FEATURE UPS & STANDBY POWER Having a strategy in place
Modern UPS systems are highly effective at protecting enterprise information and communications technology (ICT) equipment from electrical mains power disturbances and outages. There is, however, always a risk that a power outage could extend beyond the UPS’s battery autonomy. In this article, Kenny Green, technical support manager at Uninterruptible Power Supplies, looks at the choice of such strategies available to UPS operators
A
ny UPS installation must include a strategy that allows for the
possibility of extended power outages. Of the options available, a generator-based approach is the only way of keeping ICT equipment safely operational and online, irrespective of the outage’s duration. Today, most commercial and industrial enterprises use UPS systems to protect their information processing and communications equipment from electrical mains problems or failures. Given their critical role, it is worth examining how UPSs support the IT load during power failures, and when they should be supplemented by a generator back-up system. If the electricity supply does fail, the
UPS can only support its critical load for the duration of its battery autonomy. Beyond this, if no further strategy is in place, the load will lose its power just as traumatically as if it had been connected directly to the incoming mains. Accordingly, a UPS installation must be prepared for the eventuality of an extended power outage. Three alternative strategies exist, as described below.
STRATEGIC CHOICES Firstly, if the enterprise can accept the loss of the load’s data processing resource during the power outage, the UPS battery autonomy can be used to shut down the load in a safe and orderly fashion. If the UPS detects that it has been running on battery power for longer than a predetermined time, it signals the load accordingly - allowing it to shut down within the battery autonomy of typically around ten minutes. The equipment and its stored data are safe but the data processing resource is no longer available. The second approach involves adding
additional battery capacity to provide longer autonomy. Depending on the system size, extra battery cabinets can be connected, extending this protection to several hours. Service continues through a power outage, however the load remains exposed to the possibility that duration of the power outage will still exceed the battery’s autonomy. Additionally, the batteries and cabinets take more space, and incur greater
16 MARCH 2014 | ELECTRICAL ENGINEERING
costs in terms of purchasing, maintenance and final disposal. The third solution is to use
a generator to back-up the UPS. Although the most costly to implement and maintain, it is the only way of ensuring that the data processing resource and equipment is kept safely online throughout an extended power blackout. In this arrangement, the generator starts when it detects a mains failure. The UPS battery gives it time to start, stabilise and switch over to the UPS. If the generator is correctly sized for the application, the UPS will accept it as a ‘mains replacement’ and start battery recharging while continuing to support the critical load throughout the outage.
GENERATOR CONSIDERATIONS The generator typically comprises an engine driving an alternator. Since mains power failures are unpredictable, the generator must be ready to immediately start supporting the UPS. Accordingly, its engine, which is usually diesel powered, must be well maintained, with an adequate fuel supply, a healthy battery for initial start-up and adequate coolant and oil. Standby generator engines are usually kept warm with mains powered engine water heaters, while their starter batteries are trickle charged from a mains powered battery charger. The generator’s alternator converts the
engine’s mechanical power into AC electrical power. In the UK this is typically either single-phase 230V or 3-phase 400V. The value of the output voltage is set by how the alternator is ‘wound’. The amplitude and stability of the alternating output voltage is controlled by an automatic voltage regulator (AVR) and the generator’s engine speed determines the output voltage frequency, which is usually 50Hz in the UK. The engine speed and therefore voltage
frequency is stabilised by a governor that regulates the engine’s fuel supply. Either mechanical or electronic governors can be
used - mechanical types are lower cost, while electronic governors are highly responsive and offer very stable engine speed. Electronic governors are
favoured and used in standby applications as UPSs and their loads cannot handle an excessively wide generator voltage frequency
range or a frequency changing too fast. If a UPS detects this behaviour in
Above: typical protected power installation with mains failure detection and change over
a generator output, it will warn the operator that the load will not transfer from the UPS to the raw generator supply if a fault occurs. The generator has an automatic mains
failure (AMF) panel to monitor the mains power and signal the engine to start. To avoid this happening for every mains disturbance, the AMF signal should be delayed until the mains supply has failed for typically two to ten seconds. Once enabled, the AMF signal usually controls both the generator start-up and connection of its output supply as a mains replacement. Similarly, the generator should not be shut down immediately mains power is detected again. This should be delayed for at least two minutes to ensure that the supply has been reliably restored. The generator should be sized sufficiently
to run not just the UPS but also air conditioning, emergency lighting, communications and alarm systems. Additionally, the load should not be switched onto the generator in a single step, especially if its total exceeds half the generator’s standby rating. Instead, a soft- start method should be used to introduce the load gradually to the generator. Only with the system described above
is your critical load truly protected for almost any eventuality, so the question remains, how critical is your load and what level of protection do you need to meet your commitments?
Uninterruptible Power Supplies (UPSL)
www.upspower.co.uk T: 0800 731 3269
Enter 207 / ELECTRICALENGINEERING
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