Availability also concerns task reliability. The system's mission profile is also provided by module UPS with  normal AC output.

The UPS system is a voting model, with three parameters, equivalent MTBF, MTTR and availability characterizing the system’s availability.

System MTBFeqS1, MTTReqS1 and availability A S1 are shown in Formula 6, Formula 7 and Formula 8 below.

Traditional wardrobe UPS style is relatively heavy. If its capacity is 100KVA, its generally weighs around 1000kg and its size exceeds 2m3. If we selected 250KVA or more powerful UPS, problems like packaging / transport / handling / computer room door or relatively small aisle / not enough lift truck / not enough floor laminates and may occur.

The structure of traditional powerful UPS structure decides that only vertical packaging and vertical transport can be used,, which largely restricts the choice of transport vehicles, while if we choose modular UPS, only 2 ~ 4 persons can handle handling rack, 1 to 2 people can install the module, and modules can be packaged separately and the rack can be horizontal packaging and transportation.

(5) maintainability of modular UPS greatly enhanced

The cost of traditional UPS maintenance is high and it takes a long time before the arrival of spare parts. In the case of serious failure, it is hard to be transported to factories. tThe most important is that it will influence the normal use of load equipment. If you choose modular UPS, users can backup a module, so if the equipment fails, it won’t take a long time to airfreight a modular. And the UPS has the fature of hot-swappable function, so you can replace a failed module without interrupting the load power supply and at the same time ensuring personal, which makes maintenance easy and low cost.

Compared with on-site maintenance,back to factory maintenance has a lower maintenance costs, because the factory has an excellent monitoring measures. In addition, the quality of can be guaranteed. Maintenance procedures are smooth simple.

(6) Modular UPS can save the user investment

It is calculated if 10KVA capacity modules "4 +2" system provide power to 40KVA power load, the average annual failure rate is0.054 percent, while the average annual failure rate of two 40KVA single "1 +1" parallel system is 0.09%, higher than the former. With technological advances and modular UPS volume, the cost of total capacity 60KVA should be lower than that of 80KVA.

According to statistics, the user's actual load is generally much smaller than the design value with the load rate is less than 50%, while design load rate is 50%,If the "4 +2" system changed into a "2 +6" system, the average annual failure rate will be reduced by 3 orders of magnitude, or the user only need "2 +2" modular system to meet the load requirements, then failure rate was 0.011%, less than one-eighth of "1 +1" system..

(7) Modular UPS can be easily expanded (scalability)

If the system is not fully configured in initial stage, it will be easy to expand in the future, At initial stage, we need to ensure that this configuration can increase with the gradual expansion of the load, and take full consideration of uninterrupted power supply to original load in power system expansion process.

(8) Modular UPS can shorten the installation cycle

When different model of modules make up different capacity systems, since rack-mounted parallel power cables and signal cables are laid out at the factory, the installation works on the spot is so simple. The size of traditional UPS of different capacity is different, it is trouble some to connect.

(9) modular UPS makes it easy for manufactures to launches series product.

20KVA modules system can be formed into 20/40/60/80/100KVA ... the system. In contrast, if it is a single system, you need to develop the capacity system separately, which means manufacturers need to invest more in developing and time and these cost will transferred to to users.

(10) modular UPS makes it easy for manufacturers to procure materials and backups.

Delivery is guaranteed: For high-power UPS, it is difficult to which kind of specific power the user need. Suppliers may have 60KVA machine, but the user order a 80KVA.,The production cycle of high-power UPS is relatively long (usually takes 1 month ), but the user needs it within ten to 15 days or even spot good. If it is modular UPS, there is no problem, as long as there are a certain number of modules and universal rack.

convenient parts stocking for manufacture: single UPS of different capacity needs different power devices and magnetic components (IGBT / SCR / contactor / transformer / inductor different capacity), but factory inventories of these components bring troubles for troubles mass customization. The different capacity of a modular UPS system uses similar components, which is easy to mass procurement to reduce procurement costs and ultimately benefit help UPS customers. Modular UPS has made great progress in production, cost savings, product quality and delivery speed

We should also note that mass production still has another important purpose: in addition to be easy to product assembly, another advantage is that parts areinterchangeable the production of parts.

(11) to facilitate after-sales maintenance of modular UPS

If users have spare module, they can detect and replace. If the user does not have spare modules, it is easy for factory to mail modules or factory maintenance personnel can carry them. Module in modular UPS can be replaced without opening the case.

For UPS system user that use the same type of modules to make up several different capacities modular, module's common use brings convenience to users. It is easy to place the failured module.

modular UPS adopts Hot-swappable technology, which can limit faults within the minimum condition of power supply system. Therefore, system don’t need to stop the normal operation during maintenance.

(12) modular UPS is energy-saving

In general, the efficiency UPS is high only within a certain load, and its efficiency is low when there is less load. According to research, the possibility that the user’s load rate can reach 10-40% is roughly 61%. Assume that users use modular UPS, the system can sleep one or two module n(can be manual or automatic) according to load size while guarantying redundancy, so UPS systems can work at a more efficient area.
If UPS efficiency is 85% when modules‘ load rate is 20% UPS and if UPS efficiency is 90% when modules‘ load rate is 40% UPS , for first phase of the actual 40KVA/32KW final design capacity 100KVA/80KW system, there are two kinds of configurations:

1. With two UPS machines of 100KVA / 80KW, its normal load at the first stage was 20% and if there is a failure at the final stage, the load rate was 100% (if the load rate of one machine is higher than 80%, the system needs to be expanded). At the first stage, normal power consumption of system is32KW/80% = 40KW;

2. If we make "5 +2" 20KVA/16KW modules in parallel, it can be caculated that the avialbility of,"5 +2" system is relatively higher than the "1 +1" system (at full load conditions, the availability ratio is 9 / 8 or so. If the load ratio is small, it is equivalent to "2 +5" with higher availability proportion). At the first phase, two modules hibernate under normal condition. It was the "3 +2" system and the load is 40KVA/100KVA = 40%. At the final stage, all of the modules can be put into use. At this moment, normal power consumption is 32 KW/90% = 35.55KW.

The power consumption of the second configuration is 4.45KW lower than the first configuration.

(13) modular UPS is suitable toplacet together with the main equipment and take up less room area

Operators are increasingly concerned with the overall operating costs "TCO (Total Cost of Ownership)". In addition to initial cost of equipment procurement, there are a rent, electricity cost, maintenance cost and other comprehensive cost. The rent cost of equipment rental has also become an important part of contract bid evaluation. 10 years’ of rent cost way surpasses pure equipment price, so when choosing power supply solutions, operators tend to choose those occupying a small room area. A highly frequent DC power communications system with modular rack up to 4000A takes up the same area as a traditional power supply system with a rack of 400A.. If the UPS can be put side by side with main equipment,and take up less area, UPS will be more competitive.

Due to the limitation of capacity/transformer, traditional UPS with different capacity often differs in height. If placed in the main room, they did not look very neat and beautiful. If you choose a modular UPS, you can choose main equipment with same size (most of load in current communication field is embedded 19-inch 42U rack server). This will make everything in the room integrated into a whole..

(14) good compatibility with oil machine

Compared with the traditional phase-controlled UPS, the input power factor of modular UPS can reach 0.99 and input current harmonic can lower than 5%, because it uses active power factor correction Therefore, it is easy to be compatible with the oil machine, reduce the ratio of oil machine capacity and UPS volume so as to reduce users’ total investment in power protection system.

(15) Modular UPS also applies to dual power mode. In the same magnitude of availability, its cost is less than traditional dual power supply scheme

Server and other key equipments now sdopts dual AC power to protect. When one power supply fails, the server can work. For the traditional UPS, a"1 / / 1 +1 / / 1)" system made up by 4 UPS unitsis adopted to load. When the load capacity is less than 1 UPS capacity, up to twice backup availability index can be achieved. For modular UPS,

two UPS systems are close to the availability index of double backups. The cost of the latter is half less that of the former.

3, problems of modular UPS

3.1 customer acceptance of modular UPS is not high in customers

The parallel connection of DC power supply is simple, as long as voltage, amplitude and polarity are same, it can be done. Because of the one-way current and easy flow technology, there are circulation problems. The parallel connection of 2-way or multiple AC has the same frequency, same phase, same waveform and other same parameters, two-way flow caused by the current itself will be very complicated. The availability of modular UPS is several times higher than the traditional.

However, during its 10 years on market, some modular UPS do have a high failure rate, therefore, modular UPS hasn’t been highly recognized by customers, especially by high-end customers. Therefore, increasing the reliability and availability of modular UPS is the necessary premise to increase customers’ acceptance of modular UPS

3.2 high price

Modular UPS adopts difficult technology and its research and development cost is pretty high., In order to recover the cost, manufacturers put a price on modular UPS that is higher than traditional UPS.. With the R & D investment of domestic manufactures, technological advances will bring about an increasing of reliability. Modular UPS is suitable for mass customization, so the cost will show a decline trend.

3.3 problem of no output isolation transformer
Isolation transformer refer to the connection between the input and output.Modular UPS generally do not have an internal frequency isolation transformer.
3.3.1isolation transformer in traditional UPS cannot solve the problem when N-PE voltage is not zero.
UPS has a static switch to change into bypass function. Static switches only switch between wire on inverter load voltage and bypass voltage switch, but do not switch the center line. It means that the output center and input center of UPS is connected by the electrical center line. Since it is an electrical connection, UPS itself can not put down UPS’output N-PE voltage.

3.3.2 add isolation transformers in load end can solve the problem of high zero ground voltage.

Traditional UPS can not resolve the problem of high zero field voltage. The reason why zero field voltage is high is that three-phase load is unbalanced or there is harmonic current in load , while the zero line has resistance. There are four ways to reduce zero field voltage: First, equal configuration of three-phase load, the second is to reduce the load of harmonic current, the third is to thicken zero line., The most thorough way is to add an isolation transformer in front of the load, which is effective than adding an isolation fransformer at the side of UPS (Otherwise, if there is input harmonic current in load, there will be zero line voltage reduction between UPS output to the load input voltage, which would result in zero field voltage) .

3.3.3 UPS with no isolation transformer is used more and more widely

(1) The requirement and address of computer load to live wire and center line is the same

International Office product safety rules (such as IEC950 and UL1950) prohibits taking different measures to deal with live Wire and center line.. if we look at computer power supply Schematic, we will know that live wire and center line are connected to the same circuit with same method, and they are interchangeable. This shows that the required N-PE voltage is low, and L-PE-voltage cannot be low.

(2) in computers, center line can not be used as a logical reference point

In all safety regulations such as UL, center line is not allowed to be taken as the logical point of reference. Safety regulation stipulates that logical point of reference should have a 1/2cm distance from live wire or center line and no circuit connection is allowed.

(3) isolation transformer can not eliminate the problem of ground wire

International Office product safety rules (such as IEC950 and UL1950) require s that isolation transformers can only be used to separate central line and live fire and protective earth wire doesn’t pass isolation transformer. Because the computer circuit (data communication circuits) is connected with protective earth wire, but not central wire, isolation transformer and UPS with isolation transformer have no effect on addressing the problem of ground wire.

(4) the noise of central wire won’t be coupled to computer circuit
International Office product safety rules (such as IEC950 and UL1950) prohibits any coupled circuit between the power line and computer circuits. However,they may be coupled with each other through electromagnetic. The RF interference can only work on wavelength that equals to or smaller than the true size of computer. Frequency can reach tens of MHZ. Common-mode filter can be used to filter such interference.

(5) replace"Copper Steel" with "Silicon” is another developing trend of UPS

the cost of metallic materials is increasing, which makes it imperative to use semiconductor "Silicon" transformation technology to improve performance and the replacement of bulky "Copper Steel"isolation transformers meets the trend of technology. An example is that many manufacturers have begun promoting a high-power high-frequency machine without isolation transformer.

4, research topic

In order to improve the reliability of modular UPS, and promote the development of modular UPS, China Communications Standards Association issued a document and approved the project, to carry out the research project "communications with the modular uninterruptible power supply" (Project Number: 2007B13) (see SGS issued document No. 19 of 2007 on the issuance of China Communications Standards Association Technical Report 2007, the first research project to develop the project and project plan letter). We will focus on the following five questions:

1）need to address isolation fault of each module;

2）need to address the components drift’s influence on the system;

3）to improve shock resistance capability of modular UPS system;

4）battery management of modular UPS;

5）limitations of parameters such as parallel circulation.

5, Conclusion

Compared with the traditional UPS, Modular UPS has many advantages and represents one of UPS's development, but there are disadvantages, like high costs and the actual reliability of some products is not as high as the theoretical value, The research on the modular UPS will help to promote the reliability of modular, modular UPS to enhance the customer's acceptance and accelerate the development of modular UPS.