PT (Potential Transformer) disconnection is a common fault in power systems that can significantly impact the accuracy of protection, measurement, and metering. PT disconnection can occur on either the primary or secondary side, leading to abnormal voltages in the secondary circuit, which affects the normal operation of power equipment.

Characteristics

1.Primary Side Disconnection:

Complete Disconnection: The secondary voltage is zero, and there is no voltage in the open delta.

Asymmetric Disconnection: One phase's secondary voltage is absent, while the others remain unchanged, with voltage still present in the open delta.

2.Secondary Side Disconnection:

The open delta has no voltage, and the disconnected phase voltage is zero.

Important Considerations

In ungrounded neutral systems, when a single phase is grounded, the other two phases' ground voltages increase, while the grounded phase's ground voltage becomes zero. In this case, the criteria for determining PT disconnection should distinguish between single-phase grounding faults and asymmetric disconnection.

Judgement Criteria

1.Three-phase Loss of Voltage Criteria: 

Loss of voltage across all three phases with current flow in the line indicates disconnection.

2.Asymmetric Disconnection Criteria:

Criterion One: Negative sequence voltage greater than 8V.

Criterion Two: The vector sum of the three-phase voltages exceeds 18V, with at least one line voltage differing by more than 20V.

Criterion Three: The difference in the magnitude of one line voltage exceeds 18V.

Handling PT Disconnection Faults

1.For voltage transformer circuit disconnections:

Withdraw related protections according to relay protection and automation device regulations to prevent misoperation.

Check whether high and low-voltage fuses and automatic switches are normal. If a fuse has blown, determine the cause and replace it. If it blows again, handle it with caution.

Inspect all joints in the voltage circuit for looseness or breaks, and check for poor contact in the switching circuit.

2.For current transformer secondary circuit disconnections:

Immediately report to the duty dispatcher and withdraw related protections according to relay protection and automation device regulations.

Identify the fault point and attempt to short-circuit the open circuit at nearby terminals, avoiding the use of fuses during short circuits. If the open circuit cannot be resolved, consider power shutdown.