---
title: "Droop for capacity sharing: 250 and 500 MW"
subject: "Power Plant Equipment"
chapter: 2
key: "semester_7/power_plant_equipment:droop-capacity-sharing-250-500"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/droop-capacity-sharing-250-500
site: BCT Engineering
---

# Droop for capacity sharing: 250 and 500 MW

Power Plant Equipment · Chapter 2: Power/Frequency Control in Hydro Generator System

- Asked in: 2073 Shrawan, 5 marks
- Video: [Speed Load characteristics | Two Generators rated 200MW and 400MW | Solved Problem -1 | PSOC | droop](https://www.youtube.com/watch?v=MEGzoyxTT6w) — Simple n Easy Learning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/droop-capacity-sharing-250-500
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-2

## Question

Two generators are supplying power to a system. Their rating are 250 MW and 500 MW respectively. Each generator is half loaded and operating at a frequency of 60 Hz. If the system load is increased by 100 MW, the frequency drops to 59.5 Hz. What must be the individual droop of these generators so that they share the load according to their capacity?

## Short answer

Both generators must have a droop of 6.25 % on their own rating.

## Answer

To share a load change in proportion to capacity, the change on each unit must be the same fraction of its rating. Then from $R = (\Delta f/f_0)/(\Delta P/P_r)$, both droops come out equal.

**Sharing by capacity:** total capacity $= 750$ MW, $\Delta P_L = 100$ MW.
$$
\begin{aligned}
\Delta P_1 &= 100 \times \frac{250}{750} = 33.33\ \text{MW} \\
\Delta P_2 &= 100 \times \frac{500}{750} = 66.67\ \text{MW}
\end{aligned}
$$

**Frequency change:** $\Delta f = 59.5 - 60 = -0.5$ Hz, so $\Delta f/f_0 = 0.5/60 = 0.008333$ pu.

**Droop of each unit (pu on own rating):**
$$
\begin{aligned}
R_1 &= \frac{\Delta f/f_0}{\Delta P_1/P_{r1}} = \frac{0.008333}{33.33/250} = \frac{0.008333}{0.1333} = 0.0625 \\
R_2 &= \frac{\Delta f/f_0}{\Delta P_2/P_{r2}} = \frac{0.008333}{66.67/500} = \frac{0.008333}{0.1333} = 0.0625
\end{aligned}
$$

**Answer:** Each generator needs a droop of **6.25 %** on its own rating. G1 then goes from 125 to 158.33 MW and G2 from 250 to 316.67 MW, both at 63.3 % load, at 59.5 Hz.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What condition makes two generators share a load change in proportion to capacity?
   A. Equal droop in pu on their own ratings
   B. Equal droop in MW/Hz
   C. Droop inversely proportional to rating
   D. Equal initial loading only
   Answer: A. Equal change as a fraction of rating for the same df requires equal pu droop R. (Key point: Condition for capacity sharing)

2. How is the 100 MW increase split between 250 MW and 500 MW units sharing by capacity?
   A. 50 MW and 50 MW
   B. 33.33 MW and 66.67 MW
   C. 66.67 MW and 33.33 MW
   D. 25 MW and 75 MW
   Answer: B. Shares are 250/750 and 500/750 of 100 MW. (Key point: Share of the load)

3. Which expression gives droop R in pu?
   A. R = (dP/Pr)/(df/f0)
   B. R = (df/f0)/(dP/Pr)
   C. R = df/dP
   D. R = (dP/df) x Pr
   Answer: B. R is the per-unit frequency change divided by the per-unit power change. (Key point: Droop formula)

4. With df = 0.5 Hz at 60 Hz, what droop does each unit need?
   A. 3.75%
   B. 12.5%
   C. 6.25%
   D. 0.83%
   Answer: C. R = 0.008333 / 0.1333 = 0.0625, i.e. 6.25% for both units. (Key point: Droop value)

## Related in Power Plant Equipment

- [Load sharing: 250/500 MW, frequency falls to 49.5 Hz](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aload-sharing-250-500-4-2) — Power Plant Equipment, chapter 2, asked 2×
- [Droop for capacity sharing: 500 and 250 MW](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adroop-capacity-sharing-500-250) — Power Plant Equipment, chapter 2
- [Load sharing: 250 MW and 500 MW, 20%/10% droop](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aload-sharing-250-500-20-10) — Power Plant Equipment, chapter 2, asked 2×
- [Load sharing: 500 MW and 250 MW, 5%/6% droop](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aload-sharing-500-250-5-6) — Power Plant Equipment, chapter 2, asked 2×
- [Load sharing: 250 MW and 500 MW, 4%/3% droop](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aload-sharing-250-500-4-3) — Power Plant Equipment, chapter 2
- [Load decrease: 250 and 500 MW at 49 Hz](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aload-decrease-250-500-49hz) — Power Plant Equipment, chapter 2

## Related in other subjects

- [Load sharing between two stations in parallel](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aload-sharing-two-stations) — Electric Machines II, chapter 1