How to detect faults using clamp ammeters in DC grounding systems
If there is a problem with the grounding of the DC busbar system in transformers, substations, and distribution rooms, testing or searching is a very troublesome task. Usually, staff use a multimeter for segmented search, which not only affects work efficiency but also makes the search unsafe. It is necessary to measure the DC line section by section to remove some high-voltage equipment from protection, which seriously affects safety issues.
The method of online searching for DC bus fault grounding without affecting the operation of the DC system. The grounding selection of the substation DC system mainly adopts low-frequency signal injection method, DC leakage current detection method, and pull-through method. The injection method is susceptible to the influence of distributed capacitance; The pull-in method cannot select the grounding branch with parasitic circuits; The DC leakage current detection method requires the installation of DC leakage current sensors in each branch. Due to device cost constraints, sensor sensitivity cannot be guaranteed, and line selection sensitivity is limited.
None of the three methods can ensure correct line selection. Therefore, the DC leakage current detection method has been slightly improved by changing the measurement of leakage current to a high-precision DC clamp type ammeter. When using a DC clamp leakage current meter to test the grounding of the positive or negative conductors of the substation DC system, the maximum grounding leakage current is observed; When the grounding resistance is high, the leakage current is small, which may be less than 1mA. Therefore, clamp type ammeters that directly measure ground leakage current must meet the following requirements: 1. be able to measure the clamped wire core (single or multiple cores), 2. be able to test the resolution very small (microampere clamp type ammeter), 3. be high in accuracy and have small errors
Clamp the positive and negative wires of the DC line together to measure DC leakage. Measure AC leakage by clamping the live and zero wires of the AC together. Clamp the ground wire and measure the leakage current of the ground wire. Clamp the main line and measure the current of the main line.
