Selection and Sizing of DG Sets

Following is an excerpt from the article by a prominent electrical consultant .

Diesel Generator (DG) is used to feed emergency loads in case of utility supply failure . It must be clearly understood that only emergency loads are fed by DG thus a 100% standby is extremely rare .Before selecting a DG, a complete load profile study and segregation of essential loads from non-essential loads must be carried out . Otherwise it may lead to DG oversizing. In modern installations ,predominance of non-linear loads results in a higher DG size as compared to linear loads

Following points need to be considered to achieve the correct DG sizing :

 > A fully loaded generator performs far better than a lightly loaded one. In fact , light loading may cause engine damage reducing reliability .

> In case of non-linear loads like UPS, VFD, Battery chargers etc , it should be ensured that non-linear loads do not exceed 50% of the DG set capacity as higher level of harmonics will cause stator over-heating .

> Load Step sequencing - The DG is normally not rated to pick up all loads in one go . While loading the DG, it is always better to start the load with highest surge first followed by lighter loads.This will ensure optimum DG sizing .

> It is always better to have multiple smaller DGs against a larger size DG as availability and loading will be more effective .

> DG is generally selected considering provision for future loading. It is recommended that additional DG should be provided for such additions .

> In case of multiple DGs , load sharing module and load speed governor control should be mandatorily specified. There have been cases of dissimilar DGs being paralleled in absence of load sharing module where lower rating DG took all the load while running higher rated DG as synchronus motor . This has resulted in burnout with heavy damages.

> In case of DG for 100% standby requirement, the demad load in kW needs to be considered (please note DG always runs at 0.8 pf irrespective of system pf) .

e.g Consider a 500kW connected load with 50% diversity factor .

Demand load = 500 x 0.5 = 250 kW 

% DG Loading = 70%

DG kW needed = 250 / 0,7 = 357 kW

DG kVA Rating = 357/0.8 = 446 kVA .

Hence nearest high rating i.e 500kVA may be selected in this case .

If the above basic guidelines can be followed for DG selection, it will definitely result in capital savings and minimum losses .

Load profile study and System design by a competent electrical consultant will result in a strong and stable system performance .