What are the effects of reversing the direction of a current transformer (CT)?
May 27, 2026| I. Impact on electricity metering
Reversed or negative display of the electricity meter: Reversed connection reverses the phase of the secondary current by 180°, resulting in an incorrect direction of active power. The electricity meter may measure in reverse, causing abnormal electricity billing.
Significant metering error: In a three-phase four-wire system, if the polarity of one phase CT is reversed, the electricity meter reading will only be 1/3 of the correct value, meaning 2/3 of the electricity will be under-counted.
If two phase CTs are reversed, the electricity meter may reverse, resulting in a negative reading.
If all three phases are reversed, the electricity meter will continuously reverse, causing serious metering inaccuracies.
II. Impact on the relay protection system
Differential protection malfunction: Differential protection relies on the consistency of the current direction on both sides. If one CT is reversed, the differential current increases abnormally, which may be misjudged as an internal fault during normal operation, leading to circuit breaker tripping and power outage. Directional protection failure: For example, directional overcurrent and zero-sequence directional protection may fail to correctly determine the fault direction due to incorrect current phase, resulting in protection failure or false tripping.
High risk in renewable energy grid-connected systems: In photovoltaic and wind power scenarios, abnormal CT connections can cause frequent tripping and inability to close circuits, severely impacting grid connection and power transmission progress.
III. Impact on Measurement and Monitoring Systems
Abnormal instrument display: Ammeters and wattmeters display negative values, affecting operators' judgment of system status.
Power factor calculation error: Current phase reversal leads to abnormal power factor display, potentially misdiagnosed as a reactive power compensation problem.
Three-phase imbalance alarm: Reversed single-phase CT connections cause the current direction of that phase to be reversed, leading to a false three-phase imbalance alarm.
IV. Consequences under special wiring methods
Incomplete star connection (two-phase CT): If the polarity of one phase is reversed, the current in the unconnected middle phase will be √3 times higher than the other two phases, causing measurement distortion.
If both phases are reversed, although the secondary current is still balanced, it is 180° out of phase with the primary current, causing the electricity meter to reverse.
