O&M

Considerations for Selecting a Spare GSU Transformer

Careful planning should be pursued with an eye toward procuring a spare generator step-up (GSU) transformer that can be used at multiple sites.
Considerations for Selecting a Spare GSU Transformer
(Photo/Burns & McDonnell)

By David Reese and Matthew Stapf

Is your power plant financially prepared for a generator step-up (GSU) transformer failure? As one of the vital components of a power generation facility, GSUs are the interface between the power station and the electrical grid. Due to their size and complexity, these transformers are unique, customized devices that can require extensive lead times for replacement in today’s market. If a spare GSU transformer is not readily available, a generation unit can be offline for an extended time.

Utilities that operate in a regulated market are obligated to maintain enough power capacity to supply all their customers’ needs, with enough capacity in reserve to meet demand during peak usage periods. If generating units go offline and the utility cannot meet customers’ demands, they must purchase power at potentially higher rates from other power producers. 

Independent power producers (IPPs) that own large generation assets are generally compensated either through contractual power purchase agreements (PPAs) or by selling power on the open market. IPPs also may earn additional revenue via a flat rate compensating them for meeting specific reliability standards for available generating units.

It follows then that IPPs can only make money when their generating units are available. Should an IPP fail to meet its contractual power output specified under the PPA, it can lose generation revenue and face the additional risk of losing reliability payments for those units. In addition, the IPP may incur additional financial penalties through provisions for liquidated damages.

Costly Impact

Both regulated utilities and IPPs face significant financial risks from GSU failures that shut down generation assets.

As of late 2021, the typical lead time for procuring and installing a new GSU transformer ranged from 52 to 70 weeks. While the financial terms for an IPP’s PPA may vary, it is conservatively estimated that revenue losses from an outage at a 250-megawatt (MW) peaking generation unit could exceed $13 million over a 52-week period. Those losses could climb to $17.5 million if this same unit faced a 70-week outage.

In 2021, a GSU transformer for a 250-MW unit would typically cost between $2 million and $4 million. Thus, investing in a spare GSU transformer could have paid for itself many times over after a single event. This investment can be further justified if a single universal GSU transformer is designed for use at multiple generating facilities. Since no two power plants are alike, the following outlines some important factors that should be considered when selecting a universal spare GSU transformer that can be installed at more than one site.

Understanding Technical Criteria

In order for a spare GSU to be installed at multiple generating units, detailed technical criteria must be specified, including:

  • Recommended electrical ratings.
  • Plant electrical system impact.
  • Overall physical constraints.
  • General interface requirements.
  • Foundation size, firewall height, and containment area.
  • Protective Setting Changes.
  • Generator control and excitation system settings.
  • Transmission System Stability.
  • Geotechnical investigation review.

Engineers performing the evaluation should examine data from the following sources when developing specifications:

  • Site photos of existing GSU/Aux transformers, bus duct transitions, high voltage conductors, foundations, containment, fire walls, and control panel locations.
  • Site photos of MV motor nameplates.
  • Plant one-line diagrams.
  • Plant protection one-line diagrams.
  • Plant general arrangement and raceway drawings.
  • Major equipment drawings including:
    • Existing GSU nameplate, outline, and schematic diagrams.
    • Bus duct arrangement drawings, flange details, and short-circuit ratings.
    • Generator Circuit Breaker nameplate.
  • Photos of existing GSU and UAT Tap Settings.
  • Existing GSU foundation and firewall drawings.
  • Utility Interface Data including:
    • Short-circuit contribution data
    • Voltage schedule at point of interconnection
    • VAR performance requirements in interconnect agreement
  • Existing GSU test data.
  • Generator Data Sheets.
  • Generator Circuit Breaker Short-Circuit Ratings.
  • Generating unit maximum reported output (MW).
  • Site geotechnical data.
  • Existing unit protective relay settings.

Electrical System Impact Evaluation

Electrical ratings for spare GSU transformers should be evaluated so the unit achieves a range of results while also meeting the overall goals for the installation. The mega-volt ampere (MVA) rating, percent impedance (%Z), voltage ratio, and tap settings will need to be strategically balanced to achieve an optimal solution for the application. To achieve the best electrical performance, the spare GSU should be able to:

  • Continuously carry the full MW output for the generating units where it will be installed without exceeding the transformer top MVA rating.
  • Maintain the primary-to-secondary same phasor relationship as the existing GSU transformer. The most common vector group for a GSU transformer is Ynd1.
  • Provide a ground reference for the primary, high-voltage system. This is normally accomplished by using a wye-connected primary winding.
  • Allow each generating unit to export or import the voltage-ampere reactive (VAR) required while:
    • Meeting their contractual lagging and leading power factor obligations at the point of interconnection (POI) without exceeding the generator’s capability curve.
    • Maintaining a standard terminal voltage between 95% < V < 105%.

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

  • Minimize the system short-circuit contribution to the generator bus duct, generator circuit breaker (GCB), and auxiliary taps so it does not exceed short-circuit ratings.
  • Minimize the voltage drop impact through the GSU to the auxiliary system while the GCB is open and the generator is disconnected prior to synchronization.
  • Meet the insulation basic impulse level (BIL) requirements for the system.

The electrical ratings should be evaluated using Load Flow and Short-Circuit study cases in an industry-recognized power system analysis software to verify that the spare GSU transformer meets the objectives listed above.

Physical Evaluation

The spare GSU transformer must be designed for installation at an existing facility with minimal modifications required to the existing bus duct, conductors, raceway, foundations, or structural supports. Considerations should include:

  • Minimum clearance to firewalls.
  • Separation from oil filled components to nearest containment wall.
  • Height of transformer with respect to height of firewalls.

Existing foundation drawings also must be reviewed for dimensions related to containment coverage and volume, transformer pad size, firewall height, and iso-phase bus openings. Recommended foundation modifications at the facilities should also be included in the evaluation. Transformer drawings must be reviewed for information related to dimensional layout, center of gravity, weight, and oil volume.

Transformer Foundations

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

  • The spare GSU base must fit within the existing foundation pedestal dimensions.
  • The spare GSU center of gravity must be evaluated to determine where the transformer should be located on the pedestal to minimize settlement concerns on subgrade soils.

Containment

The oil containment area must also be suitable to contain the spare GSU oil volume in the event of a transformer failure. The evaluation must also consider whether standing water is present in the containment area and/or whether water may be added to the containment if fire suppression procedures are activated. This evaluation can be difficult as there are only standards recommendations but no code requirements. The evaluation needs to be discussed with the plant personnel and members of their environmental group to determine what factors should be included, based on their level of risk. This evaluation should include:

  • The potential volume from a spill of the largest single container of flammable or combustible liquids in the area.
  • The maximum number of fire hoses that could potentially be operating for a minimum of 10 minutes.
  • The maximum design discharge of fixed fire suppression systems operating for a minimum of 10 minutes.
  • Freeboard height over and above the liquid level determined from the above indicated liquids.

Firewall Height and Location

The height of the existing firewall must be evaluated to determine whether it is suitable to protect surrounding equipment if the spare GSU height exceeds that of the original GSU. If the existing firewall height is determined to be insufficient, an evaluation needs to be performed to determine if it can be retrofitted or should be replaced with a taller wall. The horizontal location of the firewall must also be far enough from the spare GSU oil cooling radiators to allow for air circulation, access, and removal.

Overhead Conductor Transitions

High-voltage conductor connections on the primary, high-voltage GSU terminals may require modifications if there are significant differences between existing and spare GSU bushing height and spacing. Any changes to high-voltage conductor transitions must be examined to verify that minimum phase-to-ground clearances are maintained between surrounding structures and equipment.

Secondary Transitions

Secondary terminals on GSU transformers may be connected through either cable or bus duct. Cable connection points may be located on either the top or side of the transformer. Bus duct termination flanges may have different diameters, bolt-hole patterns, and phase spacing. Spare GSU transformers may require custom transition boxes if a gray-market spare GSU is installed or if it is intended to be used at multiple sites.

Accessory Locations

  • Spare GSU radiators must be located so they avoid surrounding structures and allow for maintenance access. The manufacturer’s recommended horizontal clearance from the existing firewalls should be maintained to allow for adequate cooling.
  • Control panels should be located to match the existing electrical raceway if possible. If the control panel location must be different from the existing GSU, it should ideally avert the need for longer control cable lengths.

Other Considerations

  • Instrument Transformers — Current transformer (CT) quantities and ratings must be selected to accommodate the protection scheme for the existing generation unit. Phase CT ratios and accuracy classes should be selected for the anticipated unit output while avoiding saturation for external faults.
  • Alarms — The spare GSU’s instrumentation and alarms should be selected to match the plant control system’s existing input and output signals. If any differences exist, the plant control system logic must be modified to avoid any nuisance trips or alarms when the spare GSU is put into service.
  • Control Power — The spare GSU’s control power voltage and load must be coordinated with the available power supply circuits feeding the existing GSU transformer. The spare GSU’s auxiliary power specification should include not-to-exceed load requirements to minimize the need for upgrading control power source breakers or cables.

Conclusion

Because GSU transformers are critical to plant reliability and overall financial performance, having a readily available spare GSU transformer on standby should be considered much like an insurance policy. Careful planning should be pursued with an eye toward procuring a spare GSU that can be used at multiple sites.

Though some modifications may be necessary for equipment or foundations at specific locations, these considerations should amount to only incremental costs — particularly when considering the total financial impact of loss of revenue during the interim while a new GSU transformer is being manufactured, shipped and installed. However, engaging a structural engineer early in the study is critical to capture foundation modifications both from a cost and schedule perspective.

David Reese is a senior electrical engineer in the Energy Division at Burns & McDonnell. He has more than 21 years’ experience designing electrical systems for power generation facilities including fossil, hydro, renewables, and nuclear energy technologies.

Matthew Stapf is an associate structural engineer in the Energy Division at Burns & McDonnell. He has more than 15 years of design and project management experience in the power plant industry as well as the oil and gas industry.