Mining and mineral processing plants can place demanding and highly variable loads on the electrical distribution system. Crushers, grinding mills, conveyors, slurry pumps, ventilation fans, dewatering systems, and processing equipment may operate simultaneously, start at different times, or change load as production conditions vary.
For facilities connected to a 35kV network, the power transformer is a key part of the plant’s electrical infrastructure. It needs to provide the required voltage transformation while supporting continuous production, large motor loads, changing demand, and the environmental conditions found at many mining sites.
HONLE manufactures 35kV oil-immersed power transformers for industrial facilities, substations, mining projects, and other power distribution applications. For mining and mineral processing plants, we usually look at the transformer together with the actual load profile and distribution system rather than focusing only on the rated capacity.
Capacity, voltage ratio, motor starting conditions, transformer impedance, voltage regulation, ambient temperature, altitude, cooling, and future expansion can all influence the final transformer configuration.
Why Do Mining Plants Place Heavy Demands on Power Transformers?
Mining facilities rarely operate with a completely stable electrical load.
A crushing section may start while pumps and conveyors are already operating. Grinding equipment can remain heavily loaded for long periods, while ventilation, dewatering, control systems, and auxiliary equipment continue running in the background.
Common mining and mineral processing loads include:
- Primary and secondary crushers
- SAG and ball mills
- Conveyor systems
- Slurry and process pumps
- Dewatering pumps
- Ventilation fans
- Air compressors
- Flotation equipment
- Screening equipment
- Thickener drives
- Material handling systems
- Workshops and maintenance facilities
- Lighting and auxiliary distribution
This combination means the power transformer may experience high continuous loading together with relatively large changes in demand.
A suitable transformer therefore needs to match the way the plant actually operates, including which loads run together and how frequently large equipment starts.
Where Does a 35kV Power Transformer Fit in a Mining Electrical System?
In many industrial power distribution systems, a 35kV power transformer is installed in the upstream part of the plant network.
A typical arrangement may be:
Utility or mine power supply → 35kV substation → 35kV power transformer → medium-voltage distribution → large motors and downstream transformers → low-voltage equipment
The secondary voltage depends on the electrical system used by the project.
For example, a 35kV transformer may step the incoming voltage down to 10kV, 6.6kV, 6kV, or another medium-voltage level specified by the plant.
The downstream system may then supply large motors directly or feed additional transformers for low-voltage equipment.
Because the transformer sits close to the center of the plant distribution system, it is useful to consider its ratings together with switchgear, cables, protection equipment, motors, and downstream transformers.
Large Motor Loads Are Important in Transformer Selection
Mining and mineral processing plants often contain large motors for crushers, mills, conveyors, pumps, and fans.
During starting, these motors can draw considerably more current than during normal operation. The actual starting current and duration depend on the motor characteristics and the starting method.
Common arrangements include:
- Direct-on-line starting
- Soft starters
- Variable-frequency drives
- Reduced-voltage starting
- Other project-specific motor control methods
When a large motor starts, the temporary increase in current can cause a voltage drop across the transformer and upstream electrical network.
This does not automatically mean that the project needs a significantly oversized power transformer.
It is more useful to look at the motor starting condition together with the transformer and system parameters.
| Factor | Why It Matters |
|---|---|
| Largest motor rating | Influences starting demand |
| Starting method | Affects starting current and acceleration |
| Existing operating load | Shows how heavily the transformer is loaded before starting |
| Transformer impedance | Influences voltage drop and fault current |
| Upstream network | Affects available short-circuit power and voltage stability |
| Acceptable voltage dip | Important for other operating equipment |
| Starting frequency | Relevant to repeated thermal and electrical loading |
For projects with several large motors, information from the motor list or electrical design can help us understand how the transformer will behave during normal production and equipment starting.
Transformer Capacity Should Reflect the Real Plant Load
Installed equipment capacity and actual power transformer demand are not always the same.
A mineral processing plant may contain many motors, but not every motor operates at full load at the same time. Some equipment runs continuously, some operates intermittently, and some is installed as standby capacity.
Transformer sizing can therefore consider several operating conditions.
Continuous Load
This includes equipment expected to operate for long periods during normal production.
Maximum Simultaneous Load
This considers which machines and process sections may realistically operate at the same time.
Large Motor Starting
The transformer should also support the expected starting sequence without creating unacceptable voltage disturbance elsewhere in the plant.
Production Peaks
Short periods of increased processing demand can temporarily raise transformer loading.
Future Expansion
A mine may later add another crusher, conveyor, pump station, mill, or processing line.
If that expansion is already planned, some additional transformer capacity can be considered during the initial project stage.
The objective is not simply to select the largest possible power transformer. A more balanced approach is to provide suitable operating margin while also considering investment cost, transformer losses, and expected plant development.
Fluctuating Mining Loads Can Affect Transformer Operation
Mining loads can change throughout the production cycle.
Ore hardness can influence crusher and mill loading. Pump demand may change with water levels or process conditions. Conveyors may start and stop according to material flow, while different processing sections operate at different production rates.
Because of this, transformer selection can benefit from understanding the expected operating range rather than looking only at one maximum load figure.
Useful factors include:
- Typical operating demand
- Expected peak demand
- Major individual loads
- Large load changes
- Equipment operating cycles
- Motor starting sequences
- Planned production expansion
Even approximate information can be useful at an early project stage. The transformer configuration can then be refined as the electrical design becomes more detailed.
Transformer Impedance Affects Voltage Performance and Fault Current
Percentage impedance is another important power transformer parameter.
A transformer with higher impedance generally limits downstream short-circuit current more effectively, but it can also produce greater voltage drop when the load changes.
Lower impedance can improve voltage performance under heavy load, but it may increase the available fault current on the secondary side.
This means impedance should be considered together with the wider electrical system rather than selected independently.
Relevant equipment can include:
- Medium-voltage switchgear
- Circuit breakers
- Protection relays
- Busbars
- Power cables
- Large motors
- Downstream transformers
If two transformers are expected to operate in parallel, characteristics such as voltage ratio, vector group, impedance, and tap position also need to work together.
HONLE can review these parameters as part of the transformer specification when the project information becomes available.
VFD Loads Can Influence Transformer Design
Variable-frequency drives are widely used in mining facilities because pumps, fans, conveyors, and processing equipment often benefit from speed control and smoother motor acceleration.
At the same time, VFDs and other power-electronic equipment can introduce harmonic currents into the distribution system.
The actual effect depends on factors such as:
- VFD topology
- Rectifier configuration
- Percentage of VFD load
- Operating load level
- Number of drives
- Reactors or harmonic filters
- Transformer impedance
- Other nonlinear equipment
A power transformer does not automatically remove harmonics from the system.
For plants with a large proportion of VFD or rectifier loads, reviewing the expected harmonic conditions can help determine whether any additional transformer considerations are useful.
These may include transformer losses, winding heating, current loading, impedance, and coordination with external harmonic mitigation equipment.
If detailed harmonic data is not yet available, the project can still be discussed based on the type and approximate size of the major drive loads.
Voltage Regulation Can Be Considered for Changing Plant Conditions
Voltage can change as production demand varies or as upstream network conditions change.
Depending on the project, the transformer may use either an off-circuit tap changer or an on-load tap changer.
Off-Circuit Tap Changing
An off-circuit tap arrangement can be suitable when voltage adjustment is only needed occasionally.
The transformer is de-energized before the tap position is changed.
This can be a practical solution where the incoming voltage remains relatively stable and frequent adjustment is unnecessary.
On-Load Tap Changing
An on-load tap changer allows voltage adjustment while the transformer remains energized.
It may be considered where:
- Incoming voltage changes noticeably
- Plant demand varies significantly
- Downstream voltage needs closer regulation
- Production equipment benefits from more stable supply voltage
An OLTC is not necessary for every 35kV mining transformer.
The most appropriate arrangement depends on how the electrical network is designed and how much voltage variation the plant expects. HONLE can discuss the tap range and regulation method together with the other transformer parameters.
Mining Site Conditions Should Be Included in the Transformer Design
Mining equipment may operate in environments that are hotter, dustier, higher, or more exposed than a typical industrial facility.
These conditions are worth considering when configuring the power transformer.
High Ambient Temperature
Higher ambient temperature reduces the transformer’s ability to dissipate heat.
For mining projects in hot regions, the expected site temperature can therefore influence cooling and temperature-rise considerations.
High Altitude
Many mining operations are located at elevated sites.
Lower air density can affect cooling performance and external insulation conditions, so installation altitude is useful information when confirming the transformer design.
Dust and Contamination
Crushing, conveying, and mineral handling can produce considerable dust.
Although the active parts of an oil-immersed power transformer are enclosed, external components such as bushings, radiators, terminal areas, control boxes, and cooling accessories are still exposed to the surrounding environment.
Outdoor Installation
Many mining transformers are installed outdoors as part of a substation.
Depending on the project location, the specification may consider:
- Ambient temperature
- Humidity
- Rainfall
- Dust
- Corrosion conditions
- Pollution level
- Altitude
- Seismic requirements
- Terminal arrangement
- Cable connection method
Customers do not need to finalize every environmental parameter before discussing a project with us. Basic site information is usually enough to begin narrowing down the suitable transformer configuration.
Cooling Should Support Long Operating Hours
Mineral processing plants often operate for long periods, sometimes continuously.
Transformer cooling therefore needs to support the expected duty and site conditions.
The suitable cooling arrangement depends on factors including:
- Transformer capacity
- Normal operating load
- Maximum expected load
- Ambient temperature
- Installation altitude
- Available installation space
- Required operating margin
- Project preference
For larger transformer ratings, forced cooling may also be considered depending on the design.
The objective is to maintain suitable transformer temperature during the operating conditions expected at the mine, rather than adding unnecessary cooling equipment without considering the real duty.
Protection and Monitoring Depend on the Transformer and Substation Design
A main power transformer can supply a large part of the production facility.
For that reason, transformer protection and monitoring are normally coordinated with the overall substation protection system.
Depending on transformer rating and project requirements, the system may include functions or accessories related to:
- Oil temperature
- Winding temperature
- Oil level
- Pressure
- Gas accumulation
- Cooling system operation
- Overcurrent protection
- Differential protection
- Earth fault protection
- Surge protection
- Tap changer monitoring
Not every mining transformer needs exactly the same accessory package.
The appropriate arrangement depends on transformer size, electrical system configuration, protection philosophy, and local project requirements.
HONLE can match transformer accessories and interface requirements to the project specification rather than applying one fixed configuration to every installation.
Transformer Losses Matter in Long-Term Mining Operations
A mining power transformer may remain energized throughout most of the year.
This makes transformer losses relevant over the full operating life of the equipment.
Transformer losses are generally considered in two main categories.
No-load losses occur whenever the transformer is energized.
Load losses increase as transformer current increases and become increasingly important in heavily loaded industrial systems.
For a continuously operating processing plant, transformer evaluation can therefore include both initial purchase cost and expected long-term operating efficiency.
Useful considerations include:
- Average transformer loading
- Annual operating hours
- Electricity cost
- Required capacity
- Expected service life
- Project efficiency targets
Where efficiency is an important part of the project, loss requirements can be discussed and included in the technical specification.
The Transformer Should Work with the Complete 35kV Substation
A 35kV power transformer is only one part of the mining electrical system.
Its electrical characteristics need to coordinate with the equipment installed around it.
This can include:
- 35kV switchgear
- Medium-voltage switchgear
- Circuit breakers
- Current transformers
- Voltage transformers
- Protection relays
- Surge arresters
- Power cables
- Busbars
- Grounding system
- Downstream transformers
- Large motors
- VFD systems
Transformer rated current, impedance, voltage ratio, insulation level, and protection requirements can influence other parts of the substation.
For new mining projects, discussing these interfaces early can simplify equipment coordination. For replacement projects, existing transformer nameplate information and system drawings can also provide a useful starting point.
What Project Information Is Helpful When Discussing a 35kV Mining Transformer?
Customers can contact HONLE at different stages of a project.
Some projects already have a complete technical specification, while others may still be confirming transformer capacity, secondary voltage, motor loads, or site conditions.
Either situation is suitable for an initial discussion.
You can share whatever information is currently available, such as:
| Available Information | Example |
|---|---|
| Primary voltage | 35kV system |
| Secondary voltage | 10kV, 6.6kV, 6kV or another project voltage |
| Approximate capacity | Existing estimate or load study result |
| Frequency | 50Hz or 60Hz |
| Main equipment | Crushers, mills, pumps, conveyors |
| Large motor information | Motor rating and starting method |
| Operating pattern | Continuous or changing production load |
| Drive loads | Approximate VFD load if known |
| Installation conditions | Indoor or outdoor |
| Site environment | Temperature, altitude, humidity or dust |
| Voltage regulation | Existing tap requirement if already specified |
| Drawings | Single-line diagram or existing transformer datasheet |
There is no need to have every value confirmed before contacting us.
For an early-stage project, even a basic voltage level, approximate capacity, application, and installation location can help us understand the general requirement. As the project develops, HONLE can work with the available electrical data to gradually refine the power transformer configuration.
Factory Testing Can Be Matched to the Project Requirements
Testing requirements can vary between projects, customers, and applicable standards.
For a project-specific 35kV power transformer, the testing scope can be confirmed together with the approved technical specification.
Depending on the project, factory testing and documentation may cover items such as:
- Voltage ratio
- Winding resistance
- Insulation characteristics
- No-load losses
- Load losses
- Transformer impedance
- Functional checks
- Accessories
- Nameplate information
- Test reports
Some customers may also require witness inspection, additional documentation, or specific factory acceptance procedures.
These requirements can be discussed during the project process. If a customer already has an FAT specification, HONLE can review it together with the transformer technical requirements.
Working with HONLE on a Mining Power Transformer Project
Mining transformer projects can involve more coordination than standard distribution applications because the electrical system may include large motors, long operating hours, variable production demand, VFD loads, outdoor installation, and future plant expansion.
HONLE manufactures 35kV oil-immersed power transformers for industrial and substation applications and can support both new installations and transformer replacement projects.
Depending on the stage of the project, we can discuss factors such as:
- Voltage ratio
- Transformer capacity
- Frequency
- Impedance
- Vector group
- Tap arrangement
- Cooling
- Installation conditions
- Large motor loads
- VFD loads
- Protection and monitoring accessories
- Applicable standards
- Testing and documentation
You do not need to finalize all of these items before starting a conversation with us.
If the project is still at an early stage, we can begin with the information already available and gradually confirm the remaining transformer requirements as the electrical design progresses.
Conclusion
A 35kV power transformer for a mining or mineral processing plant needs to support much more than basic voltage conversion.
Large crushers and grinding mills can create demanding motor loads. Pumps, conveyors, and processing equipment can cause substantial changes in plant demand. VFDs introduce additional electrical considerations, while temperature, altitude, dust, and outdoor installation can influence the transformer configuration.
For this reason, capacity, impedance, cooling, voltage regulation, environmental conditions, protection, and future expansion are best considered together.
HONLE manufactures 35kV power transformers for mining, mineral processing, industrial power distribution, and substation projects.
Whether you are planning a new mining facility, expanding an existing processing plant, replacing an older transformer, or adding new production equipment, you can start by sharing the project information already available. We can then discuss the application and help refine the transformer configuration as the project moves forward.
Frequently Asked Questions
Is a 35kV power transformer suitable for a mining plant?
Yes. A 35kV power transformer can be used where the mine or mineral processing plant has a 35kV incoming or internal distribution network. The secondary voltage and transformer configuration can be matched to the plant’s electrical system.
How should a power transformer be sized for large mining motors?
The transformer can be selected by considering the normal plant load, simultaneous operating loads, major motor ratings, motor starting methods, expected voltage drop, and future expansion. This gives a more realistic basis than simply adding together all equipment nameplate ratings.
Can a 35kV transformer supply crushers, mills, and large pumps?
Yes. A properly configured power transformer can supply these types of mining loads through the plant’s medium-voltage distribution system. Large motor starting and operating conditions are useful factors to consider when confirming capacity and impedance.
Do VFD loads require a special power transformer?
Not necessarily. It depends on the proportion of VFD load, drive configuration, harmonic conditions, transformer loading, and overall electrical system. If detailed harmonic information is available, it can help with the design review, but it is not necessary to have a complete harmonic study before beginning a project discussion.
What information can I send HONLE for a mining transformer quotation?
You can share whatever project information is currently available, such as system voltage, approximate transformer capacity, frequency, major equipment, installation conditions, or a single-line diagram. Even if some details are still being confirmed, HONLE can discuss the project with you first and help refine the transformer requirements as the specification develops.