Lab 4 - Application Example – Uninterruptible Power Supply (UPS)

One application of voltage source converters is in an uninterruptible power supply (UPS). A UPS provides short-term emergency power to a critical load when the primary input power fails. A simplified one-line diagram of a UPS is shown below in figure 5. The system consists of a rectifier circuit to convert the three-phase AC input into DC and an inverter to convert the DC-link power back to the required AC waveforms to feed the load. Both the PWM rectifier and inverter circuits employ a 2-level voltage source converter and are highlighted in bold. The PWM rectifier is synchronized to the AC primary input and controlled to maintain the DC-link to a fixed reference voltage set-point, whereas the output inverter is controlled to maintain the voltage and frequency of the power delivered to the load. The passive circuit design and converter switching frequency of the voltage source converters are tailored to optimize the power converter performance and meet the waveform requirements. It is common that the input of the rectifier and output of the inverter are filtered to adhere to both the grid-side and load power quality requirements.

The DC-DC converter is connected to the DC-link of the UPS. Under normal operating conditions, the bidirectional DC converter operates in buck mode and charges the battery at the specified charging current until the threshold battery voltage is met. In the event of a line outage, the PWM rectifier no longer provides the DC-link power, and the DC converter operates in boost mode to maintain the DC-link of the inverter to the specified voltage. The capacity of the battery is typically expressed in kilo-watt-hours (kWh) and is rated in accordance with the load requirements. Refer to reference 2 for a more detailed diagram of a commercial UPS and for further information.

Figure 5: UPS single line diagram comprising of a PWM rectifier and voltage source inverter