Generator set power system problem
2 high-voltage factory with automatic backup power supply switching problem
The high-voltage factory uses the standby power supply to automatically switch (the standby power supply is automatically put into operation). There are two ways to design the Chinese: one is the time-free fast switching, and the other is the low-voltage blocking time-limited switching (that is, the slow switching).
(1) Analysis of the voltage that the high-voltage factory motor is subjected to when the standby power is turned on. After the power supply of the high voltage plant trips for some reason, all the motors on the busbar will be decelerated, and the residual voltage will be generated on the busbar due to the back electromotive force of the motor. This residual pressure will decay over time, i.e., the magnitude of the residual pressure is gradually decreasing, and the phase difference with the system changes over time.
1) When the standby power supply is switched to the bus, a voltage K△U will be suddenly applied to the motor. If the ΔU is too large, the motor will cause a large impact current. If the switching time is shorter, the smaller the △U, the smaller. The switch is more advantageous. In Section 22-1 of the Electrical Engineering Electrical Design Handbook (2), it is stated that when K=0.67, it is safe to switch before 0.3s.
2) The change of the angle θ with time is mainly related to the capacity and characteristics of the large motor on the bus. Therefore, for the different capacity of the generator set θ angle changes with time, that is, the same capacity generator set also varies with the selected main auxiliary machine.
3) Switching is most dangerous when the angle θ is approximately 120° to 240°. The motor will withstand the maximum surge voltage, which may cause motor damage, damage or overcurrent protection trips.
4) If the switching time is too long, on the one hand, the auxiliary machine speed of the unit is reduced too much, which causes the boiler operating conditions to deteriorate. In addition, the residual voltage of the busbar is too low, and the self-starting of the motor after switching is difficult, which may cause disturbance or even stoppage of the operating conditions of the unit, which is undesirable.
(2) Problems with fast switching. According to the motor condition of the auxiliary machine selected by the unit, the high-voltage power consumption switching of some units is designed to be time-limited. Since the selected circuit breaker is a domestically produced oil-less switch, its closing time is about 0.2s, and with the relay action time, the switching time will be as long as about 0.3s, so switching is not safe. The power plant that has been put into operation is the switching mode. The power switch on the busbar of the high-voltage factory should be changed to the vacuum circuit breaker or SF6 circuit breaker with fast closing speed as soon as possible (the closing time is 0.08~0.2s). Switching is safer. Some imported units will limit the fast switching to less than 150ms. If the switching fails and automatically converts to slow switching, this design is more reasonable.
(3) Problems with slow switching. The automatic switching of the domestic high-voltage power plant adopts slow switching. There are also many low-voltage verification and time-locking switching modes, that is, the switching condition has 2:1 bus voltage below a certain value to allow switching; 2 switching is limited to Completed within a certain time. The question now is how to set the low voltage verification relay and the time lock relay. It is necessary to ensure that the switching is successful once, and that the high-voltage factory motor is not subjected to over-voltage surge during switching, and the switching is performed as soon as possible under the voltage that the motor is allowed to withstand.
The setting of the low voltage verification relay must take into account the worst case, that is, the system voltage and the factory bus residual voltage are just reversed when switching. If the voltage drop of the system voltage on the high-voltage factory transformer is about 30V (the specific voltage drop value can be calculated or tested), the voltage of the high-voltage factory motor applied to the busbar does not exceed 1.1Ue when switching. Considering that the low-voltage verification relay contact is closed to the standby power supply for a short period of time, the factory bus voltage is still decreasing, so the setting value of the low-voltage verification relay can be set to 45V, but generally should not be greater than 45V.
The setting of the time-locking relay must ensure that the switching can be completed reliably within the set time, and the setting time cannot be too long, so that the switching of the excessively low voltage may be caused, so that the factory motor cannot be self-started. The correct setting value is best determined by experiment; for example, when the recording unit is in the normal operation mode, the residual voltage of the factory busbar changes with time when the high-voltage power supply is tripped, so as to obtain the lockout under the setting value of a certain low-voltage verification relay. The setting value of the time relay.
