Published on Wednesday, Mar 10 2010 by
Changing business priorities, the risk and consequence of power cuts, and a dependency on technology have heightened the need for secure and integrated power protection.
Providing continuous power to critical processing and communication applications such as data centres, online banking and emergency services is essential, and often nothing less than 24/7 availability can be tolerated.
Consequently, the recognised solution for continuous power is the 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 a possible ‘blackout’ period when exceeding the UPS battery autonomy will always remain a reality.
The National Grid is under pressure to meet growing demand with an aging infrastructure and uncertainty about future energy sources. Growing dependence on power for business critical systems adds impetus to the need for robust protection – and standby generation capacity - to cope with deteriorating supply, more frequent interruptions, and longer term power cuts. This firmly places integrated UPS and generators at the frontline of business continuity.
Tandem back-up
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.
During normal operation the utility mains feeds the critical load via the UPS, whilst 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 from 2 to 10 seconds, the Automatic Mains Failure (AMF) panel sends a signal to the generator to start-up.
The UPS battery serves as an alternative source of supply to support the critical load while the generator starts. Once the generator has stabilised, the UPS accepts the generator as a mains replacement, continuing to supply the critical load and recharges the battery for the duration of the mains failure. When the AMF senses that the mains supply is restored and stable it shuts-down 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 co-operation. Although reliability is of course a critical consideration, it is not the only one. There are significant cost implications in ensuring that the generator is optimally sized for the UPS, as well as many physical and environmental factors to consider.
UPS considerations
Constant 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 selected UPS system.
UPS Compatibility
There can sometimes be compatibility problems between generators and UPS systems. 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.
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 UPS systems on the market 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 guidelines
In practice, it is usually necessary to size the generator to handle more than just the UPS. As shown in figure 1, the generator may also be required to power emergency lighting, air conditioning, building alarm systems etc. As a very general ‘rule of thumb’, when assessing the rating of a generator the following multipliers should be applied:
†With UPSL’s transformerless PowerWAVE 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.
The above are only guidelines and advice should always be sought from the UPS and generator supplier when attempting to size the generator.
Step Loading
Most generators cannot accept 100 percent 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 greater than 60 percent 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.
Location and installation
A 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.
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. Whether the generator is installed indoors or outdoors the local authority and fire officer should be advised.
As 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, therefore the generator should be located as close as practically possible to the AMF panel and/or the essential services board.
Uptime in a downturn
Turnkey supply and installation of the UPS and generator delivers valuable integration benefits, ensuring seamless interaction between systems. This avoids the problem of demarcation between different suppliers and eliminates potential points of failure. Individually sourced units can compromise system autonomy and present a risk of mis-sizing, causing installation and commissioning problems.
With a packaged solution, the complexities of matching a UPS and generator are taken care of, removing the responsibility from the consultant and contractor and alleviating any concerns for the end user. Significant cost savings and operating efficiency can also be gained.
Power failures can be catastrophic, particularly in a recession when businesses can least afford productivity and revenue losses through system downtime. Fully matched UPS and standby generators - correctly configured and installed - ensure a true ‘no-break’ solution in the event of an extended power failure, maximising the protection of critical loads and assuring uptime.
This article has been published in the following:
Electrical Review
Electrical Engineering