33kV and 35kV Substation Equipment: Ratings, Applications and Key Considerations

Introduction

33kV and 35kV substations are widely used in utility distribution, industrial power systems and renewable energy projects. Although these projects may share similar switching and protection functions, the required equipment ratings depend on more than the nominal system voltage.

This guide looks at the main equipment used in 33kV and 35kV substations, including vacuum circuit breakers, disconnect switches, surge arresters and high-voltage fuses, with a focus on voltage class, application and key technical considerations.

Understanding Equipment Ratings in 33kV and 35kV Substations

One of the most important points in a 33kV or 35kV substation project is that the nominal system voltage is not always the same as the rated voltage shown on the equipment nameplate.

For example, Fenarro’s existing VCB guidance notes that a 33kV project may require 36kV- or 40.5kV-class equipment, depending on the applicable standard, insulation level and installation conditions. Fenarro’s vacuum circuit breaker range also includes indoor and outdoor models up to 40.5kV, including products used in 35kV distribution applications.

For this reason, equipment for a “33kV substation” should not be selected from the nominal voltage alone. The complete electrical specification must be checked before confirming the equipment class.

High-Voltage Vacuum Circuit Breakers for 33kV and 35kV Substations

High-voltage vacuum circuit breakers are used to control power flow and interrupt fault current in substations. When a short circuit or other abnormal condition is detected by the protection system, the circuit breaker opens the affected circuit to help protect transformers, feeders and other electrical equipment.

In medium- and high-voltage distribution applications, vacuum circuit breakers may be installed in incoming feeders, transformer circuits and outgoing feeders. The required voltage class and electrical ratings depend on the actual system voltage, short-circuit level, insulation requirements and project conditions.

Fenarro high-voltage vacuum circuit breakers are available for indoor and outdoor substation applications, with different voltage ratings, breaking capacities and operating configurations to meet project requirements.

How to Select a High-Voltage Vacuum Circuit Breaker

Selecting the right high-voltage vacuum circuit breaker requires careful consideration of the system voltage, rated current, short-circuit level and installation conditions.

1. System Voltage and Equipment Voltage Class – confirm the nominal system voltage and the required rated voltage of the circuit breaker. For example, 33kV applications may require 36kV- or 40.5kV-class equipment depending on the applicable standard and insulation requirements.

2. Rated Current – select according to the expected continuous operating current of the feeder or transformer circuit.

3. Short-Circuit Breaking Current – the circuit breaker must be able to safely interrupt the calculated fault current at the installation point.

4. Insulation Level – check the required power-frequency withstand voltage and lightning impulse withstand levels for the project.

5. Indoor or Outdoor Installation – installation conditions affect the breaker structure, insulation design and environmental protection requirements.

6. Operating and Control Requirements – confirm the operating mechanism, control voltage, interlocking and remote operation requirements where applicable.

Typical Performance of Fenarro High-Voltage Vacuum Circuit Breakers

Fenarro provides indoor and outdoor vacuum circuit breakers for different substation and power distribution applications. Performance varies by model, but the current Fenarro VCB range includes the following reference characteristics:

Vacuum circuit breakers generally require relatively low routine maintenance because the arc-interruption chamber is sealed and does not require gas monitoring or replenishment. For Fenarro VCBs, basic mechanical inspection and lubrication may be carried out every 5 years or 5,000 operations, while routine visual and functional checks should be performed more frequently according to the operating environment, switching frequency and maintenance plan.

High-Voltage Disconnect Switches in 33kV and 35kV Substations

High-voltage disconnect switches provide a visible isolation point between energized and de-energized parts of a substation. They are typically operated after the circuit breaker has interrupted the current, allowing transformers, busbars, feeders or other equipment to be safely isolated for inspection and maintenance.

Unlike circuit breakers, disconnect switches are not designed to interrupt normal load or short-circuit current. Their main function is to provide reliable electrical isolation and clearly indicate that a section of the system has been disconnected.

Fenarro high-voltage disconnect switches provide reliable isolation for substations and power distribution systems, with options for different voltage levels and installation conditions.

How to Select a High-Voltage Disconnect Switch

Selecting a suitable high-voltage disconnect switch requires consideration of both the electrical rating and the installation conditions.

1. Rated Voltage – the voltage class should match the system voltage and insulation requirements of the substation.

2. Rated Current – the disconnect switch must be able to carry the expected continuous current of the connected feeder, transformer or busbar without excessive temperature rise.

3. Short-Time Withstand Current – although a disconnect switch is not designed to interrupt short-circuit current, it must be able to withstand the thermal and mechanical stresses produced by the specified fault current for the required duration.

4. Indoor or Outdoor Installation – indoor disconnect switches are typically installed within switchgear or enclosed electrical systems, while outdoor models require suitable insulation and environmental performance for exposure to weather, pollution, temperature changes and other site conditions.

5. Operating Mechanism – depending on the application, the disconnect switch may use manual or motorized operation. Projects requiring remote control or automation should also confirm the operating mechanism and control arrangement.

6. Earthing Switch Configuration – some applications require an integrated or coordinated earthing switch to ground the isolated section during maintenance. The interlocking arrangement should prevent incorrect operation between the disconnect switch and earthing switch.

High-Voltage Surge Arresters for 33kV and 35kV Overvoltage Protection

High-voltage surge arresters protect transformers, switchgear and other substation equipment from transient overvoltages caused by lightning and switching operations. By limiting the voltage reaching equipment insulation, they help reduce the risk of insulation failure and equipment damage. For effective protection, surge arresters should be installed as close as practical to the equipment being protected, with short and direct line and grounding connections to help reduce additional surge voltage.

Surge arresters are typically installed at locations where incoming surges may threaten critical equipment, including:

• Incoming Lines – to limit surges entering the substation from overhead lines.

• Transformer Terminals – to protect transformer insulation from lightning and switching overvoltages.

• Switchgear and Busbars – to reduce overvoltage stress on switching and distribution equipment.

• Cable Terminations – to protect insulation at transitions between cables and other high-voltage equipment.

Fenarro high-voltage surge arresters are available for different substation and power distribution applications, helping protect transformers, switchgear and other critical equipment from lightning and switching overvoltages. For more detailed guidance on arrester voltage rating, MCOV, grounding conditions and installation considerations, see our High-Voltage Surge Arrester Selection Guide.

High-Voltage Fuses in 33kV and 35kV Distribution Applications

High-voltage fuses provide overcurrent and short-circuit protection for selected transformers, feeders and distribution circuits. When fault current exceeds the designed protection level, the fuse element operates to interrupt the circuit and help limit damage to connected electrical equipment.

Unlike circuit breakers, high-voltage fuses are typically used where simple, localized protection is sufficient and frequent switching or automatic reclosing is not required.

High-voltage fuses are commonly considered in applications such as:

• Distribution Transformer Protection – protecting transformers against overcurrent and short-circuit faults.

• Medium-Voltage Distribution Circuits – providing localized protection for selected feeders or electrical equipment.

• Outdoor Overhead Distribution – drop-out fuse cutouts provide protection while also giving a visible indication after the fuse operates.

• Switchgear and RMU Applications – high breaking capacity fuses may be used where compact, fast fault interruption is required.

Fenarro currently provides both High Breaking Capacity Fuses and Drop-Out Fuse Cutouts for different transformer and distribution applications. Its fuse range is designed for fast overcurrent protection, with selected products offering high breaking capacities for fault-current interruption. Not sure which type is more suitable for your application? See our Drop-Out Fuse Cutout vs High Breaking Capacity Fuse guide for a detailed comparison.

Where 33kV and 35kV Substation Equipment Is Commonly Used

33kV and 35kV substation equipment is used across utility, industrial and renewable energy projects where reliable medium-voltage power distribution, switching and protection are required. The equipment configuration varies according to load conditions, fault levels, installation environment and project layout.

• Utility Distribution Substations

In utility distribution networks, 33kV and 35kV equipment may be used for transformer bays, incoming and outgoing feeders, busbar sections and other primary distribution functions. Circuit breakers, disconnect switches and surge protection devices are coordinated to support reliable operation and maintenance.

• Industrial Power Systems

Industrial facilities such as mining, steel, chemical and manufacturing plants often require robust substation equipment to handle high loads, demanding operating conditions and significant short-circuit levels. Equipment ratings and protection arrangements are therefore selected according to the specific power system and process requirements.

• Renewable Energy Substations

Wind and solar projects may use 33kV or 35kV systems for power collection and substation connections. In these applications, equipment selection may need to consider switching frequency, transient overvoltages, outdoor exposure and coordination with transformers and collection feeders.

• Urban and Compact Substations

Projects with limited installation space may use more compact substation arrangements while maintaining the required voltage, insulation and short-circuit performance. Compact 40.5kV-class equipment can be suitable for selected urban, industrial and other space-constrained substation applications.

Key Considerations for 33kV and 35kV Substation Projects

Equipment selection for 33kV and 35kV substations depends on the overall system design rather than voltage alone. The following factors should be reviewed before confirming the final equipment configuration.

• Equipment Voltage Class – confirm the required rated voltage based on the nominal system voltage, applicable standard and insulation coordination. A 33kV or 35kV project may use a higher equipment voltage class depending on the system requirements.

• Rated Current and Load Conditions – equipment should be selected according to the expected continuous current, transformer capacity and feeder load.

• Short-Circuit Level – circuit breakers and other equipment must have suitable breaking or withstand capability for the calculated fault current at the installation point.

• Insulation Coordination – power-frequency withstand voltage, lightning impulse withstand level and surge protection should be coordinated across the substation.

• Indoor or Outdoor Installation – the installation environment affects insulation design, enclosure requirements, weather resistance and equipment structure.

• Environmental Conditions – altitude, ambient temperature, humidity, pollution level and corrosion exposure may influence equipment selection and insulation requirements.

• Operating and Interlocking Requirements – manual or motorized operation, remote control, earthing switches and interlocking arrangements should be confirmed according to the project design.

• Applicable Standards and Project Specifications – final equipment ratings and testing requirements should follow the applicable IEC standards and the technical specification of the project.

Conclusion

Selecting equipment for a 33kV or 35kV substation requires more than matching the nominal system voltage. Circuit breakers, disconnect switches, surge arresters and fuses must be considered according to their electrical ratings, application and actual installation conditions.

For projects with specific voltage classes, fault levels or operating requirements, Fenarro provides technical consultation and equipment selection support based on the project specifications.