When the generator is not operating in full-phase mode, it often occurs during the process of disengagement or when the main circuit breaker gets stuck or splits during closing or opening. In some cases, only one or two phases of the main circuit breaker are operated, which may lead to a loss of pressure and cause operational issues.
The phenomenon of non-full phase operation in a generator can be identified through several key indicators:
a. Abnormal three-phase current readings on the high-voltage side of the main transformer, with one phase showing zero current.
b. Severe imbalance in the three-phase stator current of the generator.
c. A significant increase in the negative sequence ammeter reading.
d. Two of the three-phase currents are equal or nearly equal, while the third phase shows about half the value. This suggests that the main transformer's high-voltage side switch has disconnected on two phases.
e. If the main transformer neutral point knife is closed, two of the generator’s three-phase currents are equal or close, and the third phase current is zero or near-zero. Alternatively, if the neutral point knife is open, the three-phase current could also be zero or near-zero, indicating a two-phase disconnection in the main circuit.
f. The presence of zero-sequence current (or voltage) at the generator set’s neutral point.
To address this issue, the following measures should be taken:
If the generator is not protected by full-phase protection and the circuit breaker failure protection activates, the unit should be shut down immediately. Otherwise, follow these steps:
a. Immediately switch the AVR to manual mode.
b. If one or two phases of the generator circuit have tripped, causing the generator to run in an unbalanced state, reduce reactive power and load current to safe levels.
c. Do not close the main steam valve or disconnect the de-excitation switch before opening the generator’s main switch.
d. If asymmetric overload protection trips, handle it as an emergency.
e. When the generator’s main switch fails to fully connect or disconnect, quickly reduce the output to a two-phase state and attempt to isolate the main switch. If it cannot be fully disconnected, report to the control room and perform a reverse bus operation using the bus-coupled switch to separate the generator from the system.
f. After experiencing negative sequence current impact, if the I²t exceeds allowable limits, inspect the rotor thoroughly for damage. Metal Channel Letter,Mirror Finish Metal Letters,Brushed Metal Channel Lettering,Brushed Metal Letter Signs Wuxi Motian Signage Co., Ltd , https://www.makesignage.com