The so-called uninterruptible power supply is that when the input of the AC power grid is abnormal or interrupted, it can continue to supply power to the load and ensure the quality of power supply, so that the power supply to the load is not affected. This type of power supply device is called an uninterruptible power supply device, or an uninterruptible power supply system, abbreviated as UPS (Uninterruptible Power System). Uninterruptible power supply devices can be divided into two types based on whether they provide AC or DC to the load, namely DC uninterruptible power supply system and AC uninterruptible power supply system. However, it is customary for people to refer to AC uninterruptible power supply system as UPS. That's why this book also uses this customary term and abbreviates AC uninterruptible power supply as UPS.

Working principle of dynamic UPS

The so-called dynamic UPS refers to a device that relies on kinetic energy to achieve uninterrupted power supply, which is an early device that uses diesel generators, electric motors, and generator sets to convert electrical energy. Its structure is shown in Figure 1-1. When the power grid supply is normal, AC power drives the AC motor to rotate, thereby driving the AC generator and the coaxial inertia K-wheel to run at the same speed, and the generator generates electricity to supply the load. When there is voltage fluctuation or some peak interference in the power grid, due to the action of the inertia flywheel, the short-term voltage fluctuation or short-term interference does not affect the normal operation of the inertia flywheel, thus ensuring the stability of the voltage on the load; Once the power supply of the grid is interrupted, the electric motor stops running. At this time, the generator relies on the huge kinetic energy stored on the coaxial flywheel to continue generating electricity, extending the power supply time of the load for on-site personnel to protect the site. However, in this scheme, relying on the flywheel that stores kinetic energy to extend the power supply time during power outages is bound to be limited.

Static UPS

Compared with dynamic (rotary) UPS, static (static) UPS does not have rotating bodies such as electric motors, generators, and flywheels. It converts electrical energy through a main circuit composed of power semiconductor devices. Nowadays, the concept of UPS in people's minds is static UPS. The classic solution of static UPS is shown in Figure 1-2. Its principle is that when the power grid is normal, the mains power is converted into DC through a rectifier, then converted into AC through an inverter, and finally sent to the load through a transfer switch; When the power grid is abnormal, the battery provides DC power to the inverter, which is then converted into AC power and sent to the load; When the rectifier, inverter, or battery unit fails, the mains power can be bypassed to the load through a transfer switch.