How the Online Uninterruptible Power Supply Works

How the Online Uninterruptible Power Supply Works
Today, There are 3 types of UPS. 
Off-Line UPS, Line Interactive UPS and Online UPS.
In this article, we will focus on Online UPS and mention the other two types briefly. 

Offline UPS
The UPS block diagram is shown below.
 

It provides very basic protection including surge protection and battery backup. Load is normally connected via AC filter directly to incoming utility power.
When the utility voltage falls below a predetermined level, the UPS turns on its internal DC-AC inverter circuitry  which is powered from an internal storage battery to supply energy to loads. The switchover time can be below 10 ms.

Line Interactive UPS
This type of UPS is able to tolerate continuous undervoltage brownouts and overvoltage surges without consuming the limited reserve battery power.

This cheap and economical UPS type works similarly to Offline UPS. Loads are directly connected to the grid through the filter and automatic voltage regulator. The output voltage is kept constant within specified limits. While the mains voltage is present, the batteries are charged, but the Inverter is passive. When the mains fails, the inverter switches on and the necessary sinus voltage is produced and the loads are fed. The sine voltage produced is in the Simulated Sine form. 

Battery capacity is limited in this type of UPS, which is produced at low powers up to 2 kVA, and it can supply the loads for a short time to ensure safe shutdown. During the power cut, the transition time from battery to operation is around 4 ms.

The UPS block diagram is shown below.
 

 

Online UPS
Online UPS has a more complex structure compared to the other two UPS groups and includes Rectifier / Charger, Inverter, Static ByPass Switch, Manual ByPass Switch, Battery Pack, DC and AC Filters and Control Circuit.

A simple block diagram of this type of UPS, which is most used today, is shown in the figure below. 
 

Online UPS Operation Principle
Online UPS is based on a double conversion technology. Regardless of the status of the utility power in Online Double Conversion UPSs, the load is continuously fed through the Inverter. When the mains voltage is present, the batteries are kept ready for possible power outages and mains voltage problems by being subjected to a continuous and controlled charging process. AC voltage of mains is first converted to DC and this DC voltage is provided to charge the batteries. Then, the desired output voltage is obtained by converting this DC voltage to AC voltage by means of Inverter circuitry.

System Operations
Online Mode:  The inverter shall supply AC power continuously to the critical loads. The inverter output shall be synchronized with the bypass AC power source provided that the bypass AC power source is within the specified frequency range. The rectifier/charger shall convert the normal AC input power to DC power for the inverter and for float charging the battery. 

Loss of Normal AC Input Power:  The battery shall supply DC power to the inverter so that there is no interruption of AC power to the critical loads whenever the normal AC input power  source  of  the  UPS  module  deviates  from  the  specified  tolerances  or  fails completely.  The battery shall continue to supply power to the inverter for the specified protection time.

Return of Normal AC Input Power Source: The rectifier/charger shall start and assume the DC load from the battery when the normal AC input power source returns. The rectifier/charger shall then simultaneously supply the inverter with DC power and recharge the battery. This shall be an automatic function and shall cause no disturbance to the critical load.

Transfer to Bypass AC Power Source: If the static bypass switch senses an overload, an
inverter shutdown signal or degradation of the inverter output, then it shall automatically transfer the critical loads from the inverter output to the bypass AC power source without an interruption of power. If the bypass AC power source is above or below normal voltage limits, then the transfer shall be inhibited.

Retransfer to Inverter: The static bypass switch shall be capable of automatically retransferring the load back to the inverter after the inverter has returned to normal conditions. Retransfer shall not occur if the two sources are not synchronized.

Downgrade:  If the battery is taken out of service for maintenance, it shall be disconnected from the rectifier/charger and inverter by the battery circuit breaker. The UPS shall continue to function and meet the performance criteria specified herein except for the battery reserve time.

Let us explain UPS components briefly

EMI Filter: It is made by using a coil and a capacitor. The main function of this EMI filter circuit is to reduce or filter electromagnetic interference from the mains on the UPS input to 5% THD at full UPS load.
Rectifier Circuit: After the input mains electricity is filtered, it is converted to direct current using the Rectifier circuit. For AC-DC Conversion, Thyristor and IGBT are used as well as Standard Diodes according to the UPS power and type. 

DC Filter Circuit: DC filter circuit is used to filter the DC coming from the rectifier circuit and obtain a clean DC voltage.

Inverter Circuit: The DC Voltage obtained is converted back to AC voltage by using the inverter circuit. MOSFET Transistor, Thyristor or IGBT Transistors are used for Inverter Circuit.

AC Filter Circuit: To obtain below than 1% output ripple for 0 to l00% resistive load,  AC Voltage coming from the inverter circuit is filtered by AC filter circuit.

Static Transfer Switch: A static switch is connected between the AC filter Circuit and Critical Load. If the inverter fails or overloaded over its capacity during the UPS operation, the UPS output is transferred to the ByPass Source (Utility Power) via the Static Transfer Switch. This feature is called uninterrupted Static By-Pass feature.

Control Circuitry: The operation mode selection, AC-DC and  DC-AC conversion and Battery Charging operations in the UPS is usually made by the Control Circuit containing a Digital Processor. 

Manual ByPass Switch: However, for a safe repair, at least 15 minutes should be waited for the capacitors to discharge  or the capacitors should be discharged in a controlled manner. It should be kept in mind that there are dangerous voltages in the batteries even when the UPS is completely turned off. 

Battery: It is the most important component of the system. When mains voltage is off, output voltage is taken from the Inverter by using Battery Energy and loads are fed. In case of online operation, batteries should be charged in a controlled manner and kept ready at all times. Generally, 12 V VRLA type Maintenance Free Dry type batteries are used in UPSs. The number and capacity of the Battery to be used is determined by calculating according to the UPS power, UPS Battery Charge Capacity and the required Back-up Time.

As ISTPOWER, we provide consultancy to our customers in this regard. Pls contact us for your battery requirements.