Power Transformer Installation and Operation Guide

This guide serves as a comprehensive technical document for the transportation, installation, acceptance, and initial operation of oil-immersed power transformers. It covers the full lifecycle from factory delivery to successful commissioning, incorporating both manufacturer-specific requirements and applicable industry standards. This document is intended for installation contractors, commissioning engineers, and substation operators involved in power transformer projects.

Applicable Standards:

• GB 50148-2010: Code for Construction and Acceptance of Power Transformers, Oil-immersed Reactors, and Instrument Transformers
• GB 50150-2016: Standard for Hand-over Test of Electrical Equipment
• DL/T 572-95: Power Transformer Operation Code
• GB 26860-2011: Safety Code for Electric Power Industry-Part of Power Plants and Substations
• GB/T 6451-2015: Technical Parameters and Requirements for Oil-immersed Power Transformers

For transformers of 110kV and above, the manufacturer’s factory test records, operation instructions, and other technical documents shall be reviewed and retained as part of the project archive.

2.1 Lifting Requirements

When lifting the main body, all lifting lugs on the tank must be lifted synchronously. The angle between each lifting rope and the perpendicular shall be ≤30°. The product’s center of gravity must align with the vehicle loading center to ensure stability during transit.

2.2 Road Transport Conditions

The construction party shall verify that the route from the public road to the installation site is suitable for transformer transportation. The following conditions must be met:

• Straight road width: ≥3.5m
• Curved road width: ≥18m
• Overhead line clearance: ≥4.6m
• Road gradient: ≤15°
• Road surface must be flat with no straight slopes
• Road must be capable of bearing the total transportation weight.

2.3 Inclination Limits and Speed

During road transport or manual handling, the inclination angle of the vertical axis ≤15°, and the horizontal axis ≤10°. Manual handling speed ≤5m/min, and rail pulling speed ≤5m/min. Braking devices must be disengaged before pulling the transformer on rails, with wheels adequately lubricated.

2.4 Nitrogen-Filled vs. Oil-Filled Transport

Nitrogen-filled transport: Nitrogen dew-points < -40°C, nitrogen purity ≥99.9%, inner tank pressure ≥30kPa. During transit, tank pressure must remain positive and be controlled above 10kPa at normal temperature; replenishment with qualified gas is required if pressure drops.

Oil-filled transport: Fill with qualified oil of the same brand as specified in the outline drawing, keeping the oil line approximately 100mm below the tank top. All valves must be sealed with blind plates; oil leakage is strictly prohibited.

2.5 Impact Recorder Requirements

For transformers equipped with impact recorders, the acceleration g value shall be recorded: the largest vertical value shall not exceed 4g, and other directions shall not exceed 3g. The recording paper shall be kept in the user’s archives after being signed and verified by both parties.

3.1 Site Preparation

Before the transport vehicle reaches the positioning platform, the bearing capacity of the ground must be assessed. Relevant measures shall be adopted to prevent excessive tilting during unloading.

3.2 Lifting for Unloading

Lifting rope shall be tied to the designated lifting lugs for the whole body; tying to tank lugs is strictly prohibited. If the lifting angle exceeds 30°, a lifting beam must be used to ensure safety. If unloading by manual handling, use jack plates on the tank, ensuring each jack bottom receives equal power, then lift synchronously.

3.3 Positioning

The horizontal moving speed of the main body shall be strictly controlled, with each droop height ≤30mm. There is a 1.5% gradient requirement for the top connecting pipe of the transformer; no shimming is needed on the conservator side when positioning.

For wheel-equipped transformers, verify that the screw holes align with the holes of the fixed plates under the tank. Check wheel spacing against rail spacing before installation, then complete the braking device.

4.1 Main Body and Accessories Inspection

Upon arrival, the installation company and user shall immediately inspect the main body and accessories for:

• Specification verification: Check nameplate specifications, type, and quantity against purchase requirements.
• Physical condition: Check for damage, deformation, cracking, or displacement between truck and transformer.
• Accessory quantity: Verify packages and accessories against the transfer list and packing list, including thermometers, bushings, and other components.
• Spare parts: Check spare parts and accessory equipment against contract requirements.

Any damage or abnormalities shall be recorded in detail with on-site photographs. Provide pictures, defect lists, and record copies to the manufacturer and transportation company promptly for cause analysis and timely resolution.

4.2 Insulation Oil Acceptance

Visual inspection of insulation oil upon arrival to check for foreign material mixing. Test results must meet:

• Breakdown voltage: ≥40kV/2.5mm
• Water content: ≤20ppm

Different brands of transformer oil shall be stored and kept separately.

5.1 Nitrogen-Filled Product Storage

For nitrogen-filled products, immediately check that the nitrogen pressure meets the requirement (≥10kPa at normal temperature). If stored beyond 15 days, oil filling is required. The residual oil at the tank base must meet breakdown voltage ≥40kV/2.5mm and water content ≤20ppm.

During nitrogen-filled storage, inspections shall be conducted at least twice daily, with records of tank pressure and nitrogen additions. If pressure drops rapidly, check for leakage and address it promptly to prevent moisture ingress.

5.2 Oil-Filled Product Storage

For oil-filled products, take oil samples upon arrival and test. Performance must meet: breakdown voltage ≥40kV/2.5mm. If the oil sample fails, determine whether moisture has entered and contact the manufacturer.

5.3 Component Storage

• Removal parts (conservator, radiator, cooler): Protect against rain, dust, and fouling. Storage sites must be solid and approximately 100mm above surrounding ground; bottom should be heightened about 200mm to prevent rust.
• Bushings and accessories (measuring instruments, relays, terminal boxes, control cabinets, on-load tap changer mechanisms, cables, seals, etc.): Store in dry, ventilated rooms.

5.4 Dampness Determination

A transformer is considered not damp if the following conditions are met (tested after oiling):

• Insulation resistance value ≥70% of factory data
• Core-to-ground insulation resistance ≠0
• Moisture content in nitrogen ≤100ppm
• Moisture content in oil ≤20ppm

If these requirements cannot be met, the transformer shall not be installed or operated. Contact the manufacturer for guidance.

6.1 Pre-installation Checks

• Verify accessories are complete and in good condition. Pre-installation tests include: dielectric dissipation factor of oil-paper condenser bushings, polarity and ratio of bushing current transformers, gas relay inspection and adjustment, and temperature controller inspection and adjustment.
• Prepare oil filling, draining, and filtering equipment. Verify sufficient quantity and quality of transformer oil.
• Prepare installation devices: vacuum oil filter (oiling machine), oil tank, oil tubes, crane, and other tools.

6.2 Equipment and Tools Required

6.3 Environmental Requirements for Core Lifting Inspection

• Humidity <65%: exposure time of active parts ≤16h
• Humidity 65%–75%: exposure time of active parts ≤12h
• Ambient temperature ≥5°C; body temperature should be 5–10°C above ambient
• If ambient temperature <0°C, raise active parts temperature above 10°C by hot oil circulation in advance
• Humidity ≥75%: core lifting inspection or nitrogen drainage shall not be performed

7.1 Preparation for Cover Lifting

If the contract specifies no need to lift the tank cover, this operation shall not be performed. However, if the transformer experienced impact or unexpected accidents during transportation, core lifting inspection shall be conducted regardless.

Before performing lifting tank cover inspection:

• Complete all work routines and requirements as per preparation guidelines
• Loosen or remove mechanical connections between the body and oil tank
• Disassemble on-load tap changer according to its instruction manual

7.2 Lifting Procedure

Disassemble joint bolts of the oil tank edge. According to assembly drawings, select the appropriate sling to hang in the clamping unit of the lifting lug. Perform intermittent trial lifting to adjust hook position so that the center of gravity aligns with the same vertical line. After successful trial, lift the oil tank and place it on cleaned supports.

7.3 Internal Inspection Checklist

• Overall condition: Whether the body has any displacement or serious distortion
• Top pad of winding: Check for any movements
• Lead wires: Check for open welds or breaks; verify lead wire insulation remains in good condition
• Press bolts and screws: Check for any loosening
• Standard fasteners: Check for any loosening
• Oil tank and body interior: No obstacles or water allowed
• Tap changer contact: Check contact with wire connections; ensure proper tightening
• Core grounding: Verify core is grounded at one point and insulation is in good condition

7.4 Tool Control During Internal Inspection

All tools used by inspectors must undergo ID registration. Personnel pockets are not allowed to contain metal objects. Dust and debris are strictly prohibited from entering the tank. After completion, operating personnel must confirm no debris remains inside, and all tools shall be checked against the registration.

7.5 Tank Reassembly

Clean residual oil from the bottom tank and verify no tools or debris remain. Check that the sealing gasket is intact and correctly installed. Lifting of the oil tank must be stable; lowering speed should be gentle to avoid bumping leads and the transformer body. As the oil tank approaches the gasket, use sticks to guide bolt alignment. After verifying gasket placement, insert all bolts and fasten diagonally in stages to achieve even tightness.

8.1 General Installation Provisions

Count all components to be installed (including bolts, nuts, washers). Carefully inspect, clean accessories, and remove obstacles, dirt, and dust from joint tubes.

Parts with installation signs (high-voltage, medium-voltage, low-voltage bushings, pedestals, pressure release devices) must follow instructions for reinstallation, paying attention to relative position and angle to the tank.

For 110kV and above, lead wire insulation inclined tips must be packed into the voltage-sharing ball mouth of the bushing.

When assembling conservator joint pipes, start from the gas relay side. Fasten bolts incompletely until all parts are assembled, then begin tightening from the gas relay side.

Thermometer pedestals should be filled with 2/3 transformer oil.

For components with air drain plugs (LV bushing, gas relays, oil conservator, radiators, etc.), open the upper air drain plug after oil injection to release accumulated air; do not close until oil overflows.

Before sealing gaskets are placed, keep metal surfaces clean. Sealing surfaces must be grease-sealed with the sealing gasket correctly seated. Fasten bolts diagonally with uniform stress.

8.2 Bushing Installation

Key considerations:

• Crane location should facilitate bushing lifting and installation
• Use nylon rope or wire rope with protection sleeve; prevent crashing with packing containers or other objects
• Check oil leakage after removing bushing from box; verify oil level in oil meter is normal
• Clean flange surface and interior of bushing pedestal with clean white cloth before installation
• Bushing slant angle must match pedestal slant angle

For 110kV and above condenser bushing installation:

• Remove top terminal block and covers before lifting
• After bushing is placed in the mounting pedestal, observe through tank wall holes to confirm the cone tip of lead wire is pressed into the voltage-sharing ball
• Ensure inner lead wire is straight with no twisting or bending
• Avoid dead lifting when straightening lead wire to prevent insulation damage or wooden clip pull-out

8.3 Cooling Equipment Installation

Butterfly valves should be in the off state when dismounting transportation sealing plates. Coolers have been checked for leakage under pressure and oil-washed before ex-work. Open the transportation sealing plate to check interior cleanliness; determine if additional oil-washing is needed. Install cooler according to part numbers and installation marks. When lifting, keep the cooler upright and move it into position, then assemble with tank connecting pipes.

8.4 Pressure Relief Valve Installation

Remove transportation sealing plates and installation hole covers. Wipe inner walls with white cloth until no smudge remains. Install on flange; require good sealing and reliable installation. If equipped with a cover, the oil outlet should be directed toward open ground. If equipped with an oil-guiding pipe, install it accordingly.

8.5 Gas Relay Installation

Check appearance is in good condition and test signal contact and trip contact for reliability. Remove transportation sealing plates and clear installation flange. Remove cotton rope securing the float cup during transport. Install the gas relay between the oil tank and oil conservator, following the arrow direction on the device (arrow pointing to oil conservator). Install pipes for all bushing pedestals so that all inner gas collects into the connecting pipe to the gas relay.

Note: The transformer top cover shall have a slope of 1.0%–1.5% along the gas relay flow direction to facilitate gas collection and alarm activation.

8.6 Conservator and Moisture Absorber (Breather) Installation

Lift conservator bracket and fix it to the tank bearing with bolts. Install the conservator on the bracket. Connect pipelines and install the gas relay on the butterfly valve beneath the conservator. Install gas outlet, oil inlet, and oil outlet pipelines with stop valves at bottom ends. Install moisture absorber connecting pipe, then install the breather at the bottom end. The sealing gasket for storage and transport must be removed. Fill the oil sealing box with transformer oil so that the oil level immerses the blocking air ring.

8.7 Thermometer Installation

The thermometer holder is located on the oil tank cover near the conservator. Fill a small amount of transformer oil into the holder before thermometer installation. Screw the thermometer into the holder gently. For pressure gauge thermometers, coil redundant conduits like a tray and fix to the tank wall.

8.8 On-Load Tap Changer Installation

After installing the oil tank, check and clean the selector switch oil chamber again, then install the oil suction pipe. Lift the tap changer with a special hanger plate, reconnect the middle flange and support flange, and ensure good sealing. Carefully place the selector switch into the oil chamber, rotating the insulation axle gently to align the coupling and ensure correct positioning. Install the position indicator and locking plate, then reassemble the switch head cover board. Operate positive and reverse change by hand to verify symmetry, then by motor. Check each tap position to ensure the tap indication and motor device indicator match.

9.1 Oil Quality Requirements

Transformer oil shall be refined at the plant to minimize on-site refining workload. For long-distance transportation or long-term storage, oil drums shall be used. Before oiling, an oil analysis test must be conducted. For vacuum injection, the oil must meet the following standards:

Note: For 330kV–750kV transformers, water content shall not exceed 10mg/L; for 110(66)kV, water content shall not exceed 20mg/L; for 220kV, water content shall not exceed 15mg/L.

9.2 Vacuum Filling Procedure

Before vacuum filling, all equipment grounding points and connection pipes must be reliably grounded. Oil should be injected from the lower oil valve. Vacuum pumping shall be maintained throughout the filling process, with residual pressure ≤20Pa. The oil temperature shall be higher than the body temperature; oil outlet temperature from the filter shall be 55°C–65°C. Filling speed shall not exceed 100L/min. During the entire process, vacuum filter inlet and outlet pipes shall not be switched in open-air conditions.

9.3 Vacuum Hold and Oil Level

Stop filling when the oil level is 100–200mm below the tank top. Maintain vacuum: for 110kV transformers, hold vacuum for at least 2 hours; for 220kV transformers, for at least 4 hours.

9.4 Post-Filling Air Removal

After oil filling, open air-bleeder plugs on coolers, bushings, gas relays, conservators, and radiators to release trapped air. Do not close the air drain plugs until oil overflows. For forced oil circulation transformers, start all cooling devices and circulate for at least 4 hours to fully remove residual air.

9.5 Hot Oil Circulation

For 330kV and above transformers, hot oil circulation is mandatory after vacuum filling. The hot oil circulation process uses a high-vacuum filter, with outlet oil temperature not below 50°C nor above 80°C; tank interior temperature not below 40°C. Circulation time: 220–330kV transformers not less than 24 hours; 500kV transformers not less than 48 hours. The actual circulation time should allow the entire oil volume to circulate at least twice.

9.6 Oil Level Adjustment

After filling, verify the oil level indicator at regular intervals. The oil level shall match the marks on the conservator corresponding to the ambient temperature. 1–2 hours after filling, check the oil level again. If the oil height has dropped significantly, add oil to bring it to the matching temperature line.

9.7 Static Settling

After filling, the transformer shall undergo a static settling period to allow oil to fully settle and remove suspended impurities and bubbles. For 110kV transformers, static settling time ≥24 hours; for 220kV transformers, ≥48 hours.

Transformer oil is a flammable liquid. Smoking and open flames are strictly prohibited at the installation site. If gas welding or electric welding is required, fire prevention measures must be strictly observed. Fire protection equipment shall be set up before installation, with designated personnel in charge. Paper sheets and cotton cloth immersed in transformer oil shall be collected with waste oil and disposed of uniformly after installation.

Additional safety measures:

• Before installation, the commander-in-chief and safety principal shall convene all installers for work safety meetings, provide safety education, and declare safety rules
• Irrelevant personnel must not enter the installation field
• Tools and materials passed up and down shall be tethered; no throwing allowed
• When installing or removing parts prone to falling at height, two persons shall work together
• When using mobile lighting, 220V lamps shall not be taken into the oil tank, nor shall temporary power be laid with poorly insulated conductors

11.1 Pre-Commissioning Checks

Before the transformer is put into trial operation, a comprehensive inspection shall be conducted:

• Main body and accessories fully assembled and in good condition; phase positions and markings correct
• Oil tank and all parts wiped clean, no oil leakage
• Conservator and bushing oil level indicators clear and accurate, with differences within measurement error
• Cooling devices operate normally; oil pumps and fans run correctly; control circuits function properly
• Gas relay installed in correct direction; oil level normal; gas fully drained
• Moisture absorber filled with allochroic silica gel; oil cup has clear oil at suitable level
• Tap changer operates flexibly; target indication accurate; three-phase tap positions identical
• Core grounding lugs, oil tank grounding, bushing grounding, and ground shield complete and reliably grounded
• No-load terminals of bushing-type current transformers short-circuited and grounded

Phasing check: Transformer phase and winding connection group shall meet parallel operation requirements before energization.

11.2 Electrical Tests

All electrical tests shall be conducted per GB 50150-2016 before commissioning. The mandatory test items include:

For 66kV and above transformers, dissolved gas chromatographic analysis of oil shall be performed three times: after oil filling and settling, 24 hours after withstand and partial discharge tests, and 24 hours after impact closing and operation at rated voltage. Total hydrocarbon content in new transformer oil shall not exceed 20μL/L, H₂ not exceed 10μL/L, and C₂H₂ not exceed 0.1μL/L.

11.3 Impact Closing Test

The transformer shall undergo 5 times of no-load full-voltage impact closing. After the first energization, the duration shall not be less than 10 minutes. The magnetizing inrush current shall not cause protection device misoperation. For neutral-point-grounded systems, the neutral point must be grounded during impact closing.

11.4 Trial Operation

Following successful impact closing, the transformer shall be placed into trial operation, typically running at the maximum load available from the system at that time for 24 continuous hours. After 24 hours of successful operation with no abnormalities, the trial run is considered complete, and the transformer can be formally placed into service.

11.5 Documentation to Be Handed Over

Upon acceptance completion, the following documents shall be transferred:

• Installation technical records, core inspection records, drying records, quality inspection reports, and electrical hand-over test reports
• Construction drawings and design change documentation
• Manufacturer’s product instruction manual, test records, certificate of conformity, and installation drawings
• Lists of spare parts, accessories, special tools, and test instruments

This guide focuses on large power transformers in substations and industrial plants. For other transformer configurations commonly found in distribution networks and outdoor environments, please consult the corresponding documents below.

• Substation Transformer Installation and Operation Guide
• Pole Mounted Transformer Installation and Operation Guide
• Pad Mounted Transformer Installation and Operation Guide
• Dry Type Transformer Installation and Operation Guide
• Compact Substation Installation and Operation Guide

Proper installation and commissioning are critical to the safe and reliable long-term operation of power transformers. This guide consolidates manufacturer requirements with applicable national standards. All personnel involved need strictly adhere to the procedures described herein, maintain thorough documentation throughout the installation process, and consult the manufacturer when any uncertainty arises.