Uninterruptible Power Supply (UPS)
Uninterruptible Power Supply units are used for uninterrupted supply of electrical loads. The server rooms and data centers are using the type of online power supply units, which alone can ensure continuity of supply for sensitive IT equipment. Online power supply units isolate UPS input from its output, acting as a double conversion – AC voltage into DC voltage and later DC into AC again as illustrated in the following diagram:
What is UPS used for?
Power supply units are applied to receivers that we want to protect from power failure. For the most part these are computers, servers, telephone switchboards, control panels, pumps, air-conditioners, and much, much more.
If it is necessary to protect equipment against long-term outages UPS are generally combined with power generators in complex systems.
The main parameters you should look for

Requirements for modern power systems are growing at a rate similar to the requirements of IT systems. The selection of the UPS units is based on the parameters and features that until recently were taken into account only by a small circle of specialists.
Today, they are the most important criteria:

  • Nominal power – it is important to properly select the power of supply unit as the UPS should not be overloaded because then it cannot fulfill its function, but it shouldn’t also be underloaded as this will cause deterioration of its parameters.
  • Parallel work of the UPS, which allows virtually any expansion of the system in terms of power and redundancy
  • Back-up time – the power supply back-up time is selected depending on the connected devices, external parameters (frequency and length of interruptions), standards and other. Back-up time is the parameter which typically is calculated as a time that the UPS can operate on battery power for 100% load power (nominal power) during powerfailure.
  • Efficiency – the parameter that determines the amount of heat emitted by the device, as well as the cost of its operation. The cost of energy is a hidden and paid in installments component of the device price. A few percent differences in the efficiency of equipment can generate savings of several thousand per month. On the efficiency i.e. on heat production in a very large extent depends the product life and average time between failures
  • Input power factor  determines the nature of the load transferred to the network by the supply unit and has impact on the size of the power adjustment factor for the used in the system current generator. It has a tremendous impact on the cost of the set and its operation.
  • Dynamic parameters, namely the time and the size of the output voltage distortion given in percent in the case of load step change from 0 to 100%. Dynamic response time is a parameter shamefully omitted by many manufacturers.

Modular and ‘stand-alone’ systems
We can divide UPS for two basic types of UPS systems ‘stand-alone’ and modular.
‘Stand-alone’ systems are known and used for many years. These are single freestanding units – ‘one block’ is separate UPS unit. There is also the possibility of paralleling operation which gives the option to increase power or redundancy.
In order to shorten the repair time, increase the flexibility and reliability a concept of modular system was born. It is already used and known solution because first modular system was already produced in 2000. The precursor was our partner – Swiss company Newave, which was the first to introduce these products to mass production.
Real modular systems are systems devoid of single points of failure. Each module is a separate stand-alone UPS unit having its booster, rectifier, inverter and control system and LCD display. Systems are located inside the UPS cabinet having a female socket (and the modules have a male plug). Replacing modules during maintenance or breakdown is based on the “HOT SWAP” principle.
Cutting-edge modular systems are built on a decentralized basis with dynamic master / slave system. In case of failure of the master module another unit in uninterruptible mode takes over the management. Many competitors try to sell semi-modular systems that contain some advantages of modular UPS but don’t offer a full elimination of so-called SPOF (Single Point Of Failure).
Advantages of modular systems:

  • great flexibility,
  • easy to maintain,
  • short repair times not feasible for stand-alone systems,
  • expandability (‘pay as you grow’),
  • very high power factor per square meter,
  • high compactness, low weight.

Modular systems are used in small and medium-sized data centers. At very high powers, modular systems no longer make sense, in view of the theory of probability relating to failure. Maximum power modules available on the market are 100kVA. At higher powers (about 1MVA) it is necessary to use a large number of modules (stand-alone UPS units), which increases the likelihood of failure and increases the overall cost of ownership. In such cases, the advantages of modular systems no longer prevail over cons of using a large number of autonomous units in the system. A lot more sense is to use stand-alone units with higher power in parallel.