Causes of Failure in Off-Circuit (De-energized) Tap Changers

I. Faults in Off-Circuit (De-energized) Tap Changers

1. Causes of Failure

• Insufficient spring pressure on tap changer contacts, uneven roller pressure reducing effective contact area, or inadequate mechanical strength of the silver-plated layer leading to severe wear—ultimately burning out the tap changer during operation.

• Poor contact at tap positions, or poor connections/welding of leads, unable to withstand short-circuit current surges.

• Incorrect tap position selection during switching, causing overheating and burnout.

• Insufficient phase-to-phase clearance among three-phase leads or low dielectric strength of insulation materials, resulting in insulation breakdown under overvoltage and causing inter-phase short circuits at the tap changer.

2. Fault Handling

Operators should immediately collect an oil sample for gas chromatography analysis, based on observed changes in current, voltage, temperature, oil level, oil color, and abnormal sounds, to determine the fault nature and implement appropriate corrective actions.

II. Faults in On-Load Tap Changers (OLTC)

1. Oil Leakage from the Tap Changer Oil Compartment

Causes:

• The drain valve at the bottom of the OLTC oil tank is not tightly sealed, allowing oil to mix between the OLTC compartment and the main transformer tank.

• Poor assembly or substandard sealing materials between the two oil compartments.

• Inadequate sealing of the central drive shaft oil seal.

Handling:
Remove the tap changer from the oil compartment, drain and clean the compartment thoroughly, then locate the leak source—commonly at tap lead bolts or rotating shaft seals—and apply targeted repairs.

2. Broken or Loose Transition Resistors

Causes:
If a transition resistor is already broken and a load tap change is attempted, the load current will be interrupted. Full phase voltage appears across the open contacts and resistor gap, causing:

• Breakdown of the resistor gap,

• Intense arcing between moving and fixed contacts,

• Short-circuit between adjacent tap positions, potentially burning out high-voltage winding tap segments.

Handling:
During transformer maintenance, thoroughly inspect all transition resistors for mechanical damage, looseness, or poor connections to prevent localized overheating and burnout during switching.

3. Overheating of Tap Changer Contacts

Causes:
Frequent voltage regulation causes severe electrical erosion, mechanical wear, and contamination of contacts. In transformers with high load currents:

• Joule heating weakens contact spring elasticity, reducing contact pressure,

• Contact resistance increases, generating more heat,

• This accelerates oxidation, corrosion, or mechanical deformation of contact surfaces, creating a vicious thermal cycle.

Handling:
Before commissioning, perform DC resistance tests at all tap positions. During hood-lift inspections, check contact plating integrity and measure contact resistance. To remove oil films or oxides, manually cycle the tap changer through multiple positions to ensure clean, firm contact.

4. Tap Changer "Run-On" (Continuous Operation)

Causes:

• Malfunction of AC contactors (e.g., oil contamination, residual magnetism causing delayed de-energization) or faulty sequence switches.

• Unreliable AC contactors or micro-switches; loose screws or insufficiently long stop tabs on the tap changer mechanism.

Handling:
Inspect contactors for sticking or delay; verify sequence switch logic. Realign components, use contactors with lower residual magnetism, or add a capacitor in series to suppress residual flux. Clean oil/contaminants from contactors and tighten all loose fasteners.

5. Tap Changer Exceeding Limit Positions

Causes:

• Rust on mechanical limit screws, rendering them ineffective in stopping shaft rotation.

• Insufficient height of positioning blocks, failing to trigger the electrical limit switch even at extreme positions.

Handling:
Manually adjust upper/lower limit blocks and verify that position indicators match actual tap settings. If mismatched, disconnect the motor drive, manually crank the tap changer to a mid-position, then re-engage electric control.

6. Tap Changer Failure to Operate (Refusal to Switch)

Causes:

• Excessive or insufficient spring tension in the fast-acting mechanism (leading to breakage or sluggish action).

• Loose flexible connectors; overly tight sealing between the central shaft and oil compartment base, preventing full insertion of contacts.

Handling:
Check for incomplete engagement between motor drive and tap changer:

• Verify interlock switch continuity and spring reset.

• Inspect for poor contact between fixed and moving contacts.
  If failure occurs in both directions, focus on:

• Reset status of manual crank interlock switch,

• Contact integrity of control switches,

• Normalcy of three-phase power supply.
  For delayed or incomplete switching, investigate:

• Weakened, fatigued, or broken energy-storage springs,

• Mechanical binding.
  Repair or replace faulty mechanical components or springs as needed.