The role of generator start-off protection and mispower protection

Start and shut down protection

　　During the generator start or shutdown, the protection of interphase faults and the protection of stator ground faults are provided. Since the stator voltage frequency is very low during the generator start or shutdown, the protection uses an algorithm that is not affected by the frequency, ensuring the protection of the generator during the start and shutdown. The above start-stop protection input can be locked by low-frequency components or through circuit breaker position auxiliary contacts.

Power-on protection by mistake

1. When the generator is rotating, no excitation is added, and the circuit breaker is accidentally closed, causing the generator to start asynchronously.

2. During the start and stop of the generator, the excitation has been added, but the frequency is lower than a certain value, and the circuit breaker is incorrectly closed.
   The low-frequency criterion delay is used to input 0.2s, and the frequency criterion delay t1 returns, and the time should ensure the completion of the tripping process.

3. During the start and stop of the generator, excitation has been added, but the frequency is greater than a certain value, and the circuit breaker is incorrectly connected or inconsistent.
   The circuit breaker position contact is used, and the control word can be cast and withdrawn. The criterion delay is 0.2s investment (considering the circuit breaker opening time), and the time delay t1 exits should ensure the completion of the tripping process. When the generator is closed during the non-simultaneous period, if the potential difference between the two sides of the generator circuit breaker is around 180°, the non-simultaneous period is too large, and the tripping is likely to cause damage to the circuit breaker. At this time, the circuit breaker outlet is locked and the magnetic demagnetization switch first. When the circuit breaker current is less than the fixed value, then the exit switch will be operated.

　　For generator sets of 300MW or above, mispower protection must be installed to prevent misoperation during start and shutdown of the generator. When the generator wheel or rotor is stationary, the rotating magnetic field generated by the current of the stator (positive sequence current) in the air gap will induce the current at the power frequency or close to the power frequency in the rotor body, which will cause the rotor to overheat and lose.

　　The principle of mispower protection is to divide mispower into two stages. Taking the power-on as an example, the first stage: from power-on to closing the magnetic field switch. During this period, due to no excitation, the generator cannot perform grid-connected operation. Therefore, it is required that the generator circuit breaker close and the stator have current, which will inevitably be powered on by mistake and instantaneous tripping; the second stage: from the magnetic field switch to grid-connected. During this period, impedance components are used to distinguish between grid connection and mispower. Incorrect power-on generally, tripping can be achieved within 0.5 seconds. The more serious the error power-on, the faster the tripping.

　　The power-on protection automatically exits the operation after the generator is connected to the grid, and automatically starts to run after decoupling. Protect to introduce the three-phase current of the generator and the two-phase current and two-phase voltage on the main transformer high voltage side or generator side.