Providing continuous power to critical processing and communication applications such as data centres, online banking and emergency services is essential, and nothing less than 24/7 can be tolerated. However, the user’s confidence in the reliability of the utility power supply has deteriorated due to the fragility of the electrical supply infrastructure and potential supply and demand issues. Consequently the recognised solution nowadays to meet 24/7 continuous power lies in using a combination of a standby generator and an uninterruptible power supply (UPS). While a UPS alone will protect against short term utility power loss and supply quality problems the UPS will eventually shutdown when its batteries reach the end of their discharge period. Increasing the size or number of batteries will of course extend the autonomy time but this is becoming more expensive as the rising cost of lead directly affects battery pricing. Irrespective of this, the exposure to blackout times exceeding the UPS battery autonomy will always remain a reality. Conversely, using a standby generator alone will serve as an alternative source of power but will not provide a no-break solution in the event of a loss of mains power. However, a generator with a secure fuel supply can provide a source of power, far exceeding the duration of a long term mains blackout and the back-up capability of a static UPS system. It is therefore clear that the respective limitations of utilising a UPS or a standby generator alone are fully overcome by operating these two different sources of back-up power in tandem as shown below. White paper: UPS and generators together – the only true 24/7 power solution Fully integrated power protection solutions for today’s critical loads. Figure 1 – tandem back up system UPS 245-01-00 Enclosed generator set PowerWAVE 9000DPA UPS Enclosed generator set PowerWAVE 9000DPA UPS
Transcript
Providing continuous power to critical processing and communication applications such as data centres, online banking and emergency services is essential, and nothing less than 24/7 can be tolerated.
However, the user’s confidence in the reliability of the utility power supply has deteriorated due to the fragility of the electrical supply infrastructure and potential supply and demand issues. Consequently the recognised solution nowadays to meet 24/7 continuous power lies in using a combination of a standby generator and an uninterruptible power supply (UPS).
While a UPS alone will protect against short term utility power loss and supply quality problems the UPS will eventually shutdown when its batteries reach the end of their discharge period. Increasing the size or number of batteries will of course extend the autonomy time but this is becoming more expensive as the rising cost of lead directly affects battery pricing. Irrespective of this, the exposure to blackout times exceeding the UPS battery autonomy will always remain a reality.
Conversely, using a standby generator alone will serve as an alternative source of power but will not provide a no-break solution in the event of a loss of mains power. However, a generator with a secure fuel supply can provide a source of power, far exceeding the duration of a long term mains blackout and the back-up capability of a static UPS system.
It is therefore clear that the respective limitations of utilising a UPS or a standby generator alone are fully overcome by operating these two different sources of back-up power in tandem as shown below.
White paper: UPS and generators together – the only true 24/7 power solutionFully integrated power protection solutions for today’s critical loads.
Figure 1 – tandem back up system
UPS 245-01-00
UPS 245-01-00
Enclosed generator set
PowerWAVE 9000DPA UPS
Enclosed generator set
PowerWAVE 9000DPA UPS
During normal operation the utility mains feeds the critical load via the UPS, during which time the UPS battery is also float charged. The UPS will protect against breaks and disturbances in the mains supply and after a pre-set time, typically between 2 to 10 seconds, the Automatic Mains Failure (AMF) panel sends a signal to the generator to start. The UPS battery serves as an alternative source of supply to support the critical load while the generator starts up. Once the generator has stabilised the UPS accepts the generator as a mains replacement, continuing to supply the critical load and recharge the battery for the duration of the mains failure. When the AMF senses that the mains supply is restored and stable it switches back from the generator, with the UPS battery once again covering the power interruption caused by the changeover. This sequence ensures uninterrupted clean power to the load and demands no user intervention.
However, a standby generator and UPS do not have natural plug ‘n’ play compatibility; they must be carefully matched with one another to ensure reliable operation. Although reliability is of course a critical consideration, it is not the only one. There are large cost implications in ensuring that the generator is optimally sized for the UPS, as well as many physical and environmental factors.
UPS ConsiderationsConstant voltage and frequency are directly proportional to the size and type of generator. The generators used by the power generation companies produce consistent power because they are extremely large. A standby generator, by comparison, is quite small and cannot supply such consistent power. Any increase in electrical load requires an instantaneous increase in mechanical power to supply it and whereas in a large generator many of these variations are ‘absorbed’ by the inertia of the rotating parts, a small generator set with less inertia will actually slow down until the engine governor compensates. The resultant instability in generator voltage and frequency must be accommodated by a correctly chosen UPS system.
UPS CompatibilityThere can sometimes be compatibility problems between the generator and the UPS system.
The generator output voltage may be acceptable to the UPS, but often the generator’s frequency range is wider than the UPS is designed to accept. In the worst case the frequency variations of the generator will be such that the UPS cannot synchronise with it, either because the frequency is outside limits or it is varying too quickly for the UPS to follow (slew rate) causing, in some cases, the UPS to revert to battery supply and putting the integrity of its output supply in jeopardy.
This problem can be overcome by ensuring that the generator is properly matched to the UPS and fitted with an electronic governor that allows the generator to operate within tight frequency tolerances.
UPS 245-01-00
Close up of battery
The type of load presented to the generator by the input stage of the UPS can also cause problems. Typically a UPS utilises a phase controlled rectifier which imposes notches on the power feed, interfering severely with some types of generator control systems. Phase controlled rectifiers also draw a non sinusoidal input current, creating harmonics, measured as total harmonic distortion (THDi). These can cause excessive heating in the generator alternator, especially as some manufacturers’ UPS generate up to 30% input THDi.
Some UPS manufacturers introduce a capacitor and inductor filter at the input to the UPS to attenuate the level of input current harmonic distortion. However, this is often a passive filter tuned to optimum load conditions, which can create a leading power factor if the UPS is subsequently lightly loaded. When a leading power factor is presented to the generator the alternator over-excites and the output voltage rises. To compensate, the voltage regulator reduces exciter power to reduce the strength of the magnetic field. Under such circumstances the magnetic field can fail causing the generator to shutdown or even result in catastrophic alternator damage.
Generator Sizing GuidelinesIn addition to the UPS load, the generator may also be required to power emergency lighting, air conditioning, building alarm systems etc. (see Figure 1). As a very general ‘rule of thumb’, when assessing the rating of a generator the following multipliers should be applied:
1.5† x the nominal UPS capacity should be allowed for a transformerless UPS
2.0 x the nominal UPS capacity should be allowed for a transformer based UPS
3.0 x the nominal air conditioning running capacity should be allowed for the air conditioning.
Data centres rely on 24/7 power protection
† With the UPSL transformerless 9000DPA series, this figure drops to 1.2 times the nominal UPS capacity, thanks to the much reduced THDi imposed on the generator. Significant savings in both cost and space arise from this reduction in generator size.
UPS 245-01-00
UPS 245-01-00
In practice, it is usually necessary to size the generator to handle more than just the UPS. While the UPS is typically running data processing equipment, the air conditioning power to cool the equipment must be maintained. Emergency lighting, communications and alarm systems must also stay operational.
The above are only guidelines and it is recommended that advice should be sought from the UPS and generator supplier if independently attempting to size the generator.
Step LoadingMost generators cannot accept 100% of their load rating in one single step. For example, an Auto Mains Fail (AMF) panel cannot present a 100kVA load to a 100kVA generator in one single ‘hit’. A generator’s ability to take large load steps is a function of its design and turbo charged generators can, typically, take larger load steps than standard generators.
It is good practice to not introduce the entire load to the generator when this load is >60% of the generator's standby rating. This can be achieved by either over-sizing the generator, which is a potentially expensive option, or by ensuring that load equipment such as the UPS and air conditioning has a ‘soft start’ facility or by the use of time delay contactors on the essential services distribution board.
Generator LocationA key decision involves whether to site the generator within the user’s building or outdoors. There are pros and cons to both choices. Indoors, a generator requires significant environmental management; for example, exhaust ducting, adequate airflow for cooling, noise attenuation, space etc. Outdoors, these requirements can be easier to handle, but the generator itself must be environmentally protected. Smaller generators of up to 750 kVA usually have a combined weatherproof and acoustic canopy, while larger generators are typically installed into a container for out of building operation.
Uninterruptible Power Supplies Ltd. provides full site survey and installation
UPS 245-01-00
ContactUninterruptible Power Supplies LtdBacchus HouseCalleva ParkAldermastonBerkshireRG7 8EN
The fuel storage tank must be double skinned or bunded, or have a drip tray to contain oil spillage in accordance with environment contamination regulations. Most standby generators have a base tank, often referred to as a ‘day tank’ fitted as standard. For longer autonomies a separate bulk fuel storage tank is required which adds to the cost, space and complexities of the installation.
Electrical InstallationAs a minimum, a power cable rated to carry full generator power and a signal cable to carry generator start/stop signals must be run between the generator and the AMF panel and/or the essential services board. If the cable from the generator is long then it may be necessary to increase the cable cross sectional area to reduce the ‘volt drop’ along it. This increases the electrical installation costs of the generator, so the generator should be located as close as practically possible to the AMF panel and/or the essential services board.
Uninterruptible Power Supplies Ltd. provides full commissioning and maintenance services
ConclusionThere are obvious reasons and advantages for choosing a single source vendor such asUninterruptible Power Supplies Ltd (UPSL) who are experienced in supplying both UPS andgenerator equipment and the associated installation and environmental requirements. Theycan ensure that the complexities of matching a UPS and generator are taken care of,removing the responsibility from the consultant and contractor and alleviating any concernsfor the end user. In addition to providing a one-stop solution, features within the UPSLproducts also contribute to improving operating efficiency, thereby offering significantsavings in running cost.
A correctly configured and installed UPS and generator combination can give years oftrouble free service and true 24/7 year round power availability even in the event ofextended power failures.
