Dry Type Transformer Installation and Operation Guide

Introduction

This guide provides comprehensive instructions for the installation, operation, and maintenance of dry-type power transformers. It is applicable to non-excitation dry-type power transformers and on-load regulation epoxy resin dry-type power transformers with capacities up to 6300 kVA and voltage ratings up to 35 kV. The instructions contained herein cover transportation, storage, installation, pre-service inspection, testing, operation, and maintenance.

Applicable Standards: Dry-type transformers covered in this guide are designed and manufactured in accordance with the following standards: GB 6450-86 Dry-type Power Transformers, GB 10228-1997 Technical Parameters and Requirements for Dry-type Power Transformers, and GB 1094.1-1996 Power Transformers-Part 1: General. For international applications, reference may also be made to IEC 726 and the latest GB/T 1094.11-2022 *Power Transformers-Part 11: Dry-type Transformers*.

Dry-type transformers are particularly well-suited for applications where fire safety is a primary concern, including high-rise buildings, commercial centers, sports venues, petrochemical facilities, subway systems, airports, and other locations with stringent fire protection requirements.

2.1 Normal Service Conditions

Dry-type transformers are designed for operation under the following standard conditions:

When ambient temperatures exceed 40°C or altitudes exceed 1000 m, the transformer rating must be adjusted in accordance with GB 6450 requirements.

2.2 Protection Classes

Dry-type transformers are available with three standard protection classes:

IP00: No enclosure protection; suitable for installation within dedicated electrical rooms or switchgear assemblies.

IP20: Provides protection against solid foreign objects larger than 12 mm in diameter. This is the most commonly used indoor enclosure, effectively preventing accidental contact with live parts and preventing small animals (rodents, snakes, cats, birds, etc.) from entering and causing short circuits.

IP23: In addition to IP20 protection, IP23 enclosures prevent water ingress from droplets falling at an angle up to 60° from vertical. This makes them suitable for outdoor installations or environments with light water spray exposure. However, note that IP23 enclosures reduce transformer cooling capacity, requiring derating considerations.

2.3 Cooling Methods

Two cooling methods are available for dry-type transformers:

AN (Air Natural): Natural convection cooling without forced airflow.

AF (Air Forced): Forced air cooling using fans. When operating in confined or poorly ventilated spaces, AF fittings must be added. The required supplementary air volume is 4 m³/min per 1 kW of total loss (P₀ + Pₖ).

Proper transformer selection is critical for system reliability and energy efficiency.

3.1 Determining Rated Capacity

The transformer’s rated capacity (kVA) should be determined based on the calculated load of the connected equipment. First, calculate the actual load by summing equipment power ratings and applying a diversity factor. The rated capacity should then be selected to slightly exceed this calculated load to accommodate future load growth.

3.2 Key Performance Parameters

When selecting a transformer, the following parameters should be carefully evaluated:

Voltage Ratio: Must match the supply voltage and the voltage requirements of the connected equipment. For example, a 10 kV supply serving 0.4 kV loads requires a 10 kV/0.4 kV transformer.

No-load Loss and Load Loss: These parameters directly impact energy efficiency. Lower values indicate better efficiency and reduced operating costs.

Impedance Voltage: Affects voltage regulation and short-circuit current levels.

Noise Level: For installations in noise-sensitive environments, low-noise models (typically <55 dB) are preferred.

4.1 General Transport Requirements

Transformers may be delivered via railway, shipping, motor vehicle, or air transport. Transport vehicles and containers must be kept clean and dry.

4.2 Loading and Securing

Transformers must be securely fixed within transport vehicles using trailers, clamps, and joint hinges.

Do not bind or haul the coil, insulator, plates, or leads, as this may damage the product.

The inclination of the transformer during transport must not exceed 30°.

4.3 Stacking and Storage During Transit

Do not stack transformers during transfer or at terminal points.

Wooden crossbeams (crossies) must be placed beneath the transformer, with a height not exceeding 100 mm.

4.4 Accompanying Documents and Accessories

All accessories, spare parts, and ex-works documentation shall be delivered together with the transformer.

Upon receipt of the transformer, the customer shall perform the following acceptance checks immediately:

5.1 Nameplate Verification

Verify that the data described on the nameplate (rated capacity, voltage ratings, tap positions, vector group, impedance voltage, etc.) conforms to the specifications stated in the purchase contract.

5.2 Documentation Check

Verify that all ex-works documents (certificate of conformity, test reports, packing list, general outline drawing, etc.) are complete and conform to contract requirements.

5.3 Accessories and Components

Check all dismantled parts and accessories against the packing list to confirm there is no damage or missing items.

5.4 Physical Condition Inspection

Inspect the transformer and its components for any visible damage or displacement.

Check connection lines for looseness or rupture.

Inspect insulators for cracks or other damage.

Remove any wooden crossbeams if present.

5.5 Storage After Inspection

If the transformer is not to be placed into service immediately after inspection, it must be repacked and stored appropriately.

6.1 Storage Conditions

Products and accessories requiring storage should not have their cases opened except for inspection and acceptance. After inspection, repack immediately.

For long-term storage, place transformers and accessories in a dry, clean, and well-ventilated room.

Do not store chemicals or corrosive substances in the same room.

Do not stack products.

6.2 Indoor vs. Outdoor Storage

Dry-type transformers should be stored indoors. If short-term outdoor storage is unavoidable due to site constraints:

Do not open the casing.

Check and maintain the cover for waterproofing.

Place crossbeams beneath the transformer (height ≤ 100 mm).

6.3 Storage Duration Limits

Outdoor storage shall not exceed two months. Ambient temperature during outdoor storage must not fall below -10°C.

7.1 Pre-Installation Preparation

Before commencing installation:

Study the installation specifications, nameplate data, and general outline drawing carefully.

Prepare appropriate lifting equipment (cranes, forklifts, truck cranes) and rigging.

7.2 Lifting and Handling

Proper lifting procedure is critical to prevent equipment damage:

Lifting must be performed using all lifting rings on the transformer simultaneously-never lift by individual rings alone.

When lifting an unpackaged transformer or removing it from its package, the lifting angle must not exceed 60°.

For transformers shipped with on-load tap changers (OLTC) attached, lift using the transformer lifting rings only-do not lift by the OLTC.

When the center of gravity is offset, adjust the length of lifting wires accordingly to maintain balance.

Handle with care throughout the assembly process.

7.3 Site Layout and Clearances

7.4 Minimum Safety Distances (High-Voltage Coil to Ground)

The minimum safety distance between energized conductors and ground must comply with GB 10237 Insulation Level and Insulating Test-External Insulation Air Clearance:

For 12 kV systems, the air insulation net distance should be ≥125 mm; for 40.5 kV systems, ≥360 mm. The creepage distance shall be ≥18 mm/kV for porcelain insulation and ≥20 mm/kV for organic insulation.

7.5 Foundation and Mounting

Transformer foundations shall have embedded anchor bolts positioned in accordance with the general outline drawing to align with the transformer’s mounting holes.

Concrete foundations shall have a bearing capacity ≥1.5 times the transformer weight, with horizontal level deviation ≤2 mm/m.

For seismic or vibration-sensitive installations, rubber vibration-damping pads (thickness ≥10 mm) shall be installed at all four corners.

For transformers equipped with idler wheels, the wheels can be rotated 90° in the turning direction. After positioning, wheels must be removed or locked in place, and the transformer fixed via anchor bolts.

The trailer (shipping base) shall be dismantled after positioning.

7.6 Cable and Busbar Connections

Incoming and outgoing cables or busbars shall be properly supported and clamped to prevent mechanical stress on the transformer terminals.

Copper busbar joints must be polished and coated with conductive grease before connection.

Torque requirements for fasteners:

7.7 Grounding

Proper grounding is essential for safety:

The transformer enclosure, core, and neutral point must be independently grounded.

Grounding conductors shall use copper cable with cross-sectional area ≥50 mm².

Grounding resistance shall be ≤4Ω.

Before placing the transformer into service, the following inspections shall be performed:

8.1 Mechanical Inspection

Check all fasteners and connectors; re-tighten as necessary.

Verify that all components are properly installed and secured.

Remove any foreign materials or accumulated dust (refer to Section 11.2 for cleaning procedures).

8.2 Cooling System and Temperature Controller Inspection

Check fans to ensure normal operation.

Critical check: Verify the rotation direction of three-phase transformer fans. The airflow must come from beneath the coil. If the direction is incorrect, change the phase sequence in accordance with specifications.

Check temperature controllers and temperature display units for normal operation.

Ensure proper grounding of temperature controllers and accessories in accordance with their specifications.

Verify that temperature control sensors are properly inserted into the winding temperature measurement holes (insertion depth ≥80 mm).

8.3 Default Temperature Controller Parameters

Typical default parameters for dry-type transformer temperature controllers are:

The following tests shall be performed before the transformer is placed into service:

9.1 DC Resistance Measurement

Measure DC resistance of windings at all tap positions to verify proper contact and identify any loose connections.

9.2 Voltage Ratio and Vector Group Test

Measure voltage ratio at all tap positions and verify the vector group designation.

9.3 Grounding Verification

Check that the transformer body and core are permanently grounded.

9.4 Insulation Resistance Measurement (Windings)

Test conditions: Temperature 20–30°C, relative humidity ≤90%

Insulation resistance values will decrease in humid conditions. Under damp conditions, the resistance value should be no less than 2 MΩ per 1000 V of rated voltage (reading taken at 25°C for one minute). For transformers that have become damp or show signs of condensation/frost, drying treatment must be performed before service and withstand voltage testing, regardless of measured winding resistance values.

9.5 Core Insulation Resistance Test

Disconnect the core grounding plate before testing.

Test conditions: Temperature 20–30°C, relative humidity ≤90%

Under humid conditions, measured values will decrease. After drying treatment, the resistance value shall be ≥0.1 MΩ before placing the transformer into service.

9.6 On-Load Tap Changer (OLTC) Inspection

For transformers equipped with OLTC, inspect and test in accordance with the OLTC manufacturer’s specifications before operation.

9.7 Power Frequency Withstand Voltage Test

Apply power frequency withstand voltage at 85% of the factory test voltage.

9.8 Acceptance Test Summary

Upon completion of all tests, the following acceptance documentation shall be prepared: test reports (including partial discharge test results, ≤10 pC), installation records, and temperature controller commissioning data.

10.1 Tap Position Adjustment (Non-Excitation)

For transformers with off-circuit tap changers (e.g., voltage rating 10,000 V ± 2 × 2.5%):

If the local power system voltage is 10 kV, connect taps at position 4–5.

If the supply voltage exceeds 10,500 V, adjust to the higher tap position after disconnecting high voltage.

If the supply voltage is below 9,500 V, adjust to the lower tap position after disconnecting high voltage.

All tap adjustments must be performed with the high voltage disconnected.

10.2 OLTC Operation

For transformers with on-load tap changers, refer to the specific OLTC specification. The transformer may be placed into service only after the OLTC is correctly adjusted. Disconnect power before making any adjustments.

10.3 Temperature Controller and Display

For transformers equipped with temperature controllers and temperature displays:

First energize the transformer, then power the temperature controller and display.

Configure the equipment according to the corresponding temperature controller specifications.

10.4 Initial Energization

Perform no-load tripping-closing operations. The highest tripping-closing inrush current peak may reach 8–10 times the rated current-this is normal.

Gradually increase load from small to large while monitoring for abnormal sounds.

10.5 No-Load Operation

No-load operation shall comply with IEC 905 Regulation for Dry-Type Power Transformers. Short-term overload operation is permissible based on ambient temperature and initial load conditions.

10.6 Overload Capability

When ambient temperature is lower than the rated value, output power can be appreciably higher than rated values, and vice versa.

The overload capability of dry-type transformers depends on ambient temperature, pre-load conditions, insulation heat dissipation characteristics, and thermal time constant. Overload curves should be obtained from the manufacturer for specific applications.

10.7 Special Operating Conditions

Dry-type transformers can be placed into service immediately after initial energization without special treatment.

However, if the transformer has been operating in damp air or shows signs of condensation/frost, drying treatment must be performed before subsequent operation.

10.8 First Energization Procedure

Perform no-load energization three times, with 5-minute intervals between each operation, and monitor inrush current.

Operate with load ≤30% for 24 hours to verify performance.

Winding temperature rise shall not exceed 100 K for Class F insulation.

11.1 Inspection Frequency

Newly commissioned transformers should receive their first inspection after 2–3 months of operation, followed by annual comprehensive inspections thereafter. Comprehensive overhaul is recommended every 3–5 years, or as advised by the manufacturer based on equipment condition.

11.2 Cleaning

Check and clean accumulated dust to ensure proper ventilation and prevent insulation breakdown.

Pay special attention to insulators and lower padding blocks.

Use compressed dry air to blow out pipes and cooling ducts (compressed air pressure ≤0.2 MPa).

11.3 Visual Inspection

Check the following items during each inspection:

Fasteners, connectors, and conductor parts for rust or corrosion.

Insulating surfaces for creepage marks and carbonization.

Support insulators for cracks or discharge marks.

Cooling fans for normal operation.

High and low voltage connections for overheating signs.

Core and winding for discoloration, odor, or other abnormalities.

11.4 Periodic Insulation Testing

During scheduled maintenance (approximately every five years), measure insulation resistance in accordance with Sections 9.4 and 9.5 to determine whether the equipment can continue in service. Routine insulation resistance testing is also recommended every 6–12 months.

Dry type transformers are commonly used indoors and in moisture‑sensitive environments. For outdoor, oil‑filled, or pole‑mounted applications, please refer to the following complementary installation and operation guides.

• Substation Transformer Installation and Operation Guide
• Power Transformer Installation and Operation Guide
• Pole Mounted Transformer Installation and Operation Guide
• Pad Mounted Transformer Installation and Operation Guide
• Compact Substation Installation and Operation Guide

13.1 Enclosure and Guarding

Transformers operated without an enclosure must be installed with a separate protective fence or guard to prevent unsafe accidents.

For outdoor or semi-outdoor installations, fixed guardrails of at least 1.5 m in height must be provided around the transformer, with a minimum clearance of 0.5 m between the transformer outline and the guardrail.

13.2 Prohibited Actions

No person may touch the transformer while it is energized. All electrical work must be performed with the transformer properly de-energized, locked out, and tagged out.

13.3 Qualified Personnel

All tests, insulation checks, and maintenance work on the transformer shall be performed by trained and qualified electrical professionals.

13.4 First Aid Preparedness

An emergency response plan, including first aid procedures for electrical shock, shall be established and personnel trained accordingly.