Synchronous Generator Excitation Interview Questions

Q1. What is excitation in a synchronous generator and what is its purpose?

Excitation refers to supplying DC current to the rotor field winding of a synchronous generator to create the rotating magnetic flux that induces EMF in the stator windings. Without excitation, the generator produces no output voltage — the rotor must carry typically 1–5A at 100–300VDC for a medium-scale machine like a 10MVA unit. The excitation current magnitude controls the output terminal voltage and, more importantly, determines whether the machine absorbs or supplies reactive power to the connected grid.

Follow-up: How does changing the excitation current affect the power factor of the synchronous generator?

Q2. What is the effect of over-excitation and under-excitation on a synchronous generator?

An over-excited synchronous generator operates at a leading power factor from the grid's perspective and supplies reactive power (acts as a capacitor); an under-excited machine absorbs reactive power and operates at a lagging power factor. In a 220kV grid-connected alternator at a NTPC plant, over-excitation is used during low-load periods to support bus voltage by injecting VArs. Under-excitation risks loss of synchronism because the internal EMF is insufficient to maintain the air-gap flux needed to hold synchronous speed.

Follow-up: What is the V-curve of a synchronous machine and what does it show?

Q3. What are the different types of excitation systems used in synchronous generators?

The main types are: DC exciter (a small DC generator on the same shaft), static excitation (AC source rectified by thyristors, fed directly to rotor slip rings), and brushless excitation (AC exciter with rotating rectifiers, no slip rings). Modern large generators at BHEL like the 500MW units use static thyristor excitation for fast response (< 0.1 second) needed for grid stability. Brushless systems are preferred in environments where maintenance of slip ring brushes is difficult, such as offshore or mining applications.

Follow-up: What are the advantages of brushless excitation over slip ring-based excitation systems?

Q4. What is the synchronous reactance and how does it relate to excitation?

Synchronous reactance Xs is the effective reactance of the stator winding that limits armature current and determines the voltage regulation characteristics under load. For a 3-phase 11kV, 10MVA generator with Xs = 1.2 pu, the terminal voltage drop under full-load lagging power factor is significant, requiring increased excitation to maintain rated voltage. The phasor relationship V_t = E_f − jI_a×Xs shows that increasing E_f (via excitation) compensates for the reactive voltage drop.

Follow-up: What is the per-unit system and why is synchronous reactance expressed in per-unit?

Q5. Explain the V-curves of a synchronous generator.

V-curves plot armature current (Ia) versus field current (If) at constant real power output for different power levels — the minimum armature current at each curve corresponds to unity power factor operation. At the left of the minimum (under-excited), the machine absorbs reactive power with lagging pf from the grid's perspective; at the right (over-excited), it supplies reactive power at leading pf. The V-curves of a 100MVA turbogenerator resemble the letter V, with armature current rising on both sides of the unity pf minimum.

Follow-up: What is the significance of the leading side (over-excitation) edge of the V-curve in terms of machine heating?

Q6. What is the Automatic Voltage Regulator (AVR) and what does it do?

The AVR is a closed-loop control system that continuously adjusts the excitation current to maintain the generator's terminal voltage at a set reference value despite load changes, speed variations, or network disturbances. A modern digital AVR like the ABB UNITROL 6800 measures terminal voltage, compares it to the set-point, and fires thyristors in the excitation circuit within milliseconds to correct deviations. Without the AVR, terminal voltage would sag on load application and rise on load rejection, causing equipment damage.

Follow-up: What is voltage droop in an AVR and why is it needed for parallel generator operation?

Q7. How is reactive power sharing controlled between parallel synchronous generators?

Reactive power sharing between parallel generators is controlled by adjusting the excitation (field current) of each machine — increasing excitation on one machine makes it supply more reactive power while the other absorbs less, without directly changing real power sharing (which is controlled by the governor/prime mover). For two 5MVA generators in parallel at a 33kV bus, if Generator 1's AVR set-point is raised by 2%, it will take on more reactive load, measurable as a shift in its armature current toward the over-excited V-curve region. Droop setting in the AVR ensures stable reactive sharing.

Follow-up: How does real power sharing between parallel generators differ from reactive power sharing in terms of control mechanism?

Q8. What is the excitation limit and why is it important for generator protection?

The maximum excitation limit (over-excitation limiter, OEL) prevents excessive field current that would overheat the rotor field winding, while the minimum excitation limit (MEL) prevents under-excitation that risks loss of synchronism or stator end-core heating. For a 210MW generator rotor with a field winding rated at 400A continuous, the OEL typically allows 150% for 30 seconds before tripping the excitation. The MEL is set based on the stability limit curve of the machine to keep operation within the steady-state stability boundary.

Follow-up: What is the steady-state stability limit of a synchronous generator and how is it related to excitation?

Q9. What is field forcing in synchronous generator excitation?

Field forcing is the brief application of ceiling voltage (maximum excitation voltage, typically 2–3× rated field voltage) to rapidly increase field current and recover terminal voltage during sudden large load increases or post-fault recovery. A static exciter like the Siemens THYRIPOL can apply ceiling voltage of 3× rated in < 20ms, quickly restoring flux after a nearby fault is cleared. Field forcing improves transient stability by rapidly re-establishing the internal EMF needed to maintain synchronism.

Follow-up: What is the ceiling voltage of an excitation system and how is it specified?

Q10. Explain the phasor diagram of a synchronous generator under lagging power factor load.

Under lagging power factor load, the armature current I_a lags the terminal voltage V_t by angle φ; the internal EMF E_f is found by adding the voltage drops jI_a×Xs and I_a×Ra (armature resistance, usually small) to V_t vectorially. For an 11kV generator at 0.8 pf lagging with Xs = 10Ω and I_a = 500A, E_f is larger than V_t (over-excited to compensate), confirming that lagging load demands higher excitation. The angle δ (between E_f and V_t) is the torque angle, and P = (E_f×V_t/Xs)×sinδ.

Follow-up: How does the torque angle δ change as real power output is increased while excitation is held constant?

Q11. What is armature reaction and how does it affect the excitation requirement?

Armature reaction is the effect of the stator current's magnetic field on the main rotor flux — at lagging power factor, the armature MMF demagnetizes (opposes) the rotor field, requiring increased excitation to maintain terminal voltage. For a salient-pole generator like a 50MVA hydro-unit, armature reaction under full-load 0.8 pf lagging may require 30–40% more field current than at no-load. At leading power factor, armature reaction aids the field (magnetizing), so excitation must be reduced to prevent terminal overvoltage.

Follow-up: Why is armature reaction analysis more complex in salient-pole generators than in cylindrical-rotor machines?

Q12. What is voltage regulation of a synchronous generator and how is it calculated?

Voltage regulation (VR) is the percentage change in terminal voltage when rated load is removed at constant excitation and speed: VR = (V_NL − V_FL)/V_FL × 100%. For an 11kV generator with V_NL = 12.5kV and V_FL = 11kV, VR = (12500−11000)/11000 × 100% = 13.6%. Lagging power factor gives positive VR (voltage rises on no-load), unity is smaller positive, and leading pf gives negative VR (voltage falls on no-load), which is characteristic of over-excited machines.

Follow-up: What is the EMF method for calculating voltage regulation and what approximations does it make?

Q13. What is the excitation system's role in transient stability of a power system?

A fast-acting excitation system with high ceiling voltage improves transient stability by quickly boosting the generator's internal EMF after a fault, increasing the electrical power output and reducing the accelerating torque imbalance that drives the rotor out of synchronism. IEEE transient stability studies for a 400kV interconnected grid show that upgrading from a slow DC exciter (time constant 1–2s) to a static exciter (time constant < 0.1s) can increase the critical clearing time for nearby faults by 100–200ms. This is why modern grid codes mandate fast-response excitation for large generators.

Follow-up: What is the critical clearing time for a fault and how does excitation system speed affect it?

Q14. How do you perform the short-circuit test on a synchronous generator and what does it determine?

In the short-circuit test, the generator is run at rated speed with armature terminals short-circuited and the field current is increased from zero until rated armature current flows — the required field current If(SC) is recorded. The synchronous reactance (unsaturated) is Xs = V_OC / I_SC, where V_OC is the open-circuit voltage at the same field current from the OCC (open circuit characteristic). For a 6.6kV, 2MVA alternator, if If(SC) = 4A gives I_SC = 175A (rated), and the OCC at 4A gives V_OC = 4.5kV, then Xs = 4500/(√3 × 175) ≈ 14.85Ω.

Follow-up: What is the short-circuit ratio (SCR) of a synchronous machine and what does a high SCR imply?

Q15. What is the difference between a cylindrical rotor and a salient pole rotor in terms of excitation analysis?

A cylindrical (round) rotor has uniform air-gap, so its synchronous reactance Xs is the same in all rotor positions — a single Xs suffices for analysis. A salient-pole rotor (used in hydro generators and low-speed machines) has non-uniform air-gap, requiring two reactances: Xd (direct-axis) for flux aligned with the rotor pole, and Xq (quadrature-axis) for flux between poles — the two-reaction theory. For a 100MVA salient-pole hydro generator, Xd may be 1.0 pu and Xq = 0.65 pu, and the phasor diagram is more complex than for a turbogenerator.

Follow-up: How does the reluctance torque arise in a salient-pole generator and how is it expressed mathematically?