Jiangsu Hai'an Silk Printing and Dyeing Factory (226611) Ma Deyu 1 circuit breaker protection in low-voltage power distribution system 1.1 Protective circuit breaker between circuit breaker and low-voltage network breaker is simple in structure, cheap in price and strong in breaking capacity. It is placed on the power supply side of the circuit breaker, so that its characteristics match properly, and better protection can be obtained. For example, the RT-600 fuse can break the short-circuit current of 50kA. When it is matched with the DWs-400 type circuit breaker with a breaking capacity of only 20kA, it can be used in the circuit with short-circuit current up to 50kA. Special attention should be paid to the cooperation between the protective properties, and the fitting test should be carried out when conditions permit. Fuse for backup protection is ideal for current limiting fuses such as the NT series. It has a fast action when breaking the fault current, and can bear most of the arc energy, so that the circuit breaker on the load side is subjected to minimal damage. At the same time, the requirements for dynamic and thermal stability of conductive circuits such as bus bars are also reduced.
1.2 The circuit breaker and the wire and cable cooperate to make the circuit breaker reliably operate when the circuit is short-circuited or overloaded, and ensure that the current-carrying conductor (mainly cable or insulated wire) connected to it is not damaged by short-term overheating, usually It is required that the setting current value of the circuit breaker overcurrent release should be well matched with the allowable overload current of the conductor. Expressed as: When the short-circuit current is less than the "bounce current" of the contactor, the voltage loss release time of the series AC contactor is longer than the total disconnection time of the circuit breaker. When a short circuit occurs, the AC contactor is disconnected from the rear of the circuit breaker, so that selective action can be achieved. But because of C,. The series contactors have a short-time release time, which may break the short-circuit current before the circuit breaker has been disconnected, and thus non-selective action will occur. This situation should be noted in the use. 1.4 The comprehensive cooperation between the circuit breaker and its protective device in terms of motion selectivity is now and exemplified. The most common way to cooperate is circuit breakers, fuses, AC contactors, and thermal relays. It can be seen that when the motor is overloaded and the terminal of the terminal is short-circuited. 1.3 The circuit breaker and the AC contactor cooperate in the load branch. At this time, the contactor (or magnetic starter) is used as the operating device of the circuit, and the thermal relay is Used as overload protection and circuit breakers for short circuit protection. In addition, when the short-circuit current value is less than IkA, the magnetic starters FR, KM should be disconnected, the fault is removed, and the rest of the network is still powered normally. If the short-circuit current at point S1 is greater than IkA and the point is also short-circuited, the fault should be removed by the circuit breaker 2QF to protect the contactor of the starter and the bimetal of the thermal relay FR so that it is not over-current burn. However, at this time, in order to ensure the normal operation of the network, the fuse FU should not operate. When s, the point is short-circuited, the fuse FU should disconnect the fault circuit, so that the entire system is still unaffected. If the circuit breaker 1<3F has no short-delay overcurrent relay, when the short-circuit current is greater than 4kA, the circuit breaker may malfunction, and Table 1 allows the short-time overload current multiple circuit breaker release to set the current network laying conditions and Wire and cable types allow short-time overload current multiples 々 Network operation characteristics Electromagnetic tripper Instantaneous current setting value Any 4.5 No overload protection rubber and plastic insulated wires are required to be laid in places with explosion and flammability. 0.8 requires overload protection as above, but no explosion and ignition hazard. 1.0 Same as paper insulated cable, laid in places with explosion and flammable danger. 1.0 Same as the rated current of the trip unit with non-circumferential inverse time-limited current characteristics (regardless of whether it is fast-breaking action) Any 1.0 rated current of the trip unit with adjustable inverse time-limited current characteristics (not limited by the multiple of its quick-acting action current) Any 1.5 does not require overload protection rubber and plastic insulated wires and cables 1.0 requires overload protection paper insulated cable 0.8 The same electrical and electrical equipment causes the transformer-level low-voltage network to be completely out of power. However, the circuit breaker 1QF has a delay time of 0.4s, so the short-circuit current above 4kA is cut off by the fuse. Only when the short circuit occurs at the point, and the short-circuit current is greater than 2kA, the circuit breaker 1QF will instantaneously trip. . This ensures that the low voltage network can selectively operate in the event of a fault. 2 Circuit breaker protection characteristics and selection principle 2.1 Selection of power distribution circuit breakers When selecting the power distribution circuit breakers, in addition to considering the general selection principles, this paper mainly introduces the selection principle of system selective breaking and characteristic matching. The long delay action current setting value of the circuit breaker is not greater than the allowable current carrying capacity of the wire. In the case of wire and cable, the wire cable is allowed to be 0.8 times the current carrying capacity. 3 The long-delay action current setting value can be returned no less than the maximum starting current of the motor in the line. The line calculates the load current; k - the starting current multiple of the motor; /dm - the rated current of the largest motor. The time ladder of short delay depends on the segmentation of the power distribution system. The general time ladder is mostly 2~3. The short delay time difference between each stage is 0. 〗 0.2s, depending on the accuracy of the short delay mechanism of the circuit breaker. After selecting the short delay step, it is best to check the thermal stability of the object to be protected. 2.2 Motor protection circuit breaker selection Motor protection circuit breaker can be divided into two categories: (1) the circuit breaker is only used for overload protection without normal operation; (2) the circuit breaker needs to double protection for frequent operation. In the latter case, the operating conditions and the life of the circuit breaker must be considered. The selection principle of the circuit breaker for motor protection is that the long delay current setting value is equal to the rated current of the motor. Instantaneously set the current. The instantaneous setting current of the circuit breaker for protecting the cage motor is equal to (8~15) times the rated current of the motor: the instantaneous setting current of the circuit breaker for protecting the winding type motor is equal to (3~6) times the rated current of the motor. The return time of the 6 times long delay current setting value is not less than the actual starting time of the motor. According to the light weight of the load at the start, you can select one of the 1, 3, 5, 8, and 15s when you return. 2.3 The long delay setting value of the single-pole circuit breaker for industrial and residential building lighting and living is not greater than the line calculation load current. The instantaneous action value is equal to (6~20) times the line calculates the load current. 3 The matching requirements between the circuit breaker and the protection characteristics of the upper and lower electric appliances When considering the cooperation between the circuit breaker and the protection characteristics of the upper and lower electric appliances, it is better to draw the protection characteristics of each electric appliance on the coordinate paper to compare the matching of the characteristics. The following principles must be considered: the long delay characteristics of the circuit breaker are lower than the allowable overload characteristics of the protected object (such as wires, cables, motors, transformers, etc.). The difference between the circuit breaker short delay release for the main switch and the high voltage side overcurrent protection relay is. 4~0.7s, depending on the type of different protection relays on the high voltage side. The main switch breaker overcurrent release protection characteristics are lower than the melting characteristics of the high side fuse. When the circuit breaker is matched with the fuse, the general fuse is used as backup protection. The switching current / (current value corresponding to the intersection of the operating characteristics of the two overcurrent protection devices) should be selected to be less than the rated short-circuit breaking current of the circuit breaker, and when the short-circuit current is greater than ', the fuse should be operated. When the circuit breaker is used in series with a backup device (fuse or another circuit breaker) that is used for short-circuit protection when it exceeds its breaking capacity, the following two cases must be distinguished: handover current / B, and when this value is exceeded The maximum overcurrent value that the backup device must operate; the selective current limit / s, below which the maximum overcurrent value of the circuit breaker alone. That is, below this value, the requirement for selective breaking can be satisfied. The protection characteristics of the upper circuit breaker and the protection characteristics of the lower circuit breaker cannot be crossed. In the cascade protection mode, it can be crossed, but the short-circuit current at the intersection should be 80S of the on-off current of the lower-level circuit breaker. The short-delay setting current of the upper-level circuit breaker is not less than 1.2 times the short delay or instantaneous of the lower-level circuit breaker (see below) The stage has no short delay) setting current. If the circuit breaker with short circuit delay has an undervoltage release, the undervoltage release must be delayed, and the delay time is not small. (Series: Gray Zhongliang) pays for the area, and its energy-saving effect is also the best. 3 Practical application results and precautions The application of this technology to the cage asynchronous motor with power of 45~130kW has been successfully tested. Under the same load and the same peripheral conditions, the modified motor can achieve the following effects: - the active power including the distribution line is reduced by more than 6; all the transformation costs including the addition of capacitors can be used in long-term operation. Recycling within 3~4 months. Since the present technology has a special process in which the capacitance energy extends into the winding of the motor, the following points must be noted in practical applications: the phase sequence direction of the power source and the arrangement direction of the winding must be consistent in the geometric position of the stator; When large, the compensation capacitors can be divided into multiple groups, and the automatic control loop can be used to adjust randomly; with its hexagonal wiring characteristics, the starting equipment such as the autotransformer can be cancelled; if the compensation capacitors are all disconnected for any reason, the operation is in operation. The motor load factor must not exceed 75% of the nameplate value to ensure that the equivalent secondary winding is not burned. (Editor: Going to Ping Ping) (Finish)
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