Sizing A Generator For Standby Power Protection Plans

A standby generator does not run constantly but must be correctly sized to ensure it does not start-up and then close down due to an overload condition. Failure to pick-up the load on start-up can be a common fault when loads are undersized but what is the right way to size a generator?

Selecting the right generator type, size and configuration including the alternator type and remote communications configuration is a critical aspect when designing a power continuity plan. Correct generator sizing is vital if a site is to avoid unexpected system failures and capacity overloads. Sizing a generator correctly, within a UPS installation can also help to ensure optimized operating lifetime (working life) and the overall energy efficiency of the entire power protection system.

The first task is to select between a single-phase or a three-phase generating system. This is easily done knowing your site power requirements, load characteristics (usually a mixture of resistive and inductive loads), the application area (datacentre, industrial, commercial and even domestic) and the configuration with any other associated standby power systems – including uninterruptible power supplies. Fuel type and fuel storage is also a consideration to comply with your environmental policy as will be noise attenuation and exhaust gassing. Overall the installation will require electrical works by a certified electrical contracting company and installation by a generator supplier with mechanical and electrical engineers.

To size the generator correctly, it is important to calculate the power rating of all the loads that need to be generator powered. This information can be usually accumulated from site power surveys, electrical cable load measurements using handheld meters and gleaned from any on site monitoring – Data Centre Infrastructure Management (DCIM) software and intelligent Power Distribution Units (PDUs). Manufacturer’s documentation can also be useful but the figures in O&M Manuals and technical specifications may not be accurate in relation to your datacentre’s actual configuration and load profile. What is important is to ensure all load measurements are in the same unit of measure: Watts or Amperes so that the right kW, kVA or even MW and MVA load size can be calculated. Remember:

Wattage = Amperes x volts (for resistive loads)
Wattage = (Amperes x volts) x load factor (for reactive loads)

A resistive load is one that has no initial switch on surge and the current steadily rises to a stable running state. A inductive load (also known as an reactive or capacitive) has a potentially high inrush current, which could cause system overload if not accounted for in sizing a generator.

UPS synchronization and generator pick-up are also important considerations because these can be affected by load harmonic characteristics. Most modern UPS systems have very low harmonic profiles thanks to advances in rectifier development. However the greater the harmonic content produce the more the generator may have to be oversized and/or some form of harmonic filter introduced to guarantee load acceptance. Static bypass considerations are also important considerations as the UPS must be able to synchronise with the generator output waveform for acceptable load pickup. As a rule of thumb, generators are typically sized from 125% times the UPS size but this can be reduced to close to 110% or less dependent upon the UPS rectifier’s harmonic profile.

A further consideration when generator sizing is the presence of essential and non-essential loads. Critical loads are those vital to the operation of the datacentre – server racks. Essential loads are crucial to operation and can include air conditioning for example. This cooling system requires power on mains failure to ensure the operating temperature of the datacentre facility itself. Non-essential loads would be those related to other facilities areas including lighting and security. Whether loads are classed as critical, essential or non-essential depends on the installation and site but what is important is to identify them and decide which will require generator standby power and which will not.

In terms of resilience it is also important to consider whether to put in place a parallel configuration of generators. This is dependent upon the Tier-rating of the rating datacentre (1-4). Having a second generator on-site, on a parallel configuration provides capacity for generator maintenance whilst guaranteeing standby power availability. A second generator also provides protection should the first generator fail to start first time – due to a failed starter battery or fuel flow problem.

Generator sizing should be a relatively easy calculation process but it is just one aspect of a generator installation that will require planning and project management by a generator specialist company like ECO Power Supplies.