Tettex MIDAS 2881, Operating Instructions Manual

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Applications Guide 137
Balance’ should be adjusted to make sure that the balance pointer stays in the green zone (to keep
the resonance inductor in proper resonance). The ‘Resonating Inductance Balance’ display which
button (L+ or L-) should be pressed to return to a proper balance.
When measuring relatively low capacitance values at high voltages, with the resonance inductor (e.g.
50-180nF), it is advised to add in parallel a 200nF(or bigger) capacitance to keep the forces under
control. High voltages with low capacitance values require a very small airgap inside the resonating
inductor, causing extreme forces between the 2 core-halves, slamming them together from time to
time. This causes loss of resonance due to the high current peaks distracted from Midas.
14.4 Liquid Insulation
To test liquid insulation a special oil test cell has been constructed. The oil test cell is basically a
capacitor with a liquid insulation as a dielectric constant. The test cell is supplied in an insulated case
for simple transportation and for use as insulation of the cell from ground during the test. After each
test the cell should be cleaned. If the same type of liquid will be tested, it is sufficient to flush the cell
by a portion of the new oil sample, or other oil of the same type. If the cell will be used to test a
different type of liquid insulation or is dirty, it should be cleaned with a suitable solvent properly. After
cleaning with solvent the cell should be dried. The cell shouldn’t be wiped out with rags to avoid cotton
fibers, etc., to be left in the cell and affect the test results of the sample.
To test a representative sample of liquid insulation any dirt or water in the sample should be
avoided.
The volume of the test cell is approximately one liter. It should be filled until there is about 2cm of
liquid above the top of the cylinder inside the cell; when the cover is replaced, the cylinder of the inner
cell should be covered with liquid. If there is an insufficient amount of liquid in the cell, sparking may
take place above the liquid level.
The test cell should be placed either at the bottom of the plastic case, or on a suitable insulating
material. The reason for undesirable breakdown could be caused by air bubbles, water, and other
foreign material in the cell. To prevent such breakdowns the sample should be allowed to settle down
before testing. Air bubbles could evaporate and any foreign particles can settle to the bottom.
By rotating slowly the seated inner cell, air bubbles can be released through holes in the inner
cylinder.
The test cell is built on the “Outer Cell Electrode” and the
removable “Inner Cell Electrode with Cover”
Dissipation factor test cell 6835 for liquid insulation including transportation case
14.4.1 Test Procedure
The high-voltage should be connected to the handle on the inner cell by using the high voltage cable.
The V-potential should be connected to the metallic ring on the inner cell cover, using delivered V
connection. The outer cylinder should be insulated from ground and connected either to channel A or
B of the measuring bridge by using special connection cables. A clearance of several centimeters
should be maintained between the HV connection and the ring which is connected to V-potential, so
that flashover will not occur between these parts.
The test voltage should be raised to 10 kV. The radial electrode spacing of the cell is about 6.7 mm,
the sample should not break down at this voltage unless it is in very poor condition. If a breakdown
occurs before 10 kV is reached, then attempt a measurement at some lower voltage (e.g. 2 kV).
Before the sample is tested, its temperature should be taken. If MIDAS is used the actual temperature
of the sample should be set in Setup – Condition – Temperature and a Temperature Correction Factor