22/10/2025
what is the cause of voltage hunting?
5 posible reason of voltage hunting..
This is a classic and critical issue in marine engineering. Voltage hunting on a ship's generator is a sustained or dampened oscillation of the generator's output voltage (and often frequency), where the voltage constantly rises and falls instead of remaining at a stable setpoint (e.g., 440V).
It is almost always a symptom of a problem in the Automatic Voltage Regulator (AVR) feedback control loop.
In simple terms, the AVR is constantly "over-correcting." It sees a small voltage drop, injects too much excitation, sees the voltage go too high, cuts too much excitation, and this cycle repeats, causing the "hunt."
Here are the primary causes, categorized from most common to more specific:
1. Incorrect AVR Stability (Gain) Settings (The Most Common Cause)
This is the fundamental control theory problem. The AVR is a controller that needs to be "tuned" for the specific generator.
· Gain Set Too High (Proportional Band Too Narrow): If the AVR's gain is too high, it reacts too aggressively to tiny changes in voltage. This over-sensitive response is the direct cause of hunting. It's like an inexperienced driver constantly over-steering to stay in a lane.
· Instability Potentiometer Misadjusted: Many AVRs have a dedicated "stability" or "damping" potentiometer. If this is incorrectly set during commissioning or maintenance, it removes the natural damping from the system, allowing oscillations to build up.
2. Issues with the Sensing Circuit
The AVR can only be as good as the signal it receives.
· Loose Connections: Loose terminals in the voltage sensing lines from the main generator output to the AVR can cause intermittent signals, confusing the AVR and triggering corrective actions.
· Faulty Potential Transformer (PT): A failing PT can provide an inaccurate or noisy voltage signal to the AVR.
3. Problems in the Excitation Field Circuit
The AVR's commands are executed here.
· Loose Connections in the Exciter Field: Loose connections at the brushes, slip rings, or in the rotating rectifier assembly (on brushless generators) can create a variable resistance. This causes erratic changes in the main field strength, which the AVR then tries to correct, leading to instability.
· Worn or Sticking Brushes (on brushed generators): Poor contact or brushes that don't move freely in their holders can cause intermittent excitation.
· Faulty Rotating Diodes (on brushless generators): A shorted or open diode in the rotating rectifier will cause uneven and pulsating DC to the main field, directly causing voltage ripple and hunting.
4. Interaction with Other Generators or Loads
· Incorrect Reactive Power Sharing: When two or more generators are running in parallel, their AVRs must work together. If the reactive power (kVAR) sharing is incorrect, the generators will "fight" each other for the reactive load. One generator will try to take more kVAR, causing its voltage to rise, while the other backs off, causing its voltage to fall. This creates a system-wide hunting condition between the generators.
· Large, Rapidly Fluctuating Loads: Sudden application or removal of large inductive loads (e.g., a large cargo pump motor starting, a bow thruster cycling) can shock the system. If the AVR response is not tuned to handle such rapid changes, it can oscillate. This is often a transient hunt that settles down.
· Non-Linear Loads: Modern ships with large variable frequency drives (VFDs) for propulsion or cargo handling can generate significant harmonics (especially 5th and 7th). These harmonics can distort the voltage waveform and interfere with the AVR's sensing circuitry.
5. General Mechanical and Electrical Issues
· Poor Engine Governor Performance: While the governor controls speed/frequency, the two are linked. If the engine speed is hunting (due to a faulty governor or fuel issue), the generator frequency will hunt, and the AVR will try to compensate for the resulting voltage changes, often making the overall instability worse.
· Damaged AVR Components: Internal failure of components like capacitors or transistors within the AVR itself can cause erratic control behavior.
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Troubleshooting Summary: How to Find the Cause
1. Isolate the Generator: First, take the generator off the switchboard and run it on a steady, non-fluctuating load (like the ship's service load).
· If hunting stops: The problem is likely external (e.g., reactive power sharing with other generators or the influence of large, specific loads).
· If hunting continues: The problem is internal to that generator set.
2. For Internal Problems:
· Visual Inspection: Check all connections for tightness (sensing lines, exciter field, brushes).
· Check Brushes and Slip Rings: Look for wear, cracking, or poor contact.
· AVR Settings: Verify the gain and stability settings against the manufacturer's manual. Often, a slight reduction in gain can resolve the issue.
· Test Rotating Diodes: On a brushless generator, test the diodes with a multimeter during a maintenance period.
3. For External Problems (Parallel Operation):
· Check Reactive Power (kVAR) Meters: Ensure the kVAR is being shared equally (or proportionally) between generators. Adjust the AVR's droop settings if necessary.
· Observe Load Changes: See if the hunting coincides with the operation of specific large machinery.
In summary, voltage hunting is a control loop instability. The most frequent culprit is an improperly tuned AVR, but it's crucial to systematically check the entire chain—from the voltage sensing input, through the AVR itself, to the exciter field output, and finally, the external network conditions—to correctly diagnose and resolve the issue. Always consult the specific generator and AVR manufacturer's manual before making adjustments.
