For Maintenance & Protection Engineers, there are very useful FAQ about Secondary Burden & 10% Error Curve Analysis. Through them, we can learn more about the CT PT measurement.
FAQ 1: What is "Burden" in simple terms?
Q: Why do we compare CT Burden to a truck's load?
A: CT acts as a delivery truck; Burden is the "roughness" and "weight" of the road (wires/relays). High impedance forces the CT to produce more voltage, leading to core saturation.
The Physics:
• High Burden = High Secondary Voltage (Vs = Is * Z).
• High Vs = High Magnetic Flux in the core.
Result: Excessive burden makes the CT "lie" during faults by reporting less current than reality.
FAQ 2: Rated Burden vs. Actual Load
Q: My Actual Burden is 2.5 VA, but the nameplate says 30 VA. Is this a problem?
A: No, this is ideal. Rated Burden is the maximum capacity; Actual Burden is the current weight. Large margins ensure accuracy and stability.
The Modern Shift:
• Old electromagnetic relays used 10-15 VA.
• Modern digital relays use < 0.5 VA.
Value: This massive safety margin makes CTs much more reliable in modern digital substations.
FAQ 3: Significance of the "10% Error Curve"
Q: Why is 10% the "Life-Line" for protection class CTs?
A: Relays work correctly if error is < 10%. If a CT saturates and the error hits 50-70%, the relay won't see the fault, and the equipment explodes.
Decision Matrix:
• Error < 10%: Relay trips on time. ✅
• Error > 10%: Relay "blinded" by saturation. ❌
Definition: The curve plots the Maximum Allowed Burden vs. Primary Fault Current.
FAQ 4: Predictive Testing - Seeing the 10,000A future
Q: How can the analyzer know the 10% curve without injecting huge currents?
A: It uses Mathematical Extrapolation based on the non-linear "Magnetic Model" of the CT.
The Logic Engine:
❶ Measure Core Saturation: Done during Day 2 (Excitation test).
❷ Measure Field Burden: Measures the actual wire resistance.
❸ Simulation: The computer calculates Vs needed at 20x current. If Vs < Saturation V, it PASSES.
Benefit: Laboratory-grade safety verification without any high-current stress on the system.