All online double-conversion UPS has 4 main sections:
- The rectifier/battery charger
- The UPS sealed lead-acid batteries
- The Inverter
- The static bypass switch
The UPS DC Rectifier 1st conversion AC to DC
The rectifier performs several key functions. The first function is to convert the input utility power from ESKOM in the form of AC mains power (Alternating Current) to DC power (Direct Current). Its second main function is to recharge the batteries, with the DC power supplied to the input stage of the inverter simultaneously.
Depending on the size or design of the UPS, the rectifier section may include the battery charger. With smaller UPS systems (usually 3kVA and below), the battery charger is often a separate component to the rectifier. In this case, a UPS has a rectifier to feed the inverter DC input and a battery charger to charge the battery with DC separately.
UPS rectifiers are designed to work under a huge range of input supply voltages so that the UPS system can handle overloads or surges without using valuable battery backup power.
UPS Battery backup bank
The UPS system backup batteries ensure uninterrupted power availability to the inverter and its connected critical load. They are the sole source of emergency backup power when the AC input mains supply from Eskom is lost during a rolling blackout or power failure. Depending on the UPS design, the battery charger/rectifier will always ensure that the battery is fully charged and ready to backup the inverter.
Battery backup in a UPS consists of at least one series string of batteries, with the number of batteries being determined by the UPS DC bus voltage, which is ultimately determined by the UPS design. So if one battery in the series string fails, it puts the entire string out of service, which means no emergency backup power for the UPS inverter.
UPS AC Inverter
The UPS AC inverter is the second conversion taking place in the operation of a UPS system by way of converting the DC power from the battery bank or rectifier into AC mains power to supply the critical connected loads and prevent them from dropping when ESKOM utility fails. The conversion processes of AC to DC and then DC to AC, i.e., two conversions or double conversion, as well as filtering, smooths out events such as spikes, ripple, blackouts, brownouts, sags, surges, and other electricity noises, ensuring the critical load is supplied with a pure sinewave.
Static Bypass Switch (SBS)
The static bypass switch is there to act as a backup so that a failure of a component within the UPS, or in the case of an output overload or short circuit, the mains power is available to keep the critical inverter load up and running. In the event of a UPS fault, the static bypass switch immediately connects the critical load on the UPS inverter to the mains supply while bypassing the UPS inverter and all main UPS components.
Connecting the critical inverter load to the utility mains supply is not ideal as the load will not receive clean, filtered power like a UPS inverter would supply. However, at least this allows the critical connected inverter load to operate until the UPS fault is repaired.
Other UPS Components
Almost all UPS systems have wearing components that need periodic replacement during the service life of the UPS unit. These include fans, capacitors, and sometimes contactors and switches