In many critical power applications, it’s common for the electrical system design to incorporate both an uninterruptible power supply (UPS) and some form of secondary power generation, most commonly a diesel genset.
Assuming the system has been designed and sized correctly, it should provide an optimized combination of ride-through power for shorter outages (thankfully most power losses tend to be momentary or very short in nature) together with the protection afforded by a genset to support the load in the event of a more prolonged black-out.
The question arises whether we need both types of equipment to provide effective protection. If either can be eliminated from the design, there is the opportunity to reduce one-off capital and installation budgets, as well as on-going operating expenses.
Because UPS power is uninterrupted, runtime – or autonomy – ensures that battery power is available so that the load (e.g., computers, process control or other machinery) can be shutdown gracefully and data, equipment and processes protected and kept running. Alternatively, the UPS can keep systems alive and operating normally whilst secondary power comes online. The caveat, of course, is that the batteries are properly maintained and have been sized correctly.
In addition to power back-up, the UPS can provide useful power quality features such as line conditioning, which can be in applications where sensitive equipment or processes are being protected. However, it is the uninterrupted nature of the power the UPS provides which in some cases has made it mandatory for critical applications such as airport runway lighting backup, or marine on board navigation systems.
There are a variety of reasons why diesel generators are not always favoured as the sole source of power back-up to ensure safety of people, or the security of systems (or both). For starters, unlike UPS, generators are not uninterruptible and there is always a break in supply when the automatic transfer switches (ATS) from mains to generator supply – and vice-versa.
Secondly, there are environmental considerations when using a diesel generator. The diesel engine is considered to be one of the largest contributors to environmental pollution globally. Its exhaust contains many toxic air contaminants, including some which are known or suspected to be cancer-causing substances, as well as nitrogen oxide – currently the most ozone-depleting of all emissions.
Thirdly there are mechanical considerations; e.g., it’s not good for gensets to be run at low load as this can cause excessive engine degradation, pollution and maintenance requirements. As a result, they may not also reach full power when required.
Fourthly, the generator start is fully dependent on a starter battery, which might be non operational in case of lack of maintenance. Then, a monthly check is required, implementing the full start procedure.
It’s also not good for the generating equipment to be shut down too quickly once in operation as this can create a risk of thermal shock to the component parts. In any event, stopping the engine immediately after it has been working under load can result in overheating and accelerated wear. Where power events come in a series, this can cause problems for some generators which either cannot, or should not be restarted after shutdown.
When considering power back-up requirements in critical power applications, regulatory guidance or industry standards are in place to maximise safe and secure operations. Thereafter, runtime requirements need to be taken into account, with the estimated duration of back-up guiding whether a battery bank or other form of energy storage is a pragmatic alternative to secondary generation. Finally, or primarily, depending on your company’s view, there are environmental considerations to be taken into account.
For more details, Schneider Electric white paper 52 provides some useful analysis of the economics of batteries versus generators for runtime.