--- title: "Types of excitation systems" subject: "Power Plant Equipment" chapter: 3 key: "semester_7/power_plant_equipment:excitation-system-types" source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/excitation-system-types site: BCT Engineering --- # Types of excitation systems Power Plant Equipment · Chapter 3: Var/Voltage Control in Hydrogenerating Systems - Asked in: 2069 Chaitra, 8 marks - Video: [lesson 11: Generator Excitation System](https://www.youtube.com/watch?v=7MvjbC4f5Aw) — International Engineering Training - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/excitation-system-types - Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-3 ## Question What is excitation system? Explain the various types of excitation system employed in power plant on the basis of their performance with suitable connection diagram. ## Short answer An excitation system provides regulated DC field current. Types: DC (shaft-driven DC generator, slow, brush maintenance); AC (brushless with rotating diodes, medium response, low maintenance); static (thyristor bridge from generator terminals, very fast, high ceiling voltage but needs slip rings). ## Answer An **excitation system** supplies the DC field current of a synchronous generator and controls it to hold the terminal voltage and the reactive power output at the desired values. On the basis of their power source and performance, excitation systems are of three types. #### 1. DC excitation system A shaft-driven **DC generator** (main exciter), often with a pilot exciter, supplies the field through slip rings. ```text Pilot exc --> Main DC exc ==slip rings==> Gen field ^ | +------ AVR <---- PT <-----+ ``` - Slow response (exciter time constant 0.5 to 1 s); commutator and brushes need maintenance. - Used in old and small units. #### 2. AC excitation system An **AC exciter** (alternator) is on the same shaft and its output is rectified. (a) **Stationary rectifier:** rectifiers are static, output reaches the field through slip rings. (b) **Rotating rectifier (brushless):** the exciter has stationary field and rotating armature; diodes mounted on the shaft feed the field directly, so **no brushes or slip rings**. ```text Stationary | Rotating parts on shaft | AVR -> exc | exciter rotating gen field | field ======>| armature -> diodes --> winding | (PMG supply) | (3-ph AC) (DC) | ``` - Medium response; brushless type is almost maintenance-free and suits large turbo-generators and hazardous areas. - Field current cannot be measured or de-excited directly. #### 3. Static excitation system Power is taken from the generator terminals through an excitation transformer and rectified by a **thyristor bridge**; DC goes to the field through slip rings. The AVR controls the firing angle. ```text Gen bus -> Exc. transformer -> Thyristor bridge | (AVR firing) v slip rings -> Gen field ``` - Very fast response and high ceiling voltage; best for transient stability. - Needs field flashing at start; supply falls during close-in faults. #### Comparison of performance | Feature | DC | AC (brushless) | Static | |---|---|---|---| | Response speed | Slow | Medium | Very fast | | Brushes/slip rings | Yes, plus commutator | None | Slip rings | | Maintenance | High | Low | Low | | De-excitation | Slow | Slow | Fast (inversion) | | Typical use | Old, small units | Large turbo units | Modern hydro and thermal | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the excitation system of a synchronous generator do? A. Supplies DC field current and controls it to hold terminal voltage and reactive power B. Supplies AC to the stator to run the generator at synchronous speed C. Controls turbine speed to hold frequency constant D. Steps up the generator voltage for transmission Answer: A. The exciter feeds the rotor field with DC and the AVR varies it to keep terminal voltage and MVAr output at the desired values. (Key point: Purpose of excitation) 2. Why does a rotating-rectifier (brushless) AC exciter need no brushes or slip rings? A. The exciter is a DC machine with a permanent-magnet field B. The diodes are mounted on the shaft and feed the generator field directly C. The field is supplied from the generator terminals through a transformer D. The rectifier is stationary and connected by a flexible cable Answer: B. Exciter armature and diodes rotate with the shaft, so DC reaches the field without any sliding contact. (Key point: Brushless excitation) 3. How does a static excitation system obtain and control the field power? A. A shaft-driven DC generator is controlled through its commutator B. A rotating AC exciter feeds diodes on the shaft C. Power is taken from the generator terminals through a transformer, rectified by a thyristor bridge whose firing angle the AVR controls D. A pilot exciter feeds the field through a diesel-driven set Answer: C. Static systems have no rotating exciter; thyristor firing-angle control gives very fast response. (Key point: Static excitation) 4. Which ranking of response speed is correct? A. DC very fast, AC medium, static slow B. DC medium, AC slow, static very fast C. All three have the same response D. DC slow, AC brushless medium, static very fast Answer: D. DC exciters have a large time constant (0.5 to 1 s); static systems respond fastest and have a high ceiling voltage. 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