UPS Working Principle

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Working Principle of UPS- Uninterrupted Power Supply

UPS -uninterrupted power supply that is used in power plants, working principle, and operation are explained in detail in this post with various modes of operation.

Why is UPS required?

An Uninterrupted Power Supply is employed for critical loads which cannot be powered directly by utility supply (mains).

UPS is able to protect the equipment from the following problems associated with power supply they are Voltage variation, Frequency variation, Superimposed Noise, Interruption, and Failure of supply.

Let us know, the typical application includes Computers, Process Control, and Monitoring, Telecommunications, Hospitals, and Security Systems.

Working Principle and Parts of UPS:

Components of Power Plant UPS System :

1.     Mains Distribution Unit

2.     UPS Module

3.     Battery Module

4.     Output Distribution Module

5.     Front Panel-LED Mimic Console

Parts of UPS for Working Principle
Parts of Power Plant UPS

1. Mains Distribution Unit:

It distributes the Mains (utility) or Generator power to the facility and will also supply input power to the UPS system. The mains input power connection is separate for bypass supply and rectifier inputs.

The safety “earth” connection for the UPS system is also considered to be part of the Mains distribution unit.

2. UPS Module:

It consists of UPS (without Battery). Depending upon the configuration selected, one or more UPS modules can be employed.


The job of the Rectifier is to convert AC to DC current, by means of a 6- or 12-pulse thyristor to provide supply to both Battery and Inverters. The advantage of using a 12-pulse thyristor rectifier is the main current distortion is reduced to nearly half compared to a 6-pulse thyristor rectifier.

UPS Rectifiers


The job of the inverter is to convert DC to AC current by means of an IGBT – pulse width modulated (PWM) inverter module.



A Static Switch is powered by either mains or battery power, when the static switch is in on UPS state, connecting the inverter output to the load. The static switch automatically transfers the load back to bypass in cases of high inrush or fault on the inverter. This is an important aspect, especially for critical loads where even 5–10 ms switchover time from bypass to inverter could be critical.

The advantage of the static switch is the transfer can happen in less than four milliseconds.


A manual bypass switch is used to isolate the UPS system from the rest of the circuit in the case of doing maintenance of UPS.

3. Battery Module:

This module comprises the battery pack for supplying power to the UPS module in the event of a mains failure. There are various types of batteries – SMFB, LATB, NI-CD, etc.

UPS Battery Module
UPS Battery Module

4. Output Distribution Module:

The output of the UPS system needs to be distributed to various loads. Such a module generally comprises switches, fuses, etc. for each load. The coordination of fuses is important to avoid faults in a particular load from affecting the other loads supported by the UPS.

5. Front Panel-LED Mimic Console:

The user-specific mimic diagram shows the system’s current operation status, indicating which system part is supplying the load at the time and which is in standby mode.

UPS Front Panel Display
UPS Front Panel Display

Power Plant UPS Principle of Operation and Working Modes:

Uninterrupted Power Supply UPS operates in the following modes based on the type of supply available.

UPS Working in Normal Mode:

When the Mains are normal, the UPS powers the load through the rectifier and inverter and charges the batteries at the same time, as shown in the above figure. This is called Normal mode.

UPS Working Principle-Normal Mode of Operation
UPS-Normal Mode of Operation

UPS Working in Battery Mode:

When the Mains supply fails, the UPS system will switch to battery mode without interruption and the battery will power the load through the inverter. The UPS can return to normal, as shown in the below figure.

UPS Working Principle-Battery Mode of Operation
UPS-Battery Mode of Operation

UPS Working in Bypass Mode:

In the event of inverter overload which lasts longer than the typical time, an output short circuit, or a fault on the inverter, the UPS transfer the load to bypass.

Bypass mode can be operated in two ways. In the first kind, the UPS can be set to return to normal mode automatically when the fault is cleared. In the second kind, the UPS is set to return to normal mode only with the manual transfer.

UPS Working Principle-Bypass Mode of Operation
UPS-Bypass Mode of Operation

UPS Working in Maintenance Mode:

When the UPS has to be repaired or has to undergo routine maintenance, the UPS can be set to maintenance mode by switching on the maintenance bypass circuit breaker. The load will be powered from the maintenance bypass supply without interruption.

UPS Working Principle-Maintenance Mode of Operation
UPS-Maintenance Mode of Operation

UPS Parallel Operation Mode:

When the several UPS are in parallel operation mode they will share the load automatically amongst themselves. When one unit fails the other share the load equally. If the overload occurs, the UPS will transfer to bypass.

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