Hi,
Can anyone please explain about the subject matter with reference documents for GE or other Manufacturers Gas Turbine Generator?
Apart from that "Is Power Factor control mode not normally recommended for large capacity generator??"

Regards,
RU

 

In a combined GT generator system (Frame 6, 7, 9), the generator’s excitation system regulates terminal voltage and reactive power (Vars).
Control modes generally available:

1. Voltage Control Mode (Automatic Voltage Regulator – AVR) → Keeps generator terminal voltage constant.​

2. Power Factor (PF) Control Mode → Maintains a fixed PF by adjusting excitation to vary Vars.​

3. Var Control Mode → Keeps Vars constant.​

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2. Why PF Control Is Not Normally Recommended for Large Generators
For large capacity GT generators (e.g., 47 MW Frame 6B up to 250 MW+ Frame 9FA), Voltage Control is the standard mode. PF control is rarely used as the primary mode because:
1. Grid Stability & Compliance
Most grid codes require voltage regulation at the point of interconnection.
PF control can allow voltage to drift if system conditions change, risking non-compliance with grid operator requirements.
2. Dynamic Response in Disturbances
During load changes or system disturbances, AVR in voltage mode responds faster to maintain stability.
PF mode reacts more slowly, risking unstable voltage swings in the network.
3. Coordination with Other Units
In multi-unit plants or interconnected grids, voltage control ensures coordinated reactive power sharing.
Fixed PF operation can cause unequal Var sharing and overload another generator’s field.
4. Over/Under Excitation Risk
In PF mode, sudden real power (MW) swings can demand reactive power changes beyond capability limits.
This can trigger Loss-of-Field, Overexcitation (OEL), or Underexcitation (UEL) trips, especially on large frames.
5. Thermal Stress on Rotor
Large machine field windings are sensitive to rapid Var swings.
PF mode can allow larger Var oscillations → higher rotor heating and potential insulation life reduction.
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3. Typical GE Recommendation
Normal Operation:
→ Voltage Control (AVR) at generator terminals or high-voltage bus.
→ Set voltage reference per grid code / plant coordination study.

PF Control Use Case:
→ Acceptable for smaller industrial generators or where the grid operator explicitly demands a fixed PF (rare in large utility-scale GTs).
→ If used, PF setpoint should be inside generator capability curve, with OEL/UEL limits enforced.
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GE Reference:
See GEK 110483 & generator O&M manuals (section on “Excitation Control Modes”) – they clearly note PF mode is optional and normally used only in special dispatch scenarios.


"I am not an electrical engineer—I specialize in operations, particularly in power plants and oil refinery utilities. Therefore, all my responses are based on hands-on operational experience in these environments."

 

@Kahlil Gaser Reda,

You may have posted your response in the wrong thread--operating the generator excitation system in voltage-, power factor- or VAr control modes has nothing to do with prolonged turning gear operation (which is the cooldown mode performed at very low speed operation usually without any generator excitation).

 

@Kahlil Gaser Reda,

You may have posted your response in the wrong thread--operating the generator excitation system in voltage-, power factor- or VAr control modes has nothing to do with prolonged turning gear operation (which is the cooldown mode performed at very low speed operation usually without any generator excitation).

This was side question by the poster , try to educate yourself using good wording.

 

Regarding Prolonged TG Operation – Potential Impacts on Generator
For F-class (e.g., 7FA.05) and H-class (e.g., 9HA) machines, extended TG running beyond OEM recommendations can cause:
a. Bearing & Seal Wear

• Journal and thrust bearings in the generator are still carrying rotor weight.
• Continuous oil film shear from prolonged TG hours accelerates wear, especially if lube oil cleanliness or temperature control is suboptimal.
• Seal oil system (hydrogen-cooled generators) runs continuously, increasing wear on seals and raising the risk of hydrogen leakage.

b. Lubrication & Oil System Stress

• Extended running means continuous main lube oil and seal oil pump operation → increased operating hours and earlier maintenance.
• If oil temp control is not optimized, prolonged low-speed operation can lead to:
  • Oil varnish formation in cool zones
  • Moisture ingress from seal systems during long idle periods

c. Rotor & Stator Temperature Cycling

• TG keeps rotor turning but does not actively heat or cool the generator — temperature gradients can still develop, especially in high-inertia H-class units.
• If space heaters are not energized during prolonged TG runs, internal condensation risk increases, especially in humid environments.

d. Hydrogen System Management (if applicable)
Hydrogen purity control must continue.
Extended TG operation without active load means slower gas circulation and possible stratification → can lead to localized moisture pockets and insulation degradation.
e. Increased Auxiliary Power Use
TG drives, oil pumps, and seal oil systems consume significant auxiliary power.
For large plants, prolonged TG operation increases parasitic load and operating cost.
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OEM Guidance & Field Best Practice
GE Guidance:
1. Typical TG operation post-shutdown: 6–24 hours (F-class) until rotor thermal stabilization.
2. Prolonged TG use (>48 hours) is not recommended unless required for operational constraints (e.g., hot standby, delayed cool-down before outage).
3. Reference: GEK 110563 (Generator O&M) & turbine-specific TILs for F/H-class.

In the field, I’ve seen:

• F-class generators suffer accelerated thrust bearing wear when TG was left running for >10 days awaiting parts.
• H-class units experience hydrogen seal degradation due to extended seal oil operation in TG mode without proper oil temp control.
• Rotor bow prevention is valid, but unnecessary TG run time is a hidden reliability and cost risk

 

@Kahlil Gaser Reda,

I re-read the original post and see that I missed the part about power factor control mode. Sorry if I offended you--it was not intentional I assure you.

 

@Kahlil Gaser Reda,

I re-read the original post and see that I missed the part about power factor control mode. Sorry if I offended you--it was not intentional I assure you.

No worries Bro , Best of luck.

 

Thank you very much. Appreciate your supportive issue.