What Are The Differences Between Transformer-Based ...

Also referred to as transformer-free, transformerless UPS systems were first developed in the s and offered a number of benefits over traditional transformer-based systems in terms of higher efficiency, reduced size and weight, and cost savings. 

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Transformerless uninterruptible power supplies are now common in data centre environments and with smaller installations. They are the typical technology for the smallest power ratings (below 10 kVA) and are available up to around 300 kVA at the higher end of the spectrum. Riello UPS's range of transformer-free solutions includes the Sentryum, Multi Sentry, and NextEnergy series. 

Available from 10 kVA and above, transformer-based UPS are still a popular choice in industrial process environments or installations requiring galvanic isolation. 

A transformer is a wound component consisting of windings around a core, with iron sheet laminates that can be used to change voltage levels and provide galvanic isolation. 

How Do Transformer-Based And Transformer-Free UPS Work?

In a traditional transformer-based UPS, the power flows via the rectifier, inverter and transformer to the output, with the transformer used to step up the AC voltage levels, protect the UPS from load disruptions and provide galvanic isolation. 

Transformerless UPS or transformer-free UPS operate in the same way, apart from one key difference. It uses insulated-gate bipolar transistors (IGBTs) that are capable of dealing with high voltages, eliminating the need for a step-up transformer after the inverter. This improves the energy efficiency of transformer-free uninterruptible power supplies.  

Thanks to R&D and technological improvements, the latest transformer-based UPS can achieve similar levels of efficiency as many transformerless systems (95-96%), although the latter still has the efficiency edge when carrying lower loads as it delivers a flatter efficiency curve. 

What Are The Advantages Of Transformer-Based UPS? 

There are two main benefits of a transformer-based UPS. Firstly, it is generally accepted that they are more robust &#; there are less points of failure. Secondly, the transformer provides galvanic isolation, a separation of the input and output supplies, which protects the load from any spikes, surges, or electrical noise. 

Transformer-based UPS are the typical technology for 100 kVA and above and the choice to achieve large kW sizes or provide redundancy.  

Main benefits of Transformer-based UPS: 

• Galvanic isolation 
• Independent mains power supplies 
• Dual load protection from DC voltage 
• Providing a higher phase-neutral inverter short circuit current than a phase-phase short circuit current 
• Superior power protection when presented with power quality problems 
• Greater robustness with respect to back feed protection 

What Are The Advantages Of Transformer-Free UPS? 

The obvious benefit of a transformerless UPS is the lack of a big, bulky, and heat-generating transformer. Transformers are expensive too, so eliminating them reduces initial capital costs. 

Main benefits of Transformeless UPS: 

• Physical: reduced size and weight (a factor for space-restricted data centres) 
• Operational: higher energy efficiency (particularly at lower loads), lower noise levels, and less heat 
• Cost: lower purchase, installation, and running costs (i.e. needs less air conditioning) 

One of the main drawbacks with transformer-free UPS systems is they can&#;t clear and isolate internal faults as well as a transformer-based unit.  

A solution to this is installing isolation transformers to mirror the strength of a transformer-based system, but this would significantly increase cost and footprint, while also introducing additional points of failure. 

Another issue with transformerless UPS power supplies is their power strength limitations. To achieve larger kW size or redundancy, several transformer-free UPS modules need to be paralleled together &#; the more modules (and components), the greater the likelihood of failure. 

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Further Reading: 

An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails. *Note: A UPS will beep continuously when power is off. This is normal. The UPS is only a temporary power ..

UPS - uninterrupted power supplies to power computers and laptops

An uninterruptible power supply, also uninterruptible power source, UPS or battery/flywheel backup, is an electrical apparatus that provides emergency power to a load when the input power source, typically mains power, fails.

*Note: A UPS will beep continuously when power is off. This is normal.

The UPS is only a temporary power backup (5 to 15 minutes) and it is not intended for continuous use.  You will need to switch over to a generator or power your PC down.

An inverter is meant for continuous power and will not beep continuously.

Offline UPS

Offline UPS connects the computer to the ESKOM's power grid (or to a generator) until the voltage falls below a certain level, in which case, the UPS switches over to battery backup. The switch-over time can be as long as 25 milliseconds depending on the amount of time it takes the standby UPS to detect the lost utility voltage. The UPS will be designed to power certain equipment, such as a personal computer, without any objectionable dip or brownout to that device.

Online UPS

Online UPS connects the computer directly to the battery backup at all times. This type of UPS is able to tolerate continuous under-voltage brownouts and over-voltage surges without consuming the limited reserve battery power. It instead compensates by automatically selecting different power taps on the auto-transformer.

Double conversion UPS

The primary power path is the inverter / battery and it is not the AC mains. This means that the output power is supplied by the battery all the time and no switch-over happens if power fails (0ms switch over)

If the battery / inverter fails, the AC is used as the backup power.

The advantage is that there is no switch over if AC fails, but the disadvantages are additional heat, lower efficiency and reduced life-span of the components in the UPS.

Other UPS features may include:

• AVR - Automatic Voltage Regulator
• Monitoring ports and software
• Built-in Surge Protection
• Audible alarms for power outage and low battery

What is the difference between VA and Watts

UPS are rated in VA (Volt-Amps).

This is a basic rating for UPS and is the Voltage multiplied by the Amperage the UPS handles.

At 220Volts, a UPS that can supply 1Amp would be rated 220VA.

This however is not the real power for AC devices because AC power rating requires the power factor to be taken into account.

The industry standard rule-of-thumb: plan for a power factor of 60% (this is an average conservative power factor).  The power that AC products can draw from the UPS is therefore approximately 60% of the VA rating.

A VA UPS would therefore have an AC power rating of approximately 600Watts.

What is power factor?

In AC circuits, the power factor is the ratio of the real power that is used to do work and the apparent power that is supplied to the circuit.

The power factor can get values in the range from 0 to 1.

When all the power is reactive power with no real power (usually inductive load) - the power factor is 0.

When all the power is real power with no reactive power (resistive load) - the power factor is 1.

Typical power factors
Resistive load = 1
Fluorescent lamp = 0.95
Incandescent lamp = 1
Induction motor full load  = 0.85
Induction motor no load  =  0.35
Resistive oven  = 1
Synchronous motor = 0.9

UPS keeps switching on and off while using a generator

UPS are sensitive to the voltage and frequency on its input - this is the way it detects whether it should switch to battery backup mode or not. This is normal operation for a UPS.

Some generators, especially the cheaper "home use" generators, are not always calibrated accurately when it comes to frequency (50Hz in South Africa), while the voltage seems to be in range. If the frequency is border-line, the UPS tends to switch over to battery mode whenever it falls out of range, and then switches off when it is in range again.

The best way to correct this on/off behaviour is to set the throttle on the generator to a point where the UPS becomes stable.

Note: A Voltage regulator / AVR will not correct the frequency and will do little to fix this behaviour. An AVR is all about Voltage, and the AVR will fix voltage fluctuations from a generator, protecting sensitive equipment from Voltage dips and spikes.

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