Problems that are easy to occur in the operation of power transformer differential protection and its treatment methods - News - Global IC Trade Starts Here.

Probe current voltage pin 420*4450 head diameter 5.0 over current current and voltage pin

PESD1CAN electrostatic protection SOT23-3 24V two-way

Power transformer differential protection is the most complex of all electrical components differential protection. In the course of operation, the phenomenon of protection misoperation also occurred from time to time. In the incorrect operation statistics of transformers of 220kV and above in North China Power Grid in 1997, a total of 18 incorrect actions were performed, of which the transformer longitudinal differential protection was incorrectly operated 5 times, accounting for about 5 times. 1/3 of the incorrect action. Analysis of the causes of misoperation and refusal of the differential protection, problems in operation and maintenance and management, as well as manufacturing, installation and design problems. This article analyzes and describes the problems that are easy to occur in the operation. 1. Unbalanced current transformer generated by transformer magnetizing inrush current During normal operation, the excitation current only flows through the power supply side, and an unbalanced current will be generated in the differential protection. Under normal circumstances, the excitation current is 3% to 8% of the rated current, and the excitation current of the large transformer is relatively small, generally less than 1%. In the case of an external fault, the excitation current is also reduced due to the voltage drop, and the effect is small. However, when the transformer is unloaded and the voltage is restored after the external fault is removed, a large magnitude of magnetizing inrush current may be generated, which may be 6 to 8 times the rated current of the transformer. The magnetizing inrush current contains a large number of non-periodic components and higher harmonic components, of which the second harmonic is dominant, and a discontinuity angle also appears in the waveform.
The method of reducing the influence of magnetizing inrush current in transformer longitudinal differential protection: (1) Using BCH type relay with fast saturation converter, when external fault occurs, the maximum unbalance current of non-periodic component can make fast saturation converter The core is quickly saturated, making it difficult for the unbalanced current to propagate to the coil of the differential relay, ensuring that the differential protection does not malfunction. In the case of internal faults, although the primary coil of the fast-saturated converter also has a non-periodic component, its attenuation speed is very fast. Generally, the attenuation is completed in two cycles, and all the currents passed through the converter are periodic short-circuit currents. The relay can be sensitive. (2) The differential protection of the microcomputer type transformer adopts the second harmonic braking principle. For example, the differential protection malfunctions when the external fault is removed, and the maximum phase braking mode can be set when the protection is the split phase braking mode. The OR gate braking mode, and based on the OR gate braking mode, reduces the second harmonic braking coefficient, which can be 10% to 12%. Considering the large capacity of the system and the fact that the fifth harmonic is also large when the external short circuit fault is removed, a fifth harmonic braking measure can be added. If a transformer has two sets of differential protection, the waveform symmetry principle can be used to identify the magnetizing inrush current. Because the principle of waveform symmetry is more sensitive and reliable than the second harmonic braking to identify the inrush current.
2. Current transformer secondary circuit wiring error One of the reasons for transformer longitudinal differential protection malfunction over the years has always been the accident of the secondary terminal polarity of the transformer. This shows that the basic protection of the relay protection personnel at the grassroots level is not solid. The construction personnel did not follow the correct design drawings, and the commissioning personnel did not carefully implement the commissioning regulations.
When the new installation, periodic test or secondary circuit is changed, before the transformer longitudinal differential protection is officially put into operation, the unbalanced voltage of the differential circuit must be measured with a high internal resistance voltmeter under the load condition of the transformer. Should meet the requirements of the regulations; should also measure the magnitude and phase of the secondary current on each side of the transformer, make a hexagonal vector diagram, check that the vector sum of the phase current of the same name on each side should be zero or close to zero, which means that the wiring is correct before it can be put into operation. Transformer differential protection.
3. Differential protection current transformer secondary circuit bad contact or open circuit The phenomenon of misalignment protection caused by poor contact or open circuit of the secondary circuit of the transformer occurs every year. Therefore, the operation monitoring of the differential circuit differential flow and the maintenance of the protection device should be strengthened. After the installation and commissioning of the protection device, or after the transformer is overhauled, the secondary circuit of the current transformer should be carefully checked and the wiring in the secondary circuit should be tightened. The screw of the terminal, and the spring washer or anti-vibration piece should be on the screw. It is also possible to use two cables in parallel as the secondary lead of the differential protection to prevent the secondary circuit from being opened.
4. Grounding problem of transformer longitudinal differential protection secondary circuit Some units have not been able to seriously implement the system countermeasures. There are two points of grounding in the current loop. One grounding point is inside the protection cabinet and the other is in the switch terminal box. The ground potential between the two grounding points is quite different. When there is lightning welding in or near the lightning weather, it is easy to cause a differential current in the differential element to cause the protection to malfunction.
Therefore, the countermeasures should be carefully implemented. Only one reliable grounding point should be set in the secondary current loop of the differential protection. The grounding point should be selected in the protective cabinet.
5. Current transformer secondary cable insulation problem In the project, the secondary transformer insulation of the current transformer is reduced, causing the phenomenon of misalignment protection to occur. This is mostly because the construction unit does not follow the specified construction during the construction process. When the cable is laid, the cable casing is cut. When the cable length is not enough, the two cables are used for docking. When the cable protection is welded, the cable is welded to burn the cable. This kind of phenomenon has occurred in the project, which has hidden hidden dangers for the protection and safe operation.
Therefore, during the inspection of the main equipment, the insulation between the core wires of the secondary circuit of the current circuit and the core wires is regularly checked. Measured with a 1000V insulation resistance meter, each insulation resistance value should meet the requirements of the regulations. In addition, the exposed portion of the terminal wire should be as short as possible to avoid grounding or phase-to-phase short circuit due to vibration or the like.
6. Selection of current transformer for transformer differential protection The current transformer used for transformer differential protection involves different voltage levels, different ratios, and different models on each side. This results in inconsistent transient transient characteristics of each transformer. It may cause misoperation or refusal of protection. Generally, the current transformer on the 500kV side selects the TP stage considering the transient characteristics, and the current transformer on each side of the 220kV and below generally adopts the P stage. The core of the TP-class transformer has air gaps, which reduce the core remanence to less than 10% of the saturation magnetic density, which greatly improves the transient transmission performance of the transformer. The core of the P-class transformer has no air gap, large residual magnetism, easy to saturate, and its transient transmission performance is poor.
The selection of current transformers for transformer differential protection should consider economics and practicability. TP-class transformers are technically the best, but expensive, and the low-voltage side TP-class transformers are bulky, in closed-structure busbars. Hard to install. Therefore, if there is no special requirement during the selection, and the P-class transformer can meet the actual needs, first consider the use of P-class transformers, avoiding economic waste and installation difficulties.
In addition, when selecting the secondary cable, the cross section of the secondary circuit cable core of the current transformer should be large enough. For long cables, the core cross section should be no less than 4mm2.