CN108390571B - Tundish electromagnetic heating power supply constant temperature control method - Google Patents

Tundish electromagnetic heating power supply constant temperature control method Download PDF

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CN108390571B
CN108390571B CN201810187409.7A CN201810187409A CN108390571B CN 108390571 B CN108390571 B CN 108390571B CN 201810187409 A CN201810187409 A CN 201810187409A CN 108390571 B CN108390571 B CN 108390571B
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CN108390571A (en
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罗安
岳雨霏
郭鹏
徐千鸣
姜捷
黎燕
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Hunan University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
    • H02M5/00—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases
    • H02M5/02—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc
    • H02M5/04—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters
    • H02M5/22—Conversion of ac power input into ac power output, e.g. for change of voltage, for change of frequency, for change of number of phases without intermediate conversion into dc by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/01—Arrangements for reducing harmonics or ripples
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
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    • Y02E40/40—Arrangements for reducing harmonics

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Abstract

The invention discloses a tundish electromagnetic heating power supply constant temperature control method, which aims at a tundish electromagnetic heating power supply adopting a preceding stage three-phase rectification and a subsequent stage full-bridge cascade structure, respectively establishes a three-phase current input and single-phase current output mathematical model, and adopts a power and current double-ring control mode to realize closed-loop control of single-phase load power and system input and output current; meanwhile, in order to inhibit three-phase input harmonic current generated by a variable frequency load, a harmonic suppression controller is added in input current closed-loop control, so that the electric energy quality of an input side is improved; and the feedforward control of the temperature of molten steel is added into the system, so that the output current of the electromagnetic heating power supply can be accurately tracked, the constant temperature control of the molten steel can be realized, and the continuous casting yield and the production efficiency of steel and iron can be improved.

Description

Tundish electromagnetic heating power supply constant temperature control method
Technical Field
The invention relates to a control method of a tundish electromagnetic heating power supply for refining continuous casting steel in the metallurgical industry. .
Background
In the metallurgical industry, tundish channel type induction heating and refining equipment is key equipment which is urgently needed for improving the quality of continuous casting steel in China and reducing the energy utilization rate, and has great significance for the development of national economy in China. The molten steel heating process of the continuous casting tundish is an important link in the production process of continuous casting steel, and the main problems of the link are temperature control and refining. Continuous casting practices show that constant-temperature pouring with low superheat plays an important role in stabilizing operation and improving quality of a casting blank, and the constant-temperature pouring with low superheat is beneficial to improving production quality of steel, however, among many continuous casting operation parameters, molten steel temperature or superheat is one of the most unpredictable variables, and how to control the variable is the goal pursued by continuous casting technology, and the only means at present is to use an external heat source to regulate and control the temperature of molten steel. Meanwhile, with the development of modern power electronic technology and power semiconductor technology, a high-power high-efficiency variable frequency power supply is gradually applied to steel induction heating technology, wherein the induction heating power supply serves as an electric energy conversion device to connect a three-phase power grid and an induction heater, so that the harmonic pollution to the power grid is reduced, and the low harmonic characteristic of output load current is ensured. The topology based on the full-bridge cascade multilevel converter structure has the characteristics of modularization, low harmonic wave, high redundancy and the like, so that the topology is widely concerned in medium and high voltage power supply occasions, the structure can achieve the purpose of alternating-alternating frequency amplitude modulation, and the topology is a research hotspot in the field of the topology aiming at the problems of how to quickly track input current and output current, ensure constant temperature heating of molten steel load and the like of a tundish electromagnetic heating system for continuous casting steel preparation.
Disclosure of Invention
The invention aims to solve the technical problem that the prior art is not enough, and provides a constant temperature control method for a tundish electromagnetic heating power supply, which is used for completing the rapid tracking of input and output currents, ensuring the constant temperature heating of molten steel in the continuous casting process and ensuring the stable and reliable work of the electromagnetic heating power supply.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows: a tundish electromagnetic heating power supply constant temperature control method comprises the following steps:
1) based on the basic structure of the tundish electromagnetic heating power supply, differential equations of a front-stage three-phase rectification circuit and a rear-stage single-phase inversion circuit are respectively established according to kirchhoff voltage law and kirchhoff current law;
2) discretizing the differential equation to obtain a discrete mathematical model;
3) based on the discrete mathematical model, the pre-stage three-phase rectification circuit is subjected to load power feedforward and input current closed-loop control, voltage ripples generated by output are superposed in a reference value of direct-current side capacitor voltage, the influence of direct-current side voltage control on input current control is reduced through proportional-integral control, harmonic components in the input current are suppressed, and the electric energy quality of the power grid current is improved; aiming at a rear-stage single-phase inverter circuit, a load power and temperature double feedforward control strategy is adopted to obtain an output current reference value, and closed-loop control of the output current is realized through proportion-resonance control, so that the effects of accurately tracking an output current instruction and ensuring constant temperature of a molten steel load are achieved.
The differential equation expression of the preceding stage three-phase rectification circuit is as follows:
Figure BDA0001590698000000021
wherein u issxAnd isxThree-phase AC input voltage and three-phase AC input current of the preceding three-phase rectifier circuit, respectively, L is inductance value connected to AC input side of the preceding three-phase rectifier circuit, U isdcIs the capacitor voltage on the DC side of the preceding three-phase rectification circuit, scxAn equivalent switching function of the three-phase input of the preceding-stage three-phase rectification circuit;
Figure BDA0001590698000000022
sxdefining the equivalent switching function of each phase of bridge arm of the preceding-stage three-phase rectification circuit; x is a, b, c.
The differential equation expression of the rear-stage single-phase inverter circuit is as follows:
Figure BDA0001590698000000023
wherein u isinvFor outputting voltage, L, to a subsequent single-phase inverter circuitoAnd RoOutput filter inductance and resistance u of the rear-stage single-phase inverter circuitoAs the load voltage, ioIs the load current.
The discrete mathematical model of the preceding stage three-phase rectification circuit is as follows:
Figure BDA0001590698000000024
wherein u iscx(k) Three-phase AC input voltage s representing the preceding three-phase rectification circuit at time kcx(k) Equivalent switching function for three-phase input at time k, usx(k) Is the three-phase grid voltage at time k,
Figure BDA0001590698000000025
and isx(k) Respectively serving as a reference value and an actual value of input current of a preceding-stage three-phase rectification circuit at the moment k; t is the sampling period.
The discrete mathematical model of the rear-stage single-phase inverter circuit is as follows:
Figure BDA0001590698000000031
wherein u isinv(k) The output voltage of the rear-stage single-phase inverter circuit at the moment k,
Figure BDA0001590698000000032
and io(k) Outputting a reference value and an actual value of the inverter current for the rear-stage single-phase inverter circuit at the moment k respectively; u. ofo(k) The load voltage at time k.
The implementation process of the closed-loop control method for the front-stage three-phase rectification circuit by adopting load power feedforward and input current comprises the following steps:
1) deducing ripple components delta u of capacitor voltage on the direct current side of a preceding-stage three-phase rectification circuit according to the fluctuation amount of output load power, calculating the power on the direct current side, deducing harmonic current generated on the input side of the preceding-stage three-phase rectification circuit due to the voltage ripple components delta u according to the modulation principle of the preceding-stage three-phase rectification circuit, and calculating the capacitor voltage ripple components delta u through the load power:
Figure BDA0001590698000000033
where δ is the amplitude of the voltage ripple, ωoIn order to output the frequency of the radio frequency,
Figure BDA0001590698000000034
is the initial phase angle of the voltage ripple Δ u.
2) And the delta u is superposed in a direct-current side capacitor voltage outer ring control system to realize compensation of voltage ripple components, and the input harmonic current is restrained through a proportional-integral controller, so that the power quality of the power grid side is improved.
The output of the proportional-integral controller is represented as:
Figure BDA0001590698000000035
wherein e isPIIs the output quantity, k, of a voltage closed-loop PI controllerpAnd kiProportional coefficient and integral coefficient of the PI controller,
Figure BDA0001590698000000036
is a capacitor voltage reference value u on the DC side of a preceding-stage three-phase rectification circuitdcThe actual capacitor voltage at the direct current side of the preceding-stage three-phase rectification circuit.
For the rear-stage single-phase inverter circuit, the concrete implementation process of the control strategy adopting double feedforward of load power and temperature comprises the following steps: obtaining an output current reference value meeting the load power required by the system according to the load power and the relation between the power and the current, simultaneously, taking the constant temperature control of the load into consideration, and superposing a feedforward component generated by temperature closed-loop control in the output current reference value to obtain a final output current reference value
Figure BDA0001590698000000037
Will actually output a current ioAnd its reference value
Figure BDA0001590698000000038
Comparing, and realizing output current i by closed-loop control of currentoThe accurate tracking of the load can ensure that the load operates under the constant temperature working condition.
Compared with the prior art, the invention has the beneficial effects that: the invention aims at a tundish electromagnetic heating power supply system of a three-phase PWM rectification and single-phase cascade multilevel inverter structure, firstly, a PWM rectification circuit with three-phase input and single-phase output and a mathematical model of a full-bridge cascade multilevel inverter circuit with single-phase intersection-alternating frequency output are established, discretization processing is carried out on the mathematical models of the rectification system and the inverter system by taking input and output currents as research objects, and an input and output current closed-loop control method based on load power and temperature feedforward is provided. The input current closed-loop control adopts load power feedforward and input harmonic compensation, the load power feedforward control realizes the real-time supply of energy required by the input to the load heating, the input harmonic compensation control can compensate the harmonic current generated by the load operation on the three-phase input side of the rectifier circuit, the distortion rate of a power grid is reduced, the stable operation of the system is considered, and the electric energy quality is improved; the output current closed-loop control adopts load power and temperature feedforward control, realizes accurate tracking of system output power, and simultaneously ensures constant temperature heating of continuous casting molten steel.
Drawings
Fig. 1 is a structure diagram of a tundish electromagnetic heating power supply based on a three-phase PWM rectification and single-phase full-bridge cascade multilevel inversion structure.
Fig. 2 is a block diagram of the overall control of the tundish electromagnetic heating power supply in an embodiment of the present invention.
Fig. 3 is a control block diagram of the input current of the tundish electromagnetic heating power supply in an embodiment of the invention.
Fig. 4 is a control block diagram of the output current of the tundish electromagnetic heating power supply in an embodiment of the invention.
Detailed Description
FIG. 1 is a structure diagram of a tundish electromagnetic heating power supply of a three-phase PWM rectification and single-phase full-bridge cascade multilevel inversion structure on which the invention is based, wherein ucx、isx(x ═ a, b, C) are the three-phase ac input voltage and current, respectively, L is the ac input side connection inductance, C1And C2A capacitor is connected in series on the rectifying side usxThe voltage before the three-phase alternating current is input into the inductor is isolated between the power grid side and the three-phase PWM rectifier through a multi-winding transformer, and the phase of each winding of the transformer is shifted by 30 degrees; u. ofoAnd ioRespectively output load voltage and current, CcomThe rear-stage single-phase cascade multilevel inverter circuit takes a full-bridge submodule as a power unit and cascades 6 power unit modules in total, a carrier phase-shift modulation strategy is adopted, and the voltage of a single submodule is assumed to be ucThen a single submodule may output-uc,0,ucThree levels, a cascade system can output-13 uc～13ucThere are 27 levels.
Firstly, a mathematical model of a topological structure of the tundish electromagnetic heating power supply is established, and a differential equation of a preceding-stage three-phase PWM rectifier is as follows:
Figure BDA0001590698000000051
in the formula usx(x＝a,b,cThe same applies below) and isxThree-phase AC input voltage and input current of the rectifier circuit, L is an AC input side connected inductor, UdcIs the DC side capacitor voltage, scxThe equivalent switching functions of the three-phase input of the rectification circuit can be respectively expressed as
Figure BDA0001590698000000052
Wherein s isxDefined as the equivalent switching function for each phase leg.
The differential equation for establishing the equivalent mathematical model of the rear-stage single-phase full-bridge cascade multilevel inverter is as follows:
Figure BDA0001590698000000053
in the formula uinvFor a single-phase inverter circuit output voltage, LoAnd RoOutput filter inductance and resistance u of inverter circuit, respectivelyoIs the load voltage.
Discretizing differential equations of a front-stage system and a rear-stage system of the tundish electromagnetic heating power supply, wherein a discretization mathematical model of the differential equations of the front-stage PWM rectifier is expressed as follows:
Figure BDA0001590698000000054
wherein u iscx(k) Representing the three-phase AC input voltage, s, of the rectifier circuit at time kcx(k) Equivalent switch for three-phase input at time kFunction usx(k) Is the three-phase grid voltage at time k,
Figure BDA0001590698000000055
and isx(k) The reference value and the actual value of the input current of the three-phase rectification circuit at the moment k are respectively, and T is a sampling period.
The discretization mathematical model of the differential equation of the rear-stage single-phase inverter circuit is as follows:
Figure BDA0001590698000000056
wherein u isinv(k) For the output voltage of the single-phase inverter circuit at the moment k,
Figure BDA0001590698000000057
and io(k) Respectively outputting a reference value and an actual value u of the inverter current at the moment ko(k) T is the sampling period for the load voltage at time k.
Fig. 2 is a block diagram of the overall control of the tundish electromagnetic heating power supply in an embodiment of the present invention. The tundish electromagnetic heating power supply control system provided by the invention mainly comprises three control targets: the method mainly comprises the following steps of:
aiming at the closed-loop control of the input current of the preceding-stage three-phase PWM rectifier, a control method of load power feedforward and direct-current side harmonic ripple voltage compensation is adopted, and the reference value of the input current
Figure BDA0001590698000000061
The control method is obtained by load power feedforward control and direct-current side capacitor voltage closed-loop control. In the input current control system of figure 2,
Figure BDA0001590698000000062
for the load power command value, the power and current relation Po＝f(is) The magnitude of the input current command value to meet the load power demand can be obtained,
Figure BDA0001590698000000063
and udcReference and actual values, i, of the DC-side capacitor voltage, respectivelysxIs the actual value of the input current to the three-phase rectifier.
Aiming at the closed-loop control of the output current of the rear-stage single-phase full-bridge cascade multilevel inverter, the control method of load power feedforward and load temperature feedforward is adopted to output the current reference value
Figure BDA0001590698000000064
Obtained by feed-forward control of the load power and closed-loop control of the load temperature. In the output current control system of FIG. 2, the relationship P is also usedo＝f(io) The amplitude value, T, of the output current instruction value meeting the load power requirement can be obtained*And T is a temperature reference value and a temperature sampling value of the electromagnetic heating load, ioAnd outputting the actual current value for the electromagnetic heating power supply.
In the output current closed-loop control system, the control method is implemented by controlling the output current command value
Figure BDA0001590698000000065
The feedforward output of the load temperature is superposed to realize the constant temperature control of the heating process of the load molten steel and the constant temperature overheating control of the production process of the continuous casting steel, and the constant temperature control of the system is realized by a load temperature feedforward control module in the figure 2.
Fig. 3 is a control block diagram of the input current of the tundish electromagnetic heating power supply in an embodiment of the invention. The control system of the input current of the tundish electromagnetic heating power supply mainly realizes two control targets: the method is characterized in that the input current accurately tracks and improves the power quality of a power grid side, and the input current distortion rate is reduced, and the specific implementation method comprises the following steps:
firstly, according to the relation P of load power and input currento＝f(is) Obtaining the input current instruction amplitude I meeting the load power requirementpExpressed as follows:
Figure BDA0001590698000000066
Ucx(rms)is an effective value of the AC input voltage of the rectifier,
Figure BDA00015906980000000610
the current amplitude is the load power instruction valuepAdding the instruction current I as load feedforward compensation to the output of the DC side capacitor voltage closed-loop PI controllerdcIn (1).
The current instruction value amplitude I can be obtained by the closed-loop PI control of the direct current side capacitor voltagedcIf the input harmonic suppression control based on the direct-current side voltage ripple calculation unit is not considered, the voltage ripple component in the direct-current side voltage is superposed to the input current command through the PI control
Figure BDA0001590698000000067
The specific principle analysis is as follows:
let the output voltage and current be
Figure BDA0001590698000000068
UomAnd IomIs the amplitude of the output voltage, current, omegaoAnd
Figure BDA0001590698000000069
for the initial phase angle of the output frequency and current, the fluctuation component of the output power is
Figure BDA0001590698000000071
While the output power fluctuates
Figure BDA0001590698000000072
Mainly embodied on the dc-side capacitance, and therefore,
Figure BDA0001590698000000073
resulting in a capacitor voltage udcGenerating 2 omegaoRipple voltage Δ u, and thus u, of the frequencydcCan be expressed as: u. ofdc＝Udc+Δu,UdcIs a steady-state value of the direct current of the voltage,the ripple can be expressed as
Figure BDA0001590698000000074
δ and
Figure BDA0001590698000000075
amplitude and initial phase angle of ripple Δ u, respectively, due to IdcRegulated by a voltage PI controller, so that the delta u generates harmonic components in the input current reference value
Figure BDA0001590698000000076
The input harmonic current can be expressed as
Figure BDA0001590698000000077
Omega is the angular frequency of the grid, kpIs the proportionality coefficient of the voltage PI controller. As can be seen from the formula (9),
Figure BDA0001590698000000078
containing 2 omegaoHarmonic current component of frequency + -omega, amplitude of (k)pDelta)/2, the current distortion rate of the power grid is increased, and the improvement of the power quality is not facilitated.
In order to suppress the harmonic component of the input current, the ripple component Δ u calculated by the superposition formula (8) in the voltage closed-loop control system shown in fig. 3 is needed to avoid the voltage harmonic generated by the output from entering the current reference value of the power grid, and the input current command is analyzed based on the principle
Figure BDA0001590698000000079
Current amplitude I ofsxCan be expressed as
Figure BDA00015906980000000710
Second, combining the input current command
Figure BDA00015906980000000711
And the actual value isxWhen the three-phase discrete mathematical model of the formula (4) is converted into a two-phase static αβ coordinate system to control the input current, the discrete mathematical model of the rectifier under the αβ coordinate system is
Figure BDA00015906980000000712
In the formula (11), the reaction mixture is,
Figure BDA00015906980000000713
and the actual value isα(k)，isβ(k) Respectively an input current instruction and an actual value at the moment k in an αβ coordinate systemsα(k)，usβ(k) And ucα(k)，ucβ(k) Respectively, the grid-connected voltage and the ac input voltage of the rectifier under the αβ coordinate system at time k in fig. 3, PR is a Proportional Resonance (PR) controller,
Figure BDA00015906980000000714
distributing a trigger pulse signal s of a power switch device in a three-phase PWM rectifier for a modulation wave of a three-phase bridge arm through carrier phase shift modulation1～s6。
Fig. 4 is a control block diagram of the output current of the tundish electromagnetic heating power supply in an embodiment of the invention. The control system for the output current of the tundish electromagnetic heating power supply mainly realizes two control targets: the accurate tracking of the output current ensures the constant temperature overheating control of the molten steel load, and the specific implementation method comprises the following steps:
setting the load impedance of molten steel as ZoThen output current command to meet load power demand
Figure BDA0001590698000000081
Effective value
Figure BDA0001590698000000082
This can be calculated by equation (12).
Figure BDA0001590698000000083
Meanwhile, in order to ensure that the molten steel load can operate in a constant-temperature and overheating environment, the method is characterized in that
Figure BDA0001590698000000084
Middle-superposition load temperature closed-loop PItAn output signal of the controller is
Figure BDA0001590698000000085
Amplitude of
Figure BDA0001590698000000086
Can be obtained by the calculation of equation (13).
Figure BDA0001590698000000087
In formula (13), T*And T is the reference value and the actual sampling value of the load temperature, kptAnd kitAnd a PR controller is adopted for the output current for feeding forward the proportional and integral coefficients of the PI controller for temperature, so that the accurate tracking of the instruction current and the constant temperature control of the molten steel are realized.

Claims (5)

1. A tundish electromagnetic heating power supply constant temperature control method is characterized by comprising the following steps:
1) based on the basic structure of the tundish electromagnetic heating power supply, differential equations of a front-stage three-phase rectification circuit and a rear-stage single-phase inversion circuit are respectively established according to kirchhoff voltage law and kirchhoff current law; the differential equation expression of the preceding stage three-phase rectification circuit is as follows:
Figure FDA0002359629260000011
wherein u issxAnd isxThree-phase AC input voltage and three-phase AC input current of the preceding three-phase rectifier circuit, respectively, L is inductance value connected to AC input side of the preceding three-phase rectifier circuit, U isdcIs a front stageDC side capacitor voltage s of three-phase rectification circuitcxAn equivalent switching function of the three-phase input of the preceding-stage three-phase rectification circuit;
Figure FDA0002359629260000012
sxdefining the equivalent switching function of each phase of bridge arm of the preceding-stage three-phase rectification circuit; x is a, b, c; the discrete mathematical model of the preceding stage three-phase rectification circuit is as follows:
Figure FDA0002359629260000013
wherein u iscx(k) Three-phase AC input voltage s representing the preceding three-phase rectification circuit at time kcx(k) Equivalent switching function for three-phase input at time k, usx(k) Is the three-phase grid voltage at time k,
Figure FDA0002359629260000014
and isx(k) Respectively serving as a reference value and an actual value of input current of a preceding-stage three-phase rectification circuit at the moment k; t is a sampling period;
the discrete mathematical model of the rear-stage single-phase inverter circuit is as follows:
Figure FDA0002359629260000015
wherein u isinv(k) The output voltage of the rear-stage single-phase inverter circuit at the moment k,
Figure FDA0002359629260000016
and io(k) Outputting a reference value and an actual value of the inverter current for the rear-stage single-phase inverter circuit at the moment k respectively; u. ofo(k) Load voltage at time k, LoAnd RoThe output filter inductor and the output filter resistor of the rear-stage single-phase inverter circuit are respectively;
2) discretizing the differential equation to obtain a discrete mathematical model;
3) based on the discrete mathematical model, the pre-stage three-phase rectification circuit is subjected to load power feedforward and input current closed-loop control, voltage ripples generated by output are superposed in a reference value of direct-current side capacitor voltage, the influence of direct-current side voltage control on input current control is reduced through proportional-integral control, harmonic components in the input current are suppressed, and the electric energy quality of the power grid current is improved; aiming at a rear-stage single-phase inverter circuit, a load power and temperature double feedforward control strategy is adopted to obtain an output current reference value, and closed-loop control of the output current is realized through proportion-resonance control, so that the effects of accurately tracking an output current instruction and ensuring constant temperature of a molten steel load are achieved.
2. The tundish electromagnetic heating power supply constant temperature control method according to claim 1, wherein the differential equation expression of the later stage single-phase inverter circuit is as follows:
Figure FDA0002359629260000021
wherein u isinvFor outputting voltage, L, to a subsequent single-phase inverter circuitoAnd RoOutput filter inductance and resistance u of the rear-stage single-phase inverter circuitoAs the load voltage, ioIs the load current.
3. The tundish electromagnetic heating power supply constant temperature control method according to claim 1, wherein the closed loop control method of load power feedforward and input current for the preceding stage three-phase rectification circuit is implemented by the following steps:
1) deducing ripple components delta u of the direct-current side capacitor voltage of the preceding-stage three-phase rectification circuit according to the fluctuation amount of the output load power, calculating the direct-current side power, deducing the capacitor voltage ripple components delta u according to the modulation principle of the preceding-stage three-phase rectification circuit to cause harmonic current to be generated on the input side of the preceding-stage three-phase rectification circuit, and calculating the capacitor voltage ripple components delta u through the load power:
Figure FDA0002359629260000022
where δ is the amplitude of the voltage ripple, ωoIn order to output the frequency of the radio frequency,
Figure FDA0002359629260000023
is the initial phase angle of the voltage ripple wave delta u;
2) and the delta u is superposed in a direct-current side capacitor voltage outer ring control system to realize compensation of voltage ripple components, and the input harmonic current is restrained through a proportional-integral controller, so that the power quality of the power grid side is improved.
4. The tundish electromagnetic heating power supply thermostatic control method according to claim 3, wherein the output of the proportional-integral controller is expressed as:
Figure FDA0002359629260000024
wherein e isPIIs the output quantity, k, of a voltage closed-loop PI controllerpAnd kiProportional coefficient and integral coefficient of the PI controller,
Figure FDA0002359629260000025
is a capacitor voltage reference value u on the DC side of a preceding-stage three-phase rectification circuitdcThe actual capacitor voltage at the direct current side of the preceding-stage three-phase rectification circuit.
5. The tundish electromagnetic heating power supply constant temperature control method according to claim 1, wherein the specific implementation process of the control strategy of adopting double feedforward of load power and temperature for the rear-stage single-phase inverter circuit comprises the following steps: obtaining an output current reference value meeting the load power required by the system according to the load power and the relation between the power and the current, simultaneously, taking the constant temperature control of the load into consideration, and superposing a feedforward component generated by temperature closed-loop control in the output current reference value to obtain a final output current reference value
Figure FDA0002359629260000031
Will actually output a current ioAnd its reference value
Figure FDA0002359629260000032
Comparing, and realizing output current i by closed-loop control of currentoThe accurate tracking of the load can ensure that the load operates under the constant temperature working condition.
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