Insulation resistance and DC resistance test

        Under the action of the DC voltage, a current will pass through the insulation, as shown in Figure 2-1, from the beginning of the high current drop to a stable value. The total current in the figure can be divided into three currents.

        (1) Leakage current i _L . This current is generated by the movement of ions, the size of which depends on the conductivity of the dielectric. As the current increases, the insulation resistance decreases, which is essentially independent of time.

        (2) Capacitance current i _C . This current is formed by rapid polarization, which is rapidly reduced over time up to zero since the fast polarization is done instantaneously.

        (3) Absorb current i _B . This current is a charging current formed by slow polarization (ion movement) which slowly decreases with time, which is related to the moisture content of the device under test.

        The synthesis of the above three currents is the total current i. Figure 2 shows the equivalent circuit of the tested insulation. Let R _L denote the pure resistance of the leakage loop; C denote the geometric capacitance of the dielectric; R _B and C _B denote the resistance and capacitance of the absorption loop; E denotes the applied DC voltage.

        It can be seen from the absorption curve (see Figure 2-1) that the capacitive current and the sink current approach zero after a period of time, so i approaches i _L . The so-called insulation resistance is the ratio of the DC voltage applied to the sample to its leakage current, which is related to the temperature. Its calculation formula is

        i _L — leakage current corresponding to E, μA.

        As can be seen from Figure 2-1, the ratio of the initial current to the steady-state current can be used to indicate the degree of moisture in the insulation. In actual engineering, the ratio of the insulation resistance at 60 s and 15 s is expressed as the absorption ratio. R _{60s is} generally required, /R _15s ≥1.3. For large-capacity equipment with a long absorption process, the polarization index can be used to judge whether the insulation is damp, that is, the ratio of the insulation resistance at the time of pressurization for 10 min and 1 min. Since the polarization index is measured for a long time, it is independent of temperature.

        2. Insulation resistance tester (also known as megohmmeter , commonly known as shaker) category and wiring method

        The instrument for measuring the insulation resistance and absorption ratio is a megohmmeter. There are four types according to the voltage level: 500, 1000, 2500, 5000V, etc., which can be selected according to the requirements of the "pre-regulation"; according to the structure, there are three types: hand-operated, transistor-based and digital.

        Generally, the megohmmeter has three terminals: one terminal is marked with "L", which is called a line terminal, which outputs a negative DC high voltage, which is connected to the high voltage conductor of the test object during measurement; the other terminal is marked with "E". It is called the grounding terminal, which outputs positive DC high voltage. It is usually connected to the outer casing or ground of the test object when measuring. The third terminal is marked with "G", which is called the shielding terminal, and is connected to the shielding ring of the test object during measurement. Above, used to eliminate the effects of surface or other leakage currents that do not need to be measured. Figure 2-3 shows the three wiring methods for the insulation resistance of the cable with a megohmmeter.

        3. The test procedure of the megohmmeter (take the hand megger as an example)

        (1) Select the megohmmeter. It should be selected according to the requirements of the “pre-regulation”. For equipment with rated voltage below 1000V, use a megger of 1000 V; equipment with rated voltage of 1000 V and above, use megohm of 2500V. Table; for special conditions, a 5000V megger can be used.

        (2) Check the megohmmeter. The inspection method is as follows: first open the connection between the terminals of the megger, and shake the handle according to the rated speed of the watch (about 120r/min) (the opponent shakes the megohmmeter), the hands should refer to "∞"; then the "L" and The “E” terminal is short-circuited, the handle is shaken, and the hands should be "0". If the hands are not correct, they need to be replaced or repaired before use.

        (3) Power off and discharge the equipment under test. Before testing the equipment in operation, verify that the equipment has been powered down and then adequately discharge the ground. For large capacity equipment, the discharge time is not less than 2 minutes.

        (4) Wiring. Wire as described above. The connection between the megohmmeter and the equipment under test should be as short as possible, and the connection between the line and the ground terminal should be well insulated from each other.

        (5) Measure the insulation resistance and absorption ratio. Keep the megohmmeter at the rated speed, evenly shake the handle, observe the indication of the hands, read the insulation resistance of 15s and 60, respectively, and use R _60s as the insulation resistance value of the equipment under test. After the reading is completed, disconnect the line terminal of the meter from the test object and stop the swing. Otherwise, the megger may be damaged due to the reverse charging of the test object. Pay more attention to this for large capacity devices.

        (6) Discharge of the test object. After the measurement, the test object is fully discharged in response to the ground. For devices with large capacity, the discharge time should be no less than 2 minutes.

        (7) Recording. Immediately after the measurement, the contents shall include the equipment name, serial number, nameplate, operating position, insulation temperature of the test, humidity on site, measured insulation resistance value and absorption ratio.

        (3) Various influencing factors (such as humidity, temperature, surface contamination, effects of residual charge of the equipment, etc.) should be fully considered and corrected.

Second, the measurement of equipment DC resistance

        The measurement of DC resistance of electrical equipment is a very important and indispensable test item in the Pre-Regulation. Although this test is not directly related to insulation, it is an effective way to find conductor defects in electrical equipment. There are many methods for measuring DC resistance, each with its own characteristics and measurement range. This section mainly introduces two commonly used measurement methods, namely voltmeter-current meter method and bridge method.

        Voltmeter-current meter method

        When the DC resistance is measured by the voltmeter-current meter method, the accuracy of the measurement result is not high, and the measurement process is complicated. However, this measurement method has a great advantage, that is, its measurement condition is basically the same as the actual working condition of the measured resistance, and is particularly suitable for measuring the nonlinear resistance.

        There are two types of wiring for measuring DC resistance with a voltmeter-amplifier method, as shown in Figure 2-4. In the actual measurement, no matter which measurement wiring is used, since the meter itself always has a certain internal resistance, after measuring the voltage U and the current I value, the DC resistance Rx obtained by the relationship of Rx=Ux/Ix is not true. It is the value of the side resistance R, but there is an error due to the measurement wiring. The measurement errors produced by the two wirings in Figure 2-4 are discussed separately below.

        Figure 2-8 shows a measurement wiring for a portable Kelvin bridge that is suitable for measuring 0.0001 to 2020 resistance in the field. The measurement methods and steps are as follows:

        1) Unplug the galvanometer lock and let the galvanometer pointer swing freely to zero.

        3) First estimate the approximation value. Refer to Table 2-1 to put the bridge's override switch to the corresponding position.

        4) Press the battery button E of the bridge first, then press the galvanometer button P, and rotate the slide dial to adjust the galvanometer indicator to zero.

        5) Read the dial reading and multiply the reading by the selected bridge multiplier. The resulting value is the value of the measured resistance Rx. It should be noted that when the slide dial is rotated, the galvanometer pointer is always adjusted to zero, but it is greatly deviated to the left or right, indicating that the original selected magnification is not suitable. The override switch should be turned slightly larger or slightly. The small first gear is re-adjusted until the galvanometer pointer points to zero.

        6) When measuring the DC resistance of the motor or transformer winding, special attention should be paid: press the battery button E first, then press the galvanometer button P; when disconnecting, the P must be released first to avoid the inrush current damage to the galvanometer. .

        7) After the measurement is finished, first release the button P, then release the button E to remove the measuring lead. Be sure to push the galvanometer lock on the pointer so that the pointer is no longer deflected to prevent damage to the pointer due to sloshing of the galvanometer loop when moving the bridge.