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Comparison of modern static and rotary UPS. Static UPSs have reached their limit?

The IT industry market is the largest consumer of uninterruptible power supplies (UPS), using approximately 75% of all manufactured UPS. The annual global sales of UPS equipment to all types of data centers, including corporate, commercial and super-large, is $ 3 billion. At the same time, the annual increase in sales of UPS equipment in data centers is approaching 10% and it seems that this is not the limit.

Data centers are becoming more and more large and this, in turn, creates new challenges for the energy supply infrastructure. While there is a long debate about how static UPSs are superior to dynamic ones and vice versa, there is one thing that most engineers agree with is that the higher the power, the more electric machines are suitable for working with it: namely generators used to generate electrical energy in power plants.

All dynamic UPSs use motor generators, however they have different designs and, certainly, differ in properties and characteristics. One of these rather common UPSs is a solution with a mechanically connected diesel engine - a diesel-rotary UPS (DRIBP). However, in the world of data center construction, there is real competition between static UPSs and other dynamic UPS technology - rotary UPSs, which is a combination of an electric machine that generates a sinusoidal voltage of a natural shape and power electronics. Such rotary UPSs are in electrical communication with energy storage devices, which can be either batteries or flywheels.

Modern advances in control technology, reliability, efficiency and power density, as well as reducing the unit cost of UPS power, are factors inherent not only for static UPSs. The recently introduced Piller UB-V series is a worthy alternative.

Next, we consider some key criteria for evaluating and choosing a UPS system for a modern large data center in the context of which technology looks more preferable.

1. Capital costs


It is true that static UPSs can offer a lower price per 1 kW for smaller UPS systems, but this advantage evaporates quickly when it comes to high power systems. The modular concept, which the manufacturers of static UPSs were inevitably forced to apply, revolves around the parallel connection of a large number of UPSs of small nominal power, for example, 250 kW in size as in the example below. This approach allows you to achieve the desired value of the given output power of the system, but due to the complexity of many duplicated elements it loses 20-30% of the price advantage compared to the cost of a solution based on rotary UPS. Moreover, even this parallel connection of modules has limitations on the number of blocks in one UPS system, after which the parallel modular systems themselves must be parallel,which further increases the cost of the solution due to additional switchgears and cables.

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. 1. 48. UB-V .

2.


In recent years, data centers have become increasingly commoditized enterprises, while reliability is increasingly taken for granted. In this regard, fears are increasing that this will lead to problems in the future. As operators strive for maximum fault tolerance (the number of “9”) and it is assumed that the disadvantages of static UPS technology are best overcome due to the short repair time (MTTR) due to the ability to perform quick and “hot” replacement of UPS modules. But this argument can be self-destructive. The more modules are involved, the higher the probability of failure and, more importantly, the higher the risk that such failure will lead to loss of load in the overall system. Better not to have glitches at all.

An illustration of the dependence of the number of equipment failures on the value of the MTBF during normal operation is shown in Fig. 1 and corresponding calculations.

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Fig. 1. The dependence of the number of equipment accidents on the MTBF.

The probability of failure Q (t) of the equipment during normal operation, in section (II) of the graph of the normal failure curve, is fairly well described by the exponential distribution law of random variables Q (t) = e- (λx t), where λ = 1 / MTBF is the intensity failures, and t is the operating time in hours. Accordingly, after time t, in the trouble-free state there will be N (t) settings from the initial number of all settings N (0): N (t) = Q (t) * N (0).

The average MTBF of static UPSs is 200,000 hours, and the MTBF of rotary UPSs of the UB-V Piller series is 1,300,000 hours. The calculation shows that over 10 years of operation 36% of static UPSs will be in an accident, and only 7% of rotary UPSs. Given the different amounts of UPS equipment (Table 1), this means 86 failures out of 240 static UPS modules and 2 failures out of 20 Piller rotary UPSs on the same data center with a useful IT load of 48 MW over 10 years of operation.

The experience of operating static UPSs at data centers in Russia and in the world confirms the reliability of the calculations based on the statistics of failures and repairs available from open sources.

All Piller rotary UPSs, and in particular the UB-V series, use an electric machine to generate a pure sine wave and do not use power capacitors and IGBT transistors, which very often cause failure in all static UPSs. Moreover, a static UPS is a complex part of the power supply system. Complexity reduces reliability. UB-V rotary UPSs have fewer components and a more robust system design (motor-generator), which increases reliability.

3. Energy Efficiency


Modern static UPSs have much better energy efficiency online (or “normal” mode) than their predecessors. Typically, with peak efficiencies of 96.3%. Higher numbers are often given, but this is achievable only when the static UPS works by switching between online and alternative modes (for example, ECO-mode). However, when using the alternative energy-saving mode, the load works from an external network without any protection. For this reason, in most data centers, in most cases, only the online mode is used.

The Piller UB-V series of rotary UPSs does not change state during normal operation, while providing efficiency up to 98% online for a load level of 100% and at 97% when loading 50%.

This difference in energy efficiency makes it possible to obtain significant savings in electricity during operation (Table 2).

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Tab. 2. Saving energy costs at the data center 48 MW IT load.

4. Space occupied


General-purpose static UPSs have become significantly more compact with the transition to IGBT technology and the exclusion of transformers. However, even taking into account this circumstance, the rotary UPS of the UB-V series give a gain of 20% or more in terms of the occupied space per unit of power. The resulting space savings can be used both to increase the power center capacity, and to increase the “white”, useful, space of the building to accommodate additional servers.

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Fig. 2. The occupied place of the UPS for 2 MW of different technologies. Real scale installations.

5. Availability


One of the key indicators of a well-designed, built and operated data center is its high fault tolerance. Although 100% uptime is always a goal, reports indicate that more than 30% of data centers in the world experience at least one unplanned outage per year. Many of them are caused by human errors, but energy infrastructure also plays an important role. The UB-V series uses the proven technology of the Piller rotary UPS in a monoblock design for years, the reliability of which is significantly higher than all other technologies. Moreover, for the UB-V UPS itself in data centers with a properly controlled environment, their annual shutdown is not required to perform maintenance.

6. Flexibility


Often, IT systems of data centers are updated and modernized within 3-5 years. Therefore, the infrastructure of power supply and cooling systems must be universal enough to meet this and have a sufficient future perspective. Both conventional static UPS and UB-V UPS can be configured in various ways.

However, the composition of the solutions based on the latter is wider, and, generally speaking, since this is beyond the scope of this article, it allows one to be able to implement uninterruptible power supply systems at an average voltage of 6-30 kV, to work on networks with renewable and alternative generation sources, to build cost-effective ultra-reliable systems with isolated parallel bus (IP Bus), corresponding to the level of Tier IV UI in the configuration N + 1.

As a conclusion, several conclusions can be made. The more data centers develop, the more difficult it becomes to optimize them when it is necessary to simultaneously monitor economic indicators, aspects of reliability, reputation, and minimizing environmental impact. Static UPSs have been and will be used in the future in data centers. However, it is indisputable that there are alternatives to existing approaches in the field of power supply systems, which have significant advantages over the "good old statics".

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