Current protection (current protection) is a protection based on the principle that the fault current increases when a short-circuit fault occurs in the power system. The current protection consists of a relay of current, time, intermediate, signal, etc., and acts when the current through the current relay is greater than the set value. Current protection is the most basic protection method for power systems. It can be used as protection for phase-to-phase short-circuit faults between book stores and generators, transformers, motors and other power components. When an asymmetric grounding short circuit fault occurs in an effectively grounded power system, a zero sequence current occurs, which can constitute zero sequence current protection. Zero-sequence current protection is also a basic relay protection method for power systems, and can be used as a protection against grounding short-circuit faults of power components such as transmission lines and transformers. Current protection is used for the protection of the same short-circuit faults of various voltage grade transmission lines and power components. The current protection of the transmission line generally adopts a three-stage structure, that is, no delay current and quick current protection. In some cases, protection consists of only one or two of them. For example, in a terminal line of a single-sided power supply, usually only a delay-free current quick-break and over-current protection are required; and in an ultra-high-voltage power grid, although the main protection configuration is advanced protection such as high-frequency protection, a quick cut is usually installed. Non-delay current quick-break protection for near-end short-circuit faults to improve the system's stable operation. The current protection can form a directional current protection together with the direction discriminating component to ensure the selectivity of the double-end power supply or the ring network current protection; and can also cooperate with the voltage relay to form a current-voltage interlocking quick-break protection to meet the change of the power system operation mode. Larger needs.
Transformers and generators typically use overcurrent protection as a backup protection for their phase-to-phase short-circuit faults. In order to improve the sensitivity, the low-voltage relay can be used to block the over-current protection of the low-voltage starting; or the low-voltage relay and the negative-sequence over-voltage relay can be combined to block the over-current protection of the composite voltage starting.
The no-delay current quick-break protection only operates with the inherent time of the current relay and the outlet relay. The current of the current relay is set by the maximum current of protection when the short-circuit is shorter than the end time of the protected line, ensuring short-circuit fault at the head of the next adjacent component. Things don't move. Therefore, the no-delay current quick-break protection acts quickly, but it cannot protect the full length of the line away from the top. The sensitivity of the no-delay current quick-break protection is expressed as a percentage of the length of the protection range fish line, and its value is always less than one. The smaller the source impedance ratio (the ratio of the source impedance to the fish line impedance), the longer its protection range.
The delay current quick-break protection relies on the current relay action current, the time relay action delay and the next adjacent component protection to meet the selectivity requirements. For example, when the next adjacent line of the fish has no delay current and quick-break protection, the current of the current relay is greater than the maximum current protected by the line when the short-circuit of the protection range at the end of the protection range of the next adjacent component without the delay current is applied. The setting is to ensure that the protection range of the delay current quick-break protection does not exceed the protection range of the next adjacent line without delay current quick-break protection; and the time relay is adjusted by adding a delay of Δt=0.5s. The sensitivity of the time-delay current quick-break protection is expressed by the ratio of the minimum current current regulator current setting current through the protection of the opposite end of the protected line. After the protection is set, the sensitivity check must be performed to ensure that the requirements of the procedure are met. The general procedure requires that the value be greater than 1.2. Therefore, the sensitivity-qualified delay current quick-break protection can protect the full length of the line than the petite delay.
Overcurrent protection has both timed over current protection and inverse time over current protection.
The time-limited overcurrent protection relies on the time relay delay to meet the selectivity requirements. The operating current of the current relay is set according to the maximum load current of the line; the time relay delay tr is matched with the next adjacent component overcurrent protection delay ts, usually tr=ts+Δt, Δt=0.5s, That is, the overcurrent protection delay of the next adjacent component is greater than Δt=0.5s. The time-limited overcurrent protection is usually used as backup protection. The near-standby sensitivity 表示 is the ratio of the minimum current that is protected by the protection to the current of the current relay when the phase-to-phase short circuit occurs at the end of the line. After the protection is set, the sensitivity test must be carried out to meet the requirements of the regulations. The general regulations require that the value be greater than 1.5. The definite time overcurrent protection can be used as the near backup of the line, and can also be used as the far backup protection of the next adjacent component. Therefore, the time-limited overcurrent protection generally has higher sensitivity, but the action delay is longer.
Inverse time overcurrent protection is generally constructed using an inverse time current relay. The anti-time limit current relay action time automatically changes with the decrease of the passing current, that is, the larger the current, the shorter the action time, the smaller the current, and the longer the overcurrent protection of the action time. The closer the fault point is to the protection installation, the larger the fault current through protection, and the shorter the anti-time limit overcurrent protection action time. Therefore, proper selection of the inverse time characteristic can obtain better protection performance than the time-limited overcurrent protection.
Zero-sequence current protection is used as protection for grounding short-circuit faults of power components such as transmission lines and transformers. The principle structure of fish current protection is basically the same. The zero-sequence current protection of the transmission line generally adopts a three-stage structure, with no delay, zero-sequence current quick-break, delayed zero-sequence current-break, and zero-sequence overcurrent protection. Zero-sequence current protection can also be combined with zero-sequence power directional components to form zero-sequence power direction current protection. Since the zero sequence component does not appear in theory when the system is in normal operation, the zero sequence current protection has higher sensitivity; the zero sequence current protection does not reflect the system oscillation and overload, the wiring is simple and reliable, and the speed protection range is relatively stable. The current protection delay is small. Many of the above advantages make zero-sequence current protection widely available in an efficient power grid. Effectively, transformers in power systems are also required to be equipped with zero-sequence overcurrent protection as a backup protection for transformer ground short-circuit fault backup protection and adjacent component ground short-circuit faults.
The overload protection band has a delay applied to the signal. Generally, generators, transformers and feeder lines are continuously installed with overload protection. Generators that are not directly cooled by the stator windings shall be equipped with a set time limit overload protection; for generators with 50 MW and above It ≥ 10, a set time limit negative sequence overload protection shall be installed. For transformer overload protection, the installation location of the selected protection should reflect the over-conformity of all windings of the transformer. For example, for a double-winding step-down transformer, the overload protection should be installed on the high-voltage side; for the three-winding step-down transformer or the tie transformer of the power supply on both sides, overload protection should be installed on all three sides.
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