CSS sahara_02_04_instructions.doc
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1) To prevent oxidation of contact surfaces of the off-load tap changer it has to be moved from
side to side a few times, at least once a year.
2) function of alarms and tripping contacts of thermometers.
Inspection Measurements
In addition to the above instructions the following inspection measurement may be carried out:
1. Insulation resistance of windings
HV Winding to Earth
LV Winding to Earth
HV Winding to LV Winding
Reading below 75 megaohms should be reported to the manufacturer.
2. Transformer ratio test
Approximately 220 volts or 400 volts should be supplied from an isolating transformer via a
regulating transformer to one winding of the High-Voltage side of the transformer to be tested. HV
and LV readings can be taken by means of a general-purpose meter. The measurements must be
carried between each phase. The transformation ratio calculated from the voltage reading must be
compared to the ratio of the rated voltages and to the values from the test report. Due to inaccuracy
of the general-purpose meter a difference of a few percentages may be possible. It is advisable to
use a transformation ratio bridge for the measurement.
3. No-load current
A small voltage fed between each phase and the star point (in a star connected transformer) via an
isolating and regulating transformers will result in the reading of a series connected ammeter being
only a ten of milliamperes.
With the same voltage reading the current taken by other phases can be measured. The excitation
currents of different phases should be approximately the same magnitude. In a damage phase the
current can rise to tens of amperes thus the regulating transformer has to be protected by a suitable
fuse.
4.
DC resistance of windings
A satisfactory large DC source and an accurate resistance bridge meter are needed for the
resistance measurement. When disconnecting the measuring current instrument dangerous voltage
can be induced. Therefore a skilled person must perform the measurement.
Opening/Resealing Sealed type Transformers
Sealed type transformers must not be opened unnecessarily as this could cause the oil to come out or air
to go in depending on the temperature of the oil. This results in disturbances of the normal pressure
variations causing increased stress on the tank due to under-pressure or over-pressures.
When opening the transformer, for example to replace a bushing, opening and resealing must be
performed according to these instructions.
Opening
1) If the temperature of the transformer oil is higher than +20...25°C, approximately 5% of oil
must be drained through the drain valve to relieve the pressure.
2) If the temperature of the transformer oil is below +20 °C, no draining is needed as there is
under-pressure inside the tank.
3) Filling plug can then be opened. If necessary the oil level can be lowered by draining through the
drain valve before carrying out the planned maintenance. (If the transformer is provided with an oil
level indicator, it also acts as the filling plug).
Resealing
