Current Transformer (CT), and CT selection for Active Harmonic Filter (AHF)

1. Current Transformer (CT)

A Current Transformer is an instrument transformer used to measure alternating current (AC) by stepping down high currents to a lower, measurable value.

Key Functions:

• Current Measurement: Steps down high primary current (e.g., 1000A) to a standardized secondary current (e.g., 5A or 1A) for meters and relays.

• Isolation: Provides galvanic isolation between high-voltage circuits and measurement devices.

• Protection: Used in relay circuits for overcurrent and fault detection.

Types of CTs:

• Wound CT: Primary winding is physically connected.

• Toroidal (Window) CT: No primary winding; conductor passes through a window.

• Bar-type CT: Uses a solid bar as the primary conductor.

• Rogowski Coil: Measures AC without a magnetic core (used for transient currents).

Applications:

• Energy metering

• Power system protection

• Load monitoring

• Harmonic measurement (used with Active Harmonic Filter)

2. CT Requirements for AHF Applications

Choosing the right Current Transformer (CT) for an Active Harmonic Filter (AHF) is critical for accurate harmonic detection and effective filtering. Below are the key factors to consider:

3.Types of CTs Suitable for AHFs

A. Conventional Iron-Core CTs

• Best for: Standard AHFs with fundamental (50/60Hz) + harmonic compensation.
• Pros:

• High accuracy at fundamental frequency.
• Cost-effective.

• Cons: Limited high-frequency response (may attenuate higher harmonics).

B. Rogowski Coils (Air-Core CTs)

• Best for: High-speed AHFs, transient loads, and systems with very high harmonics.
• Pros:

• No saturation risk (ideal for distorted waveforms).
• Wide frequency range (up to 1 MHz).
• Lightweight and flexible installation.

• Cons:

• Requires an integrator circuit.
• Lower accuracy at very low currents.

C. Split-Core CTs

• Best for: Retrofit installations where shutdown is not possible.
• Pros: Easy to install without disconnecting cables.
• Cons: Slightly lower accuracy than solid-core CTs.

D. High-Frequency (HF) CTs

• Best for: AHFs compensating very high-order harmonics (e.g., in data centers, EV chargers).
• Pros:  Excellent response up to 10 kHz+.
• Cons:  More expensive.

4. Key Selection Criteria

(1). Accuracy Class

• Class 0.5 (standard for AHFs).
• Class 0.2 (premium AHFs, critical applications).

(2). Frequency Range

• Standard AHFs: 50/60Hz to 2.5 kHz (up to 50th harmonic).
• High-performance AHFs: Up to 10 kHz (for switching frequencies in VFDs, inverters).

(3). Phase Shift

• < 1° phase error at harmonic frequencies to avoid compensation delays.

(4). Burden & Output

• Low burden (to avoid signal distortion).
• Output: 1A, 5A, or mV (Rogowski).

(5). Saturation Characteristics

• Must not saturate under high harmonic currents.
• Rogowski coils are ideal for highly distorted waveforms.

(6). Mounting & Installation

• Solid-core: Highest accuracy but requires cable disconnection.
• Split-core: Easier installation, slightly lower accuracy.

5.Common Mistakes to Avoid

❌ Using a standard protection CT (Class 5P, 10P) → Poor harmonic accuracy.

❌ Ignoring phase shift → Delayed compensation.

❌ Installing CT too far from AHF → Signal degradation.

❌ Overlooking saturation effects → Under-filtering of harmonics.

For most AHFs, a Class 0.5 iron-core CT with a 2.5 kHz bandwidth is sufficient.

For high-frequency harmonics, Rogowski coils or HF CTs are better.

Always verify phase accuracy, burden, and saturation limits before selection.

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