Synchronous-machine excitation systems
DC exciter (legacy), brushless AC (rotating diodes), static thyristor (<100 ms, 3-4× ceiling). AVR closed-loop with OEL/UEL/V-Hz limiters. PSS for rotor-swing damping. IEEE 421.5 standardized models.
Step 1 — Excitation system supplies DC field current to the rotor
Reference notes
The excitation system supplies the DC field current I_f to a synchronous machine's rotor field winding. It does four essential jobs: steady-state field supply, automatic voltage regulation (AVR), rapid field forcing during faults, and rotor-oscillation damping via Power System Stabilizer (PSS). Use Next → to walk through the three excitation technologies (DC exciter, brushless AC, static thyristor), the AVR loop, and the PSS damping enhancement.
Four jobs of an excitation system
- Steady-state DC field current — matched to operating power factor and reactive output.
- AVR (Automatic Voltage Regulator) — closed-loop control of terminal voltage as load varies.
- Field forcing — rapidly raise field to ceiling voltage during faults to maintain internal EMF E and transient stability.
- PSS (Power System Stabilizer) — modulate AVR setpoint to damp 0.5–3 Hz rotor swing modes.
Three technologies compared
| Type | Architecture | Response | Ceiling | Maintenance |
|---|---|---|---|---|
| DC exciter (legacy) | Shaft-mounted DC generator → slip rings + brushes → field | 1–2 s | ~1.6× | High (brushes + commutator) |
| Brushless AC exciter | Shaft AC alternator (field on stator, armature on rotor) → rotating diodes → field. No slip rings. | 0.5–1 s | ~1.8× | Low |
| Static excitation | Excitation transformer from terminals → thyristor bridge → slip rings + brushes → field | < 100 ms | 3–4× | Moderate |
DC exciter (legacy)
- Small DC generator on the shaft supplies field current via slip rings + brushes.
- Pilot exciter feeds the main exciter's field; AVR commands the pilot exciter.
- Three-stage cascade response: AVR → pilot → main → field. Slow (1–2 s).
- Brush wear, commutator maintenance. Phased out after 1965 for new builds.
Brushless AC exciter
- AC exciter alternator on the shaft with FIELD on the stator and ARMATURE on the rotor (inverse of normal construction).
- AC armature output → rotating-diode bridge mounted on the shaft → DC into the synchronous machine's rotor field.
- All on the rotating side — no slip rings, no brushes.
- Pilot AC exciter using a permanent-magnet alternator typically supplies the stationary field of the main AC exciter.
- Response 0.5–1 s, ceiling ~1.8× rated, low maintenance. Common on large turbogenerators where shaft-end space is limited.
Static excitation
- No rotating exciter. Excitation transformer taps generator terminals, secondary feeds a 6-pulse thyristor bridge, bridge output through slip rings to rotor field.
- AVR commands thyristor firing angle. Fully solid-state.
- Response < 100 ms (10× faster than DC exciter, 5× faster than brushless AC).
- Ceiling 3–4× rated → enables aggressive field forcing during faults.
- Drawback: bridge draws power from generator terminals; auxiliary backup paths needed for low-voltage fault ride-through.
- Modern preferred technology for large units, units in weak networks, units serving HVDC.
AVR — Automatic Voltage Regulator
Closed-loop terminal-voltage control. Inputs: V_t feedback, V_t setpoint, optionally I and Q for compensation. Output: excitation command (firing angle or pilot field).
- Inner loop: PI compensator with time constant 100–500 ms.
- Reactive droop: 4–6 % typical, so paralleled generators share Q without hunting.
- Line-drop compensation: regulate FAR-END voltage on long radial lines.
- Over-excitation limiter (OEL): prevents field thermal overload during sustained low-PF operation.
- Under-excitation limiter (UEL): prevents loss-of-field that would cause loss-of-stability.
- V/Hz limiter: protects against ANSI 24 overexcitation during startup or post-fault.
- IEEE 421.5 standardized models: DC1A/DC2A, AC1A/AC8B, ST1A/ST3A families.
PSS — Power System Stabilizer
Fast AVRs without PSS can REDUCE damping on inter-area rotor swing modes (~0.3–0.7 Hz) and local modes (~1–2 Hz). PSS adds a supplementary signal to the AVR setpoint that is shaped to be in phase with rotor SPEED:
- Δω-based PSS — uses rotor speed deviation directly.
- ΔP-based PSS — uses electrical power deviation computed from V·I.
- Integral-of-accelerating-power PSS — modern variant immune to mechanical-side oscillations.
- Lead-lag compensator shapes the phase so the PSS output produces a damping torque.
NERC requires PSS on all generators > 100 MVA in North America after the 1996 Western Interconnect blackouts identified inadequate damping as a contributor.
Field forcing during faults — the stability link
During a fault, terminal voltage V_t collapses. Without intervention, internal EMF E drops, P_max = V·E/X drops, and the equal-area criterion's A2_max shrinks. The AVR senses V_t dip and commands maximum field — limited by exciter ceiling. High ceiling (3–4× for static) allows field current to rise quickly through V_ceiling / L_field, supporting E during the fault and enlarging A2_max. Faster excitation = larger transient stability margin.