Method for eliminating residual magnetism of large power transformer

Abstract

The application provides a residual magnetism elimination method for a large power transformer, and relates to the field of transformer residual magnetism elimination.The device comprises the following steps: S1, when the generator of the generating and transforming group reaches the rated rotating speed, and after checking that the excitation system and the cooling system of the generator are normal, the transformer is turned on, the generator is used to perform zero-step voltage rise on the transformer, the initial voltage rise is performed to 5% of the rated voltage of the generator, then the voltage rise is continuously performed to 50% of the rated voltage of the generator, then the voltage drop is performed to 5%, and the voltage rise-voltage drop cycle is repeatedly performed at least twice; S2, the excitation circuit of the generator is cut off, the rated voltage of the generator is reduced to zero, then the voltage is raised to 5% of the rated voltage of the generator, and the transformer is subjected to impact demagnetization; the application adopts the mode of step-by-step and phased slow voltage rise and drop, can smoothly reduce the hysteresis loop of the iron core, avoids the problem that the magnetic domain turning is uneven under a single voltage level, and the demagnetization effect is better than that of the simple no-load closing or direct-current decay method.

Description

Technical Field

[0001] This invention relates to the field of residual magnetism elimination technology for transformers, and more particularly to a method for eliminating residual magnetism in large power transformers. Background Technology

[0002] After a DC test, a power transformer will generate residual magnetism. The amount of residual magnetism depends on the intensity and duration of the DC current flowing through the transformer windings. Residual magnetism is a manifestation of the hysteresis loss of ferromagnetic materials. Hysteresis loss is the result of ferromagnetic materials absorbing electrical energy and converting it into magnetic energy. In AC circuits, it manifests as part of the iron loss and together with eddy current loss, constitutes the iron loss of the transformer. In other words, hysteresis loss is the result of energy conversion. Therefore, it is related to the input power and time. That is to say, the greater the electrical power input to the transformer windings and the longer the time, the greater the amount of residual magnetism, and vice versa. The ability of a transformer to self-demagnetize relies primarily on the molecular thermal motion of the core material. Therefore, the self-demagnetization speed of a transformer is related to the oil temperature (ambient temperature) and time. However, the maintenance and de-energization time for large power transformers cannot be extended indefinitely. If the transformer body oil temperature (ambient temperature) is too low, the transformer's self-demagnetization speed will be slower. If time does not allow for forced energization, in addition to causing the transformer's heavy gas protection to trip, it may also cause light gas to send a signal and the total hydrocarbon content in the oil color analysis to rise. Therefore, this invention proposes a method for eliminating residual magnetism in large power transformers to solve the problems existing in the prior art. Summary of the Invention

[0003] To address the aforementioned problems, this invention proposes a method for eliminating residual magnetism in large power transformers. This method employs a stepped, phased, and slow voltage increase / decrease approach, which can smoothly reduce the hysteresis loop of the iron core and avoid the problem of uneven magnetic domain orientation under a single voltage level. The demagnetization effect is superior to simple no-load switching or DC attenuation methods.

[0004] To achieve the objectives of this invention, the invention is implemented through the following technical solution: a method for eliminating residual magnetism in a large power transformer, comprising the following steps: S1. When the generator of the generator-transformer unit reaches the rated speed and after checking that the excitation system and cooling system of the generator are normal, connect the transformer and use the generator to perform zero-start voltage boosting of the transformer. Initially boost the voltage to 5% of the generator's rated voltage, then continue to boost the voltage to 50% of the generator's rated voltage, and then reduce the voltage to 5%. Repeat this boost-deboost cycle at least twice. S2. Disconnect the generator excitation circuit to reduce the generator rated voltage to zero, then boost the voltage to 5% of the generator rated voltage, perform impulse demagnetization on the transformer, and repeat this boost-buck cycle at least twice. S3. The generator again boosts the transformer from zero voltage to 75% of the generator's rated voltage, then reduces the voltage to zero, and repeats this boost-deboost cycle at least twice. S4. The generator again boosts the transformer from zero voltage to 100% of the generator's rated voltage, then reduces the voltage to zero, and repeats this boost-deboost cycle at least twice.

[0005] A further improvement is that, in steps S1-S4, a stepped voltage increase is adopted during the voltage increase process. When the generator's rated voltage is in the range of 0-50%, the voltage increase is 2.5% of the generator's rated voltage each time, and when the generator's rated voltage is in the range of 50-100%, the voltage increase is 1% of the generator's rated voltage each time.

[0006] A further improvement is that, in steps S1-S4, a stepped voltage reduction is also adopted during the voltage reduction process. When the generator's rated voltage is in the range of 50-100%, the voltage reduction is 1% of the generator's rated voltage each time. When the generator's rated voltage is in the range of 0-50%, the voltage increase is 2.5% of the generator's rated voltage each time.

[0007] A further improvement is made in the following steps: In steps S1-S4, the transformer excitation current waveform is continuously monitored using an oscilloscope. If a sudden change or distortion in the current waveform is detected during the voltage boosting process, the voltage boosting is paused, the current voltage level is maintained, and the subsequent voltage boosting operation is continued after the current value stabilizes.

[0008] A further improvement is that the signal monitored by the oscilloscope comes from the secondary circuit of the transformer high-voltage side CT. When the current signal on the secondary side of the CT is too small, a variable resistor is connected in series in the secondary circuit of the CT. The waveform change of the excitation current is equivalently reflected by monitoring the voltage waveform across the variable resistor.

[0009] A further improvement is made in the following steps: If, in any step, the waveforms of the three-phase excitation current of the transformer are all stable sine waves and the amplitudes of the three phases are completely symmetrical, then it is determined that the residual magnetism has been eliminated. If the waveform contains even-order harmonic components or the three phases are asymmetrical, then it is determined that the residual magnetism has not been eliminated and the demagnetization step needs to be performed.

[0010] A further improvement is that the neutral point on the high-voltage side of the transformer is reliably grounded to provide a discharge path for zero-sequence current.

[0011] A further improvement is made in step S1, when the initial voltage is increased to 5% of the generator's rated voltage, the excitation system current, excitation system voltage, generator voltage, generator stator current, and insulation monitoring voltage are recorded, and the generator-transformer unit no-load voltage increase curve is recorded for each voltage increase.

[0012] A further improvement is that when the generator boosts the voltage to the rated voltage, the generator maintains the rated voltage for no less than 5 minutes.

[0013] A further improvement is that the residual magnetism elimination method is applicable to transformers with generator-transformer unit connections.

[0014] The beneficial effects of this invention are as follows: This invention adopts a stepped, phased, and slow voltage increase and decrease method, which can smoothly reduce the hysteresis loop of the iron core and avoid the problem of uneven magnetic domain orientation under a single voltage level. The demagnetization effect is better than simple no-load closing or DC attenuation methods. This invention directly uses the generator set as the AC power source, eliminating the need to purchase additional large-scale variable frequency demagnetization power supplies or equipment, thus saving costs. Moreover, the operation process is highly consistent with the no-load voltage increase test of the power plant, making it easy for on-site personnel to master. At the same time, by monitoring waveform changes with an oscilloscope, the time for zero-start voltage increase and demagnetization can be reduced. Because during the smooth voltage increase process, if the current changes abruptly at a certain point, the generator voltage can be stabilized at that point. After the current value stabilizes, the voltage can continue to increase until the rated voltage is reached. This avoids the process of repeated voltage increases and decreases, improving efficiency and saving energy. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the demagnetization process of the present invention. Detailed Implementation

[0016] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0017] according to Figure 1 As shown in the figure, this embodiment proposes a method for eliminating residual magnetism in a large power transformer, including the following steps: S1. When the generator of the generator-transformer unit reaches the rated speed and after checking that the excitation system and cooling system of the generator are normal, connect the transformer and use the generator to perform zero-start voltage boosting of the transformer. Initially boost the voltage to 5% of the generator's rated voltage, then continue to boost the voltage to 50% of the generator's rated voltage, and then reduce the voltage to 5%. Repeat this boost-deboost cycle at least twice. S2. Disconnect the generator excitation circuit to reduce the generator rated voltage to zero, then boost the voltage to 5% of the generator rated voltage, perform impulse demagnetization on the transformer, and repeat this boost-buck cycle at least twice. S3. The generator again boosts the transformer from zero voltage to 75% of the generator's rated voltage, then reduces the voltage to zero, and repeats this boost-deboost cycle at least twice. S4. The generator again boosts the transformer from zero voltage to 100% of the generator's rated voltage, then reduces the voltage to zero, and repeats this boost-deboost cycle at least twice.

[0018] In steps S1-S4, a stepped voltage increase is adopted during the voltage increase process. When the generator's rated voltage is in the range of 0-50%, the voltage increase is 2.5% of the generator's rated voltage each time. When the generator's rated voltage is in the range of 50-100%, the voltage increase is 1% of the generator's rated voltage each time.

[0019] In steps S1-S4, a stepped voltage reduction is also adopted during the voltage reduction process. When the generator's rated voltage is in the range of 50-100%, the voltage reduction is 1% of the generator's rated voltage each time. When the generator's rated voltage is in the range of 0-50%, the voltage increase is 2.5% of the generator's rated voltage each time.

[0020] In steps S1-S4, the transformer excitation current waveform is continuously monitored using an oscilloscope. If a sudden change or distortion in the current waveform is detected during the voltage boosting process, the voltage boosting is paused, the current voltage level is maintained, and the subsequent voltage boosting operation is continued after the current value stabilizes.

[0021] The signal monitored by the oscilloscope comes from the secondary circuit of the transformer high-voltage side CT. When the current signal on the secondary side of the CT is too small, a variable resistor is connected in series in the secondary circuit of the CT. The waveform change of the excitation current is equivalently reflected by monitoring the voltage waveform across the variable resistor.

[0022] In steps S1-S4, if the waveforms of the three-phase excitation current of the transformer are all stable sine waves and the amplitudes of the three phases are completely symmetrical, it is determined that the residual magnetism has been eliminated. If the waveform contains even harmonic components or the three phases are asymmetrical, it is determined that the residual magnetism has not been eliminated and the demagnetization step needs to be performed.

[0023] The neutral point on the high-voltage side of the transformer is reliably grounded to provide a discharge path for zero-sequence current.

[0024] In step S1, when the initial voltage is increased to 5% of the generator's rated voltage, the excitation system current, excitation system voltage, generator voltage, generator stator current, and insulation monitoring voltage are recorded, and the generator-transformer unit no-load voltage increase curve is recorded for each voltage increase.

[0025] When the generator is boosted to the rated voltage, the generator maintains the rated voltage for no less than 5 minutes.

[0026] The residual magnetism elimination method is applicable to transformers with generator-transformer unit connections.

[0027] The residual magnetism elimination method of this large power transformer adopts a stepped, phased, and slow voltage increase / decrease approach, which can smoothly reduce the hysteresis loop of the iron core and avoid the problem of uneven magnetic domain orientation under a single voltage level. The demagnetization effect is better than simple no-load closing or DC attenuation methods. This invention directly uses the generator set as the AC power source, eliminating the need to purchase additional large-scale variable frequency demagnetization power supplies or equipment, thus saving costs. Moreover, the operation process is highly consistent with the no-load voltage increase test of the power plant, making it easy for on-site personnel to master. At the same time, by monitoring waveform changes with an oscilloscope, the time for zero-start voltage increase and demagnetization can be reduced. Because during the smooth voltage increase process, if the current changes abruptly at a certain point, the generator voltage can be stabilized at that point. After the current value stabilizes, the voltage can continue to increase until the rated voltage is reached. This avoids the process of repeated voltage increases and decreases, improving efficiency and saving energy.

[0028] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for eliminating residual magnetism in a large power transformer, characterized in that: Includes the following steps: S1. When the generator of the generator-transformer unit reaches the rated speed and after checking that the excitation system and cooling system of the generator are normal, connect the transformer and use the generator to perform zero-start voltage boosting of the transformer. Initially boost the voltage to 5% of the generator's rated voltage, then continue to boost the voltage to 50% of the generator's rated voltage, and then reduce the voltage to 5%. Repeat this boost-deboost cycle at least twice. S2. Disconnect the generator excitation circuit to reduce the generator rated voltage to zero, then boost the voltage to 5% of the generator rated voltage, perform impulse demagnetization on the transformer, and repeat this boost-buck cycle at least twice. S3. The generator again boosts the transformer from zero voltage to 75% of the generator's rated voltage, then reduces the voltage to zero, and repeats this boost-deboost cycle at least twice. S4. The generator again boosts the transformer from zero voltage to 100% of the generator's rated voltage, then reduces the voltage to zero, and repeats this boost-deboost cycle at least twice. S5. Continuously monitor the winding induced voltage and excitation current of the transformer through voltage transformers and current transformers, and display the excitation current waveform through an oscilloscope. If the three-phase excitation current waveforms of the transformer are all stable sine waves and the three-phase amplitudes are completely symmetrical in any step, it is determined that the residual magnetism has been eliminated. If the waveform contains even harmonic components or the three phases are asymmetrical, it is determined that the residual magnetism has not been eliminated and the demagnetization step needs to be continued. S6. The residual magnetism of the transformer is calculated by setting the calculation module and displayed externally by the display module; S7. Based on the combined results of S5 and S6, determine whether the residual magnetism of the transformer has been completely eliminated.

2. The method for eliminating residual magnetism in a large power transformer according to claim 1, characterized in that: In steps S1-S4, a stepped voltage increase is adopted during the voltage increase process. When the generator's rated voltage is in the range of 0-50%, the voltage increase is 2.5% of the generator's rated voltage each time. When the generator's rated voltage is in the range of 50-100%, the voltage increase is 1% of the generator's rated voltage each time.

3. The method for eliminating residual magnetism in a large power transformer according to claim 1, characterized in that: In steps S1-S4, a stepped voltage reduction is also adopted during the voltage reduction process. When the generator's rated voltage is in the range of 50-100%, the voltage reduction is 1% of the generator's rated voltage each time. When the generator's rated voltage is in the range of 0-50%, the voltage increase is 2.5% of the generator's rated voltage each time.

4. The method for eliminating residual magnetism in a large power transformer according to claim 1, characterized in that: In step S6, the method for calculating the residual magnetism of the transformer includes: Step 1: Conduct a current-voltage characteristic experiment to obtain the saturation magnetic flux of the transformer. Furthermore, current and voltage are monitored during the transformer's step-up and step-down processes. Based on Faraday's law of electromagnetic induction, the induced voltage in the open-circuit winding is equal to the rate of change of magnetic flux. The formula for calculating magnetic flux is obtained by integrating the formula for calculating the induced voltage in the winding. ； Where N is the number of turns in the winding, U(t) is the induced voltage in the winding, and t is the monitoring duration; Step 2: In S1-S4, calculate the residual magnetic flux after each demagnetization step. ; Step 3: Calculate the residual magnetism rate : ； in The demagnetization is complete and displayed on the display module. If the demagnetization is incomplete, continue the demagnetization process.

5. The method for eliminating residual magnetism in a large power transformer according to claim 4, characterized in that: The signal monitored by the oscilloscope comes from the secondary circuit of the transformer high-voltage side CT. When the current signal on the secondary side of the CT is too small, a variable resistor is connected in series in the secondary circuit of the CT. The waveform change of the excitation current is equivalently reflected by monitoring the voltage waveform across the variable resistor.

6. The method for eliminating residual magnetism in a large power transformer according to claim 5, characterized in that: In steps S1-S4, if a sudden change or distortion in the current waveform is detected during the voltage boosting process, the voltage boosting is paused, the current voltage level is maintained, and the subsequent voltage boosting operation is continued after the current value stabilizes.

7. The method for eliminating residual magnetism in a large power transformer according to claim 6, characterized in that: The neutral point on the high-voltage side of the transformer is reliably grounded to provide a discharge path for zero-sequence current.

8. The method for eliminating residual magnetism in a large power transformer according to claim 7, characterized in that: In step S1, when the initial voltage is increased to 5% of the generator's rated voltage, the excitation system current, excitation system voltage, generator voltage, generator stator current, and insulation monitoring voltage are recorded, and the generator-transformer unit no-load voltage increase curve is recorded for each voltage increase.

9. The method for eliminating residual magnetism in a large power transformer according to claim 8, characterized in that: When the generator is boosted to the rated voltage, the generator maintains the rated voltage for no less than 5 minutes.

10. A method for eliminating residual magnetism in a large power transformer according to claim 9, characterized in that: The residual magnetism elimination method is applicable to transformers with generator-transformer unit connections.

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