Rethinking Altitude Derating: Insulation Coordination for Medium-Voltage Systems

At high altitude, the right answer is not always higher-rated equipment. Sometimes the smarter answer is proper insulation coordination at lower cost

Brief No.: VEB-005
Category: Protection / Equipment Design / Capacitor Banks / Harmonic Filter Banks

1. Introduction: Optimizing Insulation Coordination for High-Altitude Power Systems

High-altitude applications for medium-voltage equipment (2.4 kV to 38 kV) often lead to the automatic assumption that metal-enclosed capacitor banks, harmonic filter banks, and related upstream switchgear must be specified with higher insulation ratings.  In many engineering environments, this conservative approach results in inflated equipment costs, larger physical footprints, and suboptimal system designs.

However, when insulation coordination and surge arrester application are rigorously evaluated, higher insulation ratings are not always required. Using established IEEE and IEC methodologies, this VarStec Engineering Brief demonstrates that properly selected surge arresters can effectively limit transient overvoltages to levels safely below the altitude-derated dielectric strength of the equipment. By applying these standards-based mitigation strategies, it is possible to utilize standard equipment insulation ratings at elevations significantly exceeding 1,000 meters (3,300 feet) without sacrificing system reliability or compliance.

2. Why Insulation Coordination Matters for EPCs and Specifying Engineers

For EPCs, the common practice of automatically specifying higher BIL equipment at altitude can add unnecessary cost, increase equipment footprint, and reduce the number of suppliers able to meet the requirement. It can also push the project into less common equipment ratings with longer lead times and more difficult procurement.

When surge arresters are properly applied and insulation coordination is formally evaluated, standard equipment ratings may still be acceptable at elevation under both IEEE and IEC methods. That can lead to a lower-cost, more compact, and more competitive project without sacrificing compliance or reliability.

3. Technical Discussion: IEEE and IEC Methodologies for Altitude Derating

Both IEEE and IEC standards support the conclusion that higher equipment insulation ratings are not automatically required for high-altitude applications.  When surge arresters are properly selected and applied, arrester lead lengths are minimized, and insulation coordination is properly evaluated, standard equipment ratings can often be used without unnecessary upsizing.

3.1 IEEE Standards for High-Altitude Insulation and Switchgear

IEEE C37.010, IEEE C37.20.2, and IEEE C37.20.3 formally validate that properly coordinated surge arresters can safely mitigate the need for physical altitude derating.

• IEEE C37.010, Section 3.2.2: Directly challenges blind upsizing by stating that derating the dielectric capabilities is “not always necessary and usually not the most economical approach”. It confirms that if at least a 20% margin exists between the arrester protective level and the equipment’s Basic Lightning Impulse Insulation Level (BIL), “no derating should be necessary”.
• IEEE C37.20.2, Section 8.1.3 & IEEE C37.20.3, Section 8.1.4: Both switchgear standards explicitly mandate the evaluation of this mitigation strategy, stating that for applications above 1,000 meters (3,300 feet), the use of surge arresters on each circuit selected to “keep transient voltages below the reduced levels should be considered”.

3.2 IEC Standards for High-Altitude Insulation and Switchgear

IEC TR 62271-306, Section 4.5.1.2 and IEC 60871-1, Section 18.1. These documents support the use of overvoltage limiting devices as part of insulation coordination and confirm that arrester application may reduce the required insulation level of the installation.

• IEC TR 62271-306: States that the “Application of surge arresters with the intent to lower the insulation level of the substation may result in a more advantageous choice of equipment.”.
• IEC 60871-1: Confirms that insulation choice must account for “the type of overvoltage limiting devices” utilized in the system.

4. Engineering Considerations: Comparing IEEE and IEC Coordination Methods

IEEE C62.22 and IEC 60071-2 use different methods to evaluate insulation coordination at altitude. IEEE starts with the equipment rating and derates downward to confirm adequate protective margin, while IEC starts with the arrester protective level and calculates the minimum required equipment withstand level upward. Both methods are intended to verify that transient overvoltages remain within the dielectric capability of the equipment at elevation.

4.1 Derating Equipment According to IEEE C62.22 Protective Margins

IEEE C62.22 mandates a minimum Protective Margin (PM) of 20% (Protective Ratio) when coordinating surge arresters with equipment insulation.

• LPL Margin (PM_L2): Must be ≥ 20% between the equipment’s Basic Lightning Impulse Insulation Level (BIL) and the arrester’s Lightning Protective Level (LPL).
• FOW Margin (PM_L2): Must be ≥ 20% between the equipment’s Chopped Wave Withstand (CWW) and the Front-of-Wave (FOW) protective level.
• Calculated Stress: Both protective margins must properly account for the connecting lead inductive voltage drop.

4.2 Derating Equipment According to IEC 60071-2 Surge-Up Methodology

Unlike the IEEE “Equipment-Down” margin approach, IEC 60071-2 mandates a “Surge-Up” methodology. It requires engineers to calculate a Required Withstand Voltage (U_rw) based on the arrester’s protective level and verify that the equipment’s standard Nameplate Rating is greater than or equal to this calculated value.

• Coordination Withstand Voltage (U_cw): The baseline transient stress at the equipment is established using the arrester’s Lightning Impulse Protective Level (U_pl). This calculation must account for the added voltage drop of the connecting lead wires and separation distance effects.
• Required Withstand Voltage (U_rw): The U_cw is then multiplied upward by a mandatory Safety Factor (K_s) to account for insulation aging and manufacturing dispersion. IEC 60071-2 recommends Ks = 1.15 for internal insulation and K_S = 1.05 for external insulation. For high-altitude installations, an Atmospheric Correction Factor (K_a) is also multiplied into this value.

U_rw = U_cw x K_s x K_a

• Verification: To achieve a compliant design, the equipment’s standard Lightning Impulse Withstand Voltage (LIWV or BIL) must be strictly greater than or equal to the calculated Required Withstand Voltage (U_rw).

5. Standards and Additional Resources

• IEEE C62.22 and IEC 60071-1 / 60071-2: Provide the core methodologies and protective margin verification formulas for applying metal-oxide surge arresters and insulation coordination.
• IEEE C37.010, C37.20.2, and C37.20.3: Provide high-altitude application guidelines, explicitly stating that derating a breaker’s dielectric capabilities is not always necessary and that surge arresters should be considered to keep transient voltages below reduced levels.
• IEC TR 62271-306 and IEC 60694: Confirm that the final selection of rated insulation levels must purposefully account for overvoltage limiting devices, noting that applying surge arresters may result in a more advantageous choice of equipment

6. VarStec Perspective: The Impact of Arrester Lead Length on Coordination

VarStec’s view is that high-altitude applications should not automatically default to higher insulation ratings without first performing a formal insulation coordination review. Even when a project is governed primarily by either IEEE or IEC requirements, evaluating the design against both methods provides a stronger and more broadly accepted engineering basis for final equipment selection. In many cases, standard equipment insulation ratings can still satisfy both IEEE and IEC criteria when surge arresters are properly selected and applied, arrester lead length is minimized, and the transient duty at the equipment is rigorously evaluated. This approach can reduce project cost, improve equipment availability, and avoid unnecessary upsizing without sacrificing compliance or reliability.

Arrester lead length is one of the most important factors in insulation coordination because it directly increases the transient voltage seen by the equipment. For this reason, physical arrester placement must be treated as a critical design parameter, not a secondary layout detail. VarStec’s designs maintain total arrester lead length, including both line-side and load-side connections, at less than 1 foot (0.3 m). When evaluating alternative manufacturers, it is important to obtain equipment drawings and verify how total arrester lead length has been measured. This can be a major driver in whether the equipment passes or fails the insulation coordination evaluation.

7. Key Takeaway: Pass/Fail Screening for Standard Equipment Insulation Ratings

Do not assume that high altitude automatically requires higher insulation ratings. First perform the insulation coordination study. VarStec’s Medium-Voltage Altitude Derating and Insulation Coordination Tool can be used for preliminary pass/fail screening under both IEEE and IEC methods. When IEEE and IEC criteria are satisfied, and surge arresters are properly applied with minimum practical lead length, standard equipment insulation ratings may still be acceptable.

8. Engineering Guidance: Altitude Derating and Insulation Coordination Support

Insulation coordination at altitude can be a complex subject, and navigating both IEEE and IEC requirements can be tedious. VarStec has developed the Medium-Voltage Altitude Derating and Insulation Coordination Tool to simplify that evaluation by reducing the analysis to practical pass/fail criteria under both standards. The tool helps engineers assess whether standard equipment insulation ratings may still be acceptable at elevation when surge arrester performance, lead length, and transient overvoltage limits are properly considered.

For engineering guidance, application review, or support related to altitude derating, insulation coordination, capacitor banks, harmonic filter banks, or related medium-voltage equipment, contact VarStec. VarStec combines practical engineering expertise with equipment solutions for medium-voltage reactive compensation and power quality applications.

About the Author

Paul B. Steciuk is Co-Founder and President of VarStec Power Solutions and a subject matter expert in medium-voltage reactive compensation and power quality, specializing in the design, development, and manufacturing of fully integrated metal-enclosed capacitor banks and harmonic filter banks. His technical expertise spans the entire project scope, from initial harmonic and power system analysis through to the physical design, manufacturing, and final commissioning of specialized reactive compensation power equipment.

Before co-founding VarStec, Paul co-founded and led Northeast Power Systems, Inc. (NEPSI) for more than 25 years, supporting consultants, EPCs, utilities, and industrial users worldwide. He earned his degree in electric power engineering from Rensselaer Polytechnic Institute (RPI), began his career at Power Technologies, Inc. and Commonwealth Sprague Capacitor, Inc., and has authored more than 30 technical articles and white papers on reactive compensation, harmonic analysis, and harmonic filter applications.