Generator Set Selection Process

Generator Set Selection Process

Description

The purpose of to this bulletin is to give technical staff a basic understanding on the selection of engine and alternator combination to form a generator set (genset) that is sized to create 37kVA of electric capacity, Prime (3B) rated. Engine 3B rating allows unlimited hours per year at 75% load factor, with 10% overload allowance for 1 in every 12 hours as opposed to engine Standby (3D) rating that allows the engine operation for 500 hours per year at 85% load factor and no overload.    

Background

A generator set consists of an engine that creates mechanical power (kWm) at the flywheel, that drives an alternator that creates an electrical power (kWe). The purpose of this bulletin is to present a correlation between mechanical and electrical power and to discuss aspects such as equipment derating and the differences between gross and net power.

Selection of Engine

From the Deutz engine range, I have chosen the BF4M2011C which provides Engine (3B Rated) 56.1kWm Gross power at the flywheel at 20oC ambient conditions. No emissions ratings are required for this project.




In practice, for Australian Conditions, we need to consider at least 45oC as a maximum ambient temperature.  According to Deutz-published derating table, from Figure 1 below, Flywheel Power becomes 0.88 x 56.1 = 49.4 kWm at 45oC.



Now we need to also consider ancillary loads on the engine, when being used as a genset. (Notice that datasheet data in Figure 1 only includes for engine mechanically driven fan to achieve Net flywheel power.)

 

Mechanically, would consist of the following:

-       Engine-driven fan:             2.1 kWm (From Datasheet)

-       Charging Alternator:          0.25 kWm (Estimate)


Electrically, we consider the following ancillary loads. These would be subtracted once we consider the alternator selection.

-       Enclosure Ventilation Fan:    2 kWe (From Design)

-       Battery Charger:                    0.25 kWe (Estimate)

-       Coolant Pre-Heater:              0 kWe (Not being used for this application)

 

So, Net Flywheel =     Gross Power    –    Ancillary Power

                              =           49.4            –          2.4

                                  =     47 kWm.


Selection of Alternator

I have chosen the Mecc Alte EPC32 1M4C as an alternator. Alternator kVA @100% load is 50 kVA (0.8pf), 40oC ambient, for 415V, Class H Temperature Rise. Refer to Figure 3.



To calculate actual Alternator kVA that the alternator can produce at 40oC ambient, we need alternator efficiency. This can be calculated via the alternator efficiency table shown in Figure 4. 

Load pu, is required kVA/(kVA at 100% load) = 37kVA/50kVA = 0.74. 

From Figure 4, Alternator Efficiency Curve (pf = 0.8), for 0.74 p.u., we get 88.8% efficiency.

So, from the 100% load figure of 50kVA for this alternator, it can produce the following kVA.

Actual Alternator kVA at 40oC     =          50         x          0.888               

                                                         =          44.4 kVA



From Figure 5, to check for alternator ambient temperature derating capacity, the Deration Co-efficient for 45oC < 1000m altitude, the value is 0.96.

                              So actual kVA =            0.96      x           44.4      =          42.6 kVA. 

As 42.6 kVA > 37kVA (Tender requirement), alternator has capacity to produce the required kVA for derating at an ambient of 45oC.



Checking Engine – Alternator Match

An alternator can only produce the necessary kWe (where kWe = kVA x pf) when the engine can provide the necessary kWm to do so. In a physical sense, we will never have a situation where kWm = kWe as, when an alternator is rotating, it will lose energy via heat convection, noise, friction effects, etc.


To convert this kWm to kWe, as the motor will give the kWm at 45oC ambient, we calculate as follows:

kWe      =          Alternator eff.    x           Net kWm
              =         0.888                  x            47 kWm
              =         41.7 kWe.

Derived from the formula above, kVA = kWe/pf, and pf = 0.8, the actual kVA that this engine - alternator can produce is as follows:

kVA      =          kWe        /           pf

           =          41.7      /           0.8

           =          52.1 kVA           at 45oC ambient.

Now we subtract the auxiliary electrical loads      =          52.1kWe           -           2.25kWe
                                                                                   =          49.9 kVA at 45oC ambient


As the project calls for 37 kVA, we can easily achieve our genset kVA at 45oC ambient. 


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