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

Author： GOZ Electric Time：2024-08-08 09:26:57 Read：12

    Generally speaking, the impulse grounding impedance is a complex number, which is the sum of the component currents in resistance, inductance, and capacitance that can affect the current carrying strength of the grounding circuit.

    At present, the various methods and equipment used in the test to measure the grounding resistance are mainly used to measure the power frequency grounding impedance. Long-term monitoring shows that the grounding devices of poles, power plants, and substations receive the most lightning current. Since the lightning current is a very steep wave head and contains an instantaneous single pulse shock wave with extremely rich frequencies, it is necessary to use the impulse current test method to test the grounding impedance of the grounding system.

1 Power frequency grounding resistance test of substation grounding network

    The substation grounding network plays an important protective role for the power frequency grounding network. When the resistance value of the grounding network is large, the voltage strength of the normal phase and the intermediate node when the grounding fault occurs is higher than the standard, and the external insulation protection layer of the equipment will be damaged; in the process of being struck by lightning, the current will increase instantly, which will cause the instantaneous voltage on the line to be too large, causing equipment failure and endangering the safety of operators. Therefore, it is of great significance to regularly detect the grounding resistance of the substation ground grid, evaluate the grounding level of the substation ground grid, and explore the response characteristics of the substation ground grid under the action of impulse current to ensure the safe operation of the substation.

    This paper focuses on exploring the impulse characteristics of the substation ground grid and the transmission line tower, so as to reveal the general change law of the impulse grounding resistance and the impulse coefficient, and provide a certain theoretical reference for practical engineering applications.

1.1 Grounding resistance test method

    In the actual test process, the farther the current electrode is arranged, the better. Under normal circumstances, the distance between the current electrode and the monitored grounding device is d12, and the value should be more than 4 times the diagonal length D of the tested grounding device. If the resistivity in the soil is uniformly distributed, the value should be more than ⒉ times. The length of the wire on the voltage electrode should be 0.618 times the length of the current electrode lead.

1.2 On-site test plan

    When testing the substation ground grid, first untie the connecting bolts between the overhead ground wire and the ground grid. The test was conducted by measuring the grounding resistance of the ground grid using the current-voltage method. To ensure the accuracy of the test, we chose to conduct the test at a relatively long distance. [2] We used 6mm2 current wires to wrap all the lines, and then laid the lines from the measurement entry point to the front of the substation. In order to reduce the grounding resistance, we used puddles on the roadside as the location of the current pole.

1.3 Test results and analysis

    According to the previous measurement scheme, the results of the power frequency grounding resistance of the substation ground grid were measured.

    By observing the results of the long-distance measurement of the small current applied at the power frequency, we can draw the following conclusions:

1. Using the 8000 type power frequency grounding grid to test the system, when the test power frequency is 47Hz, 53Hz, 48Hz and 52Hz, the interference voltage value is 0, and the signal-to-noise ratio reaches the ideal level. The accuracy of the measurement results can be improved by simply increasing the current by 8A.

2. There is a clear relationship between the impedance value of the power grid and the frequency-related inductance part. If the power frequency measurement rule with a deviation of more than 50Hz is used to measure the current intensity, it will inevitably lead to errors in the measurement results. Therefore, it is necessary to use symmetrical frequency to optimize the measurement process and improve the accuracy of the measurement.

    From the results, it can be seen that after the small measurement current increases by 47Hz, 53Hz, 48Hz, and 52Hz, the measured resistance value is relatively stable. For the sake of strictness, 0.538 is selected as the equivalent power frequency grounding resistance measurement value of the grounding grid.