How to Select a High-Voltage Surge Arrester for Transformer and Substation Protection

Introduction

As power grids expand to support rising electricity demand and renewable energy integration, reliable overvoltage protection is becoming increasingly important. Yet many selection errors still begin with one shortcut: matching the arrester only to the nominal system voltage.

A suitable arrester must also match the system grounding method, MCOV, temporary overvoltage conditions, equipment insulation level, and installation environment. This guide explains how to evaluate these factors and turn project requirements into a practical surge arrester specification for transformer and substation protection.

Identify the Protected Equipment and Installation Location

Surge arrester selection should begin with the equipment being protected and the intended installation point. Transformers, substation busbars, switchgear, cable terminals, and overhead lines are exposed to different surge conditions and may require different protection arrangements.

Transformer Protection

For transformer protection, the surge arrester should be installed as close as practical to the high-voltage terminal. Long connecting leads can add voltage during a fast surge, increasing the voltage that reaches the transformer winding and reducing the available protective margin. Short, direct line and grounding connections help limit this additional voltage and improve the effectiveness of the arrester.

Surge arresters should be placed as close as possible to the protected equipment, and the connecting cables should be kept as short as possible.

Surge arresters and high-voltage fuses protect transformers against different electrical risks.

• The arrester limits lightning and switching overvoltages

• The fuse interrupts overload and short-circuit currents.

These devices are complementary rather than interchangeable within a transformer protection scheme.

For overcurrent protection selection, see Drop-Out Fuse Cutout vs High Breaking Capacity Fuse: How to Choose the Right Fuse for Transformer Protection.

Substation and Switchgear Protection

Within a substation, surge arresters may be used to protect busbars, switchgear, cable terminations, transformers, and other equipment connected to the same system. The appropriate installation point depends on where an incoming surge may enter and which equipment has the most critical insulation requirements.

Circuit breakers, busbars, cable terminations, and transformer feeders are common protection targets in substations and distribution systems. Contact Fenarro for More Equipment and Protection Options →

For switchgear and cable applications, the arrester should be positioned so that the surge path to ground is short and direct. The equipment insulation level and the distance between the arrester and the protected terminal should also be checked before determining the required protective level.

Overhead Line Protection

Overhead lines are more directly exposed to lightning activity than enclosed substation equipment or underground cables. Arresters may therefore be installed at line entrances, pole-mounted transformers, cable transition points, or other locations where lightning surges could enter the connected equipment.

Line applications may require greater attention to repeated lightning duty, outdoor pollution, mechanical installation, and the grounding condition at each mounting point.

Before selecting an arrester rating, confirm:

• The equipment to be protected

• The intended installation point

• The system voltage at that location

• Whether the installation is indoor or outdoor

• The equipment insulation withstand level

These details define the protection requirement before rated voltage, MCOV, TOV capability, and discharge current are evaluated.

Confirm the System Voltage and Grounding Configuration

The nominal system voltage only identifies the general voltage class. It does not show the highest power-frequency voltage that the arrester may experience during normal operation or a system fault. Before selecting a model, the maximum system voltage, phase-to-ground voltage, grounding method, and fault-clearing time should be confirmed.

Nominal Voltage vs Maximum System Voltage

A project may be described as an 11 kV, 33 kV, or 66 kV system, but the actual operating voltage can be higher than the nominal value. Since the arrester is connected between phase and ground, its continuous voltage capability must be checked against the maximum voltage expected at the installation point—not the nominal voltage alone.

This distinction is important when comparing the rated voltage and continuous operating voltage of different arrester models.

System Grounding Method

The neutral grounding method determines how much the voltage on the healthy phases may rise during a ground fault.

• In a solidly grounded system, the voltage rise is generally limited.

• In a resistance- or impedance-grounded system, the arrester may experience a higher phase-to-ground voltage until the fault is cleared.

• In an ungrounded system, the healthy-phase voltage can remain elevated for a longer period.

Therefore, two systems with the same nominal voltage may require different arrester ratings. Fenarro reviews the grounding arrangement together with the system voltage when matching its HY5WZ, HY5W, and HY5WS surge arrester options to a project.

Fenarro's surge arrester products.

Temporary Overvoltage Conditions

Temporary overvoltage, or TOV, may result from ground faults, load rejection, resonance, or other abnormal system conditions. Unlike a lightning impulse, which lasts for a very short time, TOV can remain across the arrester until the system returns to normal or the fault is cleared.

The arrester must withstand both the expected TOV magnitude and its duration without thermal instability. Selecting too low a rating may overstress the arrester, while selecting an unnecessarily high rating may increase the protective level and reduce the insulation margin of the connected equipment.

Before confirming the model, provide:

• Nominal and maximum system voltage

• System frequency

• Neutral grounding method

• Maximum ground-fault duration

• Expected TOV magnitude and duration

Fenarro can review these project conditions and help identify a suitable configuration from its Lightning Arrester Matching Products for 3–220 kV power systems. The final arrester rating and protective level should also be checked against the insulation withstand level of the transformer, switchgear, cable terminal, or other protected equipment.

Beyond the Arrester: A Complete Protection, Isolation, and Monitoring System

A surge arrester should not be treated as an isolated component. Fenarro extends the protection chain beyond overvoltage limitation by combining surge arresters, fault-disconnection devices, and operating-condition monitoring within one coordinated product family.

This allows project teams to configure protection around three essential functions: limiting the surge, safely isolating a failed arrester, and tracking its operating condition throughout service.

Fenarro's complete Lightning Arrester Matching Products.

Surge Arresters

Fenarro’s HY5WZ, HY5W, and HY5WS surge arrester series support power systems from 3 kV to 220 kV, with impulse-current capability of up to 40 kA and leakage-current control of 50 μA or less. Configurations are available for transformers, substations, transmission and distribution networks, capacitor banks, and industrial power systems.

Surge Arresters (HY5WZ / HY5W / HY5WS Series)

Safe Fault Isolation

The TLB Thermal Explosive Disconnector is designed to separate a failed arrester when abnormal current occurs. Its current-dependent operating characteristics provide rapid response under high-current conditions, while its compact and maintenance-free design supports reliable long-term installation.

In the event of an overcurrent, the TLB can safely disconnect the faulty surge arrester from the system.

Operation Tracking and Online Monitoring

Fenarro also provides arrester counters and online monitors that record surge operations and, on selected models, monitor current conditions. Available options range from basic outdoor counters to cabinet-mounted split designs and advanced current-monitoring models, allowing the monitoring level to match the project’s maintenance strategy.

Fenarro’s arrester counters and online monitors record the number of surge arrester operations, supporting routine inspection and operating-condition assessment.

Instead of sourcing and matching each component separately, project teams can work with Fenarro to configure the arrester, disconnector, counter, and monitoring device as one coordinated protection package. Customized solutions are available based on system voltage, installation conditions, monitoring requirements, and project-specific protection needs.

What to Confirm Before Ordering a Surge Arrester

Fenarro can configure the surge arrester, TLB disconnector, counter, and online monitor as one coordinated package. To prepare a suitable customized solution, the following project information should be confirmed before ordering.

Share your available system and installation data with Fenarro before ordering. Our technical team can review the key parameters, identify missing information, and help reduce the risk of an unsuitable arrester selection.

Conclusion

Selecting a high-voltage surge arrester requires more than matching a model to the nominal system voltage. The protected equipment, installation point, grounding configuration, temporary overvoltage conditions, and connection arrangement must be considered together.

Fenarro can provide a customized configuration that combines HY5WZ, HY5W, or HY5WS surge arresters with TLB disconnectors, arrester counters, and online monitors. By reviewing the available project data, our technical team can help match the protection, fault-isolation, and monitoring functions to the actual operating requirements.

FAQ

Can a surge arrester be selected by nominal system voltage alone?

No. Nominal voltage only defines the general voltage class. The selection should also consider the maximum system voltage, phase-to-ground voltage, neutral grounding method, expected temporary overvoltage, fault duration, and the insulation level of the protected equipment.

What is the difference between arrester rated voltage and MCOV?

The rated voltage indicates the arrester’s operating-duty and temporary overvoltage capability. MCOV, or Maximum Continuous Operating Voltage, is the highest power-frequency voltage that can be applied continuously without overstressing the arrester. Both values must be matched to the actual system conditions.

When should a TLB disconnector or arrester monitor be included?

A TLB disconnector should be considered when a failed arrester must be safely separated from the system. An arrester counter or online monitor is useful when the operator needs to record surge operations, support routine inspections, or monitor current conditions. Fenarro can configure these devices according to the project’s protection and maintenance requirements.