CONSIDERATIONS FOR REPLACING TRANSFORMERS
Should a transformer show signs of aging, there are many steps in the replacement process that need to be considered to minimize facility downtime and provide an efficient installation. Without detailed planning, improper transformer physical designs, inadequate ancillary systems and unrecognized site conditions can lead to unanticipated schedule and cost impacts.
CAPITAL COST DEVELOPMENT
When planning a major transformer replacement, it is important to recognize ancillary costs that are in addition to the basic cost of the transformer itself. Even turnkey supplier installations will require additional scope that must be executed by others. Some of the scope items that could represent additional costs include:
- Disposal of the existing transformers and oil.
- Facility owner project management, safety and technician support.
- Fire protection deluge system modifications or upgrades.
- Integration of new advanced transformer monitoring instrumentation into the facility control system.
- Isolated phase bus modifications and/or refurbishment.
- Third-party electrical testing.
- Transformer protection modifications or upgrades due to new transformer characteristics, such as megavolt-ampere (MVA) rating, impedance, etc.
MANUFACTURING LOCATION AND TRANSPORTATION
An important part of the transformer procurement process is identifying where each bidder’s transformers are manufactured to evaluate risks associated with shipping logistics. Many high-quality transformers are manufactured both domestically and internationally, but shipping methods and the associated logistics need to be considered early in the procurement cycle. Understanding logistics and planning accordingly can help to maintain delivery schedules and minimize potential outage delays.
Special care should be taken during the transformer design phase when reviewing physical arrangements submitted by the manufacturer. It is common for many transformers to have fixed primary and/or secondary bushing interfaces, such as isolated phase or nonsegregated bus duct, that can require costly, time-consuming field modifications if new bushing heights or locations do not align with field conditions. Control panel locations might be fixed in situations where auxiliary power and control conduits are embedded in the existing transformer foundation. To minimize potential design errors and mitigate overall risk it is important to perform a detailed 3D scan of the existing transformer area to obtain safe, accurate measurements without deenergizing the transformer. These measurements are critical in performing a proper review of the new transformer’s physical design characteristics.
SUPERVISION AND SAFETY
Facility owners should consider requiring the manufacturer to supply on-site supervision for the duration of the installation effort, including project managers and/or superintendents responsible for subcontractor coordination and site safety. Many turnkey solutions provided by manufacturers will require demolition, heavy-haul and electrical subcontractors. This approach provides the owner with a single point of contact for any coordination or issues that may arise and simplifies the installation process.
FACILITY SPACE CONSTRAINTS
While transformer replacements are relatively short in duration, they require the use of a large amount of space during installation, typically in high-traffic areas of the facility. Space will be needed for the following and should be clearly identified on a site plan of the facility outage:
- Crane staging area
- Heavy-haul rig parking for offloading
- Laydown space for transformer accessories (bushings, conservator tank, radiators, etc.)
- Space for oil processing rigs and equipment
For facilities where space is at a premium, such as hydroelectric plants, alternate installation methods may need to be considered. Where a crane cannot be staged in a convenient location and is unable to reach the existing transformer pad, the use of a “jack-and-slide” installation technique may be required, using hydraulic skidding systems to push the transformer into place.
Where transformer deluge systems are required, special consideration should be made early in the procurement process regarding whether the existing deluge piping should be reused or replaced. In many cases, new transformer geometries will require a complete redesign and replacement of the deluge piping system to maintain proper spray coverages. At a minimum, the deluge piping will need to be removed in preparation for the installation of the new transformer(s). Additionally, modern firewall requirements should be reviewed and oil containment volumes analyzed if the new transformer oil quantity data differs from the existing to confirm that fire protection measures comply with the latest National Fire Protection Association (NPFA) standards. NPFA 851 provides fire protection recommendations and best practices for large power transformer installations at hydroelectric generating facilities and should be used for guidance through this process.
Many replacement transformers that are larger in capacity due to unit uprates, or of a more robust design, may weigh more than the units being replaced. In this case, a structural evaluation must be performed to determine whether foundation modifications are required. For facilities where sister units must remain online during a transformer replacement outage, the following items may need to be considered as part of constructability review and planning efforts:
- Adjacent, energized transformers and bus ducts
- High-voltage line clearances
- Operations and maintenance pathways
Likewise, if key transformer design parameters, such as base MVA rating, winding voltages/taps, or impedance, change during a replacement, a detailed electrical system study should be completed. The scope of the study should include not only the new transformer, but also the electrical equipment associated with it. Examples of electrical system parameters that may change during a transformer replacement include:
- Arc flash hazards (i.e., incident energy levels)
- Breaker interrupting capabilities
- Bus short circuit withstand levels
- Protective relay settings
Prior to placing a replacement power transformer in service, it is common to perform a no-load “soak” of the unit in which the transformer primary is back fed from a utility source with the secondary disconnected or unloaded. The soak typically occurs for 24 hours and is used to monitor the transformer for any abnormalities — such as high temperature, excessive vibration and liquid leaks — prior to the application of load.
For generator step-up transformers that do not have a low-side generator circuit breaker, extra caution and planning must be performed during this process, as the generator is required to be disconnected from the transformer through removal of flexible links in the isolated phase bus. Additionally, control system and electrical protection system modifications may also be required to allow closure of the high-side generator circuit breaker during the test. For these reasons, it is recommended that a detailed back feed procedure be developed with all stakeholders before transformer installation.