Evaluation of transformer-free UPS design vs transformer-based

QUARTINO, 21.03.2017 WHITE PAPER Evaluation of transformer-free UPS design vs transformer-based by Carlo Kufahl, Product Manager, 3-phase UPS 1/1

Evaluation of transformer-free UPS design vs transformer-based The real function of the transformer in a UPS The first double-conversion UPS where born in the 1970s and at that time the semiconductors available, on top of being bigger and less performing; they were not compatible with the voltage levels as today. Consequently, the UPS system at that time needed a transformer to step-up the voltage of the UPS output in order to provide to the critical load the same voltage rating of the input (mains). This is the original and only reason why there was the need of having a transformer integrated in the UPS. Evolution of double-conversion UPS UPS are subject to technology evolution on a 3-5 year rate, which is the same rate for the electronics and power-electronics evolution. Among many others, there has been one very important step of technology, which is worth explaining in this context because 1) its customer benefits are significant as we will cover in a moment and 2) it permitted to change from transformer-based design into transformer-free. It is the topology change of the power converters (rectifier and inverter), which started to happen at the very beginning of this century, from thyristor-based design first 6-pluse and later 12-pulse thyristor bridges into the today topology based on IGBT (Insulated Gate Bipolar Transistor). Today virtually all doubleconversion UPS have the power converters based on IGBT and are transformer-free. Differences between transformer-free and transformer-based design and its effect on the customer benefits Before we look into the design differences and its consequences/impact, let us briefly explain how the two topologies work, by means of single line diagrams (Figure 1 and Figure 2). Transformer-based UPS The UPS has two AC inputs, the rectifier and bypass inputs, one AC output and a DC connection for the battery. The rectifier and bypass inputs can be connected together (inside the UPS) in order to have one common input. During the normal working conditions, when mains is available, the normal power flow goes through the rectifier, inverter and transformer to the output and supplies the critical load. This working-mode is called double-conversion mode. In double-conversion mode, the battery is constantly kept at full charge (floating). During long or short power outages (mains at input side is not available) the battery feeds the inverter which, supplies power the critical load trough the transformer with no interruption. 2

The static bypass path acts as emergency path, coming in the game when there is a problem on the double-conversion path such as overload, over temperature or short circuit at the output side of the UPS. In some cases, the static bypass path becomes the primary power path. This working mode is commonly known as eco-mode; despite the fact that each UPS manufacturer is implementing small differences and is calling it differently, it is practically identical for all: the load is still protected in case of power outages but there is an interruption of typically 5-8ms during which the UPS has to transfer from static bypass to inverter. Finally, there is a maintenance bypass switch, can be UPS integrated or external when having a parallel UPS system. Its function is to bypass the unit connecting input to output directly and thanks to the other switches the unit can be isolated and serviced. Figure 1: single line diagram of a transformer-based UPS Transformer-free UPS The working principle of transformer-free UPS is the same as for the transformer-based, with the main difference that there is no transformer integrated; the newer IGBT technology allows working with higher voltages, hence there is no need of stepping up the voltage after the inverter. This has enormous effect on the energy efficiency improving it from 90% to 96%, which is industry-standard today. Furthermore, as it is easy to imagine transformer-free UPS are much lighter and occupy less footprint; less space means less investment cost and less weight means easier handling, which also ends up with a cost difference for the installation work. 3

Figure 2: single line diagram of a transformer-free UPS Also, because of working with higher voltage, an additional converter between the DC bus and the battery was introduced. It allows to keeping precise control over the batteries constantly and providing clean DC voltage, with no ripple, which maximizes the battery lifetime. But the advantages do not stop here. The total harmonic distortion gets dramatically low and the input power factor is resistive thanks to the active control of the input currents. This means that the devices which are upstream the of the UPS (e.g. generators) must not be oversized by a factor 1.5 or even more as it usually is/was with transformerbased UPS. Finally, some other differences can be identified at the output stage: the output impedance and the dynamic response in case of unbalanced load are better on the transformer-free UPS thanks to the direct control of the output sinus and the fact that each phase is controlled independently. On the contrary, the output short circuit capability on inverter results to be stronger on the transformer-based UPS. The same cannot be confirmed for the short circuit capability on bypass: the two designs achieve the same levels having similar if not equal circuitry for the bypass. Bear in mind that the fact that designers take into account also the short circuit of the inverter is because of power-supply design rules. In reality, it is more likely that a short circuit happens when mains is available which means that the short circuit gets cleared by the bypass hence the capability on the bypass is what counts. 4

Major technical specifications differences (typical) Let us summarize the major technical differences between the two topologies. Table 1: summary of the major technical differences between the two topologies. # transformer-based transformer-free 1 Efficiency on double conversion mode 90-92% 95-96% 2 Efficiency on eco-mode 99% 99% 3 Current Total Harmonic Distortion (THDi) 30% (6-pluse thyristor-based rectifier) 12% (12-pluse thyristor-based rectifier) 3-4% (IGBT rectifier) 3-4% 4 Input Power Factor low on partial load 0.99 0.97 at full and partial load 5 AC ripple on Battery without battery charger more than 5% with battery charger 0.2% 0.2% 6 Allowed number of battery blocks in series (12V) fix (typically 40) variable from 30 to 50 7 Output impedance high (worse) low (better) 8 Output fault clearing capability on inverter up to 5 x In (better) up to 3 x In (worse) 9 Output fault clearing capability on bypass up to 10 x In up to 10 x In 10 Dynamic response poor, unbalanced loads affect output voltages ideal, direct control of the output sinus, each phase is controlled independently, thus unbalanced loads are not affecting the output voltages 11 Weight (one 500kVA unit) 2.2 2.6 tons 1 ton 12 Footprint (one 500kVA unit) 1.8 2.0 sqm 1.5 1.6 sqm Major differences for the customers (effect on the customer benefits) Table 2: summary of the major differences between the two topologies for the customers. # transformer-based transformer-free 1 Investment cost 2 Operational cost $$$ due to higher UPS cost, higher installation cost (oversize of system upstream the UPS) and larger footprint. $$$ due to lower efficiency means higher energy costs for both UPS and cooling. $$ $ Higher efficiency reduced power losses and less cooling. During many years, the saving is significant. 3 Environmental impact for producing the product and transport to final location Higher than transformer-free due to more components (transformer) and bigger mechanical size for the unit Lower than transformer-based thanks to the same arguments 4 Environmental impact for operating the product Lower efficiency means higher power losses and more energy needed for cooling. Higher efficiency means less power losses and less energy needed for cooling 5

5 Battery life (Lead-Acid) may be dramatically reduced due to AC ripple up to 12 years From the above comparisons, it clearly results that the transformer-free UPS design has more advantages on his part and those that count today. This is somewhat obvious, otherwise we would not see that the IGBT based and transformer-free UPS design market has replaced the old designs. However, it is correct to reminding that both designs provide the fundamental functionalities of a doubleconversion UPS and both designs maintain the key characteristics such as availability, reliability and the expected life-time of the UPS is equal for the two. The non-topics for having a transformer-based UPS In addition to what was just explained, some people in the industry are still convinced that a transformer in a UPS has other functions. Isolate input from output Sometimes it is required that the UPS input and/or output are completely isolated from the rest of the electrical installation. At first, a transformer-based UPS seems to be ideal solution for that, having the isolation transformer inbuilt at the output of the rectifier and before the output terminals. Is this really so? In reality the isolation isn t there, looking back at Figure 1, we immediately see that when the UPS is in static-bypass mode or maintenance bypass mode the isolation between input and output is not there. Moreover, the Neutral-line is always not isolated, also in double-conversion mode. As the figure 3 shows, Neutral-line goes through from input to output without isolation. This is valid for virtually all UPS systems. Figure 3: 4-wire diagram of a transformer-based UPS. In order to have a complete and true isolation from the input of the UPS to the output, the best way would be to add external isolation transformers as showed in the example in Figure 4. It becomes clear that when there is the need of isolate input from output, the best choice results to 6

be a transformer-less UPS (higher efficiency, smaller footprint and lower weight) plus external isolation transformers. Figure 4: 4-wire diagram of a transformer-based UPS. Better filtering As the UPS itself is a device which works with switching frequency and PWM (rectification and creation of the output sinus by means of switching ON and OFF semiconductors), there are unwelcome phenomena, like harmonic distortion, which need to be filtered out and the input Power Factor. The main indicators of the quality of the power are: 1. Total Harmonic Distortion of the input current (THDi) 2. Total Harmonic Distortion of the output voltage (THDU) 3. Input Power Factor (input PF) Now, the full explanation and theory related to these three UPS characteristics could be topic for a separate paper. Regarding our topic of transformer-based vs transformer-less UPS design it is important to understand whether there are significant differences between the two designs, for what concerns the quality of the power. 1. THDi, the lower, the better. As already summarized in Table 1 at item #3, the THDi depends on the topology of the rectifier. If the rectifier is IGBT based and the UPS has proper passive LC filtering, then the THDi will be around 3-4%, regardless if the UPS is transformer-based or not. However, it is worth mentioning that old thyristor-based UPS, which generate high THDi, are also transformer-based and are still present in the portfolio of some manufacturers. In other words, choosing a transformer-less UPS means excluding automatically thyristor based rectifier UPS. 2. THDU, like the THDi, the lower, the better. Let s analyze the transformer-less design first; as the switching frequency of the transformer-less UPSs is usually around or above 15 khz, the output voltage includes few low-order harmonics (the low-order are the ones 7

with higher amplitude, hence the one generating more total harmonic content) and the passive LC filtering is more than enough to keep THDU low. 2% is today the TDHU value of transformer-less UPS and such value is more than OK, meaning that there is no effect on the load side. On the contrary, transformer based UPS are generating more low-order harmonic content as their switching frequency ranges from 1 khz to 3kHz; so the transformer is used as a filter for those low-order harmonics and the THDU at the output results to be also around 2%. As a conclusion, we can say that the output THDU of UPS is today practically equal among the most manufacturers and it isn t worth to have an output transformer (maybe external) whose sole purpose is to lower the THDU further down, if the UPS already guarantees 2%. 3. Input PF, the closer to 1.0 (restive) the better. Like for the THDi, the responsible device for the input Power Factor is the rectifier (and the input filters). As already mentioned in Table 1 at item #4, on the transformer-based UPS the PF typically varies with the amount of load (aggravates with the decreasing of the load) due to passive filtering numb rectifiers (without active PF correction). As UPS systems are practically never fully loaded, but typically loaded between 25% and 75%, the input PF results to be capacitive and this is a big disadvantage as anyone in the UPS industry knows. transformer-less UPS typically have rectifiers with active PF correction, thus have the PF very close to resistive (0.99) at all load levels. Fault tolerant Some people in the market are still claiming that when having a transformer in the UPS it will be better able to handle internal and external faults or short circuits. This may have been true two decades ago, but today s this is not the case anymore. Transformerless UPSs have internal circuitry, fast fusing and fast digital controls that allow the machine to effectively handle internal shorts such as e.g. a short on the inverter bridge. Like many UPS manufacturers, ABB has on every UPS series the necessary circuitry and fuses to protect the power converters including the battery charger. 8

Conclusions When designing medium to large power supply systems, there are numerous factors to be taken into account, which can be grouped into the following categories: safety, availability, flexibility, impact on environment and cost. Now, if in theory, all is nice and easy and we want to have all the above mentioned categories matching 100%, in practical life the best solution is the best compromise. This is valid for all applications. It may be type of application, or the criticality of the business connected to that power supply, or the business-model of the end-user, etc. that determine which is the best compromise for each installation. Now that the pro and cons of the two UPS designs have been explained in this paper, we can conclude that the use of transformer-free UPS allow to achieve the best compromise by fulfilling by far the majority of the power supply design factors. For the customers it means that the modern transformer-free UPS design matches today s customer requirements much more widely than the transformer-based. 9