CN112134475A - Modularized multi-level rectifier without bus voltage sensor and controllable rectification method thereof - Google Patents
Modularized multi-level rectifier without bus voltage sensor and controllable rectification method thereof Download PDFInfo
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- H—ELECTRICITY
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- 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
- H02M7/00—Conversion of ac power input into dc power output; Conversion of dc power input into ac power output
- H02M7/02—Conversion of ac power input into dc power output without possibility of reversal
- H02M7/04—Conversion of ac power input into dc power output without possibility of reversal by static converters
- H02M7/12—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode
- H02M7/21—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal
- H02M7/217—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only
- H02M7/219—Conversion of ac power input into dc power output without possibility of reversal by static converters using discharge tubes with control electrode or semiconductor devices with control electrode using devices of a triode or transistor type requiring continuous application of a control signal using semiconductor devices only in a bridge configuration
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- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract
The invention relates to a modularized multi-level rectifier without a bus voltage sensor and a controllable rectification method thereof, belonging to the technical field of power electronics. When the modular multilevel converter is used as a rectifier, an output end direct current voltage sensor is removed, and efficient closed-loop control of system output voltage and input current is also achieved. The method comprises the following steps: obtaining a direct current bus voltage value through a submodule capacitor voltage value of an MMC-Rectifier and a prediction algorithm, and replacing the traditional method of obtaining the direct current bus voltage value by using a high-voltage direct current voltage sensor; the decoupling and closed-loop control of the input active power and the reactive power are realized by distributing the current corresponding to the active power to the q-axis current and distributing the current component corresponding to the reactive power to the d-axis current. The method can reduce the use of the sensing device, reduce the cost, ensure the normal work of the MMC-Rectifier when the direct current bus voltage sensing device fails, greatly enhance the system reliability and have popularization and application values.
Description
Technical Field
The invention belongs to the technical field of power electronics, and particularly relates to a modular multi-level rectifier without a bus voltage sensor and a controllable rectification method thereof.
Background
The Modular Multilevel Converter (MMC) can be used as a rectifier and is a core device for realizing grid-connected connection of an AC power grid and a DC power grid. It has a number of advantages including: modular structure, evenly distributed device voltage, little filter inductance, low harmonic distortion, and the like. At present, a Rectifier (MMC-Rectifier) based on an MMC is gradually applied to the fields of new energy grid connection, medium and high voltage motor drive, static synchronous compensator (STACOM), flexible alternating current and direct current transmission (FACTs) and the like. When the MMC is used as a high-performance controllable rectifier bridge, the input end is alternating current power grid voltage, the output end is direct current voltage, and the system needs to realize two indispensable control targets: 1) wide-range adjustable control of the bus voltage at the output end, and 2) decoupling control of input active power and reactive power.
In the prior MMC-Rectifier system or rectifiers with other topological structures, in order to achieve decoupling control of direct current bus voltage and active/reactive power, a voltage sensing device is required to be arranged on a direct current bus at an output end to acquire an output voltage signal. However, the output voltage level of the actual MMC-Rectifier system is over several kilovolts or even tens of kilovolts, the bus high-voltage sensor needs to solve the problems of strong electrical insulation, strong electromagnetic interference and unstable signal transmission, and the manufacturing cost is high and the installation is complex.
Disclosure of Invention
In view of the above, the present invention provides a modular multi-level rectifier without a bus voltage sensor and a controllable rectifying method thereof.
Through research, the invention adopts the following technical scheme:
1. a modular multilevel current regulator without a bus voltage sensor, comprising:
each phase unit comprises an upper bridge arm and a lower bridge arm, and each bridge arm comprises N sub-modules and a bridge arm inductor L which are connected in seriesarmComposition is carried out;
the submodule consists of two switching devices and a capacitor;
a modular multi-level Rectifier (MMC-Rectifier) Rectifier bridge sensor detection system comprises a line current sensor, a line voltage sensor, a submodule capacitor voltage sensor and a bridge arm inductive current sensor, wherein an output end direct current bus voltage sensor is removed;
3 PI controllers, labeled PI1、PI2、PI3，PI1Closed loop control, PI, for DC bus voltage2Closed-loop control, PI, for the input of MMC active power current components of the power grid3Closed-loop control for the power grid input MMC-Rectifier reactive power current component;
the modular multilevel Rectifier (MMC-Rectifier) is characterized in that the input end of the modular multilevel Rectifier (MMC-Rectifier) is connected with the voltage of a power grid through a filter inductor, the output end of the modular multilevel Rectifier is a direct current bus voltage side, the power grid is a three-phase alternating current power grid, and the output end of the modular multilevel Rectifier is connected with a load or a direct current power grid.
Wherein, the line current sensor is a power grid input current sensor; the line voltage sensor is a power grid input voltage sensor.
2. In the method for the controllable rectification of the modular multilevel rectifier without the bus voltage sensor, the predicted voltage value output by the direct current bus is calculated according to the following formulas (1) to (4):
in the formula, Vcap_a(i),Vcap_b(i),Vcap_c(i)The capacitance voltage value of the ith sub-module of each phase in the three phase units of a, b and c, N is the number of modules on each bridge arm, and Vave_a,Vave_b,Vave_cIs the average value of the capacitor voltage of each phase submodule in a, b and c three-phase units, Vdc_out_preAnd outputting a real-time voltage value for the predicted direct current bus, wherein the capacitance voltage of the ith sub-module of each phase is measured by a sub-module capacitance voltage sensor in the MMC rectifier bridge sensor detection system.
The voltage value of the bus side can be accurately obtained by utilizing the voltage of the internal capacitor of the system through a bus voltage prediction algorithm, so that the high-voltage sensor on the output side is removed.
Preferably, the method further comprises a filtering algorithm, wherein the filtering algorithm is an average value of the direct-current voltage at the output end of the direct-current bus in one period and is used as a feedback value of the direct-current bus output voltage in the next control period, and the fluctuation of the direct-current voltage at the output end obtained by the prediction algorithm can be reduced through the filtering algorithm, so that the harmonic quantity of the input current of the power grid is reduced. Considering the influence of capacitance voltage fluctuation and signal interference on bus voltage and the predicted value of the bus voltage, a filtering algorithm needs to be added to reduce the voltage fluctuation amount, so as to obtain a more stable bus voltage value Vdc_out_filter. Otherwise, V with large fluctuation amountdc_out_preSignificant harmonic distortion of the grid input current can result.
Preferably, the DC voltage is determined by the value of the DC voltage required to be output by the rectifierThe bus voltage reference value is subtracted from the direct current bus voltage reference value by the direct current bus voltage feedback value, and the voltage difference value is used as PI1Input of controller, from PI1The output end of the controller obtains a reference quantity i of the current component of the active powerq_refTo make PI1The controller realizes closed-loop control of the voltage of the direct current bus and is used for adjusting the size of the output direct current voltage.
Preferably, the q-axis current reference, i.e. the active power current component reference i, is obtained by controlling and outputting the voltage of the direct current bus in a closed loop mannerq_refObtaining q-axis current feedback quantity i through Park conversion with three-phase power grid currentqThe difference value of the two is taken as q-axis PI2The input end and the output end of the controller are q-axis voltage reference quantity v required by MMC-Rectifier controlq_refTo make PI2The controller realizes closed-loop control of the MMC-reciifier active power current component input by the power grid, and is used for adjusting the input active power and the corresponding current component. Wherein, the input three-phase current i is obtained by a three-phase power grid input current sensora、ib、icThe grid angle wt can be obtained through a three-phase input grid voltage sensor, and the d-axis current i under the dq coordinate system is obtained through Park conversiondAnd q-axis current iq。
Preferably, the reactive power of the MMC-Rectifier is determined according to the reactive power compensation quantity required by the power grid, and the reference quantity of the d-axis current is id_refAnd obtaining d-axis current feedback quantity i through Park conversion with three-phase power grid currentdThe difference between the two is taken as d-axis PI3The input end and the output end of the controller are d-axis voltage reference quantity v required by MMC-recitifier controld_refTo make PI3The controller realizes the closed-loop control of the MMC-reciifier active power current component input by the power grid so as to adjust the magnitude of the input reactive power current component.
Obtaining the reference quantity v of the q-axis voltageq_refReference quantity v of d-axis voltaged_refAnd obtaining the three-phase output reference voltage V of the MMC-reciifier converter through Park inverse transformation at the power grid angle wta_ref,Vb_ref,Vc_ref(ii) a Will Va_ref,Vb_ref,Vc_refThe conversion is carried out to obtain the reference voltage on each bridge arm of the MMC-recitifier, the number of the access sub-modules on each bridge arm is obtained in a phase-shifting carrier mode, the balance control of the capacitance voltage is realized through a capacitance balance control algorithm, and a PWM control signal of each sub-module switch is generated.
The invention has the beneficial effects that:
1) the novel MMC-Rectifier system structure and the system control method provided by the invention can realize the stable regulation control of the direct current voltage at the output end of the system on the one hand and the decoupling control of the input active and reactive power on the other hand while not needing an output bus terminal voltage sensor, thereby realizing the independent control of the active power and the reactive power of an MMC-Rectifier transmitted by a power grid and reducing the harmonic quantity of the current of the power grid;
2) according to the novel MMC-Rectifier system control method provided by the invention, a high-voltage detection device at the direct-current voltage output side is removed through a bus voltage prediction algorithm, the voltage value at the output bus side is accurately obtained by using the voltage of a capacitor in the system, and the closed-loop control of the bus voltage is achieved;
3) the novel MMC-Rectifier system control method provided by the invention is simple and feasible, can reduce the use, installation and maintenance of a sensor, reduces the system cost, ensures that the MMC-Rectifier has the capability of normal work when a direct current bus voltage sensing device fails, greatly enhances the system reliability, and has wide popularization and application values.
Drawings
FIG. 1 is a new topology of the MMC-Rectifier of the present invention;
FIG. 2 is a schematic diagram of the MMC-Rectifier of the present invention showing the DC bus voltage to achieve stable closed loop control;
FIG. 3 is a schematic diagram of a MMC-Rectifier active current closed-loop control of a grid input of the MMC-Rectifier of the present invention;
FIG. 4 is a schematic diagram of a grid input MMC-Rectifier reactive current closed-loop control of the MMC-Rectifier of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention are clearly and completely described below with reference to the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 1, a modular multi-level Rectifier (MMC-Rectifier) system architecture includes:
each phase unit comprises an upper bridge arm and a lower bridge arm, and each bridge arm consists of N (N is 5) sub-modules connected in series and a bridge arm inductor LarmComposition is carried out; bridge arm inductance Larm0.15 mH;
the submodule consists of two switching devices and a capacitor; the capacitance value of the capacitor is 0.6 mF;
a modular multi-level Rectifier (MMC-Rectifier) Rectifier bridge sensor detection system comprises a power grid input current sensor, a power grid input voltage sensor, a sub-module capacitance voltage sensor and a bridge arm inductance current sensor, wherein an output end bus voltage sensor is removed;
3 PI controllers, PI1Closed loop control, PI, for DC bus voltage2And PI3The closed-loop control method is used for the closed-loop control of the MMC-Rectifier active power current component and the reactive power current component of the power grid input respectively;
the input terminal of the modular multilevel Rectifier (MMC-Rectifier) passes through a 1.7mH filter inductor (L)a,Lb,Lc) The output end of the direct current bus is connected with the voltage of a power grid, the output end of the direct current bus is a direct current bus voltage side, the input power grid is a 400V/60Hz three-phase alternating current power grid, and the output end of the direct current bus voltage side is connected with a load or a direct current power grid.
The MMC-Rectifier output direct-current voltage closed-loop control and power grid input current control method comprises the following steps:
(1) through an output direct current bus voltage prediction algorithm, on the basis of removing a bus high-voltage sensor, a voltage value of an output bus side is accurately obtained by using the voltage of a capacitor in a system, wherein the algorithm is as follows:
wherein, Vcap_a(i),Vcap_b(i),Vcap_c(i)The voltage value of the ith sub-module of each phase in the three-phase unit of a, b and c, N is the number of modules on each bridge arm, and Vave_a,Vave_b,Vave_cIs the average value of the capacitor voltage of each phase submodule in an a, b and c three-phase unit, Vdc_out_preThe method is a predicted real-time voltage value of the direct current bus, and the capacitance voltage of each submodule is measured by an MMC-Rectifier system inherent voltage sensor.
Wherein, N is 5, Vcap_a(i),Vcap_b(i),Vcap_c(i),Vave_a,Vave_b,Vave_cWithin the voltage range of 200 +/-20V, Vdc_out_preIn the range of about 1000 + -100V.
Directly applying a predicted value V of the bus voltage due to the influence of capacitance voltage fluctuation and signal interferencedc_out_preSignificant harmonic distortion of the input current results. The bus voltage value required by the control method needs to be added with a filtering algorithm to reduce the fluctuation, so that more stable direct current bus voltage is obtained and defined as Vdc_out_filter；
The filtering algorithm is to calculate V in a long time perioddc_out_preIs used as the stable output DC voltage value V of the next perioddc_out_filter. In this embodiment, 2ms is used as the calculation period, and 5kHz is used as the sampling frequency to sample 10V samples in one calculation perioddc_out_preData points, labeled Vdc_out_pre1,Vdc_out_pre2,…,Vdc_out_pre10,Vdc_out_filterThe final value is within 1000 + -50V, which can be calculated by the following formula.
(2) Obtaining input three-phase current i through three-phase power grid input current sensora,ib,ic(wherein, the actual value of the current is different due to different operating conditions), the grid angle wt can be obtained by inputting the grid voltage sensor in three phases, and the d-axis current i under the dq coordinate system can be obtained by park transformationdAnd q-axis current iq；
(3) Reference value V of DC bus voltage in this embodimentout_refAt 1000V, let Vout_refSubtracting the direct current voltage V in the step (1)dc_out_filterObtaining a voltage difference value; the voltage difference is taken as PI1Input of controller, PI1The output end of the controller obtains a q-axis current reference value iq_ref；PI1K of controllerpA value of 3, KiA value of 10; the iq_refThe value is used for adjusting the active current component of the power supply input MMC-Rectifier of the power grid, so that the input size of active power is adjusted.
(4) Defining the d-axis current reference value as id_refI of thed_refThe value is determined according to the input reactive power of the system, and in order to realize the effective control of the input current, two PI controllers (PI)2And PI3) The closed-loop control of (a) is applied to the d-axis and q-axis currents, respectively;
the method specifically comprises the following steps: the q-axis current reference i obtained in the step (3)q_refQ-axis electricity minus actual grid input currentStream iqAs q-axis PI2Controller input, q-axis PI2The output end of the controller is the q-axis voltage component vq_ref；
D-axis current reference i in step (4)d_refSubtracting the actual grid input current d-axis current idAs d-axis PI3Controller input, d-axis PI3The d-axis voltage component v at the output of the controllerd_ref(ii) a d. The q-axis current versus PI controller parameters are shown in table 1 below.
TABLE 1
| PI controller | PI2 | PI3 |
| Kp | 10 | 10 |
| |
50 | 50 |
(5) D-axis voltage component v obtained in the step (4)d_refAnd q-axis voltage component v in step (3)q_refAnd (4) obtaining the three-phase input reference voltage V of the MMC-Rectifier converter through park inverse transformation at the power grid angle wt in the step (3)a_ref,Vb_ref,Vc_ref。
FIG. 2 is a schematic diagram of the MMC-Rectifier system according to the present invention for achieving stable closed-loop control of the DC bus voltage. Wherein, the target value of the dc bus voltage is set to 1000V, and as can be seen from the analysis in fig. 2, the present invention realizes that the MMC-Rectifier system can complete the system pre-charging and control the dc bus voltage to be stably output at 1000V under the condition that the bus voltage sensing device is removed. The observed upper and lower bridge arm capacitor voltage is selected to be stabilized at a rated voltage value (1.0 per unit value), so that the closed-loop control of the output voltage of the MMC-Rectifier system is realized.
Fig. 3 is a schematic diagram of a closed-loop control of a power grid of the MMC-Rectifier system of the present invention to deliver a pure active current (power factor of 1) to the MMC-Rectifier. From the analysis in the figure, when the voltage output end of the direct current bus is loaded with 30kW from no load to the active load: 1) the output voltage is still stabilized at about 1000V of the reference value through closed-loop control; 2) the q-axis current is automatically adjusted to about a target value of 50A when an active load is loaded; 3) the input current of the power grid and the voltage of the power grid have no angle difference in the same phase and are pure active power; 4) the grid current exhibits a high quality sinusoidal waveform. Therefore, the method realizes the closed-loop control of the pure active current transmitted to the MMC-Rectifier by the power grid, and has the characteristic of low harmonic wave.
Fig. 4 is a schematic diagram of closed-loop control of a power grid of the MMC-Rectifier system to supply reactive current (power factor 0) to the MMC-Rectifier. From the analysis in the figure, when the output end is from no load to loaded reactive load: 1) the output voltage is still stabilized at about 1000V of the reference value through closed-loop control; 2) the q-axis current is automatically adjusted to about a target value of 50A when the reactive load is loaded; 3) the input current of the power grid and the voltage of the power grid have a phase difference of 90 degrees and are pure reactive power; 4) the grid current exhibits a high quality sinusoidal waveform. Therefore, the invention realizes the closed-loop control of the pure reactive current transmitted to the MMC-Rectifier by the power grid and has the characteristic of low harmonic wave.
It should be understood that the above description is only a preferred embodiment of the present invention, and not intended to limit the present invention, and although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that modifications and equivalents may be made in the technical solutions described in the foregoing embodiments, or some technical features may be substituted. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (6)
1. A modular multilevel rectifier without a bus voltage sensor, comprising:
each phase unit comprises an upper bridge arm and a lower bridge arm, and each bridge arm comprises N sub-modules and a bridge arm inductor L which are connected in seriesarmComposition is carried out;
the submodule consists of two switching devices and a capacitor;
a modular multi-level Rectifier (MMC-Rectifier) Rectifier bridge sensor detection system comprises a line current sensor, a line voltage sensor, a submodule capacitor voltage sensor and a bridge arm inductive current sensor, wherein an output end direct current bus voltage sensor is removed;
3 PI controllers, labeled PI1、PI2、PI3，PI1Closed loop control, PI, for DC bus voltage2Closed-loop control, PI, for the active power current component of the network input MMC-Rectifier3Closed-loop control for the power grid input MMC-Rectifier reactive power current component;
the modular multilevel Rectifier (MMC-Rectifier) is characterized in that the input end of the modular multilevel Rectifier (MMC-Rectifier) is connected with the voltage of a power grid through a filter inductor, the output end of the modular multilevel Rectifier is a direct current bus voltage side, the power grid is a three-phase alternating current power grid, and the output end of the modular multilevel Rectifier is connected with a load or a direct current power grid.
2. The method for controllable rectification of a modular multilevel rectifier without a bus voltage sensor according to claim 1, wherein the predicted voltage value of the dc bus output is calculated according to equations (1) to (4):
in the formula, Vcap_a(i),Vcap_b(i),Vcap_c(i)The capacitance voltage value of the ith sub-module of each phase in the three phase units of a, b and c, N is the number of modules on each bridge arm, and Vave_a,Vave_b,Vave_cIs the average value of the capacitor voltage of each phase submodule in a, b and c three-phase units, Vdc_out_preAnd outputting a real-time voltage value for the predicted direct current bus, wherein the capacitance voltage of the ith sub-module of each phase is measured by a sub-module capacitance voltage sensor in the MMC rectifier bridge sensor detection system.
3. The method for controllable rectification of the modular multilevel rectifier without the bus voltage sensor according to claim 2, further comprising a filtering algorithm, wherein the filtering algorithm is an average value of the dc voltage at the output end of the dc bus in one period and is used as a feedback value of the dc bus output voltage in the next control period, and the filtering algorithm can reduce the fluctuation of the dc voltage at the output end obtained by the prediction algorithm, so as to reduce the harmonic amount of the input current of the power grid.
4. The method of claim 3, wherein the DC bus voltage reference is determined by the DC voltage value required to be output by the rectifier, the DC bus voltage feedback value obtained by the filtering algorithm is subtracted from the DC bus voltage reference, and the voltage difference is used as PI1Input of controller, from PI1The output end of the controller obtains the current component of the active powerReference quantity iq_refTo make PI1The controller realizes closed-loop control of the voltage of the direct current bus and is used for adjusting the size of the output direct current voltage.
5. The method of claim 4, wherein the q-axis current reference, i.e., the active power current component reference i, is obtained by outputting the DC bus voltage in a closed-loop control mannerq_refObtaining q-axis current feedback quantity i through Park conversion with three-phase power grid currentqThe difference value of the two is taken as q-axis PI2The input end and the output end of the controller are q-axis voltage reference quantity required by MMC-Rectifier control, so that PI is realized2The controller realizes closed-loop control of the MMC-Rectifier active power current component input by the power grid, and is used for adjusting the input active power and the corresponding current component.
6. The method of claim 5, wherein the MMC-Rectifier reactive power is determined by the amount of reactive power compensation required by the grid, and the MMC-Rectifier reactive power is divided by the grid voltage d-axis voltage to obtain a d-axis current reference, id_refAnd obtaining d-axis current feedback quantity i through Park conversion with three-phase power grid currentdThe difference between the two is taken as d-axis PI3The input end and the output end of the controller are d-axis voltage reference quantity v required by MMC-Rectifier controld_refTo make PI3The controller realizes the closed-loop control of the input MMC-Rectifier reactive power current component of the power grid and is used for adjusting the input reactive power current component.
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