The functions of generator start-stop protection and mistaken power-on protection

Start and shutdown protection

　　During the starting or stopping process of the generator, protection for phase-to-phase faults and stator ground fault protection are configured.Since the stator voltage frequency is very low during the starting or stopping process of the generator, the protection adopts an algorithm that is not affected by frequency to ensure the protection of the generator during the starting and stopping process. The input of the above start-stop protection can be blocked through low-frequency components or through the auxiliary contact of the circuit breaker position.

Mistaken power-on protection

1. When the generator was cranking, excitation was not applied and the circuit breaker was mistakenly closed, causing the generator to start asynchronously.

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

3. During the starting and stopping process of the generator, the excitation has been added, but the frequency is greater than a certain value, the circuit breaker is misclosed or out of synchronization.
   The circuit breaker position contact is used and can be switched on and off via control words. The criterion is that the delay time is 0.2s (taking into account the circuit breaker opening time), and the time delay t1 for exiting should ensure the completion of the tripping process. When the generator is closing non-synchronously, if the potential difference between the two sides of the generator circuit breaker is approximately 180°, the non-synchronous closing current will be too large, and tripping will easily cause damage to the circuit breaker. At this time, the trip circuit breaker outlet will be blocked and the excitation switch will be tripped first. When the current of the circuit breaker is less than the set value, the exit switch will be activated.

　　Generator units of 300MW and above are generally equipped with mispower-on protection to prevent misoperation when the generator starts and stops.When a misclosing operation occurs when the generator is cranking or the rotor is stationary, the rotating magnetic field generated by the stator's current (positive sequence current) in the air gap will induce a power frequency or close to power frequency current in the rotor body, causing the rotor to overheat. loss.

　　The principle of mistaken power-on protection is to divide mistaken power-on into two stages. Take 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 be connected to the grid. Therefore, if the generator circuit breaker is required to be closed and the stator has current, it must be powered on by mistake and trip instantaneously. The second stage: from closing the magnetic field switch to paralleling. net. During this period, impedance components are used to distinguish between grid connection and mistaken power-on. Mistaken power-on can generally cause tripping within 0.5 seconds, and the more serious the mispower-on situation, the faster the tripping.

　　The accidental power-on protection will automatically exit the operation after the generator is connected to the grid, and automatically put into operation after being disconnected. The protection introduces the three-phase current of the generator and the two-phase current and two-phase voltage on the high-voltage side of the main transformer or the generator side.