Auxiliary Transformer — Maintenance, POH, Oil Testing & DGA Fault Diagnosis

Maintenance Schedules — Monthly, Quarterly, Half-Yearly, Yearly

Periodic Overhaul (POH)

Overhauling (POH) can be done either in a repair shop or through trade. Before opening, tests performed: Insulation Resistance (HV-to-Earth by 2.5kV megger, LV-to-Earth by 500V megger, HV-to-LV by 2.5kV megger); Continuity Test (primary and secondary windings via multimeter); Tap Changer condition/operation at all positions; BDV of oil (automatic BDV kit); Polarization Ratio (2500V DC applied continuously between winding and earth, IR noted at 10 sec and 60 sec); DGA of oil; further oil tests (specific resistance/resistivity, water content, Tan Delta, flash point, neutralization value, sludge); and a Ratio test.

• General cleaning FIRST: remove as much dirt as possible around tank, radiator, HV & LV bushing, neutral link block covers; blow dust off with compressed air.
• Before opening, disconnect: drain oil, disconnect all leads, remove tank top cover/accessories/conservator/fitting, disconnect HV & LV bushings (store safely), disconnect earth leads from primary earth bushing terminal link block, disconnect power cable connections from secondary winding side bushing.
• AFTER opening: measure winding IR; check all core support bolts for cracks, replace if required; check core/winding/lamination condition; check for water content/sludge accumulation inside the tank; check earth connections; check soldered joints; check insulation paper for damage, reinsulate if necessary; check tap changer contact tips at all positions.
• Details of work: clean winding/core with new filtered oil; tighten core bolts and fasteners, replace if worn; BAKE the winding and core in an oven at 60°C — observe DAILY IMPROVEMENT in winding IR values; clean tank thoroughly inside/outside, check for flash/hit marks; varnish tank inside with COPAL VARNISH; paint tank outside with BATTLESHIP GRAY paint; change drain-cock washers; check secondary terminal block studs (oil-leak-proof); clean HV bushing fitting surface; replace all gaskets/O-rings/rubber items; replace worn tap-changer contacts.
• Fittings inspection: silica gel breather reactivated by heating in a shallow pan at 150-200°C for 2-3 hours until crystals regain their original blue tint. Radiator cleaned with compressed air/high-pressure water jet. Conservator and glass indicator cleaned thoroughly.
• Testing after assembly: meggering, continuity test, ratio test, voltage output at different taps, magnetizing current.
• Final checking: check for oil leakage from any point/coupling/joint; check unusual sound; ensure conservator oil at MAXIMUM mark; release air from top screw of HV bushing; measure final IR and compare with previous values; measure BDV and moisture content (ppm); check arcing horn gap.

Factors Affecting Auxiliary Transformer Life

• Moisture: highly undesirable — affects dielectric properties of oil AND the paper insulation (highly hygroscopic). When oil oxidizes in service, acids form; these acids INCREASE moisture solubility of oil; acids + moisture further decompose the oil, forming MORE acids and moisture — a self-accelerating deterioration cycle.
• Oxygen: reacts with and decomposes cellulose insulation, forming an oil-soluble organic acid and sludge that blocks free oil circulation — its adverse effect may be aggravated by catalytic action between hot oil and bare copper, increasing operating temperature.
• Solid Impurities: reduce dielectric strength considerably — remedy is periodic filtering.
• Varnishes: some have an oxidizing effect, reacting with oil and precipitating sludge on windings; synthetic varnishes with acid-inhibiting properties DELAY natural acid/sludge formation.
• Slackness of Winding: after a few months of service, transformer coils may suffer natural settling — wearing conductor insulation in places, leading to inter-turn failure. Coils may also displace under load/short-circuit, causing electrical/magnetic unbalance and GREATER displacement. Good practice: lift core and windings at the FIRST major schedule to take up any slackness.

Transformer Oil — Testing Standards

Auxiliary transformers are supplied complete with insulating oil per IS:335 — shipped with windings/core UNDER oil, plus 10% EXTRA oil in non-returnable sealed steel drums, from RDSO-approved manufacturers who submit test certificates. Pre-energizing oil characteristics must conform to IS:1866.

Oil Purification: object is to remove all contaminants (water, carbon deposits, dirt, sludge, dissolved moisture/gases) — the most important quality preserved is dielectric strength, affected by moisture presence. Moisture is absorbed through defective breathers, gaskets, and addition of untreated make-up oil. Purifying machine per RDSO spec ETI/PSI/103.

• Oil Sampling precautions: use clean, dry, amber glass bottles per IS:6855-1973; take samples preferably in DRY weather to avoid external contamination; oil sample shall be AT LEAST AS WARM AS ambient air.
• Sampling procedure: first drain at least 1 LITRE to eliminate drain-cock contamination; rinse bottles with the oil being sampled; ensure cock is correctly closed after sampling; check label marking correct/complete; store samples in a DARK place; ensure sampling is done by an EXPERIENCED PERSON; fill each bottle to 95-98% of capacity.

Common Failures & Remedies

• Oil leakage — location-specific causes: screw joints (foreign material in threads, poor threads, improper assembly); gasket joints (insufficient/uneven compression, improper gasket prep, old gaskets); weld joints (shipping strains, imperfect weld); couplings (cracks, defective coupling joints); drain plugs (defective threads, defective oil seal).
• Low BDV — causes: moisture contamination from inactive (pink) silica gel (remedy: reactivate/replace gel, purify oil); leaks around cover accessories letting in breathing air (remedy: attend leaks, regasket, purify oil); humid rainy-season atmosphere (remedy: purify oil, check BDV/water content).
• Bushing failure — HV flashover: lightning discharge/over-voltage, insufficient arcing horn gap (remedy: measure SPLIT-TYPE arcing horn gap, minimum 82.5mm), dirty bushing, bird electrocution (remedy: ensure porcelain cleaning during each inspection). Porcelain petticoat broken/cracked: external hitting (remedy: proper cleaning/visual checking each inspection).
• Winding failures: primary lead open/earthed from overload short circuit (check winding for overheating/charring signs); bulging and inter-turn/layer/coil shorts from coil shrinkage or insulation failure (investigate overloading, correct accordingly); LV-HV shorting from insulation failure (during manufacture/rewinding, coils should be pressed, heated, cooled repeatedly until height stabilizes); flash marks on core/support from dead short circuit due to coil lateral displacement (remedy: Nomex paper insulation sheet between HV & LV coils, must not obstruct oil flow) or winding loose on core (remedy: lift core, check thoroughly, repair if possible).
• Excessive overheating: temperature rise from internal fault (short-circuited core, core bolts/clamps insulation failure — remedy: replace transformer, lift core for thorough checking) OR low oil level in conservator (remedy: top up) OR sludged oil (remedy: purify to remove sludge).
• Low IR value: moisture in oil (remedy: purify with high vacuum plant, test electrical strength/water content); insulation failure between winding and core (remedy: replace transformer, check winding thoroughly); internal connection lead insulation damage (remedy: check leads, re-tape damaged insulation).
• Humming sound: loose core (remedy: lift active part, tighten all pressure/clamping bolts); winding loose from coil shrinkage (remedy: press/heat/cool repeatedly during manufacture until height stabilizes; tighten pressure/clamping bolts during FIRST POH after commissioning to address shrinkage).

Condition Monitoring & DGA Fault Diagnosis (Ratio Method)

Three maintenance approaches exist: Routine (day-to-day — oil level, silica gel colour, abnormal sound, oil leakage), Periodic (long-term — oil testing, winding IR measurement), and Condition-Based Monitoring. It is established that OVER-maintenance leads to MORE failures — the trend should be to MINIMIZE maintenance via condition-based monitoring. A transformer rarely fails suddenly except from short-circuiting or lightning surges — faults normally develop GRADUALLY, and DGA is a very powerful tool to catch them at an incipient stage.

Ratio-coding convention: ratios LESS THAN 1.0 are designated 0; ratios GREATER THAN 1.0 are designated 1 — this binary coding is what makes the ratio combinations above unambiguous to apply.

Do's and Don'ts

• DO: replace DO fuse periodically; use only RDSO-approved transformer oil suppliers; check silica gel regularly; clean bushings periodically; attend oil leakage at the earliest; clean conservator thoroughly before refilling; test oil periodically; maintain transformer/oil records transformer-wise; check DO assembly barrel spring tension; use proper-size cable lugs; use proper fuse ratings on both sides; use silica gel breather of MINIMUM 250 GRAMS capacity; use a MOTORISED MEGGER for IR measurement.
• DON'T: use under-capacity pulley/tirfor for unloading; leave loose connections; meddle with the protection system; allow conservator oil to fall below 1/3 (minimum) level; over-tighten nuts/bolts to stop leakage; use fuses higher than prescribed ratings; tamper with earthing connections; keep breather pipe open/exposed; re-energize a faulty transformer unless oil test report (including DGA) is satisfactory; impinge hot oil jets on windings for cleaning; use RECLAIMED oil; MIX old and new transformer oils; energize without commissioning checks; overload the transformer; use cotton waste for cleaning INSIDE parts or the bushing.