Power network model and voltage stability analysis under impulse charge (Part 2)

Author： GOZ Electric Time：2024-08-10 09:20:51 Read：10

1.4 Field test plan

    The test loop of the substation ground grid impulse grounding resistance is shown in the figure. The center point of the ground grid is selected as the impulse injection point, and the return point is 150m away from the edge of the substation. The voltage reference point is selected 300m away from the site as the common reference ground. The entire impulse loop is connected to the injection pole and the return pole, and the rest is all floating. Because the current generator during the impulse process is not connected to other lines, it can be regarded as an impulse current between the ground grid and the return electrode.

    Calibrate each measuring device, the effective voltage divider ratio of the resistor divider is 1195.35, the standard open voltage is 200kV; the coil is Rogowski, the maximum sensitivity is 101.416A/V; the detection target of the oscilloscope is mainly the intensity of current and voltage, the number of detections is two, the oscilloscope model is Tek2024, and the maximum bandwidth range is 200MHz.

    The impulse generator uses a portable impulse generator, which can generate a standard current waveform with a wave of 2.6us. The generator capacitor is charged by the battery in conjunction with the inverter, and the test voltage levels are 30kV, 45kV, and 60kV respectively.

1.5 Test results and analysis

    According to the test plan, the substation ground grid is tested for impact grounding resistance. From the test results, it can be seen that:

    When the impact voltage is applied to the substation ground grid, the voltage and current waveforms of the ground grid show oscillation attenuation as a whole. This is because the substation ground grid has a large area, and the equivalent ground capacitance and line inductance form an oscillation circuit. Under the action of the impact voltage, an LC oscillation circuit is formed, which causes the ground grid voltage and current to oscillate. This paper selects the impact voltage and current waveform data of the first half cycle as the calculation data of the substation ground grid impact grounding resistance.

    Before the measurement, the grounding wire should be separated from the grounding device first, and the points to be tested should be cleaned. Then install 4 square grounding electrodes with a length of 1m in the site, each with a buried depth of 1m and a diameter of 50mm.

    The impulse current generator is connected to the grounding line, and the number of negative grounding electrodes is 4. After combination, a current return structure is formed. The entire impulse circuit is connected to the injection electrode and the return electrode, and the rest are all floating. The impulse current generator during the test was not connected to other lines. Therefore, it can be determined that the impulse current intensity in the monitored grounding body and the return electrode is consistent, but the phase is different.

1.6 Test results and analysis

    According to the test plan, the impulse grounding resistance of the selected tower is measured. By comparing the changes in the impulse coefficient under the impulse voltage of different amplitudes and the impulse current, it can be found that:

    1. Under the test conditions of this article, the impulse grounding resistance of the transmission line tower grounding body is greater than the power frequency grounding resistance, that is, the impulse coefficient is greater than 1. This is because the tower grounding body shows a strong inductance effect under the action of the impulse current, which makes the potential under the action of the impulse current rise higher than the power frequency current.

    2. Compared with the impulse grounding resistance test of the substation grounding grid, the area of the substation grounding grid is larger, and the corresponding inductance effect is stronger than the transmission line tower grounding body. Therefore, the impulse coefficient of the substation grounding grid is generally greater than the tower grounding body, and the impulse coefficient obtained by the test (about 3) is much greater than the impulse coefficient of the transmission line tower grounding device (slightly greater than 1).

    3. The impulse coefficient of the output circuit tower grounding body generally decreases slightly with the increase of the impulse current amplitude. This shows that the tested tower grounding body has produced partial ionization under the action of the impulse current, thereby reducing the impulse grounding resistance to a certain extent.

    4. In order to further study this property, subsequent research needs to be carried out using a large impulse generator to break through the output power limit of the portable impulse generator, so as to more comprehensively reflect the response characteristics of the grounding device under the action of the impulse current.