PT stands for Potential Transformer (voltage transformer). PT disconnection, a relatively common fault in electrical power production, occurs when the voltage transformer loses connection. Once the PT is disconnected and loses voltage, it critically affects the accuracy and reliability of protection, metering, and measurement operations. This can be summarized in the following aspects:
1. Protection device voltage deviations:
A PT disconnection causes deviations in the voltage measurement used by protection devices. Accurate voltage measurements are a prerequisite for the proper functioning of distance protection, directional overcurrent protection, and overcurrent protection with low-voltage initiation elements.
2. Impact on metering devices' accuracy:
The metering device relies on accurate voltage data, a foundational electrical measurement parameter for power metering. Additionally, the voltage provides a power source for certain meters. A PT disconnection compromises the reliability and accuracy of metering data.
3. Accurate monitoring of voltage values:
PT disconnection causes reliable data loss for monitoring voltage levels, making voltage monitoring during operation unreliable.
Characteristics of PT Disconnection
PT disconnections can occur on either the primary or secondary side. Regardless of which side is disconnected, the result is abnormal voltage in the PT secondary circuit.
1. Primary-side PT disconnection:
If all phases are disconnected, there will be no secondary voltage, and the open delta winding will show no voltage.
If only one phase is disconnected asymmetrically, the affected phase will lose its secondary voltage while the other phases maintain their normal voltage. The open delta winding will show some voltage.
2. Secondary-side PT disconnection:
In this case, there will be no voltage in the open delta winding, and the phase-to-phase voltage of the disconnected phase will drop to zero.
Important Considerations for PT Disconnection
In ungrounded neutral systems, a single-phase ground fault exhibits the following characteristics: the voltage of the grounded phase drops to zero, while the voltage of the other two phases to ground increases by a factor of √3. However, the three phases' line currents and voltages remain symmetrical. Therefore, in protection systems for ungrounded neutral systems, the PT disconnection criterion should distinguish between a single-phase ground fault and an asymmetric PT disconnection.
Criteria for Detecting PT Disconnection
The judgment criteria for a three-phase PT voltage loss (symmetrical disconnection) are generally consistent among manufacturers, typically based on detecting no voltage in all three phases, but with the presence of current flow in the line.
For asymmetric PT disconnection, however, different manufacturers use different criteria. Below are three examples:
1. Negative-sequence voltage greater than 8V:
This criterion uses the presence of negative-sequence voltage during asymmetric PT disconnection, while the negative-sequence voltage is zero during a single-phase ground fault.
2. Sum of the three-phase voltage vectors greater than 18V, and at least one phase-to-phase voltage difference greater than 20V:
The sum of the three-phase voltage vectors exceeding a specified value (18V) is a key feature of asymmetric disconnection. The condition that at least one phase-to-phase voltage difference exceeds 20V helps distinguish asymmetric PT disconnection from a single-phase ground fault in an ungrounded system, where the line voltages remain symmetrical.
3. One phase-to-phase voltage difference greater than 18V:
Similar to the second criterion, this relies on the phase-to-phase voltage difference to detect asymmetric PT disconnection and differentiate it from a single-phase ground fault.
Handling PT Disconnection Faults
When dealing with a PT circuit disconnection, follow these three steps:
1. Disable relevant protections according to relay protection and automatic device regulations to prevent misoperations.
2. Check the high- and low-voltage fuses and automatic switches for proper operation. If a fuse has blown, identify the cause and replace it immediately. If the fuse blows again, proceed cautiously.
3. Inspect all voltage circuit connections for loose or broken wires, and check the switching circuits for poor contact.
For current transformer (CT) secondary circuit open circuits, follow these steps:
1. Immediately report the issue to the dispatcher and disable relevant protections as per relay protection regulations.
2. Identify the fault location, and, under safe conditions, attempt to short-circuit the open terminal nearby. When short-circuiting, fuses should not be used. If the open circuit cannot be resolved, consider a shutdown to address the issue.