Disclosure of Invention
The invention aims to provide a method for solving hysteresis loss of a traction transformer by taking iron core magnetic circuit grading into account, which is realized by the following technical means:
1) because the existence of power electronic equipment and the nonlinear characteristic of an iron core, the exciting current has obvious low-order harmonic component, because the exciting current function meets the Dirichlet sufficiency condition, Fourier transform is carried out on the exciting current function for analyzing the harmonic property of the exciting current function, and the expansion formula is as follows:
in the formula of exciting current I
hIs decomposed into DC components
And fundamental wave and each subharmonic a orthogonal to each other
ncosn ω t and b
nsinn ω t. The current of the power supply system has no direct current component and has periodicityIn one period of the exciting current, the following equation can be simplified:
in the formula InCharacterised by the amplitude, I, of the fundamental and each harmonic of the exciting currentnThe values are found from the fourier decomposition properties:
because the power grid is a balanced three-phase system, even harmonics are mutually offset in the balanced three-phase system, the even harmonics of the exciting current can be approximately ignored, and the expression is simplified as follows:
in the formula, k belongs to {0,1,2,3 … }, since the harmonic amplitude is inversely proportional to the harmonic frequency and the higher harmonic amplitude is small, the calculation only considers the action of the fundamental wave and the third harmonic, the expression is further simplified:
Ih(t)≈I1cosωt+I3cos3ωt
considering that the magnetic hysteresis loss calculation needs to integrate the product of the magnetic field intensity and the magnetic flux density, the frequency of the magnetic field intensity is equal to the frequency of the exciting current, the frequency of the magnetic flux density is equal to the frequency of the exciting voltage, the exciting voltage is not distorted and is always the standard power frequency, and the integration of the product of physical quantities with different frequencies is equal to zero, so that only the fundamental component of the exciting current for determining the magnetic field intensity can be considered in the magnetic hysteresis loss calculation, and the exciting current expression is further simplified as follows:
Ih(t)≈I1cosωt
in the formula Ih(t) represents the excitation current of the traction transformer, I1、InRespectively representing the amplitude of the fundamental component and the nth harmonic component of the excitation current after Fourier decompositionAmplitude, ω is the angular frequency, which satisfies: ω is 2 pi f, f is the excitation frequency, t is the time;
2) because the silicon steel sheet adopted by the traction transformer iron core is in a cold-rolled orientation type, the silicon steel sheet is consistent with the optimal direction of the magnetic conductivity no matter in a core column, an iron yoke or a corner in the winding process, and the full current law can be realized
Normalized to
In the formula, H is the magnetic field intensity, N is the number of turns of the coil, and L is the length of a magnetic circuit where the geometric center of the cross section of the traction transformer iron core is located.
Meanwhile, the lengths of magnetic circuits where the geometric centers of all levels of cross sections of the iron core of the traction transformer are located are different, the magnetic field strengths of all levels of the iron core are respectively calculated, and the excitation current expression is substituted to obtain the magnetic field strength expression of all levels of the iron core of the traction transformer:
in the formula, Hi(t) represents the magnetic field strength of the ith stage of the traction transformer core, a and b represent the magnetic path length and the magnetic path width of the traction transformer core, RiRepresents the radius of the ith-level magnetic circuit arc segment;
3) the electromotive force formula induced by the coil electromagnetism is
In the formula E
1Is an effective value of the primary side induced voltage of the transformer
mRepresents the maximum value of the magnetic flux; due to the lower voltage drop at the primary side of the transformer, E is provided
1And U is approximately equal to U, wherein U is the effective value of the excitation voltage of the transformer. The two formulas are combined and the magnetic induction intensity is related to the magnetic flux
Substituting to obtain:
in the formula, BmRepresenting the maximum value of the magnetic flux density of the traction transformer iron core, S representing the cross section area of a magnetic circuit, and w and d representing the width of each stage of the traction transformer iron core and the thickness of a silicon steel sheet respectively;
4) because the magnetomotive force F-NI of each stage of magnetic circuit of the iron core of the traction transformer is equal, the cold-rolled oriented silicon steel sheet has one-way magnetic permeability, an insulating layer is arranged between each stage of the iron core, and the topology of each stage of the iron core can be regarded as parallel connection on the surface of the magnetic circuit. Parallel magnetic circuit, magnetic flux and reluctance RmInversely proportional, the reluctance expression is:
in the formula, mu is the permeability of the transformer core material, and because the magnetic circuits at different levels are the same in material and equal in cross-sectional area, the magnetic flux at each level of the core can be regarded as being inversely proportional to the length of the magnetic circuit with the geometric center, namely:
in the formula B
i(t) is the magnetic flux density of each core stage, R
1Representing the radius of the 1 st order magnetic circuit arc segment. The shortest primary magnetic circuit and the largest magnetic flux are considered, and the main magnetic flux lags behind the phase of the exciting current
The first stage magnetic circuit flux density may be expressed as:
further, substituting the first-stage magnetic circuit magnetic flux density expression into each-stage magnetic circuit magnetic flux density relational expression to obtain each-stage magnetic flux density expression of the traction transformer:
the magnetic flux density only has a fundamental frequency component because the magnetic flux density is determined by the excitation voltage, and the transformer excitation voltage is generally the power frequency voltage;
5) substituting the above formula into the definition of hysteresis loss of electromagnetism to obtain the average hysteresis loss P calculation formula of the traction transformer considering the grading of the iron core magnetic circuit under the high humidity environment:
the method has the advantages that a magnetic hysteresis loss calculation mode of the traction transformer considering magnetic circuit classification, which is more in line with material physical properties and operation conditions, is provided, and necessary data guarantee can be provided for the production optimization design and service performance evaluation of the traction transformer.
Detailed Description
The following describes the process of the present invention in detail with reference to the accompanying drawings. Because the existence of power electronic equipment and the nonlinear characteristic of an iron core, the exciting current has obvious low-order harmonic component, because the exciting current function meets the Dirichlet sufficiency condition, Fourier transform is carried out on the exciting current function for analyzing the harmonic property of the exciting current function, and the expansion formula is as follows:
in the formula of exciting current I
hIs decomposed into DC components
And fundamental wave and each subharmonic a orthogonal to each other
ncosn ω t and b
nsinn ω t. Because the current of the power supply system has no direct current component and has periodic property, in one period of the exciting current, the formula can be simplified as follows:
in the formula InCharacterised by the amplitude, I, of the fundamental and each harmonic of the exciting currentnThe values are found from the fourier decomposition properties:
because the power grid is a balanced three-phase system, even harmonics are mutually offset in the balanced three-phase system, the even harmonics of the exciting current can be approximately ignored, and the expression is simplified as follows:
in the formula, k belongs to {0,1,2,3 … }, since the harmonic amplitude is inversely proportional to the harmonic frequency and the higher harmonic amplitude is small, the calculation only considers the action of the fundamental wave and the third harmonic, the expression is further simplified:
Ih(t)≈I1cosωt+I3cos3ωt
the product of magnetic field strength and magnetic flux density is required to be integrated in consideration of hysteresis loss calculation, the frequency of the magnetic field strength is equal to the frequency of exciting current, the frequency of the magnetic flux density is equal to the frequency of exciting voltage, the exciting voltage is not distorted and is constant to standard power frequency, and the product integral of physical quantities with different frequencies is equal to zero, so that only the fundamental component of the exciting current for determining the magnetic field strength can be considered in the hysteresis loss calculation, and the exciting current expression is further simplified into Ih(t)≈I1cosωt
In the formula Ih(t) represents the excitation current of the traction transformer, I1、InRespectively represent the fundamental component amplitude and the nth harmonic component amplitude of the excitation current after Fourier decomposition, omega is angular frequency, and meets the following requirements: ω is 2 pi f, f is the excitation frequency, and t is the time.
Because the silicon steel sheet adopted by the traction transformer iron core is in a cold-rolled orientation type, the silicon steel sheet is consistent with the optimal direction of the magnetic conductivity no matter in a core column, an iron yoke or a corner in the winding process, and the full current law can be realized
Normalized to
In the formula, H is the magnetic field intensity, N is the number of turns of the coil, and L is the length of a magnetic circuit where the geometric center of the cross section of the traction transformer iron core is located.
Meanwhile, the magnetic field intensity of each level of the iron core is respectively calculated by considering the different lengths of the magnetic circuits of the geometric centers of the cross sections of each level of the iron core of the traction transformer.
Fig. 1 is a schematic diagram of a traction transformer core magnetic circuit in a hierarchical manner, wherein 8 levels are taken as an example in the diagram, and as can be seen from the diagram, each level of magnetic circuit is composed of four rectangles and four quarter circles, and the magnetic circuit length can be regarded as the sum of twice the core magnetic circuit length, twice the core magnetic circuit width and the circumference, so that an excitation current expression is substituted to obtain a magnetic field intensity expression of each level of the traction transformer core:
in the formula, Hi(t) represents the magnetic field strength of the ith stage of the traction transformer core, a and b represent the magnetic path length and the magnetic path width of the traction transformer core, RiRepresents the radius of the ith-order magnetic circuit arc segment.
The electromotive force formula induced by the coil electromagnetism is
In the formula E
1Is an effective value of the primary side induced voltage of the transformer
mRepresents the maximum value of the magnetic flux; due to the lower voltage drop at the primary side of the transformer, E is provided
1And U is approximately equal to U, wherein U is the effective value of the excitation voltage of the transformer. The two formulas are combined and the magnetic induction intensity is related to the magnetic flux
Substituting to obtain:
in the formula, BmThe maximum value of the magnetic flux density of the traction transformer iron core is represented, S represents the cross section area of a magnetic circuit, and w and d respectively represent the width of each stage of the traction transformer iron core and the thickness of a silicon steel sheet.
Because the magnetomotive force F-NI of each stage of magnetic circuit of the iron core of the traction transformer is equal, the cold-rolled oriented silicon steel sheet has one-way magnetic permeability, an insulating layer is arranged between each stage of the iron core, and the topology of each stage of the iron core can be regarded as parallel connection on the surface of the magnetic circuit. FIG. 2 is a topological diagram of the parallel connection of the iron core stages of the traction transformer in the invention, and the parallel magnetic circuit, the magnetic flux and the magnetic resistance R can be known from the diagrammInversely proportional, the reluctance expression is:
in the formula, mu is the permeability of the transformer core material, and because the magnetic circuits at different levels are the same in material and equal in cross-sectional area, the magnetic flux at each level of the core can be regarded as being inversely proportional to the length of the magnetic circuit with the geometric center, namely:
in the formula B
i(t) is the magnetic flux density of each core stage, R
1Representing the radius of the 1 st order magnetic circuit arc segment. Considering the first stage with shortest magnetic path and maximum magnetic fluxMagnetic flux lag excitation current phase
The first stage magnetic circuit flux density may be expressed as:
further, substituting the first-stage magnetic circuit magnetic flux density expression into each-stage magnetic circuit magnetic flux density relational expression to obtain each-stage magnetic flux density expression of the traction transformer:
the only fundamental frequency component of the magnetic flux density is because the magnetic flux density is determined by the excitation voltage, which is generally the line frequency voltage.
Substituting the above formula into the definition of hysteresis loss of electromagnetism to obtain the average hysteresis loss P calculation formula of the traction transformer taking the iron core magnetic circuit grading into account:
the method has the advantages that a magnetic hysteresis loss calculation mode of the traction transformer considering magnetic circuit classification, which is more in line with material physical properties and operation conditions, is provided, and necessary data guarantee can be provided for the production optimization design and service performance evaluation of the traction transformer.