Substation construction planning
All actions involving substation construction require heavy planning. This is especially true of a program of substation uprating or expansion. The trend is toward the assessment of existing substations and individual equipment to develop a predictive maintenance and substation life extension program. There are many figures that need to be carefully planned.
This approach implements a planned program for evaluating 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For instance, major substation uprating or expansion planning might include the replacement of existing outdated protection relays with modern microprocessor relays for improved substation protection and monitoring.
Maintenance planning should be a part of the early stages of uprating or expansion projects. Such planning includes visual inspections, periodic testing, maintaining spare parts inventories, logging of equipment test results, and logging of misoperations and maintenance records.
New vs. Uprating or Expansion Existing Substation
Cost 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 speaking, substation designers should 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, designers should include in cost estimates potential impacts due to underground obstructions and environmental concerns.
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.
- Substation uprating
- Substation Expansion
In 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 manufacturer’s headquarters engineering staff as to technical feasibility of the uprating, the cost of such work, and where the work can be done – field or manufacturing plant.
It may be necessary for the work to be performed at the manufacturer’s facilities or by its field service personnel to obtain a 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.
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.
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.
Figure 1 – Power transformer
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.
New contacts and bushings may possibly overcome any full load current deficiency. Replacement of interrupter units could safely handle the increased interrupting duty.
The application 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.
Figure 2 – 33Kv Oil circuit breaker
Current transformers should be evaluated for thermal rating under the uprating program by the equipment manufacturer when the apparatus is being assessed. If determined inadequate, a 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 a 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.
Figure 3 – Uprating current transformers (CTs)
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 require replacement.
Figure 4 – Uprating a Wave trap
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.
Note that a voltage transformer (VT) is in the same category as a CCVT relative to uprating.
Figure 5 – Uprating a Coupling Capacitor Voltage Transformer (CCVT)
Two factors enter the uprating considerations regarding the substation bus system:
- The current-carrying capacity of the conductors and connections, and
- Fault current capability of the conductor support systems
An increase in bus current is directly proportional to the increase in substation MW capacity. However, the increase in bus heating is proportional to the current squared (I2R). This heat increase has to be considered. Additional heat may, by conduction, affect the connected apparatus.
Also, it becomes progressively more difficult to maintain good bolted joints, free from deterioration, as the temperature increases. For these reasons, a good practice generally indicates rating the bus for a 30°C (54°F) rise over a 40°C (104°F) ambient under full load conditions.
If strain bus, possibly only the drops need changing to a larger size. If the substation uprating is a measure to buy time prior to a more extensive program to serve load growth, possibly the bus need not be replaced.
The fault currents associated with a substation, in the case of a 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.
Figure 6 – Substation busbars
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.
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 records or 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.
Figure 7 – 110kV oil switch disconnectors
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 the 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.
Figure 8 – High voltage surge (lightning) arresters
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.
Figure 9 – Substation ducts with cables
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 to increase 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.
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.
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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.
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Substation expansion is the addition of transmission, sub-transmission, 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). Figure 10 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.
Installing 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.
Figure 10 – Substation expansion layout with the detail
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 far outweigh 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.
When land availability is a concern, gas-insulated substations (GIS) are a compact, though costly, the 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.
Figure 11 – Gas-insulated substations
If 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.
Take 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.
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If the existing substation employs an underground duct system, this does not in itself mandate the expansion of this method. A 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 the expansion area.
In substations 230 kV and above, there may be a concern with the shielding of control cables. Make an effort to provide appropriate shielding and segregation of cables routed in cable trench beneath the high-voltage buses.
Figure 12 – Substation underground cable duct system
Unless 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. The layout of the house should take into consideration the optimum arrangement of control panels to facilitate operations.
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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. Substation designers should design the bus and insulators at this level taking into account contemplated full load bus current.
Suggested Reading – EHV substation layouts for busbar systems (up to 400 kV)
The selection of transformers and circuit breakers together with their associated isolating switches is very important for the expansion of the substation. 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.
Figure 13 – Substation expansion; 220kV circuit breakers
Carrier equipment, surge arresters, and voltage devices should be specified at the operating voltage. However, foundations and supporting structures can and should be designed for the higher voltage for the reasons set forth previously.
Check and possibly revise or increase in capacity of several items in the auxiliary systems to successfully expand an existing substation:
- Auxiliary transformer capacity
- Throwover switch ratings, full load, and momentary
- Low-voltage AC and DC panel circuit capacity and adequacy of mains
- Low-voltage switchgear circuit capacity
- Battery and charger capacity
Redesign or modification of the auxiliary system of the expanded substation is accomplished by summing existing loads with the expansion loads. 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.
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.
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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.
A situation such as this is practically identical to a completely new design and should be treated accordingly.
Suggested Reading – Schematics and docs needed for communication systems of substation protective relaying system
New vs. Uprating or Expansion Existing Substation
Successful substation uprating will require a high degree of technical cooperation between the cooperative, 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 of the future program.
These comments apply largely to power transformers and, if the history of operation shows a minimum of operation above rated temperature, this life estimate can be quite reassuring.
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.
- Design Guide for Rural Substations by the United States Department of Agriculture
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