Uninterruptible static power supplies, protection of persons

Continuity of electrical power

The primary function of uninterruptible power supplies is to ensure continuity of electrical power. Even when the line supply fails, they are able to provide the necessary power via their integrated batteries, a backup power source, or even an emergency standby source in the event of a fault. Static transfer switches can be used to supply power to a load via two independent power sources. In the event of a fault, they automatically transfer the loads from one source to the other.

However, the number of power sources and the multiplicity of possible configurations, as well as the different types of earthing system increase the complexity of setting up these devices in accordance with the requirements of installation standards IEC 60364 and NF C 15-100.

This “Cahier Technique” summarizes these difficulties and offers explanations to assist with the selection of the most suitable solutions for the various case scenarios.

Double conversion UPS (known as online UPS)

In normal operation, the load is supplied with power permanently via the inverter. The DC voltage required by the inverter is supplied by the rectifier if the voltage of the main power supply lies within the tolerance limits permitted by the rectifier, or by the battery if it does not. In this system (see Fig. 3 ), the battery is permanently represented on the DC section of the inverter and the rectifier also functions as a battery charger.

In some configurations, the battery has a separate charger and is connected to the inverter directly via a switch when the main power supply fails. In other configurations, in particular in the event of low battery voltage, a DC/DC converter is inserted between the battery and the DC section of the inverter.

This configuration is the most popular, in particular for medium or high-power UPSs, as it optimizes the quality of the voltage supplied to the load. It is therefore suitable for all types of load, even the most sensitive.

UPS in passive stand-by operation

In normal operation, the load is supplied with power directly via the line (see Fig. 5 ). If the line voltage strays outside the permissible load tolerance limits, the switch can be activated in order to continue to supply the load with power via the inverter.

The switch is often a relay on which the breaking time is less than 10 ms on source transfer, as this system is usually reserved for low power ratings. This configuration is designed for low-sensitivity loads such as personal computers, for which the voltage quality of the main power supply (line supply or possible replacement sources) is considered sufficient to ensure the correct operation of the devices.

The main function of this type of UPS is therefore to operate when there is no voltage or when the voltage strays outside the permissible tolerance limits.

AUTHOR: Schneider Electric expert | Jean-Noël FIORINA