1. What is Current Transformer (CT) Saturation?

• Drastic reduction in inductance.

• Distorted secondary current waveforms.

• Loss of accuracy and potential protection system failure.

2. Causes of CT Saturation

2.1 DC Offset in Primary Current

• Root Cause:

• Fault currents (e.g., short circuits) often contain a DC component (offset) that drives the core into saturation.

• Example: A phase-to-ground fault may produce a decaying DC offset superimposed on the AC fault current.

• Effect on Core:

• The DC component shifts the operating point of the core, reducing the available linear flux range.

2.2 Excessive Primary Current

• Root Cause:

• Continuous overloads or high-magnitude fault currents (e.g., 10x–20x rated current) exceed the core’s flux-carrying capacity.

• Inadequate CT sizing for the application (e.g., using a 500A CT for a circuit with 800A continuous current).

• Effect on Core:

• The core reaches its saturation flux density ($B_{sat}$), typically 1.5–1.8 T for silicon steel.

2.3 High Secondary Burden

• Root Cause:

• Excessive impedance in the secondary circuit (e.g., long cables, multiple relays) increases the burden (VA), forcing the core to draw more magnetizing current.

• Formula: $\text{Burden (VA)}=I_2^2\times Z_{load}$. A higher $Z_{load}$ requires more magnetizing current to maintain $I_2$.

• Effect on Core:

• Increased magnetizing current leads to core heating and eventual saturation.

3. Effects of CT Saturation

4. Detection of CT Saturation

4.1 Waveform Analysis

• Oscilloscope Measurement:

• In saturated CTs, the secondary current waveform becomes flat-topped or clipped (see Figure 1 below).

• Harmonic content increases, with dominant 3rd and 5th harmonics.

• FFT Analysis:

• Use a power quality analyzer to detect high harmonic distortion (THD >5% in protection CTs).

4.2 DC Offset Measurement

• Digital Multimeter (DMM):

• Measure the DC component in the secondary current during a fault. A DC offset >10% of the AC component indicates saturation risk.

4.3 Thermal Imaging

• Overheating in the core or windings (e.g., hotspot temperature >80°C) may signal prolonged saturation.

5. Mitigation Strategies

5.1 Design-Level Solutions

• Select Appropriate CT Ratings:

• Continuous Thermal Current (CTC): Ensure the CT’s CTC ≥ 1.2x the maximum continuous primary current.

• Short-Time Thermal Current (STC): Choose a CT with STC ≥ the expected fault current (e.g., 31.5kA for a 10kV system).

• Use Larger Cores or Higher Permeability Materials:

• Amorphous metal cores offer lower coercivity and higher linearity than silicon steel.

• Example: Amorphous cores reduce saturation by 50% in low-load applications.

• Reduce Secondary Burden:

• Shorten cable lengths or use higher-conductivity wires (e.g., copper instead of aluminum).

• Limit the number of devices connected to a single CT (e.g., use a multi-output CT for metering and protection).

5.2 Operational and Protective Measures

• DC Offset Filtering:

• Install passive filters (e.g., RC circuits) in the secondary circuit to attenuate DC components.

• Adaptive Relaying:

• Use modern numerical relays with saturation detection algorithms (e.g., comparing primary current estimates from voltage and impedance).

• CT Saturation Testing:

• Perform excitation curve tests (伏安特性试验) during commissioning to verify core integrity. A flat curve indicates saturation at low voltages.

5.3 System-Level Adjustments

• Differential Protection Enhancements:

• Use harmonic restraint (e.g., blocking on 2nd harmonic) to avoid false tripping during inrush currents (e.g., transformer energization).

• Zoning and Coordination:

• Install CTs closer to the fault source to reduce the DC offset component (e.g., at the transformer primary rather than the feeder).

6. Case Study: Saturation in a 33kV Feeder

• Problem: A 33kV feeder’s protection relay failed to trip during a bolted fault, leading to a transformer burnout.

• Analysis:

• Post-fault waveform analysis showed severe CT saturation (secondary current clipped at 30% of expected value).

• Root cause: Inadequate CT sizing (500A/5A CT used for a circuit with 800A maximum load).

• Solution:

• Upgraded to a 1000A/5A CT with a larger core and Class 5P20 accuracy.

• Added a harmonic filter to the relay input to mitigate DC offset.

• Outcome: Successful fault clearing in subsequent tests, with no saturation observed.

7. Best Practices for Avoiding Saturation

1. Load and Fault Analysis:

• Conduct short-circuit studies to determine maximum fault currents and DC offset magnitudes.

2. Core Material Selection:

• Use silicon steel for protection CTs (better saturation tolerance during faults).

• Use ferrite or amorphous cores for metering CTs (prioritize low losses at rated current).

3. Regular Maintenance:

• Test CT excitation curves every 5–10 years to detect core degradation.

• Inspect secondary connections for looseness, which can increase burden.

4. Digital Solutions:

• Deploy IoT-enabled CTs with real-time saturation alerts (e.g., using strain gauges to monitor core stress).

Conclusion