Coordination control method for stability of DC voltage of two-stage grid-connected converter

Abstract

The invention relates to a two-stage grid-connected converter DC voltage stabilization coordination control method, which comprises the steps of initializing an adaptive filtering algorithm and a PI controller, sampling or reconstructing voltages and currents at two ends of an LC filter, and calculating a prediction coefficient matrix so as to obtain prediction values of an inductive current and a capacitor voltage. And calculating the output power of the DAB converter by using the predicted values, and obtaining the compensation amount of the phase shift ratio between the bridges through a feed-forward coefficient. Meanwhile, PI control is carried out on the intermediate direct-current link voltage to obtain a DAB converter inter-bridge shift ratio, and the DAB converter inter-bridge shift ratio and the compensation amount are added to obtain a final inter-bridge shift ratio. And in combination with an in-bridge phase shift ratio obtained by an inductive current stress optimization method, eight switching tubes of the DAB converter are controlled through an expansion phase shift method, so that quick response of output power and voltage stability of an intermediate direct-current link are realized. According to the method, the minimum mean square error adaptive filtering algorithm is introduced, so that the prediction precision and the response speed are improved, and the system stability is ensured.

Description

Technical Field

[0001] The invention belongs to the technical field of power electronics and relates to a coordinated control method for stabilizing direct current voltage of a two-stage grid-connected converter. Background Art

[0002] The two-stage circuit topology consisting of a dual active bridge (DAB) DC-DC converter and a T-type three-level grid-connected converter cascaded has attracted widespread attention in the fields of battery energy storage, bidirectional charging piles, and power electronic transformers for low-voltage power distribution. Keeping the intermediate DC link voltage of the cascade system stable at all times has a great impact on the dynamic and static performance of the system. The control of the intermediate DC link voltage is usually the responsibility of the DAB converter, and the T-type three-level grid-connected converter usually uses PQ control to achieve energy exchange with the grid. When a large power fluctuation occurs on the grid side, the instantaneous power imbalance between the two-stage converter causes a large fluctuation in the intermediate DC link voltage. If the DAB converter cannot quickly suppress the DC link voltage fluctuation, it may cause stability problems in the cascade system. Therefore, how to improve the control strategy of the DAB converter and improve the stability of the intermediate DC link voltage of the cascade system and the dynamic performance of the DAB converter has become one of the current research hotspots.

[0003] At present, the control strategies used to improve the output DC link voltage stability and dynamic performance of DAB converters mainly include direct inductor current control, load current feedforward control and output power feedforward control. Among them, direct inductor current control, also known as inductor current peak control, can effectively improve the power transmission efficiency of DAB converters, but this method requires sampling of inductor current, which increases hardware cost and needs to overcome the influence of noise, which reduces the dynamic performance of DAB converters to a certain extent. Load current feedforward control requires sampling of DC link current, which also increases hardware cost. Output power feedforward control reduces the parameter sensitivity of DAB converters. Compared with direct inductor current control and load current feedforward, it saves additional current sensors and reduces hardware cost, which is a good choice. However, considering the instantaneous power change on the grid side, there is a certain loss between the transmission power of DAB converter and the output power of T-type three-level grid-connected converter, and the accuracy of transmission power calculation is further reduced due to the influence of control delay, which affects the performance of output power feedforward control. In order to solve some problems caused by the output power feedforward control adopted in the above-mentioned cascade system, the present invention introduces the instantaneous power of the filter inductor of the T-type three-level grid-connected converter in the process of calculating the transmission power, and uses an adaptive filtering algorithm to reduce the influence of delay control, thereby improving the stability and rapidity of the cascade system. Summary of the invention

[0004] The purpose of the present invention is to provide a coordinated control method for stabilizing the DC voltage of a two-stage grid-connected converter, so as to achieve power balance between a two-stage grid-connected converter cascaded with a DAB converter and a T-type three-level converter. A variable prediction method of a minimum mean square error adaptive filtering algorithm is introduced, and the predicted inductor current and capacitor voltage are used as the output of the adaptive filtering algorithm. The output power of the DAB converter is obtained by predicting the output, and the compensation amount of the bridge shift phase ratio of the DAB converter is obtained by using power feedforward. The intermediate DC link voltage is subjected to PI control to obtain the bridge shift phase ratio of the DAB converter, and the bridge shift phase ratio and the compensation amount of the bridge shift phase ratio are added to obtain the final bridge shift phase ratio of the DAB converter. The eight switch tubes of the DAB converter are subjected to PWM action by an extended phase shift method, so as to achieve rapid output power response and ensure the stability of the intermediate DC link voltage.

[0005] The technical solution adopted by the present invention is: a coordinated control method for stabilizing the DC voltage of a two-stage grid-connected converter, comprising the following steps:

[0006] Step 1: At the initial moment, the adaptive filtering algorithm and the PI controller are initialized, and the initial prediction coefficient matrix H(0) of the adaptive filtering algorithm is calculated by formula (6);

[0007]

[0008] Step 2: In each control cycle, the voltage and current across the LC filter are sampled or reconstructed to obtain u oi (k), i Li (k) and u Ci (k), i gi (k), the predicted value obtained from the previous control cycle And the sampled u Ci (k), i Li (k) are subtracted respectively, and the prediction coefficient matrix H(k+1) is calculated by formula (7);

[0009]

[0010] Where k is the current step number, H(k) is the prediction coefficient matrix of the current k-th step, and μ is the learning rate, which is 10 -5 ;

[0011] Step 3: Based on the prediction coefficient matrix obtained in step 2, use formula (8) to calculate the predicted values ​​of the inductor current and capacitor voltage;

[0012]

[0013] Step 4: Use formula (9) to calculate the output power of the DAB converter. Substitute the output power P of the DAB converter calculated by formula (9) into DAB_out Multiplying by the feedforward coefficient m, the compensation amount Δθ of the DAB converter bridge phase shift ratio is obtained;

[0014]

[0015] In the formula, is the current prediction value of the filter inductor of phase i in this control cycle; is the predicted current value of the filter inductor of phase i in the previous control cycle; is the voltage prediction value of the filter capacitor of phase i in this control cycle; is the voltage prediction value of the filter capacitor of phase i in the previous control cycle;

[0016] Step 5: Set the expected value of the intermediate DC link voltage U 2_ref Subtract the actual sampling value U2, and then use formula (10) to obtain the DAB converter bridge offset ratio θ0, and then add it to the compensation amount Δθ of the DAB converter bridge offset ratio obtained in step 4 to obtain the final DAB converter bridge offset ratio θ;

[0017]

[0018] In the formula, k p , k i are the proportional coefficient and integral coefficient of the PI controller respectively;

[0019] Step 6: Obtain the bridge displacement ratio by using formula (13): The bridge between the two sides is shifted to θ, the bridge inside is shifted to The PWM signals g1 to g8 of the eight switch tubes of the DAB converter are obtained by the extended phase shift method;

[0020]

[0021] In the formula, the voltage conversion ratio K = U1 / nU2, n is the transformation ratio of the primary and secondary sides of the transformer.

[0022] The beneficial effects of the present invention are as follows: compared with the original method, the method of the present invention introduces the instantaneous power of the filter inductor, can accurately calculate the transmission power of the cascade system, adds the variable prediction of the adaptive filtering algorithm, compensates for the control delay of the system, thereby obtaining the optimal control amount of the output power of the DAB converter, and realizes the improvement of the dynamic performance of the cascade system under the output power feedforward control, so that the cascade system can also operate stably and efficiently when the power on the grid side changes suddenly. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1It is a cascade main circuit diagram of a DAB converter and a T-type three-level grid-connected converter corresponding to the present invention;

[0024] Figure 2 It is a power feedforward coordinated control block diagram for realizing voltage stabilization of the intermediate DC link of a two-stage grid-connected converter according to the present invention;

[0025] Figure 3 The present invention is a flow chart of the power feedforward coordination control steps for realizing voltage stabilization of the intermediate DC link of a two-stage grid-connected converter.

[0026] The symbols are defined as follows:

[0027] U1 is the primary DC voltage of the DAB converter; U2 is the intermediate DC link voltage; K is the voltage conversion ratio; n is the transformation ratio of the transformer T; L r is the transformer series inductance; f s is the switching frequency of the DAB converter; θ is the bridge-to-bridge shift phase of the DAB converter; is the bridge internal shift of the DAB converter; C1 and C2 are the intermediate DC link filter capacitors; L a =L b =L c =L is the grid-side filter inductance; C a =C b =C c =C is the grid-side filter capacitor; ω f is the resonant frequency of the LC filter; u oi is the input voltage of the i-th phase filter inductor, i Li is the i-phase filter inductor current, u Ci is the voltage of the filter capacitor of the i-th phase, i gi is the i-th phase grid current, u i is the i-th phase grid voltage, i = a, b, c. T s is the switching period of the T-type three-level grid-connected converter. L is the instantaneous power on the filter inductor; P out is the instantaneous power output of the T-type three-level grid-connected converter; P DAB_out Output instantaneous power for DAB converter. DETAILED DESCRIPTION

[0028] Figure 1 This is the cascade main circuit diagram of the DAB converter and T-type three-level grid-connected converter corresponding to the present invention. This circuit topology is used to realize bidirectional reversible AC-DC conversion, which can not only transmit the electric energy on the DC side to the power grid or load through the converter, but also transmit the electric energy of the power grid to the DC side through the converter.

[0029] The two ends of the DC voltage U1 and the supporting capacitor C inThe source of the switch tube S1 is connected to the drain of the switch tube S3, serving as the output terminal X1 of the first bridge circuit. The drain of the switch tube S1 is connected to the drain of the switch tube S2, and is also connected to the positive electrode of the DC voltage U1. The source of the switch tube S2 is connected to the drain of the switch tube S4, serving as the output terminal X2 of the second bridge circuit. The source of the switch tube S3 is connected to the source of the switch tube S4, and is also connected to the negative electrode of the DC voltage U1. X1 is connected to the inductor L r One end of the inductor is connected to r The other end of is connected to the same-name end of the primary side of transformer T, and X2 is connected to the opposite-name end of the primary side of transformer T. The source of switch tube S5 is connected to the drain of switch tube S7, serving as the output end Y1 of the third bridge circuit. The drain of switch tube S5 is connected to the drain of switch tube S6, and is connected to the P end of capacitor C1. The source of switch tube S6 is connected to the drain of switch tube S8, serving as the output end Y2 of the fourth bridge circuit. The source of switch tube S7 is connected to the source of switch tube S8, and is connected to the N end of capacitor C2. Capacitor C1 and capacitor C2 are connected in series and intersect at the O end. Switch tube S a1 The collector and switch tube S b1 The collector of the switch tube S c1 The collector of the switch tube S a1 The emitter and switch tube S a3 The collector is connected to the inductor L a The switch tube S a3 The emitter and switch tube S a2 The emitter of the switch tube S b1 The emitter and switch tube S b3 The collector is connected to the inductor L b The switch tube S b3 The emitter and switch tube S b2 The emitter of the switch tube S c1 The emitter and switch tube S c3 The collector is connected to the inductor L c The switch tube S c3 The emitter and switch tube S c2 The emitter of the switch tube S a2 The collector of the switch tube S b2 The collector of the switch tube S c2 The collector of is connected to the O terminal. The filter inductor L a The other end a2 and the filter capacitor C a , grid voltage u a Filter inductor L b The other end b2 is connected to the filter capacitor C b , grid voltage u b Filter inductor L cThe other end c2 and the filter capacitor C c , grid voltage u c The other end of the filter capacitor is connected to the other end of the grid voltage and is grounded.

[0030] Figure 2 This is a power feedforward coordinated control block diagram for realizing voltage stability of the intermediate DC link of a two-stage grid-connected converter according to the present invention, including a variable prediction module of an adaptive filtering algorithm, a power calculation module, a PI controller and an extended shift phase calculation module, a pulse generator module and a DAB converter. Among them, the DAB converter is cascaded with a T-type three-level converter, that is, the output end of the DAB converter is connected to the input end of the T-type three-level converter, which is the intermediate DC link, and the voltage signal at this point is U2. The output power of the DAB converter is obtained by the adaptive filtering algorithm, and then multiplied by the power feedforward coefficient m to obtain the compensation amount Δθ of the shift phase between the bridges of the DAB converter. The intermediate DC link reference voltage U 2_ref The difference between the actual intermediate DC link voltage U2 is processed by the PI controller to obtain the bridge-to-bridge shift phase ratio θ0 of the DAB converter. The bridge-to-bridge shift phase ratio θ0 and the compensation amount Δθ of the bridge-to-bridge shift phase ratio are added to obtain the final bridge-to-bridge shift phase ratio θ of the DAB converter. The existing inductor current stress optimization method is used to obtain the bridge-to-bridge shift phase ratio of the DAB converter. Set θ, Input into the pulse generator, and obtain the PWM signals g1~g8 of the eight switch tubes of the DAB converter through the extended phase shift method, so as to control the operation of the DAB converter. The specific implementation process of each module is as follows:

[0031] 1) Variable prediction module of adaptive filtering algorithm:

[0032] The adaptive filtering algorithm is divided into two basic algorithms: recursive least squares (RLS) algorithm and least mean square error (LMS) algorithm. The RLS algorithm is complex in calculation, requires large storage capacity, and is not conducive to microcontroller implementation. The LMS algorithm has a simple structure, small amount of calculation, and is easy to implement in a microcontroller. Therefore, the LMS adaptive filtering algorithm is used to select the filter inductor current. and filter capacitor voltage As the predicted output of the adaptive filtering algorithm, the specific process is as follows:

[0033] Set the appropriate initial value of the prediction matrix H(0) and establish the differential equation of the single-phase equivalent circuit of the LC filter circuit of the T-type three-level converter. Where L represents the filter inductor, C represents the filter capacitor, and the differential equation expression of the equivalent circuit of the i-th phase is shown in formula (1).

[0034]

[0035] In the formula, i = a, b, c, iLi is the i-phase filter inductor current, u Ci is the voltage of the filter capacitor of the i-th phase, i gi is the i-th phase grid current, T s is the switching period of the T-type three-level grid-connected converter, u oi is the input voltage of the i-th phase filter inductor, which is switched by the i-th phase switch function S oi And the intermediate DC link voltage U2 is reconstructed and synthesized. The switching function is defined as follows:

[0036]

[0037] So we can get u oi As shown in formula (3).

[0038] u oi =S oi ·U2 (3)

[0039] By discretizing equation (1), we can get equation (4):

[0040]

[0041] In the formula, ω f represents the resonant frequency of the LC filter, as shown in equation (5).

[0042]

[0043] Therefore, the initial value of the prediction matrix H(0) can be selected according to formula (6).

[0044]

[0045] In each control cycle, the voltage and current at both ends of the LC filter are first sampled or reconstructed to obtain u oi (k), i Li (k) and u Ci (k), i gi (k), the predicted value obtained from the previous control cycle And the sampled u Ci (k), i Li (k) are subtracted to obtain the error signal e(k), and then the prediction coefficient matrix H(k+1) is calculated by formula (7).

[0046]

[0047] Where k is the current step number, H(k) is the prediction coefficient matrix of the current k-th step, and μ is the learning rate, which is 10 -5 .

[0048] The predicted values ​​of the inductor current and capacitor voltage are calculated using H(k+1) obtained from equation (7), as shown in equation (8).

[0049]

[0050] because The phase is ahead of u Ci (k), i Li (k), so the phase lag of the control signal of the T-type three-level grid-connected converter caused by the control delay is compensated, so that the output power calculation of the T-type three-level grid-connected converter is more accurate.

[0051] 2) Power calculation module:

[0052] In the steady state, the energy of the filter inductor of the T-type three-level grid-connected converter remains basically unchanged. However, the inductor current will change with the load, so in practice, the output power P of the DAB converter DAB_out The output instantaneous power P of the T-type three-level grid-connected converter can be out And the instantaneous power of the filter inductor P L The filter inductor current of the T-type three-level grid-connected converter is predicted by the adaptive filtering algorithm. and filter capacitor voltage Then the output power of the DAB converter is shown in formula (9).

[0053]

[0054] In the formula, is the current prediction value of the filter inductor of phase i in this control cycle; is the predicted current value of the filter inductor of phase i in the previous control cycle; is the voltage prediction value of the filter capacitor of phase i in this control cycle; is the voltage prediction value of the filter capacitor of phase i in the previous control cycle.

[0055] 3) PI controller and extended shift ratio calculation module:

[0056] The input of the PI controller is the expected value of the intermediate DC link voltage U 2_ref The output of the PI controller is the bridge shift phase θ0 of the DAB converter, which is the error of the actual sampling value U2, as shown in formula (10).

[0057]

[0058] In the formula, k p , k i are the proportional coefficient and integral coefficient of the PI controller respectively.

[0059] The DAB converter output power P calculated by formula (9) DAB_out Multiplying by the feedforward coefficient m, which is usually 0.006, we get the compensation amount Δθ of the DAB converter bridge phase shift, as shown in equation (11).

[0060] Δθ=mP DAB_out (11)

[0061] The final DAB converter inter-bridge phase shift ratio θ is obtained by adding equation (10) and equation (11), as shown in equation (12). Using the existing DAB converter inductor current stress optimization algorithm, the bridge intra-bridge phase shift ratio θ is obtained by As shown in formula (13).

[0062] θ=θ0+Δθ (12)

[0063]

[0064] In the formula, the voltage conversion ratio K = U1 / nU2, n is the transformation ratio of the primary and secondary sides of the transformer.

[0065] 4) Pulse generator module:

[0066] The function of this module is to compare the inter-bridge shift ratio to θ and the intra-bridge shift ratio to Converted into PWM signals for driving switch tubes S1 to S8. Taking the PWM signal of the first bridge as the reference, the PWM signal of the second bridge lags behind the PWM signal of the first bridge. The PWM signal of the third bridge lags behind the PWM signal of the first bridge by θ switching cycles, and the PWM signal of the fourth bridge lags behind the PWM signal of the first bridge by θ cycles. The PWM signals of the given switches S1 to S8 are all square wave signals with a duty cycle of 50%. The PWM signal of the switch S1 is represented by g1. The switches S1 and S3 are two switches with complementary drive signals of the same bridge arm. The PWM signal g3 of the switch S3 lags behind the PWM signal of the switch S1 by 1 / 2 switching cycle. The PWM signal g2 of the switch S2 lags behind the PWM signal of the switch S1. Switching cycles, switch tubes S2 and S4 are two switch tubes with complementary driving signals of the same bridge arm, PWM signal g4 of switch tube S4 lags behind PWM signal of switch tube S2 by 1 / 2 switching cycle, PWM signal g5 of switch tube S5 lags behind PWM signal of switch tube S1 by θ switching cycles, switch tubes S5 and S7 are two switch tubes with complementary driving signals of the same bridge arm, PWM signal g7 of switch tube S7 lags behind PWM signal of switch tube S5 by 1 / 2 switching cycle, PWM signal g6 of switch tube S6 lags behind PWM signal of switch tube S1 Switching tubes S6 and S8 have complementary driving signals for the same bridge arm, and the PWM signal g8 of switch tube S8 lags behind the PWM signal of switch tube S6 by 1 / 2 switching cycle. In summary, the PWM signals of eight switching tubes are obtained, thereby controlling the normal operation of the DAB converter.

[0067] In summary, Figure 3 A flowchart of the coordinated control method for realizing DC voltage stability of a two-stage grid-connected converter according to the present invention is provided, and the method is implemented according to the following steps:

[0068] Step 1: Initialization: At the initial moment, the adaptive filtering algorithm and PI controller have not yet been implemented, so the algorithm and controller need to be initialized; the initial prediction coefficient matrix H(0) of the adaptive filtering algorithm is calculated by formula (6).

[0069] Step 2: In each control cycle, the voltage and current across the LC filter are sampled or reconstructed to obtain u oi (k), i Li (k) and u Ci (k), i gi (k), the predicted value obtained from the previous control cycle And the sampled u Ci (k), i Li (k) are subtracted respectively, and the prediction coefficient matrix H(k+1) is calculated by formula (7).

[0070] Step 3: Use equation (8) to calculate the predicted values ​​of the inductor current and capacitor voltage.

[0071] Step 4: Use formula (9) to calculate the output power of the DAB converter. Substitute the output power P of the DAB converter calculated by formula (9) into DAB_out Multiplying by the feedforward coefficient m, we can get the compensation amount Δθ of the phase shift between bridges of the DAB converter.

[0072] Step 5: Set the expected value of the intermediate DC link voltage U 2_ref Subtract the actual sampling value U2, and then use formula (10) to obtain the DAB converter bridge offset ratio θ0, and then add it to the compensation amount Δθ of the DAB converter bridge offset ratio obtained in step 4 to obtain the final DAB converter bridge offset ratio θ.

[0073] Step 6: Obtain the bridge displacement ratio by using formula (13): The bridge between the two sides is shifted to θ, the bridge inside is shifted to The PWM signals g1 to g8 of the eight switching tubes of the DAB converter are obtained by an extended phase shift method.

Claims

1. A coordinated control method for stabilizing DC voltage of a two-stage grid-connected converter, characterized in that: The following steps are involved: Step 1: At the initial moment, the adaptive filtering algorithm and the PI controller are initialized, and the initial prediction coefficient matrix H(0) of the adaptive filtering algorithm is calculated by formula (6); Step 2: In each control cycle, the voltage and current across the LC filter are sampled or reconstructed to obtain u oi (k), i Li (k) and u Ci (k), i gi (k), the predicted value obtained from the previous control cycle And the sampled u Ci (k), i Li (k) are subtracted respectively, and the prediction coefficient matrix H(k+1) is calculated by formula (7); Where k is the current step number, H(k) is the prediction coefficient matrix of the current k-th step, and μ is the learning rate, which is 10 -5 ; Step 3: Based on the prediction coefficient matrix obtained in step 2, use formula (8) to calculate the predicted values ​​of the inductor current and capacitor voltage; Step 4: Use formula (9) to calculate the output power of the DAB converter. Substitute the output power P of the DAB converter calculated by formula (9) into DAB_out Multiplying by the feedforward coefficient m, the compensation amount Δθ of the DAB converter bridge phase shift ratio is obtained; In the formula, is the current prediction value of the filter inductor of phase i in this control cycle; is the predicted current value of the filter inductor of phase i in the previous control cycle; is the voltage prediction value of the filter capacitor of phase i in this control cycle; is the voltage prediction value of the filter capacitor of phase i in the previous control cycle; Step 5: Set the expected value of the intermediate DC link voltage U 2_ref Subtract the actual sampling value U2, and then use formula (10) to obtain the DAB converter bridge offset ratio θ0, and then add it to the compensation amount Δθ of the DAB converter bridge offset ratio obtained in step 4 to obtain the final DAB converter bridge offset ratio θ; In the formula, k p , k i are the proportional coefficient and integral coefficient of the PI controller respectively; Step 6: Obtain the bridge displacement ratio by using formula (13): The bridge between the two sides is shifted to θ, the bridge inside is shifted to The PWM signals g1 to g8 of the eight switch tubes of the DAB converter are obtained by the extended phase shift method; In the formula, the voltage conversion ratio K = U1 / nU2, n is the transformation ratio of the primary and secondary sides of the transformer.

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