--- title: "Static excitation pros and cons" subject: "Power Plant Design" chapter: 4 key: "semester_8/power_plant_design:static-excitation-pros-cons" source: https://www.bctengineer.com/q/semester_8/power_plant_design/static-excitation-pros-cons site: BCT Engineering --- # Static excitation pros and cons Power Plant Design · Chapter 4: Electric System Design of a Power Plant - Asked in: 2081 Shrawan, 6 marks - Video: [PE Lec 36: Static Excitation System For Alternator](https://www.youtube.com/watch?v=RAiVAMDltUY) — Polytech Plus - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/power_plant_design/static-excitation-pros-cons - Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-4 ## Question Explain static excitation system. Also highlight its pros and cons in comparison to its best possible alternative. ## Short answer Static excitation feeds the field from the generator terminals through a thyristor bridge and slip rings. Its pros over brushless are very fast response and fast field suppression; the main con is loss of excitation during severe close-in faults. ## Answer A **static excitation system** is one in which the generator field is fed with DC from a **thyristor (SCR) rectifier** that has no rotating parts. The power is taken from the generator terminals through an excitation transformer and reaches the rotor through **slip rings and brushes**. #### Working 1. An **excitation transformer** connected to the generator terminals steps the voltage down. 2. A fully controlled **3-phase thyristor bridge** rectifies it. The AVR changes the firing angle $\alpha$ to control the field voltage, $V_f = 1.35\,V_{ac}\cos\alpha$ (where $V_{ac}$ is the line voltage at the bridge input). 3. The DC goes to the field through slip rings. A **field breaker** and a **discharge resistor/crowbar** are provided for de-excitation. 4. At start the terminal voltage is zero, so **field flashing** from the station battery or an AC supply builds up the initial voltage. ```text Gen terminals -> Exc. transformer -> Thyristor bridge | PT, CT -> AVR (firing angle) --+ v Battery -> Field flashing -> Field CB -> Slip rings | Main field ``` #### Best alternative The main alternative is the **brushless excitation system**: an AC exciter with rotating diodes mounted on the shaft. #### Pros (compared with brushless) - **Very fast response**: the field voltage changes almost at once. A high ceiling voltage (often 1.6–2 times rated or more) improves **transient stability**. - **Fast de-excitation**: the bridge can work in inverter mode (negative field voltage) to kill the field quickly during internal faults. - No rotating exciter, so the **shaft is shorter** and the machine is simpler. Easy to fit in an existing plant. - Field voltage and current can be measured directly, and a PSS and limiters can be added easily. - Thyristors are easy to reach for maintenance, and redundant bridges can be used. #### Cons (compared with brushless) - Needs **slip rings and brushes**, so there is brush wear, carbon dust and regular maintenance. - The supply comes from the generator terminals, so during a **close-in three-phase fault** the terminal voltage collapses and excitation is lost unless there is compound (CT) support. The fault current may decay before the relays act. - Needs field flashing for start. - The thyristors produce **harmonics** and shaft voltages. Cooling and a large excitation transformer are needed. | Point | Static | Brushless | |---|---|---| | Response | Very fast | Slow | | Brushes | Required | None | | Field suppression | Fast | Slow | | Fault-time support | Weak | Good | | Typical use | Large hydro/thermal | Small–medium units | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Where does a static excitation system take its power from? A. A diesel set B. The exciter armature C. A PMG on the shaft D. The generator terminals through an excitation transformer Answer: D. An excitation transformer connected to the generator terminals feeds the thyristor bridge. (Key point: Power source) 2. Why does static excitation improve transient stability? A. It has a slow time constant B. It has no field breaker C. Very fast response and a high ceiling voltage (1.6-2 times rated) D. It uses no AVR Answer: C. Quick forcing of the field voltage helps retain synchronism after a disturbance. (Key point: Fast response) 3. How can a static system kill the field quickly in an internal fault? A. Open brushes B. Use field flashing C. Run the thyristor bridge in inverter mode for negative field voltage D. Increase the firing delay to zero Answer: C. Inverter operation gives negative field voltage, forcing the field current down quickly. (Key point: Fast de-excitation) 4. What is the main weakness of static excitation during a close three-phase fault? A. It has no brushes B. Terminal voltage collapses so excitation is lost unless CT support exists C. The exciter is too slow D. It cannot be measured Answer: B. Because the supply comes from the terminals, voltage collapse reduces excitation. (Key point: Drawback) ## Related in Power Plant Design - [Brushless excitation pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-pros-cons) — Power Plant Design, chapter 4 - [Brushless versus static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-vs-static-excitation) — Power Plant Design, chapter 4, asked 5× - [Why recommend brushless over static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-over-static-logic) — Power Plant Design, chapter 4 - [Working of brushless and static excitation](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Abrushless-static-working) — Power Plant Design, chapter 4 ## Related in other subjects - [Static excitation system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Astatic-excitation-system) — Power Plant Equipment, chapter 3 - [Distribution layouts, pros/cons, improvements](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Adistribution-layouts-improvements) — Water Supply Engineering, chapter 7, asked 4× - [Collective intelligence pros and cons](https://www.bctengineer.com/question.md?key=semester_8%2Finformation_system%3Acollective-intelligence-pros-cons) — Information System, chapter 4 - [Phase shift method of SSB generation: pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Assb-phase-shift-pros-cons) — Communication Systems, chapter 2, asked 2×