Uninterruptible Power Supplies - What is a True On Line UPS System?

An immediate difference between this design and the previously described off-line system is that the battery charger is replaced by a ‘rectifier/charger’ black. The rectifier/charger may be two separate units or a combined power black. When the mains supply is present this black float charges the battery and supplies the inverter with a stable de voltage. In the absence of the mains supply the charger shuts down and the inverter dc supply is provided by the battery, which begins to discharge. The connection between the rectifier/battery and inverter is often known as the dc busbar, or dc bus. As part of its control function the rectifier/charger generally includes an input current limit feature to provide overload protection an a dc overvoltage shutdown mechanism to protect the battery/inverter and dc filter components This UPS deign, which is sometimes also referred to as a double conversion UPS, offers the greatest degree of critical supply integrity in that the load is supplied with processed power at all times. That is, when the UPS input mains supply is resent the rectifier, charger and inverter power blocks are all active and the load is connected to the inverter output via the static switch. As the load is powered from the inverter under normal circumstances it is well protected from input supply aberrations because the rectifier and inverter act as a barrier to mains borne noise and transient voltage excursions, in addition to providing a well regulated output voltage. If the input supply goes outside a preset voltage range (typically +10% to -20%), or suffers a total failure, the inverter continues operating from battery power and the event is totally transparent to the load as there is no transfer operation involved. When operating from battery power the inverter supplies the same degree of supply regulation as when the main is present. If the mains is not restored before the battery reaches its end-of-discharge voltage the inverter shuts down and, in some models, the static switch may attempt to transfer the load to the bypass line. The result of the transfer action depends on whether or not the module’s bypass line is connected to the same mains supply as the modules rectifier, and if the bypass supply is live. (Known as a split bypass system.)

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