What is the working principle of an online ups

What is the working principle of an online ups

The working principle of an online ups is that when the voltage and frequency of the mains power supply match the input parameter set values of the UPS, it is converted into a DC power supply through an AC/DC rectifier. This DC power supply serves as the DC input power for the next-stage DC/AC inverter and also provides charging current for the backup battery pack.

The inverter converts the input DC voltage through PWM full-bridge transformation and then outputs a power frequency sinusoidal voltage with high-frequency components superimposed through a static switch. Subsequently, the high-frequency components on the sinusoidal voltage are filtered out by the output filter circuit, and a sinusoidal voltage with a smaller degree of distortion is obtained at the output end.

When the mains power is interrupted or fails to meet the input requirements of the UPS, the input AC/DC rectifier of the UPS will shut down. At this time, the backup battery will supply power to the inverter in a no-switching time manner to ensure that the output load receives continuous, stable and high-quality power supply. When the mains power is restored, the battery pack stops supplying power to the inverter. At this time, the internal charger replenishes the consumed electricity of the battery pack for reuse.

When the mains power is present and within the allowable range of the UPS input, if the DC/AC inverter malfunctions or the load power exceeds the rated output power of the inverter, to ensure that the load can still operate normally, the static switch at the inverter output end switches to the bypass power supply state. At this time, the mains power directly supplies power to the load through the input filter circuit and the static switch at the output end. When the inverter returns to normal or the load power drops within the rated output power of the inverter, the output static switch will automatically switch from bypass power supply to inverter power supply.

The output and input filters of UPS are generally passive filters. The filters on the side of the municipal power grid mainly filter out high-frequency interference and common-mode noise voltage in the power grid. The output filter mainly filters out the high-frequency components on the output sinusoidal voltage.

The control circuit of UPS is rather complex, especially for medium and large-sized UPS with three-phase input and three-phase output, which have very complete alarm and protection functions. This makes its identification, control and protection circuits rather complex and large. Moreover, the mutual actions and controls among these circuits must conform to the designed timing and logical relationships.

The components of the control circuit mainly include: Input AC voltage, frequency, phase sequence, battery pack voltage and capacity detection circuit, output frequency and phase tracking circuit, DC/AC inverter PWM control circuit, static switch status recognition and drive circuit, start control circuit, various alarm signal processing and protection drive circuit, operation status display circuit, control circuit of battery charger, etc.

Because this type of online UPS has two converters, namely the rectifier and the inverter, it is also called a double-conversion online UPS. The number of input and output phases of this type of UPS can be classified as single-input single-output, three-input single-output, and three-input three-output. The output power of a single-input single-output UPS is generally below 10kVA, while that of a three-input single-output ups is around 20kVA. When this type of three-input single-output UPS switches to bypass power supply, the load is only powered by one phase of the mains power. Excessive power can cause an imbalance in the local three-phase mains power, so the usage of usp is not large.