Selection of Low-Voltage Circuit Breakers and Selection of Low-Voltage Circuit Breakers The selection of low-voltage circuit breakers is based on the application, ie the protection requirements of the protected power-receiving equipment. In the circuit for power distribution of the motor, because of the thermal relay for load protection, a circuit breaker with only an instantaneous overcurrent release is generally selected. However, for small motor circuits and long circuits, the protection sensitivity of the minimum short-circuit current at the end is not satisfactory. In design selection, this problem is often overlooked because these loops are not important. Therefore, in the above circuit, the backup protection of both the instantaneous overcurrent release and the long delay current release should be selected. The action sensitivity of the release should be satisfied: the operating time is not greater than the cable current carrying capacity. At the same time, the following conditions are met, the unit is A; /zdl - long delay overcurrent release setting current, the unit is A; /re - thermal relay setting current, the unit is A; I, - cable current carrying capacity The unit is A. The selective overcurrent release of the selective low-voltage circuit breaker cannot protect the full length of the line, sometimes even small or no protection range (for example, when the upper and lower switches are close together), there is no instantaneous overcurrent release. When the protection range is small, the three-stage low-voltage circuit breaker should not be selected, but the two-stage low-voltage circuit breaker should be used. However, the instantaneous overcurrent release of the non-selective low-voltage circuit breaker cannot be selective and the timing should be: flow, the unit is A; â–¡ Huang Jianhua / 'zd, - the maximum setting current of the next-stage overcurrent release The unit is A. Because this still does not achieve selectivity, it will also reduce the scope of protection due to the increase. Therefore, the setting current of the instantaneous overcurrent release of the non-selective low-voltage circuit breaker should be as small as possible. Whether it is a selective or non-selective low-voltage circuit breaker, the long-delay overcurrent release should be guaranteed to be selective at the timing, that is, the maximum setting current of the following conditional device is satisfied, A; t-long delay overcurrent Buckle action time, s; t' - the maximum operating time of the next stage long delay overcurrent release, s. The application loop coordination of the low voltage circuit breaker.
Since the normal operation of the unit coordinated control system is the premise of the AGC mode put into operation, we have done a lot of work around the coordinated control of the unit and the stable operation of the AGC.
Because the coordinated control system is actually a feedforward-feedback control system, the purpose of feedforward control is mainly to compensate the dynamic delay and inertia of the boiler. In order to better utilize the feedforward control and eliminate the instability of the air volume control, we are sending In the wind control, the total powder supply signal is removed, and the actual load of the unit is used as the air volume setting value, which improves the control quality of the coordinated control system.
In order to eliminate the excessive disturbance and damage of the coordinated control system due to the instability of the wind signal, we replaced the secondary fan wing measuring device with unstable measurement signal and changed it to a reliable double venturi measuring device to improve the stability of the air volume control.
In order to ensure the automatic stability of the fan, we increase the protection of the blower air volume p and S value, and increase the blower blade command, feedback command, the draft fan vane command and feedback deviation protection, and improve the automatic reliability of the blower fan.
Replace the output relay that does not meet the requirements of the original WDPF system, ensuring that the WDPF to DEH load increase and decrease relay output signal is accurate and reliable, effectively avoiding the disturbance of the unit running in the AGC mode.
According to the actual operation of the unit AGC, we gradually adjust the DEC system response AGC rate, and repeatedly test the data that matches the operating conditions, and copy this value on the DEH system chip. Ensure the stable operation of the unit AGC.
The important transmitters and temperature measuring components of the AGC system are strictly verified, and the three-level inspection quality inspection points are set up, and the verification report and the abnormality record are established to ensure that the AGC system does not have a problem in one measurement.
In order to make the DEH system A and B mainframes operate reliably, the main board is not affected by the heat dissipation of itself or other card parts, and the ambient temperature is too high. The DEH system A and B main boards are equipped with cooling fan boards.
Modify the DEH sequence valve switching and valve test procedures to make the AGC freely and stable.
Regularly disturb and characterize the automatic adjustment system of the unit, continuously sum up experience, accumulate analytical data, comprehensively optimize and perfect the optimal parameters of automatic control of the unit, and all AGCs are stable and reliable.
Modifying the original inconvenient unit operation control mode, there are only five control modes of BASE, TF, BF, CCS and AGC. They can be switched without disturbance under certain conditions, which greatly facilitates the monitoring and operation of the operating personnel.
The AGC of our factory's 6th and 7th units has achieved good results after being commissioned and commissioned. In the AGC mode, the maximum deviation of the main steam pressure is less than 0.25 MPa, the maximum deviation of the main steam temperature is less than 4. The maximum deviation of the drum water level is less than 20 mm, and the control parameters of the unit are within the allowable range. The control system is operating normally. The unit load smoothly follows the mid-range command, and the AGC mode is accurate and reliable. (Received 2001 30 隹 gas era 35kV cable terminal insulation fault analysis □ Fujian Nanjing Shipyard Xishui Power Plant Cheng Fu Jian Wei Qi caused by large humidity) and continue to operate. When checking gas, such as 3, NO, etc., the exact cause of the oxidation of the insulating material should be analyzed and effective measures should be taken to record the viable record. It is known that the line is connected and the impact of charged particles before the incident, so that the air gap is put into operation. For the long-term grounding signal that appears in operation, it cannot be reset. The three relative ground voltages are gradually deteriorated, and the electrical performance is degraded. (Received 2001 On August 13, 2000, the 35kV South Star Line (non-grounded system) outdoor cable terminal suddenly ignited and ignited, and the operating personnel immediately disconnected the power supply to extinguish the fire. The on-site inspection found that the B phase was under the third rain skirt. There is a mung bean-sized breakdown point with burning marks around it. After the phase-insulated heat-shrinkable tube is cut, it is found that there is a large area of ​​tree-like discharge trace on the cross-linked polyethylene insulation surface near the end of the cable semiconductor layer. It is a hole-like carbonization breakdown point, and it has been faintly visible that the cable core line and a penetrating discharge trace from the grounding steel sill to the breakdown point along the semiconductor layer constitute a complete discharge channel.
In April 2000, the heat-shrinkable cable terminal was made, and no abnormality was found in the handover test. It was put into operation on May 9, 2000. After three months of operation, the cable was pre-tested on August 7, 2000. The weather was bright on the day, but there was a rain on the day before the test. The relative humidity of the site was large, and the leakage current value measured when DC78kV was applied was 42; (although it was significantly higher than the 29f/A at the time of handover but did not attract attention (test The personnel believe that the low-voltage circuit breaker can “improve†its breaking capacity through the cascade technology. Due to the limit characteristic of the upper and lower stages, the breaking capacity of the circuit breaker of the lower-level protection power receiving device is allowed to be lower than the expected short-circuit current at that point. The current is limited by the current limiting circuit breaker, so the actual breaking capacity of the lower circuit breaker of the current limiting circuit breaker is relatively "enhanced"
For two adjacent upper and lower protection devices, their actual breaking capacity is not limited by their rated breaking capacity. When the level is connected, the upper and lower circuit breakers can be installed in different switch rejections. Therefore, when the breaking capacity of the circuit breaker is less than the expected short-circuit current at this point, cascade technology can be used. Of course, the upper level swings, the PT triangle opening voltage is 60V, and the duration is as long as 26h. Since the voltage value to the ground is only slightly increased (after special analysis is 1/2 frequency resonance), it did not cause enough attention of the operators at that time. With the results of on-site anatomy and related information, the author believes that the main causes of the cable end failure are as follows: When making the cable terminal, the cable stripping does not meet the technical requirements. On-site anatomy shows that the semiconductor layer is up to 1 cm (standard 2 cm), and a strong distortion electric field is generated at the end of the severely long semiconductor layer. Therefore, the cable termination head initially leaves the problem of uneven electric field due to process variation.
Inappropriate baking of the cable terminal head causes uneven shrinkage of the insulated heat-shrinkable tube and stress tube. After the summer sun and rain (stimulation and rapid cooling), thermal expansion and contraction, the dielectric layer degumming and the number of air gaps increase. Defects such as volume expansion are gradually deteriorating. Due to the large difference in dielectric coefficient, the electric field is obviously distorted. The field strength in the air gap is much higher than the field strength in the adjacent solid medium, and the electrical strength is significantly lower than that in the solid medium. Therefore, partial discharge is first generated in the air gap. The discharge results in an increase in the air gap temperature, which increases the degree of partial discharge and produces a vicious circle. The breaking capacity of the active circuit breaker due to the accompanying discharge process must be greater than or equal to the expected short circuit current at that point. The two cascaded circuit breakers shall comply with the following standards: "The two protective devices shall coordinate their actions, that is, the electric power energy passing through the higher-level protective devices shall be within the tolerance of the lower-level protective devices, and these protective devices shall be Can protect the cable from damage."
In the two-stage cascade connection of the drawing, the model of the upper circuit breaker B is NC100H (breaking capacity is 20kA), the expected short-circuit current of the output terminal is L=15kA, and the model of the lower-level circuit breaker A is DPN (breaking capacity is 4.5kA). , the expected short-circuit current / B at the output (10kA. through the series connection of the upper and lower stages to open the circuit may induce dendritic discharge and eventually lead to main insulation breakdown.
The frequency-divided resonant overvoltage further contributes to the deterioration of the cable termination head insulation. Although the overvoltage of the ground overvoltage multiple of the crossover resonance is only 26h, the overvoltage of 26h has caused a serious threat to the cable terminal with large defects. It is known from the flow injection theory that under the action of a certain voltage, the air gap between the dielectric layers will escape the initial electrons, and the electric field will accelerate to the positive electrode to continuously cause impact ionization to generate a new electron collapse. When a resonant overvoltage occurs, the electron collapse is more intense. Once an air gap voltage value is higher than the breakdown voltage, the electron collapse in the air gap first turns into a flow and advances until the breakdown, so that other series air gaps are subjected to The voltage rises and breaks down successively, causing the field strength at the end of the semiconductor layer to rise, accelerating the end tip discharge of the semiconductor layer and the electron collapse inside the main insulation of the cable, and finally developing under the combined effects of light, heat and decomposition gas. A continuous discharge is generated to form a continuous arc, and the arc temperature reaches the ignition point of the heat-shrinkable tube, causing a fire.
The facts remind us that the cable terminal must be made in strict accordance with the technical regulations. The test data shows that the breaking capacity of the abnormal device A is greatly “enhancedâ€, and the enhanced breaking capacity is 10kA. The luck of the picture in 2001 is W)
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