A method for suppressing inrush current of a core-type transformer

Abstract

The present invention provides an inrush current suppression method for a core-type transformer, which includes obtaining the magnetizing current value of the transformer; modeling the data in the DC magnetizing process to calculate the closing angle; obtaining the relationship between the closing angle and the magnetizing current, establishing a magnetizing model, and outputting the magnetizing current and the optimal closing angle after the transformer parameters are input; judging whether the output magnetizing current exceeds the overcurrent protection value; calculating the voltage value of the transformer element according to the magnetizing current to judge whether it exceeds the overvoltage protection value; and performing phase-controlled closing according to the closing angle output after the magnetization is completed, so that the bias magnetic energy generated by the closing is offset by the residual magnetism in the iron core after magnetization, thereby suppressing the inrush current. The present invention calculates the relationship between the magnetizing current and the transformer magnetic flux, uses the magnetizing current to control the three-phase iron core magnetic flux of the transformer within a specified range, and then controls the transformer to close at a specific angle, thereby achieving the suppression of the three-phase inrush current.

Description

Technical Field

[0001] The invention belongs to the technical field of transformers, and in particular relates to a method for suppressing inrush current of a core-type transformer. Background Art

[0002] In actual engineering commissioning, the total magnetic flux in the transformer core exceeds the saturation magnetic flux after closing, resulting in a large inrush current. Due to the characteristics of the transformer core, the total magnetic flux of the transformer is different when closing at different angles. When the magnetic flux of the transformer core exceeds its saturation magnetic flux, its excitation current will surge. At this time, the total magnetic flux is approximately proportional to the excitation current. Due to the particularity of the excitation inrush current, the peak value of the largest inrush current in the three-phase sequence is usually used as an indicator when evaluating the inrush current impact. The excitation inrush current has a great harm to the power system, such as causing protection malfunction, generating and responding inrush current, and damaging the transformer, which seriously affects the safe and stable operation of the power system. With the development of transformer excitation inrush current technology and the promotion of residual magnetism algorithms, the inrush current suppression problem under continuous switching conditions has been basically solved. The problem of the first closing of the transformer (hereinafter referred to as the first closing) has become one of the most urgent and difficult problems in the field of inrush current suppression. When the transformer is under maintenance or in standby for a long time, its residual magnetism state is in an unknown state. Putting the transformer into operation under such a state is called the first closing of the transformer. Due to the 90° difference between magnetic flux and voltage, when the transformer is first closed, it is generally closed at the voltage angle corresponding to the minimum magnetic flux of the reference phase (i.e., the closing phase voltage is 90°). However, since this method cannot eliminate the influence of transformer residual magnetism, this type of first closing problem is usually not effective.

[0003] There are two general solutions to solve the problem of transformer first-closing excitation inrush current. The first is to install a closing resistor to increase the branch resistance to achieve the purpose of current suppression. When closing, the closing resistor is put into use to limit the current. After a period of time, the resistor is short-circuited and exited. This solution can achieve inrush current suppression, but due to cost and reliability considerations, it is not widely used in actual sites. The second solution is to demagnetize the transformer. After the transformer is overhauled, the transformer is demagnetized to demagnetize the transformer core flux to zero. This solution will be demagnetized after the transformer is overhauled on the actual site. Generally, all external wiring of the transformer needs to be removed. When the phase control is performed, the inrush current can be suppressed to a certain extent. The solution of installing a closing resistor requires modification of the primary wiring, which requires large investment, complex maintenance, and certain risks. In actual use, there are certain restrictions on the installation space. The demagnetization solution requires the removal of all external wiring of the transformer. The operation process is cumbersome and requires changes in the primary wiring, which takes a long time to prepare. Moreover, since most sites use three-phase interlocking circuit breakers, phase control cannot be performed. During interlocking control, demagnetization cannot make the surge current in each of the three phases very small. There will always be one or two phases with large surge current, so it cannot be applied to most scenarios. Summary of the invention

[0004] The object of the present invention is to provide a method for suppressing inrush current of a core-type transformer, aiming to solve the problem that the existing method for suppressing inrush current requires adding components to the transformer, the operation process is cumbersome and the inrush current of all three phases cannot be reduced.

[0005] The present invention is implemented as follows: a method for suppressing inrush current of a core-type transformer, the method comprising the following steps:

[0006] Step 1: Obtain the magnetizing current value of the transformer;

[0007] Step 2: Model the data during DC magnetization. The DC magnetization flux value is flux s = flux DC -flux sj , where flux DC The flux in the iron core of the constant magnetic field created by the applied DC is calculated from the magnetizing current. sj It indicates the attenuation of magnetic flux after the DC magnetizing current is removed. Since the magnetic flux decays exponentially and the decay speed is very fast, it is considered that flux sj is a function related to the magnetizing flux sj =f(flux sj ), at the moment of closing, the total magnetic flux is the sum of the charged magnetic flux and the closing bias magnetic flux, that is, flux z = flux s +flux p, where flux z Expressed as total magnetic flux, flux p It is expressed as the bias magnetism at the moment of closing, which is calculated based on the closing angle. The closing angle is calculated based on the principle of mutual restraint between residual magnetism and bias magnetism.

[0008] Step 3: The relationship between the closing angle and the magnetizing current is obtained through step 2. By analogy, a magnetizing model is established. After the transformer parameters are input, the magnetizing current and the optimal closing angle are automatically output;

[0009] Step 4: Determine whether the magnetizing current output in step 3 exceeds the overcurrent protection value. If so, return to step 2 to recalculate. If not, execute step 5.

[0010] Step 5: Calculate the voltage value of the transformer element according to the magnetizing current output in step 3, and determine whether it exceeds the overvoltage protection value. If it exceeds, return to step 2 to recalculate; if it does not exceed, execute step 6;

[0011] Step 6: Perform phase-controlled closing according to the closing angle output after magnetization is completed, so that the bias magnetic energy generated by closing the circuit can offset the residual magnetism in the iron core after magnetization, thereby suppressing the inrush current.

[0012] A further technical solution of the present invention is: in step 2, flux DC The size is positively correlated with the current size, that is, flux DC =K DC i dc , K DC is the coefficient of magnetizing current and magnetic flux, i dc is the magnetizing current, the DC magnetizing flux value flux s =K DC i dc -f(flux sj ).

[0013] A further technical solution of the present invention is: in step 2, flux sj Through the formula flux sj =B*a t , a is the attenuation constant, B is the magnetic flux of the magnetic field established by the DC current, and t is the time.

[0014] A further technical solution of the present invention is: in step 2, flux p The calculation formula is flux p =cos(α), where α is the closing angle.

[0015] A further technical solution of the present invention is: in step 2, the closing angle is expressed as α=arccos(-flux s ).

[0016] A further technical solution of the present invention is: after the magnetization in step six is ​​completed, a magnetization report and an optimal closing angle of phase control are output.

[0017] The beneficial effects of the present invention are as follows: the present invention calculates the relationship between the magnetizing current and the transformer flux, uses the magnetizing current to control the three-phase core flux of the transformer within a specified range, and then controls the transformer to close at a specific angle so that the total flux of the three-phase sequence can be controlled within a smaller range, thereby achieving the suppression of the three-phase inrush current. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a flow chart of the method of the present invention. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0020] Figure 1 A method for suppressing inrush current of a core-type transformer provided by the present invention is shown, and the method comprises the following steps:

[0021] Step 1: Obtain the magnetizing current value of the transformer;

[0022] Step 2: Model the data during DC magnetization. The DC magnetization flux value is flux s = flux DC -flux sj , where flux DC The flux in the iron core of the constant magnetic field created by the applied DC is calculated from the magnetizing current. sj It indicates the attenuation of magnetic flux after the DC magnetizing current is removed. Since the magnetic flux decays exponentially and the decay speed is very fast, it is considered that flux sj is a function related to the magnetizing flux sj =f(flux sj ), at the moment of closing, the total magnetic flux is the sum of the charged magnetic flux and the closing bias magnetic flux, that is, flux z = flux s +flux p , where flux z Expressed as total magnetic flux, flux p It is expressed as the bias magnetism at the moment of closing, which is calculated based on the closing angle. The closing angle is calculated based on the principle of mutual restraint between residual magnetism and bias magnetism.

[0023] Step 3: The relationship between the closing angle and the magnetizing current is obtained through step 2. By analogy, a magnetizing model is established. After the transformer parameters are input, the magnetizing current and the optimal closing angle are automatically output;

[0024] Step 4: Determine whether the magnetizing current output in step 3 exceeds the overcurrent protection value. If so, return to step 2 to recalculate. If not, execute step 5.

[0025] Step 5: Calculate the voltage value of the transformer element according to the magnetizing current output in step 3, and determine whether it exceeds the overvoltage protection value. If it exceeds, return to step 2 to recalculate; if it does not exceed, execute step 6;

[0026] Step 6: Perform phase-controlled closing according to the closing angle output after magnetization is completed, so that the bias magnetic energy generated by closing the circuit can offset the residual magnetism in the iron core after magnetization, thereby suppressing the inrush current.

[0027] Preferably, in step 2, flux DC The size is positively correlated with the current size, that is, flux DC =K DC i dc , K DC is the coefficient of magnetizing current and magnetic flux, i dc is the magnetizing current, the DC magnetizing flux value flux s =K DC i dc -f(flux sj ).

[0028] Preferably, in step 2, flux sj Through the formula flux sj =B*a t , a is the attenuation constant, B is the magnetic flux of the magnetic field established by the DC current, and t is the time.

[0029] Preferably, in step 2, flux p The calculation formula is flux p =cos(α), where α is the closing angle.

[0030] Preferably, in step 2, the closing angle is expressed as α=arccos(-flux s ).

[0031] Preferably, after the magnetization in step six is ​​completed, a magnetization report and an optimal closing angle of phase control are output.

[0032] The present invention provides an inrush current suppression method for a core-type transformer, providing transformer parameter input, calculating a rated current value through transformer parameters, inputting the rated current value into a magnetizing model to calculate the magnetizing current, wherein the magnetizing model is mainly trained by collecting the relationship between the magnetizing current and the three-phase magnetic flux, the magnetizing model calculates the magnetizing current based on the transformer parameters, outputs the magnetizing current through a magnetizing device, and outputs a magnetizing report and a phase-controlled optimal closing angle after the magnetization is completed.

[0033] For core-type transformers, since the three-phase windings are wound on the same iron core, the magnetization method is to perform magnetization operations through a certain phase sequence (any one of the three phases) and the N phase. When current flows through a certain phase sequence winding, a magnetic field will be generated around it. Due to the special design of the transformer, most of its magnetic flux flows through the iron core. When the current is stable, the magnetic flux flowing in the iron core is also relatively stable. When the current is withdrawn, the attenuation coefficient is different due to the different magnetic fluxes of each phase sequence. The attenuation coefficient is calculated by the magnetizing current stabilization time to control the three-phase magnetic flux to meet the symmetrical state as much as possible (120° electrical symmetrical state). At this time, when the reference angle is selected for closing (the reference angle is calculated by the charged magnetic flux), the bias magnetism and the charged magnetic flux generated when each sequence is closed offset each other, and the total superimposed magnetic flux does not exceed the saturation magnetic flux of the transformer, thereby achieving the purpose of inrush current suppression.

[0034] In order to ensure that the magnetization process is simple and effective, and to minimize the change in the relevant characteristics of the transformer, it is preferred to use DC to magnetize the transformer. DC magnetization is defined as the process of using a DC power supply to provide magnetization for the iron core of the transformer. In this process, the change in magnetic flux (i.e. the flow of magnetic field through a certain surface) is important. The whole process has about 6 steps, from magnetization-stabilization-remanence.

[0035] Initial state: Before the start of DC magnetization, the iron core of the transformer is in an unmagnetized state, and there is some unknown magnetic flux on the iron core; Start of magnetization: When the DC current starts to flow through the winding of the transformer, a magnetic field is generated around it, in which the main magnetic circuit is in the iron core, and part of the leakage magnetic flux is transmitted through the air as a medium; Increase of magnetic flux: As the current increases, the magnetic field formed by the current passes through the iron core, causing an increase in magnetic flux. Since a DC power supply is used, the magnetic field and magnetic flux rise rapidly to a stable value in the early stage. The magnitude of the magnetic flux is proportional to the current passing through the coil. This increase is nonlinear because the magnetic permeability of the core material changes with the increase in magnetization; Steady state: As the magnetic flux increases, a stable magnetic field is generated in the iron core until the current stabilizes, and the change in magnetic flux tends to be stable. At this time, the magnetic field in the transformer core remains basically unchanged unless the DC power supply changes; After power failure: When the DC power supply is disconnected, the magnetization of the core will decrease, but it usually does not disappear completely, which leads to the so-called residual magnetism phenomenon. Due to the proportional relationship between current and magnetic field, controlling the size of DC current can adjust the size of magnetic flux flowing in the iron core. Different magnetic fluxes have different attenuation speeds and attenuation amounts after the DC current disappears.

[0036] Hysteresis means that when the magnetic flux changes, the magnetization state in the material does not immediately follow the change in magnetic flux. Specifically, in the transformer core, once the applied DC current or DC magnetic field is removed, the magnetic flux does not immediately drop to zero, but gradually decays exponentially. Influencing factors include the characteristics of the material, the applied magnetic field strength (DC), time, etc.

[0037] Flux attenuation is usually caused by the following factors: ① Hysteresis loss: When the transformer's core cycles between different magnetization states, hysteresis loss occurs due to the magnetic hysteresis of the core material. Hysteresis loss causes part of the magnetic energy to be converted into heat energy, resulting in a reduction in magnetic flux. ② Eddy current loss: Under the action of the alternating magnetic field, eddy currents are generated in the core. These eddy currents generate an anti-magnetic field that resists changes in the original magnetic field, causing the magnetic flux to weaken. ③ Aging of core materials: Long-term operation and environmental factors (such as temperature changes and mechanical stress) may cause the performance of the core material to degrade, thereby affecting its magnetic permeability and causing flux attenuation.

[0038] Based on the above analysis, DC magnetization is mainly divided into two processes. The first is the process of DC establishing a magnetic field. This process is relatively fast, even if its magnetization process is nonlinear. The main reason is that the magnetic permeability of the core material changes with the increase of magnetization degree. The magnitude of the magnetic flux is proportional to the current passing through the coil. After the magnetic field is removed, the attenuation of the magnetic flux in the core is mainly affected by the characteristics of the material, the maximum magnetization intensity, the time of magnetic field application, etc. Generally speaking, the attenuation rate is faster at the beginning, then gradually slows down, and finally stabilizes. The default magnetization flux stability section is proportional to the magnetization current. After the magnetization current is removed, it enters the exponential decay mode, and the decay is exponential.

[0039] The data in the DC magnetization process is modeled. The DC magnetization flux value is: flux s = flux DC -flux sj . where flux DC The flux in the iron core of the constant magnetic field created by the applied DC is positively correlated with the current, that is: flux DC =K DC i dc , K DC is the coefficient of magnetizing current and magnetic flux, i dc is the magnetizing current; flux sj It means the attenuation of magnetic flux after the DC is removed. The attenuation is exponential. In the same transformer, the parameters such as temperature, material, and magnetic field history are not important. The main parameter is the attenuation coefficient, which is a function of time. sj =B*a t, a is the attenuation constant, B is the initial value, that is, the flux of the magnetic field established by the DC current, t is the time, flux sj is the decaying magnetic flux as a function of time.

[0040] However, in actual engineering use, since the flux decays exponentially, the decay speed is very fast. After the magnetizing current stops, and then in the process of closing the transformer, it usually decays to a stable value, so it can be considered as a function independent of time, and is only a function related to the magnetizing flux. sj =f(flux sj ).

[0041] In summary, the DC magnetizing flux value can be expressed as flux s =K DC i dc -f(flux sj ), at the moment of closing, the total magnetic flux is the sum of the charged magnetic flux and the closing bias magnetic flux, that is, flux z = flux s +flux p , where flux z Expressed as total magnetic flux, flux p It is expressed as the bias magnetism at the moment of closing, which can be calculated based on the closing angle, that is, flux p =cos(α), where α is the closing angle. Therefore, the principle of mutual restraint between residual magnetism and bias magnetism can be used, that is, the closing angle can be expressed as α=arccos(-flux s ).

[0042] After the above model is derived and verified through laboratory experiments, the residual magnetism can be calculated through the phase-controlled closing angle, so the relationship between the residual magnetism (optimal closing angle) and the magnetizing current can be found. By analogy, a magnetizing model is established to realize different transformers. After the transformer parameters are input, the magnetizing current and the optimal closing angle are automatically output.

[0043] After calculating the rated current and other parameters according to the parameters of the transformer, in order to prevent the impact on the transformer, the corresponding overvoltage and overcurrent protection are designed. When the calculated current value is too large, its protection logic is activated and the magnetizing current value is recalculated. Since the transformer winding is an inductive load, when the magnetizing current is too large, according to the characteristics of the inductive element, a large induced voltage may be generated. The voltage value of the transformer-related parameter calculator is used to check whether it meets the normal voltage range. When the overvoltage and overcurrent parameters are met, the magnetizing current and the optimal closing angle are output, and finally the closing is performed through phase control.

[0044] Phase-controlled closing is performed according to the optimal closing angle output after magnetization is completed, so that the bias magnetic energy generated by closing the switch is offset by the residual magnetism in the iron core after magnetization, thereby achieving the effect of suppressing inrush current.

[0045] The present invention calculates the magnetizing current through a magnetizing current model, outputs the best closing angle after magnetizing, and closes the circuit breaker through a phase-controlled manner, thereby effectively suppressing the inrush current. By applying a corresponding direct current to the transformer winding, a corresponding magnetic field can be established on the transformer core. When the current disappears, due to the ferromagnetic characteristics of the transformer, some magnetic flux remains on the core. By using the magnetizing method reasonably, the residual magnetism of the transformer after maintenance is in a preset state. At the moment of closing, the three-phase residual magnetism and the bias magnetism cancel each other out so that the total magnetic flux of the three-phase sequence is kept at a small state, thereby effectively suppressing the inrush current.

[0046] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for suppressing inrush current of a core-type transformer, characterized in that: The method comprises the following steps: Step 1: Obtain the magnetizing current value of the transformer; Step 2: Model the data during DC magnetization. The DC magnetization flux value is flux s = flux DC -flux sj , where flux DC The flux in the iron core of the constant magnetic field created by the applied DC is calculated from the magnetizing current. sj It indicates the attenuation of magnetic flux after the DC magnetizing current is removed. Since the magnetic flux decays exponentially and the decay speed is very fast, it is considered that flux sj is a function related to the magnetizing flux sj =f(flux sj ), at the moment of closing, the total magnetic flux is the sum of the charged magnetic flux and the closing bias magnetic flux, that is, flux z = flux s +flux p , where flux z Expressed as total magnetic flux, flux p It is expressed as the bias magnetism at the moment of closing, which is calculated based on the closing angle. The closing angle is calculated based on the principle of mutual restraint between residual magnetism and bias magnetism. Step 3: The relationship between the closing angle and the magnetizing current is obtained through step 2. By analogy, a magnetizing model is established. After the transformer parameters are input, the magnetizing current and the optimal closing angle are automatically output; Step 4: Determine whether the magnetizing current output in step 3 exceeds the overcurrent protection value. If so, return to step 2 to recalculate. If not, execute step 5. Step 5: Calculate the voltage value of the transformer element according to the magnetizing current output in step 3, and determine whether it exceeds the overvoltage protection value. If it exceeds, return to step 2 to recalculate; if it does not exceed, execute step 6; Step 6: Perform phase-controlled closing according to the closing angle output after magnetization is completed, so that the bias magnetic energy generated by closing the circuit can offset the residual magnetism in the iron core after magnetization, thereby suppressing the inrush current.

2. The inrush current suppression method for a core-type transformer according to claim 1, characterized in that: In step 2, flux DC The size is positively correlated with the current size, that is, flux DC =K DC i dc , K DC is the coefficient of magnetizing current and magnetic flux, i dc is the magnetizing current, the DC magnetizing flux value flux s =K DC i dc -f(flux sj ).

3. The inrush current suppression method for a core-type transformer according to claim 1, characterized in that: In step 2, flux sj Through the formula flux sj =B*a t , a is the attenuation constant, B is the magnetic flux of the magnetic field established by the DC current, and t is the time.

4. The inrush current suppression method for a core-type transformer according to claim 1, characterized in that: In step 2, flux p The calculation formula is flux p =cos(α), where α is the closing angle.

5. The inrush current suppression method for a core-type transformer according to claim 1, characterized in that: In step 2, the closing angle is expressed as α = arccos (-flux s ).

6. The inrush current suppression method for a core-type transformer according to claim 1, characterized in that: Step 6: After the magnetization is completed, the magnetization report and the optimal closing angle of the phase control are output.

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