Tools › Building Systems › Lighting and Electrical › Generator and UPS Sizing › Example
Standby generator for a 480 V facility: running load with growth, step loading with a soft-started 100 hp chiller, voltage dip from X′_d, fuel and tank
| Tool | Generator and UPS Sizing |
| Source | U.S. Department of Defense (2014). Engine-Driven Generator Systems for Prime and Standby Power Applications. UFC 3-540-01, Appendix D; U.S. Department of the Army (1984). Electric Power Plant Design. TM 5-811-6, 4-16c and 7-6; U.S. Department of Defense (2004). Diesel-Electric Generating Plants. UFC 3-540-04N (MIL-HDBK-1003/11, 5.3 Table 8 and 5.4.6); U.S. Environmental Protection Agency (2010). Exhaust and Crankcase Emission Factors for Nonroad Engine Modeling: Compression-Ignition. EPA-420-R-10-018, Table A2. |
| Level | Professional |
Problem
A 480 V facility must be carried by a standby set: 30 kW of lighting (pf 0.95) in step 1, 30 kW of HVAC (pf 0.85) and a 100 hp chiller motor (efficiency 0.93, pf 0.87) on a soft starter limited to 3 × running kVA in step 2, and a 50 kVA UPS input (47.5 kW at 0.95) in step 3, with 15 % growth. The alternator has X′_d = 0.25 per unit and the dip at any step may not exceed 15 % (the 85 % motor terminal voltage of TM 5-811-6 4-16c). Find the running load, the starting kVA and dip of each step, the alternator kVA the dip requires, the kW the set must carry, whether a 400 kW / 500 kVA set serves, its full-load current, and the fuel rate and 72-hour tank from a specific-consumption curve ending at the EPA value 0.367 lb/hp·h.
Inputs to Enter
- Schedule: Lighting L 30 kW pf 0.95 step 1 priority 1; HVAC H 30 kW pf 0.85 step 2 priority 2; Chiller M 100 hp η 0.93 pf 0.87 SS step 2 priority 2; UPS input U 47.5 kW pf 0.95 step 3 priority 1
- Demand factor 1.0, growth 15 %, rated pf 0.8, X′_d 0.25, X″_d 0.15, allowable dip 15 %, locked-rotor 6 ×, soft-start 3 ×, drive 1.2 ×, starting pf 0.3, nonlinear multiplier 1.0, set 400 kW, 480 V, ATS 800 A, alternator efficiency 0.94, 1,000 ft and 95 °F (no derating)
- Fuel curve 0.45 / 0.40 / 0.375 / 0.367 lb/hp·h at 25 / 50 / 75 / 100 %, 7.1 lb/gal, 72 h runtime, 0.95 usable, 2,000 gal existing tank, 8 h day tank
Answers
- Running load 187.68 kW, 207.93 kVA (phasor sum; 209.04 arithmetic), pf 0.903; with growth 215.84 kW / 239.12 kVA
- Step 2 starting block 311.8 kVA at pf 0.36 (chiller 276.5 + HVAC 35.3); dip on the 500 kVA set 12.8 % by the divider 1 − z/√(z² + 2 z X′_d sin φ + X′_d²) (13.5 % by the reactive form X′_d S_st/(S_gen + X′_d S_st)); alternator for 15 %: 416.3 kVA (441.7 reactive form)
- Governing: motor starting dip; kW required 333.0 (416.3 kVA × 0.8); the 400 kW / 500 kVA set carries the running load (54.0 %), the nonlinear-adjusted kVA (nonlinear share 23.9 %, below the 25 % X″_d rule) and every dip: meets every criterion
- Full-load current 601.4 A (S/(√3 V)); conductors 115 % = 691.6 A (NEC 445.13); 800 A transfer switch adequate
- At 54.0 % load: shaft power 229.6 kW, 0.396 lb/hp·h, 17.18 gal/h; main tank 1,302 gal for 72 h (MIL-HDBK-1003/11 form 1,593 gal); day tank 236 gal; the 2,000 gal tank runs 110.6 h
Notes
The tool's default inputs are this facility; press Submit. The schedule is illustrative (stated on the page); the printed values the test reproduces are MIL-HDBK-1003/11 Table 8 (the plant sub-tool's default: 2,400 / 1,500 / 600 kW, firm 3,000 kW, 1,000 kW units, 5,000 kW installed), the TM 5-811-14 Appendix G locked-rotor currents (119 / 198 / 237 / 395 A) and the EPA fuel value. Checked by test_generator_ups_sizing.php (1,096 checks) against the numpy / scipy script References/Generator-UPS/checks/genset_check.py (complex divider with brentq, solve_ivp Peukert discharges).