Introduction to the working principle of UPS uninterruptible power supply

This article introduces the basic concept of uninterruptible power supply (UPS), including its definition, classification, and working principle. Elaborated in detail on three types of UPS: backup, online interactive, and online, and compared their characteristics, application scenarios, and technical advantages.

A. Definition

UPS, also known as uninterruptible power supply, is a system equipment that connects batteries (mostly lead-acid maintenance free batteries) to the host and converts DC power into mains power through module circuits such as the host inverter. In European and American countries, the English prefix for Uninterruptible Power System is abbreviated as UPS. Due to the fixed voltage and frequency characteristics of the AC output of uninterruptible power supply systems, known as Constant Voltage Constant Frequency, some people in Japan also use its English prefix CVCF for short.

B、 Classification and principle:

Divided by the number of input and output phases: single input single output, three input single output, and three input three output.

Classified by power level: micro (<3KVA), small (3KVA~10KVA), medium (10KVA~100KVA), and large (>500KVA).

According to the circuit structure form, there are backup type, online interactive type, online type, etc.

According to the different output waveforms, there are two types: square wave and sine wave.

Now, according to the classification of circuit structure forms, they are described as follows:

1. Backup type

Principle:

When the mains power is normal, the mains power is stabilized and directly output to the load. At the same time, the AC power is converted into DC power by the rectifier to charge the battery. At this time, the inverter does not work.

When the mains power fails, the battery provides power and the inverter outputs the load. This switching time is approximately 4-10ms.

Features:

Power range: 300-2000VA

Job characteristics: Perform simple voltage boosting and filtering processing on the mains power and directly supply it to the load. Only when the input power does not meet the requirements will it be powered by the battery. Most of the time, the load is powered by mains electricity.

Advantages: Simple structure, affordable price, small size, low noise, high efficiency.

Disadvantages: The conversion time between mains and battery power is 4-10ms, with low output accuracy, poor output waveform, and a square wave output waveform.

Main application object: Power outage protection for a single computer system

2 Online Interactive

Online interactive, also known as hybrid or three port.

Principle:

When the mains power is normal, after being stabilized, the load is output by the conversion switch, and at the same time, the AC power is converted into DC power by the rectifier to charge the battery. At this time, the inverter has been activated but has not output.

When the mains power fails, the battery provides power and the inverter outputs the load.  

Features:

Power range: 1~800KVA

Job characteristics: The mains power is rectified and converted into direct current, which is modulated by an inverter to produce a stable sine wave.

Advantages: The load end and the mains input end are in an excellent "electrical isolation" state, the output waveform is good, the output voltage quality is high, and it exhibits excellent load characteristics regardless of load sudden changes or load stability.

Disadvantages: Low efficiency, high cost

Main application objects: power-off protection for computer and network systems, precision instruments and meters, and industrial systems.

3 Online:

Principle:

When the mains power is normal, the AC power is converted into DC power by the rectifier to charge the battery, and then output to the inverter to convert the DC power into sinusoidal AC power, which is then output to the load through the conversion switch.

When the mains input is abnormal, it is output from the battery to the load through the inverter.

When the inverter fails or the output power is insufficient (due to overload, overheating, etc.), the transfer switch will automatically switch to the static bypass route to continue power supply from the mains. If the bypass is caused by overload, the UPS will trip back to the normal output of the inverter when the load is below 100%. If the bypass is caused by overheating, the UPS will trip back to normal output when the temperature is below the alarm point.

Features:

Whether the mains power supply is normal or interrupted and powered by the battery inverter, the inverter is always in working condition, which fundamentally eliminates the impact of voltage fluctuations and interference from the grid on the load.

Due to the synchronous operation of the inverter and the mains power, the automatic switching time between the inverter and the mains power through the transfer switch should be less than 2ms. Therefore, it has high reliability and is suitable for places with high power quality requirements.