Generator Controls Disturbance

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A synchronous machine with IEEE DC1A excitation and a TGOV1 governor riding through a 100 ms bolted three-phase fault.

The machine's two regulators close around it: terminal voltage $V_t$ feeds the DC1A exciter, which returns field voltage $E_{\mathrm{fd}}$; shaft speed $\omega$ feeds the TGOV1 governor, which returns mechanical torque $T_m$. A 10 mΩ ABCG fault switches onto the 13.8 kV bus at $t=4$ s and clears 100 ms later. The source is no longer infinite — 0.038 Ω $+j0.38$ Ω gives the bus a 500 MVA short-circuit level, which is what lets the fault actually collapse the voltage.

This is the open-loop machine sample with its two regulators attached, following the standard wiring contract: machine Vt into the exciter's Vt, exciter EFD back into the machine's Efd, machine speed into the governor's speed, governor Tm back into the machine's Tm. The AVR holds vref0 = 1.03 pu and the governor holds pref0 = 0.4 pu. Then a bolted three-phase-to-ground fault lands on the bus at t = 4 s and clears 100 ms later. The plots follow Pgen and Pmech, the excitation pair VC / EFD, and the three fault-branch currents.

For the first four seconds the regulators just hold station: VC stays inside 0.99–1.02 pu, EFD drifts only between 0.98 and 1.30 pu as the field flux finishes settling, Pgen tracks the 40 MW load and Pmech holds 0.399 pu while the rotor angle pulls in to 31.6°. The fault changes that in one time step. Terminal voltage collapses from 0.99 to 0.048 pu, the fault branch carries 33–43 kA rms as the subtransient decays — eight to ten times the machine's 4.2 kA rated current, with a first-cycle peak near 99 kA once the DC offset is counted — and Pgen falls to 14 MW, because a machine with almost no terminal voltage has almost nothing to push against. The AVR responds the way an exciter is supposed to: seeing a badly depressed VC, it drives EFD from 1.30 up to 2.41 pu, well short of the VRMAX = 5 ceiling, forcing field current in to rebuild flux. On clearing, that boosted field pulls the voltage back fast — 0.82 pu within 50 ms, 0.97 pu by 4.3 s, and back on setpoint by 6 s. The rotor takes one swing to 32.4° at 4.74 s and settles at 29.1° without pole-slipping.

Watch how little the governor does: across the whole disturbance Pmech moves by 0.002 pu and the valve by 0.006. That is correct, not a bug — a 500 MVA system source fixes the frequency, so the speed error a droop governor feeds on never really develops, and the disturbance is the AVR's to handle. Give the governor something to do by weakening the source or islanding the machine with its load. Other levers worth pulling: cut KA from 46 to a few and the voltage recovery turns sluggish; raise fault_resistance for a partial sag instead of a collapse; stretch fault_duration and the first swing grows — 200 ms reaches 37.7°, 300 ms reaches 54°, 400 ms reaches 78°, which is a direct read on how much margin the machine has before it loses synchronism. The run is 10 s at a 20 µs step, fine enough to resolve the fault-current waveform and long enough to see the recovery finish.