Battery pack or generator?

Robin Koffler, Commercial Managing Director at Advance Galatrek, considers the dilemma of choice between battery packs and generators to supplement UPS power.

Nowadays, managers are concerned to ensure that power outages do not interrupt the production process. It is well known that many manufacturing processes cannot tolerate any form of power disturbance without the loss of an entire batch or production run. In the light of the importance of ensuring power continuity, therefore, many production operations are turning towards specifying multiple sources of electrical power for their entire facilities.

Recent research from the United States suggests that over 90% of manufacturing plants will experience power sags of at least 20% deviation from their nominal voltage, and that there will be in excess of thirty incidents of at least 10% deviation each year. Although the reasons themselves are diverse, these disturbances can result either from natural events such as weather-based phenomena, as well as other occurrences elsewhere in the power chain.

Against this backdrop, it is clear that escalating demand, utility deregulation and an ageing infrastructure all militate against the availability of consistent, high quality mains power.

Traditionally, commercial and industrial organisations have relied on the UPS to provide emergency cover in the event of a power interruption. This is especiallytrue in the case of IT managers, where the UPS is a critical component, given the need to supply instantaneous power to sensitive microprocessor-based equipment. Because even the shortest power disturbance to these systems can cause costly information loss, programming errors and data corruption, lead-acid batteries have typically been used to support the UPS, by supplying instantaneous power the moment an outage is detected.

However, lead acid cells are not the only choice as an auxiliary power source for a UPS installation. Depending on a variety of factors, including cost, size of installation, space available and need to power other devices, a generator might be a more suitable alternative to a traditional battery pack. As a general rule, the bigger the UPS, and the longer the runtime duration required, the more cost effective it becomes to specify an auxiliary generator, especially for larger three phase applications.

As such, batteries have historically been used to cover only shorter duration power outages. If a power cut lasts for longer than the specified battery capacity, such a system will fall foul of your business continuity expectations. In such circumstances, the most that a user can expect is to have sufficient backup time to save work in progress, and complete an orderly shutdown of the affected systems.

Because many commercial users have a very high expectation of reliability and continuity, this has led organisations to search for alternative auxiliary power supplies that will stand behind the ubiquitous UPS, and guarantee longer term backup power. As a result, many facilities now use engine-based generators, which can be powered by diesel, petrol or gas, to complement their UPS installation. Make no mistake, some form of power generator is essential for larger institutions looking to avoid missing a key element in their critical power distribution system. Nonetheless, many argue that generators still have their drawbacks.

One of the most significant disadvantages of using a large generator as a standby power source is that it cannot be run indoors. This is a major drawback for a lot of organisations where there is simply insufficient external space available in which to site a separate generator. Moreover, the practical restrictions on siting such a unit often dictate that a dedicated housing has to be constructed for the equipment.

Other disadvantages in running an auxiliary generator include the cost of installation and the high level of routine maintenance that such a system entails. In the first instance, a static generator will generally require expert installation, which can be costly in itself. On top of this is the expense of regular routine maintenance to ensure that the system will work reliably in the event of the very mains failure that it is supposed to protect against. Added to this is the fact that potentially hazardous fuelsneed to be stored, often in critical environments, as well as the likelihood of both noise and emissions pollution occurring.

In contrast, the main advantages of battery technology read very much like the converse of the above. Batteries have zero emissions, relatively low maintenance requirements, quiet operation and do not use toxic or hazardous fuel. For many locations, these will be compelling reasons to use a battery set as opposed to a fuel based generator. However, the choice between generator and battery pack is not quite as clear-cut as simple issues such as size and duration of operation alone.

Although in principle, when a mains power failure occurs, the auxiliary generator steps in to provide an alternate power source, UPS batteries are still needed to provide the initial power for the load. Once the generator is fully operational, an automatic transfer device switches the power to that supplied by the generator. What this requires, however, is careful planning to make sure that the generator is properly specified for the job in hand. While it is pointless specifying a generatorthat has a far greater capacity than is strictly needed for the application, it is equally foolish to install equipment that is only just adequate for the equipment to be protected. After all, circumstances can alter, and it would be costly exercise to purchase a new generator only a few months after originally specifying your business continuity systems.

More importantly, there are some quirks of operation that must be taken into account when considering the UPS/generator interface. Many types of UPS rectifier represent a non-linear load. Because of this, the generator may typically exhibit poor output voltage regulation, caused by reflected current distortion from the rectifier switching. Another common problem is a fluctuation in the frequency supplied by the generator, due to external physical factors, such as sudden load steps from equipment like conditioner compressors. These may also be caused by the rectifier phase controlsadjusting to voltage deviations.

The cumulative effect of these voltage and frequency issues can be significant for the UPS itself. Most conventional UPSs can accept a voltage variation of plus or minus ten to fifteen percent. However, if the input voltage varies much outside this range, the UPS will revert to battery operation, defeating the object of specifying a generator in the first place. And, with enough of these events, the UPS batteries can easily become completely discharged, leaving the critical load without any protection.

Equally, although electronic equipment such as PCs, servers and comms hardware can tolerate AC frequency shifts, this can also compromise UPS performance. In fact, a sufficiently wide frequency deviation can cause a UPS rectifier to shutdown due to high DC protection and the inability to maintain output voltage regulation. Once again, this would cause the UPS to switch to battery power instead of the generator. With these issues in mind, there are several guidelines that any organisation considering a UPS/generator combination should be aware of.

Firstly, the generator should be oversized compared to the UPS rating. This will allow the generator to supply a more pure output to the UPS, especially where driving non-linear or dynamic loads. For this reason as well, the generator should ideally incorporate a form of frequency governor and voltage regulator.

Secondly, any UPS that partners a generator will benefit from a low input current distortion. However, expert advice should be sought when selecting the type of UPS, as many incorporate filters that cause leading input power factors at a light loading. The problem here is that this can reduce generator performance, and even cause resonant conditions with the power feed during inconsistent mains conditions.

In summary, therefore, while there is a general consensus among industry and commerce that an uninterruptible power system is an essential step on the road to business continuity, there is still some debate as to what represents the best means of auxiliary power to support the UPS itself. In practice, this all boils down to what users needs from their systems. Both battery sets and generators have their inherent merits and disadvantages, especially when it comes to the equations of cost, size and performance.

However, because technology is progressing all the time, battery life is improving while their footprint continues to reduce and, all the while, generators become smaller, lighter and more efficient. Nevertheless, the compromises inherent in each technology still mean that businesses have to make a choice between cost versus complexity, and between the securing of critical data and longer term business continuity.

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