Shielding Design and Anti-Interference Measures for Secondary Windings of Current Transformers

1. Introduction

2. Mechanisms of Electromagnetic Interference in CT Secondary Windings

2.1 Types of Interference Sources

• Radiated interference: Electromagnetic fields emitted by nearby high-voltage conductors, switchgear operations, or radiofrequency (RF) devices (e.g., communication antennas). These fields induce voltages/currents in secondary windings via capacitive or inductive coupling.

• Capacitive coupling: Occurs between the secondary winding (or its leads) and high-voltage primary conductors, forming a parasitic capacitor that transfers high-frequency noise.

• Inductive coupling: Alternating magnetic fields from nearby current-carrying conductors induce eddy currents in secondary windings or loops, following Faraday’s law of induction.

• Conducted interference: Noise transmitted through physical connections, such as:

• Ground loops formed by multiple grounding points in secondary circuits, where potential differences drive interference currents.

• Power supply noise from auxiliary devices (e.g., transformers, inverters) propagating into secondary leads.

• Transient pulses from switching operations (e.g., circuit breaker tripping) conducted via cables.

• Electrostatic discharge (ESD) and electromagnetic pulses (EMP): Sudden voltage spikes from ESD or EMP (e.g., lightning strikes) can damage insulation or disrupt signal integrity.

2.2 Impact of Interference

• Measurement errors: High-frequency noise overlaps with the 50/60Hz fundamental current, causing inaccuracies in energy metering or power quality analysis.

• Relay maloperation: Protective relays may misinterpret interference signals as fault currents, triggering unnecessary tripping or failing to respond to actual faults.

• Insulation degradation: Sustained high-voltage interference can weaken winding insulation, reducing CT lifespan.

3. Shielding Design for Secondary Windings

3.1 Shielding Materials

• Electric field shielding: Requires materials with high electrical conductivity to reflect or absorb electric fields.

• Copper: Excellent conductivity (58 MS/m) and malleability, ideal for forming continuous shields.

• Aluminum: Lightweight and cost-effective, suitable for large-area shields (e.g., cable sheaths).

• Copper-clad steel: Combines conductivity with mechanical strength, used in harsh environments.

• Magnetic field shielding: Requires materials with high magnetic permeability to divert magnetic flux (for low-frequency fields, <1kHz).

• Mu-metal (nickel-iron alloy): High permeability (up to 80,000 μ₀) for shielding low-frequency magnetic fields from power lines.

• Silicon steel: Used in CT cores to reduce magnetic interference but less effective for secondary winding shielding.

• Ferrite: Effective for high-frequency magnetic fields (e.g., RF noise), often used in cable shields.

• Composite shields: For mixed interference, multi-layer shields (e.g., copper + mu-metal) combine electric and magnetic shielding capabilities.

3.2 Shielding Structures

• Winding layer shielding: A conductive foil (copper or aluminum) wrapped around the secondary winding, isolated from the winding by insulation (e.g., polyimide film). This blocks capacitive coupling between primary and secondary windings and suppresses internal electric fields.

• Design consideration: The foil must be tightly wrapped with overlapping seams to avoid gaps, ensuring a continuous Faraday cage.

• Cable shielding: Secondary leads (connecting CTs to meters/relays) are prone to interference, requiring shielded cables:

• Braided shields: Flexible copper braids (coverage >85%) provide good electric field shielding and mechanical durability.

• Foil shields: Thin aluminum/polyester laminates (coverage >98%) offer superior high-frequency shielding but are less flexible.

• Combined braid-foil shields: Balance flexibility and shielding efficiency, suitable for harsh industrial environments.

• Enclosure shielding: CTs installed in metal enclosures (e.g., stainless steel) benefit from additional shielding. The enclosure acts as a barrier against external radiated interference, with performance enhanced by conductive gaskets at seams to prevent field leakage.

3.3 Shield Grounding Techniques

• Single-point grounding: The shield is grounded at one end (e.g., CT terminal or relay panel).

• Advantages: Eliminates ground loops, which are major sources of conducted interference.

• Application: Low-frequency systems (<1MHz) or short cable runs (<30m).

• Two-point grounding: The shield is grounded at both ends (CT and relay sides).

• Advantages: Provides better high-frequency shielding by reducing shield impedance.

• Risks: May create ground loops if ground potentials differ; use only when ground potentials are equal (e.g., same substation ground grid).

• Floating shielding: The shield is insulated from ground, used in low-interference environments or with differential amplifiers to minimize noise pickup.
• Grounding standards: IEC 61936-1 recommends single-point grounding for CT secondary circuits, with ground leads as short and thick as possible (≤4mm² copper) to reduce impedance.

4. Comprehensive Anti-Interference Measures

4.1 Circuit Optimization

• Twisted-pair secondary leads: Twisting reduces inductive coupling by canceling magnetic fields induced in adjacent wires. The twist rate (e.g., 10-20 twists per meter) increases with interference levels.

• Balanced circuits: Using differential amplifiers in relays/metering devices rejects common-mode interference (noise present on both leads), while amplifying the differential-mode signal (the desired current).

• Surge protection devices (SPDs): Metal-oxide varistors (MOVs) or gas discharge tubes (GDTs) installed at CT terminals clamp transient voltages (e.g., from lightning) to safe levels (<2kV).

4.2 Wiring and Installation Practices

• Routing: Secondary cables should be routed away from high-voltage busbars, transformers, or motor leads (minimum distance ≥0.5m). Parallel runs with power cables should be avoided; if necessary, cross at 90° angles to minimize coupling.

• Cable separation: Separate secondary cables from control or communication cables using dedicated cable trays or conduits. Shielded barriers can further isolate sensitive circuits.

• Insulation integrity: Ensure secondary windings and cables have adequate insulation (e.g., 1kV dielectric strength) to resist capacitive discharge from primary circuits.

4.3 Grounding System Design

• Low-impedance ground grid: Substation ground grids with multiple interconnected electrodes (copper rods, strips) reduce ground resistance (<5Ω) and equalize potentials, minimizing ground loops.

• Separate grounding for analog and digital circuits: Analog (CT secondary) and digital (relay logic) grounds are isolated to prevent digital noise from coupling into analog signals, connected at a single point (star grounding) to avoid loops.

• Ground bus design: A dedicated copper bus (≥10mm²) for secondary circuit grounding ensures low-impedance paths for noise currents.

4.4 Signal Processing and Filtering

• Low-pass filters: RC or LC filters (cutoff frequency ~1kHz) installed at relay inputs attenuate high-frequency noise while preserving the 50/60Hz fundamental signal.

• Digital filtering: Modern microprocessor-based relays use algorithms (e.g., Fourier transform, wavelet analysis) to distinguish fault currents from interference, enhancing noise immunity.

• Burden matching: Ensuring the secondary burden (impedance of meters/relays) matches the CT’s rated burden minimizes signal distortion and reduces susceptibility to noise.

5. Standards and Validation

5.1 Relevant Standards

• IEC 61869-2: Specifies requirements for CTs, including immunity to electromagnetic disturbances (e.g., 10V/m RF field immunity).

• IEEE C57.13: Mandates testing of CT secondary circuits for noise rejection, with limits on error under specified interference levels.

• EN 61000-6-2: Defines industrial environment EMC requirements, guiding shielding and grounding design.

5.2 Testing and Validation

• EMC testing: CTs undergo radiated immunity tests (30MHz–1GHz, 10V/m) and conducted immunity tests (0.15–80MHz, 1V) to verify performance under interference.

• Ground loop testing: Measuring current in secondary loops with a clamp meter identifies excessive ground loop currents (>10mA), indicating poor grounding.

• Field monitoring: Post-installation, using spectrum analyzers to measure noise levels in secondary circuits ensures compliance with design specifications.

6. Case Studies

6.1 Substation Interference Mitigation

• Adding mu-metal shields around secondary windings.

• Replacing unshielded cables with braided-shield twisted pairs, grounded at the relay panel.

• Installing low-pass filters (cutoff 500Hz) at relay inputs.

• Result: Interference-induced errors reduced from ±5% to <±0.5%, eliminating false trips.

6.2 Industrial Plant Noise Reduction

• Enclosing CTs in ferrite-lined metal enclosures.

• Using two-point grounded foil-braid cables (grounded at CT and PLC panels, with equalized ground potentials).

• Implementing digital wavelet filtering in metering systems.

• Result: Noise floor reduced by 20dB, enabling accurate energy monitoring.

7. Conclusion