top of page

Technical Fundamentals of Surge Arresters

Often overlooked, surge arresters are critical components in the power grid, particularly in Medium and High Voltage (MV/HV) power systems operating above 1 kV AC or 1.5 kV DC. Although similar devices are used in low-voltage applications, they are typically referred to as Surge Protective Devices (SPDs) due to design differences, as SPDs may not rely solely on a simple stack of Metal Oxide Varistors (MOVs).

​

The term “surge arrester” is officially recognized in all major international standards and reference documents, including those from IEC, IEEE, and CIGRE. However, alternative terms such as “lightning arrester” (commonly used in the USA and India) are sometimes employed, though they refer to the same technology. Occasionally, misspellings like "surge arrestors" or "lighting arrestors" appear, which can create confusion.

​

Surge arresters are indispensable for MV, HV, and Ultra-High Voltage (UHV) electrical systems. They play a vital role in maintaining, as far as possible, the uninterrupted supply of electricity to users by mitigating overvoltages caused by lightning and switching surges.

Fundamentals of Surge Arresters: Resources and Tips

Key takeaways: Surge Arresters

  • Connection: Surge arresters are typically connected from phase to ground, with some exceptions.

  • Function: They act as a "static switch" to limit overvoltages and safely divert excess energy to the ground.

  • Construction: The active component is a stack of Metal Oxide Varistors (MOVs).

  • MOV Microstructure: The microstructure of MOVs comprises numerous miniature electronic switches, enabling precise overvoltage control.

  • Purpose: Surge arresters are essential for maintaining proper insulation coordination, offering protection against:

    • Critical transient overvoltages that could compromise dielectric strength.

    • Lightning impulses resulting from atmospheric conditions.

    • Switching impulses caused by breaker reclosing operations.

Fundamentals of Surge Arresters: Text
Untitled design.png

The Role of Surge Arresters in Insulation Coordination

Surge arresters are indispensable for achieving proper insulation coordination in electrical power supply systems. To understand their importance, consider the various voltage magnitudes that can occur in a high-voltage power system, categorized by their duration:

  1. Fast-front overvoltages: Lightning-induced overvoltages occurring in the microsecond range.

  2. Slow-front overvoltages: Switching-induced overvoltages occurring in the millisecond range.

  3. Temporary overvoltages (TOV): Sustained overvoltages lasting in the second range.

  4. Continuous system operation voltage: The highest voltage the system operates under normal conditions.

These voltage ranges in the figure below represent the dielectric stresses that the system and its equipment must endure. For a solidly grounded transmission system, typical voltage values are provided as approximations. The insulation withstand voltage of equipment, such as power transformers, represents its maximum capacity to endure these stresses.

Without surge arrester protection, the insulation of equipment would be unable to withstand these dielectric stresses, potentially leading to breakdown and system failure. Surge arresters play a critical role in maintaining an adequate safety margin, ensuring that transient surges remain below the withstand voltage of the equipment.

The protection performance of surge arresters is determined by their voltage ratings, specifically in relation to lightning and switching overvoltages. However, it is important to note that surge arresters are not designed to limit temporary overvoltages. Instead, they must be capable of withstanding TOV and continuous system operation voltage without sustaining damage.

In conclusion, effective insulation coordination means that all expected transient surges are kept well below the equipment's withstand voltage, ensuring system reliability and protection. Surge arresters are a fundamental part of achieving this goal.

Insulation Coordination Surge Arresters, principle lightning arrestor

Surge Arrester: A Stack of MOVs

The core of every modern surge arrester is simply a stack of MOVs. These MOVs are strategically stacked to form one or multiple columns, each designed according to specific requirements for protection levels (residual voltage), energy absorption capabilities, charge transfer ratings, and voltage ratings to handle temporary overvoltages.

 

While the number of MOVs in a single column typically correlates with voltage ratings, a full MOV stack can comprise multiple columns in parallel to achieve specific energy or charge ratings. Supporting current flow depends on having the right number of parallel paths within the MOV(s). Ratings for energy, charge, or protection levels are controlled by selecting an appropriate MOV diameter or the total number of parallel columns. For construction reasons, the diameter is generally limited to around 120 mm per MOV column. Multiple MOV columns may be housed within one or more surge arrester housings, depending on the application. In summary, the MOV diameter directly influences its ability to handle high current densities from transient surges without sustaining damage.

 

Voltage ratings are essential for ensuring the long-term stability of the MOV stack under continuous operating voltage or specific temporary overvoltage conditions. Although MOVs are not always under continuous voltage, this remains the case for most applications today.The breakdown voltage of a boundary layer between two ZnO grains is approximately 3V, depending on the structure. To reach the desired breakdown voltage for a specific application, an MOV must contain an appropriate number of serial grain boundaries. However, due to production and testing limitations, MOV height is generally restricted to about 45 mm across manufacturers. Therefore, in high-voltage surge arresters, multiple MOV blocks are stacked vertically to meet the necessary performance requirements.

Lightning Data (LinkedIn Post).png

Understanding Leakage Current in MO Surge Arresters

MO surge arresters have a complex impedance comprising resistive and capacitive components. When subjected to alternating current (AC) voltage, this leads to two superimposed currents:

  • A sinusoidal capacitive current that is phase-shifted by -90° relative to the voltage signal.

  • A resistive current that is in phase with the voltage and appears as a periodic pulse signal.

​

The total leakage current of the arrester includes two critical parameters: peak current (It) and third harmonic current or resistive current (Ir). The peak current is influenced by the dominant component of the current, which may vary depending on operating conditions.

​

  1. Normal conditions: At low voltage levels, the peak current (It) is primarily determined by the peak value of the capacitive component (Ic​).

  2. Stress conditions: At higher voltage levels, elevated temperatures, or when the MOV structure is damaged, the peak current (It​) is largely determined by the peak value of the resistive component (Ir​).

​

Between these two extremes, the peak current is affected by harmonic distortion resulting from the increasing resistive component. This creates a behavior with low sensitivity to changes in voltage.

  • The capacitive current corresponds to the current flowing through the series capacitance of the surge arrester and varies proportionally with changes in voltage.

  • The resistive current is highly sensitive and exhibits logarithmic growth across the leakage current region of the voltage-current curve, making it a reliable indicator of the surge arrester's condition.

​

The resistive current is often analyzed using its third harmonic content, which is extracted from the leakage current spectrum through a Fourier transformation algorithm. This third harmonic component serves as a key diagnostic value for assessing the condition and performance of MO surge arresters.

Lightning Data (LinkedIn Post).png
Fundamentals of Surge Arresters: Image

Terms & Definitions

Main electrical ratings for gapless types

Rated Voltage (Ur)

Duty-Cycle Voltage (IEEE)

Specific to surge arresters, should not be compared with other equipment's ratings. It is basically a TOV rating for 10 sec with prior duty that is verified during a thermal recovery period.

IEC: Maximum permissible 10 s power frequency r.m.s. overvoltage that can be applied between the arrester, as verified in the TOV test and the operating duty test.

Continuous operating voltage (Uc)

MCOV (IEEE)

Basically the maximum phase-to-ground voltage that the surge arrester can withstand across itself over its service life without degradation of performance and power losses.

IEC: designated permissible r.m.s. value of power-frequency voltage that may be applied continuously between the arrester terminals in accordance with 8.7

Temporary Overvoltages TOV

for 1sec / 10 sec

You must pay attention to those values, there are often underestimated.

First, the better you control the TOV in your system, the better is your protection level with surge arresters. It even helps reducing clearances in some cases.

Then, you need to consider carefully if you define TOV with prior duty or without. The "prior duty" rating is a proper verification of the performance under worst case preconditioning and prestress, clearly defined in the standards.

Nominal Discharge Current (In)

Classifying current (IEEE)

A standardized lightning current amplitude/waveform to help classifying the different Metal-Oxide Varistors/Surge Arresters designs. A Nominal Discharge Current should not be considered as a maximum lightning current discharge capability. It is rather a soft lightning impulse associated with its protection level.

IEC: peak value of lightning current impulse, which is used to classify an arrester

Residual Voltage (Protection Level)

Discharge Voltage (IEEE)

Residual voltages can be considered as the most essential values for surge arresters. There are often misunderstood since there are very specific to the non-linear characteristics of the Metal-Oxide Varistors. The residual voltage is the resulting voltage across the surge arresters when standard discharge currents are applied to it. Each type of transient overvoltage (fast-front, lightning, switching) is limited by the surge arresters. The protection margin is the difference between the protection level (residual voltages) and the withstand levels of the equipment to be protected. Therefore, the lower the residual voltage, the higher is the protection margin.

IEC: peak value of voltage that appears between the terminals of an arrester during the passage of 
discharge current 

Repetitive Charge Transfer Rating (Qrs)

Single impulse charge transfer (IEEE)

A recently introduced concept of energy withstand classification. It is a statistical approach to verify the capacity of varistors to absorb an electrical charge in a repetitive manner. The objective is to guarantee its physical integrity and its electrical performance after undergoing a succession of discharges.

IEC: maximum specified charge transfer capability of an arrester, in the form of a single event or  group of surges that may be transferred through an arrester without causing mechanical failure or unacceptable electrical degradation to the MO resistors.

Thermal Energy (Wth)

Thermal energy withstand (IEEE)

An important energy rating to verify the thermal stability of the arresters after injecting critical current impulses. The idea is to consider the worst case scenario in the context of the Operating Duty Test (high temperature, provoked ageing, temporary overvoltages and max phase to ground voltage) where we look for its best energetic performance while avoiding a thermal runaway.

IEC: maximum specified energy, given in kJ/kV of Ur, that may be injected into an arrester or arrester section within 3 minutes in a thermal recovery test without causing a thermal runaway.

Fundamentals of Surge Arresters: List

The non-linear Voltage-Current characteristics of gapless Metal-Oxide Surge Arresters

Voltage Current VI Characteristics Surge Arresters, gapless, MOV, MO Resistors
Porcelain Station Class Surge Arresters, Power Transformer, Surge Protection, Valuable Assets, Protection Level

Station Class Arresters

Protection of valuable assets as power transformers

To protect valuable equipment such as power transformers. Primary winding is universally protected since the transformer is the highest value asset in a substation and often has the lowest surge withstand voltage.

Both primary and secondary transformer bushings are protected by default using the same arresters that protect transformer windings. Essential to protect neutral points bushing going through neutral grounding resistor.

​

System Voltage Parameters 

Neutral grounding is essential for the voltage ratings selection

​

Example 420 kV System :

Solidly grounded neutral

Earth fault factor = 1.4

UL-L = 420 kV

UL-G = 420 kV / √3 = 242 kV

Uc ≥ 1,05 × Us ⁄ √3

=1,05 × 420 kV ⁄ √3 = 255 kV

Uc ≥ 1,05 × Us  ⁄ √3 = 1,05 × 420 kV ⁄ √3 = 255 kV
Ur1 = Uc × 1,25=255kV×1,25 = 319kV

1,25 is inherent to MOV manufacturing


TOV consideration  10 sec:
Ur2=TOV⁄T_10s =(1,4×Um⁄√3)÷T_10s=(1,4×420kV⁄√3)÷1,075= 316 kV

​

Typical Ur value is 336kV or 360kV

Ur Rated Voltage = TOV capability 10sec with prior duty  

Fundamentals of Surge Arresters: Welcome
Cable Termination, Transition Point Surge Arresters

Cable Terminations

Protection of sensitive transition points

Often protected with arresters. For such applications, the protection in generally dedicated to the cable since the cable itself is not a self-restoring insulation. Potential damages on the cable could be significantly costly

Fundamentals of Surge Arresters: Welcome
Fundamentals of Surge Arresters: Image
bottom of page