Surge Arrester Selection And Configuration for 10kV And Below Distribution Systems: A Comparative Guide To IEC And IEEE Standards

1. Introduction to Metal-Oxide Surge Arresters

Modern surge protection for distribution systems is almost exclusively provided by Zinc-Oxide (ZnO) metal-oxide surge arresters (MOSA). Without a series spark gap, they offer superior performance, faster response times, and excellent energy absorption capabilities. The core of selection lies in choosing an arrester whose operational characteristics match the specific parameters of the power system it is intended to protect.

To major families of standards govern this process:

· IEC Standards: Predominantly used in Europe, Asia, Africa, and most other parts of the world. The key standard is IEC 60099-4:2014, Surge arresters - Part 4: Metal-oxide surge arresters without gaps for a.c. systems.

· IEEE Standards: Primarily used in North America and several countries influenced by its practices. The key standard is IEEE C62.11-2020, IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV).

Understanding both frameworks is essential for global projects and for making informed design choices.

2. Key Selection Parameters: A Side-by-Side Comparison

The following parameters form the cornerstone of arrester selection. While the concepts are similar, the terminology and specific requirements differ between the standards.

Parameter IEC 60099-4 IEEE C62.11 Technical Explanation

System Voltage Rated Voltage (Ur) Maximum Continuous Operating Voltage (MCOV) This is the maximum power-frequency voltage that can be continuously applied to the arrester without causing thermal instability. Ur (IEC) is typically chosen to be equal to or greater than the highest temporary overvoltage (TOV) the system might experience. MCOV (IEEE) is the crest value of the maximum 60Hz voltage divided by √2. For most solidly grounded 10kV systems, MCOV ≈ System Line-to-Ground Voltage.

Protection Level Lightning Impulse Protective Level (LIPL)   Switching Impulse Protective Level (SIPL) Lightning Impulse Discharge Voltage (LIV)   Switching Impulse Discharge Voltage (SIV) This is the maximum voltage that will appear across the arrester terminals during a discharge event. It must be well below the Basic Lightning Impulse Level (BIL) and Basic Switching Impulse Level (BSL) of the protected equipment. A lower protective level offers better protection but may require a larger, more expensive arrester.

Energy Handling Line Discharge Class (e.g., Class 1, 2, 3, 4, 5) Duty Cycle (Distribution, Intermediate, Station) and Energy Absorption Capability This quantifies the arrester's ability to absorb the energy from a long-duration switching surge without damage. IEC uses a standardized test based on a stored energy line. IEEE classes (Distribution, Intermediate, Station) are broader categories with specific test requirements, with Station class having the highest energy capability.

Normal Current Rated Current Pressure Relief Rating This is not a current-carrying rating but a short-circuit current rating. It ensures the arrester will safely withstand internal faults and rupture in a controlled manner (without exploding) if it fails, thus maintaining system safety.

3. Step-by-Step Selection Guide

Step 1: Determine System Characteristics

· Nominal System Voltage (Un): e.g., 10kV, 6.6kV.

· System Earthing (Grounding): Effectively grounded (ungrounded, impedance grounded, solidly grounded). This drastically affects the temporary overvoltages (TOV).

· Highest System Voltage (Um): The maximum voltage the system operates under normal conditions (e.g., 12kV for a 10kV system).

Step 2: Select the Continuous Operating Voltage (IEC Ur / IEEE MCOV)

· For a 10kV, Solidly Grounded System:

 · IEC: Calculate the expected Temporary Overvoltage (TOV). For a solidly grounded system, a common practice is to select a Ur ≥ 1.25 * Um / √3. For Um=12kV, this gives Ur ≥ 1.25 * 12 / √3 ≈ 8.7kV. A standard 10kV rated arrester is typically chosen.

 · IEEE: MCOV must be ≥ the maximum line-to-ground voltage. For a 10kV system (line-to-line), MCOV ≥ 10kV / √3 ≈ 5.8kV. The maximum system voltage is often 12.47kV, so MCOV ≥ 12.47 / √3 ≈ 7.2kV. An arrester with an MCOV of 8.4kV or 9kV is common.

Step 3: Determine the Required Protection Level

· The protective level (LIPL/LIV) must be lower than the BIL of the protected equipment (e.g., a transformer with a 95kV BIL).

· Protective Margin is calculated as: Margin (%) = [(BIL / Protective Level) - 1] * 100%. A typical margin is 20% or more.

· Example: If a transformer's BIL is 95kV, the arrester's LIPL/LIV should be ≤ 79kV to achieve a 20% margin.

Step 4: Choose the Energy Handling Capability (Duty Class)

· IEC: For most distribution applications (feeders, transformers), a Class 2 arrester is sufficient. For critical substation entries or long lines with high fault currents, Class 3 or 4 may be required.

· IEEE: Distribution Class arresters are standard for poles and underground distribution equipment. Station Class is reserved for high-value, critical equipment inside substations due to its superior energy handling and lower protective levels.

Step 5: Verify Pressure Relief (Short-Circuit) Performance

· The selected arrester must have a pressure relief rating higher than the available symmetrical fault current at the installation point.

4. Configuration and Placement Guidelines

The principles of configuration are largely universal and are derived from engineering practice rather than a single standard.

1. As Close As Possible: Install the arrester directly at the terminals of the equipment to be protected (transformer, switchgear, cable box). Long lead wires between the arrester and equipment increase the inductive voltage drop (V = L*di/dt) under surge conditions, degrading the protection level.

2. Grounding is Paramount: The arrester ground must be connected to the equipment's ground terminal with a short, low-impedance connection. The overall grounding system must have a low impedance to earth to dissipate the surge energy effectively.

3. Locations:

  · Substation Incoming Feeder: To protect the entire substation.

  · On Each Distribution Transformer (pole-mounted or ground-mounted).

  · On Underground Cable Terminations: To protect the transition from overhead lines to underground cables.

  · On Automatic Circuit Reclosers (Reclosers) and Sectionalizers.

5. Conclusion and Key Takeaways

While both IEC and IEEE standards achieve the same goal—reliable system protection—they approach the problem with different terminology and methodological nuances.