Three related sizing/analysis tools for generator and transformer plant: standby/prime generator kVA sizing from running load plus a motor-starting voltage-dip check, transformer loss/efficiency/regulation analysis from routine test data, and (advanced) neutral earthing resistor/transformer sizing for high- or low-resistance grounded systems.
Standards followed
ISO 8528 (reciprocating internal combustion engine driven AC generating sets, referenced for generator sizing conventions)IEEE 142 (grounding of industrial and commercial power systems — the Green Book, referenced for NGR sizing R = V_LN/I_fault)Kapp's approximate voltage regulation formula (classical transformer theory, from open/short-circuit test data)
What it can do
- Generator Sizing: running load (linear + non-linear with derating) and motor-starting voltage-dip requirement, with the recommended genset rating from whichever constraint governs, plus a fuel-consumption estimate.
- Transformer Losses: total loss and efficiency at any loading fraction from OC/SC test data, the loading point of maximum efficiency, %R/%X and Kapp's voltage regulation formula, and an optional annual loss-cost estimate.
- Advanced (NGR/NET Sizing): neutral earthing resistor sizing with HRG/LRG classification, power/energy dissipated, charging-current design check, and neutral earthing transformer secondary resistor sizing.
Example problem
Size a generator for a facility with mixed linear/non-linear running load plus a large motor starting event, and separately analyze an existing transformer's efficiency from test data.
- Generator: combine linear kW/PF and derated non-linear kVA into a total running kVA requirement.
- Generator: compute motor-starting voltage dip = (motor starting kVA × Xd) / genset kVA × 100, and find the genset size that keeps this under the allowable dip.
- Generator: the recommended size is whichever of running-load kVA or starting-dip-driven kVA is larger (the governing constraint).
- Transformer: from no-load loss Pfe, load loss Pcu and loading fraction x, compute total loss = Pfe + x²×Pcu and efficiency = output/(output+loss).
- Transformer: find the loading fraction where efficiency peaks (where core loss equals copper loss) and compute voltage regulation via Kapp's formula from %R and %X.
Default scenario: generator sizing governed by the 502.94kVA running-load requirement (versus a 240kVA voltage-dip requirement); transformer losses show 98.516% efficiency at full load, 98.964% maximum efficiency at x=0.405, and 4.140% voltage regulation.
Want the full step-by-step version?
Standby/prime generator sizing for a facility is fully implemented here as the Generator Sizing tab — this is the same functionality sometimes requested as a standalone 'Generator Sizer'.