UPRATING AND EXPANDING EXISTING SUBSTATIONS

UPRATING AND EXPANDING EXISTING SUBSTATIONSBy Egbodor Peter Uwhubetine1

1.1 APPLICABILITY

All substation design and construction including uprating and expanding has to be based on sound practices to ensure safe and reliable operation. While it may not always be practical in uprating to attain every desired recommended clearance and spacing, the minimum applicable standards where established have to be met or exceeded.Modern practice requires that certain environmental and safety issues be addressed in any substation uprating or expansion project, even though the existing substation may not have been designed with such issues in mind. A suitable oil spill prevention plan, possibly including oil containment facilities, has to be implemented. Fire protection methods (including physical separation, barrier walls, and sprinkler systems) should be weighed against the safety concerns and the costs of fire insurance to arrive at an appropriate design. Several other environmental issues should be considered, as applicable: noise abatement, aesthetics, disposal and containment of hazardous materials, and containment of electromagnetic fields.221.2 FEASIBILITYCost is usually a primary factor when determining a course of action: construction of a new facility versus uprating and/or expanding an existing facility. Prepare construction cost estimates for the schemes under consideration. Generally choose the plan with the most favorable cost/benefit ratio, provided that such action is consistent with the near- and long-range system plan.With facility expansion or new construction, include in cost estimates potential impacts due to underground obstructions and environmental concerns.Consider substation uprating as an alternative where increased capacity is required and routine expansion is hindered due to lack of land area. During the initial planning of an uprating program, it may become apparent, after discussions with manufacturers, that such a program is not cost-effective. In this case, expansion or new construction is usually the most desirable course of action.31.3 SUBSTATION UPRATINGIn uprating substation equipment, the cooperation of the equipment manufacturer is usually required.Although an agent or distributor for the equipment vendor may initially be contacted, obtain final determinations from the manufacturers headquarters engineering staff as to technical feasibility of the uprating, the cost of such work, and where the work can be donefield or manufacturing plant. It may be necessary for the work to be performed at the manufacturers facilities or by its field service personnel to obtain proper warranty of the uprated equipment.When equipment uprating is being considered, only the capacity is increased. The voltage level remains the same. Normally the location of incoming or outgoing circuits remains the same although they may be reconductored for increased capacity.41.3.1 Major Equipment Uprating1. Power Transformer: In the initial phase of a planned substation uprating, furnish the power transformer manufacturer with complete nameplate data. Additionally, supply original purchase information, such as purchase order number and date. This information will make it possible for the manufacturer to retrieve the original design calculations to determine the possible additional capacity.If the original design was conservative, some additional capacity may be possible. A loading history may be necessary to confirm this. If the unit is oil insulated, self-cooled, the addition of radiators and fans should provide added capacity. If the unit is fan-cooled, additional or larger fans or radiators may add to available capacity. Insulating oil pumping, or additional pumping, may be necessary to further increase the rating. In some cases, internal leads may require inspection, testing, and even replacement.There are variations between manufacturers but, in general, a 15 to 20 percent increase in MVA capacity may be possible.5Major Equipment Uprating contd.2. Oil Circuit Breaker: Increasing the MVA capacity of a substation may necessitate increased circuit breaker ratings. Breakers may be inadequately rated for increased continuous and momentary currents and interrupting duty. Consequently, determine the fault and continuous current requirements of all associated breakers.The existing oil circuit breakers may be adequate for the increased full load current but inadequate for the interrupting duty to be imposed.Give the manufacturer of the breakers complete nameplate and purchase data together with the ultimate full load current and asymmetrical fault current expected from the uprating program.From this data the feasibility of the program can be determined as far as the breakers are concerned.New contacts and bushings may possibly overcome any full load current deficiency. Replacement of interrupter units could safely handle the increased interrupting duty. 6Application of capacitors on a substation bus causes severe capacitive current switching duty. Compare rated capacitive switching current for the existing breakers with the anticipated duty to determine the need for breaker mechanism modifications. Consult the breaker manufacturer to determine the need for such modifications

6Major Equipment Uprating contd.3. Current Transformer (CT): Current transformers should be evaluated for thermal rating under the uprate program by the equipment manufacturer when the apparatus is being assessed. Ifdetermined inadequate, replacement will be necessary. Next determine the ratio suitability. For example, a 3000/5 multi-ratio CT, being operated on the 1200/5 tap, can be reconnected for 2000/5 service.Application of multi-ratio CTs on lower rated taps results in less accuracy and can lead to saturation of the CTs (with associated error) under heavy fault conditions. Consider these features in the CT evaluation when fault currents are increased.

4. Wave Trap: Since a wave trap or line trap is a current-rated device, it is undesirable to operate such equipment above the nameplate rating. In most cases of uprating, wave traps will requirereplacement.7Major Equipment Uprating contd.5. Coupling Capacitor Voltage Transformer (CCVT): A CCVT is a voltage-rated device as is the associated line coupling tuner when the CCVT is equipped with carrier current accessories.Replacement will not be required for a capacity uprating program unless the addition of new metering or relaying exceeds the loading limits of the device.

6. Voltage Transformer (VT): A VT is in the same category as a CCVT relative to uprating.

7. Bus System: Two factors enter the uprating considerations regarding the substation bus system:Current-carrying capacity of the conductors and connectionsFault current capability of the conductor support systems8Major Equipment Uprating contd.9(a). An excellent publication of this nature, Aluminum Electrical Conductor Handbook , is available from The Aluminum Association, 750 Third Avenue, New York, NY 10017.

9Major Equipment Uprating contd.The fault currents associated with a substation, in the case of rigid bus mounted with apparatus insulators on structures, cause stress in the insulators and structures. With the added capacity and consequent increase of the fault current, calculate these stresses be to determine if insulators or structures are adequate.(a)The insulator cantilever strength will most likely be the weak element under the uprated condition. Several courses are open to remedy this situation. Insulators of increased cantilever strength can be installed on the center phase only. However, it may be necessary to change all insulators to higher strength, depending on the calculated forces. Additional bus structures to reduce bus span length may be an answer, although probably a costly solution. An alternative solution may be the addition of interphase, fiberglass insulators. Coordination with manufacturers is necessary to find a device that will work properly. Calculations are needed to verify that the additional weight that would be added to the bus is acceptable to the existing design.10(a). Methods of calculation are described in Chapters 4 and 7.10Major Equipment Uprating contd.8. Disconnecting Switches: The increased current of the uprated substation will require that the disconnecting switches be examined for full load rating. This can be done from the substation recordsor the switch nameplates. Also check the momentary current capability. If either the full load or momentary currents are found inadequate, consult the original equipment manufacturer. It may be possible to uprate the switches by additions or replacement of the current-carrying parts and insulators. If this is not possible or the switch vendor no longer manufactures this product, replace the units.9. Surge Arresters: Since the voltage level or substation BIL is not usually increased in the uprating program, the surge (lightning) arresters need not be changed. However, if the existing units are of old and outdated design, it is advisable to replace, in particular, those positioned for power transformer protection. Generally, silicon carbide arresters should be replaced with metal oxide arresters for the improved protection characteristics that are available.1111Major Equipment Uprating contd.10. Raceway System: Essentially, the only changes in the raceway system would be provisions for additional transformer fan and oil pump circuits. If the system is underground and spare raceways or ducts have not been provided, new direct burial plastic conduits can be installed above or beside existing duct banks, thus using the present routing.11. Auxiliary Systems: In an uprating program the essential addition to the auxiliary systems will probably include new ac circuits for transformer fans and oil pumps. Consider these circuits as critical or essential loads and assign them a 100 percent demand factor. It is doubtful that the auxiliary system transformers, panelboards, and service conductors will need increasing in size. Normally these are specified conservatively. In addition, the operating history of the substation may indicate that the existing loads were assigned a demand factor in excess of the true factor. However, check the auxiliary system capacity nevertheless for adequacy. An additional panelboard may be required to provide for additional circuits. Consider fault current ratings of equipment downstream of an uprated auxiliary system transformer.1212Major Equipment Uprating contd.The most important equipment check to make of the ac system in an uprating program is the capacity of the automatic transfer switch. This switch may have to be replaced with a unit having a larger rating,both full load and momentary. It is unlikely that the battery and charger system will be affected by a substation uprating, but also check these components to verify their adequacy.

12. Relaying and Metering: Unless the relaying scheme is being changed concurrently with the substation uprating program, the changes to existing relays will usually consist of revising the settings. Higher fault current ratings may result in the need for complete re-coordination of feeder and bus relaying. Some current transformers may have to be reconnected or replaced for different ratios both for relaying and metering. Since there is usually no voltage change in an uprating program, potential transformers and other voltage devices generally can remain the same.13131. GeneralSubstation expansion is the addition of transmission, subtransmission, or distribution circuits to existing substations. These additional circuits may be required on the primary or secondary side. In some cases modifications to the switching scheme may be necessary or desirable. At the same time, capacity may be increased with the installation of an additional transformer(s). The figure below shows a substation expansion adding 69 kV line, a 69/12 kV transformer, and a 12 kV distribution structure to an existing substation consisting of 69 kV line, a 69/34.5 kV transformer, and a 34.5 kV distribution structure.A planned expansion is also the time to consider the possibility of a different voltage level, for example, whether the expansion of a 132 kV substation be designed for future 330 kV. Phase-to-phase rigid bus spacing is nominally 2.13 meters (7 feet) and 3.35 meters (11 feet), respectively.

1.4 SUBSTATION EXPANSION

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SUBSTATION EXPANSION contd.

1515Installing structures and buswork for a higher voltage spacing and clearance with operation at the present voltage may be warranted when the long-range system plan indicates increasing the voltage at a later date. When the expansion goes to the higher voltage, this portion could be coupled to the existing voltage through a suitable transformer or completely divorced from the lower voltage installation, depending on system configuration.If a higher voltage construction is decided for the expansion and the higher voltage is contemplated within the near term (less than 10 years), design and install foundations for the higher voltage equipment. The advantages of the monolithic pour over the modification of a smaller foundation at a later date faroutweigh the higher cost.Reasonable equipment dimensions and weights for the higher voltage equipment are readily available from equipment manufacturers. The trend is to smaller, not larger, equipment so this risk is reasonable.If future bus extensions are anticipated, it may be advantageous to install disconnect switches on the ends of the bus to facilitate the future construction with minimal outages.

SUBSTATION EXPANSION contd.

1616With the switches open, future bus extensions can be made on the dead side of the switch without de-energizing the existing bus.When land availability is a concern, gas-insulated substations (GIS) are a compact, though costly, solution to restricted space requirements. Typically, such installations become more economical in the 230 kV and higher voltages, but contact equipment vendors to determine applicability for a given installation.

2. Site WorkIf the expansion land area was originally set aside for a lower voltage, it has to be enlarged to accommodate the future higher voltage.Obtain additional soil data in the expansion area. It would be an invalid assumption to take for granted that conditions in the existing site carried on to the expansion area.

SUBSTATION EXPANSION contd.

17173. GroundingTake ground resistivity measurements in the expansion area. These can often be obtained along with the soil data.A reasonable estimate of ground fault current can be calculated for the proposed higher voltage. Design the grounding system for this higher voltage using the required standard methods.4. Raceway SystemIf the existing substation employs an underground duct system, this does not in itself mandate the expansion to this method.It should be noted that cable trench has certain advantages over ducts. A large handhole can be designed to interface the existing ducts to a trench and the advantages of trench used throughout theexpansion area. If the expansion area is later separated from the existing area, the handhole becomes an ideal point of electrical separation.When the higher voltage level is built, the trench can be paralleled with the other trench for the increased cable requirements with segregation usually occurring at this level.

SUBSTATION EXPANSION contd.

18In substations 230 kV and above, there may be concern with shielding of control cables. Make an effort to provide appropriate shielding and segregation of cables routed in cable trench beneath the high-voltage buses.

183. GroundingTake ground resistivity measurements in the expansion area. These can often be obtained along with the soil data.A reasonable estimate of ground fault current can be calculated for the proposed higher voltage. Design the grounding system for this higher voltage using the required standard methods.4. Raceway SystemIf the existing substation employs an underground duct system, this does not in itself mandate the expansion to this method.It should be noted that cable trench has certain advantages over ducts. A large handhole can be designed to interface the existing ducts to a trench and the advantages of trench used throughout theexpansion area. If the expansion area is later separated from the existing area, the handhole becomes an ideal point of electrical separation.When the higher voltage level is built, the trench can be paralleled with the other trench for the increased cable requirements with segregation usually occurring at this level.

SUBSTATION EXPANSION contd.

19In substations 230 kV and above, there may be concern with shielding of control cables. Make an effort to provide appropriate shielding and segregation of cables routed in cable trench beneath the high-voltage buses.

195. Control HouseUnless substation expansion was planned in the original design and the control house sized accordingly, it will probably require enlarging. Design the enlargement with the higher, future voltage in mind.Expansion of the existing control house may or may not be feasible because of physical obstructions or limitations in the construction methods originally used. It may be necessary to build a separate control house, interconnected with the original house by the necessary cable and raceway. Expansion of the existing control house is the preferred method, since it allows for all controls within the same building.Layout of the house should take into consideration the optimum arrangement of control panels to facilitate operations.

SUBSTATION EXPANSION contd.

20In substations 230 kV and above, there may be concern with shielding of control cables. Make an effort to provide appropriate shielding and segregation of cables routed in cable trench beneath the high-voltage buses.

206. Equipment (A) Bus System: Make a conservative estimate of expected fault currents at the higher voltage level and establish the bus BIL along with ground clearances to personnel, roads, and fencing. Following the methods outlined in other chapters, design the bus and insulators at this level taking into account contemplated full load bus current. (B) Transformers and Circuit Breakers: The selection of transformers and circuit breakers together with their associated isolating switches is detailed in other chapters of this guide. Specify this equipment for the operating voltage. Design foundations and switch structures for the higher, future voltage. When the higher voltage becomes a reality, cutover will be more orderly and less time consuming. Specify disconnecting switches with the phase spacing of the higher level. (C) Carrier Equipment, Surge Arresters, and Voltage Devices: Specify this equipment at the operating voltage. However, foundations and supporting structures can and should be designed for the higher voltage for the reasons set forth previously.

SUBSTATION EXPANSION contd.

21In substations 230 kV and above, there may be concern with shielding of control cables. Make an effort to provide appropriate shielding and segregation of cables routed in cable trench beneath the high-voltage buses.

21 (D) Auxiliary Systems: Check and possibly revise or increase in capacity several equipment items in the auxiliary systems to successfully expand an existing substation:i. Auxiliary transformer capacityii. Throw-over switch ratings, full load and momentaryiii. Low-voltage ac and dc panel circuit capacity and adequacy of mainsiv. Low-voltage switchgear circuit capacityv. Battery and charger capacityRedesign or modification of the auxiliary system of the expanded substation is accomplished by summing existing loads with the expansion loads and proceeding. A review of the operating history of the ac system may reveal that the originally assigned demand factors were overly conservative, and the existing capacity may be adequate for the substation expansion.The same could be true regarding the throw-over switch. In the interest of reliability, any deficiency, however slight, indicates replacement of this switch.

SUBSTATION EXPANSION contd.

22as outlined in Chapter 15, AC and DC Auxiliary Systems, for a new installation.

22Well-designed ac and dc systems should have provided ample spare panel circuits and adequate mains.This may not have been done because no expansion was ever considered possible at the particular installation under consideration. A new panel can be tied directly to the existing panel by doubling themain lugs of the existing unit. Locate the new panel close to the existing and full-ampere capacity cable installed.Low-voltage switchgear falls into the same category as the panels. Additions can be made in the same way using individual fused switches or circuit breakers.The dc battery and charger, if not originally specified for equipment additions and/or if found inadequate, should be replaced for the substation expansion.

SUBSTATION EXPANSION contd.

2323 (5) Relaying, Metering and Control: If the same relaying scheme as existing is applied to the substation expansion, the only requirement is the addition of relay panels for the expansion together with associated control panels. In this situation, the metering scheme would undoubtedly remain the same with equipment duplicating the existing equipment.The different loading conditions of the substation with the expansion may require resetting of the relays of the existing portion. Re-coordination of feeder and bus relaying, as well as evaluation of CT ratios, may be required.The reason for the expansion program may dictate more complex, sophisticated protective relaying, both for the existing and the expanded substation. A situation such as this is practically identical to a completely new design and should be treated accordingly.

SUBSTATION EXPANSION contd.

2424All programs involving substation construction require planning. This is especially true of a program of uprating or expansion.The trend is toward assessment of existing substations and individual equipment to develop a predictive maintenance and substation life extension program. This approach implements a planned program forevaluating substation components and making modifications or individual equipment replacements to improve reliability and extend the overall substation life.Such a program can be operated in conjunction with uprating or expansion planning to optimize the replacement and maintenance of substation equipment. For instance, major substation uprating orexpansion planning might include the replacement of existing electromechanical relays with microprocessor relays for improved substation protection and monitoring.Reliability analysis is being implemented in many maintenance programs to assess the probability of failures and prioritize modifications based on safety, economics, obsolescence, and power quality.1.5 PLANNING FOR UPRATING OR EXPANSION2525Maintenance planning should be a part of the early stages of uprating or expansion projects. Such planning includes visual inspections, periodic testing, maintaining of spare parts inventories, logging ofequipment test results, and logging of misoperations and maintenance records.Consider safety issues during the planning stages of any project. Provide and maintain proper tools, personnel protective equipment, safety procedures, and safety training.A Critical Path Method (CPM) or similar method is recommended for scheduling the actual uprating or expansion activities. Include the detailed activities of engineering, material specification, procurement, manufacturing, and delivery times together with itemized construction activities. The construction work may need to be performed in phases to minimize outage time on particular circuits. Plan required service outages to cause the least revenue loss and customer inconvenience. Factor into the program adequate time to account for contingent delays that can and will occur. Inform customers of forthcoming service outages so they can plan their activities around the outages.PLANNING FOR UPRATING OR EXPANSION contd.2626Once the program or plan is developed, assign it to a qualified person to monitor the actual activities, both office and field.The program will probably require revision as time passes, but with a detailed plan, future problem areas can be detected and appropriate action taken before they become crisis areas.PLANNING FOR UPRATING OR EXPANSION contd.2727Successful substation uprating will require a high degree of technical cooperation between the utility company, the engineer, and the manufacturers staff.If uprating is just a stop-gap measure to favor a future program, ask the equipment manufacturer to provide a reasonable life estimate of the uprated equipment. This will assist in the priority assignment ofthe future program.These comments apply largely to power transformers and, if history of operation shows a minimum of operation above rated temperature, this life estimate can be quite reassuring.New substation construction obviously causes the least disturbance, electrically, to the customers and the system. In the case of a small installation, expansion can consist of duplicating the existing installation and making a hot cutover or otherwise placing the new section in service with minimum outage. An expansion to existing facilities is on a par with uprating as to disturbance, but with good planning and management of all phases of the program this can be kept to a minimum.1.6 COMPARISONS-NEW VS. UPRATING OR EXPANSION2828Substation upgrading by itself is difficult to justify because of the extent and cost of the modifications normally required. However, when coupled with concurrent substation expansion, upgrading can often become the best choice compared with construction of a completely new facility.Substation modifications or upgrading are warranted when conditions affecting safety or security are evident. Substations, particularly those of early vintage, may not meet current minimum recommendedrequirements for insulation, electrical clearances, or structural integrity. In these instances, make a thorough examination to determine the most efficient and economical method to improve the situation.Construction of a new installation with ample provisions for future expansion may be the best choice, particularly if extensive modifications are required.

1.7 SUBSTATION UPGRADING

29It should be noted that, because a standards group has lowered permissible operating temperatures or made other standards changes to certain equipment or materials, if no trouble has been experienced and maintenance is properly scheduled on existing equipment installed under the older standards, this equipment need not be arbitrarily replaced.

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