltage attains at shorter charging time. For comparison, the results for the case of 500V in Figs.7-8 are combined and shown in Figure 9.
10
1 0 0
10
20
30 40 50 60 70 Charging time (s)
80
90 100
Fig.9: Polarization spectrum (500V, 800C). Figures 8, 10 and 11 show the effect of temperature on the spectra. The actual temperatures recorded on the conductor, paper and oil for the three different tempeature conditions are shown in the Table below: Condition
Conductor
Paper
Oil
Ambient
21.00C
24.90C
24.80C
Normal Overloading
0
0
76.9 C
79.6 C
79.70C
139-1430C
114-1190C
83-880C
Table 1: Measured temperatures of test conditions. The results are combined and shown in Figure 12 for the case of 500V charging voltage. It can be seen that the recovery voltage decreases with increasing temperature. However, there are no significant changes in the dominant time constant which is unexpected. The argument is that the moisture exchange between paper and oil and the polarisation process tend to be more active at higher temperature [5], resulting in more free charges in the interface. Consequently, the conductivity is higher and the resistance is lower, leading to maximum recovery
8 7 6 5 4 3 2 1 0
20 deg.C 80 deg.C 140 deg.C
0
10
20
30 40 50 60 70 Charging time (s)
80
90 100
Fig.12: Polarization spectrum (500V, 265ppm). One possible explanation for the failure to obtain a reduction in the time constant with increasing temperature is that, following the injection of water into the oil, the test cell was not allowed to stand for long enough so that the paper could absorb the moisture. Further tests are intended to be carried out to investigate this problem. 5.
CONCLUSIONS
Based on the results obtained from the experiments conducted on the oil-impregnated paper insulation laboratory models, it was observed that the peak of the polarization spectrum shifted to a smaller time constant with the increase of moisture content. The study also indicates a good linearity response in variation of charging voltage.
Uneven distribution of moisture or other ageing products may complicate the evaluation of the measurements of recovery voltage. Therefore, moisture equilibrium of the test object plays an important role in determining the conditions of the insulation system. Overall, recovery voltage measurement is an insulation diagnostic technique, capable of assessing the conditions of the aged transformer. On top of that, it can also be used as a tool for the planning of maintenance for future use. With the introduction of LabVIEW program in the recovery voltage measurement system, the whole measurement process becomes easier and straightforward. It provides data storage capability and allows a data bank, which can be used for future references and researches. As LabVIEW offers online monitoring over the Internet using TCP/IP, future implementation of this feature would be studied to improve the development of a computer based data logger for recovery voltage measurement. 6.
REFERENCES
[1] M. Darveniza, T. K. Saha, D. J. T. Hill, T. T. Le, “Assessment of Insulation in Aged Power Transformers by Interfacial Polarization Spectrum and its Correlation with Chemical Properties”, Conf. on Electrical Insulation and Dielectric Phenomena, 1992, pp.671-678. [2] H. Yoshida, Y. Ishioka, T. Suzuki, T. Yanari and T. Teranishi, “Degradation of Insulation Materials of Transformers”, IEEE Trans. on Electrical Insulation, 1987. [3] Working Group Report, “Background Information on High Temperature Insulation for Liquid Immersed Power Transformers”, IEEE Trans. on Power Delivery, Vol.9, No.4, 1994, pp.1892 –1906. [4] S. Itahashi, H. Mitsui, T. Sato and M. Sone, “Analysis of Water in Oil Impregnated Kraft Paper and its Effect on Conductivity”, IEEE Trans. on Dielectrics and Electrical Insulation, Vol.2, No.6, Dec. 1995, pp.1111 –1116. [5] Gusztáv Csépes, István Hámos, Roger Brooks, Volker Karius, “Practical Foundations of the RVM (Recovery Voltage Method for Oil/Paper Insulation Diagnosis)”, Conf. on Electrical Insulation and Dielectric Phenomena, Vol.1, 1998, pp.345-355. [6] P. R. S. Jota, S. M. Islam, F. G. Jota, “Modeling the Polarization Spectrum in Composite Oil/Paper Insulation Systems”, IEEE Trans. on Dielectrics and Electrical Insulation, Vol.6, No.2, April 1999, pp.145 –151. [7] LabVIEW on-line tutorial.
