8/10/2019 DG Set Sizing Methodology Concepts

1/1

Sizing Methodology

Conventional sizing mode:Although most applications may be operated with loads appliein multiple steps, its not uncommon a generator set at commissioning be required todemonstrate full load acceptance as per NFPA 110 (NFPA 110 is a North Americastandard). Depending on the specifics of the installation, governor and/or voltage regulatoradjustments may be needed, and frequency and voltage dips may be significant, but all will

be capable of demonstrating full load acceptance. This concept is at the core of ourConventional sizing methodology. Conventional sizing would equate to "competitive"sizing, since this sizing methodology permits the user to select a range of allowable voltagedip and frequency dip typically between 5% to 35%. A requirement of NFPA 110 is thegenerator set pick up full load and recover... and not stall. Excessive frequency dip can stala generator set. The Conventional sizing mode in SpecSizer uses transient response datafrom factory tested generator sets on warm engines. It is far less restrictive than theFrequency Limited sizing mode. Most sizings in SpecSizer are based upon the Conventionasizing mode, since it provides the user more options on setting different loads' allowablefrequency and voltage dips.

.Frequency Limited sizing mode:It is also not uncommon for generator sets to be held tomore rigorous standards, such as limited (or restrictive) frequency or voltage dip during loadacceptance. The most common example is ISO 8528. (note: a sized generator set may alsomeet the ISO 8528 standard using the Conventional mode, if the user defined frequency anvoltage dip percentage allowances are set in accordance with the four ISO 8528 "G"classifications - G1, G2, G3 or G4.) Therefore, SpecSizer has included the concept ofFrequency Limited sizing. A generator set sized using this methodology will respond to itsload steps based purely on governor performance, with little or no dependence onturbocharger acceleration. This will limit frequency dip to the minimum value possible,

depending on engine displacement and governor design. User selection of the FrequencyLimited sizing mode will set or reset each user specified load's frequency dip to 10%. TheFrequency Limited sizing mode in SpecSizer uses transient response data from factorytested generator sets that do not exceed a 10% frequency dip on a cold engine. TheFrequency Limited sizing mode will almost always oversize the generator set, particularlywhen some loads in the load scenario do not require a restrictive 10% frequency dip andstart earlier in the load step scenario.

Site demonstration specifications: Note that if specifications require both single-stepstarting and Frequency Limited sizing, a large generator set selection may result.

However, if the loads are carefully divided into multiple steps, both of these sizingmethodologies may at times select similar generator sets.