Power outages: How much backup time do you require?

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By Jack Ward, MD of Powermode

One of the ways to deal with South Africa’s unstable and unreliable power service is to install an uninterruptible power supply (UPS) system. There are many options available, so users should take time to decide on the most appropriate offering in order to save unnecessary expenditure.

The majority of power cuts experienced by South Africans are not momentary – such as those caused by grid switching. Today we are faced with long periods of ‘load-shedding’ or what can pass for load-shedding; service interruptions due to ‘infrastructure maintenance’.

Therefore, if vital equipment such as security systems and surveillance cameras or mission-critical computers must be powered on a 24 x 7 basis, the sizing of the UPS serving them is vital, particularly if there is no standby generator to supply replacement power.

In determining the total number of hours of back-up time available, accurate calculations relating to the size of the UPS and number of batteries in its battery-pack play key roles.

Importantly, one of the advantages of a UPS system – compared to a generator or power inverter – is its ‘power conditioning’ effect or the removal of so-called spikes and brown-outs from the power supply. This is a fundamental requirement for sensitive electronic equipment.

Central to a UPS system selection include factors such as the maximum possible load required, the average likely load and the level of redundancy in runtime needed.

In the past, before the load-shedding era, between 10 and 30 minutes was considered to be a reasonable run time. Today, between two and four hours of backup is considered mandatory.

To achieve this goal, an appropriate battery pack must be installed. When considering its sizing a good rule of thumb is to see a recharge to 80% within twenty-four hours. This is based on the non-linear recharge curve associated with deep-cycle lead acid batteries.

Simplistically, the calculation is: Backup Time = Battery AH x 12V x N x Efficiency of Battery / Load in Watts.

  • Battery AH = Amp-hour capacity of battery
  • N = number of 12 volt batteries needed.
  • Efficiency of battery is generally calculated at 0.8.

One of the pitfalls to avoid when selecting a UPS is to over-specify and opt for the largest UPS system available, to meet a theoretical maximum load at some accepted point in the future.

This often results in an overly large UPS system being installed which would run at a significantly less-than-maximum capacity when called upon to do duty. It would also deliver a less-than-optimum capacity from a cost efficiency perspective as a UPS system runs at its highest efficiency when it is near its maximum rated capacity. As load level drops, so does efficiency.

One of the most important benefits of new-generation UPS systems – featuring compact transformer design techniques – is their modularity. This gives them the ability to scale to meet future anticipated loads in N+N redundancy-type configurations with the inherent advantages of failover transparency which contributes towards hitting a continual (100%) uptime target.

This feature also gives users the benefits of flexibility to mirror increasing power demands without placing undue stress on a traditional single (non-modular) system that might find itself overwhelmed by corporate growth and expansion thus requiring a ‘forklift upgrade’ to remain viable.

A significant benefit linked to this technology is the ability to ‘hot-swap’ faulty UPS modules, boosting overall system redundancy. With a ‘cold’ spare module available, mean time to repair (MTTR) benchmarks could be reduced to minutes.

Modern modular UPS systems can be configured – and re-configured – ‘on the fly’ in order to keep them close to capacity and operating at the highest efficiency levels.

Efficiency levels of 95+% can thus be reliably achieved. This is considerably better than the 80 to 85% range that traditional systems can expect to reach under ideal conditions.

This ‘right-sizing’ concept – continually matching the capacity of the UPS system to its critical load – is already delivering major reductions in electricity consumption and CO2 emissions. An increasingly important spin-off is a significant reduction in the user’s carbon footprint. Another key advantage of this approach is a reduction in the impact of maintenance on uptime and budget allocations.

In the latest UPS systems, load-sharing control systems are able to intelligently distribute the workload among the UPS modules without the need for direct synchronisation links between them. This means that any module can provide backup support for any other module without interruption to the power supply or downtime. In an over-capacity situation, modules can be temporarily shut down.

Modern technological advances are also playing a role in improving battery longevity – recognised as an Achilles Heel of all standby power devices. Newly released on the local market is an SA-designed and branded ‘long-run’ battery pack geared to obviate this problem.

The battery pack’s design is based on the principle that the key to battery longevity lies in a thorough an understanding of the status of batteries in terms of their duty cycles and load factors.

In the case of this long-run battery pack, the knowledge is facilitated by in-built ‘smart’ technology. This includes a computerised, automatic battery balancing harness that also reports – via a ‘cloud-based’ portal – on a range of parameters associated with individual batteries in the pack.

Data streams containing information critical to the well-being of individual batteries, including temperature, state of charge and depth of discharge are monitored and a tally of the number of discharge/charge cycles is recorded.

The manufacturers of this technology are backing it with a battery performance guarantee. Surely a ‘first’ for this industry sector in South Africa?

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