CC BY
36/ TECHNICAL SCIENCE
the Boundaries of the Layer. - Journal of Mechanical Engineering. - 2019. - Vol. 22, N 2. - P. 4452. https://doi.org/10.15407/pmach2019.02.044
14. Miroshnikov V. Yu. Determination of the stress state of a layer with a cylindrical cavity located УДК 621.314
Kazanskiy S.
Ph.D., Associate Professor of electric networks and systems,
Mossakovskiy V. TF of electric networks and systems National Technical University of Ukraine «Igor Sikorsky Kyiv Polytechnic Institute»
DOI: 10.24411/2520-6990-2019-10643 COMPARATIVE ANALYSIS OF STATE STANDARDS FOR LOADING CAPACITY OF THE OIL-
IMMERSED POWER TRANSFORMERS
on an elastic base and specified boundary conditions in the form of stresses. Colloquium-journal.- №18 (42). -2019. - P.50-55 DOI: 10.24411/2520-6990-201910610
Abstract
It was stated the necessity of increasing the operational reliability of oil-immersed power transformers in electric networks. It was analysed the state standards which provides guidance for loading capacity of power transformers. It is pointed on some differences in the criteria of determining the actual thermal mode, in particular, the winding temperature insulation and the thermal life-time.
Key words: oil-immersed power transformer, loading capacity, winding temperature insulation.
Oil-immersed power transformer is one of the most important elements of electric power transmission systems. There is a significant shortage of transforming power in electrical networks, and this impedes the connection of new consumers and hinders the networks development. Therefore, increasing the load capacity of oil-immersed power transformers is an application task. Performing this task, the requirements for safe and reliable operation must be strictly observed [1].
Table 1 presents the results of the analysis of statistical information on 536 failures and failures of oil-
Statistical analysis of damages of step-down
1996 -
immersed power transformers of different voltage classes, which occurred in the period 1996-2010 in 58 utility companies in 21 countries [2].
Table 2 presents the results of a failure analysis of 200 lower power transformers from 100 to 500 kV that occurred during 2000 and 2010 in 32 countries [3]. The results summaries indicate that problems with winding and insulation account for between 36% and 50% of the total number of causes, with thermal effects accounting for 16% of failures. Therefore, one of the most common causes of failures of oil-immersed power transformers is damage to the winding insulation due to thermal overload.
Table 1
transformers with voltage over 100 kV during 2010 [2]
Failure cause Percentage Failure cause Percentage
HV winding 19,4 Electrical screen 0,56
MV winding 5,6 HV bushings 13,99
LV winding 5,6 MV bushings 2,8
Tapping winding 3,36 LV bushings 0,37
HV lead exit 3,17 Core & magnetic circuit 2,43
MV lead exit 1,68 Flux shunts 0,37
LV lead exit 1,12 Tank 0,75
Phase to phase isolation 0,75 Cooling unit 1,12
Winding to ground isolation 1,31 Tap changer 31,16
Winding to winding isolation 0,37 Current transformer 0,37
Table 2
Statistical analysis of damages of 200 step-down transformers with voltage from 100 to 500 kV during _2000 - 2010 [3]_
Failure cause Percentage Failure cause Percentage
Tap changer 30,0 HV winding 6,5
Cooling unit 1,0 MV winding 5,0
Tank 1,0 LV winding 11,5
Core and magnetic circuit 4,0 Tapping winding 2,0
LV bushings 0,5 HV lead exit 3,5
MV bushings 5,0 MV lead exit 3,5
HV bushings 12,0 MV lead exit 2,5
TECHNICAL SCIENCE /
Electrical screen 0,5 Isolation 1,5
General requirements for determining the thermal mode and load capacity of oil-immersed power transformers are set out in several guidelines. These include state standards:
• MTOCT 14209-97 (IEC 354-91). Loading guide for power transformers [4];
• IEC 60076-7. Power Transformers - Part 7: Loading guide for oil-immersed power transformers
[5];
• IEEE C57.91-2011. IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators [6];
• IEEE C57.119-2018. IEEE Recommended Practice for Performing Temperature Rise Rest on Liquid-Immersed Power Transformers at Load Beyond Nameplate Ratings [7].
These guidelines include, in particular, methods for calculating the hot-spot temperature &h(t) on the winding surface of oil-immersed power transformers with different types of cooling systems. It is noted that this temperature is functionally dependent on the value of current load, oil temperature and ambient temperature and can be determined by indirect calculation or by simulation [4]. In addition, the values of &h(t) and the life-time integral value of the windings insulation Lh, are criteria for the permissible and duration of overload. However, the methods for determining these values are differ in early mentioned standards.
The temperature influence leads to decrease of the electrical strength of the insulation and the mechanical characteristics of the insulation change, which leads to a decrease in its resistance to the dynamic effects of currents [3]. It is also stated in [3] that the value of the winding hot-spot temperature imposes restrictions on the current overload of the transformer, so it is necessary to determine the value of this temperature as precisely as possible.
Since the late 80's the implementation of direct temperature measurements on the winding surface through fiber-optic sensors has become commonplace [4]. Usually they are located on the upper conductors of the winding, because that is where the influence of the leakage flux is greatest, and the oil temperature is the highest possible value. Also, some of sensors are positioned at the lowest level of the winding for determining the temperature gradient. It should be noted that in
[6] the sensitivity of the sensors to transients during the start-up of the transformer is shown, the necessity of their periodic calibration, as well as the decrease of the quality level of the signal with the operation time of the transformer.
Unfortunately, there is no information regarding the calibration time of such sensors and the performers that would perform this procedure. There is also no information on the widespread use of these sensors; there are only prototypes and bench tests.
On this basis, it is necessary to use the system of exponential equations for describing the process of heating and cooling based on the load schedule, as well
as the system of differential equations used to account for the dynamic change of the factors influencing the temperature level, to calculate the hot-spot temperature of the winding. Mathematical models based on the statement that the maximum value of the hot-spot temperature of the winding for paper insulation is 98°C, for thermally upgraded paper insulation is 110 °C. In [5, 6] the permissible values of currents and temperature levels of the hot-spot, metal parts and top-oil for normal cyclic loading and emergency loading are given. The [7] additionally provides empirical correlation coefficients that take into account the need to change the load under the following conditions.
Conclusion. Oil-immersed power transformers are the most important elements of the transmission and distribution of electrical energy. The failure rate of power transformers is quite high, with a significant proportion of thermal damage. The load capacity of the oil-immersed power transformers is regulated by several standards that have some certain differences. The comparative analysis results of the standards for the overload capacity of oil-immersed power transformers show that the hot-spot temperature of the winding insulation is the most important criterion for the permissibility of current above nameplate rating. However, these standards recommend different methods for determining this temperature. The authors suggest that the indirect determination of this temperature by real time dynamic simulation is optimal.
Determining the temperature of the winding insulation hot spot during operation will allow more efficient use of the rated power and significantly increase the reliability of oil power transformers in electrical networks.
References
1. Kazansky S. Reliability of power systems: training aid / S. Kazansky, Y. Mateyenko, B. Serdyuk. - K.: NTUU «KPI», 2011. - ISBN 978-966-622-453-1.
2. Electronic resource. Access mode: https://www.modernpowersystems.com/ features/fea-turea-new-assessment-of-power-transformer-reliabil-ity-6045374.
3. Farzaneh Vahidi, Stefan Tenbohlen. Statistical Failure Analysis of European Substation Transformers // ETG-Fachtagung Diagnostik elektrischer Betriebsmittel. November, 2014.
4. MGOST 14209-97 (IEC 354) Manual load power oil transformers. Date of introduction 2002.01.01.
5. IEC 60076-7: 2005 Power transformers - Part 7: Loading guide for oil-immersed power transformers. Released: 2005-12-15.
6. IEEE C57.91-2011. IEEE Guide for Loading Mineral-Oil-Immersed Transformers and Step-Voltage Regulators.
7. IEEE C57.119-2018. IEEE Recommended Practice for Performing Temperature Rise Rest on Liquid-Immersed Power Transformers at Load Beyond Nameplate Ratings.
