Secondary Burden Impact on Current Transformer Accuracy Classes: Mechanism Analysis and Calibration Strategy

2026-02-28

Secondary Burden Impact on Current Transformer Accuracy Classes: Mechanism Analysis and Calibration Strategy

Problem Definition: Accuracy Class Degradation in Field Applications

In industrial power systems, current transformers (CTs) specified with 0.2S or 0.5 accuracy classes frequently exhibit measurement errors exceeding their rated specifications during actual operation. Field investigations reveal that the primary culprit is not manufacturing defects but rather secondary burden mismatch between design assumptions and real-world installations. When CTs are installed with secondary cable lengths exceeding 100 meters or connected to multiple metering devices simultaneously, the total burden can easily surpass the rated burden, causing accuracy class degradation from 0.2S to worse than 1.0 class.

Standard Requirements: IEC 61869-2 Burden Classification and Testing

IEC 61869-2 standard defines accuracy classes based on specific burden conditions. For 0.2S class CTs, the standard requires error limits of ±0.2% for ratio error and ±10 minutes for phase displacement at 100% rated current, but crucially, these specifications are valid only when operating at the rated burden (typically 10VA, 15VA, or 30VA). The standard mandates burden testing at multiple points: 25%, 100%, and 120% of rated burden to verify performance across the operational range.

The burden itself is defined as Z_b = R_wire + R_instrument + X_L, where R_wire is the resistance of secondary cables, R_instrument is the resistance of connected devices, and X_L is the inductive reactance. For accurate classification, the power factor of the burden must also be considered, with standards typically assuming cosφ = 0.8 lagging.

Mechanism Analysis: How Burden Affects Magnetic Circuit Performance

The relationship between secondary burden and CT accuracy is governed by fundamental electromagnetic principles. The CT’s magnetic circuit must satisfy the equation:

V_sec = I_sec × Z_b = 4.44 × f × N_sec × B_max × A_core

Where V_sec is secondary voltage, I_sec is secondary current, Z_b is burden impedance, f is frequency, N_sec is secondary turns, B_max is maximum flux density, and A_core is core cross-sectional area.

When burden increases beyond design specifications, V_sec must increase proportionally to maintain the same I_sec. This forces B_max higher, potentially driving the core closer to saturation. Near saturation, the magnetizing current I_m increases non-linearly, creating additional phase shift and ratio error.

The impact is particularly severe at low current levels (5-20% of rated current), where the signal-to-noise ratio is already compromised. At 5% rated current with excessive burden, a 0.2S class CT can exhibit errors exceeding 2.0%, rendering it useless for revenue metering applications.

Design Trade-offs and Customization: Optimizing for Real-World Burden Conditions

Manufacturers face critical trade-offs when designing CTs for specific burden conditions:

Core Size vs. Cost: Larger cores provide better performance under high burden but increase material costs and physical dimensions
Secondary Turns vs. Voltage Rating: More turns improve accuracy but increase secondary voltage, potentially exceeding insulation limits
Core Material vs. Frequency Response: High-permeability materials improve low-current performance but may have limited frequency response for harmonic-rich environments

Customization strategies include:

Burden-Specific Design: Specify actual field burden during ordering rather than using standard burden ratings
Dual-Ratio Windings: Provide multiple secondary taps to match different burden conditions
Enhanced Core Materials: Use grain-oriented silicon steel or amorphous metal cores for improved low-burden performance
Compensated Windings: Implement turn compensation to offset burden-induced errors

Engineering Implementation: Burden Calculation and Verification Protocol

Field Burden Calculation Checklist:

Measure secondary cable resistance (R_cable = 2 × ρ × L / A, where ρ = 0.0172 Ω·mm²/m for copper)
Sum instrument burdens from all connected devices (meters, relays, transducers)
Calculate total burden: Z_total = √[(R_cable + R_instruments)² + X_L²]
Verify Z_total ≤ CT rated burden at specified power factor

On-Site Verification Procedure:

Perform burden test at 100% and 5% rated current using calibrated test equipment
Measure actual ratio error and phase displacement
Compare results against accuracy class specifications
If errors exceed limits, either reduce burden or replace with higher-rated CT

Troubleshooting Common Burden-Related Issues:

Symptom: Errors increase with temperature
Cause: Cable resistance increases with temperature
Solution: Use larger cable gauge or shorter runs
Symptom: Errors worse at light loads
Cause: Magnetizing current dominates at low currents
Solution: Specify 0.2S or 0.5S class for metering applications
Symptom: Inconsistent readings between phases
Cause: Unequal cable lengths or connections
Solution: Balance secondary circuits and verify connections

Conclusion: Matching CT Specifications to Actual Field Conditions

Accuracy class specifications are meaningless without proper burden matching. The key to reliable CT performance lies in accurate burden calculation during design phase and verification during commissioning. For critical metering applications, always specify actual field burden conditions rather than relying on standard burden ratings. When burden cannot be reduced, consider custom-designed CTs with enhanced core materials and optimized winding configurations. Remember that a properly specified 0.5 class CT with matched burden will outperform an improperly applied 0.2S class CT every time.