Why have modular UPS systems become popular?

The advent of modular topology has arisen from the conjunction of three factors: The developments that have made it a practical proposition, the technical and commercial benefits it bestows, and the changes in the business environment that have made those benefits important. Enabling technology Modular topology ultimately owes its existence to advances in the semiconductor industry. The monolithic double conversion on-line UPS systems that first appeared in the seventies were referred to as transformer based UPSs. They used a rectifier to convert incoming raw AC mains to a DC voltage, which was used to charge the UPS backup battery and to feed an inverter for conversion back to a clean AC output waveform. However an output transformer was needed to step up the inverter output to the level needed for the critical load. By the mid nineties however, advances in power semiconductor technology and the arrival of the Insulated Gate Bipolar Transistor (IGBT) allowed a different, transformerless approach. In a typical design, an IGBT based DC converter boosts the rectifier output to a much higher level, allowing the inverter to directly produce an AC voltage sufficient to drive the critical load. Many UPS advantages derive directly from transformerless design. These include greater efficiency, higher input power factor, lower input current harmonic distortion (THDi), reduced capital and operating costs, lower audible noise and enhanced battery life. But elimination of the transformer also yields very significant reductions in physical size and weight. For example a 120 kVA system footprint shrinks from 1.32m2 to 0.53m2, while the weight is reduced from 1200Kg to 370Kg. This scale of reduction and cost saving allows a different, modular configuration in which the critical load demand is met by a number of smaller UPSs operating in parallel rather than one large monolithic unit. This modular topology offers further improvements in efficiency as well as great benefits in resilience, availability, uptime and easier maintenance. An ever more demanding business climate The arrival of these benefits is very timely. Even when businesses mainly used computers as internal tools to automate commercial, manufacturing and engineering functions, losing data processing capability to a power outage or transient voltage spike would still have been serious; Today, when enterprises must typically support 24/7 online transactions with external customers and suppliers, such a power event would be catastrophic or even fatal in business terms. Accordingly, ever since technology rendered modular systems possible, their development has been driven hard by customer demand for the highest achievable power availability. Similarly, the improved energy efficiency of modular systems is of vital importance to users facing continually rising energy costs together with increasing legislative and social pressure to cut carbon emission. A modular configuration example We can see how users can best access these benefits by looking at a typical example. Suppose that a data centre has a load requirement of 80kVA, and because of its critical nature, a redundant UPS configuration is essential – i.e. a configuration that will continue to deliver power even if one UPS unit fails. Such a requirement could be fulfilled by two 80kVA standalone UPS cabinets sharing the load. If either cabinet should fail, the other has sufficient capacity to support the 80kVA load until the faulty unit can be repaired. Alternatively, a single rack containing three modular rack mounting 40kVA UPSs can be installed. This is also a redundant system, because if a single 40kVA module fails, the remaining two modules together have a capacity of 40+40=80kVA – enough to drive the critical load. In fact both systems can be referred to as N+1 redundant systems, where N is the number of UPS units required to meet the critical load demand.