Design of HV Switching for Polarization & Depolarization Current Measurement

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1 Design of HV Switching for Polarization & Depolarization Current Measurement N. F. Kasri, M. A. M. Piah, A. A. Suleiman, N. A. M. Jamail, N. Bashir and N. A. Muhamad Institute of High Voltage & High Current (IVAT) Universiti Teknologi Malaysia (UTM) Skudai, Johor, Malaysia Abstract- The method of assessing the insulation condition in high voltage (HV) electrical equipments can never be overemphasized. Over the years, several method of assessing the insulation condition have been researched & used by many researchers. One of these methods is using the dielectric response of the materials to determine the Polarization and Depolarization Current (PDC). PDC is a non-destructive assessment method that combines a sequence of switching operations & current measurement for time domain response. In this paper, the HV switching sequence for a PDC assessment was designed & built to fulfill the PDC assessment basic theory. The switch was applied on three materials used in HV insulation to determine their polarization current (i p ) & depolarization current (i d ). Test duration for both i p & i d were seconds and the test results are presented as a current pattern. The current pattern is consistent with the theoretical current wave PDC pattern from literatures thereby confirming the validity of the built switching device. Fig. 1. PDC Voltage & Current wave pattern [7] I. INTRODUCTION All electrical equipment is as good as their insulation and so the assessment of electrical insulators is very important for the continuous operations of the equipment. Several methods of insulation testing (destructive & non-destructive) are available and they include breakdown voltage, type testing, DGA, PDC, FDS and RVM [1-5]. Polarization & depolarization current measurement is one of such non-destructive insulation assessment methods. PDC measurement can be defined as a non-destructive dielectric diagnostic technique to obtain the conductivity and moisture content of insulation materials [5-7]. This technique is gaining immense popularity throughout the globe as it is a new dielectric diagnostic technique which potentially confines all the information gathered from FDS and RVM [8]. The basic theory of the PDC is that a charging voltage U 0 is applied across the insulation sample for a period of t p and the resulting current i p is measured. When the voltage is removed after t p, the sample is short circuited for a period of t d (t p is usually equal to t d ) and the current i d is also measured [5, 7, 9-11] as shown in Fig 1. In this paper, the HV switching sequence for a PDC assessment was designed & built to fulfill the PDC assessment basic theory presented above. The electrometer used is Keithley 6517B and LabVIEW software computer program is used for the switching operation. The PDC test setup is shown in Fig 2 while the anticipated current wave pattern is shown in Fig 1. Fig. 2. PDC assessment laboratory set up II. DESIGN OF THE SWITCH The switching device was designed and operated based on the PDC measurement technique as recorded in many previous general literatures [1-5]. It consists of hardware (the switch; Fig 5), software (the computer program, LabVIEW), computer & the voltage source/ammeter. The integration of all the components was done by the LabVIEW program and user interaction is through the GUI display on the computer monitor. The switching operations are automated & the 35

2 measurements and recordings done by the instrument meter are also automatically recorded and stored in the computer. The design of the switch begins with the simulation done using NI-Multisim 10 software. Fig 3 shows the switching circuit that was tested by simulation. Based on the result obtained, it shows that this type of switching is valid and the plotted graph (Fig 4) is similar with the typical wave pattern as shown in Fig 1. the test cell while the switch S2 is used to trigger or bypass the electrometer (Keithley 6517B). The purpose of bypassing the electrometer is to eliminate all the transient current. A 2kΩ resistor, which is connected in series with test cell and electrometer, was used to avoid possible high currents during a potential low impedance defective connection [13]. The switching command & control includes the NI-DAQ Data Acquisition (NI USB-6210) and the LabVIEW computer program, linked by a USB type communication. The LabVIEW has dual function of switching voltage source on & off as well as obtaining & storing measured outputs like current, resistance & charge from the Keithley meter. The development of PDC switching algorithm was also developed using the LabVIEW software. Fig. 3. Simulation circuit in NI-Multisim 10 software Fig. 5. HV switch relay & circuit board Fig. 4. Simulation result by using NI-Multisim 10 software A. PDC Switch Board & Circuit Design In this switch, there are two units of G45C HV relays from GIGAVAC indicates by S1 & S2 as shown in Fig 2. Each relay can handle a peak dc voltage of 6kV while during normal operations, they handle 5 kv dc voltages each [12]. The relay S1 has 3-terminal electrodes to charge or discharge B. PDC switching Software Design LabVIEW is the software used for this switching because it has the advantage of supporting instrument hardware. The graphic node circuit developed in the LabVIEW is used to drive both the switch and the Keithley meter & also save the measured currents; thus interfacing them both. The software design is based on the structure of state machine programming. It is used because the PDC switching operations has many state actions that need to be run in sequence. The first approach included the design of state diagram and at this stage any possibilities of the state actions were determined. Fig 6 shows an example of the state diagram before proceeding to develop the practical state machine programming in LabVIEW. The next step was to implement the designed/developed state diagram in the LabVIEW environment as shown in Fig 7. The development of the programming start from the first state until the last state (including the input and the output of NI- DAQ, graph plotting and saving data) and the cognitive information from literature guided this programming structure. 36

A Standard Commands for Programmable Instruments (SCPI) code is added in order to control the Keithley meter which use a standard short-range digital interconnection namely GPIB (General Purpose

3 A Standard Commands for Programmable Instruments (SCPI) code is added in order to control the Keithley meter which use a standard short-range digital interconnection namely GPIB (General Purpose Interface Bus). The combination of the methods mentioned above produce software which capable of controlling both switch board and Keithley meter. Fig. 6. A very simple state diagram for the system to operate Fig. 8. The GUI for the PDC Assessment III. TESTING THE SWITCHING DEVICE The designed switch was tested on three insulator samples; Dry Kraft Paper, Kraft Paper immersed in Red Palm Oil and LLDPE-Natural Rubber. As the currents expected to be measured are in scales of between micro to nano Ampere. Some steps must be taken to eliminate noise from the source voltage and the samples placed inside Faraday cage and subsequently discharged of any residual charges before the test. Furthermore provision is made in the switching program to discharge for a period that is dependent of the sample used; a period of discharge/delay as shown in Table 1. Fig. 7. State machine in LabVIEW environment C. System and Graphical User Interface (GUI) development The GUI is a simple user friendly graphic window that is designed to suit multiple test sample parameters as any operator the switching assembly may require; Fig 8. The GUI is divided into three major sections, PDC PARAMETER(S) SETTING, KEITHLEY INSTRUMENT SETTING & GRAPH. 1mm thick Dry Kraft Paper 1mm thick Kraft Paper immersed in red palm oil 3mm LLDPE- Natural Rubber Applied dc Voltage (V) TABLE I Charging time, t p (s) Discharging time t dp (s) Discharge/ Delay before Charging (s)

4 From the measured current stored, the PDC current patterns were plotted. The wave plots have shown that each of the insulating materials had current wave pattern that is appropriate in the material. When they are compared, the polarization current (i p ) wave plots for the dry Kraft paper as in Fig 9 is lower than the one for the Kraft paper immersed in red palm oil as in Fig 11. This is consistent with the anticipated wave plot because the presence of moisture diffused into the paper will increase the free charged molecules. Similarly, the free charged molecules are lower in the LLDPE-Natural Rubber and hence the polarization current in it will be lower compared to the ones in both Kraft paper sample as shown in Fig 13. All the depolarization current wave plots in the three samples were also consistent with the anticipated plots as in Figs 10, 12 & 14. Fig. 11. Polarization current for 1mm thick Kraft Paper immersed in Red Palm oil Fig. 9. Polarization current for 1mm thick dry Kraft paper Fig. 12. Depolarization current for 1mm thick Kraft Paper immersed in Red Palm oil Fig. 10. Depolarization current for 1mm thick dry Kraft paper Fig. 13. Polarization current for LLDPE- Natural Rubber 38

5 Fig. 14. Depolarization current for LLDPE- Natural Rubber The current wave plots drawn from the obtained results have shown that the PDC switching device can successfully be used in any PDC assessment for HV insulation. IV. CONCLUSION The design of this HV switching for PDC measurement technique is reliable since the obtained results are similar with the typical nature of PDC's waveform as shown in Fig 1. The pattern of generated waveform is identical where if it is evaluated in the view of theory of PDC measurement, the results are valid and can be analyzed to get the conductivity and moisture content of the insulator materials. Hence, the results obtained are conclusive and achieved the research objective. Furthermore, PDC is one of the insulation condition assessment methods gaining popularity due to its ability to distinctively react the properties of the material [7]. This salient quality of PDC makes it a suitable assessment technique where equipment insulation is not easily accessible; like the Kraft paper insulation in power transformers. This PDC assessment device can be compacted into a handy test kit/analyzer that can be used of on-site testing of electrical equipment. [2] H. A. P. Silva, et al., "Non-invasive Ageing Assessment by Means of Polarization and Depolarization Currents Analysis and its Correlation with Moisture Content for Power Transformer Life Management," presented at the IEEUPES Transmission 8 Distribution Conference & Exposition Latin America, [3] T. K. Saha, et al., "An Attempt to Correlate Time & Frequency Domain Polarisation Measurements for the Insulation Diagnosis of Power Transformer," IEEE Power Society General Meeting, vol. 2, pp , [4] T. K. Saha and P. Purkait, "Investigations of Temperature Effects on the Dielectric Response Measurements of Transformer Oil-Paper Insulation System," Power Delivery, IEEE Transactions on, vol. 23, pp , [5] E. David, et al., "PDC measurements to assess machine insulation," Dielectrics and Electrical Insulation, IEEE Transactions on, vol. 17, pp , [6] T. K. Saha and P. Purkait, "Investigation of polarization and depolarization current measurements for the assessment of oil-paper insulation of aged transformers," Dielectrics and Electrical Insulation, IEEE Transactions on, vol. 11, pp , [7] N. A. M. Jamail, et al., "Comparative study on conductivity using Polarization and Depolarization Current (PDC) test for HV insulation," in Electrical Engineering and Informatics (ICEEI), 2011 International Conference on, 2011, pp [8] N. A. Muhamad, et al., "Polarization and Depolarization Current (PDC) tests on biodegradable and mineral transformer oils at different moisture levels," in Power Engineering Conference, AUPEC Australasian Universities, 2009, pp [9] T. Gradnik, et al., "Estimation of water content in power transformers in service by polarization and depolarization current measurements," presented at the IEEE International Conference on Dielectric Liquids 2008, ICDL, [10] N. A. Muhamad, et al., "Polarization And Depolarization Current (PDC) Tests On Biodegradable And Mineral Transformer Oils At Different Moisture Levels," presented at the Australasian Universities Power Engineering Conference, AUPEC 2009., [11] T. K. Saha and P. Purkait, "Investigation of Polarization and Depolarization Current Measurements for the Assessment of Oil-paper Insulation of Aged Transformers," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 11, pp , [12] GIGAVAC, "G45C Make & Break Load Switching," GIGAVAC, Ed., ed. Santa Barbara, CA: GIGAVAC, [13] C. Ekanayake, et al., "Application of polarization based measurement techniques for diagnosis of field transformers," in Power and Energy Society General Meeting, 2010 IEEE, 2010, pp ACKNOWLEDGMENT The authors gratefully acknowledge the Malaysia Ministry of Higher Education (MOHE) and University Technology Malaysia under grant Vote no 4F097 and 4L014. REFERENCES [1] U. Gafvert, et al., "Dielectric spectroscopy in time and frequency domain applied to diagnostics of power transformers," in Properties and Applications of Dielectric Materials, Proceedings of the 6th International Conference on, 2000, pp vol.2. 39

Correlation between time and frequency domain polarisation measurements for transformer moisture assessment

Australasian Universities Power Engineering Conference (AUPEC 24) 26-29 September 24, Brisbane, Australia Correlation between time and frequency domain polarisation measurements for transformer moisture