Power transformers are the central voltage-conversion equipment in many industrial substations, but they do not operate as independent units. A complete substation also requires switching, protection, distribution, monitoring, grounding, and connection equipment to deliver electricity safely from the incoming utility supply to the facility’s loads.
Depending on the installation environment and project requirements, industrial substations may use oil-immersed power transformers for outdoor or higher-capacity applications, or dry-type transformers for indoor electrical rooms and locations with stricter fire-safety requirements. However, selecting the transformer type and capacity is only one part of the project.
Engineers must also determine how the transformer will be connected, isolated, protected, monitored, and coordinated with the upstream and downstream electrical equipment. A mismatch between these components can cause installation delays, inadequate fault protection, excessive equipment ratings, or difficulties during future maintenance.
Understanding what equipment is required around power transformers helps project owners, consultants, EPC contractors, and electrical engineers prepare a more complete technical specification before requesting quotations.
Why Can Power Transformers Not Be Specified Separately?
The main function of a power transformer is to convert electricity from one voltage level to another. In a typical industrial substation, the transformer may receive medium-voltage power from the utility network and reduce it to a voltage suitable for motors, production lines, HVAC equipment, pumps, control panels, and general facility loads.
The transformer itself, however, does not provide all the switching and distribution functions needed by the substation.
Upstream equipment controls the incoming supply.
Before electricity reaches the transformer, medium-voltage equipment is normally required to connect or disconnect the incoming feeder. This equipment may also detect faults, interrupt fault current, measure electrical values, and isolate the transformer for inspection.
Downstream equipment distributes the transformed power.
After the transformer reduces the voltage, low-voltage switchgear receives the transformer output and divides it among different feeders. Each feeder must be protected according to its load, cable size, starting current, and operating characteristics.
Auxiliary systems support safe operation.
Power transformers also depend on grounding, surge protection, temperature monitoring, cable or busbar connections, ventilation, fire protection, and control power. Oil-immersed units may require oil containment arrangements, while dry-type units require suitable airflow and clearance around the enclosure.
A simplified industrial substation arrangement may therefore look like this:
Utility or Generator Supply
→ Medium-Voltage Incoming Equipment
→ Power Transformer
→ Low-Voltage Main Switchboard
→ Distribution Feeders
→ Industrial Loads
This arrangement may be installed in a conventional substation building or integrated into a factory-assembled prefabricated substation. HONLE’s prefabricated substation range, for example, combines high-voltage switchgear, a transformer compartment, and low-voltage distribution equipment in one compact structure.
How Does Medium-Voltage Equipment Control and Protect Power Transformers?
The medium-voltage side connects power transformers to the utility feeder, generator, renewable energy system, or another upstream substation. Its configuration depends on the system voltage, available fault current, transformer capacity, utility requirements, and required level of operational flexibility.
Incoming switchgear
Medium-voltage switchgear forms the main switching and protection section before the transformer. It may contain a circuit breaker, disconnector, earthing switch, current transformer, voltage transformer, protection relay, and metering equipment.
For larger industrial substations, metal-enclosed medium-voltage switchgear is commonly used when the project requires more complete protection, measurement, interlocking, and operational control.
HONLE’s product range includes medium- and low-voltage switchgear alongside power transformers, allowing these two parts of the substation to be considered within the same project configuration.
Ring main unit
A ring main unit may be selected for compact medium-voltage distribution systems, especially where space is limited or where the transformer is connected to a ring network.
The RMU may provide:
- Incoming and outgoing feeder switching;
- Transformer feeder protection;
- Load isolation;
- Earthing functions;
- Network sectionalizing;
- Space-efficient installation.
The decision between an RMU and conventional medium-voltage switchgear should be based on the single-line diagram, fault level, protection requirements, network structure, and required switching sequence.
Medium-voltage circuit breaker or load break switch
A circuit breaker is able to interrupt normal load current and specified fault current when coordinated with suitable protection relays. A load break switch is generally intended for switching normal operating current, while fault protection may be provided through fuses or another upstream device.
The correct arrangement depends on the transformer rating and the protection philosophy of the project. Larger power transformers or substations requiring more advanced protection usually need a circuit-breaker-based arrangement.
Surge arrester
Surge arresters help limit overvoltage caused by lightning or switching events. They are especially important for outdoor substations, overhead incoming lines, and locations exposed to frequent lightning activity.
The arrester voltage rating and protection level must match the system voltage, grounding arrangement, and transformer insulation level.
Metering equipment
Current transformers and voltage transformers may be required for utility metering, protection, monitoring, or energy management. Their ratio, accuracy class, burden, and installation location should be confirmed before manufacturing the switchgear.
These values cannot be selected independently from the transformer and system data. The rated current, fault current, voltage ratio, grounding method, and protection scheme must be reviewed together.
Which Power Transformer Configuration Should the Substation Use?
Once the incoming system has been defined, the power transformer must be selected according to the actual load and operating environment.
The first questions normally concern rated capacity and voltage ratio, but a complete specification should cover considerably more information.
Transformer capacity
The selected capacity must support the expected operating load without unnecessary oversizing. Engineers should consider:
- Connected load;
- Actual demand factor;
- Maximum simultaneous load;
- Motor-starting conditions;
- Cyclic or intermittent loads;
- Harmonic-producing equipment;
- Ambient temperature;
- Future expansion;
- Required redundancy.
A transformer that is too small may experience excessive temperature rise and reduced service life. A power transformer that is significantly oversized may increase initial investment, no-load losses, switchgear ratings, and available short-circuit current.
Voltage ratio
The primary voltage must match the incoming power system, while the secondary voltage must match the low-voltage distribution system.
Common project configurations may include medium-voltage input and 400 V, 415 V, 440 V, or another specified low-voltage output. The exact values depend on the country, utility network, equipment requirements, and project standard.
Oil-immersed or dry-type design
Oil-immersed power transformers are commonly selected for outdoor substations and industrial projects requiring effective heat dissipation or higher capacity.
Dry-type transformers are often installed indoors, including factories, commercial buildings, data facilities, hospitals, transport infrastructure, and other locations where fire safety and indoor installation conditions are important.
The choice should not be based only on transformer price. Installation space, fire separation, ventilation, environmental exposure, maintenance planning, noise, and local requirements must also be considered.
HONLE’s transformer range includes both dry-type and oil-immersed distribution solutions for different installation conditions.
Transformer impedance
Percentage impedance affects voltage regulation and available short-circuit current on the low-voltage side.
A lower impedance may improve voltage performance under load but can result in higher fault current. A higher impedance can limit fault current but may increase voltage drop during heavy loading or motor starting.
For this reason, transformer impedance must be reviewed together with the breaking capacity and short-time withstand rating of the downstream switchgear.
Tap changer
An off-circuit tap changer may be suitable where voltage adjustment is only required occasionally and the transformer can be de-energized before changing the tap position.
An on-load tap changer may be required where voltage must be adjusted while the transformer remains energized. This arrangement is more common in substations where system voltage varies significantly or where continuous voltage control is required.
How Does Low-Voltage Switchgear Distribute Transformer Output?
The secondary terminals of power transformers normally connect to a low-voltage main switchboard. This switchboard receives the transformed power and distributes it to the facility’s downstream loads.
The equipment must be selected according to the transformer secondary voltage, full-load current, short-circuit current, neutral arrangement, number of outgoing feeders, and required form of separation.
Main incoming circuit breaker
The main incomer provides switching and protection between the transformer and the low-voltage busbar.
Its rated current must be sufficient for the transformer output, but rated current alone is not enough. Engineers must also verify:
- Breaking capacity;
- Short-time withstand current;
- Making capacity;
- Overload protection range;
- Instantaneous protection setting;
- Earth-fault protection;
- Selective coordination;
- Communication requirements.
When the transformer and low-voltage switchgear are supplied by different manufacturers, incomplete data exchange may result in an incorrectly selected main circuit breaker or busbar system.
Main busbar
The switchboard busbar carries power from the incoming breaker to the outgoing feeders. Its current rating must account for the transformer output, ambient temperature, enclosure conditions, ventilation, and possible future expansion.
The busbar must also withstand the thermal and mechanical forces produced during a short circuit. These forces can be substantial when a large transformer or multiple parallel transformers supply the same busbar.
Outgoing feeders
Each outgoing feeder supplies a specific load group, motor control center, distribution board, process system, or building area.
The number and rating of outgoing feeders should be confirmed according to:
- Load schedule;
- Cable size;
- Load type;
- Motor-starting current;
- Required protection;
- Spare feeders;
- Future expansion;
- Operational grouping.
Bus coupler
A bus coupler may be required when two power transformers supply separate busbar sections. Under normal conditions, each transformer may operate independently. If one transformer is unavailable, the coupler may allow selected loads to be transferred to the remaining transformer.
However, the operating logic must prevent unacceptable transformer overloading or excessive fault current when the bus sections are connected.
Power factor correction and harmonic considerations
Facilities containing motors, variable-frequency drives, rectifiers, welding equipment, UPS systems, and other nonlinear loads may require power factor correction or harmonic control equipment.
These systems should be evaluated based on actual load characteristics. Capacitor banks, reactors, and filtering equipment should not be specified without checking the harmonic environment, switching conditions, and transformer loading.
How Do Protection and Monitoring Systems Support Power Transformers?
Protection around power transformers should detect abnormal conditions quickly enough to limit equipment damage while avoiding unnecessary interruption during normal operating events.
The required functions depend on transformer capacity, voltage level, construction, connection group, grounding arrangement, and project standard.
High-voltage overcurrent protection
Overcurrent protection detects excessive current on the transformer primary side. Settings must allow normal energization and transformer inrush current while still responding to genuine faults.
If the setting is too sensitive, the transformer may trip during energization. If it is too high or too slow, the transformer may not receive adequate protection.
Earth-fault protection
Earth-fault protection responds to current flowing through an unintended path to earth. Its design depends heavily on the system grounding method and transformer winding connection.
The protection arrangement must be coordinated across the medium-voltage system, transformer, low-voltage main breaker, and downstream feeders.
Differential protection
Larger or more critical power transformers may use differential protection. This system compares the current entering and leaving the transformer protection zone.
Current transformer ratios, connection compensation, transformer vector group, inrush restraint, and relay settings must be configured correctly. Otherwise, the protection may operate incorrectly during normal transformer energization or external faults.
Temperature monitoring
Oil-immersed transformers may include oil temperature and winding temperature indicators. Dry-type transformers may use temperature sensors embedded in the windings.
Temperature signals can be connected to local indicators, alarms, fans, trip circuits, or a supervisory control system.
Oil and pressure protection
Depending on the oil-immersed transformer design and capacity, protection and monitoring may include:
- Oil level indication;
- Pressure relief device;
- Gas-actuated relay;
- Sudden-pressure protection;
- Oil temperature indication;
- Winding temperature indication;
- Alarm and trip contacts.
The required accessories should be specified before manufacturing because they affect the transformer control box, terminal arrangement, cabling, and interface with the switchgear or control system.
Automatic recloser
For substations supplied from medium-voltage overhead distribution lines, an automatic recloser may be installed upstream to interrupt faults and sectionalize the feeder.
It is not a replacement for transformer-specific protection. Instead, it forms part of the wider distribution protection system.
HONLE’s SQ automatic recloser is designed for medium-voltage distribution networks up to 38 kV and provides switching, protection, sectionalizing, metering, and automation functions.
How Are Power Transformers Connected to the Rest of the Substation?
Even when the main transformer and switchgear ratings are correct, installation problems can occur if the physical and electrical interfaces have not been coordinated.
Medium-voltage cable connection
The project should define:
- Cable type and size;
- Number of cables per phase;
- Cable entry direction;
- Termination type;
- Required clearance;
- Phase sequence;
- Cable support arrangement;
- Earthing connection;
- Surge arrester location.
The transformer terminal design, switchgear cable compartment, and site cable route must be compatible.
Low-voltage cable or bus duct connection
Large power transformers may produce secondary currents that make multiple parallel cables difficult to install. In these cases, a bus duct or busbar connection may be more practical.
The project team should confirm:
- Terminal orientation;
- Transformer-to-switchboard distance;
- Bus duct dimensions;
- Flexible connection requirements;
- Phase and neutral arrangement;
- Enclosure protection;
- Thermal expansion;
- Short-circuit mechanical forces.
These details should be reviewed before both the transformer and switchboard enter production.
Grounding system
The transformer tank or enclosure, switchgear enclosure, neutral point, cable screens, surge arresters, and metallic structures must be connected to the grounding system according to the project design.
The neutral grounding arrangement may involve:
- Solid grounding;
- Resistance grounding;
- Reactance grounding;
- An isolated neutral;
- Another project-specific method.
This decision affects earth-fault current, protection settings, insulation requirements, and downstream equipment.
Auxiliary power
Substation equipment may need auxiliary AC or DC power for:
- Protection relays;
- Circuit-breaker operation;
- Control circuits;
- Indication lamps;
- Temperature controllers;
- Cooling fans;
- Communication equipment;
- Remote monitoring;
- Emergency lighting.
The auxiliary voltage, source, backup duration, and terminal requirements should be included in the project specification.
Ventilation and heat dissipation
Power transformers and switchgear generate heat during operation. Indoor substations require sufficient ventilation to keep the ambient temperature within the equipment’s specified operating range.
Dry-type transformers need adequate airflow around the windings and enclosure. Oil-immersed transformers installed indoors may require additional fire separation and ventilation measures.
Oil containment and drainage
Outdoor oil-immersed power transformers may require an oil collection pit, bund, drainage system, or fire barrier. The arrangement depends on transformer oil volume, installation layout, environmental requirements, and local regulations.
How Should the Main Substation Equipment Be Coordinated?
The following table summarizes the main equipment installed around power transformers and the information that must be coordinated.
Table 1: Equipment Required Around Power Transformers
| Equipment | Main Function | Information to Coordinate |
| MV incoming switchgear | Connects and isolates the incoming supply | System voltage, rated current, fault level and protection scheme |
| RMU | Provides compact MV switching and transformer feeder control | Network structure, feeder rating, protection method and cable interface |
| MV circuit breaker | Interrupts load and fault current | Breaking capacity, relay settings and operating sequence |
| Surge arrester | Limits lightning and switching overvoltage | Rated voltage, grounding method and transformer insulation level |
| CTs and VTs | Provide metering and protection signals | Ratio, accuracy class, burden and relay requirements |
| Power transformer | Converts voltage and supplies the load | Capacity, voltage ratio, impedance, vector group, losses and cooling |
| LV main circuit breaker | Protects the transformer secondary and main busbar | Full-load current, short-circuit current and coordination settings |
| LV switchgear | Distributes power to downstream loads | Busbar rating, feeder schedule, fault rating and spare capacity |
| Bus coupler | Connects separate busbar sections | Transformer loading, interlocking and parallel-operation conditions |
| Protection relay | Detects abnormal electrical conditions | CT data, transformer characteristics, grading and communication |
| Monitoring system | Provides alarms and operational data | Temperature, oil, pressure, breaker status and communication protocol |
| Grounding system | Provides a controlled path for fault current | Neutral arrangement, earth resistance and protection philosophy |
| Cables or bus duct | Connect the transformer to switchgear | Current rating, terminal orientation, route and short-circuit strength |
| Auxiliary supply | Powers control and monitoring equipment | AC/DC voltage, capacity, backup time and distribution method |
| Ventilation or cooling | Removes equipment heat | Transformer losses, room dimensions and ambient temperature |
This coordination should begin with the single-line diagram and project data sheet. It should not be postponed until the equipment arrives at the installation site.
Should the Equipment Be Supplied Separately or as a Complete Substation?
Some projects purchase power transformers, switchgear, protection equipment, and auxiliary systems from separate suppliers. This approach may be appropriate when the consultant or EPC contractor has a detailed design and manages all equipment interfaces directly.
Other projects may benefit from a more integrated supply arrangement.
A coordinated transformer and switchgear package can help reduce uncertainty involving:
- Voltage and current ratings;
- Short-circuit levels;
- Terminal orientation;
- Busbar or cable interfaces;
- CT and relay data;
- Control voltage;
- Alarm and trip contacts;
- Mechanical layout;
- Factory documentation;
- Installation sequence.
A prefabricated substation provides an even higher level of integration by placing high-voltage equipment, the transformer, and low-voltage distribution equipment within a factory-assembled enclosure.
HONLE’s YBP(M)-12/40.5 prefabricated substation supports transformer capacities from 100 kVA to 1600 kVA in the listed standard configuration and combines high-voltage, transformer, and low-voltage sections within one system. Actual project selection should still be based on the required voltage, capacity, short-circuit level, environmental conditions, and layout.
An integrated package does not remove the need for engineering review. The project must still provide accurate load information, system parameters, protection requirements, site conditions, and applicable standards.
Its main advantage is that more of the interfaces can be identified and reviewed before manufacturing and site installation.
What Information Should Be Provided When Requesting a Quotation?
A quotation for power transformers and related substation equipment will be more accurate when the request includes complete electrical and site information.
The most important documents and parameters normally include:
- Single-line diagram;
- Transformer quantity;
- Rated capacity;
- Primary and secondary voltage;
- System frequency;
- Phase configuration;
- Transformer type;
- Percentage impedance;
- Vector group;
- Tap range and tap changer type;
- Cooling method;
- Installation location;
- Indoor or outdoor environment;
- Maximum and minimum ambient temperature;
- Altitude;
- Utility fault level;
- Grounding arrangement;
- Required transformer accessories;
- MV switchgear configuration;
- LV switchgear feeder schedule;
- Protection and metering requirements;
- Cable or bus duct connection;
- Communication protocol;
- Applicable technical standard;
- Required routine and special tests;
- Documentation language;
- Delivery schedule.
When some of these values have not yet been confirmed, the available load list, utility information, and preliminary single-line diagram can be provided first. The manufacturer can then identify which missing parameters may affect equipment selection or pricing.
As a manufacturer of power transformers and related electrical distribution equipment, HONLE can review transformer requirements alongside medium-voltage switchgear, low-voltage panels, automatic reclosers, and prefabricated substation configurations. This helps the project discussion remain connected to the actual electrical system rather than treating each device as an isolated purchase.
Frequently Asked Questions
Can power transformers be connected directly to a utility feeder?
A power transformer may be physically connected to an incoming feeder, but appropriate switching, isolation, surge protection, and fault protection are normally required. The exact equipment depends on the voltage level, transformer capacity, utility rules, and protection design.
Does every power transformer need medium-voltage switchgear?
Not always. Smaller distribution transformers may use a fused switch, pole-mounted protection, or another simplified arrangement. Larger industrial power transformers and substations with more complex operational requirements generally use dedicated medium-voltage switchgear.
How is the low-voltage main breaker selected?
The breaker must be selected according to the transformer secondary current, available short-circuit current, overload requirements, earth-fault protection, and coordination with downstream feeders. Selecting it only by normal operating current is not sufficient.
Can two power transformers operate in parallel?
They can operate in parallel when important characteristics such as voltage ratio, vector group, phase sequence, impedance, and tap position are compatible. The switchgear and busbar must also be rated for the resulting load and fault current.
Is a prefabricated substation suitable for an industrial project?
It can be suitable when the required transformer capacity, voltage level, feeder arrangement, environmental conditions, access, ventilation, and maintenance space can be accommodated. Prefabricated substations can reduce on-site installation work, but the configuration must still be designed for the specific project.
Conclusion
Power transformers form the core of voltage conversion in an industrial substation, but reliable operation depends on the equipment installed around them.
Medium-voltage switchgear controls and protects the incoming supply. Low-voltage switchgear distributes the transformer output. Circuit breakers, relays, surge arresters, monitoring devices, grounding systems, cables, bus ducts, ventilation, and auxiliary power complete the system.
The most important principle is coordination. Transformer capacity, voltage ratio, impedance, vector group, terminal arrangement, protection data, and installation conditions must be matched with the upstream and downstream equipment before manufacturing begins.
HONLE supplies oil-immersed and dry-type power transformers together with medium- and low-voltage switchgear, automatic reclosers, and prefabricated substation equipment for industrial and power distribution projects. Welcome to contact us with your single-line diagram, transformer data sheet, load information, site conditions, and switchgear requirements. Our team will review the available information and discuss a suitable equipment configuration for your project.