1.
Introduction for a
Diagnostic of Power Transformers
It is in principle about tests applied for test equipment with an aim to detect weak points in insulation system and to determine stage of progressive devaluation of this system as the whole. They are specific terms, which considering the necessity of short-time shut-down of equipment from an operation (it is not recommended to use time demanding methods) and minimisation of number of an operations, which are required to make a machine preparation for a individual measurement.
2. Diagnostic Testing Methods
a) Insulation Oil Analysis:
·
Breakdown Voltage - On the basis of the analysis results we can assume if oil contains
emulsified water, over-saturated gas or conductive impurities. It does not indicate
the stage of operation ageing (fig. 1).
·
Dissipation Factor
- Loss factor indicates presence of polar and ion
substances in oil. Therefore, it indicates oil ageing. Heat dependency (tgd) may indicate presence of foreign soluble matter in oil.
·
Resistivity
- Resistivity indicates foreign matter of conductive
character (including water) present in oil.
·
Relative Permitivity
- This factor provides rough information on oil ageing
degree, but not moistening.
·
Ageing Factor - determines
the quality of new oil. Considering operation it serves to define the ageing
degree.
·
Interfacial Tension of
oil against water - serves
for new oil quality assessment. Considering operation it
serves to define the ageing degree. It reacts sensitively onto soluble sediments creation.
·
Density - represents a
quality factor considering both new and used oil. It is essential for surface
tension calculation.
Fig.1 Measurement of oil breakdown voltage by testing
equipment Megger
b) Thermal and Electric Defects Detection in Oil Transformers:
·
Total Gas Content in Oil - Considering new transformers this factor impacts assessment of
degasification of the insulation system of the machine. Considering hermetic
machines it verifies the sealing effectiveness. It represents factor significant
for detection of kinetics of the thermal defect (fast or slow process) if we
take machines with thermal defects into consideration.
·
Chromatographic Analysis
of Gases Dissolved in Oil - method used for determination of thermal and electric defects in
transformers filled with oil. It enables to detect the defect location,
intensity of the process and kinetics of development (that means the
rate of risk taken during consequent operation of the transformers).
·
Chromatographic Analysis
of Gases from Gas Relay - method used for determination of thermal defects
c) Measurement of Power Transformers Insulation Characteristics - measurement of power transformers insulation characteristics (measurement of loss factor, capacities, insulation resistance, polarization index calculation, non-temporal constants in various connections of measured winding) is to detect the operation ageing degree, insulation systems moisturizing and to detect defect creation location in each section of insulation systems.
d)
Short-circuit Impedance Measurement
under Reduced Voltage - the
assessment of short-circuit impedance changes enables to detect potential
winding deformations. The maximum of method response deals with radial
deformation. Considering axial deformations the response is low.
3. The electric fortresses oil examination
The preparation of samples
-
Directly before filling
examinational container is the sample in
container little by shake and the container is turn over a few times up
bottom like this, so that is provide by decision what the most homogenity diversion messies in
the fluid without production of air bubbles.
-
Is needs to exclude useres exposure samples
to outside´s air.
The examinational container filling.
-
directly before early in the test oneself empty the examinational container
and her walls, elektrodes and others sections is rinse
with testing fluid. The container is emptying and again is slowly filled with testing
fluid, like this so that it excludes the generation air bubbles.
-
Measurement container is placed
into examinational machinery and the mixing is running.
The Voltage Connecting
-
the first voltage connecting is begining about
5 min. after overfilling of testing fluid and after her infus,
whether there are not visible bubbles in a blank between electrodes.
-
The voltage is connecting at
electrodes and equaly is going up by 0 with speeds of
2,0 kV/s ± 0,2
kV/s, while non sequitur breakdown
voltage. The breakdown voltage is maximum achieved voltage and circuit is automaticly interrupted by it.(arc
formation).
-
Measuring data is recording.
-
To execute cycle 6 breakdown
voltage with per se sample with aborted least on 2 minutes every breakdown
voltage ago frequently connection voltage. If a blender is used, this
blender has to be repeated during the test all the time.
-
To calculate the average value of 6 breakdowns and introduction them by Kilovoltage (kV).