How does an uninterruptible power supply work?

An uninterruptible power supply is a power supply device that contains an energy storage device. It is mainly used to provide uninterrupted power supply to some equipment that requires high power stability.

When the mains input is normal, the uninterruptible power supply stabilizes the mains and supplies it to the load. At this time, the uninterruptible power supply is an AC voltage stabilizer, and it also charges the battery in the machine; when the mains power is interrupted (accidental power outage), the uninterruptible power supply immediately uses the battery's DC power to continue to supply 220V AC power to the load through the inverter switching conversion method, so that the load maintains normal operation and protects the load software and hardware from damage. Uninterruptible power supply equipment usually provides protection against overvoltage or undervoltage.

1. What is an uninterruptible power supply:

An uninterruptible power supply (UPS) is a system device that connects a battery (mostly a lead-acid maintenance-free battery) to a host and converts DC power into mains power through module circuits such as the host inverter. It is mainly used to provide a stable and uninterrupted power supply to a single computer, computer network system or other power electronic equipment such as solenoid valves, pressure transmitters, etc., to ensure the uninterrupted operation of these equipment and instruments, and to prevent computer data loss, telephone communication network interruption or instrument loss of control.

2. Application of uninterruptible power supply:

Uninterruptible power supply is widely used in: mining, aerospace, industry, communications, national defense, hospitals, computer business terminals, network servers, network equipment, data storage equipment, emergency lighting systems, railways, shipping, transportation, power plants, substations, nuclear power plants, fire safety alarm systems, wireless communication systems, program-controlled switches, mobile communications, solar energy storage and energy conversion equipment, control equipment and its emergency protection system, personal computers and other fields.

3. Working principle of uninterruptible power supply

A. Composition of uninterruptible power supply system:

The basic structure of a typical uninterruptible power supply system is a set of rectifiers and chargers that convert AC power into DC power, and an inverter that converts DC power into AC power. The battery stores energy and maintains a normal charging voltage when the AC power is normally supplied. Once the mains power supply is interrupted, the battery immediately supplies power to the inverter to ensure the AC output voltage of the uninterruptible power supply.

(1) When the mains power is normally supplied within the range allowed by the uninterruptible power supply, the mains power is directly provided to the output end equipment and the battery is charged.

(2) When the main power fails, that is, the mains power is cut off, the power will instantly be switched to the inverter to convert the DC power stored in the battery into the AC power required by the equipment for use by the output end equipment.

(3) When the load is too large or the inverter in the power supply fails, the uninterruptible power supply will open the bypass route and supply power directly from the main power supply.

4. Uninterruptible power supply connection method:

(1) A single uninterruptible power supply can supply all loads. The connection method is relatively simple. Directly connect the uninterruptible power supply input terminal, battery terminal, and output terminal to the corresponding equipment respectively.

(2) The total load power requires two or more uninterruptible power supplies. This connection method is called parallel connection.

(3) The total capacity of two or more uninterruptible power supplies is greater than the total capacity of the load, but the system must have the capacity of one more uninterruptible power supply than the total capacity of the load. When any uninterruptible power supply fails, the remaining uninterruptible power supplies can still supply all loads. This connection method is called parallel connection with backup.

(4) After the entire uninterruptible power supply system fails, the mains disconnects the uninterruptible power supply and directly connects the load without affecting the maintenance of the uninterruptible power supply system. This method is called bypass mode.

When the uninterruptible power supply system is not faulty, the maintenance switch is opened, and the input switch and output switch are closed; when the uninterruptible power supply system fails, the maintenance switch is closed, and the input switch and output switch are opened. In this way, the normal operation of the power load is guaranteed without affecting the online maintenance of the uninterruptible power supply system failure.

5. Functions of uninterruptible power supply:

As the requirements of computer application systems for power supply become higher and higher, uninterruptible power supply has received increasing attention and has gradually developed into a power protection system with functions such as voltage stabilization, frequency stabilization, filtering, anti-electromagnetic and radio frequency interference, and anti-voltage surfing. Especially when the quality of the power grid lines and power supply is not very high, the anti-interference technology is backward, and the computer system has high requirements for power supply, the role of uninterruptible power supply becomes more obvious.

The protective function of uninterruptible power supply is first manifested in stabilizing the voltage of the mains power supply. The input voltage range of the uninterruptible power supply is relatively wide, generally from 170V to 250V, and the quality of the output power supply is quite high. The output voltage of the backup uninterruptible power supply is 5%~8%, and the output voltage of the online uninterruptible power supply is stable within 3%. When the mains power is normal, the uninterruptible power supply is equivalent to an AC mains voltage stabilizer; at the same time, the mains power charges the battery, which is also equivalent to a charger. In the event of a sudden power outage, the UPS automatically switches to battery power, allowing the computer to maintain normal operation and protecting the software and hardware from damage.

6. Classification of UPS:

Uninterruptible power supplies are classified into the following three categories according to their structure and operating principle according to the new standard IEC (International Electrotechnical Commission):

(1). Passive backup UPS:

Inverters are connected in parallel between the mains and the load and are simply used as a backup power source. For this type of UPS, when the mains is normal, the load is completely and directly powered by the mains, the inverter does not perform any power conversion, and the battery is powered by an independent charger; when the mains is abnormal, the load is completely powered by the inverter.

The passive backup UPS has the advantages of simple structure and the lowest price, and is used for some non-important loads, such as home computers. However, when the mains is out of power, the relay switches the inverter to the load, and the switching time is relatively long, generally requiring a few milliseconds, so slightly important computer equipment should not use a passive backup UPS.

(2). Online interactive uninterruptible power supply:

Inverter is connected in parallel between the mains and the load, acting as a backup power supply, and the inverter acts as a charger to charge the battery. Through the reversible operation mode of the inverter, it interacts with the mains, so it is called interactive. For this type of uninterruptible power supply, when the mains is normal, the load is powered by the improved mains, and the inverter acts as a charger to charge the battery. At this time, the inverter acts as an AC/DC converter; when the mains fails, the load is completely powered by the inverter. At this time, the inverter acts as a DC/AC converter.

The online interactive uninterruptible power supply has the advantages of simple structure, convenient implementation, easy parallel connection, easy maintenance and repair, high efficiency, low operating cost, high reliability of the whole machine, etc. The performance meets certain load requirements and is particularly suitable for systems where some computer equipment in the network adopts distributed power supply. The disadvantage of this power supply is that the voltage regulation performance is not high, especially the dynamic response speed is low. Secondly, the anti-interference ability is not strong, and the circuit will generate harmonic interference and modulation interference.

(3). Double conversion uninterruptible power supply:

Inverters are connected in series between the AC input and the load, and the power supply is continuously supplied to the load through the inverter. The power supply mode of this uninterruptible power supply is as follows: when the mains power is normal, the mains power is supplied to the load through the rectifier and inverter; when the mains power is abnormal, the battery is supplied to the load through the inverter.

Double conversion uninterruptible power supply is the mainstream product of uninterruptible power supply, with advantages such as good performance, high voltage stability and frequency stability, strong functions, hot backup connection and parallel redundant connection. Its disadvantage is that when the capacity is less than 10kVA, its overall efficiency is not high, generally around 85%.

7. Uninterruptible power supply technical indicators:

The inverters in the new generation of uninterruptible power supplies mostly use PWM technology, and use quartz crystal oscillation to control the frequency of the inverter, and ensure the stability of the output voltage through the voltage negative feedback circuit. It has a series of advantages of switching power supply, and ensures stable power output by accurately adjusting the pulse width. At the same time, the application of switching power supply technology also greatly reduces its own loss. The main technical indicators are as follows:

(1) Rated output power and maximum output power;

(2) Switching time;

(3) Output voltage stability, reference value ±0.5%~±2%;

(4) Output frequency stability, reference value ±0.01%~±0.5%;

(5) Pure output waveform (sine wave output), voltage distortion less than 1%, no submerged wave distortion problem;

(6) High efficiency and low loss. The reference index is higher than 90%;

(7) Trouble-free working time. Due to the adoption of high technologies such as microprocessor monitoring technology and advanced IGBT-driven SPWM, the current uninterruptible power supply has reached an extremely high level of reliability. For large-scale uninterruptible power supplies, the average annual trouble-free working time (MTBF) of a single machine has exceeded 200,000 hours. If a multi-machine "redundant" uninterruptible power supply system with dual bus output is used, its MTBF can even reach the order of 10 million hours.