In my previous article, we have seen what is a UPS system and its necessity in data center infrastructure. Now let us discuss what are the different types of UPS systems and its various advantages. The varied types of uninterruptible power supplies (UPS) and their attributes often cause confusion in the data center industry. For example, it is widely believed that there are only two types of UPS systems, namely standby UPS and on-line UPS. These two commonly used terms do not correctly describe many of the UPS systems available. Many misunderstandings about UPS systems are cleared up when the different types of UPS topologies are properly identified. UPS topology indicates the basic nature of the UPS design. Various vendors routinely produce models with similar designs, or topologies, but with very different performance characteristics. Commonly there are five different types of UPS systems used which are,

• Standby(Offline)
• Line Interactive
• Standby-Ferro
• Double Conversion On-Line
• Delta Conversion On-Line

All five basic uninterruptible power supply (UPS) technologies have their place in protecting today’s distributed IT infrastructure especially on the network edge. Each technology has its advantages and each may be necessary for configuring cost effective power protection, especially in complex systems. Selecting a UPS for your particular application requires an examination of a number of factors. The load size, location and criticality of the equipment to be protected are key, as well budgetary considerations, when choosing a UPS for power backup.

Understanding how these various UPS designs work is critical to choosing the best UPS for a particular application. Let us discuss each of these in detail now,

Standby UPS(Offline UPS)

Offline UPS – also referred to as VFD (Voltage and Frequency Dependent) or Standby UPS – offer the most basic level of power protection. Standby UPS are the most basic models and designed for use in small, non-critical applications that require protection against momentary loss of power such as personal computers. The operating principal behind the standby UPS is that it contains a transfer switch which by default, uses filtered AC power as the primary power source. 

When AC power fails, the UPS switches to the battery by way of the transfer switch. As the battery-to-AC power converter, also known as the inverter is switched off when the UPS is operating normally, the term ‘Offline’ is given to any UPS of this design.

High efficiency, small size, and low cost are the main benefits of this design. With proper filter and surge circuitry, these systems can also provide adequate noise filtration and surge suppression. The limitations are that this type of UPS uses its battery during brownouts, which degrades overall battery life. Also, it is an impractical solution over 2kVA. It is not advised to use a VFD UPS to protect critical load or sensitive electrical equipment.

Line-Interactive UPS

Line Interactive UPS systems – also referred to as VI – Voltage Independent – operate similarly to an Offline UPS, with the addition of a built-in Automatic Voltage Stabilizer(AVS) which would generally be a transformer. The AVS ensures the output voltage remains within a pre-defined voltage window regardless of any voltage variations on the mains input supply. Voltage regulation is an important feature when variable voltage conditions exist, otherwise the UPS would frequently transfer to battery and then eventually down the load. This more frequent battery usage can cause premature battery failure. This enables Line Interactive UPS to provide protection against power sags, surges, and brownouts.

The Line Interactive UPS is the most common design used for small business, Web, and departmental servers. 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 reduces switching transients when compared with the Standby UPS topology.

The primary benefits of the Line-interactive UPS topology include high efficiency, small size, low cost and high reliability. Additionally, the ability to correct low or high line voltage conditions make this the dominant type of UPS in the 0.5-5kVA power range. The Line-interactive UPS is ideal for rack or distributed servers and/or harsh power environments. Over 5kVA, the use of a line –interactive UPS becomes impractical.

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. 

In the case of a power failure, the transfer switch is opened, and the inverter picks up the output load. 

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. The transformer has a special “Ferro-resonant” capability, which 

provides limited voltage regulation and output waveform “shaping”. The isolation from AC power transients provided by the Ferro transformer is as good as or better than any filter available. But the Ferro transformer itself creates severe output voltage distortion and transients, which can be worse than a poor AC connection.

Even though it is a standby UPS by design, the Standby-Ferro generates a great deal of heat because the Ferro-resonant transformer is inherently inefficient. These transformers are also large relative to regular isolation transformers; so standby-Ferro UPS are generally quite large and heavy.

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. 

The primary benefit of this design is high reliability and excellent line filtering. The limitations include 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. 

Double Conversion On-Line UPS

The Double Conversion On-Line UPS is the most common type of UPS above 10kVA. The design configuration is the same as the Standby UPS, except that the primary power path is the inverter instead of the AC main. 

The Double Conversion On-Line UPS converts AC power to DC and then converts the DC back to AC to power the connected equipment. The batteries are directly connected to the DC level. This effectively filters out line noise and all other anomalies from the AC power.

Failure of the AC Power does not cause activation of the transfer switch, because the input AC is charging the backup battery source which provides power to the output inverter. Therefore, during an AC power failure, on-line operation results in no transfer time.

There are certainly benefits and limitations of this UPS. A benefit is that it provides nearly ideal electrical output performance, with no transfer time. But the constant wear on the power components reduces reliability over other designs. Also, both the battery charger and the inverter convert the entire load power flow, resulting in reduced efficiency and increased heat generation. Additionally, the inefficiency of electricity energy consumption is a significant part of the life-cycle cost of the UPS. 

Delta Conversion On-Line

We have mentioned that the double conversion UPS has some drawback due to the load power is always drawn from the rectifier and inverter which reduces power efficiency. Thus the delta conversion technology is to eliminate those drawbacks. The UPS components are the same as double conversion, but it has one more element which is delta transformer. This design is available in ranges from 10 KVA up to 1 MVA.

As the primary power varies away from its normal value the inverter comes to life to make up the difference. 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.

In the Delta Conversion On-Line design, the Delta Converter acts with dual purposes. 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. 

This input power control makes the Delta Conversion On-Line UPS compatible with generators and reduces the need for wiring and generator over sizing. 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.

The benefits of Delta Conversion On-Line UPS include high efficiency, excellent voltage regulation, and overall reduction in life-cycle costs of energy in large installations. It is impractical in installations under 5kVA. 

I would strongly recommend you to watch the video by Titan Power Inc which gives you the atmost simple ways to understand these terminologies.

The following table shows some of the characteristics of the various UPS types. 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.

What do the different UPS Topologies Protect Against?

Online UPS are the preferred choice for mission-critical applications as they ensure ‘no-break switchover’ in the event of any mains power supply failure and an automatic internal bypass for safe failure to mains in the event of a fault.

EN/IEC 62040-3 defines three generic Static UPS topologies. Each topology is then further classified based on its output waveform and dynamic performance:

• VFI – Voltage and Frequency Independent: more generally known as ‘Online UPS‘ or ‘Double Conversion‘, where the UPS output is completely independent of any voltage and/or frequency abnormalities on the incoming mains supply. The highest classification for an Online UPS is VFI-SS-111. There are two main approaches to Online UPS – transformer-based or transformerless. It is known as ‘Double Conversion’ because of its two voltage conversion stages: rectifier (AC to DC) and inverter (DC to AC).
• VI – Voltage Independent: typically referred to as ‘Line Interactive UPS‘, where voltage fluctuations are stabilized by built-in regulation devices. Output frequency tracks the input frequency when the mains supply is present. 
• VFD – Voltage and Frequency Dependent: more commonly referred to as ‘Offline UPS‘ or ‘Standby UPS‘, where the output of the UPS output tracks the voltage and frequency of the input mains supply. When the mains supply fails, there’s a brief (4-8msec) break in supply while the inverter switches on and powers the load from the batteries.

The table below offers an easy comparison of the protection provided by Offline UPS, Line Interactive UPS, and double-conversion online UPS.

Knowledge Credits: Schneider Electric, Riello UPS

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