CN110311407B - Double-mode seamless switching control method for cascade inverter based on voltage closed loop - Google Patents
Double-mode seamless switching control method for cascade inverter based on voltage closed loop Download PDFInfo
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Abstract
The invention discloses a voltage closed loop-based dual-mode seamless switching control method for a cascade inverter. When the current source mode is switched to the voltage source mode, closed-loop tracking of modulation wave voltage is completed through amplitude synchronization and phase synchronization, and seamless switching of the inverter from the current source mode to the voltage source mode is realized on the basis of the closed-loop tracking by adopting current feedforward control. When the voltage source mode is switched to the current source mode, the modulation wave voltage synchronous regulator is used for completing closed loop tracking of the modulation wave voltage, and on the basis, current feedforward control is adopted to realize seamless switching of the inverter from the voltage source mode to the current source mode. The method can realize seamless switching of two control modes and can also realize maximum tracking of photovoltaic power generation power.
Description
Technical Field
The invention relates to a voltage closed loop-based dual-mode seamless switching control method for a cascaded inverter, and belongs to the technical field of control over cascaded photovoltaic inverters.
Background
Grid-connected photovoltaic power generation is receiving attention because it provides clean energy and is environmentally friendly. In order to solve the problems of improving the efficiency of a photovoltaic system, reducing the power generation cost and the like, the cascaded H-bridge multi-level inverter becomes a research hotspot due to the advantages of easy modularization expansion, high system efficiency, small total harmonic distortion of grid-connected current and the like.
The output of the cascade H-bridge photovoltaic inverter is directly merged into the power grid, so that a step-up transformer at the grid-merging end is omitted, and the overall efficiency of the photovoltaic power generation system is improved. However, a large-scale new energy power generation system is often installed in a remote area, and the permeability of the new energy power generation system is also increasing, so that the new energy power generation system is often connected to a terminal weak power grid, and the impedance of the local power grid is often changed due to the changes of factors such as line impedance, the number of grid-connected units, load, and system operation mode. Under the weak power grid with impedance change characteristics, the cascaded H-bridge photovoltaic inverter is easy to oscillate by adopting a traditional current source mode grid connection due to small output impedance. At this time, by adopting the voltage source grid-connected mode based on droop control, stable control of inverter grid connection can be realized. Therefore, the method for researching the dual-mode seamless switching control method of the cascade inverter has outstanding engineering significance.
At present, scholars at home and abroad have related researches on a dual-mode seamless switching control method of a current source and a voltage source of an inverter. For example, beam-building steel, jinxinmin, wu scholarly and childhood are incorporated in 2014 4-month "switching technology of VCS mode and CCS mode of microgrid inverter" published in power grid technology "volume 38, phase 4. The article researches a mutual switching technology of a microgrid inverter based on a three-phase PWM rectifier topological structure between a PQ-controlled current source mode and a droop-controlled voltage source mode aiming at distributed power sources such as an energy storage device in a microgrid, and provides a method for closed-loop tracking mutual switching between different modes. However, the method aims at the problem of mutual switching between the current source mode and the voltage source mode of the inverter caused by two working conditions of grid connection and grid disconnection of the inverter, and how to realize the dual-mode seamless switching of the current source and the voltage source when the inverter is connected to the grid is not researched. Furthermore, the method is directed to a centralized inverter and seamless switching between cascaded inverter current source and voltage source modes is not investigated.
In the virtual synchronous generator power control strategy based on adaptive mode switching, which is published in 6 months in 2017 in the journal of electrotechnical science, volume 32, stage 12, of Shirongliang, Zhangxing, Xuhaizhen and the like, a control mode for switching a microgrid energy storage inverter according to whether the power grid frequency fluctuates sharply or not is proposed. When the frequency of the power grid fluctuates greatly, the inverter works in a current source mode, and the overshoot or over-discharge of the energy storage battery is prevented to prolong the service life of the energy storage battery. When the frequency of the power grid is normal, a voltage source mode based on droop control is adopted, and current sharing of the parallel micro-grid energy storage inverter is achieved. However, the method aims at a three-phase centralized energy storage inverter, and a cascade type inverter is not researched. In addition, since the dc side is an energy storage cell, maximum tracking of photovoltaic generated power is not studied.
In 2018, the document "A Novel Stability Improvement Strategy" for a Multi-Inverter System in a week Grid Utilizing Dual-Mode Control "Ming Li, Xing Zhang and Wei Zhao," energies, 2018, 11(8), 2144-. But the method does not study how the inverter achieves maximum tracking of the photovoltaic generated power in either current source mode or voltage source mode. Furthermore, the method is directed to a three-phase concentrated inverter, not involving a cascade-type photovoltaic inverter.
In summary, the current source and voltage source dual-mode seamless switching control method of the cascaded H-bridge photovoltaic grid-connected inverter mainly has the following problems:
(1) the current source and voltage source dual-mode switching control method of the inverter researched by the prior art is mainly used for solving the problem of mutual switching between current source and voltage source modes of the inverter caused by two working conditions of grid connection and grid disconnection of the inverter, and is less related to how to realize the dual-mode seamless switching of the current source and the voltage source when the inverter is connected to the grid.
(2) The current source and voltage source dual-mode switching control method of the inverter researched by the prior art is mainly used for a three-phase centralized energy storage inverter and cannot relate to a cascade type photovoltaic inverter.
(3) The current source and voltage source dual-mode switching control method of the inverter researched by the prior art is mainly used for a stable power source on the direct current side of the inverter, and the maximum power tracking of photovoltaic power generation is not considered.
Disclosure of Invention
The invention aims to solve the problem of overcoming the limitation of the scheme, and provides a voltage closed loop-based dual-mode seamless switching control method for a cascaded inverter aiming at the problem of dual-mode switching of a current source and a voltage source of the cascaded photovoltaic grid-connected inverter. The method adopts a voltage closed-loop control mode, so that the dual-mode seamless switching of the current source and the voltage source of the cascade type photovoltaic grid-connected inverter can be realized, and the maximum power tracking of photovoltaic power generation of each H-bridge unit can be realized.
In order to solve the technical problem, the invention provides a voltage closed loop-based dual-mode seamless switching control method for a cascade inverter s And a filter capacitor C f The control method comprises a current source mode seamless switching voltage source mode control method and a voltage source mode seamless switching current source mode control method:
the current source mode seamless switching voltage source mode control method comprises the following steps:
step 3, obtaining N H bridges according to the step 1Actual value V of DC side voltage of unit PVi And the direct current side voltage instruction values V of the N H-bridge units obtained in the step 2 PVi * Calculating the active power P of each H-bridge unit in the current source mode through the direct-current voltage regulator in the current source mode Ci Wherein i ═ 1, 2, 3.. N, calculated as:
wherein, K CVP Is the proportional coefficient, K, of a DC voltage regulator in a current source mode CVI The integral coefficient of the direct-current voltage regulator in the current source mode is 1, 2, 3.. N, and s is a Laplace operator;
step 4, obtaining the active power P of the N H-bridge units in the current source mode according to the step 3 Ci Calculating the sum of the active power of N H-bridge units in the current source mode and recording as P CT The calculation formula is as follows:
step 5, the actual value V of the voltage of the filter capacitor sampled in the step 1 is processed o Phase locking is carried out to obtain the voltage amplitude V of the power grid m And phase θ g (ii) a Converting the actual value V of the filter capacitor voltage sampled in the step 1 through virtual synchronous rotation coordinate o Converting the voltage into the active component V of the current source mode filter capacitor under the rotating coordinate system Cod And the voltage reactive component V of the current source mode filter capacitor Coq (ii) a The actual value I of the filter inductance current sampled in the step 1 is converted through a virtual synchronous rotation coordinate L Conversion into active component I of current source mode filter inductor current under rotating coordinate system CLd And current source mode filtering inductive current reactive component I CLq ；
Step 6, according to the sum P of the active powers of the N H-bridge units in the current source mode obtained in the step 4 CT And step 5, obtaining the voltage amplitude V of the power grid m Calculating to obtain a reference value of active components of the filter inductor current in the current source mode
The calculation formula is as follows:
and 7, filtering the active component I of the inductor current in the current source mode according to the current source mode obtained in the step 5 CLd Current source mode filter inductive current reactive component I CLq And 6, obtaining the reference value of the active component of the filter inductance current in the current source mode
Respectively calculating to obtain a d-axis PI regulation value E in the current source mode through an active current regulator in the current source mode and a reactive current regulator in the current source mode Cd Q-axis PI regulation value E in sum current source mode Cq The calculation formula is respectively:
wherein, K CiP Is the current regulator proportionality coefficient, K, in current source mode CiI Is the current regulator integral coefficient in the current source mode;
step 8, obtaining a d-axis PI regulating value E under the current source mode according to the step 7 Cd Q-axis PI regulation value E in current source mode Cq And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Obtaining the total modulation wave voltage V of the inverter in the current source mode through the virtual synchronous rotation inverse coordinate transformation Cr ；
Step 9, obtaining the active power P of the N H-bridge units in the current source mode according to the step 3 Ci And step 4, the sum P of the active power of the N H-bridge units in the current source mode is obtained CT Calculating the power distribution coefficient Factor of each H-bridge unit in the current source mode Ci 1, 2, 3.. N, calculated as:
step 11, before switching, latching the reference value of the active component of the filter inductor current in the current source mode of the last period as
Latching the power distribution coefficients of N H-bridge units in the last periodic current source mode and recording the power distribution coefficients as Factor Cmi And calculating the feedforward control quantity I of the direct-current voltage regulator in each voltage source mode VFeedi 1, 2, 3.. N, calculated as:
step 12, before switching, obtaining a d-axis PI regulating value E under the current source mode according to the step 7 Cd Q-axis PI regulation value E in current source mode Cq And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Calculating to obtain the amplitude V of the total modulation wave voltage of the inverter in the current source mode Crm And phase theta Cr The calculation formula is as follows:
step 13, the actual value V of the voltage of the filter capacitor sampled in the step 1 is used o Converting the virtual synchronous rotating coordinate into a voltage active component V of a voltage source mode filter capacitor under a rotating coordinate system Vod Voltage reactive component V of sum voltage source mode filter capacitor Voq ；
Step 14, the actual value I of the power grid current sampled in the step 1 is used s Converting the virtual synchronous rotating coordinate into a voltage source mode power grid current active component I under a rotating coordinate system VSd And reactive component I of voltage source mode grid current VSq ；
Step 15, obtaining the active component V of the voltage source mode filter capacitor according to the step 13 Vod Voltage reactive component V of voltage source mode filter capacitor Voq And step 14, obtaining the active component I of the voltage source mode grid current VSd Voltage source mode network current reactive component I VSq The average active power P output by the inverter under the voltage source mode is obtained by calculation and filtering through a first-order low-pass filter Vo And average reactive power Q Vo The calculation formula is as follows:
wherein τ is a first order low pass filter time constant;
step 16, obtaining the actual value V of the DC side voltage of the N H-bridge units according to the step 1 PVi And D, obtaining the direct-current side voltage instruction values V of the N H-bridge units obtained in the step 2 PVi * And step 11, the feedforward control quantity I of the direct-current voltage regulator under the N voltage source modes is obtained VFeedi Calculating the active power P of each H-bridge unit in the voltage source mode through the direct-current voltage regulator in the voltage source mode vi Wherein i ═ 1, 2, 3.. N, calculated as:
wherein, K VVP Is the proportionality coefficient, K, of a DC voltage regulator in voltage source mode VVI The integral coefficient of the direct-current voltage regulator in a voltage source mode is 1, 2, 3.. N;
step 17, obtaining the active power P of the N H-bridge units in the voltage source mode according to the step 16 Vi Calculating to obtain the sum of the active powers of N H-bridge units in the voltage source mode and recording as P VT The calculation formula is as follows:
step 18, according to the sum P of the active powers of the N H-bridge units in the voltage source mode obtained in step 17 VT And step 15, the average active power P of the inverter output under the voltage source mode is obtained Vo Calculating to obtain the output angular frequency omega of the inverter in the voltage source mode through an active power-frequency droop control equation Vo Angular frequency ω output in voltage source mode Vo Obtaining the output phase angle theta of the inverter in the voltage source mode through integration Vo The active power-frequency droop control equation is as follows:
ω Vo ＝ω * +m(P VT -P Vo )
wherein omega * The synchronous angular frequency of the power grid is adopted, and m is an active droop coefficient;
step 19, outputting the average reactive power Q by the inverter under the voltage source mode according to the obtained step 15 Vo The reference value of the active component of the voltage of the filter capacitor under the voltage source mode is obtained through calculation of a reactive power-voltage droop control equation
Reference value of voltage reactive component of filter capacitor in sum voltage source mode
The reactive power-voltage droop control equation is as follows:
wherein E is reference electromotive force, n is reactive droop coefficient, and Q * Giving a reactive power instruction for an upper layer;
wherein, K VSP1 For the proportional coefficient, K, of the amplitude synchronous regulator in voltage source mode VSI1 Integrating coefficient, K, of amplitude synchronous regulator in voltage source mode VSP2 Is the proportional coefficient, K, of the phase synchronous regulator in voltage source mode VSI2 Is the integral coefficient of the phase synchronous regulator in the voltage source mode;
step 21, obtaining the active power P of the N H-bridge units in the voltage source mode according to step 16 Vi And the sum P of the active powers of the N H-bridge units in the voltage source mode obtained in the step 17 VT Calculating the power distribution coefficient Factor of each H-bridge unit in the voltage source mode Vi 1, 2, 3.. N, calculated as:
step 22, obtaining the actual value V of the DC side voltage of the N H-bridge units according to the step 1 PVi And the amplitude V of the total modulation wave voltage of the inverter in the switching time voltage source mode obtained in the step 20 Vrm And phase θ Vr And N H-bridge unit power distribution coefficients Factor in the voltage source mode obtained in step 21 Vi Calculating to obtain the modulation wave signal M of each H-bridge unit in the voltage source mode during switching Vri 1, 2, 3.. N, calculated as:
the control method for the seamless switching of the current source mode in the voltage source mode comprises the following steps:
firstly, before switching, the modulating wave voltage synchronous regulator is started, and the reference value of the active component of the filter capacitor voltage in the voltage source mode obtained in the step 19 is used
Step 7, obtaining d-axis PI regulating value in last period current source mode
And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Obtaining a reference value of active components of filter inductor current in a current source mode before switching through a modulating wave voltage synchronous regulator
The calculation formula is as follows:
wherein, K CSP For modulating wave voltage synchronous regulator proportionality coefficient under current source mode, K CSI The integral coefficient of the modulation wave voltage synchronous regulator under the current source mode;
secondly, after the voltage synchronization of the modulated wave is completed,latching the reference value of the active component of the filter inductor current in the last period voltage source mode
Latching the power distribution coefficients of N H-bridge units in the last periodic voltage source mode and recording the power distribution coefficients as Factor Vmi Calculating to obtain feedforward control quantity I of the direct-current voltage regulator under each current source mode CFeedi 1, 2, 3.. N, calculated as:
finally, the feedforward control quantity I of the direct-current voltage regulator in the current source mode is converted into the feedforward control quantity I CFeedi The output of the direct-current voltage regulator in each current source mode is superposed, and the active power P 'of each H-bridge unit in the current source mode during switching is calculated' Ci Wherein i ═ 1, 2, 3.. N, calculated as:
compared with the prior art, the double-mode seamless switching control method of the cascade type inverter based on the voltage closed loop, disclosed by the invention, realizes the seamless switching of the double-mode control of the cascade H-bridge photovoltaic grid-connected inverter by adopting a voltage closed loop control strategy, and has the beneficial effects that:
1. the method provided by the invention can realize seamless switching of the cascade type photovoltaic inverter between two control modes of the current source and the voltage source.
2. The method provided by the invention can enable the cascade photovoltaic inverter to realize the maximum tracking of the photovoltaic power generation power under two control modes of the current source and the voltage source.
3. The dual-mode switching method provided by the invention is simple and easy for engineering realization.
Drawings
Fig. 1 is a block diagram of a main circuit topology of the cascaded inverter of the present invention.
Fig. 2 is an overall control block diagram of the dual-mode switching of the cascaded inverter of the present invention.
FIG. 3 shows the grid current I of the cascaded inverter switched from the current source mode to the voltage source mode when the control method of the present invention is applied s And the DC side voltage waveform of each H-bridge unit.
FIG. 4 shows the grid current I of the cascade inverter switched from voltage source mode to current source mode by the control method of the present invention s And the DC side voltage waveform of each H-bridge unit.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more clearly and clearly understood, the present invention will be further clearly and completely described below with reference to the accompanying drawings and embodiments.
Fig. 1 is a topology of a cascaded inverter according to an embodiment of the present invention, and as shown in the diagram, the cascaded inverter includes N H-bridge units with photovoltaic modules, and a filter inductor L s And a filter capacitor C f And (4) forming. Specifically, the direct current sides of the N H-bridge units are sequentially connected with N photovoltaic cell panels PV1, PV2 2 The maximum power point voltage is 30.40V, each photovoltaic cell panel is connected with each H-bridge unit through a 14.1mF capacitor, and a cascade system is connected with each H-bridge unit through a 1.5mH filter inductor L s And 55uF filter capacitor C f Is connected to the power grid.
The general control block diagram of the dual-mode switching of the invention is shown in fig. 2.
The current source mode seamless switching voltage source mode control method comprises the following steps:
In this embodiment, five H-bridge units are taken as an example, and the actual value of the dc-side voltage of each H-bridge unit at the beginning is V PV1 ＝V PV2 ＝V PV3 ＝V PV4 ＝V PV5 ＝35V。
in this embodiment, when the initial time T is 0.8s, each H-bridge unit operates at a rated temperature T of 25 ℃, and a rated illumination intensity E 1 ＝E 2 ＝E 3 ＝E 4 ＝E 5 ＝1000W/m 2 Under the condition of (3), obtaining a direct-current side voltage command value V of each H-bridge unit PV1 * V PV2 * ＝V PV3 * ＝V PV4 * V PV5 * 30.40V. When t is 1s, the temperature is kept unchanged, the light intensity of the 3 rd, 4 th and 5 th H bridges is kept unchanged, and the light intensity of the 1 st and 2 nd H bridges is respectively changed into E 1 ＝E 2 ＝800W/m 2 Obtaining the direct-current side voltage instruction value V of each H-bridge unit PV1 * ＝V PV2 * ＝30.57V，V PV3 * V PV4 * ＝V PV5 * ＝30.40V。
Step 3, obtaining the actual value V of the direct-current side voltage of the N H-bridge units according to the step 1 PVi And the direct current side voltage instruction values V of the N H-bridge units obtained in the step 2 PVi * Calculating the active power P of each H-bridge unit in the current source mode through the direct-current voltage regulator in the current source mode Ci Wherein i ═ 1, 2, 3.. N, calculated as:
wherein, K CVP Is the proportional coefficient, K, of a DC voltage regulator in current source mode CVI Is electricityThe direct current voltage regulator integral coefficient in the flow source mode, i is 1, 2, 3.. N, and s is a Laplace operator. The direct-current voltage regulator proportional coefficient and integral coefficient under the current source mode are designed according to a conventional grid-connected inverter, in the embodiment, K CVP ＝1，K CVI ＝10。
Step 4, obtaining the active power P of the N H-bridge units in the current source mode according to the step 3 Ci Calculating the sum of the active power of N H-bridge units in the current source mode and recording as P CT The calculation formula is as follows:
step 5, the actual value V of the voltage of the filter capacitor sampled in the step 1 is processed o Phase locking is carried out to obtain the voltage amplitude V of the power grid m And phase theta g (ii) a Converting the actual value V of the filter capacitor voltage sampled in the step 1 through virtual synchronous rotation coordinate o Converting the voltage into the active component V of the current source mode filter capacitor under the rotating coordinate system Cod And the voltage reactive component V of the current source mode filter capacitor Coq (ii) a The actual value I of the filter inductance current sampled in the step 1 is converted through a virtual synchronous rotation coordinate L Conversion into active component I of current source mode filter inductor current under rotating coordinate system CLd And current source mode filtering inductive current reactive component I CLq The calculation formula is respectively:
wherein k is 1 Is a gain factor. In this example, k 1 ＝0.5。
Step 6, according to the sum P of the active powers of the N H-bridge units in the current source mode obtained in the step 4 CT And step 5, obtaining the voltage amplitude V of the power grid m Calculating to obtain a reference value of active components of the filter inductor current in the current source mode
The calculation formula is as follows:
and 7, filtering the active component I of the inductor current in the current source mode according to the current source mode obtained in the step 5 CLd Current source mode filter inductive current reactive component I CLq And 6, obtaining the reference value of the active component of the filter inductance current in the current source mode
Respectively calculating to obtain a d-axis PI regulating value E in the current source mode through an active current regulator in the current source mode and a reactive current regulator in the current source mode Cd Q-axis PI regulation value E in sum current source mode Cq The calculation formula is respectively:
wherein, K CiP Is the current regulator proportionality coefficient, K, in current source mode CiI Is the current regulator integral coefficient in current source mode. The proportional coefficient and the integral coefficient of the current regulator in the current source mode are designed according to a conventional grid-connected inverter, in the embodiment, K CiP ＝4，K CiI ＝20。
Step 8, according to the d-axis PI regulating value E in the current source mode obtained in the step 7 Cd Q-axis PI regulation value E in current source mode Cq And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Obtaining the total modulation wave voltage V of the inverter in the current source mode through the virtual synchronous rotation inverse coordinate transformation Cr The calculation formula is as follows:
V Cr ＝(E Cd +V Cod )cosθ g +E Cq sinθ g
step 9, obtaining the active power P of the N H-bridge units in the current source mode according to the step 3 Ci And step 4, the sum P of the active powers of the N H-bridge units in the current source mode is obtained CT Calculating the power distribution coefficient Factor of each H-bridge unit in the current source mode Ci 1, 2, 3.. N, calculated as:
step 11, before switching, latching the reference value of the active component of the filter inductor current in the current source mode of the last period as
Latching the power distribution coefficients of N H-bridge units in the last periodic current source mode and recording the power distribution coefficients as Factor Cmi Calculating the feedforward control quantity I of the direct-current voltage regulator in each voltage source mode VFeedi 1, 2, 3.. N, calculated as:
step 12, before switching, obtaining a d-axis PI regulating value E under the current source mode according to the step 7 Cd And q-axis PI in current source modeRegulating value E Cq And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Calculating to obtain the amplitude V of the total modulation wave voltage of the inverter in the current source mode Crm And phase theta Cr The calculation formula is as follows:
step 13, the actual value V of the voltage of the filter capacitor sampled in the step 1 is used o Converting the virtual synchronous rotation coordinate into a voltage source mode filter capacitor voltage active component V under a rotation coordinate system Vod Voltage reactive component V of sum voltage source mode filter capacitor Voq The calculation formula is as follows:
wherein θ' Vo Is the output phase angle, k, of the inverter in the last cycle voltage source mode 2 Is a gain factor. In this example, k 2 ＝0.5。
Step 14, the actual value I of the power grid current sampled in the step 1 is obtained s Converting the virtual synchronous rotating coordinate into a voltage source mode power grid current active component I under a rotating coordinate system VSd And reactive component I of voltage source mode grid current VSq The calculation formula is as follows:
wherein k is 3 For the gain factor, k in this embodiment 3 ＝O.5。
Step 15, obtaining the active component V of the voltage source mode filter capacitor according to the step 13 Vod Voltage reactive component V of voltage source mode filter capacitor Voq And step 14, obtaining the active component I of the voltage source mode grid current VSd Voltage source mode network current reactive component I VSq The average active power P output by the inverter under the voltage source mode is obtained by calculation and filtering through a first-order low-pass filter Vo And average reactive power Q Vo The calculation formula is as follows:
where τ is the first-order low-pass filter time constant, and in this embodiment, τ is 1e-4 s.
Step 16, obtaining the actual value V of the DC side voltage of the N H-bridge units according to the step 1 PVi And D, obtaining the direct-current side voltage instruction values V of the N H-bridge units obtained in the step 2 PVi * And step 11, the feedforward control quantity I of the direct-current voltage regulator under the N voltage source modes is obtained VFeedi Calculating the active power P of each H-bridge unit in the voltage source mode through the direct-current voltage regulator in the voltage source mode Vi Wherein i ═ 1, 2, 3.. N, calculated as:
wherein, K VVP Is the proportional coefficient of a DC voltage regulator in voltage source mode, K VVI I is the integral coefficient of the dc voltage regulator in voltage source mode, 1, 2, 3.. N. The direct-current voltage regulator proportional coefficient and integral coefficient under the voltage source mode are designed according to a conventional grid-connected inverter, in the embodiment, K VVP ＝0.05，K VVI ＝40。
Step 17, obtaining the active power P of the N H-bridge units in the voltage source mode according to the step 16 Vi Calculating to obtain the sum of the active powers of N H-bridge units in the voltage source mode and recording as P VT The calculation formula is as follows:
step 18, obtaining according to step 17The sum P of the active powers of N H-bridge units in the voltage source mode VT And step 15, the average active power P of the inverter output under the voltage source mode is obtained Vo Calculating to obtain the output angular frequency omega of the inverter in the voltage source mode through an active power-frequency droop control equation Vo Angular frequency ω output in voltage source mode Vo Obtaining the output phase angle theta of the inverter in the voltage source mode through integration Vo The active power-frequency droop control equation is as follows:
ω Vo ＝ω * +m(P VT -P Vo )
wherein omega * And m is the power grid synchronous angular frequency, and the active droop coefficient. Grid synchronous angular frequency omega in this embodiment * 100 pi rad/s, and the active droop coefficient m is 6.28e-3 rad/W.
Step 19, outputting the average reactive power Q by the inverter under the voltage source mode according to the obtained step 15 Vo The reference value of the active component of the voltage of the filter capacitor under the voltage source mode is obtained through calculation of a reactive power-voltage droop control equation
Voltage reactive component reference value of filter capacitor in voltage source mode
The reactive power-voltage droop control equation is as follows:
wherein E is reference electromotive force, n is reactive droop coefficient, and Q * The reactive power command is given to the upper layer. In the embodiment, the reference electromotive force E is 120V, the reactive droop coefficient n is 5E-3V/Var, and the upper layer gives the reactive power instruction Q * ＝0Var。
wherein, K VSP1 For the proportional coefficient, K, of the amplitude synchronous regulator in voltage source mode VSI1 For amplitude synchronous regulators in voltage source mode, K VSP2 Is the proportional coefficient, K, of the phase synchronous regulator in voltage source mode VSI2 Is the phase synchronous regulator integral coefficient in voltage source mode. In this example, K VSP1 ＝0.5，K VSI1 ＝2，K VSP2 ＝40，K VSI2 ＝200。
Step 21, obtaining the active power P of the N H-bridge units in the voltage source mode according to the step 16 Vi And the sum P of the active power of the N H bridge units in the voltage source mode obtained in the step 17 VT Calculating the power distribution coefficient Factor of each H-bridge unit in the voltage source mode Vi 1, 2, 3.. N, calculated as:
step 22, obtaining the actual value V of the DC side voltage of the N H-bridge units according to the step 1 PVi And the amplitude V of the total modulation wave voltage of the inverter in the switching time voltage source mode obtained in the step 20 Vrm And phase theta Vr And N H-bridge unit power distribution coefficients Factor in the voltage source mode obtained in step 21 Vi Calculating to obtain the modulation wave signal M of each H-bridge unit in the voltage source mode during switching Vri ，i＝N, calculated as:
the control method for the seamless switching of the current source mode in the voltage source mode comprises the following steps:
firstly, before switching, the modulating wave voltage synchronous regulator is started, and the reference value of the active component of the filter capacitor voltage in the voltage source mode obtained in the step 19 is used
Step 7, obtaining d-axis PI regulating value in last period current source mode
And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Obtaining a reference value of active components of filter inductor current in a current source mode before switching through a modulating wave voltage synchronous regulator
The calculation formula is as follows:
wherein, K CSP For modulating wave voltage synchronous regulator proportionality coefficient under current source mode, K CSI And the integral coefficient of the modulation wave voltage synchronous regulator is adjusted under the current source mode. In this example, K CSP ＝0.2，K CSI ＝20。
Secondly, after the voltage synchronization of the modulation wave is finished, the reference value of the active component of the filter inductance current in the mode of the voltage source with the last period is latched and recorded as
Latching power distribution coefficients of N H-bridge units in the last periodic voltage source mode and recording as Factor Vmi Calculating to obtain the current source modeFeedforward control quantity I of DC voltage regulator CFeedi 1, 2, 3.. N, calculated as:
finally, the feedforward control quantity I of the direct-current voltage regulator in the current source mode is converted into the feedforward control quantity I CFeedi The output of the direct-current voltage regulator in each current source mode is superposed, and the active power P 'of each H-bridge unit in the current source mode during switching is calculated' Ci Wherein i ═ 1, 2, 3.. N, by the formula:
FIG. 3 shows the grid current I in the mode of switching the current source mode of the cascade inverter from the current source mode to the voltage source mode when the control method of the present invention is adopted s And DC side voltage waveforms of the H-bridge units. The voltage source mode is switched by the current source mode in 1.5s, the switching is completed in 1.54s, the current of the power grid has no impact, and the inverter is switched seamlessly. After the switching is finished, the direct current side voltage of each H bridge unit is controlled to be the direct current side voltage instruction value V PVi * I ═ 1, 2, 3.. N, i.e. at the maximum power point of each H-bridge unit.
FIG. 4 shows the grid current I of the cascade inverter switched from the voltage source mode to the current source mode when the control method of the present invention is adopted s And DC side voltage waveforms of the H-bridge units. And the current source mode is switched by the voltage source mode in 2s, the switching is completed in 2.04s, the current of the power grid has no impact, and the inverter is switched seamlessly. After the switching is finished, the direct-current side voltage of each H bridge unit is controlled to be at the direct-current side voltage instruction value V PVi * N, i.e. at the maximum power point of each H-bridge unit.
Claims (1)
1. A voltage closed loop based dual-mode seamless switching control method for a cascade inverter is disclosed, wherein the cascade inverter comprises N H-bridge units with photovoltaic components and a filter inductor L s And a filter capacitor C f The control method comprises a current source mode seamless switching voltage source mode control method and a voltage source mode seamless switching current source mode control method:
the current source mode seamless switching voltage source mode control method comprises the following steps:
step 1, sampling the direct current side voltage of each H-bridge unit, filtering the direct current side voltage by a 100Hz wave trap in sequence to obtain the actual direct current side voltage values of N H-bridge units, and recording the actual direct current side voltage values as V PVi N, · 1, 2, 3,. N; sampling the actual DC side current values of N H-bridge units and recording as I PVi N, · 1, 2, 3,. N; sampling the actual value of the filter inductance current and recording the actual value as I L (ii) a Sampling the actual value of the voltage of the filter capacitor and recording the actual value as V o (ii) a Sampling the actual value of the current of the power grid and recording the actual value as I s ；
Step 2, the actual value V of the direct-current side voltage of each H-bridge unit is obtained PVi Carrying out maximum power point tracking control to obtain direct current side voltage instruction values of N H-bridge units and recording the direct current side voltage instruction values as V PVi * Wherein i is 1, 2, 3.. N;
step 3, obtaining the actual value V of the direct-current side voltage of the N H-bridge units according to the step 1 PVi And the direct current side voltage instruction values V of the N H-bridge units obtained in the step 2 PVi * Calculating the active power P of each H-bridge unit in the current source mode through the direct-current voltage regulator in the current source mode Ci Wherein i ═ 1, 2, 3.. N, calculated as:
wherein, K CVP Is the proportional coefficient, K, of a DC voltage regulator in a current source mode CVI The integral coefficient of the direct-current voltage regulator in the current source mode is represented by i ═ 1, 2, 3.. N, and s is a Laplace operator;
step 4, obtaining the active power P of the N H-bridge units in the current source mode according to the step 3 Ci Calculating to obtain N H-bridge units under the current source modeIs denoted as P CT The calculation formula is as follows:
step 5, the actual value V of the voltage of the filter capacitor sampled in the step 1 is processed o Phase locking is carried out to obtain the voltage amplitude V of the power grid m And phase theta g (ii) a Converting the actual value V of the filter capacitor voltage sampled in the step 1 through virtual synchronous rotation coordinate o Converting the voltage into the active component V of the current source mode filter capacitor under the rotating coordinate system Cod And current source mode filter capacitor voltage reactive component V Coq (ii) a The actual value I of the filter inductance current sampled in the step 1 is converted by virtual synchronous rotation coordinates L Current source mode filter inductance current active component I converted into rotating coordinate system CLd And current source mode filtering inductive current reactive component I CLq ；
Step 6, according to the sum P of the active powers of the N H-bridge units in the current source mode obtained in the step 4 CT And step 5, obtaining the voltage amplitude V of the power grid m Calculating to obtain a reference value of active component of filter inductance current in a current source mode
The calculation formula is as follows:
and 7, filtering the active component I of the inductor current in the current source mode according to the current source mode obtained in the step 5 CLd Current source mode filtering inductive current reactive component I CLq And 6, obtaining the reference value of the active component of the filter inductance current in the current source mode
Respectively pass active current regulation in current source modeCalculating a d-axis PI regulating value E of the reactive current regulator in the economizer and current source mode Cd Q-axis PI regulation value E in sum current source mode Cq The calculation formula is respectively:
wherein, K CiP Is the current regulator proportionality coefficient, K, in current source mode CiI Is the current regulator integral coefficient in the current source mode;
step 8, obtaining a d-axis PI regulating value E under the current source mode according to the step 7 Cd Q-axis PI regulation value E in current source mode Cq And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Obtaining the total modulation wave voltage V of the inverter in the current source mode through the virtual synchronous rotation inverse coordinate transformation Cr ；
Step 9, obtaining the active power P of the N H-bridge units in the current source mode according to the step 3 Ci And step 4, the sum P of the active powers of the N H-bridge units in the current source mode is obtained CT Calculating the power distribution coefficient Factor of each H-bridge unit in the current source mode Ci 1, 2, 3.. N, calculated as:
step 10, obtaining the actual value V of the DC side voltage of the N H-bridge units according to the step 1 PVi And 8, obtaining the total modulation wave voltage V of the inverter under the current source mode Cr And the power distribution coefficients Factor of the N H-bridge units in the current source mode obtained in the step 9 Ci Calculating the modulated wave signal M of each H-bridge unit in the current source mode Cri 1, 2, 3.. N, calculated as:
step 11, before switching, latching the reference value of the active component of the filter inductor current in the current source mode of the previous period as
Latching N H-bridge unit power distribution coefficients under the last periodic current source mode and recording as Factor Cmi And calculating the feedforward control quantity I of the direct-current voltage regulator in each voltage source mode VFeedi 1, 2, 3.. N, calculated as:
step 12, before switching, obtaining a d-axis PI regulating value E under the current source mode according to the step 7 Cd Q-axis PI regulation value E in current source mode Cq And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Calculating to obtain the amplitude V of the total modulation wave voltage of the inverter in the current source mode Crm And phase theta Cr The calculation formula is as follows:
step 13, the actual value V of the filter capacitor voltage sampled in the step 1 is processed o Converting the virtual synchronous rotating coordinate into a voltage active component V of a voltage source mode filter capacitor under a rotating coordinate system Vod Voltage reactive component V of sum voltage source mode filter capacitor Voq ；
Step 14, the actual value I of the power grid current sampled in the step 1 is used s Converting the virtual synchronous rotating coordinate into a voltage source mode power grid current active component I under a rotating coordinate system VSd And voltage source mode network current reactive component I VSq ；
Step 15, obtaining the active component V of the voltage source mode filter capacitor according to the step 13 Vod Voltage reactive component V of voltage source mode filter capacitor Voq And step 14, obtaining the active component I of the voltage source mode grid current VSd Voltage source mode network current reactive component I VSq The average active power P output by the inverter in the voltage source mode is obtained by calculation and filtering through a first-order low-pass filter Vo And average reactive power Q Vo The calculation formula is as follows:
wherein τ is a first order low pass filter time constant;
step 16, obtaining the actual value V of the DC side voltage of the N H-bridge units according to the step 1 PVi And D, obtaining the direct-current side voltage instruction values V of the N H-bridge units obtained in the step 2 PVi * And step 11, the feedforward control quantity I of the direct-current voltage regulator under the N voltage source modes is obtained VFeedi Calculating the active power P of each H-bridge unit in the voltage source mode through the direct-current voltage regulator in the voltage source mode Vi Wherein i ═ 1, 2, 3.. N, calculated as:
wherein, K VVP Is the proportional coefficient of a DC voltage regulator in voltage source mode, K VVI The integral coefficient of the direct-current voltage regulator in a voltage source mode is 1, 2, 3.. N;
step 17, obtaining the active power P of the N H-bridge units in the voltage source mode according to the step 16 Vi Calculating to obtain the sum of the active powers of N H-bridge units in the voltage source mode and recording as P VT The calculation formula is as follows:
step 18The sum P of the active powers of the N H-bridge units in the voltage source mode obtained in step 17 VT And step 15, the average active power P of the inverter output under the voltage source mode is obtained Vo Calculating to obtain the output angular frequency omega of the inverter in the voltage source mode through an active power-frequency droop control equation Vo Angular frequency ω output in voltage source mode Vo Obtaining the output phase angle theta of the inverter in the voltage source mode through integration Vo The active power-frequency droop control equation is as follows:
ω Vo ＝ω * +m(P VT -P Vo )
wherein ω is * The synchronous angular frequency of the power grid is adopted, and m is an active droop coefficient;
step 19, outputting the average reactive power Q by the inverter under the voltage source mode according to the obtained step 15 Vo The reference value of the active component of the filter capacitor voltage in the voltage source mode is obtained through calculation of a reactive power-voltage droop control equation
Reference value of voltage reactive component of filter capacitor in sum voltage source mode
The reactive power-voltage droop control equation is as follows:
wherein E is reference electromotive force, n is reactive droop coefficient, and Q * Giving a reactive power instruction for an upper layer;
step 20, obtaining the amplitude V of the total modulation wave voltage of the inverter in the current source mode according to the step 12 Crm And phase theta Cr The phase angle θ of the output of the inverter in the voltage source mode obtained in step 18 Vo And step 19, obtaining the reference value of the active component of the filter capacitor voltage in the voltage source mode
Respectively calculating to obtain the amplitude V of the total modulation wave voltage of the inverter in the voltage source mode during switching through a phase synchronous regulator and an amplitude synchronous regulator in the voltage source mode Vrm And phase theta Vr The calculation formula is as follows:
wherein, K VSP1 For the proportional coefficient, K, of the amplitude synchronous regulator in voltage source mode VSI1 Integrating coefficient, K, of amplitude synchronous regulator in voltage source mode VSP2 Is the proportional coefficient, K, of the phase synchronous regulator in voltage source mode VSI2 Is the integral coefficient of the phase synchronous regulator in the voltage source mode;
step 21, obtaining the active power P of the N H-bridge units in the voltage source mode according to the step 16 Vi And the sum P of the active powers of the N H-bridge units in the voltage source mode obtained in the step 17 VT Calculating the power distribution coefficient Factor of each H-bridge unit in the voltage source mode Vi 1, 2, 3.. N, calculated as:
step 22, obtaining the actual value V of the DC side voltage of the N H-bridge units according to the step 1 PVi And the amplitude V of the total modulation wave voltage of the inverter in the switching time voltage source mode obtained in the step 20 Vrm And phase theta Vr And N H-bridge unit power distribution coefficients Factor in the voltage source mode obtained in step 21 Vi Calculating to obtain the modulation wave signal M of each H-bridge unit in the voltage source mode during switching Vri 1, 2, 3.. N, calculated as:
the control method for the seamless switching of the current source mode in the voltage source mode comprises the following steps:
firstly, before switching, the modulating wave voltage synchronous regulator is started, and the reference value of the active component of the filter capacitor voltage in the voltage source mode obtained in the step 19 is used
Step 7, obtaining d-axis PI regulating value in last period current source mode
And step 5, obtaining the active component V of the current source mode filter capacitor voltage Cod Obtaining a reference value of active components of filter inductor current in a current source mode before switching through a modulating wave voltage synchronous regulator
The calculation formula is as follows:
wherein, K CSP For modulating wave voltage synchronous regulator proportionality coefficient under current source mode, K CSI The integral coefficient of the modulation wave voltage synchronous regulator under the current source mode;
secondly, after the voltage synchronization of the modulation wave is finished, the reference value of the active component of the filter inductance current in the mode of the voltage source with the last period is latched and recorded as
Latching the power distribution coefficients of N H-bridge units in the last periodic voltage source mode and recording the power distribution coefficients as Factor Vmi Calculating to obtain feedforward control quantity I of the direct-current voltage regulator under each current source mode CFeedi 1, 2, 3.. N, calculated as:
finally, the feedforward control quantity I of the direct-current voltage regulator in the current source mode is converted into the feedforward control quantity I CFeedi The output of the direct-current voltage regulator in each current source mode is superposed, and the active power P 'of each H-bridge unit in the current source mode during switching is calculated' Ci Wherein i ═ 1, 2, 3.. N, calculated as:
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