Unacceptable frequency ranges
Consistent voltage and frequency are directly proportional to the size and type of generator. Standby generators generally specified for the protection of IT applications are smaller than generators that would support equipment within other areas of a building or manufacturing facilities. These smaller generators, unlike those used in utilities, produce less consistent power. Although the voltage range may be acceptable, the frequency range can often be wider than off-line or line-interactive UPS are designed to accept. As a result, it’s almost impossible to make an off-line or a line-interactive UPS work with a standby generator. Here’s why:
Off-line and line-interactive UPS technology
An off-line (including line-interactive) UPS passes mains power to the critical load. An off-line UPS (Uninterruptible Power Supply) may have some surge suppression and filtering but will have no power conditioning. A line-interactive UPS will have some power conditioning functions but will focus entirely on correcting voltage variations by means of a transformer and an automatic tap-switching feature. This has no effect on frequency variations, so the line-interactive UPS reacts to out-of-spec frequencies the same way as the off-line UPS, i.e. it will transfer to UPS battery. When the battery is depleted, the off-line or line-interactive UPS will shut down the critical load.
Why on-line UPS technology works best with generators
On-line or double conversion UPS designs are ideal for use with generators because they accept input power with relatively wide variations in voltage and frequency. An on-line UPS actually re-develops the waveform. The AC input from either the mains or a standby generator is first converted to DC by the rectifier. The DC current is used to charge the battery. During an outage, DC current from the batteries is reformed by an inverter back to perfect sine wave AC for the critical load. Newer models also use power factor correction to provide an even wider input voltage range.
Because of this double conversion (AC to DC, then DC to AC), variations in the input frequency are of little concern. Therefore the same frequency variations that would cause an off-line or a line-interactive UPS to transfer to battery power have no effect on the on-line UPS.
Sizing the generator
A standby generator must be sized adequately for the UPS it is supplying.
As well as being capable of handling the step load of the UPS, the generator must have sufficient capacity to recharge the UPS batteries and cover the conversion losses of the UPS. The maximum step load of most generators is around 65% of the total rated capacity of the generator. Conversion losses and charging current can add a further 15% to the rated UPS power draw. A 100kVA UPS can, therefore, draw 115kVA and to accommodate a step load of 65% the generator must be sized at 172.5kVA, or 72.5% larger than the UPS.
It’s normal not to connect air conditioning equipment to the UPS, but instead to support it on the generator (since the air conditioning can handle a 30-second break while the generator starts). This load can be timed to reconnect once the step load of the UPS is supported, making full use of the generator capacity.
The UPS itself embodies a switched-mode power supply. This will create harmonics in any electrical circuit, but a supply of sufficient capacity will provide a ‘damping’ effect. Although this can be reduced by fitting a 12-pulse rectifier or using a unity input factor UPS, generous sizing of the generator will often provide a cost-effective alternative.
A further reason to allow plenty of generator capacity is that when running it is normal to experience a 10° rise in temperature within the proximity. When exposed to high ambient temperatures, generators will not run efficiently and can experience losses in power output.
High running temperatures can also cause damage to both the turbocharger and exhaust system. If you anticipate this situation, it is common practice to increase the overall size of the generator installation, since a generator will run cooler when not working to its full capacity. Good practice is to load the generator to 75% of its overall capacity for constant running. There is nothing wrong with running a standby generator at 100% of its output capability for short periods, but they are not generally designed for this. A ‘prime power’ generator should be specified where this is likely.
The generator circuit should also include two relatively inexpensive peripheral devices. The generator itself should be equipped with a good electronic governor. This will reduce the magnitude of frequency deviations on its output. There should also be an automatic transfer switch, ahead of the generator, which should have an in-phase monitor.
The load transfer switch prevents the generator power from back-feeding the mains supply. The in-phase monitor ensures that the two power sources (generator and mains power) are not out of synchronisation when mains is restored as the transfer switch cuts away from the generator in favour of mains. Out-of-sync power sources could result in a potentially destructive power spike to the UPS and unprotected connected equipment.
Larger Data Centres
As with UPS equipment, larger and/or more critical sites can utilise generators connected in parallel, either for capacity or redundancy. Use of (say) three generators where two can handle the load ensures that even during scheduled maintenance visits, full protection is available to the site. Synchronisation is achieved via the use of additional equipment, and generally, this can be retrofitted. This helps protect the original investment where growth has dictated an upgrade.
In summary, a matched UPS and generator for IT mission-critical applications requires an on-line UPS that can accommodate most frequency variations. There should also be a properly sized generator that will absorb most voltage variations. Further insurance can be provided by an electronic governor on the generator and an in-phase monitor on the automatic transfer switch.