The number that matters is the closing-angle error, but the recorded frequency difference shows how quickly that angle was moving. A 25 MW, 14-pole hydrogenerator was connected while the bus measured 50.3 Hz and the machine measured 47.85 Hz. The resulting 2.45 Hz slip drove stator current to a recorded 3 kA—twice the 1.5 kA rating—produced a heavy mechanical grunt, and operated the under-impedance protection.
Electrical and mechanical severity
At 2.45 Hz slip, the electrical phase relationship moved at 882 electrical degrees per second. Frequency alone cannot reveal the phase angle at contact closure. Recover the actual angle from synchronized voltage waveforms, breaker auxiliary-contact timing, and disturbance records.
For a 14-pole machine, synchronous speed is n = 120f/P. The recorded frequencies correspond to approximately 431.1 rpm at 50.3 Hz and 410.1 rpm at 47.85 Hz, a speed difference of about 21 rpm. These are derived values assuming both recorded frequencies represent their respective electrical fundamentals.
| Quantity | Event value | Engineering significance | Where to confirm |
|---|---|---|---|
| Generator rating | 25 MW, 14 poles | Defines the machine scale and synchronous-speed relationship | Nameplate and design file |
| Frequency mismatch | 50.3 - 47.85 = 2.45 Hz |
Phase angle moved 882 electrical degrees per second | Time-synchronized disturbance recorder channels |
| Speed mismatch | Approximately 21 rpm | Rotor acceleration or deceleration follows the synchronizing torque impulse | Speed pickup and governor record |
| Stator current | 3 kA recorded; 1.5 kA rated | Twice rated current; first-order electromagnetic force comparison is approximately four times because force varies roughly with current squared | Recorder scaling, CT ratio, and whether the value is peak or RMS |
| Thermal duty | Unknown I²t
|
Cannot be calculated from one current magnitude without waveform and duration | Oscillography and relay event record |
| Protection | Under-impedance trip | High current and depressed apparent impedance can enter the relay operating characteristic | Relay targets, settings, and impedance trajectory |
The closure creates a synchronizing torque impulse between the rotor field and the bus field. It also drives slip-frequency currents in the rotor damper system. The torque can load the shaft, rotor attachments, bearings, stator end windings, frame, anchor bolts, grout, and foundation. This is heat, not logic: current magnitude, current duration, closing angle, and mechanical response determine damage.
Inspection approaches compared
| Approach | Advantage | Exposure | Decision criterion |
|---|---|---|---|
| Reset and restart | Fastest return to service | Can miss burned damper bars, displaced end-winding blocking, slip-ring damage, loose rotor hardware, or foundation cracking | Unsuitable when abnormal current, a mechanical impact, or a new noise was recorded |
| External inspection and electrical tests | Finds accessible damage without full dismantling | May not expose hidden rotor or internal bracing damage | Use as the initial scope while reviewing oscillography and OEM limits |
| Targeted internal inspection | Directly examines the components exposed to electrical heating and torque | Requires a longer outage and controlled disassembly | Preferred here because current reached twice rating, the machine produced a heavy grunt, and a twice-rotational-frequency exciter noise appeared afterward |
The recommended path is a targeted internal inspection supported by event reconstruction. A protection trip proves that a relay characteristic was crossed; it does not clear the rotor, windings, shaft train, or foundation for service.
Event preservation and preliminary checks
- Keep the unit isolated until the new beating or flapping noise from the unexcited DC exciter armature is identified. A component moving at twice rotational frequency can indicate runout, looseness, rub, or periodic magnetic or mechanical forcing.
- Export raw oscillography before records roll over. Collect all three stator currents, terminal and bus voltages, field current and voltage, speed, breaker command, breaker contact status, protection elements, and trip timing.
- Validate recorder scaling and channel definitions. Determine whether 3 kA is RMS, peak, fundamental RMS, or another calculated quantity.
- Reconstruct the voltage phase angle at actual main-contact closure. Breaker command time alone is insufficient because operating time and pole scatter separate the command from electrical contact.
- Review the under-impedance element pickup, operating trajectory, and clearing sequence. Check whether any other protection started without completing a trip.
- Record shaft position, bearing clearances, visible movement indicators, oil condition, and pre-event versus post-event vibration data before disturbing hardware.
Rotor, exciter, and bearing inspection
Inspect damper bars and their connections for localized heating, discoloration, arcing, cracks, looseness, or burned joints. Slip during an out-of-phase connection can drive substantial damper current even though the event is brief.
Examine rotor fixing bolts, keys, pole attachments, retaining features, and accessible shaft-coupling hardware for witness marks or movement. Check mechanical runout and air-gap uniformity against the machine’s baseline or manufacturer limits.
Inspect slip rings and every brush path. Shaft bounce can momentarily unload brushes unevenly, leaving one or two brushes to carry a disproportionate share of excitation current. Look for burned tracks, pitting, uneven film, damaged brush faces, altered spring loading, and displaced holders.
Check bearings for distress, shifted components, abnormal clearances, and oil contamination. During controlled rotation and any approved restart, trend bearing temperature and vibration by location rather than relying on a single overall value.
Stator, frame, and foundation inspection
Electromagnetic force rises approximately with current squared, so the reported two-per-unit current deserves a close end-winding examination. Inspect coil ends, blocking, bracing, ties, connections, and phase leads for looseness, abrasion, cracked supports, dust marks, or evidence of movement.
Inspect the stator core and frame interfaces, frame welds, supports, and accessible fasteners. Use the plant’s qualified examination method where a visual result is inconclusive. Record every indication before tightening or repair so its direction can be compared with the expected torque reaction.
Survey anchor-bolt regions, grout, and surrounding concrete for fresh cracking, separation, spalling, or movement. Foundation damage can occur even when the generator shows no obvious internal failure, particularly where aged grout or concrete already has defects.
Electrical testing and return-to-service verification
Start with non-destructive insulation and winding checks appropriate to the machine: compare phase-to-phase winding resistance, insulation resistance, and other established baseline tests. Investigate asymmetry or a meaningful departure from historical values before applying a high-potential test.
A high-potential test is not the first diagnostic for a torque event. Select its voltage, duration, and acceptance criteria from the generator manufacturer’s service guidance and the insulation system’s condition; an unnecessarily severe test can turn a weakened area into a failure without locating the mechanical cause.
- Resolve all visual, dimensional, electrical, and protection-record discrepancies.
- Perform a controlled mechanical rotation or approved low-risk run while measuring shaft runout, exciter behavior, vibration, and bearing response.
- Confirm that the twice-rpm noise is absent or has a documented benign cause.
- Verify synchronizing instrumentation, breaker closing-time compensation, and the complete permissive path.
- Install or restore an independent synchronism-check function before routine manual synchronization. Test it with simulated unacceptable voltage, frequency, and phase-angle conditions using settings approved for the unit.
- On the first approved return to service, trend all bearing temperatures, vibration, stator current balance, field quantities, and protection indications through speed rise, excitation, synchronization, and loading.
Frequently asked questions
What happens if a hydrogenerator closes 2.45 Hz below the bus?
The voltage angle moves at 882 electrical degrees per second, so breaker operating time can place the contacts far from the intended closing angle. The actual torque severity comes from the phase angle and voltages at main-contact closure, which must be reconstructed from oscillography.
What happens if stator current reaches 3 kA on a 1.5 kA generator?
The recorded current is two per unit. Electromagnetic force scales approximately with current squared, giving a first-order force comparison near four times the rated-current condition, while heating still requires waveform duration to calculate I²t.
What happens if damper bars carry out-of-phase current?
Slip-frequency rotor current can produce localized heating, burned connections, looseness, or cracking. Inspect the bars and joints directly rather than treating a cleared protection trip as proof that they are intact.
What happens if the shaft bounces during synchronization?
Brushes can unload unevenly, concentrating excitation current in the remaining contacts and burning a slip-ring track. Inspect brush faces, holders, spring loading, ring surfaces, bearing condition, and shaft runout.
When should a hydrogenerator synchronization event go to official support?
Stop recommissioning if the twice-rpm noise remains, inspections find movement or burning, electrical results depart from baseline, the closing angle cannot be reconstructed, or the manufacturer’s allowable event duty is unavailable. Escalate the recorder files, relay report, photographs, dimensional readings, and test results to the generator and protection-equipment manufacturers through their official support channels.