--- title: "Isochronous governor and parallel operation problem" subject: "Power Plant Equipment" chapter: 2 key: "semester_7/power_plant_equipment:isochronous-governor-parallel" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/isochronous-governor-parallel site: BCT Engineering --- # Isochronous governor and parallel operation problem Power Plant Equipment · Chapter 2: Power/Frequency Control in Hydro Generator System - Asked in: 2081 Baisakh, 5 marks; 2080 Baisakh, 4 marks; 2078 Bhadra, 4 marks; 2071 Chaitra, 4 marks; 2073 Shrawan, 3 marks; 2071 Shrawan, 8 marks (asked 6 times) - Video: [Speed Droop and Isochronous Mode Explained in Urdu / Hindi. Speed Droop Formula.](https://www.youtube.com/watch?v=9ZmBCw6nGzE) — Asad Electrical - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/isochronous-governor-parallel - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-2 ## Question What is isochronous governor? Explain with its closed loop control system. Why it is not suitable for two generating units operating in parallel? ## Short answer An isochronous governor maintains exactly constant speed for all loads using integral control, eliminating steady-state frequency error. It is unsuitable for parallel units because each tries to enforce its own setpoint, causing fighting and unstable load sharing; only one unit may run isochronous, and others must use speed droop. ## Answer An **isochronous governor** is a speed governor that keeps the turbine speed (frequency) **exactly constant** at the set value for every load. "Isochronous" means "same speed". It uses integral action: the gate keeps moving as long as there is a speed error. #### Closed loop control system ```text w_ref + error +------+ dY +-------+ dPm ---->(+)-------->| K/s |--->|turbine|---+ ^ - +------+ +-------+ | | v | -dPL -->(+) | +-----------+ | +------| 1/(2Hs+D) |<--------------+ speed +-----------+ ``` Governor law: $$ \Delta Y(s) = -\frac{K}{s}\,\Delta\Omega(s) $$ **Operation:** 1. Load increases, $P_e > P_m$, so the rotor slows down. 2. The speed error $(\omega_{ref} - \omega)$ is integrated; the integrator output opens the gate. 3. $P_m$ increases; once it exceeds $P_e$ the rotor accelerates back. 4. The integrator stops changing only when the error is zero. So in steady state $\omega = \omega_{ref}$ and $\Delta P_m = \Delta P_L$: zero steady-state frequency error. ```text speed w0 |-----. .---------- w0 (restored) | \ / | '.___.' +-------------------------> t Pm rises until Pm = Pe ``` #### Why it is not suitable for units in parallel - Units in parallel are tied to the same frequency. Each isochronous governor tries to hold the frequency at **its own** set value. - The set points can never be exactly equal. If unit 1 is set at 50.01 Hz and unit 2 at 50.00 Hz, unit 1 keeps opening its gate to raise frequency while unit 2 keeps closing, so they **fight** each other. One unit is driven to full load and the other to zero (or motoring). - With integral control there is **no unique load sharing**: the load can be divided in any ratio and drifts. The operating point is not stable. - Therefore only one unit in a system (or an isolated unit) may run isochronous. Other units use a **speed droop** characteristic, which gives a definite sharing of load in proportion to $1/R$. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does an isochronous governor do? A. Lets speed fall in proportion to load B. Keeps speed exactly constant at the set value for every load, using integral action C. Keeps power output constant D. Controls only the voltage Answer: B. Integral action keeps moving the gate while any speed error exists, so steady-state error is zero. (Key point: Meaning of isochronous) 2. Which governor law describes isochronous control? A. Delta Y = -(1/R) Delta Omega B. Delta Y = -K Delta Omega C. Delta Y = -(K/s) Delta Omega D. Delta Y = K / (1 + sT) Answer: C. The gate position is the integral of the speed error, giving zero steady-state error. (Key point: Governor law) 3. Why do two isochronous units in parallel fail to work properly? A. They have different voltages B. Each tries to hold its own slightly different frequency set point, so they fight, driving one to full load and the other to zero C. They always share load equally D. They draw too much reactive power Answer: B. Set points cannot be exactly equal, so one unit keeps opening while the other keeps closing. (Key point: Why not in parallel) 4. How do units in parallel share load properly? A. All run isochronous B. Speed droop characteristic gives definite sharing in proportion to 1/R C. Sharing is random D. Using the surge tank Answer: B. Droop gives a unique operating point and load sharing inversely to R; only one unit may be isochronous. (Key point: Droop alternative) ## Related in Power Plant Equipment - [Isochronous governor and speed droop model](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aisochronous-droop-model) — Power Plant Equipment, chapter 2 - [Speed response without governor and isochronous governor](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Aspeed-response-isochronous-block) — Power Plant Equipment, chapter 2, asked 2× ## Related in other subjects - [Define control system and closed loop block roles](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Adefine-cs-bd-role) — Control Systems, chapter 1, asked 5× - [Closed loop control system block diagram explained](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-closed-loop) — Control Systems, chapter 1, asked 4× - [Steps to design closed loop control system](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Aclosed-loop-design-steps) — Embedded System, chapter 7 - [Control system block diagram and closed loop advantages](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Awhat-is-cs-bd-cl-advantages) — Control Systems, chapter 1