Electrical, Power

Seven tips for transformer design in industrial buildings

Proper knowledge of transformer design for both new and replacement installations is essential to building operation, specifically in industrial applications.
By Jared Markle, PE, RMF Engineering, Baltimore December 18, 2018
Figure 2: A cooling package for a dry-type cast coil transformer consists of several small fans pointed directly at the core-and-coil assembly. The fans have a dedicated temperature controller and, in this case, increase the capacity of the transformer from 2,000 to 2,667 kVA.

Learning Objectives 

  • Understand the two main categories of transformers with subcategories that suit different applications.
  • Become familiar with a number of outside factors that can affect a transformer design, including national and local codes, insurance, and utility requirements.
  • Learn about several design tricks that can be used to make a system safer and more efficient, including high-resistance grounding and transformer cooling packages.

As the building industry continues to evolve and focus shifts more toward reliability and efficiency, electrical system designs must follow suit. The design of any electrical distribution system or building service starts with the transformer. Proper knowledge of transformer design is essential to building operation, specifically in industrial applications. There are many challenges with transformer design and installation, both physically and operationally. Here are seven tips that address common hurdles in the design process that will lead to a successful project.

1. Understand transformer types and applications 

Specifying the proper transformer type is the first step in the transformer design process. Many factors are taken into account when choosing the type of transformer including location, cost, and system configuration. Industrial transformer types can be divided into two main categories: dry-type and liquid-filled.

In general, dry-type transformers are used for smaller indoor applications, less than 5 MVA, and liquid-filled transformers are used for pad-mounted exterior and/or larger applications. Dry-type transformers are subdivided into three types: vacuum pressure impregnated (VPI), vacuum pressure encapsulated (VPE), and cast coil.

VPI transformers typically are the most affordable of the three and the best for installation in electrical rooms that are properly conditioned, dry environments. Cast coil transformers are often the most expensive of the three and are suitable for use in harsher environments due to their completely sealed core-and-coil construction. VPE transformers fall in between VPI and cast coil both in cost and in environmental suitability.

Figure 1: Pictured is a liquid-filled substation-style transformer rigging and placement at Rutgers University. Coordinated shop drawings were provided to verify that both the primary and secondary conduit stub-ups aligned with the transformer’s termination compartments to ensure a successful installation. All graphics courtesy: RMF Engineering Inc.

Although dry-type transformers have the advantage of being installed indoors, often as part of integrated substations, there are several disadvantages, most notably the need to design ventilation systems sized for the significant heat rejection associated with the transformer and plan for rigging paths and methods for future replacement.

Liquid-filled transformers use several different types of liquids for cooling—identified in NFPA 70: National Electrical Code (NEC)-2017, Article 450, as less flammable, nonflammable, and oil—and are either station-type or pad-mounted. The type of cooling liquid chosen can trigger both code and insurance requirements for containment, relative location to buildings, and room requirements if installed within a building. Station-type transformers typically have a large capacity, greater than 5 MVA, and are installed in outdoor substation yards with overhead exposed busing. Pad-mounted transformers are commonly mounted outside of the building that they serve and are accompanied by either an internal or separately mounted medium-voltage switch and fuse for isolation and protection.

There are several disadvantages to consider with liquid-filled transformers when compared with dry-type transformers, most notably meeting the appropriate code and insurance requirements for onsite locations, containment, and physical protection from damage.

A key specification item for transformers is the temperature rise rating. Dry-type transformers have three standard temperature-rise values of 80°C, 115°C, and 150°C. The standard temperature-rise values for liquid-filled transformers are 55°C and 65°C. The temperature-rise rating represents the maximum increase in temperature above ambient that the transformer will incur when operating at rated full load.

The industry standard rating for transformer insulation is 220°C, and the lower temperature-rise ratings result in a larger margin between the insulation rating and the maximum temperature of the transformer. Lower temperature-rise transformers require higher initial costs, both for dry-type and liquid filled, but they result in more efficient operation and a better ability to accommodate overloads due to the lower operating temperature.

Figure 2: A cooling package for a dry-type cast coil transformer consists of several small fans pointed directly at the core-and-coil assembly. The fans have a dedicated temperature controller and, in this case, increase the capacity of the transformer from 2,000 to 2,667 kVA.

2. Know the codes

To properly design transformer installations, the specifying engineer must have a full understanding of the governing codes. Several of the chief codes that identify transformer installation requirements are NEC, International Building Code (IBC), and International Fire Code (IFC). A summary of common requirements identified in these codes can be found in Table 1.

A caveat to the requirements identified in the referenced codes is that they can be superseded by state building codes and even the local authority having jurisdiction (AHJ). In addition to national and international code requirements, the owner’s property insurance provider may have special requirements or recommendations for the transformer installation. For example, FM Global provides a property loss prevention data sheet that includes recommendations for fire protection, physical location, electrical protection, testing, and containment for critical transformer installations.

To provide a transformer that will be approved by the AHJ as well as be insured by the owner’s insurance provider, a code-compliance checklist is recommended for each individual project.

3. Optimize size 

Appropriate transformer sizing for both new and replacement installations is essential for efficient and reliable operation and can result in substantial savings both in initial capital and operational costs. When designing a transformer that serves a building, both the connected load, usually measured in kilovolt-amperes (kVA), and demand load are identified. The connected load is typically much larger than the rating of the transformer so the demand load is a lesser value that represents the engineer’s best estimation of the peak load the transformer will realistically see. For new buildings, the demand load is determined based on NEC Article 220 and is used to size the transformer, typically yielding a higher value than the real-world metered peak load.

When replacing an existing transformer, NEC Article 220.87 allows the engineer to use the metered peak-load data over a 1-year period when sizing the transformer, often resulting in a reduced transformer size. In the event of 1-year load data not being available, NEC also allows the engineer to use metered data over a 30-day period plus the maximum anticipated demand of the heating or cooling equipment if the metering did not take place during a peak season. Oversizing of either a replacement or new transformer will result in monetary losses for the owner associated with transformer efficiency. Smaller, appropriately sized transformers also result in lower incident energy levels, potentially helping to reduce arc flash hazards within an electrical system.

Transformer efficiency is calculated by taking the ratio of input power to output power, where the transformer losses are subtracted from the input power. Transformer losses consist of no-load losses, which are constant, and load losses that vary with transformer loading. Oversized transformers will carry larger no-load losses that result in an unneeded decrease in efficiency. In addition to efficiency losses, oversized transformers carry additional negative consequences including increases in the feeder sizing, distribution equipment sizing, and overcurrent protective device rating.

The three best ways to optimize transformer sizing are specifying a cooling package, taking advantage of NEC-allowed metering for replacements, and developing detailed load calculations that represent realistic peak-loading scenarios for new installations. Transformer cooling packages primarily include natural air or liquid cooling, forced air or liquid cooling, or gas-insulated cooling. A transformer cooling package increases the capacity of dry-type transformers by up to 133% of the base rating and can increase the capacity of liquid-type transformers by a similar amount depending on the number and type of cooling stages used. The additional capacity provided by a cooling package allows the engineer the ability to appropriately size the base rating of the transformer and to use the cooling rating for either peak conditions or for added redundancy for multiple transformer topologies.

Figure 3: Pictured is a liquid-filled pad-mounted transformer installation at a New York State correctional facility. Several items were specified to promote safe personnel interaction and to be environmentally friendly including dead-front construction, key interlocking between the medium-voltage compartment and fused switch, and edible seed-based oil for the cooling liquid.

4. Review utility requirements

Industrial buildings often have a dedicated electrical service from the local utility and can house the electrical distribution equipment for campus microgrids. The point of demarcation with the electrical utility will typically occur either on the primary or secondary side of the building transformer, resulting in the need for compliance with utility requirements.

Typical utility requirements for transformer installations include:

  • Metering potential transformer and current transformer locations and specifications.
  • Protective device requirements including differential protection and redundant relaying.
  • Transformer configurations including wye or delta primary.
  • Grounding means and methods including resistance or reactance grounding.

The utility requirements for a project can be easy to include during the design but have monetary and schedule impacts when implemented during construction. The utility will often have published standards that reference the requirements for connecting transformers, but meetings with the utility during the design phase of the project go a long way toward successful installations.

5. Consider high-resistance grounding 

Industrial buildings often contain process equipment that is valuable to the owner while in operation or critical to the operation of a campus. A number of measures are taken in the design of such buildings to ensure reliable operation, but an often-overlooked design option is the fault tolerance of the electrical system. An electrical fault can halt an industrial process anywhere from a few hours to days or weeks depending on the nature of the damaged equipment. The majority of faults in electrical systems are line-to-ground faults, which can be managed via a high-resistance grounded (HRG) secondary for the building transformer.

An HRG system is achieved by adding a continuously rated resistor between the transformer secondary neutral and ground, sized to limit the ground-fault magnitude to a lower value, typically 5 amps. A ground-fault detection panel is provided with the associated resistor, which notifies the owner when a fault occurs and provides the means to identify the fault location within the electrical system. In addition to limiting the magnitude of ground-fault current from potentially thousands of amps to 5, the electrical system will remain in service in the event of a ground fault in lieu of tripping as in a solidly grounded system. Outside of the obvious safety benefits, this allows the company to diagnose the fault on its own terms without losing valuable operation time.

Several other considerations must be taken into account when an HRG transformer is introduced into an electrical system, including system rating, single-phase loads, and owner operations and maintenance (O&M) procedures. A ground fault occurs when a single phase is connected to ground. In a high-resistance grounded system, the remaining two phase-to-ground voltages elevate to the phase-to-phase voltage, increasing by 173%. Because of the potential elevated voltages on the system, devices that are connected from phase to ground, most notably surge and lightning arrestors, must be rated for the phase-to-phase voltage in lieu of phase-to-ground voltage as designed in solidly grounded systems.

HRG systems are not suitable for connection of single-phase loads, therefore additional transformers are required to re-establish a solidly grounded system as neutral. A typical HRG system will include main distribution equipment connected to the secondary of the transformer to serve the building’s 3-phase loads and a delta-wye transformer to serve the single-phase distribution equipment.

The last consideration when designing an HRG system is the owner’s operational procedures. Because the system can operate with a single ground fault indefinitely, diligence is required by the operations staff to diagnose and schedule repair of a fault. In the event that the owner does not have full-time or properly trained operations staff, an HRG system may not be appropriate to specify.

Figure 4: A liquid-filled substation-style transformer is typically used for either large-capacity (>5 MVA) and/or higher voltage (>15 kV) applications. The pictured transformer steps the utility voltage from 115 to 13.2 kV and is equipped with differential relaying and sudden pressure-relaying protection.

6. Plan installation and future replacement

Service transformers can be among the heaviest and largest pieces of electrical equipment found in a building. Designing an electrical room to meet code clearances is required; designing rigging paths and lifting measures for equipment replacement is essential. The typical lifespan of a transformer is listed as 30 to 40 years, according to most manufacturers, so replacement will be required at some point during the life of the building. To add complexity to an already difficult task, transformer sizes change as efficiency requirements and technology evolves. For example, the Department of Energy (DOE) enacted a requirement in 2016 for greater efficiency in all transformers sold in the United States, resulting in larger footprints on average for identically rated transformers.

The rigging path for a transformer not only has to take the space within an electrical room into account, but also the path throughout the building from a suitably sized building entrance as well. If an electrical room is located at ground level, a 10-ft overhead door or double door is a good design practice for this access. When the electrical room is located either in a basement or on a higher floor, large access hatches or panels and trolley beams with hoists are suitable design practices.

Coordination of transformer access (and all other large equipment access) with the architect and structural engineers is a vital part of the design process and can be accomplished by modeling the building in 3-D and holding regular coordination meetings.

7. Protect your investment

The electrical system in a building is an asset that requires diligent attention and sound design to operate as intended for the life of the equipment. One of the responsibilities of an engineer is to provide a design that protects the client’s investment and to inform the client of equipment-maintenance requirements after the project is complete. Transformers, like all other electrical equipment, are susceptible to electrical faults, manufacturer defects, and poor maintenance practices, which can all adversely affect the life of the equipment.

Protection of transformers from electrical faults starts with meeting the overcurrent protective device (OCPD) requirements outlined in NEC Article 450. Implementation of proper OCPD settings that protect the transformer’s damage curve is required by code and is the first line of defense to a fault within the electrical system. Additional protection and suitable applications for transformers are outlined in Table 2.

Factory quality control measures generally include impedance and open- and short-circuit tests, but even with the most careful designs, equipment defects still exist and can derail a project during start-up or right after turnover of the project. A few ways that the engineer can minimize these effects is to specify comprehensive field-testing procedures to be carried out by an InterNational Electrical Testing Association (NETA)-certified testing agent and to require extended warranties where applicable. Most manufacturers offer a standard 1-year warranty after installation of equipment, but this can be extended in most cases to 3 or more years.

Finally, requiring the manufacturer to provide O&M manuals that identify all required routine maintenance as well as training to the owner is the best way to protect the long-term investment in a transformer. Maintenance items range from monthly visual inspections to periodic oil sampling to infrared images of coil assemblies. Several items can be specified to make routine maintenance easier for the owner including infrared inspection windows for dry-type transformers and external oil sampling enclosures for liquid-filled transformers. Performance of the required maintenance is ultimately an owner’s responsibility, and it is the engineer’s job to stress the importance and provide all information required for a successful maintenance program.

In summary, a number of factors are included in the design and implementation of service transformers in industrial buildings, and following these seven tips will result in a successful project:

  1. Understand transformer types and applications.
  2. Know the codes.
  3. Optimize the size.
  4. Review utility requirements.
  5. Consider high-resistance grounding.
  6. Plan installation and future replacement.
  7. Protect your investment.

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Jared Markle, PE, RMF Engineering, Baltimore
Author Bio: Jared Markle is an electrical project engineer at RMF Engineering Inc.