The University of Texas at Austin's recent US$45.7 million utility infrastructure capital improvement initiative expanded the total electrical power-delivery capability of the campus to 112 MW. The initiative included projects to replace all of the main distribution switchgear, add a remote monitoring control and metering system, install a 25 MW steam turbine generator and upgrade the existing utility interconnect substation.

The Harris Substation is the fail-safe reliability component of the electrical system that provides the interconnection point between the University of Texas (UT; Austin, Texas, U.S.) and Austin Energy (AE; Austin) electrical grids. Should UT lose any level of generation output, the Harris Substation can still furnish the needed electrical power. Harris Substation had major modifications in the mid-1980s that were based on the projected 20-year electrical load growth. The growth projections were exceeded, requiring additional upgrades.

In a joint effort with AE, UT upgraded the existing two 50 MVA Harris Substation to a four 50 MVA facility. A consulting firm acted as the design engineers, commissioning agent and construction administrators. AE is responsible for the transmission side of the substation, while UT provides the site, transformers and ancillary equipment. Both AE and UT serve distribution loads from the Harris Substation facility. Limited location options necessitated that the upgraded substation be built at the site of the existing substation.

Improvement Needs

UT generates all of its own electrical power to meet the needs of its 160-building, 15.5-million sq ft (1.4 million sq m) campus. Peak electrical demand reached 60 MW in 2003 and is projected to reach 75 MW by 2010. A 100 MW peak load is anticipated by 2030.

Even though self-generation is the primary electrical source for the campus, consideration had to be given for maintaining service without generation. This required that the interconnection substation with AE be capable of supplying the projected load, so in the event of a failed Harris Substation transformer, electrical service to the campus could be maintained. Four 50 MVA transformers were installed to meet this load and provide the required redundancy.

Austin Energy has a 69-kV transmission loop that feeds UT's two existing 50 MVA transformers. This transmission line goes through a sensitive urban area. Ideally, the easiest construction approach for a new substation would be to build on a new site relatively close to the transmission line. For UT, there simply was nowhere else to build that was affordable. The existing site was the only logical choice for the expanded substation after considering transmission line access and reconnection requirements to the existing UT electrical grid. Unfortunately, the site is located in close proximity to several buildings, including a parking garage. Gas-insulated switchgear (GIS) technology was the only option that would work in this compact site.

Included in UT's growing demand are several large (4000-hp) chillers that are located on the extremities of the campus electrical grid. Starting these units on the existing electrical system and maintaining an acceptable voltage profile was becoming more difficult with the load growth. Upsizing the substation and some feeder reconductoring enables UT to maintain acceptable voltage levels even with no online generation. These upgrades allow operational flexibility to meet future load growth.

Project Installation

The existing site for Harris Substation is on a footprint that is approximately 112 ft by 112 ft (34 m by 34 m) and located on a cutout hillside. The maximum area that was available for construction was approximately 1 acre (0.4 hectares). Typically, an open-air type outdoor substation of this size would require a footprint of 3.5 to 4 acres (1.4 to 1.6 hectares). The only possibility of fitting into the existing site was to use GIS technology for the transmission-side switchgear.

GIS technology uses SF₆ as the insulating medium for both the breaker operation and all connecting bus work. All energized parts are enclosed except the connections at the transformer high-voltage bushings. The limited exposure of energized equipment decreases outage possibilities.

Long-term maintenance costs are minimized with the use of the SF₆ gas-filled metal enclosures. However, initial costs are higher using GIS technology. A construction agreement was developed in which UT was liable for much of the additional cost of using this design. AE would still be responsible for all maintenance and repair needs.

A ring-bus transmission-side breaker was possible because of the compactness of the GIS design. All of the GIS equipment is housed in a building (see white building in photos) that is approximately 30 by 60 ft (9 to 18 m). Even though the bus was energized at 69 kV, it was rated for future 138-kV operation. An eight-breaker scheme allowed for a future third 138-kV feed. Additional costs were incurred with the dual 138/69-kV transformer rating, but the higher transmission voltage simply was not available at this time.

When paralleling transformers, controlling circulating current and maintaining the system-voltage profile is always a design consideration. Being able to control the load tap changers (LTCs) on four transformers and minimize the circulating current is difficult. A firm that specializes in LTC controls and applications was hired to develop the programming necessary to operate the system. The programming was incorporated into the supervisory control and data acquisition system (SCADA). The system was also designed to remain operational at 100% capacity in the event of a transformer loss. The LTC operation allows the remaining transformers to divide the load equally and to minimize the current that circulates among them.

The increase in MVA size of the substation caused the available fault current levels to increase significantly. Load-flow and fault-analysis studies were performed using specially designed software. All of the main distribution switchgear is being replaced. The campus loads will not tolerate any outages, so the switchgear is being replaced in stages, allowing electrical service to be maintained and the campus generators to be kept on-line.

The new monitoring system allows real-time access to actual operating conditions. All currents, voltages, watts and VARs can be viewed for each bus and each feeder. Trending can be set up for any of these parameters and used for forecasting. Power plant operators can make adjustments to generator outputs or tap positions and see the effects on the overall system.

This is an essential tool in determining VAR level requirements for each generator. The monitoring system provides the ability to synchronize any piece of equipment back into the electrical grid should that device become disconnected. Any disturbances on the electrical system can be reviewed through the sequence of events data that are stored with a time stamp. Each piece of equipment can be evaluated for the appropriate response, and adjustments can be made as needed. The oscillograph capability of the relays can be used for troubleshooting purposes and to review the system's response in more detail.

The construction delivery process was fairly complex, because AE and UT used their own general contractors and then AE's contractor performed some work for UT. A single contractor would have made managing the project easier, but the funding available for both parties and the time constraints of the project made a single contractor delivery difficult. Dual general contractors required that the construction documents detail the scope of the work for each contractor and that boundaries and responsibilities be clearly defined. Joint construction meetings were held weekly, and schedules and issues were reviewed and confirmed.

The substation was finished in July 2004. It was completed on time, within budget and without a service interruption to the campus. The substation has N-1 redundancy with SCADA that will trend, archive operating data and allow for future control considerations.

The monitoring system allows for the recording of sequence of events in the distribution system, the evaluation and resolution of problems, campus electrical equipment to be operated with or without generation, expanded capability and future automated switching, completely meeting all campus electrical needs and expectations through 2030.

Albert Schuman is an associate director in the Utilities and Energy Management Department at the University of Texas at Austin with responsibilities over the electrical distribution and the elevator sections. He has a BSEE degree from the University of Texas and is a registered engineer in the state of Texas. Schuman has been at UT for 21 years and has previous experience in the engineering consulting and utility industries. He has served as project manager for the utility infrastructure capital improvement projects that totaled about US$45 million.
Al.Schuman@austin.utexas.edu

High-Tech Hybrid SCADA Design

The SCADA package at the University of Texas at Austin (UT) is a morph of equipment from the utility and telecommunications worlds.

System status (breaker in/out, open/closed), as well as general and specific system alarms, are implemented using a PLC that is rated for Class I, Division II hazardous locations and has the ability to provide sequence of events reporting by time-stamping all events with a 1 msec signal provided by an IRIG-B satellite signal. Each PLC operates independently; are set to function as MODBUS slaves across a MODBUS TCP network; and are connected to utility-grade, 100-MB Ethernet switches with 100-MB single-mode, fiber-uplink ports. All Ethernet switches are connected in a ring topology similar in design to the SONET rings that have been used in the telecommunications industry for many years. This allows for multiple data paths back to the MODBUS master servers. These rings allow for path failure without data loss, and for system administration and maintenance without scheduling SCADA system outages.

System event reporting is available using utility-standard equipment. A MODBUS interface to provide the information to the SCADA master servers via the same Ethernet equipment. Employing software provided by equipment manufacturers, UT can use an engineering workstation to query information from revenue-quality meters, protective relays, as well as 1 msec time-stamped SOE data from the PLCs. This allows UT engineers to rapidly and accurately diagnose potential problems and provide safe, efficient solutions in a rapid manner.