Nanocrystalline Current Transformer Cores: Class 0.2s Accuracy
High-permeability Nanocrystalline cores designed for Class 0.2s accuracy metering. Saturation flux density: 1.2T. Frequency range: 50Hz-400Hz. Phase angle error: <10 minutes at rated burden.
Technical Specifications
| Parameter | Nanocrystalline | Mu-Metal | Silicon Steel |
|---|---|---|---|
| Initial Permeability (μi) | 80,000 | 20,000 | 2,000 |
| Saturation Flux Density (Bs) | 1.2T | 0.8T | 1.8T |
| Remanence (Br) | 0.05T | 0.3T | 1.2T |
| Core Loss @ 50Hz, 0.1T | 2 W/kg | 5 W/kg | 8 W/kg |
| Phase Angle Error | <10 minutes | 15-30 minutes | 30-60 minutes |
| Temperature Stability | ±2% (-40°C to +130°C) | ±5% | ±8% |
| Hysteresis Loss | Minimal | Low | Moderate |
Why Nanocrystalline Excels for CT Applications:
The combination of high permeability (80k μi) and low remanence (0.05T) enables accurate measurement at low primary currents while maintaining linearity up to 1.2T saturation. This wide dynamic range is critical for Class 0.2s metering accuracy across 5-120% of rated current.
CT Series Product Specifications
Toroidal cores for Class 0.2s accuracy metering and protection applications
| Core Code | Dimensions (OD×ID×H mm) | Ae (mm²) | le (mm) | Ve (mm³) | Primary Current Range | Accuracy Class | Applications |
|---|---|---|---|---|---|---|---|
| CC-NC-CT-001-26 | 26×16×10 | 50 | 66 | 3300 | 5-100A | 0.2s, 0.5s | Revenue metering |
| CC-NC-CT-002-40 | 40×30×15 | 75 | 110 | 8250 | 50-500A | 0.2s, 0.5s, 1.0 | Metering, Protection |
| CC-NC-CT-003-50 | 50×30×15 | 150 | 126 | 18900 | 100-2000A | 0.2s, 0.5s, 1.0 | HV Protection, GIS |
| CC-NC-CT-004-40G | 40×30×15 | 75 | 110 | 8250 | 50-500A | 0.2s, 0.5s, 1.0 | Metering, Protection |
CC-NC-CT-001-26
26×16×10 mm
50 mm²
66 mm
3,300 mm³
CC-NC-CT-002-40
40×30×15 mm
75 mm²
110 mm
8,250 mm³
CC-NC-CT-003-50
50×30×15 mm
150 mm²
126 mm
18,900 mm³
CC-NC-CT-004-40G
40×30×15 mm
75 mm²
110 mm
8,250 mm³
Magnetic Parameters
- • Initial permeability: 80,000 ± 25%
- • Saturation induction: 1.2T @ 800 A/m
- • Remanence: <0.05T
- • Coercivity: <1.5 A/m
Electrical Performance
- • Phase angle error: <10 minutes
- • Ratio error: <0.2% (Class 0.2s)
- • Secondary burden: 2.5-15 VA
- • Frequency range: 50-400 Hz
Environmental
- • Operating temp: -40°C to +130°C
- • Humidity: 95% RH non-condensing
- • Vibration: IEC 60068-2-6
- • Insulation: 3kV AC (1 min)
Performance Characteristics
Permeability vs Frequency Response
μi remains >60k up to 10kHz
Flat response enables wideband CT accuracy
- Initial permeability: 80,000 ± 25% @ 1kHz
- Maintains >60k μi up to 10kHz
- Enables accurate harmonic measurement
- Superior to ferrite above 1kHz
B-H Hysteresis Loop (50Hz)
Bs = 1.2T, Br = 0.05T
Minimal remanence for CT accuracy
- Saturation induction: 1.2T @ 800 A/m
- Remanence: <0.05T (excellent linearity)
- Coercivity: <1.5 A/m
- Core loss: 2 W/kg @ 50Hz, 0.1T
Phase Angle vs Current Loading
Phase error <10 minutes
5-120% current range (Class 0.2s)
- Phase angle error: <10 minutes @ rated burden
- Ratio error: <0.2% (5-120% In)
- Composite error: <0.5% @ 5% In
- Meets IEC 61869-2 Class 0.2s requirements
Temperature Stability
±2% drift (-40°C to +130°C)
Stable across full operating range
- Permeability drift: ±2% over temp range
- Operating: -40°C to +130°C
- Storage: -55°C to +150°C
- Humidity: 95% RH non-condensing
Typical Applications & Case Studies
400kV GIS Revenue Metering
Integrated into 400kV Gas Insulated Switchgear for protection and metering. CC-NC-CT-003-50 cores achieve Class 0.2s accuracy for 1000/1A ratio with 2.5VA burden at 50Hz.
Environment: SF6 gas, -25°C to +40°C
Performance: Phase error <8 minutes, ratio error <0.15%
Differential Protection Relay
CC-NC-CT-002-40 cores in busbar protection systems requiring fast, accurate fault detection. Maintains linearity during 20x overcurrent conditions.
Fault Current: 40kA symmetrical
Response Time: <2ms to 95% accuracy
Smart Grid Power Quality
Real-time monitoring of harmonics up to 50th order in renewable energy integration. CC-NC-CT-001-26 maintains accuracy across 5-400Hz bandwidth.
Dynamic Range: 1% to 120% of rated current
Sampling Rate: Compatible with 10kHz ADCs
DC-Tolerant Protection
CC-NC-CT-004-40G gapped cores for HVDC converter stations and renewable energy systems with DC bias components up to 10% of AC rating.
Saturation Margin: >2x at maximum DC+AC
Applications: Wind farms, solar inverters, HVDC
Engineering FAQ: CT Core Selection
How to choose a CT core for high-precision metering?
For Class 0.2s accuracy, calculate the required turns ratio using Np/Ns = Is/Ip. Ensure the core's saturation flux density (1.2T for nanocrystalline) provides adequate margin above the maximum flux: Φmax = (Ip × √2 × Zb) / (2πf × Ns)
Φmax = (1000 × 1.414 × 2.5) / (2π × 50 × 1000) = 0.011 Wb
Required Ae ≥ 0.011/1.2 = 9.2 mm² (CC-NC-CT-002-40 provides 15 mm²)
What accuracy classes can nanocrystalline CT cores achieve?
Nanocrystalline cores readily achieve IEC 61869 Class 0.1, 0.2s, and 0.5s for revenue metering. The low remanence (0.05T) and high permeability (80k μi) enable accurate measurement from 5% to 120% of rated current with phase errors typically <10 minutes.
How does DC bias affect CT core performance?
Standard nanocrystalline cores tolerate DC bias up to 5% of AC rating without significant accuracy degradation. For higher DC content (renewable energy, HVDC), use gapped cores like CC-NC-CT-004-40G which maintain linearity with DC bias up to 10% of AC rating.
What's the difference between Class 0.2 and Class 0.2s?
Class 0.2s has tighter phase angle requirements (±10 minutes vs ±30 minutes) and extended accuracy range (5-120% vs 20-100% of rated current). This makes 0.2s suitable for revenue metering where accurate measurement at light loads is critical.
How to calculate secondary burden for CT design?
Total burden = Meter burden + Lead resistance + Safety factor. For revenue metering:
Where: ρ = 0.0175 Ω·mm²/m (copper), L = lead length, A = conductor area
Need CT Core Design Support?
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About the Author: Rajesh Kumar is Lead Magnetics Engineer at CenturaCores with 15+ years experience in current transformer design for utility and industrial applications. He holds an M.Tech in Power Systems from IIT Delhi and is a member of IEEE Power & Energy Society.
Standards Compliance: All CenturaCores CT products are designed and tested to IEC 61869-2, ANSI/IEEE C57.13, and relevant national standards.