TL;DR
Elevated stator winding temperature with normal load usually means reduced cooling water flow or a blocked cooling passage. Check cooling water flow rate, inlet temperature, and compare RTD readings across all stator slots.
What you might see
- stator RTD reading above alarm threshold
- cooling water outlet temperature elevated
- real or reactive load unchanged but temperature rising
- generator protection alarm for over-temperature
Likely causes
Reduced cooling water flow from a throttled valve, fouled heat exchanger, or failing pump
Cooling water inlet temperature above design limit due to upstream heat exchanger fouling
Single blocked stator cooling channel creating a local hot spot
Sustained operation above rated MVA or power factor limit
Required tools
- DCS or relay panel for RTD readout
- Cooling water flow meter readout
- Thermometer or DCS trend data
Safety first
- Large generator stator cooling systems contain high-purity water under pressure. Opening connections requires the unit to be offline and the system to be depressurized and locked out.
- Hydrogen-cooled generators contain flammable gas at elevated pressure. Follow all hydrogen safety procedures before approaching any generator enclosure penetration.
Procedure
- 1
Compare all stator slot RTD readings on the excitation system panel. An isolated high RTD points to a blocked cooling channel; a global rise suggests a cooling system issue.[1]
- 2
Check the stator cooling water flow rate on the cooling system flow transmitter against the design flow specification.
Warning: Generator stator cooling water systems on large hydrogen-cooled generators operate at pressures that require trained personnel. Do not open any cooling water connection without first confirming the unit is offline and the system is depressurized. - 3
Measure the cooling water inlet and outlet temperatures. Compare the delta-T to the design curve for your current load.
- 4
Inspect the stator cooling water heat exchanger for fouling. If delta-T is higher than normal, schedule a tube-side cleaning during the next outage.
- 5
Verify the cooling water treatment chemistry is in specification (deionized or treated water as required by GE). Conductivity out of range accelerates corrosion and scale.
- 6
If a single RTD is high and others are normal, report a blocked cooling channel to the OEM. This typically requires an offline high-pressure water flow test to locate and clear the block.
- 7
Reduce generator load if temperature approaches the high-high alarm limit and the root cause has not been resolved.
Sources
GE GE Gen6 / Gen8 Generator (Large) general technical documentation, GE
Large electric generator maintenance and cooling system operation, general power generation references (general)
More guides for GE GE Gen6 / Gen8
How to diagnose high shaft vibration and rotor unbalance on a GE Gen6 / Gen8 generator
A 1x vibration increase that tracks with load or temperature is consistent with thermal rotor bow or slow mass unbalance. Compare vibration vectors at multiple load points and contact the OEM if the vector changes.
How to detect and respond to a hydrogen gas leak on a GE Gen6 / Gen8 generator
Declining hydrogen purity or elevated makeup demand signals a leak. Check the shaft seal oil system first since shaft seals are the most common hydrogen leak path on large generators.
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