The components and principles of the protection device of Qu Yanfang (Suan'an Group Sima Coal Co., Ltd., Changzhi 047105, Shanxi, China) were briefly analyzed, and the development of the protection of the electrical equipment and power supply system in the coal mine was prospected.
Coal mines have poor working conditions, large fluctuations in load, unstable working conditions, and gas accumulation, dripping, and other phenomena will gradually reduce the insulation strength, and often cause faults such as electrical leakage. Therefore, in the safety production of coal mines, the protection of electric equipment and power supply systems in coal mines plays a very important role. At present, relay protection devices are mostly used for the protection of electrical equipment and power supply systems in coal mines. With the continuous development of computer control technology, microelectronics technology, information technology, and network communication technologies, intelligent protection systems have been successfully developed, making the entire system reliable. Has greatly improved, thus ensuring the quality of production.
1 Underground types of protection The types of underground electrical equipment are classified into general electrical equipment for mining and flameproof electrical equipment for mines. The former does not have explosion-proof performance and the latter has explosion-proof and explosion-proof properties. At the same time, according to the working voltage level, electrical equipment can be divided into low-voltage electrical equipment and high-voltage electrical equipment, most of downhole electrical equipment is a type of load and second-class load, work current, voltage are larger, its protection is to ensure the work The key to reliability. Overcurrent protection, leakage protection and grounding protection are the three major protections in downhole, and other protections include loss of pressure protection, undervoltage protection, overvoltage protection, etc. 2. 1.1 Leakage protection In addition to underground leakage, equipment will be further damaged, resulting in a short circuit accident. Can also lead to personal shock and leakage sparks detonating gas, coal dust. Stray currents can also corrode nearby metal pipes and armored cable metal sheaths, shortening the life of metal pipes and armored cables. It may also cause an early explosion of electric detonators and threaten the safety of personnel. Therefore, leakage protection devices must be installed in the underground power supply system to achieve insulation monitoring, leakage protection, and compensation for capacitive current flowing through the body. According to its realization of protection functions are divided into non-selective leakage protection and selective leakage protection.
1.1.1 Selective leakage protection The principle of zero-sequence current protection is used. The zero-sequence current signal is obtained by a zero-sequence current transformer. When no leakage occurs, the three-phase current on the primary side is symmetrical, the current phasor sum is 0, and the secondary side has no current output; when the leakage occurs, the primary side three-phase current is asymmetric and the current phasor is not 0. Secondary current output. The use of the device in combination with a shunt switch has the advantages of reducing the power outage range and facilitating the finding of faulty lines and is therefore widely used.
1.1.2 Non-selective leakage protection The protection principle of the additional DC power supply is used. The auxiliary power supply is also called the auxiliary power supply, and the auxiliary power supply (auxiliary) DC power supply is included in the detection circuit that includes the insulation resistance against the ground. The change of the DC current is monitored. If there is leakage after the leakage, the voltage terminal discharges to the ground, and there will be voltage between the ground and the voltage terminal. Voltage drop and current exist, if the voltage drop direction, current direction, and unbalanced voltage signal detected by the three-phase reactors are in the same direction, then the leakage tripping is judged, so as to achieve the purpose of monitoring the insulation resistance. The device must be used with a low-voltage automatic feed master switch. The disadvantage is that the scope of power outage is large and it is not easy to judge the leakage circuit, but the structure is simple and the work is reliable, so it is still in use.
1.2 Over current protection The main cause of electrical fires is the over-current of the power grid, which is caused by short-circuit, overload and phase failure. Therefore, the method of preventing electrical fire is to prevent over-current. So over-current protection includes short circuit protection, overload protection and phase failure protection.
1.2.1 Overload Protection Overload means that the operating current of the motor or the operating current of the electrical equipment is greater than its rated current but less than the multiple of the rated current, usually within 1.5 times of the rated current. There are many causes for motor or electrical equipment overload, such as a sudden increase in load, phase-failure operation, and grid voltage drop. The operating time of the overload protection is related to the size of the overload current, and its action value setting is less than the action value of the short-circuit protection.
The operation delay depends on the degree of overload. The greater the degree of overload, the shorter the delay; the smaller the degree of overload, the longer the delay, this feature is called inverse time characteristics. The delay link is composed of time relays. When the overload occurs, the current relay acts. The contact relay turns on the time relay coil. After the time delay, the time relay contact operates to cause the actuator to act, cut off the main circuit power supply, and send an overload signal at the same time. Overload protection can be achieved by electromagnetic relays, electronic relays and thermal relays.
1.2.2 Short-Circuit Protection The short-circuit current generated by the short circuit can reach ten to several dozen times of the rated current, causing electrical damage to electrical equipment or distribution lines due to over-current, and even causing fire due to arcing. The operation time of the short-circuit protection is shorter, and the action value is set larger, and the power supply is cut off in a short time. Both electromagnetic relays and electronic relays can be short-circuit protected.
1.3 Phase-failure protection During the operation of the motor, the lack of a phase of the symmetrical three-phase power supply due to a power grid fault or a phase fuse blown, the motor will run at a low speed or stall after a phase loss, the stator current is very large, and the motor insulation is easily caused. Winding damage. Phase failure protection is a kind of protection method that prevents the application of power to the protected equipment by relying on the disappearance of the current in one phase conductor of the polyphase circuit to disconnect the protected equipment or rely on the loss of the phase or several phases of the polyphase system. . The phase failure protection adopts the principle of negative sequence protection, and the electronic relay and thermal relay can realize phase protection.
1.4 Ground Protection The ground protection can limit the current through the body to within the limit current. The key to protective grounding is to reduce the grounding resistance of the protective grounding device within the specified range. Coal mine safety regulations stipulate that the grounding resistance of any protective grounding point on the grounding grid must not exceed 2a. At the same time, the distance between the main grounding pole and the auxiliary grounding pole should not be less than 5m. 2 Relay protection relay protection device is protected by one or more The components and logic components are grouped together to respond to a specific protection function in the power system fault and act on the automatic device that the circuit breaker trips or signals. Fuses, relays, and contactors can all be used to protect the device. However, due to the continuous increase of normal operating current and short-circuit current in coal mine systems, fuses can no longer meet the requirements of selectivity and rapidity. Relays and contactors are currently used. More extensive protection devices. Its component mainly has 4: The scene signal input part. Field signals sent to the relay protection device must undergo the necessary pre-processing, such as the use of optical isolation technology to eliminate interference signals; level conversion circuit makes low signal strong signals easy to handle; low-pass filtering removes high-frequency signals and lines The wave voltage, etc., enable the relay to effectively detect the physical quantities at each site.
Measurement section. It is to measure the physical quantity related to the object to be protected after being processed by the field signal input circuit, and compare it with the given set value or the criterion that is automatically generated in real time, and gives “Yes†or “Non†according to the comparison result. That is, a set of logic signals or level signals of 0 or 1 nature, determined to determine whether protection is enabled.
The logical judgment section. It is based on the size of the measurement part of the output, the nature of the logic state, the output sequence and other information, according to a certain combination of logic, operation, and finally determine whether the circuit breaker should trip or send a signal, and the relevant command to the implementation part . The commonly used logic generally has "and", "or", "non-" delay", "memory" and other functions.
Execution section. It is based on the export signal sent by the logic judgment part to complete the final task of the protection device. It is mainly responsible for the isolation, connection, level conversion, power output of the exit trip, and the return of the status information of the field device. In order to make the relay protection device work reliably: The electrical equipment and electrical equipment will trip in the event of a fault, and signals will be sent out during abnormal operation. The ideal state will not be activated in normal operation.
3 Conclusion The protection of coal mine underground electrical equipment and power supply system is of great significance to the safe production of coal mines. With the development of the power system and the continuous improvement of the requirements for the safe operation of coal mines, in addition to the need for enhanced stability, reliability, and rapid responsiveness, data protection will become even more important in future protection devices. Intelligent, flexible, fast and accurate features of the protection and control system will enable the continuous improvement and improvement of the protection technology of electrical equipment and power supply systems.
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