A-137
2012 IEEE International Conference on Condition Monitoring and Diagnosis 23-27 September 2012, Bali, Indonesia
Partial discharge behavior of mineral oil and oil-board insulation systems at HVDC Jürgen Fabian, Michael Muhr
Stefan Jaufer, Wolfgang Exner
Institute of High Voltage Engineering and System Management Graz University of Technology Inffeldgasse 18, 8010 Graz, Austria
[email protected]
WEIDMANN Electrical Technology AG Neue Jonastrasse 60, 8640 Rapperswil, Switzerland
This paper deals with the specific dielectric stresses of insulation systems of HVDC converter transformers. The field distribution at AC and DC is discussed as well as parameters of influence on oil-board insulation systems and the behavior of partial discharges at HVDC applications.
Abstract—Power transformers have been built successfully with oil-board insulation systems for many years. Oil has the function of being a coolant as well as an insulator, whereas the board serves for electrical and mechanical purposes. When dealing with solid insulating materials, special focus has to be set on partial discharges (PD), as they have a destructive nature. On the one hand they may supply charge carriers for breakdown mechanisms and on the other hand they can lead to a carbonization of the organic insulation.
II.
In general, an insulating system behaves differently depending on whether it is stressed with AC or DC. In the case of AC stress the field distribution is determined by the materials permittivities. Otherwise, for DC applications the electrical conductivity (respective resistivity) plays the major role, as shown in Figure 1.
In the case of AC, phase-resolved patterns, so called finger prints, can be used to interpret the type of partial discharge. These methods are in use for many years and have proven successful in interpreting possible PD sources. However, in the case of DC, no phase information is available. Therefore other methods for interpreting the behavior of PD at HVDC have to be found. In this paper, a non-uniform test setup for mineral insulating oil and a small oil-board model was used. The different observations were evaluated to discuss the issues of PD measurements at HVDC. In addition to conventional electrical partial discharge measurements also optical detection of partial discharges was used. Furthermore, a scanning electron microscopy (SEM) was used for the high voltage electrodes to gain information about surface changes of the needle electrodes.
Figure 1: Dielectric stress in an oil-board barrier system for AC and DC voltages, adapted from [2]
Keywords-HVDC; converter transformer; oil-board insulation system; field distribution; conductivity; partial discharge (PD)
I.
Special focus must be set on the insulation interfaces of converter transformers, because of the field distribution and the resulting polarization. Different mineral oil types may have strongly differing values of conductivity (about a factor of 10 or more), whereas the conductivity of solids hardly changes at the same conditions. Due to these facts, designing HVDC converter transformers needs special attention, depicted in Figure 2 [2]-[5].
INTRODUCTION
Nowadays our living standard is based on the consumption of energy. Due to the increasing need of electrical energy, many transmission lines have to be built or upgraded to transport the required energy. For the bulk transmission of electrical power over long distances, HVDC transmission systems are getting more and more important. One of the benefits of this technology is the lower transmission loss per unit of length compared to overhead AC transmission lines. Nevertheless, HVDC brought some new challenges for engineers, because of the specific characteristics of the HVDC insulation system of converter transformers. Converter transformers are essential components in HVDC transmission systems. Because of pulsating DC currents flowing in the transformer’s valve windings, these transformers have to cope with different electrical, mechanical, and thermal stress compared to conventional power transformers [1].
978-1-4673-1018-5/12/$31.00 ©2012 IEEE
FIELD DISTRIBUTION AT AC AND DC
Figure 2: Electric fields related to HVDC transformers insulation design, left side: AC field distribution; right side: DC field distribution [6]
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The knowledge of material parameters and their behavior is essential for setting up models and computer simulations to properly design electrical equipment. However, different materials can depend on and vary in a complex manner strongly from parameters, which are difficult to capture. III. FACTORS OF INFLUENCE ON OIL-BOARD-INSULATION-SYSTEMS Conductivities (respective partial discharges) of oil and board can differ by many orders of magnitude and are dependent on many parameters in a complex correlation. Factors of influence are, among others, the materials themselves, temperature, aging, and moisture. Furthermore, additional effects occur by the application of an electrical DC field, which are in many cases not investigated or their effects are not considered [7]-[9]. Figure 3 gives an overview of different factors of influence. Temperature
Others
Humidity
Due to the fact that it is difficult to distinguish between disturbances and PD caused by the test object at DC voltage (no phase correlation possible), the test setup was built up in a fully shielded room. Therefore, ground noise was lower than 0.5 pC. Added shields to the components contributed to an improved field distribution, hence the test setup was PD free up to 70 kV DC. With the appropriate capacitors of 10 nF and two low pass filters, the voltage ripple was lower than 1 % (note: according to IEC 60060-1 the voltage ripple should not be greater than 3 % for tests longer than 1 minute).
Time
Oil-Board-Insulation
Quality of Cellulose
Figure 5: Practical HVDC test setup inside the fully shielded climate chamber at the Institute of High Voltage Engineering and System Management, Graz University of Technology [10]
Voltage (AC vs. DC)/ Field strength
Oxygen
The voltage was raised up in steps (“up”), each step not more than 10 % of the maximum applied voltage and was held 5 minutes for each step. The voltage between the steps was slowly increased with approximately 300 V/s in order to avoid high displacement currents. The same applies for decreasing the voltage (“down”), to gain information about the extinction behavior of PD.
Figure 3: Factors of influence on oil-board-insulation systems [8]
IV.
EXPERIMENTAL SETUP
To get information about the parameters (type of material, electric field strength, temperature, time of stress) laboratory measurements were carried out by varying the above mentioned parameters. Using a test vessel (illustrated in Figure 4) and proper measurement equipment, it was possible to monitor all relevant parameters (oil temperature, water content of oil, applied high voltage). To get comparable results all board samples and the oil had been prepared under special conditions as explained in [10].
V.
EXPERIMENTAL RESULTS AND CONCLUSIONS
A. Conventional PD measurement according to IEC 60270 Exemplarily, some obtained results are given in Figure 6 and Figure 7 (mineral oil without board), where the number of PD pulses (PD counts) and the so called NQS-values (N: number, Q: charge, S: per second) are depicted. The recording of PD activity was performed over the time, because in the case of DC no phase information is available. Measurement time was 5 minutes for each voltage level, whereas the analysis was carried out for the 1-, 3-, and 5minutes-values.
Figure 4: Schematic view of test vessel for partial discharge measurement (conventional and optical) (left), electrode set-up (right), adapted from [10]
As stated before, partial discharges are depending on those parameters, therefore it is essential to keep those conditions constant during the measurement time. In consequence, all measurements were carried out in a climate chamber (see Figure 5).
Figure 6: Number of PD pulses at HVDC (negative polarity), needle-to-plate electrode arrangement with new and dry (about 5 to 10 ppm) mineral oil, 50 mm gap distance [10]
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Figure 7: NQS-values at HVDC (negative polarity), needle-to-plate electrode arrangement with new and dry (about 5 to 10 ppm) mineral oil, 50 mm gap distance [10]
Figure 9: Optical PD measurement: PD signals (yellow: electrical signal, green and purple: optical signals) [10]
At DC, space charge builds up an electric field, which counteracts the applied field. Therefore, PD can extinguish after a certain time of voltage application. Hence, an inception voltage is hard to define. In case of evaluating PD according to the amplitude and the number of pulses, a critical threshold value has to be defined which determines the pass or fail of electrical equipment during a HVDC test. According to IEC 60270 [11], no limit is defined, but is left open to the manufacturer and consumer to specify. However, there are specific standards how to perform high voltage tests at HVDC equipment, e.g. IEC 61378-2: Converter Transformers – Part 2: Transformers for HVDC Applications.
C. Scanning Electron Microscopy (SEM) A scanning electron microscopy was carried out to compare changes in the surface topology of the unused and used electrode material of the needles. Additionally the tip radius of the needles was determined to ensure that the needles did not get blunt during the performed high voltage test. It could be observed that in the case of the needle-to-plate electrode arrangement within pure mineral oil, the needles’ surface got little “craters” due to the PD activity (depicted in Figure 11).
Another parameter to describe the behavior of partial discharges is the NQS-value, as this value gives the average current over a certain time. This energy based parameter integrates the amplitude and number of partial discharges and relates them to the measurement time. Because of the above mentioned reasons, it makes sense for HVDC applications to use energy related parameters for diagnostic purposes [10]. B. Optical PD measurement In addition to electrical measurements also optical measurements were carried out in order to prove the results with a different method [12], [13]. Those optical measurements lead to the same conclusions as the electrical method did. However, with the benefit of using two orthogonally arranged sensors, it was also possible to get the direction of wave propagation within the vessel, yet without precise localization as only two sensors were used (shown in Figure 8 and Figure 9).
Figure 10: Unused needle electrode before high voltage test [10], [14]
Figure 11: Electrode with new and dry (about 5 to 10 ppm) mineral oil, HVDC voltage (positive polarity), 10 mm gap distance [10], [14]
Figure 8: Practical setup of the test vessel with two orthogonally arranged sensors for optical PD measurement [10]
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Interestingly it turned out that there are very small changes in the needles topology at the creepage arrangement, although there was no detection at the electrical PD measurement system or the optical system. So it can be assumed that the current of partial discharge is flowing along the surface of the board producing a single hot spot where the needle touches the board where the metallic material of the needle is molded together with some board fibers (see Figure 12) [10].
ACKNOWLEDGMENT The work described in this paper was part of a scientific cooperation with WEIDMANN Electrical Technology AG, Rapperswil, Switzerland, which was initiated and implemented thanks to the endeavour of Christoph Krause. The authors would like to thank all involved partners for enabling this research project, their support, and the assistance during the performance of the measurements. Special thanks to Robert Schwarz (Siemens Transformers Austria Weiz) for his support and interest on this topic. Many thanks to Hartmuth Schröttner and his team (Institute for Electron Microscopy, Graz University of Technology) for carrying out the scanning electron microscopy of the electrode material. Thanks a lot to Katharina Barth, Philipp Pretis, and Bettina Wieser, Institute of High Voltage Engineering and System Management, for their valuable assistance of evaluating the measurement data. REFERENCES [1]
[2]
Figure 12: Oil-board-creepage electrode-arrangement, new and wet (about 20 to 25 ppm) mineral oil, new impregnated high-density pressboard, HVDC voltage (positive polarity), 10 mm distance between high voltage electrode and grounded electrode [10], [14]
VI.
[3]
CONCLUSION
[4]
The electrical DC field distribution of an oil-pressboard insulation system is completely different when compared to AC or impulse stress. Whereas the latter are dominated by the materials’ permittivities, a stationary DC field distribution is mainly given through its conductivities. However, partial discharges depend significantly on the field distribution and local inhomogeneities.
[5]
[6]
[7]
It is well known that the electrical conductivity of insulating liquids and board can differ by many orders of magnitude. Additionally, the conductivities and PD behavior of both materials are dependent on many parameters, such as time, temperature, humidity, aging, and local field strength. For these reasons, it is difficult to determine the field distribution of a complex oil-board insulation system with high precision. In addition, PD interpretation is more difficult compared to AC, as a phase relation is missing at DC. Because disturbances are difficult to identify at DC using only a conventional electrical measurement system, other methods for locating PD are suggested, such as acoustical or optical PD detection. One approach for a better description of the PD behavior at DC is to use an energy based parameter (NQS-value), because of its integrating character.
[8]
[9]
[10]
[11]
To sum up, the knowledge of material parameters and their behavior is essential for setting up models and computer simulations to properly design a HVDC converter transformer. Optimization of these transformers is a challenge, yet operational safety has to be guaranteed or even increased. In general, a better understanding of insulation problems and insulating materials at HVDC has still to be gained [2], [5], [7].
[12]
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[14]
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