【20150820】How are the protection values for stage 1, stage 2, and stage 3 of high-voltage electricity set?

1. Zero-sequence current primary protection: Its setting must meet the following requirements: (1) It must be able to handle the maximum zero-sequence current flowing through the line in the event of single-phase or two-phase ground faults at the end of the line (or at both busbars) under normal operating conditions as well as during normal maintenance operations. (2) Avoid the maximum zero-sequence current during full-phase operation within the single-phase reclosing cycle. If unlimited zero-sequence current protection is required during partial-phase operation of the line, and the maximum zero-sequence current under partial-phase operation is greater than that in the case of a ground fault at one end (or both ends), then two types of unlimited first-stage zero-sequence current protections must be installed: one that is sensitive enough to detect end faults, and another that is less sensitive in order to handle partial-phase operation conditions. Since the zero-sequence current that occurs during partial-phase operation cannot be avoided, one of the protection sections must be taken out of service during the single-phase reclosing cycle, so that only the less sensitive protection section continues to operate during partial-phase operation. It must be noted that during partial-phase operation, it is necessary to retain a time-independent first-stage protection capable of withstanding partial-phase zero-sequence currents. This protection serves as the foundational level for the step-by-step coordination of the backup protections in adjacent lines, and by remaining active during partial-phase operation, it ensures that the second-stage protections associated with those adjacent lines are properly coordinated, thereby preventing unsolicited, selective tripping. Under this premise, a sensitive section is set up to expand the protection range of the time-limitless protection section in the event of the first fault. Therefore, the zero-sequence current direction protection device used in conjunction with integrated reclosing must also have the capability to implement two sections. 2. Zero-sequence current two-stage protection 1) To improve sensitivity in the event of a fault at the end of the line and to reduce the setting time, the setting can be determined with consideration only for coordination with the less sensitive first-stage protection of adjacent lines. With such a configured two-stage current protection, if, during the single-phase reclosing cycle of this line, neither the current setting value nor the setting time can avoid the condition of partial-phase operation, then it must be taken out of service. 2) In some systems, so-called insensitive two-stage protection is installed on certain short circuits; its operating current value is set to accommodate the maximum zero-sequence current that occurs during partial-phase operation. However, since it cannot handle end-fault conditions, a time delay element is included. During single-phase reclosing on this line, the insensitive second stage remains in operation. Setting a less sensitive two-stage protection is mainly aimed at improving the setting coordination conditions between adjacent backup sections. In the absence of a non-sensitive second stage, the second stage of adjacent lines can only operate in conjunction with the non-sensitive first stage of the same line, whereas the third-stage protection of adjacent lines must work together with the third stage protection after reclosing of that same line. If a non-sensitive second stage is provided, the setting of the second or third stage for adjacent lines can be coordinated with it, thereby improving the protection performance of those adjacent lines. In some cases, the effect of this improvement is quite noticeable. Therefore, the zero-sequence current protection device must consider the possibility of implementing two stages. However, for the device, although it is required to be able to have two first sections as well as two second sections, a non-sensitive section 1 and a non-sensitive section 2 cannot both exist in the same protection device. 3. Zero-sequence current backup protection sections: The various sections of the zero-sequence current direction protection device are tuned in accordance with a step-by-step coordination principle; it is necessary to have four protection sections. Although some domestic power systems can meet the setting requirements using three-stage protection, for the protection installed on bypass circuit breakers, since these breakers are meant to replace multiple line protections, several additional stages are generally required for ease of operation. To this end, an additional current element and a time element are also provided within the protection device, allowing for further protection to be added when necessary.