Video: TAND220A

Video transcript

What tan delta and resistivity say about an insulating oil, how the TAND220A measures them, and how the test is done. The conductivity of an insulating oil rises with the ionic impurities that get into it, or that form inside it as it ages. Dissolved moisture, on the other hand, hardly changes it. That is why conductivity, and tan delta with it, describe the quality of the oil so well. Four figures describe the oil.

Conductivity, in picosiemens per metre. Resistivity, which is simply its inverse. Relative permittivity, how much the oil stores charge compared with vacuum. And tan delta, the dissipation factor: how much of the current is loss. In a perfect insulator, the current leads the voltage by exactly 90 degrees. Losses add a small resistive current, in phase with the voltage. Delta is how far the total current falls short of 90 degrees;

tan delta is the resistive current over the capacitive one. The oil is excited with a square wave of plus and minus 30 volts at half a hertz, with no direct component. At each reversal, a short capacitive pulse gives the capacitance. Between reversals, only the flat resistive current flows, and it gives the conductance. This square wave method is far more sensitive than the classic bridge. It resolves conductivities down to 0.01 picosiemens per metre,

and tan delta down to one in a 1000000 at 50 hertz, so the oil can be characterised at room temperature. Conductivity from 0.01 to 20000 picosiemens per metre, in 4 ranges, within one percent plus one digit. Relative permittivity up to 5. 2.6 kilos, on AA batteries in the field or on mains in the lab. Two displays: relative permittivity on the left, conductivity on the right. Below them, the calibration knob and the range selector. A switch sets the permittivity scale, below or above two.

The cell connects by two coaxial cables, with the thermometer in its lid. The cell has two parts, a container and an active part. Two coaxial stainless steel electrodes hang from the lid, one and a half millimetres apart, each with its own BNC connector. The oil fills it to 23 millimetres below the rim, about 210 millilitres. Six steps. Clean the container and the active part, which is never taken apart. Adjust the empty cell on the calibration knob. Fill with 210 millilitres. Close it and insert the thermometer. Connect both cables.

Choose the range, and read. Typical conductivities at room temperature give a first reading. New laboratory oil, up to 0.05 picosiemens per metre. New industrial oil, up to 0.1. Lightly used, up to one. Middle use, up to 5. Above 5, the oil is heavily used and in bad condition. A spreadsheet program supplied with the unit takes the temperature, conductivity and permittivity, and gives resistivity, tan delta, power factor and the discharge time constant at the temperatures of interest.

That extrapolation depends on the oil's activation energy. A typical value can be used, but over large temperature steps the uncertainty grows. The optional heater holds the sample at 90 degrees, within a 10th of a degree: with one reading at room temperature and one at 90, the software computes the oil's own values. For continuous monitoring there is a flow through cell, all in stainless steel, for indoor or outdoor use, to IEC 61 6 20 and 62 47. Oil enters at the bottom and leaves at the top.

The recommended circuit. The main oil flow runs along the bottom, for example a transformer cooling circuit. A bypass branches off through a shut off valve and rises through the cell. It rejoins the main flow through a second valve, at point one to 3 litres per minute. A purge valve sits at the top. It is used to test and document insulating liquids in transformers, bushings, cables and capacitors, in manufacturing and maintenance,

as a failure alert, and in ageing and purity studies of oils. Insulating oil testing, from Amperis.