Method for reducing residual magnetism of large generator iron core

Abstract

The invention relates to the field of large-scale generators, and discloses a method for reducing residual magnetism of a large-scale generator iron core, which comprises the following steps of: selecting equipment such as a direct-current electric welding machine, an air switch and a voltage and current instrument, applying direct current in a reverse direction to a rotor exciting winding, and reducing the direct current in a stepped manner to reduce the residual magnetism of the iron core in a three-minute power-on and one-minute power-off manner; by reducing the residual magnetism of the iron core, the voltage of the residual magnetism is below 30V, so that equipment maintenance and routine inspection of a large generator are facilitated.

Description

Technical Field

[0001] The invention relates to the field of large generators, in particular to residual magnetism of a large generator core, and more particularly to a method for reducing residual magnetism of a large generator core. Background Art

[0002] The magnetic circuit of a large generator is composed of a rotor core, a stator core and an air gap. A direct current is passed through the rotor excitation winding to generate a magnetic field. The main magnetic flux of the magnetic field flows along the rotor core, the air gap, the stator core, and then returns to the closed loop of the rotor core in one direction. The stator winding cuts the rotating magnetic field and induces a voltage. The greater the excitation current, the greater the induced voltage. In actual operation, when the rotor excitation current is reduced to zero, the stator winding still has an induced voltage, which is called the residual voltage, also known as residual voltage. The residual voltage is generated because after the rotor and stator core are magnetized in the magnetic field, although the external power supply is removed, the core still has a certain residual magnetization strength because the magnetic domain direction of the core magnet part remains fixed. If the generator rotates, the stator winding cuts the residual magnetic field, and the stator winding will induce a residual voltage.

[0003] Equipment maintenance, routine inspections, and some electromagnetic parameter tests of large generators all require the reduction of residual magnetic voltage. The residual magnetic voltage of large generators is generally 100V to 300V. Such a high voltage brings dangerous factors to maintenance work; large generator stator and rotor windings often use test instruments for routine inspections such as DC resistance and insulation resistance, and residual magnetism and residual magnetic voltage will cause harm to the test instruments; when conducting some electromagnetic parameter tests of large generators, excessive residual magnetic voltage will affect the test data and test results. It is necessary to reduce the residual magnetism of the core to facilitate equipment maintenance and routine inspections of large generators. Summary of the invention

[0004] In view of this, the present invention provides a method for reducing residual magnetism of the core of a large generator, by passing a step-wise decreasing reverse direct current through the rotor winding to reduce the residual magnetism of the core, thereby reducing the residual magnetism voltage, so as to facilitate the maintenance and routine inspection of large generator equipment.

[0005] The purpose of the present invention is achieved by following steps:

[0006] S1. Detection of original residual magnetism;

[0007] S2. Selection of degaussing equipment;

[0008] S3. Connection of degaussing equipment;

[0009] S4, demagnetization process;

[0010] S5. Inspection of demagnetization effect.

[0011] In the above-mentioned method for reducing residual magnetism of the core of a large generator, the method for detecting the original residual magnetism in S1 is: determining the direction of the residual magnetization intensity by the positive and negative DC voltage of the original power supply input to the rotor excitation winding; and detecting the magnitude of the residual magnetization intensity by testing the residual magnetization voltage induced by the stator winding.

[0012] In the above method for reducing residual magnetism of the core of a large generator, the method for selecting the demagnetization equipment in S2 is: the demagnetization equipment includes a DC welding machine, a power cable, an air switch, a shunt, a millivoltmeter, and a DC voltmeter.

[0013] In the above-mentioned method for reducing residual magnetism of the core of a large generator, the method for connecting the demagnetization equipment in S3 is as follows: the positive and negative outputs of the DC welding machine are connected to the air switch through two power cables, and then connected to the negative and positive inputs of the original power supply voltage of the generator rotor excitation winding, the shunt is connected in series in a power cable, the millivoltmeter tests the DC current through the secondary side of the shunt, and the DC voltmeter directly tests the DC voltage.

[0014] In the above method for reducing residual magnetism of a large generator core, the demagnetization process in S4 is as follows:

[0015] S41. Close the air switch, connect the demagnetization circuit, adjust the welding machine so that the DC current input to the rotor excitation winding is 100A, and power on for three minutes;

[0016] S42, open the air switch, cut off the demagnetization circuit, so that there is no power input to the rotor excitation winding, and cut off the power for one minute;

[0017] S43, reduce the DC current in a stepwise manner, 10A each time; input 90A, 80A, 70A...10A to the rotor excitation winding in sequence; perform a demagnetization cycle of powering on for three minutes and powering off for one minute each time;

[0018] S44. When the demagnetization process of the minimum DC current 10A is completed, the demagnetization process ends.

[0019] In the above-mentioned method for reducing residual magnetism of the core of a large generator, the method for testing the demagnetization effect in S5 is: the generator is idling at rated speed, and the residual magnetism voltage of the stator winding of the generator after demagnetization is tested; when the residual magnetism voltage is less than 30V, the purpose of reducing the residual magnetism of the core is achieved.

[0020] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] Effect 1. Eliminates the potential safety hazard caused by residual magnetic voltage: During the maintenance of large generators, the stator winding needs to be equipped with a ground wire and discharged to the ground; however, due to the existence of residual magnetic voltage, it is equivalent to live work, with the risk of electric shock. After the residual magnetism is eliminated, the residual magnetic voltage is less than 30V, which is a safe voltage and will not cause harm to people. Reducing residual magnetism is not only necessary for the personal safety of maintenance personnel, but also for the normal use of test instruments for routine inspections, electromagnetic parameter tests, and other work needs.

[0022] Effect 2. Detected the direction and magnitude of the residual magnetization intensity: The residual magnetization intensity of the iron core is not easy to test directly. By using the characteristics of large generators, the direction and magnitude of the residual magnetization intensity are indirectly reflected by testing the electrical quantity signal. The direction of the residual magnetization intensity is determined by the positive and negative input of the original DC voltage of the rotor excitation winding; the magnitude of the residual magnetization intensity is detected by testing the residual voltage of the stator winding. The detection of the direction and magnitude of the residual magnetization intensity is very necessary for implementing the demagnetization plan and analyzing the cause of magnetization.

[0023] Effect 3. The demagnetization method is simple and convenient: when the generator is stationary, the residual magnetism is reduced. When the unit is stationary, the magnetic circuit still exists, and the direction and path of the magnetic flux are the same as when it is running. The demagnetization effect when stationary is the same as that when it is running, except that the induced voltage of the stator winding cannot be detected at any time when the demagnetization is stationary. Demagnetization when stationary does not require the unit to be run, and the method is simpler and more convenient.

[0024] Effect 4. Current unidirectional demagnetization: The residual magnetization intensity is formed by the original unidirectional magnetic field. The original excitation current forms the N pole and S pole of the rotor magnetic pole, and the direction of the magnetic flux in the magnetic circuit remains unchanged. It only needs to input a DC current in the opposite direction of the original magnetic field; unidirectional current demagnetization does not require reversing the switch or reversing the line, and the demagnetization process is simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The following is a brief introduction to the drawings required for use in the embodiments of the present invention.

[0026] Figure 1 Wiring diagram for degaussing equipment.

[0027] Among them: 1- DC welding machine, 2- power cable, 3- air switch, 4- shunt, 5- millivoltmeter, 6- DC voltage, 7- excitation winding, 8- stator winding. DETAILED DESCRIPTION

[0028] To make the purpose, technical solution and beneficial effects of the present invention clearer, further explanation will be given below in conjunction with the accompanying drawings and specific embodiments. The specific embodiments described in the accompanying drawings are exemplary and are only used to explain the present invention, and cannot be understood as limiting the present invention.

[0029] Specific implementation method 1: A method for reducing residual magnetism of a large generator core, the method is implemented through the following five steps: detection of original residual magnetism, selection of demagnetization equipment, connection of demagnetization equipment, demagnetization process, and inspection of demagnetization effect.

[0030] In the present embodiment, a method for reducing residual magnetism of the core of a large generator has the following steps: first, the residual magnetism of the core is detected to determine the direction and direction of the residual magnetization intensity; secondly, a DC welding machine, an air switch, a test instrument and other equipment are selected; then the equipment is connected according to the positive and negative outputs of the DC welding machine and the negative and positive inputs of the original power supply voltage; then, the DC current is reduced in a step-like manner in the opposite direction, and the demagnetization process is performed in the form of power-on for three minutes and power-off for one minute; finally, the residual magnetism voltage after demagnetization is tested to verify the demagnetization effect.

[0031] Specific implementation method 2: In this implementation method, the detection of the original residual magnetism of S1 determines the direction of the residual magnetization intensity by the positive and negative DC voltage of the original power supply input to the rotor excitation winding; and detects the magnitude of the residual magnetization intensity by testing the residual magnetization voltage induced by the stator winding.

[0032] In this embodiment, the residual magnetization intensity of the core is not easy to test directly. By utilizing the characteristics of the large generator itself and testing the electrical quantity signal, the direction and magnitude of the residual magnetization intensity are indirectly reflected.

[0033] Specific implementation method three: In this implementation method, the selection of the degaussing device in S2 is as follows: Figure 1 As shown, the degaussing equipment includes a DC welding machine 1, a power cable 2, an air switch 3, a shunt 4, a millivoltmeter 5, and a DC voltmeter 6.

[0034] In this embodiment, the DC welding machine 1 outputs an adjustable DC current, and the maximum output current can reach 100A; the power cable 2 has an insulating sheath, a withstand voltage level of 500V, and a current carrying capacity of 100A; the air switch 3 has a withstand voltage level of 500V and a breaking current of 100A; the shunt 4 has a primary test of 100A and a secondary output of 75mV; the millivoltmeter 5 is connected to the shunt 4 and can test 75mV; the DC voltmeter 6 has a maximum range of 500V.

[0035] Specific implementation method 4: In this implementation method, the method of connecting the degaussing device in S3 is as follows: Figure 1 As shown, the positive and negative outputs of the DC welding machine 1 are connected to the air switch 3 through two power cables 2, and then connected to the negative and positive inputs of the original power supply voltage of the generator rotor excitation winding 7. The shunt 4 is connected in series in one power cable 2, the millivoltmeter 5 tests the DC current through the secondary side of the shunt 4, and the DC voltmeter 6 directly tests the DC voltage.

[0036] In this embodiment, the positive and negative output terminals of the DC welding machine 1 should be connected to the negative and positive input terminals of the original excitation power supply of the rotor excitation winding 7. The air switch 3 is in the electrical circuit to close and open the demagnetization circuit; the millivoltmeter 5 is connected to the shunt 4 to measure the corresponding DC current value; and the DC voltmeter 6 tests the DC voltage.

[0037] Specific implementation method 5: In this implementation method, the demagnetization process in S4 is as follows:

[0038] a. Close the air switch 3, connect the degaussing circuit, adjust the DC welding machine 1 so that the DC current input to the rotor excitation winding 7 is 100A, and power it on for three minutes;

[0039] b. Open the air switch 3, cut off the demagnetization circuit, so that the rotor excitation winding 7 has no power input, and cut off the power for one minute;

[0040] c. Reduce the DC current in steps, 10A each time; input 90A, 80A, 70A…10A to the rotor excitation winding 7 in sequence; perform a demagnetization cycle of powering on for three minutes and powering off for one minute each time;

[0041] d. When the demagnetization process with the minimum DC current of 10A is completed, the demagnetization process ends.

[0042] In this embodiment, the residual magnetism is reduced when the generator is stationary. When the unit is stationary, the magnetic circuit still exists, and the direction and path of the magnetic flux are the same as when it is running. The demagnetization effect when stationary is the same as that when running, except that the induced voltage of the stator winding cannot be detected at any time during demagnetization when stationary. In this embodiment, the current demagnetizes in one direction. The residual magnetization intensity is formed by the original unidirectional magnetic field, and the original excitation current forms the N pole and S pole of the rotor pole, which remain unchanged, and the direction of the magnetic flux flowing in the magnetic circuit remains unchanged. To demagnetize the magnetic field, it is only necessary to input a DC current in the opposite direction of the original magnetic field; for unidirectional current demagnetization, there is no need to reverse the switch or the line.

[0043] Specific implementation method six: In this implementation method, the demagnetization effect is tested in S5; after the demagnetization process is completed, the generator is idling at rated speed to test the residual magnetism voltage of the generator stator winding 8 after demagnetization; when the residual magnetism voltage is less than 30V, the purpose of reducing the residual magnetism of the iron core is achieved.

[0044] In this embodiment, by comparing the residual magnetism after demagnetization with that before demagnetization, it is tested whether the residual magnetism voltage is significantly reduced to check the effect of core demagnetization; the residual magnetism of the core is reduced to make the residual magnetism voltage less than 30V, and 30V is a safe voltage that does not cause harm to people; the lower residual magnetism of the core has little effect on the inspection and test of the generator.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for reducing residual magnetism of a large generator core, which is characterized by comprising the following steps: S1. Detection of original residual magnetism; S2. Selection of degaussing equipment; S3. Connection of degaussing equipment; S4, demagnetization process; S5. Inspection of demagnetization effect.

2. The method for reducing residual magnetism of a large generator core according to claim 1, characterized in that: The detection of the original residual magnetism of S1 also includes determining the direction of the residual magnetization intensity by the positive and negative of the DC voltage of the original power supply input to the rotor excitation winding; and detecting the magnitude of the residual magnetization intensity by testing the residual magnetization voltage induced by the stator winding.

3. The method for reducing residual magnetism of a large generator core according to claim 1, characterized in that: The selection of the S2 degaussing device also includes that the degaussing device includes a DC welding machine (1), a power cable (2), an air switch (3), a shunt (4), a millivoltmeter (5), and a DC voltmeter (6).

4. The method for reducing residual magnetism of a large generator core according to claim 1, characterized in that: The connection of the S3 degaussing device also includes that the positive and negative outputs of the DC welding machine (1) are connected to the air switch (3) through two power cables (2), and then connected to the negative and positive inputs of the original power supply voltage of the generator rotor excitation winding (7), the shunt (3) is connected in series in one power cable (2), the millivoltmeter (5) tests the DC current through the secondary side of the shunt (4), and the DC voltmeter (6) directly tests the DC voltage.

5. The method for reducing residual magnetism of a large generator core according to claim 1, characterized in that: The S4 degaussing process also includes: S41, close the air switch (3), connect the demagnetization circuit, adjust the DC welding machine (1) so that the DC current input to the rotor excitation winding (7) is 100A, and power on for three minutes; S42, open the air switch (3), cut off the demagnetization circuit, so that the rotor excitation winding (7) has no power input, and the power is cut off for one minute; S43, reducing the DC current in a stepwise manner, each time by 10A; inputting 90A, 80A, 70A ... 10A to the rotor excitation winding (7) in sequence; each time executing a demagnetization cycle of powering on for three minutes and powering off for one minute; S44. When the demagnetization process of the minimum DC current 10A is completed, the demagnetization process ends.

6. The method for reducing residual magnetism of a large generator core according to claim 1, characterized in that: The inspection of the demagnetization effect of S5 also includes idling the generator at rated speed and testing the residual magnetism voltage of the stator winding (8) of the generator after demagnetization; when the residual magnetism voltage is less than 30V, the purpose of reducing the residual magnetism of the core is achieved.

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