First, the analysis of differential principle Differential protection is known as fast protection with absolute selectivity. The principle is based on Kirchhoff's current law. According to Kirchhoff's current law, the phasor of the current flowing into and out of any node in the circuit is zero. The sum of the phasors of the current flowing into the component and the current flowing out of the component is called the differential current. When the transformer is operating normally or an out-of-zone fault occurs, the differential current is zero and the differential protection does not operate. When a fault occurs in the zone, the differential current is not zero and the differential protection action. Differential protection will cause unbalanced current due to factors such as current transformer saturation and transformer ratio. For the unbalanced current caused by various factors, the introduction of the braking current is adopted, so that the differential protection does not malfunction. It is called the ratio difference based on the ratio of the differential current to the braking current ratio to determine whether the protection is active. Since the rated current of each side of the main transformer is different, and the ratio of the current transformers on each side is different, the differential protection shall convert the secondary currents on each side according to the transformer ratio and the ratio of the current transformers on each side to make the differential The current can truly reflect the actual primary differential current. Second, the accident analysis Tangshan Xiaomafang 110kV substation, there are two main transformers in the station, and the accident of tripping of the two sides of the circuit breaker occurs during the #1 main transformer operation. The operation mode of the station before the accident is #1 main transformer in the no-load operation state, #2 The main transformer is in a cold standby state. After the accident, the substation operator went to the site to check. The breakers on both sides of the site #1 main transformer were in the opening position. Check the main transformer protection device and find the differential protection action of #1 main transformer protection device. The device displays the message "differential Protection action", showing fault current A phase 0.32A, phase B 0.35A, phase C 0.41A; view #2 main transformer protection device, device also shows "differential protection action", fault current A phase 0.39A, phase B 0.35 A, C phase 0.38A; check fault recording and other monitoring devices without fault information. Differential protection is the main protection of the transformer. The principle is the difference between the current flowing through the reactor and the current flowing out of the protected transformer. The main function of the transformer in the power system is to change the voltage to facilitate the transmission of electric power and the distribution of electric energy. It is a bridge and link between power plants, power grids and users. In order to prevent the safety and reliability of the power system from being affected by the failure of the transformer, various protection measures are taken for the power transformer, and the transformer differential protection misoperation is the most common one. However, during system operation, it is found that the differential protection and the inrush current often cause the differential protection to malfunction. More importantly, the differential protection misoperation often affects the safe and reliable operation of the entire power system. Therefore, the research on the problem of transformer differential protection misoperation is of great significance and value. Third, the inspection situation and cause analysis According to the protection action situation, the site is fully inspected and analyzed. The preliminary analysis may cause differential protection actions for the following reasons: 1 an abnormality occurs in the primary equipment within the differential protection range; 2 the differential protection device is malfunctioning; 3 the current loop error wiring. According to the analysis results, the inspection is carried out one by one, and the analysis of the system operation before and after the protection action is as follows: 1 The overall inspection of the primary equipment within the differential protection scope is found to be abnormal, the insulation test is qualified, the relevant test items are qualified, and the fault record is recorded. It can detect the fault-free information of other monitoring devices, so that it can confirm that there is no abnormality in the equipment at one time; 2 to carry out comprehensive verification of the differential protection device with the relay protection tester, and with the circuit breaker drive test, the protection device operates correctly; 3 check the differential Protect the CT secondary wiring and the polarity is correct. No abnormalities were found in the above inspection items. Finally, the insulation resistance is measured after the CT grounding point of the high-voltage side terminal box of the transformer is removed. As a result, the insulation value is displayed as zero, thereby determining that the differential current loop has two points grounded. The inspection found a second grounding point in the differential protection panel. According to the "relay protection anti-accident measures", the secondary circuit of the current transformer must be grounded separately and only one point. A separate current transformer secondary circuit that is not electrically connected to other transformer secondary circuits should be grounded at the circuit breaker site. From the differential protection two-point ground current flow diagram, when the power system has a ground fault, there is a large ground current flowing into the substation ground network, and a large potential difference will occur between the two points, because even if the grounding point is in the same The grounding network is not absolutely equipotential, so there will be a potential difference between the two points. When the differential protection current secondary circuit is grounded at two points, the current generated by the potential difference is high. The ground potential flows through the differential relay to the low ground potential. When the current value is greater than the fixed value set by the differential relay, no matter what state the transformer is in ( Operation, no-load, cold standby, hot standby) can cause differential protection malfunction. Therefore, the secondary circuit of the differential protection current must be grounded only a little. From the above analysis, it can be seen that the #1 main transformer no-load operation and the #2 main transformer are in the cold standby state. The reason for the differential protection misoperation is due to the two-point grounding of the current secondary circuit and the ground fault due to the system. The fault current flows through the substation ground grid. Fourth, the treatment measures The direct cause of the accident was due to the two-point grounding of the #1 and #2 main transformer differential current secondary circuits, which were not strictly implemented in accordance with the requirements of relay protection and anti-accident measures during the construction and acceptance process. The big safety hazard finally caused the malfunction of the transformer differential protection after commissioning. Based on the above problems, combined with the analysis, the following preventive measures were proposed. 1. In the new or expanded or rebuilt project, the current secondary circuit used for differential protection must carefully check the drawing design and drawings, including the initial design of the drawings, the review, etc. Each link must be carefully examined and sloppy. Especially in the project of protection transformation or equipment replacement, we should pay attention to the change design of the relevant protection secondary circuit, which is the premise and basis for eliminating the hidden dangers. 2. During the construction and commissioning process, the technical personnel of the construction personnel shall be strictly in accordance with the anti-accident measures and relevant regulations. If there is any error, the project contact list shall be submitted in time to prevent the phenomenon of “arbitrary construction and barbaric constructionâ€. 3. The grounding of the primary and secondary equipment must be separated, and the secondary circuit of the current transformer must be grounded separately and only one point. Independent current transformer secondary circuit that has no electrical connection with other transformer secondary circuits should be grounded at the circuit breaker site to ensure reliable connection between relay protection device and grounding grid. Grounding in relay protection room The copper bus net and the grounded copper wire mesh are reliably connected to the main grounding net. 4. During the process of adjusting the test, each commissioning and inspection personnel should have a strict working attitude and be meticulous. When performing a grounding check on the current secondary circuit, remove the grounding point and measure the insulation resistance. After the inspection, the circuit must be restored to the normal state. V. Measures to solve the malfunction of the differential protection device 1. The wiring of the secondary circuit of the microcomputer protection device should be minimized; the process level of the microcomputer protection device should be improved; the plug-in integration, component placement, and each module are relatively independent to improve the stability of the system. 2. Separate the strong and weak electrical leads of the protection device to reduce the influence of strong electromagnetic fields on the weak electrical signals; add decoupling capacitors at both ends of the power supply pins of the IC components to absorb the high-frequency signals generated when the IC components operate at high speed; Decoupling capacitors should use monolithic or ceramic capacitors with high frequency characteristics, and the soldering pins should be short; add anti-interference capacitors between current and voltage signals from high noise areas and ground, and absorb surge voltage and spikes. Pulse; add magnetic ring to the input and output parts of the power supply plug to reduce the influence of DC power supply ripple voltage. 3. The current transformer is disconnected by the current transformer, and the detection component detects the high voltage side of the transformer. The lock is protected when the secondary side of the transformer is disconnected; the protection device is effectively prevented from malfunctioning. 4. Check the ratio of the current transformer of the traction substation, and check the protection setting value to prevent the actual ratio of the current transformer from being inconsistent with the calculation ratio and causing the protection device to malfunction. 5. For the dedicated line traction substation, the discharge of the low-voltage side vacuum circuit breaker of the transformer will cause the transformer differential protection device to malfunction. It is necessary to change the second harmonic blocking criterion in the differential protection to the integrated harmonic blocking to solve the phenomenon that the differential protection device malfunctions. The transformer differential protection is used to reflect the phase-to-phase short circuit of the transformer winding, the inter-turn short-circuit fault of the winding, the neutral point ground fault and the phase-to-phase short-circuit fault of the lead-out line, and the ground fault of the neutral-point ground-side lead-out line. Under normal operating conditions, the unbalanced current flowing through the differential protection differential relay should be zero, so the differential protection does not operate. However, due to the unbalanced current caused by various operations of the transformer, the differential setting action current is increased, thereby reducing Protection sensitivity. With the large-capacity units and new substations being put into operation in the grid, the power system is constantly increasing, and the principle structure of relay protection is becoming more and more complicated. Differential protection has its unique advantages and is widely used in the main protection of transformers. Due to natural disasters or human factors, such as protection setting error, secondary circuit wiring is not standardized, current transformer polarity is reversed, etc., the transformer differential protection malfunction occurs sometimes, causing users to power off in large areas. Affect the safe and stable operation of the power grid. However, in the state of no-load operation of the transformer, the two-point grounding of the secondary circuit of the differential current will also cause the malfunction of the differential protection. Such safety hazards deserve our attention and prevention in the future work. smart Touch Screen whiteboard for meeting room and classroom,smart Interactive Whiteboard for conference video system and teaching;Science is changing our world.with the development of touch screen technology,LCD touch screen becomes an important part of our life.Our LCD interactive Smart board are widely used for conference video system,teaching multimedia equipment and advertising display.Smart digital whiteboard,android and windows dual system,10-20 points infrared touch screen,built-in wifi module,LAN internet access,4K HD display images,digital smart board,movable bracket,mobile PC laptop projection,wired and wireless connection,HDMI VGA AV-IN AUDIO IN/OUT USB Touch and all mainstream interfaces.a smart interactive whiteboard can help us work and teach more efficiently.Companies,schools,universities,exhibition centers,business centers,our smart electrical board can be used everywhere. smart board,smart interactive flat panel,smart touch screen interactive,smart interactive whiteboard Jumei Video(Shenzhen)Co.,Ltd , https://www.jmsxdisplay.com