Measuring Reliability For Uninterruptible Power Supplies and Power Protection Plans

The criticality for which uninterruptible power supplies (UPS) were created means that their reliability requires some form of measure to give customers a means of comparing different manufacturers and UPS. The purpose being to shield the loads the UPS is protecting from vulnerability, therefore, reliability should not be guessed at.

Mean Time Between Failure

MTBF or Mean Time Between Failure is one such measure – an indicator of the reliability of an uninterruptible power supply. It is the average operational time between powering up and system shutdown due to failure (not power failure in this sense but failure of the UPS system itself). It is represented by a measurement of hours.

Average failure rate is another measure of reliability. This is the total number of failures in a given time period. The failure rate over the lifetime of any UPS system, therefore, is inversely proportionate to its MTBF.

Uninterruptible power supplies are no different to any other electronic equipment in that the rate at which they fail is not constant. There are three distinct periods associated with UPS failure (which are often represented by a bathtub curve diagram showing a) infant mortality failures, b) random failures and c) wear out failures).

Infant Mortality UPS Failures

Infant mortality failures correspond to failures early on the life of the uninterruptible power supply. IT-sized uninterruptible power supplies can suffer what is termed ‘dead-on-arrival’. This could be due to a component manufacturing defect or transportation damage. A sudden shock or jolt in transportation may weaken a soldered joint, for example. Whilst UPS manufacturers strive to reduce these incidents as much as possible through stringent quality checks and testing processes, they do happen. Various processes can be applied to minimise the chances of it happening. UPS from 10kVA, for example, can be run for short burn-in periods (up to 48 hours) at high ambient temperature to reduce the potential for such failures.

Random UPS Failures

Random failures happen less often. During the normal working life of a UPS, the rate of these is low and fairly constant.

Wear Out Failures

Wear out failures at the end of an uninterruptible power supply’s working life are more common (and this is where the curves is steeper). Here, battery problems account for 98 percent of UPS wear out failures. Particularly where uninterruptible power supply has been subjected to high ambient temperatures over long periods, internal cabling insulation becomes brittle and breaks down. There are other consumable items that should be part of a regular monitoring regime, such as fans and capacitors, which will also eventually wear out with use.

Just because a manufacturer shows you some favourable MTBF stats does not necessarily mean that their products are the most reliable. Like most things, these can be massaged into looking more relaxed than they actually are. The important question to ask is: what was the basis for their calculation? There are two primary approaches:

1) A record of the total number of failures for a particular UPS size over a given time period.

Commonly adopted by UPS manufacturers, this is a valuable approach if the field population is large and the time period long enough (more than the typical life expectancy of a UPS, which is five to ten years).

2) A system value calculated from the known MTBF values of components and assemblies.

Obviously, this approach is more complex and relies on following standardised calculation formats.

Mean Time to Repair

Mean Time to Repair (Mean Time to Restore) is the time taken to return an uninterruptible power supply to normal operation from shutdown.

Online UPS are designed to fail safely to mains; therefore, the MTBF calculation of the mains power supply is also an important consideration along with mean time to repair (or average repair time).

As it is highly unlikely for a service engineer to be onsite at the very moment a UPS fails, MTTR needs also to include a travel time element. This also assumes the service engineer is carrying the required parts needed to fix the problem in a single visit, which is sometimes not the case. Uninterruptible power supply manufacturers may only provide a figure based on the actual repair time. Although this may be a satisfactory comparison tool, it is not a true representation of reliability. A degree of scepticism is sometimes necessary when comparing marketing data from some manufacturers.

Uninterruptible Power Supplies and Harmonics

Harmonic pollution is a growing problem in Europe and one that designers of power continuity programmes and manufacturers of UPS (uninterruptible power supplies) cannot ignore. Typical harmonic problems include the distortion of mains power supply voltage, overheating of wiring, neutral conductors, supply transformers and switchgear and nuisance tripping of breakers. Harmonics can also cause disruption to equipment on the same supply and lead to random failures.

Harmonics are caused by voltage or current waveforms with frequencies that are multiples of the fundamental frequency – in Europe, 50Hz (50 cycles per second). The multiples are always ordered in a specific sequence: for example, the 2nd harmonic is 100Hz (2x50Hz), the third 150Hz and the fourth 200Hz and so on.

The particular problem of Triplens (third order) harmonics. Harmonics are a particular issue for power continuity management due to the large number of switch mode power supply (SMPS) loads being connected to modern electrical distribution networks – and their associated UPS systems. These are the most common form of power supply unit (PSU) in use today. As a non-linear load, they draw their power in regular modulated pulses of current from a mains power supply rather than as a continuous linear supply. This can lead to SMPSs generating high levels of harmonics, especially when many are being supplied from a single three-phase mains power supply.

In particular, system designers must be aware of the potentially damaging Triple-Ns (or Triplens) whose harmonic order numbers are multiples of three and include the notorious third-harmonics as well as ninths and fifteenths. Thirds are probably the most challenging harmonic in terms of neutral conductor loading within a three-phase system. Whereas other harmonics cancel each other out, third-harmonics are in phase with each other and exhibit a summing effect which greatly increases the current – potentially overloading conductors and switchgear.

Harmonics and total power factor – implications for UPS sizing. Harmonics are also closely related to power factor management – and another key aspect of uninterruptible power supply system design and implementation. The displacement power factor is only applicable to the fundamental frequency (50Hz in Europe) and therefore does not take into account the power factor generated by any harmonics induced into the mains power supply by the load itself (referred to as the distortion power factor and produced by the harmonics produced by non-linear loads). The combination of the displacement power factor and the distortion power factor gives what is known to UPS systems experts as the true power factor. When correctly sizing a UPS, an understanding of this is critical.

Mitigation of total harmonics distortion. Harmonics issues need to be addressed at the design stage of any power continuity plan. Not least, because consumers are responsible for the harmonic levels introduced into their three-phase mains power supply.

A UPS can sometimes be fitted with a harmonic filter (post installation) but this can be a costly and inelegant solution as extensive internal wiring changes may be required. For a transformer-based UPS, using a 12-pulse rectifier in place of a 6-pulse set will reduce the levels of THDi (total harmonic distortion). Coupling this with a passive filter will provide further reduction to around 4%.

For a transformerless uninterruptible power supply, THDi levels of less than 4% can be achieved by installing an active harmonic filter. However, levels as low as 3% can now be achieved by some designs whose rectifiers are IGBT (Insulated Gate Bipolar Transistor) based. This can remove the need for an additional active harmonic filter and simplify the UPS design process. Such designs are expected to become the norm: not only do they reduce initial costs, but they allow a smaller UPS system footprint whilst increasing input power factors.

Active harmonic filters reduce the impact of leading power factors. When designing a power continuity plan and UPS system, various methods can be applied to reduce the impact of leading power factors (where the current waveform leads the voltage waveform): ensuring that leading power factors represent a smaller percentage of the UPS load, installing power factor correction between the UPS and the load, increasing UPS size (and that of any standby generation capacity) and specifying a UPS with leading power factor capabilities.

A popular approach to reduce the effect of leading power factors on a UPS installation is to use an active harmonic filter with power factor correction on the UPS output. This presents the UPS with a more acceptable load, but results in higher capital and installation costs, lower efficiency and a greater footprint.

Familiar territory for UPS manufacturers. Although many aspects of harmonics must be considered when specifying a UPS system, reassurance can be gained from the fact that this is familiar territory for UPS manufacturers such as Riello UPS. End users and their professional advisers can certainly be confident that this specialized aspect of UPS application will be thoroughly addressed during the modern consultative sales and specification process.