UPS Types and How They Work: A Comprehensive Overview

Schematic structure of a Voltage Independent UPS (USV VI)

“Online”, “Double-Conversion”, “Continuous Operation” or “Double Conversion”

Class 1: VFI (Voltage and Ffrequency Independent from mains supply, voltage and frequency independent from the mains)

In a UPS of this category, the input is connected directly to a rectifier (GR), which supplies the accumulators. The output is supplied exclusively by an inverter (WR), which draws the necessary energy from the rectifier during normal operation, i.e. when there is mains voltage at the UPS input, and is supplied from the battery system (accumulators) in the event of a power failure.

The alternating voltage at the output is always generated from the direct voltage of the so-called intermediate circuit via a downstream inverter - regardless of the quality of the input voltage. To increase the security of supply, a VFI UPS has a so-called bypass circuit that is connected in parallel to the rectifier/inverter combination. In the event of overloads at the UPS output or an internal error in the rectifier/inverter branch, the connected consumer is switched to this bypass branch "without interruption" and thus continues to be supplied. Since the rectifier and inverter are constantly loaded with the full operating current, they must be of particularly high quality, making this type of construction the most expensive. In addition, losses occur in both the direct and the alternating direction, which reduces the efficiency.

The product descriptions of UPS systems usually state the efficiency at full load. However, since most UPSs are not used at full capacity, the generally lower efficiency at partial load is crucial for correctly calculating power consumption and costs. An efficiency of more than 95% at full load can now be described as standard for a VFI UPS.

In addition to protecting against the consequences of a power failure, undervoltage and overvoltage, a VFI UPS also protects against frequency fluctuations and harmonics. They also offer sporadic protection against lightning strikes and voltage distortions (bursts). VFI UPS are preferably used in areas of application that have high criteria for tolerable downtimes, such as the power supply of a data center.

It should be noted that VFI UPSs have the highest internal power requirements compared to VFD or VI UPSs. For example, while a 650 VA VFD UPS consumes around 5 W when idle (without connected loads), and an 850 VA VI UPS consumes around 15 W, a VFI UPS requires considerably more power even when idle (a rule of thumb is that it requires around 10% of its rated power, i.e. an 850 VA VFI UPS consumes around 85 W when idle).

Schematic structure of a Voltage Independent UPS (USV VI)

“Line-Interactive”, “network-interactive”, “single-conversion”, “delta-conversion” or “active follow-through operation”

Class 2: VI (Volder Independent from mains supply, voltage independent from the mains)

In a UPS of this category, a bidirectional inverter is used as the central component. Depending on requirements, it generates the direct voltage for charging the batteries from the alternating voltage at the input or the alternating voltage at the output from the direct voltage of the batteries.

Because the converter also continuously limits the level of the voltage at the output, this is largely independent of the level of the voltage at the input. If a voltage is present at the input, its frequency determines the frequency of the voltage at the output. The switchover time in the event of a power failure is shorter than with VFD UPS and is around 2 to 4 ms. The switchover to mains operation after restoration takes place without any time delay. The input voltage is synchronous with the output voltage.

VI-UPS not only protect against the consequences of a power failure, but also against undervoltage and overvoltage.

Schematic structure of a Voltage and Frequency Dependent UPS – VFD

"Offline", "Stand by" or "passive"

Class 3: VFD (Voltage and Ffrequency Dependent on mains supply, voltage and frequency dependent on the mains)

A UPS of this class transfers the power directly from the input to the output during normal operation. The input also supplies a rectifier that charges the batteries. If the mains supply fails, the output is switched to an inverter that is powered by the batteries.

Depending on the model, switching takes place with a delay of up to 10 milliseconds (ms). In addition, according to EN 62040-3, voltage fluctuations of less than 16 ms and voltage peaks between 4 and 16 ms are compensated. For some very sensitive devices, this may already be too long. In normal operation, the level and frequency of the output voltage is directly dependent on the input voltage.