In high-load-grade environments (Class 1 load), the commonly used power source is the Uninterruptible Power Supply (UPS), which ensures continuous power supply to devices that cannot be interrupted, such as communication systems; computer systems; monitoring systems; DCS systems; and critical electric valves in industry. Now, UPS outlets are also common in hospital rooms and hotels. It can be said that the application scope of UPS is increasingly widespread. As electrical professionals, we should understand the working principle, operation methods, and simple troubleshooting of UPS. This article provides a brief introduction to the composition, working principle, and functional requirements of UPS.

The working principle of an Uninterruptible Power Supply (UPS)

UPS systems typically consist of a rectifier, battery, inverter, static switch, and control system. Online UPS systems are commonly used. They first convert the AC power from the main supply into a stable DC power supply, which is then supplied to the battery and inverter. The inverter reconverts it into a stable, pure, and high-quality AC power. It can completely eliminate any power issues that may arise in the input power supply, such as voltage fluctuations, frequency fluctuations, harmonic distortion, and various interferences.

2. Non-Intermittent Power Supply (UPS) Functional Requirements

The switching time for a static bypass switch is typically 2~10ms and should possess the following functions:

When an inverter unit fails or requires maintenance, it should be promptly switched to grid (mains) power supply.

b. In the event of a sudden fault causing a short circuit in the branch loop, with the current exceeding the predetermined value, switch to the grid (city backup) power to increase the short-circuit current, allowing the protective device to operate rapidly. After the fault is removed, restart the inverter to supply power.

c. Uninterruptible Power Supply (UPS) units with frequency monitoring capabilities should automatically disconnect from the power grid when the grid frequency or voltage fluctuates beyond the rated value, and re-connect to the grid once the frequency and voltage return to normal.

When using the city power bypass, the inverter's frequency and phase should be synchronized with the city power lock.

3. The unbalanced negative voltage of three-phase output, the difference between the fundamental root mean square (RMS) current of the first phase and the second phase loads, should not exceed 25% of the uninterruptible power supply's rated current, and the line current should not exceed its rated value.

4. The unbalance factor of the three-phase output system's voltage detection (the ratio of negative sequence component to positive sequence component) should not exceed 5%. The total waveform distortion of the output voltage should not exceed 5% (10% for single-phase output is permissible).