Maintenance of Electrical
Equipment
By
Eng. D G R Fernando
Module 3
Transformers and Power Supplies
Things you should know to keep your
transformer healthy
Transformer types
Yearly checks and more frequent checks
Testing of transformers
Vacuum pressure device
Pressure relief device
Sudden pressure relay (Buchholz)
Gaskets
Paints
Transformer Types
Dry type
transformers are
Dry Type
Oil Filled Transformers
Mineral
With
•Sealed Tanks
Oil Filled
Varnished Synthetic
•Conservator Coils and
System oil filled
PCB
•With filled
Nitrogen(No more –Banned)
Blanket
Resin cast windings
Transformer cooling
OF – Oil forced
ON – Oil Natural
AF – Air Forced
AN – Air Natural
IEC Standards for transformers
Maintenance Checks for Transformer
Tests
1. Insulation Resistance
2. Power angle (Loss Angle) test for insulation
3. Thermo graphic test
4. Open circuit test
5. Short circuit test (if needed)
6. Sound Levels
Component Maintenance
Maintaining oil level of the transformer
Maintaining silica gel breather
Check oil level and any trapped air in the
buchholz relay
Check temperature maximum and minimum
Check for leaks
Check paint condition
Check for top of transformer and radiators and
clean it
Clean bushings
Test Insulation Resistance
For a Transformer over 50 kVA the
benchmark value is taken as 50 mega ohms
Test for
However the thumb rule of Loss Angle
the insulation can
be used for windings that is R = kV + 1
I = Leakage Current
Tan Ф = Loss Factor
Ф =Loss Angle
V
Thermo-graphic Tests on
Transformers
Open Circuit Test and Short Circuit
Test
400 V on HV and the voltage on LV
measured and the turns ratio calculated and
the secondary voltage calculated with turns
ratio
Short Circuit Test cannot be performed to its
methods at site however LV Terminals can
be short circuited and 400 V can be applied
to HV. And matched with calculated values.
Oil Level Gauge and Buchholz Relay
Oil level gauge is mounted on the
Conservator Tank. The level differs with
temperature of oil. If Buchholz
Relay operates.
Checking of air
Over trapped
filling of oil in buchholz shall
If the collected be
gases
are not combustible the
done with bleeding
may causeof oil from oil release
transformer is healthy. If
valve bleeding of oil through
of the relay. Combustible the
breather pipe transformer should
not be switched
on again
Temperature
Temperature meters are not very common on
distribution transformers.
Over 1 MVA Transformers are with
temperature meters and maximum
temperature is recorded in the meter
The oil temperature can go up to about 80 0 C
to 900 C hot spot temperature is around 108 0
C. above such temperatures the oil will start
charing
Painting
Galvanized transformers can be touched up
with zinc rich paint
Acrylic paints can be touched
Synthetic oils up with similar
paints such as Si Oil can
withstand a temperature
Powder coats are touched up with various
of higher order
anti-corrosives and matching insulation
with Si oil can
Metal filler compounds can
Withstand be used if
Higher
needed. Temperature
Oil Leaks
WeekTypes
points are welded
of the edges and flanges
gaskets
tightened with gaskets, such as top cover
•Cork
and the bushings etc.
•Corkprene
Remedial measures are replacing gaskets oil
•Nitrile Rubber
seals.
•Viton Rubber
Oil leaks on the radiators can be treated with
metal fillers.
Cleaning
General cleaning on transformer.
Contamination on the transformer
The unit for measurement of contaminationbusing can
is called Equivalent Salt Deposit Degree
be measured. ESDD, and given as mg/m 2
When ESDD is higher the surface
flash over occurs. Therefore the
Insulators to be cleaned well.
The insulators flashed over can be
repaired by painting with Sylguard
from Dow Corning or CSL
Transformer Oil Tests and
Maintenance
Tests
Oil Analysis –Dielectric, Water content, Flash
point, Surface tension, Acidity, Loss angle
etc. Types of Oil
Gas in Mineral Oil
Oil Analysis.
Silicon Based Oil
Synthetic Oil
Ester/ Vegetable Oil
Categories of Equipment
Pg No 51 of IEC 60422 – Table 2 –Frequnch
recommended for Break Down Voltage
Sound Level according to VDE Standards
Rated Power Sound Level Db Sound Level Db
KVA for oil immersed for Cast Resin
transformer transformer
50 55
100 59 59(51)
160 62 62(54)
250 65 65(57)
400 68 68(60)
630 70 70(62)
1000 73 73(65)
1600 76 76(76)
2500 81 79(79)
Recovery of Oil
On line oil purifier with vacuum filter
Filter removes particles larger than
3 micro meter
Vacuum filter removes moisture up
to 3 ppm
85 % of gasses will be removed.
Gas in oil test
Chromatographic test – Analysis of dissolved
gasses in oil.
High Temperature of
Partial Discharges
oil formsInternal
Methanearcing
CH forms
formsAging and Heating 4
Ethane Acetylene
C2H6 and
H2 Forms C2H2
Ethylene
CO and COC2H4
2
Life of Transformers
Over 30 years some f the transformers of
utilities are operated to failure.
Loading guides and standards suggest the
calculation of reduction of life span due to
over loading and other factors
Loading Guide – IEEE C 57.115 and IEC
Life time calculations IEEE C 57 .91
Power Supplies and UPS
Maintenance of power supplies are simple
Battery packs are nowadays sealed type and
maintenance free
Self trouble indications are avilable
Features of the ups‘s
Monthly automatic battery test – by this the data of battery to be
changed is given.
Deep calibration test – this gives an battery actual capacity
Boost charging -enables fast recharging of batteries
Temperature compensated battery charger – Keeps battery life
longer
Load Dependant end of discharge voltage -Discharge voltage
depends on discharge current. However the voltage is lowered
without allowing over discharge.
No load shut down and on load pick up by by-pass
arrangement.
On board event logging
Input power factor 1
Available for 3 phase input and single phase output
Thank You
