A solar power plant does not use only one transformer for one purpose. Depending on the plant capacity, inverter arrangement, collection voltage, grid-connection voltage, and auxiliary power demand, several types of power transformers may be installed at different points in the electrical system.
Near the photovoltaic arrays, transformers collect and step up the AC output from solar inverters. Inside the medium-voltage collection network, they help combine multiple generation blocks. At the main substation, larger power transformers raise the voltage again so that electricity can enter the regional distribution or transmission grid. Separate transformers may also supply the plant’s control, lighting, cooling, communication, and maintenance systems.
This means that the phrase solar power transformers can describe several different transformer applications rather than one fixed product. A transformer designed for an inverter station does not necessarily have the same capacity, winding arrangement, insulation level, cooling system, or protection requirements as the main grid-connection transformer.
Solar-generation transformers can be liquid-filled or dry-type and are generally designed according to the generation capacity, collection-system voltage, applicable standards, environmental conditions, and customer specifications.
Understanding where each transformer is installed helps project engineers avoid undersizing, unnecessary oversizing, unsuitable insulation design, incorrect winding connections, and poor coordination between inverters, transformers, switchgear, and the grid.
Where Are Power Transformers Located in a Solar Energy System?
The electrical path through a utility-scale photovoltaic plant can generally be represented as:
Solar modules → DC collection system → inverter → inverter step-up transformer → medium-voltage collection network → main power transformer → utility grid
Additional auxiliary transformers may be connected to the medium-voltage or low-voltage system to supply the solar plant itself.
The exact arrangement differs from project to project. A small commercial rooftop installation may connect directly to an existing low-voltage distribution system and may not need a dedicated large power transformer. A utility-scale solar plant, however, may include hundreds of inverter units, many step-up transformers, several collection feeders, and one or more main power transformers.
The main transformer locations normally include:
- The inverter output side
- The photovoltaic power block
- The medium-voltage collection station
- The main grid-connection substation
- The plant auxiliary power system
- The energy storage connection point, when battery storage is included
Each location has a different electrical purpose.
What Does an Inverter Step-Up Transformer Do?
An inverter step-up transformer is installed after the solar inverter.
Solar panels produce direct current, but a conventional transformer requires alternating current. The inverter therefore converts the DC electricity into AC before the transformer raises the voltage.
For example, an inverter may deliver low-voltage AC while the solar plant’s internal collection network operates at medium voltage. The inverter step-up transformer increases the inverter output voltage so that power can be transmitted through the site more efficiently.
Raising the voltage reduces the current required to transmit the same amount of power. Lower current can reduce cable losses, voltage drop, and the conductor size required for long collection routes.
Where is it installed?
The inverter step-up transformer is usually installed close to the inverter or inverter station. It may be supplied as part of an integrated power conversion station containing:
- Solar inverters
- Low-voltage switchgear
- Step-up transformer
- Medium-voltage switchgear
- Protection and monitoring devices
- Auxiliary power equipment
- Enclosure or skid structure
Placing the transformer close to the inverter keeps the low-voltage cable route relatively short. This is important because low-voltage circuits carry higher current than medium-voltage circuits of the same power rating.
What configurations are available?
An inverter step-up transformer may have:
- One low-voltage winding and one medium-voltage winding
- Two low-voltage windings and one medium-voltage winding
- Several low-voltage windings connected to multiple inverter outputs
- An additional tertiary winding for a specific system function
The correct configuration depends on how many inverters must be connected and whether their outputs should remain electrically separated.
A two-winding transformer is suitable for a relatively straightforward connection between one inverter group and one medium-voltage feeder. A three-winding or multi-winding transformer can collect the outputs of several inverter groups in one unit.
What operating conditions must it withstand?
An inverter transformer operates with a power-electronic source rather than a conventional rotating generator. Its design may therefore need to consider:
- Harmonic currents
- Rapid power variations
- Switching-related voltage stress
- Repeated daily energization
- Partial-load operation
- High ambient temperature
- Uneven loading between windings
- DC components or common-mode effects
- Grounding and insulation coordination
The inverter manufacturer’s electrical data should be reviewed before the transformer design is finalized.
Why Are Multi-Winding Transformers Used in Solar Power Blocks?
Large photovoltaic plants are commonly divided into power blocks. Each block contains a defined amount of solar generation capacity, several inverter inputs, transformer equipment, and a medium-voltage connection.
A multi-winding transformer can connect several inverter groups to one common medium-voltage winding.
For example, a transformer may contain:
- Two low-voltage windings and one medium-voltage winding
- Four low-voltage windings and one medium-voltage winding
- Other customized winding combinations
The independent low-voltage windings allow separate inverter circuits to transfer power through a common transformer without directly connecting every inverter to the same low-voltage bus.
What are the benefits?
Multi-winding solar power transformers may help:
- Reduce the total number of transformer units
- Reduce medium-voltage connection points
- Create more compact power stations
- Integrate multiple inverters into one power block
- Shorten some cable routes
- Standardize repeated plant sections
However, fewer transformer units do not always mean a better design. A larger multi-winding transformer can be more complicated to manufacture, protect, transport, replace, and maintain.
If it fails, several inverter groups may be disconnected at the same time. The design must therefore balance equipment integration against plant availability.
What must be checked?
The following details are particularly important:
- Capacity assigned to each winding
- Maximum simultaneous loading
- Impedance between individual windings
- Permitted load imbalance
- Inverter output compatibility
- Fault-current behavior
- Harmonic interaction
- Temperature rise
- Protection coordination
A multi-winding transformer should not be specified only by adding together the rated power of all connected inverters.
What Is the Role of Transformers in the Medium-Voltage Collection Network?
After the inverter transformers raise the voltage, electricity enters the solar plant’s medium-voltage collection network.
This network collects power from multiple photovoltaic blocks and delivers it to the main substation. Depending on the plant design, the collection system may contain underground cables, overhead lines, ring-main units, switchgear, feeder protection, and collection substations.
Not every plant needs an additional transformer within the collection network. In many projects, the inverter step-up transformers feed the collection voltage directly.
However, an intermediate transformer may be needed when:
- Different parts of the plant use different collection voltages
- Several collection networks must be interconnected
- A remote section requires another voltage transformation
- The plant is expanded using equipment with a different voltage rating
- A separate substation is used to combine large generation zones
- Electrical isolation is required between system sections
Why must the voltage level be coordinated?
The collection voltage affects cable current, losses, insulation cost, switchgear rating, and the number of feeders needed.
A lower collection voltage may use simpler equipment but require higher current and more cable circuits. A higher voltage can reduce current but may increase insulation, switchgear, and transformer requirements.
The transformer voltage ratio must therefore be selected as part of the entire collection-system design.
What Does the Main Power Transformer Do?
The main power transformer is usually the largest transformer in a utility-scale solar project.
It is installed at the plant’s main substation and connects the medium-voltage collection system to the utility distribution or transmission network.
For example, the solar plant may collect electricity at medium voltage but connect to a much higher grid voltage. The main power transformer raises the collection voltage to the required point-of-interconnection voltage.
Large power-station step-up transformers form a critical link between generating facilities and high-voltage networks.
How is it different from an inverter transformer?
An inverter transformer normally serves one photovoltaic power block or a limited group of inverters. The main power transformer receives the combined output of many power blocks.
| Item | Inverter step-up transformer | Main power transformer |
| Typical location | Near inverter station | Main grid substation |
| Main input | One or several inverter groups | Entire collection system or major plant section |
| Output connection | Medium-voltage collection network | Distribution or transmission grid |
| Relative capacity | Smaller | Much larger |
| Quantity | Multiple units may be used | Often one or several large units |
| Main design focus | Inverter compatibility and block integration | Grid connection, high-voltage insulation and plant output |
The main transformer may include an on-load tap changer to help maintain the required voltage under changing grid and generation conditions.
What information is needed?
The main transformer specification normally includes:
- Rated plant output
- Collection-system voltage
- Grid-connection voltage
- Frequency
- Vector group
- Short-circuit impedance
- Insulation level
- Tap range
- Cooling class
- Grid fault level
- Overload requirements
- Reactive power operating range
- Harmonic limits
- Environmental conditions
- Noise limits
- Protection requirements
Because the main transformer connects the solar plant to the external grid, its design must comply with utility and grid-code requirements.
A large solar substation may contain not only high-voltage power transformers but also circuit breakers, instrument transformers, reactors and distribution transformers.
Why Might a Solar Plant Use Two Main Transformers?
Some large solar power plants use more than one main power transformer.
The reasons may include:
- Plant capacity is too large for one practical transformer
- Generation is divided into separate collection zones
- Redundancy is required
- Transportation limits restrict transformer dimensions or weight
- The project is built in stages
- Different grid-connection circuits are used
- Maintenance must be possible without shutting down the entire plant
Using two transformers can improve operational flexibility, but it also requires more switchgear, foundations, protection, cables, and coordination.
The project should compare:
- Initial investment
- Energy losses
- Reliability
- Maintenance access
- Spare capacity
- Outage impact
- Future expansion
- Grid requirements
Two smaller transformers may provide better redundancy, while one larger unit may reduce equipment count and simplify the substation.
What Is a Station Service Transformer?
A solar power plant consumes electricity even though its main purpose is to generate electricity.
Control systems, switchgear, communication equipment, cooling fans, pumps, lighting, security systems, maintenance sockets, fire protection, heating, ventilation, and monitoring devices all require power.
A station service transformer, also called an auxiliary transformer, reduces the plant’s medium voltage to a usable low-voltage level.
It may supply:
- Inverter station auxiliaries
- Transformer cooling equipment
- Control building loads
- SCADA and communication systems
- Lighting
- Heating and ventilation
- Security systems
- Battery chargers
- Maintenance equipment
- Fire pumps or other safety loads
Where does its input power come from?
The auxiliary transformer may be connected to:
- The medium-voltage collection network
- The main substation bus
- A dedicated utility supply
- A backup generator
- Another independent plant source
Critical solar plants may use two auxiliary sources so that essential systems remain available when one supply is unavailable.
Why is auxiliary power important at night?
Solar generation falls to zero at night, but many plant systems still need electricity.
Protection relays, communication equipment, environmental control, battery chargers, security systems, and substation devices must continue operating.
The station service system must therefore be designed for both generating and non-generating periods.
Are Earthing Transformers Used in Solar Plants?
An earthing transformer may be used when the collection system does not have a suitable neutral point.
Its purpose is not primarily to transfer solar generation power. Instead, it provides a neutral connection for system grounding and earth-fault protection.
Common configurations include zigzag or star-delta arrangements, depending on the system design.
An earthing transformer may help:
- Establish a neutral point
- Limit earth-fault current when used with a resistor
- Support earth-fault protection
- Control temporary overvoltage
- Improve grounding consistency
Whether one is needed depends on the winding connection of the inverter transformers, the main transformer vector group, the collection network, and the project’s grounding philosophy.
It should not be added automatically. A grounding and fault study should determine the correct solution.
Are Isolation Transformers Needed in Solar Energy Systems?
Electrical isolation is already provided by many power transformers, but some solar systems may use a dedicated isolation transformer for a particular circuit.
Possible reasons include:
- Separating sensitive equipment
- Meeting inverter manufacturer requirements
- Creating a specific grounding arrangement
- Reducing the transfer of certain disturbances
- Supplying control or measurement systems
- Connecting equipment with different earthing methods
However, not every transformer marketed for solar use is a separate “isolation transformer.” Many inverter step-up transformers already provide galvanic isolation between the low-voltage and medium-voltage windings.
Adding another transformer creates additional losses, cost, space requirements, and protection needs. It should be used only when the system design requires it.
What Transformers Are Used When Solar Plants Include Battery Storage?
Many modern solar projects include a battery energy storage system, but the battery system is not always connected at the same point as the photovoltaic generation.
The batteries produce DC electricity. A power conversion system converts it into AC, after which a power transformer may connect the battery system to the plant’s medium-voltage network or grid substation.
The storage transformer may be:
- A dedicated step-up transformer
- A multi-winding transformer shared by several power conversion systems
- Part of a containerized storage station
- Connected to the solar collection bus
- Connected separately at the main substation
The correct arrangement depends on whether the battery is used for:
- Energy shifting
- Peak shaving
- Grid support
- Frequency response
- Voltage control
- Curtailment reduction
- Backup operation
Battery systems can charge as well as discharge. Their transformers must therefore be suitable for bidirectional power flow.
The transformer specification should also consider harmonic currents, rapid changes in power, frequent operating cycles, partial loading, and the control characteristics of the power conversion system.
How Do Dry-Type and Oil-Immersed Transformers Fit into Solar Plants?
Both dry-type and oil-immersed solar power transformers can be used, but they are suited to different conditions.
Oil-immersed transformers
Oil-immersed units are widely used outdoors and at higher capacities.
Their possible advantages include:
- Efficient cooling
- High capacity range
- Strong outdoor suitability
- Compact dimensions for a given rating
- Established use in substations and generating plants
The project must consider:
- Oil containment
- Fire protection
- Leak detection
- Maintenance
- Environmental regulations
- Separation distances
Dry-type transformers
Dry-type transformers may be selected for indoor, enclosed, or fire-sensitive locations.
Their possible advantages include:
- No liquid insulating oil
- Lower oil-leak risk
- Suitability for some indoor installations
- Reduced oil-related fire concerns
Their design must account for:
- Ventilation
- Dust
- Humidity
- Condensation
- Ambient temperature
- Enclosure protection
- Noise
- Installation clearance
The choice should be based on the transformer location and operating conditions rather than using one construction type for the entire solar plant. Solar transformer portfolios commonly include both liquid-filled and dry-type solutions.
How Should Transformer Capacity Be Selected at Each Stage?
Capacity selection should begin with the actual power flow.
For an inverter transformer, engineers should check:
- Number of connected inverters
- Rated AC output of each inverter
- Maximum current
- Power factor range
- Overload capability
- Expected clipping or curtailment
- Ambient-temperature derating
- Harmonic losses
- Future expansion
For the main transformer, the assessment should include:
- Total installed AC generation capacity
- Maximum export limit
- Reactive power requirements
- Auxiliary load
- Grid voltage
- Power factor operating range
- Plant expansion
- Transformer redundancy
- Expected operating profile
Simply matching the transformer’s MVA rating to the solar module MWp value is not accurate.
The DC capacity of a photovoltaic field may be greater than the inverter AC capacity. The grid export limit may also differ from both values. Transformer selection should therefore be based primarily on the expected AC power and reactive power duty at the transformer location.
A transformer that is too small may overheat, limit plant output, or age prematurely. A transformer that is unnecessarily oversized may have higher purchase costs and less favorable losses during long periods of partial-load operation.
Why Do Solar Transformers Need Special Attention to Partial Loading?
Solar output changes throughout the day.
A transformer may operate at:
- Zero or very low load before sunrise
- Increasing load during the morning
- High load near peak irradiance
- Fluctuating load during cloud movement
- Decreasing load in the evening
- No generation at night
This operating pattern differs from a conventional industrial transformer serving a relatively stable load.
Both no-load loss and load loss matter.
No-load loss occurs whenever the transformer remains energized, even when little or no solar power is being exported. Load loss rises as transformer current increases.
The most suitable design should therefore be evaluated over the expected annual generation profile instead of comparing efficiency at only one operating point.
What Environmental Conditions Affect Solar Power Transformers?
Solar plants are often built in open areas with demanding conditions.
Transformers may face:
- High ambient temperature
- Strong solar radiation
- Dust and sand
- High altitude
- Salt contamination
- Humidity
- Condensation
- Heavy rain
- Flooding risk
- Strong wind
- Seismic activity
- Large daily temperature changes
Solar-generation transformers are designed for a wide range of environmental installations, but the manufacturer still needs accurate site data to produce an appropriate design.
Why does altitude matter?
At higher altitudes, lower air density can reduce cooling performance and external insulation strength.
The transformer may require:
- Capacity derating
- Improved cooling
- Larger clearances
- Higher insulation coordination
- Modified enclosure design
Why does temperature matter?
High ambient temperature reduces the transformer’s ability to release heat.
The design may need:
- Lower temperature rise
- Increased cooling surface
- Fans or pumps
- Larger conductors
- Temperature monitoring
- Reduced operating capacity
Using standard conditions for a hot solar site can result in inadequate thermal performance.
What Protection Should Be Coordinated with Solar Power Transformers?
Each transformer must be coordinated with the protection system at its location.
Depending on voltage and capacity, protection may include:
- Differential protection
- Overcurrent protection
- Earth-fault protection
- Restricted earth-fault protection
- Winding temperature protection
- Oil temperature protection
- Pressure relief
- Gas-operated protection
- Sudden pressure protection
- Overfluxing protection
- Surge arresters
- Buchholz relay for applicable oil-filled units
The protection philosophy for a small inverter transformer may differ from that of the main grid transformer.
The main transformer usually requires more comprehensive protection because its failure may disconnect a large part or all of the solar plant.
Protection settings must consider inverter fault-current characteristics. Solar inverters often limit fault current differently from traditional synchronous generators, so conventional assumptions should not be applied without checking inverter data.
What Should Be Confirmed Before Ordering Solar Power Transformers?
Before ordering, the project should provide complete technical information for each transformer location.
Electrical requirements
- Rated capacity
- Input and output voltage
- Frequency
- Number of phases
- Number of windings
- Capacity of each winding
- Vector group
- Short-circuit impedance
- Insulation level
- Tap range
- Power factor range
- Harmonic data
- Grounding method
- Fault level
Mechanical and environmental requirements
- Indoor or outdoor installation
- Ambient-temperature range
- Altitude
- Humidity
- Pollution level
- Dust and sand exposure
- Corrosive atmosphere
- Seismic level
- Noise limit
- Enclosure rating
- Transportation limitations
- Foundation requirements
Operational requirements
- Expected loading profile
- Maximum overload
- Partial-load operation
- Uneven winding loads
- Number of daily energizations
- Bidirectional power flow
- Future expansion
- Redundancy strategy
- Maintenance requirements
The transformer manufacturer should also receive the plant single-line diagram, inverter data, grid requirements, and protection philosophy.
How Can the Right Transformer Arrangement Improve a Solar Plant?
Correct transformer placement can improve the entire electrical system.
A well-planned arrangement can:
- Reduce collection losses
- Keep low-voltage cable routes short
- Limit the number of medium-voltage feeders
- Improve equipment accessibility
- Simplify repeated power blocks
- Support protection selectivity
- Improve plant availability
- Allow future expansion
- Avoid unnecessary transformer capacity
- Reduce lifecycle energy losses
The goal is not to use as many transformer types as possible. It is to place the correct transformer at each point where voltage conversion, electrical isolation, power collection, grounding, or auxiliary supply is actually needed.
Conclusion
Different stages of a solar energy system require different transformer functions.
Inverter step-up transformers raise the low-voltage AC output from solar inverters to the plant’s collection voltage. Multi-winding transformers can integrate several inverter groups into one power block. The main power transformer connects the combined solar output to the distribution or transmission grid. Auxiliary transformers supply the plant’s own electrical loads, while grounding, isolation, and battery storage transformers may be used when required by the system design.
These solar power transformers cannot be selected from one common specification. Their capacity, voltage ratio, winding arrangement, impedance, insulation, cooling, protection, and environmental design must match their exact position in the plant.
HONLE provides customized transformer solutions for photovoltaic generation, medium-voltage collection, grid connection, auxiliary power, and solar-plus-storage projects. By reviewing the inverter arrangement, plant single-line diagram, voltage levels, site environment, and grid requirements before production, HONLE can help configure solar power transformers for reliable and efficient operation across the complete solar energy system.