The DC loop operation is a critical task for substation and power plant operators. It involves checking the grounding point in the DC system when a fault occurs, as well as managing the activation or deactivation of relay protection and automatic devices. However, improper execution of such operations can lead to malfunctions in protective systems, making it essential to follow strict safety procedures.
1. **Operation of DC Control Fuses (i.e., Circuit Breaker Fuses)**
(1) When removing a DC control fuse, always remove the positive terminal first, followed by the negative one. When reinstalling, reverse the order—start with the negative terminal, then the positive. This prevents parasitic loops that could cause unintended tripping of protection devices. Fuses should be handled quickly without repeated on-off cycles. There should be at least a 5-second interval between removal and reinstallation.
(2) Before deactivating the DC power supply for an active protection device, make sure to disable the protection outlet connection piece first. The restoration process follows the reverse order.
(3) For bus differential and failure protection systems, the outlet connection piece must be disabled before the DC fuse is removed. After restoring the DC circuit, check if the entire device is functioning properly. If necessary, use a high-impedance voltmeter to measure the voltage across the outlet connection piece before reconnecting it.
(4) When performing a power outage on a circuit breaker, the control fuse should only be removed after the circuit breaker has been opened and proper safety measures (like grounding or insulating covers) are in place. Removing the fuse before opening the isolating switch may prevent the circuit breaker from tripping in case of an accidental closure, which could worsen the situation.
(5) During a power-up operation, the circuit breaker’s control fuse should be installed before removing the safety measures. This allows for a quick check of the protection and control circuits. If any issues are found, they can be addressed while the safety measures are still in place. Moreover, having the protection system ready ensures that it can respond effectively in case of an unexpected fault. Installing the fuse after closing the isolating switch might result in the circuit breaker failing to trip during a load condition, potentially leading to more severe consequences.
2. **Closing Fuse Operation**
The closing fuse refers to the fuse used in electromagnetic operating mechanisms. When a circuit breaker is being powered off, the closing fuse should be removed only after the circuit breaker has been disconnected. This prevents accidental closing due to external factors. If the fuse is removed before the isolating switch is opened, there is a risk of pulling the isolating switch under load, which could lead to serious accidents.
Similarly, during a power-up operation, the closing fuse should be installed after the isolating switch has been closed but before the circuit breaker is engaged. This ensures that the protection system is ready and reduces the risk of faults caused by incorrect sequence of operations.
Following these guidelines not only enhances operational safety but also minimizes the chances of equipment damage and system failures. Always ensure that all steps are carefully reviewed and executed according to established protocols.
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