During the transformer testing process, as a power engineer, we may encounter various problems during the operation. Through the following questions and answers about CT PT measurment, we can acquire very important knowledge and information.
FAQ 1: What is Ratio Error, and why are 0.2s Class CTs so strict?
Q: Ratio error of 0.05%? Why does a metering CT need to be so much more accurate than a protection CT?
A: Ratio error is the % difference between nameplate and measured. 0.2s means "Special" accuracy down to 1% current. It is about the "Honesty" of the electric bill.
FAQ 2: Polarity - The "Veto" in Differential Protection
Q: What happens if my ratio is perfect but polarity (S1/S2) is reversed?
A: The protection system will commit "Suicide." The relay will see a massive false fault current.
Differential Logic (87 Relay):
• Correct Polarity: 100A In - 100A Out = 0A difference. ✅
• Reversed Polarity: 100A In - (-100A Out) = 200A difference. ❌
• Result: Instant false trip. You cannot energize the station.
FAQ 3: Phase Displacement (Angle Error) and the Electric Bill
Q: Why does a small "Phase Error" (e.g. 15 minutes) matter if the magnitude is correct?
A: Because Active Power P = V * I * cos(phi). Phase displacement shifts the timing of the current wave.
The Accuracy Trap:
• Meters measure the ANGLE between V and I.
• On low-power-factor loads (e.g. cos phi = 0.5), a 15-min error causes 1% error in power.
FAQ 4: Modeling Method - verifying 1000A with only 120V
Q: Traditional tests push 1000A through the primary. How can the analyzer be accurate without it?
A: It uses the "Modeling Method." It separates physical geometry (turns) from magnetic losses.
How it works:
❶ Turns Ratio: High-precision voltage bridge measures Ns/Np.
❷ Magnetic Model: Excitation curve captures exactly how much current the iron core "steals."
❸ Math Prediction: Computer combines Turns + Loss + Burden to predict 1000A error.
FAQ 5: Field Reliability - 0.1% accuracy in a noisy substation
Q: Won't substation interference ruin a 0.1% precision measurement?
A: Analyzers use Narrow-band Digital Filtering and Frequency Shifting to stay quiet.
Noise Rejection Tactics:
• Frequency Shifting: Injects signals at 49.7 Hz or 50.3 Hz. Ignores 50 Hz background noise.
• Internal Reference: Self-calibrates against ultra-stable internal voltage references.
• Kelvin 4-Wire: Eliminates the resistance of your own test leads.