Automatic switching device and switching time of EPS power supply

　　In order to realize automatic switching between AC power supply and inverter power supply, the EPS power supply must be a backup type according to national standards. Therefore, the automatic switching device is an indispensable component of the EPS power supply and one of the key components affecting the reliability of the EPS power supply. Therefore, EAST specifically conducts the following analysis and explanation on the automatic switching device.

　　According to the output capacity and load requirements of the EPS power supply, the automatic switching device can be composed of power relays, AC contactors, mutual switches, solid-state switches (thyristors), etc. The switching time requirements for EPS are diverse. For example, the general fire emergency lighting requires a switching time of less than 5s, but the fire emergency lighting used in high-risk areas requires a switching time of less than 0.25s. When the high-pressure gas discharge lamp is powered without extinguishing, the switching time is required to be in the order of milliseconds. When the loads such as fans, pumps, rolling doors, elevators, etc. are powered, the switching time will also be in the order of milliseconds according to different application requirements.

　　The difference between EPS power supply and UPS power supply is that most applications do not have very strict requirements on switching time, and the requirement for switching time is not the shorter the better. Under the premise of meeting the application requirements, switching slowly can benefit in other aspects, such as reducing losses, reducing transient shocks, improving reliability, etc., and avoiding load malfunctions due to momentary power outages. When the mains power is normal, the inverter of EPS power supply is basically in standby state, and there are two working modes: cold backup and hot backup. In cold backup, only the control part of the inverter is in working state, and the power part is in power-on standby state, but not started; in hot backup, the entire inverter is in normal operation, but does not bear the load. When the inverter is hot backup, the shortest switching time is basically determined by the action time of the switching device used; and when the inverter is cold backup, the shortest switching time is also subject to the constraints of the inverter's action time. Especially for EPS power with large capacity, if it starts too fast, the inverter transformer and low-pass filter will produce a large transient impact, and may even damage the IGBT power device. Therefore, the inverter generally has a soft start feature, and the greater the power, the slower the start. The start time of a large-capacity EPS inverter can be as long as several seconds. If a faster switching time is required, only hot backup can be adopted. At this time, the standby loss of EPS will naturally increase a lot, and the overall efficiency will be lower.

　　As for which switching device to use, it is mainly based on the requirements for switching time. If the switching time is required to be in the millisecond level, then only solid-state switches (thyristors) can be used for switching, and the inverter must be in hot standby state. Compared with mechanical switching switches of the same capacity, the cost of solid-state switches (thyristors) is much higher, and the on-state loss will be relatively more. In applications where there are no stringent requirements for switching time, mechanical switching switches are generally used for switching. EPS power supplies with smaller capacity generally use power relays, and EPS with larger power usually use interlocked AC contactors or automatic mutual switching switches. Compared with AC contactors, automatic mutual switching switches are slower, but because mutual switching switches have mechanical self-holding characteristics, they have more advantages in long-term reliability for infrequent switching.

　　Although the technology of using solid-state switches (thyristors) to achieve fast switching between mains power and inverter output has been used in UPS power supplies for many years, it is also different. UPS uses power relays (or contactors) and solid-state switches (thyristors) to combine into a bypass (BYPASS) switching device. The solid-state switch (thyristor) is mainly used for instant overload bypass (BYPASS) switching, which instantly enables the inverter and the power grid to have a short parallel operation, thereby obtaining instant power supply without switching time (making up for the transit time of the power relay or contactor). The key is to achieve phase-locked operation of the inverter and rapid detection and tracking of the instantaneous voltage of the mains. It is not a real power-off switching, because this method is used by "online" UPS. The solid-state switch (thyristor) does not participate in the work during the real power-off switching. When it is used for EPS power supply, it is the solid-state switch (thyristor) that truly participates in the power-off switching. EPS power supplies are all backup type and do not have bypass contactors. That is, when the AC power is normal and the inverter is operating normally, it is technically possible to perform uninterrupted switching. However, the actual situation is that the switching needs to be performed when the AC power is suddenly interrupted or fails. Since the time of the AC power interruption or failure is random and unpredictable, it takes time to detect and confirm the AC power failure. The switching time at this time cannot be less than the time required to detect and confirm the AC power failure.

　　In order to prevent malfunction caused by various power supply interference, the detection time cannot be too short. Practice has shown that when the detection time is less than 2ms, the detection reliability will be significantly reduced. Therefore, a switching time of less than 2ms is not advisable. Among the various loads used by EPS power supplies, the most demanding switching time should be high-pressure gas discharge lamps. Although this type of lamp is not allowed to be used for fire emergency lighting, it is used in many large venues due to its high-intensity and high-efficiency performance characteristics. But there is such a problem, that is, once this type of lamp goes out, it needs to cool down before it can be restarted. In order to ensure that the lighting is not interrupted, the EPS power supply must have a fast switching capability. According to the test of a variety of high-pressure gas discharge lamp products, if appropriate follow-up measures are not taken, a 5ms power interruption may cause the light to go out, and some products may even go out with a 3ms power interruption. For some elevator loads, millisecond-level switching is obviously unnecessary, but the instantaneous power interruption during switching may also cause the elevator control system to enter a protection state. In this case, the switching time needs to be appropriately increased through the delay of the EPS power control system to ensure that the elevator continues to operate normally after the emergency power supply. In some applications, in order to achieve zero switching, the EPS is required to be designed for online operation. In this case, the EPS power supply has actually become a dedicated UPS, and the inverter works under long-term load.