New features in hybrid rotary ups give it the flexibility of the static, as well as potentially saving space and enhancing system integrity.
It may not be obvious why a mission critical installation would use anything other that static ups technology – yet clients often opt for a rotary ups when they need high reliability, longevity and load handling characteristics.

The argument for static technology is invariably one of cost and rarely one of superiority. But looking at whole-life costs, the rotary ups stands up very well, especially in modern data centre and telecommunications facilities. After all, a rotary ups represents state-of-the art power generation – at least until power stations adopt the approach of generating an artificial ac waveform by the chopping of a dc source.

Modern data centre and telecommunications facilities demand increasing power unaccompanied by any improvement in the complex load profile. Typical large data centres have moved to power requirements of 1 MVA to 5 MVA or more. Large-scale power is not particularly well handled by the multiple parallelling of static units (800 kVA maximum), where load balancing between modules becomes harder and fault clearing more complex to handle.

Not least large-scale static systems, which require a large-scale static switch bypass that reduces reliability and represents a single point of failure in an otherwise carefully engineered and expensive capital cost project.

The question is then, why do the most discerning clients continue to select rotary technology and how do they balance the cost to the reliability and performance equations?

Hybrid rotary ups
The hybrid rotary ups has been around for more than 20 years. The latest generation brings new features including the option of removing unnecessary air-conditioning requirements, but the fundamentals of operation remain unchanged.

The common static ups in its basic form employs a double conversion system (a rectifier and inverter) coupled to an isolation transformer. The inverter controls the output voltage and the rectifier inverter combination provides a stable frequency/voltage output for varying input frequency/voltage.

The isolation transformer is essential both for decoupling and filtering/isolation purposes. See figure 1.

The poor overload characteristics of the inverter forces the use of a static switch bypass for fault clearing functions. Under this mode of operation, ups function is lost. Additionally, to provide a reasonable quality output sinusoid, complex commutation circuits and power capacitors are required.

By replacing the isolation transformer and transistor inverter with a more rugged motor generator and thyristor inverter combination, these drawbacks are overcome. The thyristors are more tolerant of overload and are naturally commutated by the motor back electromotive force, eliminating the need for complex commutation circuits. This, coupled with the generator inertia, removes the need for bypassing the ups under fault conditions. The motor generator also serves to provide the galvanic isolation previously afforded by the isolation transformer. See figure 2.

However, both these solutions are relatively inefficient, involving both double conversion and isolation transformer losses. If the motor generator can be fed directly from the utility, then efficiency can be greatly improved. This is achieved by the addition of a second ups path in the form of a static switch. This second path is commonly mistaken as equivalent to that of static bypass in a static ups. However, this is a ups conditioning path and has no similar function. There is, also a third path that is the bypass, essentially for maintenance purposes. See Figure 3.

By employing this technology, the Piller UNIBLOCK hybrid rotary ups functions in the same manner as a static ups but with some significant built in advantages:

  • five times more short circuit fault clearing capacity
  • enhanced efficiency
  • up to ten times the mean time between failure through simplified design and elimination of power capacitors
  • lower input harmonics in normal operation
  • dual input capability for separate transformer feeds (for added resilience)
  • battery-free option
  • direct coupled diesel option
  • direct coupled cooling option
Alternatives to batteries
In the majority of ups systems, the interim energy store used to support the load until a diesel engine can be started is a battery.

The battery takes up considerable space, requires exacting environmental control and has a relatively short life. It is also unpredictable. The only real way to establish the energy available from a battery is to discharge it. This is difficult to and often puts the load at risk.

But there are alternatives to batteries. For example, The Piller Powerbridge can be used as a direct substitute for a battery used in any conventional static or rotary ups system. With a footprint of just 2·2 m x 1·3 m, this battery alternative stores 16·5 MW of energy, the equivalent of two minutes support at 150 kVA or 15 seconds for a 1 MVA load – more than sufficient time to start a diesel back-up. Not only this, but such an approach increases net-lettable space and reduces costs.

Working without air con
Every ups requires cooling of both the ups set itself and of the associated batteries (if an alternative is used). The amount of cooling will be several hundred kW for high kVA systems. Invariably, air conditioning provides the cooling for a ups room. This system must be redundant to maintain a level of resilience in the system as a whole. This is expensive and uses space as well as increasing electrical and mechanical infrastructure requirements. Every additional element in the system compromises reliability.

Again, it is possible to find alternative methods to air conditioning. For example, Piller has introduced direct heat exchanger cooling (dhc) as an option with the latest generation of hybrid rotary ups. Air is passed around a closed circuit within the ups cabinet itself, driven by the integral impeller within the rotary machine. Heat from the circulating air is extracted via the dhc attachment, which is connected to the building chilled water supply. Each ups set has its own dhc, thereby removing the interdependency of conventional systems. Dampers are also incorporated to provide an orderly method for ups shutdown in the event of chilled water failure: In conventional systems, the time for this shutdown is limited due to reaching thermal limits in the room very quickly.

Benefits of this system include:

  • no cooling units, switchgear and cabling
  • reduced space considerations
  • silent operation
  • suitable for polluted and dirty environments.
  • mezzanine design is possible
  • reduced maintenance costs
  • single building management system interface
  • reduced maintenance regime
  • reduced running costs
One major investment bank has installed more than 20 such units and has been able to save the capital and running costs of almost 70, re-circulating air conditioning units with the associated infrastructure.

Summary
The hybrid rotary ups offers the flexibility of static ups. When the entire system is considered, larger scale systems can be less costly while maintaining the superior technical and reliability characteristics. With the latest additions, the UNIBLOCK ups can also save space, add flexibility and improve system integrity.