Thread Content
This post was last edited by chenjinfeng on 2020-2-19 at 20:39. As a common fault in power systems, the ability to detect a PT disconnection promptly and effectively is a prerequisite for the proper operation of relay protection devices. Considering the characteristics of PT disconnection, after analyzing the criteria from different manufacturers and drawing on a practical field example, the shortcomings in the existing criteria were identified, and a practical criterion for PT disconnection was proposed. The line protection device developed based on this criterion has been put into use in the field, demonstrating its engineering practical value. Line protection PT open-circuit criterion: An open-circuit fault in the PT in a substation is a common type of failure. Once the PT wire breaks and voltage is lost, it causes a deviation in the voltage readings of the protection device; accurate voltage measurement is a prerequisite for the proper operation of distance protection, protection with direction locking, and overcurrent protection that includes low-voltage activation elements. In a neutral-ungrounded system, a single-phase ground fault presents the following characteristics: the voltage of the grounded phase with respect to ground becomes zero, while the voltages of the other two phases increase by a factor of sqrt(3) with respect to ground. Meanwhile, the load currents and line voltages among the three phases remain symmetric. Therefore, in the line protection devices of ungrounded neutral systems, the criterion for detecting PT disconnection should be able to distinguish between single-phase ground faults and asymmetric disconnections. The determination of PT three-phase voltage loss (symmetrical open circuit) is basically the same across different manufacturers, with the judgment being based on the absence of voltage in all three phases while current is present in the circuit. However, the case of asymmetric breakage in PT is different. After analyzing the characteristics of PT wire breaks, this paper specifically addresses the criteria for detecting asymmetric PT wire breaks from different manufacturers. By referring to actual on-site accidents, it points out the potential shortcomings of these criteria when applied in practice, and proposes a practical criterion for detecting PT wire breaks. After being applied in real scenarios, the correctness and engineering utility of this criterion have been verified. 1 Characteristics of PT device disconnection PT disconnection can generally be divided into disconnection on the primary side of the PT and disconnection on the secondary side; in either case, the voltage in the PT’s secondary circuit will become abnormal. When there is a break in the primary side of PT, it can be a complete break; in such a case, there is no voltage on the secondary side at all, and neither is there any voltage in the open delta connection ； The other type is asymmetric open circuit; in this case, there is no phase voltage on the secondary side of the corresponding phase, the secondary voltage of the phases that are not open circuited remains unchanged, while a voltage is present in the open delta connection. When the secondary side of the PT is open-circuited, there is no voltage in the open delta connection of the PT, and the voltage of the open-circuited phase is zero. 2 Several different criteria for asymmetrical PT open circuits Since the criteria for symmetrical three-phase PT open circuits are basically the same, this paper mainly analyzes the criteria for asymmetrical PT open circuits. Currently, domestic manufacturers have different criteria for determining PT asymmetric open circuits; the following three criteria are given as examples. Criterion 1: The negative sequence voltage is greater than 8 V. This criterion is used to determine PT asymmetric disconnection by taking advantage of the fact that negative-sequence voltage exists in the case of PT asymmetric disconnection, whereas it is zero in the case of single-phase ground fault. Criterion 2: The vector sum of the three-phase voltages is greater than 18 V, and the difference in the magnitudes of at least one line voltage is greater than 20 V. The vector sum of the three-phase voltages being greater than a specified value (18 V) is a key characteristic of an asymmetric open circuit; \"the difference in the magnitudes of at least one pair of line voltages being greater than 20 V\" is used to take into account the fact that, in a system with an ungrounded neutral point, the line voltages remain symmetric in the event of a single-phase ground fault, thereby allowing for the distinction between a single-phase ground fault and an asymmetric open circuit. Criterion 3: There exists a difference in the magnitudes of the line voltages greater than 18 V. Similar to Criterion Two, this criterion also uses the difference in the magnitudes of the line voltages as a criterion for detecting asymmetrical open circuits in PTs; this difference is used to distinguish between single-phase ground faults and asymmetrical open circuits. After analysis, and taking into account the characteristics of PT asymmetric breaks, it can be seen that all three different criteria for identifying PT asymmetric breaks are valid, and their performance in practical operation is quite good. It can operate correctly in the case of an asymmetric open circuit on the primary side of the PT; under normal circumstances, it can also operate correctly when there is an asymmetric open circuit on the secondary side of the PT. During the development of a microcomputer-based line protection device, the author referred to and utilized the PT asymmetry open-circuit criterion shown in Criterion 2. Since its commissioning, the device has never experienced any incidents involving false or failed issuance of alarm signals due to PT open circuits. However, in 1999, during the commissioning of this microcomputer-based line protection device at a substation in Shandong Province, it failed to issue an alarm signal when testing for two-phase disconnection on the PT secondary side.