Analysis of Key Control Points in Large Power Transformer Installation and Commissioning

From Delivery to Operation, These 7 Steps Are Where “Assuming” Gets You Into Trouble

Electric Power Substation

Installing and commissioning a large power transformer isn’t exactly rocket science, but anyone who’s actually done it knows one thing: problems rarely come from complex theory — they come from overlooked basics.

Below, the process is broken down into 7 key control points, each with “common pitfalls” and “things you must take seriously.”

Control Point 1: Equipment Receiving and Unpacking Inspection

A lot of people think “just unload the gear as soon as it arrives” — then pay for it later.

Key Action	Common Issue	What You Must Check
Visual inspection	Only check the tank, ignore the shock recorder	Any shock over 3g must be reported — do not sign off
Unpacking & inventory	Missing small parts (bushings, radiators, bolts)	Check off item by item, take photos, document shortages on the spot
Insulating gas pressure	Assume factory pressure is fine	Record SF6 or N2 pressure — if below 0.05 MPa, leak-check

Real talk: The 10 minutes you save by rushing unpacking can cost you two days later figuring out who’s at fault.

Control Point 2: Positioning and Foundation Check

If the transformer isn’t level, every measurement after that is wrong.

Foundation levelness: Allowable deviation ≤ 3mm over full length. Use a precision level — don’t fake it with a carpenter’s level.
Vibration isolation & embedded plates:
- Embedded plates must be level and coplanar
- Isolation pads or rollers must align with drawings — no offset
Center of gravity & shipping braces: For gas-filled transport, confirm internal bracing is removed before positioning (many accidents happen right here).

Control Point 3: Internal Inspection (Tank Entry)

This is the step most likely to get rushed — and the riskiest.

Do it on a dry, low-humidity day. But on site, schedules always push back. These three rules are non-negotiable:

Dew point control
Ambient humidity ≤ 75%. Maximum exposure time of the active part:
- Humidity ≤65% → max 16 hours
- 65% < humidity ≤75% → max 12 hours
What to check
- Core clamping bolts — tight?
- Lead insulation — damaged or displaced?
- Foreign objects inside (screws, welding slag) — use a strong flashlight + wipe with a clean white cloth
Last step before sealing
Two-person cross-check: no debris, no tools left inside, gaskets seated correctly.

Control Point 4: Vacuum Oil Filling and Hot Oil Circulation

If oil processing is done wrong, the transformer will develop insulation problems within three years.

Step	Control Parameters	Watch Out For
Vacuum pulling	Residual pressure ≤ 133 Pa (for ≥35kV), hold ≥8 hours	Don’t trust pump runtime — read the vacuum gauge
Oil filling	Oil temp 50–70°C, fill rate 3–5 tons/hour	Too fast = trapped bubbles
Hot oil circulation	≥ 24 hours, inlet-outlet temp difference 10–15°C	Stopping early = wasted effort
Settling	≥24h for 110kV class, ≥48h for 220kV class	No meggering or hi-pot testing during settling

Control Point 5: Electrical Testing (Staged)

Don’t wait until everything is fully assembled to test — if something’s wrong, it’s a nightmare to take apart.

Stage 1: Winding & Core Tests (before oil filling)

DC resistance: phase difference ≤ 2%, line difference ≤ 1%
Insulation resistance (2500V megger): absorption ratio ≥ 1.3 (or PI ≥1.5)
Core insulation: typically > 100 MΩ to ground

Stage 2: Insulating Oil Tests (after hot oil circulation)

Breakdown voltage: ≥ 50 kV (for 110kV class and above)
Moisture content: ≤ 15 ppm
Dielectric dissipation factor (90°C): ≤ 0.5%

Stage 3: Pre-commissioning Tests (fully assembled)

AC withstand (80% of factory test value)
Partial discharge measurement: ≤ 100 pC (for 220kV class)
Winding deformation test (sweep frequency response) — no significant change between phases

Control Point 6: Cooling System and Auxiliaries Commissioning

Too many people stare at the main tank and forget the radiators and fans.

Oil pumps and fans
Start each one individually — listen for abnormal noise, measure three-phase current balance (deviation ≤10%)
Buchholz relay (gas relay)
- Mounting direction must be correct (arrow pointing toward conservator)
- Perform trip test: light gas at 25–35ml, heavy gas at specified flow setting (typically 0.6–1.2 m/s)
Oil level gauge and thermometer
Analog outputs must match the SCADA reading (replace if error > ±2°C)

Control Point 7: Energization and Trial Run

The last step — and the most intense. Don’t pack up too soon.

Item	Operation	Why It Matters
Energization (inrush)	5 times, ≥5 minutes between, run 24h after last shot	Checks mechanical strength under inrush and protection misoperation
No-load current	Record three-phase no-load current — deviation from factory value ≤15%	High current may indicate core short circuit
Trial run listening	Use a stethoscope rod against the tank	High-frequency “hissing” = possible partial discharge
Infrared thermography	Check bushing connections, lead joints, radiator inlet/outlet	ΔT >10°C = poor contact

An Overarching Rule That’s Easy to Forget

All key control points ultimately come down to three things:

Cleanliness — Oil piping internals, active part surface, tools. Dirty = hidden trouble.
Traceability — Every vacuum reading, oil temperature, time stamp, and test result must be logged and filed.
No rushing — Vacuum hold time, settling time, hot oil circulation time. One hour short today may cost one year of life later.

Transformer installation and commissioning is basically error-proofing — locking down every possible mistake in advance with procedures. Nail these 7 points, and you’ll avoid at least 80% of post-commissioning failures.