Abstract:
The present invention relates to an apparatus and method for improving the transportation and installation of portable step-up power transformers for a power generating station due to failure or unavailability of installed step up transformers. A large MVA portable transformer may be used to alleviate or minimize these issues by being purpose built for transportability and commissioning speed. The portable transformer is transportable by road or lighter rail. Its size and weight are arranged to overcome restrictions such was weight limits, bridge constraints, and power line constraints. This device may also provide for rapid commissioning and decommissioning. This transformer may be a set of three single-phase units of sufficient size to be used as a generator step up transformer in three-phase operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates generally to improving the continuity and reliability of electric power delivery and more specifically to a large-scale portable power transformer trailerable over public roads and a method for using the portable power transformers to improve the continuity and reliability of power delivery for the electric utility user. 
         [0002]    A transformer is a device in which, by electromagnetic induction, an alternating current (AC) of one voltage is transformed to another voltage, without change of frequency. Transformers are widely used in electrical apparatus of all kinds, and in particular in power transmission where high voltages and low currents are utilized. 
         [0003]    A transformer has two coils, a primary for the input and a secondary for the output, wound on a common iron core. The ratio of the primary to the secondary voltages is directly proportional to the number of turns in the primary and secondary coils; the ratio of the currents is inversely proportional. A step-up type of transformer has more turnings on the secondary coil than on the primary coil and therefore the voltage induced in the secondary coil is larger than the primary coil voltage. If the number of turns on the primary coil is N P  and the number of turns on the secondary coil is N S , and the respective voltages are V P  and V S , then N S /N P =V S /V P . For example, if there are 200 turnings on the primary coil and 2,000 turnings on the secondary coil, then the voltage induced in the secondary coil is ten times larger than the primary coil voltage. 
         [0004]    Large power transformers are an essential component in the delivery of electric power by an electrical utility. Delivery of power is initiated by a power generating station, which may include a fossil fuel station, a nuclear power plant or a renewable power source such as a windfarm. Electric power is provided from the generating station or a primary substation as three-phase ac power at different voltages, ranging from 13.1 kV to 22 kV. The electric power source may be connected to the low voltage (LV) or primary side of a three-phase transformer that steps up the voltage on its high voltage (HV) or secondary side to about 230 kV for a transmission grid. Other secondary side voltages that may be provided by different transformer configurations may include 115 kV or 345 kV. 
         [0005]    The transformers must have sufficient capacity to accommodate the amount of power being provided by the generating station. The megawatt (MW) power rating of the generating station or the primary substation utilizing step-up transformers vary widely, but may include power generation in the 25 MW to 100 MW range and even up to over 500 MW. Transformer capacity may be rated in volt-amperes (VA), which is generally speaking a product of the magnitudes of voltage and current on the input side of the transformer. The large step-up transformers for a power station may be rated in Mega VA (MVA). 
         [0006]    Because the step-up transformer occupies an in-line circuit path for the delivery of power from the generating station, unavailability of the transformer prevents the generating station from delivering power. Failure to deliver power results in loss of revenues and may also result in added costs to obtain replacement power from other power suppliers, potentially at higher per unit costs. Further, inability to deliver power reliably, as contracted, damages the reputation of the utility. Unavailability of the transformer may result from failure of installed transformers or the need by the utility to take the installed transformers out-of-service for preventive maintenance or modification. 
         [0007]    In the event of failure of an installed generating station transformer, either a repair must be implemented or a replacement provided. These larger transformers are too heavy to be transported quickly and require significant permitting and logistics planning due to the large size and weight. They are not considered portable. They are often shipped on special purpose-built rail cars and with far too much weight to be transported by road. These transformers are also complex in their commissioning at site. 
         [0008]    Mobile substation transformers have been used for lower MVA ranges, but these are not suitable for the large power generating station step-up transformers. Larger, more financially strong utilities have used spare transformers, either a full spare or an additional single-phase transformer, as a replacement of one of individual single-phase units that would be in service. However, for economic and other reasons it may not be desirable to maintain a spare unit on hand to address transformer outages. 
         [0009]    Accordingly, there is a need to provide and utilize large-scale portable power transformer units that can be quickly and easily delivered to and installed at electric utility to restore electric power delivery for unplanned and planned outages of existing generator stepup transformers. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0010]    The present invention relates to an apparatus and method for improving the transportation and installation of portable generator step-up (GSU) power transformers for a power generating station due to failure or unavailability of installed step up transformers. A large MVA portable transformer may be used to alleviate or minimize these issues by being purpose built for transportability and commissioning speed. The portable transformer is transportable by road or lighter rail. Its size and weight are arranged to overcome restrictions such was weight limits, bridge constraints, and power line constraints. This device may also provide for rapid commissioning and decommissioning. This transformer may be a set of three single- phase units of sufficient size to be used as a generator step up transformer in three- phase operation. 
         [0011]    Briefly in accordance with one aspect of the present invention, a method is provided for maintaining power transmission supplied normally through an installed generator stepup (GSU) transformer adapted for serving a primary generating station or a primary substation. The method includes providing portable spare single-phase GSU transformer units of suitable rating to replace at least one phase of an installed GSU transformer. The portable spare single-phase GSU transformer is sized to fit aboard an individual on-the-road trailer according to national transportation standards for on-road transport. For the United States, the national standards include U.S. Department of Transportation (U.S.D.O.T.) height, width and weight standards for on road transport. The method also includes storing the portable spare single-phase GSU transformer in at least one central location. 
         [0012]    In accordance with another aspect of the present invention, a method is provided for maintaining power transmission normally supplied through an installed generator stepup (GSU) transformer, where the GSU transformer is adapted for serving a primary generating station or a primary substation. The method includes establishing an arrangement between a service provider and an operator for supporting continuity of power transmission through an installed GSU transformer. The method also includes maintaining an availability for one or more phases of a portable single-phase GSU transformer for replacing one or more phases of an installed GSU transformer. For a normal three-phase installed GSU transformer, replacement of all three phases with portable units is required. The method further includes providing services to the power generating entity for maintaining operability of the installed GSU transformer and expediting installation of at one or more phases of the portable single-phase GSU transformer in place of one or more phases of the installed GSU transformer. 
         [0013]    In accordance with a third aspect of the present invention, s portable single- phase generator stepup (GSU) transformer arrangement is provided. The portable single-phase transformer arrangement is adapted for replacing the phases of a three- phase GSU transformer that may be used in a primary generating station or a primary substation. The GSU transformer arrangement includes a portable single-phase GSU transformer for at least one of a power generating station and a primary substation, sized to fit aboard an on-the-road trailer according to national transportation standards. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0014]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0015]      FIG. 1  illustrates a front elevation view for an embodiment of the inventive portable single-phase GSU transformer unit; 
           [0016]      FIG. 2  illustrates a top view for an embodiment of the inventive portable single-phase GSU transformer unit; 
           [0017]      FIG. 3  illustrates an embodiment of the inventive portable single-phase GSU transformer arranged for transport atop an on-the-road trailer. 
           [0018]      FIG. 4  illustrates an embodiment for the inventive portable single-phase GSU transformer unit to supply power to a transmission line from a power generating station. 
           [0019]      FIG. 4A  illustrates an embodiment for three portable single-phase GSU transformers configured as a replacement for a failed installed three-phase GSU step- up transformer; 
           [0020]      FIG. 5  illustrates an arrangement of portable transformer units installed for supplementing load capacity of an installed and functioning GSU step-up transformer, which may be overloaded when operating alone; 
           [0021]      FIG. 6  illustrates an arrangement of portable transformer units installed for temporarily replacing one installed three phase step-up transformer unit of a multi- unit step up transformer; 
           [0022]      FIG. 7  illustrates an embodiment of the portable single-phase transformer unit to accommodate a generating station with rated MW and MVA output significantly below the rated MVA capacity of the replacement transformer units by matching impedance using current limiting reactors; 
           [0023]      FIG. 8  illustrates another embodiment of an operational application for the portable single-phase transformer unit allowing continued operation with reduced MW output of a power generating station, which is above the combined rated MVA capacity of three portable single-phase transformer units operating in parallel; 
           [0024]      FIG. 9  illustrates a further embodiment for the inventive portable single-phase transformer unit stacking multiple units in parallel to obtain a higher MVA capacity. in a configuration requiring higher MVA capacity; 
           [0025]      FIG. 10  illustrates a connection for the portable single-phase GSU transformers to the installed iso-phase bus infrastructure; 
           [0026]      FIG. 11  illustrates a connection for the portable single-phase GSU transformers to an installed iso-phase bus infrastructure; and 
           [0027]      FIG. 12  illustrates an alternate connection between an existing iso-phase infrastructure to the portable single-phase GSU transformers using cable connections. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    The following embodiments of the present invention have many advantages, including providing rapid access to large portable MVA generator step-up (GSU) transformer units quickly, thereby allowing replacement power for unavailable phases of an installed step-up transformer for a power generating station to be restored. 
         [0029]    The inventive portable single-phase transformer unit may be provided in a range of outputs for augmenting or temporarily replacing installed equipment. Usually the portable transformers may be installed quickly, once on site. A present embodiment may be rated at about 70 MVA, however, other embodiments may up to 100 MVA or even larger. 
         [0030]    This portable single-phase GSU transformer is transportable via road, rail, barge or other standard means so as to be utilized as a short cycle, return-to-service equipment replacement solution for generator step-up transformers and autotransformers. An advanced design permits optimal MVA and voltage selection within the lightest weight and smallest size possible. At the same time, the inventive portable single-phase transformer unit meets safety and reliability targets and shipping constraints. Unlike mobile substations, the inventive portable single-phase transformer units do not have primary and secondary switchgear, allowing the maximum shipping advantage for large MVA. Units are delivered without oil and require assembly and vacuum filling to be placed in service. 
         [0031]    The portable single-phase GSU transformer unit may be used to provide a one-for-one replacement for one or more unavailable individual phases of an installed step-up transformer configured as three single-phase GSU transformers in parallel to provide a three-phase output. The portable single-phase GSU transformer unit may be used to provide a complete replacement for a unitary three-phase step-up transformer by providing three portable single-phase transformer units. This transformer solution is unique in that it is purpose-built for this application and configurable for multiple sites with disparate electrical requirements 
         [0032]    Another application of the portable single-phase transformer unit is to maintain continuity of operation for overloaded multi-unit substations by adding the portable single-phase transformer units in parallel with the overloaded components. 
         [0033]    In addition, the portable transformers may be provided as a supplement to installed step-up transformer units for service at peak demand as a temporary solution. The portable units may also permit overloaded two-unit substations to take a maintenance outage, with the load shifted to the temporarily installed portable transformer units. 
         [0034]    Other benefits also include shorter outages. The portable single-phase transformer units can be transported quickly and easily. The portable single-phase transformer was designed to meet the strictest federal, state, and local transportation regulations to keep transportation permitting at a minimum. Transformers can be delivered on site in less than one week with an estimated installation time of two weeks from the time of deployment. 
         [0035]    The inventive portable transformers may be fabricated to fit many typical applications, and customers can now replace transformers for an MVA size that previously wasn&#39;t available to them, including a small generator step-up or large primary substation.. 
         [0036]      FIG. 1  illustrates a front elevation section for an embodiment of the portable single-phase transformer.  FIG. 2  illustrates a top view for the embodiment of the portable single-phase transformer. A casing  105  for the transformer  100  houses a core  110  with a primary winding  115  and a secondary winding  120 . Off-load tap changers  125  are provided for minor variation on the secondary voltages. Low voltage bushings  130  provide for connection to the output from the generator of the generating station (not shown). The low voltage bushings  130  are also internally connected through low voltage (primary) bus bars  135  to the primary windings  115 . A low voltage neutral bushing  132  is also provided for the secondary voltage. Secondary windings  120  are connected to a high voltage bushing  140  that provides for connection to the transmission system for the site (not shown). The portable transformer  100  is provided with a cooling system  150 . Transformer oil is circulated through an oil cooler  155  by oil circulating pumps  160  and though a tank  165  which houses the core  110 , primary windings  115  and secondary windings  120 . An oil preservation tank  170  maintains a head of oil on the casing  105 . The transformer  100  may further include a control cabinet  175  for monitoring and control of transformer performance including transformer oil circulation and temperature monitoring. 
         [0037]    The dimensions of the transformer are such that when three units are placed side by side they will have a minimal footprint so as to allow placement in reasonable proximity to the existing transformer connecting infrastructure. 
         [0038]    Transformer tare weight (transformers less coolers and oil but including trailer and tractor weighs less than 20,000 lbs per axle. Other embodiments can be smaller or larger depending on transportation restrictions and methods. Each transformer will be 15 ft long, 7 ft wide, 13.5 feet tall to avoid interference with bridges, overpasses, and power lines. The shipping weight of each transformer is approximately 90,000 lbs (without oil). The total weight, with tractor and trailer, will be less than 20,000 lbs/axel. Loads under 20,000 lbs/axle require minimal permitting in most states. Transformers are designed to ship on a standard drop deck commercial trailer. The use of a standard commercial trailer reduces cost and allows the service provider flexibility in shipping transformers. 
         [0039]      FIG. 3  illustrates, an embodiment of the inventive portable single-phase GSU transformer arranged for transport atop an on-the-road trailer. The portable single-phase GSU transformer  310  is shipped aboard the on-the-road trailer  320  without several components that would make transport unwieldy or potentially result in damage to the component. For example, components that are not installed for shipping include the high voltage bushing; the low voltage bushings; the neutral bushing; the oil preservation tank; and the oil cooler, pumps and piping. 
         [0040]    The inventive portable single-phase GSU transformers may be used to replace one or more unavailable phases of the three-phase installed step-up transformer for the power generating stations. A transformer casing may include an external control panel for control of cooling and alarm functions associated with transformer 
         [0041]    As previously described, the one embodiment of the portable single-phase GSU transformer may include a rating of about 70 MVA. The method for employing the portable transformer describes configurations based on a 70 MVA rating, however it should be understood, that other embodiments for the portable transformer are not limited to a 70 MVA output per phase. 
         [0042]      FIG. 4  illustrates an embodiment of an application for the inventive portable single-phase transformer. Three portable single-phase transformer units  410  are configured in parallel, acting to step-up an 18 kV output  415  of 210 MVA  420  from a 180 MW power generating station  425  and to provide a three-phase output at 230 KV  430  on the high voltage side to a transmission grid  440 . Overall, the configuration may accept 210 MVA or about 180 MW at a power factor of about 0.85. The high voltage side may also provide high voltage taps capable of providing outputs of −5%, −2.5%, +2.5% and +5% of the nominal output voltage. 
         [0043]    Further, the nominal primary and secondary voltages for the portable single-phase transformer are not limited to  18  kv on the primary and 230 kv on the secondary. Variants of the exemplary transformer may incorporate combinations of nominal primary side voltages of 13.1 kV-13.8 kV-15 kV, 18 kV and 22 kV with nominal secondary side voltages of 69 kV, 115 kV, 138 kV, 161 kV, 230 kV, 345 kV, and 500 kV. 
         [0044]      FIG. 4A  illustrates an embodiment for three portable single-phase GSU transformers configured as a replacement for a failed installed three-phase GSU step-up transformer. The three portable single-phase GSU transformers  410  are connected to the grid  400  on the high voltage side through high voltage bushings and to the power plant generators  425  on the low voltage side through the low voltage bushing  412  via temporary iso-bus  460 . 
         [0045]      FIG. 5  illustrates an embodiment of an arrangement of the portable single-phase GSU transformers installed for supplementing load capacity of an installed and functioning step-up transformer, which may be overloaded when operating alone. The portable single-phase GSU transformers  410  are connected in parallel between the generating station  424  and the grid  440  with the installed three-phase transformer  455 , which may be overloaded. 
         [0046]      FIG. 6  illustrates an embodiment for arrangement for one portable single-phase GSU transformer  410  installed for temporarily replacing one installed single-phase step-up transformer  471  unit of a multi-unit single-phase step up transformer  475 . The normally installed single-phase step-up transformer  471  is disconnected on the primary side and the secondary side and otherwise electrically isolated. The portable single-phase GSU transformer unit  410  is connected in its place to provide replacement load capacity. The temporary replacement may be made for failure or a planned maintenance outage on the installed transformer unit. 
         [0047]      FIG. 7  illustrates an embodiment of the portable single-phase transformer unit to accommodate reduced MVA loads. Three portable single-phase GSU transformers  710  may be configured in parallel acting as a replacement for an unavailable three-phase step-up transformer unit that receives power from a power generation station  725  or primary substation with a rated MW and MVA output significantly below the rated MVA capacity of the replacement transformer units. Impedance, however becomes an issue. As the impedance goes down, the stress on the system, if a fault occurs, will go up. If a fault occurs with the portable single-phase transformer units installed, stress on the system may be many times that which would result in a normal configuration impedance. Current-limiting reactors  750  may be provided in line with the primary winding of the replacement transformers to limit fault current to acceptable levels. 
         [0048]      FIG. 8  illustrates another embodiment of an operational application for the inventive single-phase transformer unit. For a power generating station  825  with a 450 MW capacity operating at nominal power factor of about 0.85, may generate about 383 MVA, which is above the combined rated MVA capacity of three portable single-phase transformers  810  operating in parallel and individually rated at 70 MVA  850 . In this circumstance, use of the three portable single-phase GSU transformers  810  may allow restored or continued operation of the power generating station albeit with reduced MW output for delivery to the grid  840 , while satisfying the MVA limitations of the portable GSU transformers  810 . 
         [0049]      FIG. 9  illustrates a further embodiment for the inventive portable single-phase transformer unit in a configuration requiring higher MVA capacity. A power generating station  925  may output a power in the range of about 540 MW requires a step-up transformer for which the rated MVA capacity for each phase exceeds the capacity of the portable single-phase GSU transformer operating. In such a circumstance more than one portable single-phase transformer unit may connected in parallel for each unavailable phase of the installed step-up transformer to provide the required MVA capacity. As illustrated, each of phase A, phase B and phase C may be provided with the two portable single-phase GSU transformers  910  operating in parallel. Each phase will then be capable of transforming 140 MVA (combined rating of two portable single-phase transformer units). 
         [0050]    A further aspect of the present invention includes a method for providing a temporary GSU spare for major or catastrophic failures of installed GSUs. The method is designed to solve key business issues for an operator of the GSU transformer, such as but not limited to an operator of an electric utility, an operator of a primary substation, and an operator of a power generating station, associated with a GSU failure. Specifically the method provides for mitigating the risk of prolonged business interruption and the associated revenue loss, reducing downtime when a failure occurs, and providing a spare option without a major capital expenditure. 
         [0051]    The method may include a transformer reserve program (TRP) wherein an arrangement is established between a service provider and a power generating entity for supporting continuity of power transmission through a GSU transformer. The arrangement may provide for maintaining availability of a replacement GSU for replacing at least one phase of an installed GSU of the power generating entity. The arrangement may also include providing services to the power generating entity for preventing failure of an installed GSU transformer and for expediting replacement of one or more phases of the installed GSU) 
         [0052]    The TRP may incorporate a contractual arrangement for a term. For economic viability, the term may extend from about 5 years to about 20 years and may be expected to average about 10 years. The arrangement may include a periodic fee for maintaining availability of one or more phases of a replacement GSU transformer and for expediting installation of one or more phases of a replacement GSU transformer. The periodic fee may include an an annual membership fee per each covered installed transformer. The arrangement may further include a periodic use fee, such as a weekly use fee, when using the temporary spare GSU transformer. The arrangement may also include charging a fee for services including one or more of assessing initial condition, continuing inspection, continuing monitoring, and performing preventive maintenance for the operator&#39;s installed GSU transformer, and providing technical advisory services for removal of the failed GSU transformer and replacement of the replacement GSU transformer. 
         [0053]    The program may also provide availability type guarantee options for customers. Such guarantees may include, but not be limited to one or more features, such as “return to service” time (days) and “on-site delivery” time (days). In addition, the agreement could include all applicable terms and conditions, such as liquidated damages, limits of liability, etc. 
         [0054]    Although such a method may be concentrated on a long-term “contractual lease” a one-time transactional rental may be provided, dependent upon the needs of the power generating entity and an ability to provide such support. 
         [0055]    The TRP may include also provide a variety of services for fee to the power generating entity. A first service may include transformer condition assessments. The assessments may include a thorough analysis and report of the transformer condition. A transformer expert may review and evaluate the pertinent current data and documentation against baseline data to establish the health of each transformer. The health of each transformer may be used to prioritize the maintenance and monitoring needs of each unit. The assessment may rank probability factors against consequence factors to determine a transformer risk index (TRI). 
         [0056]    The TRP may further include continued transformer monitoring, which will collect, analyze and diagnose certain parameters at a designated frequency to assess routine and preventative maintenance needs and other remedial actions. 
         [0057]    The step of providing services may further include providing outage planning for the installed GSU transformer, wherein an outage includes a failure of the installed GSU transformer or a planned removal of at least one phase of the installed GSU transformer from service. 
         [0058]    Outage planning provides a detailed plan of action at the facility level for each transformer included under the arrangement. The outage plan includes all necessary steps that need to be taken, and all information required, should a transformer fail. This includes logistical information, such as contact information for initiating unit repair and replacement activity, all unit data necessary for such activity, all necessary transportation details, permits, limitations, etc. Further, the service provider may perform all necessary pre-engineering required to expedite the removal of the failed unit for repair/replacement, and the movement, placement, and electrical connection of any lease transformer, as applicable. 
         [0059]    The TRP may additionally include transformer condition-based maintenance, including performing preventive maintenance, inspection, testing and reporting on each of the member transformers. 
         [0060]    The TRP may provide for all aspects of transportation of the replacement GSU transformer to the operator&#39;s site, including necessary permits for the replacement GSU transformer. Such preparation may include obtaining pre-permits for emergency transportation of the portable GSU from the applicable transportation agencies to minimize cycle time required for the permit process. Also, provide the transportation to/from customer sites via roadways or other modes of transportation (railroad). 
         [0061]    The TRP may also provide technical advisory services for all deployment events, such as disassembly/removal of customer GSU, installation of portable GSU, electrical tests/energize/deenergize units, repair/replacement proposals, etc. 
         [0062]    As part of the TRP lease program, the temporary lease transformers are stored and maintained by the service provider. Also, the TRP lease program may be maintained by personnel to manage the various levels of transformer coverage provided under the program, monitor transformer maintenance and reporting, and to act as the single point of contact for facilities with member transformers, and the various service provider transformer repair, engineering, and headquarters project management teams. 
         [0063]      FIG. 10  illustrates a flowchart representing a method for maintaining power transmission through a generator stepup (GSU) transformer. In step  1010  an agreement is established between a service provider and an operator for supporting continuity of power transmission through an installed GSU transformer. The arrangement may include charging a periodic fee failure coverage including maintaining availability of at least one phase of a replacement GSU transformer and for expediting installation of at least one phase of a replacement GSU transformer for a failed unit of the operator. The agreement may also include charging a rental fee for use of the replacement GSU transformer. In addition to the failure coverage, the agreement may also provide for charging a fee for services including at least one of assessing initial condition of the installed GSU transformer; continuing inspection; continuing monitoring of the installed GSU transformer; performing preventive maintenance on the installed GSU transformer; and providing technical advisory services for installation and removal of the replacement GSU transformer. 
         [0064]    In step  1015 , it is determined if failure coverage is in place, then the service provider will maintain a spare replacement transformer for the operator in step  1020 . In step  1025  the service provider will also perform transformer replacement planning in preparation for the occurrence of a failure. 
         [0065]    If no failure occurs in step  1030 , the service provider will continue maintaining the spare transformer per step  1020  available. It will also be determined in step  1075  whether the arrangement between the service provider and the operator has other covered services, as previously described. The covered services will be performed by the service provider on the operator&#39;s transformers in step  1080 . The services may include diagnostic, continuing monitoring, and continuing maintenance, which will assist in maintaining the transformer in a safe and continued operating condition. Further under covered services, the service provide will evaluate the installed transformer condition in step  1085  and determine in step  1090  if the installed transformer needs to be taken out of service. If removal from service is not required, then the service provide continues with performance of the other covered services per step  1080 . 
         [0066]    If removal from service is required for the installed transformer in step  1090  or a failure has occurred in step  1030 , then in step  1035 , the spare transformer is transported to the operator site. For a scheduled replacement, the transformer will be shipped to the operator site in advance. In step  1040 , the installed transformer is removed. In step  1045 , the spare transformer is installed. In step  1050 , operation with the spare transformer is restored. It is determined in step  1055  if the operator&#39;s transformer is available for return to service. If the operator transformer is available, then in step  1060  the spare transformer is removed. In step  1065 , the operator&#39;s transformer is installed and in step  1070 , normal operation is resumed. If, in step  1055 , the operator transformer is not available, then operation continues with the spare transformer until such time as the operator transformer becomes available through repair or procurement. 
         [0067]    Pre-engineering is intended to expedite installation of the portable single- phase GSU transformers when they reach the operator&#39;s power plant site. Typically, the low voltage, high-current generator side of the installed GSU transformer is connected to the power plant generator by iso-phase buswork.  FIG. 11  illustrates a connection for the portable single-phase GSU transformers to the installed iso-phase bus infrastructure. The existing iso-phase bus infrastructure  1110  includes a bus  1115  for each output phases A  1120 , B  1121 , C  1122  of an installed generator  1125 . The bus  1115  includes a center conductor  1130  supported within the iso-phase conduit  1131  by insulators  1132 . Shorting plates  1133  tie the iso-phase conduit together at a generator end and at a junction end. The junction  1135  provides for a mechanical connection (internal-not shown) of the iso-phase bus  1115  from the generator  1125  to the installed transformer (not shown). 
         [0068]    When an installed GSU transformer must be removed to make way for a portable replacement, the iso-phase bus between the junction and the installed GSU transformer is removed. A temporary connection plate  1155  is attached to the junction for accepting temporary iso-phase buses for each output phase A  1140 , B  1141  and C  1142  for the portable single-phase GSU transformer  1150 . The portable single-phase GSU transformers  1150  are mounted on pads  1151  or other suitable placement for connection to the junction. The iso-phase buses are connected from the low voltage bushing  1153  for each the portable single-phase GSU transformers to the junction, where a connection is made with respective iso-phase buses on the generator side. A supporting structure  1160  may be provided for each iso-phase bus. Shorting plates  111170  tie the iso-phase conduit together at a generator end and at a junction end 
         [0069]    As an alternative, the connections between the existing iso-phase infrastructure may be made with cable connections as illustrated in  FIG. 12 . One or more high capacity cables  1190  may be connected between the low voltage bushing  1153  for each of the portable single-phase GSU transformers  1150  and the respective connection at the junction  1135  with the existing iso-phase infrastructure  1110  through the temporary connection plate. One or more cable trays  1195  may be provided to maintain the cables  1190  off the ground. 
         [0070]    Pre-engineering may provide for layout and preparation of drawings for the iso-phase bus connections between the existing infrastructure  1110  and the portable single-phase GSU transformers  1150 . Further, some operators may find it advantageous to have the connection hardware built and stored on site to limit replacement time for failed installed units.