We examine the inherent complexities surrounding parallel redundant ups systems and the internal/external static switch paradox.
In the uninterruptible power supply (ups) arena, the name of the game is power quality and load protection. Imagine that you are running a London-based data centre responsible for a large internet service provider's (isp) server network. A storm cripples the power grid, and the centre suffers a power outage. If there is no ups infrastructure in place and the servers' power supply is severed, then the loss of revenue to both the isp and data centre could be catastrophic. Having planned for such an occurrence and by operating a ups system, this scenario can be averted. A ups provides both protection against and assurance that any loss of power will not detrimentally affect power-reliant operations.

Now, imagine the above happening and the ups failing, or it being the subject of a regular maintenance visit. This does not bear thinking about. However, despite a UPS offering 99·999% power availability, such vital applications should not be entrusted to a solitary ups.

To protect against the second scenario, organisations are demanding parallel redundant ups systems, where multiple ups are configured in parallel incorporating an external static switch (by-pass) thus providing a multi-module support system with increased fault-clearing capacity (fcc), compared to the internal static switch of the individual ups module.

To protect and serve
The premise behind this model is that such a parallel system offers increased reliability; should one ups fail, additional units are available to support the load, thereby offering 99·999% power availability. Within the industry, the rule for the configuration of such parallel systems is n+1, where (for example) a 1200 kVA load would be supported by three 400 kVA (n) units plus an additional 400 kVA machine (+1), thus increasing the fcc and mean time between failure (mtbf).

While the implementation of a parallel redundant system is widely acknowledged as a means of offering increased mtbf, high power availability and integrity, the means by which the fault or short circuit sharing capacity is managed is one that creates controversy amongst manufacturers.

Traditionally, ups vendors implementing such parallel redundant systems either utilise an internal, integral ups module static switch from each of the units to operate in parallel, or utilise an external load rated static switch with a very high kA rating. However, with the likes of data centres and air-traffic control installations requiring secured 99·999% power availability, the overall reliability and functionality of the internal static switch means that it cannot consistently satisfy such high demands during short circuits on the load side.

External versus internal debate
The premise behind the implementation of parallel redundant systems is that they will offer maximum power protection against virtually all eventualities. However, due to the technical nature of the internal integral static switch, such protection cannot be provided. If we take the example given earlier (the London-based data centre) and relate this to the arguments that follow, it will become apparent that the two methodologies differ greatly.

To protect against an outage, the data centre has in place a parallel redundant system (n+1) operating with integral internal static switch methodology – four units each at 400 kVa (n+1) protecting a 1200 kVA load. Let us assume that one of the units fails resulting in reduced fcc values. At this point, the 1200 kVA load is only supported by a parallel implementation of 3 x 400 kVA. The fcc is now subject to the limited individual capacity of the internal static switches (400 kVA each) and the cable impedances. In order to clear the short circuit, the fault must be routed to each of the supporting ups via their individual internal static switch. However, as all static switches cannot operate simultaneously (nor can the input electro-mechanical breakers for each ups), the load is therefore exposed and without either ups or mains support.

It is clear that such systems require an extremely detailed study with regard to the selection of input side circuit breaker sizes, thus ensuring that the distribution network discrimination is not left to chance. Albeit for a millisecond, such a moment of exposure could bring down the data centre, potentially costing the organisation millions.

Should the data centre have installed the exact same parallel system, but this time using the external static switch methodology, no exposure would have occurred. Since an external static switch can be rated to any load desired and is not reliant on individual ups module static switch limitations, the application in question could have been designed with four 400 kVA units (n+1) supporting a 1200 kVA load with a 1200 kVA external static switch. At the point of solitary ups failure, the fcc of the system would not have been compromised as the external static switch instantaneously transfers the short circuit to the bypass supply, thus avoiding any risk to the load or exposing the centre.

Ultimate protection
In this example, we can see that an external static switch can provide an organisation with a far greater degree of assurance than that provided by an internal integral static switch. Due to its independent stance, the external static switch functions as a separate fail-safe in the event of a ups malfunction or down stream short circuit. Due to its ability for expandability, the external static switch offers a far greater degree of load protection. However, internal static switch technologies can be adapted to provide similar levels of protection, but with higher component counts etc.

If one looks at a system operating five 200 kVA units (n+1) supporting a load of 800 kVA, the external static switch in place would be 800 kVA, thus supporting the total load in the unlikely event of ups failure (if it were a super critical application, a second 800 kVA static switch could be installed to offer additional protection). To provide the same level of assurance with an internal static switch, this model would have to implement four static switches each being a 800 kVA rated ups, resulting in a total rating of 3200 kVA… far greater than necessary.

Such practice would be nonsensical, as this renders the system impractical from an economic stance.

Form over function
Nevertheless, within the competitive ups market, vendors providing parallel redundant systems to ventures such as data centres do enjoy two tangible advantages: footprint size and cost.

Despite the inherent technical and functional disadvantages, the internal static switch type system does offer a slightly smaller footprint as all components are housed within each ups module. In the eyes of the data centre, this means that the ups infrastructure requires less space, and as such allows more server-racks to be rented thus bringing in more revenue. This, coupled with the lower initial purchase price has allowed the internal static switch model to retain a prevalent position in the market place despite its flaws.

To combat such thinking, providers of ups systems with external static switch technology need not issue special discounts to entice customers, or instil the virtues of the technical advantages of its offering, but ask a simple question: how much indemnity cover does your consultant possess? Not a strange question in reality if we take an air traffic control centre and imagine that a design consultant has specified a parallel ups system using internal static switch methodology.

Naturally, to facilitate purchase, a professional recommendation has been made that the system suggested will meet the protection needs of the installation in question (99·999% availability). Now imagine that a short circuit on the load side takes place and the millisecond taken to send the fault upstream causes data exposure, radar screens going blank and planes being placed on hold position… Does the consultant have the indemnity cover to protect against this eventuality? They had better hope so. Despite the minimal cost saving, the risk to passengers, loss of revenue and faith could be in the millions.

In such a scenario, users face a situation where they cannot realistically argue in favour of lower capital outlay and increased risk. Instead, common sense must prevail and the external static switch type design should be chosen.

Conclusion
With today's demand for clean and protected power getting greater by the hour, ups providers are facing an increasingly complex battle to ensure that vital applications are protected against outage. Since outage could be catastrophic to many organisations, for example an isp or financial institution, organisations are implementing parallel redundant systems to ensure that should even the ups fail, back-up is on hand.

Despite reduced fcc values, some organisations are still entrusting system integrity to internal static switch ups system technology. With its reduced load and short circuit transfer capabilities and reliability, internal static switch systems should be viewed as a Trojan Horse.

As such, vendors should seek to adopt and recommend external static switch ups system technology. In line with the ethos of the ups, this methodology ensures 24 h/day, seven days/week, 365 days/year, 99·999% power availability via instantaneous load or short circuit transfer in the event of ups malfunction. Despite the slightly higher capital outlay, such systems protect against the need for a good lawyer and the loss of reputation caused by data exposure and down-time.

Programme-critical networking systems

Airbus UK has installed a ups infrastructure to safeguard the power supply to programme-critical networking equipment utilised in the design and production of wings for the Airbus family of aircraft at Filton in Bristol.

With the important WAN, LAN and networking equipment installed in the main computer room at Filton, it was crucial that 24 h/day, seven days a week systems availability was maintained.

Designed, built and installed within a four week period, four Galaxy 60kVA units (three phase on-line double conversion units, each providing 60 minutes back-up time) ensure that continuous load protection is provided by the inverter, irrespective of whether the power supply originates from the ac input source or the battery.

To ensure comprehensive monitoring and reporting of all power-related events at the site, Airbus UK plans to implement Solution Pac, Management Pac and HP Openview software with SNMP capabilities. The software package, designed and configured by MGE UPS Systems is an automated management system that continually assesses power supply and demand across the entire system. The software will enable remote site monitoring and can also be used to co-ordinate the sequential shutdown of all mainframe operating systems.