UPS - Modular UPS systems – why are they so popular?

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Originally, UPS systems were implemented as a standalone, monolithic design. In  today's business climate, where pressure on and demand for quality electrical power has become much greater, modular UPS systems have become increasingly popular. Alan Luscombe of Uninterruptible Power Supplies looks at how this topology has evolved, and why users like it 

The advent of modular topology has arisen from the conjunction of three factors: The technology 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 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; one in the standalone example and two for the modular systems. The extra ‘1' provides the UPS installation's resilience, as a single UPS unit failure will be invisible to the load. Extra redundancy or resilience can be provided if the load warrants. Systems with N+n redundancy can be built, where n is the number of redundant modules.

The first and most obvious advantage of the modular system is that it is smaller, with an implementation in a single rack rather than two cabinets. This is an important saving for modern data centres where floor space is at an increasing premium.  However, there are also many further benefits, of which energy efficiency is one. Each UPS unit in the standalone example supplies half the load, 40kVA, during normal operation so is therefore 50% loaded. By contrast, each 40kVA module is more heavily loaded at 66.7% of its capacity. Because UPS efficiency increases with loading, the modular units run with 96% efficiency compared with 91% for the standalone units. This improved efficiency not only reduces direct energy cost; it brings further savings through reduced cooling costs. The total energy savings in this example would amount to £16,700 over five years - or possibly more, depending on electricity pricing.

Increased availability is another benefit. Each UPS unit's availability can be defined as a ratio between its mean time betweenfailures (MTBF) and mean time to repair (MTTR). And, whereas a standalone unit takes typically six hours to repair, some modules can be simply swapped in less than half an hour. This reduced MTTR gives a ‘hot swap' module an availability of 99.9999% (‘six nines') even before allowing for the resilience provided by the N+1 configuration. This level of power protection is key to users, but cost savings accrue as well. Inventory cost for specialist parts is reduced, and the need for highly skilled on site technicians is eliminated.

During the UPS installation's operational life, scalability can emerge as a further advantage of modular topology. Suppose transaction traffic growth increases our example's load from 80 to 110kVA. A brief effort in slotting another 40kVA module into a spare rack location will restore the system's N+1 redundancy status, without degrading the UPS loading too severely and with no interruption of power to the load. The UPS remains ‘rightsized'. Further growth in load demand can be conveniently accommodated by further modular increments of the UPS system capacity. The rack's capacity for further modules is known as the UPS system's vertical scalability. If this should become exhausted, horizontal scalability can be achieved through the addition of further racks.

By contrast, adding another standalone 80kVA standalone unit always means having to find more floor space, laying more cabling and carrying out a nontrivial installation exercise. The gap between the load kVA and the UPS units' rating would also widen, to the detriment of the UPS system's energy efficiency.

Lifetime savings and benefits
A modular system can cost more than a standalone installation in terms of initial capital cost. But this will be offset by the modular system's reduced operating costs, especially when factors such as initial transportation and infrastructure costs, spares and maintenance are taken into account. In addition to reduced costs, the modular approach offers a smaller footprint, greater flexibility, easier manageability, inherently greater availability, and scalability throughout its operational life.