Original scientific paper
CHECKING OF THE CONDITION OF TRANSFORMERS AND THE EFFICENCY OF OIL REGENERATION WITH RVM (RETURN VOLTAGE MEASUREMENT)
Bálint Németh
orcid.org/0000-0003-4473-1713
; Budapest University of Technology and Economics
Gusztáv Csépes
orcid.org/0000-0001-9896-3726
; Diagnostics Ltd.
István Kispál
; Diagnostics Ltd.
Zsolt Laczkó
; MVM OVIT Ltd.
Abstract
Up to now the insulation in HV power transformers has been made from oil/paper therefore the
diagnosis of this kind of insulation will be also important in the next forty-fifty years. The lifetime of this
equipment strongly depends on the condition of the insulation system. The ageing process of oil/paper
insulating systems is a very complex and complicated phenomenon. In order to get a well-supported
decision on the further operation of aged transformers, relevant information would be necessary on the
condition of the oil-paper insulation. We also know that the oil-paper insulation has almost always
inhomogeneous condition considering the temperature, moistening and ageing processes. The classical
methods (insulation resistance, loss tangent, etc.) characterise the insulation by single measured value.
This single value is not sufficient for relevant characterisation and diagnosis of such a complex system
with lots of tons of insulation and with almost always inhomogeneous distribution of temperature, moisture
and ageing product. If we measure the polarization spectra in three states (new insulation in equilibrium
and uniform polarization spectrum, new insulation with not uniform distribution of polarization spectrum,
later reaching again the uniform distributions) the shape of three polarization spectra will be different.
Comparing the polarization and classical methods of this three case we can realize that sometimes the
“classical single values” are almost the same but the polarization spectra are different.
The three response methods provide very practical information about the insulation system (e.g.
moisture, ageing). The moistening and accumulation of ageing by-products change the distribution of
interfacial polarisation in the range of long time-constants. These promising response methods measure
the polarisation distribution in the range of long time-constant (with other words, in the low frequency
range). The three test methods were: Return Voltage measurement (RVM), the C and tgδ measurement
in range of some tens of mHz to 50 Hz (FDS=Frequency Domain Spectroscopy), and the measurement
of DC charging and discharging currents (PDC=polarisation and depolarization currents) up to some
thousands of sec. These equivalent methods (RVM, FDS and PDC) are able to follow the changing of
condition of insulation contrary to classical methods. Therefore a little bit surprising that until now the
convenient standards is missing considering the polarization methods. In an earlier Hungarian research
work (Budapest University of Technology) almost all the necessary measurements have been realized,
consequently we are in possession of fundamental data considering the polarization methods. This paper
would like to show a short review about the RVM technique, the correct interpretation of RVM data and a
case study for the checking of the efficiency of the oil reclamation with RVM technique.
Keywords
transformers; Dielectric Response Methods; Return Voltage Measurement (RVM); condition assessment; ageing; moistening; life management; oil regeneration
Hrčak ID:
199221
URI
Publication date:
15.8.2017.
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