⚙ Governor PID Tuning
Proportional-Integral-Derivative Control System
⚠️ MANDATORY SAFETY PROTOCOL
STOP. READ THIS FIRST.
- Grid Synchronization: Never adjust PID parameters while unit is synchronized to grid without dispatcher approval.
- Load Stability: Perform tuning tests under stable load conditions (avoid sudden load changes during testing).
- Emergency Stop: Ensure Emergency Stop button is accessible and functional before starting tests.
- Communication: Notify operations center before initiating response tests.
The Governor controls the position of 20 wicket gates to maintain constant turbine speed (typically 500 RPM for 12-pole generator at 50 Hz).
Control Loop
Control Output = (P × Error) + (I × ∫Error dt) + (D × dError/dt)
Proportional (P): Immediate Response
What it does: Reacts proportionally to the current speed error.
- High P: Fast response, but may cause overshoot and oscillations.
- Low P: Slow response, but stable.
- Typical Range: 2.0 - 6.0 (dimensionless gain)
💡 Simple Analogy: Like a driver turning the wheel harder the further off course they are.
Integral (I): Steady-State Corrector
What it does: Eliminates steady-state error by accumulating error over time.
- High I: Fast elimination of offset, but can cause "wind-up" and instability.
- Low I: Slow correction, but prevents overshoot.
- Typical Range: 0.5 - 2.0 seconds (integration time)
💡 Simple Analogy: Like a driver remembering how long they've been off course and correcting accordingly.
Derivative (D): Predictive Damper
What it does: Predicts future error based on rate of change and applies damping.
- High D: Strong damping, but sensitive to noise (can cause jittering).
- Low D: Less damping, smoother response.
- Typical Range: 0.1 - 0.5 seconds (derivative time)
💡 Simple Analogy: Like a driver anticipating a curve and starting to turn before reaching it.
What is Dead-Band?
Dead-Band is a small speed range (e.g. ±0.1 RPM) where the governor ignores speed deviations and does not move the wicket gates.
Why It Exists
- Prevent Wear: Constant micro-adjustments cause excessive wear on servomotors, linkage pins, and gate bearings.
- Reduce Hunting: Without dead-band, governor would constantly "hunt" for exact setpoint, causing oscillations.
- Noise Immunity: Filters out small speed fluctuations caused by grid noise or sensor jitter.
Dead-Band Configuration
| Parameter | Typical Value | Effect if Too High | Effect if Too Low |
|---|---|---|---|
| Dead-Band | ±0.05 - 0.15 RPM | Slow response to load changes | Excessive wear, hunting |
What is Step Response Test?
Controlled test where load is changed abruptly by a fixed amount (typically 10% rated power) to observe how governor responds.
Test Procedure
- Initial Conditions: Unit synchronized to grid, stable load (e.g. 5.0 MW).
- Record Baseline: Monitor speed, gate position, and power output for 2 minutes.
- Apply Step Change: Increase load setpoint by 10% (e.g. 5.0 MW → 5.5 MW).
- Observe Response: Record speed and gate position for 5 minutes.
- Analyze: Measure overshoot, settling time, and oscillation frequency.
Ideal Response Characteristics
| Metric | Target Value | Interpretation |
|---|---|---|
| Overshoot | < 5% | Speed must not exceed setpoint by more than 5% |
| Settling Time | < 10 seconds | Time to stabilize within ±1% of setpoint |
| Oscillations | < 2 cycles | Should damp out quickly without sustained hunting |
Ideal: Fast rise → Small overshoot → Quick settling
Bad: Slow rise OR Large overshoot OR Sustained oscillations
What is Governor Hunting?
Hunting is continuous oscillation of wicket gates around setpoint, causing speed and power to fluctuate.
Symptoms
- Speed oscillates ±0.2 - 0.5 RPM at regular frequency (e.g. every 3-5 seconds).
- Gate position oscillates ±1-2% continuously.
- Audible "clicking" or "thumping" from servomotors or linkage.
- Increased wear on linkage pins and bushings.
Root Causes
1. Mechanical Linkage Backlash
This is a problem identified during pin inspection:
- Worn Pins: Gap > 0.5 mm between pin and bushing.
- Loose Joints: Bolts not torqued to spec.
- Effect: Servomotor moves, but gates don't respond immediately, causing late feedback loop.
Solution: Replace worn pins and bushings, retorque all linkage bolts.
2. Excessive Integral Gain (I)
- Cause: I value too high, causing "integral wind-up".
- Effect: Controller overcorrects, causing oscillations.
Solution: Reduce I gain by 20-30% and retest.
3. Insufficient Derivative Damping (D)
- Cause: D value too low, insufficient oscillation damping.
- Effect: System overshoots and oscillates.
Solution: Increase D gain by 10-20% and retest.
4. Too Small Dead-Band
- Cause: Dead-band < 0.05 RPM.
- Effect: Governor responds to noise, causing constant micro-adjustments.
Solution: Increase dead-band to 0.10 - 0.15 RPM.
Diagnostic Procedure
- Mechanical Inspection: Check all linkage pins for play (target < 0.2 mm gap).
- Monitor Oscillation Freq: Fast oscillations (< 2 sec)=mechanical issue. Slow oscillations (> 5 sec) = PID issue.
- Isolate Cause: Temporarily increase dead-band to 0.3 RPM. If hunting stops, problem is PID tuning. If continues, problem is mechanical.
Step-by-Step Tuning Process
- Start with Conservative Values: P=3.0, I=1.0, D=0.2, Dead-Band=0.10 RPM.
- Perform Step Response Test: Apply 10% load change, record response.
- Adjust P: If response too slow, increase P by 0.5. If overshoot excessive, decrease P by 0.5.
- Adjust I: If steady-state error persists, increase I by 0.2. If hunting occurs, decrease I by 0.2.
- Adjust D: If oscillations poorly damped, increase D by 0.1. If response jittery, decrease D by 0.05.
- Iterate: Repeat steps 2-5 until optimal response achieved.
- Document: Record final PID values and response graphs in unit log.
- Software: Access to Governor HMI (password protected).
- Monitoring: SCADA system with speed, power, and gate position trends.
- Documentation: Response recording software, unit log.
- Mechanical Tools: Calipers (for measuring linkage play), torque wrench.
- Safety: Communication radio, emergency stop access.