Premium Membership ♕

CYBER WEEK OFFER 💥 Save 20% on PRO Membership Plan and Video Courses with the coupon CYM23 and learn from experienced engineers.

Home / Technical Articles / Uprating and expanding existing power substations (good practice and recommendations)

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.

Uprating and expanding existing power substations (good practice and recommendations)
Uprating and expanding existing power substations (good practice and recommendations)

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For instance, major substation uprating or expansion planning might include the replacement of existing outdated protection relays with modern microprocessor relays for improved substation protection and monitoring.

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.

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.

The Cost

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.

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.

Table of contents:

  1. Substation uprating
    1. Major Equipment Uprating
      1. Power Transformer
      2. Oil Circuit Breaker
      3. Current Transformer (CT)
      4. Wave Trap
      5. Coupling Capacitor Voltage Transformer (CCVT)
      6. Bus System
      7. Disconnecting Switches
      8. Surge Arresters
      9. Raceway System
      10. AC/DC Auxiliary Systems
      11. Relaying and Metering
  2. Substation Expansion
    1. Site Work
    2. Grounding
    3. Raceway System
    4. Control House
    5. Substation Equipment
      1. Bus System
      2. Transformers and Circuit Breakers
      3. Carrier Equipment, Surge Arresters, and Voltage Devices
      4. AC/DC Auxiliary Systems
      5. Relaying, Metering and Control
  3. Conclusion

1. Substation uprating

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.

Go back to the Contents Table ↑

1.1 Major Equipment Uprating

1.1.1 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.

Figure 1 – Power transformer

Power transformer uprating
Figure 1 – Power transformer uprating

Go back to the Contents Table ↑

1.1.2 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.

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

33Kv Oil circuit breaker
Figure 2 – 33Kv Oil circuit breaker (photo credit: Raymond d’ orsoy de Flines via Flickr)

Go back to the Contents Table ↑

1.1.3 Current Transformer (CT)

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)

Uprating current transformers (CTs)
Figure 3 – Uprating current transformers (CTs) – photo credit:

Go back to the Contents Table ↑

1.1.4 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 require replacement.

Figure 4 – Uprating a Wave trap

Uprating a Wave trap
Figure 4 – Uprating a Wave trap

Go back to the Contents Table ↑

1.1.5 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.

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)

Uprating a Coupling Capacitor Voltage Transformer (CCVT)
Figure 5 – Uprating a Coupling Capacitor Voltage Transformer (CCVT)

Go back to the Contents Table ↑

1.1.6 Bus System

Two factors enter the uprating considerations regarding the substation bus system:

  1. The current-carrying capacity of the conductors and connections, and
  2. 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.

Under emergency conditions consider a 25 percent maximum bus current increase. These loadings should, however, be limited to a couple of days’ duration. For heat rise computations, the necessary data and mathematical relations are available from conductor manufacturers and industry associations. Once the thermal considerations of the uprated bus have been calculated, decide if the existing conductor should remain or be replaced.

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

Substation busbars
Figure 6 – Substation busbars (photo credit: Edvard CSANYI)

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.

Go back to the Contents Table ↑

1.1.8 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 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

110kV oil switch disconnectors
Figure 7 – 110kV oil switch disconnectors

Go back to the Contents Table ↑

1.1.9 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 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

High voltage surge (lightning) arresters
Figure 8 – High voltage surge (lightning) arresters

Go back to the Contents Table ↑

1.1.10 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.

Figure 9 – Substation ducts with cables

Substation ducts with cables
Figure 9 – Substation ducts with cables

Go back to the Contents Table ↑

1.1.11 AC/DC 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 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.

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.

Suggested Reading – Learn how to analyze and check factory wiring diagrams of an MV switchgear

Learn how to analyse and check factory wiring diagrams of a MV switchgear

Go back to the Contents Table ↑

1.1.12 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.

Suggested Reading – Coordinating study in protection of utility supply line, transformer and plant main bus/feeder

Coordinating study in protection of utility supply line, transformer and plant main bus/feeder

Go back to the Contents Table ↑

2. Substation Expansion

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.

A planned expansion is also the time to consider the possibility of a different voltage level, for example, whether the expansion of a 115 kV substation is designed for the future 230 kV. Phase-to-phase rigid bus spacing is nominally 2.13 meters (7 feet) and 3.35 meters (11 feet), respectively.

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

Substation Expansion Layout
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.

If future bus extensions are anticipated, it may be advantageous to install disconnect switches on the ends of the bus to facilitate future construction with minimal outages. With 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, 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

Gas-insulated substations (GIS)
Figure 11 – Gas-insulated substations (GIS) (photo credit: ABB)

Go back to the Contents Table ↑

2.1 Site Work

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.

Go back to the Contents Table ↑

2.2 Grounding

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.

Suggested Reading – Practical steps in the design of a substation grounding

Practical steps in the design of a substation grounding

Go back to the Contents Table ↑

2.3 Raceway System

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.

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.

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

Substation underground cable duct system
Figure 12 – Substation underground cable duct system

Go back to the Contents Table ↑

2.4 Control House

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.

Suggested Reading – Control house at HV/EHV switchyards and substations

Control house at HV/EHV switchyards and substations (construction, layout and functions)

Go back to the Contents Table ↑

2.5 Substation Equipment

2.5.1 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. 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)

EHV substation layouts for busbar systems (up to 400 kV)

Go back to the Contents Table ↑

2.5.2 Transformers and Circuit Breakers

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

Substation expansion - 220kV Circuit breakers
Figure 13 – Substation expansion; 220kV circuit breakers

Go back to the Contents Table ↑

2.5.3 Carrier Equipment, Surge Arresters, and Voltage Devices

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.

Go back to the Contents Table ↑

2.5.4 AC/DC Auxiliary Systems

Check and possibly revise or increase in capacity of several items in the auxiliary systems to successfully expand an existing substation:

  1. Auxiliary transformer capacity
  2. Throwover switch ratings, full load, and momentary
  3. Low-voltage AC and DC panel circuit capacity and adequacy of mains
  4. Low-voltage switchgear circuit capacity
  5. 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.

Well-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 the main 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.

Suggested Reading – Learn how to read and analyze control circuits of MV GIS

Learn how to read and analyze control circuits of MV gas insulated switchgear (GIS)

Go back to the Contents Table ↑

2.5.5 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.

Suggested Reading – Schematics and docs needed for communication systems of substation protective relaying system

Schematics and docs needed for communication systems of substation protective relaying system

Go back to the Contents Table ↑

3. Conclusion

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.

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 the minimum outage. In this case, if transformers are being paralleled, other chapters in this guide should be consulted for guidelines.

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.

Go back to the Contents Table ↑


  1. Design Guide for Rural Substations by the United States Department of Agriculture

Premium Membership

Get access to premium HV/MV/LV technical articles, electrical engineering guides, research studies and much more! It helps you to shape up your technical skills in your everyday life as an electrical engineer.
More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Joao Fulumbi
    Jul 20, 2023

    Hi Edvard, very good topic i appreciated. I would like to know when expanding or upgrading a substation like adding a new bay, do we need a new lightning study ? while the old one exists.

  2. Santhanam Gopalan
    Jun 19, 2023

    Dear Edvard,

    what are the procedures to be followed in US (NA) for the testing and commissioning of

    a process plant comprising MV and LV electrical generation and distribution system in an new industry.

  3. Kamal Azin
    Jan 23, 2022

    Thank you Edvard for such a generous knowledge sharing.

    Sep 19, 2021

    I’m very glad to reading your documents .
    Because of sanctions and very bad economic condition, there isn’t good job here then I would like to have work in Electrical field like as substation , power plant, solar power system in any country except of IRAN and can study , research.
    I hope you can introduce me for obtaining a good job in power.

  5. Erinosho Jeremiah O.
    Jul 24, 2021

    Please, I am a starter in the study of transformers – construction and working principles.

    I need a detailed (Practical) explanations on the Vector (clock type) groupings of a transformer and its use, why must there be a vector grouping and its applications.

    Thank you sir.

  6. frederico reis da silva
    Jul 06, 2021

    I like posts that can increase knowledge in the electrical field

  7. Goran Uzelac
    Jul 05, 2021

    Vrlo dobro

  8. Goran Uzelac
    Jul 05, 2021

    Veoma dobro.

    • Edvard
      Jul 07, 2021

      Thank you, Goran.

      • Ibrahim Liman Yusuf
        Jan 22, 2022

        Please do you have any certificate for member’s

  9. Ramakanth
    Jul 05, 2021

    Sir I need your help, I am working in distribution, I have a question regarding , what is the protection if one of the line broken and touches the ground or any human being or animal. During that time to save the life and avoid unwanted trippings further
    How can we protect the system( I faced the incident one male elephant electrocuted on 11kv line and feeder not tripped even if elephant died,it is about 17 kms radius from feeder location) and second incident one line snatched and grounded after feeder trips but after 10 minutes it holds even one conductor grounded , due to this one cow died and it is dangerous for humans also)

    • Edvard
      Jul 07, 2021

      You must check the settings of ground-fault protection, or if there is any such protection.

Leave a Reply to Ramakanth

Tell us what you're thinking. We care about your opinion! Please keep in mind that comments are moderated and rel="nofollow" is in use. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Let's have a professional and meaningful conversation instead. Thanks for dropping by!

four  ×  8  =  

Learn How to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!

Subscribe to Weekly Newsletter

Subscribe to our Weekly Digest newsletter and receive free updates on new technical articles, video courses and guides (PDF).
EEP Academy Courses - A hand crafted cutting-edge electrical engineering knowledge