Power Up To Protect

Barrie Miller of Scorpion Power Systems takes a look at standby power protection

Imagine you are the manager of a 1,000-user network, it is Friday afternoon and you are looking forward to a relaxing weekend away from the office. Suddenly, the overhead lights flicker and within seconds your pager displays a digital message ‘application server power lost’. You dash to a nearby PC, quickly bring up a web-based interface to the power-management software located on the network, and check the status of the affected server. Your display illustrates graphically that your standby generator has taken over and is providing a temporary supply of power to the server.

Nowadays, anyone managing a network should know the importance of having an effective power-management strategy to prevent data corruption and ultimately save their company time and money. If the power to a server is turned off without properly shutting down the server, data can be corrupted, so all servers and any PCs that run critical applications should be protected from power outages. The cost to the company in the time that it takes to restore the data from backups, plus the interruption of service while the server is rebooted, can easily run to thousands or even hundreds of thousands of pounds per hour.

The first step in providing power protection is to understand the leading causes of power-related problems. A myth to dispel is that power is perfect.

Research carried out on power supply disturbances conducted throughout Europe shows great differences from place to place and between different types of installation. On average a computer site experiences around 20 blackouts each year and these are generally far more common in rural areas. Most blackouts arise in the local, low-voltage distribution network.

Many companies still have a picture in their mind of generators being noisy, dirty machines that frequently breakdown. However, this is far from the truth.

The development of generators over the past few years has been immense. Today it is possible to obtain small units that can power up a single appliance or large systems that are suitable for all major standby power applications including the server, security and telecom markets.

The advances in high-volume, high-specification, relatively low-power standby generators with AC or DC output has significantly reduced the capital cost of these facilities and with the advent of mobile communications, telephone companies are now protecting nodal sites and even individual base transmitter stations with standby power generation. Where the cost of installing mains power to remote sites is excessive, the option of generators fuelled by diesel oil or propane gas as the prime power source is now a very viable proposition.

Combined with UPS

Standby generators provide additional backup to a conventional UPS (uninterruptible power supply). Installed as part of an SPS (standby power system), the generator takes over before the batteries of the UPS run out, smoothly transferring the network over as part of the SPS. When mains power returns the software will automatically switch back to conventional power after it has ensured that the power will remain constant. The inclusion of an SPS into a power-protection plan significantly reduces the headaches associated with complete loss of power.

AC or DC generators

The mix of equipment making up the load will most likely decide the choice between AC and DC power for sites. Where air-conditioning equipment is a large and essential part of the site load, then AC power generation is really the only option.

When selecting an AC generator a number of points need to be considered. A large proportion of the load is likely to consist of non-linear switch-mode equipment and this can adversely affect the alternator’s performance because of harmonic distortion reflected to the generator output. In extreme cases the engine speed, and consequently the electrical frequency of the generator, can become severely unstable as the governing system overreacts to instantaneous load changes. Over-speed protection devices can also be fooled by the harmonic ‘peaks’, causing the generator to shut down under a false alarm.

Oversizing the alternator capacity can greatly assist in meeting this problem and a minimum factor of 1.6 should be applied to the load rating to calculate the minimum alternator size. A true load characteristic printout should be obtained to accurately calculate the alternative size.

Where the bulk of the site load is DC, then a DC standby generator is a better option. It brings the advantage of freedom from problems with harmonics, with no need for changeover switching equipment. Protection against rectifier failure as well as mains power failure is also included.

These units are now available down to very small outputs and larger units can be used in parallel to provide redundancy and greater power outputs. For powering test equipment and other light AC loads, a small commercially available inverter can be added.

The best option — diesel or LPG

Diesel fuel has been the traditional fuel for standby generators, offering the advantages of low cost, easy availability and readily accessible engines. In recent years, however, environmental issues have made diesel a less attractive fuel to store and dump. For applications in the telecoms market problems with light-load running of diesel engines are difficult to eliminate since the theoretical maximum load of a site with fully discharged batteries is likely to be dramatically greater than the steady-state running load.

A better alternative is to run gas-engine generators, either with mains natural gas or LPG (propane) in cylinders or bulk tanks. This fuel is stored in sealed containers, cannot become contaminated and burns extremely cleanly, giving very low emissions, freedom from light-load running problems and around 1,000h between oil changes. For prime power applications this is a considerable bonus, giving greatly reduced service costs. The ability to run continuously on light load gives significant commercial and environmental advantages over deep-cycling larger battery banks.

Comprehensive remote-monitoring systems are now very cost-effective and true protection against fluctuations and failure of the mains power supply network is viable for almost any site.

Scorpion Power Systems, Davenport House, Davenport Gate, McAdam Way, Portway Industrial Estate, Andover, Hants SP10 5SQ; tel: (01264) 321100.