| Most critical power protection solutions, incorporating | | | | Pressure (BMEP) of the engine. This is a function of |
| uninterruptible power supplies (UPS), today are | | | | engine speed, number of cylinders and the swept |
| interfaced with an alternative source of back-up | | | | volume of each cylinder. |
| power (standby power) which could be a fuel cell or | | | | Synchronisation |
| flywheel but more usually it is a diesel generator. | | | | For load acceptance to occur, a UPS must be able to |
| Generator sizing and UPS compatibility are | | | | synchronise to the voltage waveform supplied by the |
| fundamental to power continuity and must be taken | | | | generator. Uninterruptible power supplies tend to |
| into account at the outset of any power protection | | | | have fairly wide input voltage windows and generator |
| plan. | | | | output is usually well within this. Its frequency, |
| Power Rating | | | | however, can vary, which can be problematic. This is |
| A generator must be sized correctly so that when | | | | overcome by widening the UPS operating parameters |
| it’s required to do so it will be able to power the | | | | to accept a broader range. This may not always be |
| UPS (taking into account any allowance for harmonics | | | | sufficient, particularly for poorly maintained or |
| that the UPS’s rectifier will generate) and the | | | | undersized generators. Their output frequencies could |
| load/s that the UPS is supplying. Generators are | | | | drift and make it impossible for the UPS to |
| typically rated in two ways: | | | | synchronise. |
| Prime Power Rating (PPR) – whereby the | | | | A generator can never be matched on a 1:1 aspect |
| generator supplies power as an alternative to the | | | | ratio with an uninterruptible power supply. A UPS will |
| mains power supply, but on an unlimited basis. | | | | at times be drawing additional current to charge its |
| Standby Power Rating (SPR) – whereby the | | | | battery set. Generator sizing may also have to take |
| generator supplies power as an alternative to the | | | | into account the powering of essential loads, |
| mains power supply but for a short duration, typically | | | | air-conditioning, for example, and emergency lighting. |
| one hour out of every twelve. | | | | As already mentioned, a UPS rectifier can generate |
| A generator rated under SPR can be as much as 10 | | | | harmonics and this needs also be the taken into |
| percent larger than one sized using PPR. This provides | | | | consideration when sizing the generator. |
| an overload capability for a short duration, perhaps to | | | | Ambient Temperature |
| meet sudden load demand changes, for example. | | | | The ambient temperature around a generator is |
| For an uninterruptible power supply installation, PPR is | | | | important. It is usual for the engine room |
| the more suitable method of rating. It is extremely | | | | temperature to rise by around 10 degrees centigrade |
| important, for achieving greater resilience (fault | | | | when a generator is in operation. Things can get quite |
| tolerance), that a generator and its UPS are suitably | | | | hot if the outside temperature is also hot. High |
| matched. Not only must a generator be able to | | | | ambient temperatures can degrade generator |
| accept the load of the uninterruptible power supply | | | | performance and cause damage to turbo-chargers |
| but the UPS rectifier and static bypass supplies must | | | | and exhaust systems. In such instances, it is normal |
| be able to operate with, and synchronise to, the | | | | to de-rate and increase the overall size of the |
| output of the generator. | | | | generator installation. |
| Generator set manufacturers have four recognised | | | | Recommended practice is to oversize a generator by |
| categories of load acceptance: one = 100%, two = | | | | a factor of one-and-a-quarter to two-times the size |
| 80%, three = 60% and four = 24%. Categories two, | | | | of the uninterruptible power supply and to increase |
| three and four are used in practice for PPR-rated | | | | this to three-times or more when additional essential |
| generators. Load acceptance is closely related to the | | | | loads are to be powered. |
| turbo charging system and the Break Mean Effective | | | | |