Transformer Surge Arrester Selection: ZnO vs. SiC, Rated Voltage, Residual Voltage & BIL Coordination with ≥20% Protective Margin

Abstract

The surge arrester is the last line of defense between a lightning strike or switching surge on the transmission line and the transformer winding insulation. A correctly selected arrester clamps the overvoltage to a level that the transformer insulation can withstand with a defined protective margin; an incorrectly selected arrester either fails to protect (arrester rated voltage too high, residual voltage too close to BIL) or self-destructs under temporary overvoltage (TOV) conditions (arrester rated voltage too low). This article provides a production-engineer-level selection methodology: the physical difference between zinc-oxide (ZnO) gapless and silicon-carbide (SiC) gapped arresters, the step-by-step calculation of arrester rated voltage (U_r) and continuous operating voltage (U_c) from system parameters, the interpretation of residual voltage (U_res) from the manufacturer's data sheet, and the verification that the protective margin meets the ≥20% criterion per IEC 60071-2. All references align with IEC 60099-4 and IEEE C62.11.

1. ZnO vs. SiC: The Technology Dichotomy

1.1 Silicon Carbide (SiC) Arresters — Legacy Technology

SiC arresters consist of silicon carbide valve elements (nonlinear resistors, α ≈ 4-6) connected in series with spark gaps. At normal operating voltage, the spark gaps isolate the valve elements from the system voltage — no continuous current flows. When an overvoltage exceeds the sparkover voltage, the gaps flash over and the SiC blocks conduct, limiting the voltage.

Advantages (historical):

• Mature technology with 60+ years of field data
• Spark gaps provide galvanic isolation under normal conditions

Disadvantages (why SiC is obsolete for new installations):

• Spark gaps have an inherent response time (0.5-2 μs) — not instantaneous
• Sparkover voltage varies with polarity, waveform steepness, and environmental conditions (altitude, humidity)
• Gap erosion over repeated operations changes the sparkover voltage
• SiC blocks have a higher nonlinearity exponent (α ≈ 4-6) compared to ZnO, meaning they draw higher follow current after a discharge and require series gaps to interrupt it
• Pressure-relief design for internal fault is more complex due to the spark-gap assembly

Status today: SiC arresters are no longer manufactured for new installations; they are encountered only in legacy substations built before ~1990. When a SiC arrester fails, replacement with a ZnO arrester is mandatory.

1.2 Zinc Oxide (ZnO) Arresters — Current Standard

ZnO arresters are gapless — the ZnO varistor blocks are permanently connected between line and ground. At normal operating voltage, the varistors draw only a small capacitive leakage current (typically <1 mA peak). During an overvoltage, the varistors transition into a highly conductive state, with a characteristic approximated by:

I = k × V^α

Where α (nonlinearity exponent) is 20-50 for modern ZnO varistors — dramatically higher than SiC (α ≈ 4-6). This means:

• At 0.8 × U_r: Leakage current ~0.1 mA
• At 1.0 × U_r (rated voltage): Reference current typically 1-10 mA
• At 1.5 × U_r: Current increases by a factor of 10⁶-10⁸

Advantages:

• Gapless — no sparkover delay, instantaneous voltage clamping
• No follow-current problem — the high α means the arrester immediately returns to a high-resistance state after the surge passes, with no power-frequency current to interrupt
• Simpler construction, smaller size, lower weight (no gap assembly)
• Excellent energy absorption capability (typically 3-15 kJ/kV of U_r)
• No performance degradation from gap erosion

Disadvantages:

• Continuous leakage current — the ZnO blocks are under voltage stress 24/7, leading to gradual aging (increase in resistive leakage current, decrease in reference voltage)
• Thermal runaway risk — if the leakage current increases beyond the arrester's thermal dissipation capability, the arrester enters positive-feedback thermal runaway and fails (typically explosively with the pressure-relief device operating)
• ZnO blocks are moisture-sensitive — any seal failure leads to rapid degradation

2. Arrester Rated Voltage (U_r) Selection

2.1 Definition

The rated voltage U_r is the maximum permissible power-frequency voltage (RMS) between the arrester terminals at which the arrester is designed to operate correctly under temporary overvoltage (TOV) conditions. It is not the nominal system voltage.

2.2 Selection Based on System Earthing

The temporary overvoltage (TOV) that the arrester must withstand during an earth fault depends on the system earthing arrangement:

System Earthing	Earth Fault Factor (k_e)	Maximum TOV (per unit of U_m/√3)	Typical U_r Selection
Solidly earthed (effective)	≤1.4	1.4 pu	U_r ≥ 0.8 × U_m
Resistance-earthed	1.4-1.73	1.4-1.73 pu	U_r ≥ 0.9 × U_m
Isolated neutral	1.73-2.0+	1.73-2.0+ pu	U_r ≥ U_m
Compensated (Petersen coil)	1.73-2.0+	1.73-2.0+ pu	U_r ≥ U_m

2.3 Calculation Example

For a 145 kV system (U_m = 145 kV), solidly earthed:

• Phase-to-ground voltage: U_m / √3 = 145 / 1.732 = 83.7 kV
• Maximum TOV (k_e = 1.4): 83.7 × 1.4 = 117.2 kV
• TOV duration: Consider the earth-fault clearing time. If the protection clears a single-line-to-ground fault in 1 second, the arrester must withstand 117.2 kV for 1 second.
• Arrester TOV capability curve (from manufacturer): At 1 second, the arrester can typically withstand 1.15-1.25 × U_r.
• Required U_r: U_r ≥ 117.2 / 1.20 = 97.7 kV
• Select standard rated voltage: U_r = 108 kV (next standard value above 97.7 kV per IEC 60099-4)

2.4 Standard Rated Voltage Values (IEC 60099-4)

Common standard U_r values for ZnO arresters (kV RMS): 3, 6, 9, 12, 15, 18, 21, 24, 30, 36, 42, 48, 54, 60, 66, 72, 84, 90, 96, 108, 120, 132, 144, 168, 180, 192, 198, 210, 228, 240, 258, 264, 276, 288, 300, 312, 330, 336, 360, 372, 396, 420, 444, 468, 480, 588, 600.

3. Continuous Operating Voltage (U_c)

U_c is the maximum permissible power-frequency voltage that can be applied continuously between the arrester terminals. It must be at least equal to the highest continuous phase-to-ground voltage:

U_c ≥ U_m / √3

For a 145 kV system: U_c ≥ 145 / 1.732 = 83.7 kV

The manufacturer will have a stated U_c for each U_r. For a U_r = 108 kV arrester, typical U_c ≈ 84-86 kV. Verify: 86 kV > 83.7 kV → acceptable.

4. Residual Voltage and Protective Margin

4.1 Residual Voltage (U_res)

Residual voltage is the voltage appearing across the arrester terminals during the passage of discharge current. It is specified at:

• Lightning impulse current: 8/20 μs waveform, typically at 5 kA, 10 kA, or 20 kA
• Steep current impulse: 1/5 μs (or 1/10 μs) for testing the response to extremely fast-front overvoltages
• Switching impulse current: 30/60 μs or longer, typically at 500 A, 1 kA, or 2 kA

Example data sheet extract for a 108 kV ZnO arrester:

Current (kA peak)	Waveform	U_res (kV peak)
0.5 (switching)	30/60 μs	225
1 (switching)	30/60 μs	234
5 (lightning)	8/20 μs	265
10 (lightning)	8/20 μs	280
20 (lightning)	8/20 μs	302

4.2 Protective Margin Calculation

The protective margin (PM) for lightning impulse is:

PM_LIPL = (BIL / U_res_LI) - 1 ≥ 0.20 (20%)

For a 145 kV transformer with BIL = 650 kV and a U_r = 108 kV arrester with U_res = 280 kV at 10 kA:

PM = (650 / 280) - 1 = 1.321 - 1 = 0.321 → 32.1% ≥ 20% → ACCEPTABLE

The protective margin for switching impulse:

PM_SIPL = (SIL / U_res_SI) - 1 ≥ 0.15 (15%)

For a 145 kV transformer with SIL = 550 kV (typically 85% of BIL for this voltage class):

PM = (550 / 225) - 1 = 2.444 - 1 = 1.444 → 144.4% ≥ 15% → ACCEPTABLE

4.3 Adding Lead Length Voltage Drop

The arrester residual voltage at the manufacturer's test terminals does not include the voltage drop in the arrester lead connections. In the actual installation, the voltage at the transformer terminal during a surge is:

V_transformer = U_res + L × di/dt

Where:

• L = lead inductance ≈ 1 μH per meter of lead length
• di/dt = rate of rise of discharge current ≈ 5-10 kA/μs for a typical lightning surge

For 5 meters of lead length: ΔV = 5 μH × 10 kA/μs = 50 kV

The effective protective level at the transformer terminals is: 280 + 50 = 330 kV

Recalculating protective margin: PM = (650 / 330) - 1 = 0.97 → 97% ≥ 20% → still acceptable for this example, but for a lower BIL or longer lead, the margin erodes quickly.

Practical rule: Keep arrester leads as short as physically possible. Connect the arrester ground terminal to the transformer tank ground pad directly — do not route through the substation ground grid. Each meter of lead adds approximately 10 kV to the effective protective level for a 10 kA lightning surge.

5. Energy Handling Capability

The arrester must absorb the energy of the surge without exceeding its thermal capacity. Energy absorption is specified in two ratings:

Single-impulse energy (kJ): Typically specified at the high-current (4/10 μs) and long-duration (2,000 μs) impulses. For transmission-class arresters (U_r ≥ 120 kV), the single-impulse rating is typically 5-15 kJ/kV of U_r.

Thermal energy rating (line discharge class per IEC 60099-4):

Class	Discharge Energy (kJ/kV U_r)	Application
1	2.5	Distribution, cable systems
2	4.0	Transmission lines ≤ 100 km
3	6.0	Transmission lines 100-200 km
4	8.0	Long transmission lines >200 km
5	11.0	Extra-long lines, capacitor bank switching

For a 145 kV substation on a 150 km transmission line: Class 3 → 6.0 kJ/kV × 108 kV = 648 kJ. Verify the manufacturer's rated single-impulse energy exceeds this.

FAQ

Q: What is the difference between arrester rated voltage (U_r) and system nominal voltage (U_n)?

U_r is the arrester's maximum continuous temporary overvoltage withstand capability. U_n is the nominal phase-to-phase voltage of the power system (e.g., 132 kV, 220 kV). For a 132 kV solidly earthed system, U_r is typically 96-108 kV — significantly lower than the 132 kV system voltage. This is correct and intentional: U_r is based on the phase-to-ground voltage (U_n / √3 = 76 kV for 132 kV) multiplied by the earth fault factor (1.4 for a solidly earthed system → 106 kV). Selecting an arrester with U_r = U_n would provide no protection because the residual voltage would exceed the transformer BIL.

Q: Why do we need 20% protective margin? Can I accept 15%?

The 20% margin for lightning impulse (IEC 60071-2, Clause 5.3) accounts for multiple uncertainties: (1) arrester lead length voltage drop (which is not included in the nameplate U_res), (2) arrester aging (U_res may increase by 1-3% over the arrester's 30-year life), (3) distance between the arrester and the transformer (separation effect — the further the arrester, the higher the voltage at the transformer due to traveling wave reflection, typically adding 10-50 kV per meter of separation at lightning frequencies), (4) manufacturing tolerance on U_res (±5%), and (5) transformer BIL degradation over its service life. Reducing the margin below 20% requires a detailed insulation coordination study that quantifies each of these factors for the specific installation. For a standard substation without such a study, maintain ≥20%.

Q: Can I use the same arrester for both the HV and LV sides of a transformer?

No. The HV and LV sides require separate arresters rated for their respective system voltages. The HV arrester protects the HV winding from surges originating on the HV transmission line; the LV arrester protects the LV winding from surges that are transferred through transformer capacitance (transferred surge) from the HV side, and from lightning strikes on the LV distribution or tertiary system. A standard practice for generator step-up transformers is to install ZnO arresters on both the high-voltage and low-voltage bushings, with the LV arrester U_r selected for the generator voltage class (typically 12-24 kV).

Q: What happens if an arrester is overstressed and fails?

A ZnO arrester overstressed beyond its energy absorption capability enters thermal runaway: the increased temperature reduces the ZnO grain-boundary barrier height, increasing leakage current, which generates more heat, further increasing leakage current. Within seconds to minutes, the arrester body reaches temperatures of 300-500 °C. At this point: (1) the pressure relief device (PRD) operates — a weakened section of the porcelain/polymer housing ruptures in a controlled manner, venting the internal arc gases in a designated direction to prevent explosive fragmentation, (2) the internal arc establishes a line-to-ground fault, and (3) the upstream protection operates to clear the fault. The transformer is left without surge protection until the failed arrester is replaced — which is why critical substations install two parallel arresters per phase (redundancy), or specify a monitoring system that detects arrester degradation before failure.

Q: How do I know if an arrester is aging and needs replacement?

ZnO arrester aging is monitored by measuring the resistive component of the leakage current at operating voltage. A new, healthy arrester has a resistive leakage current of 50-200 μA (the total current is dominated by the capacitive component, typically 0.5-2 mA). As the ZnO blocks age, the resistive component increases — the third harmonic component is particularly sensitive because the ZnO V-I characteristic's nonlinearity generates harmonics proportional to the resistive current. Field test methods: (1) third-harmonic resistive current measurement (using a clamp-on CT and a harmonic analyzer — typically a Doble M4100 or similar), (2) reference voltage measurement (U_ref at 1 mA DC applied — a decrease in U_ref >5% from the nameplate value indicates degradation), and (3) thermography — an aging arrester shows a higher surface temperature (2-5 °C above adjacent phases) due to increased resistive heating. If any of these methods show a trend of increasing resistive current or decreasing U_ref, plan for replacement.

Q: What is the difference between station-class and distribution-class arresters?

Station-class arresters (IEC 60099-4: Class SH, SM, SL; IEEE C62.11: Station class) are designed with higher energy absorption capability, higher pressure-relief fault current rating (typically 40-80 kA symmetrical), and longer service life (30+ years). They are used for substation transformer protection, bus protection, and cable riser pole applications. Distribution-class arresters (IEC 60099-4: Class DH; IEEE C62.11: Distribution class) have lower energy ratings and are designed for pole-mounted transformer protection and overhead line protection. For a power transformer ≥10 MVA in a substation, always specify station-class arresters. Using a distribution-class arrester on a substation transformer is a false economy — the energy rating is typically 2-5× lower, and failure of a distribution arrester under a multi-stroke lightning flash leaves the transformer unprotected for the subsequent strokes.

References / Standards

Reference	Title
IEC 60099-4:2014	Surge arresters — Part 4: Metal-oxide surge arresters without gaps for a.c. systems
IEC 60071-1:2019	Insulation co-ordination — Part 1: Definitions, principles and rules
IEC 60071-2:2018	Insulation co-ordination — Part 2: Application guidelines
IEC 60071-4:2004	Insulation co-ordination — Part 4: Computational guide to insulation co-ordination and modelling of electrical networks
IEEE C62.11-2020	IEEE Standard for Metal-Oxide Surge Arresters for AC Power Circuits (>1 kV)
IEEE C62.22-2009	IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems
CIGRE TB 544	Metal Oxide (MO) Surge Arresters — Stresses and Test Procedures

*Authored by Du Fu, Production Engineer at ZY POWER. Surge arrester selection is a safety-critical insulation coordination decision — every selection must be verified against the specific system earthing, expected TOV duration, and transformer BIL, with the protective margin calculated explicitly rather than assumed.*