Also in: Disaster Response and Recovery
Paralleled Generator Droop Load Sharing
Temporary power often means paralleling whatever units arrived -- different sizes, different governors.
Example
You enter
- Unit 1 rating (kW) 100
- Unit 1 droop (%) 3
- Unit 1 no-load frequency setpoint (Hz) 61.8
- Unit 2 rating (kW) 200
- Unit 2 droop (%) 5
- Unit 2 no-load frequency setpoint (Hz) 63
- Unit 3 rating (kW, 0 if none) 0
- Unit 3 droop (%) 0
- Unit 3 no-load frequency setpoint (Hz) 0
- Rated frequency (Hz) 60
- Total bus load (kW) 180
You get
- Unit1 (kW/Hz) 55.5556
- Unit2 (kW/Hz) 66.6667
- Bus frequency 60.98 Hz
- Unit 1 45.5 kW (45.5% of rating; 55.6 kW/Hz)
- Unit 2 134.5 kW (67.3% of rating; 66.7 kW/Hz)
- Unit1 load (%) 45.4545
- Unit2 load (%) 67.2727
- First full load (kW) 300
- Matched bus (Hz) 60.96
- With matched settings 60
Checked against published worked examples from: Woodward.
Details, formula, and sources
Scopeon an isolated bus. With every governor in droop the units settle on one frequency, and each carries whatever load its droop line gives there, so the share is set by the governor's droop percentage and no-load setpoint, not by the nameplate. Following Woodward Application Note 01302, the calculator gives each unit a stiffness in kW per Hz, solves for the common bus frequency that makes the shares add to the load, and reports each unit's kW and percentage of rating, an overload flag per unit, the total at which the first unit reaches its rating, and what matched settings would do. A 100 kW unit at 3% droop (61.8 Hz no load) with a 200 kW unit at 5% (63.0 Hz) carrying 180 kW settles at 60.98 Hz: 45.5 kW (45%) and 134.5 kW (67%). Raise the large unit's no-load setpoint to 63.5 Hz and it reaches its rating when the bus carries only 272 of 300 kW -- it trips, the small unit inherits everything, and the bus goes down. Matched at 4% and 62.4 Hz both run at 60%. It does not model isochronous sharing, utility paralleling, transients, reactive sharing, or synchronizing, and real droop is only approximately linear. The governor and switchgear manufacturers, the qualified technician setting them up, and the AHJ govern.
Formula or tablestiffness k_i = P_rated,i / (droop_i x f_rated) in kW per Hz; bus frequency f = (sum k_i x f_nl,i - P_load) / sum k_i; unit load P_i = k_i x (f_nl,i - f); the first unit to reach its rating is the one with the highest f_nl,i - P_rated,i / k_i; matched comparison at the mean droop with no-load = f_rated x (1 + droop).
Source and editionWoodward Application Note 01302, Speed Droop and Power Generation, named as the method (droop is a straight-line function of speed reference against fuel position); the governor and paralleling-controls manufacturers named as governing.
Free accessA linear system solved on droop settings and ratings the user enters; no manufacturer curve is reproduced.
What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.
API field namesunit1_kw, unit1_droop_pct, unit1_no_load_hz, unit2_kw, unit2_droop_pct, unit2_no_load_hz, unit3_kw, unit3_droop_pct, unit3_no_load_hz, rated_hz, load_kw, unit1_kw_per_hz, unit2_kw_per_hz, bus_hz, unit1_load_kw, unit2_load_kw, unit1_load_pct, unit2_load_pct, first_full_load_kw, matched_bus_hz, matched_load_pct
- Straight-line droop frequency falls linearly from the no-load setpoint by droop x rated frequency at full loadWoodward Application Note 01302
- Isolated bus, all units in droop no load-sharing controller, no isochronous unit, no utility tieWoodward Application Note 01302
- Matched comparison every unit at the mean entered droop with a no-load setpoint giving rated frequency at full loadproject method