Optical storage system bidirectional DC-DC converter control method based on feedforward self-coupling proportional integral control

Abstract

The invention discloses an optical storage system bidirectional DC-DC converter control method based on feedforward self-coupling proportional integral control. The method comprises the following steps: constructing an optical storage system which comprises a photovoltaic power generation unit, a load, a power grid, a bidirectional DC-DC converter and an energy storage unit; constructing a bidirectional DC-DC converter mathematical model according to the bidirectional DC-DC converter topology circuit; a bidirectional DC-DC converter control system is constructed, the bidirectional DC-DC converter control system comprises a voltage outer loop subsystem and a current inner loop subsystem, the voltage outer loop subsystem adopts feedforward self-coupling proportional-integral control, and the current inner loop subsystem adopts PI control. According to the invention, stable energy transmission between the storage battery and the DC bus is ensured, and the problems of DC bus voltage fluctuation and poor system anti-interference capability caused by photovoltaic power generation uncertainty and frequent load disturbance are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy, and in particular, relates to a control method for a bidirectional DC-DC converter of a light storage system based on feedforward self-coupled proportional integral control. BACKGROUND

[0002] The output characteristics of a photovoltaic power generation unit are significantly affected by external factors such as light intensity and ambient temperature, and the randomness and intermittence of the power output of the photovoltaic power generation unit easily cause a dramatic fluctuation of the direct current bus voltage. Such fluctuation not only reduces the power transmission efficiency, but also causes an impact on key equipment such as an energy storage device and a power electronic converter, and even causes a misoperation of a system protection device in a serious case. In order to maintain stable operation of the system, an energy storage unit and a bidirectional DC-DC converter are usually configured to form a buffer link, and the power fluctuation is smoothed through bidirectional energy flow.

[0003] A proportional integral (PI) control strategy is usually used in a traditional energy storage system to realize voltage stabilization control of the bidirectional DC-DC converter, but the inherent limitations of the PI control strategy gradually appear in actual application. On the one hand, the fixed parameters of the PI controller are difficult to adapt to the rapid change of the photovoltaic output power and the dynamic switching of the load demand, resulting in voltage overshoot and recovery lag of the system in the light mutation or load step scene; on the other hand, the traditional PI control has a dimension conflict problem of physical quantities, and the parameters of the voltage loop and the current loop need to be repeatedly coordinated, and the debugging process is complex and easy to cause control mismatch. In addition, the PI control has limited suppression ability for nonlinear characteristics and coupled disturbances of the system, and the steady-state error is easily accumulated in a complex working condition, which seriously affects the control accuracy of the bus voltage.

[0004] In view of the above problems, scholars have proposed improved strategies such as active disturbance rejection control, model predictive control and sliding mode control in recent years. For example, the sliding mode control based on the disturbance observer can enhance the robustness of the system, and the model predictive control improves the dynamic response speed through the rolling optimization strategy. However, these methods face significant challenges in actual application: the active disturbance rejection control depends on an accurate disturbance estimation model, and is easily affected by parameter drift in engineering implementation; the model predictive control needs to solve an optimization problem online, and has a very high requirement for the processor computing power; the sliding mode control has strong robustness, but the inherent high-frequency chattering phenomenon of the sliding mode control aggravates the device loss. Especially for embedded application scenarios such as the light storage system, the existing algorithms generally have high computational complexity and large hardware resource occupation, and are difficult to be efficiently deployed in low-cost microcontrollers, which seriously restricts the industrialization and popularization potential of the light storage system.

[0005] Therefore, the application provides a control method for a bidirectional DC-DC converter of a light storage system based on feedforward self-coupled proportional integral control. SUMMARY

[0006] The present application aims to overcome the deficiencies of the prior art, and proposes a light storage system bidirectional DC-DC converter control method based on feedforward self-coupling proportional integral control, so as to achieve the following purposes: ensure the stability of energy transmission between the battery and the DC bus, and improve the problems of DC bus voltage fluctuation and poor system disturbance resistance caused by photovoltaic power generation uncertainty and frequent load disturbance.

[0007] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows: a light storage system bidirectional DC-DC converter control method based on feedforward self-coupling proportional integral control, the method comprising the following steps:

[0008] Step S1, constructing a light storage system, including a photovoltaic power generation unit, a load, a power grid, a bidirectional DC-DC converter and an energy storage unit, the energy storage unit being connected to a DC bus through the bidirectional DC-DC converter, the DC bus being further connected to the photovoltaic power generation unit, the load and the power grid respectively;

[0009] Step S2, constructing a bidirectional DC-DC converter mathematical model according to the topology circuit of the bidirectional DC-DC converter;

[0010] Step S3, constructing a bidirectional DC-DC converter control system, including a voltage outer loop subsystem and a current inner loop subsystem, wherein the voltage outer loop subsystem adopts feedforward self-coupling proportional integral control, and the current inner loop subsystem adopts PI control.

[0011] Preferably, in the step S2, the bidirectional DC-DC converter mathematical model includes a mathematical model in Boost mode, that is, when the switching tubes S1 and S2 are turned off, the anti-parallel diode D2 is turned off, and the diode D1 is turned on, the state space equation of the circuit is obtained:

[0012] (2) ;

[0013] Among them, Indicates the current of the energy storage side inductance; Indicates the energy storage side voltage; Indicates the DC bus voltage; L indicates the energy storage side filter inductance; Indicates the filter capacitance on the bus side; Indicates the equivalent load on the DC bus side;

[0014] When the switching tube S2 is turned on, S1 is turned off, and the diodes D1 and D2 are turned off, the state space equation of the circuit is obtained:

[0015] (3) ;

[0016] The state space average method is applied to the energy storage bidirectional DC-DC converter, and the state space equation of the circuit in Boost mode is obtained:

[0017] (4);

[0018] wherein, is the duty ratio in Boost mode;

[0019] The small signal modeling and Laplace transform are performed on the energy storage bidirectional DC-DC converter to obtain the transfer function in Boost mode:

[0020] (5);

[0021] wherein, represents the filter capacitor on the energy storage side; s is a complex frequency; is the control quantity; is the transfer function of the inductor current on the energy storage side to the Boost control quantity; is the transfer function of the DC bus voltage to the inductor current on the energy storage side in Boost mode.

[0022] Preferably, in the step S2, the mathematical model of the bidirectional DC-DC converter further comprises a mathematical model in Buck mode, and the corresponding state space average equation and transfer function are respectively:

[0023] (6);

[0024] wherein, is the duty ratio in Buck mode;

[0025] (7);

[0026] wherein, is the equivalent resistance on the energy storage side; is the transfer function of the inductor current on the energy storage side to the Buck control quantity; is the transfer function of the DC bus voltage to the inductor current on the energy storage side in Buck mode.

[0027] Preferably, in the step S3, the feedforward self-coupling proportional integral control is composed of an SC-PI controller and a feedforward controller, the SC-PI controller is used to adjust the system output in real time to offset external disturbances, and the feedforward controller offsets external disturbances by introducing a positive feedback channel in the input signal of the SC-PI controller, and the reference value V dcref of the DC bus voltage is subtracted from the sampled actual value V dc of the DC bus voltage and offset by a feedforward control coefficient k ffc .

[0028] Preferably, the design of the SC-PI controller includes establishing a controlled error energy storage bidirectional DC-DC converter system model, i.e.:

[0029] A controlled energy storage bidirectional DC-DC converter system is represented as follows:

[0030] (8);

[0031] wherein, and are state variables of the system; is a voltage input of the system; is a current reference value output of the system; is a system function; is an external disturbance;

[0032] The total disturbance d is defined as the sum of the system uncertain dynamics and the external disturbance, and its expression is:

[0033] (9);

[0034] wherein, is an estimated value of the control gain, which is not equal to 0;

[0035] According to equation (8) and equation (9), the rewritten energy storage bidirectional DC-DC converter system is:

[0036] (10);

[0037] Let the inductance current target value of the controlled energy storage bidirectional converter voltage outer loop be r, and define the tracking error as:

[0038] (11);

[0039] The integral error is defined as:

[0040] (12);

[0041] Combining equation (10) and equation (11) gives:

[0042] (13);

[0043] Solving equation (11)-(13) gives the controlled error energy storage bidirectional DC-DC converter system model as:

[0044] (14).

[0045] Preferably, the design of the SC-PI controller comprises establishing a SC-PI controller model according to the controlled error energy storage bidirectional DC-DC converter system model, and the SC-PI controller model is expressed as follows:

[0046] (15) ;

[0047] The inductance current target value r is a constant, so formula (15) is rewritten as:

[0048] (16) ;

[0049] In formula (15) and (16), The speed factor of the SC-PI controller is expressed as follows.

[0050] Preferably, the speed factor The gain setting rule of the speed factor

[0051] (17) ;

[0052] Wherein, The proportional coefficient is expressed as follows. The integral coefficient is expressed as follows. The proportional and integral two different properties of physical links are tightly coupled to form a cooperative control signal.

[0053] Preferably, the speed factor is set by introducing a speed factor setting model, which is expressed by the formula as follows:

[0054] (18) ;

[0055] Wherein, The adjustment coefficient is expressed as follows. ; The sampling period of the bidirectional DC-DC converter is expressed as follows.

[0056] Preferably, the feedforward controller is designed as follows:

[0057] u (19) ;

[0058] Wherein, The feedforward control coefficient is expressed as follows. The reference value of the current loop control is expressed as follows. The reference value of the DC bus voltage is expressed as follows. The actual value of the DC bus voltage is expressed as follows. The output of the SC-PI controller model is expressed as follows.

[0059] The technical effects of the present application are as follows:

[0060] This invention uses SC-PI to compensate the system, and couples the proportional-integral link by introducing a speed factor. At the same time, it introduces feedforward control, which enhances the robustness of the system while ensuring that the gains of SC-PI control have the same physical dimensions. This solves the problem of insufficient dynamic response capability in bidirectional DC-DC converters for energy storage based on traditional proportional-integral control.

[0061] Meanwhile, experimental verification shows that applying the FFSC-PI control strategy of this invention to a bidirectional DC-DC converter can not only maintain the stability of the DC bus voltage of the system, but also has better dynamic response capability compared with the FF-PI control strategy and SC-PI control strategy, effectively improving the system's anti-disturbance capability. Attached Figure Description

[0062] Figure 1 This is a schematic diagram of the photovoltaic-storage system structure;

[0063] Figure 2 This is a topology diagram of an energy storage bidirectional DC-DC converter.

[0064] Figure 3 The block diagram of the voltage outer loop control for FFSC-PI control;

[0065] Figure 4 This is a waveform diagram of the bus voltage under load disturbance.

[0066] Figure 5 The waveform of the DC bus voltage under photovoltaic output disturbance;

[0067] Figure 6 A schematic diagram comparing experimental results with the input-side voltage increase;

[0068] Figure 7 A schematic diagram comparing the experimental results of input-side voltage reduction;

[0069] Figure 8 Add a diagram showing the comparison of experimental results to the load;

[0070] Figure 9 A diagram showing the comparison of experimental results for reducing load. Detailed Implementation

[0071] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. This is to help those skilled in the art to have a more complete, accurate, and in-depth understanding of the inventive concept and technical solutions of the present invention, and to facilitate its implementation. It should be noted that the terms "first," "second," etc., used in this application are only for the convenience of describing the technical solutions and to distinguish components; the corresponding component configurations may be the same or different, and are not intended to limit the scope of this application. To make the technical solutions of the present invention clearer, the present invention will be explained and illustrated through the following embodiments.

[0072] In photovoltaic-storage systems, to ensure stable energy transfer between the battery and the DC bus and to improve the problems of DC bus voltage fluctuations and poor system immunity caused by uncertainties in photovoltaic power generation and frequent load disturbances, this embodiment provides a control method for a bidirectional DC-DC converter in a photovoltaic-storage system based on a small-signal mathematical model of the energy storage bidirectional DC-DC converter. The method includes the following steps:

[0073] Step S1: Construct a photovoltaic-storage system, including a photovoltaic power generation unit, a load, a power grid, a bidirectional DC-DC converter, and an energy storage unit. The energy storage unit is connected to a DC bus through the bidirectional DC-DC converter. The DC bus is also connected to the photovoltaic power generation unit, the load, and the power grid.

[0074] Step S2: Construct a mathematical model of the bidirectional DC-DC converter based on the bidirectional DC-DC converter topology.

[0075] Step S3: Construct a bidirectional DC-DC converter control system, including an outer voltage subsystem and an inner current subsystem. The outer voltage subsystem adopts feedforward autotransformer proportional-integral control, and the inner current subsystem adopts PI control.

[0076] Specifically, a typical photovoltaic-storage system structure is as follows: Figure 1 As shown, it includes a photovoltaic power generation unit, a load, a power grid, a bidirectional DC-DC converter, and an energy storage unit. Figure 1 The relationship between the variables is shown in equation (1):

[0077] (1);

[0078] in, Output power for the energy storage unit (battery); The power required by the power grid; For load power; This refers to the output power of photovoltaics.

[0079] Referring to step S2, a bidirectional DC-DC converter topology for energy storage is as follows:Figure 2 As shown, where I bat I represents the inductor current on the energy storage side. dc L is the output current; C is the energy storage side filter inductor; L is the output current; C is the energy storage side filter inductor; C is the output current; L ... d For the filter capacitor on the bus side; C b For the filter capacitor on the energy storage side; V bat It is the voltage on the energy storage side; V dc R is the DC bus voltage; dc This represents the equivalent load on the DC bus side. S1 and S2 represent a pair of complementary conducting IGBT switches; D1 and D2 represent anti-parallel diodes. The switching frequency of the energy storage bidirectional DC-DC converter is much higher than the rate of change in the microgrid state, allowing the power on the DC bus side to be considered constant for a short period, equivalent to a constant power load. The terminal voltage on the energy storage side remains stable for a short time and can be considered an ideal constant voltage source.

[0080] Energy storage bidirectional DC-DC converters exhibit high nonlinearity and discontinuity, making the establishment of accurate mathematical models challenging. Currently, widely used mathematical models for energy storage bidirectional DC-DC converters employ the state-space averaging method, neglecting inductance, capacitance, and equivalent resistance, and selecting the DC bus voltage V... dc and the inductor current I on the energy storage side bat As state variables, mathematical models of the circuit are established in Boost mode and Buck mode respectively.

[0081] First, establish the mathematical model for Boost mode, that is, when switches S1 and S2 are off, anti-parallel diode D2 is cut off, and diode D1 is on, the state-space equation of the circuit is obtained:

[0082] (2);

[0083] in, This represents the current in the inductor on the energy storage side; Indicates the voltage on the energy storage side; Indicates the DC bus voltage; L represents the energy storage side filter inductance; This refers to the filter capacitor on the bus side; This represents the equivalent load on the DC bus side;

[0084] When switch S2 is on, S1 is off, and diodes D1 and D2 are off, the state-space equation of the circuit is obtained as follows:

[0085] (3);

[0086] Applying the state-space averaging method to the energy storage bidirectional DC-DC converter, the state-space equation of the circuit in Boost mode is obtained:

[0087] (4);

[0088] in, This refers to the duty cycle in Boost mode.

[0089] Small-signal modeling and Laplace transform of the energy storage bidirectional DC-DC converter are performed to obtain the transfer function in Boost mode:

[0090] (5);

[0091] in, This represents the filter capacitor on the energy storage side; s is the complex frequency. To control the quantity; This is the transfer function of the energy storage side inductor current to the Boost control quantity; This is the transfer function of DC bus voltage to energy storage side inductor current in Boost mode.

[0092] The working state of Buck mode is similar to that of Boost mode. In Buck mode, the corresponding state-space average equation and transfer function are as follows:

[0093] (6);

[0094] in, This refers to the duty cycle in Buck operating mode.

[0095] (7);

[0096] in, The equivalent resistance of the energy storage side; This is the transfer function of the energy storage side inductor current to the Buck control quantity; This is the transfer function of DC bus voltage to energy storage side inductor current in Buck mode.

[0097] To address the insufficient dynamic response capability of bidirectional DC-DC converters for energy storage based on traditional proportional-integral (PI) control, this embodiment combines the SC-PI control strategy with feedforward control, designing an FFSC-PI (Feedforward Autocoupled Proportional-Integral) control strategy, which is applied to the bidirectional DC-DC converter for energy storage. The FFSC-PI control algorithm not only improves the dynamic response capability of the bidirectional DC-DC converter under complex operating conditions but also effectively solves the problems of physical dimension conflicts and parameter inconsistencies faced by traditional PI control, as well as the high computational load, structural complexity, and tuning difficulties required by other control algorithms.

[0098] Referring to step S3, the bidirectional DC-DC converter control system of this embodiment includes a voltage outer loop subsystem and a current inner loop subsystem. The voltage outer loop subsystem adopts feedforward autotransformer proportional-integral control to enhance the bus voltage's anti-disturbance performance; the current inner loop subsystem adopts PI control to further improve the system's response speed.

[0099] The feedforward autotransformer proportional-integral control in this embodiment consists of two parts: an SC-PI controller and a feedforward controller. Figure 3 As shown, the SC-PI controller is used to adjust the system output in real time to counteract external disturbances, thereby achieving accurate dynamic response and stability; the feedforward controller introduces a positive feedback channel into the input signal of the SC-PI controller, which then converts the reference value V of the DC bus voltage into a positive feedback path. dcref Compared with the actual value of the sampled DC bus voltage V dc The difference is calculated and passed through the feedforward control coefficient k. ffc To counteract external disturbances.

[0100] Specifically, the first step is to design the SC-PI controller, including establishing a controlled error energy storage bidirectional DC-DC converter system model, namely:

[0101] From equations (5) and (7), it is known that the characteristic equation of the bidirectional DC-DC converter system with voltage as the system input is missing a term, and the system is unstable in the open loop. Closed-loop control is required to stabilize the system. A controlled system of a bidirectional DC-DC converter is represented as follows:

[0102] (8);

[0103] in, and It is a state variable of the system; It is the system's voltage input; This is the system's current reference value output; It is a system function; It is an external disturbance;

[0104] The total disturbance d is defined as the sum of the system's uncertain dynamics and external disturbances, and its expression is:

[0105] (9);

[0106] in, It is an estimated value of the control gain, which is not equal to 0;

[0107] According to equations (8) and (9), the rewritten bidirectional DC-DC converter system for energy storage can be obtained as follows:

[0108] (10);

[0109] Let the target value of the inductor current in the outer loop of the controlled energy storage bidirectional converter be r, and define the tracking error as:

[0110] (11);

[0111] The integral error is defined as:

[0112] (12);

[0113] Combining equations (10) and (11), we get:

[0114] (13);

[0115] Combining equations (11) to (13), the controlled error energy storage bidirectional DC-DC converter system model is as follows:

[0116] (14).

[0117] Next, based on the controlled error energy storage bidirectional DC-DC converter system model, the SC-PI controller model can be obtained as follows:

[0118] (15);

[0119] The target value of the inductor current r is a constant, therefore equation (15) can be rewritten as:

[0120] (16);

[0121] In equations (15) and (16), This represents the speed factor of the SC-PI controller.

[0122] According to the traditional proportional-integral control and the speed factor described in equation (14) The gain tuning rule is as follows:

[0123] (17);

[0124] in, Indicates the proportionality coefficient; Indicates the integral coefficient; By tightly coupling two physical components with different properties, proportional and integral, a coordinated control signal is formed. When the velocity factor... When the value is greater than 0, the closed-loop control system composed of SC-PI controllers is not only stable over a wide range, but also has good robustness against disturbances.

[0125] As the speed factor increases, the anti-interference capability also becomes stronger. However, if the speed factor is too large, the integral gain weight will be too large, leading to overshoot and oscillation during the dynamic response of the bus voltage. To solve this problem, a speed factor tuning model is introduced to adjust the speed factor. The tuning is performed, expressed by the following formula:

[0126] (18);

[0127] in, This represents the adjustment coefficient. ; This is the sampling period for the bidirectional DC-DC converter. Because the energy storage bidirectional DC-DC converter is a fast system... The larger the value, the faster the response speed and the stronger the ability to resist disturbances.

[0128] The feedforward controller responds immediately upon the occurrence of an disturbance, without waiting for the system to develop a deviation. Therefore, feedforward control is more timely and effective than traditional feedback control. Since the feedforward controller does not depend on the system's output error, it can compensate for disturbances as soon as they occur, effectively avoiding the effects of system lag. The expression for the feedforward controller is as follows:

[0129] u (19);

[0130] in, These are the feedforward control coefficients; This is the reference value for current loop control; This is a reference value for the DC bus voltage; is the actual value of the DC bus voltage; u represents the output of the SC-PI controller model.

[0131] To verify the effectiveness of the proposed strategy, this invention constructed the following in the MATLAB / Simulink environment: Figure 1 The photovoltaic-storage system is shown, and experiments simulating photovoltaic power output disturbances and load surges are conducted. To verify the effectiveness and feasibility of the proposed FFSC-PI control strategy, this invention compares and analyzes the DC bus voltage waveforms under the dual-closed-loop FF-PI control strategy, SC-PI control strategy, and FF-SCPI control strategy. The parameters of the three control strategies have been optimized, and the simulation parameters of the system are shown in Table 1.

[0132]

[0133] Table 1

[0134] Under experimental conditions where the illuminance was maintained at 500 W / m², the load power experienced a disturbance. At t=4 seconds, the load power increased from 4 kW to 8 kW, increasing the demand for DC bus voltage support and forcing the battery to release energy to maintain power balance. The bidirectional DC-DC converter switched to Boost mode, effectively transferring the energy stored in the battery to the DC bus, thereby maintaining the bus voltage. At t=6 seconds, the load power decreased from 8 kW to 4 kW, leading to an increase in excess power on the DC bus and consequently causing the bus voltage to rise. The bidirectional DC-DC converter switched to Buck mode, transferring excess energy to the battery to suppress and stabilize the DC bus voltage. Under load disturbance conditions, the DC bus voltage waveform is as follows: Figure 4 As shown in Table 2, the overshoot of the bus voltage is shown in Table 3.

[0135]

[0136] Table 2

[0137]

[0138] Table 3

[0139] When the load power suddenly increases, the output voltage waveform is as follows: Figure 4 As shown in (b). With the FFSC-PI control strategy, the output voltage overshoot is 0.8%, the response time is 0.05 seconds, and the steady-state error is 0.12V. With the SC-PI control strategy, the output voltage overshoot is 3.8%, the response time is 0.15 seconds, and the steady-state error is 0.15V. With the FF-PI control strategy, the output voltage overshoot is 3.9%, the response time is 0.65 seconds, and the steady-state error is 0.25V. Compared to the SC-PI control strategy, the FFSC-PI control strategy reduces the overshoot by 3.0% and shortens the settling time by 0.1 seconds. Compared to the FF-PI control strategy, the FFSC-PI control strategy reduces the overshoot by 3.1% and shortens the settling time by 0.6 seconds. The output voltage waveform during a sudden load reduction is as follows: Figure 4As shown in (c). With the FFSC-PI control strategy, the output voltage overshoot is 0.7%, the response time is 0.04 seconds, and the steady-state error is 0.32V. With the SC-PI control strategy, the output voltage overshoot is 3.8%, the response time is 0.17 seconds, and the steady-state error is 0.51V. With the FF-PI control strategy, the output voltage overshoot is 4.3%, the response time is 0.7 seconds, and the steady-state error is 0.61V. Compared to the SC-PI control strategy, the FFSC-PI control strategy reduces the overshoot by 3.1% and shortens the settling time by 0.13 seconds. Compared to the FF-PI control strategy, the FFSC-PI control strategy reduces the overshoot by 3.1% and shortens the settling time by 0.66 seconds.

[0140] Experimental results show that, under load disturbance conditions, the FF-SCPI control strategy effectively reduces the bus voltage change and shortens the response time compared to the SC-PI and FF-PI control strategies.

[0141] Under constant load power, the photovoltaic output changes. At t=4 seconds, the irradiance increases from 500W / m² to 800W / m², the photovoltaic system output power increases, and the bidirectional DC-DC converter switches to Buck mode. At t=7 seconds, the irradiance decreases from 800W / m² to 500W / m², the photovoltaic output power decreases, and the DC bus voltage cannot meet the demand, failing to compensate for the power gap caused by the decrease in photovoltaic power. The system needs to introduce an additional energy source to maintain dynamic power balance. Under this condition, the battery will compensate for the power gap caused by the reduction in photovoltaic power, and the bidirectional DC-DC converter switches to Boost mode, causing the battery to enter the discharge state, thereby transferring the stored energy to the DC bus. The DC bus voltage waveform under photovoltaic output disturbance is shown in Figure 5, the overshoot of the bus voltage is shown in Table 4, and the settling time of the bus voltage is shown in Table 5.

[0142]

[0143] Table 4

[0144]

[0145] Table 5

[0146] When the photovoltaic output power suddenly increases, the output voltage waveform is as follows: Figure 5As shown in (b). When using the FFSC-PI control strategy, the output voltage overshoot is 0.6%, the response time is 0.04 seconds, and the steady-state error is 0.44V. When using the SC-PI control strategy, the output voltage overshoot is 3.3%, the response time is 0.16 seconds, and the steady-state error is 0.48V. When using the FFPI control strategy, the output voltage overshoot is 3.8%, the response time is 0.63 seconds, and the steady-state error is 0.51V. Compared to the SC-PI control strategy, the FFSC-PI control strategy reduces the overshoot by 2.7% and shortens the settling time by 0.12 seconds. Compared to the FF-PI control strategy, the FFSCPI control strategy reduces the overshoot by 3.2% and shortens the settling time by 0.59 seconds. When the photovoltaic output power suddenly decreases, the output voltage waveform is as follows: Figure 5 As shown in (c). With the FFSC-PI control strategy, the output voltage overshoot is 0.7%, the response time is 0.06 seconds, and the steady-state error is 0.58V. With the SC-PI control strategy, the output voltage overshoot is 3.2%, the response time is 0.15 seconds, and the steady-state error is 0.68V. With the FF-PI control strategy, the output voltage overshoot is 3.9%, the response time is 0.51 seconds, and the steady-state error is 0.82V. Compared to the SC-PI control strategy, the FFSC-PI control strategy reduces the overshoot by 2.5% and shortens the settling time by 0.09 seconds. Compared to the FF-PI control strategy, the FFSC-PI control strategy reduces the overshoot by 3.2% and shortens the settling time by 0.55 seconds.

[0147] Experimental results show that under photovoltaic output disturbance conditions, the SC-PI control strategy is superior to the FF-PI control strategy. Compared with the SC-PI control strategy, the FFSC-PI control strategy has smaller voltage change and faster response time.

[0148] Experimental results on load disturbance and photovoltaic output disturbance show that the DC bus voltage stabilizes at 400V after fluctuations using the FFSC-PI control strategy, SC-PI control strategy, and FF-PI control strategy. Compared with the FF-PI control strategy and SC-PI control strategy, the FFSC-PI control strategy reduces the fluctuation amplitude of the DC bus voltage and shortens the response time.

[0149] To further verify the effectiveness of the FFSC-PI control strategy, comparative experiments were conducted with the FF-PI control strategy and the SC-PI control strategy, respectively. Figure 6 , Figure 7 , Figure 8 and Figure 9 The four operating conditions are: sudden increase / decrease in bus voltage, and sudden increase / decrease in load. The output voltage of the energy storage bidirectional DC-DC converter is used to evaluate its disturbance rejection capability by the maximum voltage change and the settling time.

[0150] When the input voltage suddenly increases, the output voltage waveform is as follows: Figure 6 As shown. When using the FFSC-PI control strategy, the maximum voltage change is 0.65V, the settling time is 0.015 seconds, and the steady-state error is 0.03V. When using the SC-PI control strategy, the maximum voltage change is 0.82V, the settling time is 0.019 seconds, and the steady-state error is 0.05V. When using the FF-PI control strategy, the maximum voltage change is 1.16V, the settling time is 0.063 seconds, and the steady-state error is 0.07V. Compared to the SC-PI control strategy, the FFSC-PI control strategy reduces the maximum voltage change by 0.48% and shortens the settling time by 0.04 seconds. Compared to the FF-PI control strategy, the FFSC-PI control strategy reduces the overshoot by 1.5% and shortens the settling time by 0.048 seconds.

[0151] When the input voltage decreases, the output voltage waveform is as follows: Figure 7 As shown. When using the FFSC-PI control strategy, the maximum voltage change is 0.56V, and the settling time is 0.020 seconds. When using the SC-PI control strategy, the maximum voltage change is 0.71V, and the settling time is 0.026 seconds. When using the PI control strategy, the maximum voltage change is 1.23V, and the settling time is 0.058 seconds. Compared to SC-PI control, FFSC-PI control reduces the maximum voltage change by 0.43% and shortens the settling time by 0.06 seconds. Compared to traditional PI control, FFSC-PI control reduces the overshoot by 1.9% and shortens the settling time by 0.038 seconds.

[0152] When the load increases, the output voltage waveform is as follows: Figure 8 As shown. When using the FFSC-PI control strategy, the maximum voltage change is 0.81V, and the settling time is 0.028 seconds. When using the SC-PI control strategy, the maximum voltage change is 1.51V, and the settling time is 0.043 seconds. When using the PI control strategy, the maximum voltage change is 1.95V, and the settling time is 0.1 seconds. Compared to SC-PI control, FFSC-PI control reduces the maximum voltage change by 2% and shortens the settling time by 0.015 seconds. Compared to traditional PI control, FFSC-PI control reduces the overshoot by 3.3% and shortens the settling time by 0.072 seconds.

[0153] When the load decreases, the output voltage waveform is as follows: Figure 9As shown. When using the FFSC-PI control strategy, the maximum voltage change is 0.61V, and the settling time is 0.054 seconds. When using the SC-PI control strategy, the maximum voltage change is 0.91V, and the settling time is 0.069 seconds. When using the PI control strategy, the maximum voltage change is 1.31V, and the settling time is 0.12 seconds. Compared to SC-PI control, FFSC-PI control reduces the maximum voltage change by 0.86% and shortens the settling time by 0.015 seconds. Compared to traditional PI control, FFSC-PI control reduces the overshoot by 2.2% and shortens the settling time by 0.066 seconds.

[0154] Experimental results show that, compared with SC-PI control and traditional PI control, FFSC-PI control can effectively limit the fluctuation range of output voltage and shorten the adjustment time under four operating conditions: increase and decrease of bus voltage, and increase and decrease of load.

[0155] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution; or the direct application of the inventive concept and technical solution to other situations without modification, are all within the protection scope of the present invention.

Claims

1. A control method of a bidirectional DC-DC converter for a photovoltaic energy storage system based on feedforward self-coupled proportional-integral control, characterized in that: The method comprises the following steps: Step S1, constructing a light storage system, comprising a photovoltaic power generation unit, a load, a power grid, a bidirectional DC-DC converter and an energy storage unit, the energy storage unit being connected to a DC bus through the bidirectional DC-DC converter, and the DC bus being connected to the photovoltaic power generation unit, the load and the power grid respectively; Step S2, constructing a bidirectional DC-DC converter mathematical model according to a bidirectional DC-DC converter topology circuit; Step S3, constructing a bidirectional DC-DC converter control system, comprising a voltage outer loop subsystem and a current inner loop subsystem, wherein the voltage outer loop subsystem adopts feedforward self-coupling proportional integral control, and the current inner loop subsystem adopts PI control.

2. The control method of bidirectional DC-DC converter for optical storage system based on feedforward self-coupled proportional-integral control according to claim 1, characterized in that: In the step S2, the bidirectional DC-DC converter mathematical model comprises a mathematical model in a Boost mode, that is, when the switching tubes S1 and S2 are turned off, the anti-parallel diode D2 is turned off, and the diode D1 is turned on, the state space equation of the circuit is obtained: （2）； wherein, represents the current of the energy storage side inductance; represents the voltage of the energy storage side; represents the DC bus voltage; L represents the energy storage side filter inductance; represents the bus side filter capacitance; represents the equivalent load on the DC bus side; When the switching tube S2 is turned on, the switching tube S1 is turned off, and the diodes D1 and D2 are turned off, the state space equation of the circuit is obtained: （3）； The state space average method is applied to the energy storage bidirectional DC-DC converter to obtain the state space equation of the circuit in the Boost mode: （4）； wherein, Duty cycle in Boost mode of operation; Small signal modeling and Laplace transformation are performed on the energy storage bidirectional DC-DC converter to obtain the transfer function in the Boost mode: （5）； wherein, represents the filter capacitor on the energy storage side; s is the complex frequency; is the control quantity; is the transfer function of the energy storage side inductor current to the Boost control quantity; is the transfer function of the DC bus voltage to the energy storage side inductor current in the Boost mode.

3. The control method of bidirectional DC-DC converter for optical storage system based on feedforward self-coupled proportional-integral control according to claim 2, characterized in that: In the step S2, the bidirectional DC-DC converter mathematical model further comprises a mathematical model in a Buck mode, and the corresponding state space average equation and transfer function are as follows: （6）； wherein, Duty cycle in Buck mode of operation; （7）； wherein, is the equivalent resistance on the energy storage side; is the transfer function of the energy storage side inductor current to the Buck control variable; is the transfer function of the DC bus voltage to the energy storage side inductor current in Buck mode.

4. The control method of bidirectional DC-DC converter for optical storage system based on feedforward self-coupled proportional-integral control according to claim 1, characterized in that: In the step S3, the feedforward self-coupling proportional integral control consists of two parts, SC-PI controller and feedforward controller, the SC-PI controller is used to adjust the system output in real time to offset external disturbance; the feedforward controller introduces a positive feedback channel to the input signal of the SC-PI controller, and the reference value V dcref of the DC bus voltage is subtracted from the actual value V dc of the sampled DC bus voltage, and the difference is offset by the feedforward control coefficient k ffc to offset external disturbance.

5. The control method of bidirectional DC-DC converter for optical storage system based on feedforward self-coupled proportional-integral control according to claim 4, characterized in that: The design of the SC-PI controller comprises establishing a controlled error energy storage bidirectional DC-DC converter system model, that is: An energy storage bidirectional DC-DC converter controlled system is represented as follows: （8）； wherein, and are state quantities of the system; is a voltage input of the system; is a current reference value output of the system; is a system function; is an external disturbance; The total disturbance d is defined as the sum of the system uncertain dynamic and external disturbance, and the expression is as follows: （9）； wherein is an estimate of the control gain, which is not equal to 0; According to the formula (8) and the formula (9), the energy storage bidirectional DC-DC converter system after rewriting is as follows: （10）； The inductance current target value r of the controlled energy storage bidirectional converter voltage outer loop is defined as the tracking error, that is: （11）； The integral error is defined as: （12）； Combined with the formula (10) and the formula (11), the following is obtained: （13）； The controlled error energy storage bidirectional DC-DC converter system model is obtained by combining the formula (11) to (13), that is: （14）。 6. The control method of bidirectional DC-DC converter for optical storage system based on feedforward self-coupled proportional-integral control according to claim 5, characterized in that: The design of the SC-PI controller comprises establishing an SC-PI controller model according to the controlled error energy storage bidirectional DC-DC converter system model, and the SC-PI controller model is represented as follows: （15）； The inductance current target value r is a constant, so the formula (15) is rewritten as: （16）； In formulas (15) and (16), denotes the speed factor of the SC-PI controller.

7. The control method of bidirectional DC-DC converter for optical storage system based on feedforward self-coupled proportional-integral control according to claim 6, characterized in that: The speed factor The gain setting rule is: （17）； wherein, represents a proportional coefficient; represents an integral coefficient; The proportional and integral properties are tightly coupled to form a synergistic control signal.

8. The control method of bidirectional DC-DC converter for optical storage system based on feedforward self-coupled proportional-integral control according to claim 7, characterized in that: The speed factor setting model is introduced to set the speed factor and is expressed by the following formula: （18）； wherein, represents a regulation coefficient, ; is the sampling period of the bidirectional DC-DC converter.

9. The control method of bidirectional DC-DC converter for optical storage system based on feedforward self-coupled proportional-integral control according to claim 8, characterized in that: The feedforward controller is designed as follows: u （19）； wherein, is a feedforward control coefficient; is a reference value for current loop control; is a reference value for DC bus voltage; is an actual value of DC bus voltage; u denotes the SC-PI controller model output.

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