THE DIFFERENCE BETWEEN TRANSFORMER BASED AND TRANSFORMERLESS UPS
Total power loss can be one of the most devastating events for any business. Critical systems can no longer function, manufacturing ceases and, perhaps worst of all, crucial data is at risk. The solution is at hand however, by specifying a suitable uninterruptible power system (UPS). But, with so many different alternatives to choose from, which is most appropriate for your circumstances?
Nowadays, the traditional transformer-based on-line UPS is being usurped by the latest transformerless design, at power ratings below 4OkVA. The advent of sophisticated microprocessor controlled electronics and advancements in power electronics have allowed UPS manufacturers to produce ever more capable devices at lower costs. And, because the latest breed of transformerless on-line UPSs dispense with conventional bulky and heavy transformers, they offer a highly compact and cost-effective solution for many modern power protection environments.
Make no mistake, in most circumstances these smaller, lighter and lower cost UPSs offer an ideal solution for any organisation looking to provide back up power for their computer and telecoms equipment. As with most things, however, there is a trade off between certain characteristics, some of which are an essential consideration for particular applications, while others assume a lesser importance.
An average transformerless on-line UPS with a reasonable specification can provide adequate power protection for a wide range of critical devices. Because of the recent proliferation of transformerless UPSs, most customers will be able to find a suitable device within their budget, boasting a specification that appears to meet their needs. However, comparative specifications can be misleading and users that operate in high risk mission critical sectors such as telecoms and military environments, or those operating highly sensitive equipment such as mobile communications base stations and file servers, should examine exactly what they require from their UPS, before committing to a particular technology
Briefly, a transformer-based on-line UPS boasts several intrinsic advantages compared to the more recent transformerless technology For many years, experienced electrical engineers have been aware of the fact that transformerless UPS devices are susceptible to interference from spikes and transients caused by a variety of devices such as photocopiers, laser printers and motors. The ‘dirty’ mains that this produces can be transferred through a UPS, and feeds the connected load.
This means that, in practice, a transformerless UPS has to incorporate some sort of electronic mains filter. Otherwise, it can do nothing to improve the quality of raw mains that can be variable to say the least.
In contrast, because the transformer itself constitutes a physical barrier, a transformer-based on¬line UPS completely isolates the load from the worst vagaries of the mains supply Therefore, in circumstances where the load is likely to be affected by a very large variation in its power supply, a transformer-based UPS provides a safer and more robust solution than transformerless technology —simply because its size and construction affords some inertia between the input and output waveforms, with no additional electronic filtering required.
The above has nothing to do with transformerless, would make sense when comparing double conversion with standby/line interactive topology (3 and 5 series)
Also, because transformer-based UPSs inherently contain Galvanic Isolation, the power supply fed to the load is invariably superior to the mains supply itself. This attribute alone can be a major consideration for a number of crucial applications and installations. In fact, the latest electrical standards for medical installations (BS-EN 60601 and 61558.2) require that critical devices be connected through a Galvanic Isolation transformer, rather than directly to the raw mains.
A transformer based UPS rarely provides galvanic isolation, with the exception of a few Powerware 9315 models and some older topologies (magnetically stabilized) like the Ferrups. The reason for this is, that when a static bypass switch is incorporated, the neutral of the bypass is connected to the UPS output, thus bypassing the galvanic isolation. When galvanic isolation is desired, a isolation transformer is equally needed for a transformerless and a transformer based UPS.
Throughout this paper, transformer-based is incorrectly confused with galvanically isolated and/or magnetically stabilized (ferroresonant) operation.
Any electrical installation that requires the utmost reliability needs to deploy proven solutions to minimise the likelihood of potential equipment failure. One of the major advantages of transformer-based UPS devices is their higher mean time before failure (MTBF). Because these devices use traditional electrical engineering principles instead of a high concentration of electronic components, they are inherently more reliable than a corresponding electronic circuit board assembly
The above applies mainly to magnetically stabilized technology
By contrast, transformerless UPSs employ sophisticated electronic circuitry to mimic the on-line characteristics of a transformer-based device, as well as providing suitable standards of mains filtering. As such, they can be more prone to failure on sites where there is a high degree of mains borne pollution such as transients, spikes and electrical noise.
That’s why most experts will only recommend a transformer-based design within industrial and/or critical environments such as medical and transportation infrastructure.
A design can be more or less robust independently of a single component like a transformer.
However, it should still be noted that, because transformerless designs use plug-in circuit boards, they are actually quicker to repair than transformer-based units. In contrast, faults with transformer-based designs can be more difficult to diagnose on-site.
The above cannot be applied generally, this has to be referring to some particular design.
Of course there are some very compelling reasons to specify a transformerless UPS. Among these is the fact that conventional transformer-based uninterruptible power systems are significantly larger than their transformerless brethren. The main reason for this is that the transformer itself takes up a large amount of space within the cabinet, which results in the transformer-based UPS being significantly heavier, and therefore not as convenient or portable as the transformerless variant.
Moreover, transformer-based uninterruptible power systems operate typically 10% less efficiently than their transformerless stable mates, as the transformer itself causes heat to build up within the vicinity of the device, leading to greater care in terms of airflow and air conditioning. Some transformerless on-line UPSs also incorporate an extra economy mode to further boost efficiency up to 99%.
The potential anti-social characteristics of the transformer-based UPSs are heightened by the noise they can make—frequently a loud and intrusive hum— which can often be compounded by their cooling fan noise. In practice this means that this type of UPS is often unsuitable for siting in an office or small computer and telecoms rooms, particularly where the levels of noise and heat emitted might cause a problem for staff.
Transformerless UPSs are quieter in operation, and run considerably cooler. Conversely the heat and noise generated by transformer-based UPSs makes them significantly less efficient in operation than transformerless devices. This makes them more costly to run, with a higher current drain than the electronic switching circuitry in a transformerless UPS.
Although transformerless UPSs are more complex in design, because the majority of components are electronic, they are cheaper to manufacture than the transformer-based variety Rather like mass-produced electronics, many transformerless UPS manufacturers can take advantage of significant economies of scale in their manufacturing processes, further reducing the costs of designing, developing and producing this type of uninterruptible power supply Transformer-based UPS devices differ significantly in that they are more specialist electrical items, and therefore cannot benefit to the same degree from mass produced off the shelf electronic components. This makes them typically up to 30% more expensive to purchase than transformerless devices.
What this suggests therefore is that, while the transformer-based UPS might be the ideal choice in certain circumstances, economic considerations, together with issues of space and environmental impact, may sway the decision towards a transformerless model. There is no doubt that transformer-based devices are ideal for sites that experience heavily polluted mains supplies —particularly industrial, rural and complex infrastructure locations, such as hospitals. In these circumstances, any UPS would be expected to offer dependable long-term protection from repetitive transients and electrical noise.
Despite containing sophisticated electronic circuits that smooth out such anomalies, transformerless UPSs cannot fully isolate the load from the mains, unlike a transformer-based device. This is why several transformerless on-line UPS manufacturers offer additional cabinets to house filters and isolation transformers so that their systems can approach the benefits required for industrial and heavily polluted environments.
To achive a separately derived source with independent grounding, an isolation transformer is needed whether the UPS employs a transformer based design or not.
In addition, a traditional transformer-based UPS will prove more reliable over an extended period than sophisticated but sensitive electronic component based filters. This makes the transformerless UPS more suitable for circumstances where the incoming mains supply is less polluted. Equally they are more suited to modern office-based environments where cost and footprint are more significant considerations, and where the UPS may be likely to be sited in full view of employees and visitors alike.
Again, the above is not relevat to most designs.
Of course, there are certain situations where a transformerless UPS will have an advantage. For example, where there are sudden step load changes on the UPS, a transformerless device can react immediately while a transformer-based unit will take a little time to respond. Whether this dynamic weakness affects the load depends very much on its individual characteristics.
Applies to ferroresonant technology. Normal transformer based inverters designed since the mid 1980’s have the same superiour voltage control as the transformerless ones.
In addition, a transformerless unit can provide input power factor correction capabilities as an integral part of its electronic circuitry This is a major advantage for certain types of load. In this case, power factor leading or lagging can be caused by the connected load, which a transformer-based UPS tends to delay As such, this type of UPS requires a separate filter or power factor correction device.
The corollary of this is that — because of the principle of Galvanic Isolation — a transformer-based unit is incapable of generating DC current that might be passed to the connected load. In contrast, a transformerless UPS will pass on the DC current. Because DC current causes up to a lOx heating effect compared to an alternating current, this inevitably stresses the unit. Additionally a transformer-based UPS will allow a load that has a high neutral current requirement to take such a current, by simply passing this through from its input.
That statement is confusing. One of the major benefits of the standard three phase transformer based UPS design is, that the transformer will take care of all triplen harmonics of the load, and not passing it through to the input or to the inverter IGBTs.
This gives a higher capability of supplying non linear load conpared to a transformerless design with the same inverter sizing.
Additionally a transformer based three phase phase inverter provides 1.7 times the short circuit current into a short circuit. This will help when planning a selective downstream fusing scheme.
In summary therefore, choosing the most suitable UPS for your needs is rather like deciding between a diesel and petrol car. When selecting the right technology to employ the user has to balance short and long term benefits against budget and overall life cycle costs. Consultation from a supplier that can offer an impartial recommendation from both technologies should lead to the adoption of a secure and robust power solution.
