1). Transformer based UPS

2). Transformer less UPS

3). Transformer less UPS with external input/output transformer

The answer to this fundamental question is determined by the technical necessities of downstream electrical infrastructure, which are based on the following factors.

• The capacity to join two separate sources for increased availability.
• DC (Battery) galvanic isolation from AC output
• UPS Short Circuit Characteristics
• Design Safety Margin
• Neutral grounding on a local level
• Effectiveness

Transformer-Based UPS

In a transformer-based UPS, the transformer is incorporated into the inverter’s output and provides galvanic isolation across the rectifier and the UPS output. This allows to connect two distinct sources to the rectifier & bypass without affecting the existing earthing system.

Working of a Transformer UPS

Transformer-based UPS systems are the most commonly used power supply system for 100 kVA & above, with the ability to produce enormous kW sizes or provide redundancy. 

To ensure the safety of the load, the machine features an isolated DC function. This transformer efficiently isolates the inverter output’s DC component from the load, enhancing load protection, particularly for switches & inductive loads. 

Transformer-based UPS have a number of benefits of stability, reliability, large overload capacity, & impact resistance over transformer less UPS. Furthermore, because the UPS is transformer-based, it can be put in tougher working situations than transformer less systems.

When dependability and stability are critical, a transformer-based UPS system is recommended in finance, data center building, transportation, & basic industrial) applications that involve oil, steel, and metal.

Advantages of Transformer-Based UPS

A transformer-based UPS has two primary advantages. 

• For initially, it is widely acknowledged that they tend to be more robust – there are fewer sources of failure. 
• Second, the transformer gives galvanic isolation, which is a separation of the input & output supplies that protects the load from spikes, surges, and electrical noise. 

Transformer-based UPS are the most common technology for 100 kVA and higher, and they are the best option for achieving huge kW sizes or providing redundancy.  

The following are the primary advantages of transformer-based UPS: 

• Isolation using galvanic current.
• independent primary electricity supplies.
• DC voltage protection for two loads.
• A phase-neutral inverter short circuit current is greater than a phase-phase short circuit. current. When faced with power quality issues, superior power protection is provided. 
• increased robustness in terms of back feed protection.

Transformer Less UPS

Because there is no galvanic isolation transformer that connects the rectifier input and output in a transformer less UPS, an external isolation transformer is needed either at the rectifier input or at the bypass input to allow for the flexibility of connecting two distinct sources to the rectifier & bypass without changing or disrupting the existing earthing system.

It is banned to connect two distinct sources for rectifier mains & bypass because the neutrals of both sources will get connected and cause a short circuit.

• ELCB nuisance tripping upstream of UPS.
• Ground fault detectors are inactive.
• EMC is a problem caused by circulation current on neutral of both sources.

Once a transformer is connected to the UPS bypass, the co-ordination of the downstream safety mechanisms is disrupted and must be re-coordinated depending on the transformer’s short circuit current capability.

Working of a Transformer Less UPS

A transformer-free UPS operates in the same way as a transformer-based UPS, with the difference that the transformer-free UPS employs an insulated-gate bipolar transistor (IGBT) capable of handling high voltages, removing the requirement for a step-up transformer after the inverter. 

This industrial standard increases energy efficiency from 90% to 96%. Importantly, transformer-free UPSs are smaller and lighter, cutting investment and running expenses.

The weight, space, and cost advantages of transformer-free UPS systems at 200 kW power frequently outweigh the superior capacity capabilities and resilience of transformer-based systems. 

All under-200 kW applications offer high efficiency & efficient input power conditioning because of their transformer-free design. 

Because of their modular architecture and hence scalability, transformer-free UPS can prevent excessive provisioning of power despite retaining operating efficiency.

Advantages of Transformer-Less UPS

• The absence of a large, bulky, & heat-generating transformer is an obvious advantage of a transformer less UPS. Transformers are also expensive, therefore eliminating them lowers the initial capital expenditures. 
• Decreased size and weight (important for data centers with limited space). 
• Higher operational efficiency (especially at less loads), reduced levels of noise,  less heat. 
• Purchase, installation, and operating costs are cheaper (i.e., less air conditioning is required).

Disadvantages of Transformer-Less UPS

• One of the primary disadvantages of transformer-free UPS systems is that they cannot clear and isolate internal problems as effectively as transformer-based units.  
• Using isolation transformers to replicate the robustness of a transformer-based system is one solution, but it would considerably increase cost and footprint while also creating extra sources of failure. 
• Another difficulty with transformer-less UPS power sources is their limited power strength. To reach a higher kW capacity or redundancy, many transformer-free UPS modules must be paralleled together – the more modules (as well as components) there are, the greater the possibility of failure.

Technical Difference Between Transformer-Based UPS and Transformer-Less UPS

Features	Transformer-Based UPS	Transformer-less UPS
Double conversion mode efficiency	90-93%	95-97%
Eco-friendly efficiency	98%	99%
THDI (total harmonic distortion of the input current)	(IGBT-based rectifier) 3-4% (thyristor-based rectifier with 12 pulses) 12% (6-pulse thyristor) 30%	3-4%
Total harmonic distortion of Output voltage (THDU)	2%	2%
Input PF or input power factor	inadequate partial load	0.99 to 0.97 at full & reduced load
AC ripple on battery	greater than 5% without a charge	0.2% when a battery charger is used.
Bypass function	Switch for static bypass	Switch for static bypass, yet the inverter’s AC output must match the bypass AC input
Output Impedance	High	Low
Dynamic response	Poor and imbalanced loads have an impact on output voltages	The output sine wave is directly controlled in the ideal case, with independent control of each phase. Unbalanced loads therefore have no impact on the output voltages

Which type of UPS is superior, transformer-based or transformer-free?

Following a discussion of the technical distinctions between transformer-free & transformer-based UPS, the following section will contrast the two UPS designs from the standpoint of the consumer and explain each one’s benefits.

1). Adaptability

A transformer-free UPS offers more adaptability and flexibility for handling ambiguous future requirements. 

A UPS without a transformer is typically smaller in size than one with a transformer that can handle the same amount of power. 

It allows for more expansion in a provided space. Additionally, large transformer-free UPS systems currently use a modular architecture and a small footprint so that additional capacity can be added with ease if the total load need is uncertain or in the event of future needs.

2). Reliability 

A UPS without a transformer is unable to isolate internal defects the same way a transformer-based UPS can. 

Although this issue can be resolved by including an isolation transformer, doing so may also result in more failure spots and higher unit service rates. 

As a result of generations of application experience & technological advancements, transformer-based UPS designs exhibit improved reliability and robustness. 

The isolation transformer inside a transformer-based UPS design improves the reliability of the DC link.

3). Availability

Typically, transformer-based UPSs offer the maximum availability while streamlining fault current control, particularly for high power data centers & other crucial applications. 

Transformer-free designs currently also increase their accessibility to a respectable level, albeit less so than transformer-based variants. 

Transformer-free UPS is therefore more practical for small and medium-sized organizations when availability shouldn’t be the main concern.

4). Battery life 

The battery’s AC ripple and battery life are related. A second converter is typically added to the transformer-free design because to the high voltage in transformer-free UPSs, which allows for accurate battery control and a ripple-free, clean DC circuit. 

Thus, the transformer-free UPS has a battery life that can last up to 12 years, which is better than the transformer-based UPS.

5). Cost

 The initial purchase price and ongoing operational costs for UPS power units must both be taken into account. 

Transformer-free UPSs naturally benefit from being less expensive than transformer-based ones because they don’t have a transformer place inside of them. 

Additionally, transformer-free UPS are smaller than transformer-based UPS, necessitating less additional room in the data center. 

Included in running costs are expenses for power, maintenance, and other things. Power losses are decreased by transformer-free UPS units’ better efficiency.

Summary

Both transformer-based & transformer-free UPS options provide basic functionality while retaining their primary characteristics. 

It is apparent that transformer-free UPS leads the market these days; transformer-free UPS meets the majority of today’s power supply design needs; however, transformer-based UPS is the ideal choice for applications needing the highest dependability and safety, such as medical fields.