Utility substations rarely remain unchanged throughout their service life. Growing electricity demand, new industrial loads, renewable energy connections, urban development, and aging equipment can all push an existing substation toward its original capacity limit.
In many cases, the next step is not to build an entirely new substation. The utility may instead add another transformer bay, replace an existing transformer with a higher-capacity unit, reinforce the 110kV network, or reorganize the substation so that it can carry more load with greater operational flexibility.
That makes power transformer selection more complicated than simply choosing a higher MVA rating.
As a 110kV power transformer manufacturer, HONLE works with projects where the new transformer must fit into an electrical system that already exists. Voltage ratio, impedance, tap-changing requirements, protection settings, short-circuit conditions, cooling arrangement, dimensions, terminal configuration, and future load growth all need to be considered together.
A successful substation expansion therefore begins with the network—not just the transformer nameplate.
Why Are Utility Substations Being Expanded Instead of Completely Rebuilt?
For an existing utility network, expanding a substation can often be more practical than developing a completely new site.
The original substation may already have:
- 110kV incoming transmission or sub-transmission lines
- established land and access
- switchgear and busbar infrastructure
- protection and control systems
- outgoing distribution feeders
- auxiliary power systems
- grounding infrastructure
- operating and maintenance procedures
If the basic station arrangement remains suitable, increasing transformer capacity can provide additional network capability while making use of existing infrastructure.
Typical projects include:
- adding a second or third 110kV power transformer
- replacing an aging transformer with a larger unit
- upgrading a 110kV substation to support increased downstream demand
- reinforcing the grid around new industrial or residential developments
- increasing capacity for renewable energy integration
- improving N-1 operating capability
- replacing older transformers with lower-loss equipment
- reorganizing transformer loading between neighboring substations
Although these projects may look similar from the outside, the transformer requirements can be very different.
A Larger Power Transformer Is Not Automatically the Right Upgrade
One of the first questions in a substation expansion is usually capacity.
If an existing 31.5MVA transformer is becoming heavily loaded, for example, it may appear straightforward to replace it with a larger unit. But the new power transformer changes electrical conditions throughout the substation.
Higher transformer capacity can affect:
- short-circuit current
- busbar loading
- circuit breaker duties
- protection settings
- cable and conductor ratings
- downstream transformer loading
- cooling requirements
- auxiliary systems
- foundation loads
- oil containment arrangements
The transformer rating therefore needs to be considered as part of the complete network upgrade.
Future load growth is equally important. Selecting a transformer only for the present peak load can leave little margin for new feeders or further expansion. Oversizing the transformer excessively, however, may increase investment and create unnecessary no-load losses.
The target should be a power transformer capacity that fits both the present operating requirement and a realistic future development plan.
Existing Substation Parameters Should Be Checked Before the Transformer Is Specified
A greenfield substation offers more freedom because equipment can be designed together from the beginning.
Expansion projects are different. The new transformer normally has to connect with equipment that may have been operating for years.
Before we configure a 110kV power transformer, several existing system parameters should therefore be confirmed.
| Parameter | Information to Confirm |
| HV system | 110kV network configuration |
| LV/MV side | Required secondary voltage |
| Capacity | Existing and planned MVA |
| Frequency | 50Hz or 60Hz |
| Impedance | Required % impedance |
| Tap control | OLTC/DETC and required range |
| Cooling | Required cooling method |
| Connection | Vector group |
| Installation | Outdoor conditions and altitude |
| Interface | Bushings, cable or bus connection |
| Space | Foundation and transport limits |
These details provide a much better starting point than simply sending a request for “one 110kV transformer.”
Transformer Impedance Becomes Especially Important During Grid Expansion
Impedance deserves particular attention because it affects several parts of the electrical system at the same time.
The impedance of a power transformer influences the short-circuit current that can flow through the transformer. This directly affects the fault duty seen by downstream switchgear and other electrical equipment.
Lower impedance generally allows higher fault current.
Higher impedance helps limit fault current but can also produce greater voltage drop under load.
For a new substation, the entire system can be coordinated around the selected value. In an existing station, however, the new transformer often needs to work with equipment that is already installed.
That means the required impedance should be checked against:
- existing circuit breaker interrupting ratings
- busbar short-circuit withstand capability
- protection coordination
- voltage regulation requirements
- parallel transformer requirements
- downstream network fault levels
A transformer that matches the correct voltage and MVA rating but has unsuitable impedance can still create significant integration problems.
Parallel Operation Requires More Than Matching Two Voltage Ratings
Adding another 110kV power transformer is a common way to expand substation capacity.
The two transformers may operate separately under normal conditions, or the substation may require some degree of parallel operation depending on the bus arrangement and operating philosophy.
Where transformers will operate in parallel, compatibility needs to be checked carefully.
Important parameters include:
Voltage Ratio
The transformers need compatible transformation ratios. A meaningful difference in secondary voltage can cause circulating current even before substantial external load is applied.
Vector Group
The phase displacement must be suitable for the intended parallel arrangement.
Impedance
Large differences in percentage impedance can cause unequal load sharing. One transformer may approach its capacity limit while another remains more lightly loaded.
Tap Position
Tap-changing control needs to be coordinated so that the transformers do not work against each other.
Capacity Ratio
Different MVA ratings can operate together in some designs, but expected load sharing should be evaluated rather than assumed.
For this reason, when a new power transformer will work alongside an existing unit, we recommend providing the existing transformer nameplate and available technical data during the specification stage.
The goal is not necessarily to reproduce the old transformer exactly. It is to make sure the new equipment can work correctly within the intended operating arrangement.
OLTC Requirements Should Reflect How the 110kV Network Is Actually Operated
Voltage regulation is another major consideration in utility substations.
Grid voltage changes throughout the day as generation, demand, power flow, and network configuration change. The downstream distribution network still needs to remain within the required operating range.
For applications requiring adjustment while energized, an on-load tap changer (OLTC) allows the transformer ratio to be changed without taking the transformer out of service.
When defining the OLTC arrangement for a 110kV power transformer, the project should specify:
- required tap range
- number and size of tap steps
- regulated winding
- control method
- remote control requirements
- automatic voltage regulation requirements
- interface with substation control systems
This should be defined according to the grid operating philosophy rather than selected only from a standard transformer configuration.
In an upgraded substation, OLTC settings may also need to coordinate with existing transformers, capacitor banks, reactive power equipment, distributed generation, and downstream voltage-control strategies.
Short-Circuit Strength Matters as Grid Capacity Increases
Grid reinforcement usually increases available power, but it can also increase fault levels.
During a severe short circuit, transformer windings experience substantial electromagnetic forces. The winding structure, clamping system, insulation arrangement, conductors, core, and mechanical assembly must be designed to withstand the stresses associated with the specified system.
This is particularly important for a 110kV power transformer used in a utility substation, where higher system capacity can also mean higher fault levels and greater mechanical stress on the windings during a short circuit. For this reason, short-circuit withstand capability needs to be considered as part of the transformer design rather than treated as a secondary specification.
In our 110kV range, the SZ11-63000/110 power transformer has passed the required national tests as well as a short-circuit withstand capacity test. This provides a practical reference for utility substation expansion and grid reinforcement projects where the transformer may be exposed to demanding fault conditions. Even so, each project should still be evaluated according to its actual short-circuit level, system impedance, protection arrangement, and operating conditions.
For an actual project, however, transformer design should still be based on the specified network conditions rather than assuming that one standard configuration is suitable for every 110kV substation.
Losses Matter More When the Transformer Will Operate for Decades
The purchase price of a power transformer is only one part of its long-term cost.
Utility transformers operate continuously for long periods, so both no-load loss and load loss can accumulate into substantial energy costs over the equipment life.
This becomes particularly important when a utility is replacing an older transformer.
The project may be increasing capacity, but it is also an opportunity to evaluate whether the new transformer can improve the operating efficiency of the station.
No-Load Loss
No-load loss occurs whenever the transformer is energized, even when load demand is low.
It is therefore particularly important for transformers that remain energized continuously.
Load Loss
Load loss increases with current and becomes increasingly important when the power transformer operates at higher loading levels.
A realistic loss evaluation should therefore consider the expected loading profile rather than comparing transformer efficiency at only one operating point.
For utilities managing multiple substations, even relatively small differences in transformer losses can become meaningful when multiplied across many years of operation.
Physical Compatibility Can Decide Whether an Expansion Project Is Practical
Electrical parameters receive most of the attention during transformer selection, but physical constraints can be just as important in an existing substation.
A replacement 110kV power transformer may need to fit an existing transformer bay with limited room for structural modifications.
Important dimensions include:
- overall transformer length and width
- total height
- wheel gauge
- foundation arrangement
- HV and LV bushing positions
- conservator position
- radiator arrangement
- control cabinet location
- maintenance clearance
The route into the substation must also be checked.
A larger power transformer can exceed the limitations of existing access roads, gates, bridges, turning areas, cranes, or unloading positions.
These issues are much easier to solve during engineering than after the transformer reaches the project site.
Existing Busbars, Switchgear, and Protection Must Be Included in the Upgrade Review
Installing a higher-capacity transformer does not automatically increase the capacity of the complete substation.
The transformer may be capable of carrying the new load while another part of the system becomes the limiting factor.
The project should therefore check whether existing:
- 110kV switchgear
- medium-voltage switchgear
- busbars
- CTs
- circuit breakers
- disconnectors
- surge arresters
- power cables
- control and protection systems
remain suitable for the new operating conditions.
Protection settings are particularly important.
Changing transformer capacity or impedance can change fault-current levels and coordination between upstream and downstream protective devices.
Transformer differential protection, overcurrent protection, earth fault protection, temperature protection, Buchholz protection, pressure protection, and other functions need to be coordinated according to the final system design.
For a grid upgrade, the power transformer should therefore be treated as part of the complete protection system rather than as an isolated piece of equipment.
Outdoor Conditions Should Be Defined Before Final Transformer Design
Most 110kV utility power transformers are installed outdoors, where the transformer may remain in service through substantial environmental variation.
The specification should identify conditions such as:
- maximum and minimum ambient temperature
- altitude
- humidity
- pollution level
- coastal or salt-fog exposure
- wind conditions
- seismic requirements where applicable
- unusual dust or industrial contamination
Altitude and ambient temperature are particularly relevant to insulation and cooling performance.
A power transformer intended for a moderate climate at low altitude should not automatically be assumed suitable for a high-altitude site or an environment with unusually high ambient temperatures.
Providing site conditions during the quotation stage allows the transformer configuration, insulation coordination, cooling arrangement, surface treatment, and accessories to be evaluated before production begins.
Cooling Should Be Selected Around Real Loading Conditions
As transformer loading increases, heat has to be removed effectively to keep winding and oil temperatures within acceptable limits.
Oil-immersed 110kV power transformers may use different cooling arrangements depending on capacity and operating requirements.
The correct arrangement depends on factors including:
- transformer capacity
- expected continuous loading
- overload requirements
- ambient temperature
- redundancy requirements
- site noise limitations
- maintenance philosophy
A utility expecting substantial future demand growth may also consider whether the cooling system should provide additional operating flexibility.
The important point is that cooling should not be treated as an accessory decision made after transformer capacity has already been selected. Thermal performance is part of the overall power transformer design.
Grid Upgrades Should Leave Room for the Next Expansion
A substation expansion normally solves an immediate capacity problem, but grid development rarely stops with one project.
New industrial parks may appear. Residential demand may continue increasing. EV charging, data centers, renewable generation, energy storage systems, or additional distribution feeders may connect later.
When specifying a new power transformer, it therefore makes sense to consider the likely network configuration several years ahead.
That does not mean installing unnecessarily oversized equipment.
It means considering whether today’s decisions will restrict tomorrow’s options.
For example:
- Can another transformer bay be added later?
- Can the bus arrangement support additional feeders?
- Is the selected impedance suitable for future short-circuit levels?
- Can the protection system accommodate another transformer?
- Is the OLTC arrangement suitable for expected voltage-control requirements?
- Will the new transformer be able to operate with future units?
These questions can prevent a transformer selected for today’s problem from becoming tomorrow’s constraint.
What Should You Send to a 110kV Power Transformer Manufacturer?
The more complete the project information, the easier it is to develop an appropriate transformer configuration.
For a utility substation expansion, an RFQ should ideally include the available electrical and site information rather than only voltage and capacity.
Useful information includes:
- rated capacity
- HV and LV rated voltage
- system maximum voltage
- frequency
- vector group
- impedance requirement
- tap-changing range
- OLTC requirements
- insulation level
- cooling method
- loss requirements
- noise requirements
- altitude and ambient temperature
- connection arrangement
- protection and monitoring accessories
- applicable IEC or local utility standards
- existing transformer data where parallel operation is planned
- dimensional restrictions for replacement projects
Where some parameters have not yet been finalized, the single-line diagram and existing transformer nameplate can provide useful starting information.
As a power transformer manufacturer, we would rather clarify these conditions before production than discover an interface conflict during installation.
HONLE 110kV Power Transformers for Substation Expansion Projects
For utility substations and high-voltage grid projects, HONLE supplies 110kV oil-immersed power transformers as part of our broader transformer range. Our transformer portfolio also covers other voltage classes and distribution applications, allowing project requirements to be considered according to the actual network rather than forcing every project into the same transformer configuration.
For a substation expansion, our focus begins with the project data: voltage ratio, MVA requirement, impedance, regulation method, insulation level, losses, cooling, installation conditions, terminal arrangement, and interface with the existing station.
This is particularly important for replacement and grid reinforcement projects, where a new power transformer has to fit both the electrical system and the physical site.
If you are preparing a 110kV utility substation expansion or grid upgrade, send us the single-line diagram, required transformer capacity, voltage levels, existing transformer data, and relevant utility specifications. We can use this information to evaluate the required 110kV power transformer configuration for the project.
FAQ About 110kV Power Transformers for Utility Substations
Can a higher-MVA 110kV power transformer directly replace an existing transformer?
Not always. Capacity can be increased only after checking impedance, short-circuit levels, switchgear ratings, protection, busbars, foundation dimensions, cooling, and connection interfaces.
Can two 110kV power transformers with different capacities operate in parallel?
It may be possible, but voltage ratio, vector group, impedance, tap positions, and expected load sharing must be evaluated before parallel operation is approved.
Why is transformer impedance important in a substation expansion?
Transformer impedance affects short-circuit current, voltage regulation, protection coordination, and load sharing between parallel transformers. It is therefore a key parameter in grid upgrade projects.
Does a 110kV utility transformer normally need an OLTC?
Many utility applications use OLTCs because the transformer needs to regulate downstream voltage while remaining energized. The exact tap range and control arrangement should follow the network operating requirements.
What information should be provided when requesting a 110kV power transformer quotation?
At minimum, provide capacity, voltage ratio, frequency, impedance, vector group, tap requirements, cooling, installation conditions, applicable standards, and connection details. For an expansion project, existing transformer data and the substation single-line diagram are also very useful.