Selection of breaking capacity of generator magnetic circuit breaker

At present, the accidental demagnetization of generators at home and abroad, most of them still use the magnetic field circuit breaker and the non-destructive system of the non-linear resistance in the actual operation of the power station. The accident of burning the magnetic circuit breaker during the demagnetization has often caused the main cause of these accidents. First, because the magnetic field breaker can not break the DC power circuit of the generator rotor (the connection circuit between the rotor and the excitation power rectifier) ​​when the demagnetization is performed under certain demagnetization conditions, that is, the magnetic circuit breaker fails to meet the generator in a certain The analysis of the requirements for the breaking capacity of the magnetic circuit breaker during the de-excitation under some operating conditions shows that the current and voltage applied to the main contacts of the magnetic circuit breaker are different when the generator is de-excited under different operating modes. That is, the requirements for the breaking capacity of the magnetic circuit breaker are different. The breaking capacity of the magnetic field circuit breaker shall be selected according to the requirements of the generator to eliminate the demagnetization under the severe de-excitation conditions considered, so that the generator can be reliably de-excited under these working conditions. The current and voltage of the main contact are shown in the self-excited system of the generator using SC or linear de-excitation resistor. In the arcing process after the main contact of the generator magnetic field breaker QF is disconnected, the arc of the main contact The current A has the following relationship with the rotor current If and the current Ir on the demagnetizing resistor: the maximum current Am flowing through the main contact during the arcing process (also the maximum current flowing through the main contact during the demagnetization process) is the rotor at the start of demagnetization The current flowing through the main contact before the reverse induced voltage occurs, at this time Ir=0, therefore: after the main contact is separated, the voltage W across the contact is: when the demagnetization uses a nonlinear resistor, the A current is not Linear resistance value; u is the nonlinear coefficient, 0 1.3Ugr/nb, Ugr is the rated voltage of the generator, and nh is the voltage ratio of the primary and secondary sides of the excitation transformer. When the calculation takes 3 segments, the demagnetization starts. When the rotor current /f(0)= /3) is approximated, power can also be used. The rotor's steady-state current /f0) at 1.3 times rated voltage and mis-excitation is used as the rotor current at the start of demagnetization.

During the demagnetization process, the maximum output voltage Udm of the excitation power rectifier is the rotor voltage at the start of demagnetization, that is, the output voltage Ug of the power rectifier when the excitation system is erroneously excited under the terminal voltage Ug(0) at the start of demagnetization ( 0) The terminal voltage corresponding to the rotor current /f(0) at the start of demagnetization can be found from /f(0) from the no-load characteristic of the generator. The maximum current and maximum voltage of the main contact of the circuit breaker are calculated according to the calculation of the maximum current and maximum voltage of the main contact of the magnetic circuit breaker during the de-excitation process, and the magnetic load condition of the no-load mis-excitation is similar to the protection before the trip. The generator load characteristics (and T/calculation) are used. When the generator is tripped and the airborne characteristic is approximated, it can also be considered that the generator is forced to be excited under the stator voltage corresponding to the setting current of the rotor overcurrent protection. The rotor steady-state current corresponding to the rotor voltage is used as the rotor current at the start of demagnetization. In the transient process of the generator three-phase short-circuit, the stator short-circuit current will generate an induced current induced current on the rotor winding. The sub-transient component A// and the transient component A/f/ and the periodic component A/fc are respectively attenuated with time by the generator's T, /', T, / and Ta time constants after the short circuit occurs. When the generator is operating, the induced current in the rotor is attenuated but still large. If the short-circuit fault is not cut off when the generator is de-energized, the current that the magnetic field breaker needs to break is the induced current and excitation power of the stator short-circuit current on the rotor. The sum of the rotor current supplied by the rectifier and the calculation of the induced current in the rotor winding during the three-phase short-circuit of the machine can be found in the engineering design or the ANSUIEEEC37.18 standard. The rotor at the start of the three-phase short-circuit demagnetization of the salient-pole generator Current, the standard is recommended to take /f(t)=/f(0)=the three-phase short circuit of the machine, the output voltage of the excitation power rectifier is 0, but if the short circuit point is cut off during the arcing of the main contact of the magnetic circuit breaker The higher recovery voltage will appear on the machine end and the excitation system will be in a strong excitation state when the rated end is short-circuited within a small time delay, and the excitation regulator will keep the strong excitation state until the terminal voltage returns to a higher level. Voltage level In the engineering design, the output voltage of the excitation power rectifier can be considered according to the rated voltage of the terminal and the excitation state of the excitation system. When the three-phase short circuit of the machine is used, the maximum current and maximum current of the main contact of the magnetic circuit breaker are The voltage can be calculated as: Table 1 Table 2 is the calculation result of several de-excitation conditions of a certain two power station generators. Table 1 Calculation results of de-excitation process (power station 1) De-excitation condition Main contact rectifier output de-excitation voltage Remarks Rated no-load demagnetization rated load de-excitation normal strong-excitation tripping de-magnetic no-load mis-excitation excitation magnetic (115 °C) protection delay 0 load mis-excitation excitation magnetic (115 ° C) protection delay 0 machine three-phase three-phase Short circuit demagnetization (115C) takes V, excitation transformer voltage ratio: 18kV/1.014kV; number in parentheses corresponds to protection without delay calculation results Table 2 demagnetization process calculation results (power station 2) demagnetization condition main contact rectifier output off Magnetic voltage remarks rated no-load demagnetization rated load degaussing normal strong excitation tripping de-magnetic no-load mis-excitation excitation magnetic (115C) protection delay ft load mis-excitation excitation magnetic (115C) protection delay ft machine three-phase short circuit Demagnetization (115C) takes kV/0.932kV; numbers in parentheses The same electrical appliance, under different breaking voltages or different from Table 2 of Table 1, can be seen, when the excitation system is normal, the three-phase short-circuit demagnetization of the machine is the most severe de-excitation condition of the generator, considering the excitation system failure (mis-excitation) For the calculated power station 1, the most severe de-excitation condition of the generator is the load mis-excitation excitation magnetic condition; for the calculated power station 2, the most severe de-excitation condition of the generator is the no-load mis-excitation excitation magnetizer Condition 2 The relationship between the breaking capacity of the magnetic circuit breaker and the definition of the breaking capacity in the low-voltage electrical appliance At present, the general-purpose generator magnetic circuit breaker belongs to the low-voltage electrical appliance series according to the low-voltage electrical appliance standard, and the breaking capacity of the low-voltage electrical appliance has two definitions of rated breaking capacity and short-circuit breaking capacity. . Rated breaking capacity (ratedbreakingcapacity) is the current value that can be broken under the specified breaking conditions. The product standard or technical documentation should clearly specify the breaking condition h (g characteristic circuit rated rated short-circuit breaking capacity (ratedshort-circuitbreakingcapacity) is The test of the rated breaking capacity of the electrical equipment capable of breaking the short-circuit current value of the electrical appliance under the product standard or the voltage rated frequency specified by the manufacturer and a certain power factor or time constant is performed under the specified load and a certain voltage of the circuit breaker. The short-circuit breaking capability is performed when the breaker load terminal is short-circuited.

In the product standard or technical documentation of the electrical appliance, the rated breaking capacity is the negative load current that the electrical appliance can break under a certain voltage (ie, the recovery voltage) is the rated short-circuit breaking capacity given in different electrical product standards or technical documents. Is the value of the short-circuit current that can be broken by the appliance at a certain voltage (ie, the recovery voltage).

For general low-voltage electrical appliances, the test circuit for rated breaking capacity is carried out according to IEC or GB standard with series resistance of inductive resistors. The circuit time constant is selected according to the standard (for example, the time constant is 15ms). For magnetic field circuit breakers, according to ANSI/IEEEC37.18 standard The test load of the rated breaking capacity shall be carried out according to the magnetic field circuit or equivalent circuit of the actual generator.

When the generator is demagnetized, the magnetic circuit breaker is broken under the condition of the rotor and the de-excitation resistance load. After the arc of the main contact of the magnetic circuit breaker is extinguished, the two ends of the main contact are still subjected to the rotor and the excitation power rectifier. The output voltage (ie, the recovery voltage) can be seen from the working environment when the magnetic circuit breaker is disconnected. The breaking capacity of the magnetic circuit breaker required by the generator during the demagnetization is the rated breaking capacity of the circuit breaker, that is, the magnetic circuit breaker should be The rated breaking capacity satisfies the demagnetization requirement of the generator's severe de-excitation condition. The selection of the rated breaking capacity of the magnetic field circuit breaker can be seen from the above analysis. The requirement of the de-excitation of the generator to the breaking capacity of the magnetic circuit breaker is accompanied by the de-energizer of the generator. The difference in the values ​​is quite different in value. Therefore, when selecting a magnetic circuit breaker, the severe demagnetization condition that the generator should consider should be analyzed and determined first, and then the maximum contact of the main contact of the magnetic circuit breaker when de-excitation under such severe de-excitation conditions should be calculated. The rated breaking capacity of the circuit breaker selected by the current Am and the maximum voltage Ubm shall not be lower than the breaking capacity of the breaking load current Ibm when the breaking voltage (ie, the recovery voltage) is Ubm, and the rated rating of the selected circuit breaker product The breaking capacity shall be the breaking capacity determined according to the test conditions specified in the ANSU IEEE C37.18 standard. For example, for the power station 1 calculated in this paper, when the rated breaking capacity of the magnetic circuit breaker does not take into account the de-excitation condition of the excitation system failure, it shall be pressed. The three-phase short-circuit de-excitation of the machine requires the selection of the magnetic field circuit breaker, and its rated breaking capacity should not be lower than the load current value of 8175A at the breaking voltage of 4316V. When the magnetic field circuit breaker is selected in consideration of the excitation system fault (missing excitation), it shall have a rated breaking capacity of not less than the load current value of 11 530 A at a breaking voltage of 5 137 V, and an excitation system using ZnO de-excitation resistors. It can be seen from equation (6) that when the selected magnetic field circuit breaker is demagnetized under the severe de-excitation conditions under consideration, the rotor current can be reliably transferred to the ZnO de-magnetization resistance. The excitation system, before the magnetic circuit breaker contacts are separated to the ZnO de-excitation resistors, the load of the circuit breaker is only the rotor winding; the load side of the circuit breaker after the ZnO is turned on is similar. Therefore, for a magnetic field circuit breaker using ZnO demagnetization resistance, the test of the rated breaking capacity of the product must take into account this change in the load of the circuit breaker.

In general, it is not necessary to consider the more serious situation of multiple fault overlap to select the rated breaking capacity of the magnetic circuit breaker, such as the three-phase short circuit of the machine when the rotor is overloaded, and the three-phase short circuit of the machine when the power is excited. Problems and Suggestions a For the calculation of the rated breaking capacity and other parameters of the magnetic circuit breaker, there is no comprehensive standard in China; the requirements for the demagnetization time of the generator related to the selection of the rated breaking capacity have not seen the relevant standards. Or the specification is to design and use the magnetic field circuit breaker. It is recommended that according to the situation in China and related standards at home and abroad (such as ANSI/IEEEC37.18), the corresponding breaking standard of the national standard or standard magnetic circuit breaker is designed. Selecting and using the important parameters of the magnetic circuit breaker, this parameter has been clearly defined in the ANSUIEEE C37.1 low-voltage electrical appliance standard, and the test methods and requirements of the parameter are given, but some magnetic field breaks at home and abroad. In the product technical specifications of the device, the parameters of the magnetic circuit breaker are not given in accordance with the provisions of these standards. When designing and selecting, we should first understand the meaning of the parameters given in the product technical specifications and the test conditions to ensure the correct selection.

The domestic excitation system fault (missing strong excitation) is still not uncommon today, and the domestic magnetic circuit breaker has a lower breaking capacity. When the rated breaking capacity of the selected magnetic circuit breaker can not meet the severe de-excitation conditions under consideration, In order to ensure the safety and reliability of the de-excitation system, it is recommended to make the following considerations: Prolong the de-excitation time to reduce the voltage on the non-linear resistance during de-excitation, and the current de-excitation time of the domestic hydro-generator Most of the requirements are about 1s, while some foreign companies have suggestions for using 2s~3s demagnetization time. For excitation systems that may generate false excitation, consider improving the start-up loop of the action demagnetization, or set the action when the excitation is strong. De-excitation quick-acting protection to avoid de-excitation in the case of severe over-current of the rotor during mis-excitation

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