UPS power supply system
Time of issue:2021-11-29
4. UPS power supply system
4.1 UPS working principle
UPS is mainly composed of rectifier filter circuit, charger, inverter, output transformer and filter, static switch, battery pack and control, monitoring, display alarm and protection circuit.
When the mains power is normal, the input voltage passes through the rectifier and filter circuit to provide voltage to the inverter all the way, and send it to the charger to charge the battery all the way. At this time, the static switch is switched to the inverter, and the inverter completes the functions of voltage stabilization and frequency tracking.
When the mains fails, the UPS is working in a backup state, and the static switch is still switched on the inverter side. The inverter converts the DC voltage of the battery into an AC voltage and outputs it to the load through the static switch.
When the mains power is normal, the inverter fails or the output is overloaded, the UPS works in the bypass state, the static switch is switched to the mains end, and the mains directly supplies power to the load.
4.2 Four elements of UPS
The four elements of a high-availability UPS: reliability, functionality, availability, and fault tolerance.
Reliability: UPS modules, static switches and power distribution equipment must be very reliable, as measured by MTBF. In addition, system equipment should be as simple as possible to minimize single points of failure.
Functionality: It should be able to protect the load from all mains power interference. The interference that UPS of different technologies can protect is different.
Availability: All power supplies in the system must be allowed to be maintained at the same time. When some components of the system are maintained, the system can still supply power to the load normally. The true maintainability is related to the redundancy of the system, but the system should have internal or external maintenance bypass.
Fault tolerance: The system must have fault tolerance to handle the failure of system components without affecting the power supply of the load equipment.
Reliability and functionality mainly depend on the internal technology of UPS, that is, the use of passive standby, line interactive, double onversion and other technologies.
Availability and fault tolerance mainly depend on the redundancy mode of the UPS and the power distribution circuit scheme
4.3 UPS classification
Commonly used UPS systems are generally divided into two categories: standby redundant systems and parallel redundant systems.
In a backup redundant system, one power supply is supplied with power and the others are in standby. Once the running power supply fails, the backup power supply is immediately put into operation.
In a parallel redundant system, multiple power supply devices are supplied in parallel, and under normal working conditions, the output power of each power supply device is lower than its rated output power.
4.4 UPS working mode
Stand-alone working mode
Serial backup working mode
Parallel redundant working mode
4.4.1 UPS stand-alone working mode
Stand-alone working mode is the most common and basic working mode of UPS. It is generally used in general load occasions where power can not be cut off, and its reliability is poor.
The UPS stand-alone system has no capacity redundancy and cannot protect the internal module itself from failure. Nor can it protect equipment from malfunctions. Therefore, the internal modules, systems, and power distribution of the UPS cannot be maintained at the same time; there is no fault tolerance for the internal modules and power distribution. Therefore, the stand-alone system is only suitable for the load that allows the UPS to stop for 2 to 4 hours for maintenance, during which time it can be directly powered by the mains power supply with various interferences. For applications requiring higher availability, the double-conversion UPS stand-alone system is not suitable.
4.4.2 UPS series backup working mode
The dual-system hot backup is also to greatly improve the reliability of the power supply system. It is the same as the dual-system parallel connection, and it is also used in particularly important occasions.
Its working mode is: UPS2's output is used as the bypass input of UPS1. Normally, UPS1 is in the main state and assumes 100% of the load, and UPS2 is in the hot backup state; when UPS1 fails, UPS2 is converted to the main state and assumes the full load; If both UPS1 and UPS2 are faulty, the utility power will directly supply power to the load through the static bypass switch.
Disadvantages: The aging degree of the main and standby machines is different, which is easy to cause the switchover to fail. Or need to be switched regularly.
4.4.3 UPS parallel redundant operation mode
The necessary conditions for the parallel connection of two UPSs are the same frequency, same phase, and equal amplitude, so there must be a parallel controller, which mainly completes the functions of synchronizing phase locking, current sharing and parallel management.
The purpose of UPS parallel connection is to greatly improve the reliability of the power supply system. It is often used in particularly important occasions, such as communications, satellite launch centers, petroleum, chemical, electric power, steel, finance, and radio and television systems. These systems will cause power outages. Cause huge economic losses, so the power supply system is required to be absolutely reliable.
Its operating mode is: when both UPSs are normal, each bears 50% of the load; when one of the UPS fails, the other one bears 100% of the load; when both UPSs are faulty, the mains is passed through the static side The switch directly supplies power to the load.
4.4.4 Parallel redundant UPS-single bus power supply system
4.4.5 Number of redundant units in parallel
Manufacturers generally promise that 6 (8) UPSs can be connected in parallel. However, when the number of parallel single UPS systems increases, the degree of improvement in the availability of parallel redundant systems will decrease. When N is very large, the increase in availability of the parallel redundant system is not obvious. Moreover, in practical applications, the failure rate of an N+1 parallel redundant system with a larger N is higher. Therefore, 1+1 parallel redundant UPS system should be used as far as possible when investment permits. If the system capacity is very large, and N+1 parallel redundant UPS system must be used, it should be noted that the number of parallel single units should not be too many. Generally, it is recommended that N≤3.
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