Classification and Types of UPS – Part Five

In the previous article “Classification and Types of UPS – Part One”, we stated that UPS is classified according to:

  1. Voltage range,
  2. No. of phases,
  3. Mobility,
  4. Technological design,
  5. Physical Size/capacity,
  6. Form factor/ configurations,
  7. Topology,
  8. Distribution Architecture,
  9. Use of transformers.


We already explained the first six classifications in the Previous articles (see table in the end of Article).

Today, we will continue explaining other Classification and Types of UPS.



Seventh: according to UPS Topology




To meet the input voltage requirements for the equipment that will be plugged into the UPS, a variety of UPS topologies provide specific levels of power protection against power loss and disturbances in the electrical system.

Therefore, the UPS is classified to different topologies based on how the UPS’s output voltage is dependent upon the quality of the input voltage which is called the Input dependency Characteristic (AAA).


The input dependency Characteristic (AAA) consists of 2 or 3 letters and describes the relationship between the UPS output and the UPS input during normal operation and Three classifications are offered:

  1. VFD Voltage and Frequency Dependent,
  2. VI Voltage Independent and
  3. VFI Voltage and Frequency Independent.


Each class has different capabilities for solving power problems such as power interruptions, voltage sags and swells, etc. and


The selection between them depend on:

  • The level of sensitivity of the equipment you want to plug in the UPS.
  • The quality of UPS output voltage and frequency,
  • Waveform of the output voltage
  • The output performance under transient loading conditions. (These conditions are defined in IEC 62040-3, the international standard for testing of UPS performance).


Table-1 indicates how a UPS’s input dependency performance relates to seven common types of power problems.

From Table-1, it’s clear that a VFI UPS is the highest performance among the three classifications because it can provide protection from power interruptions and perform both voltage and frequency regulation.


7 types of power problems

Problems addressed by classifications




Power interruption

Voltage sags and swells

Voltage transients


Sustained over and under voltages


Voltage waveform distortion



Voltage fluctuations



Frequency variations





*some VFD UPSs may include surge suppression and filtering components to address voltage transients.

** within the  VI category, only on-line UPSs are able to address voltage fluctuation and waveform distortion problems, such as harmonics, inter-harmonic, notching, etc.




Based on the above three classifications (VFD, VI & VFI), A variety of static UPS types are available in the market, and each has distinct performance characteristics. The most common types are as follows:

  1. Off-line or Standby UPS,
  2. Line Interactive UPS,
  3. Standby-Ferro UPS,
  4. Online Double Conversion UPS,
  5. The Delta Conversion On-Line UPS.





1- Off-line or Standby UPS




The offline or Standby UPS is the most common type used for Personal Computers. The topology of Offline UPS is classified as VFD as per IEC 62040-3 which its output voltage & frequency is dependent on input. Offline UPS passes the AC mains supply directly to the output load if the AC Mains supply is present. Only in case of power failure, it switches to inverter within few milliseconds to ensure uninterrupted power to load until mains supply returns, the inverter only starts when the power fails, hence the name "Standby." See fig.1


Fig-1 offline or Standby UPS

The main benefits of this design are High efficiency, small size, and low cost. In addition, with proper filter and surge circuitry, these systems may provide adequate noise filtration and surge suppression. See fig.2


Fig-2 offline or Standby UPS




2- Line Interactive UPS




The Line Interactive UPS, indicated in Fig-3, is the most common design used for small business, Web, and departmental servers. The topology of Line interactive UPS is classified as VFI as per IEC 62040-3 that its output voltage is independent from input.


fig-3 Line Interactive ups

In this design, the battery-to-AC power converter (inverter) is always connected to the output of the UPS. Operating the inverter in reverse during times when the input AC power is normal provides battery charging.

When the input power fails, the transfer switch opens and the power flows from the battery to the UPS output. With the inverter always on and connected to the output, this design provides additional filtering and yields reduced switching transients when compared with the Standby UPS topology.

In addition, the Line Interactive design usually incorporates a tap-changing transformer. This adds voltage regulation by adjusting transformer taps as the input voltage varies. Voltage regulation is an important feature when low voltage conditions exist, otherwise the UPS would transfer to battery and then eventually down the load. This more frequent battery usage can cause premature battery failure.

However, the inverter can also be designed such that its failure will still permit power flow from the AC input to the output, which eliminates the potential of single point failure and effectively provides for two independent power paths.

The main benefits of this design are High efficiency, small size, low cost and high reliability coupled with the ability to correct low or high line voltage conditions

They are available with ratings up to 10 kVA, allowing wide input voltage tolerances.






3-    The Standby-Ferro UPS




The Standby-Ferro UPS was once the dominant form of UPS in the 3-15kVA range. This design depends on a special saturating transformer that has three windings (power connections). The primary power path is from AC input, through a transfer switch, through the transformer, and to the output (see Fig-4)


Fig-4 The Standby-Ferro UPS

In the Standby-Ferro design, the inverter is in the standby mode, and is energized when the input power fails and the transfer switch is opened and the inverter picks up the output load.

Standby-Ferro UPS systems are frequently represented as On-Line units, even though they have a transfer switch, the inverter operates in the standby mode, and they exhibit a transfer characteristic during an AC power failure. Figure 3 indicate this Standby-Ferro topology.



  • High reliability,
  • Excellent line filtering: The isolation from AC power transients provided by the Ferro transformer is as good as or better than any filter available. The transformer has a special "Ferro-resonant" capability, which provides limited voltage regulation and output waveform "shaping".



  • The design has very low efficiency combined with instability when used with some generators and newer power-factor corrected computers, causing the popularity of this design to decrease significantly. The principal reason why Standby-Ferro UPS systems are no longer commonly used is that they can be fundamentally unstable when operating a modern computer power supply load. All large servers and routers use “Power Factor Corrected” power supplies which draw only sinusoidal current from the utility, much like an incandescent bulb. This smooth current draw is achieved using capacitors, devices which ‘lead' the applied voltage, Ferro resonant UPS system utilize heavy core transformers which have an inductive characteristic, meaning that the current 'lags' the voltage. The combination of these two items form what is referred to as a 'tank' circuit. Resonance or 'ringing' in a tank circuit can cause high currents, which endanger the connected load.
  • The Standby-Ferro generates a great deal of heat because the Ferro-resonant transformer is inherently inefficient.
  • The Ferro transformer itself creates severe output voltage distortion and transients, which can be worse than a poor AC connection.
  • These transformers are also large relative to regular isolation transformers; so standby-Ferro UPS are generally quite large and heavy.





4-    Online Double Conversion UPS




The topology of Online UPS is classified as VFI as per IEC 62040-3 which is output voltage & frequency is independent from input. Online Double conversion UPS is the most widely used UPS topology to protect the critical loads above 10kVA. As shown in the below Fig-5, the online double conversion UPS has:

  1. Rectifier Converts AC-DC and charges the battery,
  2. Inverter Converts DC-AC and sup ports the loads connected to it,
  3. Bypass Secondary power, normally the mains power to support in case of emergency.


Fig-5 Double Conversion UPS

Under normal conditions, the mains power with all the impurities as voltage variation, frequency variation etc are converted to DC by the rectifier and from the DC source an AC Voltage is generated by the inverters.

The battery is connected to the DC bus of UPS and in the event of power failure the battery takes over the load immediately and there is no change over or transfer time. As there is two conversions of power AC-DC and DC-AC, this topology is widely called as double conversions UPS.



  • The double conversion on-line system provides complete isolation of the load from mains service, thus providing high-quality conditioned power to the loads.



  • The constant wear on the power components reduces reliability over other designs.
  • The input power drawn by the large battery charger is often non-linear and can interfere with building power wiring or cause problems with standby generators.
  • the increased cost.
  • As all power in both normal and backup modes of operation is channelled through the UPS, there is a decrease in efficiency, resulting in an increase in heat output.





Operating Principle of Double Conversion UPS


The Double Conversion UPS has three modes of operations (see Fig-5):

  1. Normal mode,
  2. Battery mode,
  3. Bypass Mode.


1- Normal mode:

  • This is the most frequent operating condition and During normal operation of a double-conversion UPS system,
  • Power flows from the input source, through the rectifier, across the DC Bus to the inverter.
  • The inverter provides power to the critical load.
  • The inverter is constantly synchronized with the auxiliary mains to enable load transfer to bypass (due to an overcurrent or inverter shutdown) without any break in the power supply to the load.
  • If the batteries are charging, power will also flow from the DC Bus (rectifier) into the batteries to maintain or recharge them.


2- Battery mode:

  • In the event of input power failure (micro interruptions or extended black-outs), power is drawn from the batteries through the DC Bus to the inverter. This is true for all topologies of UPS systems, not just double conversion.
  • The UPS system keeps the user constantly informed on the status of battery and on the remaining back-up time according to the battery capacity and based on connected loads in the UPS.
  • Online Double conversion UPS is the only type of UPS used widely as they have many advantages over the other topologies.
  • The output voltage & frequency is completely independent from the mains supply.
  • No break transfer to battery mode or bypass mode


3- Bypass Mode:

This mode may occur in the following situations:

  1. In the event of a temporary overload, the inverter continues to power the load. If the condition persists, output is switched onto the auxiliary mains via the automatic bypass.
  2. When the voltage generated by the inverter goes out of tolerance due to a major overload or a fault on the inverter
  3. When the internal temperature exceeds the maximum value allowed

It’s important to remember that when a system is in bypass, the load is not protected. If the input power is lost, the load will lose power.

There are two types of bypasses (see Fig-6):

  1. Static bypass,
  2. Maintenance bypass.


Fig-6 UPS bypass switches 

1- Static bypass:

  • It gets its name from the original UPS systems using a “static switch” to bypass the rectifier and inverter when a fault occurs. This allows the load to continue to operate from unprotected power.
  • The switch is called static because it is an electronic switch that is solid state, not mechanical.
  • The static switch provides a method to bypass the rectifier and inverter when maintenance must be completed without shutting down the load.


2- Maintenance bypass:

  • It Is external manual switch separated from the UPS, It is usually a wrap-around switch either of rotary or MCB design , this switch ensures a continuous flow of power while your UPS is undergoing maintenance or repair. 
  • Without it, your network may suffer significant downtime, leading to lost revenue and productivity. 






5- The Delta Conversion On-Line UPS




This UPS design, illustrated in Fig-7, is a newer, 10 year old technology introduced to eliminate the drawbacks of the Double Conversion On-Line design and is available in sizes ranging from 5kVA to 1.6MW.

Similar to the Double Conversion On-Line design, the Delta Conversion On-Line UPS always has the inverter supplying the load voltage. However, the additional Delta Converter also contributes power to the inverter output.


Fig-7 Delta Conversion On-Line UPS  

A simple way to understand the energy efficiency of the delta conversion topology is to consider the energy required to deliver a package from the 4th floor to the 5th floor of a building as shown in Fig-8. Delta Conversion technology saves energy by carrying the package only the difference (delta) between the starting and ending points. The Double Conversion On-Line UPS converts the power to the battery and back again whereas the Delta Converter moves components of the power from input to the output.


Fig-8 Comparison between Delta and Double Conversions  

In the Delta Conversion On-Line design, the Delta Converter acts with dual purposes as follows:

  • The first is to control the input power characteristics. This active front end draws power in a sinusoidal manner, minimizing harmonics reflected onto the utility. This ensures optimal utility and generator system compatibility, reducing heating and system wear in the power distribution system.
  • The second function of the Delta Converter is to control input current in order to regulate charging of the battery system.


The Delta Conversion On-Line UPS provides the same output characteristics as the Double Conversion On- Line design. However, the input characteristics are often different. Delta conversion on-line designs provide dynamically-controlled, power factor corrected input, without the inefficient use of filter banks associated with traditional solutions.

The most important benefit is a significant reduction in energy losses.

The input power control also makes the UPS compatible with all generator sets and reduces the need for wiring and generator oversizing.

Delta Conversion On-Line technology is the only core UPS technology today protected by patents and is therefore not likely to be available from a broad range of UPS suppliers.

During steady state conditions the Delta Converter allows the UPS to deliver power to the load with much greater efficiency than the Double Conversion design.






Comparison of UPS Topologies




The following table in Fig-9 shows comparison of UPS topologies in handling power quality issues. Some attributes of a UPS, like efficiency, are dictated by the choice of UPS type. Since implementation and manufactured quality more strongly impact characteristics such as reliability, these factors must be evaluated in addition to these design attributes.


Fig-9 Comparison of UPS Topologies


In the next Article, I will continue explaining other Classifications of UPS Systems like:

8- Distribution Architecture,

9- Use of transformers.

So, please keep following.


Subject Of Pervious Article


Applicable Standards for UPS Systems

What is a UPS?

Why do we need a UPS?

UPS Rating

Classification of UPS:

1-Voltage range,

2-No. of phases,

3- Mobility,

4- Technological design,

Classification and Types of UPS – Part One

5- Physical Size/capacity,

6- Form factor/ configurations:

6.1- “N” System Configuration

Classification and Types of UPS – Part Two

6.2- “N+1” System Configuration, which includes:

  • Isolated Redundant Configuration (N +1)
  • Parallel Redundant Configuration (1+1)
  • Parallel Redundant Configuration (N +1)
  • Parallel Redundant Configuration (N +2) and so on

6.3- Parallel Redundant with Dual Bus Configuration (N+1 or 1+1)

Classification and Types of UPS – Part Three


6.4- Parallel Redundant with STS Configuration

  • Parallel Redundant Configuration (1+1) + STS
  • Parallel Redundant Configuration (N+1) + STS 

6.5- System plus System 2(N+1), 2N+2, [(N+1) + (N+1)], and 2N

Classification and Types of UPS – Part Four




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