A, overcurrent protection --- is in accordance with the maximum load current that may arise in the protected equipment or line to avoid the setting. Such as large motor starting current (short time) and traversing short-circuit current and other non-fault current to ensure the normal operation of equipment and lines. In order to make the upper and lower levels of overcurrent protection can obtain selectivity, in the time limit has a corresponding level difference.
B, current breakage protection --- is in accordance with the protected equipment or the end of the line may appear the maximum short-circuit current or transformer secondary side of the three-phase short-circuit current and the set. Quick-break protection action, theoretically, current flow protection has no time limit. That is, zero seconds and the following time limit action to cut off the circuit breaker.
Over-current protection and current-flow protection is often used in conjunction as the main protection of the equipment or line and adjacent line backup protection.
C, fixed-time overcurrent protection --- in normal operation, the protected line flows over the maximum load current, the current relay should not be acted on, and when a fault occurs on the line at this level, the current relay should be reliably acted on; fixed-time overcurrent protection consists of a current relay, a time relay and a signal relay three components (the current relay on the secondary side of the current transformer to measure the size of the current→ the time relay to set the time of action→ the signal relay to set the time of action→ the signal relay to set the time of action→ the current relay to measure the size of the current. The time relay sets the action time → the signal relay sends out the action signal); the action time of fixed-time overcurrent protection has nothing to do with the size of the short-circuit current, and the action time is constant. (Human setting)
D, inverse time overcurrent protection --- the action time of relay protection is inversely proportional to the size of the short-circuit current, i.e., the larger the short-circuit current, the shorter the action time of the relay protection, the smaller the short-circuit current, the longer the action time of the relay protection. In 10KV system commonly used induction type overcurrent relay. (GL-type)
E, no time limit current break --- can not protect the full length of the line, it can only protect a part of the line, the system changes in the mode of operation, will affect the current break the scope of protection, in order to ensure that the action of the selectivity of the starting current must be according to the maximum mode of operation (that is, the current through the line for the maximum mode of operation) to set, but This shortens the protection range for other modes of operation, and the regulations require that the minimum protection range should not be less than 15% of the total length of the line. In addition, the length of the protected line also affects the characteristics of the fast-break protection, when the line is longer, the protection range is larger, and less affected by the operation of the system, and vice versa, when the line is shorter, the impact is greater, and the protection range will even be shortened to zero.
②, voltage protection: (according to the system voltage abnormality or fault when the change in the action of relay protection)
A, over-voltage protection --- to prevent the voltage rise may lead to damage to electrical equipment and installed. (Lightning, high potential intrusion, accidental overvoltage, operational overvoltage, etc.) 10KV open and close the end of the transformer high-voltage side of the lightning arrester is mainly used to protect the switchgear, transformer; transformer low-voltage side of the lightning arrester is used to prevent the lightning wave intrusion by the low-voltage side of the insulation of the transformer and the set.
B, undervoltage protection --- to prevent a sudden drop in voltage resulting in damage to the normal operation of electrical equipment and set up.
C, zero sequence voltage protection --- to prevent a phase of the transformer insulation damage caused by single-phase ground fault relay protection. Mainly used in three-phase three-wire system neutral point insulation (not grounded) in the power system. Zero sequence current transformer primary side of the line to be protected (such as cable three phase), the core set in the cable, the secondary winding connected to the current relay; cable phase line must be insulated to ground, the cable head of the grounding wire must also be passed through the zero sequence current transformer; Principle: Normal operation and short-circuiting between the phases, the zero-sequence current on the primary side of the zero (phase and), the secondary side of the very small imbalance within the current. When a single-phase grounding occurs in the line, the grounding zero sequence current is reflected in the secondary side, and flows into the current relay, when it reaches or exceeds the set value, action and signal. (Transformer zero sequence current transformer series connected to the zero line terminal out of the copper row)
③, gas protection: oil-immersed transformer internal fault, short-circuit current produced by the arc so that the transformer oil and other insulating material decomposition, and produces gas (gas), the use of gas pressure or impulse to make the gas relay action. The nature of the fault can be divided into light gas and heavy gas, when the fault is serious (heavy gas) gas relay contact action, so that the circuit breaker trips and sends an alarm signal. Light gas action signal is generally only signal alarm and not issued trip action.
Transformer initial input, long-distance transportation, refueling, oil change and other reasons, the oil may be mixed into the gas, accumulated in the upper part of the gas relay (glass window can be seen in the oil level drop, indicating that there is a gas), encountered such a situation can be used on top of the gas relay bleeder valve (screws unscrewed) bleed, until the gas relay is full of oil. Considering the safety, it is better to bleed the gas when the transformer is out of power. Capacity in 800KVA and above transformer should be equipped with gas protection.
4 differential protection: this is a kind of power system, according to the protected equipment short-circuit fault, in the protection of the differential current and action of a protective device. Commonly used to do the main transformer, generator and shunt capacitor protection device, according to its device can be divided into:
A, crosslinking differential protection --- commonly used for generator short-circuit protection and shunt capacitor protection, the general equipment for each phase of the double winding or double busbar, the use of this differential protection.
B, longitudinal differential protection --- generally used as the main transformer protection, is specialized in the protection of transformer internal and external faults of the main protection .
5 high frequency protection: this is a kind of high reliability relay protection device as the main system, high voltage long line. At present, China has built a number of 500KV ultra-high voltage transmission lines require the use of this feasibility, selectivity, sensitivity and rapid action of the protective device. High-frequency protection is divided into phase difference high-frequency protection; direction high-frequency protection.
Phase difference high-frequency protection of the basic principle is to compare the phase of the two ends of the current protection. Provides that the direction of current flow from the bus to the line is positive, from the line to the bus is negative. That is, when the line internal fault, both sides of the current with the phase and external fault, both sides of the current phase difference of 180 degrees. Direction of high-frequency protection of the basic working principle is to compare the direction of the power at both ends of the line being protected, to identify the internal or external faults in the transmission line of a protective device.
6 distance protection: this relay protection is also the main system of high reliability, high sensitivity relay protection, also known as impedance protection, this protection is in accordance with the long line fault point of different impedance values and the rectification.
7 balance protection: this is a high-voltage shunt capacitor protection device. Relay protection has a higher sensitivity, for the use of double star-shaped wiring of shunt capacitor bank, the use of this protection is more appropriate. It is according to the shunt capacitor fault generated by the unbalance current and action of a protective device.
8 negative sequence and zero sequence protection: this is as a three-phase power system in the occurrence of asymmetric short-circuit faults and ground faults in the main protective device.
9 direction protection: this is a directional relay protection. For the ring grid or double back line power supply system, a part of the line failure, and the direction of the fault current in line with the direction of the current relay protection set, the protection device reliably act to remove the fault point.
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