What is UPS? – Uninterruptible Power Supply

What is the Role of UPS (Uninterruptible Power Supply)?

UPS, which stands for “Uninterruptible Power Supply”, is a power supply system that includes energy storage devices and consists mainly of rectifiers and inverters. It provides uninterruptable power supply with constant voltage and frequency to equipment such as monitoring systems, automation instruments, and long-distance communication systems in substations.

What is UPS? – Uninterruptible Power Supply
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What is UPS system serves two main purposes:

  1. It serves as a reliable power source for essential equipment, preventing damage and disruptions caused by sudden power outages.
  2. It eliminates power disturbances such as power surges, momentary high/low voltages, electrical noise, and frequency deviations in the utility power supply. It improves power quality and provides high-quality power to devices such as computers.

What are the classifications of UPS?

UPS systems can be categorized into three types: standby, line-interactive, and online.

  1. Standby: This type of UPS provides automatic voltage regulation, power outage protection, and has a conversion time of around 10ms. The AC output from the inverter is a square wave. It has a simple structure and is cost-effective.
  2. Line-interactive: This type of UPS includes filtering functions and has strong resistance to utility power interference. It has a conversion time of less than 4ms, and the AC output from the inverter is an analog sine wave. It is more affordable compared to online UPS.
  3. Online: This type of UPS has a more complex structure and offers complete and uninterrupted output of pure sine wave AC power. It can handle all power-related issues such as spikes, surges, and frequency drift. Online UPS is more expensive and is typically used in environments with demanding power requirements, such as critical computers and network devices.

What combines UPS Structure?

A typical UPS system in a substation consists of three main components: the UPS main unit, bypass voltage regulator cabinet, and output feeder cabinet (or sometimes combined into one unit for low power).

The schematic diagram below illustrates the structure, and here are explanations of a few important parts:


It serves two primary functions:

  1. It converts AC power to DC power, which is then filtered and supplied to the inverter.
  2. It charges the batteries, acting as a charger.


It converts DC power to AC power for the loads.


UPS utilizes UPS battery backup to store energy. When there is a power outage, the DC power from the batteries is inverted to provide uninterrupted power to the loads. The capacity of the batteries determines the duration of the discharge (uninteruptable power supply) time.

Static Switch:

Also known as a static transfer switch, it is an AC switch composed of two controllable silicon-controlled rectifiers (SCRs) connected in reverse parallel. Its closing and opening are controlled by a logic controller.

Isolation Transformer and Voltage Regulator:

They isolate AC and DC in the input or output section. The voltage regulator also stabilizes the voltage.

What are the Four Operating Modes of UPS?

UPS operates in four modes, and the simplified schematic diagram of the UPS system is as follows:

Normal Operation Mode:

Under normal AC power supply, the rectifier converts AC power to DC power, eliminating the “power pollution” in the utility power while charging the batteries. The inverter then converts the DC power to AC power, providing stable power to the loads.

Power Failure Mode:

When there is an abnormality in the AC power supply or a failure in the rectifier or reactor, the battery group provides DC power to the inverter, ensuring uninterrupted AC output and protecting the loads.

Standby Power Mode:

In the event of an abnormality in the inverter, such as a blown fuse or a short circuit, the inverter automatically shuts down to prevent damage. If the bypass AC power supply is normal at this time, the static switch transfers the power supply from the inverter to the standby power source.

Maintenance Bypass Mode:

When UPS needs maintenance or battery replacement without interrupting the power supply to the loads, the inverter switch is first turned off. Then, the maintenance bypass switch is activated, followed by the disconnection of the rectifier and bypass switch. The AC power supply is then supplied to the loads through the maintenance bypass switch, allowing safe maintenance of the UPS system.

What are the Requirements for Power System UPS?

  1. Monitoring systems and long-distance communication systems are crucial for substations, so the UPS system used as the power source requires high reliability.
  2. Since most loads in the system are single-phase loads, power system-specific UPS power supplies are mostly three-phase/single-phase input with single-phase output, typically with a capacity within the range of 60kVA.
  3. The bypass static transfer switch should have both automatic and manual operation modes to achieve uninterrupted switching.
  4. As substations have 220V or 110V DC systems with a DC charging panel for batteries, power system-specific UPS does not come with batteries. It directly uses the DC system as the UPS’s DC input and does not require a charging function.
  5. The DC input terminal of the power system-specific UPS generally requires a noise suppression device, such as a reverse-blocking diode, to minimize the UPS’s impact on the DC bus.
  6. After accommodating all the equipment, the UPS power supply should have a power capacity reserve of more than 40%. The UPS should provide uninterrupted power supply for a duration of not less than 60 minutes after a power outage.

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