With electric deregulation becoming a reality in an increasing number of states, several of our larger clients have approached us for help in restructuring their electric service to best take advantage of the deregulated environment.
With electric deregulation becoming a reality in an increasing number of states, several of our larger clients have approached us for help in restructuring their electric service to best take advantage of the deregulated environment. In many cases, having access to the interconnected utility grid-and thus to a wide variety of potential sellers-requires the migration of electric service from the serving utility's distribution system to their transmission system. In doing so, the industrial facility must assume all of the design and operational responsibilities traditionally handled by the utility. In most cases, however, the rewards for converting are well worth the effort.
Distribution service voltages in the United States are generally in the range of 5 kilovolts (kV) to 35 kV, phase to phase, while subtransmission and transmission voltages are 69 kV and higher. Except for those with the largest loads, most industrial consumers are supplied at a distribution voltage, even though a transmission line often exists in close proximity to the industrial property.
In the traditional operation of the regulated electric utility, this was the mode of operation: the vertically integrated utility provided a complete service package from generation to utilization voltage.
Bypassing the utility
In the deregulated environment, the customer can select a preferred generation source and, in effect, use the electric transmission system as a common carrier to get energy to his facility, bypassing altogether the local utility's distribution system. The decision to bypass the distribution system is primarily economic (see "Distribution vs. Transmission: Comparing Tariffs," page 51). The decision also hinges on other key factors, such as:
If, however, any significant length of transmission line has to be constructed to serve the facility, both the cost and environmental impact considerations of that line can quickly become overwhelming.
Utilities are very reluctant, however, to allow any but the largest loads to interconnect with their very-high-voltage system. Also, the cost of a transformer to interconnect at these voltages is quite high.
Once an approved single-line connection diagram is obtained from the serving utility, they will usually provide sufficient data to allow the engineer to design a substation that will serve the needs of the industrial facility and still be fully compatible with the utility's system operation.
The first key piece of information is the short-circuit capability of the utility system at the point of interconnection. In the eastern United States, the interconnected transmission system is capable of producing high short-circuit currents into system faults. Detailed short-circuit computer simulations are done to ensure that the substation equipment can withstand the electrical and mechanical stresses placed on it under both normal and abnormal operating conditions.
Secondly, the utility must fully describe the protective-relaying equipment that has been installed on the transmission line. The industrial substation designer must provide a substation protective-relaying system that is completely coordinated with the utility's relaying system such that the two systems essentially function as one.
Orientation and grounding
Once these initial parameters have been determined, the physical layout of the substation can begin. First, the general orientation of the substation depends heavily on the direction of the primary and secondary circuits coming into and out of the substation and whether those circuits are underground or overhead construction. It is also appropriate at this time to consider the aesthetics of the installation. If the substation is in close proximity to a residential area, it may be wise to consider a low-profile design that utilizes SF6 insulated equipment. If there are no aesthetic considerations, a standard air-insulated bus design is more cost effective. If there are space restrictions or if the substation is to be located indoors, the SF6 option offers great flexibility.
Grounding the substation is the next major consideration, one that in too many instances is not given the attention it deserves. With more and more computers and robots being installed in the modern manufacturing facility, a poor or wandering ground is a recipe for disaster, especially when there is a nearby problem on the electric transmission system. The consulting engineer should encourage the client to employ a robust grounding system for all installations.
Once all of the general design criteria are established, the detailed equipment specifications are developed. In North America, broad safety issues are governed by standards of design, manufacturing and testing as promulgated by National Electrical Manufacturers Association (NEMA), the American National Standards Institute (ANSI) and the Institute of Electrical and Electronics Engineers (IEEE). Within these standards, there is a wide array of choices in insulating and interrupting media, interrupting ratings and fault-current withstand capabilities, circuit- breaker operating speed and transformer kilovolt-ampere (kVA) ratings.
Low- and medium-voltage circuit breakers today are available with air, vacuum, SF6 and oil interrupters. At the higher voltages, the designer's choices are reduced to oil and SF6. From here, the designer must choose from a wide array of options in mechanical and electrical withstand and interrupting capabilities, duty cycle and physical layout of the facility.
The choice of substation transformer is based on available voltages, megavolt-ampere (MVA) rating and the insulation level required. Station voltages are dictated by the local utility and the utilization voltages required in the industrial facility. The transformer MVA rating is usually given by three values:
The rating with no forced cooling, called OA.
The rating with forced-air cooling, called FA.
The rating with forced-oil and forced-air cooling, FOA.
Typical distribution transformers are rated 12/16/20 MVA, indicating that the full-load rating is 12 MVA with no cooling. Turning on the fans on the transformer radiators will increase the rating to 16 MVA; lastly, turning on the fans and the oil pump will further increase the unit's capability to 20 MVA.
Base impulse level , referred to as BIL, is a measure of the transformer's ability to withstand short-duration, high-magnitude electrical surges such as lightning or switching surges. The BIL level of the transformer is determined by the level of expected switching surges on the transmission system as well as by the frequency of lightning strikes in the geographic area. The BIL level of the transformer must be coordinated with the rating of the surge arresters connected to the high-voltage side of the transformer.
Transformer efficiency can vary depending on the size of the transformers and on the manufacturer. A small 5-kVA transformer may have an efficiency of 94 percent while a large 20-MVA transformer may have an efficiency as high as 99.4 percent. A simple economic analysis comparing the increased cost of a more efficient transformer with lower losses will quickly reveal if the improved efficiency is cost effective.
The final major design decision to be made on the transformer is the need for a voltage regulator . This feature becomes critical for certain heating processes or if the facility has critical voltage requirements for process control. If the transmission-system voltage experiences fluctuations or if the secondary load varies over a wide range, the voltage regulator will keep the load voltage within a narrow range.
Two types of regulators are available: the first is a load-tap changer, which is an integral part of the transformer; the second is a set of three induction-voltage regulators located separately from the transformer. The primary difference between the two is that the induction units can regulate the three-phase voltages individually while the operation of the tap changer requires that all three phase voltages move together.
With this information, the substation layout can begin. The layout of a gas-insulated substation is generally more straightforward than for a comparable air-insulated substation. There are fewer component choices in gas-insulated substation components and the layout choices tend to be concentrated in a layering format.
While air-insulated substations offer much more latitude for creativity, good engineers try to channel that creativity toward economy of design, as these conversion projects often have slim margins. In air-insulated designs, tubular designs are favored by leading engineers for switchyard equipment over the lattice designs, because the tubular designs lend themselves more to shop fabrication rather than field fabrication.
Throughout the design process, constructability should be stressed at all costs. The small cost premium associated with systems that are factory preassembled or prewired, or both, is generally more than offset by the reduction in field assembly time.
The final area that should be stressed in any designs is the ability to remove equipment if a failure occurs. The consulting firm should provide detailed removal instructions for all major equipment in the substation-while still keeping the remainder of the station in operation.
In the detailed design of the station, snow, ice and wind loading of the outdoor structures should be a concern. For projects in California, seismic considerations should be included as well. In general, engineers should design for the so-called 100-year weather event. In the case of tornadoes, it is advisable to design for either an F4 or F5 tornado, depending on the location and the client's desires.
After the design process is complete, the consulting engineer should monitor key areas of construction, particularly those areas such as grounding and high-voltage cable terminations that become hidden by later construction. It is also critical to witness key testing procedures on circuit breakers and transformers to ensure that specifications are fully met.
This article and the recommendations it contains are not exhaustive in the area of substation design. However, this overview nevertheless provides a framework for consulting electrical engineers to evaluate the merits of converting an existing distribution substation to transmission service to take advantage of the new world of opportunities in the deregulated electric market.