A primary goal of electric utilities is to maintain reliable and stable service. Therefore utilities forecast and evaluate future power system scenarios and implement the necessary improvements to mitigate adverse impacts. To support reliability and reinforcement improvements to their 138 kV system, a plan was developed to provide an additional source of power into the 138 kV system from the 345 kV system. This involved bisecting an existing 345 kV transmission line and constructing a new 345 kV to 138 kV substation. This paper summarizes engineering solutions to the many challenges involved in a complex transition from old to new facilities. The project was completed with no adverse events and met the stated project objectives.
Power industry, Electricity supply industry, Substations, Construction
The project objective was to augment the power sources into the 138 kV system by providing new connections from the 345 kV transmission system. The project was to also provide greater flexibility for future upgrades of the transmission system in the area, and to increase the amount of available reactive power.
The optimum solution was to build a new 345 kV to 138 kV substation. The 345 kV source to the new substation would be by bisecting an existing double circuit 345 kV transmission line. Existing 138 kV transmission line and generation connections would be transferred to the new substation.
II. DESIGN CONSIDERATIONS
A. Flexibility for Multiple Scenarios
With the future of generation being unknown, the new facility had to incorporate near term and long term possibilities. One long term scenario was the complete closure of another station. The intermediate scenario was retirement of one additional unit. Another scenario was that remaining units continued at full operation.
B. Siting of the new facilities
A criterion of the project was to construct the new facilities without the acquisition of additional property or right of way. At the site of an existing Generation Station, the utility owned multiple parcels of land including the existing substation property, property outside the existing substation within the generating station perimeter security fence and properties adjoining the generating station outside the fence. Due to property constraints, only 149,735 sq. meters of space was available to construct the new substation and transmission line corridors for construction.
C. Environmental constraints
The selected property was the former site of commercial business built in 1917 and operated through early 1973. The tanning process produced wastes that were disposed of on the property. The site had been previously remediated of high level of wastes but areas of lower level waste remained.
The soil properties within the top three to five meters were unsuitable for load bearing. Also the water table at the property was approximately one meter below existing grade.
A. Substation Design
The new 345 kV substation would consist of 345 kV circuit breakers consisting of two three breaker ring buses connected by a bus tie breaker to feed two 300 MVA transformers and four 345 kV lines. The new 138 kV substation would consist of four busses with various bus tie capabilities.
The large footprint for the design of the substations along with the 345 kV and 138 kV transmission corridors dictated that a number of options be evaluated. The objective was to minimize the disturbance of soil and groundwater, while providing the volume of storm water detention. Several general arrangement drawings were prepared and compared to determine the best solution. Air Insulated Switchgear (AIS) and Gas Insulated Switchgear (GIS) were evaluated.
GIS was not selected for the 138 kV switchyard because a) seven of the breakers would be unused if the generation station was closed and AIS breakers could be relocated and used elsewhere, unlike 138 kV GIS breakers, and b) the overall area required for this station was dictated by the 138 kV transmission dead-end take-off structures. However, to minimize the area required for these line terminations, a steel box structure was designed which incorporated the dead-ends in the overall structure. This allowed for a 40% reduction in the overall width from standard AIS arrangement. The final box structure design was 55 meters wide by 168 meters long by 18 meters high, including lightning masts.
AIS versus GIS designs were compared for the 345 kV substation. As the GIS footprint was substantially smaller than the AIS footprint (only 18% of the AIS footprint), it saved over 5,800 m3 of water detention and resulted in a significant savings in soil remediation costs, GIS was selected for the 345 kV switchyard. Swapping the locations of the 138 kV and 345 kV switchyards from their original designed locations also allowed utilization of overhead air-insulated conductors for the incoming 345kV lines in lieu of the underground lines that were originally proposed.
The GIS selected was ASEA Brown Boveri (ABB) ELK-3/420 type switchgear that is more compact than the standard 345 kV switchgear. Since each breaker bay was completely pre-assembled and factory tested prior to shipment installation time was reduced.
The 345 kV GIS equipment was installed in a new precast building 13.4 m wide by 45.3 m long and provided with two 4.5 metric ton capacity cranes with an 8.5 m hook height. Since all of the GIS cable exits were designed to exit the building with GIS bus and SF6-to-Air bushings, a cable vault was not required which allowed for the use of a slab-on-grade building foundation that remained above the water table.
The building was designed and constructed with sufficient space for the future build out even though only seven 345 kV circuit breakers were being installed with this project. To mitigate the impact on the existing station during the future expansion, the original one-line was revised to allow for the installation of the seven breakers in the center of the building, leaving space for future expansion at each end of the building via an overhead door. In addition, a 345 kV GIS disconnect switch and additional (buffer) gas zone was added at each of the two areas designated for future expansion to allow the new bus positions to be installed while the remainder of the bus is energized.
Another key requirement with GIS equipment is the provision of permanent accessibility to all of the GIS view ports and switch operators for the performance of GIS monitoring and maintenance. To meet this requirement an elaborate two-tier (at 1560 mm and 4150 mm) catwalk platform was installed the entire length of the GIS equipment. Even so, there were still some areas that were not accessible via the permanent catwalk so for several locations camera remote junction boxes were installed and at a few others portable rolling ladders were provided.
B. Foundation Elements
Several factors and additional constraints had to be considered in the selection of foundation types used for this project. The high water table along with waste cost of disposing of the contaminated soils and the unsuitable bearing capacity of the upper layer of soil were concerns for all of the foundations with the exception of the capacitor bank structures foundations. Outside the substation where the majority of the transmission line structures were located the high water table and the soil profiles were the controlling factors in the foundation selection process. For one of the new transmission structures located within the existing substation, the existence of multiple buried obstructions and existing foundations played a role in the selection process for this foundation.
C. Heavily Loaded Substation Foundations
For the heavily loaded 345 kV dead end structure, 345 kV GIS building, transformer/containment structure and the 138 kV box structure, after considerable review of possible soil improvement methods and alternate foundation systems, it was determined that large diameter reinforced concrete filled pipe piles would be the most effective.
The use of this type of foundation was very beneficial to the project. The steel pipes were easily driven into the loose upper sandy soil and were pounded in a harder dense clayey layer located below the unsuitable load bearing strata. These 40.6 centimeter diameter piles are displacement piles which eliminated the need to excavate and dispose of existing soils. Eliminating the need to excavate also eliminated any need for dewatering. The pipes also acted as a form for the reinforced concrete that was required to transfer the large vertical and lateral loads to the load bearing strata. A total of 931 of these piles were utilized in the substation. These piles were designed to resist substantial vertical tensil and lateral loads.
The foundation for the 345 kV GIS building consisted of four rows of piles that support reinforced concrete grade beams built integral with a reinforced concrete slab which supported the 345 kV GIS equipment and its enclosure building.
The foundation for the transformer/containment structure consisted of an 11 by 39 grid of piles which support a reinforced concrete mat. The mat in turn supports four transformers, four reinforced concrete transformer support pedestals and 140 meters of reinforced concrete firewalls.
The foundations for the 70 columns of the 138 kV box structure each consisted of four piles that support a pile cap with a concrete pier on which the box structure columns are attached by four to eight anchor rods. The columns are on a 14 meter grid pattern that covered an area 168 meters long by 55 meters wide.
D. Lightly Loaded Substation Structures
For the lightly loaded exterior gas insulated bus (GIB), the 345 kV and 138 kV control buildings, the 138 kV circuit breakers and the 138 kV lightning arrestor and capacitance coupled voltage transformer (CCVT) support structures foundations, smaller 23 centimeter diameter concrete filled pipe piles were selected. The selection of the small diameter pipe piles was based on the same concerns as the large diameter pipe piles and allowed for an installation that would not require excavation of waste impacted soils and the handling of contaminated water. These piles were designed to resist a vertical load of 9 metric tons, a tensile load of 9 metric tons and no lateral load capacity per pile. Passive earth pressure provides the lateral load resistance of the foundations that utilize these piles. A total of 700 small diameter piles were used on this project.
E. Special Case Transmission Structure
One transmission structure was located within the existing substation in the middle of four existing transformers. A below grade investigation was performed by hydro excavation and numerous below grade obstructions were discovered. With the layout of the below grade obstructions and the size constraint due to the proximity to the existing foundations, it was determined that the foundation for this structure would be a reinforced concrete mat supported by a grid of auger cast piles. The installation of auger cast piles would have little impact on the operation of the active substation, unlike a driven pile that would induce a substantial amount of vibration. The location of the auger cast piles were selected to fit between the numerous obstructions and were designed to support the loads applied by the structure.
The 345 kV GIS Building is of a similar design to numerous other GIS buildings that Sargent & Lundy has successfully used in the past. The building has a structural steel frame designed to support the building roof as well as the girders and rails for two cranes used for the installation and maintenance of the GIS equipment. The exterior walls of the building are precast concrete panels that act as the lateral load resisting system of the building. Overhead doors located on either end of the building were installed to facilitate current installation and the future expansion of the GIS equipment. This has proven to be a very efficient design that can be erected quickly which was a benefit to this project due the condensed construction schedule. The overall size of this building is 700 square meters and a roof that is 11 meters above grade.
G. Transformer/Containment Structure
This structure was designed to accommodate the two 345 kV to 138 kV transformers. There are four 15 meter wide by 15 meter deep transformer bays in which a reinforced concrete transformer plinth and three GIB support plinths are located. On three sides of each bay is a 9 meter high firewall. The forth side of each bay was open to allow for the installation and maintenance of the transformers. Spanning the open side of each bay, a horizontal lattice truss was installed to support the 138 kV strain bus needed to connect the transformers to the 138 kV box structure.
Due to the high water table, the depth of the containment area around the transformers was restricted to an average depth of 46 centimeters. This shallow depth was not sufficient to meet oil retaining requirements. To comply with the requirements, each bay was connected to a collector pipe which drains the transformer bays into a 374 square by 1.2 meter deep containment basin.
H. 138 kV Box Structure
The size of the typical bay for the box structure was dictated by the requirement that the incoming and outgoing 138 kV transmission lines be spaced at 4.6 meters apart at their termination points. This required that the typical bay width would need to be 13.7 meters. At the two bus tie bays, where no transmission lines are attached, the bay width was decreased to 9.1 meters. The structure is four bays wide to accept the strain bus coming from the low side of the transformers. Along the length there are 12 bays that accept incoming and outgoing transmission lines and two bus tie bays. This resulted in a box structure that is 55 meters wide by 168 meters long. Due to the extreme length of this structure two thermal expansion joints at approximately one third spacing were incorporated in its design.
Due to the compressed construction schedule and overall size of the structure, a steel fabricator was consulted early in the design process to brainstorm ideas to simplify and standardize the design for quick and easy construction. Ideas that were incorporated into the design and fabrication included; providing setting plates for the anchor rods with cross hairs to insure the proper placement, designing the moment connections with end plates to eliminate field welding, adjusting the column heights to utilize standard mill rolling lengths and standardizing all of the framing to be repetitive to reduce the quantity of different members that would need to be fabricated and facilitate field erection of the structure.
The new 138 kV substation replaced an existing substation, and required the transfer of seven 138 kV transmission lines, three 138 kV connections to Waukegan Generating Station, and four 138 kV feeds to distribution transformers. The relay protection and communications were also upgraded at twelve remote substations and customer sites.
Commissioning (outage planning and construction sequencing) began in early 2012 to support the regional transmission organization (RTO) transmission outage scheduling requirements and to provide input to early design decisions. A high level cut-over approach was developed: energize the new 345 kV GIS substation, then the 345/138 kV transformers and finally the new 138 kV substation. A commissioning plan was developed that detailed the outage planning and construction sequencing for the project. The commissioning plan is a multiple page document using a one-line diagram depiction with each page detailing the chronology of the construction sequencing, outage cut-overs and other important project evolutions. This commissioning plan was modified and updated throughout the project as necessary to document the current plan details.
An example of a construction sequencing decision based on commissioning plan development is the transmission line cut-over sequence. The location of the existing seven 138 kV transmission lines was in a corridor north of the new 138 kV substation and running in an east to west direction. A “south to north” transmission line cut-over sequence was selected from a constructability standpoint. The assumption was, as transmission lines were cut-over, the structures and conductors which were identified to be demolished were removed to make room for the next evolution of transmission line demolition and installation. This assumption as all other transmission system configuration modifications was analyzed by Transmission Planning group.
The goal to reduce the duration of outages was considered during all major design decisions. When required for physical and electrical clearance, short duration outages were utilized to install equipment (foundations, structures, etc.) to minimize the duration of cut-over outages. Another example of early project outage planning was related to the installation of the new 345 kV conductors over the existing 138 kV transmission line corridor. A 138 kV outage plan was developed to install these conductors safely and to minimize system risk. This early installation minimized the actual 345 kV cut-over outage durations.
During the cut-over sequencing discussion it was determined that 138 kV connections would need to be installed to connect the existing 138 kV substation and the new 138 kV substation during the period of the 138 kV transmission line and the generating unit cut-overs. Based on the contingency analysis, four “temporary” 138 kV connections were required between the existing and new 138 kV substations. These “temporary” connections” were on average 700 meters long. The commissioning plan included the installation of these “temporary” 138 kV connections. The overall design approach to create these “temporary” connections was to utilize existing substation 138 kV circuit breakers which were made available after their respective transmission line was cut-over. The transmission line paths for these “temporary” connections were designed using a combination of existing and new structures and existing and new 138 kV conductors. As the 138 kV generating and distribution services were to be cut-over later in the overall sequencing, these circuit breakers at the new substation were used for the “temporary” 138 kV connections. A line differential relay scheme was used to protect these “temporary” connections. For two of the “temporary” connections the new existing relaying was able to be used with only relay setting modification. For the other two connections new relaying had to be procured for the “temporary” connections.
These “temporary” connections were energized at the required times during the 138 kV cut-over sequence and all four were in-service during the summer of 2014.
Five 138 kV transmission lines were transferred to the new substation by June 2014. The transfer of the remaining two 138 kV transmission lines and distribution service connections were completed by December 2014. The transfer of the generating station services took place during their respective outages in 2015. During these final cut-overs the “temporary” connections were removed.
Safety was the highest priority on the project. Actions to communicate and support the emphasis for safety included:
- An overall project safety plan to communicate expectations.
- Specific safety plans by each work group.
- Twice daily job briefings by each work group on site.
- A “safety council” with representation by each work group conducted a weekly walk down of the site and in-progress activities.
- Lift plans for all major crane operations.
- A weekly audit by an independent safety professional.
- Daily observation by the site construction managers.
The project was completed without any Lost Work Days or Occupation Safety and Health Administration (OSHA) reportable events.
Environmental compliance was also a high priority. Due to the magnitude of contaminated soil that would be disturbed, a National Pollutant Discharge Elimination System (NPDES) permit was obtained, and a Stormwater Pollution Prevention Plan (SWPPP) was in effect.
With the potential for the presence of Blanding’s Turtles, a “Fact Sheet” was provided to each worker on site. The sheet provided pictures to assist in identification of the turtles and what actions to take if detected. Inspections for the turtles were conducted with the routine SWPPP inspections.
Due to the waste in the soils, an environmental consultant performed frequent observations to assure proper worker precautions and that soils were not migrated into improper locations.
The project was completed in full compliance of all environmental requirements and regulations.
The schedule for the project was very condensed with only one year to complete the initial phase to reach substantial completion and be available for service. The project broke ground on June 6, 2013. The 345 kV system was energized on April 4, 2014 and 138 kV system was energized and carrying load on April 6, 2014, ten months after breaking ground. The project was substantially complete and available for service on May 29, 2014. Project construction completed in December 2014 with all physical work and system configuration changes complete. All schedule milestones for the project were achieved.
The project was a massive undertaking. It was one of the largest substation projects at the time. The project successfully fulfilled the objectives for both the initial phase and at final completion. By providing an additional source of power into the 138 kV system, voltage stability and system operational flexibility has greatly increased. Although not discussed in this paper, additional benefit was achieved by replacing the means of relay communications from telephone line to fiber optic; and the replacement or removal of aging 138 kV equipment at the original substation thereby reducing emergent issues and maintenance.
Edward L Crockett, James T Kerkhoven, SE Doug Eakins, Brian J. Smith, Carl M. Formento