Feeder CT Ratio Error After Relay Retrofit
After a relay retrofit on a 13.8 kV feeder, the new feeder relay displays 480 A while a calibrated clamp-on meter on the same phase reads 520 A. The CT is nameplate 600:5, multi-ratio 1200/600:5, connected on the 600:5 tap. The relay's CT ratio setting is 600:5. Secondary current measured at the relay terminals with a true-RMS meter is 4.00 A. The CT burden is 0.8 Ω at 60 Hz, and a CT excitation test shows a knee point of 95 V, which is lower than the manufacturer's 140 V minimum for this rating. The feeder load is stable and sinusoidal. The relay is a microprocessor overcurrent relay. The technician wants to change the relay ratio to 650:5 to make the displayed current match the clamp-on meter.
1. Calculate the actual CT ratio and ratio error. Is changing the relay ratio setting to 650:5 appropriate?
Read the worked answer
The actual ratio implied by the measurements is primary current divided by secondary current: 520 A / 4.00 A = 130:1, which is equivalent to 650:5. The nameplate ratio is 600:5, or 120:1. The ratio error is (130 - 120) / 120 = 8.3%, which is large for a metering-class CT at normal load.
Changing the relay setting to 650:5 is not appropriate as a first response. It would force the relay display to match the clamp-on meter while hiding the real cause. The clamp-on meter itself has accuracy limits, but an 8.3% discrepancy is well outside normal measurement error. The correct approach is to determine whether the CT is on the wrong tap, has a turns defect, is affected by burden, or whether the secondary circuit has a wiring or grounding problem. The relay setting should reflect the actual installed CT ratio after that is verified, not be adjusted to mask a test discrepancy.
2. Interpret the knee-point and burden data. What tests should be performed, and what is the likely disposition?
Read the worked answer
The knee point of 95 V is below the manufacturer's 140 V minimum. A low knee point means the CT saturates at a lower secondary voltage than expected and may not maintain accuracy under fault current or higher burden. The burden of 0.8 Ω is not extreme for a 5 A CT, but the combination of low knee point and normal-load ratio error suggests the CT may be damaged, misconnected, or of the wrong rating.
Perform a primary-injection ratio test at several points, such as 10%, 50%, and 100% of rated primary current, and record the secondary current. Measure the secondary winding resistance, burden voltage, and excitation curve on each connected tap. Verify polarity with a DC kick test or an AC polarity test. Inspect the secondary wiring for shorts, multiple grounds, and incorrect tap connections. If the excitation curve remains below the manufacturer's minimum and the ratio error is confirmed, plan to replace the CT rather than compensate with relay settings.
3. Outline the safe test plan for ratio, polarity, and burden, including how to avoid open-circuiting the CT.
Read the worked answer
De-energize and lock out the feeder, then verify absence of voltage. Because a CT secondary must never be open-circuited while primary current can flow, use a shorting terminal block or shorting switch to short the CT secondary before changing any test connections. Confirm the short is applied with a clamp meter or by measuring near-zero voltage across the shorted secondary.
For ratio testing, inject primary current with a primary-injection set and measure secondary current at the relay terminals; compare the ratio on every tap. For polarity, use a DC kick test with a battery and analog meter, or an AC polarity test, and confirm the polarity mark is consistent with the relay settings. Measure burden by injecting secondary current and measuring the voltage drop across the connected burden; calculate burden in ohms and volt-amperes. After testing, remove the short only after the primary circuit is de-energized and the test set is disconnected. Restore the secondary circuit, verify the relay readings, and document all results.