Authors: David Reese and Matthew Stapf

Is your power plant financially prepared for a generator step-up (GSU) transformer failure? As one of the important components of power generation facilities, GSU ​​is the interface between the power station and the power grid. Due to their size and complexity, these transformers are unique customized devices that can take a long time to replace in today's market. If the backup GSU transformer is not available, the generator set may be offline for a long time.

Utilities operating in a regulated market are obliged to maintain sufficient power capacity to meet the needs of all customers, and have sufficient reserve capacity to meet demand during peak usage periods. If the generator sets are offline and the utility cannot meet the needs of the customers, they must purchase electricity from other electricity producers at a possibly higher price. 

Independent power producers (IPPs) with large power generation assets usually receive compensation through contractual power purchase agreements (PPA) or selling power on the open market. IPPs can also earn additional income through flat rates to compensate them for meeting the specific reliability standards of available generator sets.

Therefore, IPP can only make money when their generator sets are available. If IPP fails to meet the contractual power output required by the PPA, it may lose power generation revenue and face the additional risk of losing payment for the reliability of these units. In addition, IPP may incur additional financial penalties due to liquidated damages clauses.

Both regulated utility companies and IPPs face significant financial risks from the shutdown of power generation assets due to GSU failure.

By the end of 2021, the typical delivery time for the procurement and installation of new GSU transformers is 52 to 70 weeks. Although the financial terms of IPP's PPA may vary, it is conservatively estimated that the loss of revenue caused by the outage of the 250 megawatt (MW) peak generator set may exceed 13 million U.S. dollars in 52 weeks. If the same unit faces a 70-week power outage, these losses could climb to $17.5 million.

By 2021, the cost of GSU transformers for 250 MW units is usually between US$2 million and US$4 million. Therefore, after an incident, investing in a spare GSU transformer may recover a lot of costs. If a single general-purpose GSU transformer is designed for multiple power generation facilities, this investment can be further justified. Since no two power plants are the same, the following outlines some important factors that should be considered when selecting a universal backup GSU transformer that can be installed at multiple sites.

In order to install a backup GSU on multiple generator sets, detailed technical standards must be specified, including:

The engineer performing the evaluation should check the data from the following sources when formulating the specification:

The electrical rating of the spare GSU transformer should be evaluated so that the device can achieve a range of results while meeting the overall goals of the installation. Megavolt-ampere (MVA) ratings, impedance percentages (%Z), voltage ratios, and tap settings need to be strategically balanced to achieve the best solution for the application. In order to achieve the best electrical performance, the backup GSU should be able to:

This requirement is based on the VAR performance requirements listed in the site interconnection agreement. This evaluation will determine the selection and range of GSU tap settings.

The power flow and short-circuit study cases in the industry-approved power system analysis software should be used to evaluate the electrical ratings to verify whether the standby GSU transformer meets the above objectives.

The spare GSU transformer must be designed to be installed in an existing facility with minimal modifications to existing bus ducts, conductors, pipes, foundations or structural supports. Considerations should include:

The existing basic drawings must also be checked for dimensions related to containment coverage and volume, transformer pad dimensions, firewall height, and in-phase bus openings. The assessment should also include the proposed infrastructure improvements. The drawings of the transformer must be checked for information related to the size layout, center of gravity, weight, and oil volume.

The existing GSU foundations must be evaluated to determine whether they can support the weight of the spare GSU without structural damage or settlement. The assessment must include existing basic design parameters and site geotechnical data. If existing design parameters and geotechnical data are not available, it may be difficult to evaluate the existing soil to support future loads and a new geotechnical evaluation may be required. Considerations should include:

The oil storage area must also be suitable for accommodating the spare GSU oil volume in the event of a transformer failure. The assessment must also consider whether there is water in the containment area and/or whether water can be added to the containment if the fire extinguishing process is initiated. This evaluation can be difficult because there are only standard recommendations but no code requirements. The assessment requires discussions with plant personnel and members of the environmental team to determine which factors should be included based on their risk level. The assessment should include:

If the height of the backup GSU exceeds the height of the original GSU, the height of the existing firewall must be evaluated to determine whether it is suitable for protecting the surrounding equipment. If it is determined that the existing firewall is not high enough, an assessment is required to determine whether a higher wall can be retrofitted or replaced. The horizontal position of the firewall must also be far enough away from the spare GSU oil-cooled radiator for air circulation, entry and removal.

If there is a significant difference between the existing and spare GSU bushing height and spacing, the high-voltage conductor connections on the primary and high-voltage GSU terminals may need to be modified. Any changes in the transition of high-voltage conductors must be checked to verify that there is a minimum relative ground clearance between surrounding structures and equipment.

The secondary terminals of the GSU transformer can be connected via cables or busbars. The cable connection point can be located on the top or side of the transformer. The busway terminal flanges may have different diameters, bolt hole patterns, and phase spacing. If a gray market spare GSU is installed or intended to be used at multiple sites, the spare GSU transformer may require a customized conversion box.

Since GSU transformers are critical to plant reliability and overall financial performance, spare GSU transformers that are readily available should be considered as an insurance policy. Care should be taken to plan and focus on purchasing spare GSUs that can be used at multiple sites.

Although some modifications to the equipment or foundation at a particular location may be required, these considerations should only be incremental costs—especially when considering the total financial impact of lost revenue during the manufacturing, transportation, and installation of the new GSU transformer. However, hiring a structural engineer early in the research is critical to capture the fundamental changes from a cost and schedule perspective.

David Reese is a senior electrical engineer in the Burns & McDonnell Department of Energy. He has more than 21 years of experience in designing electrical systems for power generation facilities including fossil, hydropower, renewable energy, and nuclear technology.

Matthew Stapf is an associate structural engineer with Burns & McDonnell's Department of Energy. He has more than 15 years of design and project management experience in the power plant industry and the oil and gas industry.