How to Stop Leakage Current in Power Distribution Transformers

Why this matters

Leakage current to ground shortens transformer life, triggers nuisance protection, raises safety risks, and can indicate hidden insulation failure. Fixing leakage quickly and correctly saves downtime and expensive repairs. 

1 — What creates leakage current

Leakage current is a symptom, not a root cause. Common contributors are:

• Contaminated external surfaces (salt, oil, dust) creating conductive creepage paths.

• Moisture or degraded oil/insulation that lowers dielectric strength.

• Damaged or cracked bushings and terminations allowing surface leakage or internal paths.

• Capacitive coupling from long cable runs and grounded shields that create measurable AC currents to earth.

• Poor or intermittent earthing that routes stray currents through unintended paths.

• Partial discharge or internal faults that slowly degrade insulation and increase leakage over time.

2 — Fast detection steps

1. Isolate safely: Lock out and follow your LOTO procedures before any hands-on checks.

2. Measure earth return current: Use a clamp meter around the transformer earth conductor and neutral to quantify the leakage.

3. Phase checks: Clamp around each phase conductor to see which phase(s) contribute to leakage.

4. Insulation resistance (IR) test: Use a megohmmeter phase-to-earth and phase-to-phase and record the polarization index.

5. Visual sweep: Inspect bushings, terminations, tank top, conservator, and cable joints for deposits, cracks, or oil trails.

6. Take oil & bushing samples (if suspect): Send for moisture and gas analysis when oil condition looks questionable.

3 — Immediate (on-site) fixes that often work

If the leakage is not caused by an internal fault, the following often bring immediate improvement:

• Clean insulating surfaces and bushing skirts with approved solvent and lint-free wipes.

• Retorque electrical and earth connections and fix any loose clamps or corroded lugs.

• Replace or reseal damaged gaskets and cable terminations to stop moisture ingress.

• Temporary insulating shields or boots can be used on heavily contaminated terminations until a planned repair.

• Restore proper earthing — correct loose or corroded earth leads and verify continuity.

4 — Medium-term repairs

When cleaning and retorquing don’t solve it:

• Oil filtration and dehydration (vacuum treatment) to remove dissolved moisture and gasses.

• Drying procedures for windings (vacuum drying or controlled bake) if paper insulation absorbed moisture.

• Replace faulty bushings or compromised cable terminations rather than patching them repeatedly.

• Refresh or replace silica gel in breathers — a saturated breather accelerates moisture ingress.

5 — Long-term solutions

To prevent repeated leakage issues across the fleet:

• Improve grounding and equipotential bonding. A low-impedance earth grid reduces stray current paths.

• Use screened or armoured HV/LV cable assemblies and grounded screens to reduce capacitive leakage into earth.

• Upgrade bushings to designs with higher creepage and improved hydrophobic coatings for coastal or polluted sites.

• Add continuous monitoring: online leakage current sensors, PD monitors, and oil sensors for trend analysis.

• Reevaluate neutral grounding strategy with protection engineering — sometimes adding or modifying an NGR reduces nuisance currents (requires system study).

6 — Test & acceptance after repair

Before returning the transformer to service, perform:

• IR and polarization index: ensure values meet OEM specs or baseline levels.

• Dielectric loss (tan δ) on bushings/windings if available.

• Dissolved gas analysis (DGA) after any oil work.

• A final clamp measurement of earth return current to confirm the leak is resolved.
  Document readings and compare to historical trends.

7 — Maintenance checklist

• Monthly: visual inspection of bushings, seals, and breathers.

• Quarterly: torque check on accessible connections, silica gel inspection.

• Semi-annual: IR spot checks and trending.

• Annual: oil sampling (moisture & DGA), thermal imaging, and surface cleaning.

• On any unusual rise in leakage: stop-gap isolate → clamp measurement → oil & bushing sampling → corrective action.

8 — How to prioritize fixes

1. Is leakage small and stable vs. sudden large increase?

• Small / stable → add monitoring + clean surfaces + trend.

• Large / sudden → isolate, test, sample oil, visual inspection.

2. Do IR and DGA point to internal breakdown or moisture?

• Yes → schedule vacuum drying or rewind depending on severity.

• No → focus on bushings, terminations, earthing, and external contamination.

3. Is the site environment corrosive (coast, industrial)?

• Yes → prioritize polymeric bushings with hydrophobic coatings and more frequent surface maintenance.

FAQ

Q: How high is “too high” for leakage current?
A: There is no universal number — compare to transformer manufacturer specs and historical baselines. Sudden increases or currents that interfere with protection are “too high.”

Q: Can capacitive coupling from long cables look like leakage?
A: Yes — long cable runs and ungrounded shields create capacitive currents to earth. Use phase clamps and cable shield grounding to diagnose.

Q: Will oil filtration always fix leakage?
A: Not always. Filtration/dehydration helps when moisture or dissolved contaminants are the cause. If solid insulation or bushings are compromised, further work is required.