How Long Does It Take to Replace a 35kV Oil-Immersed Power Transformer?

Time：2026-01-15    Auther：ZTelec-www.ztelectransformer.com

In power system operation and maintenance, the 35kV oil-immersed power transformer is a core piece of equipment in distribution networks and industrial power systems. When transformers age, malfunction, or no longer meet system capacity requirements, timely replacement is essential to ensure power supply safety and stability. A common question for operators and maintenance teams is: how long does it actually take to replace a 35kV oil-immersed transformer? This article provides a systematic analysis to help users plan power outage schedules and construction timelines efficiently.

Overall Timeline for Replacing a 35kV Oil-Immersed Transformer

Under standard conditions, the replacement process for a 35kV oil-immersed transformer—from preparation to commissioning—typically takes approximately 3 to 15 days. The wide range depends on project complexity, transformer capacity, site conditions, and power supply system characteristics. The replacement process generally includes old equipment removal, new transformer transportation, on-site installation, testing and inspection, and final power transmission.

Key Factors Affecting Replacement Time

Transformer Capacity and Weight

Transformers of smaller capacities (below 10,000 kVA) are generally faster to replace due to easier handling and lighter weight. For large transformers above 20,000 kVA, heavy-duty hoisting equipment is required, which significantly increases construction time.

Site Conditions

Installation site conditions strongly influence replacement duration. Key considerations include whether the site is an outdoor substation or an indoor distribution room, space constraints along hoisting paths, and whether the equipment foundation requires modification. Replacing a transformer with the same specification and model is typically the fastest, while model or size changes require secondary design and planning work.

Power Supply System Complexity

The complexity of the power supply system also affects replacement time. Single-circuit systems are simpler and faster to manage, while multi-circuit switching or networked systems require careful coordination and temporary power arrangements to maintain safety and reliability.

Standard Replacement Process of a 35kV Oil-Immersed Transformer

The typical replacement process includes the following stages:

Power Outage and Safety Isolation

Planned power outage and proper safety isolation procedures must be conducted before beginning the replacement work to ensure personnel and equipment safety.

Draining and Removal of the Old Transformer

Oil must be drained from the old transformer to safely remove it from the site. Proper handling and storage of transformer oil are critical to environmental compliance and safety.

Hoisting and Positioning of the New Transformer

The new transformer is lifted and positioned using cranes or specialized hoisting equipment. Precision in placement ensures mechanical stability and alignment with existing busbars or foundation points.

Accessory Installation

Cooling systems, bushings, and protective devices are installed. All auxiliary components must be checked for compatibility and correct installation before proceeding.

Vacuum Oil Filling and Settling

Vacuum oil filling is a critical step for insulating the transformer and preventing air bubbles. The settling and defoaming process ensures the transformer oil reaches operational stability. This stage typically requires 12 to 24 hours and is crucial for long-term reliability.

Electrical Testing

After installation, comprehensive tests are performed, including insulation resistance measurement, DC resistance, turning ratio verification, oil withstand voltage test, and partial discharge detection if necessary. These tests are mandatory to ensure operational safety and compliance with standards.

Power-On Commissioning

Once all testing is complete and the transformer meets design criteria, it is energized and commissioned. Operational parameters are monitored during initial power-on to confirm system stability.

How to Effectively Shorten Replacement Time

Refined Pre-Project Management

Begin equipment bidding and production immediately after project confirmation. Conduct on-site surveys and simulation exercises to ensure all tools, equipment, and personnel are ready. Within safety regulations, coordinate multiple teams to work simultaneously for different stages of the replacement.

Selecting a Professional Team

Experienced construction teams can efficiently handle on-site challenges, avoid rework, and ensure adherence to procedures. Using advanced machinery such as vacuum oil filters and high-efficiency hoisting equipment also improves replacement speed and reliability.

Adopting Modular or Prefabricated Solutions

For critical loads, temporary power solutions like mobile substations or transformer trucks allow “uninterrupted power replacement.” Prefabricated components such as cable heads, busbars, and connectors reduce on-site work time and improve efficiency.

Key Tests Required After Replacement

Before a 35kV oil-immersed transformer can be officially commissioned, the following tests must be completed:

Insulation resistance measurement, DC resistance test, voltage ratio and phase test, oil withstand voltage test, and partial discharge detection if applicable. Each test ensures the transformer is safe and ready for operation.

Frequently Asked Questions

Q1: Can a 35kV oil-immersed transformer be replaced without a power outage?

No. Main transformer replacement must be conducted during a planned power outage or with a backup power supply switching plan in place.

Q2: Is it necessary to drain the oil during replacement?

Yes. Oil-immersed transformers require draining or partial pumping of oil to safely remove and handle the old transformer.

Q3: Can the new transformer be installed immediately after delivery?

No. After delivery, visual inspection, oil sample testing, and verification of factory data must be completed before installation.

Replacing a 35kV oil-immersed power transformer is a complex process influenced by transformer capacity, site conditions, power system configuration, and required testing standards. With proper planning, professional teams, and prefabricated solutions, the replacement process can often be completed efficiently within 2 to 3 days for standard transformers. However, critical stages like vacuum oil filling and settling must not be rushed, as they are essential for operational reliability. Understanding these factors allows operators to plan outages, reduce downtime, and ensure safe, stable power supply.