Control method for cascaded power electronic transformers based on small capacitors

Abstract

This invention discloses a control method for a cascaded power electronic transformer based on small capacitors. The application topology of this control is a cascaded power electronic transformer, including an input stage cascaded H-bridge, an intermediate stage dual active bridge, and a load. The control method involves: decomposing the three-phase voltage and current on the grid side into positive and negative sequence values, and using the phase-to-phase DC side voltage values ​​after command correction, rectifier feedforward correction, and PI correction as inputs for positive sequence current decoupling and dq0-abc transformation, thereby obtaining the modulation wave signal; subtracting the average phase-to-phase DC side voltage from a reference value, correcting it with PI and DC-DC conversion feedforward to obtain a common shift ratio, and subtracting this from the correction of the independent shift ratio of the phase-to-phase DC side voltage after PIR correction to obtain the independent shift ratio. This invention features low hardware cost, accurate control, and wide applicability. It can reduce the size of the capacitor in the topology, effectively suppress second harmonics in the system, and also balance the phase-to-phase voltages on the three-phase DC side.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of power electronics, and particularly relates to a control method for a cascaded power electronic transformer based on small capacitance. BACKGROUND

[0002] In order to work in a medium or high power occasion, the power electronic transformer must improve the voltage resistance and power transmission capacity of the device itself, and it is difficult to achieve this by using a single power electronic transformer. The design of the power electronic transformer can solve the above problems by using a cascaded H-bridge. The input of the power electronic transformer is a rectifier H-bridge, and the middle stage is a dual active bridge. Since the power electronic transformer adopts a cascaded structure, the voltage stress of each switching tube is greatly reduced, which can ensure the power supply quality and reliability.

[0003] The ideal operating condition of the cascaded power electronic transformer is that each module of the cascaded H-bridge input stage and the dual active bridge of the middle stage uses the same parameter element, and the phase shift ratio of all switching tubes is the same. At this time, the cascaded power electronic transformer operates in a stable state, and the voltages of the cascaded H-bridge input stage are balanced. In actual work, the actual parameters of each module of the cascaded power electronic transformer are not the same, and if the same phase shift ratio is given to each switching tube, the voltages of each stage of the cascaded power electronic transformer will fluctuate, which will reduce the power supply quality and even damage the entire system. This is a common problem of the cascaded power electronic transformer, which is called voltage imbalance, mainly caused by the difference in parameters between the modules.

[0004] In order to deal with the secondary ripple in the cascaded power electronic transformer, there are two ways to suppress it, namely topology suppression and control strategy suppression. Topology suppression can filter out voltage ripples by adding additional hardware, which has good effect, but will increase the additional investment, and also increase the space volume and weight of the device and power loss. In order to avoid the defects brought by topology suppression, control strategy suppression is usually used to eliminate the secondary ripple in the power electronic transformer, so as to reduce the use of capacitors in the topology, reduce the production cost, and reduce the size of the device. The PI controller commonly used in closed-loop control has good tracking effect on direct current components, but the compensation effect on double-frequency components is not good. The R controller can be used to track the double-frequency alternating current signal, thereby making up for the deficiency of the PI controller. SUMMARY

[0005] The purpose of the present application is to provide a control method for a cascaded power electronic transformer based on small capacitance.

[0006] The technical solution for achieving the object of the present application is: a small-capacitance-based cascaded power electronic transformer control method. Its application occasion is a cascaded power electronic transformer, and the topology structure includes an input stage cascaded H-bridge, an intermediate stage double active bridge, a load, and a driving circuit. The small-capacitance-based cascaded power electronic transformer control method is composed of input stage total DC voltage control and output stage voltage equalization control.

[0007] The input stage total DC voltage control scheme is as follows:

[0008] Step 1, measure the three-phase AC current i flowing into the device on the grid side a 、 b 、 c , a 、 b 、 c , perform positive and negative sequence dq decomposition on i ,

[0009] The decomposition quantity formula of the three-phase positive sequence current on the grid side into the rotating coordinate system is:

[0010]

[0011] , , wherein i ,

[0012] Step 2, measure the three-phase AC voltage e flowing into the device on the grid side a 、 b 、 c , a 、 b 、 c , perform dq decomposition on e ,

[0013] The decomposition quantity formula of the three-phase AC voltage on the grid side into the rotating coordinate system is:

[0014]

[0015] Step 3, average value of inter-phase DC side voltage , and average value command value of inter-phase DC side voltage , the difference value is added to the value of the rectification feedforward quantity d multiplied by the correction coefficient m1 to obtain the DC conversion feedforward quantity i g .

[0016] Step 4, i g , after PI correction, is added to the decomposition quantity of the three-phase positive sequence current in the rotating dq coordinate system and the command value of the positive sequence current q phase The values calculated after the positive sequence current decoupling control are added to the decomposed values of the three-phase input voltage in the rotating dq coordinate system After transformation from the rotating dq0 coordinate system to the stationary abc coordinate system, the three-phase total modulation wave signals are generated The three-phase current output by the input stage H-bridge is balanced to the intermediate stage.

[0017] The modulation wave signal in the rotating coordinate system is transformed into the modulation wave signal in the stationary coordinate system according to the formula:

[0018]

[0019] Step 5, the number of cascaded units in each phase is n, and the total modulation wave signal After the average processing, the modulation wave signal of each cascaded unit in each phase is obtained The working state of each switch tube of the input stage H-bridge is controlled by the drive circuit.

[0020] The output stage voltage balancing control scheme is as follows:

[0021] Step 1, the intermediate stage load voltage u dc2 and the intermediate stage load voltage command value The difference value is corrected by PI, and the DC transformation feedforward value i g is added to the value multiplied by the correction coefficient m2 to obtain the rectification feedforward value d.

[0022] Step 2, the inter-phase DC side voltage u dcxn of each cascaded unit in each phase is subtracted from the average value of the inter-phase DC side voltage The difference value is corrected by the R controller to obtain the correction amount Δd xRn .

[0023] Step 3, the inter-phase DC side voltage u dcxn of each cascaded unit in each phase is subtracted from the average value of the inter-phase DC side voltage The difference value is corrected by the PI controller to obtain the correction amount Δd xPIn .

[0024] Step 4, the correction amount Δd xRn of each cascaded unit in each phase is added to the correction amount Δd xPIn and d to obtain the individual phase shift ratio d of each cascaded unit in each phase xn , which is output to the drive circuit of each unit.

[0025] The PI controller has good tracking effect on the DC component, but the compensation effect on the double-frequency component is not good. The R controller can be used to track the double-frequency alternating current signal to make up for the deficiency of the PI controller.

[0026] The transfer function of the PI controller is:

[0027]

[0028] The transfer function of the R controller is:

[0029]

[0030] Among them, K P K I K R Different values ​​are required for different PI and R controllers. Attached Figure Description

[0031] Figure 1 This is a block diagram of the control method for cascaded power electronic transformers based on small capacitors proposed in this invention.

[0032] Figure 2 This is a general application topology diagram of the cascaded power electronic transformer control based on small capacitors proposed in this invention.

[0033] Figure 3 This is a phase-to-phase DC side voltage diagram under the control method of a cascaded power electronic transformer based on small capacitors in an embodiment of the control method of the present invention.

[0034] Figure 4 This is an embodiment of the control method of the present invention, in which only the phase-to-phase DC side voltage diagram is controlled by the total DC voltage of the input stage.

[0035] Figure 5 In this embodiment of the control method of the present invention, the input stage total DC voltage control is adopted, and the phase-to-phase DC side voltage diagram is obtained when the input stage capacitor is increased to four times its original value. Detailed Implementation

[0036] The present invention will be further described below with reference to the accompanying drawings.

[0037] exist Figure 2 The cascaded power electronic transformer topology shown in the figure applies the control method proposed in this invention. The topology includes an input stage cascaded H-bridge, an intermediate stage dual active bridge, and a load. The grid-side current of each phase flows into the cascaded power electronic transformer through a filter inductor. Figure 2 The cascaded power electronic transformer shown adopts a star connection structure. The parameters of the input stage cascaded H-bridges are exactly the same, and the parameters of the primary and secondary H-bridges of the intermediate stage dual active bridges are also exactly the same. The turns ratio and inductance of the intermediate-stage dual active bridge intermediate frequency transformers in the three phases are slightly different to simulate the voltage imbalance problem caused by different system parameters.

[0038] The embodiment is simulated in the Matlab / Simulinik2018b environment, the control method of the small-capacitance-based cascaded power electronic transformer is simulated, the circuit is built according to Figure 2 , and the closed-loop control is according to Figure 1 . The simulation parameter table is shown in Table 1.

[0039] Table 1 Simulation parameters of three-phase star type cascaded PET

[0040]

[0041] The input stage total DC voltage control process is as follows:

[0042] Step 1, the positive and negative sequence dq decomposition is carried out on i a , i b , i c , and the decomposition amount of the three-phase positive sequence current in the rotating dq coordinate system is obtained

[0043] The decomposition amount formula of the three-phase positive sequence current on the grid side to the rotating coordinate system is:

[0044]

[0045] Step 2, dq decomposition is carried out on e a , e b , e c , and the decomposition amount of the three-phase AC input voltage in the rotating dq coordinate system is obtained

[0046] The decomposition amount formula of the three-phase AC voltage on the grid side to the rotating coordinate system is:

[0047]

[0048] Step 3, the average value of the inter-phase DC side voltage and the average value of the inter-phase DC side voltage command value The difference value is added to the value of the rectifier feedforward amount d multiplied by the correction coefficient m1 to obtain the DC conversion feedforward amount i g .

[0049] Step 4, i g is corrected by PI, and the decomposition amount of the three-phase positive sequence current in the rotating dq coordinate system and the command value of the positive sequence current q phase are added to the values calculated after the positive sequence current decoupling control, and the decomposition amount of the three-phase input voltage in the rotating dq coordinate system is added, and the transformation from the rotating dq0 coordinate system to the stationary abc coordinate system is carried out to generate the three-phase total modulation wave signal The three-phase current output balanced by the input stage H-bridge is given to the intermediate stage.

[0050] The modulation wave signal in the rotating coordinate system is transformed into the modulation wave signal in the stationary coordinate system.

[0051]

[0052] Step 5, the number of cascaded units of each phase is n, and the total modulation wave signal After the average processing, the modulation wave signal of each cascaded unit of each phase is obtained The working state of each switch tube of the input stage H-bridge is controlled by the driving circuit.

[0053] The output stage voltage balancing control process is as follows:

[0054] Step 1, the intermediate stage load voltage u dc2 and the intermediate stage load voltage instruction value The difference value is added to the DC conversion feedforward value i g multiplied by the correction coefficient m2 to obtain the rectification feedforward value d.

[0055] Step 2, the inter-phase DC side voltage u dcxn of each cascaded unit of each phase is subtracted from the average value of the inter-phase DC side voltage The difference value is corrected by the R controller to obtain the correction amount Δd xRn .

[0056] Step 3, the inter-phase DC side voltage u dcxn of each cascaded unit of each phase is subtracted from the average value of the inter-phase DC side voltage The difference value is corrected by the PI controller to obtain the correction amount Δd xPIn .

[0057] Step 4, the correction amount Δd xRn of each cascaded unit of each phase is added to the correction amount Δd xPIn and d to obtain the individual phase shift ratio d xn of each cascaded unit of each phase, which is output to the driving circuit of each unit.

[0058] Through simulation verification, under the same parameter conditions, the control method proposed in the application can realize the balance of the inter-phase DC side voltage in the three simulation test cases. At the same time, Figure 3 、 Figure 4 、 Figure 5 The simulation results show that the cascaded power electronic transformer control method based on small capacitors can effectively reduce the size of the capacitor. In this simulation test, the capacitor can be reduced from 8000 μF to 2000 μF while maintaining the original secondary ripple size.

Claims

1. A small-capacitance-based control method for a cascade-type power electronic transformer, characterized by, The application occasion is a cascade power electronic transformer, a topology structure comprises an input stage cascade H-bridge, an intermediate stage double active bridge, a load and a driving circuit, and a control method comprises input stage total direct current voltage control and output stage voltage balance control; The input stage total direct current voltage control comprises the following control steps: Step 1.

1. Measure the three-phase current i of the grid-side in-flow device a b c a b c ​​​​​​ Step 1.2, measuring three-phase voltage e of the grid-side inflow device a b c a b c dq-decomposing the three-phase voltage e to obtain decomposition quantities of the three-phase input voltage in a rotating dq coordinate system ​​​​​ Step 1.3, average value of inter-phase DC side voltage and average value of inter-phase DC side voltage command value The difference value is added to the value of the rectification feedforward amount d multiplied by the correction coefficient ml to obtain the DC conversion feedforward amount i g ; Step 1.4, direct current conversion feedforward amount i g After correction by the proportional integral controller, the decomposed amounts of the three-phase positive sequence currents in the rotating dq coordinate system and the command value of the positive sequence current q phase After calculation by the positive sequence current decoupling control, the values are added to the decomposed amounts of the three-phase input voltages in the rotating dq coordinate system After conversion from the rotating dq0 coordinate system to the stationary abc coordinate system, the three-phase total modulation wave signals are generated Step 1.5, the number of cascaded units of each phase is n, and the total modulation wave signal After the averaging process, the modulation wave signal of each cascaded unit of each phase is obtained The working state of each switch tube of the input stage of the driving circuit controls the input stage of the H-bridge. The output stage voltage balance control comprises the following control steps: Step 2.1, intermediate stage load voltage u dc2 and the intermediate stage load voltage command value the difference value, corrected by a PI, and the DC conversion feedforward value i g the value multiplied by the correction coefficient m2 is added to obtain the rectification feedforward value d; Step 2.2, each phase each level of the interphase DC side voltage u dcxn The average value of the interphase DC side voltage The difference value, after being corrected by the resonance controller, obtains the correction amount Ad xRn ; Step 2.3, each phase each level of the interphase DC side voltage u dcxn The average value of the interphase DC side voltage The difference value, after being corrected by a PI controller, obtains a correction amount Ad xPIn ; Step 2.

4. Correction amount Ad of each phase and each cascade unit xRn and the correction amount Ad xPIn and d to obtain the individual phase shift d of each phase and each cascade unit xn , output to the drive circuit of the intermediate stage dual active bridge.

2. The small-capacitance-based cascade-type power electronic transformer control method according to claim 1, characterized by, The transfer function of the PI controller is: where K P is a proportional gain coefficient, K I is an integral gain coefficient; The transfer function of the R controller is: where K R is the resonance gain coefficient; The formula from the static abc coordinate system to the rotating dq coordinate system is: where x a , x b , x c are the decomposed quantities of any vector parameter x in the stationary abc coordinate system, and x d , x q are the decomposed quantities of x in the rotating dq coordinate system; The formula from the rotating dq coordinate system to the static abc coordinate system is:

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