How to determine the moisture content of the insulation of electric motors and transformers

Determination of moisture content in the insulation

The moisture content of the insulation is usually determined to decide whether it is necessary to dry the hygroscopic insulation of electrical machines and transformers. Methods for determining the degree of humidity of insulation are based on the physical processes occurring in the insulation when voltage is applied to it.

Insulation capacity can be presented geometric capacity determined by the geometric dimensions of the insulation, and absorption capacity, i.e. a container formed in the thickness of the insulation by inhomogeneities of the insulation material, as well as by various inclusions in the form of air gaps, moisture, pollution etc.

When a voltage is applied, a charging current with a geometric capacitance flows through the insulation at the first moment, which quickly stops due to the charging process of this capacitance.

The absorption capacity does not appear immediately after the voltage is applied to the insulation, but some time after the loading of the geometric capacity as a result of the subsequent redistribution of charges in the thickness of the insulation and their accumulation at the boundaries of the individual layers, which, due to inhomogeneities, form a circuit anyway of series connected capacitances. The charging of the respective individual containers (polarization) leads to an absorption current in the insulation.

After termination of polarization, i.e. charge of the absorption capacity, the absorption current becomes zero, but the leakage current continues to flow through the insulation (leakage current), the value of which is determined by the resistance of the insulation to current.

Determination of moisture by absorption coefficient based on a comparison of megohmmeter readings taken at various intervals after applying voltage.

Cab = R60 / R15

where R.60 and R15 — the insulation resistance measured 60 and 15 s, respectively, after the application of the megohmmeter voltage.

For a non-moistened coil at a temperature of 10 — 30 ° C, Kab = 1.3-2.0, and for a moistened coil, the absorption coefficient is close to unity. This difference is explained by the different charging time of the absorption capacity of dry and wet insulation.

The value of the absorption coefficient is strongly dependent on the temperature of the insulation, so values ​​measured or reduced to the same temperature should be used for comparison. The absorption coefficient is measured at a temperature not lower than + 10 ° C.

The determination of humidity by capacity and frequency is mainly carried out when testing power transformers.It is based on the fact that the capacitance of non-wetted insulation changes less (or not at all) with a change in frequency than the capacitance of wetted insulation.

Insulation capacity is usually measured at two frequencies: 2 and 50 Hz. When measuring insulation capacitance at a frequency of 50 Hz, only the geometric capacitance, which is the same for dry and wet insulation, has time to appear. When measuring the insulation capacity at a frequency of 2 Hz, the absorption capacity of the wet insulation has time to appear, while in the case of dry insulation it is less and charges slowly. The temperature during the measurements should not be lower than + 10 ° C.

The ratio of the measured capacitance at 2 Hz (C2) to the capacitance at 50 Hz (C60) is about 2 for wet insulation and about 1 for non-wet insulation.

Determination of moisture content of insulation transformers by power and temperature

Insulation can be considered non-humidified if (C70 — C20) / C20 < 0.2

The capacitance of the coils can be measured either using a P5026 type bridge at the same time as the measurement dielectric loss tangent, or with a voltmeter — an ammeter. The temperature of the transformer windings is measured with a thermometer installed in the upper layers of the oil or is set by the resistance of the copper winding.

Determination of moisture content in the insulation of power transformers by increasing the capacitance for 1 s.

Charging the insulating capacitance and then discharging it, measure the capacitance of object C and the increase in capacitance dC in 1 s due to the absorptive capacity, which has time to appear in 1 s for wet insulation and no time for dry insulation.

Behavior dC / C characterizes the degree of moisture content of the insulation of the transformer windings. Behavior dC / C depends on the temperature of the insulation and must be measured at a temperature not lower than + 10 ° C.